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

Abstract

Can a single oxygen spillover species on Pt explain the effect of electrochemical promotion of catalysis (EPOC or NEMCA, non-Faradaic electrochemical modification of catalytic activity, effect) with O 2À conductors such as YSZ (yttria-stabilized ZrO 2 ), as suggested in a recent paper published in this journal by Toghan, Rçsken and Imbihl, [1] or is it necessary to use the longtested sacrificial promoter mechanism, [2][3][4][5] which involves two oxygen species or-equivalently-two different oxygen adsorption states, a regular chemisorbed oxygen and a more ionic, more strongly bonded and less reactive spillover oxygen species (or state), commonly denoted aspoles, in the EPOC literature? [2][3][4][5] This question has been addressed in great detail by numerous older [6][7][8] and more recent [9][10][11][12] papers, which were unfortunately not cited by the authors of ref. [1]. For example, Figure 1 from ref. [9] shows oxygen temperature-programmed desorption (TPD) spectra obtained from a Pt-catalyst electrode deposited on YSZ and using 18 O 2 for gaseous O 2 adsorption. One observes that the more weakly bonded state b 2 is populated primarily by 18 O oxygen atoms resulting from the adsorption of 18 O 2 from the gas phase while the more strongly bonded state b 3 , which desorbs at 100 K higher temperature, is occupied almost exclusively by 16 Oatoms resulting from the spillover of lattice 16 O from the YSZ support. The existence of the two different states is also evident from the 16 O 18 O desorption peak, resulting from some isotopic scrambling in the catalyst surface, which takes place during the long timescale (~1400 s) of the experiment.[9] This time is much longer than the time (inverse of turnover frequency, TOF) associated with the catalytic process, which is electropromoted and is typically in the 0.1-10 s range. [3,10] Thus, this figure and many others [9,10] leave little room for any reasonable doubts about the validity of the sacrificial promoter, that is, a two-oxygen-species (or states) EPOC mechanism. [3,9] However, it is always useful to examine alternate mechanisms if data contradicting the sacrificial promoter mechanism are obtained. Þvalues up to 2.4] only under fuel-rich conditions where in situ X-ray photoelectron spectroscopy (XPS) showed that the surface is predominantly covered by a carbonaceous CH x layer [1] resulting from the adsorption of C 2 H 4 . They proposed that their data can be explained by an "ignition" mechanism, where "electrochemical O 2À pumping causes an ignition from an unreactive carbon-poisoned state of the surface to an active state with less carbon and clarified that the term "ignition" just refers to "a transition initiated by perturbation of a metastable state, but not to a temperature increase accompanying this transition". The authors criticized the currently accepted sacrificial promoter two-oxygen-species mechanism of electrochemical promotion [3][4][5] not for failing to explain their data, which is not at all the case as discussed Figure 1. Temperature-programm...

Ambient pressure X-ray photoelectron spectroscopy during electrochemical promotion of ethylene oxidation over a bimetallic Pt–Ag/YSZ catalyst

“…One main motivation for this study has been the possibility that at high pressure an oxygen spillover species might develop which is different from chemisorbed oxygen. The existence of such a special spillover species has been postulated by Vayenas and co-workers in order to explain the non-Faradaic nature of the EPOC effect but so far no valid experimental proof exists [4,5,9]. Also here no indication for the existence of O species with  close to 2 was found.…”

“…A second issue discussed controversely in the literature is the origin of the non-Faradaic behavior in EPOC experiments. A "sacrificial promoter mechanism" based on the existence of a special spillover oxygen species was postulated by Vayenas et al [3,4] Alternatively, an ignition mechanism has been proposed by Toghan et al [9,11,12]. The ignition mechanism does not require a special oxygen spillover species and instead assigns the spillover species the role of triggering the reactive removal of an inhibitory adlayer.…”

Ionically Conducting Ceramics as Active Catalyst Supports

“…This is particularly important when carrying out NEMCA experiments under high vacuum. 234,359,360 4.9.2. Scale-Up: Multipellet, Multiplate, and Bipolar EPOC Reactors.…”

Electrochemical promotion of catalysis: the case of ethylene oxidation

“…One should therefore leave the possibility open that in the future some further experiments will be able to demonstrate that at high coverage/high pressure such a special spillover species exists or not. So far there is a quite agreement as the identity of chemisorbed oxygen and the spillover species at low coverage/low pressure is accepted by Vayenas and Imbihl, i. e. under these conditions only one oxygen species exists [81,92].…”

“…responsible for the non-Faradaic rate increase [89][90][91]. A third possibility has been introduced recently by Toghan et al who suggested that ignition mechanism [48,49,[57][58][59]92]. The ignition mechanism does not require a special oxygen spillover species and instead assigns the spillover species the role of triggering the reactive removal of an inhibitory adlayer.…”