Abstract:
The reaction of CO2 hydrogenation is of high environmental interest since it allows for the transformation of the logistically challenging H2, gained from renewable sources, to the much more manageable hydrocarbons.
CO2 hydrogenation takes place mainly through the following two reactions:
xCO2 + (2x-z+y/2)H2 --> CxHxOz + (2x-z)H2O
and
CO2 + H2 --> CO + H2O
The first reaction directly produces hydrocarbons whereas the second one, also known as RWGS, produces syngas which is useful in the synthesis of several hydrocarbons.
With CO2 being a rather inert molecule, the reaction of CO2 hydrogenation requires high pressures and temperatures, as well as the existence of a good catalyst. The development of an efficient catalyst is a requirement for the extensive application of a strategy where renewable energy is stored as HCs. An important parameter for the development of an efficient catalyst is the metal-support interactions. Those interactions have been closely identified as the underlying reason for Electrochemical Promotion of Catalysis [1-5]. Conversely, EPOC has proven itself as a valuable tool for the study of metal support interactions. Promoters of catalysts alter the catalytic activity and selectivity by modifying the bonds of the reactants on the active sites and the work function of the catalytic surface. Electropositive promoters enhance the chemisorption of electron-acceptors and weaken the bonds of electron donors. Electronegative promoters have the opposite effect [1-5]. Ruthenium is a catalyst widely used to produce methane from CO2. In this study, we present an example of how electrochemical promotion of catalysis (EPOC) can elucidate the role of solid electrolytes (YSZ, BZY), supporting Ru porous films or nanoparticles.
The results of the study have shown that the electrolytic features of the support (anionic or cationic or mixed conductor) can have a very pronounced and dominant effect on the activity and selectivity of the supported metal nanoparticles. The mechanism of the interaction can be studied conveniently via EPOC and then the support can be chosen accordingly. Nucleophilic EPOC behavior suggests that the reaction will be enhanced when using an anionic catalyst support, such as YSZ, and electrophilic EPOC behavior suggests that the reaction will be enhanced using a cationic support, such as BZY. Thus, one may conclude, again, that EPOC (or NEMCA effect) and MSI are functionally identical and only operationally different [1, 2] since they both rely on ion spillover. The use of EPOC can significantly facilitate the choice of catalyst support.
References:[1] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-Support Interactions, Kluwer Academic/Plenum Publishers, New York, 2001.
[2] 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, Chemical Reviews, 113 (2013) 8192-8260.
[3] A. Katsaounis, Recent developments and trends in the electrochemical promotion of catalysis (EPOC), Journal of Applied Electrochemistry, 40 (2010) 885-902.
[4] D. Tsiplakides, S. Balomenou, Milestones and perspectives in electrochemically promoted catalysis, Catalysis Today, 146 (2009) 312-318.
[5] A. De Lucas-Consuegra, J. Gonzalez-Cobos, Y. Garcia-Rodriguez, A. Mosquera, J.L. Endrino, J.L. Valverde, Enhancing the catalytic activity and selectivity of the partial oxidation of methanol by electrochemical promotion, Journal of Catalysis, 293 (2012) 149-157.
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