The Partial Oxidation of Methane Over Pd/Al2O3 Catalyst Nanoparticles Studied In-Situ by Near Ambient-Pressure X-ray Photoelectron Spectroscopy

Rachel Price,Liam Deacon,Georg Held,Richard Smith,Tuǧçe Eralp-Erden,Sana Rani,Ethan Crumlin,Sonia Garcia,Chanan Euaruksakul

doi:10.1007/s11244-015-0520-8

Abstract

Near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to study the chemical state of methane oxidation catalysts in-situ. Al2O3–supported Pd catalysts are prepared with different particle sizes ranging from 4 to 10 nm. These catalysts were exposed to conditions similar to those used in the partial oxidation of methane (POM) to syn-gas and simultaneously monitored by NAP-XPS and mass spectrometry. NAP-XPS data show changes in the oxidation state of the palladium as the temperature increases, from metallic Pd0 to PdO, and back to Pd0. Mass spectrometry shows an increase in CO production whilst the Pd is in the oxide phase, and the metal is reduced back under presence of newly formed H2. A particle size effect is observed, such that CH4 conversion starts at lower temperatures with larger sized particles from 6 to 10 nm. We find that all nanoparticles begin CH4 conversion at lower temperatures than polycrystalline Pd foil.

Highlights

Instead of flaring off large quantities of unused natural gas around the world, it could be used in a more environmentally friendly way as fuel for automotive engines
Due to the catalyst preparation method and storage, Pd shows some degree of oxidation for all samples at 500 K, but metallic Pd peaks at 335:0 Æ 0:2 eV are dominant at these lower temperatures
As the temperature is increased to 600 K and again to 650 K, an X-ray photoelectron spectroscopy (XPS) signal characteristic of a PdOx species at 336.0–336.2 eV becomes dominant for most particle sizes

Summary

Introduction

Instead of flaring off large quantities of unused natural gas around the world, it could be used in a more environmentally friendly way as fuel for automotive engines. Natural gas vehicles (NGVs) operate under lean conditions with low methane concentrations (500–1000 ppm) and working temperatures typically under 823 K (550 °C). The beneficial outcome of these working conditions is that relatively ’’clean’’ products are produced (CO2, H2, H2O) in comparison to other fossil fuels producing harmful nitrogen and sulphur containing compounds. The C–H bonds in aliphatic hydrocarbons have high dissociation energies (439.3 kJ molÀ1 in CH4) and the absence of functional groups leaves the molecule with zero polarity and no sites for either nucleophilic or electrophilic attack. This makes small hydrocarbons, such as methane, very difficult to oxidise without a catalyst at low temperatures [4]

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Discussion

Conclusion