Abstract

The search to discover a suitable catalyst for complete combustion of methane at low temperature continues to be an active area of research. We prepared a 5 wt % PdO-PdOx/γ-Al2O3 catalyst by a modified Vortex-assisted Incipient Wetness Method. X-ray Photoelectron Spectroscopy showed that the original catalyst contained PdO (38%) and PdOx (62%) on the surface and indicated that PdOx originated from the interaction of PdO with the support. Scanning Transmission Electron Microscopy confirmed the catalyst had an average particle size of 10 nm and was well-dispersed in the support. The catalyst exhibited exceptional low-temperature activities with 90–94% methane conversion at 300–320 °C. The catalyst was active and stable after several catalytic runs with no signs of deactivation by steam in this narrow temperature range. However, the conversion decreased in the temperature range 325–400 °C. The surface composition changed to some extent after the reaction at 325 °C. A tentative mechanism proposes PdOx (Pd native oxide) as the active phase and migration of oxide ions from the support to PdO and then to PdOx during the catalytic oxidation. The high methane conversion at low temperature is attributed to the vortex method providing better dispersion, and to catalyst–support interaction producing the active phase of PdOx.

Highlights

  • Methane is a greenhouse gas with a global warming potential 25 times higher than carbon dioxide

  • The high methane conversion at low temperature is attributed to the vortex method providing better dispersion, and to catalyst–support interaction producing the active phase of Pd native oxide (PdOx)

  • We propose a new mechanism for the catalytic reaction at lower temperatures below 325 ◦ C involving the migration of oxide ions from PdOx to adsorbed CH4 and from the support (γ-Al2 O3 ) to PdO to form PdOx

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Summary

Introduction

Methane is a greenhouse gas with a global warming potential 25 times higher than carbon dioxide. Under some situations, the oxygen from the aluminum oxide could migrate to the palladium, forming PdO or some form of PdOx , and/or migrate to the gas-phase molecule providing oxygen for the combustion [22] This mechanism is underexplored and is one that this research seeks to advance evidence to support or reject. We developed a catalyst containing PdO-PdOx on a gamma-alumina support that showed low-temperature activity at 275–325 ◦ C. Another objective was to identify the surface compounds and their role in catalytic activity and the reaction mechanism. We propose a new mechanism for the catalytic reaction at lower temperatures below 325 ◦ C involving the migration of oxide ions from PdOx to adsorbed CH4 and from the support (γ-Al2 O3 ) to PdO to form PdOx

Catalyst Preparation
Scanning
2–6. HAADF-STEM
X-ray Powder
Physisorption and Chemisorption
Activity Studies
11. Activity
Active Phase of the Catalyst and Proposed Mechanism
Catalyst Synthesis
Catalyst Characterization
Activity Measurements
Conclusions
Full Text
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