Non-Faradaic Electrochemical Modification Of Catalytic Activity Research Articles

First-Principles Studies for Optimal Model of the Ni/YSZ Triple Phase Boundary in Solid Oxide Cells

Non-Faradaic electrochemical modification of catalytic activity (NEMCA) with electric field applications in solid oxide cells (SOCs) is thought to be induced by spillover effects of lattice oxygen from the bulk, although the detailed mechanism has not still been clear. In SOCs, important phenomena such as fuel decomposition, charge transfer, etc. occur at the triple phase boundary (TPB) as a highly active site that consists of catalyst, electrolyte, and gas phases. NEMCA is expected to be also induced strongly by the surface mechanism on TPB, and understanding surface reactions on TPB is essential for improving catalyst and cell performances. However, the reliable TPB model has not been still uniquely defined to discuss the property of TPB although various studies have been reported. Therefore, in this study, we have focused on the TPB model comprising Ni catalyst cluster; YSZ electrolyte; and gas phase, and aimed to identify a reliable TPB model for theoretical studies by using first-principles calculations as an initial step. In concrete, we identified firstly the stable structures of YSZ surface models by using DFT calculations taking into account oxygen vacancy positions, yttrium atom arrangements, yttria concentration, and other factors. Thereafter, we discussed a reliable Ni/YSZ interface model based on the most stable YSZ model proposed above results by evaluating the Ni structure, interface stability, and so on.In this study, DFT calculations with a plane-wave basis set were implemented using CASTEP, and GGA-PBE exchange-correlation functional was used. The plane-wave cutoff energy was set as 489.8 eV, the OTFG-ultrasoft was used as the pseudopotentials, and the spin-polarization is considered because YSZ is a ferromagnetic substance. In YSZ surface models, yttria concentrations are set to 4.35 mol% and 9.1 mol% which shows the maximum ion conductivity of ZrO2. The three-layer YSZ (111) slabs with 15 Å vacuum layer with 2×2 and 2×4 unit cells were used for repeated slab models. The DFT+U method is used to obtain the correct electric structure of metal oxides with partially filled d or f-orbital shells, and k-points were set to 4×2×1. In Ni/YSZ interface models, we considered Ni cluster and Ni belt type models based on the most stable 2×4 YSZ surface model (9.1 mol%) proposed in this study. We also considered both cases of (111) and (100) facets for the contact interface. In the case of Ni/YSZ interface models, the DFT+U method was not considered to improve the calculation convergence and k-points were set to 4×2×1. A schematic diagram of the Ni/YSZ interface model is shown in the attached Figure.In the case of the 2×2 YSZ model with the yttria concentration of 9.1 mol%, the YSZ model is stabilized when oxygen vacancy is on the second O atom layer and the second neighbor to Y atoms, indicating that improvement in the geometry instability of ZrO2 for 8-coordination is more important than keeping local electron neutrality. We have also found that oxygen vacancy positions are more sensitive to the YSZ surface stability than Y atom arrangements. In the case of the 2×4 YSZ model with the yttria concentration of 4.35 mol%, the YSZ model where there are Y atoms on the first layer is stabilized. The crystal structure achieves a more stable structure by varying bond lengths when Y3+ with the larger ionic radius is replaced with Zr4+. Therefore, the structure is easier to stabilize when the Y atom exists on the surface than in the bulk due to the higher degree of freedom of the Y atom. The most stable structure of the 2×4 YSZ model with the yttria concentration of 9.1 mol% given based on the above results is 0.18 eV more stable than the previously reported structure [1]. This is because the number of Y atoms on the first layer with the second neighbor from oxygen vacancy is larger than the previously reported structure. We evaluated then the structural stability of the Ni/YSZ interface model based on the above YSZ model. As a result, we have found that adhesion energy between Ni and YSZ is independent of the relative position of Ni atoms. In addition, the larger the number of Ni atoms is, the more stable the structure is. This is because the electronic property of Ni atoms approaches the metal as increasing the number of Ni atoms. Other results and a detailed discussion will be reported in our meeting publications (ECS Transactions) and the presentation.[1] M. Shishkin and T. Ziegler, Phys. Chem. Chem. Phys., 16, 1798-1808 (2014). Figure 1

Non-Faradaic Electrochemical Modification of Catalytic Activity: LSCF As Electrocatalyst for Nitrous Oxide Electrochemical Reduction

Nitrous oxide (N2O) mitigation is of high importance as this gas is responsible for several environmental problems, such as photochemical smog, acid rain, ozone layer depletion and global warming. While N2O is not the principal contributor to global warming (~ 6 %), it has 310 and 21 times the Global Warming Potential (GWP) of the most common anthropogenic greenhouse gases, CO2 and CH4, respectively [1]. European Union directives currently aim at a reduction in the emissions of these gases by 42 % from their 2013 levels by 2030 [2].The current work aims to bridge solid-state chemistry and heterogenous catalysis, by studying the electrochemical promotion of catalysis (EPOC), also known as non-Faradaic electrochemical modification of catalytic activity (NEMCA), of the N2O reduction reaction. Here, the catalytic activity of an electrode surface is radically changed upon polarization, typically applying large electrical potentials (> 2 V) that lead to considerable changes in the conversion and the selectivity of the products [3].For this work, the well-known mixed electronic and ionic conductor La0.6Sr0.4Co0.2Fe0.8O3-δ LSCF) was utilised as the electrocatalyst, due to its high availability and excellent performance as a cathode in solid oxide fuel cells (SOFCs). This composition exhibits a very high oxygen surface exchange coefficient (K* = 1.1 x 10-3cm s-1 at 800 °C) as well as high electronic (230 S cm-1 at 900 °C) and ionic conductivities (0.2 S cm-1 at 900 °C), essential requisites for this application [4].A 3-probe cell-based reactor was developed, composed by a solid oxide-ion-conducting electrolyte substrate made of 8 % mol Y2O3 – stabilized ZrO2, a working electrode made of LSCF and counter and reference electrodes made of inert gold (Au), to avoid any influence of the latter parts in the electrocatalytic activity of the main LSCF electrocatalyst. A buffer layer of Ce0.9Gd0.1O2-δ (CGO) was additionally used between the working electrode and the substrate, to prevent the inter-reaction between the YSZ and LSCF.The key aspect of this work was the use of an integrated zirconia oxygen sensor in the studies, to determine variations in the values of oxygen partial pressure (pO2 ) upon polarisation, which were subsequently correlated with the extent of N2O conversion. This methodology proved to be highly effective and avoids the need for additional techniques, such as gas chromatography.From the experiments, it was found that by applying cathodic polarisation (i.e -0.05 and -0.25 A), an increase of the catalytic rate (r O2 mol s-1) was observed, resulting in a significant increase of N2O conversion. An electrophobic behaviour of the electrocatalyst, i.e., a reaction rate that increases upon cathodic polarization, was observed for all temperatures with a maximum rate enhancement ratio (ρ=r/r0) up to 3.47 was achieved at -0.05 A at 600 ºC. The Faradaic efficiency (Λ) was also calculated for the tested experimental conditions, and showed values that exceed unity, evidencing non-Faradaic electrochemical behaviour. The highest Λ value of 8.8 was obtained at the lower temperature and currents tested at 600 °C. The nature of the mechanism was also assessed, showing a completely reversible effect, where the electrochemically promoted reaction rate returns to its open circuit value upon removal of polarisation. This work, therefore, highlights that non-Faradaic activation of heterogenous catalysis can be an effective tool to promote the reductive treatment of this greenhouse gas, with great environmental importance.

A Discussion on the Unique Features of Electrochemical Promotion of Catalysis (EPOC): Are We in the Right Path Towards Commercial Implementation?

The phenomenon of “Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA)” or “Electrochemical Promotion of Catalysis (EPOC)” has been extensively studied for the last decades. Its main strength, with respect to conventionally promoted catalytic systems, is its capability to modify in-situ the activity and/or selectivity of a catalyst by controlling the supply and removal of promoters upon electrical polarization. Previous reviews have summarized the main achievements in this field from both the scientific and technological points of view. However, to this date no commercial application of the EPOC phenomenon has been developed, although numerous advances have been made on the application of EPOC on catalyst nanostructures (closer to those employed in conventional catalytic systems), and on the development of scaled-up reactors suitable for EPOC application. The main bottleneck for EPOC commercialization is likely the choice of the right chemical process. Therefore, from our point of view, future efforts should focus on coupling the latest EPOC advances with the chemical processes where the EPOC phenomenon offers a competitive advantage, either from an environmental, a practical or an economic point of view. In this article, we discuss some of the most promising cases published to date and suggest future improvement strategies. The considered processes are: (i) ethylene epoxidation with environmentally friendly promoters, (ii) NOx storage and reduction under constant reaction atmosphere, (iii) CH4 steam reforming with in-situ catalyst regeneration, (iv) H2 production, storage and release under fixed temperature and pressure, and (v) EPOC-enhanced electrolysers.

Electrochemical promotion of propane oxidation at Pt/BiCuVOx-YSZ thin-film cells: Economical use of YSZ

Abstract Electrochemical promotion of catalysis (EPOC) or Non-faradaic electrochemical modification of catalytic activity (NEMCA) is a phenomenon that may be exploited in catalytic converters to reduce the usage of precious metal catalysts. Nevertheless, the conventional structure of NEMCA cells employs a thick piece of relatively expensive solid electrolyte such as yttria-stabilised zirconia (YSZ). Minimising the amount of this costly material may bring EPOC closer to commercial application. NEMCA cells were fabricated with thin films of YSZ or BiCuVOx-YSZ as the electrolytes and sputtered Pt as the working catalyst electrodes. The catalytic activity for propane combustion was studied at 200, 300, 400, and 500 °C for cells with YSZ, but only at 400 °C for cells with BiCuVOx-YSZ. The effects of working electrode potentials and BiCuVOx contents were also studied. Most cells made with only YSZ could not function as electrochemical cells. Nevertheless, only one cell survived and exhibited NEMCA. On the other hand, all BiCuVOx-YSZ cells functioned properly. Electrophobic NEMCA behaviour was observed from both cell types. Addition of BiCuVOx improved sintering and resulted in films of higher conductivity. Nonetheless, diminished surface areas and suppressed faradaic efficiencies were observed. Undesirable electronic conductivity was also introduced by BiCuVOx and hence an optimum BiCuVOx content of 3.5 wt % was observed to offer the highest rate enhancement ratio, ρ, and the highest faradaic efficiency, Λ, of 5.3 and 339, respectively, at 400 °C.

First-Principles Investigation of Promoting Effect in CO2 Methanation with an Electric Field in SOEC

Non-faradaic electrochemical modification of catalytic activity (NEMCA) by impression of electric field is one of the methods to improve catalyst performance. In this study, we have theoretically investigated the mechanism of NEMCA in CO2 methanation in solid oxide electrolysis cell using the density functional theory (DFT). DFT calculations have been performed for hydrocarbon species on Ni (111) facet with direct electric field or co-adsorbed oxygen atoms, and their dependence for the rate determining steps (RDSs) in CO2 methanation from our kinetic simulation has discussed. We have found that direct effects of electric field in surfaces adsorption are different in each intermediate in RDSs and all RDSs accelerate with electric field. While, all intermediates bind on the surface less strongly as increasing the number of co-adsorbed oxygen atoms and some reactions decelerate and the other accelerate with co-adsorbed oxygen atoms.

Electrochemical Promotion of Bimetallic Nanoparticles with Low-Noble Metal Content Interfaced with Yttria-Stabilized Zirconia Solid-Electrolyte

Heterogeneous catalysts play a central role in production of a variety of useful chemicals, energy generation and greenhouse gas emissions reduction. A heterogeneous catalyst usually consists of fine metal or metal oxide particles in the nanometer range dispersed on a high surface area supports. The catalytic activity and/or selectivity of the catalyst could be tuned in by using chemical promoters that are added to the catalyst during its fabrication or by choosing a suitable catalyst supports via metal-support interaction (MSI) effect. Another way to efficiently promote catalytic activity and in some instances catalyst selectivity is the use of electrochemistry via Electrochemical Promotion of Catalysis (EPOC), also called Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) phenomenon [1, 2]. EPOC or NEMCA is ascribed to significant changes in the catalytic properties of metal or metal oxide catalysts deposited on solid-electrolytes caused by the application of small current (in the range of a few μA) or potential (up to 2V). The application of electric stimuli results in the supply of ionic species from the solid-electrolyte to the active surface, where the catalytic reaction takes place. These ionic species act as promotes and their addition results in significant catalytic rate increase due to changes in the catalyst work function [2-4]. This phenomenon has been applied to over 100 catalytic systems and reactions and continues to make an impact to surface science, catalysis and solid-state electrochemistry. The application of EPOC to nanostructured, highly dispersed catalysts has attracted significant interest in the last decade and several studies have demonstrated dramatic EPOC effect with noble metal (Pt, RuO2, Pd, etc.) nanoparticles (NPs) as low as 1.5 nm in size and metal loading down to 0.1 mg metal/cm2 [5-9]. Despite this tremendous progress, further improvement in metal utilization for practical advancement of EPOC is essential. In the present work, we investigated EPOC of bi-metallic, highly dispersed Ni90Pd10 and Ru45Fe55 (at. %) nanoparticles for two environmentally important reactions: CH4 combustion and CO2 hydrogenation. Both CH4 and CO2 are greenhouse gases that have negative impact on the environment and contribute to climate change, therefore methane conversion to CO2 and CO2 transformed to useful chemicals have several advantages. Ni90Pd10 and Ru45Fe55 NPs were synthesized using polyol reduction method by using ethylene glycol as reducing and stabilizing agent. TEM/SEM and XRD analysis were carried out to determent the particle and crystallite size, as well as alloy formation in bi-metallic NPs. The resulting colloidal solution containing NPs were deposited on yttria-stabilized zirconia (8 mol % Y2O3-ZrO2, (YSZ)) solid-electrolyte disk (D = 19 mm, thickness = 1 mm) with loading of 0.2 - 0.3 mg metal/cm2 and served as a catalyst-working electrode. Inert gold counter and pseudo-reference electrode were deposited on the opposite side of YSZ disk. Gold mesh served a current collector for NPs. The solid-electrolyte cell was placed in the CSTR-type quartz reactor (atm. pressure) [9] for CH4 combustion (T = 400 – 500 oC) and CO2 hydrogenation (T = 275 – 400 oC) reactions. Galvanostatic and potentiostatic EPOC transient experiments were carried out under various gas compositions and temperatures. The EPOC results on bi-metallic nanoparticles will be presented and the role of Ni and Fe on the catalytic activity and EPOC efficiency will be discussed. References Stoukides, C.G. Vayenas, J. Catal., 70 (1981), 137-146G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625–627.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. TsiplakidesElectrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-support Interactions, Kluwer Academic/Plenum, New York (2001)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) 8192–8260.Kambolis, L. Lizarraga, M.N. Tsampas, L. Burel, M. Rieu, J.P. Viricelle, Vernoux Electrochem. Commun., 19 (2012), pp. 5-8.A.E. Dole, E.A. Baranova, Implementation of Nanostructured Catalysts in the Electrochemical Promotion of Catalysis in Handb. Nanoelectrochemistry M. Aliofkhazraei, H.A.S. Makhlouf (Eds.), Springer International Publishing, Cham (2015), 1-27A.E. Dole, A. Costa, M. Couillard, E.A. Baranova, J. Catal. 333 (2016) 40–50.M. Hajar, K.D. Patel, U. Tariq, E.A. Baranova, J. Catal. 352 (2017) 42–51.

Electrochemical Promotion of Catalysis for CO2 Hydrogenation on Ru-Based Catalyst Using Ionically Conducting Ceramics

The release of carbon dioxide (CO2) into the atmosphere has led to effects of climate change resulting in an increase in global temperature, ocean acidification and many other environmental issues. Hydrogenation of CO2 into synthetic hydrocarbons is a promising solution in decreasing anthropogenic dependence on fossil fuels and providing an energy source that is carbon-neutral. The reverse water gas shift (RWGS) reaction is a feasible hydrogenation reaction that requires a 2-electron transfer to yield syngas (CO + H2) to be used in the Fischer-Tropsch reaction to synthesize synthetic hydrocarbons. In previous work (submitted to the Journal of CO2 Utilization), the conversion of CO2 into CO using Ru-nanostructured metal nanoparticles dispersed on ionically conducting ceramic supports like ceria (CeO2), doped-ceria (x-CeO2) and yttria-stabilized zirconia (YSZ) was studied with promising results. The activity of the Ru-based nanoparticles was improved due to the ionically conductive properties of the support, which contain oxygen (Oδ-) ionic species that promote the reaction. This promotional effect is known as the metal-support interaction (MSI) where nanoparticles are dispersed on a powder support, allowing Oδ- species to migrate from support to nanoparticle by an increase in temperature [1,2]. The MSI effect has been observed using the best Ru-based powder catalyst supported on samarium-doped ceria (SDC) - Ru45Fe55/SDC (2 wt.%), which yielded high CO amounts between 300-750°C. Current research aims at improving the overall RWGS reaction at lower temperatures through the utilization of the electrochemical promotion of catalysis (EPOC) or non-faradaic electrochemical modification of catalytic activity (NEMCA) effect [3,4]. EPOC allows to control in-situ the migration of ionic species to and from the metal surface through the application of a potential difference or current between the catalyst-working electrode and an inert counter electrode. This migration of species leads to the formation of a neutral double layer encapsulating Ru nanoparticles, promoting the reaction. The catalyst setup resembles an electrocatalytic cell where metal nanoparticles act as the working electrode deposited on a solid support in the form of a disc. The support (YSZ in this case) represents a fixed layer of electrolytes that conducts Oδ- ions to migrate to and from the active catalyst. As shown in Fig. 1, a promotional effect is observed for Ru on YSZ at 350°C under constant potential of 0.25 V, favoring the formation of CO through an enhancement ratio of ~2 and Faradaic efficiency of ~19, which is attributed to the synergistic effect between the metal and promoted ionic species Oδ­-. Additionally, density functional theory (DFT) calculations are being conducted for the hydrogenation of CO2 on Ru nanoparticles and will be discussed in correlation with the experimental findings to confirm the mechanisms occurring during the reaction. [1] P. Vernoux, M. Guth, X. Li, Ionically Conducting Ceramics as Alternative Catalyst Supports, Electrochem. Solid-State Lett. 12 (2009) E9–E11.[2] S. Ntais, R.J. Isaifan, E.A. Baranova, An X-ray photoelectron spectroscopy study of platinum nanoparticles on yttria-stabilized zirconia ionic support: Insight into metal support interaction, Mater. Chem. Phys. 148 (2014) 673–679.[3] D. Vayenas, C.G., Bebelis, S., Pliangos, C., Brosda, S., Tsiplakides, Electrochemical Activation of Catalysis, Springer US, (2001).[4] 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, Ionically Conducting Ceramics as Active Catalyst Supports, Chem. Rev. 113 (2013) 8192–8260. Figure 1

Electrochemical Promotion of Catalysis

Electrochemical Promotion of Catalysis (EPOC), also called Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA), is a promising concept for boosting catalytic processes and advancing the frontiers of catalysis [1]. This innovative field aims to modify in-operando both the activity and the selectivity of catalysts, in a reversible and controlled manner. EPOC utilizes solid electrolyte materials (ionically conducting ceramics) as catalytic carriers. Ions contained in these electrolytes are electrochemically supplied to the catalyst surface and act as promoting agents to modify the catalyst electronic properties in order to achieve optimal catalytic performance. It thus provides a unique means of varying promoter levels at the metal surface under reaction conditions by simply changing the potential of the catalyst film. The main advantage of EPOC is that the electrochemical activation magnitude is much higher than that predicted by Faraday’s law. Therefore, EPOC requires low currents or potentials. Moreover, promoting species such as O2- cannot be formed via gaseous adsorption and cannot be easily dosed by chemical ways. The EPOC technology consists in the implementation of catalysis in an electrochemical cell by using electrochemical catalysts which consist of a catalytic layer interfaced on an ionically conducting ceramic support. The latter serves as an electrically controlled source or sink of ionic promoter species that activate and modulate the behavior of the catalytic surface. Figure 1a displays a typical EPOC reactor. The catalytic layer, typically 40 nm to 10 mm thick, is deposited on a dense solid electrolyte membrane (Figure 1a). This catalytic layer must be electronic conductor to allow the polarization. Therefore, the catalyst is also an electrode and is then named catalyst-electrode. A counter-electrode and a reference electrode, both catalytically inert, are deposited on the other side of the membrane (Figure 1a). An electrical current density (1-500 µA/cm2) or potential (±2 V) is applied between the catalyst and the counter electrode. The reactants are co-fed over the porous electrochemical catalytic layer. Figure 1b displays a typical example of an EPOC experiment for the propane deep oxidation. The electrochemical catalyst was composed of a Pt thin film deposited by Physical Vapor Deposition (PVD) on a dense membrane of Yttria-Stabilized Zirconia (YSZ), an O2- ionic conductor. A positive current (from +10 up to +500µA) can strongly increase the propane conversion with a non-Faradaic manner, since the activation is up to 300 times higher than that predicted by the Faraday’s law through the electrochemical oxidation of propane. This latter reaction will be used to highlight recent advances of the EPOC technology both for the understanding (18O2 isotopic exchange) of the process and its applications via the proof of concept of electropromoted nanoparticles. [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. Figure 1

High Faradaic Yields of Non-Faradaic Electrochemical Modification of Catalytic Activity of Propane Oxidation at Pt-YSZ

Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) of propane oxidation at Pt dispersed on yttria-stabilized zirconia (YSZ) discs by strong electrostatic adsorption (SEA) and wet impregnation (WI) in wireless configuration was studied and compared. With a stoichiometric gas mixture of propane and oxygen diluted in helium, an exceptionally large faradaic yield of 5.2 × 104 was achieved from a cell prepared by SEA at a reaction temperature of 200°C and a cell voltage of 6 V, compared to a smaller value of 1.0 × 104 obtained from a cell prepared by WI. A conceptual mechanism explaining the dependence of the faradaic yields and the rate enhancement ratios on Pt particle sizes is proposed.

Recent Progress on the Application of BASE in Energy Storage and Environmental Protection

Beta-alumina solid electrolyte (BASE) is a fast ion conductor material which shows high conductivity of sodium ions and low electronic conductivity. Recent Progress on the Application of BASE in energy storage such as Na Based Batteries (NBBS) and Alkali metal thermal to-electric converter (AMTEC), and Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) in environmental protection were reviewed.

Methane Conversion Assisted by Plasma or Electric Field

Direct conversion of methane to other valuable products such as ethylene, methanol, benzene, carbon, hydrogen, and syngas has been widely investigated. Such conversion requires high temperatures because of the strong C–H bond in CH4, and such high-temperature reactions present various problems: sequential reaction to decrease the selectivity to target products, need for multiple heat exchangers to use or recover high-temperature waste heat, materials that are stable at high temperatures, etc. To solve these problems, several trials have been undertaken to lower reaction temperatures using plasma, Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA), and electric fields. This review describes recent trends in methane conversion, and describes our methods to lower the reaction temperature.

ChemInform Abstract: Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) . Part 4. The Use of β′′-Al2O3 as the Solid Electrolyte.

ChemInform Abstract: Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) . Part 4. The Use of β′′-Al2O3 as the Solid Electrolyte.

Electrochemical catalysis for propane combustion using nanometric sputtered-deposited Pt films

The catalytic activity of sputtered-Pt films onto Y 2O 3-stabilized ZrO 2 (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.

Temperature programmed desorption of oxygen from Pd films interfaced with Y2O3-doped ZrO2

The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or Electrochemical Promotion was investigated via temperature-programmed-desorption (TPD) of oxygen, from polycrystalline Pd films deposited on 8 mol%Y2O3–stabilized–ZrO2 (YSZ), an O2− conductor, under high-vacuum conditions and temperatures between 50 and 250 °C. Oxygen was adsorbed both via the gas phase and electrochemically, as O2−, via electrical current application between the Pd catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives two adsorbed atomic oxygen species desorbing at about 300 °C (state β1) and 340–500 °C (state β2). The creation of the low temperature peak is favored at high exposure times (exposure >1 kL) and low adsorption temperatures (Tads < 200 °C). The decrease of the open circuit potential (or catalyst work function) during the adsorption at high exposure times, indicates the formation of subsurface oxygen species which desorbs at higher temperatures (above 450 °C). The desorption peak of this subsurface oxygen is not clear due to the wide peaks of the TPD spectra. The TPD spectra after electrochemical O2− pumping to the Pd catalyst film show two peaks (at 350 and 430 °C) corresponding to spillover Oads and \( O^{{\delta - }}_{{\rm ads}} \) according to the reaction: $$ O^{{2 - }}_{{\rm (YSZ)}} \to {\text{O}}^{{\delta - }}_{{{\text{ads}}}} \to {\text{O}}_{{{\text{ads}}}} $$

On the work function changes and other properties of the gas-exposed electrode surface in the NEMCA effect

A simple model is developed to describe the backspillover of ionic species at the solid/gas interface as observed in non-Faradaic electrochemical modification of catalytic activity (NEMCA) systems. This model predicts a linear relationship between work function changes and changes in the potential difference at the metal/solid electrolyte interface, as observed experimentally. It also predicts a linear dependence of the surface coverage by ionic species on the potential difference at the metal/solid electrolyte interface, and a linear dependence of the binding energy of adsorbed species on the work function of the gas-exposed catalyst surface.

Electrochemical promotion of the oxidation of propane on Pt/YSZ and Rh/YSZ catalyst-electrodes

The effect of electrochemical promotion (EP) or non-faradaic electrochemical modification of catalytic activity (NEMCA) was studied in the catalytic reaction of the total oxidation of propane on Pt and Rh films deposited on Y2O3-stabilized-ZrO2 (or YSZ), an O 2) conductor, in the temperature range 420–520 � C. In the case of Pt/YSZ and for oxygen to propane ratios lower than the stoichiometric ratio it was found that the rate of propane oxidation could be reversibly enhanced by application of both positive and negative overpotentials (‘‘inverted volcano’’ behavior), by up to a factor of 1350 and 1130, respectively. The induced rate increase Dr exceeded the corresponding electrochemically controlled rate I/2F of O 2) transfer through the solid electrolyte, resulting in absolute values of the apparent faradaic efficiency K=Dr/(I/2F) up to 2330. The Rh/YSZ system exhibited similar EP behavior. Abrupt changes in the oxidation state of the rhodium catalyst, accompanied by changes in the catalytic rate, were observed by changing the O2 to propane ratio and catalyst potential. The highest rate increases, by up to a factor of 6, were observed for positive overpotentials with corresponding absolute values of faradaic efficiency K up to 830. Rate increases by up to a factor of 1.7 were observed for negative overpotentials. The observed EP behavior is explained by taking into account the mechanism of the reaction and the effect of catalyst potential on the binding strength of chemisorbed reactants and intermediates and on the oxidative state of the catalyst surface.

Coupling catalysis to electrochemistry: a solution to selective reduction of nitrogen oxides in lean-burn engine exhausts?

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 N 2. Such an improvement can be obtained using low overpotentials, applying −100 mV to the Pt catalyst-electrode enhances the selectivity to N 2 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.

The oxidation of ethanol over Pt catalyst-electrodes deposited on ZrO 2 (8 mol% Y 2O 3)

The reaction of ethanol oxidation has been examined over a Pt polycrystalline film deposited on an yttria-stabilized zirconia (YSZ) electrochemical reactor. Experiments were carried out between 300 and 350 °C at atmospheric total pressure and under open and closed circuit conditions. It was found that the electrochemically induced change in the catalytic rate of ethanol consumption is four orders of magnitude higher than the rate of ion transport through the solid electrolyte. The galvanostatic transient behaviors of the consumption rates of ethanol and oxygen and of the formation rates of carbon dioxide and acetaldehyde have also been recorded. Finally, the increase in the rate of the electrochemical supply of oxygen ions through the solid electrolyte was found to induce an almost 700% increase on the yield of acetaldehyde. The results are interpreted on the basis of the theory of non-Faradaic electrochemical modification of catalytic activity (NEMCA).

Selective oxidation of methane in an SOFC-type reactor: effect of applied potential

The effect of applied potential on the selective oxidation of methane in a solid oxide fuel cell (SOFC)-type reactor was investigated. A tube-type YSZ was used as an electrolyte where a La 1.8Al 0.2O 3 anode and a conventional La 0.85Sr 0.15MnO 3 cathode were deposited on the inner and outer surfaces of the tube, respectively. The fuel cell-type temperature-programmed desorption (FC-TPD) measurements revealed that the amount of adsorbed oxygen on the anode catalyst was altered with the applied potential. Increasing the applied potential increased the amount of weakly adsorbed oxygen at the “oxygenate site”. Two operating modes of the SOFC-type reactor were carried out. In the normal mode, it was found that the rate of C 2 formation was not affected by an applied potential. The positive potential increased the rate of carbon oxides formation, especially the product CO, while the negative potential suppressed the rates of CO and CO 2 formation. For the mixed-flow mode, even though the electrochemically promoted rates did not show significant improvement over the open-circuit rates, the catalytic activities of the anode catalyst to different products were altered by the applied potential with non-Faradaic manner. The change in the selectivity of the active site gave a new aspect to non-Faradaic electrochemical modification of catalytic activity (NEMCA) phenomena.

Electrochemical promotion of conventional and bipolar reactor configurations for NO reduction

The reduction of NO by CO or C3H6 in the presence of O2, a reaction of great technological importance, was investigated on porous polycrystalline Rh catalyst films using a single-chamber and a wireless bipolar cell configuration. In the latter case the Rh catalyst films were deposited on the inner side of a YSZ tube, while two Au films deposited on the outer side of the tubes were used to polarize the Rh catalyst surface. The experimental conditions used in this study were close to those in the exhaust of a lean burn or diesel engine, i.e., high flowrates and space velocities and in some cases, considerable excess of oxygen. It was found that both direct (conventional) and indirect (wireless) polarization of the catalyst causes significant enhancement in the reaction rates (up to a factor of 20) and in the reactant conversion. These rate increases are strongly non-Faradaic with apparent Faradaic efficiencies, A, in the order of 100, manifesting the effect of Electrochemical Promotion or Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA). The Rh catalyst films were subsequently promoted in a classical way, via dry impregnation with NaOH, followed by drying and calcination. The thus Na-promoted Rh films were found to exhibit higher catalytic activity than the unpromoted films, with a considerable decrease in their light-off temperature. The effect of Electrochemical Promotion was then studied on these, already Na-promoted Rh catalysts. The results showed that the effect of Chemical and Electrochemical Promotion on the catalyst performance can be synergetic and their combination may lead to interesting practical applications. This is further supported by the fact that such bipolar tube configurations: (a) do not need electrical connection to the catalyst and (b) can be adapted easier to commercial exhaust units.