Methane activation with nitric oxide at low temperatures on supported Pt catalysts: effects of the support

Abstract

Platinum nanoparticles supported on various metal oxides (MgO, (θ + γ)-Al₂O₃, SiO₂, TiO₂, Y₂O₃, ZrO₂, and CeO₂) were examined for the catalytic activation of methane with nitric oxide at low temperatures (300–400 °C) and atmospheric pressure. The catalysts exhibited similar levels of CH₄ and NO conversion, except for those supported on the basic oxides (MgO and Y₂O₃). Notably, hydrogen cyanide (HCN), a widely used chemical intermediate, was detected in an appreciable amount only on the (θ + γ)-Al₂O₃-supported Pt catalyst. A more detailed comparison of the Pt nanoparticles dispersed on SiO₂, TiO₂, and (θ + γ)-Al₂O₃ indicated that the difference in the catalytic behavior for C–N coupling was not related to the electronic and geometric properties of Pt, but rather stems from the relative concentrations of adsorbed CH_x and NO species. A combination of reactivity and in situ spectroscopic results suggest that a much higher surface concentration of NO relative to CH_x may be responsible for the easier reduction of NO to form N₂ and N₂O on Pt/SiO₂ and Pt/TiO₂. Along the same vein, comparable concentrations of NO and CH_x may be responsible for the formation of HCN on Pt/(θ + γ)-Al₂O₃. A further comparison of Pt catalysts supported on Al₂O₃ with different crystal structures (α-Al₂O₃, θ-Al₂O₃, (θ + γ)-Al₂O₃, and γ-Al₂O₃) confirmed the favorable properties of Al₂O₃ in HCN formation, although a preponderance of surface acidic OH groups on the supports promoted subsequent hydrolysis of HCN to NH₃.