Non-classical kinetics in hydrogen isotope exchange reaction on supported platinum catalysts: Experiments and simulations.

Abstract

Non-classical kinetic behavior of the hydrogen isotope exchange reaction on supported platinum catalysts has been investigated by experimental and computer simulation studies. Monte Carlo simulations of the reaction A2 + B2 $\to$ Products have provided several interesting models for experimental study. Anomalous reaction kinetics and reaction orders are exhibited at early time on a one-dimensional "large lattice" system. Non-classical reaction orders have also been found at steady state on "low dimensionality", one- and two-dimensional many-island systems. The former is attributed to strong initial reactant correlation which dissipates in time due to surface diffusion whereas the latter is attributed to a combination of effective "low dimensionality" and low concentration. A new analytical approach, which monitors neighbor pair densities instead of conventional surface concentration, has produced a clearer understanding of the non-classical results above by providing the nearest neighbor pair distance distributions, the pair orders, and the "local" vs. global concentrations. A computer algorithm scheme, which successfully monitors pair distributions, has also been developed for this simulation study. Four different platinum catalysts, with average metal island sizes ranging from 10 to 45 A, have been studied. The metal catalyst island sizes have been characterized by transmission electron microscopy. In situ analysis of both reactant and product gas concentrations have been provided by time-of-flight mass spectrometry. A theoretical methodology which correlates the surface reaction rate with the bulk gas pressure change rate for each reactant and product has been developed. Three catalysts with smaller metal islands have exhibited an early time tail in their product gas pressure curves which could not be fitted by the equation above whereas one catalyst, with an average of 45 A metal island, produced a bulk product gas pressure curve which could be fitted nicely due to the absence of the tail. The results indicate the possibility that the non-classical surface reaction rate, signified by the early time tail in the bulk gas pressure curve, may be attributed to low dimensionality of the system. It is interesting that both computer simulation and experimental study results show "dimensionality effects" on non-classical kinetics.