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Article

“PdO vs. PtO”—The Influence of PGM Oxide Promotion of Co3O4 Spinel on Direct NO Decomposition Activity

by
Gunugunuri K. Reddy
*,
Torin C. Peck
and
Charles A. Roberts
*
Toyota Research Institute of North America, Ann Arbor, MI 48105, USA
*
Authors to whom correspondence should be addressed.
Catalysts 2019, 9(1), 62; https://doi.org/10.3390/catal9010062
Submission received: 17 November 2018 / Revised: 22 December 2018 / Accepted: 5 January 2019 / Published: 9 January 2019
(This article belongs to the Special Issue Emissions Control Catalysis)
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Abstract

:
Direct decomposition of NO into N2 and O2 (2NO→N2 + O2) is recognized as the “ideal” reaction for NOx removal because it needs no reductant. It was reported that the spinel Co3O4 is one of the most active single-element oxide catalysts for NO decomposition at higher reaction temperatures, however, activity remains low below 650 °C. The present study aims to investigate new promoters for Co3O4, specifically PdO vs. PtO. Interestingly, the PdO promoter effect on Co3O4 was much greater than the PtO effect, yielding a 4 times higher activity for direct NO decomposition at 650 °C. Also, Co3O4 catalysts with the PdO promoter exhibit higher selectivity to N2 compared to PtO/Co3O4 catalysts. Several characterization measurements, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (H2-TPR), and in situ FT-IR, were performed to understand the effect of PdO vs. PtO on the properties of Co3O4. Structural and surface analysis measurements show that impregnation of PdO on Co3O4 leads to a greater ease of reduction of the catalysts and an increased thermal stability of surface adsorbed NOx species, which contribute to promotion of direct NO decomposition activity. In contrast, rather than remaining solely as a surface species, PtO enters the Co3O4 structure, and it promotes neither redox properties nor NO adsorption properties of Co3O4, resulting in a diminished promotional effect compared to PdO.

Graphical Abstract

1. Introduction

Nitrogen oxides (NOx) formed by combustion from fixed and mobile sources cause severe detrimental environmental problems, such as acid rain and photochemical smog [1,2,3]. Effectively controlling the emission of NOx is the topic of much research and has led to the introduction of many new catalyst technologies, such as three-way catalysts (TWC), NOx storage-reduction (NSR), and selective catalytic reduction (SCR) for NOx gas removal from mobile sources, and SCR and selective non-catalytic reduction (SNCR) for NOx gas removal from fixed sources [4,5,6]. Among various deNOx strategies, direct decomposition of NO (NO→1/2O2 + 1/2N2) has been considered to be the most desirable method because this reaction is thermodynamically favorable at low temperatures and does not need any reductants, such as NH3, H2, CO, or hydrocarbons. However, kinetic studies have indicated that the reaction needs to overcome a large activation energy (~335 kJ mol−1) barrier [4,5,6,7,8,9,10,11,12,13,14,15]. Accordingly, there is an apparent need for a suitable catalyst to decompose NOx at a given temperature, and therefore, significant research has been undertaken towards development of active and stable catalysts.
Since the pioneering work of Jellinek on the catalytic decomposition of NO in 1906, much research has been reported on NO direct decomposition over several materials, including perovskites, rare earth oxides, and Cu-zeolites [2,3,4,5,6,7]. Numerous metal oxides have also been examined as candidates for NO decomposition catalysts [16] and Co3O4 is often recognized as a significant component in many active catalysts at higher reaction temperatures [17,18,19,20,21]. However, Haneda et al. recently reported that NO decomposition takes place slowly, if at all, over pure Co3O4 at temperatures below 650 °C [18]. They reported that the presence of small amounts of alkali metals were essential to activate NO decomposition over Co3O4 oxide by enhancing NO adsorption [18,19,20]. This interesting effect of alkali metals, particularly Na, was also reported by Kung et al. [21], but dependence on alkali metals is not feasible for practical applications due to their volatile nature at temperatures above 600 °C.
Metal oxide supported platinum group metals (PGM metals) were also one of the earliest types of NO decomposition catalysts studied, and the results have been widely reported; mainly Au, Pt, Pd, and Ir at temperatures higher than 700 °C [22,23]. Suzuki et al. [24] synthesized a porous CaZrO3/MgO/Pt composite and found that this catalyst could obtain a NO conversion rate of about 52% at 900 °C in the absence of O2. Haneda et al. [25] found that the addition of Pt improved the direct NO decomposition performance of rare earth oxides. They [26,27] also compared the activity of [Pd(NH3)4] (NO3)2, Pd(NO3)2, Pd(CH3COO)2, and (NH4)2-[PdCl4] as palladium precursors for NO decomposition in a Pd/Al2O3 catalyst at 700 °C, and the activity was found to decrease in the order of Pd(NO3)2 > [Pd(NH3)4] (NO3)2 > Pd(CH3COO)2 >> (NH4)2[PdCl4]. Almusaiteer et al. [28] reported that compared to Pd/Al2O3, the Pd/C (activated carbon) catalyst was found to be more beneficial for O2 desorption, but both have similar activity. Oliveira et al. [29] investigated the catalytic performance of palladium and copper catalysts loaded in mordenite (MOR) and found that these catalysts were more active for NO decomposition than alumina supported catalysts. However, the reports on supported PtO catalysts for direct NO decomposition at temperatures below 700 °C are very limited in the literature. Similarly, metallic Pd catalysts always deactivate over time due to oxidation of Pd metal to PdO at temperatures below 650 °C [30].
To the best of our knowledge, Co3O4 supported PGM catalysts have never been explored for direct NO decomposition, likely due to the inactivity of the individual components at lower temperatures. However, the need for enhanced NO adsorption on Co3O4 suggests that the addition of PGM promotion can lead to increased low temperature activity. Hence, the present study aims to investigate the promotional effect of PdO vs. PtO on the Co3O4 for direct NO decomposition. The activity measurements show that the optimum PdO/Co3O4 catalyst exhibits 4 times higher activity than PtO/Co3O4 catalysts. Several characterization techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2-Temperature programmed reduction (H2-TPR), and in situ FT-IR are employed to understand the influence of PdO and PtO on the structural and surface properties of Co3O4.

2. Results and Discussion

2.1. Direct NO Decomposition Activity Measurements

To qualify as direct NO decomposition, the catalyst must decompose NO into just two products: N2 and O2. The possibility for unwanted N2O and NO2 formation as side products cannot be neglected during the analysis of this reaction. Therefore, high NO conversion is desired, but not sufficient; it is also very important to maximize selectivity towards N2 rather than N2O or NO2. Considering all possible products, the reaction can be written as:
2 NO→N2+ O2
4 NO→2N2O+ O2
NO + [O]→NO2 ([O]: catalyst lattice oxygen)
The selectivity to N2 can be defined as:
N2 selectivity (%) = 2 × [N2]/(2 × [N2] + 2[N2O] + [NO2])
Also, when NO dissociates on the catalyst surface, it is possible for N2 production to occur, but without simultaneous release of the stoichiometric amount of the O2 product. This situation can occur quite frequently and is the topic of a previous study from some of us [30]. Rather than desorb as the O2 product, oxygen atoms either remain strongly adsorbed on the surface or chemically react with catalyst material, oxidizing both surface and bulk, and changing the catalyst composition. In either case, the catalyst deactivates over time. Hence, it is very important to confirm that the catalyst releases both N2 and O2 as products.
Direct NO decomposition measurements were performed over the pure spinel oxide Co3O4 and over the PdO- and PtO-promoted Co3O4 catalysts, denoted PdO/Co3O4 and PtO/Co3O4, respectively. Activity was measured for 2 h at 400, 450, 550, and 650 °C. The direct NO decomposition activity to N2 of PdO/Co3O4 and PtO/Co3O4 with varying PGM loading is presented in Figure 1 as a function of temperature and compared to the pure Co3O4 spinel. All numeric values of NO conversion and N2, N2O, NO2 ppm product concentrations of the Co3O4, PdO/Co3O4, and PtO/Co3O4 catalysts at various reaction temperatures are presented in Tables S1 and S2. The raw NO conversion profiles (NO converted to all the products) of Co3O4, 3PdO/Co3O4, and 4 PtO/Co3O4 during the steady state direct NO decomposition obtained from FT-IR detector in the temperature region 400 to 650 °C are presented in Figure S1. High values for NO conversion at lower temperatures may seem counterintuitive, however, most of the NO conversion at low temperature is simply due to thermodynamically favorable NO oxidation to NO2. For example, as shown in Table S1, the NO conversion values of Co3O4, 3 PdO/Co3O4, are 3 PtO/Co3O4 are 3.15, 2.92, and 7.66, respectively, giving the impression that 3 PtO/Co3O4 is the most active catalyst at 400 °C. However, for 3 PtO/Co3O4, the NO conversion specifically to N2 is lower than that of 3 PdO/Co3O4 because most of the NO conversion is due to NO oxidation. Hence, for NO decomposition to N2, it is more instructive to consider NO converted into N2 rather than total NO conversion. The activity values were calculated in this manner and are presented in Figure 1 in units of micromoles of NO converted to N2 per gram per second.
NO and Ar partial pressure values obtained by mass spectrometry (MS) during the steady state measurements are presented in Figure S2 and compared with data obtained in the absence of the catalyst, which serves as a baseline. Inert Ar gas was introduced as a tracer to monitor for potential systematic variation in signal intensity during the experiment. As shown in Figure S2, no change in Ar signal intensity was observed during the experiment, however, the intensities of NO signal changed based on catalyst identity and reaction temperature. These measurements confirm the change in the NO signal is due to catalytic conversion of NO and not due to artifacts. Also, the MS signal for NO (m/z = 30) tracks with the conversion reported by the FT-IR measurements (Figure S1). Figures S3–S5 present the MS partial pressure values of the N2, N2O, and NO2 products, respectively, during direct NO decomposition and are also compared to the MS partial pressures obtained in the absence of the catalyst. As shown in Figure S3, the N2 and O2 signal intensities are higher compared to the background signals, which confirms the simultaneous release of N2 and O2 as expected for NO decomposition. Furthermore, the MS intensity of the N2 signal qualitatively tracks with the N2 concentrations calculated by nitrogen mass balance from the FTIR measurements (Tables S1 and S2), lending additional confidence in the activity results. Similarly, good correlation between the FT-IR detection of N2O and NO2 and the MS signals was observed (compare Tables S1 and S2 to Figures S3 and S4). Finally, the release of oxygen as a product (Figure S3) and stable NO conversion (Figures S1 and S2) during the steady state measurements suggest that the catalysts were not poisoned by the irreversible chemical adsorption of oxygen. Thus, the good correlation between the FT-IR data and MS signals suggests that the calculation of N2 production from the FT-IR is reliable and can be used for the calculation of activity from NO conversion to N2. As shown in Figure 1b, NO decomposition activity to N2 increases slightly with temperature up to 550 °C for the pure Co3O4 catalyst. Further increase in the temperature to 650 °C results in decreased activity. This result suggests that the Co3O4 spinel is not a good catalyst for NO decomposition at temperatures below 650 °C, as indicated by Hamada et al. [21]. The addition of PdO and PtO to the Co3O4 spinel improves the direct NOx decomposition activity of Co3O4. Direct NOx activity increases with temperature for all palladium and platinum loadings, but unlike the pure Co3O4, deactivation was not observed above 550 °C for the any of PdO or PtO catalysts.
The direct NO decomposition activity of PdO/Co3O4 and PtO/Co3O4 to N2 at various temperatures is presented as a function of the weight percent loading of the respective PGMs in Figure 2. For PdO/Co3O4, activity increases with palladium loading up to 3 wt%, but further increase in the palladium loading leads to decreased activity (Figure 2a). For PtO/Co3O4, activity increases with PtO loading up to 4 wt%, however, the overall effect on activity is significantly diminished compared to PdO/Co3O4 (Figure 2b). The optimum loading of each PGM was found to be 3 wt% PdO/Co3O4 and 4wt% PtO/Co3O4. Interestingly, PdO-promoted catalysts exhibit higher activity than the PtO-promoted catalysts. At 650 °C, the optimum PdO/Co3O4 catalyst exhibits 4 times higher activity compared to the optimum PtO/Co3O4 catalyst.
To confirm the reaction is indeed direct NO decomposition to N2 rather than the unwanted production of N2O or NO2, the selectivity to N2 was calculated. The N2 selectivity is presented as a function of PGM loading from 400–650 °C for PdO/Co3O4 and PtO/Co3O4 in Figure 3a,b, respectively. As expected, pure Co3O4 (0 wt% PGM loading) exhibited relatively low selectivity to N2 (≤20%) at 400 and 450 °C. The N2 selectivity increased to 80% at 550 °C and to 100% at 650 °C. Formation of N2O was not observed and only N2 and NO2 products are detected during direct NO decomposition over the PtO- and PdO-promoted Co3O4 catalysts (Tables S1 and S2). Thus, the product distribution measurements suggest NO oxidation (NO2 formation) is more favorable at lower reaction temperatures (≤450 °C), and at higher reaction temperatures, NO decomposition (N2 formation) is predominant. Remarkably, the addition of 1 wt% PdO to the Co3O4 improves the N2 selectivity from 1 to 40% at 400 °C and from 21 to 70% at 450 °C. The N2 selectivity further increased to 50% at 400 °C and 75% at 450 °C with increasing PdO loading to 3 wt%. Increasing the PdO loading from 3 wt% to 4 wt% lead to only a slight decrease in the N2 selectivity. When also considering the activity measurement in Figure 2, the N2 selectivity measurements confirm 3 wt% PdO as the optimum loading on Co3O4 for direct NO decomposition.
Regarding N2 selectivity as a function of PtO loading (Figure 3b), the addition of 1 wt% PtO yielded less improvement at 400 °C (1 to 16%) compared to the addition of 1 wt% PdO (1 to 40%). Furthermore, the N2 selectivity decreases with increasing PtO loading, dropping to 9% for the 4wt% PtO/Co3O4 catalyst. Moreover, there is no improvement in the selectivity observed at reaction temperatures at and above 450 °C. Therefore, the direct NO decomposition measurements show that the addition of PdO to Co3O4 improves the decomposition activity and N2 selectivity and 3wt% is the optimum Pd loading over Co3O4, whereas PtO loaded on Co3O4 leads to only slight improvement in the activity and almost no influence on the N2 selectivity.

2.2. Catalyst Characterization

2.2.1. Structural and Textural Properties

These catalysts have been evaluated using several characterization techniques, like XRD, XPS, H2-TPR, BET surface area, and in-situ FT-IR during NO adsorption, to understand the influence of PdO and PtO on the structural and surface properties of Co3O4 and to explain the greater promoter effect of PdO on Co3O4 compared to PtO during NO decomposition. The palladium and platinum loadings of the studied catalysts prepared by impregnation were verified with XRF spectrometry. For the nominal 1.0, 2.0, 3.0, and 4.0 wt% of PdO, the experimental values were 0.83, 1.94, 2.80, and 4.15, respectively (Table 1). The experimental values for PtO doped Co3O4 catalysts were 0.93, 2.12, 3.23, and 4.02, respectively. Differences may be due to surface heterogeneity or incomplete precursor dispersion during the impregnation procedure or the inherent uncertainty related to the employed XRF method, which did not utilize a standard material to aid the data analysis.
The BET surface area values of PdO/Co3O4 and PtO/Co3O4 catalysts and the pure Co3O4 are presented in Table 1. The pure Co3O4 catalyst exhibits a BET surface area of 36 m2/g. Little change in the surface area is observed after impregnating Co3O4 with 1, 2, and 3 wt% Pd. However, increasing Pd loading from 3 to 4 wt% on to Co3O4 lead to a decrease in the surface area from 33 to 26 m2/g. These values suggest that PdO dispersed very well on the surface of Co3O4 until 3wt% and further increase in the loading to 4wt% likely leads to surface agglomeration, which can block access to the active surface. The BET surface area measurements are corroborated by the activity measurements, which showed that the activity of Co3O4 increases with increasing palladium doping only until 3 wt%. Further increase in the Pd loading to 4 wt% lead to a decrease in the activity. Little change in surface area was observed for the Co3O4 with platinum impregnation (Table 1), as only a slight decrease in the surface area was observed at the highest loading. As suggested above, the blockage of the active surface may be the cause of the decrease in the activity for the 4 wt% PdO/Co3O4 catalyst. It is hypothesized that the formation of PdO crystallites is responsible for this behavior, and this hypothesis will be investigated below.
The X-ray diffraction (XRD) patterns of the fresh PdO/Co3O4 and PtO/Co3O4 catalysts are shown in Figure 4a,b, with the pattern of the pure Co3O4 for reference. The X-ray diffraction lines characteristic of the cubic cobalt spinel structure were indexed within the Fd3m space group (JCPDS card no. 01-080-1533) in the case of the pure Co3O4 catalyst [31]. As shown in Figure 4a, all the PdO/Co3O4 catalysts exhibit peaks due to Co3O4. The diffractograms provide evidence that the spinel structure was preserved after Pd impregnation, revealing no observable structural changes compared to the pure Co3O4 carrier. Diffraction peaks related to Pd or PdO were not detected on samples up to a nominal Pd loading of 3 wt%. For the 4 wt% Pd sample, a low-intensity diffraction peak indicative of tetragonal PdO (0 0 2) (JCPDS card no. 75-584) was visible at 2θ of 33.6° [32]. These measurements suggest that PdO is well-dispersed on the Co3O4 support up to 3 wt%, and above this loading, crystalline PdO forms on Co3O4 surface. As stated above, the BET surface decreases from 33 to 26 m2/g with increasing Pd loading from 3 wt% to 4 wt%. XRD measurements confirm that the decrease in the surface area is due to the formation of crystalline PdO on the surface of Co3O4 and blocking of the active surface. The X-ray diffraction patterns of PtO/Co3O4 catalysts are shown in Figure 4b and only peaks due to Co3O4 are present at all Pt loadings, suggesting the absence of bulk metallic Pt or PtO with long-range order. However, unlike PdO/Co3O4, the XRD peaks of the PtO/Co3O4 samples were shifted to higher values relative to the pure Co3O4 spinel for all Pt loadings. The shift in the peak position to higher values indicates that Pt is likely incorporating into the Co3O4 spinel structure in contrast to PdO/Co3O4, where the absence of the peak shift indicates that Pd remained dispersed on the Co3O4 surface.
Figure 5a,b display X-ray diffraction patterns of the spent PdO/Co3O4 and PtO/Co3O4 catalysts after direct NO decomposition. As shown in Figure 5a, the spent Co3O4 exhibits only peaks due to spinel structure. There are no peaks due to either CoO or metallic Co, suggesting the Co3O4 spinel is structurally stable during direct NO decomposition. In addition to the Co3O4 spinel peaks, the spent PdO/Co3O4 catalysts with 1, 2, and 3 wt% Pd loading also exhibit peaks at 2θ values of 40.3, 46.79, and 68.4°. These peaks are due to the (1 1 1), (2 0 0), and (2 2 0) facets of Pd metal (JCPDS no: 46-1043). The XRD measurements of the spent catalysts show that the dispersed PdO reduced to Pd metal during direct NO decomposition. Similar to the XRD pattern for the fresh 4 wt% PdO/Co3O4, the spent XRD pattern also exhibits peaks due to PdO along with the Pd metal and Co3O4 peak, which suggests that the crystalline PdO remains even after direct NO decomposition. The X-ray diffraction patterns of the spent PtO/Co3O4 catalysts after direct NO decomposition are displayed in Figure 5b. The spent PtO/Co3O4 catalysts exhibit peaks due only to Co3O4 after direct NO decomposition. In contrast to the metallic phase observed in the spent PdO/Co3O4 catalysts, there are no peaks due to metallic Pt observed in the spent PtO/Co3O4 catalysts.
The XRD measurements show that in the case of PdO/Co3O4 catalysts, the PdO reduced to metallic Pd during direct NO decomposition and promotes the activity of Co3O4 catalysts. On the other hand, no metallic Pt formation occurred in the PtO/Co3O4 catalysts, leading to a greatly diminished promoter effect compared to the PdO/Co3O4 catalysts. Also, the NO decomposition measurements show that the catalytic activity decreases with increasing Pd loading from 3 wt% to 4 wt%. The formation of crystalline PdO and decrease in the surface area (blocking of the active surface) explains the lower activity of 4 wt% sample compared to 3 wt% sample. Hence, XRD and BET surface area measurements corroborate with the activity measurements. The spent PtO/Co3O4 catalysts also exhibit a shift in the peak positions to higher 2θ values compared to the pure Co3O4 catalyst, which suggests the incorporation of Pt into the Co3O4 spinel structure even after direct NO decomposition.

2.2.2. Redox Properties

The influence of PdO and PtO on the redox properties of Co3O4 are investigated using H2-temperature programmed reduction (H2-TPR) measurements. The H2-TPR profiles of PdO/Co3O4 and PtO/Co3O4 catalysts are presented in Figure 6a,b, along with that of the pure Co3O4 for comparison. Several authors reported that the reduction behavior of Co3O4 is strongly dependent on the preparation method, catalyst composition, and dispersion on a support [33,34]. The reduction behavior of Co3O4 was widely accepted as a stepwise process, including the reduction of Co3+ to Co2+ and Co2+ to metallic Co. There are three well-defined reduction peaks in the TPR profile of Co3O4 (Figure 6). The peak at 235 °C is attributed to the reduction of surface oxygen species. The other two peaks are for the stepwise reduction of Co3O4 to metallic cobalt. According to the literature, the second reduction peak centered at 275 °C is due to the reduction of Co3O4 to CoO, and the third peak at the region of 305 °C is due to the reduction of CoO to metallic cobalt [33,34].
Co3O4 + H2→3CoO + H2O
CoO + H2→Co + H2O
The addition of 1 wt% PdO to the Co3O4 leads to a drastic change in the redox profile of Co3O4 (Figure 6). No peaks were observed in the 250–310 °C temperature region. Both PdO and Co3O4 were reduced at much lower temperature and all reduction events completed below 150 °C. These measurements show that PdO promotes the reduction of Co3O4. Two reduction peaks were observed in the 1 wt% PdO/Co3O4 H2-TPR profile at 79 and 104 °C. The first reduction peak at 79 °C is due to the reduction of PdO to metallic Pd and the second reduction peak is due to the reduction of Co3O4. The H2-TPR profiles for 1, 2, and 3 wt% PdO/Co3O4 were all very similar (Figure 6). The promotion of Co3O4 reduction by Pd observed in H2-TPR is possibly ascribed to hydrogen spillover and the synergistic effect between Pd species and Co3O4. The synergistic effect can weaken the Co-O bond. Chen et al. [35] also reported a similar promotional effect for PdO impregnated on Co3O4 catalysts with different morphologies, and the synergistic effect between Pd and Co existed, regardless of Co3O4 morphology. In the present study, the intensity of the first reduction peak increases with increasing PdO loading from 1 to 3 wt%, and the increase is accompanied by a slight shift in the peak temperature from 79 to 85 °C. This may be due to the increase in the PdO loading on the Co3O4 surface. The reduction profile of 4 wt% PdO/Co3O4 is slightly different from the PdO promoted catalysts of lower loading. Along with the peaks due to Co3O4 and PdO, a small additional peak is observed at 220 °C. Given the identification of crystalline PdO in the XRD pattern of the 4 wt% PdO/Co3O4, it is reasonable to assign this peak to reduction of crystalline PdO.
The H2-TPR profiles of PtO/Co3O4 catalysts are presented in Figure 6b. Two types of reduction features were observed in the case of PtO promoted Co3O4 catalysts, one from 130 to 190 °C and another from 200 to 325 °C. The first feature corresponds to reduction of PtO to metallic Pt and the second is reduction of Co3O4. Unlike PdO/Co3O4, little to no shift in the Co3O4 reduction temperature of PtO/Co3O4 catalysts was observed relative to the pure Co3O4. The reduction of Co3+ to Co2+ occurred in the 260–275 °C temperature region for Co3O4 and PtO/Co3O4 catalysts, irrespective of PtO loading, and the reduction of PtO occurred separately at a distinctly lower temperature. Yang et al. [36] observed similar behavior for Pt promoted Co3O4/Al2O3 catalysts, wherein both PtO and Co3O4 reduced separately in distinct temperature regions. Even though the Co3O4 reduction shifted to slightly lower temperatures at higher Pt loadings in their study, a synergistic effect, similar to Pd and Co in PdO/Co3O4, was not observed between Pt and Co. This lack of synergistic effect by Pt on the reduction of Co3O4 is consistent with the smaller promotional effect of Pt on direct NO decomposition activity compared to Pd promotion. Conversely, the decreased reduction temperature of Co3O4 observed in H2-TPR measurements of PdO/Co3O4 illustrates how Pd can promote direct NO decomposition by enhancing the reducibility of the catalyst.

2.2.3. Surface Properties

The X-ray photoelectron spectroscopy (XPS) was used to investigate the surface elemental compositions, metal oxidation states, and adsorbed oxygen species of the as-prepared and spent samples. The O1s XPS spectra of fresh PdO- and PtO-promoted Co3O4 catalysts are presented in Figure 7, with that of the pure Co3O4 for comparison. The pure Co3O4 exhibits two peaks in the O1s spectra. The large peak at lower binding energy (BE = 530.2–530.7 eV) is attributed to the surface lattice oxygen in Co3O4 (denoted as Olat) [37]. The shoulder at higher BE (532.0–532.7 eV) is associated with oxygen atoms present as surface adsorbed oxygen or surface hydroxyl groups or defect oxide (denoted as Oad). The PdO- and PtO-promoted Co3O4 samples also exhibit two peaks in their O1s spectra due to the Olat and Oad species., however, little difference in the peak energies is observed. This is may be due to lower loadings of promoters.
The fitted Co2p XPS spectra of the fresh PdO- and PtO-promoted Co3O4 catalysts are presented in Figure 8, with that of the pure Co3O4 for comparison. In the pure Co3O4 XPS spectrum, the main peak in the BE range of 780.7–782.2 eV is assigned to Co2p3/2, and the shoulder at 795.9–797.9 eV is attributed to Co2p1/2. Pure Co3O4 exhibits peaks due to both Co3+ and Co2+ and their satellites. The main Co2p3/2 feature can be further resolved into two components, with BE values centered at 778.7–780.4 eV and 779.8–781.6 eV, and corresponding to Co3+ and Co2+, respectively [38]. Furthermore, the presence of the satellite peaks also confirms the presence of Co2+ in the catalysts. As expected for samples containing Co3O4 spinel, all catalysts exhibited peaks and satellites due to both Co3+ and Co2+, irrespective of Pd or Pt promoter loading. Also, no significant change in the position of the peaks was observed upon impregnation of Co3O4 with PdO or PtO. The spent catalysts also exhibit peaks due to the Co3+ and Co2+ ions, irrespective of promoter identity or loading. The Co2p XPS measurements show that the Co3O4 spinel is very stable during direct NO decomposition, which agrees with XRD measurements.
The Pd3d XPS spectra of the fresh and spent 2, 3, and 4 wt% PdO/Co3O4 are presented in Figure 9a,b. In general, Pd may exist as Pd0 (335.1–335.4 eV [39]), Pd2+ (336.8.1–337.2 eV or 336.3–336.8 eV [40,41,42,43,44]), Pd4+ (337.8–339.3 eV), or a combination thereof. All the PdO/Co3O4 catalysts exhibit peaks due to the Pd2+ and Pd4+ after calcination at all PdO loadings. However, XRD measurements show no peaks corresponding to PdO or PdO2 up to 3 wt% loading, which indicates that the PdO present on Co3O4 is in amorphous form and dispersed very well on the surface. In agreement with the above XRD analysis of the spent samples (see Figure 6a), XPS indicates PdO/Co3O4 catalysts exhibit peaks due to PdO and metallic Pd after direct NOx decomposition. These results suggest that some of the PdO reduced to metallic Pd during direct NOx decomposition, which corroborates the evidence from H2-TPR and XRD of the promotional effect of Pd on the activity of Co3O4 spinel catalysts. The intensity of the metallic Pd increases with increasing PdO loading from 2 to 3 wt%, however, the intensity of the metallic Pd peak decreases drastically with further PdO loading from 3 to 4 wt%. This is due to the formation of the separate bulk PdO phase in the spent 4 wt% PdO/Co3O4 sample, which is clearly observed in the spent 4 wt% PdO/Co3O4 Pd3d spectrum. The formation of a separate PdO phase leads to less reduction of PdO to metallic Pd during direct NOx decomposition and is the likely cause of the lower activity compared to the 3 wt% PdO/Co3O4 catalyst. These results agree with the conclusions made based on the spent XRD patterns (see Figure 5a), further strengthening the evidence that 3 wt% Pd is the optimum loading for promoting activity.
Figure 10a,b present the Pt4f XPS spectra of the fresh and spent 2, 3, and 4 wt% PdO/Co3O4. All fresh and spent PtO/Co3O4 catalysts only exhibit peaks due to Pt2+ (72.3, 74.1 eV) at all Pt loadings [45]. There are no observed peaks due to either Pt4+ or metallic Pt0 in contrast to the PdO/Co3O4 catalysts, wherein the PGM underwent significant changes in oxidation state with exposure to reaction conditions. Overall, XPS measurements show that the support Co3O4 is very stable during the promoter impregnation, as well as during the direct NO decomposition. PdO reduced to metallic Pd during the direct NO decomposition and improves the direct NO decomposition activity of Co3O4. On the other hand, PtO stays in an oxidized state (no metallic Pt formation) during the direct NO decomposition and exhibits less promotional effects compared to PdO.

2.2.4. NO Adsorption Properties

The adsorption of NO and formation of surface intermediates is essential to establishing activity for direct NO decomposition. In situ FT-IR spectroscopy was performed on pure Co3O4 and the PdO- and PtO-promoted Co3O4 catalysts to understand the interaction of NO with the catalyst during adsorption. In situ FT-IR measurements were collected during NO adsorption over the catalysts at 300 °C. Before NO adsorption, all the catalysts were pretreated at 350 °C in the presence of 10% O2 in a helium balance and cooled to 300 °C in the presence of helium. All the spectra collected were normalized with respect to the gas phase NO peak at 1874 cm−1. The in situ FTIR spectra of Co3O4, PdO- and PtO-promoted Co3O4 catalysts during NO adsorption are presented in Figure 11a,b. As shown in Figure 11a, little to no NO adsorption occurs over the pure Co3O4 spinel oxide at 300 °C, as no clear peaks were present relative to the noise level. Interestingly, impregnating PdO over Co3O4 leads to a formation of chelating surface nitrate intermediates (1577 and 1254 cm−1) during the NO adsorption [46]. The intermediate formation was observed for all the catalysts irrespective of loading and the intensity of the peak increases with PdO loading. On the other hand, PtO-promoted Co3O4 catalysts do not produce spectroscopically relevant amounts of intermediates during the NO adsorption and exhibit spectra similar to the pristine Co3O4 catalyst. The catalysts in the current study exhibit activity at temperatures ≥400 °C, however, at these temperatures, no spectroscopically relevant surface NOx species were observed by in situ FTIR (not shown). This observation indicates that neither the surface chelating nitrate nor any other surface NOx species is the most abundant reactive intermediate in the direct NO decomposition mechanism. The in situ FTIR results at 300 °C are, therefore, interpreted as a probe of the affinity of the NO reactant molecule to interact with the catalyst surface. In this interpretation, it is concluded that the presence of PdO improves the affinity of the catalyst to interact with NO compared to PtO or the pure Co3O4 support.
The direct NO decomposition measurements show that PdO promotes direct NO decomposition activity of Co3O4 much better compared to the PtO. The Co3O4 is a normal spinel with AB2O4 formula, where A (Td) sites are occupied by Co2+ ions and B (Oh) sites are occupied by Co3+ ions. According to the general mechanism proposed by Haneda et al. [18], initially NO adsorbs on the surface and decomposes into N and O. The oxygen atoms adsorb on Co2+ ions and are oxidized to Co3+. Then, Co3+ ions reduce back to Co2+ upon release of the oxygen as a product. Hence, NO adsorption and oxygen release (redox) properties are very important for direct NO decomposition. The in-situ FT-IR results reveal that PdO increases the affinity of the catalyst to form surface NOx species compared to PtO or a pure Co3O4 support. The H2-TPR studies in our work show that the Co3O4 reduction temperature is significantly decreased by the presence of dispersed PdO, thus suggesting a more facile reduction of Co3+ to Co2+ to release O2 during direct NOx decomposition is possible. The improvement in the NOx adsorption properties and ease of cobalt reduction explains the better direct NO decomposition activity of PdO catalysts compared to PtO catalysts.

3. Materials and Methods

3.1. Catalyst Synthesis

Palladium and platinum promoted Co3O4 catalysts were synthesized using the wet impregnation method. Commercial Co3O4 was purchased from Sigma-Aldrich (St. Louis, MO, USA) (99.5% trace metal basis) and used as received without any further modification for the synthesis. In a typical synthesis procedure, 5 g of commercial Co3O4 were mixed with 50 mL of water. Then, the required quantity of palladium nitrate hydrate (Sigma-Aldrich), or tetraamine platinum (II) nitrate (Sigma-Aldric, 99.995% trace metal basis) was dissolved separately in deionized water and combined with the Co3O4 suspension. The mixture was heated to 80 °C with continuous stirring. The obtained powder was then dried in an oven at 120 °C for 12 h under air. Finally, the catalyst was calcined at 400 °C for 4 h with a 1 °C/min ramp. Different loadings of palladium and platinum on Co3O4 (nominally 1 to 4 wt%) catalysts were prepared by varying the amount of palladium nitrate or platinum nitrate. For reference, the commercial Co3O4 support was also calcined at 400 °C for 4 h.

3.2. Catalyst Characterization

X-ray diffraction: X-ray powder diffraction (XRD) patterns were obtained using a Rigaku SmartLab X-ray diffractometer (Rigaku, The Woodlands, TX, USA) using Cu Kα radiation (1.5405 A). A glass holder was used to support the sample. The scanning range was from 10 to 80° (2θ) with a step size of 0.02° and a step time of 1 s. The XRD phases present in the samples were identified with the help of ICDD-JCPDS [31] data files.
BET Surface Area Measurements: The surface area of the PdO and PtO promoted Co3O4 materials were measured using a Micromeritics 3Flex surface characterization instrument (Micromeritics, Atlanta, GA, USA). N2 physisorption isotherms was conducted at −196 °C, and the surface area was measured by the BET method. Prior to the analyses, the samples were outgassed at 300 °C under vacuum (5 × 10−3 Torr) for 3 h.
X-ray Fluorescence Measurements: XRF was collected using a Rigaku ZSX, primus II X-ray spectrometer (Rigaku, The Woodlands, TX, USA). Impurities in the crystals were gained by X-ray fluorescence in operation of spectrometer in standard fewer modes with coverage of a full element. The amount of any elements and oxides particles was detected by the XRF experiment.
H2-Temperature Programmed Reduction (H2-TPR) Measurements: The redox properties of the PtO/Co3O4 and PdO/Co3O4 catalysts were studied using H2 temperature programmed reduction (H2-TPR) experiments. H2-TPR experiments were performed using a Micromeritics 3Flex surface characterization instrument (Micromeritics, Atlanta, GA, USA) equipped with a thermal conductivity detector. Before the experiment, the catalysts were preheated to 300 °C in the presence of 20% O2/He (30 mL/min). After the pretreatment, the temperature was decreased to 50 °C. The H2-TPR measurements were performed by heating the catalyst from 50 to 600 °C in the presence of 10% H2/Ar (30 mL/min).
X-ray Photo Electron Spectroscopy: The XPS measurements were performed using a PHI 5000 Versa Probe II X-ray photoelectron spectrometer (Physical Electronics, East Chanhassen, MN, USA) using an Al Kα source. Charging of the catalyst samples was corrected by setting the binding energy of the adventitious carbon (C1s) to 284.6 eV [47]. The XPS analysis was performed at ambient temperature and at pressures typically on the order of 107 Torr. Prior to the analysis, the samples were outgassed under vacuum for 30 min.
In Situ FTIR Spectroscopy Measurements during NO Adsorption: The NO adsorption properties were measured using in situ Fourier transform infrared (FTIR) spectroscopy. The Harrick High Temperature Cell with environmental (gas flow) and temperature control was used for in situ diffuse-reflectance FTIR spectroscopy. Spectra were recorded using a Thermo Scientific Nicolet 8700 Research FT-IR Spectrometer (Thermo Scientific Fidher, Waltham, MA USA) equipped with a liquid N2 cooled MCT detector. Spectra were obtained with a resolution of 2 cm−1 and by averaging 64 scans. In situ diffuse-reflectance FTIR spectra were collected during NO adsorption at 300 °C. Prior to NO adsorption, the sample was first pretreated at 350 °C in 30 mL/min of 10% O2/He. The background spectrum (64 scans) was of the catalyst after cooling to 300 °C in 30 mL/min UHP He. Adsorption of NO was achieved by flowing 30 mL/min of 10,000 ppm NO over the catalyst for 25 min. Adsorption of NO proceeded for 25 min, while spectra were obtained every minute using a series collection. To compare peak intensities among different catalyst samples, the adsorption spectra were normalized to the NO gas phase peak at ~1876 cm−1.

3.3. Direct NO Decomposition Measurements

Direct NO decomposition measurements were performed in a fixed bed flow reactor. A gas mixture of ~1% NO in helium balance was used with a gas hourly space velocity of ~2100 h−1 and in the temperature region of 450–650 °C. Before the reaction, catalysts were pretreated at 500 °C in the presence of 20% O2/He. After pretreatment, the bed temperature was decreased to 400 °C and direct NO decomposition measurements were collected. The measurements were performed at 400, 450, 550, and 650 °C, with 2 h of steady state at each temperature. The NO, N2O, and NO2 concentrations were analyzed with a FTIR detector (CAI 600 SC FTIR California Analytical Instruments, Inc., Orange County, CA, USA). The N2 concentration was calculated by mass balance of the total nitrogen species. The raw NO conversion (NO converted to all the products) during the steady state measurements are presented in Figure S1 of the supporting information and activity to N2 was reported in Figure 1 of the manuscript. The steady state direct NO decomposition measurements were also performed in a reactor, which was equipped with the mass spectroscopy (MKs, Cirrus 2). The changes in the NO, N2, O2, N2O, and NO2 signal intensities were monitored during the reaction (Figures S2–S5). The inert Ar gas was introduced as a tracer to monitor for potential systematic variation in signal intensity during the experiment (Figure S2).

4. Conclusions

The direct NO decomposition measurements show that PdO promotes the activity of Co3O4 and is 4 times more active compared to PtO at 650 °C. Also, the activity increases with increasing PdO loading until 3 wt% and further increase in the loading leads to a decrease in the activity. On the other hand, only a slight increase in the activity was observed with increasing PtO loading up to 4 wt%. Surface area measurements indicated that both PdO and PtO have little to no influence on the surface area of Co3O4, except for a decrease in surface area for 4 wt% PdO/Co3O4. The X-ray diffraction measurements show that Pt incorporated into the Co3O4 structure during the synthesis and PdO stays mostly on the surface. The diffraction measurements also suggested that PdO is in an amorphous form up to 3 wt% over Co3O4 surface and crystalline PdO forms at 4 wt% loading, whereas PtO mostly stays as amorphous from or incorporated into Co3O4 structure until 4 wt%. Due to the synergistic effect between Pd species and Co3O4, an improvement in the redox properties of Co3O4 was observed in the case of PdO/Co3O4 catalysts. Conversely, PtO do not have any influence on the redox properties of Co3O4. The X-ray photo electron spectroscopic measurements reveal that PdO reduced to Pd metal during the direct NO decomposition reaction and Pt was in 2+ oxidation state before and after the direct NO decomposition reaction. In situ NO adsorption measurements show that PdO improve the NO adsorption properties of Co3O4 by forming the nitrate ion intermediates, whereas PtO/Co3O4 do not form any intermediates during the NO adsorption at 300 °C. Overall, PdO ease the redox properties of Co3O4 and forms surface adsorbed species during NO adsorption and improves the direct NO decomposition activity of Co3O4. On the other hand, PtO do not have any influence on the redox or NO adsorption properties of Co3O4 and exhibits lesser promotional effects compared to PdO. For PdO/Co3O4 catalysts, the PdO remains in amorphous form until 3PdO/Co3O4 and improves the activity of Co3O4 with loading. However, further increase in the loading to 4 wt% leads to formation of crystalline PdO, which reduces separately during H2-TPR and inhibits the PdO reduction to metallic Pd during direct NO decomposition and exhibits lesser activity compared to 3 wt% PdO/Co3O4.
Supporting information: The NO conversion, and N2, N2O, and NO2 ppm values of various PdO/Co3O4 and PtO/Co3O4 catalysts are presented in Tables S1 and S2. The total NO conversion profiles of the Co3O4, 3PdO/Co3O4, and 4PtO/CoO calculated from the FT-IR detector during the steady state direct NO decomposition measurements are presented in Figure S2. NO, Ar, N2, O2, N2O, and NO2 partial pressure values (obtained from mass spectroscopy) of Co3O4, 3 wt% PdO/Co3O4, and 3 wt% PtO/Co3O4 during steady state direct NO decomposition in the 400 to 650 °C temperature region are presented in Figures S3 and S4.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/9/1/62/s1. Table S1: NO conversion, N2, N2O, NO2 ppm values of the Co3O4 and various PdO/Co3O4 catalysts in the temperature region 400–650 °C. Table S2: NO conversion, N2, N2O, NO2 ppm values of the Co3O4 and various PtO/Co3O4 catalysts in the temperature region 400–650 °C. Figure S1: Steady state NO conversion values of the Co3O4, 3PdO/Co3O4, and 4PtO/Co3O4 catalysts during the direct NO decomposition in the temperature region 400 to 650 °C. Figure S2: NO and Ar M.S. partial pressures of the (a) Co3O4, (b) 3PdO/Co3O4, and (c) 3PtO/Co3O4 catalysts during the steady state direct NO decomposition in the temperature region 400 to 650 °C. Figure S3: N2 and O2 M.S. partial pressures of the (a) Co3O4, (b) 3PdO/Co3O4, and (c) 3PtO/Co3O4 catalysts during the steady state direct NO decomposition in the temperature region 400 to 650 °C. Figure S4: N2O M.S. partial pressures of the (a) Co3O4, (b) 3PdO/Co3O4, and (c) 3PtO/Co3O4 catalysts during the steady state direct NO decomposition in the temperature region 400 to 650 °C. Figure S5: NO2 M.S. partial pressures of the (a) Co3O4, (b) 3PdO/Co3O4, and (c) 3PtO/Co3O4 catalysts during the steady state direct NO decomposition in the temperature region 400 to 650 °C.

Author Contributions

This study was conducted through contributions of all authors. G.K.R. Reddy designed the study, performed the experiments, and wrote the manuscript. T.C.P. was involved in performing the experiments. C.A.R. checked and corrected the manuscript.

Funding

This research received no external funding.

Acknowledgments

The authors thank Hongfei Jia from Toyota Research Institute—North America and Naoto Nagata from Toyota Motor Corp. for their support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Direct NO decomposition activity to N2 as a function of reaction temperature for (a) PdO- and (b) PtO-promoted Co3O4 catalysts with varying PGM loading. The pure Co3O4 support is included for comparison. (Gas hourly space velocity (GHSV) = 2100 h−1, 1% NO/He).
Figure 1. Direct NO decomposition activity to N2 as a function of reaction temperature for (a) PdO- and (b) PtO-promoted Co3O4 catalysts with varying PGM loading. The pure Co3O4 support is included for comparison. (Gas hourly space velocity (GHSV) = 2100 h−1, 1% NO/He).
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Figure 2. Direct NO decomposition activity to N2 as a function of Pd and Pt loading for (a) PdO- and (b) PtO-promoted Co3O4 catalysts at varying temperature. The pure Co3O4 support is included for comparison. (GHSV = 2100 h−1, 1% NO/He).
Figure 2. Direct NO decomposition activity to N2 as a function of Pd and Pt loading for (a) PdO- and (b) PtO-promoted Co3O4 catalysts at varying temperature. The pure Co3O4 support is included for comparison. (GHSV = 2100 h−1, 1% NO/He).
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Figure 3. N2 selectivity as a function of PdO and PtO loading for (a) PdO- and (b) PtO-promoted Co3O4 catalysts at varying temperature. The pure Co3O4 support is included for comparison. (GHSV = 2100 h−1, 1% NO/He).
Figure 3. N2 selectivity as a function of PdO and PtO loading for (a) PdO- and (b) PtO-promoted Co3O4 catalysts at varying temperature. The pure Co3O4 support is included for comparison. (GHSV = 2100 h−1, 1% NO/He).
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Figure 4. X-ray diffraction patterns of fresh (a) PdO- and (b) PtO-promoted Co3O4 catalysts. The pattern for the fresh pure Co3O4 support is included for reference.
Figure 4. X-ray diffraction patterns of fresh (a) PdO- and (b) PtO-promoted Co3O4 catalysts. The pattern for the fresh pure Co3O4 support is included for reference.
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Figure 5. X-ray diffraction patterns of spent (a) PdO- and (b) PtO-promoted Co3O4 catalysts. The pattern for the spent pure Co3O4 support is included for reference.
Figure 5. X-ray diffraction patterns of spent (a) PdO- and (b) PtO-promoted Co3O4 catalysts. The pattern for the spent pure Co3O4 support is included for reference.
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Figure 6. H2- Temperature programmed reduction profiles of fresh (a) PdO- and (b) PtO-promoted Co3O4 catalysts. The pure Co3O4 profile is shown for comparison.
Figure 6. H2- Temperature programmed reduction profiles of fresh (a) PdO- and (b) PtO-promoted Co3O4 catalysts. The pure Co3O4 profile is shown for comparison.
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Figure 7. O1s XPS spectra of fresh (a) PdO- and (b) PtO-promoted Co3O4 catalysts. Co3O4 spectrum is shown for reference.
Figure 7. O1s XPS spectra of fresh (a) PdO- and (b) PtO-promoted Co3O4 catalysts. Co3O4 spectrum is shown for reference.
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Figure 8. Co2p XPS profiles of fresh (a) PdO- and (b) PtO-promoted Co3O4. The pure Co3O4 spectra is presented for reference.
Figure 8. Co2p XPS profiles of fresh (a) PdO- and (b) PtO-promoted Co3O4. The pure Co3O4 spectra is presented for reference.
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Figure 9. Pd3d XPS spectra of (a) fresh and (b) spent PdO-promoted Co3O4 catalysts.
Figure 9. Pd3d XPS spectra of (a) fresh and (b) spent PdO-promoted Co3O4 catalysts.
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Figure 10. Pt4f XPS profiles of (a) fresh and (b) spent PtO-promoted Co3O4 catalysts.
Figure 10. Pt4f XPS profiles of (a) fresh and (b) spent PtO-promoted Co3O4 catalysts.
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Figure 11. In-situ FT-IR spectra of Co3O4, PdO (a), and PtO (b) promoted Co3O4 catalysts during NO adsorption at 300 °C.
Figure 11. In-situ FT-IR spectra of Co3O4, PdO (a), and PtO (b) promoted Co3O4 catalysts during NO adsorption at 300 °C.
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Table
Table 1. Co/M (M = Pd, Pt) and BET surface area values of PdO/Co3O4 and PtO/Co3O4 catalysts.
Table 1. Co/M (M = Pd, Pt) and BET surface area values of PdO/Co3O4 and PtO/Co3O4 catalysts.
Catalyst LoadingPGM Loading (wt%) *BET Surface Area (m2/g)
Pd wt%Pt wt%PdO/Co3O4PtO/Co3O4
0--3636
10.830.933635
21.942.123539
32.83.233334
44.154.022633
* As measured by XRF.

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K. Reddy, G.; C. Peck, T.; A. Roberts, C. “PdO vs. PtO”—The Influence of PGM Oxide Promotion of Co3O4 Spinel on Direct NO Decomposition Activity. Catalysts 2019, 9, 62. https://doi.org/10.3390/catal9010062

AMA Style

K. Reddy G, C. Peck T, A. Roberts C. “PdO vs. PtO”—The Influence of PGM Oxide Promotion of Co3O4 Spinel on Direct NO Decomposition Activity. Catalysts. 2019; 9(1):62. https://doi.org/10.3390/catal9010062

Chicago/Turabian Style

K. Reddy, Gunugunuri, Torin C. Peck, and Charles A. Roberts. 2019. "“PdO vs. PtO”—The Influence of PGM Oxide Promotion of Co3O4 Spinel on Direct NO Decomposition Activity" Catalysts 9, no. 1: 62. https://doi.org/10.3390/catal9010062

APA Style

K. Reddy, G., C. Peck, T., & A. Roberts, C. (2019). “PdO vs. PtO”—The Influence of PGM Oxide Promotion of Co3O4 Spinel on Direct NO Decomposition Activity. Catalysts, 9(1), 62. https://doi.org/10.3390/catal9010062

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