FTIR and reaction kinetics studies of the hydrodehalogenation of CF2Br2, CH2Br2, and CD2Br2 over a Pd/γ-Al2O3 catalyst

A combination of FTIR and chemical reaction kinetics studies was employed to investigate the hydrodehalogenation of CF2Br2, CH2Br2, and CD2Br2, catalyzed by Pd/gamma-Al2O3. The products of the CF2Br2+H-2 reaction (C2FH2, CF3H, C-1-C-3 hydrocarbons) are typical for a reaction mechanism with the participation of :CF2 and :CH2 carbenes as the reaction intermediates. It was not possible to detect :CF2 carbenes by FTIR during the adsorption studies. It is proposed that the carbenes are unstable on the Pd surface in the absence of H-2 and readily decompose to form carbonaceous deposits and adsorbed halogen atoms. Thus, room-temperature hydrogenation of the surface species formed during adsorption of CF2Br2 on Pd/gamma-Al2O3 at room temperature yields only CH4, HBr, and HF. The :CD2 and :CH2, carbenes formed from dissociation of CD2Br2 and CH2Br2 adsorbed on the Pd surface are stable at room temperature, as indicated by the formation of CD2H2 upon hydrogenation of adsorbed species by H-2 and D-2, respectively.