Molecular insights into the hydrodenitrogenation mechanism of pyridine over Pt/γ-Al2O3 catalysts

The hydrodenitrogenation of nitrogen-containing heterocycles over noble metals has fundamental importance for energy and environmental science. To develop more efficient catalyst, mechanistic investigation has been conducted with a method combining in situ Fourier transformation infrared experiments and density functional theory calculations in this work. The in situ experiments indicate flatly adsorbed pyridine molecules convert to pyridinium and alpha-pyridyl species at higher temperature on metallic Pt of Pt/gamma-Al2O3 catalysts. Pyridine hydrogenation distinctly takes place at 150 degrees C with the appearance of methylene stretching vibrations, and a stepwise mechanism is identified as the temperature further increases. The adsorption, hydrogenation and hydrogenolysis of pyridine on Pt are studied in detail by theoretical calculations. In line with these findings, the geometry optimization confirms pyridine preferentially adsorbs on Pt(111) and Pt(211) both in a parallel configuration. Based on the Langmuir-Hinshelwood mechanism, the results show successive hydrogenation markedly lowers the energy barrier for subsequent hydrogenolysis. The C-N bond cleavage occurs via nucleophilic attack of pentahydropyridine, rather than piperidine, which determines the reaction products, including piperidine, n-pentylamine and n-pentane. The comparative study reveals both hydrogenation and hydrogenolysis are kinetically and thermodynamically more competitive on Pt(211) than Pt(111). Especially for hydrogenolysis, the coordinatively unsaturated Pt step atoms play an essential role in C-N bond cleavage. Thus, hydrogenolysis is more geometric-dependent than hydrogenation. This provides instructive information for the design of catalysts with adjustable product selectivity.