Seminars Archive

Near-ambient pressure XPS/NEXAFS at Diamond Light Source

Georg Held - email: georg.held@diamond.ac.uk
Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, Oxfordshire, UK

Abstract
The near-ambient-pressure beamline B07 (Versatile Soft X-ray beamline) at Diamond Light Source opened for users in July 2017. It features two branch lines with endstations covering the pressure range from UHV to atmosphere. The energy range 50/170 - 2200/2800 eV allows accessing a wide range of core levels and is optimised for the electron kinetic energy range necessary to penetrate gas environments in the 10 mbar range. The differentially pumped beamline entrance and analyser of the NAP endstation enable measurements routinely up to 10-30 mbar. A small NEXAFS endstation is separated from the beamline by a window and allows pressures up to 1 bar. The talk will discuss the beamline designs and performance and present results of experiments in ambient-pressure conditions, which demonstrate the research possibilities it offers in areas, such as catalysis, electrochemistry, materials science, etc.
A particular focus of our in-house research in recent years was on cluster-size effects in heterogeneous catalysis, whose importance has been recognized for some time. Results will be presented of Pd nano-catalysts in the range between 1 and 10,000 atoms, which have been studied by NAP-XPS under dry and wet reaction conditions for methane oxidation (CH4 + O2 [+ H2O]). Under dry reaction conditions large Pd clusters appear to oxidize fully (almost only Pd II) whereas smaller particles show a mix of oxidation states (Pd 0 and Pd II). This clearly shows compositional changes as function of particle size when all other reaction parameters are kept the same and demonstrates the possibilities of fine-tuning catalytic activity if better size-control can be achieved in catalyst synthesis.