Experimental Evidence for Alloying Effects in Au-Pt-Catalyzed Low-Temperature CH₄ Activation with NO

The activation of methane with nitric oxide at low temperatures (300-400 °C) and atmospheric pressure was studied on Au-Pt nanoparticles with different Au/Pt ratios supported on alumina. The reaction forms HCN, a valuable chemical intermediate, via concurrent or successive C-H bond cleavage, NO dissociation, and C-N coupling reactions. The catalysts with different Au/Pt ratios had similar particle morphology and bulk properties, as evidenced by scanning transmission electron microscopy imaging and X-ray absorption near-edge structure spectroscopy, respectively. However, they exhibited different electronic and geometric surface properties, according to in situ diffuse reflectance infrared Fourier transform spectra of preadsorbed CO, a likely consequence of Au surface enrichment. These differences had catalytic implications, where the relative proportion of electron-deficient Pt sites on the surface of the catalysts was found to track with activity, expressed as turnover frequency for the total CH₄ conversion and HCN formation. Based on the activity trend, kinetic comparison of reaction orders, temperature-programmed desorption experiments, and comparisons to the literature, it is proposed that the CH₄-NO catalysis occurred mainly on contiguous Pt sites, that the higher C-H activation activity of electron-deficient Pt species may be related to weaker adsorption of CH₄, and that the higher NO dissociation activity of metallic Pt species may have contributed to the complete oxidation of CH₄ to CO₂. Nonetheless, contact time and in situ spectroscopic studies at isothermal conditions revealed only subtle differences in the reaction scheme by surface modifications, indicating that Au affected the reactivity indirectly.