Efficient Removal of Methane over Cobalt-Monoxide-Doped AuPd

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Efficient Removal of Methane over the Cobalt Monoxide Doped AuPd Nanocatalysts Shaohua Xie, Yuxi Liu, Jiguang Deng, Simiao Zang, Zhenhua Zhang, Hamidreza Arandiyan, and Hongxing Dai Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03983 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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Environmental Science & Technology

Efficient Removal of Methane over Cobalt Monoxide Doped AuPd Nanocatalysts

Shaohua Xie,† Yuxi Liu,*,† Jiguang Deng,*,† Simiao Zang,† Zhenhua Zhang,‡

Hamidreza Arandiyan,§ and Hongxing Dai*,†

†

Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of

Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials, Education Ministry of China, and Laboratory of Catalysis Chemistry and Nanoscience, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China ‡

Institute of Microstructure and Property of Advanced Materials, Beijing University

of Technology, Beijing 100124, China §

Particles and Catalysis Research Group, School of Chemical Engineering, The

University of New South Wales, Sydney, NSW 2052, Australia

ACS Paragon Plus Environment


ABSTRSCT: In order to overcome deactivation of Pd-based catalysts at high temperatures, we herein design a novel pathway by introducing a certain amount of CoO to the supported Au–Pd alloy nanoparticles (NPs) to generate high-performance Au–Pd–xCoO/3DOM Co3O4 (x is the Co/Pd molar ratio) catalysts. The doping of CoO induced formation of PdO–CoO active sites, which was beneficial for the improvement in adsorption and activation of CH4 and catalytic performance. The Au–Pd–0.40CoO/3DOM Co3O4 sample performed the best (T90% = 341 oC at a space velocity of 20 000 mL/(g h)). Deactivation of the 3DOM Co3O4-supported Au–Pd, Pd–CoO, and Au–Pd–xCoO nanocatalysts resulted from water vapor addition was due to formation and accumulation of hydroxyl on the catalyst surface, whereas deactivation of the Pd–CoO/3DOM Co3O4 catalyst at high temperatures (680–800 °C) might be due to decomposition of the PdOy active phase into aggregated Pd0 NPs. The Au–Pd–xCoO/3DOM Co3O4 nanocatalysts exhibited better thermal stability and water tolerance ability, as compared to the 3DOM Co3O4-supported Au–Pd and Pd–CoO nanocatalysts. We believe that the supported Au–Pd–xCoO nanomaterials are promising catalysts in practical applications for organics combustion.


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■ INTRODUCTION As a main component of natural gas, CH4 is a greenhouse gas, with the global warming potential being dozens of times higher than that of CO2.1 Therefore, much attention has been paid on reducing the emission of CH4 from industrial and transportation activities, which can give rise to substantial environmental and economic benefits.2–4 Catalytic total oxidation of CH4 at a relatively low temperature over the precious metal (e.g., Pd-, Rh- or Pt)-based catalyst is believed to be a potential technology for the combustion of CH4. The best-performing Pd-based catalysts for CH4 combustion tend to deactivate at temperatures above 600 °C because the active PdOy phase is easily reduced into aggregated metallic Pd nanoparticles (NPs). Therefore, it is highly desired to develop novel catalysts with high activity at low temperatures and good stability at high temperatures. Due to the limited resources, many efforts have been made to reduce the use of noble metals, in which the focus is the preparation of single atom and base metal-promoted noble metal catalysts.5–9 However, the current catalytic materials are still hard to meet the combined requirements of high performance, good durability, and

low

cost.

Recently,

Zheng

and

coworkers

fabricated

iron–nickel

hydroxide–platinum NPs with a size of below 5 nm, and observed that these nanomaterials were highly efficient for carbon monoxide oxidation at room temperature.9 Such hybrid metal NPs could allow to separate reaction steps to take place in close proximity at different metal sites (the transition metal–OH–Pt interface), thus enhancing the catalytic activity and noble metal utilization efficiency. Due to the



outstanding catalytic property, bimetallic Pd-based catalysts have recently gained much attention,10–15 in which the supported Au−Pd alloys showed a high efficiency in oxidation or reduction reactions. Miao et al. prepared a series of Co3O4-supported Au, Pt, and Pd catalysts for methane oxidation, and found that the activity decreased in the order of Pd/Co3O4 > Pt/Co3O4 > Au/Co3O4, indicating that Pd was more active than Pt or Au.10 The results of our previous works have demonstrated that the supported Au–Pd alloy catalyst possessed much better thermal stability than the supported Au or Pd catalysts, in which a big growth of Au or Pd particles but no significant growth of Au–Pd alloy particles were detected after the pretreatment at a N2 flow and 600 oC for 3 h.14 These results suggest that the supported Au–Pd catalysts would be stable for methane oxidation, especially at higher temperatures. In terms of effective utilization of noble metals, the ideal strategy is to make most of the noble metal atoms locate on the surface of a support. Doping a metal oxide into the Au−Pd alloys may be a facile way to obtain a high effective utilization of noble metals. As a versatile transition-metal oxide, cobalt oxide is highly active for methane oxidation.10,16–18 Nowadays, three-dimensionally ordered macroporous (3DOM) materials as catalyst or support showed high activities for soot and volatile organic compounds (VOCs) abatement due to their easy transportation and diffusion properties.18–25 Previously, our group adopted the polymethyl methacrylate (PMMA)-templating and polyvinyl alcohol (PVA)-protected reduction strategies to successfully generate a series of 3DOM structured materials.20–24 We herein report for the first time the Au–Pd–xCoO/3DOM Co3O4 (x is the Co/Pd molar ratio) catalysts by


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introducing a certain amount of CoO to the Au–Pd alloy NPs. It is found that the Au–Pd–xCoO/3DOM Co3O4 materials were highly active for methane oxidation, which was due to formation of the unique PdO–CoO interface in the Au–Pd alloy NPs for CH4 activation and the improvement of CH4 adsorption ability. In addition, Au NPs in the samples also played a key role in stabilizing the interface structure against sintering. ■ EXPERIMENTAL SECTION Catalyst Preparation. Au–Pd–xCoO NPs (x is the Co/Pd molar ratio) were prepared using a modified PVA-protected reduction method. A desired amount of PVA (noble metal/PVA mass ratio = 1.0 : 1.5) was added to a mixed aqueous solution of HAuCl4 and PdCl2 (Au/Pd molar ratio = 0.54 : 1.00) in an ice bath under stirring for 15 min, and then a desired amounts of CoCl2 (1.5 mmol/L) and HCl (3.0 mol/L; Pd/HCl molar ratio = 1.0 : 5.0) aqueous solutions were added to the mixed noble metal solution. After vigorous stirring for 30 min, a NaBH4 aqueous solution (2.0 g/L; noble metal/NaBH4 molar ratio = 1.0 : 5.0) was rapidly injected to form a dark brown suspension, obtaining the Au–Pd–xCoO NPs after further stirring for 1 h. 3DOM Co3O4 was prepared via the PMMA-templating route20 and served as a support for loading of Au–Pd–xCoO NPs. In a typical fabrication, 20 mmol of Co(NO3)2⋅6H2O was dissolved in 10 mL of polyethylene glycol (PEG) and methanol (MeOH) solution (PEG/MeOH volumetric ratio = 1.0 : 9.0) at room temperature (RT) under stirring for 1 h, obtaining a mixed transparent solution. Then, 2.0 g of the PMMA template was soaked in the above mixed solution for 4 h. After being filtered



and dried, the obtained powders were calcined in N2 and air flow, respectively, thus generating the 3DOM Co3O4 support. For comparison purposes, the 3DOM Mn2O3 and 3DOM Al2O3 supports were also fabricated using the PMMA-templating method reported previously (See the Supporting Information).22,23 The 3DOM Co3O4-, 3DOM Mn2O3-, and 3DOM Al2O3-supported Au−Pd−xCoO (x = 0, 0.19, 0.40, 0.90 and 3.61) samples were prepared using the gas bubble-assisted adsorption strategy. A desired amount of the 3DOM support was added to the obtained Au−Pd−xCoO suspension with the theoretical Au−Pd−Co loading of 2.0 wt%. Such a suspension was subjected to ultrasonic (60 kHz) treatment for 10 min. The gas bubble-assisted adsorption operation with three outlets was used to make the reaction homogenous. After bubbling the suspension with N2 (100 mL/min) for 4 h, the wet solid was filtered, washed with 2.0 L of deionized water, dried at 80 oC for 12 h, and calcined in air at a ramp of 10 oC/min from RT to 400 oC and kept at 400 oC for 1 h, thus obtaining the supported Au−Pd−xCoO samples. The results of inductively coupled plasma atomic emission spectroscopic (ICP−AES) investigations reveal that the real loading (1.95−1.99 wt%) of Au−Pd−Co was rather similar in each of the samples. All of the chemicals (A.R. in purity) are purchased from Beijing Chemical Reagents Company and used without further purification. Catalyst Characterization. All of the samples were characterized by means of techniques, such as ICP−AES, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark


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field (HAADF) and elemental mapping, X-ray photoelectron spectroscopy (XPS), and hydrogen

temperature-programmed

reduction

(H2-TPR),

methane

temperature-programmed desorption (CH4-TPD), and in situ diffuse reflectance Fourier transform infrared spectroscopic (DRIFT). The detailed characterization procedures are described in the Supporting Information. Catalytic Performance Evaluation. Catalytic activities of the samples were evaluated in a continuous flow fixed-bed quartz tubular microreactor (i.d. = 6.0 mm). To minimize the effect of hot spots, 50 mg of the sample (40–60 mesh) was diluted with 0.25 g of quartz sands (40–60 mesh). Before the test, each sample was treated in an oxygen flow of 30 mL/min at 300 oC for 1 h. The reactant mixture was composed of 2.5 vol% CH4 + 10.0 vol% O2 + 87.5 vol% N2 (balance), and the total flow was 16.6–166.0 mL/min, thus giving a space velocity (SV) of 20 000–200 000 mL/(g h). In the case of water vapor introduction, 1.0, 2.0, and 5.0 vol% H2O was introduced by passing the feed stream through a water saturator at different temperatures. Reactants and products were analyzed online by gas chromatography (GC-14C, Shimadzu) equipped with a flame ionization detector (FID) and a thermal conductivity detector R column (30 m in length) and a Carboxen 1000 column (3 (TCD), using a stabilwax○

m in length). The balance of carbon throughout the catalytic system was estimated to be 99.5 %. Catalytic activities of the samples were evaluated using the temperatures (T10%, T50%, and T90%) required for achieving methane conversions of 10, 50, and 90 %, respectively. CH4 conversion was defined as (cinlet–coutlet)/cinlet × 100 %, where the cinlet and coutlet were the inlet and outlet CH4 concentrations in the feed stream, respectively.



It is reasonable to suppose that the combustion of methane in the presence of excessive oxygen (CH4/O2 molar ratio = 1/4) would obey a first-order reaction mechanism with respect to methane concentration (c): r = −kc = (−A exp(−Ea/RT))c, where r, k, A, and Ea are the reaction rate (mol/s), rate constant (s-1), pre-exponential factor, and apparent activation energy (kJ/mol), respectively. The k values could be calculated from the reaction rates and reactant conversions at different SVs and reaction temperatures. 3. RESULTS AND DISCUSSION The STEM images (Figures S1(a–e) and S2(A and C)) reveal that the Au–Pd–3.61CoO NPs showed a good distribution and their average particle size was 2.8 nm (Figure S1(f)). The Co 2p3/2 XPS spectra show that the Co in Au–Pd–3.61CoO NPs existed in divalency (Figure S2(B)). The results of STEM–EDS investigations demonstrate that the O atoms were well adhered to the Co atoms (i.e., generating a CoO phase), and such a CoO phase and Au or Pd were well mixed in the Au–Pd–3.61CoO NPs (Figure S2(D–H)). The formation of CoO might be due to the oxidation of Co generated from the reduction of CoCl2 by NaBH4 when the alloy NPs was exposed to the air. The loading of Au–Pd–xCoO NPs in each of the samples was ca. 2.0 wt%. Figure S3 shows the XRD patterns of the as-prepared samples. Compared to the XRD pattern of the Co3O4, Mn2O3, and Al2O3 support, the loading of Au-Pd-xCoO NPs did not lead to any changes in crystal structure. The XRD patterns of the samples clearly reveal that the 3DOM supports crystal structures could be indexed to the cubic Co3O4


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(JCPDS PDF# 42-1467), Mn2O3 (JCPDS PDF# 41-1442), and Al2O3 (JCPDS PDF# 10-0425) phases. According to the Scherrer equation, the grain sizes of Co3O4, Mn2O3, and Al2O3 were 34−36, 47, and 27 nm, respectively. High-quality 3DOM entities can be observed in the Au–Pd–xCoO/3DOM Co3O4 (x = 0.19, 0.40, 0.90, and 3.61) and Au–Pd–3.61CoO/3DOM MOy (MOy = Mn2O3 or Al2O3) samples (Figure S4). The macropore sizes were in the range of 115–185 nm, smaller than the average size (ca. 300 nm) of the PMMA microspheres. This might be due to the partial carbonization of PMMA microspheres during the calcination process in a N2 flow at 300 oC, which could lead to the shrinking of the PMMA microspheres, hence giving rise to pore sizes smaller than the PMMA microspheres. A number of Au–Pd–xCoO NPs (average size = 2.6–3.1 nm) were highly dispersed on the surface of the 3DOM support (Figures 1(A–D), S5, and S6, and Table S1). As shown in Figure S5(g), the interplanar spacings (d values) of Au–Pd–3.61CoO were measured to be ca. 0.235 and 0.288 nm (in good consistence with those of the (111) and (110) crystal planes of the standard Au sample), ca. 0.194 and 0.225 nm (responding to those of the (200) and (111) crystal planes of the standard Pd sample), and ca. 0.301 and 0.426 nm (well agreeing with those of the (110) and (100) crystal planes of the standard CoO sample (JCPDS PDF # 43-1004)). These results are in good agreement with those of the STEM–EDS and XPS investigations, suggesting the presence of a mixed phase of Au, Pd, and CoO in the NPs. In our previous studies,20-22 we found that the loading of noble metal NPs had a less effect on the surface area of the support. Therefore, we did not investigate the effect of PMMA microsphere size on surface area of the 3DOM support. The



H2-TPR experiments were carried out to investigate the reducibility of Au–Pd–xCoO/3DOM Co3O4. When Au–Pd–xCoO NPs were loaded on the surface of 3DOM Co3O4, the reduction peaks due to the reduction of cobalt oxides were shifted to the lower temperatures (Figure S7), indicating the presence of interaction between Au–Pd–xCoO NPs and 3DOM Co3O4;14 the total H2 consumption also increased (Figure 1E and Table S2). The XPS characterization suggests the existence of surface Co2+, lattice oxygen, and adsorbed oxygen species, and the surface Co2+/Co3+ and Oads/Olatt molar ratios of Au–Pd–xCoO/3DOM Co3O4 increased with the loading of CoO (Figures 1E and S8(A and B) and Table S2). The loading of CoO did not significantly affected the surface Auδ+ concentration but greatly enhanced the formation of Pd2+ species (Figures 1F and S8(C and D) and Table S2). It was reported that PdO was the active site for methane oxidation.26 Therefore, the Co in Au–Pd–xCoO NPs of Au–Pd–xCoO/3DOM Co3O4 was present in Co2+, and such a CoO promoted the formation of PdO–CoO interface and the enhancement of adsorbed oxygen species concentration, which would be beneficial for the improvement in catalytic activity. The Au–Pd–xCoO/3DOM Co3O4 samples exhibited much better methane oxidation activity than the Au–Pd/3DOM Co3O4 and 3DOM Co3O4 samples (Figure S9(A) and Table 1). Among all of the samples, the Au–Pd–0.40CoO/3DOM Co3O4 sample showed the highest methane conversion of 89 % at 340 oC, ca. 38 and 59 % higher than those over the supported Au–Pd and 3DOM Co3O4 samples, respectively. Among the Au–Pd–3.61CoO/3DOM MOy samples, the 3DOM Co3O4-supported sample


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performed much better than the other samples, and the catalytic performance decreased in a sequence of 3DOM Co3O4 > 3DOM Mn2O3 > 3DOM Al2O3 (Figure S9(B)). These results are in agreement with those of transition-metal oxides for CH4 combustion reported by Paredes et al.,27 who believed that apart from the surface area, the discrepancy in catalytic performance of different metal oxides was due to the difference in nature and availability of oxygen species, which played an important role in the adsorption and activation of CH4. The decrease in apparent activation energy (Ea) was in good agreement with the enhancement in catalytic performance of the samples (Figure S10(A) and Table 1). Figure S10(B) shows the effect of SV on catalytic activity of the Au–Pd–0.40CoO/3DOM Co3O4, Au–Pd/3DOM Co3O4, and Pd–3.61CoO/3DOM Co3O4 samples. Methane conversion decreased with the rise in SV from 20 000 to 120 000 mL/(g h), but no significant loss in activity was observed with a further rise in SV from 60 000 to 200 000 mL/(g h). It is generally believed that as the SV increases, a reactant conversion is influenced by two factors: shortening of residence time and increasing of thermal power.28 In one of our previous works, the catalysts with a 3DOM structure performed better than the ones with a bulk structure at high SVs for CO or toluene oxidation.22 A similar effect would also occur in the case of 3DOM Co3O4-supported samples. With a rise in SV, the contact time decreased, giving rise to a drop in methane conversion; however, the higher the SV, the higher was the thermal power (a result due to a reduction in heat loss from the reactor), inevitably leading to a higher CH4 conversion. As we know, hot spots and intra-pellet/intra-particle thermal gradients might greatly



affect kinetics for methane oxidation, due to the fast and exothermic nature of the reaction. To minimize the effect of hot spots, 50 mg of the sample (40‒60 mesh) was well diluted with 0.25 g of quartz sands (40‒60 mesh). It is well known that the bypass and axial dispersion effects could be neglected, when the reactor diameter/particle diameter ratio is higher than 10 and the catalyst bed length/particle diameter ratio is higher than 50.29 In the present study, the reactor diameter/particle diameter ratio and the catalyst bed length/particle diameter ratio is 21 and 65, respectively. That is to say, the mass transfer limitation might be absence. In addition, making a Weisz−Prater analysis is a facile way to evaluate the presence of mass transfer limitation for the porous catalysts. According to the Weisz-Prater criterion, when the effectiveness factor η ≥ 0.95 and reaction order n = 1, the dimensionless Weisz-Prater parameter (NW−P) value is less than 0.3, which can be considered a sufficient condition for the absence of significant pore diffusion limitation.27 For the present Au–Pd–0.40CoO/3DOM Co3O4 sample at 280 oC, the NW−P value was calculated to be 0.0197, which was much less than 0.3. Therefore, we deduce that there was no significant heat or mass transfer limitation under the present reaction conditions. For the transition metal oxide-supported noble metal catalysts, it is hard to define the active sites and to calculate the number of active sites. In the present study, all of the species, such as Au, Pd, and Au–Pd–CoO NPs, and the interface between Au–Pd–CoO NPs and 3DOM Co3O4, might act as the active sites for methane oxidation. That is to say, the accurate calculation of TOFs might be impossible. For the convenience of comparison, we use the amounts of Pd in the supported catalysts to


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calculate TOFs according to TOFPd = xC0/nPd, where x is the methane conversion, C0 is the initial methane concentration, and nPd is the molar amount of Pd. Methane oxidation rates at 280 oC could be calculated according to the activity data and amounts of Pd in the Au–Pd–xCoO/3DOM Co3O4 samples, and the results are summarized in Table 1. Among all of the samples, Au–Pd–3.61CoO/3DOM Co3O4 exhibited the highest TOFPd (0.0118 s–1) and methane oxidation rate (110.5 µmol/(gPd s)) at 280 oC, much higher than those (0.0053 s–1 and 49.9 µmol/(gPd s)) over Au–Pd/3DOM

Co3O4.

The

TOFPd

and

methane

oxidation

rates

over

Au–Pd–3.61CoO/3DOM Co3O4 and the catalysts reported in the literature are summarized in Table S3. It can be seen that the methane oxidation rate at 280 oC (110.5 µmol/(gPd s)) over Au–Pd–3.61CoO/3DOM Co3O4 was much higher than that (7.9 µmol/(gPd s)) over 5.0 wt% Pd/p-Mn3O4,30 that (46.2 µmol/(gPd s)) over 0.98 wt% AuPd1.93/3DOM CoCr2O4,31 and that (74.9 µmol/(gPd s)) over 1.0 wt% Pd/Co3O4,32 similar to that (104.0 µmol/(gPd s)) over 1.0 wt% Pd/(SiO2 + TiO2),3 but lower than that (161.9 µmol/(gPd s)) over 1.97 wt% Au0.45Pd/meso-Co3O433 and that (254.5 µmol/(gPd

s))

over

1.0

wt%

Pd@CeO2/H-Al2O3.34

Compared

with

the

Au–Pd–3.61CoO/3DOM Co3O4 sample, the Au–Pd–0.40CoO/3DOM Co3O4 sample showed a lower TOF but better low-temperature activity, which might be due to the lower surface PdO species concentration and higher Pd loading. To examine water vapor tolerance ability of the typical sample for methane oxidation, we conducted 30 h on-stream methane oxidation in the presence of 1.0, 2.0, and 5.0 vol% water vapor. It is found that the supported Au–Pd–xCoO samples



exhibited better water vapor tolerance ability than the supported Au–Pd sample, indicating that doping a small amount of CoO in the Au–Pd NPs was beneficial for enhancing the hydrothermal stability (Figure 2A). Furthermore, the partial deactivation of the samples induced by water vapor addition was reversible. Such a negative effect of moisture was associated with the existence of the equilibrium PdO + H2O → Pd(OH)2, where the PdO represents an active phase whereas the Pd(OH)2 an inactive phase, and the moisture adsorption on the support, which restricting oxygen mobility on the sample surface.35 The improved moisture tolerance might be due to the inhibition of hydroxyl formation by CoO inclusion. As shown in Figure 2B, the introduction of a more amount of water vapor gave rise to a great decrease in methane conversion. With the addition of 5 vol% water vapor, methane conversion at 320 oC decreased more than that at 340 oC. It is understandable that a high water vapor concentration and a low reaction temperature favor the formation and accumulation of hydroxyl on the surface of the catalyst. As can be seen from Figure 3, Au–Pd–3.61CoO/3DOM Co3O4 showed no significant decrease in activity when the temperature rose or dropped, indicating that this sample was catalytically stable for CH4 oxidation in the entire temperature range (400–800 °C). It is also detected that methane conversion with temperature drop was higher than that with temperature rise (300–400 oC) over Au–Pd–3.61CoO/3DOM Co3O4 for methane oxidation. This phenomenon is commonly observed in the case of temperature drop and rise experiments of methane oxidation in the literature.34,36 It is generally believed that Pd was easily oxidized into the PdOy species at relatively high temperatures (< 600 oC).


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That is to say, a more amount of the PdOy species might be formed at higher temperatures when the reaction temperature decreased, whereas a less amount of the PdOy species could be generated when reaction temperature increased. Hence, the above two phenomena could result in a higher conversion with temperature drop for methane oxidation over Au–Pd–3.61CoO/3DOM Co3O4. On the contrast, the Pd–3.61CoO/3DOM Co3O4 sample clearly showed a usual transient decrease in CH4 conversion when the temperature rose or dropped in the range of 680–800 °C. The deactivation at high temperatures might be due to the PdOy active phase was decomposed into aggregate Pd0 NPs.34 To understand the details, TEM and XPS characterization was conducted for the samples after catalytic methane oxidation at 650 °C. From Figure S11, it is obviously seen that there were no apparent increases in particle size of Au–Pd–3.61CoO/3DOM Co3O4 (from 3.0 to 3.6 nm) and Au–Pd/3DOM Co3O4 (from 2.6 to 3.7 nm), but a slight particle growth of Pd–3.61CoO/3DOM Co3O4 (from 3.8 to 5.9 nm). Figure S12 shows the Pd 3d XPS spectra of the samples. It can be observed that there were almost no changes in the nature of Pd species on the Au–Pd–3.61CoO/3DOM Co3O4 and Au–Pd/3DOM Co3O4 samples after methane oxidation at 650 °C, but a large decrease in Pd2+ species on the Pd–3.61CoO/3DOM Co3O4 sample was detected after methane oxidation at 650 °C. These results suggest that the deactivation of Pd–3.61CoO/3DOM Co3O4 at high temperatures might be the decomposition of the PdOy active phase. As reported, the supported core-shell Pd@ZrO236 and Pd@CeO234 catalysts exhibited good stability at high temperatures. These authors believed that the unique hierarchical core-shell



structure and metal−oxide interaction between Pd and CeO2 or ZrO2 might be beneficial for maintaining an oxidized Pd, thus showing good catalytic stability. In the case of the Au–Pd–3.61CoO/3DOM Co3O4 sample without a core-shell structure, however, the unique role of the Au in the sample in stabilizing the active phase might be attributable to the effect of gold atom clusters or/and Pd–O–Co phase on the catalyst surface.37,38 Generally speaking, Co3O4 is more thermodynamically stable than CoO under the reaction conditions in the presence of excessive oxygen. Due to the small size and low loadings of Au–Pd–CoO NPs, it is hard to directly determine the cobalt oxide phase in the samples. According to the results of Co 2p3/2 XPS and STEM–EDS (Figure S2) investigations, we deduce that the phase of cobalt oxide was CoO in Au–Pd–3.61CoO NP. In order to find the role of CoO, we have investigated the catalytic performance of the 3DOM Co3O4-supported Au–Pd, Au–Pd–0.40CoO, and Au–Pd–3.61CoO samples pretreated in a flow of O2 at 500 oC for 3 h. As shown in Figure 4A, the surface Co2+/Co3+ molar ratio over the supported Au–Pd catalyst remained almost unchanged, whereas those over the supported Au–Pd–0.40CoO and Au–Pd–3.61CoO catalysts decreased, in the meanwhile the Pd2+ species concentration increased (Figure 4B).39 That is to say, most of the CoO in the supported Au–Pd–0.40CoO and Au–Pd–3.61CoO samples were oxidized into Co3O4. The fresh and pretreated supported Au–Pd samples exhibited similar catalytic activities, but the pretreated supported Au–Pd–0.40CoO and Au–Pd–3.61CoO samples showed lower catalytic activities than the fresh counterparts. Such results confirm that the cobalt oxide was


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CoO in the supported Au–Pd–0.40CoO and Au–Pd–3.61CoO samples, and the addition of CoO was favorable for the complete oxidation of methane. Furthermore, the Au–Pd–3.61CoO/3DOM Co3O4 sample was catalytically stable for CH4 oxidation in the entire temperature range (400–800 °C), as shown in Figure 3. In other words, CoO in the Au–Pd–3.61CoO/3DOM Co3O4 catalyst could be maintained under the present reaction conditions, possibly related to the interaction between Au–Pd and CoO NPs. Of course, the real reason needs further study in future work. According to the literature, activation of the first C–H bond in methane was the rate-determining step in methane oxidation.26 Ability of methane adsorption has been regarded as a key factor influencing the catalytic performance of a sample. As shown in the CH4-TPD profiles (Figure 4C) of the typical samples, a peak of methane desorption below 400 o

C was recorded over the supported Au–Pd–0.40CoO and Au–Pd–3.61CoO samples

but not over the supported Au–Pd sample, which could be assigned to the desorption of methane adsorbed on the CoO surface within the Au–Pd–xCoO NPs. The desorbed temperature (345 oC) over the supported Au–Pd–0.40CoO sample was lower than that (370 oC) over the supported Au–Pd–3.61CoO sample, possibly due to a stronger interaction between PdO and CoO in the latter sample. The addition of CoO to the Au–Pd NPs facilitated the adsorption of methane since the enhanced adsorbed oxygen species induced by CoO doping were helpful for methane adsorption.40 The methane molecules adsorbed on the CoO surface could easily be activated on the PdO surface of the PdO–CoO interface within the Au–Pd–xCoO NPs, and react with the adsorbed oxygen species to produce CO2 and H2O. The undermining of the PdO–CoO interface



was related to the drop in catalytic activity after the thermal treatments. In situ DRIFT experiments were conducted to determine the methane oxidation species on the sample surface by flowing an O2/CH4 mixture gas (O2/CH4 molar ratio = 4) through the sample at different temperatures. Figures S13 and S14 show the in situ DRIFT spectra of the supported Au–Pd, Au–Pd–0.40CoO, and Au–Pd–3.61CoO samples. There were the species, such as H2O (at 1640 and 3450 cm–1), gas CH4 (at 1305 and 3018 cm–1), CO2 (at 2331 and 2368 cm–1), CH3– (at 1402 cm–1), H2CO (at 1125–1185 cm–1), and HCOO– (at 1150–1163, 1050, and 980–1020 cm–1)40,41 on the supported Au–Pd, Au–Pd–0.40CoO, and Au–Pd–3.61CoO samples. However, the CH2OH– species (at 1050 and 1250 cm–1)40,41 were detected on the supported Au–Pd–3.61CoO samples (Figure 4D), again confirming that CoO was the sites for CH4 adsorption and CH4 oxidation might take place at the interface between CoO and PdO. The absorption band intensity of adsorbed CO2 species at 180 oC reached the highest and then dropped with the rise in temperature. A similar scenario occurred for the H2O species (the highest absorption band intensity appeared at 220 oC), but other species in absorption band intensity increased with the rise in temperature. These results suggest that methane oxidation took place and CO2 and H2O were desorbed from the samples when the temperature rose. Based on the above in situ DRIFT results as well as those reported in the literature,39–41 we tentatively propose the following methane oxidation mechanisms: PdO–CoO + O2 → PdOy*–CoO + O* PdOy*–CoO + CH4 → PdO–CoO + CH3* + OH*


Page 18 of 34

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CH3* + O* → H2COH* H2COH* + OH* → H2CO + H2O H2CO → HCO* + OH* HCO* + O* → HCOO* HCOO* + OH* → CO2 + H2O In summary, 3DOM Co3O4 and Au–Pd–xCoO/3DOM Co3O4 were prepared via the PMMA-templating and modified PVA-protected reduction routes, respectively. The 3DOM Co3O4-supported Au–Pd–xCoO catalysts exhibited high performance for methane oxidation, with the Au–Pd–3.61CoO/3DOM Co3O4 catalyst performing the best (TOF = 0.0076 s–1 at 280 oC). The introduced CoO enhanced methane oxidation by promoting the formation of PdO–CoO active sites and the adsorption of methane molecules. Our present work not only suggests a new route of introducing transition metal oxide (CoO) into noble metal alloy (Au–Pd) NPs to significantly improve its catalytic performance for the removal of methane, but also supplies a strategy to synthesize novel supported uniform tri-component catalysts, which is useful in designing high-performance catalytic nanomaterials. ■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acs.est.xxxxxxx. Catalyst preparation, catalyst characterization and calculation of Weisz-Prater Criterion (NW-P); physical and chemical properties, catalytic activities of various



Page 20 of 34

catalysts reported in the literature; representative bright-field and HADDF−STEM images and elemental mapping images of typical samples, particle size distribution of Au−Pd−3.61CoO NPs; SEM images of the samples; TEM and HADDF−STEM images of Au−Pd−0.40CoO/3DOM Co3O4 and Au−Pd−3.61CoO/3DOM Co3O4; H2-TPR profiles and XPS spectra of the samples; catalytic performance of the prepared samples; Arrhenius plots for methane oxidation over the samples; methane conversion versus reaction temperature over the samples at different SVs; TEM images, particle distributions, and Pd 3d XPS spectra of Au−Pd−3.61CoO/3DOM Co3O4, Au−Pd/3DOM Co3O4, and Pd−3.61CoO/3DOM Co3O4 after methane oxidation at 650 oC, and Pd−3.61CoO/3DOM Co3O4 before reaction; and in situ DRIFT of the typical samples (PDF). ■ AUTHOR INFORMATION Corresponding Author *Tel.

No.:

+86-10-6739-6118;

[email protected]

(Y.X.

Liu);

Fax:

+86-10-6739-1983;

[email protected]

E-mail (J.G.

addresses:

Deng);

and

[email protected] (H.X. Dai). Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21377008 and 21677004), National High Technology Research and Development Program ("863" Program) of China (Grant No. 2015AA034603), Ph.D.


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Program Foundation of Ministry of Education of China (20131103110002), Beijing Nova Program (Z141109001814106), Natural Science Foundation of Beijing Municipal Commission of Education (KM201410005008), Foundation on the Creative Research Team Construction Promotion Project of Beijing Municipal Institutions, and Scientific Research Base Construction–Science and Technology Creation Platform National Materials Research Base Construction. ■ REFERENCES (1) Chen, J. H.; Shi, W. B.; Zhang, X. Y.; Arandiyan, H.; Li, D. F.; Li. J. H. Roles of Li+ and Zr4+ cations in the ctalytic performances of Co1–xMxCr2O4 (M = Li, Zr; x = 0–0.2) for mthane combustion. Environ. Sci. Technol. 2011, 45, 8491. (2) Hui, K. S.; Kwong, C. W.; Chao, C. Y. H. Methane emission abatement by Pd-ion-exchanged zeolite 13X with ozone. Energy Environ. Sci. 2010, 3, 1092. (3) Chenakin, S. P.; Melaet, G.; Szukiewicz, R.; Kruse, N. XPS study of the surface chemical state of a Pd/(SiO2 + TiO2) catalyst after methane oxidation and SO2 treatment. J. Catal. 2014, 312, 1. (4) Ab Rahim, M. H.; Forde, M. M.; Jenkins, R. L.; Hammond, C.; He, Q.; Dimitratos, N.; Lopez-Sanchez, J. A.; Carley, A. F.; Taylor, S. H.; Willock, D. J.; Murphy, D. M.; Kiely, C. J.; Hutchings, G. J. Oxidation of methane to methanol with hydrogen peroxide using supported gold–palladium alloy nanoparticles. Angew. Chem. Int. Ed. 2013, 52, 1280. (5) Yang, X. F.; Wang, A. Q.; Qiao, B. T.; Li, J.; Liu, J. Y.; Zhang, T. Single-atom catalysts: A new frontier in heterogeneous catalysis. Accounts Chem. Res. 2013, 46,



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Captions of Table and Figures Table 1. Catalytic activities and apparent activation energies (Ea) of the samples for methane oxidation at SV = 20 000 mL/(g h). Figure 1. (A–C) HADDF–STEM images and (D) particle size distribution of Au–Pd–3.61CoO/3DOM Co3O4, (E) hydrogen consumption and Co2+/Co3+ molar ratio, and (F) Pd2+/Pd and Auδ+/Au molar ratios of (a) Au−Pd/3DOM Co3O4, (b) Au−Pd−0.19CoO/3DOM

Co3O4,

(c)

Au−Pd−0.40CoO/3DOM

Co3O4,

(d)

Au−Pd−0.90CoO/3DOM Co3O4, and (e) Au−Pd−3.61CoO/3DOM Co3O4. Figure 2. Methane conversion as a function of reaction time in the absence or presence of (A) 5.0 vol% water vapor in the feedstock over the samples at 340 oC, and (B) different water vapor in the feedstock at different reaction temperatures over Au−Pd−3.61CoO/3DOM Co3O4. Figure 3. Methane conversion as a function of temperature when the reaction temperature increased (solid) or decreased (open) over 3DOM Co3O4-supported (a) Au−Pd−3.61CoO, (b) Au−Pd, and (c) Pd−3.61CoO at SV = 20 000 mL/(g h). Figure 4. (A) Methane conversion at 340 oC and Co2+/Co3+ molar ratio, (B) Pd 3d XPS spectra, (C) CH4-TPD profiles, and (D) in situ DRIFT spectra during methane oxidation at 320 oC of (a) Au−Pd/3DOM Co3O4, (b) Au−Pd−0.40CoO/3DOM Co3O4, (c) Au−Pd−0.90CoO/3DOM Co3O4, (d) Au−Pd−3.61CoO/3DOM Co3O4, (e) Au−Pd−3.61CoO/3DOM Mn2O3, and (f) Au−Pd−3.61CoO/3DOM Al2O3.


Page 29 of 34


Table 1. Catalytic activities and apparent activation energies (Ea) of the samples for methane oxidation at SV = 20 000 mL/(g h).

Methane oxidation at 280 oC

Methane combustion Sample

Ea (kJ/mol)

T10% (oC)

T50% ( oC )

T90% ( oC )

Conversion

TOFPd (s−1)

Reaction rate (× 10−6 mol/(gPd s))

Au−Pd/3DOM Co3O4

284

337

379

9.0 %

0.0053

49.9

86

Au−Pd−0.19CoO/3DOM Co3O4

274

315

353

11.3 %

0.0072

68.1

69


260

312

341

14.0 %

0.0094

88.0

63


280

318

356

10.4 %

0.0077

71.9

70


281

320

362

9.6 %

0.0118

110.5

77

Au−Pd−3.61CoO/3DOM Mn2O3

365

430

500

—

—

—

101

Au−Pd−3.61CoO/3DOM Al2O3

420

543

625

—

—

—

132



A

C

B

60

30 20 10 0

1.5−2.5 2.5−3.5 3.5−4.5 4.5−5.5 nm nm nm nm

E

1.6

13.4

1.4

13.2 1.2 13.0 1.0

12.8

0.8

12.6 12.4

0.6

(a)

(b)

(c)

(d)

(e)

0.8

1.0

F

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0

(a)

(b)

(c)

(d)

Figure 1. (A–C) HADDF–STEM images and (D) particle size distribution of Au–Pd–3.61CoO/3DOM Co3O4, (E) hydrogen consumption and Co2+/Co3+ molar ratio, and (F) Pd2+/Pd and Auδ+/Au molar ratios of (a) Au−Pd/3DOM Co3O4, (b) Au−Pd−0.19CoO/3DOM

Co3O4,

(c)

Au−Pd−0.40CoO/3DOM

Co3O4,

Au−Pd−0.90CoO/3DOM Co3O4, and (e) Au−Pd−3.61CoO/3DOM Co3O4.


(d)

(e)

Auδ +/Au molar ratio

40

13.6

1.0

1.8

Pd2+/Pd molar ratio

50

Co2+/Co3+ molar ratio

13.8

d = 3.0 nm H2 consumption (mmol/gcat)

Frequency (%)

D

Page 30 of 34

Page 31 of 34


100

B

80


60


Au−Pd/3DOM Co3O4

40

5 vol% H2O in feedstock 20

H2O on

2.0 vol% H2O on

o

80

Methane conversion (%)


A

100

60

340 C

H2O off

1.0 vol% H2O on o

320 C

5.0 vol% H2O on

40 5.0 vol% H2O off

5.0 vol% H2O on 20

H2O off


0

0 0

5

10

15

20

Time on stream (h)

25

30

0

5

10

15

20

25

30

Time on stream (h)

Figure 2. Methane conversion as a function of reaction time in the absence or presence of (A) 5.0 vol% water vapor in the feedstock over the samples at 340 oC, and (B) different water vapor in the feedstock at different reaction temperatures over Au−Pd−3.61CoO/3DOM Co3O4.



Page 32 of 34

100


80

(a)

60

Au Pd

40 20 0 100 80 60 40 20 0 100 80 60 40 20 0

200

Au–Pd–CoO CoO

3DOM Co3O4 support

(b) Au –Pd 3DOM support 3DOMCo Co 3 O4support

(c) Pd–CoO CoO

Pd

3DOM Co3O4 support

300

400

500

600

700

800

o

Temperature ( C)

Figure 3. Methane conversion as a function of temperature when the reaction temperature increased (solid) or decreased (open) over 3DOM Co3O4-supported (a) Au−Pd−3.61CoO, (b) Au−Pd, and (c) Pd−3.61CoO at SV = 20 000 mL/(g h).

32


Page 33 of 34


B

2.0

o

Methane oxidation at 340 C

80

1.6 60

1.4 1.2 1.0

20

0.8

Fresh catalysts Calcined catalysts

0 (a)

0.6

2+

3+

40

Calcined Au−Pd−3.61CoO/3DOM Co3O4 Au−Pd−3.61CoO/3DOM Co3O4

Calcined Au−Pd−0.40CoO/3DOM Co3O4


329

(d)

(b)

Pd 3d

Intensity (a.u.)

1.8

Co /Co molar ratio


A100

333

337

341

345

349

Binding energy (eV) Methane desorption (a.u.)

C

D

o

370 C

Au Pd

Intensity (a.u.)

(f) Au–Pd–3.61CoO CoO 3DOM Co3O4

o

Au Pd

345 C

Au–Pd–0.40CoO CoO 3DOM Co3O4

Au–Pd

140

230

320

410

o

320 C

β

γ

(e) (d) (c) (b) (a)

3DOM Co3O4

50

o

α

500

1700 1500 1300 1100

900

700

-1

Temperature ( C)

Wavenumber (cm )

Figure 4. (A) Methane conversion at 340 oC and Co2+/Co3+ molar ratio, (B) Pd 3d XPS spectra, (C) CH4-TPD profiles, and (D) in situ DRIFT spectra during methane oxidation at 320 oC of (a) Au−Pd/3DOM Co3O4, (b) Au−Pd−0.40CoO/3DOM Co3O4, (c) Au−Pd−0.90CoO/3DOM Co3O4, (d) Au−Pd−3.61CoO/3DOM Co3O4, (e) Au−Pd−3.61CoO/3DOM Mn2O3, and (f) Au−Pd−3.61CoO/3DOM Al2O3.

33



Page 34 of 34

Efficient Removal of Methane over Cobalt Monoxide Doped AuPd Nanocatalysts

Shaohua Xie,† Yuxi Liu,*,† Jiguang Deng,*,† Simiao Zang,† Zhenhua Zhang,‡ Hamidreza

Arandiyan,§ and Hongxing Dai*,†

Doping CoO to Au–Pd/3DOM Co3O4 can significantly enhance its catalytic performance for methane combustion. Au–Pd–CoO/3DOM Co3O4 shows excellent catalytic stability, and no significant decrease in activity is observed when the temperature increases or drops at 400–800 °C.


Efficient Removal of Methane over Cobalt-Monoxide-Doped AuPd

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