The Effect of Polarization and Reaction Mixture on the Rh/YSZ Oxidation State During Ethylene Oxidation Studied by Near Ambient Pressure XPS
- 98 Downloads
In this study, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is applied to investigate an electrochemical cell consisting of a rhodium thin film catalyst supported on an yttria-stabilized zirconia (YSZ) solid electrolyte under various ethylene-oxygen reaction mixtures. The aim of the study is twofold: first to show how the surface oxidation state of the Rh catalyst is correlated with the reactants feed composition and the temperature, and second, to reveal the effect of the anodic polarization on the stability of Rh oxides and the implications on the electrochemical promotion of catalysis. It is clearly shown that even under reducing conditions part of the Rh electrode remains oxidized at temperatures up to 250 °C. Reduction of the oxide can take place by increasing the temperature under C2H4 excess, something which is not happening under oxidizing reaction mixtures. Moreover, anodic polarization, i.e. oxygen ion supply to the surface, facilitates reduction of oxidized Rh electrodes over a broad range of ethylene–oxygen reaction mixtures. Remarkably, under mildly reducing conditions a stable ultrathin Rh surface oxide film forms over metallic Rh. This surface Rh oxide film (RhOx) is associated to higher cell currents, counterintuitive to the case of bulk Rh oxides (Rh2O3).
KeywordsElectrochemical promotion of catalysis (EPOC) Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) Rh oxide Ethylene oxidation
We thank AC department of FHI and in particular M. Hävecker and A. Knop-Gericke for the opportunity to use the ISISS beamline and HZB for the allocation of synchrotron radiation beamtime. We acknowledge the support of Dr. T. Dintzer for the SEM images and Dr. Y.T. Law during the beamtime measurements.
- 3.Vayenas CG, Bebelis S, Pliangos C et al (2001) Electrochemical activation of catalysis: promotion, electrochemical promotion, and metal-support interactions. https://doi.org/10.1007/b115566
- 25.Carberry JJ (1976) Chemical and catalytic reaction engineering. McGraw-Hill, New YorkGoogle Scholar
- 35.Grass ME, Zhang Y, Butcher DR et al (2008) A reactive oxide overlayer on rhodium nanoparticles during CO oxidation and its size dependence studied by in situ ambient-pressure X-ray photoelectron spectroscopy. Angew Chem Int Ed 47:8893–8896. https://doi.org/10.1002/anie.200803574 CrossRefGoogle Scholar
- 43.Katsaounis A, Nikopoulou Z, Verykios XE, Vayenas CG (2004) Comparative isotope-aided investigation of electrochemical promotion and metal-support interactions: 2. CO oxidation by 18O2 on electropromoted Pt films deposited on YSZ and on nanodispersed Pt/YSZ catalysts. J Catal 226:197–209. https://doi.org/10.1016/j.jcat.2004.05.009 CrossRefGoogle Scholar