Active Sites in Heterogeneous Catalysis

This chapter focuses on the active sites in heterogeneous catalysis. The chapter reviews a surface structure and chemisorption characteristics of crystals cut along different crystallographic orientations. A chemical reaction is studied at low pressure to establish correlations between reactivity and surface structure and composition. The same catalytic reaction is studied at high pressures and the pressure dependence of the reaction rate is determined using the same sample over the nine orders of magnitude range. The rates and product distributions that were determined at high pressures on single-crystal surfaces are compared with the reactivity of polydispersed small-particle catalyst systems. At low pressures, a quadrupole mass spectrometer is used as a detector of both the chemical reaction rates and the product distributions. At high pressures, a gas chromatograph is employed that is as sensitive as a mass spectrometer that is used at low pressures. Small-surface-area single-crystal catalyst samples can readily be used in studies as long as the reaction rate is greater than 10−6 product molecules/surface atom/sec. The rate defined is called “turnover number” in the field of catalysis.