Electrochemical Promotion of Catalysis for CO2 Hydrogenation on Ru-Based Catalyst Using Ionically Conducting Ceramics

Abstract

The release of carbon dioxide (CO2) into the atmosphere has led to effects of climate change resulting in an increase in global temperature, ocean acidification and many other environmental issues. Hydrogenation of CO2 into synthetic hydrocarbons is a promising solution in decreasing anthropogenic dependence on fossil fuels and providing an energy source that is carbon-neutral. The reverse water gas shift (RWGS) reaction is a feasible hydrogenation reaction that requires a 2-electron transfer to yield syngas (CO + H2) to be used in the Fischer-Tropsch reaction to synthesize synthetic hydrocarbons. In previous work (submitted to the Journal of CO2 Utilization), the conversion of CO2 into CO using Ru-nanostructured metal nanoparticles dispersed on ionically conducting ceramic supports like ceria (CeO2), doped-ceria (x-CeO2) and yttria-stabilized zirconia (YSZ) was studied with promising results. The activity of the Ru-based nanoparticles was improved due to the ionically conductive properties of the support, which contain oxygen (Oδ-) ionic species that promote the reaction. This promotional effect is known as the metal-support interaction (MSI) where nanoparticles are dispersed on a powder support, allowing Oδ- species to migrate from support to nanoparticle by an increase in temperature [1,2]. The MSI effect has been observed using the best Ru-based powder catalyst supported on samarium-doped ceria (SDC) - Ru45Fe55/SDC (2 wt.%), which yielded high CO amounts between 300-750°C. Current research aims at improving the overall RWGS reaction at lower temperatures through the utilization of the electrochemical promotion of catalysis (EPOC) or non-faradaic electrochemical modification of catalytic activity (NEMCA) effect [3,4]. EPOC allows to control in-situ the migration of ionic species to and from the metal surface through the application of a potential difference or current between the catalyst-working electrode and an inert counter electrode. This migration of species leads to the formation of a neutral double layer encapsulating Ru nanoparticles, promoting the reaction. The catalyst setup resembles an electrocatalytic cell where metal nanoparticles act as the working electrode deposited on a solid support in the form of a disc. The support (YSZ in this case) represents a fixed layer of electrolytes that conducts Oδ- ions to migrate to and from the active catalyst. As shown in Fig. 1, a promotional effect is observed for Ru on YSZ at 350°C under constant potential of 0.25 V, favoring the formation of CO through an enhancement ratio of ~2 and Faradaic efficiency of ~19, which is attributed to the synergistic effect between the metal and promoted ionic species Oδ­-. Additionally, density functional theory (DFT) calculations are being conducted for the hydrogenation of CO2 on Ru nanoparticles and will be discussed in correlation with the experimental findings to confirm the mechanisms occurring during the reaction. [1] P. Vernoux, M. Guth, X. Li, Ionically Conducting Ceramics as Alternative Catalyst Supports, Electrochem. Solid-State Lett. 12 (2009) E9–E11.[2] S. Ntais, R.J. Isaifan, E.A. Baranova, An X-ray photoelectron spectroscopy study of platinum nanoparticles on yttria-stabilized zirconia ionic support: Insight into metal support interaction, Mater. Chem. Phys. 148 (2014) 673–679.[3] D. Vayenas, C.G., Bebelis, S., Pliangos, C., Brosda, S., Tsiplakides, Electrochemical Activation of Catalysis, Springer US, (2001).[4] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou, E.A. Baranova, Ionically Conducting Ceramics as Active Catalyst Supports, Chem. Rev. 113 (2013) 8192–8260. Figure 1