Spatiotemporal patterns in a porous catalyst during light-off and quenching of carbon monoxide oxidation

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O₂ concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature.As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO₂ production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO₂ peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO₂ production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO₂ production rate drops abruptly as the reaction “quenches”.Catalyst layer thickness was changed over the range 0.06-1.0 mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO₂ production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness.     

Transient and resonant behavior for no reduction by CO over a Pt/Al2O3 catalyst during forced composition cycling

The reduction of NO by CO over a Pt/Al₂O₃ catalyst has been investigated using the technique of forced concentration cycling in an isothermal recycle reactor at 485 K. Time-average conversions exhibit resonant behavior with increasing frequency. Maximum time-average NO conversion of 78%, compared with the steady-state conversion of 3.8%, was attained during out-of-phase feed concentration cycling. The effect of the phase angle between the NO and CO feed cycles has been examined. Higher conversions are obtained by decreasing the NO phase lead below 180°. The convergence to cycle-invariance was slow for high frequency cycling.     

Ammoxidation of acrolein on catalyst of Fe–Bi–P mixed oxide system

Ammoxidation of acrolein (Al) on Fe₂O₃-Bi₂O₃-P₂O₅ (44.5:44.5:11.0 mol%) catalyst was studied in a fixed bed reactor. The main products were acrylonitrile (AN), CO₂ and acetonitrile; and CO was negligibly small. The rate equations for Al conversion and product formation, together with their apparent activation energies, were obtained. The reaction rates are controlled by the chemical reaction rates at temperatures below 340°C, while the external film diffusion rates are the rate determining steps at temperatures above 380°C. Assuming the chemical reaction controlling region, the conversion rate of Al at 400°C is about 1000 times faster than that of propylene. From the effects of feed composition on selectivities, it is concluded that AN is produced according to Hadley's path.     

Preferential Oxidation of CO in H2-Rich Gases over Co-Promoted Pt-γ-Al2O3 Catalyst

Carbon monoxide is a poison to the Pt anode in proton exchange membrane fuel cell (PEMFC). Preferential oxidation (PROX) is an effective method to reduce CO in hydrogen-rich gas streams to a tolerant level. In the present work, the effect of adding cobalt to Pt/γ-Al₂O₃ on the PROX of CO was investigated. Our results showed that the addition of Co to Pt/γ-Al₂O₃ could not only improve the low-temperature activity but also reduce significantly the loading of Pt in the catalysts. Over the catalyst 3%Co/1%Pt/γ-Al₂O₃ the conversion of CO was close to 100% at 90 °C and space velocity of 8000 mL g^?1 h^?1. In addition, the Co-promoted Pt/γ-Al₂O₃ catalyst showed good resistance to H₂O and CO₂ and could be operated in a wide range of space velocity. At temperatures above 90 °C, the existence of H₂O in the feed increased the conversion and broadened the operating temperature range without worsening the selectivity. When space velocity was changed from 8000 to 80,000 mL g^?1 h^?1 and temperatures was kept between 120 and 160 °C, the conversion of CO was always over 99% and the decrease in O₂ selectivity did not exceed 10%. Furthermore, a strong opposite effect of the ratio of O₂ to CO on the conversion of CO and the selectivity of O₂ was observed. However, at the O₂/CO ratio of 1.0 and temperatures between 120 and 160 °C, a satisfied balance between conversion and selectivity could be obtained.     

Preferential CO oxidation over Pt–SnO2/Al2O3 in hydrogen rich streams containing CO2 and H2O (CO removal from H2 with PROX)

The effects of reaction gases including CO₂ and H₂O and temperature on the selective low-temperature oxidation of CO were studied in hydrogen rich streams using a flow micro-reactor packed with a Pt–SnO₂/Al₂O₃ sol–gel catalyst that was initially designed and optimized for operation in the absence of CO₂ and H₂O. 100% CO conversion was achieved over the 1 wt% Pt–3 wt% SnO₂/Al₂O₃ catalyst at 110 °C using a feed composition of 1.0% CO, 1.5% O₂, 25% CO₂, 10% H₂O, 58% H₂ and He as balance at a space velocity of 24,000 cm³/(g h). CO₂ in the feed was found to decrease CO conversion significantly while the presence of H₂O in the feed increased CO conversion, balancing the effect of CO₂.     

Plasma-Assisted Diesel Oxidation Catalyst on Bench Scale: Focus on Light-off Temperature and NOx Behavior

The effect of a non-thermal plasma reactor over a commercial Diesel oxidation catalyst (DOC) was investigated. Studies have been focused on the gas treatment efficiency together with lowering light-off temperature when a DOC catalyst was connected downstream to plasma reactor in test bench scale. Experiments have been conducted using multi-DBDs (dielectric barrier discharge) reactor in planar configuration driven by a HV AC generator (11 kV–15 kHz). The specific input energy was set to 57 and 85 J/L. Experiments were performed in gas composition simulating Diesel exhaust. Commercial DOC, monolith-supported Pt–Pd/Al₂O₃, was used at gas hourly space velocities of about 55,000 and 82,000 h^?1. CO and hydrocarbons light-off curves were determined for DOC, plasma, and plasma-DOC systems by temperature programmed surface reaction from 80 to 400 °C. Particular attention has been paid to the gas temperature between the plasma reactor and the DOC. Results show that the plasma-catalyst system provides the lowest light-off temperatures for CO and HCs. Under conditions of this study, light-off temperature improvement by about 57 °C was obtained and the plasma reactor totally oxidized NO to NO₂ at low temperature.     

Thermodynamic Equilibrium Calculations for the Reforming of Coke Oven Gas with Gasification Gas

Thermodynamic analyses of the reforming of coke oven gas with gasification gas for syngas were investigated as a function of coke oven gas‐to‐gasification gas ratio (1–3), oxygen‐to‐methane ratio (0–1.56), pressure (25–35 bar) and temperature (700–1100 °C). Thermodynamic equilibrium results indicate that the operating temperature should be approximately 1100 °C and the oxygen‐to‐methane ratio should be approximately 0.39, where about 80 % CH₄ and CO₂ can be converted at 30 bar. Increasing the operating pressure shifts the equilibrium toward the reactants (CH₄ and CO₂); increasing the pressure from 25 to 35 bar decreases the conversion of CO₂ from 73.7 % to 67.8 %. The conversion ratio of CO₂ is less than that in the absence of O₂. For a constant feed gas composition (7 % O₂, 31 % gasification gas, and 62 % coke oven gas), a H₂/CO ratio of about 2 occurs at temperatures of 950 °C and above. Pressure effects on the H₂/CO ratio are negligible for temperatures greater than 750 °C. The steam produced has an effect on the hydrogen selectivity, but its mole fraction decreases with temperature; trace amounts of other secondary products are observed.     

Effect of operating parameters on methanation reaction for the production of synthetic natural gas

Concerns about the depletion and increasing price of natural gas are generating interest in the technology of synthetic natural gas (SNG) production. SNG can be produced by the methanation reaction of synthesis gas obtained from coal gasification; this methanation reaction is the crucial procedure for economical production of SNG. We investigated the effect of operating parameters such as the reaction temperature, pressure, and feed compositions (H₂/CO and CO₂/CO ratios) on the performance of the methanation reaction by equilibrium model calculations and dynamic numerical model simulations. The performance of the methanation reaction was estimated from the CO conversion, CO to CH₄ conversion, and CH₄ mole fraction in the product gas. In general, a lower temperature and/or higher pressure are favorable for the enhancement of the methanation reaction performance. However, the performance becomes poor at low temperatures below 300 °C and high pressures above 15 atm because of limitations in the reaction kinetics. The smaller the amount of CO₂ in the feed, the better the performance, and an additional H₂ supply is essential to increase the methanation reaction performance fully.     

COMPARATIVE STUDY OF Ag2O/CO3O4 CATALYSTS PREPARED BY THE SOL-GEL AND CO-PRECIPITATION TECHNIQUES: CHARACTERIZATION AND CO ACTIVITY STUDIES

In this study effects of the preparation method on the characteristic properties and CO oxidation activities of Ag₂O/Co₃O₄ catalysts were investigated. Catalysts were prepared by two different methods: sol gel and co-precipitation. N₂ physisorption measurements, X-ray diffraction, and scanning electron microscopy measurements were used to characterize the catalysts. CO oxidation activity tests were carried out under 1% CO, 21% O₂, and the remainder He feed condition between 20° and 200°C. According to the N₂ physisorption measurements, catalysts prepared by the co-precipitation method have a higher surface area than the catalysts prepared by the sol-gel method. Co₃O₄ and AgCoO₂ phases were obtained from catalysts prepared by both techniques. In addition to these phases, metallic silver peaks were obtained by increasing calcination temperature. SEM micrographs of the catalysts showed that catalysts have uniform particles. Increasing the calcination temperature caused the formation of different-sized agglomerates and an increase in the gaps between agglomerates. The best activity was obtained from the Ag₂ O/Co₃ O₄ catalyst calcined at 200°C and prepared by the co-precipitation method. This catalyst gave 50% CO conversion at 106°C. The other two catalysts gave 100% CO conversion at a higher temperature of 200°C.     

Analysis of hydrogen production from methane autothermal reformer with a dual catalyst-bed configuration

This paper presents a performance analysis of a dual-bed autothermal reformer for hydrogen production from methane using a non-isothermal, one dimensional reactor model. The first section of Pt/Al₂O₃ catalyst is designed for oxidation reaction, whereas the second one based on Ni/MgAl₂O₄ catalyst involves steam reforming reaction. The simulation results show that the dual-bed autothermal reactor provides higher reactor temperature and methane conversion compared with a conventional fixed-bed reformer. The H₂O/CH₄ and O₂/CH₄ feed ratios affect the methane conversion and the H₂/CO product ratio. The addition of steam at lower temperatures to the steam reforming section of the dual-bed reactor can produce the synthesis gas with a higher H₂/CO product ratio.     

Synergism Between Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Au/TiO<Subscript>2</Subscript> in the Low Temperature Oxidation of Propene

CO impedes the low temperature (<170 °C) oxidation of C₃H₆ on supported Pt. Supported Au catalysts are very effective in the removal of CO by oxidation, although it has little propene oxidation activity under these conditions. Addition of Au/TiO₂ to Pt/Al₂O₃ either as a physical mixture or as a pre-catalyst removes the CO and lowers the light-off temperature (T ₅₀) for C₃H₆ oxidation compared with Pt catalyst alone by ~54 °C in a feed of 1% CO, 400 ppm C₃H₆, 14% O₂, 2% H₂O.     

Enhanced Methane Conversion Under Periodic Operation Over a Pd/Rh Based TWC in the Exhausts from NGVs

The enhancement of methane oxidation performances under periodic operation over a commercial Pd–Rh based three way catalyst (TWC) is investigated at different temperatures. Results confirm that under conditions with periodic oscillating feed around stoichiometry (λ = 1 ± 0.02), higher and more stable CH₄ conversion are obtained than under conditions with constant stoichiometric feed. In particular higher CH₄ conversion is obtained in the rich part of the cycle than in the lean one, the difference being more pronounced at high temperature. A narrow turning point for the TWC activity is finally observed under slightly rich conditions, which is characterised by a marked increase of CH₄ conversion, paralleled by total consumption of O₂ and NO and formation of small amounts of CO, H₂ and NH₃. Results suggest that the oxidation state of palladium plays a key role in the observed enhancement of catalyst performances.     

Modeling of non-catalytic partial oxidation of natural gas under conditions found in industrial reformers

Non-catalytic partial oxidation of natural gas/O₂/H₂O mixture at elevated pressures was simulated kinetically using Chemkin package incorporating detailed reaction mechanisms of methane oxidation. The dependence of reaction time was investigated as a function of inlet temperature, system pressure, and O₂/CH₄ ratio. The conversion to products was predicted to complete within a residence time of less than 0.1 ms at pressures greater than 30 atm and temperatures higher than 1450 K. A minimum O₂/CH₄ ratio of 0.64 was found necessary for a complete methane conversion at the conditions typical for the industrial reformer. The effect of O₂/H₂O in the feed gas was examined computationally, and the results suggested that adding H₂O in the feed gas could be a viable tool for adjusting the H₂/CO ratio in the products and for controlling the flame temperature. Formations of higher order hydrocarbons and soot, which may play important roles in the actual fuel-rich conversion environment, are not considered in the present study.     

Improved performance of NOx reduction by H2 and CO over a Pd/Al2O3 catalyst at low temperatures under lean-burn conditions

Selective reduction of NO_x by H₂ and CO in excess O₂ was studied using a Pd/Al₂O₃ catalyst. Incipient wetness impregnation into Al₂O₃ was done using PdCl₂ as the metal precursor. The NO_x conversion profile was maximal at 423 K (up to 95.2% for 250 ppm CO), at which temperature complete oxidation of H₂ and CO occurred. There is a very strong synergic effect when both CO and H₂ are simultaneously present in the feed. This catalyst has good selectivity towards N₂ and has a window of operation going from 400 to 530 K.     

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.     

Electrochemical activation of Pt catalyst by potassium for low temperature CO deep oxidation

This study exposes, for the first time, the effect of Electrochemical Promotion (EPOC) by potassium cations on Pt catalyst for low temperature CO oxidation process. The effect of catalyst polarization drastically increased the catalytic rate for more than 11 times by decreasing the light-off temperature by almost 40 °C. The promotional phenomenon was found to be reversible due to the formation and decomposition of potassium compounds on Pt surface that enhanced the adsorption of O₂ at the expense of CO. The achieved results demonstrated that Electrochemical Promotion of Catalysis could be applied to improve the performance of catalytic converters at lower temperatures.     

Detailed Mechanism for Reduction of N2O over Rhodium by CO in Automotive Exhaust

Elementary-steps based mechanisms of CO–O₂ and CO–N₂O over rhodium catalyst were proposed and utilized to simulate experimental data from literature. The results showed that the mechanisms possess good prediction capability. It was found that the dissociation of adsorbed N₂O is the rate limiting step of N₂O reduction under conditions characterized by high CO coverages. The rather high light-off temperature (50 % conversion) of CO–N₂O (638 K) compared to that of CO–O₂ (453 K) is explained by the high temperature to initiate N₂O dissociation to offer surface oxygen needed for CO oxidation. Removing CO out of the reaction system, the oxygen generated via the dissociation of adsorbed N₂O accumulates on the surface of Rh, and finally leads to a poisoned catalyst and termination of the N₂O reduction process. However, increasing the inlet CO concentration inhibits the adsorption of N₂O to some extent, thus the reduction rate of N₂O is lowered on the contrary. Analysis of kinetic parameters showed that facilitating CO desorption or the decomposition of adsorbed N₂O leads to higher conversion of N₂O, with the latter having larger influence.     

Effect of sintering on the catalytic activity of a Pt based catalyst for CO oxidation: Experiments and modeling

The goal of this paper was to make the link between sintering of a 1.6% Pt/Al₂O₃ catalyst and its activity for CO oxidation reaction. Thermal aging of this catalyst for different durations ranging from 15 min to 16 h, at 600 and 700 °C, under 7% O₂, led to a shift of the platinum particle size distributions towards larger diameters, due to sintering. These distributions were studied by transmission electron microscopy. The number and the surface average diameters of platinum particles increase from 1.3 to 8.9 nm and 2.1 to 12.8 nm, respectively, after 16 h aging at 600 °C. The catalytic activity for CO oxidation under different CO and O₂ inlet concentrations decreases after aging the catalyst. The light-off temperature increased by 48 °C when the catalyst was aged for 16 h at 600 °C. The CO oxidation reaction is structure sensitive with a catalytic activity increasing with the platinum particle size. To account for this size effect, two intrinsic kinetic constants, related either to platinum atoms on planar faces or atoms on edges and corners were defined. A platinum site located on a planar face was found to be 2.5 more active than a platinum site on edges or corners, whatever the temperature. The global kinetic law {r (mol m⁻² s⁻¹) = 103 × exp(−64,500/RT)[O₂]^0.74[CO]^−0.5)} related to a reaction occurring on a platinum atom located on planar faces allows a simulation of the CO conversion curves during a temperature ramp. Modeling of the catalytic CO conversion during a temperature ramp, using the different aged catalysts, allows prediction of the CO conversion curves over a wide range of experimental conditions.     

Selective methanation of CO over supported Ru catalysts

The catalytic performance of supported ruthenium catalysts for the selective methanation of CO in the presence of excess CO₂ has been investigated with respect to the loading (0.5–5.0 wt.%) and mean crystallite size (1.3–13.6 nm) of the metallic phase as well as with respect to the nature of the support (Al₂O₃, TiO₂, YSZ, CeO₂ and SiO₂). Experiments were conducted in the temperature range of 170–470 °C using a feed composition consisting of 1%CO, 50% H₂ 15% CO₂ and 0–30% H₂O (balance He). It has been found that, for all catalysts investigated, conversion of CO₂ is completely suppressed until conversion of CO reaches its maximum value. Selectivity toward methane, which is typically higher than 70%, increases with increasing temperature and becomes 100% when the CO₂ methanation reaction is initiated. Increasing metal loading results in a significant shift of the CO conversion curve toward lower temperatures, where the undesired reverse water–gas shift reaction becomes less significant. Results of kinetic measurements show that CO/CO₂ hydrogenation reactions over Ru catalysts are structure sensitive, i.e., the reaction rate per surface metal atom (turnover frequency, TOF) depends on metal crystallite size. In particular, for Ru/TiO₂ catalysts, TOFs of both CO (at 215 °C) and CO₂ (at 330 °C) increase by a factor of 40 and 25, respectively, with increasing mean crystallite size of Ru from 2.1 to 4.5 nm, which is accompanied by an increase of selectivity to methane. Qualitatively similar results were obtained from Ru catalysts supported on Al₂O₃. Experiments conducted with the use of Ru catalyst of the same metal loading (5 wt.%) and comparable crystallite size show that the nature of the metal oxide support affects significantly catalytic performance. In particular, the turnover frequency of CO is 1–2 orders of magnitude higher when Ru is supported on TiO₂, compared to YSZ or SiO₂, whereas CeO₂- and Al₂O₃-supported catalysts exhibit intermediate performance. Optimal results were obtained over the 5%Ru/TiO₂ catalyst, which is able to completely and selectively convert CO at temperatures around 230 °C. Addition of water vapor in the feed does not affect CO hydrogenation but shifts the CO₂ conversion curve toward higher temperatures, thereby further improving the performance of this catalyst for the title reaction. In addition, long-term stability tests conducted under realistic reaction conditions show that the 5%Ru/TiO₂ catalyst is very stable and, therefore, is a promising candidate for use in the selective methanation of CO for fuel cell applications.     

The Effect of Acidity on Olefin Aromatization over Potassium Modified ZSM-5 Catalysts

The preparation of light alkenes by the gas phase oxidative cracking (GOC) or catalytic oxidative cracking (COC) of model high hydrocarbons (hexane, cyclohexane, isooctane and decane in the GOC process and hexane in the COC process) was investigated in this paper. The selection for the feed in the GOC process was flexible. Excellent conversion of hydrocarbons (over 85%) and high yield of light alkenes (about 50%) were obtained in the GOC of various hydrocarbons including cyclohexane at 750°C. In the GOC process, the utilization ratio of the carbon resources was high; CO dominated the produced CO _X (the selectivity to CO₂ was always below 1%); and the total selectivity to light alkenes and CO was near or over 70%. In the COC of hexane over three typical catalysts (HZSM-5, 10% La₂O₃/HZSM-5 and 0.25% Li/MgO), the selectivity to CO _X was hard to decrease and the conversion of hexane and yield of light alkenes could not compete with those in the GOC process. With the addition of H₂ in the feed, the selectivity to CO _X was reduced below 5% over 0.1% Pt/HZSM-5 or 0.1% Pt/MgAl₂O₄ catalyst. The latter catalyst was superior to the former catalyst due to its perfect performance at high temperature, and with the latter, excellent selectivity to light alkenes (70%) and the conversion of hexane (92%) were achieved at 850°C (a yield of light alkenes of 64%, correspondingly).