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

摘要

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 CHx and NO species. A combination of reactivity and in situ spectroscopic results suggest that a much higher surface concentration of NO relative to CHx 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 CHx 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.