The effect of electrochemical oxygen pumping on the rate of CO oxidation on Au electrode-catalyst

The effect of electrochemical pumping of oxygen on the rate of carbon monoxide oxidation on Au electrode-catalyst in a solid oxygen conducting electrolyte cell has been demonstrated. The induced change in the reaction rate at the cathodic polarization of an Au electrode was an order of magnitude higher than the rate of O^2– pumping from the reaction zone through the electrolyte. The anodic polarization of the Au electrode (O^2– pumping to the reaction zone through the electrolyte) caused purely Faradaic changes in the reaction rate.     

Oxidation of methane over platinum in a solid proton-conducting electrolyte cell

Studies of the complete oxidation of methane on a Pt electrode-catalyst in the cell with a solid proton-conducting electrolyte (CH₄ + O₂, Pt ¦ SrCe_0.92Dy_0.08O₃ ¦ Pt, H₂O + N₂) were carried out. The non-Faradaic effect of electrochemical hydrogen pumping on the rate of methane oxidation has been demonstrated. The induced change in the reaction rate at anodic polarization of a Pt electrode-catalyst was over two orders of magnitude higher than the rate of hydrogen pumping from the reaction zone through the electrolyte.     

High-temperature oxidation of aluminium in various gases

The oxidation of high-purity aluminium sheet in dry oxygen, moist oxygen, carbon dioxide and carbon monoxide (at total pressure 1.333 × 10³ Nm^?2) was studied in the range 673–923°K, using a vacuum microbalance to follow weight gains. ¹⁴CO₂ and ¹⁴CO were used to elucidate the mechanism of the oxidation in these gases and to estimate the extent of carbon deposition in the oxide layer. The rate of oxidation in moist oxygen was similar to that in dry oxygen, the principle reaction being 2Al + 3H₂O ← Al₂O₃ + 3H₂. It is suggested that there are three steps in the reaction in CO₂, viz. 2Al+3CO₂ ← Al₂O₃ + 3CO, followed by 2Al + 3CO ← Al₂O₃ + 3C, and about 10% of the deposited carbon reacting further by 4Al + 3C ← Al₄C₃. Only the last two reactions are operative in carbon monoxide. The Arrhenius plots show a distinct break in the region 773–823°K for both carbon monoxide and carbon dioxide, but not for dry or moist oxygen. This is tentatively explained by a change in the rate-determining process from diffusion via grain boundaries or cracks in the oxide, to lattice diffusion. It is suggested that carbon may become mobile in the oxide film between 773 and 823°K and may tend to congregate in the grain boundaries and cracks. The oxide film remained protective throughout the duration of the experiments in all the gases.     

Kinetics of heterogeneous reactions of carbon and oxygen during combustion of porous carbon particles in oxygen

A model of combustion of a high-porosity carbon particle in oxygen is considered, which takes into account heterogeneous and homogeneous chemical reactions inside the particles and radiative heat transfer. The boundaries of the domain where the burning rate depends on the particle temperature are determined. The possibility of two combustion regimes is demonstrated: regime with a high burning rate, where the carbon-oxygen reaction proceeds in a layer adjacent to the particle surface, and regime with a low burning rate, where the reaction proceeds in the entire particle volume. In the regime with a high burning rate, the main product of the reaction between carbon and oxygen is carbon monoxide, whereas both carbon monoxide and carbon dioxide can be formed in the regime with a low burning rate. The kinetic equations of heterogeneous reactions C + O₂ = CO₂ and 2C + O₂ = 2CO are determined, which reveal the retarding effect of carbon monoxide and dioxide on the rates of these reactions. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 11–22, May–June, 2006.     

Synthesis gas production in methane conversion using the P|yttria-stabilized zirconia|Ag electrochemical membrane system

The electrochemical membrane reactor of YSZ (yttria-stabilized zirconia) solid electrolyte coated with Pd and Ag as anode and cathode, respectively, has been applied to the partial oxidation of methane to synthesis gas (CO + H₂). The Pd|YSZ|Ag catalytic system has shown a remarkable activity for CO production at 773 K, and the selectivity to CO was quite high (96.3%) under oxygen pumping condition at 5 mA. The H₂ production strongly depended on the oxidation state of the Pd anode surface. Namely, the H₂ treatment of the Pd anode at 773 K for 1 h drastically reduced the rate of H₂ production, while air treatment enhanced the H₂ production rate. From the results of the partial oxidation of CH₄ with molecular oxygen, it is considered that the reaction site of the electrochemical oxidation of CH₄ to synthesis gas was the Pd–YSZ–gas-phase boundary (triple-phase boundary). In addition, it is found that the oxygen species pumped electrochemically over the Pd surface demonstrated similar activity to adsorbed oxygen over Pd, PdO_ad, for the selective oxidation of CH₄ to CO, when the Pd supported on YSZ was used as a fixed-bed catalyst for CH₄ oxidation with the adsorbed oxygen. The difference with respect to the H₂ formation between the electrochemical membrane system and the fixed-bed catalyst reactor results from differences in the average particle size of Pd and the way of the oxygen supply to the Pd surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics of the reaction of oxygen with carbon monoxide adsorbed on palladium

A study of the kinetics of the reaction of adsorbed carbon monoxide with oxygen on polycrystalline palladium is reported in which a pressure jump method was used to induce transients in the carbon dioxide production. Through an analysis of these transients under a variety of conditions of temperature and oxygen pressure, some details of the kinetics have been delineated. At relatively low temperatures and under a significant O₂ pressure, CO(a) is desorbed more readily as CO₂, via the reaction CO(a) + O(a) → CO₂, than as CO. The reaction is first order in oxygen and the rate is limited by the rate of adsorption of oxygen onto sites which are in close proximity to CO(a). Oxygen adsorption at sites which are further than a critical distance from CO(a) are unreactive. The critical distance increases with temperature reflecting increased mobility. Under conditions where both CO(a) and O(a) are significant and both CO(g) and O₂(g) are small the rate is limited by the mobility of CO(a) and/or O(a). The amount of CO(a) during the course of the steady-state oxidation reaction can be determined by analyzing the transient CO₂ production which occurs following a pressure jump in carbon monoxide.     

Die H2S-oxidation an aktiver kohle—ein elektrochemischer prozess?

Proceeding from the observation that a galvanic cell can be formed with activated carbon as electrode material and aqueous solutions of H₂S and oxygen, a mechanism is submitted for discussion, according to which the oxidation of H₂S catalyzed by activated carbon takes place by this very formation of cells on the microscopic surfaces of the carbon. The known effects of H₂O, H⁺ ions, as well as Fe and I content of the activated carbon on catalysis may be explained by this model.The catalytic inefficiency of dry activated carbon with respect to the oxidation of H₂S with oxygen-containing gases at ambient temperature is due to the fact that electrolyte is indispensable for electrochemical processes. Carbon hexagon layers with reversibly chemisorbed O₂ behave as oxygen electrode while layers with adsorbed H₂S act as fuel electrode. Only after addition of electrolyte elemental sulphur can separate on the fuel electrode.The pH value can drop far below pH 2 due to H₂SO₄ formed in a side reaction. With such acid concentrations, H₃S⁺ ions are increasingly formed. Since H₃S⁺ is no longer accessible to electrochemical oxidation, catalysis is retarded within these pH ranges.Iron bonded in complex form to the layers behaves as an electron acceptor. By this, the layers and the adsorbed H₂S are positively polarized which allows oxidation of the H₂S to sulphuric acid. The negatively polarized iron assumes the function of the oxygen electrode.Iodine blocks the direct access of H₂S to the layer or fuel electrode respectively. By direct chemical reaction, also within the strongly acidic range, elemental iodine reacts with H₂S forming elemental sulphur. The iodide ions formed are subject to electrochemical oxidation to I₂ on the carbon layer.The special advantage of the electrochemical oxidation mechanism is that the O₂ and H₂S molecules must not necessarily be in direct contact.     

Oxygen transfer and carbon gasification in the reaction of different carbons with CO2

The oxidation of carbon in CO₂-CO mixtures can be discussed assuming the mechanism CO₂ = CO + O (adsorbed) (1) O (adsorbed) + C → CO (2) This treatment is supported and supplemented by measurements of both partial reactions at different carbons, electrode graphite, charcoal, natural graphite and iron doped graphite. From the kinetics it can be seen that the reactions (1) and (2) predominantly take place at different sites. Nevertheless the kinetics of both reactions are correlated, since a uniform oxygen activity is established at the carbon surface by the interplay of the oxygen surface diffusion and of both reactions. The rate of reaction (2) and accordingly the rate of the overall reaction is proportional to the stationary oxygen activity which can be calculated from the steady state condition for oxygen adsorption. Several rate equations are derived for the overall reaction at special reaction conditions and for different carbons and it is shown that no general explicit rate equation can be given for the oxidation of carbon in CO₂.     

Oxidation of methane on a Au + SrFeO3−δ//YSZ electrode characterised by mass spectroscopy and 18O2 pulses

A solid state electrochemical reactor is described in which reactants can be oxidised at high temperatures over an anode/catalyst using co-fed oxygen gas as well as electrochemically supplied oxygen. The setup permits injection of isotopic pulses in the reactant streams. The composition and isotopic distribution in the products are recorded with a quadrupole mass spectrometer. The use of the system is exemplified by oxidation of methane over a Au + SrFeO_3?δ//YSZ anode at 800–850°C. Pulses of ¹⁸O₂ in the stream of co-fed O₂ were used to study the reactivity and products of gaseous oxygen as distinguished from the electrochemically supplied oxygen. The results indicate that the anode used supports oxygen pumping, but is only moderately active for methane oxidation. The products are mainly CO and CO₂. The content of ¹⁸O in the products is low, indicating that methane oxidation takes place by ¹⁶O-rich lattice oxygen. In comparison, a reference Au//YSZ electrode was found to be a slower anode for oxygen pumping, but a better catalyst for the reaction between CH₄ and gaseous O₂, seemingly involving adsorbed oxygen.     

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.     

Cathodic polarization characteristics of the oxygen electrodes/stabilized Bi2O3 solid electrolyte interface

A study has been conducted on the effects of electrode materials and electrolyte stabilizers on the kinetics of the oxygen cathodic reaction at the oxygen electrode/stabilized Bi₂O₃ electrolyte interface in the temperature range of 450-700°C under dry N₂+20% O₂ gas mixture. The La_l-xM_xCoO₃ electrodes prepared by the thermal decomposition method employing the ultrasonic technique and the Ag electrodes by the rf magnetron sputtering method were used as the oxygen electrodes. The electrode/electrolyte interface behaviour was investigated by means of dc and ac measurements. The electrolyte polarization or iR drop could be distinguished from the electrode polarization using an ac method and a current interruption method with the aid of a reference electrode.The electrode process is significantly influenced by the nature of the electrode/electrolyte interface. The dc and ac polarization properties for the system of (Bi₂O₃)_0.77(Y₂O₃)_0.23/La_0.5Sr_0.5CoO₃ electrode prepared by the thermal decomposition method using the ultrasonic technique were more favorable than that of the other electrode/electrolyte systems. The impedance results for the La_0.5Sr_0.5CoO₃ electrode system were different from those for the Ag-sputtered electrode system. It can be considered that the differences are due to the interaction condition between an electrode and an electrolyte, ie the electrochemical matching condition.The reaction mechanisms were also investigated. In the La_0.5Sr_0.5CoO₃ electrode system, the diffusion process of O⁻_ad and/or V^{+ +}₀ along and/or through the interface, respectively, and in the Ag electrode system the O⁻₂ migration process along the interface play an important role in determining the reaction kinetics.     

Heterogeneously assisted oxidation of adsorbates from carbonmonoxide, methanol and ethanol by hydrogen peroxide solutions on platinum electrodes in sulphuric acid

The influence of hydrogen peroxide on the adsorption and oxidation of carbon monoxide, methanol and ethanol adlayers on porous Pt electrodes were studied in 2 M sulphuric acid solution by means of cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). The oxidation of adsorbed species is observed at electrode potentials far less negative than those required for electrochemical adsorbate oxidation. The oxidation by H₂O₂ is dependent on its concentration in solution, as well as on the adsorbates and their coverages. In all cases the isolated adlayers are oxidised by dissolved H₂O₂. However, the presence of H₂O₂ during adsorption partially inhibits adlayer formation from CH₃OH and C₂H₅OH, but avoids almost completely the adsorption of carbon monoxide. The removal of the residues from the surface by dissolved hydrogen peroxide probably occurs through O_ad species formed during the heterogeneous decomposition reaction of H₂O₂ on Pt.     

Novel oxidation reaction at ambient temperature and atmospheric pressure with electric discharge and oxide surface

We investigated a novel oxidation reaction with surface-oxygen and lattice-oxygen induced using a non-equilibrium electric discharge at ambient temperature. We employed MgO, ZrO₂, and TiO₂ for this novel reaction. Methane was oxidized easily and converted into H₂, CO, and CO₂ by the surface-oxygen and lattice-oxygen of oxide with activation of discharge at ambient temperature without gas-phase oxygen. The oxide itself was stable after the reaction. Among these oxides, the tetragonal phase and amorphous phase of ZrO₂ showed remarkably high activity for methane oxidation. Consequently, up to 8% of surface and lattice oxygen of the oxide was consumed by methane oxidation induced by electric discharge. The non-equilibrium electric discharge activated both the surface-oxygen and the lattice-oxygen of the oxides and methane molecules in the gas phase. After these reactions, the oxide surface vacant sites were recovered partially through steam post-treatment. Hydrogen formed simultaneously with steam decomposition. Other reactions were also studied by changing the reaction gas: methane into carbon monoxide, carbon monoxide with oxygen, and carbon monoxide with steam. Furthermore, the correlation of reactivity between the feed gas and surface oxygen was studied. Emission spectra under a CH₄ atmosphere with electric discharge showed complex peaks caused by carbon monoxide formation at 280-500 nm at 0-4 min, suggesting that surface oxygen on oxides was probably consumed within 4 min from the start of the reaction.     

Pt–Ru nanoparticles supported on functionalized carbon as electrocatalysts for the methanol oxidation

Platinum–ruthenium alloy electrocatalysts, for methanol oxidation reaction, were prepared on carbons thermally treated in helium atmosphere or chemically functionalized in H₂O₂, or in HNO₃ + H₂SO₄ or in HNO₃ solutions. The functionalized carbon that is produced using acid solutions contains more surface oxygenated functional groups than carbon treated with H₂O₂ solution or HeTT. The XRD/HR-TEM analysis have showed the existence of a higher alloying degree for Pt–Ru electrocatalysts supported on functionalized carbon, which present superior electrocatalytic performance, assessed by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy, as compared to electrocatalysts on unfunctionalized carbon. It also was found that Pt–Ru alloy electrocatalysts on functionalized carbon improve the reaction rate compared to Pt–Ru on carbons treated with H₂O₂ solution and thermally. A mechanism is discussed, where oxygenated groups generated from acid functionalization of carbon and adsorbed on Pt–Ru electrocatalysts are considered to enhance the electrocatalytic activity of the methanol oxidation reaction.     

CO oxidation on a Cu(100) catalyst

The oxidation of carbon monoxide by molecular oxygen on a single crystal Cu(100) catalyst was studied at 458 K using reactant gas mixtures with CO/O₂ ratios of 2/1, 10/1 and 25/1 at a total pressure of 10 Torr. The catalytic activities were found to be strongly dependent upon the CO/O₂ ratio. Under stoichiometric reaction conditions (CO/O₂ = 2), the initial CO oxidation activity decreased sharply; with a highly reducing reaction mixture (CO/O₂ = 25), the initial activity gradually increased. These changes in catalytic activities with reactant gas mixture composition correlate with changes in surface composition, namely an increase in the surface oxygen coverage. Post-reaction TPD revealed the presence of a carbonate-like species which decomposed at ca. 630 K.     

Potentiometric measurements of oxygen electrodes in molten sodium sulphate

The dependence of the potential of platinum and gold electrodes in molten Na₂SO₄ on the composition of surrounding atmospheres of SO₂ + O₂, and of N₂ + O₂ has been investigated at 900°. The electrode potential dependence on the oxide ion concentration in the electrolyte in an oxygen atmosphere is also reported. It is postulated that the potential of O₂(Au) and O₂(Pt) electrodes in basic molten sulphate is controlled by the reaction: $\text{1}\text{2}$O₂ + O^2? = O^2?₂. It is thought that the sulphur dioxide electrode reaction is very complex and further studies are required to elucidate its mechanism. However, in the light of the present work it seems reasonable to consider this electrode as an oxygen electrode which also responds to SO₂ and SO₃.     

Performance of CuO/Al2O3 Catalysts Promoted by SnO2 and ZrO2 in the Selective Oxidation of Carbon Monoxide

The effect of added SnO₂ and ZrO₂ to CuO/Al₂O₃ catalysts was investigated with reference to the oxygen spillover phenomena in the selective oxidation of carbon monoxide. The TPR and TPO analyses indicated that SnO₂ and ZrO₂ addition caused oxygen migration and induced the formation of high concentrations of active oxygen species on the SnO₂ and ZrO₂ surface. The catalytic activities of SnO₂ and ZrO₂ supported CuO/Al₂O₃ catalysts were superior to that for CuO/Al₂O₃ catalysts in the selective oxidation of carbon monoxide. Oxygen, when absorbed to the SnO₂ and ZrO₂ surface can spill over to the CuO phase and easily react with carbon monoxide. Consequently, the addition of SnO₂ and ZrO₂ led to significantly improved activities. This can be attributed to the enhanced migration of oxygen to the catalyst surface.     

Reaction pathways and poisons—II: The rate controlling step for electrochemical oxidation of hydrogen on Pt in acid and poisoning of the reaction by CO

The reaction mechanism for hydrogen molecule oxidation on platinum electrocatalysts in acid solutions is deduced by comparing kinetic rate parameters obtained electrochemically, to those rate parameters obtained in the gas phase from H₂—D₂ exchange. The electrochemical rate parameters were obtained from potentiodynamic scanning of smooth platinum and from polarisation curves of porous platinum black flooded electrode structures. The rate controlling step for hydrogen molecule oxidation on Pt is the dual site dissociative chemisorption of the hydrogen molecule H₂ → 2H (known as the Tafel reaction). Specific poisons for this reaction are chemisorbed carbon monoxide and adsorbed hydrogen, producing a simple site elimination for dissociative chemisorption of the hydrogen molecule and confirm the dual site mechanism. The electrochemical reaction rate parameters are the same on smooth platinum, unsupported platinum black and platinum crystallites supported on graphitised carbon, correlating with the H₂-H₂ exchange in the gas-phase, both with and without chemisorbed carbon monoxide and are dependent only upon the surface areas of platinum catalysts.     

A comparative investigation of chromium deposition at air electrodes of solid oxide fuel cells

Deposition processes of chromium (Cr) species were investigated for the O₂ reduction on (La,Sr)MnO₃ (LSM), Pt and (La,Sr)(Co,Fe)O₃ (LSCF) electrodes in the presence of chromia-forming alloy metallic interconnect at 900^oC under air flow. For the reaction on LSM electrodes, deposition of Cr species preferentially occurred on the zirconia electrolyte surface, forming a distinct deposit ring at the edge of the LSM electrode while at LSCF electrodes, Cr species deposited on the electrode and electrolyte surface, forming isolated Cr particles. In contrast, there was no detectable deposition of Cr species either on the electrode or electrolyte surface for the O₂ reduction reaction on Pt electrodes. The results clearly demonstrated that deposition of Cr species in solid oxide fuel cells is not an electrochemical reduction of high valent Cr vapor species to Cr₂O₃ in competition with O₂ reduction. Cr deposition at SOFC cathodes is basically a chemical dissociation reaction and is controlled by the nucleation reaction between the nucleation agent and the gaseous Cr species. The nature of the nucleation agent strongly depends on the electrode material and impurities which may be introduced during electrode and electrolyte fabrication processes.     

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.