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Recent studies have shown that the catalytic performance (activity and/or selectivity) of Pt-group metal (PGM) catalysts for the CO and hydrocarbons oxidation as well as for the (CO, HCs or H2)-SCR of NOx or N2O can be remarkably affected through surface-induced promotion by successful application of electropositive promoters, such as alkalis or alkaline earths. Two promotion methodologies were implemented for these studies: the Electrochemical Promotion of Catalysis (EPOC) and the Conventional Catalysts Promotion (CCP). Both methodologies were in general found to achieve similar results. Turnover rate enhancements by up to two orders of magnitude were typically achievable for the reduction of NOx by hydrocarbons or CO, in the presence or absence of oxygen. Subsequent improvements (ca. 30⁻60 additional percentage units) in selectivity towards N2 were also observed. Electropositively promoted PGMs were also found to be significantly more active for CO and hydrocarbons oxidations, either when these reactions occur simultaneously with deNOx reactions or not. The aforementioned direct (via surface) promotion was also found to act synergistically with support-mediated promotion (structural promotion); the latter is typically implemented in TWCs through the complex (Ce⁻La⁻Zr)-modified γ-Al2O3 washcoats used. These attractive findings prompt to the development of novel catalyst formulations for a more efficient and cost-effective control of the emissions of automotives and stationary combustion processes. In this report the literature findings in the relevant area are summarized, classified and discussed. The mechanism and the mode of action of the electropositive promoters are consistently interpreted with all the observed promoting phenomena, by means of indirect (kinetics) and direct (spectroscopic) evidences.

Abstract: This study has shown that Ir/YSZ electrochemical catalysts can reach high catalytic activity for the SCR of NO by propene in the presence of oxygen. Under stoichiometric conditions, porous and thin (about 40nm) Ir film achieves N2 yield higher than 80% between 350 and 500°C. However, iridium films require on-stream treatment at 500°C in order to develop their catalytic efficiency by increasing Ir particles size and establishing an ideal Ir0/IrO2 ratio. Under lean-burn conditions, the same sample presents high and durable NO conversion at 500°C. These results confirm that magnetron sputtering is a suitable technique for synthesis of effective thin catalytic films. Furthermore, it was found that NO reduction by propene can be electrochemically promoted on Ir/YSZ electrochemical catalysts. This promotion exhibits an electrophobic NEMCA effect, the reaction rate increasing when a positive polarisation is applied. The electropromotion efficiency was found to be structure sensitive. [Copyright &y& Elsevier]

Abstract: A honeycomb monolith of YSZ (yttria stabilized zirconia) was used, for the first time, as a solid electrolyte to perform electrochemical promotion of the deep oxidation of methane. The goal of this study was to validate the concept of Electrochemical Promotion of Catalysis (EPOC) by using Pd particles dispersed on the channels surface of a YSZ dense honeycomb monolith. The Pd catalyst was an effective material as methane conversion reached 20% at about 320°C. The catalytic properties of the monolithic electrochemical catalyst were investigated upon electrical polarization at 400°C. Non faradaic effects were observed both under positive and negative polarizations with a maximum of the faradaic efficiency of 47. [Copyright &y& Elsevier]

Abstract: Due to the great importance of automotive exhaust gas treatment, the catalytic activity was investigated in selective catalytic reduction of NO by propene on Pt films – with controlled microstructure – deposited on YSZ (8 mol% Y2O3-doped ZrO2) by electrostatic spray deposition. This technique requires low Pt loadings in order to reduce costs and also to achieve high Pt particles dispersion with good reproducibility. This kind of electrochemical catalysts was found to be effective for NO reduction by propene in the presence of oxygen. A dense Pt film was found to be the most suitable. Furthermore, we have demonstrated that these electrochemical catalysts can implement the concept of electrochemical promotion of catalysis (EPOC). [Copyright &y& Elsevier]

Abstract: The EPOC concept upon positive polarization of Pt/YSZ has been investigated with plethora of techniques. It is attributed to the presence of Oδ− species with the compensating charge in the metal catalyst. However, the origin of EPOC in the case of negative polarization is not yet well established. This study gives, for the first time, an insight of this mechanism. Isotopical labelling studies were conducted on a Pt/YSZ(O2−) electrochemical catalyst for investigating the origins of Electrochemical Promotion of Catalysis (EPOC) using propane combustion as a model reaction. [Copyright &y& Elsevier]

Abstract: Isotopical labeling studies were performed on a Pt/YSZ electrochemical catalyst for operando investigating mechanisms involved in the electrochemical activation of propane combustion. This study clearly confirms the sacrificial promoter model and highlights the significant contribution of a surface oxygen exchange mechanism in the catalytic rate both under open circuit and anodic polarization conditions. [Copyright &y& Elsevier]

Abstract: The effect of electrochemical promotion of catalysis was investigated for the water–gas shift reaction over porous Pt catalyst electrodes interfaced with 8%mol Yttria-stabilized Zirconia. A fuel cell type electrochemical reactor was used at temperatures from 300°C to 400°C, under ratio values from 2.85 to 31. A negative order dependence of the catalytic reaction rate on P CO and a positive one on was found under open-circuit and polarization conditions. Positive potential application (+2.5V), i.e., O2− supply to the catalyst surface, causes a small decrease in the catalytic reaction rate, while negative potential application (−1.5V) results in a pronounced rate increase, up to 200%, with apparent faradaic efficiency values up to 110. The rate increase obtained with negative polarization can be attributed to the weakening of the Pt–CO bond strength but also, to the increase in surface concentration of oxygen ion vacancies near the Pt-gas-support three-phase boundaries necessary for water dissociation. [Copyright &y& Elsevier]

Abstract: The oscillatory behaviour of CO oxidation was studied at 250°C and atmospheric pressure using an electrochemical catalyst composed of a thin (60nm) sputtered-Pt film interfaced with an yttria-stabilized zirconia membrane. Oscillations of CO oxidation rate showed a perfect correlation with those of the electrochemical potential values. Electrochemical promotion of catalysis was used to initiate and stop the oscillatory behaviour. Small current application induced a permanent effect on the oscillatory behaviours. An extremely small negative current (−17μA) led to a 4-fold increase of the catalytic activity and created oscillations that were stable even after current interruption. This permanent effect in the oscillatory behaviour of CO oxidation rate is observed for the first time using EPOC. This has been interpreted by the higher tendency of the nanometric-Pt particles to form PtOx in thin sputtered films. [ABSTRACT FROM AUTHOR]

Abstract: A complex route provided powders of La1−x Sr x MnO3 (x =0, 0.15, 0.3 and 0.4) from the thermal decomposition of the chelated nitrate precursors. The catalysts were characterized by XRD, BET, SEM, and temperature-programmed techniques. Electrochemical catalysts based on La0.7Sr0.3MnO3 (LSM) were deposited on yttria-stabilized zirconia (YSZ) pellets. Electrochemical characterizations of these samples evidenced two redox processes assigned to the existence of different environments around Mn4+ in LSM in good agreement with the peaks observed by temperature programmed reduction (TPR). In situ O2 temperature programmed desorption (O2-TPD) and temperature programmed oxidation (TPO) of propane were performed on LSM/YSZ to investigate the influence of the polarization on oxygen and propane chemisorption on LSM/YSZ. O2-TPD experiments have shown that the electrical polarization during adsorption process has no significant impact on gaseous oxygen adsorption on LSM thin film. TPO experiments suggest an increase in the adsorption strength of propane on LSM that could be at the origin of the electrochemical promotion effects observed upon polarization conditions. [Copyright &y& Elsevier]

Abstract: For the first time, electrochemical promotion (EP) was evidenced on a multi-layer electrochemical catalyst with a BIMEVOX ceramic used as an electrolyte. The electrochemical catalysts were tested for two environmental applications: selective catalytic reduction (SCR) of NOx by propene under lean-burn conditions and propene combustion. Thin Pt films were sputtered on BITAVOX.20 (Bi2V0.8Ta0.2O5.5−δ ) dense membranes. The thickness of the Pt films was less than 100nm which led to a very small amount of Pt. First experiments revealed the formation of a Bi x Pt1−x alloy, likely due to a slight reduction of the electrolyte. A new geometry was then proposed with the insertion of a YSZ barrier layer of a few nanometers between the catalyst and the electrolyte. Electrochemical promotion (EP) was evidenced for this system at low temperature (200–300°C) under lean-burn conditions for both reactions. In the case of propene combustion, EP was maintained even in the presence of steam. [Copyright &y& Elsevier]