Study of the low-temperature reaction between CO and O2 over Pd and Pt surfaces

Abstract

Field electron microscopy (FEM), high-resolution electron energy loss spectroscopy (HREELS), molecular beams (MB) and temperature-programmed reaction (TPR) have been applied to the study of the kinetics of CO oxidation at low temperature, and to determine the roles of subsurface atomic oxygen (O sub ) and surface reconstruction in self-oscillatory phenomena, on Pd(111), Pd(110) and Pt(100) single crystals and on Pd and Pt tip surfaces. It was found that high local concentrations of adsorbed CO during the transition from a Pt(100)-hex reconstructed surface to the unreconstructed 1·1 phase apparently prevents oxygen atoms from occupying hollow sites on the surface, and leads to the appearance of a weakly bound active adsorbed atomic oxygen (O ads ) state in an on-top or bridge position. It was also inferred that subsurface oxygen O sub on the Pd(110) surface may play an important role in the formation of new active sites for the weakly bound O ads atoms. Experiments with 18 O isotope labeling clearly show that the weakly bound atomic oxygen is the active form of oxygen that reacts with CO to form CO 2 at T~140-160 K. Sharp tips of Pd and Pt, several hundreds angstroms in diameter, were used to perform in situ investigations of dynamic surface processes. The principal conclusion from those studies was that non-linear reaction kinetics is not restricted to macroscopic planes since: (i) planes as small as~200 Å in diameter show the same non-linear kinetics as larger flat surfaces; (ii) regular waves appear under conditions leading to reaction rate oscillations; (iii) the propagation of reaction-diffusion waves involves the participation of different crystal nanoplanes via an effective coupling between adjacent planes.

Oxidation of propylene over Pd(5 5 1): Temperature hysteresis induced by carbon deposition and oxygen adsorption

“…The mechanism is complex and the catalyst activity and selectivity are influenced by variations in the process pressure, temperature, and the gas mixture composition. Moreover, under oxidizing conditions, Pd-based catalysts can exhibit an interesting dynamic behavior, including hysteresis phenomena, self-sustained rate oscillations, spatial pattern formation, and deterministic chaos; a fast deactivation of palladium is frequently observed as well [7][8][9][10][11][12][13][14][15][16][17][18]. It is obvious that in all these cases a change in the chemical state of palladium determines the catalytic performance, causing the critical phenomena or catalyst deactivation.…”

Use of molecular beams for kinetic measurements of chemical reactions on solid surfaces

“…This is believed to be due to the desorption of CO, which releases extra sites for O 2 dissociation and in turn removes more CO (hence the self-acceleration) [275]. Finally, on surfaces prone to reconstruction, the non-linearity of the rate of that process on surface coverage can cause, under specific conditions, chemical oscillations, both in time and in space [276][277][278].…”

From single crystals to supported nanoparticles in oscillatory behavior of CO+O2 reaction on platinum and palladium surfaces: Experiment and stochastic models

“…The effect of subsurface oxygen on the rate of the CO + O ads → CO 2 reaction was studied by TPR experiments [32]. It has become clear that the differentiation between adsorbed (O ads ) and subsurface (O sub ) oxygen is difficult.…”

“…The CO + O 2 reaction on the Pd(1 1 0) single crystal and Pd-tip surfaces were investigated using the PEEM, FEM, MB techniques to learn the details of the reaction dynamics [30,32,33]. At 300 K the reaction is limited by oxygen adsorption because the surface is covered with CO ads .…”

“…Then, the subsurface oxygen layer forms again and the cycle is restored. Based on our TPR, PEEM, MB and FEM data regarding CO oxidation over Pd surfaces [9,30,[32][33][34], some elementary steps have been added to the classic L-H scheme, namely in Scheme 2.…”