The Electrochemical Promotion of Ethylene Oxidation at a Pt/YSZ Catalyst

Abstract

The electrochemical promotion of the C(2)H(4)+O(2) reaction to form CO(2) and H(2)O is studied in the 10(-5) and 10(-4) mbar range with a Pt catalyst interfaced as working electrode to yttrium-stabilized zirconia (YSZ). Photoemission electron microscopy (PEEM) was used as spatially resolving method. Under open-circuit conditions, that is, without an external voltage applied, the CO(2) production exhibits a pronounced hysteresis upon cyclic variation of p(C(2)H(4)). The hysteresis is attributed to the build-up of a carbonaceous CH(x) layer inhibiting O(2) adsorption and hence poisoning the reaction. It is shown that the application of a positive potential of 1 V triggers a transition from the unreactive branch of the reaction to an active branch. The large non-Faradayicity reported in the literature for this reaction system is explained as an ignition effect caused by the partial removal of the inhibiting carbonaceaous layer.

Note on “The Electrochemical Promotion of Ethylene Oxidation at a Pt/YSZ Catalyst”

“…[1], where the rate increases by a factor of 2.7 upon increasing the temperature from 546 to 763 K, one extracts an activation energy of roughly 3.8 kcal mol À1 which again confirms that the rate is mass-transfer-controlled. When a catalytic process is mass-transfer-controlled, its rate is not dictated by the kinetics on the catalyst surface (which is what one is trying to study) but rather by the rate of mass transport from the gas phase (or in the present case, also from the YSZ solid electrolyte) to the catalyst surface.…”

“…[9,10] This is one main reason for the low values of L observed in this [1] and previous high-vacuum EPOC studies. [10,13] This is because the rate of mass transfer of a reactant, for example, O 2 , from the bulk of the gas phase to a catalyst surface, at fixed mol fraction y O 2 , is roughly proportional to the total pressure both in the Knudsen (free molecular flow) and in the bulk diffusion regime.…”

“…[3] But at high oxygen coverages with a concomitant lack of normal oxygen chemisorption sites, the spillover oxygen O dÀ does not lose its memory, as shown very clearly in Figure 1. Unfortunately ,simple inspection of the kinetic data presented by Toghan et al [1] shows that both the open-circuit rate and the electropromoted rate are mass-transfer-controlled and thus cannot be used for any mechanistic considerations. Thus, the open-circuit activation energy extracted from Figure 6 in ref.…”

“…[3] Finally, it is worth noting that the formation of the carbonaceous CH x layer itself under fuel-rich conditions [1] is due to the oxygen mass-transfer limitation, that is, due to the oxygen starvation at the catalyst surface.…”

Electrochemically induced oxygen spillover and diffusion on Pt(111): PEEM imaging and kinetic modelling

“…This might be in agreement with the observation that on some polycrystalline Pt films with a rougher topography and a few pores only parts of the surface darkened upon anodic polarization. 16 A sample having some porosity close to the TPB due to dewetting, for example, showed no clear spillover fronts.…”