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Article

Influence of Co-Precipitation Agent on the Structure, Texture and Catalytic Activity of Au-CeO2 Catalysts in Low-Temperature Oxidation of Benzyl Alcohol

by
Lukasz Wolski
1,*,
Grzegorz Nowaczyk
2,
Stefan Jurga
2 and
Maria Ziolek
1
1
Faculty of Chemistry, Adam Mickiewicz University, Uniwersytetu Poznańskiego 8, 61-614 Poznań, Poland
2
NanoBioMedical Centre, Adam Mickiewicz University, Wszechnicy Piastowskiej 3, 61-614 Poznań, Poland
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(5), 641; https://doi.org/10.3390/catal11050641
Submission received: 5 May 2021 / Revised: 14 May 2021 / Accepted: 16 May 2021 / Published: 18 May 2021
(This article belongs to the Special Issue Advance in Selective Alcohol and Polyol Oxidation Catalysis)
Browse Figures
Versions Notes

Abstract

:
The aim of the study was to establish the influence of a co-precipitation agent (i.e., NaOH–immediate precipitation; hexamethylenetetramine/urea–gradual precipitation and growth of nanostructures) on the properties and catalytic activity of as-synthesized Au-CeO2 nanocomposites. All catalysts were fully characterized with the use of XRD, nitrogen physisorption, ICP-OES, SEM, HR-TEM, UV-vis, XPS, and tested in low-temperature oxidation of benzyl alcohol as a model oxidation reaction. The results obtained in this study indicated that the type of co-precipitation agent has a significant impact on the growth of gold species. Immediate co-precipitation of Au-CeO2 nanostructures with the use of NaOH allowed obtainment of considerably smaller and more homogeneous in size gold nanoparticles than those formed by gradual co-precipitation and growth of Au-CeO2 nanostructures in the presence of hexamethylenetetramine or urea. In the catalytic tests, it was established that the key factor promoting high activity in low-temperature oxidation of benzyl alcohol was size of gold nanoparticles. The highest conversion of the alcohol was observed for the catalyst containing the smallest Au particle size (i.e., Au-CeO2 nanocomposite prepared with the use of NaOH as a co-precipitation agent).

Graphical Abstract

1. Introduction

Cerium dioxide is one of the most intensively studied model supports for various metals (e.g., Cu [1,2,3], Ag [1,4,5], Pt [6] and Au [7,8]). From among numerous ceria-based systems, the catalysts containing gold nanoparticles have attracted particular attention and have been successfully used in different catalytic processes (e.g., Water–Gas Shift reaction [9], oxidation of CO [10,11], alcohols [12,13,14] and formaldehyde [15]). In terms of selective oxidation of alcohols, it has been established that one of the most important factors affecting the activity and selectivity of Au/CeO2 catalysts is the size of the supported gold nanoparticles [11,16] and the concentration of lattice defects in the ceria support [17]. These two factors have been considered as the key guidelines on the development of new methods for the synthesis of highly active ceria-based gold catalysts. Literature data show that various strategies to meet these two criteria can be applied, e.g., development of new methods for gold deposition [18,19,20,21] or design of new supports by controlling ceria morphology [9,10].
According to literature [22,23,24,25], geometric and electronic properties of gold are strongly affected by the type of precipitation agent used during the synthesis of catalysts. Radnik et al. [23] have found that Au/Al2O3 catalysts prepared by deposition–precipitation method using two different precipitation agents, namely NaOH and urea, had not only different particle size, but also showed different electronic properties of gold species. In the catalyst prepared using urea, they observed only metallic gold species, while in the catalyst prepared with the use of sodium hydroxide, they identified two forms of gold, namely metallic gold and partially oxidized gold species. Differences in electronic properties of gold nanoparticles (Au NPs) loaded on TiO2 surface, using different precipitation agents, have been also observed by Zanella et al. [24]. The authors have found that Au NPs prepared using NaOH as a precipitation agent were more easily reducible than those prepared with the use of urea. The influence of the precipitation agent on the properties and catalytic activity of gold catalysts supported on ceria has also been studied by Chen et al. [25]. The authors have reported that the highest activity in gas phase oxidation of formaldehyde was characteristic of the sample prepared with the use of urea. High activity of this sample was attributed by these authors to smaller gold particle size and stronger metal-support interaction than that observed for the catalyst prepared with the use of sodium hydroxide. Chen et al. [25] have claimed that the efficient charge transfer from gold nanoparticles to ceria support observed for the catalyst prepared with the use of urea resulted in formation of large quantities of Au3+ and Ce3+ species. According to the same authors, the presence of this strong electronic interaction weakened the Ce-O bonds, leading to easier reduction of the ceria surface oxygen species, and this promoted high activity of this sample in gas phase oxidation of formaldehyde. In view of these results, one can conclude that there is no simple correlation between the type of precipitation agent used during the synthesis and the properties/activity of as-prepared gold species.
Recent literature reports have shown that not only NaOH and urea, but also hexamethylenetetramine (HMTA) can be successfully used for the synthesis of heterogeneous catalysts with desirable properties [26,27,28]. Similarly to urea, HMTA undergoes hydrolysis at higher temperatures leading to formation of NH3 [27]. The as-formed ammonia produces a basic environment that is necessary for the production of metal hydroxides, which can be further transformed into relevant metal oxides. To date, HMTA has been successfully used by She et al. [26] for the synthesis of gold catalysts supported on zinc oxide nanorods. These authors have discovered that polyvinylpyrrolidone (PVP)-assisted in situ nucleation and growth of ZnO nanorods in the colloidal solution of gold nanoparticles can lead to successful formation of well-defined nanostructures. A recent report by Saad et al. [29] has shown that HMTA can be used not only for the preparation of ZnO-based materials, but also for the synthesis of Ag-, Al- and Zn-doped TiO2 nanostructures. One of the most important differences between urea and HMTA is the type of side products of their hydrolysis. As mentioned previously, heating of water solution of urea or HMTA leads to formation of ammonia as the main product. However, hydrolysis of the former precipitation agent leads also to formation of carbon dioxide [30,31], while hydrolysis of the latter precipitation agent results in production of formaldehyde [27]. It is expected that in situ formation of these two different side products during the synthesis of gold catalysts may have significant impact on the growth of nanostructures and properties of as-prepared materials.
The main goal of this work was to establish the influence of the type of co-precipitation agent on the structure, texture, electronic and catalytic properties of Au-CeO2 catalysts prepared by a facile co-precipitation method. Of particular interest was to identify correlations between the properties of co-precipitation agents used during the synthesis of gold catalysts (NaOH—immediate precipitation of Au-CeO2 nanostructures vs. HMTA and urea—gradual precipitation and growth of Au-CeO2 nanostructures) and the properties of as-synthesized Au-CeO2 nanostructures. The activity of the catalysts was tested in low-temperature oxidation of benzyl alcohol as a model reaction, which allowed identification of correlations between the properties of the as-prepared Au-CeO2 composites and their catalytic performance.

2. Results

2.1. Characterization of Materials

Chemical composition of Au-CeO2 catalysts was analyzed using Inductively Coupled Plasma—Optical Emission Spectrometry (ICP-OES; see Table 1). It was found that for all catalysts synthesized in this study, high efficiency of gold introduction was achieved. The highest gold loading of 4.7 wt. % was characteristic of the catalyst prepared with the use of HMTA as a co-precipitation agent. For the catalysts synthesized with the use of NaOH and urea, the real gold loading was slightly lower and was found to be of 4.5 and 4.3 wt.%, respectively.
Structure of the materials was analyzed by X-ray diffraction measurements (XRD). It was found that all the catalysts synthesized in this study, irrespectively of the co-precipitation agent used during the synthesis, had cerianite structure (ICDD No. 00-034-0394; see Figure 1). For the gold containing catalysts, four additional diffraction peaks at 2 Θ equal to 38.2, 44.4, 64.5 and 77.5° were found. According to literature, these diffraction peaks are characteristic of (111), (200), (220) and (311) facets of metallic gold nanoparticles, respectively [32,33].
As can be seen from Figure 1A, the type of co-precipitation agent had significant impact on the size of gold nanoparticles. For the gold catalysts prepared with the use of urea and HMTA, the diffraction peaks typical of Au NPs were much sharper and more intense than that observed for the catalyst prepared with the use of sodium hydroxide, indicating larger gold particle size for the former two catalysts. The average gold particle sizes estimated from XRD patterns using the Scherrer equation were found to be of 61.8, 51.5 and 12.5 nm for Au-CeO2_U, Au-CeO2_H and Au-CeO2_N, respectively (see Table 1).
Figure 2 shows nitrogen adsorption–desorption isotherms recorded for the catalysts. It was found that all materials exhibited isotherms of type IV(a) characteristic of porous solids [34], but had different surface areas. From among all cerium dioxides synthesized in this study, the highest surface area of 49 m2/g was characteristic of the sample prepared with the use of sodium hydroxide as a co-precipitation agent. The surface area of the ceria support prepared with the use of urea was slightly lower than that observed for CeO2_NaOH (46 m2/g vs. 49 m2/g, respectively), while the lowest surface area of 11 m2/g was characteristic of the ceria catalyst prepared with the use of HMTA (see Table 1).
Detailed analysis of the hysteresis loops permitted us to conclude that the type of co-precipitation agent affected not only the surface area of ceria but also had a significant impact on the pore structure of as-synthesized materials. As can be seen from Figure 2A, CeO2_N catalyst exhibited a hysteresis loop of type H2, while CeO2_H and CeO2_U samples exhibited hysteresis loops of type H3. The H2 hysteresis loop found for CeO2_N may be associated with pore blocking effect and indicated the presence of mesopores with a narrow range of pore necks [34]. On the other hand, hysteresis loop of type H3 found for CeO2_H and CeO2_U indicated the presence of macropores in the structure of these materials. Differences in porosity of ceria catalysts were further confirmed by the pore size distributions (see Figure 2C). It was found that CeO2_N sample contained mesopores of sizes in the range from 2 to 30 nm, while the pore network of CeO2_U and CeO2_H consisted of small mesopores with the size of ca. 2 nm, but some large meso- and even some macropores were also identified.
Figure 2B,D shows that gold catalysts had similar pore structures and pore size distributions as that observed for ceria supports prepared with the use of the same co-precipitation agent. In terms of catalysts surface area, it was found that gold catalysts prepared with the use of NaOH and urea had slightly lower surface areas than that observed for the samples without gold (see Table 1). On the other hand, the surface area of gold catalyst prepared with the use of HMTA was ca. three times higher than that observed for CeO2_H (28 m2/g vs. 11 m2/g, respectively; Table 1). Detailed analysis of pore size distribution estimated for CeO2_H and Au-CeO2_H sample permitted us to observe that the presence of chloroauric acid during the synthesis of materials led to formation of additional mesopores with the sizes in the range from 5 to 10 nm.
Morphology of the catalysts was characterized using the Scanning Electron Microscopy (SEM). As shown in Figure 3, the type of co-precipitation agent had significant impact on the structure of materials obtained. Cerium dioxide prepared with the use of HMTA was composed of irregular aggregates of particles with different shapes, mostly spherical-shape particles.
On the other hand, the catalyst prepared using urea as a co-precipitation agent consisted of strips aggregated into coralloid-like structures. The highest distortions of sizes and shapes of CeO2 particles were observed for the catalysts prepared with the use of NaOH. As can be seen from Figure 3, CeO2_N was composed of large irregular aggregates of different shapes and sizes accompanied by smaller and very small irregular nanoparticles. It is important to notice that all gold catalysts had morphology similar to that of cerium oxides synthesized using the same co-precipitation agents.
To get a deeper insight into the morphology and structure of the materials, the as-prepared catalysts were analyzed with the use of High Resolution Transmission Electron Microscopy (HRTEM). As shown in Figures S1‒S3, all cerium oxides consisted of small nearly spherical, highly crystalline particles fused into larger aggregates. It is worth noting that the most condensed structures were observed for the ceria support prepared with the use of urea as a co-precipitation agent (CeO2_U; see Figure S3). The HRTEM images revealed a characteristic distance of about 3.1 Å for all the samples, attributed to the (111) CeO2 lattice planes. Further analysis of crystalline parameters calculated on the basis of Selected Area Electron Diffraction (SAED) patterns (Figures S1–S3) showed that they were in good agreement with those obtained from XRD experiments.
Detailed analysis of gold catalysts with the use of HRTEM provided a deeper insight into the role of a co-precipitation agent in controlling the size of gold nanoparticles (see Figure 4). It was found that the smallest and the most homogeneous in size gold nanoparticles were formed with the use of NaOH as a co-precipitation agent. Additionally, with this co-precipitation agent, it was frequently observed that gold particles of single nanometers size were tightly attached to CeO2 nanoparticles (see inset in Figure 4A). Gold nanoparticles formed in the presence of HMTA were much larger and less homogeneous in size than those observed for Au-CeO2_N (see Figure 4B). The largest differences in size of gold nanoparticles were noted for the catalyst prepared with the use of urea as a co-precipitation agent. It was found that Au-CeO2_U catalyst contained both relatively small Au NPs with the size below 15 nm and large gold nanoparticles with the size larger than 30 nm (see Figure 4C). To shed more light on the distribution of gold nanoparticles formed on the surface of the catalysts, the as-prepared materials were further analyzed with the use of Energy Dispersive X-ray Spectroscopy (EDS) combined with elemental mapping. The detailed analysis of EDS maps of elements recorded for Au-CeO2_N allowed drawing a conclusion that the use of sodium hydroxide as a co-precipitation agent resulted in formation of relatively small and uniform Au NPs (see Figure 5A,A’). TEM-EDS measurements confirmed also large variations in the size of gold species formed in the presence of HMTA as a co-precipitation agent (see Figure 5B’). The greatest diversity in gold particle size was observed for Au-CeO2_U. For this material, we identified both some gold nanoparticles with the size ranging from 10 to 30 nm (see Figure 5C,C’) and extremely large gold aggregates shown in Figure 5 D and D’ (the regions containing extremely large gold nanoparticles were very rare).Oxidation state of metals in the catalysts was investigated by the Ultraviolet-visible (UV-vis) spectroscopy. As can be seen from Figure 6, all materials exhibited broad absorption bands with the maximum intensity at ca. 260 and 345 nm. According to literature [35,36], these bands are attributed to O2− → Ce4+ charge transfer transitions and interband transitions, respectively. It is important to notice that the band at ca. 255 nm corresponding to the O2− → Ce3+ charge transfer transitions was not clearly observed in the UV-vis spectra [35,37]. However, it cannot be excluded that this band is overlapped with the more intense absorption bands typical of Ce4+ species. The presence of Ce3+ ions in the structure of ceria supports will be further discussed in the X-ray Photoelectron Spectroscopy (XPS) section. Figure 6B shows that the UV-vis spectra recorded for all gold catalysts exhibited additional absorption band with a maximum intensity at ca. 500–650 nm. According to literature [38,39], this absorption band is assigned to Surface Plasmon Resonance (SPR) of metallic gold nanoparticles. Thus, UV-vis studies provided us further evidence for successful deposition of metallic gold species on the surface of all the catalysts. Furthermore, significant differences in the positions of the SPR bands for Au NPs shown in Figure 6B permitted concluding that the nature of co-precipitation agent strongly affected the optical properties of as-prepared Au-CeO2 nanocomposites.
To get a deeper insight into the chemical composition of the catalysts, the samples were further characterized using XPS. Figure 7 and Figure 8 show the Ce 3d and Au 4f XP spectra of all materials. As can be seen from Figure 7A, the type of co-precipitation agent has no significant influence on the oxidation state of ceria.
In all the samples, the most intense peaks were typical of Ce4+ species (labelled as v, v″, v‴ and u, u″, u‴ [20,40]), indicating that majority of cerium species existed in the form of CeO2. The contribution of components typical of Ce3+ species or lattice defects (labelled as v0, v’ and u0, u’ [20,40]) in all the samples was noticeable, but significantly lower than that observed for Ce4+ species. This observation indicated that some Ce3+ species or lattice defects (e.g., oxygen vacancies) can be present in the structure of all the ceria-based catalysts, but the majority of cerium existed as Ce4+.
Important factors affecting the activity of gold catalysts in oxidation reactions are electronic properties of gold. As can be seen from Figure 8, in the Au 4f region of the XP spectra recorded for Au-CeO2_N, two spectral components at the binding energy of 84.1 and 87.8 eV, were observed. According to literature, these two components are characteristic of metallic gold species (spin orbitals Au 4f7/2 and Au 4f5/2, respectively [41]). For Au-CeO2_H and Au-CeO2_U, we observed the same components but at lower binding energy values. The shift of BE of the Au 4f spectral components towards lower values may result from larger gold particle size observed for these two catalyst. As mentioned in the XRD and TEM sections, Au NPs formed on the surface of the catalyst prepared with the use of HMTA and urea were much larger than that found on the surface of Au-CeO2_N. Thus, in view of all the information, one can observe that the results of XPS studies are in agreement with the conclusions drawn on the basis of XRD and TEM measurements.

2.2. Catalytic Activity

Catalytic activity of the materials was tested in low-temperature oxidation of benzyl alcohol in liquid phase. As implied by Table 2 data, CeO2_U and CeO2_H exhibited no activity in this process, while as a result of the reaction with the use of CeO2_N, traces of benzoic acid were formed. Thus, ceria itself is inactive in this reaction.
Deposition of gold on ceria resulted in a significant increase in the catalysts performance. It means that gold is the active phase in this reaction and the presence of this noble metal is essential for oxidation of the alcohol. The highest activity after 40 min of the reaction was observed for Au-CeO2_N (47% of alcohol conversion). Au-CeO2_H was less active than Au-CeO2_N (47% vs. 38% of alcohol conversion, respectively), while the lowest activity was characteristic of Au-CeO2_U (17% of alcohol conversion). To compare the activity of Au-CeO2_H and Au-CeO2_N at lower conversion of the alcohol the reaction time was reduced by half. As follows from Table 2, after 20 min of the reaction we observed the same tendency as previously described. In a shorter reaction time, Au-CeO2_N sample was still more active than Au-CeO2_H (33 vs. 20% of benzyl alcohol conversion, respectively). In terms of selectivity, it was established that at a low conversion of benzyl alcohol a significant amount of benzaldehyde was formed. However, with increasing alcohol conversion, the selectivity to benzoic acid increased. From among all the catalysts the highest selectivity to benzoic acid was shown by Au-CeO2_N sample (91% of benzoic acid selectivity after 40 min of the reaction).
To get a deeper insight into the oxidation of benzyl alcohol over the most active sample, i.e., Au-CeO2_N, the additional experiments in different reaction times and temperatures were performed. As can be seen from Figure 9A, the increase in reaction time led to an almost linear increase in the benzyl alcohol conversion. After 120 min of the reaction, Au-CeO2_N exhibited a very high activity of 98%. It is important to notice that with increasing the reaction time, the selectivity to benzoic acid also grew. According to Figure 9A, after 120 min of the reaction, benzoic acid was the only product. As far as the reaction temperature effect is concerned, it was found that the increase in reaction temperature from 40 °C to 80 °C led to the increase in the alcohol conversion from 47 to 58%, but had negligible impact on the selectivity (see Figure 9B). Au-CeO2_N exhibited very high selectivity to benzoic acid of ca. 90%, irrespectively of the reaction temperature.

3. Discussion

Results obtained in this study clearly show that the type of co-precipitation agent has a significant impact on the properties and catalytic activity of gold catalysts supported on cerium dioxide. The highest activity in low-temperature oxidation of benzyl alcohol was observed for the sample prepared with the use of sodium hydroxide as a co-precipitation agent. This catalyst had the highest surface area, the smallest gold particle size and the most uniform gold species. The latter two features of Au-CeO2_N catalyst resulted most likely from different growth mechanism of the nanostructure. Very important role in this process played different solubility of Au(OH)3 and Ce(OH)3 species. According to literature [42], the former species are much less soluble than the latter. As described in the experimental section, during the synthesis of Au-CeO2_N catalyst, the precipitation of both gold and cerium hydroxides was observed immediately after addition of NaOH because of a very fast increase in pH of the reaction mixture (high excess of NaOH). Under such reaction conditions, gold hydroxides could be adsorbed and stabilized on the surface of the as-formed cerium hydroxides during the hydrothermal treatment, and this promoted formation of small gold nanoparticles of the size of ca. 12.5 nm (see Table 1). In the case of syntheses with the use of HMTA and urea as co-precipitation agents, the concentration of hydroxide anions was very low at the beginning of the syntheses and increased during the gradual hydrolysis of the co-precipitation agents. Under such reaction conditions, gold hydroxides appeared before the cerium hydroxides because of lower solubility of the former species. Under such conditions, the aggregation of gold hydroxides in larger particles was favored, and this resulted in formation of much larger gold nanoparticles with large variations in gold particle size as observed for Au-CeO2_H and Au-CeO2_U. In view of the results obtained in this study, one can conclude that future research aiming at the synthesis of new and more active gold catalysts should be performed with the use of strong bases, such as NaOH, which cause immediate co-precipitation of metal ions.
As to the catalysts selectivity, Au-CeO2_N was observed to show the highest selectivity to benzoic acid. As described in the section on catalytic tests, Au-CeO2_N exhibited higher selectivity to benzoic at lower conversion of benzyl alcohol than Au-CeO2_H at a higher conversion of the alcohol (86% of selectivity to benzoic acid at 33% of alcohol conversion for Au-CeO2_N vs. 78% selectivity to benzoic acid at 38% of alcohol conversion for Au-CeO2_H, respectively; Table 2). Both catalysts had similar gold loading, but totally different porosity and pore structure. Au-CeO2_H had not only ca. twice larger average pores than Au-CeO2_N (12.0 nm vs. 7.4 nm, respectively; Table 1), but also much broader pore size distribution (pore structure of Au-CeO2_H sample consisted not only of mesopores, but also some macropores; see Figure 2D). Furthermore, SEM images of the catalysts clearly showed that the aggregates of Au-CeO2_N particles were much larger than those observed for Au-CeO2_H sample. In view of these facts, we concluded that the higher selectivity of Au-CeO2_N to benzoic acid may result from longer diffusion time of reactants within the pore network of Au-CeO2_N sample. It is highly probable that the longer diffusion pathway observed for Au-CeO2_N sample promoted further oxidation of the primary product of the alcohol oxidation, i.e., benzaldehyde, to benzoic acid. As far as the selectivity is concerned, it is worth noting that gold nanoparticles formed on the surface of Au-CeO2_N were significantly smaller than those found in Au-CeO2_H. Taking this fact into account, we assert that high selectivity of the former catalyst to benzoic acid may result not only from the differences in porosity of these two catalysts, but also to some extent, from the differences in gold particle size. It is highly probable that smaller gold nanoparticles found in Au-CeO2_N exhibited stronger ability to chemisorb benzaldehyde than the larger ones observed for Au-CeO2_H, and this promoted further oxidation of this primary product to benzoic acid.

4. Materials and Methods

4.1. Materials

The reagents were cerium(III) nitrate hexahydrate (Sigma-Aldrich; 99.99%), gold(III) chloride hydrate (Sigma-Aldrich, Saint-Louis, MI, USA; 99.995%), hexamethylenetetramine (Sigma-Aldrich, Saint-Louis, MI, USA; ≥99.5%), urea (Fluka, Monte Carlo; ≥99%); sodium hydroxide (POCH; analytical grade), benzyl alcohol (Sigma-Aldrich, Saint-Louis, MI, USA; 99.8%), methanol (HPLC grade; Sigma-Aldrich), deionized water. All the reagents were used without any further purification.

4.2. Synthesis of Catalysts

In a typical synthesis, 14.99 mmol (6.51 g) of cerium(III) nitrate hexahydrate was dissolved in 150 mL of deionized water. At the same time, 0.789 mmol (0.2682 g) of gold(III) chloride hydrate was dissolved in 40 mL of water and then added to the water solution of cerium source upon intensive stirring. In the next step, the as-prepared mixture containing cerium and gold sources was poured into a polypropylene bottle (of total volume = 500 mL). No precipitation at this step of the synthesis was observed. Next, 89.99 mmol of the co-precipitation agent (namely NaOH, urea or HMTA) was dissolved in 150 mL of deionized water and added upon continuous stirring to the propylene bottle containing water solution of cerium and gold sources. At this step, immediate precipitation was observed only for the synthesis with the use of NaOH. In the reaction mixtures containing HMTA and urea, no precipitate was observed to form. Then, the propylene bottle was tightly closed and transferred into the furnace heated to 90 °C. Following the heating for 18 h at 90 °C, the reaction mixture was cooled to room temperature, filtered and washed with water. The as-prepared powder was then dried at 80 °C for 20 h, crushed in a agate mortar and calcined at 400 °C for 2 h (temperature ramp: 1.66 °C/min) followed by calcination at 600 °C for 4 h (temperature ramp: 3.33 °C/min). The gold catalysts prepared with the use of NaOH, HMTA and urea as co-precipitation agents were labelled as Au-CeO2_N, Au-CeO2_H and Au-CeO2_U, respectively. Cerium oxides without gold have been also prepared to be reference materials. In the synthesis of these materials no gold source was added to the reaction mixture. The concentration of the other reagents was the same as that used for the preparation of Au-CeO2 catalysts. The as-prepared ceria supports synthesized with the use of NaOH, HMTA and urea were denoted as CeO2_N, CeO2_H and CeO2_U, respectively.

4.3. Characterization of Materials

The as-prepared materials were characterized with the use of inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray diffraction measurements (XRD), low-temperature nitrogen adsorption–desorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM), diffuse reflectance ultraviolet-visible spectroscopy (DR UV–vis) and X-ray photoelectron spectroscopy (XPS). Detailed experimental conditions are described in Supplementary Data.

4.4. Catalytic Activity Test

Oxidation of benzyl alcohol was performed using EasyMax 102 Advanced Thermostat system (Mettler Toledo). In a typical reaction, 20 mg of catalyst was added to a glass reactor (total volume of 25 mL) containing a mixture consisted of deionized water (10 mL), benzyl alcohol (20 µL) and sodium hydroxide (390 µL of 0.5 M water solution). The reaction mixture was then tightly closed in the reactor using a septum and purged with oxygen. Following the purging step, a balloon filled with oxygen was connected to the reaction vessel via a needle. Next, the reactor was placed into the EasyMax 102 Advanced Thermostat system and heated to 40 °C under continuous stirring (800 RPM; heating time from room temperature to 40 °C = 5 min). After a given reaction time (20, 40, 80 or 120 min), the reaction mixture was cooled down to 25 °C in five minutes, and then the agitation was stopped. Quantitative analyses of the reaction mixtures were made by high performance liquid chromatography (HPLC). The analysis was carried out using HPLC chromatograph (Waters) equipped with a photodiode array detector (Waters 996). Before the analysis the catalyst was removed from the reaction mixture by filtration through a 0.2 μm Millipore filter. The reactant and the products were separated on an XBridge® C18 Column (5 µm, 4.6 × 250 mm, Waters). The eluent was a mixture of methanol and water (vol. ratio of 48/52). In all the reactions carbon mass balance was equal to or higher than 95%.

5. Conclusions

Results obtained in this study clearly show that the type of co-precipitation agent has a significant impact on the structure, texture and catalytic properties of Au-CeO2 catalysts. It was found that the co-precipitation agent plays an important role in controlling the growth of gold particles. Immediate co-precipitation and growth of Au and CeO2 in the presence of NaOH was found to be more beneficial for formation of smaller and more uniform gold nanoparticles than gradual co-precipitation and growth of Au-CeO2 nanostructures in the presence of HMTA or urea. Nevertheless, taking into account relatively large gold particle size observed for the sample prepared with the use of sodium hydroxide as a co-precipitation agent (ca. 12.5 nm in diameter), one can conclude that further research aimed at optimization of synthesis conditions to obtain significantly smaller Au NPs in Au-CeO2_N nanocomposite is needed.
As concerns the catalysts activity, detailed analysis of the relationship between the properties of gold catalysts supported on cerium dioxide and their catalytic performance allowed drawing a conclusion that the activity of gold catalysts in low-temperature oxidation of benzyl alcohol is strongly affected by the size of gold nanoparticles. The presence of small metallic gold nanoparticles was a key factor for the obtainment of high activity in low-temperature oxidation of benzyl alcohol. It is expected that the new knowledge on the relationship between the type of co-precipitation agent used during the synthesis of Au-CeO2 catalysts and the properties of the as-synthesized materials may have a significant impact on design and development of new, more active and selective catalysts dedicated to low-temperature oxidation of alcohols using molecular oxygen as environmentally friendly oxidant.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11050641/s1, Extended experimental section–Characterization of materials; Figure S1. HRTEM image (a) and SAED diffraction pattern (b) of sample CeO2_N. In the inset the interplanar distance between 111 planes is denoted; Figure S2. HRTEM image (a) and SAED diffraction pattern (b) of sample CeO2_H. In the inset the interplanar distance between 111 planes is denoted; Figure S3. HRTEM image (a) and SAED diffraction pattern (b) of sample CeO2_U.

Author Contributions

Conceptualization, L.W.; methodology, L.W. and G.N.; validation, L.W. and G.N.; investigation, L.W. and G.N.; resources, M.Z. and S.J.; data curation, L.W. and G.N.; writing—original draft preparation, L.W. and G.N.; writing—review and editing, L.W., M.Z. and S.J.; visualization, L.W. and G.N.; supervision, M.Z. and S.J.; funding acquisition, L.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Centre, Poland, grant number 2018/28/C/ST5/00255.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Catalysts 11 00641 g001 550
Figure 1. XRD patterns of as-synthesized materials (A) and XRD patterns recorded for materials prepared with the use of sodium hydroxide as a co-precipitation agent shown at higher magnification (B).
Figure 1. XRD patterns of as-synthesized materials (A) and XRD patterns recorded for materials prepared with the use of sodium hydroxide as a co-precipitation agent shown at higher magnification (B).
Catalysts 11 00641 g001
Catalysts 11 00641 g002 550
Figure 2. Nitrogen adsorption–desorption isotherms (A,B) and pore size distribution (C,D) estimated for CeO2 and Au-CeO2 catalysts.
Figure 2. Nitrogen adsorption–desorption isotherms (A,B) and pore size distribution (C,D) estimated for CeO2 and Au-CeO2 catalysts.
Catalysts 11 00641 g002
Catalysts 11 00641 g003 550
Figure 3. SEM images of catalysts. Scale bars represents 10 µm.
Figure 3. SEM images of catalysts. Scale bars represents 10 µm.
Catalysts 11 00641 g003
Catalysts 11 00641 g004a 550Catalysts 11 00641 g004b 550
Figure 4. HRTEM images and Selected Area Electron Diffraction (SAED) patterns of Au-CeO2_N (A,A’), Au-CeO2_H (B,B’) and Au-CeO2_U (C,C’). The insets in (A–C) show the interplanar distance between the (111) planes of CeO2 and a single gold particle (dark region).
Figure 4. HRTEM images and Selected Area Electron Diffraction (SAED) patterns of Au-CeO2_N (A,A’), Au-CeO2_H (B,B’) and Au-CeO2_U (C,C’). The insets in (A–C) show the interplanar distance between the (111) planes of CeO2 and a single gold particle (dark region).
Catalysts 11 00641 g004aCatalysts 11 00641 g004b
Catalysts 11 00641 g005 550
Figure 5. High angle annular dark field STEM (HAADF STEM) image and EDS elemental map of Au-CeO2_N (A,A’), Au-CeO2_H (B,B’) and Au-CeO2_U (C,C’,D,D’).
Figure 5. High angle annular dark field STEM (HAADF STEM) image and EDS elemental map of Au-CeO2_N (A,A’), Au-CeO2_H (B,B’) and Au-CeO2_U (C,C’,D,D’).
Catalysts 11 00641 g005
Catalysts 11 00641 g006 550
Figure 6. (A,B) Diffuse Reflectance UV-vis spectra of catalysts.
Figure 6. (A,B) Diffuse Reflectance UV-vis spectra of catalysts.
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Catalysts 11 00641 g007 550
Figure 7. Ce 3d XP spectra of ceria supports (A) and gold catalysts (B).
Figure 7. Ce 3d XP spectra of ceria supports (A) and gold catalysts (B).
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Catalysts 11 00641 g008 550
Figure 8. Au 4f XP spectra of catalysts.
Figure 8. Au 4f XP spectra of catalysts.
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Catalysts 11 00641 g009 550
Figure 9. Activity and selectivity of Au-CeO2_N in benzyl alcohol oxidation after different reaction times at 40 °C (A) and after 40 min of reaction at different temperatures (B).
Figure 9. Activity and selectivity of Au-CeO2_N in benzyl alcohol oxidation after different reaction times at 40 °C (A) and after 40 min of reaction at different temperatures (B).
Catalysts 11 00641 g009
Table
Table 1. Characteristics of catalysts prepared with the use of different co-precipitation agents: urea (catalysts labelled as CeO2_U and Au-CeO2_U), HMTA (catalysts labelled as CeO2_H and Au-CeO2_H) and NaOH (catalysts labelled as CeO2_N and Au-CeO2_N).
Table 1. Characteristics of catalysts prepared with the use of different co-precipitation agents: urea (catalysts labelled as CeO2_U and Au-CeO2_U), HMTA (catalysts labelled as CeO2_H and Au-CeO2_H) and NaOH (catalysts labelled as CeO2_N and Au-CeO2_N).
CatalystReal Gold Loading 1 (wt.%)BET Surface Area 2 (m2/g)Average Pore Size 3 (nm)Average Gold Particle Size 4 (nm)
CeO2_U-466.9-
Au-CeO2_U4.3356.561.8
CeO2_H-1117.1-
Au-CeO2_H4.72812.051.5
CeO2_N-498.1-
Au-CeO2_N4.5457.412.5
1 estimated using ICP-OES; 2 estimated by Brunauer–Emmett–Teller (BET) method, 3 estimated from adsorption branch using Barrett–Joyner–Halenda (B.J.-H.) method; 4 estimated from XRD patterns using the Scherrer equation.
Table
Table 2. Results of benzyl alcohol oxidation at 40 °C 1.
Table 2. Results of benzyl alcohol oxidation at 40 °C 1.
CatalystReaction Time [Min]Conversion [%]Selectivity [%]
BenzaldehydeBenzoic Acid
CeO2_U40---
Au-CeO2_U40175644
CeO2_H40---
Au-CeO2_H20205248
40382278
CeO2_N40<< 1-traces
Au-CeO2_N20331486
4047991
1 Reaction conditions: catalyst (20 mg), deionized water (10 mL), benzyl alcohol (20 µL), sodium hydroxide (390 µL of 0.5 M water solution of NaOH), oxygen atmosphere (balloon filled with oxygen connected to the reactor), stirring (800 RPM).
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Wolski, L.; Nowaczyk, G.; Jurga, S.; Ziolek, M. Influence of Co-Precipitation Agent on the Structure, Texture and Catalytic Activity of Au-CeO2 Catalysts in Low-Temperature Oxidation of Benzyl Alcohol. Catalysts 2021, 11, 641. https://doi.org/10.3390/catal11050641

AMA Style

Wolski L, Nowaczyk G, Jurga S, Ziolek M. Influence of Co-Precipitation Agent on the Structure, Texture and Catalytic Activity of Au-CeO2 Catalysts in Low-Temperature Oxidation of Benzyl Alcohol. Catalysts. 2021; 11(5):641. https://doi.org/10.3390/catal11050641

Chicago/Turabian Style

Wolski, Lukasz, Grzegorz Nowaczyk, Stefan Jurga, and Maria Ziolek. 2021. "Influence of Co-Precipitation Agent on the Structure, Texture and Catalytic Activity of Au-CeO2 Catalysts in Low-Temperature Oxidation of Benzyl Alcohol" Catalysts 11, no. 5: 641. https://doi.org/10.3390/catal11050641

APA Style

Wolski, L., Nowaczyk, G., Jurga, S., & Ziolek, M. (2021). Influence of Co-Precipitation Agent on the Structure, Texture and Catalytic Activity of Au-CeO2 Catalysts in Low-Temperature Oxidation of Benzyl Alcohol. Catalysts, 11(5), 641. https://doi.org/10.3390/catal11050641

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