CO2 Hydrogenation: Na Doping Promotes CO and Hydrocarbon Formation over Ru/m-ZrO2 at Elevated Pressures in Gas Phase Media

Abstract

Sodium-promoted monoclinic zirconia supported ruthenium catalysts were tested for CO2 hydrogenation at 20 bar and a H2:CO2 ratio of 3:1. Although increasing sodium promotion, from 2.5% to 5% by weight, slightly decreased CO2 conversion (14% to 10%), it doubled the selectivity to both CO (~36% to ~71%) and chain growth products (~4% to ~8%) remarkably and reduced the methane selectivity by two-thirds (~60% to ~21%). For CO2 hydrogenation during in situ DRIFTS under atmospheric pressure, it was revealed that Na increases the catalyst basicity and suppresses the reactivity of Ru sites. Higher basicity facilitates CO2 adsorption, weakens the C–H bond of the formate intermediate promoting CO formation, and inhibits methanation occurring on ruthenium nanoparticle surfaces. The suppression of excessive hydrogenation increases the chain growth probability. Decelerated reduction during H2-TPR/TPR-MS and H2-TPR-EXAFS/XANES at the K-edge of ruthenium indicates that sodium is in contact with ruthenium. A comparison of the XANES spectra of unpromoted and Na-promoted catalysts after H2 reduction showed no evidence of a promoting effect involving electron charge transfer.

Low-Temperature CO2 Methanation over Ru Nanoparticles Supported on Monoclinic Zirconia

“…The resulting CO species, particularly Ru-CO bridged , are promptly consumed by H 2 , indicating hydrogenation. This mechanism is in line with that of Ru/PCS90­(IWI) (Figure S4) and reported Ru/ZrO 2 catalysts. , …”

“…This mechanism is in line with that of Ru/PCS90(IWI) (Figure S4) and reported Ru/ZrO 2 catalysts. 58,59 3.5. Catalytic Performance.…”

“…When the introduced gas is switched from 25% CO 2 /Ar to 25% H 2 /Ar at 200 °C (Figure b), various peaks emerge within 1 min, indicating the adsorption of CO species on Ru (2042, 1992, 1972, and 1910 cm –1 ) and formate species on ZrO 2 (2965, 2870, 1576, 1383, 1357 cm –1 ). − The peaks of the formate species (b-HCOO) appear ∼1 min after introducing the 25% H 2 /Ar mixture, and simultaneously, the peaks associated with bicarbonate/carbonate species (b-CO 3 H, i-CO 3 H, and b-CO 3 ) on ZrO 2 disappear. Hence, the bicarbonate and carbonate species on ZrO 2 are transformed to formate species.…”

Exploiting the potential of calcined sodium citrate as a novel and efficient heterogeneous catalyst for biodiesel synthesis

“…The increase in sodium concentration contributes to the enhancement of the basicity as it increases the negative charge of the oxygen atoms within the material structure. 59–62 This relation is supported by the TGA results (Fig. 2a), as the residue, related to sodium oxide, increases progressively.…”

An Outlook on the Applications of Ru-Based Nanocatalysts for CO2 Hydrogenation to CO and CH4

“…While Ni has been widely studied as the active metal for this reaction owing to its abundant number of active sites and strong capability toward dihydrogen (H 2 ) dissociation, Ru, a precious material, is another highly active candidate for the rWGS reaction, as it is more effective in H 2 dissociation than Ni and therefore is more potent in advancing rWGS closer toward equilibrium. , However, given the easier hydrogen dissociation ability of Ru, it promotes CO 2 methanation more easily than rWGS. , This problem can be addressed by using fewer active sites (e.g., 1 wt % Ru, or 1% Ru added by weight) to reduce the H 2 dissociation extent and therefore promote CO production . Other examples of such products generated via CO 2 hydrogenation that have been researched with respect to Ru catalysts, which can also include a CO side-product or intermediate to be further hydrogenated, are methanol, dimethoxymethane, or longer hydrocarbons. − Furthermore, Ru has high durability and resistance to deactivation, though it is more specifically a platinum group metal that is largely mined in South Africa and Russia and its limited abundance makes it more expensive. , However, since using less Ru promotes the rWGS reaction more, this can also reduce material costs, thereby making the process more industrially and economically viable . To stabilize the surface charge and disperse the active sites more to promote the CO 2 hydrogenation reaction more easily, Ru can be loaded onto support materials (e.g., SiO 2 , Al 2 O 3 , C).…”