Mechanism of the Reaction NO + H2 on the Pt(100)-hex Surface under Conditions of the Spatially Nonuniform Distribution of Reacting Species

M. Yu. Smirnov, Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences
D. Yu. Zemlyanov, Purdue University, Birck Nanotechnology Center
E. I. Vovk, Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences

Abstract

The interaction of hydrogen with NO ads /1 × 1 islands produced by NO adsorption on the reconstructed surface Pt(100)-hex was studied by high-resolution electron energy loss spectroscopy (HREELS) and the temperature-programmed reaction (TPR) method. The islands are areas of the unreconstructed surface Pt(100)- 1 × 1 saturated with NO ads molecules. The hexagonal phase around these islands adsorbs much more hydrogen near room temperature than does the clean Pt(100)-hex surface. It is assumed that hydrogen is adsorbed on the hexagonal surface areas that are adjacent to, and are modified by, the NO ads /1 × 1 islands. The reaction of adsorbed hydrogen atoms with NO ads takes place upon heating and has the character of so-called surface explosion. The TPR peaks of the products of this reaction—nitrogen and water—occur at T des ~ 365–370 K, their full width at half-maximum being ~5–10 K. In the case of the NO ads /1 × 1 islands preactivated by heating in vacuo above the NO desorption onset temperature (375–425 K), after the admission of hydrogen at 300 K, the reaction proceeds in an autocatalytic regime and the product formation rate increases monotonically at its initial stage. In the case of activation at 375 K, during the initial, slow stage of the reaction (induction period), hydrogen reacts with nitric oxide molecules bound to structure defects (NO def ). After activation at 425 K, the induction period is characterized by the formation and consumption of imido species (NH ads ). It is assumed that NH ads formation involves N ads atoms that have resulted from NO ads dissociation on defects upon thermal activation. The induction period is followed by a rapid stage of the reaction, during which hydrogen reacts with NO 1 × 1 molecules adsorbed on 1 × 1 areas, irrespective of the activation temperature. After the completion of the reaction, the areas of the unreconstructed phase 1 × 1 are saturated with adsorbed hydrogen. The formation of H ads is accompanied by the formation of a small amount of amino species (NH 2ads ).