A comparative study of various transition metal overlayer catalysts for low-temperature NH3 oxidation under dry and wet conditions

In this study, several nanometer-thick transition metal overlayers (M) were formed on an Fe-Cr-Al metal (SUS) foil by pulsed cathodic arc-plasma deposition. The thin-film materials prepared, exhibiting a full-coverage M overlayer (M/SUS), were applied as catalysts for NH3 oxidation in the presence of excess O-2 (300 ppm NH3, 8% O-2, and He balance). Under dry conditions (0% H2O in the gas feed), the observed NH3 oxidation activity decreased in the order of Ir > Pt > Co > Pd > Cu > Rh when the product selectivity was not taken into consideration. The activities of the materials composed of Co and Cu decreased significantly in the presence of 10% H2O, while those of the catalysts containing Pt and Ir remained nearly unchanged. Based on the kinetic analyses and X-ray photoelectron spectrometry experiments conducted, it was determined that the contrasting effect of H2O on the activity of M/SUS was related to the ease of oxidation of the non-precious metals studied. The resulting hydrophilic oxide surface was more susceptible to the competitive adsorption of H2O and thus inhibited the NH3 reaction than a metallic surface. The negative effect of H2O was evident by the decrease in the NH3 concentration in the gas feed. Notably, the metallic surface of the Pt overlayer was stable even under oxidizing reaction conditions; therefore, the oxidation of NH3 was less susceptible to the effects of high H2O concentrations.