4 A SURFACE-SENSITIVE IN-SITU EXPERIMENT

The limitations of using intrinsically bulk-sensitive methods to investigate surface catalytic processes became apparent in the previous example. Surface-sensitivity is achieved if not photons (except in special cases) but electrons are used as the carriers for the spectral information. There is a fundamental physical limitation of the application of electron probes under in-situ conditions as the mean free path of electrons of relevant energies in gases is very limited (a dimension of millimetres at millibar pressure). Complementing the developments in non-linear photon-in photon-out techniques, an international co-operation between the present group and the University of Berkeley set out to design the very useful XPS technique into a "high-pressure" in-situ version. The new idea was to use besides the conventional17 set-up a high-pressure (10 mbar) flow reactor chamber with an aperture for letting the photoelectrons produced from a highly focused synchrotron radiation source escape. This was combined with an electron optics that collects all photoelectrons through the multiple differential pumping stages.

A test case is the investigation of selective oxidation of methanol to formaldehyde over metallic copper. This process occurring as dehydrogenation and/or oxydehydrogenation was studied extensively by in-situ NEXAFS18 and earlier by surface science19. These studies brought about the relevance of unique oxygen species termed as "sub oxide"20 for the selective catalytic reaction. Conventional copper oxides were identified to catalyse the total oxidation and bare metal was found to be inactive. A target for in-situ XPS was thus the search...