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 O²⁻ 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 Δr exceeded the corresponding electrochemically controlled rate I/2F of O²⁻ transfer through the solid electrolyte, resulting in absolute values of the apparent faradaic efficiency Λ=Δr/(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 Λ 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.     

Electrochemical promotion of IrO2 catalyst for the gas phase combustion of ethylene

The catalytic activity of IrO₂ catalyst films for the gas phase combustion of ethylene can be increased by up to a factor of 10 via anodic polarization of the IrO₂ catalyst relative to a Au electrode both deposited on Y₂O₃-stabilized ZrO₂ solid electrolyte. The steady-state increase in the catalytic reaction rate is typically 200 higher than the expected rate increase of ethylene combustion calculated from Faraday's law. This is the first demonstration of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) using a metal oxide catalyst.     

The effect of catalyst film thickness on the electrochemical promotion of ethylene oxidation on Pt

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion of catalysis (EPOC or NEMCA effect) was investigated for the model catalytic reaction of C₂H₄ oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement ρ is up to 400 for thinner (0.2 μm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 μm) films. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O²⁻ species which causes electrochemical promotion.     

Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds

Catalytic oxidations of volatile organic compounds (VOCs) (benzene, toluene and o-xylene) over 1 wt% Pd/γ-Al₂O₃ catalyst were carried out to assess the properties and performance of the Pd based catalyst. The properties of the prepared catalysts were characterized by the Brunauer Emmett Teller (BET) surface area, H₂ chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. The experimental results revealed a significant increase in VOCs conversion with the lapse of the reaction time at certain reaction temperatures. On the other hand, the hydrogen pretreated 1 wt% Pd/γ-Al₂O₃ catalyst, whose shape of conversion curve is similar to the non pretreated catalyst, led the conversion curves for the total oxidation of VOCs to be shifted to lower temperature. It was also found that such increases in VOCs conversion were highly dependent on the oxidation state of Pd and the growth of Pd particles in the catalyst. In addition, in the case of the catalyst consisting of the same oxidation state (PdO/Pd²⁺ or Pd⁰), the particle sizes possibly play a more important role in the catalytic activity. The activity order of 1 wt% Pd/γ-Al₂O₃ catalyst with respect to the VOC molecule was o-xylene > toluene > benzene.     

Mechanism of low-temperature CO oxidation on a model Pd/Fe2O3 catalyst

A model Pd/Fe₂O₃ catalyst prepared by the vacuum technique has been studied in the carbon monoxide oxidation in the temperature range of 300–550 K at reagent pressures P(CO)=16 Torr, P(O₂)= 4 Torr. It has been shown that the activity of the fresh catalysts is determined by palladium. According to the XPS data, the reduction with carbon monoxide results in the formation of Fe²⁺ (formally Fe₃O₄) and appearance of the catalytic activity in this reaction at low temperatures (350 K). High low-temperature activity of the catalyst is supposed to be connected with the reaction between oxygen adsorbed on the reduced sites of the support (Fe²⁺) and CO adsorbed on palladium (CO_ads) at the metal–oxide interface.     

An XPS evidence of the effect of the electronic state of Pd on CH4 oxidation on Pd/γ–Al2O3 catalysts

In this work, we investigated the catalytic activity of 2% Pd/γ–Al₂O₃ and 2% Pd–1% Sn/γ–Al₂O₃ for CH₄ oxidation in lean conditions in the presence and in the absence of SO₂ in the reaction feed. The catalysts were studied by the Pd3d_5/2 electron binding energy values determined by XPS analysis. Sulfates formation and/or tin addition to Pd/Al₂O₃ resulted in an increase of the Pd3d_5/2 electron binding energy. Results showed a direct relation between Pd activity for CH₄ oxidation and the degree of oxidation of Pd species.     

The active phase of Au–Pd/Al2O3 for CO oxidation

Au–Pd/Al₂O₃ catalyst was prepared by modified impregnation method. It was found that the catalyst calcined in air at 473 K showed higher CO oxidation activity in comparison with the catalysts treated at other temperature. Nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) techniques were employed to study the relationship between the surface/bulk structures of these catalysts and their catalytic performance. The results indicated the higher activity was attributed to the smaller pore volume and co-existence of PdO and Au⁰ in their surface. The formation of Au_xPd_y alloy was unfavorable for the catalytic reaction.     

Sulphide-induced deactivation of Pd/Al2O3 as hydrodechlorination catalyst and its oxidative regeneration with permanganate

Pd-based catalysts have become important in environmental catalysis for their ability to hydrodechlorinate a wide range of chlorinated organic contaminants in water under ambient conditions. The success of their application in the remediation practice, e.g. for groundwater treatment, is often hindered by the sensitivity of Pd to poisoning by sulphur compounds. In this study, the stability and sulphide-induced deactivation behaviour of a highly active Pd/Al₂O₃ catalyst was investigated. The specific activity of Pd for the hydrodechlorination of chlorobenzene corresponds to rate coefficients up to k_Pd = 350 L g⁻¹ min⁻¹. The totally deactivated catalyst, resultant of sulphide poisoning, was regenerated with potassium permanganate. The pH value, as a key parameter which may influence the degree of deactivation as well as the efficiency of catalyst regeneration, was evaluated. Results show that in clean water the Pd/Al₂O₃ catalyst showed no inherent deactivation regardless of the ageing time and the pH value of the catalyst suspension. The degree of catalyst poisoning effected by 1.8–5.4 μM sulphide, corresponding to molar ratios of S:Pd_surface = 1.5–8.5, was observed to be higher under neutral and alkaline than under acidic conditions. The exposure of the catalyst to higher sulphide concentration of 14.2 μM resulted in complete catalyst deactivation regardless of the pH conditions. However, the efficacy of permanganate as oxidative regenerant for the fouled catalyst showed strong pH-dependence. A regeneration time of 10–30 min at low pH was sufficient to recover completely the high catalytic activity of Pd/Al₂O₃ for the hydrodechlorination reaction.     

Electrochemical promotion of Rh catalyst in gas-phase reduction of NO by propylene

The concept of non-faradaic electrochemical modification of catalytic activity (NEMCA) has been applied for the in situ control of catalytic activity of a rhodium film deposited on YSZ (yttria stabilized zirconia) solid electrolyte towards reduction of 1000 ppm NO by 1000 ppm C₃H₆ in presence of excess (5000 ppm) O₂ at 300 °C. A temporary heating at this feed composition results in a long-lasting deactivation of the catalyst under open circuit conditions due to partial oxidation of the rhodium surface. Positive current application (5 A) over both the active and the deactivated catalysts gives rise to an enhancement of N₂ and CO₂ production, the latter exceeding several hundred times the faradaic rate. While active rhodium exhibits a reversible behaviour, electrochemical promotion on the deactivated catalyst is composed of a reversible and an irreversible part. The reversible promotion results from the steady-state accumulation of current-generated active species at the gas exposed catalyst surface whereas the irreversible effect is due to the progressive reduction of the catalyst resulting in an increased recovery rate of lost catalytic activity. The results are encouraging with respect to application of rhodium for the catalytic removal of NO from auto-exhaust gases under lean-burn conditions.     

Direct formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in aqueous acidic reaction medium over halide-containing Pd/SiO2 catalytic system

Formation of H₂O₂ from H₂ and O₂ and decomposition/hydrogenation of H₂O₂ have been studied in aqueous acidic medium over Pd/SiO₂ catalyst in presence of different halide ions (viz. F⁻, Cl⁻ and Br⁻). The halide ions were introduced in the catalytic system via incorporating them in the catalyst or by adding into the reaction medium. The nature of the halide ions present in the catalytic system showed profound influence on the H₂O₂ formation selectivity in the H₂ to H₂O₂ oxidation over the catalyst. The H₂O₂ destruction via catalytic decomposition and by hydrogenation (in presence of hydrogen) was also found to be strongly dependent upon the nature of the halide ions present in the catalytic system. Among the different halides, Br⁻ was found to selectivity promote the conversion of H₂ to H₂O₂ by significantly reducing the H₂O₂ decomposition and hydrogenation over the catalyst. The other halides, on the other hand, showed a negative influence on the H₂O₂ formation by promoting the H₂ combustion to water and/or by increasing the rate of decomposition/hydrogenation of H₂O₂ over the catalyst. An optimum concentration of Br⁻ ions in the reaction medium or in the catalyst was found to be crucial for obtaining the higher H₂O₂ yield in the direct synthesis.     

Electrochemical promotion of RuO2 catalysts for the combustion of toluene and ethylene

The electrochemical promotion of the complete catalytic oxidation of ethylene and toluene on RuO₂Catalyst films deposited on Y₂O₃-stabilized-ZrO₂ (YSZ) has been investigated at temperatures 400 to 500°C. Anodic polarization (1.5 V), i.e. O^2– supply from the YSZ support to the catalyst, is found to enhance the rates of ethylene and toluene oxidation by a factor of 10 and 8 respectively. Cathodic polarization (–1.5 V) i.e. oxygen removal from the catalyst, enhances the oxidation rates by a factor of 3 and 4, respectively. These rate enhancements are strongly nonFaradaic. The kinetics, positive order in both reactants, and the promotional results, inverted-volcano type for both reactions, conform to the recently found rules of electrochemical promotion.*To whom correspondence should be addressed. E-mail: cat@chemeng.upatras.gr     

Electrocatalysis and electrochemical promotion of CO oxidation in PEM fuel cells: the role of oxygen crossover

The electrochemical promotion of catalysis (or NEMCA effect) was studied for the CO oxidation and water gas shift reaction on a Pt anode in a polymer electrolyte membrane (PEM) fuel cell. It was found that this phenomenon plays a significant role in a normal fuel cell operation (fuel mixture – air) but not in a hydrogen pumping operation (fuel mixture – H₂). This implies that the role of oxygen crossover in the electropromotion (EP) of CO oxidation is vital. During fuel cell operation, the increase in the rate of CO consumption is 2.5 times larger than the electrochemical rate, I/2F of CO oxidation, while for oxygen bleeding conditions (fuel mixture + O₂−air) the increase is five times larger than I/2F. This shows that the catalytic properties of the Pt anode are significantly modified by varying the catalyst potential. In order to confirm the role of oxygen crossover, Nafion membranes (117, 1135) with different thickness, were studied. The results show that upon decreasing the membrane thickness the crossover is increased and thus the electrochemical promotion effect becomes more pronounced.     

A Two-Step Reaction Mechanism of Oxygen Release from Pd/CeO2: Mathematical Modelling Based on Step Gas Concentration Experiments

A mathematical model has been developed to study the transient release of oxygen from a 1wt% Pd/CeO₂ catalyst in the 450–550 °C range based on alternate step concentration switches between CO and O₂. A two-step reaction mechanism that involves the reaction of gaseous CO with the oxygen species of PdO and of the back-spillover of oxygen from ceria to the oxygen vacant sites of surface PdO has been proven to better describe the CO and CO₂ transient response curves. The proposed mathematical model allows the estimation of the transient rates of the CO oxidation reaction and of the back-spillover of oxygen process. It also allows the calculation of the intrinsic rate constant k ₁ (s^–1) of the Eley–Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO₂. An activation energy of 10.1 kJ/mol was estimated for this elementary reaction step. In addition, an apparent rate constant k ₂ ^app (s^–1) was estimated for the process of back-spillover of oxygen.     

Electrochemically Induced Oscillations of C2H4 Oxidation Over Thin Sputtered Rh Catalyst Films

The electrochemically promoted induction of self-sustained catalytic rate and potential oscillations during C₂H₄ oxidation was studied over sputtered Rh thin (40 nm catalyst films interfaced with ZrO₂ (8 mol% Y₂O₃). The reaction rate oscillates simultaneously with the catalyst potential, and always in the opposite direction. The electrochemically induced oscillatory rate is typically 60 times larger than the open-circuit catalytic rate and 1000 times larger than the electrochemical rate of O²⁻ supply to the catalyst. The phenomenon is completely reversible and only observed under anodic polarization where the rate oscillates between the values corresponding to metallic Rh and surface Rh₂O₃. The oscillations are caused by the electrochemically controlled backspillover of O²⁻ to the catalyst surface and the concomitant, via repulsive lateral interactions, decomposition of surface rhodium oxide followed by surface reoxidation to Rh₂O₃ by gaseous O₂.     

Synthesis of a highly active carbon-supported Ir–V/C catalyst for the hydrogen oxidation reaction in PEMFC

The active, carbon-supported Ir and Ir–V nanoclusters with well-controlled particle size, dispersity, and composition uniformity, have been synthesized via an ethylene glycol method using IrCl₃ and NH₄VO₃ as the Ir and V precursors. The nanostructured catalysts were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The catalytic activities of these carbon-supported nanoclusters were screened by applying on-line cyclic voltammetry and electrochemical impedance spectroscopy techniques, which were used to characterize the electrochemical properties of fuel cells using several anode Ir/C and Ir–V/C catalysts. It was found that Ir/C and Ir–V/C catalysts affect the performance of electrocatalysts significantly based on the discharge characteristics of the fuel cell. The catalyst Ir–V/C at 40 wt.% displayed the highest catalytic activity to hydrogen oxidation reaction and, therefore, high cell performance is achieved which results in a maximum power density of 563 mW cm⁻² at 0.512 V and 70 °C in a real H₂/air fuel cell. This performance is 20% higher as compared to the commercial available Pt/C catalyst. Fuel cell life test at a constant current density of 1000 mA cm⁻² in a H₂/O₂ condition shows good stability of anode Ir–V/C after 100 h of continuous operation.     

Steady-state multiplicity phenomena during the electrochemical promotion of NO reduction by C2H4 in presence of O2 on thin Rh and Pt catalyst-electrodes in a monolithic electropromoted reactor

The electrochemical promotion of catalysis (EPOC) was used to promote the selective reduction of NO by hydrocarbons in presence of oxygen using thin (40 nm) porous Rh and Pt catalyst layers sputtered on the opposite surfaces of thin (0.25 mm) solid electrolyte (YSZ) plates serving as electrocatalytic elements of a monolithic electrochemically promoted reactor (MEPR). Using 22 Rh/YSZ/Pt type cells it was found that the reduction of NO in presence of 1.1 kPa O₂ and 0.36 kPa C₂H₄ can be efficiently electropromoted with 340% rate enhancement, reaching 95% NO conversion with 100% selectivity to N₂ in the temperature range from 280 to 340 °C. The apparent Faradaic efficiency is larger than unity for both the NO reduction and the C₂H₄ oxidation reaction.At elevated temperatures (≥300 °C) and high reactant conversions it was found that after current interruption, the catalytic rates do not return to their initial values but remain in a new highly active steady state. It appears that this highly active state is not a genuine intrinsic permanent NEMCA state but is manifestation of steady-state multiplicity in the monolithic reactor resulting from near complete gaseous O₂ consumption. Thus the low and high activity steady states corresponding to zero applied potential appear to correspond to high and low average P_O2 in the reactor. The latter is the result of the near complete reactant conversion under the preceding electropromoted operation. These highly active permanent NEMCA states may be quite useful for practical applications.     

Catalysts Ru–CeO2/Sibunit for catalytic wet air oxidation of aniline and phenol

Catalytic properties of carbon materials Sibunit (commercial samples) and ceria-promoted precious metals (Ru, Pt, Pd) supported on carbon were studied in the processes of catalytic wet air oxidation (CWAO) of aniline and phenol at elevated pressures and temperatures (T =433–473 K, P_O2 = 0.3–1.0 MPa). It was found that the activity increases when the catalyst is pretreated with hydrogen peroxide. An efficiency of Ru–CeO₂/Sibunit catalyst with a low ruthenium content (~0.6 Ru) for deep cleaning of polluted waters is demonstrated.     

Development of Monolithic Catalysts with Low Noble Metal Content for Diesel Vehicle Emission Control

Monolith washcoated catalysts with potential for diesel emission control have been developed. Two types of catalysts have been prepared for further study: (1) MnO _x supported on granulated -Al₂O₃, (2) MnO _x supported on cordierite monolith washcoated with -Al₂O₃. Both catalysts have been calcined at 500 and 900 °C and subsequently modified by doping with 0.1–1.0 wt% of Pt or Pd. The influence of the concentration of both manganese oxide (0–10 wt%) and noble metals Pt and Pd in the range 0–1.0 wt% on the catalytic activity in methane oxidation has been studied. Comparison of the catalytic activity of MnO _x /Al₂O₃ and MnO _x + Pt(Pd)/Al₂O₃ with that of a standard 1 wt%Pt/Al₂O₃ catalyst shows the existence of a synergetic effect. This effect is more pronounced for the samples calcined at 900 °C. The developed monolithic catalysts MnO _x + Pt(Pd)/Al₂O₃ demonstrate higher activity and thermal stability (up to 900 °C) compared to the commercial monolithic catalyst (TWC's).     

Single- and double-stage catalytic preferential CO oxidation in H2-rich stream over an α-Fe2O3-promoted CuO–CeO2 catalyst

Single- and double-stage catalytic preferential CO oxidation (CO-PrOx) over-Fe₂O₃-promoted CuO–CeO₂ in a H₂-rich stream has been investigated in this work. The catalyst was prepared by the urea-nitrate combustion method and was characterized by X-ray diffractometer (XRD), X-ray fluorescence (XRF), Brunauer–Emmet–Teller (BET), transmission electron microscope (TEM), and scanning electron microscope (SEM). The catalytic activity tests were carried out in the temperature range of 50–225 °C under atmospheric pressure. The results of the single-stage reaction indicated that complete CO oxidation was obtained when operating at a O₂/CO ratio of 1.5, W/F ratio of 0.36 g s/cm³, and at a reaction temperature of 175 °C. At these conditions, H₂ consumption in the oxidation was estimated at 58.4%. Applying the same conditions to the double-stage reaction, complete CO oxidation was found and H₂ consumption in the oxidation was reduced about 4.9%. When decreasing the double-stage reaction temperature to 150 °C, the results elucidated that CO could be converted to CO₂ completely while H₂ consumption in the oxidation was further reduced to 33.5%. A temperature blocking 2² factorial design has been used to describe the importance of the factors influencing the catalytic activity. The factorial design was according to the experimental results. When adding CO₂ and H₂O in feed, reduction of CO conversion for single- and double-stage reaction is obtained due to a blocking of CO₂ and H₂O at a catalytic active site. Comparing CO conversion obtained when operating with/without CO₂ and H₂O in feed for single- and double-stage reaction, less reduction is achieved when operating in double-stage reaction.     

From single crystals to supported nanoparticles in experimental and theoretical studies of H2 oxidation over platinum metals (Pt, Pd): Intermediates, surface waves and spillover

The present paper reviews our investigations concerning the mechanism of H₂ + O₂ reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti³⁺/TiO₂). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H₂ oxidation on a nanolevel. The adsorption of both O₂ and H₂ and several dissociative products (H_ads, O_ads, OH_ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H₂O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti³⁺-□_O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H₂ + O₂ steady-state reaction over Pd–Ti³⁺/TiO₂ surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H⁺/TiO_x) as a result of spillover effect: diffusion of H_ads atoms from Pd particles on a TiO_x surface. The comprehensive study of H₂, O₂ adsorption and H₂ + O₂ reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces.