Spectroscopic evidence of the simultaneous participation of rhodium carbonyls and surface formate species during the CO2 methanation catalyzed by ZrO2-supported Rh

ZrO2-supported rhodium nanoparticles prepared by impregnation of RhCl3 are active for CO2 methanation at temperatures above 180 degrees C. Infrared (IR) spectra recorded during catalysis allowed identification of Rh carbonyls and formate species bonded to the support. To verify their individual involvement in the catalysis, their transformations were investigated by IR spectra measured as a sample of ZrO2-supported Rh was treated with CO and isotopically labelled formic acid. The data indicate two coexisting reaction routes: a dissociative route in which CO2 reacts on the Rh sample to give Rh carbonyls, and an associative route in which the CO2 molecule is activated on the support, in the form of bicarbonate species that are hydrogenated to give formate species prior to methane formation. Our data show that labelled formates are transformed into Rh-13CO, thus connecting the dissociative and associative catalytic routes. The results indicate the existence of a dual mechanism for CO2 methanation.