Site-blocking effects in temperature-programmed reaction studies of surface decomposition reactions

Abstract

The starting point for most analyses of temperature-programmed reaction (TPR) spectra has been the Wigner-Polanyi expression in the form: rate = v(θ) exp(− E a(θ) kT )θ n , where θ is the surface coverage. This paper discusses the b approximation for the case of decomposition reactions where the dissociation rate is inhibited by blocking sites on the surface. It is shown that coverage dependences other than θ n that arise from site-blocking have dramatic effects on TPR spectra. These effects are illustrated both with simulations and with experimental results for the decomposition of butyl groups bound to a Cu(110) surface. While the surface coverages of filled and vacant sites must, in general, be carefully distinguished, it is shown that if adsorbate coverage is defined relative to saturation of the monolayer then regardless of the number of sites blocked per adsorbate the initial rate varies as θ 0(1 − θ 0) where θ 0 is the initial coverage. The effects of interactions between adsorbates are also discussed.