Infrared spectroscopy of transient surface species

A full understanding of a catalytic reaction requires a thorough identification of precursor and intermediate species. In this regard spectroscopies in general and infrared (IR) spectroscopy, in particular, can play an important role. However, the spectroscopic identification of precursor and intermediate species is often difficult owing to their transient nature. In this review we show how control of experimental parameters such as temperature, equilibrium pressure, and reactant-catalyst contact time allows experimentalists to alter the rates of dynamic processes and consequently to modify appreciably the relative concentrations of precursor, intermediate, and product species present under reaction conditions. In this context, time-resolved FTIR spectroscopy (with constant temperature and pressure during the experiment) and also temperature-time resolved (with both temperature and time changing simultaneously in a controlled way during the experiment) can be useful for kinetic investigations of several types of reactions. In this review several examples demonstrating the potential value of FTIR spectroscopy for the identification of surface transient species are discussed. These are classified in four main categories: (i) adsorption processes and transformations in the adsorbed state; (ii) proton-catalyzed oligomerization and polymerization of alkenes and unsaturated molecules in protonic zeolites; (iii) oligomerization reactions catalyzed by basic surface sites, and (iv) oligomerization and polymerization of alkenes on catalysts incorporating supported transition metal ions. All these reactions are discussed in the framework of a few common potential energy profiles in which the evolution from the precursor through the intermediate species to the final products is governed by the relative height of the corresponding activation energy barriers.