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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.

Nanoparticles of platinum were dispersed on an oxygen ionically conducting support, Yttria-Stabilized Zirconia (YSZ), on non-doped zirconia and on silica. The role of the support oxygen lattice in the mechanism of propane deep oxidation was investigated by using O Temperature-Programmed Desorption, catalytic activity measurements using isotopic oxygen, and isotopic exchange experiments. The results emphasize that the propane combustion mechanism on Pt/YSZ presents a different pathway compared with that involved on Pt/SiO and Pt/ZrO because, propane is preferentially oxidized by lattice YSZ oxygen species on Pt/YSZ. Strong Pt nanoparticles/YSZ interactions generate a promoted state similar to that observed on polarizations during electrochemical promotion of catalysis. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

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The origin of the electrochemical promotion of catalysis (EPOC) was investigated via oxygen temperature-programmed desorption (O2-TPD) from a polycrystalline Pt film interfaced with YSZ. TPD experiments were carried out under operating conditions similar to those used for catalytic activity measurements. This study has clearly shown that an anodic current generates the migration of “backspillover” ionic oxygen species from YSZ toward the Pt surface. These ionic species act as promoters and enable the formation of weakly adsorbed oxygen species coming from the gas phase which are more reactive and thus responsible for the activity enhancement. The effect of polarization is to carry or to remove the promoting ionic species on the Pt surface. Therefore, electrochemical promotion of catalysis can be considered as an electrically controlled metal support interaction, where the support is an O2− conducting solid electrolyte. [ABSTRACT FROM AUTHOR]

Abstract: A new NO x storage-reduction electrochemical catalyst has been prepared from a polycrystalline Pt film deposited on 8mol% Y2O3-stabilized ZrO2 (YSZ) solid electrolyte. BaO has been added onto the Pt film by impregnation method. The NO x storage capacity of Pt-BaO/YSZ system was investigated at 350°C and 400°C under lean conditions. Results have shown that the electrochemical catalyst was effective for NO x storage. When nitric oxides are fully stored, the catalyst potential is high and reaches its maximum. On the other hand, when a part of NO and also NO2 desorb to the gas phase, the catalyst potential remarkably drops and finally stabilizes when no more NO x storage occurs but only the reaction of NO oxidation into NO2. Furthermore, the investigation has clearly demonstrated that the catalyst potential variation versus temperature or chemical composition is an effective indicator for in situ following the NO x storage-reduction process, i.e. the storage as well as the regeneration phase. The catalyst potential variations during NO x storage process was explained in terms of oxygen coverage modifications on the Pt. [Copyright &y& Elsevier]

Abstract: Porous Pt–YSZ composites were prepared by impregnation of a commercial YSZ powder by a platinic precursor (H2PtCl6) followed by isostatic pressing and sintering at 1000 °C. Two different contents of platinum were introduced, i.e., 1 and 5 wt.%. Electrical and electrochemical properties of these samples were investigated between 350 and 600 °C by using impedance spectroscopy under various atmospheres including He, O2/He, and C3H8/O2/He. The catalytic activity of these samples for the propane combustion has also been investigated in the same temperature range. Dispersed platinum particles in YSZ at the surface of YSZ grains acted as an additional blocker of the ionic conductivity. This additional grain boundary was ascribed to the formation of platinum oxides or of a layer of chemisorbed oxygen on dispersed Pt particles. The interfacial process seemed to be characterized by a predominant low-frequency contribution, whatever the temperature, the Pt loading, and the atmosphere. This contribution was interpreted in terms of an oxygen concentration polarization. It was found to vary in the same way as the grain boundary process. The catalytic performances of porous Pt–YSZ composites for propane combustion were higher than those of Pt films deposited on a dense YSZ pellet. Unfortunately, we did not evidence any electrochemical promotion of the catalytic activity in our operating conditions. [Copyright &y& Elsevier]

Abstract: The development of electrochemical promotion necessitates new solid electrolytes with improved ionic conduction at lower temperatures. Among them, Na Super Ionic Conductor (NaSICon), i.e. Na3Zr2Si2PO12, is a promising Na+ conducting electrolyte. The Pt/NaSICon system was evaluated for the electrochemical promotion of propene combustion both with large excess of oxygen (for volatile organic contaminants, VOCs, abatement) and near the stoichiometry (for energy production). [Copyright &y& Elsevier]

A series of Pt/γ-Al2O3 catalysts promoted by various concentrations of Na were prepared and tested for the SCR of NO by propene and propane under lean-burn conditions. The addition of Na markedly modifies the Pt catalytic behaviors. Under a C3H8/NO/O2 atmosphere, the presence of Na leads to a drastic decrease of the C3H8 and NO conversions. In contrast, under C3H6/NO/O2, Na promotion improves the Pt catalytic performance. For high Na loadings (5 wt%), we observed an enhancement of the selectivity to N2 and a widening of the operating temperature window. The promotional phenomena observed have been explained in terms of modification of the adsorption strengths of the various reactants on the metal surface. The Na effects should be to strengthen the Pt–electron-acceptor (O and NO) bonds and to weaken the Pt–electron-donor (C3H8 and C3H6) bonds. The opposite effects observed by using propane or propene have been attributed to their different mechanisms of adsorption on Pt. [Copyright &y& Elsevier]

The non-faradaic electrochemical modification of catalytic activity (NEMCA) was investigated in selective catalytic reduction (SCR) of NO by propene on Pt films deposited on NASICON, a Na+ conducting solid electrolyte. This study was carried out under lean-burn conditions. In such a system, electrochemical promotion is shown to strongly enhance both the catalytic activity and the selectivity to N2. Such an improvement can be obtained using low overpotentials, applying −100 mV to the Pt catalyst-electrode enhances the selectivity to N2 from 41 to 61%. The use of NASICON allows work to be done at quite low temperatures (about 300 °C) compatible with the treatment of automotive exhausts. [Copyright &y& Elsevier]

The nonfaradaic electrochemical promotion modification of catalytic activity (NEMCA) was investigated in the total oxidation of propane and propene on Pt films deposited on 8 mol% Y2O3-stabilized ZrO2 solid electrolyte. It was found that propane and propene oxidation exhibit opposite behaviors with respect to the applied overpotentials. Propane oxidation exhibits an electrophobic enhancement (reaction increases upon positive polarization of the catalyst-electrode) while propene oxidation is electrophilic. Upon applied negative overpotential and under an oxygen-rich atmosphere, the rate of propane oxidation strongly decreases, with high values of faradaic efficiency, up to 10,000. Propane and propene oxidation rates were found to be controlled over a wide range of voltage in a reversible way. Catalytic activity measurements under open-circuit voltage conditions, using solid electrolyte potentiometry and temperature-programmed desorption, have been used in order to understand and investigate the origins of this electrochemical promotion. They seem related to different behaviors of propane and propene with respect to their competitive adsorption with oxygen on platinum. Propene adsorbs strongly in competition with oxygen, whereas propane adsorbs weakly. This indicates that the NEMCA behavior of a catalytic reaction is dependent on the oxygen coverage on the catalyst-electrode under open-circuit conditions. [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 catalytic activity of sputtered-Pt films onto Y2O3-stabilized ZrO2 (YSZ) pellets was investigated on the propane combustion. Catalytic performances were compared with conventional electrochemical catalysts based on Pt-paste films. Sputtered electrochemical catalysts presented higher Pt dispersion and significantly larger intrinsic catalytic activity. Furthermore, the catalytic activity of both systems was improved when electrochemical promotion of catalysis (EPOC) was applied to them. The magnitude of the electrochemical activation was higher for sputtered electrochemical catalysts than for Pt-paste films. However, no permanent Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) effect was observed on sputtered-Pt films. Nevertheless, the kinetic behaviour of the electrocatalytic activity exhibited temperature dependence, which may be related with the formation of PtO x species in the bulk of the Pt film during the anodic polarization. [ABSTRACT FROM AUTHOR]

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: 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]

Abstract: The aim of this study was to select, to test and to optimise supported Pt catalysts for nitrogen oxide sensors application in order to improve their selectivity. A series of catalysts based on platinum dispersed onto different supports: SiO2, γ-Al2O3, Al2O3–SiO2, ZrO2, TiO2, CeO2 and YSZ (formulae: (ZrO2)0.92(Y2O3)0.08) were prepared and tested for NO oxidation. The effect of the nature of the support, and of the Pt dispersion and loading on the catalytic activity has been examined under operating conditions similar to those encountered by NOx sensors, i.e. traces of NO in air. Catalytic activity and TPD measurements have clearly shown the prevailing role of the support. Silica is the most adequate support because it weakly adsorbed NO and did not store NO2 as nitrates, then favouring the reaction between NO molecules coming from the support and oxygen adsorbed on Pt. The effect of the Pt particles sizes was also important and our results have shown that larger particles exhibit the highest activity. Finally, the most effective catalyst was found to be Pt/SiO2 containing 2wt.% of platinum and presenting a low Pt dispersion. This kind of catalyst enables to fix a determined value of the NO/NO2 ratio near the sensing layer of the sensor by reaching the thermodynamic equilibrium over the whole working temperature range. [Copyright &y& Elsevier]