In many cases, the performance of heterogeneous catalysts sensitively depends on the structural properties of their surfaces. Often, size and structure of the nanometer-sized active catalyst particles are optimized empirically to provide a distribution of adsorption and reaction sites which maximizes selectivity and activity for the reaction system of interest. But how do the ensembles of different sites on a catalyst particle control the microscopic reaction kinetics and lead to size and structure dependencies? In spite of the large industrial and economical impact of this question, there is hardly any detailed understanding of the molecular origins of such effects (e.g. refs. [1, 2]). This lack of detailed knowledge is related to both serious experimental problems in studies on real catalyst and the enormous complexity of their surfaces. To overcome these difficulties, we have recently started to combine two experimental approaches: 1) using supported model catalysts with a reduced level of complexity and 2) molecular-beam techniques, which provide detailed kinetic data under most well-controlled conditions. [3] A key problem in kinetic studies on catalyst surfaces, which remains unsolved in most cases, is the spectroscopic identification of the different particle sites and the possibility to examine in situ their occupation by reactant and product species under normal reaction conditions. Herein, we present results of the NO dissociation on Pd/Al 2 O 3 , for which it was possible to monitor the reactant distribution on the catalyst nanoparticles and the related reaction kinetics under reaction conditions. We investigate adsorbed NO by TR-IRAS (timeresolved IR reflection absorption spectroscopy) and from this data draw conclusions on the distribution of dissociation products on the catalyst particles.
Recently, we investigated the decomposition of methanol on Pd nanocrystallites and were able to show how selectivity in a system of two competing decomposition reactions, CÀH and CÀO bond scission, is controlled by the presence of specific sites on the particles. [4] This was achieved using
[*] Dr.