Electrochemical Promotion Of Catalysis Research Articles

Short Term Silver Electrode Microstructure Changes Under Epoxidation Conditions for Solid Oxide Electrolysis Cells

Ethylene epoxidation is an important reaction forming ethylene oxide (EO), which is a precursor to many other critical chemicals. This study links short-term EO production to the effects on the microstructure of Ag/yttria-stabilized zirconia cells with and without electrochemical promotion of catalysis (EPOC). Nano-scale features, called striations, were observed on the silver under all epoxidation reaction conditions tested, using a scanning electron microscope. While appearing in both cases, the striations for the EPOC case are finer in size (∼150 to 250 nm) compared to the no current case (∼400 to 500 nm). These features did not appear when epoxidation conditions were not present. Striation formation was further linked to the epoxidation reaction through electrochemical impedance spectroscopy and gas chromatography. Ethylene conversion to EO declines over the course of hours as striations form, indicating that striations have a negative influence on the reaction. Distribution of relaxation times was performed to determine the effect striation formation has on the electrochemical performance of the cells. It was observed that the low-frequency peaks in the DRT analysis significantly decrease as striations formed over the course of 10 h.

Electrochemical Promotion of Bimetallic Palladium‐Cobalt Nano‐Catalysts for Complete Methane Oxidation

AbstractThe catalytic activity and electrochemical promotion of catalysis (EPOC) of the bimetallic Pd−Co nanoparticles (10 nm) deposited on yttria‐stabilized zirconia were investigated for complete methane oxidation. The reaction was conducted under open‐circuit and under polarization in a temperature range of 320–400 °C in reducing, stoichiometric, and oxidizing conditions. The catalytic activity of the catalyst nanoparticles increased to 40 % upon positive polarization in all gaseous compositions. A comparison of three reaction conditions revealed that the highest reaction rate increase (enhancement ratio, ρ=1.4) occurs under reducing conditions. The reaction rate increased upon anodic polarization, resulting in non‐Faradaic electrochemical modification of catalytic activity (Λ≫1) in reducing and oxidizing conditions and Faradaic or electrochemical enhancement (Λ<1) in stoichiometric reaction condition. This work demonstrates that the formation of different Pd and Co oxide phases can be accurately controlled by electrochemical stimuli and, in reducing conditions, result in pseudo‐capacitor behaviour.

Electrode-Supported Protonic Ceramic Electrolysis Cells for Ammonia Electrosynthesis at Intermediate Temperatures

Ammonia electrosynthesis, the electrochemical process of converting water and nitrogen into ammonia using renewable energy, has been the focus of attention in the search for a Haber-Bosch alternative. Ammonia produced using renewable energy can not only be used as a fertilizer, but also in its liquefied form as an energy-dense fuel (“energy carrier”). Protonic ceramic electrolysis cells (PCECs) are suitable for ammonia electrosynthesis at high temperatures due to their superior chemical and thermal stability and their ability to keep ammonia and water separate throughout the process, enabling efficient separation of pure ammonia after synthesis. However, on the system scale, high temperature conditions are less compatible with the fluctuating output of renewable energy generation. Additionally, as the temperature dependence of ammonia synthesis faces a trade-off between thermodynamic and kinetic favorability, intermediate temperatures, defined here as between 200-400˚C, are considered promising for ammonia electrosynthesis [1].We previously demonstrated that under a mixed cathodic gas feed of N2/H2, ammonia electrosynthesis using PCECs can be accelerated by cathodic polarization [2-3]. Non-Faradaic effects were observed; that is, the ammonia formation rate was not proportional to the current density. Our results demonstrated similar characteristics as those observed under the electrochemical promotion of catalysis (EPOC), first proposed by Stoukides et. al [4]. Recently, using electrolyte-supported PCECs operated at 600˚C, we showed that high ammonia formation rates on the order of 10-8 mol s-1 cm-2 can be attained in a single-chamber co-fed with N2 and H2 [5], which can eliminate gas sealing requirements.In this work, we aim to understand the factors needed to lower the operating temperature of ammonia electrosynthesis using PCECs co-fed with N2 and H2, from high temperatures between 500-600˚C to intermediate temperatures of 400˚C and below. Electrolyte-supported and electrode-supported cells were compared for their ammonia electrosynthesis rates and cell performances. Electrochemical testing was conducted in a single-chamber reactor co-fed with 50% H2/50% N2 at 300-600˚C. Ammonia formation rates were evaluated by bubbling the outlet gas into a dilute H2SO4 solution and measuring the resulting ammonium ion concentrations via high-performance liquid chromatography. We conducted electrochemical control tests in H2/Ar to validate that ammonia has been produced electrochemically from dinitrogen, rather than from potential traces of ammonia or NOx species (which are easier to reduce) found in the experimental environment.Preliminary experiments using electrode-supported cells with the configuration Fe|BaCe0.9Y0.1O3-δ (BCY)|Ni-BaZr0.8Y0.2O3-δ (Ni-BZY) have resulted in ammonia formation rates over 10-8 mol s-1 cm-2 at an applied voltage of -1 V, at just 400˚C, as shown in Fig. 1. This corresponds to a 150~200˚C reduction in temperature compared to that needed to obtain similar ammonia formation rates using electrolyte-supported cells of the configuration Pt|BCY|Fe. We further evaluate both types of cells in terms of their current density and electrochemical impedance, to understand how the effect of applied voltage varies with the different cell structures. The comparison suggests that Ohmic and anodic overpotentials are effectively limited in our electrode-supported cells, leading to an increased cathodic polarization that aids in accelerating ammonia formation even at intermediate temperatures. Acknowledgements This work was supported by JSPS KAKENHI Grant Numbers JP21H04938 and JP22J11546.

Electrochemical Promotion of CO2 Hydrogenation Using a Pt/YSZ Fuel Cell Type Reactor.

The hydrogenation of CO2 is a reaction of key technological and environmental importance, as it contributes to the sustainable production of fuels while assisting in the reduction of a major greenhouse gas. The reaction has received substantial attention over the years within the catalysis and electrocatalysis communities. In this respect, the electrochemical promotion of catalysis (EPOC) has been applied successfully to the CO2 hydrogenation reaction to improve the catalytic activity and selectivity of conductive films supported on solid electrolytes. However, designing an effective electrocatalytic reactor remains a challenge due to the connections required between the electrodes and the external potentiostat/galvanostat. This drawback could be alleviated if the catalytic reaction occurs in a reactor that simultaneously operates as a power generator. In this work, the Electrochemical Promotion of the CO2 hydrogenation reaction in a low-temperature solid oxide electrolyte fuel cell (SOFC) reactor is studied using yttria-stabilized zirconia (YSZ) and a platinum (Pt) electrode catalyst. The system has been studied in two distinct operation modes: (i) when the necessary energy for the electrochemical promotion is produced through the parallel reaction of H2 oxidation (galvanic operation) and (ii) when a galvanostat/potentiostat is used to impose the necessary potential (electrolytic operation). The performance of the fuel cell declines less than 15% in the presence of the reactant mixture (CO2 and H2) while producing enough current to conduct EPOC experiments. During the electrolytic operation of the electrochemical cell, the CO production rate is significantly increased by up to 50%.

Insight into Electrochemical Promotion of Cu/Co3O4 Catalysts for the Reverse Water Gas Shift Reaction

AbstractIn this study, Copper‐modified Co3O4 (Cu/Co3O4) catalysts were investigated for the reverse water gas shift (RWGS) reaction at 200–400 °C under different CO2 : H2 ratios. It was found that the electrocatalytic performance of Co3O4 was improved by loading 4 weight percent (wt.%) Cu nanoparticles (average size: 13 nm), which could be then used as an electrode in electrochemical promotion of catalysis (EPOC) studies. Under the applied voltages of +2 V and −1 V, the catalytic rates were suppressed and increased by about 40 % and 14 %, respectively. This was due to the changes in the active oxidation states of Cu and Co caused by O2− migration under polarization, as confirmed by XPS, XRD, and cyclic voltammetry (CV). The study also revealed that the exchange current density (i0) was counter‐correlated with the open‐circuit catalytic rate (r0) and could serve as an informative tool for predicting the catalytic rate, which was demonstrated for the first time in the instance of RWGS reaction.

(Digital Presentation) Iron-Based Electrode Structures for Ammonia Electrosynthesis Cells with Proton-Conducting Ceramic Electrolytes

Ammonia electrosynthesis is considered promising for the production of ammonia as an energy carrier, as the process could provide greater flexibility and compatibility to the fluctuations of renewable energy than the industrial Haber-Bosch process. One approach for enhancing the ammonia formation rate is the electrochemical promotion of catalysis (EPOC) [1], in which a non-faradaic acceleration of ammonia synthesis is observed; however, much of the mechanism remains unclear. Further mechanistic insight would aid in designing better electrode structures that are suited for efficient ammonia electrosynthesis. In this presentation, we explore the mechanism underlying the electrochemical promotion of ammonia synthesis, using electrochemical cells with iron-based cathodes on a proton-conducting BaCe0.9Y0.1O3 (BCY) electrolyte.Previously, our group reported that the ammonia formation rate of a cathodically polarized cell can be enhanced by adding H2 to the cathode gas feed (commonly pure N2) [2, 3]. Further kinetic and isotopic analysis using deuterium showed that the primary hydrogen source for the ammonia produced was the gaseous hydrogen feed, rather than the protons conducted via the electrolyte [4]. Cells with a porous pure Fe cathode resulted in higher ammonia formation rates than those with a porous 10 wt.% Fe-BCY cathode, suggesting that the pure Fe surfaces near the cathode-electrolyte interface, as opposed to the triple phase boundaries, are active for ammonia electrosynthesis [3]. However, the 10 wt.% Fe-BCY cathode, fabricated by the impregnation method, was thought to be limited by lack of Fe network formation needed for electron conduction.In this study, we fabricated a cermet cathode with the composition 56 wt.% Fe-BCY, aiming for Fe and BCY to form separate networks for electron and proton conduction, respectively. Electrochemical cells of the configurations Pt|BCY|Fe and Pt|BCY|Fe-BCY [Fig. 1a], with similar Fe mass (0.5 and 0.4 mg) were compared. Ammonia synthesis was conducted in a single-chamber reactor at 500-600˚C with a 50% H2/50% N2 feed. The gaseous ammonia produced was captured by bubbling into a dilute H2SO4 solution and quantified via high-performance liquid chromatography.Using the pure Fe electrode, an ammonia formation rate of 1.3 × 10-8 mol s-1 cm-2 (26 mmol gFe -1 h-1) was obtained at 600˚C and -1 V [Fig. 1b]. This corresponds to an almost 5-fold increase in ammonia formation compared to the catalytic reaction at 0 V. In contrast, a lower ammonia formation rate of 7.0 × 10-9 mol s-1 cm-2 (17 mmol gFe -1 h-1) was obtained using the Fe-BCY electrode at the same conditions. Despite the cermet electrode having a slightly higher current density, and despite the comparable iron loading on the two cells, the pure Fe cell still resulted in a faster ammonia formation rate per electrode area and per iron mass. As the cermet electrode is expected to have a significantly longer triple phase boundary length, from the comparison of these two electrochemical cells, the triple phase boundaries do not appear to be the primary active sites. We further analyze the roles of the bulk Fe surfaces and the Fe/BCY triple phase boundaries, by examining the effect of the electrode thickness and by post-experiment cross-sectional electron microscopy approaches.AcknowledgementsThis work was supported by JSPS KAKENHI Grant Number JP21H04938.

Iron-Based Electrode Structures for Ammonia Electrosynthesis Cells with Proton-Conducting Ceramic Electrolytes

Ammonia electrosynthesis was conducted using proton-conducting ceramic electrolysis cells in a single chamber supplied with mixed N2 and H2, to determine the active electrode regions for the electrochemical promotion of catalysis (EPOC). Cells with Fe-based cathodes of either a metal, cermet, or mixed ionic-electronic conductor (MIEC) structure were compared, to assess whether the triple-phase boundaries (TPBs) or the Fe catalyst surfaces are more effective for ammonia synthesis. Upon cathodic polarization at 600˚C, high ammonia formation rates on the order of 10−8 mol s−1 cm−2 were obtained with both Fe metal and Fe-BaZr0.8Y0.2O3 (Fe-BZY) cermet cathodes. Comparisons of the three electrode structures suggest that when H2 is also supplied to the cathode, ammonia formation occurring on bulk Fe surfaces leads to a greater promotional effect than that at TPBs. There also remains a possibility that the interfacial layer formed at the cathode-electrolyte interface due to cation interdiffusion aids ammonia formation.

Electrochemical promotion of catalysis

The Electrochemical Promotion of Catalysis (EPOC) or Non-Faradaic Electrochemical Promotion of Catalysis (NEMCA effect) is a phenomenon observed as a reversible change in catalytic rate (i.e. no net charge transfer rate) of a chemical reaction occurring on a catalyst film (or supported dispersed catalyst) deposited on an ionically conducting or mixed electronically-ionically conducting solid electrolyte support upon the application of an electrical potential between the catalyst and a second conductive film deposited on the solid electrolyte support.

A low temperature SOFC as a self-promoted reactor for CO2 catalytic hydrogenation

Low temperature operation of Solid Oxide Fuel Cells (SOFC) typically provides limited and insufficient power for practical applications. The present work demonstrates how this limited power can be utilized effectively for the promotion of a catalytic reaction taking place on a catalyst electrode with minimal energy consumption and environmental impact. A novel design of a low-temperature SOFC reactor is reported and utilized for the electrochemical promotion of CO2 hydrogenation, a reaction of both environmental and industrial importance. The power demand for the enhancement of the catalytic reaction rates is produced in situ by the cell during the parallel oxidation of hydrogen which acts both as a reactant and as a fuel. The results of the present study pave the way for the utilization of Electrochemical Promotion of Catalysis (EPOC) in a wireless configuration, using exclusively the internal power of low temperature, low pressure, SOFCs.

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.

Electrochemical promotion of CO2 hydrogenation in a monolithic electrochemically promoted reactor (MEPR)

The electrochemical promotion of the CO2 hydrogenation was investigated, using a monolithic electropromoted reactor (MEPR) loaded with nine, connected in parallel, Ru/YSZ/Au electrochemical cells. The study was carried out at ambient pressure and in the temperature range between 220°C and 370°C, using high reactant flowrates (up to 2500 cm3/min) and reducing conditions (PCO2/PH2=1/7). For an intermediate flowrate of 1000 cm3/min, the selectivity to methane (SCH4) was 100 % for temperatures up to 300°C. Further increase in temperature resulted to a decrease in selectivity to methane with a concomitant increase of the corresponding yield (YCH4) due to the higher CO2 conversion. However, the total performance of the reactor, in terms of methane selectivity and yield, was affected significantly under electropromoted conditions. At the maximum studied temperature of 370°C, the unpromoted values of SCH4 and YCH4 were 84 % and 24 % respectively, while O2− supply to the catalytic surface enhanced theses values to 92 % and 32 % respectively. The qualitative features of electropromotion, i.e. electrophobic behavior for methanation and electrophilic behavior for the RWGS reaction, were in agreement with previous EPOC studies of CO2 hydrogenation in laboratory scale reactors showing a successful scale up of Electrochemical Promotion of Catalysis (EPOC). In addition, the good performance of the monolithic reactor under severe operating conditions of high flowrates and temperatures is a promising result for further scale-up and for the practical utilization of EPOC.

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.

Electrochemical promotion of Ru nanoparticles deposited on a proton conductor electrolyte during CO2 hydrogenation

Recycling CO2 into a carbon-neutral source is a beneficial approach in reducing non-renewable energy sources. Herein, the electrochemical promotion of catalysis (EPOC) has been exploited to enhance the catalytic activity of Ru nanoparticles (0.7−1 nm) deposited on the proton conductor yttria-doped barium zirconate (BZY), as free-standing nanoparticles and supported on Co3O4 semiconductor, for CO2 hydrogenation. Under 250−450 °C and atmospheric pressure, both methanation and reverse water-gas shift (RWGS) reaction take place simultaneously over the Ru nanoparticles with a superior selectivity to CO. Under anodic polarization, free-standing Ru nanoparticles displayed an increase in CH4 and a decrease in CO production, while the opposite effect was observed under cathodic polarization. Ru supported on Co3O4 displayed a superior catalytic activity mostly due to enhanced metal–support interactions. The electronic effects induced by the pairing of Co3O4 and BZY resulted in a new approach to EPOC applications that brings it closer to industrial application.

Elucidating the role of electrochemical polarization on the selectivity of the CO2 hydrogenation reaction over Ru

The hydrogenation of CO2 into high-value fuels is a potentially effective approach to reduce anthropogenic dependence on fossil fuels and effects of climate change. In this study, we evaluated the hydrogenation of CO2 into CO and CH4 under the electrochemical promotion of catalysis (EPOC) effect through experimental and computational studies using Ru nanoparticles. Ru nanoparticles (1–2 nm) supported on yttria-stabilized zirconia (YSZ) solid electrolyte were evaluated at 250 °C at atmospheric pressure. Under positive polarization, the methanation reaction was promoted and the competitive reverse water gas shift (RWGS) reaction was impeded. On the other hand, negative polarization resulted in suppressing permanently the methanation reaction and minimally affecting the RWGS reaction. To qualitatively rationalize the tuning of selectivity via EPOC, Density Functional Theory (DFT) computations were used to model the EPOC effect induced on the Ru (0001) surface. DFT computations uncovered that electric field effects together with a change in surface electrochemical potential between intermediates are responsible for the contrasting influence of EPOC on the CH4 and CO formation over Ru catalysts.

Electrochemically Enhancing Nanostructured Fe-Oxide/Co3O4 for the Reverse Water Gas Shift Reaction

The abundant amount of CO2 in the atmosphere is a valuable resource that when managed correctly can replace energy from the fossil fuel industry and provide a carbon-neutral solution that will reduce the impacts of the climate and ecological crisis[1]. This is achieved by reducing CO2 with hydrogen (H2) through the reverse water gas shift (RWGS) reaction to produced carbon monoxide (CO). CO then acts as a source molecule in the Fischer-Tropsch synthesis to manufacture hydrocarbon fuels. Due to the high stability of the CO2 molecule, significant thermal energy is required to undertake the reaction. Iron-oxide (FeOx) has been shown to be an active and thermally stable catalysts towards the RWGS reaction. FeOx (iron-oxide) nanowires are fabricated through the polyol reduction method and have been characterized through scanning transmission electron microscopy (STEM) analysis[2]. Following the wet deposition method, the Fe nanowires are finely dispersed on cobalt-oxide (Co3O4) to act as a support and semi-conductor. The dispersion of Fe on Co3O4 enhances the electronic properties of the catalyst through the metal-support interaction (MSI) effect[3]. The MSI entails the back spillover of promoting oxygen (Oδ-) ionic species from Co3O4 to FeOx by an increase in temperature, altering the work function of FeOx and allowing to cycle oxygen from the breaking of CO2. Through the Electrochemical Promotion of Catalysis (EPOC) phenomenon the catalytic activity can be enhanced by altering the binding energy of the reactant and intermediate species on the catalytic surface[4]. The MSI and EPOC phenomena have been shown to be functionally equivalent in terms of the change in work function. The difference between them is that in EPOC the movement of ions can be controlled through the application of an electrical current or potential difference, while the MSI effect is controlled by a thermal input. EPOC entails the use of solid electrolytes, for instance yttria-stabilized zirconia (YSZ) and barium zirconate yttrium-doped (BZY) which are oxygen and proton conductors, respectively. The catalyst-working electrodes (i.e. FeOx nanowires) are deposited on one side of the electrolyte and on the opposite side, inert gold counter and reference electrodes. Through the application of a potential difference between the electrodes, promoting species migrate through the three-phase (solid-gas-catalyst) boundary in order to interact with the exposed-catalyst surface. In the case of BZY, when a potential difference is applied between the counter and working electrode, H+ are removed from the surface through the three-phase boundary (tpb) towards BZY, referred to as positive polarization. When the polarization is reversed from the working to counter electrode, H+ migrate from BZY through the tpb towards the catalytic surface. Utilizing YSZ, results in the opposite migration with O2- ions. Regardless of the type of solid electrolyte used the catalyst is oxidized under positive polarization and reduced under negative polarization. The use of the Co3O4 semiconductor allows to combine both the MSI and EPOC effect at 350°C, by finely dispersing the Fe nanowires on Co3O4 to expose active sites and lowering the rate of sintering while being conductive to close the circuit[5]. As opposed to using other supports that do not display fully conductive properties, this approach provides an electronic path for the promoters to follow. Results have shown the working-catalyst FeOx/Co3O4 on YSZ and BZY to be highly selective to CO formation under oxidizing (3CO2:H2) and reducing (CO2:7H2) conditions, with a superior activity experienced under rich reducing conditions. Furthermore, alteration in work function from the EPOC effect, has led to a promotional response for both YSZ and BZY. Combining the MSI and EPOC effect allows for high RWGS activity, establishing a promising solution in utilizing CO2 as a resource while using transition metals. Additionally, the EPOC effect will be elucidated through Fourier-Transform Infrared (FTIR) spectroscopy to provide insight on the promoting mechanism occurring during polarization. 1. Porosoff, M. D., Yan, B. & Chen, J. G. Catalytic reduction of CO2 by H2 for synthesis of CO, methanol and hydrocarbons: challenges and opportunities. Energy Environ. Sci. 2, 303 (2015).1. Baranova, E. A., Bock, C., Ilin, D., Wang, D. & MacDougall, B. Infrared spectroscopy on size-controlled synthesized Pt-based nano-catalysts. Surf. Sci. 600, 3502–3511 (2006).2. Panaritis, C., Edake, M., Couillard, M., Einakchi, R. & Baranova, E. A. Insight towards the role of ceria-based supports for reverse water gas shift reaction over RuFe nanoparticles. J. CO2 Util. 26, 350–358 (2018).3. Vayenas, C. G., Bebelis, S., Pliangos, C., Brosda, S. & Tsiplakides, D. Electrochemical Activation of Catalysis. (Springer US, 2001).4. Zagoraios, D. et al. Electrochemical promotion of methane oxidation over nanodispersed Pd/Co3O4 catalysts. Catal. Today (2019). doi:10.1016/j.cattod.2019.02.030

Over-faradaic hydrogen production in methanol electrolysis cells

A new alcohol electrolyser concept (Methanol Over-Faradaic Electrolyser: MOFE) has been developed to achieve over-faradaic H2 production rates, which were experimentally measured. In a first stage, this outstanding phenomenon was investigated in an alkaline membrane electrolyser. Secondly, a membrane-less electrolyser was used to simplify and implement this concept in practical configurations. The obtained results demonstrated that the experimentally measured over-faradaic H2 production rates on the cathode were due to the coupling of the electrocatalytic hydrogen production via methanol electrolysis (hydrogen evolution reaction) with the catalytic methanol dehydrogenation reaction. The kinetic of the latter reaction is drastically enhanced upon polarization due to the phenomenon of Electrochemical Promotion of Catalysis (EPOC), in liquid phase conditions. Hence, the K+ ions supplied to the Pd/C catalyst-cathode from the liquid electrolyte under polarization conditions promote the chemisorption of intermediate species, activating the catalytic hydrogen production via methanol dehydrogenation reaction on the active Pd sites. Thus, the synergy between these two processes (electrocatalytic + catalytic) lowers the overall energy requirements of the hydrogen produced below the typical values found for conventional alcohol electrolysis cells. These results demonstrate, for the first time, the possibility to produce an “over-faradaic” H2 production rate in a liquid phase electrolysis cell, which is of great interest in view of enhancing the energetic efficiency and the power requirements for hydrogen production via electrolysis of organic molecules.

(Invited) Tuning of Catalytic Activity By Thermoelectric Effect

Thermoelectric (TE) materials convert temperature difference directly into electrical voltage via the Seebeck effect, S= -V/ΔT where V is the voltage between the two ends of the TE material and ΔT the temperature difference, S is the Seebeck coefficient. It has been used for energy harvesting from waste heat, and for silent cooling via its converse phenomenon the Peltier effect. Here we report its novel application as a catalyst promoter. Catalyst promoters can generally be divided into structural and electronic promoters. Structural promoters enhance and stabilize the dispersion of the catalyst on the support. Electronic promoters induce changes of electronic state of the catalyst near the Fermi level. These promoters are added or fixed during the catalyst preparation, therefore can subject to degradation during catalytic process. We report here the realization of in situ, controllable, reversible, and significant modification of catalytic activity, through the generation of a Seebeck voltage. A chamber reactor which combine TE effect with catalytic chemical reaction was assembled.1, 2 The chamber was placed directly on a hot-plate, and its top cap was water cooled. A disc TE BiCuSeO (BCSO) sample of 20 mm in diameter and about 2 mm in thickness was then placed into a specific glass ceramic holder and attached to the cold surface of the cap for catalytic reaction investigation under the usual TE conditions. As the back side of the disc was in contact with the water cooled stainless steel cap, its temperature was never higher than 373 K, so a large temperature gradient therefore a large Seebeck voltage across the sample thickness was created when the front surface of the disc reached a high temperature Th. Under the reduced TE (RTE) condition, the back surface of the disc was not in contact with the cap, therefore the temperature gradient across the disc thickness and also the Seebeck voltage were much smaller. Figure 1a shows the ethylene oxidation reaction rates at different front surface temperatures Th for a sample of Pt thin film of 80 nm supported on BCSO with a large TE effect (red marks, Pt(80)/BCSO TE), and under a reduced TE effect (blue marks, Pt(80)/BCSO RTE) conditions respectively. For comparison purpose, the ethylene reaction rate on a typical catalyst for this reaction Pt thin film on Y2O3-stabilized ZrO2 (YSZ, green marks, Pt(80)/YSZ) is also depicted in Fig. 1a. For Pt(80)/BCSO TE, when the front surface Th was at 666 K, the backside temperature Tc was 349 K, so a temperature difference of 317 K existed across the BCSO thickness, which generated a Seebeck voltage -71 mV. For the same sample under the reduced TE effect (Pt(80)/BCSO RTE), at 553 K, the measured Seebeck voltage was -3.1 mV, and the reaction rate was much lower than that for Pt(80)/YSZ at a similar temperature 563 K. This was probably because the surface area of the Pt(80)/BCSO was much smaller than for Pt(80)/YSZ. However, with the increasing temperature, the reaction rates for Pt(80)/BCSO under TE and RTE conditions increased much faster than for non-TE YSZ supported catalyst. Further detailed investigation demonstrated that when the total reaction rate was lower than a certain value, a linear relationship existed between the logarithm of the reaction rate and the Seebeck voltage.1, 2 We called this thermoelectric promotion of catalysis (TEPOC). To confirm the above observed TEPOC can also be observed in other reactions, the same samples were used as catalysts for carbon dioxide hydrogenation. The inlet gases were CO2 and H2, and the reaction products observed were CO and CH4, with the vast majority (>90%) being CO. Similar as was observed for ethylene oxidation, for the same sample under the same inlet gas composition, the CO2 conversion rates r under TE conditions were much higher (between 10 to 30 times) than under RTE conditions. Again, a good linear relationship between Ln(r) and -eV/kbTh was observed (Figure 1b). The effects of the Seebeck voltage on the catalytic activity under different reaction conditions, such as fuel-lean and fuel-rich conditions for the ethylene oxidation, have also been investigated. The underlying mechanisms based on metal-semiconductor contact theory and work function change, as well as the comparison with the Electrochemical Promotion of Catalysis, will be discussed.

The role of the promoting ionic species in electrochemical promotion and in metal-support interactions

The reaction of CO2 hydrogenation is of high environmental interest since it allows for the transformation of the logistically challenging H2, gained from renewable sources, to the much more manageable hydrocarbons. Ruthenium is a catalyst widely used to produce methane from CO2. In this study we present an example of how Electrochemical Promotion of Catalysis (EPOC) can elucidate the role of solid electrolytes (YSZ, BZY) supporting Ru porous films or nanoparticles. It is found that the sign of the charge (-/+) of the current-conducting species of the electrolyte has a profound effect on the selectivity of Ru during CO2 hydrogenation. Using this insight, we have developed and tested a new supported Ru-Co catalyst for the production of higher hydrocarbons.

Electrochemical promotion of methane oxidation over nanodispersed Pd/Co3O4 catalysts

Abstract During the last two decades the Electrochemical Promotion of Catalysis (EPOC) phenomenon has been studied extensively for many catalytic reactions including hydrocarbon oxidation reactions and hydrogenations. In the majority of the studies, the catalysts/electrodes consisted of porous noble metal films (Pt, Pd, Rh) prepared, for instance, by calcination of organometallic pastes. This results in low metal dispersion and low active surface area, limiting therefore the overall catalytic activity. In view of further practical application of the EPOC phenomenon to industrial catalysts, we should be able to enhance the activity of nanodispersed materials. In this study, for the very first time, we observed an enhanced catalytic activity of a Pd nanodispersed catalyst supported on a porous Co3O4 semiconductor film. The Pd/Co3O4 composite powder was deposited on an yttria-stabilized zirconia (YSZ) solid electrolyte without the presence of an interlayer film. The observed enhancement was non-Faradaic, with Faradaic efficiency values as high as 80. The Pd/Co3O4 catalyst was characterized thoroughly by means of a wide variety of physicochemical techniques, such as TEM, SEM, TGA, ICP and BET. In addition, this material was further compared with a classical Pd/YSZ metallic film, prepared by calcination of an organometallic paste. This allows for a comparison of metal-support interactions (MSI) and the EPOC effect. Using supported catalysts as catalytic films for electrochemical promotion studies may lead to the practical utilization of EPOC in the chemical industry or in gas exhaust treatment.