Next Article in Journal
The Influence of Acetone on the Kinetics of Water Electrolysis Examined at Polycrystalline Pt Electrode in Alkaline Solution
Previous Article in Journal
Garcinia mangostana L. Leaf-Extract-Assisted Green Synthesis of CuO, ZnO and CuO-ZnO Nanomaterials for the Photocatalytic Degradation of Palm Oil Mill Effluent (POME)
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 14
Issue 8
10.3390/catal14080487
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Molecular TiO2 Modifications of Supported PPh3-Capped Pd Nanocatalysts for CO2 Hydrogenation into Formates

by
María Dolores Fernández-Martínez
and
Cyril Godard
*
Department of Physical and Inorganic Chemistry, Universitat Rovira I Virgili, C/Marcel·li Domingo s/n, 43007 Tarragona, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(8), 487; https://doi.org/10.3390/catal14080487
Submission received: 20 June 2024 / Revised: 17 July 2024 / Accepted: 26 July 2024 / Published: 29 July 2024
(This article belongs to the Special Issue Ligand-Decorated Metal Nanoparticles for Catalysis)
Browse Figures
Versions Notes

Abstract

:
TiO2-supported Pd NPs-based materials were prepared following two distinct approaches: For the first set of materials 1–8, modification of the TiO2 support was performed prior to Pd NPs deposition, while the second set (9–15) was synthesized by deposition of modifiers over pre-synthesized Pd-PPh3/TiO2. These catalysts were applied in the hydrogenation of CO2 to formate, and their performance was compared with that of the unmodified Pd-PPh3/TiO2. Modification of the TiO2 support by organosilanes provided a beneficial effect in catalysis compared with the catalyst containing unmodified TiO2 or TiO2 modified by organophosphonic acids. In contrast, in most cases, the deposition of modifiers over previously synthesized Pd NPs supported on TiO2 was not beneficial to the activity of the catalyst. Interestingly, upon recycling, the first set of catalysts suffered a rapid decrease in activity, while the anchoring of modifiers over previously formed Pd NPs showed an improved stability (TON > 500 after the third recycling).

Graphical Abstract

1. Introduction

The high concentration of CO2 in the atmosphere is nowadays regarded as a critical issue and has generated a strong interest in research for the development of non-fossil-based feedstocks. As such, in spite of its inertness, CO2 is viewed as a very attractive C1 source to produce fuels and chemicals [1,2,3,4]. Among the potential transformations of CO2, its hydrogenation into formic acid and derivatives has become a priority in view of the recent progress made in the production of H2 via water splitting [5]. Indeed, formic acid offers high hydrogen storage capacity and stability and provides a methodology for CO2 storage in a chemically stable form, closing a hydrogen energy cycle [6,7,8].
Among the heterogeneous catalysts reported for the hydrogenation of CO2 into formates, Pd was described as one of the most active metals [7,9,10,11,12], and the combination of Pd with other metals such as Au [8,13], Ag [14,15,16], and Ni was also described as efficient for this transformation [17]. The selection of the support is also significant in this process, and for instance, Pd catalysts on carbon-based supports such as reduced graphene oxide (rGO), N-doped carbon (N-C), or mesoporous graphitic carbon nitride (g-C3N4) provided excellent results for the hydrogenation of bicarbonates to formates [13,18]. In 2018, Mori et al. reported highly efficient CO2 hydrogenation using Pd-based catalysts supported onto TiO2 [16].
Heterogeneous catalysts have become a crucial part of many industrial activities, such as organic synthesis, oil refining, and pollution control [19,20,21,22,23,24]. Modern heterogeneous catalysts consist of several elements in precise proportions [21] and are optimized to obtain the greatest reaction rate, which in turn results in optimal selectivity [20,21,22]. The heterogeneous catalyst performance can be improved by modifying the support using approaches such as nanotechnology and nanoscience or controlling the pore structure [23,24,25]. However, the support must retain its specific properties, such as porosity, surface area, dispersion, selectivity, and activity [26,27,28,29,30].
In the design of new catalysts, the appropriate metal–support interactions (MSI) must be achieved, and their tuning can be completed through adjustments of either the composition and/or morphology of the support and active phase or through the modifications of their surface (Scheme 1) [31,32,33,34]. However, changes in composition and morphology can affect their nature [35,36]. Surface modifications can enhance steric control or provide hydrophilic/hydrophobic characteristics that can be suitable for the target substrates and catalysis media [37]. In this area, the use of organic self-assembled monolayers (SAMs) is of particular interest since these organic modifiers can act as spacers between NPs to minimize sintering and improve the stability/recyclability of the catalyst [38,39]. Moreover, depending on the functional groups in these modifiers, the catalyst activity can be enhanced through interactions with the substrate [40,41,42] or by conferring a hydrophobic/hydrophilic character to the system to favor catalyst–substrate interactions [43].
Modification of carbon- and silica-supported catalysts with organic molecules were previously reported to cover metallic nanoparticles with an alkylic chain [44,45] or with groups containing amine functionalities [14,46] to produce catalytic materials for CO2 hydrogenation into formate. Organosilanes are among the most used molecules to modify supports, and they constitute a type of inorganic/organic hybrid materials [47]. Aminoalkylsilanes such as (3-aminopropyl)triethoxysilane (APTES) are the most commonly reported organosilanes [48]. Modification occurs through silanization, which is a process that covers a surface such as metal oxide with chloro or alkoxysilane [49]. Using APTES, this process starts with the hydrolysis of the ethoxy groups that are catalyzed by water, leading to the formation of silanols that condense with the surface hydroxyls to form a monolayer [50]. The most accepted chemisorption of APTES onto TiO2 implies one or two Si−O−Ti bonds [48,51,52]. Moreover, APTES can also be applied directly over metallic nanoparticles for their stabilization and functionalization [53,54,55].
Liu et al. reported that APTES directly participates in the synthesis of a protonated Schiff base that covers Au NPs during the dehydrogenation of FA into CO2 and H2 [56]. Later, the same authors developed a new Au-based catalyst for the hydrogenation of CO2 into formic acid in which the SiO2 support was modified by APTES and a Schiff base, reaching a TON of 14,470 over 12 h at 90 °C [57]. The same year, Mori et al. reported PdAg NPs supported over amine-functionalized mesoporous silica for reversible CO2 hydrogenation and release of H2 [58]. DFT calculations revealed that the presence of amine affects the O-H dissociation of FA and favors the adsorption of CO2 in hydrogenation. Additionally, the catalyst could be recovered and reused for three runs without loss of activity.
Srivastava reported the preparation of Ru NPs supported over various amine organosilane-modified SBA-15 mesoporous silica and their application as a catalyst in the hydrogenation of CO2 into formic acid [59]. Primary, secondary, and tertiary amines were tested, and the use of the primary amine provided the highest catalytic activity.
Ionic liquids (ILs) constitute another type of interesting molecules for support modification [60,61], and in catalytic reactions involving CO2, it was reported that the interactions between IL and CO2 usually depend on the anion. Indeed, basic anions such as acetate in combination with 1,3-dialkylimidazolium enhanced such interactions [62], and this type of ILs was used in the hydrogenation of CO2 to formic acid [63,64]. In recent years, Leitner and co-workers reported the preparation of metallic nanoparticles in ionic liquids covalently grafted onto SiO2 by silanization [65,66,67,68,69,70,71,72,73,74]. They recently reported a Ru-based catalyst supported over an imidazolium-based supported ionic liquid phase (SILP) for the hydrogenation of CO2 into formate in the presence of NEt3 [75]. The authors first covalently modified the SiO2 surface with ILs via silanization, while the synthesis of the Ru NPs was performed in a second step using the organometallic approach [76]. Modifications of the alkyl chain and anion proved very important to modulate the NPs properties and resulted in an increase in the TON by 2- or 10-fold when compared with unmodified Ru/SiO2 catalyst. Using H/D exchange experiments, the authors determined that the modification favored the desorption of formate from the catalyst surface. However, in recycling experiments, a significant loss of activity was observed, which was attributed to the leaching of IL from the support due to the use of NEt3 + H2O as the solvent system. Other authors reported Pd NPs supported over poly(ionic liquid)s (PILs) for the hydrogenation of CO2 into formic acid [77,78]. Using this catalytic system, a TOF of 1190 h−1 was obtained in the presence of NEt3 aqueous solution, and good recyclability was observed during several runs [77].
Organophosphonic acids (PAs) constitute another common type of molecules used for the modification of support surfaces. These RPO(OH)2 compounds were especially used to modify metal oxide surfaces [79]. However, when PAs are used for surface modification, an additional annealing or aging treatment is necessary to produce condensation reactions and form strong bonds between PAs and metal oxide [39].
In a previous report from our group, ligand-capped nanocatalysts were prepared over supports of different natures (metal oxides and carbon based) and tested in Pd-catalyzed hydrogenation of CO2 to formate. The presence of stabilizing ligands proved crucial to obtain small and well-defined Pd NP. In this study, the best performance was achieved using TiO2-based catalysts [80]. However, upon recycling, a rapid decrease of activity was observed.
Here, modifications of the TiO2 support/PPh3-capped catalyst are described using Si- and P-based molecular modifiers for the first time. These new materials were characterized and tested in the catalytic hydrogenation of CO2 into formate. The effect of these modifications on the activity and recyclability of these catalysts was particularly investigated.

2. Results and Discussion

The modified catalysts were prepared according to two distinct approaches using triethoxysilanes (TESs) and organophosphonic acids (PAs) as modifiers (Figure 1).
In the first approach, the TiO2 support was modified prior to Pd NP deposition, whereas in the second approach, the Pd NPs were initially supported onto TiO2 [80], and the modifiers were reacted subsequently (Scheme 2).
The grafting of the RSi(OEt)3 modifiers onto TiO2 was carried out following the literature procedures (Figure 1) [52,81]. In a typical synthesis, the reaction was performed in an EtOH:H2O solution or absolute EtOH. The mixture was then centrifuged, and the solid was washed several times with H2O and EtOH and dried under a vacuum for several hours.
The optimization of synthesis parameters such as concentration of APTES, APTES/TiO2 ratio, temperature, and time was initially performed (Table S1). Organophosphonic acids (PAs) containing a butyl chain or a 3-propylamine group were also used as modifiers (Figure 1). The preparations were carried out using a modifier concentration of 0.01 M at room temperature. For these modifiers, an additional annealing or aging treatment of the material at 120 °C was performed.
To confirm the successful modification of TiO2, the samples were first analyzed by FT-IR analysis (Figures S2, S5 and S7), which corroborated that the condensation between surface TiO2 hydroxyl groups and silanol groups had taken place via the detection of CH2 and C-N stretching bands.
Quantification of the anchoring of the modifiers was carried out via TGA analysis through analysis of the weight losses observed in every case (Figures S3, S6, S8 and Table S2). Apart from the low-temperature (<200 °C) weight loss associated with the removal of water, a new weight loss between 200 and 450 °C was detected for the modified TiO2 samples and was attributed to the loss of anchored modifiers. Variations between 1 and 3.5 wt% were measured depending on the nature and concentration of modifier used.
In view of these results, it was concluded that a series of functionalized TiO2 supports was successfully prepared using organosilanes and organophosphonic acids as modifiers. FT-IR analysis confirmed the presence of these modifiers at the surface of the TiO2 supports (Figures S2, S5 and S7), and TGA (Figures S3, S6, S8 and Table S2) provided quantitative information about these systems.

2.1. Synthesis and Characterization of Supported PPh3-Capped Pd NPs over Modified TiO2

The synthesis of Pd NPs was carried out over the modified TiO2 supports in the presence of PPh3 as the stabilizing ligand (Scheme 3). The materials were prepared by decomposition of Pd(dba)2 under H2 pressure at room temperature using THF as the solvent. A nominal Pd loading of 4 wt% was targeted.
The newly prepared systems were characterized by TEM, HR-TEM, ICP, XPS, FT-IR, and TGA.
Samples were analyzed by TEM to obtain information about the size and morphology of the nanoparticles. TEM images of the newly synthesized materials are displayed in Figure 2. In all cases, small and crystalline Pd NPs were formed. The sizes and distributions obtained from TEM measurements and ICP results are summarized in Table S3. In all cases, Pd loadings between 2.7 and 3.5 wt% were measured.
For catalyst 1 bearing PTES as the TiO2 modifier (Figure 2a), the resulting NPs exhibited a size of 5.75 nm with a broad distribution. Some agglomerations of Pd NPs were also observed. When TiO2 modified with APTES was employed as support (Figure 2b–d), the Pd NPs sizes ranged from 2.04 nm to 2.52 nm, while the Pd content was between 2.75% and 3.36 wt%. The material 2 was also analyzed by HR-TEM (Figure S10) to confirm the size of the Pd NPs and obtain information on their crystallinity. EDS mapping (Figure S10c) provided information about the composition of the catalyst active phase and revealed the presence of Si at the surface of TiO2, which confirmed the presence of APTES. The catalyst 3b (Figure 2d), synthesized in the absence of a ligand, was also analyzed by HR-TEM (Figure S11). For this catalyst, the same edge of Si is appreciated (Figure S11c). The difference in Pd NP size observed for the PTES-modified support and those modified with APTES clearly indicated the role of the amine function in the NP stabilization. The largest NPs were obtained when the synthesis was performed in the absence of PPh3, indicating that this ligand also played a role in the NP stabilization, although to a much lesser extent than the amine group from the support.
When the support was functionalized by organosilanes containing an IL moiety, particle sizes between ca. 1.9 and 2.4 nm were measured (Figure 2e–h). The catalyst 4 containing a support modified by IL was also analyzed by HR-TEM and EDS (Figure S12). Due to the low concentration of IL-Cl employed for synthesis of this catalyst, the layer of Si around TiO2 was narrow. The presence of P was also observed, indicating that the ligand remained on the NPs. Interestingly, for the materials 5 and 6a, which contain TiO2 modified by the same organosilane but differ by the loading of organosilane at the support surface, only a slight difference in size was observed (2.43 vs. 1.99 nm). Moreover, when the synthesis of 6a was repeated in the absence of PPh3 6b (Entry 8), similar size and Pd loading were observed, indicating that the ligand was not playing an important role in the Pd NP stabilization when the IL-containing modifiers were used.
When the PA containing an alkylic chain was used for the modification of the support (Figure 2i), large Pd agglomerations were observed, and nanoparticles were detected out of the support. This indicates that using this modified support, the NP stabilization was not efficient. When the PA containing an amine group was used as the support modifier (Figure 2j), the sizes of the resulting Pd NPs were 3.41 nm, while similar Pd contents were obtained by ICP (ca. 3.3 wt%) (Table S3). Both samples revealed Pd NPs with broad distributions. The material 8, containing TiO2 modified by PAs, was analyzed by HR-TEM (Figure S13). Small agglomerations were observed by HR-HAADF STEM. The presence of phosphorus was detected over the support and over the Pd NPs due to the presence of NH2-PA and PPh3.
These results therefore indicated that when organosilanes (TESs) were used to modify TiO2, the NP stabilization was efficient, resulting in smaller Pd NPs with narrower distributions than for PA-modified supports. However, in terms of Pd loading, no relevant differences were observed. The presence of functional groups in the TESs modifiers also affected the NP stabilization since the materials containing an organosilane with a simple alkyl chain provided larger Pd NPs with broader distribution than with TESs containing either an amine function or an imidazolium group.
Moreover, when the synthesis was performed using the support with a higher concentration of APTES, smaller nanoparticles were obtained. The same effect was observed when IL-containing modifiers were used.
XPS analysis was also performed for some of the Pd-PPh3/mod@TiO2 1–8 catalysts (Table S4 and Figures S14–S16) to obtain the surface composition data, chemical state, and electronic state of different elements present in these catalysts. The data obtained were compared with those of the unmodified Pd-PPh3/TiO2 catalyst. All the spectra were referenced using the C1s signal and set at 285.0 eV. The presence of Si2p, N1s, and P2p for 4 catalyst and N1s and P2p for 8 was detected. For Pd3d, no significant differences were observed with the unmodified Pd-PPh3/TiO2 catalyst (335.0 eV). For all the samples, the relative amount of Pdδ+ also revealed similar (ca. 10%).
To conclude, the synthesis and characterization of the series of catalysts 1–8 based on PPh3-capped Pd NPs were carried out using modified supports. When organosilanes were employed to modify the support, smaller Pd NPs were obtained than when the supports were modified with organophosphonic acids. The presence of the -NH2 group in organosilanes and organophosphonic acids influences the structure/composition of the materials since in the absence of the -NH2 group, large NPs as well as unsupported NPs and agglomerations were observed.

2.2. Deposition of Modifier over TiO2-Supported PPh3-Capped Pd NPs

A new series of catalysts was synthetized by deposition of TESs and PAs modifiers over the previously synthesized Pd-PPh3/TiO2 material (Scheme 4). The newly prepared systems were characterized by TEM and ICP, and FT-IR, TGA, HR-TEM, and XPS analyses were also performed for representative examples.
PTES or APTES was reacted with the previously synthesized Pd-PPh3/TiO2 either at r.t. overnight in a mixture of 95% EtOH, 5% H2O, or at 120 °C during 4 h using EtOH as solvent. At the end of the reaction, the samples were centrifugated, washed with milli-Q H2O and EtOH, and dried overnight at 100 °C.
The samples were initially analyzed by TEM (Figure S17) and ICP, and the data are summarized in Table 1.
As described previously, the Pd NPs in the Pd-PPh3/TiO2 catalyst exhibited a mean size of 2.37 ± 0.19 nm [80]. When the catalyst was modified by deposition of PTES (Entry 1), the size of Pd NPs slightly increased to 2.65 ± 0.80 nm, and a Pd content of 3.38 wt% was measured by ICP. When APTES was used as the modifier, the mean size of the NPs varied from ca. 1.9 (Entry 3) to 2.7 nm (Entry 2), indicating little effect of the treatment on the Pd NPs. However, relevant decreases in Pd content were observed, indicating that the deposition of modifiers containing an amine group could induce the leaching of Pd from the previously synthesized material. The presence of P (from PPh3) and Si (from the organosilane) was confirmed by ICP.
When modifiers containing imidazolium groups in the alkyl chain were tested, the NPs mean diameters were between ca. 1.9 (Entry 5) and 2.4 nm (Entry 4), with distributions narrower than 1 nm. However, the Pd content was lower than expected. Surprisingly, when PAs were deposited over Pd-PPh3/TiO2, larger NP sizes were observed, as mean diameters of 4.45 ± 1.68 nm (Entry 6) and 3.00 ± 1.14 nm (Entry 7) were measured when PA and APA were employed, respectively. This therefore indicated that the deposition of PAs over Pd-PPh3/TiO2 caused a restructuration of the Pd NPs. In contrast, for these catalysts, the Pd content was almost the same as that in the original material, thus suggesting that the reaction of the previously synthesized material with PAs does not induce Pd leaching but could produce some restructuration, such as Ostwald’s ripening.
Comparing the NP sizes obtained via the two approaches described in this work, modification over the pre-synthesized Pd-PPh3/TiO2 NPs (Table S3) yielded smaller Pd NPs and narrower distributions than when the TiO2 support was modified in the first step (Table 1). However, lower Pd contents were obtained.
FT-IR and TGA analyses were also performed on 11 (Figure S18). Comparing the FT-IR spectra of this sample with that of TiO2-3 (Figure S18a) (same concentrations and conditions), the same IR signals were detected. However, the TGA analysis revealed a greater amount of organic material at the surface of TiO2-3 (Figure S18b).
From HR-TEM analysis of 11 (Figure 3 and Figure S20a), a thin layer was detected at the surface of the material. A similar observation was reported by Liu et al. using APTES as the modifier [56]. This layer was ca. 2 nm thick and wrapped both support and Pd particles. EDS mapping (Figure S20c) revealed the Si-based nature of this layer at the surface of both TiO2 and Pd NPs. Therefore, this evidenced a structural difference for the materials obtained by this approach when compared with those synthesized by reverse deposition.
When HR-TEM analysis of 12 was performed (Figure S21), agglomerations were detected in some regions of the support (Figure S21a,b). Microanalysis also confirmed the presence of P originating from the PPh3 ligands.
XPS was also employed for the analysis of some of these catalysts (Figures S24–S26). The data obtained were compared with those of the unmodified Pd-PPh3/TiO2 catalyst. All the spectra were referenced using the C1s signal and set at 285.0 eV. The presence of Si2p, N1s, and P2p for 11 catalyst and N1s and P2p for 15 was detected. The Pd spectra were studied with more detail (Figure S26b). When a P-based modifier was used (15), no significant differences with the unmodified catalyst were observed. However, for the sample containing the Si-based modifiers, the BEs observed were higher. For 11, a difference of 1 eV was observed (336.0 eV) with the reference catalyst, which could be due to the interaction with the amine functional group from APTES, as previously reported by Kim et al. [82]. They also observed an increase of BE for Pd when APTES was used on the synthesis of Pd NPs over CNTs, and this difference was attributed due to the interaction of Pd with amine functional groups and to the particle size (1.85 nm). A combination of interaction of amine and small size could produce the BE observed for 11.
The relative amount of Pdδ+ also revealed differences in samples (Table S6). In unmodified Pd-PPh3/TiO2, a value of 12.5% was measured. When the aminophosphonic acid was used in the synthesis of 15, the proportion of Pdδ+ was 21.9%. These values remained similar to that of the reference Pd-PPh3/TiO2 catalyst. However, a value as high as 64% was obtained for 11 that was attributed to the oxidation of the samples during manipulations.
To conclude, these new catalysts were obtained by deposition of organosilane and organophosphonic acid modifiers over previously synthesized Pd NPs supported on TiO2. When organosilanes were employed as modifiers, smaller Pd NPs were obtained compared with those modified by organophosphonic acids. Using APTES at different concentrations, the use of a greater amount of APTES led to lower Pd contents, which was attributed to Pd leaching.

2.3. Hydrogenation of CO2 into Formates Using Modified Catalysts

First, a screening was performed to evaluate the effect of the different modifications carried out on catalysts. These tests were performed at 80 °C using 36 bar of CO2/H2 (1:1) pressure during 15 h in the presence of KHCO3 as a base and using water as solvent. The results obtained using the Pd-PPh3/modifier@TiO2 1–8 and modifier@Pd-PPh3/@TiO2 9–15 catalysts are summarized in Table 2 and Table 3.

2.3.1. Hydrogenation of CO2 into Formates Using Catalysts Formed by Deposition of Pd NPs over Modified TiO2 Supports

In Table 2, the results obtained with systems 1–8 are displayed and compared with those provided by the unmodified Pd-PPh3/TiO2 catalyst (TON of 881, Entry 1) under the same conditions [80]. When 2 was used as catalyst, a TON of 1057 was reached (Entry 3), whereas using material 8, modified by organophosphonic acid (Entry 11), a decrease in activity was observed, and the lowest TON value of the series was measured (694). These results therefore indicated that when the catalyst is formed by deposition of the Pd NPs over the modified support, the nature of the modifier affects the catalyst activity.
Table 2. Catalytic activity of Pd-PPh3/modifier@TiO2 1–8 with different anchor atoms for CO2 hydrogenation to formate 1.
Table 2. Catalytic activity of Pd-PPh3/modifier@TiO2 1–8 with different anchor atoms for CO2 hydrogenation to formate 1.
Catalysts 14 00487 i002
EntryCatalystTON 2
1Pd-PPh3/TiO2881
21588
321057
43a868
5 33b917
641043
751030
86a776
9 36b826
107635
118694
1 Reaction conditions: 20 mg of catalyst, 4 M KHCO3, 5 mL milli-Q H2O, 80 °C, pTotal = 36 bar, p(CO2) = p(H2), and 15 h. 2 TON = mmol formate/mmol total of Pd, calculated by NMR using 1,4-dioxane as internal standard. 3 In absence of PPh3.
Next, the effect of the reaction conditions used for the support modification using APTES was investigated. The results obtained show three distinct effects compared with the unmodified catalyst (Entry 1). When low concentrations of APTES (0.01 M and 0.003 mmol/mg TiO2) were used to modify the support, a positive effect on the activity of the resulting catalyst 2 was observed (Entry 3). When the concentration of APTES was increased up to 0.1 M (up to 0.013 mmol/mg TiO2) (catalyst 3a, Entry 4), the TON obtained was similar to that obtained with the unmodified catalyst (868), suggesting that the excess of APTES “cancelled out” the positive effect observed at lower concentrations. These results are in contrast with those reported by Mori et al. [58] using a phenylamine containing organosilane to modify SiO2 since they observed an increase in catalytic activity when the amount of phenylamine was increased.
Next, the effect of the functional group at the end of the alkyl chain of the modifier was evaluated. For the catalysts formed by modifications with organosilanes, the presence of a functional group (either NH2 or imidazolium group) was clearly beneficial to the catalyst activity since an increase in TON was observed from 588 (for PTES, Entry 2) to ca. 850 for APTES and IL-OAc-modified catalysts (Entries 4 and 8). A similar effect was observed for PAs-modified catalysts (Entry 10 vs. 11), although to a lesser extent (635 vs. 694).
For the Pd NPs over TiO2 previously modified by organosilanes containing an IL functionality, the highest TON was obtained with the catalyst 5 (1030, Entry 7).
Based on previous results using organosilane modifiers (Table 2), the concentration could be the parameter responsible for this difference. This was confirmed by the results obtained with the catalysts 5 and 6a, which only differ in the concentrations used during their support synthesis (Entry 7 vs. Entry 8). Indeed, the catalyst 5 containing the support modified at the lower concentration provided a TON of 1030, while when 6a was used, a TON of 776 was obtained. These results are therefore in agreement with the trend previously observed.
The presence of the PPh3 ligand during the synthesis of the Pd NPs over supports modified with APTES and IL-OAc was evaluated (Table 2). In both cases, slightly higher TON values were obtained in the absence of PPh3 (Entry 4 vs. Entry 5 and Entry 8 vs. Entry 9). When 3a was used, a TON of 868 was obtained (Entry 4), while in the system without PPh3, 3b provided a TON of 917 (Entry 5). IL-OAc, 6a, which was synthesized in the presence of PPh3, reached a TON of 776 (Entry 8), while its analogue without PPh3, 6b, yielded a TON of 826 (Entry 9).
These catalytic results therefore show that the modification of the TiO2 support by oganosilanes provided a beneficial effect compared with the catalyst containing unmodified TiO2 or TiO2 modified by organophosphonic acids. Moreover, the modifier concentration is a key parameter during the support modification, and lower values provide catalysts with higher activities. The presence of a functional group (either NH2 or imidazolium) in the modifiers also improved the activity of the catalysts compared with those containing a simple alkyl chain.

2.3.2. Hydrogenation of CO2 into Formates Using Catalysts Formed by Modifications of Pre-Synthesized Pd-PPh3/TiO2

The catalysts synthesized by the deposition of the modifier over previously synthesized Pd-PPh3/TiO2 were evaluated in the hydrogenation of CO2 into formate (Table 3).
Table 3. Catalytic activity of modifier@Pd-PPh3/TiO2 9–15 with different anchor atoms for CO2 hydrogenation to formate 1.
Table 3. Catalytic activity of modifier@Pd-PPh3/TiO2 9–15 with different anchor atoms for CO2 hydrogenation to formate 1.
Catalysts 14 00487 i003
EntrySystemTON 2
1Pd-PPh3/TiO2881
29862
310581
411992
512388
613551
714912
815844
1 Reaction conditions: 20 mg of catalyst, 4 M KHCO3, 5 mL milli-Q H2O, 80 °C, pTotal = 36 bar, p(CO2) = p(H2), and 15 h. 2 TON = mmol formate/mmol total of Pd, calculated by NMR using 1,4-dioxane as internal standard.
When the results obtained by the deposition of organosilane and organophosphonic acid under the same conditions (same concentration, mmol modifier/mg TiO2 ratio, and amine substituent on the organosilane and organophosphonic acid) were compared, the system containing a Si as anchoring atom, 10, provided a TON of 581 (Entry 3), whereas when APA was used as modifier, in 15, a TON of 844 was reached (Entry 8).
As the unmodified catalyst provided a TON of 881, it was concluded that the deposition of the organosilane over the pre-synthesized catalyst had a detrimental effect on the CO2 hydrogenation, while the deposition of PA had no significant effect. These results are in contrast with those obtained using the catalyst where Pd NPs were deposited over the modified support, and they highlight the importance of the synthetic strategy when such catalysts are modified with organic molecules.
The results obtained for hydrogenation of CO2 to formate using catalysts formed by the deposition of APTES at different concentrations are also summarized in Table 3. As previously mentioned, the system 10 obtained at low concentration (0.01 M of APTES; 0.003 mmol/mg TiO2), and the TON obtained was 581. However, when the same modifier was deposited using a 0.13 M solution, the resulting catalyst 11 provided much higher activity with a TON of 992 (Entry 4).
The performance of catalysts modified by organosilanes containing a propyl, a propylamine, or a propyl-imidazolium groups and those modified by PAs containing a butyl and a propylamine substituent was also evaluated (Table 3). Interestingly, all these catalysts exhibited activities similar to that of the unmodified catalyst, except that modified by an organosilane containing an imidazolium function. It is noteworthy that, as described in the previous section, the catalyst 11 showed a slight positive effect compared to the reference catalyst (Entry 4 vs. Entry 1). The results described here are in clear contrast with those obtained when the support was first modified prior to the deposition of Pd NPs, for which the presence of a functional group (either amine or imidazolium) in the modifier clearly improved the performance of the resulting catalysts. This might be due to the beneficial effect of this group in the anchoring of the NPs when present at the support surface prior to the NP deposition, while such a group does not influence the catalytic activity when deposited after NP formation. This thus suggests that the presence of this group does not influence the catalysis but only the preparation of the catalyst.
The catalytic performances of the catalysts 12 and 13, modified by ionic liquid-containing modifiers with distinct anions, are summarized Table 3. The two catalysts tested provided lower TONs (388 and 551, Entries 5 and 6, respectively) than the unmodified catalyst, indicating that the deposition of IL-containing organosilanes over the Pd NPs was detrimental to the catalytic performance of the Pd-PPh3/TiO2 material.
These catalytic results therefore show that the deposition of organosilane and organophosphonic acid modifiers over previously synthesized Pd NPs supported on TiO2 was not beneficial, in most cases, to the activity of the resulting catalysts.
The TON values obtained using the catalysts described here are among the highest reported for Pd-based catalysts (range ca. 800–1200) [42,58,83,84].

2.3.3. Reusability Tests

After evaluating the activity of the newly synthesized catalysts, recyclability tests were performed using the most active systems. During these experiments, the catalysts were recovered by filtration over a Nylon membrane after each cycle, washed several times with milli-Q H2O, and dried under vacuum for several hours prior to their reuse. All the experiments described in the manuscript were carried out at least twice to confirm the reproducibility of the data.
Initially, the catalysts 2, 3b, 4, and 8 were employed to evaluate the effect of the modifiers on the reusability of the materials performing the hydrogenation reactions at 80 °C. The results obtained are summarized in Figure 4. In all cases, a strong decrease in activity was observed after two cycles. However, the catalysts 2 and 3b containing the support previously modified by APTES suffered a minimal loss of activity during the first cycle and a large decrease during the second cycle. In contrast, the catalysts 4 and 8 containing the support modified by NH2-PA and an organosilane containing an imidazolium group suffered a large loss of activity during the first recycling but maintained their activity during the second recycling. After the third recycling, a new drop in activity for these catalysts was observed. TEM analysis of the spent catalysts revealed large agglomerations in all samples, while no dramatic changes in mean size were measured on non-agglomerated Pd NPs (Figures S40, S41, S43 and S44).
Next, recyclability experiments were performed using the catalysts 11, 12, 14, and 15 under the same conditions (Figure 5). It was found that 11 was the most active system of this series, with an initial TON of 904. This catalyst provided a TON of 724 after the first recycling, 662 after the second recycling, and 524 after the third recycling.
For the catalysts in which deposition of an organophosphonic acid was performed over the previously synthesized Pd-PPh3/TiO2, distinct behaviors were observed depending on the functional groups contained in the modifiers. For 15 containing an PA with an amine function, a drastic decrease in activity was observed during the first recycling, with a drop of TON value from 873 to 332. No relevant decrease in activity was observed during the second recycling, while a drop of TON to 201 was measured for the third recycling. In contrast, when the catalyst contained a PA modifier with a butyl chain 14, the activity loss during the first recycling was less pronounced (from a TON of 827 to 723), while a sudden drop to 369 was observed during the second recycling. For the catalyst 12 modified by an organosilane containing an imidazolium group, much lower activities were measured (TONs of 405 initially and 209 after three runs).
TEM analysis of the spent catalysts after two recyclings (Figure S45) revealed that the mean sizes of the Pd NPs in these materials was in all cases in the range 3.5–4 nm (Table S19), and different degrees of agglomeration were observed. In the case of catalyst 12, for which an increase in Pd NPs mean size from 2.4 nm to ca. 4 nm was observed, the presence of agglomerations was pronounced. For the catalysts modified with PAs 14 and 15, no large variation in size was observed, but large agglomerations were detected.
These recycling experiments therefore show that the synthetic procedure used for the modification of the Pd-PPh3/TiO2 catalyst affects the reusability of these materials in the CO2 hydrogenation into formate. Indeed, the catalysts formed by modification of the support prior to Pd NP deposition suffer a rapid decrease in activity during their recycling and reuse in spite of the initial beneficial effect. In contrast, some of the materials where the modifiers were deposited over the previously anchored Pd NPs onto TiO2 show a much more gradual decrease in activity and reached a TON > 500 after the third recycling. To evidence the improvement in reusability obtained by catalyst modification in this work, the TON values obtained with the unmodified catalyst Pd-PPh3/TiO2 during the recycling experiments are compared with those of 11 in Figure 6. Despite a superior initial activity of the unmodified catalyst, both materials exhibited similar TON values after the first recycling and in the second and third recyclings, and the modified catalyst clearly showed a better performance.
Important differences were observed by TEM analyses of the spent catalysts since for 11, an increasing of size from 1.86 ± 0.69 nm to 3.53 ± 1.23 nm was measured after two cycles, but only a few agglomerations were observed. In contrast, the Pd NPs in Pd-PPh3/TiO2 were largely agglomerated after the recyclings and exhibited a mean size of ca. 5 nm. These results suggest that the presence of APTES provided additional stabilization and limited the degree of agglomerations of the Pd NPs. This is in agreement with the previously reported results where modifications of the heterogeneous catalyst by APTES improved the stability of the catalysts and hence their recovery [53,54,56,58,59].

3. Materials and Methods

Pd(dba)2, PPh3, TiO2 (Titanium (IV) oxide nanopowder, 21 nm primary particle size (TEM), ≥99.5% trace metals basis, rutile–anatase mixture, specific surface area 35–65 m2/g), and the rest of compounds employed for modification were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without any further purification. All solvents were dried from a solvent purification system (SPS) and deoxygenated. Tetrahydrofuran was further dried by refluxing in the presence of sodium/acetophenone. Milli-Q water purchased from Merck (Madrid, Spain) was employed in catalytic experiments. Any other solvent or reagent employed was reagent-grade. Hydrogen (5.0) was purchased from Carburos Metálicos (Barcelona, Spain), and CO2 (5.3) was purchased from Abelló Linde (Valencia, Spain). All the synthesis were performed using Schlenk techniques under argon and glovebox using nitrogen as the inert gas. The synthesis of nanoparticles were carried in Fischer–Porter bottles, and catalytic tests were performed in the stainless-steel high-pressure reactor Hel CAT-7 (7 × 10 mL). Characterization techniques that were employed included TEM, HR-TEM, ESEM, FESEM, ICP-OES, TGA, NMR, FT-IR, and XPS. All details and corresponding figures can be found in the Supporting Information.

3.1. Modification of TiO2 with n-Propyltriethoxysilane (PTES) and (3-Aminopropyl)triethoxysilane (APTES)

In a general synthesis of TiO2-1–3, a solution (mixture EtOH:milli-Q H2O 95:5 v/v) of the accorded concentration of PTES or APTES was prepared in a Schlenk (under Ar). Then, the corresponding amount of TiO2 (mmol (A)PTES/mg TiO2) (Table S1) was added to the solution under vigorous stirring. The reaction was stirred at room temperature overnight. Then, the mixture was centrifuged. The supernatant was removed, and the solid was washed several times with milli-Q H2O and EtOH and was dried at 80 °C under vacuum for several hours.

3.2. Modification of TiO2 with Ionic Liquids (ILs)

First, synthesis of the ILs was performed according to the literature with some variations [75,85,86,87,88,89]. The synthesis of TiO2-4 was performed according to reported procedures [86]. A suspension of 2 g of TiO2 in 80 mL of milli-Q H2O was added to a previously prepared solution of 659.08 mg (1.8 mmol) of IL-Cl in 2 mL of milli-Q H2O. The mixture was stirred at 80 °C for 12 h (Table S1). After reaction, the mixture was centrifuged and washed twice with milli-Q and once with EtOH. After that, it was dried under vacuum at 60 °C.
For the ILs with AcO− (TiO2-5 and TiO2-6) as anion, TiO2 was added to a solution of IL-OAc (mixture EtOH:milli-Q H2O 95:5 v/v), and the mixture was heated at 80 °C overnight (Table S1). The product was washed with EtOH several times and dried under vacuum at 60 °C.

3.3. Modification of TiO2 with Phosphonic Acids (PAs)

The modification of TiO2 with PAs (TiO2-7 and TiO2-8) was performed according to the literature procedures with some variations [90,91,92] (Table S1). Solutions contained 83.35 µmol of modifier per each square meter of TiO2 nanopowder, which corresponded approximately to a 10-fold excess in respect to building a monolayer on the surface of the support.
A solution of 10 mM of 3-aminopropylphosphonic acid (APA) (TiO2-8) (1.46 mmol, 202.84 mg) in 146 mL of milli-Q H2O was prepared. Then, 0.5 g of TiO2 was added, and the mixture was stirred at r.t. overnight. After this time, the suspension was centrifuged and washed with abundant milli-Q water, EtOH, and acetone. The solid was dried at 120 °C in an oven. It is necessary to produce condensation reactions to yield strong bonds between PAs and metal oxide [39]. So, in this case, after the reaction, the solid was separated by centrifugation, the solvent was removed, and the solid was placed into an oven at 100 °C overnight. After that, the solid was aged in a Quartz furnace at 120 °C under air flow during 6 h. Then, it was washed with abundant milli-Q water and centrifuged every time (5 times). The solid was dried overnight at 100 °C. When butylphosphonic acid was employed (PA) (TiO2-7), the synthesis was performed under the same conditions (Table S1). The only difference was that THF was used as the reaction media and for washing and centrifugation.

3.4. Synthesis of Pd NPs with PPh3 as Ligand over Different Modified TiO2 Supports through Organometallic Approach (1–8)

The catalysts were prepared following a previously reported methodology [80] to obtain a 4 wt% theorical content of Pd over TiO2. In a common experiment, the metal precursor (Pd(dba)2), 0.2 eq. of stabilizer (PPh3), and modified TiO2 were weighted in the glove box and charged in a Fischer–Porter bottle. Then, solvent (THF) was added; the Fischer–Porter was closed and purged with hydrogen several times and then charged with 3 bar of H2. The mixture was then heated at 60 °C and stirred at 700 rpm overnight. After the reaction, the mixture was cooled to room temperature and degassed. Samples for TEM analysis were prepared by the deposition of several drops of the reaction crude onto a copper grid. The rest of the reaction crude was concentrated and washed several times with abundant hexane. The catalyst was dried under vacuum during several hours.

3.5. Deposition of Modifier over Previously Synthesized Pd-PPh3/TiO2 Systems (9–15)

PTES and APTES (9–11): A specified concentration of PTES or APTES (and determined mmol (A)PTES/mg:Pd-PPh3/TiO2 ratio) and Pd-PPh3/TiO2 that was previously synthesized were mixed (Table S5). At r.t., the synthesis was carried out overnight in Schlenk using a mixture of 95% EtOH and 5% milli-Q H2O, while reactions at 120 °C reaction temperature were performed in an autoclave during 4 h using EtOH as solvent. In both cases, samples were dried overnight in an oven at 105 °C and stored inside the glove box.
ILs (12–13): A mixture of a specified concentration of IL–anion and the previously synthesized Pd-PPh3/TiO2 system was prepared at r.t. using a mixture of 95% EtOH and 5% milli-Q H2O, overnight (Table S5).
PAs (14–15): A mixture of a specified concentration of PAs and the previously synthesized Pd-PPh3/TiO2 system was prepared at r.t. using milli-Q H2O as solvent (for synthesis with APA) or THF (for synthesis with PA), overnight (Table S5).

3.6. Catalytic Experiments for CO2 Reduction to Formate

The stainless-steel high-pressure reactor HEL CAT-7 (7 × 10 mL) was charged with TiO2-supported palladium nanoparticles (20 mg), 20 mg of 1,4-dioxane, and 5 mL of a 4 M base solution employing milli-Q water. The reactor was first flushed with 3 cycles of hydrogen to remove the air. Then, the reactor was charged with 10 bar of H2 and heated at 40 °C under stirring for twenty minutes. At this point, the reactor was depressurized, purged several times with CO2, and charged with 18 bar of CO2 and left under stirring for 20 min. Then, the reactor was charged with 18 bar of H2 (1:1, CO2:H2) and heated to reach the temperature under 600 rpm of stirring. The experiment was left 15 h, and after this time, the reactor was allowed to cool in an ice bath. When the reactor was cooled, it was depressurized and opened. A small amount of the sample was centrifuged, and 100 µL of supernatant was analyzed by NMR using D2O as the deuterated solvent.

3.7. Recycling Experiments

After every catalytic experiment, the mixture was filtered through a Nylon membrane. The solid was washed several times with abundant milli-Q H2O and dried under vacuum for several hours. At this point, the solid was reused in an identical catalytic experiment.

4. Conclusions

A series of supported Pd NPs-based materials were successfully synthesized using modifiers of different natures (organosilanes, ILs, and PAs) following two distinct approaches. The so-called reverse deposition approach requires in the first place to modify the TiO2 support prior to Pd NPs deposition, while the second approach involves the modification of a pre-synthesized Pd-PPh3/TiO2 by deposition of the modifier over its surface. The newly prepared materials, including the modified TiO2 supports, were characterized by various techniques, such as TEM, HRTEM, EDS, FT-IR, TGA, ICP, etc.
These catalysts were tested in the hydrogenation of CO2 to formate, and their performance was compared with those of the unmodified catalyst Pd-PPh3/TiO2. The modification of the TiO2 support by oganosilanes provided a beneficial effect in catalysis compared with the catalyst containing unmodified TiO2 or TiO2 modified by organophosphonic acids, and the modifier concentration is a key parameter during the support modification.
The presence of a functional group (either NH2 or imidazolium) in the modifiers improved the activity of the catalysts. In contrast, the deposition of organosilane and organophosphonic acid modifiers over previously synthesized Pd NPs supported on TiO2 was not beneficial, in most cases, to the activity of the catalyst.
The synthetic procedure used for the modification of the Pd-PPh3/TiO2 catalyst also affected the reusability of these materials in the hydrogenation of CO2 into formate, and when the modifiers were deposited over the previously anchored Pd NPs onto TiO2, a more gradual decrease in activity was observed.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14080487/s1. Synthetic procedures, technics data, TEM, XRD, XPS, EDX, HRTEM images, and results of catalysis experiment of hydrogenation of CO2 under different conditions.

Author Contributions

Writing—original draft preparation, data curation, M.D.F.-M.; methodology, writing—review and editing, supervision, C.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Spanish Ministerio de Ciencia e Innovación (CTQ2016-75016-R and PID2019-104427RB-I00) and the Generalitat de Catalunya (2017SGR1472 and 2021SGR00163) for financial support and the FPI grant (BES-2017-081305).

Data Availability Statement

The original contributions presented in the study are included in the article (and Supplementary Material), further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors are grateful to the Spanish Ministerio de Ciencia e Innovación (CTQ2016-75016-R and PID2019-104427RB-I00) and the Generalitat de Catalunya (2017SGR1472) for financial support and the FPI grant (BES-2017-081305). Authors also thank the Servei of Recursos Cientifics i Tecnics of Universitat Rovira i Virgili for the analytical assistance and the Fundació Institut Català de Nanociència i Nanotecnologia (ICN2).

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Centi, G.; Perathoner, S. Opportunities and prospects in the chemical recycling of carbon dioxide to fuels. Catal. Today 2009, 148, 191–205. [Google Scholar] [CrossRef]
  2. Centi, G.; Quadrelli, E.A.; Perathoner, S. Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries. Energy Environ. Sci. 2013, 6, 1711–1731. [Google Scholar] [CrossRef]
  3. Dibenedetto, A.; Angelini, A.; Stufano, P. Use of carbon dioxide as feedstock for chemicals and fuels: Homogeneous and heterogeneous catalysis. J. Chem. Technol. Biotechnol. 2014, 89, 334–353. [Google Scholar] [CrossRef]
  4. Koolen, C.D.; Oveisi, E.; Zhang, J.; Li, M.; Safonova, O.V.; Pedersen, J.K.; Rossmeisl, J.; Luo, W.; Züttel, A. Low-temperature non-equilibrium synthesis of anisotropic multimetallic nanosurface alloys for electrochemical CO2 reduction. Nat. Synth. 2024, 3, 45–57. [Google Scholar] [CrossRef]
  5. Raveendran, A.; Chandran, M.; Dhanusuraman, R. A comprehensive review on the electrochemical parameters and recent material development of electrochemical water splitting electrocatalysts. RSC Adv. 2023, 13, 3843–3876. [Google Scholar] [CrossRef] [PubMed]
  6. Ohi, J. Hydrogen energy cycle: An overview. J. Mater. Res. 2005, 20, 3180–3187. [Google Scholar] [CrossRef]
  7. Lee, J.H.; Ryu, J.; Kim, J.Y.; Nam, S.-W.; Han, J.H.; Lim, T.-H.; Gautam, S.; Chae, K.H.; Yoon, C.W. Carbon dioxide mediated, reversible chemical hydrogen storage using a Pd nanocatalyst supported on mesoporous graphitic carbon nitride. J. Mater. Chem. A 2014, 2, 9490–9495. [Google Scholar] [CrossRef]
  8. Zhong, H.; Iguchi, M.; Chatterjee, M.; Ishizaka, T.; Kitta, M.; Xu, Q.; Kawanami, H. Interconversion between CO2 and HCOOH under basic conditions catalyzed by PdAu nanoparticles supported by amine-functionalized reduced graphene oxide as a dual catalyst. ACS Catal. 2018, 8, 5355–5362. [Google Scholar] [CrossRef]
  9. Park, H.; Lee, J.H.; Kim, E.H.; Kim, K.Y.; Choi, Y.H.; Youn, D.H.; Lee, J.S. A highly active and stable palladium catalyst on a g-C3N4 support for direct formic acid synthesis under neutral conditions. Chem. Commun. 2016, 52, 14302–14305. [Google Scholar] [CrossRef]
  10. Shao, X.; Miao, X.; Yu, X.; Wang, W.; Ji, X. Efficient synthesis of highly dispersed ultrafine Pd nanoparticles on a porous organic polymer for hydrogenation of CO2 to formate. RSC Adv. 2020, 10, 9414–9419. [Google Scholar] [CrossRef]
  11. Su, J.; Lu, M.; Lin, H. High yield production of formate by hydrogenating CO2 derived ammonium carbamate/carbonate at room temperature. Green Chem. 2015, 17, 2769–2773. [Google Scholar] [CrossRef]
  12. Wang, W.-H.; Feng, X.; Bao, M. Transformation of CO2 to Formic Acid or Formate over Heterogeneous Catalysts; Chapter 3; Springer: Berlin/Heidelberg, Germany, 2017; pp. 43–52. ISBN 978-981-10-3250-9. [Google Scholar]
  13. Nakajima, K.; Tominaga, M.; Waseda, M.; Miura, H.; Shishido, T. Highly Efficient Supported Palladium–Gold Alloy Catalysts for Hydrogen Storage Based on Ammonium Bicarbonate/Formate Redox Cycle. ACS Sustain. Chem. Eng. 2019, 7, 6522–6530. [Google Scholar] [CrossRef]
  14. Kuwahara, Y.; Fujie, Y.; Mihogi, T.; Yamashita, H. Hollow Mesoporous Organosilica Spheres Encapsulating PdAg Nanoparticles and Poly(Ethyleneimine) as Reusable Catalysts for CO2 Hydrogenation to Formate. ACS Catal. 2020, 10, 6356–6366. [Google Scholar] [CrossRef]
  15. Yang, G.; Kuwahara, Y.; Masuda, S.; Mori, K.; Louis, C.; Yamashita, H. PdAg nanoparticles and aminopolymer confined within mesoporous hollow carbon spheres as an efficient catalyst for hydrogenation of CO2 to formate. J. Mater. Chem. A 2020, 8, 4437–4446. [Google Scholar] [CrossRef]
  16. Mori, K.; Sano, T.; Kobayashi, H.; Yamashita, H. Surface Engineering of a Supported PdAg Catalyst for Hydrogenation of CO2 to Formic Acid: Elucidating the Active Pd Atoms in Alloy Nanoparticles. J. Am. Chem. Soc. 2018, 140, 8902–8909. [Google Scholar] [CrossRef] [PubMed]
  17. Nguyen, L.T.M.; Park, H.; Banu, M.; Kim, J.Y.; Youn, D.H.; Magesh, G.; Kim, W.Y.; Lee, J.S. Catalytic CO2 hydrogenation to formic acid over carbon nanotube-graphene supported PdNi alloy catalysts. RSC Adv. 2015, 5, 105560–105566. [Google Scholar] [CrossRef]
  18. Shao, X.; Xu, J.; Huang, Y.; Su, X.; Duan, H.; Wang, X.; Zhang, T. Pd@C3N4 nanocatalyst for highly efficient hydrogen storage system based on potassium bicarbonate/formate. AIChE J. 2016, 62, 2410–2418. [Google Scholar] [CrossRef]
  19. Wan, K.T.; Davis, M.E. Design and synthesis of a heterogeneous asymmetric catalyst. Nature 1994, 370, 449–450. [Google Scholar] [CrossRef]
  20. Shibasaki-Kitakawa, N.; Honda, H.; Kuribayashi, H.; Toda, T.; Fukumura, T.; Yonemoto, T. Biodiesel production using anionic ion-exchange resin as heterogeneous catalyst. Bioresour. Technol. 2007, 98, 416–421. [Google Scholar] [CrossRef] [PubMed]
  21. Liu, R.; Jin, R.; An, J.; Zhao, Q.; Cheng, T.; Liu, G. Hollow-Shell-Structured Nanospheres: A Recoverable Heterogeneous Catalyst for Rhodium-Catalyzed Tandem Reduction/Lactonization of Ethyl 2-Acylarylcarboxylates to Chiral Phthalides. Chem. Asian J. 2014, 9, 1388–1394. [Google Scholar] [CrossRef]
  22. Leng, Y.; Liu, J.; Jiang, P.; Wang, J. Organometallic-polyoxometalate hybrid based on V-Schiff base and phosphovanadomolybdate as a highly effective heterogenous catalyst for hydroxylation of benzene. J. Chem. Eng. 2014, 239, 1–7. [Google Scholar] [CrossRef]
  23. Cong, P.; Doolen, R.D.; Fan, Q.; Giaquinta, D.M.; Guan, S.; McFarland, E.W.; Poojary, D.M.; Self, K.; Turner, H.W.; Weinberg, W.H. High-Throughput Synthesis and Screening of Combinatorial Heterogeneous Catalyst Libraries. Angew. Chem. Int. Ed. 1999, 38, 484–488. [Google Scholar] [CrossRef]
  24. Uysal, B.; Oksal, B.S. New heterogeneous B(OEt)3-MCM-41 catalyst for preparation of α,β-unsaturated alcohols. Res. Chem. Intermed. 2015, 41, 3893–3911. [Google Scholar] [CrossRef]
  25. Yamaguchi, K.; Yoshida, C.; Uchida, S.; Mizuno, N. Peroxotungstate Immobilized on Ionic Liquid-Modified Silica as a Heterogeneous Epoxidation Catalyst with Hydrogen Peroxide. J. Am. Chem. Soc. 2005, 127, 530–531. [Google Scholar] [CrossRef] [PubMed]
  26. Planeix, J.M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P.S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P.M. Application of Carbon Nanotubes as Supports in Heterogeneous Catalysis. J. Am. Chem. Soc. 1994, 116, 7935–7936. [Google Scholar] [CrossRef]
  27. Kent, P.D.; Mondloch, J.E.; Finke, R.G. A Four-Step Mechanism for the Formation of Supported-Nanoparticle Heterogeneous Catalysts in Contact with Solution: The Conversion of Ir(1,5-COD)Cl/γ-Al2O3 to Ir(0)~170/γ-Al2O3. J. Am. Chem. Soc. 2014, 136, 1930–1941. [Google Scholar] [CrossRef]
  28. Dobrzeniecka, A.; Kulesza, P.J. Electrocatalytic Activity toward Oxygen Reduction of RuSxNy Catalysts Supported on Different Nanostructured Carbon Carriers. ECS J. Solid State Sci. Technol. 2013, 2, M61–M66. [Google Scholar] [CrossRef]
  29. Astruc, D.; Lu, F.; Aranzaes, J.R. Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis. Angew. Chem. Int. Ed. 2005, 44, 7852–7872. [Google Scholar] [CrossRef]
  30. Crudden, C.M.; Sateesh, M.; Lewis, R. Mercaptopropyl-Modified Mesoporous Silica: A Remarkable Support for the Preparation of a Reusable, Heterogeneous Palladium Catalyst for Coupling Reactions. J. Am. Chem. Soc. 2005, 127, 10045–10050. [Google Scholar] [CrossRef]
  31. Lou, Y.; Xu, J.; Zhang, Y.; Pan, C.; Dong, Y.; Zhu, Y. Metal-support interaction for heterogeneous catalysis: From nanoparticles to single atoms. Mater. Today Nano 2020, 12, 100093. [Google Scholar] [CrossRef]
  32. van Deelen, T.W.; Hernández Mejía, C.; de Jong, K.P. Control of metal-support interactions in heterogeneous catalysts to enhance activity and selectivity. Nat. Catal. 2019, 2, 955–970. [Google Scholar] [CrossRef]
  33. Liu, L.; Corma, A. Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles. Chem. Rev. 2018, 118, 4981–5079. [Google Scholar] [CrossRef] [PubMed]
  34. Sun, R.; Liao, Y.; Bai, S.-T.; Zheng, M.; Zhou, C.; Zhang, T.; Sels, B.F. Heterogeneous catalysts for CO2 hydrogenation to formic acid/formate: From nanoscale to single atom. Energy Environ. Sci. 2021, 14, 1247–1285. [Google Scholar] [CrossRef]
  35. Fan, F.; Zhang, J.; Ma, K.; Zhang, Y.; Hu, Y.-M.; Kong, L.; Jia, A.-P.; Zhang, Z.; Huang, W.; Lu, J.-Q. Ceria morphology-dependent Pd-CeO2 interaction and catalysis in CO2 hydrogenation into formate. J. Catal. 2021, 397, 116–127. [Google Scholar] [CrossRef]
  36. Zhang, Z.; Zhang, L.; Hülsey, M.J.; Yan, N. Zirconia phase effect in Pd/ZrO2 catalyzed CO2 hydrogenation into formate. Mol. Catal. 2019, 475, 110461. [Google Scholar] [CrossRef]
  37. Mori, K.; Yamashita, H. Design and Architecture of Nanostructured Heterogeneous Catalysts for CO2 Hydrogenation to Formic Acid/Formate. In CO2 Hydrogenation Catalysis, 1st ed.; Chapter 7; Himeda, Y., Ed.; Wiley-VCH GmbH: Weinheim, Germany, 2021; pp. 179–205. [Google Scholar]
  38. Schoenbaum, C.A.; Schwartz, D.K.; Medlin, J.W. Controlling the Surface Environment of Heterogeneous Catalysts Using Self-Assembled Monolayers. Acc. Chem. Res. 2014, 47, 1438–1445. [Google Scholar] [CrossRef]
  39. Jenkins, A.H.; Medlin, J.W. Controlling Heterogeneous Catalysis with Organic Monolayers on Metal Oxides. Acc. Chem. Res. 2021, 54, 4080–4090. [Google Scholar] [CrossRef] [PubMed]
  40. Xu, L.; Cui, T.; Zhu, J.; Wang, X.; Ji, M. PdAg alloy nanoparticles immobilized on functionalized MIL-101-NH2: Effect of organic amines on hydrogenation of carbon dioxide into formic acid. New J. Chem. 2021, 45, 6293–6300. [Google Scholar] [CrossRef]
  41. Chen, B.; Dong, M.; Liu, S.; Xie, Z.; Yang, J.; Li, S.; Wang, Y.; Du, J.; Liu, H.; Han, B. CO2 Hydrogenation to Formate Catalyzed by Ru Coordinated with a N,P-Containing Polymer. ACS Catal. 2020, 10, 8557–8566. [Google Scholar] [CrossRef]
  42. Masuda, S.; Mori, K.; Futamura, Y.; Yamashita, H. PdAg Nanoparticles Supported on Functionalized Mesoporous Carbon: Promotional Effect of Surface Amine Groups in Reversible Hydrogen Delivery/Storage Mediated by Formic Acid/CO2. ACS Catal. 2018, 8, 2277–2285. [Google Scholar] [CrossRef]
  43. Hao, P.; Schwartz, D.K.; Medlin, J.W. Effect of Surface Hydrophobicity of Pd/Al2O3 on Vanillin Hydrodeoxygenation in a Water/Oil System. ACS Catal. 2018, 8, 11165–11173. [Google Scholar] [CrossRef]
  44. Sun, Q.; Fu, X.; Si, R.; Wang, C.-H.; Yan, N. Mesoporous Silica-Encaged Ultrafine Bimetallic Nanocatalysts for CO2 Hydrogenation to Formates. ChemCatChem 2019, 11, 5093–5097. [Google Scholar] [CrossRef]
  45. Umegaki, T.; Satomi, Y.; Kojima, Y. Catalytic Properties of Palladium Nanoparticles for Hydrogenation of Carbon Dioxide into Formic Acid. J. Jpn. Inst. Energy 2019, 96, 487–492. [Google Scholar] [CrossRef]
  46. Yang, G.; Kuwahara, Y.; Mori, K.; Louis, C.; Yamashita, H. PdAg alloy nanoparticles encapsulated in N-doped microporous hollow carbon spheres for hydrogenation of CO2 to formate. Appl. Catal. B Environ. 2011, 283, 119628. [Google Scholar] [CrossRef]
  47. Chen, Q.; Yakovlev, N.L. Adsorption and interaction of organosilanes on TiO2 nanoparticles. Appl. Surf. Sci. 2010, 257, 1395–1400. [Google Scholar] [CrossRef]
  48. Meroni, D.; Lo Presti, L.; Di Liberto, G.; Ceotto, M.; Acres, R.G.; Prince, K.C.; Bellani, R.; Soliveri, G.; Ardizzone, S. A Close Look at the Structure of the TiO2-APTES Interface in Hybrid Nanomaterials and Its Degradation Pathway: An Experimental and Theoretical Study. J. Phys. Chem. C 2017, 121, 430–440. [Google Scholar] [CrossRef] [PubMed]
  49. Schmitt, C. Surface Modification of Oxide Nanoparticles Using Phosphonic Acids: Characterization, Surface Dynamics, and Dispersion in Sols and Nanocomposites. Material Chemistry. Ph.D. Thesis, Université Montpellier, Montpelier, France, 2015. [Google Scholar]
  50. Glaser, A.; Foisner, J.; Hoffmann, H.; Friedbacher, G. Investigation of the Role of the Interplay between Water and Temperature on the Growth of Alkylsiloxane Submonolayers on Silicon. Langmuir 2004, 20, 5599–5604. [Google Scholar] [CrossRef] [PubMed]
  51. Klaysri, R.; Tubchareon, T.; Praserthdam, P. One-step synthesis of amine-functionalized TiO2 surface for photocatalytic decolorization under visible light irradiation. J. Ind. Eng. Chem. 2017, 45, 229–236. [Google Scholar] [CrossRef]
  52. Cheng, F.; Sajedin, S.M.; Kelly, S.M.; Lee, A.F.; Kornherr, A. UV-stable paper coated with APTES-modified P25 TiO2 nanoparticles. Carbohydr. Polym. 2014, 114, 246–252. [Google Scholar] [CrossRef]
  53. Sharma, R.K.; Yadav, M.; Gaur, R.; Gupta, R.; Adholeya, A.; Gawande, M.B. Synthesis of Iron Oxide Palladium Nanoparticles and Their Catalytic Applications for Direct Coupling of Acyl Chlorides with Alkynes. ChemPlusChem 2016, 81, 1312–1319. [Google Scholar] [CrossRef]
  54. Demirelli, M.; Karaoğlu, E.; Baykal, A.; Sözeri, H.; Uysal, E. Synthesis, characterization and catalytic activity of CoFe2O4-APTES-Pd magnetic recyclable catalyst. J. Alloys Compd. 2014, 582, 201–207. [Google Scholar] [CrossRef]
  55. Didas, S.A.; Choi, S.; Chaikittisilp, W.; Jones, C.W. Amine–Oxide Hybrid Materials for CO2 Capture from Ambient Air. Acc. Chem. Res. 2015, 48, 2680–2687. [Google Scholar] [CrossRef]
  56. Liu, Q.; Yang, X.; Huang, Y.; Xu, S.; Su, X.; Pan, X.; Xu, J.; Wang, A.; Liang, C.; Wang, X.; et al. A Schiff base modified gold catalyst for green and efficient H2 production from formic acid. Energy Environ. Sci. 2015, 8, 3204–3207. [Google Scholar] [CrossRef]
  57. Liu, Q.; Yang, X.; Li, L.; Miao, S.; Li, Y.; Li, Y.; Wang, X.; Huang, Y.; Zhang, T. Direct catalytic hydrogenation of CO2 to formate over a Schiff-base-mediated gold nanocatalyst. Nat. Commun. 2017, 8, 1407. [Google Scholar] [CrossRef] [PubMed]
  58. Mori, K.; Masuda, S.; Tanaka, H.; Yoshizawa, K.; Chee, M.; Yamashita, H. Phenylamine-functionalized mesoporous silica supported PdAg nanoparticles: A dual heterogeneous catalyst for formic acid/CO2-mediated chemical hydrogen delivery/storage. Chem. Commun. 2017, 53, 4677–4680. [Google Scholar] [CrossRef] [PubMed]
  59. Srivastava, V. Amine-Functionalized SBA-15 Supported Ru Nanocatalyst for the Hydrogenation CO2 to Formic Acid. Catal. Surv. Asia 2021, 25, 192–205. [Google Scholar] [CrossRef]
  60. Gindri, I.M.; Frizzo, C.P.; Bender, C.R.; Tier, A.Z.; Martins, M.A.P.; Villetti, M.A.; Machado, G.; Rodriguez, L.C.; Rodrigues, D.C. Preparation of TiO2 Nanoparticles Coated with Ionic Liquids: A Supramolecular Approach. ACS Appl. Mater. Interfaces 2014, 6, 11536–11543. [Google Scholar] [CrossRef] [PubMed]
  61. Xin, B.; Hao, J. Imidazolium-based ionic liquids grafted on solid surfaces. Chem. Soc. Rev. 2014, 43, 7171–7187. [Google Scholar] [CrossRef] [PubMed]
  62. Gurau, G.; Rodríguez, H.; Kelley, S.P.; Janiczek, P.; Kalb, R.S.; Rogers, R.D. Demonstration of Chemisorption of Carbon Dioxide in 1,3-Dialkylimidazolium Acetate Ionic Liquids. Angew. Chem. Int. Ed. 2011, 50, 12024–12026. [Google Scholar] [CrossRef]
  63. Wu, Y.; Zhao, Y.; Wang, H.; Yu, B.; Yu, X.; Zhang, H.; Liu, Z. 110th Anniversary: Ionic Liquid Promoted CO2 Hydrogenation to Free Formic Acid over Pd/C. Ind. Eng. Chem. Res. 2019, 58, 6333–6339. [Google Scholar] [CrossRef]
  64. Qadir, M.I.; Weilhard, A.; Fernandes, J.A.; de Pedro, I.; Vieira, B.J.C.; Waerenborgh, J.C.; Dupont, J. Selective Carbon Dioxide Hydrogenation Driven by Ferromagnetic RuFe Nanoparticles in Ionic Liquids. ACS Catal. 2018, 8, 1621–1627. [Google Scholar] [CrossRef]
  65. Bordet, A.; Moos, G.; Welsh, C.; Licence, P.; Luska, K.L.; Leitner, W. Molecular Control of the Catalytic Properties of Rhodium Nanoparticles in Supported Ionic Liquid Phase (SILP) Systems. ACS Catal. 2020, 10, 13904–13912. [Google Scholar] [CrossRef]
  66. Moos, G.; Emondts, M.; Bordet, A.; Leitner, W. Selective Hydrogenation and Hydrodeoxygenation of Aromatic Ketones to Cyclohexane Derivatives Using a Rh@SILP Catalyst. Angew. Chem. Int. Ed. 2020, 59, 11977–11983. [Google Scholar] [CrossRef]
  67. Rengshausen, S.; Van Stappen, C.; Levin, N.; Tricard, S.; Luska, K.L.; DeBeer, S.; Chaudret, B.; Bordet, A.; Leitner, W. Organometallic Synthesis of Bimetallic Cobalt-Rhodium Nanoparticles in Supported Ionic Liquid Phases (CoxRh100–x@SILP) as Catalysts for the Selective Hydrogenation of Multifunctional Aromatic Substrates. Small 2021, 17, 2006683. [Google Scholar] [CrossRef]
  68. Offner-Marko, L.; Bordet, A.; Moos, G.; Tricard, S.; Rengshausen, S.; Chaudret, B.; Luska, K.L.; Leitner, W. Bimetallic Nanoparticles in Supported Ionic Liquid Phases as Multifunctional Catalysts for the Selective Hydrodeoxygenation of Aromatic Substrates. Angew. Chem. Int. Ed. 2018, 57, 12721–12726. [Google Scholar] [CrossRef] [PubMed]
  69. Luska, K.L.; Bordet, A.; Tricard, S.; Sinev, I.; Grünert, W.; Chaudret, B.; Leitner, W. Enhancing the Catalytic Properties of Ruthenium Nanoparticle-SILP Catalysts by Dilution with Iron. ACS Catal. 2016, 6, 3719–3726. [Google Scholar] [CrossRef]
  70. Goclik, L.; Offner-Marko, L.; Bordet, A.; Leitner, W. Selective hydrodeoxygenation of hydroxyacetophenones to ethyl-substituted phenol derivatives using a FeRu@SILP catalyst. Chem. Commun. 2020, 56, 9509–9512. [Google Scholar] [CrossRef]
  71. Luska, K.L.; Julis, J.; Stavitski, E.; Zakharov, D.N.; Adams, A.; Leitner, W. Bifunctional nanoparticle–SILP catalysts (NPs@SILP) for the selective deoxygenation of biomass substrates. Chem. Sci. 2014, 5, 4895–4905. [Google Scholar] [CrossRef]
  72. El Sayed, S.; Bordet, A.; Weidenthaler, C.; Hetaba, W.; Luska, K.L.; Leitner, W. Selective Hydrogenation of Benzofurans Using Ruthenium Nanoparticles in Lewis Acid-Modified Ruthenium-Supported Ionic Liquid Phases. ACS Catal. 2020, 10, 2124–2130. [Google Scholar] [CrossRef]
  73. Bordet, A.; El Sayed, S.; Sanger, M.; Boniface, K.J.; Kalsi, D.; Luska, K.L.; Jessop, P.G.; Leitner, W. Selectivity control in hydrogenation through adaptive catalysis using ruthenium nanoparticles on a CO2-responsive support. Nat. Chem. 2021, 13, 916–922. [Google Scholar] [CrossRef]
  74. Bordet, A.; Leitner, W. Metal Nanoparticles Immobilized on Molecularly Modified Surfaces: Versatile Catalytic Systems for Controlled Hydrogenation and Hydrogenolysis. Acc. Chem. Res. 2021, 54, 2144–2157. [Google Scholar] [CrossRef] [PubMed]
  75. Louis Anandaraj, S.J.; Kang, L.; DeBeer, S.; Bordet, A.; Leitner, W. Catalytic Hydrogenation of CO2 to Formate Using Ruthenium Nanoparticles Immobilized on Supported Ionic Liquid Phases. Small 2023, 19, 2206806. [Google Scholar] [CrossRef] [PubMed]
  76. Philippot, K.; Chaudret, B. Organometallic approach to the synthesis and surface reactivity of noble metal nanoparticles. C. R. Chim. 2003, 6, 1019–1034. [Google Scholar] [CrossRef]
  77. Feng, B.; Zhang, Z.; Wang, J.; Yang, D.; Li, Q.; Liu, Y.; Gai, H.; Huang, T.; Song, H. Synthesis of hydrophobic Pd-poly(ionic liquid)s with excellent CO2 affinity to efficiently catalyze CO2 hydrogenation to formic acid. Fuel 2022, 325, 124853. [Google Scholar] [CrossRef]
  78. Li, Q.; Huang, T.; Zhang, Z.; Xiao, M.; Gai, H.; Zhou, Y.; Song, H. Highly Efficient Hydrogenation of CO2 to Formic Acid over Palladium Supported on Dication Poly(ionic liquid)s. Mol. Catal. 2021, 509, 111644. [Google Scholar] [CrossRef]
  79. Queffélec, C.; Petit, M.; Janvier, P.; Knight, D.A.; Bujoli, B. Surface Modification Using Phosphonic Acids and Esters. Chem. Rev. 2012, 112, 3777–3807. [Google Scholar] [CrossRef] [PubMed]
  80. Fernández-Martínez, M.D.; Godard, C. Hydrogenation of CO2 into Formates by Ligand-Capped Palladium Heterogeneous Catalysts. ChemCatChem 2023, 15, e202201408. [Google Scholar] [CrossRef]
  81. Wanag, A.; Sienkiewicz, A.; Rokicka-Konieczna, P.; Kusiak-Nejman, E.; Morawski, A.W. Influence of modification of titanium dioxide by silane coupling agents on the photocatalytic activity and stability. J. Environ. Chem. Eng. 2020, 8, 103917. [Google Scholar] [CrossRef]
  82. Kim, Y.; Lee, H.; Yang, S.; Lee, J.; Kim, H.; Hwang, S.; Jeon, S.W.; Kim, D.H. Ultrafine Pd nanoparticles on amine-functionalized carbon nanotubes for hydrogen production from formic acid. J. Catal. 2021, 404, 324–333. [Google Scholar] [CrossRef]
  83. Masuda, S.; Mori, K.; Kuwahara, Y.; Yamashita, H. PdAg nanoparticles supported on resorcinol-formaldehyde polymers containing amine groups: The promotional effect of phenylamine moieties on CO2 transformation to formic acid. J. Mater. Chem. A 2019, 7, 16356–16363. [Google Scholar] [CrossRef]
  84. Wen, M.; Mori, K.; Futamura, Y.; Kuwahara, Y.; Navlani-García, M.; An, T.; Yamashita, H. PdAg Nanoparticles within Core-Shell Structured Zeolitic Imidazolate Framework as a Dual Catalyst for Formic Acid-based Hydrogen Storage/Production. Sci. Rep. 2019, 9, 15675. [Google Scholar] [CrossRef] [PubMed]
  85. Price, T.L., Jr.; Choi, U.H.; Schoonover, D.V.; Arunachalam, M.; Xie, R.; Lyle, S.; Colby, R.H.; Gibson, H.W. Ion Conducting ROMP Monomers Based on (Oxa)norbornenes with Pendant Imidazolium Salts Connected via Oligo(oxyethylene) Units and with Oligo(ethyleneoxy) Terminal Moieties. Macromolecules 2019, 52, 1371–1388. [Google Scholar]
  86. Dou, Q.; Liu, L.; Yang, B.; Lang, J.; Yan, X. Silica-grafted ionic liquids for revealing the respective charging behaviors of cations and anions in supercapacitors. Nat. Commun. 2017, 8, 2188. [Google Scholar] [CrossRef] [PubMed]
  87. Li, J.R.; Chen, C.; Hu, Y.L. Novel and Efficient Knoevenagel Condensation over Mesoporous SBA-15 Supported Acetate-functionalized Basic Ionic Liquid Catalyst. ChemistrySelect 2020, 5, 14578–14582. [Google Scholar]
  88. Dai, Y.; Wang, S.; Wu, J.; Tang, J.; Tang, W. Dicationic AC regioisomer cyclodextrins: Mono-6A-ammonium-6C-alkylimidazolium-β-cyclodextrin chlorides as chiral selectors for enantioseparation. RSC Adv. 2012, 2, 12652–12656. [Google Scholar] [CrossRef]
  89. Roshan, K.R.; Mathai, G.; Kim, J.; Tharun, J.; Park, G.-A.; Park, D.-W. A biopolymer mediated efficient synthesis of cyclic carbonates from epoxides and carbon dioxide. Green Chem. 2012, 14, 2933. [Google Scholar] [CrossRef]
  90. Brodard-Severac, F.; Guerrero, G.; Maquet, J.; Florian, P.; Gervais, C.; Mutin, P.H. High-Field 17O MAS NMR Investigation of Phosphonic Acid Monolayers on Titania. Chem. Mater. 2008, 20, 5191–5196. [Google Scholar] [CrossRef]
  91. Zhang, J.; Deo, S.; Janik, M.J.; Medlin, J.W. Control of Molecular Bonding Strength on Metal Catalysts with Organic Monolayers for CO2 Reduction. J. Am. Chem. Soc. 2020, 142, 5184–5193. [Google Scholar] [CrossRef]
  92. Helmy, R.; Fadeev, A.Y. Self-Assembled Monolayers Supported on TiO2: Comparison of C18H37SiX3 (X = H, Cl, OCH3), C18H37Si(CH3)2Cl, and C18H37PO(OH)2. Langmuir 2002, 18, 8924–8928. [Google Scholar] [CrossRef]
Catalysts 14 00487 sch001
Scheme 1. Modifications of catalyst surface for hydrogenation of CO2 to formates in basic media.
Scheme 1. Modifications of catalyst surface for hydrogenation of CO2 to formates in basic media.
Catalysts 14 00487 sch001
Catalysts 14 00487 g001
Figure 1. Modifying agents used for TiO2 modification.
Figure 1. Modifying agents used for TiO2 modification.
Catalysts 14 00487 g001
Catalysts 14 00487 sch002
Scheme 2. Deposition of modifier on a support prior to metal catalyst formation (reverse deposition) (a) and deposition of modifier on a previously synthesized supported metal catalyst (b).
Scheme 2. Deposition of modifier on a support prior to metal catalyst formation (reverse deposition) (a) and deposition of modifier on a previously synthesized supported metal catalyst (b).
Catalysts 14 00487 sch002
Catalysts 14 00487 sch003
Scheme 3. Synthesis of nanoparticles stabilized with PPh3 supported over modified TiO2.
Scheme 3. Synthesis of nanoparticles stabilized with PPh3 supported over modified TiO2.
Catalysts 14 00487 sch003
Catalysts 14 00487 g002
Figure 2. TEM images and size distributions of Pd NPs synthesized over modified TiO2 supports. Deposition of TESs and PAs modifiers over previously synthesized Pd-PPh3 supported over TiO2. 1 (a), 2 (b), 3a (c), 3b (d), 4 (e), 5 (f), 6a (g), 6b (h), 7 (i), and 8 (j).
Figure 2. TEM images and size distributions of Pd NPs synthesized over modified TiO2 supports. Deposition of TESs and PAs modifiers over previously synthesized Pd-PPh3 supported over TiO2. 1 (a), 2 (b), 3a (c), 3b (d), 4 (e), 5 (f), 6a (g), 6b (h), 7 (i), and 8 (j).
Catalysts 14 00487 g002
Catalysts 14 00487 sch004
Scheme 4. Deposition of TESs and PAs modifiers over previously synthesized Pd-PPh3 supported over TiO2.
Scheme 4. Deposition of TESs and PAs modifiers over previously synthesized Pd-PPh3 supported over TiO2.
Catalysts 14 00487 sch004
Catalysts 14 00487 g003
Figure 3. HR-TEM image of the catalyst 11.
Figure 3. HR-TEM image of the catalyst 11.
Catalysts 14 00487 g003
Catalysts 14 00487 g004
Figure 4. Recycling experiments with catalysts 2, 3b, 4, and 8 in CO2 hydrogenation to formate. Conditions: 20 mg of catalyst, 4 M KHCO3, 5 mL milli-Q H2O, pTotal = 36 bar, p(CO2) = p(H2), 80 °C, and 15 h. TON = mmol formate/mmol total of Pd, calculated by NMR using 1,4-dioxane as internal standard.
Figure 4. Recycling experiments with catalysts 2, 3b, 4, and 8 in CO2 hydrogenation to formate. Conditions: 20 mg of catalyst, 4 M KHCO3, 5 mL milli-Q H2O, pTotal = 36 bar, p(CO2) = p(H2), 80 °C, and 15 h. TON = mmol formate/mmol total of Pd, calculated by NMR using 1,4-dioxane as internal standard.
Catalysts 14 00487 g004
Catalysts 14 00487 g005
Figure 5. Recycling experiments with the systems 11, 12, 14, and 15 in the hydrogenation of CO2 to formate. Conditions: 20 mg of catalyst, 4 M KHCO3, 5 mL milli-Q H2O, pTotal = 36 bar, p(CO2) = p(H2), 80 °C, and 15 h. TON = mmol formate/mmol total of Pd, calculated by NMR using 1,4-dioxane as internal standard.
Figure 5. Recycling experiments with the systems 11, 12, 14, and 15 in the hydrogenation of CO2 to formate. Conditions: 20 mg of catalyst, 4 M KHCO3, 5 mL milli-Q H2O, pTotal = 36 bar, p(CO2) = p(H2), 80 °C, and 15 h. TON = mmol formate/mmol total of Pd, calculated by NMR using 1,4-dioxane as internal standard.
Catalysts 14 00487 g005
Catalysts 14 00487 g006
Figure 6. Comparation of recycling experiments with 11 and Pd-PPh3/TiO2 for CO2 hydrogenation to formate. Conditions: 20 mg of catalysts, 4 M KHCO3, 5 mL milli-Q H2O, pTotal = 36 bar, p(CO2) = p(H2), and 15 h. Temperatures employed for 11 and Pd-PPh3/TiO2 were 80 and 60 °C, respectively. TON = mmol formate/mmol total of Pd, calculated by NMR using 1,4-dioxane as internal standard.
Figure 6. Comparation of recycling experiments with 11 and Pd-PPh3/TiO2 for CO2 hydrogenation to formate. Conditions: 20 mg of catalysts, 4 M KHCO3, 5 mL milli-Q H2O, pTotal = 36 bar, p(CO2) = p(H2), and 15 h. Temperatures employed for 11 and Pd-PPh3/TiO2 were 80 and 60 °C, respectively. TON = mmol formate/mmol total of Pd, calculated by NMR using 1,4-dioxane as internal standard.
Catalysts 14 00487 g006
Table 1. Characterization data for the deposition of TESs and PAs over the previously synthesized Pd-PPh3/TiO2 system 1.
Table 1. Characterization data for the deposition of TESs and PAs over the previously synthesized Pd-PPh3/TiO2 system 1.
Catalysts 14 00487 i001
EntrySystemSize (nm) 2Pd wt% 3P wt% 3
192.65 ± 0.803.38-
2102.69 ± 0.923.040.21
3111.86 ± 0.692.57-
4122.38 ± 0.951.81-
5131.87 ± 0.612.98-
6 4144.45 ± 1.683.100.82
7 5153.00 ± 1.143.15-
1 Synthesis conditions: Pd-PPh3/TiO2 previously synthesized was added to a solution of TESs (for determined mmol TESs/mg Pd-PPh3/TiO2 ratio) on mixture EtOH:milli-Q H2O (95:5, v/v), and it was left to react at r.t. overnight. 2 Determined by TEM. 3 Determined by ICP. 4 Pd-PPh3/TiO2 system was added to a solution of PA on THF, and it was left to react r.t. overnight. 5 Pd-PPh3/TiO2 system was added to a solution of APA on milli-Q H2O, and it was left to react r.t. overnight.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Fernández-Martínez, M.D.; Godard, C. Molecular TiO2 Modifications of Supported PPh3-Capped Pd Nanocatalysts for CO2 Hydrogenation into Formates. Catalysts 2024, 14, 487. https://doi.org/10.3390/catal14080487

AMA Style

Fernández-Martínez MD, Godard C. Molecular TiO2 Modifications of Supported PPh3-Capped Pd Nanocatalysts for CO2 Hydrogenation into Formates. Catalysts. 2024; 14(8):487. https://doi.org/10.3390/catal14080487

Chicago/Turabian Style

Fernández-Martínez, María Dolores, and Cyril Godard. 2024. "Molecular TiO2 Modifications of Supported PPh3-Capped Pd Nanocatalysts for CO2 Hydrogenation into Formates" Catalysts 14, no. 8: 487. https://doi.org/10.3390/catal14080487

APA Style

Fernández-Martínez, M. D., & Godard, C. (2024). Molecular TiO2 Modifications of Supported PPh3-Capped Pd Nanocatalysts for CO2 Hydrogenation into Formates. Catalysts, 14(8), 487. https://doi.org/10.3390/catal14080487

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop