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 degrees 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.

Automated summary

This study investigates the activation of methane with nitric oxide on Au-Pt nanoparticles supported on alumina. The reaction produces HCN through C-H bond cleavage, NO dissociation, and C-N coupling reactions. The catalysts with different Au/Pt ratios have similar particle morphology and bulk properties but exhibit differences in electronic and geometric surface properties due to Au surface enrichment. The relative proportion of electron-deficient Pt sites on the catalysts' surface correlates with their activity for CH4 conversion and HCN formation.