Characterization and activity of vanadia-promoted Pt/ZrO2 catalysts for the water–gas shift reaction

Pt/ZrO₂ catalysts for the water–gas shift (WGS) were promoted with various amounts of vanadia. Analyses by XRD, N₂ adsorption, Raman, and UV–vis DRS showed that vanadia is present below monolayer coverage as monovanadate and polyvanadate, with the former dominating at lower loadings, and that following monolayer formation, VO₅ species appear, with the eventual generation of V₂O₅ and ZrV₂O₇ for a vanadia weight loading of 13%. Though in all cases vanadia induced an enhancement in WGS activity, the best catalyst, that contained 3 wt.% of vanadia, gave a rate that was nearly double that of the unpromoted Pt/ZrO₂. That superior global activity probably results from the monovanadate that is the main species at low loadings. It is believed that monovanadate promotes the WGS by rendering the support's surface more oxidizing through its VOZr bonds.     

Additive treatment effect of TiO2 as supports for Pt-based electrocatalysts on oxygen reduction reaction activity

In this study, we investigated the additive treatment effect of TiO₂ as alternative support materials to common carbon black for Pt-based electrocatalysts on electrocatalytic activity for oxygen reduction reaction (ORR). The shape of TiO₂ was varied by hydrothermal treatment with various additives, such as urea, thiourea, and hydrofluoric acid. From the results of transmission electron microscopy (TEM) images and ultraviolet-visible spectroscopy (UV-vis) spectra, it was identified that the morphology of hydrofluoric acid (HF)-treated TiO₂ was changed into a round shape having lower aspect ratio than other samples, and its band gap was decreased. Notably, the electronic state of HF-treated TiO₂ support was changed into highly reduced (electron rich) state which led to the increase of ORR activity, compared to other samples treated with different additives or before treatment. The electrocatalytic characteristics changes after treatment with various additives were investigated by using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), cyclic voltammograms (CV), and rotating disk electrode (RDE) techniques.     

Dynamics and selectivity of NOx reduction in NOx storage catalytic monolith

Several nitrogen compounds can be produced during the regeneration phase in periodically operated NO_x storage and reduction catalyst (NSRC) for conversion of automobile exhaust gases. Besides the main product N₂, also NO, N₂O, and NH₃ can be formed, depending on the regeneration phase length, temperature, and gas composition. This contribution focuses on experimental evaluation of the NO_x reduction dynamics and selectivity towards the main products (NO, N₂ and NH₃) within the short rich phase, and consequent development of the corresponding global reaction-kinetic model. An industrial NSRC monolith sample of PtRh/Ba/CeO₂/ -Al₂O₃ type is employed in nearly isothermal laboratory micro-reactor. The oxygen and NO_x storage/reduction experiments are performed in the temperature range 100–500 °C in the presence of CO₂ and H₂O, using H₂, CO and C₃H₆ as the reducing agents.The spatially distributed NSRC model developed earlier is extended by the following reactions: NH₃ is formed by the reaction of H₂ with NO_x and it can further react with oxygen and NO_x deposited on the catalyst surface, producing N₂. Considering this scheme with ammonia as an active intermediate of the NO_x reduction, a good agreement with experiments is obtained in terms of the NO_x reduction dynamics and selectivity. A reduction front travelling in the flow direction along the reactor is predicted, with the NH₃ maximum on the moving boundary. When the front reaches the reactor outlet, the NH₃ peak is observed in the exhaust gas. It is assumed that the ammonia formation during the NO_x reduction by CO and HCs at higher temperatures proceed via the water gas shift and steam reforming reactions producing hydrogen. It is further demonstrated that oxygen storage effects influence the dynamics of the stored NO_x reduction. The temperature dependences of the outlet ammonia peak delay and the selectivity towards NH₃ are correlated with the effective oxygen and NO_x storage capacity.     

Effects of precursors on the surface Mn species and the activities for NO reduction over MnOx/TiO2 catalysts

TiO₂-supported manganese oxide catalysts were prepared from two different precursors, manganese nitrate (MN) and manganese acetate (MA), and these samples were characterized by BET, XRD, TG/DTA, XPS and FT–IR. The characterization results showed that the MN precursor resulted primarily in MnO₂, accompanied with some Mn-nitrate, while the MA precursor caused mainly Mn₂O₃ species. These two different precursors also led to different surface Mn atom concentrations indicated by XPS and NH₃ adsorption. Consequently, the higher low-temperature activity of MnO_x/TiO₂ from MA precursor was attributed to higher surface Mn concentration and the surface Mn₂O₃ species.     

NOx storage/reduction catalysts based in cobalt/copper hydrotalcites

The use of materials based on hydrotalcites as NO_x storage/reduction (NSR) catalysts has been investigated, examining their activity at low temperature and their resistance to poisons such as H₂O and SO₂. The results obtained show that catalysts derived from Mg/Al hydrotalcites containing copper or cobalt is active at low temperatures, specially the samples containing 10 or 15% of Co. The addition of 1 wt% of transition metals with redox properties such as Pt, Pd, V and Ru to the hydrotalcite increases its activity because the combination of the redox properties of these metals and the acid-base properties of the hydrotalcite. The best results were obtained with the catalyst derived from a hydrotalcite with a molar ratio Co/Mg/Al = 15/60/25 and containing 1 wt% V. This material shows a higher activity, at low temperatures and in the presence of H₂O and SO₂, than a Pt–Ba/Al₂O₃ reference catalyst.     

Preparation and electrochemical characteristics of CoTMPP-TiO2NT/BP composite electrocatalyst for oxygen reduction reaction

CoTMPP-TiO₂NT/BP composites have been synthesized by preparing CoTMPP and depositing CoTMPP on carbon-supported titania nanotube (TiO₂NT/BP) using microwave irradiation method at the same time, followed by heat-treatment from 300 to 900 °C in N₂ atmosphere. The catalytic activity for oxygen reduction was evaluated by rotating disc electrode technique in half cells with 0.5 M H₂SO₄. The number of electrons exchanged during ORR and the percentage of peroxide (%H₂O₂) produced by the reaction were evaluated for catalysts by rotating ring disk electrode (RRDE) measurements. The influences of TiO₂NT doping, the heat-treating temperature and the different ratios of BP:TiO₂NT on the activity of electrocatalysts for oxygen reduction were investigated. The stability of the CoTMPP-TiO₂NT/BP electrocatalysts was studied with potentialstatic-polarization measurements in 0.5 M H₂SO₄ + 0.5 M CH₃OH. CoTMPP-TiO₂NT/BP composites show higher catalytic activity and better stability than CoTMPP/BP. The mechanism for the enhanced catalytic activity of CoTMPP-TiO₂NT/BP is discussed.     

The effect of magnetic field on the oxygen reduction reaction and its application in polymer electrolyte fuel cells

The effect of magnetic field gradients on the electrochemical oxygen reduction was studied with relevance to the cathode gas reactions in polymer electrolyte fuel cells. When a permanent magnet was set behind a cathode, i.e. platinum foil or Pt-dispersed carbon paper for both electrochemical and rotating electrode experiments and oxygen was supplied to the uphill direction of the magnetic field, electrochemical flux was enhanced and the current increased with increasing the absolute value of magnetic field. This magnetic effect can be explained by the magnetic attractive force toward O₂ gas. When magnet particles were included in the catalyst layer of the cathode and the cathode was magnetized, the current of oxygen reduction was higher than that of nonmagnetized cathode. A new design of the cathode catalyst layer incorporating the magnet particles was tested, demonstrating a new method to improve the fuel cell performance.     

A comparative study of the selective oxidation of NH3 to N2 over gold, silver and copper catalysts and the effect of addition of Li2O and CeOx

This paper describes the selective oxidation of ammonia into nitrogen over copper, silver and gold catalysts between room temperature and 400 °C using different NH₃/O₂ ratios. The effect of addition of CeO_x and Li₂O on the activity and selectivity is also discussed. The results show that copper and silver are very active and selective toward N₂. However the multicomponent catalysts: M/Li₂O/CeO_x/Al₂O₃ (M: Au, Ag, Cu) perform the best. On all three metal containing catalysts the activity and selectivity is influenced by the particle size and the interaction between metal particles and support.     

High temperature phase stabilities and electrochemical properties of InBaCo4−xZnxO7 cathodes for intermediate temperature solid oxide fuel cells

InBaCo_4−xZn_xO₇ oxides have been synthesized and characterized as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFC). The effect of Zn substitution for Co on the structure, phase stability, thermal expansion, and electrochemical properties of the InBaCo_4−xZn_xO₇ has been investigated. The increase in the Zn content from x = 1 to 1.5 improves the high temperature phase stability at 600 °C and 700 °C for 100 h, and chemical stability against a Gd_0.2Ce_0.8O_1.9 (GDC) electrolyte. Thermal expansion coefficient (TEC) values of the InBaCo_4−xZn_xO₇ (x = 1, 1.5, 2) specimens were determined to be 8.6 × 10⁻⁶ to 9.6 × 10⁻⁶/°C in the range of 80–900 °C, which provides good thermal expansion compatibility with the standard SOFC electrolyte materials. The InBaCo_4−xZn_xO₇ + GDC (50:50 wt.%) composite cathodes exhibit improved cathode performances compared to those obtained from the simple InBaCo_4−xZn_xO₇ cathodes due to the extended triple-phase boundary (TPB) and enhanced oxide-ion conductivity through the GDC portion in the composites.     

Influence of Nafion film on oxygen reduction reaction and hydrogen peroxide formation on Pt electrode for proton exchange membrane fuel cell

The influence of Nafion^® film on ORR kinetics and H₂O₂ formation on a Pt electrode was investigated using RRDE in 0.1 M HClO₄. It was found that the Nafion^®-coated Pt system showed lower apparent ORR activity and more H₂O₂ production than the bare Pt electrode system. From the temperature sensitivity, it was revealed that the apparent activation energies of ORR in the Nafion^®-coated Pt system were lower than the bare Pt electrode system, and the H₂O₂ formation was suppressed with the increase of the temperature. In order to analyze the results furthermore, other systems (0.1/1.0 M, HClO₄/CF₃SO₃H) with the bare Pt electrodes were also examined as references. It was exhibited that the ORR kinetic current, the H₂O₂ formation, and the apparent activation energies of 1.0 M CF₃SO₃H system were close to those of the Nafion^®-coated Pt system. We concluded that the orientation of anion species of Nafion^® and CF₃SO₃H to the Pt surface via water molecules, as well as a fluorocarbon polymer network of Nafion^®, might block O₂ adsorption, resulting in the smaller effective surface area of the Pt electrode for ORR, the smaller ORR kinetic current, and the more H₂O₂ production.     

Immobilization of nanocatalysts on cordierite honeycomb monoliths for low temperature NOx reduction

Noble metal nanocatalysts such as Pd, Pt, and Au were strongly immobilized on the inside walls of monolithic honeycomb-structured cordierite, in which bi-functional molecules were used as linkers for anchoring noble metal nanoparticles (NPs) on the cordierite surface. The supported nanocatalysts were characterized by ICP-MS, TEM, and X-ray powder diffraction. The efficiencies of the immobilized nanocatalysts for the removal of harmful nitrogen oxides (NO_x) have been investigated by measuring the deNO_x capability as a function of temperature. The catalytic activities depend mainly on the compositions of the nanocatalysts. The Pd/Pt bi-metal catalyst anchored on the cordierite surface shows higher NO_x conversion and better activity than the commercial emission catalyst at low temperature region, which could be due to the large portion of active surface areas of the catalysts with nanometer scale.     

Fe-based catalysts for the reduction of oxygen in polymer electrolyte membrane fuel cell conditions: determination of the amount of peroxide released during electroreduction and its influence on the stability of the catalysts

Fe-based catalysts have been prepared by pyrolyzing ClFeTMPP (Cl-Fe tetramethoxyphenyl porphyrin) or Fe acetate adsorbed on PTCDA (perylene tetracarboxylic dianhydride) or on prepyrolyzed PTCDA (p-PTCDA). The catalysts which were already well characterized in terms of active FeN₄/C and FeN₂/C catalytic sites (J. Phys. Chem. B 106 (2002) 8705) are now characterized by RRDE experiments to determine the values of the apparent number of electron transferred (n) and the percentage of peroxide (%H₂O₂) released during the oxygen reduction reaction (ORR) in H₂SO₄ at pH 1. A direct correlation is found between the relative abundance of the FeN₂/C catalytic site in these materials, their catalytic activity and the value of n. The correlation is inverse for %H₂O₂. The best catalysts at their maximum catalytic activity are characterized by n>3.9 and %H₂O₂<5%, equivalent to a value of %H₂O₂ released by a 2 wt.% Pt/C catalyst. It is shown that even low peroxide levels of the order of 5 vol% in H₂SO₄ are able to decompose the catalytic sites releasing iron ions in the H₂SO₄ solution. The loss of catalytic activity correlates directly with the loss of iron ions by these catalysts. All the catalysts have been tested at the cathode of single membrane electrode assemblies (MEAs). The slow decrease in performance in fuel cell stability tests is interpreted as the result of the detrimental effect that has H₂O₂, released during ORR, on the chemical integrity of the nonnoble metal catalytic sites at work at the fuel cell cathodes.     

Effect of support type and synthesis conditions on the oxygen reduction activity of RuxSey catalyst prepared by the microwave polyol method

Ru_xSe_y nanoparticles supported on different carbon substrates were synthesized by microwave heating of ethylene glycol solutions of Ru(III) chloride and sodium selenite at different pH and Ru/Se mole ratios. The resulting catalysts were used for the electrochemical oxygen reduction reaction (ORR) in acidic solution. The electrochemical activity was highest for the supported catalyst synthesized at pH 8. Increasing the Se concentration of the catalyst up to 15 mol% increased the catalytic activity for the ORR; at this Se concentration, the activity of the catalyst was considerably higher than that observed for pure Ru catalyst synthesized at exactly the same conditions. The influence of the type of carbon support on the activity of the electrocatalyst was also investigated. Among the different supports, including carbon black (Vulcan XC-72R) (C1), and nanoporous carbons synthesized from resorcinol- (C2) and phloroglucinol-formaldehyde (C3) resins, the Ru_xSe_y catalyst supported on C3 exhibited highest activity for ORR.     

Pt decorating of PdNi/C as electrocatalysts for oxygen reduction

A low-cost and high performance catalyst consisting of Pt decorating PdNi/C (Pt-PdNi/C) for oxygen reduction is prepared by a two-stage route. The characterization techniques considered are X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX) technique. The results show that the Pt-PdNi/C catalyst has an average diameter of ca. 5 nm. The electrochemical activity for the ORR is evaluated from steady state polarization measurements, which are carried out in an ultra-thin layer rotating disk electrode (RDE). The RDE tests show that the Pt-PdNi/C catalyst has the highest ORR activity compared to pure Pt/C, Pd/C and PdNi/C catalysts. High electrocatalytic activities could be attributed to the synergistic effect between Pt and PdNi.     

A New Simulation Model for NOx Storage Catalyst Dynamics

The NO _x storage concept has been studied experimentally and by two differently detailed numerical simulation models. The first detailed model simulates the concentration fronts of the solid components in the barium particles. It shows the slow, diffusion-hindered formation of dense nitrate layers around barium nanoparticles during NO _x storage and their rapid break-up during regeneration. Based upon this knowledge a new simplified model was developed which is able to describe well the storage and regeneration and to explain the main chemical and physical processes in the NO _x storage catalyst.     

LaNi0.9Ru0.1O3 as a cathode catalyst for a direct borohydride fuel cell

LaNi_0.9Ru_0.1O₃ as cathode catalyst for a direct borohydride fuel cell (DBFC) was synthesized and investigated for the first time. The electrochemical experiments indicated that perovskite-type oxide LaNi_0.9Ru_0.1O₃ exhibited higher electrochemical performance compared with LaNiO₃, which suggested incorporation of element Ru into LaNiO₃ could further improve the catalytic ability for oxygen reduction reaction (ORR) in alkaline solution. LaNi_0.9Ru_0.1O₃ catalyst was found to have good tolerance of BH₄⁻. Meanwhile the maximum power density of 171 mW cm⁻² was obtained at 65 °C without using any precious ion exchange membrane. A life test indicated that the DBFC displayed no significant degradation for about 70 h testing. The electrochemical data suggested that LaNi_0.9Ru_0.1O₃, which provided a simple way to construct DBFCs without using any ion exchange membrane, might be promising cathode catalyst with high performance and low cost for DBFCs.     

The effects of regeneration-phase CO and/or H2 amount on the performance of a NOX storage/reduction catalyst

The effects of regeneration-phase CO and/or H₂, and their amounts as a function of temperature on the trapping and reduction of NO_X over a model and a commercial NO_X storage/reduction catalyst have been evaluated. Overall, for both catalysts, their NO_X removal performance improved with each incremental increase in H₂ concentration. For the commercial sample, using CO at 200 °C, beyond a small amount added, was found to decrease performance. The addition of H₂ to the CO-containing mixtures resulted in improved performance at 200 °C, but the presence of the CO still resulted in decreased performance in comparison to activity when just H₂ was used. With the model sample, the presence of CO resulted in very poor performance at 200 °C, even with H₂. The data suggest that CO poisons Pt sites, including Pt-catalyzed nitrate decomposition. At 300 °C, H₂, CO, and mixtures of the two were comparable for trapping and reduction of NO_X, although with the model sample H₂ did prove consistently better. With the commercial sample, H₂ and CO were again comparable at 500 °C, but mixtures of the two led to slightly improved performance, while yet again H₂ and H₂-containing mixtures proved better than CO when testing the model sample. NH₃ formation was observed under most test conditions used. At 200 °C, NH₃ formation increased with each increase in H₂, while at 500 °C, the amount of NH₃ formed when using the mixtures was higher than that when using either H₂ or CO. This coincides with the improved performance observed with the mixtures when testing the commercial.     

Reduction of NOx Stored at Low Temperatures on a NOx Adsorbing Catalyst

Isothermal storage and reduction of NO₂ with CO, C₃H₆ and H₂ as reducing agents on a lean NO _x adsorber was investigated by temperature programmed desorption (TPD) and temperature programmed reduction (TPR) studies. The reduction of NO _x was clearly favoured with H₂ as reducing agent. Carbon monoxide and C₃H₆ showed fairly low reduction of NO _x . The NO _x reduction at low temperatures with H₂ as reducing agent was found to be effective, clearly much more effective than for CO.     

Reductive annealing for generating Se doped 20 wt% Ru/C cathode catalysts for the oxygen reduction reaction

Reductive annealing was chosen as a method for the syntheses of Se modified Ru/C catalysts. Initial preparation of a 20 wt% Ru/C was performed by impregnating RuCl₃·2H₂O on Vulcan XC72 with subsequent conditioning using H₂ at 250 °C for 4 h. Surface treatment of Ru/C by SeO₂ followed by reductive annealing produced Se modified Ru/C catalysts with a pre-determined Ru:Se = 1:0.15 and 1:1 a/o. Structural characterization was carried out using HRTEM while electrochemical characterization was performed using RDE measurements. It is concluded that the presence of Se on Ru has a positive effect on the oxygen reduction reaction of RuSe/C catalyst systems with an optimal loading of Se close to a Ru:Se ratio of 1:0.15 a/o. Overloading of selenium led to neutralization of its promoting effect.