CO and H2 oxidation on a platinum monolith diesel oxidation catalyst

This paper presents experimental and modelling results for the oxidation of mixtures of hydrogen and carbon monoxide in a lean atmosphere. Transient light-off experiments over a platinum catalyst (80 g/ft³ loading) supported on a washcoated ceramic monolith were performed with a slow inlet temperature ramp. Results for CO alone agree with earlier results that predict self-inhibition of CO; that is an increasing light-off temperature with increasing CO concentration. Addition of hydrogen to the feed causes a reduction in light-off temperature for all concentrations of CO studied. The most significant shift in light-off temperature occurs with the addition of small amounts of hydrogen (500 ppm, v/v) with only minor marginal enhancement occurring at higher hydrogen concentrations. Hydrogen alone in a lean atmosphere will oxidise at room temperature. In mixtures of hydrogen and CO, the CO was observed to react first until a conversion of about 50% was observed, at which point the conversion of hydrogen rapidly went from 0 to 100%.

Simulations performed using literature mechanistic models for the oxidation of these mixtures predicted that hydrogen ignites first, followed by CO, a direct contradiction of the experimental evidence. Upon changing the activation energy between adsorbed hydrogen and oxygen, the CO was observed to oxidise first, however, no enhancement of light-off was predicted. The effect cannot be explained by the mechanistic model currently under discussion.     

Inhibition effect of carbon dioxide on the oxidation of hydrogen over a platinum foil catalyst

This paper investigates the catalytic ignition of the H₂/O₂/CO₂ mixture on platinum in a stagnation flow at atmospheric pressure experimentally and numerically. We measure the ignition temperatures of the gas mixtures flowing towards resistively heated platinum with various composition ratios and various diluent gases of N₂, Ar and CO₂. Compared with N₂ or Ar, the CO₂ dilution shows higher ignition temperature by about 50 K, even at the same composition ratio. The ignition temperature increase is proportional to the dilution ratio. Through the numerical simulation, it is illustrated that higher ignition temperature is caused by the adsorption of CO₂ and following dissociation on platinum surface, which was to date considered negligible in catalytic combustion.     

CO oxidation over alumina supported platinum catalyst

The oxidation of CO over Pt/Al₂O₃ has been studied using combined FTIR andin situ reaction cell. During reaction the stretching frequency of the adsorbed carbonyl species remained constant over a temperature range during which a change in the CO conversion occurred. The range of conversion during which this invariance was observed was considerably greater for used catalyst than for fresh Pt/Al²O³. The formation of islands of CO and the role of these in the overall reaction mechanism is discussed.     

Light-off criterion and transient analysis of catalytic monoliths

A one-dimensional two-phase model is used to derive an analytical light-off criterion for a straight channeled catalytic monolith with washcoat, in which the flow is laminar. For the case of uniform catalyst loading and a first order reaction, the light-off criterion is given by

     

Performance of CO preferential oxidation reactor with noble-metal catalyst coated on ceramic monolith for on-board fuel processing applications

On-board fuel processors are being developed to provide hydrogen-rich gas to the polymer electrolyte fuel cell automotive propulsion systems. Whereas the anode catalyst in the fuel cell has low tolerance for carbon monoxide, 10–100 ppm, reforming of gasoline and other hydrocarbon fuels generally produces 1–2% of CO. Of the many methods of removing CO from the reformer gas, preferential oxidation (PrOx) of CO over noble-metal catalysts is practiced most frequently. In this paper, we present experimental data for CO conversion on a Pt-based catalyst that is active at room temperature and was coated on a ceramic monolith. The data is used to develop an empirical correlation for selectivity for CO oxidation as a function of CO concentration and oxygen stoichiometry at 30,000–80,000/h space velocity. The selectivity correlation is used in a model to analyze the performance of multi-stage, adiabatic PrOx reactors with heat exchange between the stages to cool the reformate to 100 °C. An optimization algorithm is used to determine the operating conditions that can reduce CO concentration to 10 ppm while minimizing parasitic loss of H₂ in the reformate stream. It is found that the 10 ppm constraint limits the maximum inlet CO concentration to 1.05% in a single-stage reactor and to 3.1% in a two-stage reactor. The results clearly show the incremental reduction in parasitic H₂ loss by addition of second and third stages.     

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.     

CO selective oxidation in hydrogen-rich gas over platinum catalysts

The kinetics of CO oxidation in hydrogen-rich gas on Pt/mordenite (Pt/MD) or Pt/Al₂O₃ were investigated over a wide range of CO (0.4–1.8%) and O₂ concentrations (0.26–1.14%). The integral flow measurements showed that both the catalysts that could remove CO from 1% to ppm-level Pt/MD had a wider operation temperature range than Pt/Al₂O₃, especially towards lower temperatures.     

The effect of the contact between platinum and soot particles on the catalytic oxidation of soot deposits on a diesel particle filter

Experiments were performed with two model soot aerosols brought into different forms of contact with Pt aerosol particles, to investigate the effectiveness of this contact in lowering the catalytic soot oxidation temperature. The contact was either generated between individual particles in the aerosol state (Pt-doped soot to simulate a fuel borne catalyst), or by sequential or simultaneous deposition of separately generated soot and Pt aerosols onto a sintered metal filter. (Formation of a soot cake on previously deposited Pt aerosol would simulate a catalyst coated diesel particle filter.) The catalytic activity was determined in all cases from temperature ramped oxidation in air of the filtered particles, and defined as the 50% conversion temperature.

It was found that Pt-doped soot and simultaneously filtered aerosols were both equally effective in reducing the oxidation temperature by up to 140–250 °C for the spark discharge soot (with 3–47 wt% Pt concentration in the soot cake), and by up to 140 °C for the pyrolysis soot (3 wt% Pt). Conversely, the deposition of a thin soot layer of 5–10 μm thickness onto Pt, or vice versa, produced only a slight temperature reduction on the order of about 13–42 °C. These results suggest that the distance between soot and Pt particles plays a key role in promoting an effective oxidation on the filter, which is consistent with the role of Pt particles as local generators of activated oxygen.     

Catalytic wet-air oxidation of carboxylic acids on carbon-supported platinum catalysts

Catalytic wet-air oxidation (CWAO) of aqueous solutions (5 g · l⁻¹) of car☐ylic acids (formic, oxalic, and maleic) was carried out with air at 293–463 K on carbon-supported platinum catalysts. Platinum was loaded on active charcoal by cationic exchange, then reduced under H₂. Homogeneous dispersions of 1–2 nm metal particles were obtained. CWAO reactions were performed at 1 or 15 bar air pressure in stirred, batch reactors. Total conversion of formic and oxalic acids into carbon dioxide was obtained under very mild conditions (air at atmospheric pressure, 326 K). The Pt/C catalyst was almost inactive for the oxidation of acetic acid but maleic acid was oxidized under moderate conditions (15 bar of air pressure, 405 K) which indicates that the degradation of this acid does not occur via acetic acid.     

Criterion for extinction of CO oxidation in a catalytic monolith

The extinction of CO oxidation occurring on the Pt catalyst in a monolithic reactor depends on the interplay of the reaction kinetics and mass and heat transfer. In particular, it may be related to a first-order kinetic phase transition from the high-reactive regime to the low-reactive regime. For this scenario, we obtain the criterion for extinction.     

Degradation of maleic acid in a wet air oxidation environment in the presence and absence of a platinum catalyst

Over a temperature range of 415–478 K, the catalytic and non-catalytic degradation of an aqueous solution of maleic acid (0.03 M) has been studied both in the presence of oxygen and under an inert atmosphere (nitrogen). These reactions were first-order for maleic acid. The non-catalytic oxidation reaction was zero-order in oxygen over a partial pressure range of 0.4–1.4 MPa. The apparent activation energies for the non-catalytic removal of maleic acid under both a nitrogen (66.7 kJ mol⁻¹) and an air (131.5 kJ mol⁻¹) environment have been calculated. The use of 0.5 wt.% platinum on γ-alumina catalyst significantly enhanced the degradation rate of maleic acid. A kinetic expression was developed accounting for both homogeneous and heterogeneous routes in maleic acid elimination. Although maleic acid removal was zero-order for oxygen concentration, the presence of oxygen is shown to result in significant chemical oxygen demand (COD) removal in both the catalytic and the non-catalytic process. Finally, the stability of a platinum catalyst has been tested for eight consecutive runs without any noticeable loss in catalyst activity.     

Flow-mediated spatial coupling during CO oxidation on a palladium-supported catalyst in a continuous-flow reactor

We consider the CO oxidation in a continuous-flow reactor containing N catalytic layers of zeolite-supported palladium. For the kinetics of the reaction in one layer, we adopt a model proposed by Slin’ko and Jaeger. Rather than studying non-uniform coverage patterns on the individual catalyst layers, we focus on the influence of flow-mediated spatial coupling between the layers provided by variations in the CO partial pressure, which are transmitted by the flow to the adjacent downstream layer. Using the flow rate and the CO partial pressure at the inlet of the reactor as bifurcation parameters, in a parameter range where the reaction in one layer exhibits relaxational rate oscillations, we find different modes of operation for the reactor. The bifurcation diagram for two layers exhibits synchronized behavior at large flow rate. At lower flow rate and small CO partial pressure, we obtain a drop of catalytic activity at the first layer followed by a compensating increase at the second layer. In the multi-layer system, an increasing number of layers works synchronously when the flow rate grows up. Then, downstream and upstream moving pulse trains of catalytic activity can develop.     

Investigation of CO oxidation and NO reduction on three-way monolith catalysts with transient response techniques

The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O₂, CO, NO and H₂) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O₂ and O₂/CO also revealed the enhancing effect of ceria. The step change experiments NO/H₂ and H₂/NO gave nitrogen as a main reduction product and N₂O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H₂. The CO oxidation transients were modeled with a mechanism which consistent of CO and O₂ adsorption and a surface reaction step. The NO reduction experiments with H₂ revealed the role of N₂O as a surface intermediate in the formation of N₂. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N₂O formation. Pt was shown to be more efficient than Rh for NO reduction by H₂.     

CO低温氧化负载金催化剂研究进展

从合成方法、制备条件和金粒子尺寸、载体效应等方面介绍了影响负载型纳米Au催化剂催化活性的因素、可能的反应机理。评述了选择适当的合成方法可以有效地控制金粒子直径,并使其均匀分散于载体之上,从而得到更好的催化活性。适当的焙烧温度与载体类型也是获得高活性催化剂的必要条件。     

Kinetic model of wet oxidation of phenol at basic pH using a copper catalyst

Catalytic wet oxidation of phenol employing a commercial copper catalyst has been studied. The pH of the media was maintained in the range 7-8 by using 500 mg/L of sodium bicarbonate as buffer solution. The use of this chemical avoids the copper leaching from the solid catalyst and presents another additional advantage since the organic oxidation intermediates obtained at these basic conditions are far less toxic than phenol. Thus, detoxification of phenolic wastewater is directly linked to the phenol conversion achieved. The kinetic model of the phenol disappearance rate has been discriminated by fitting experimental data. These data have been obtained by feeding a phenol solution of 1000 mg/L to a three phase fixed bed reactor at different catalyst weights to liquid flow rates ratios. Temperature and oxygen pressure were changed in the range from to and from 8 to 16 bar, respectively. The kinetic model proposed is based on a heterogeneous free radical mechanism which takes into account the scavenger effect of the bicarbonate on the phenoxy radicals formed on the catalyst surface by reduction of the active copper sites. A slight positive influence of the temperature, but no effect of the oxygen pressure has been found on the kinetic equation for phenol oxidation, at least in the ranges studied. These facts can be explained by the free radical mechanism proposed. Besides, a linear relationship was found between phenol and TOC disappearance that allows quite an accurate prediction of the mineralization achieved.     

Conceptual design and CFD simulation of a novel metal-based monolith reactor with enhanced mass transfer

This paper introduces a novel structured metallic catalyst that improves mass transfer performance of a monolith reactor for highly exothermic gas–solid reactions. The monolith channels are designed to have metallic substrates that consist of two layers with one of the layers being the metallic support and another layer being a foam metal annular that is tightly deposited onto the support surface by some means. Parametrical studies based on a 2D monolith reactor model showed that the present design yields an enhanced mass transfer between the bulk fluid and the catalyst layer due to a decrease in external film resistance, and an enhanced mass transfer within the solid phase mainly due to the viscous flow effect within the porous catalyst layer.     

Catalytic CO oxidation reaction studies on lithographically fabricated platinum nanowire arrays with different oxide supports

Deep-ultraviolet lithography has been coupled with size-reduction and nanoimprint lithography to create high-density arrays of 20-nm wide platinum nanowires supported on oxide thin films of silica, alumina, zirconia, and ceria. These nanowire arrays have been used as two-dimensional platinum model catalyst systems to study the effects of support on catalytic activity during the catalytic oxidation of carbon monoxide. Strong support dependence is seen for both reaction turnover frequency and the measured activation energy. In addition, the stability of the nanowire arrays under reaction conditions shows support dependence.     

Modeling of a metal monolith catalytic reactor for methane steam reforming-combustion coupling

A novel metal monolith reactor for coupling methane steam reforming with catalytic combustion is proposed in this work, the metal monolith is used as a co-current heat exchanger and the catalysts are deposited on channel walls of the monolith. The transport and reaction performances of the reactor are numerically studied utilizing heterogeneous model based on the whole reactor. The influence of the operating conditions like feed gas velocity, temperature and composition are predicted to be significant and they must be carefully adjusted in order to avoid hot spots or insufficient methane conversion. To improve reactor performance, several different channel arrangements and catalyst distribution modes in the monolith are designed and simulated. It is demonstrated that reasonable reactor configuration, structure parameters and catalyst distribution can considerably enhance heat transfer and increase the methane conversion, resulting in a compact and intensified unit.     

CO oxidation on a model Cu/Rh(100) catalyst

The reaction between carbon monoxide and molecular oxygen on a model Cu/Rh(100) bimetallic catalyst was studied at 455 K using a CO/O₂ = 2 reactant gas mixture at a total pressure of 10.0 Torr. A maximum in the initial activity was observed at a Cu coverage of 1.3 monolayers. However, the Cu overlayers were found to be unstable at the reaction conditions employed in that the Cu films interact strongly with surface oxygen to form three-dimensional CuO_x clusters. The morphological modifications were found to influence markedly the catalytic properties of the surface. However, the initial catalytic activity and surface morphology of the Cu films could be restored by flashing the sample to > 750 K.