15 - Surface reactions

Summary

Introduction

Our penultimate chapter is concerned with the chemical physics of surface reactions. More precisely, the consistent microscopic perspective adopted throughout this book demands that we ask the following question: can the experimental and theoretical methods of surface physics provide a useful account of real-life surface reaction processes? Let us emphasize the word ‘useful’. It is one thing to construct a post facto analysis which faithfully reproduces some set of observations. It is quite another to formulate general principles which provide qualitative insight and lead to quantitative predictive power. The selected examples below are intended to demonstrate that this discipline is just now passing from the former perspective to the latter.

Surface reactions are complex events which come in many guises for many purposes. Often (but not always!) one begins with some combination of species in the gas phase: the reactants. In heterogeneous catalysis, the purpose of the surface is to confine the reactants to a two-dimensional space in order to increase the probability for collision and reaction. The sought-after reaction products desorb for collection with no material change in the surface itself. Compare this to metalorganic chemical vapor deposition (MOCVD) growth of compound semiconductors (Dupuis, 1984). Therein, the surface stimulates a decomposition reaction. Unwanted species desorb and the desired species incorporate themselves into the solid.