Studies have been carried out to exploit synchrotron radiation in the soft X-ray (SXR) and vacuum ultraviolet (VUV) parts of the spectrum to examine adsorption systems of relevance to heterogeneous catalysis, particularly the strong metal support interaction (SMSI) phenomenon.
Clear evidence was found that nitrogen monoxide molecules were adsorbed onto nickel (111). At low and high coverage, and in the presence of coadsorbed oxygen, the molecules occupied hollow sites. This result led to a reassessment of the vibrational data.
Another system studied was ammonia on nickel (111). The adsorption site was determined, and off normal photoelectron diffraction showed enhanced vibrational amplitudes parallel to the surface.
Structural studies of acetylene on copper (111) provided an identification of the adsorption site and determination of the carbon to carbon bond length.
Photoelectron diffraction data can, in principle, be inverted mathematically to produce an atomic scale image of the local adsorption geometry. It was thought that very large data sets would be required, but in this work it was shown that smaller sets were adequate, where measurement was of photoelectron angular distributions at photoelectron energies corresponding to nearest neighbour backscattering interference. In that case, local structure could be obtained directly by real space triangulation.
The technique of chemical shift photoelectron diffraction was developed, whereby different atoms of the same element could be distinguished through their core level binding energy chemical shifts. Further exploitation of this technique could include quantitative elucidation of complex adsorption systems, such as those involving coadsorbed molecular fragments, which are of increasing relevance to catalytic chemistry.
A new collaboration between two groups currently involved in adsorbate structural studies on single crystals surfaces using synchrotron radiation methods (U. Warwick and Fritz-Haber-Institut) and two groups pursuing more conventional studies of catalysts (Instituto de Catalisis y Petroleoquimica and Materials of Seville) is proposed. These complementary approaches will be brought together to study adsorbate interactions (including promotor and poisoner effects) on surfaces and their role in heterogeneous catalysis, particularly through the use of synchrotron radiation spectroscopies on national facilities in the U.K. and West Germany.
Funding SchemeCSC - Cost-sharing contracts