Methane oxidation over oxide-supported Pd nanoparticles observed in-situ by ambient-pressure XPS

A fundamental microscopic understanding of heterogeneous catalysis under working conditions is key to further developments in catalyst design and control of chemical processes. Heterogeneous catalysis happens at the catalyst surface; we therefore need meaningful experimental data about the chemical composition and molecular arrangement at the surfaces under investigation. In general, surface-specific information is best provided by electron-based photoemission spectroscopy such as X-ray photoelectron spectroscopy (XPS). The property that makes electrons ideal surface probes is their short penetration depth of only a few atomic layers in solid matter. This poses, however, a big problem when these systems are to be studied under realistic pressure environments in the range of millibar or higher. The mean free path of electrons is then only a few millimetres or less and conventional electron energy analysers cannot be used anymore.
This so-called "pressure gap" between conventional electron spectroscopy and actual catalytic reactions, which usually take place at high reactant pressures or even in solution, is bridged to some extent by "near-ambient pressure electron analysers". Such instruments have become available very recently at synchrotron radiation facilities, such as the Advanced Light Source (ALS) in Berkeley, USA.

The applicant is seeking funding for travelling to ALS in Berkeley in order to perform near-ambient pressure photoemission experiments on oxide-supported Pd nanoparticle catalysts for partial oxidation of methane under reaction conditions. This reaction produces syn-gas, a mixture of carbon monoxide and hydrogen which is used as basis for a whole range of useful bulk chemicals. The significance of the planned experiments is that they involve real industrial catalysts as opposed to model catalysts. Therefore the results can be fed into improvement of chemical plants much more directly.

EPSRC has identified catalysis as a growth area, because of its key importance to UK industry. It is estimated that 30 - 40% of the world's GDP depend on heterogeneous catalytic processes, covering a range of reactions for production, conversion and removal of chemicals with particular impact on the energy sector. Partial oxidation of methane plays an important part in this contribution to the world's economy.
Every improvement of catalyst performance based on a deeper understanding of the elementary processes taking place on their surfaces will naturally have an impact on the economy. Equally, contributing to alternative more environment-friendly sources of fuel and energy will help tackling global climate change, probably the biggest challenge our society currently faces.

The main outcome of the experiments, which are at the centre of the current proposal, will be a protocol for studying industrial oxide-supported nanoparticulate heterogeneous catalysts under near-process conditions and information about changes in their chemical state depending on temperature and gas composition. Once it has been demonstrated that meaningful results can be obtained from synchrotron-based XPS studies on oxide-supported nanoparticulate catalysts, it can be expected that a wider community of industrial research groups will adopt this protocol and use ambient-pressure XPS facilities to carry out similar experiments.

In order to make this as widely known as possible within the synchrotron radiation communities and beyond, we will use the established communication channels of Diamond Light Source. The PI has a long-standing collaboration with Diamond Light Source through his role as Principal Beamline Scientist of the VERSOX beamline, which is devoted to ambient-pressure experiments and will start operation in 2016. Other means of dissemination and promoting impact are direct meetings with catalyst manufacturers, joint research proposals and projects, and engagement in the UK catalysis Hub.