Washcoating of metallic monoliths with a MnCu catalyst for catalytic combustion of volatile organic compounds

A ready-made MnCu catalyst showing a good performance in the combustion of volatile organic compounds has been deposited on FeCrAlloy^® metallic monoliths by means of washcoating. With the aim of depositing a homogeneous and well-adhered layer of catalyst on the monoliths, the effects of a pre-coating with colloidal alumina as a primer, the solid concentration in the slurry, the addition of a stabilizer to the slurry, and the immersions number of the monolith in the slurry were studied. The addition of a stabilizer to the slurry was necessary to prevent sedimentation and to achieve reproducible washcoatings. The pre-coating of the monolith with the primer improved the washcoating adherence obtained with one impregnation but a better adherence did not compensate a lower solid load due to a lower surface roughness. A higher amount of deposited material was obtained with the slurry at 35 wt% making two impregnations. The obtained monoliths showed excellent catalytic activity in the combustion of ethyl acetate and toluene. The activity as well as the surface area and pore volume of each monolith increased with the amount of retained solid.     

Electrochemical promotion of heterogeneous catalysis

The catalytic activity and selectivity of metals interfaced with solid electrolytes can be altered dramatically and reversibly via potential application. The increase in catalytic rate can be several orders of magnitude higher than that anticipated from Faraday's Law. This new phenomenon of electrochemical promotion is of considerable theoretical and potentially practical importance in heterogeneous catalysis. In this paper the main phenomenological features of electrochemical promotion (or NEMCA effect) are surveyed and the origin of the effect is discussed in view of recent surface spectroscopic and quantum mechanical studies.     

Electrochemical promotion of YSZ monolith honeycomb for deep oxidation of methane

A honeycomb monolith of YSZ (yttria stabilized zirconia) was used, for the first time, as a solid electrolyte to perform electrochemical promotion of the deep oxidation of methane. The goal of this study was to validate the concept of Electrochemical Promotion of Catalysis (EPOC) by using Pd particles dispersed on the channels surface of a YSZ dense honeycomb monolith. The Pd catalyst was an effective material as methane conversion reached 20% at about 320 °C. The catalytic properties of the monolithic electrochemical catalyst were investigated upon electrical polarization at 400 °C. Non faradaic effects were observed both under positive and negative polarizations with a maximum of the faradaic efficiency of 47.     

Electrochemical promotion of deep oxidation of methane on Pd/YSZ

Electrochemical catalysts based on Pd deposited by Physical Vapour Deposition on YSZ were used for methane deep oxidation. Different thicknesses of Pd films varying from 11 to 75 nm were catalytically characterized between 150 and 750 °C. The Pd loadings were extremely low. Catalytic and EPOC experiments were carried out on those electrochemical catalysts. Their catalytic activities were compared with the performances of a reference catalyst. It was found that the catalytic activity can be in situ tuned by applying an anodic polarization thus supplying oxygen ions at the surface of the catalyst. Faradaic efficiency values up to 258 were observed and the induced modifications of the catalytic rate were typically 100 times higher than the corresponding ionic current. The influence of the polarization on the temperature of decomposition of the palladium oxide was also examined. The polarization was found to enhance the thermal stability of the oxide and turn palladium oxide into metallic palladium at higher temperatures.     

Catalytic combustion of toluene on platinum-containing monolithic carbon aerogels

Monolithic organic aerogels were prepared by the sol–gel procedure from the polymerisation reaction of resorcinol and formaldehyde in water. The organic aerogels were heat treated in inert atmosphere at either 500 or 1000 °C to obtain the carbon aerogels. The catalysts were prepared by impregnation with an aqueous solution of [Pt(NH₃)₄]Cl₂ or by dissolving this salt in the initial aerogel mixture. Supported catalysts were pretreated in He at 400 °C or H₂ at 300 °C before their characterization by H₂ chemisorption, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy or before testing their catalytic activity. Catalyst activities in toluene combustion were evaluated by conversion versus temperature (light-off curves) and conversion versus time catalytic tests. In the case of catalysts prepared by impregnation, the light-off curves for the total combustion of toluene were shifted to lower temperatures with increasing Pt particle size. This suggests that the reaction was sensitive to the Pt structure within the dispersion range of these catalysts. However, the reverse occurred with catalysts prepared by mixing the precursor in the initial aerogel mixture. Results found could be due to the different surface Pt content of these catalysts as revealed by X-ray photoelectron spectroscopy. This difference was related to the growth of large three-dimensional Pt particles on the surface of the less dispersed catalyst. This means that there is a critical Pt particle size above which the toluene combustion activity decreases with increasing Pt particle size, due to the reduction in active surface sites available for the combustion reaction. Other effect that might influence the activity of these last catalysts is the encapsulation of some Pt particles by the carbon matrix.     

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     

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

The catalytic activity for the gas phase combustion of ethylene of an IrO2 film, used as electrode in a solid electrolyte cell, can be increased by polarizing the metal–solid electrolyte interface. The catalytic activity remains higher even after current interruption. To explain the increase in the open circuit rate after polarization we speculate that the IrO2 catalyst is oxidized at the catalyst–solid electrolyte interface, forming IrO2+delta, which is transported by surface and/or bulk diffusion to the gas exposed catalyst, thus modifying its catalytic activity.     

Towards a new definition of EPOC parameters for anionic electrochemical catalysts: case of propene combustion

The electrochemical promotion of catalysis (EPOC) of propene combustion was investigated using Pt sputtered thin film on an O²⁻ conductor, 8 mol% Y₂O₃-stabilized-ZrO₂ (YSZ). In order to separate the influence of the thermal migration of the O²⁻ oxide ions from the electrolyte to the catalyst surface and the impact of an electrical polarization on the catalytic activity, several light-off experiments (cool down and heat up procedures) were successively performed under different polarizations, i.e. OCV, +2 and −2 V. These experiments have clearly shown that the presence of O²⁻ (thermally or electrochemically induced) inhibits the catalytic activity of the platinum for the propene deep oxidation. These results demonstrate the importance to define a normalized rate enhancement ratio, ρ _n , from a reference value of the catalytic rate corresponding to a Pt surface state free of O²⁻ ions.     

Electrochemical promotion of Pt impregnated catalyst for the treatment of automotive exhaust emissions

This study has shown that Pt/K-βAl₂O₃ electrochemical catalyst can reach high catalytic activity for the selective reduction of N₂O by C₃H₆. In addition it was also demonstrated that Electrochemical Promotion could be a solution to reduce the adverse effects of poisons (O₂ and H₂O) on the catalytic activity. For instance, in the presence of O₂ (1%) and H₂O (3%) in the reactive stream, electrochemical pumping of potassium ions to the Pt catalyst increased the N₂O reduction rate by a factor of 7.4 at 400 °C. We have also demonstrated that the wet impregnation procedure led to a very stable Pt film, with very good resistance to thermal sintering under real operation conditions. Therefore, the use of Pt impregnated films deposited on K-βAl₂O₃ solid electrolytes is a feasible solution for the treatment of automotive exhaust gases.     

VOC removal by synergic effect of combustion catalyst and ozone

The synergic effects of the combustion catalyst Ba---CuO---Cr₂O₃/Al₂O₃ and ozone, used as strong oxidant species in the combustion of various VOCs, were studied. The experiments were performed with five different methods by feeding the organics with air (1000 ppmV) before (methods 2 and 4) and after the ozone generator (methods 1, 3 and 5), they entered into the catalytic reactor (methods 1, 2 and 3) or were directly analyzed (methods 4 and 5). The concentration profiles of the organics were compared as a function of the reaction temperature (from 100 to 500°C) for the methods 1, 2 and 3 and as a function of the peak voltage of the ozone generator (2–10 kV), that is the ozone concentration (up to 2000 ppm) for methods 4 and 5. Ozone strongly reacted in the gas phase with aromatic molecules and organics having functional groups. Moreover, a positive ionizing effect produced by the corona discharge in the ozone generator on the destruction of the organics was found too.     

铜锰复合氧化物催化剂上甲苯的催化燃烧

采用碳酸盐共沉淀法在全范围内制备了一系列具有高比表面积的铜锰互相掺杂的复合氧化物,通过调变铜锰摩尔比考察了其在甲苯催化燃烧反应中的表现。铜锰复合氧化物催化剂,尤其是掺杂低浓度铜的氧化锰,在甲苯催化燃烧中表现出了超越单组分氧化物的性能。随掺杂量的变化,反应活性呈现火山状的趋势。XRD、H₂-TPR、XPS及BET等表征结果显示氧化物粒子的结晶度及分散度、催化剂的可还原性、表面氧浓度、催化剂的比表面积等诸多因素均对催化活性产生了一定的影响,然而复合氧化物催化性能上升的根本原因在于铜物种和锰物种之间存在的较强的相互协同作用。     

A facile preparation of hausmannite as a high-performance catalyst for toluene combustion

Mesoporous transition metal oxide catalysts are well-used in the elimination of volatile organic compounds. In this study, we developed an efficient method for the preparation of mesoporous-Mn₃O₄ (m-Mn₃O₄) without the use of templates or surfactants. In this method, KCl protects oxygen defects on the surface of fresh Mn₃O₄ crystallites. m-Mn₃O₄ shows higher ameliorative catalytic activity than bulk-Mn₃O₄ (b-Mn₃O₄) and calcined-Mn₃O₄ (c-Mn₃O₄), achieving toluene catalytic oxidation of T₁₀ and T₉₀ (the temperature at a conversion rate of about 10% and 90%) at 191 ℃ and 230 ℃, respectively (WHSV = 40,000 ml·g^-1·h^-1). Based on various characterizations, the prepared m-Mn₃O₄ has large specific surface area and abundant oxygen defects, and thus can provide more surface active sites, which give it superior toluene combustion activity.     

Improvement of toluene catalytic combustion by addition of cesium in copper exchanged zeolites

HY and HMFI zeolites exchanged with copper and cesium have been studied for the catalytic combustion of toluene (800 ppm) with air. The catalysts activity has been analyzed by comparison of light-off curves and in both Cu zeolites, the addition of Cs leads to a decrease of the light-off temperature by 50 °C. Temperature-programmed reduction (H₂-TPR) and EPR studies have evidenced clear differences in the characteristics of the copper species both in the presence and absence of Cs co-cations. These differences account for the nature of the active centers in the Cu zeolites for the toluene oxidation. The position and geometry of the copper ions in the zeolite matrix are of great significance for the redox behavior and activity for toluene oxidation. In both MFI and FAU structures, the bulky Cs co-cations are located in the more accessible main zeolite pores, forcing the copper ions to occupy the most stable, but less accessible positions within each structure. In the case of the MFI zeolite, the EPR study revealed that the Cs exchange resulted in an increased abundance in the number of square pyramidal Cu²⁺ ions relative to the other Cu environments. Cs co-cations also lead to an increase in the reducibility of the copper ions mainly due the reduction of protons in Cu, Cs-containing samples.     

Electrochemical promotion of CO oxidation on Pt/YSZ: The effect of catalyst potential on the induction of highly active stationary and oscillatory states

We have investigated the kinetics, rate oscillations and electrochemical promotion of CO oxidation on Pt deposited on YSZ using a standard oxygen reference electrode at temperatures 300–400 °C. We have found that electropromotion is small (ρ < 3) when the catalyst potential U_WR, is below 0.4 V and very pronounced (ρ  9, Λ  1500) when U_WR exceeds 0.4 V. This sharp transition in the electropromotion behavior is accompanied by an abrupt change in reaction kinetics and in catalyst potential. For fixed temperature this transition, which leads to a highly active electropromoted state, occurs at specific ratio and catalyst potential. It is shown via comparison with independent catalyst potential–catalyst work function measurements that the transition corresponds to the onset of extensive O²⁻ spillover from YSZ onto the catalyst surface, and concomitant establishment of an effective double layer at the catalyst–gas interface, which is the cause of the highly active electropromoted state.     

Comparative studies between classic and wireless electrochemical promotion of a Pt catalyst for ethylene oxidation

A comparative study between a classic and a wireless electrochemical promotion experiment was undertaken as a tool towards the better understanding of both systems. The catalytic modification of a platinum catalyst for ethylene oxidation was studied. The catalyst was supported on yttria-stabilised-zirconia (YSZ), a known pure oxide ion conductor, for the classic experiment and La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ—a mixed oxide ion electronic conductor—was used for the wireless experiment. The two systems showed certain similarities in terms of the reaction classification (in both cases electrophobic behaviour was observed) and the promotion mechanism. Significant difference was observed in the time scales and the reversibility of the induced rate modification.     

Catalytic combustion of toluene on Cu-Mn/MCM-41 catalysts: Influence of calcination temperature and operating conditions on the catalytic activity

Catalytic combustion of toluene on Cu-Mn/MCM-41 catalyst was performed in tubular flow reactor operated at atmospheric pressure. The effect of catalyst pre-treatment temperatures on the catalytic activity and stability was investigated. Some reaction variables, such as inlet concentration of toluene and oxygen, reaction temperatures and space velocities were varied over wide ranges, and the influence of different reaction conditions on toluene conversion was discussed. It is showed that the catalytic activity was significantly affected by calcination temperatures between 300 and 800 °C, and oxygen concentration, toluene concentration and space velocity are all key experimental factors to optimize the toluene combustion activities. The objective of this study was to investigate catalytic properties of Cu-Mn/MCM-41 catalysts prepared at different calcination temperatures, in order to obtain additional information to prepare an efficient and highly active catalyst at low temperature.     

Electrochemical promotion of bimetallic RhAg/YSZ catalysts for the reduction of NO under lean burn conditions

At positive overpotentials, RhAg bimetallic electrodes interfaced with yttria-stabilised zirconia (YSZ) exhibit strong electrochemical promotion in the catalytic reduction of NO by propene, even in the presence of a large excess of gaseous oxygen. Compared with the unpromoted system, at positive overpotentials, the N₂ production rate can be accelerated by a factor of eight: this is the key figure of merit. In addition the N₂ selectivity improves from 28 to 55%. The effect on reaction rates is strongly non-faradaic and persists to a significant degree in the absence of applied potential. At negative overpotentials the nitrogen-containing products exhibit faradaic behaviour, possibly reflecting self-poisoning of the system due to excessive adsorption of oxygen. The overall performance of the RhAg/YSZ bimetallic system is substantially better than that of Rh/YSZ, suggesting that the former are promising candidates for use in novel approaches to NO_x reduction under fuel lean conditions.     

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.     

Computer simulation of the effective double layer occurring on a catalyst surface under electro-chemical promotion conditions

In this work, the structural and energetic properties of two typical catalytic surfaces, Na/Pt(111) and O/Pt(111), are studied by means of quantum mechanical calculations and Monte Carlo Grand Canonical simulations. The simulations were performed with electrostatic potentials at different truncation schemes. In order to elucidate the modification of catalyst surfaces produced by the backspillover of ionic species onto Pt(111), the electrostatic field at the interface due to the electric double layer was also analyzed.     

Electrochemical promotion of a platinum catalyst supported on the high-temperature proton conductor La0.99Sr0.01NbO4−δ

The recently discovered, high-temperature proton conductor, La_0.99Sr_0.01NbO_4−δ, was used as a support for the electrochemical promotion of a platinum catalyst. Ethylene oxidation was used as a probe reaction in the temperature range 350–450 °C. Moderate non-Faradaic rate modification, attributable to a protonic promoting species, occurred under negative polarisation; some permanent promotion was also observed. In oxidative atmospheres, both the p_O2 of the reaction mixture and the temperature influenced the type and magnitude of the observed rate modification. Rate-enhancement values of up to ρ = 1.4 and Faradaic-efficiency values approaching Λ = −100 were obtained. Promotion was observed under positive polarisation and relatively dry, oxygen-rich atmospheres suggesting that some oxygen ion conductivity may occur under these conditions. Impedance spectroscopy performed in atmospheres of 4 kPa O₂/N₂ and of 5 kPa H₂/N₂ under dry and slightly humidified (0.3 kPa H₂O) conditions indicated that the electrical resistivity is heavily dominated by the grain-boundary response in the temperature range of the EPOC studies; much lower grain-boundary impedances in the wetter conditions are likely to be attributable to proton transport.