Electrochemically Driven Gas Phase Ethylene and CO Oxidation on Pt/Yttria-Stabilized Zirconia Catalysts
Authors: Baranova Elena A. , Isaifan Rima , Ntais Spyridon , Couillard Martin 
Published: February 2020

Solid electrolytes such as yttria stabilized zirconia (YSZ) and doped-ceria (Sm-CeO2 or Gd-CeO2) are widely used in the fuel cells, sensors and in heterogeneous catalysis, particularly in electrochemical promotion of catalysis (EPOC) [1, 2]. The bulk ionic O2- conductivity of YSZ is caused by the presence of oxygen vacancies and becomes significant above 600 °C. Several recent studies showed that YSZ can finely disperse and stabilize nanoparticles and strongly enhance the catalytic activity of nanostructured catalysts through the action of the thermally induced O2-promoters [2], which makes it very promising support material in heterogeneous catalysis [1-4].
Recently [3,4], we reported the oxidation of CO and ethylene over Pt nanoparticles (NPs) with the average size of 2.5 ± 0.5 nm deposited on ionic and mixed ionic-electronic conductors (MIEC) supports: YSZ, ceria (CeO2) and samarium doped-ceria (SDC) in the absence of oxygen in the gas feed. The full conversion of 909 ppm of CO and C2H4 by reaction with lattice O2- from the conductive ceramic supports was achieved in the temperature range of 120–240 °C depending on the support. The conversion was observed already at temperature as low as 70 °C indicating that surface O2- is an active reactant, because bulk ionic conductivity of YSZ, ceria and doped-ceria is insignificant below 350 °C. The proposed redox mechanism of CO and C2H4 oxidation by O2- involves formation of local nano-galvanic cells at the three-phase boundary (tpb), that is, Pt NPs/conducting ceramic support/gas phase, where anodic and cathodic processes occur simultaneously but separated in space. The anodic reaction is CO or C2H4 electro-oxidation by oxygen ions from YSZ and/or carbon oxidation that can cover Pt surface during CO and C2H4 oxidation. Whereas, the cathodic reaction is the partial surface reduction of zirconia or ceria at the three-phase boundary (tpb).
In the present work, we present the Pt particle size effect on the oxidation of CO and ethylene in the absence of O2 in the gas feed [4]. NPs of Pt with four average particle sizes, 1.9, 3.0, 4.4 and 6.7nm, were synthesized using the polyol method and ethylene glycol as a reducing and stabilizing agent. Scanning transmission electron microscopy (STEM) coupled with energy-dispersed X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used to evaluate Pt/YSZ properties. Furthermore, CO and ethylene oxidation in the presence of 3.5 kPa oxygen is studied and compared with the catalytic activity without O2 in the gas phase.
The catalytic measurements in the absence of gaseous O2 revealed a strong dependence of CO and C2H4 conversion on the mean Pt particle size. The highest intrinsic rates and turnover frequency were found for Pt NPs having the smallest size at the lowest temperature, which are also characterized by the lowest activation energy. The largest Pt NPs (6.7 nm) do not show any catalytic activity up to 400 °C for both reactions. Figure 1 shows the effect of Pt particle size on CO electro-oxidation in the absence of O2.
The observed particle size effect was attributed to (i) the larger number of nano-galvanic cells formed by the smaller Pt NPs, as was confirmed by the length of tpb estimated for each catalyst; (ii) the higher electronic interaction between the smallest Pt and YSZ (i.e., shorter charge transfer distances). XPS measurements revealed that the deposition of platinum NPs causes a change in the chemical environment of Y and Zr atoms with a change in the relative intensities of the O1s components attributed to the ‘active’ oxygen species (chemisorbed oxygen, OH- groups). In addition, the Pt4f peak shows a BEs downshift when compared to the platinum foil indicating a charge transfer from YSZ to Pt.
In conclusion, in the absence of oxygen in the gas feed, the oxidation reactions occur via nano-galvanic cell mechanism, where CO and ethylene are electro-oxidized at tpb by O2- from YSZ and zirconia undergoes partial surface reduction. In the presence of oxygen, the mechanism is different and in addition to electrochemical reactions at tpb, the backspillover of promoting O2- species from YSZ to Pt surface takes place simulatneously with the catalytic oxidation of CO and C2H4 by molecular O2.
[1] 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, Chem. Rev. 113 (2013) 8192.
[2] P. Vernoux, M. Guth, X. Li, Electrochem. Solid-State Lett. 12 (2009) E9.
[3] R.J. Isaifan, E.A. Baranova, Electrochem. Commun. 27 (2013) 164.
[4] R. J. Isaifan, S. Ntais, M. Couillard, E.A. Baranova, J. Catal. 324 (2015) 32.

Figure 1