CeO2 Catalysts for

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Mesoporous Silica-Supported Nanostructured PdO/CeO2 Catalysts for Low-Temperature Methane Oxidation Yiling Dai,† Vanama Pavan Kumar,† Chujie Zhu,‡ Mark J. MacLachlan,*,† Kevin J. Smith,*,‡ and Michael O. Wolf*,† †

Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada

‡

S Supporting Information *

ABSTRACT: Nanostructured PdO/CeO2 supported on mesoporous SBA-15 silica was synthesized using a combination of incipient wetness impregnation and surface-assisted reduction. After calcination, the materials showed good activity as catalysts for the low-temperature oxidation of methane, with a sample having 5 wt % Pd loading showing 50% conversion to CO2 at ∼290 °C and complete conversion below 360 °C. The stability of catalysts in the presence of water was studied. The formation of Pd(0) during the methane oxidation reaction increases the oxygen vacancies on the surface of catalysts, improving the catalytic activity. KEYWORDS: methane oxidation, double deposition approach, mesoporous silica, ceria, palladium oxide °C.10 Our group reported the preparation of nanostructured PdO/CeO2 by a new “surface-assisted reduction” (SAR) method.32 In this approach, reactive formate groups on the surface of nanostructured cerium formate (CF) act to reduce Pd2+ onto the surface, yielding nanostructured CeO2 substrates coated with highly dispersed PdO after calcination. These catalysts show 50% conversion for the oxidation of methane below 300 °C. The activity of heterogeneous catalysts is influenced by many factors, including particle size,33 and in many cases, nanoparticles give rise to the best catalytic performance;34 sintering, however, can reduce performance over time. Confining nanoparticle catalysts in mesoporous silica such as SBA-15 is an approach to overcome the sintering problem.35 In fact, SBA15 has been used to construct nanoparticles of various materials,36 some of which have been explored for catalysis.37 Here, we report the preparation of new nanostructured catalysts based on PdO/CeO2/PdO materials supported on SBA-15. These materials have been constructed using a novel double deposition approach, where SAR of Pd is carried out inside the channels of a mesoporous host for the first time. Importantly, the SBA-15 host constrains the size of the resulting Pd and CeO2 particles, whereas the SAR ensures excellent distribution of Pd on the nanostructured CeO2. The materials are evaluated for LTMOX catalysis.

1. INTRODUCTION Natural gas, composed primarily of methane, is a plentiful, cleaner alternative to gasoline and diesel as it results in the emission of less CO2 per unit energy produced.1−4 However, methane is a potent greenhouse gas, with a global warming potential more than 20 times greater than that of carbon dioxide.5 Consequently, managing the unburned methane expelled in the exhaust of natural gas engines is an important consideration for further expansion in the use of natural gas vehicles.6 Owing to the lack of a dipole moment and strong C−H bond strength (440 kJ mol−1),7 methane is the least reactive hydrocarbon and is not readily oxidized by conventional catalytic convertors.8 Efficient catalytic materials for lowtemperature methane oxidation (LTMOX) require low lightoff temperatures (99% conversion at 400 °C, with a 5% decrease in conversion after reaction for 50 h (gas hourly space velocity (GHSV) = 30 000 mL h−1 g−1). For the catalysts reported here with 1% Pd loading, 90% conversion is obtained at 370 °C but this recovers to 100% conversion after

oxidation, showing both Pd and PdO peaks. In contrast, only PdO was observed by XPS of the fresh catalysts. This indicates that the reaction gas reduced the PdO to Pd.44 Although Pd(0) was observed by XPS, crystalline Pd was not observed in the PXRD of the used catalyst. The Pd may be present in nanoparticles that are too small to show diffraction. Moreover, the Ce 3d peaks for the fresh sample show a pure CeO2 pattern with about 6% Ce3+.45 For the used catalyst, the percentage of Ce3+ increased to about 10%, resulting in weakening of the Ce−O bond and increased oxygen mobility.45 The formation of Pd during the reaction increases the number of oxygen vacancies on the surface. Both the PdO and H

DOI: 10.1021/acsami.7b13408 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 11. XPS spectra for PdCFPdSBA-15(M) 5% Pd before and after the thermal stability test. (a) Pd 3d peaks, (b) Ce 3d peaks (red solid circles represent Ce 3d3/2, and blue solid circles represent Ce 3d5/2) for fresh catalysts. (c) Pd 3d peaks, (d) Ce 3d peaks for used catalysts.

attributed to the small size and good distribution of Pd on CeO2 and also PdO/CeO2/PdO on SBA-15. In addition, the water stability of the catalysts was studied. After the injection of water, the activity clearly decreased. However, the activity increased gradually afterward and was even better than that for fresh catalysts after water was removed. The improved catalytic activity during the reaction is attributed to the formation of Pd(0), which increased oxygen vacancies. These results demonstrate a new way to make stable methane oxidation catalysts that may be useful for catalytic converters in natural gas engines.

Table 4. XPS Data for Catalysts of PdCFPdSBA-15(M) 5% Pd before and after the Thermal Stability Test element

catalysts

Ce

fresh used

Pd

fresh used

binding energy (eV) 883.7, 886.6, 883.3, 886.0, 337.4, 337.3, 336.2,

889.5, 899.3, 902.1, 908.3, 917.8 905.6 889.1, 899.0, 901.7, 908.0, 917.5 904.5 342.6 342.6 340.7

oxidation state +4 +3 +4 +3 +2 +2 0

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20 h of exposure to water (GHSV = 180 000 mL h−1 g−1). Yang et al.48 reported the synthesis of Au@PdOx embedded in Co3O4 nanorods with 2.44 wt % Pd loading; this catalyst achieved 90% conversion at 344 °C (GHSV = 60 000 mL h−1 g−1). The catalysts reported here with 1% Pd loading achieved 100% conversion at 355 °C after water stability tests under much higher GHSV.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13408. Silica loading calculations, IR spectra, and PXRD patterns for the catalysts (PDF)

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4. CONCLUSIONS In summary, nanoparticles of PdO/CeO2/PdO supported on SBA-15 were prepared by impregnation and surface-assisted reduction. Using the SAR method within the pores of a mesoporous host (SBA-15) allowed us to constrain the size of the catalytic nanoparticles and to obtain an even distribution of the catalyst inside the channels of silica. The catalyst system PdO/CeO2/PdO on SBA-15 shows high activity for lowtemperature methane oxidation, and the high activity is

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.J.M.). *E-mail: [email protected] (K.J.S.). *E-mail: [email protected] (M.O.W.). ORCID

Mark J. MacLachlan: 0000-0002-3546-7132 Kevin J. Smith: 0000-0001-6008-0223 Michael O. Wolf: 0000-0003-3076-790X I

DOI: 10.1021/acsami.7b13408 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada and Westport Fuel Systems Inc. for funding this research. We are especially thankful to Dr. Sandeep Munshi for helpful comments and support. Y.D. is grateful to UBC for a four year fellowship.

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DOI: 10.1021/acsami.7b13408 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX