The carbon dioxide (CO2) methanation has gained increasing interest since the power-to-gas technology has shown potential to decrease the CO2 emissions to the atmosphere. Because the reaction occurs at relatively mild conditions, it has been regarded as prototypical to understand the chemical transformations of CO2 on supported metal catalysts. In spite of its seeming simplicity, there is an ongoing debate regarding its mechanism, with a variety of proposals that are based (mostly) on infrared (IR) spectroscopic investigations. The apparently contradictory proposals can be divided in two categories: (a) those suggesting dissociative routes (with CO as an intermediate) and (b) those hinting at associative mechanisms (involving formate species). Typically, it is suggested that the reaction occurs by only one of those two general mechanisms, and the idea that both could coexist tends to be neglected. We used Ru3(CO)12 as a precursor to synthesize a family of γ-Al2O3-supported Ru carbonyls, which were active for CO2 methanation catalysis in the temperature range between 140 and 250 oC and atmospheric pressure. At low temperatures, IR spectra of the functioning catalysts indicated that CO2 was activated by (a) direct dissociation on surface Ru atoms to give Ru(I) dicarbonyl species and by (b) reacting with surface hydroxyl groups on the support to give bicarbonate species. Both CO2-derived surface species participated in the catalytic formation of methane. During catalysis, the oxidation state of Ru cycled through Ru(I) and Ru(0), as evidenced by Ru K-edge X-ray absorption near edge structure and IR spectra. At high temperatures (&gt; 220 oC), the data indicated that the CO2 activation occurred mainly via the reverse water gas shift (RWGS) reaction to give CO (probably at the metal-support interface), which was then hydrogenated to give methane. IR spectra recorded during catalysis indicate that formate species are invoved in the process, participating as intermediates during the RWGS reaction. Our results indicate that various mechanistic routes coexist during the CO2 methanation catalyzed by supported Ru and show that the relative importance of each route depends on the temperature and composition of the reactor feed.

Coexistence of multiple mechanistic routes for the CO2 methanation catalyzed by γ-Al2O3-supported Ru carbonyls

ACS Spring 2022

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