A general method for determining the role of spectroscopically observed species in reaction mechanisms: Analysis of coverage transients (ACT)

Abstract

A kinetic method is described to determine the role of adsorbed intermediates in heterogeneous reaction mechanisms. The method, denoted as analysis of coverage transients (ACT), involves comparing the time response of a spectroscopically observed species in an inert gas and a reactive gas to differentiate between adsorption–desorption processes and reaction. The method is applied in a kinetic study of pyridine hydrodenitrogenation (HDN) on a 12.2 wt% silica-supported nickel phosphide (Ni 2P/SiO 2) catalyst at 423 K and atmospheric pressure. In situ Fourier transform infrared spectroscopy (FTIR) measurements of pyridine adsorbed on Ni 2P revealed the formation of a pyridinium surface intermediate. The concentration of the pyridinium intermediate increased with pyridine partial pressure and decreased in the presence of hydrogen, suggesting that it was a reaction intermediate. However, transient and steady-state kinetic measurements showed that the rate of reaction of the intermediate did not correspond to the overall reaction rate, and it is concluded that the pyridinium intermediate is not directly involved in the HDN reaction of pyridine over Ni 2P. The studies demonstrate that mere observation of an adsorbed surface species at reaction conditions is not sufficient to prove that it is a reaction intermediate. The ACT method has potential as it can be used with any type of spectroscopy, as long as the surface coverage can be calibrated.