PEMFC contaminant tolerance limit—CO in H2

Equations are derived from a generic, transient PEMFC contamination model to predict the effect of CO contaminant concentration in H₂ on both steady state performance losses and time constants. The resulting predictions allowed determination of the CO tolerance limit for the cases of a Pt and WC catalyst under specific operating conditions. An increase of the International Organization for Standardization CO tolerance limit of 0.2 ppm is possible because the CO concentration leading to a steady state performance loss of less than 1% is estimated at 0.2-0.9 ppm. An increase in CO tolerance limit is expected to reduce the analytical verification cost (quality control). The steady state performance loss is independent of catalyst loading thus avoiding a future standard change resulting from PEMFC cost reduction activities.     

Reduction of nitrogen oxides by carbon monoxide over an iron oxide catalyst under dynamic conditions

The reduction of NO and N₂O by CO over a silica-supported iron oxide catalyst was investigated by the transient response method, with different initial oxidation states of the catalyst, i.e. completely reduced (Fe₃O₄), or oxidised (Fe₂O₃). The influence of CO pre-adsorption was also studied. From the material balance on the gas phase species, it was shown that the composition of the catalyst changes during relaxation to steady-state. The degree of reduction of the catalyst at steady-state could thus be estimated. During the transient period, CO was shown to inhibit N₂O as well as NO reductions by adsorption on reduced sites. The activity of the reduced catalyst was found to be substantially higher as compared to the oxidised catalyst for both reactions. On this basis, it was attempted to keep the catalyst in a reduced state by periodically reducing it with CO. As a result, a significant increase in the performance of the reactor with respect to steady-state operation could be achieved for N₂O reduction by CO. Finally, the dynamic behaviour of the N₂O–CO and NO–CO reactions made it possible to evidence reaction steps, the occurrence of which could not be shown during our previous investigations on the separate interactions of the reactants with the catalyst.     

Development of prox (preferential oxidation of CO) system for 1 kWe PEMFC

PROX is known as one of the most promising technologies which prevent the anode of PEMFC from being poisoned by carbon monoxide. Hence, commercialization of the PEMFC system is highly dependent on the development of the corresponding PROX system. This study is focused on the development of the PROX system for 1 kW_e PEMFC, and the results can be used to predict the performance of a higher-scaled system. Pt-Ru/Al₂O₃ catalyst made by incipient wetness method has been used for the reaction, since this catalyst shows high activity and selectivity for CO oxidation over a wide range of temperature. With the catalyst, a 1 kW_e proto-type PROX system was set up and its performance was evaluated for the steady state as well as the transient conditions. The outlet CO con-centration of the system was below 10 ppm at its steady state. Also, even at transient conditions, in which sudden flow rate change occurred, the resulting CO concentration still remained under 10 ppm. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University     

The kinetics of methanol oxidation on a supported Pd model catalyst: molecular beam and TR-IRAS experiments

Combining a multi-molecular-beam approach and in situ time-resolved IR reflection absorption spectroscopy (TR-IRAS), we investigate the kinetics of methanol oxidation on a well-defined supported Pd model catalyst. The model catalyst is prepared under ultra-high-vacuum (UHV) conditions by Pd deposition onto a well-ordered Al₂O₃ film grown on NiAl (110). In previous studies, this system has been characterized in detail with respect to its geometric and electronic structure and its adsorption properties. Crossing molecular beams of methanol and oxygen on the sample surface, we systematically probe the rate of total methanol oxidation to CO₂ as a function of surface temperature and reactant fluxes. The results are compared with equivalent experiments for the related CO oxidation reaction. Pronounced differences are observed in the kinetics of the two processes, both under steady state and under transient conditions. The dissimilarities can be related to the dehydrogenation step of methanol, which is found to be strongly inhibited at high oxygen coverage. At low oxygen fluxes, CO is formed as the main product of methanol decomposition. Via a three-beam isotope-exchange experiment combined with TR-IRAS, the kinetics of CO formation is investigated as a function of reactant fluxes and surface temperature. Mean-field simulations of the kinetics are performed in a two-step procedure. First, the kinetics of CO oxidation is described, both under steady state and transient conditions. In a second step the microkinetic model is extended to include the formation of CO formed by methanol dehydrogenation. A comparison with the experimental data indicates that the transient kinetics cannot be fully described by a mean-field approach.     

CO multipulse TAP studies of 2% Pt/CeO2 catalyst: Influence of catalyst pretreatment and temperature on the number of active sites observed

CO multipulse temporal analysis of products (TAP) experiments were used to characterize a ceria-supported platinum catalyst after various oxidative and reductive pretreatments using O₂, H₂O, CO₂, and H₂. Based on the amount of CO consumed, using the final CO-saturated catalyst composition as the common state point, the oxidatively pretreated catalyst could be described using a general scale. From a kinetic analysis of the CO multipulse responses, two kinetic regimes corresponding to two types of active sites could be identified. As the temperature was raised, the number of the most active sites did not change while the amount of the less active site increased. Comparison of the number of active sites determined from the TAP data reported herein with that determined by a previous steady-state isotope transient kinetic analysis experiment showed excellent agreement. This correlation indicates that the (very fast response) TAP experiments can provide information regarding the number and type of active sites that are relevant to a catalyst under real reaction conditions.     

Role of the surface state of Ni/Al2O3 in partial oxidation of CH4

The effect of gas phase O₂ and reversibly adsorbed oxygen on the decomposition of CH₄ and the surface state of a Ni/Al₂O₃ catalyst during partial oxidation of CH₄ were studied using the transient response technique at atmospheric pressure and 700°C. The results show that, when the catalyst surface is completely oxidized under experimental conditions, only a small amount of CO and H₂ can be produced from non‐selective oxidation of CH₄ by reversibly adsorbed oxygen which is more active in oxidizing CH₄ completely than NiO via the Rideal–Eley mechanism and both the conversions of CH₄ and O₂ and the selectivities to CO and H₂ are very low. Therefore, keeping the catalyst surface in the reduced state is the precondition of high conversion of CH₄ and high selectivities to CO and H₂. The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH₄. During partial oxidation of CH₄, no oxygen species but a small amount of carbon exists on the catalyst surface, which is favorable for maintaining the catalyst in the reduced state and the selectivity of CO. The results also indicate that direct oxidation is the main route for partial oxidation of CH₄, and the indirect oxidation mechanism is not able to gain dominance in the reaction under the experimental conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

TRANSIENT KINETIC ANALYSIS OF METHANE SYNTHESIS OVER UNSUPPORTED Co and Rh/SiO2

Concentration jump and isotopic transient experiments show that at H₂/CO = 3, one atmosphere pressure, and 523 K both unsupported cobalt and Rh/SiO₂, have near monolayer coverage of molecular CO but coverage of the primary reactive intermediate below 5% of a monolayer. Modeling the data for unsupported Co requires inclusion of a second reactive surface carbon pool about four times less active than the primary pool. Coverages of both reactive intermediates increase with temperature and with H₂/CO ratio, the latter suggesting that CO dissociation is assisted by hydrogen. On Rh/SiO₂, deactivation appears to be caused by growth of an inactive surface carbon species that blocks sites for the reactive intermediate. Initially, CO dissociation is rate determining over the Rh catalyst. Over the Co catalyst and over Rh after long reaction times or at lower temperature, hydrogcnation of the reactive intermediate produced by CO dissociation has a low enough rate constant to allow the coverage of the intermediate, relative to the rate of reaction, to attain a value which is small but sufficient to slow the dynamic response of ¹³CH₄ to a CO to ₁₃CO switch. Thus, CO dissociation is not rate determining in these systems.     

Experimental study of traveling waves in nonadiabatic fixed bed reactors for the oxidation of carbon monoxide

An experimental study of axial temperature profiles in a nonadiabatic tubular fixed bed reactor has been made under the transient operation. The catalytic carbon monoxide oxidation occuring on a Pt/alumina catalyst has been used. Unlike the adiabatic conditions the velocity of a traveling temperature wave in a nonadiabatical arrangement depends on its axial position. In certain regions of inlet concentration multiple temperature fronts have been observed. For low inlet CO concentration a downstream temperature wave results and the lower (kinetic) steady state is dominant. For high inlet CO concentration an upstream propagating front results and the upper steady state is dominant. For a downstream moving wave oscillations of wave velocity, hot spot temperature and exit conversion have been measured. For certain operating conditions periodic behavior of temperature profiles in the reactor has been observed.     

Dynamic and Steady-State Methods for Evaluation of CO-Tolerant Electrocatalysts and Membrane-Electrode-Assemblies

The operation of PEMFC stacks with impure hydrogen generated from reforming processes based on hydrocarbon fuels is associated with problems due to poisoning of the Pt based anode catalyst by carbon monoxide (CO) [1-3]. In order to address these problems, advanced CO tolerant PtRu electro-catalysts and MEAs have been developed by OMG over the past years [4-7]. In the course of this work, specific testing and evaluation methods for these products were established and implemented. During operation of PEMFC systems in vehicles and busses, transient exposure of the anode catalyst with high levels of CO can occur. To simulate these conditions in a lab environment, dynamic testing procedures simulating transient CO poisoning conditions were developed. For stationary PEMFC systems running on CO containing natural gas (NG) reformate, besides the production costs, endurance, stability and lifetime of MEAs are of primary importance. For this application field, the “CO-gain” diagnostic method was developed. This in-situ method allows to generate data and information on anode catalyst performance while performing a continuous, non-interrupted lifetime testing of the MEA. This paper describes the dynamic and steady-state testing methods developed and employed at OMG to evaluate CO tolerant electrocatalysts and membrane-electrode-assemblies (MEAs). For illustration, typical performance data obtained with the new testing procedures and methods are presented.     

铜基催化剂上甲醇合成的动态反应动力学

Adsorption, surface reaction and process dynamics on the surface of a commercial copper-based cata-lyst for methanol synthesis from CO/CO2/H2 were systematically studied by means of temperature programmed desorption (TPD), temperature programmed surface reaction (TPSR), in-situ Fourier transform-inferred spectroecopy(FTIR) and stimulus-response techniques. As a part of results, an elementary step sequence was suggested and a group of ordinary differential equations (ODEs) for describing transient conversations relevant to all species on the catalyst surface and in the gas phase in a micro-fixed-bed reactor was derived. The values of the parameters referred to dynamic kinetics were estimated by fitting the solution of the ODEs with the transient response data obtained by the stimulus-response technique with a FTIR analyzer as an on-line detector.     

The transient response study of CO,CO2, and O2 adsorption and CO oxidation over La0.4Sr0.6Co0.4Mn0.6O3

The transient behaviour caused by the change of the component concentration for CO oxidation on the perovskite‐type catalyst La_0.4Sr_0.6Co_0.4Mn_0.6O₃ was investigated. Results showed that CO was not adsorbed on the catalyst surface and CO oxidation was carried out between the surface oxygen species and gas phase CO. On the other hand, CO₂ can be adsorbed on the catalyst surface but its adsorption site was different from the forming site.     

Mathematical modelling of fluidized bed reactors—III: Axial dispersion model

An axial dispersion model has been developed for a continuous fluidized bed catalytic reactor with a cocurrent flow of the emulsion phase gas and the catalyst particles. The influence of some parameters on multiplicity of steady states has been reported. Several examples illustrating the transient behavior of the system are presented. In cases where three steady states are possible it appears that the intermediate steady state is unstable, while the lower and the upper steady states are locally stable. It was noted that the initial temperature of the emulsion phase is a predominant factor in determining which steady state will be approached.     

颗粒尺度下混合催化剂床层中CO2加氢反应体系数值模拟

CO2的有效转化对于实现“双碳”目标具有重要意义。柱形颗粒比异形颗粒具有更优良的热导性和更小的颗粒表面积,因此,为获得更优异的床层性能,采用离散元方法(DEM)对异形颗粒混合堆积床内的CO2加氢反应体系进行颗粒尺度CFD模拟计算,探究不同堆积方式对床层多物理场分布、CO2转化率及CH4产率的影响。结果表明,催化剂内部存在扩散阻力,随着反应进行,颗粒内物质由分层分布渐变为均匀分布。随机混合床与常规床相比,随机混合床稳态前热点不易波动、稳态产率更高,而柱形床径向热场更均匀且热点更高。在四种规则混合床中,底部为4孔的两种催化剂床相较于底部为柱形的两种催化剂床,整体流场更均匀、高速区少、压降大、高温区占比大、稳态前热点不易波动;2层-底4孔催化剂床的稳态出口CO2转化率和CH4产率均最大;4层-底4孔催化剂床高温区占比较高,随着反应进行,高温区占比呈上升趋势,CO2转化率和CH4产率大幅下降。     

In situ perpetual regeneration of a Ni-faujasite methanation catalyst

A prolonged lifetime of a Ni-faujasite methanation catalyst and a stable rate of methanation can be achieved by operating the fluidized catalyst bed under unsteady state conditions. A higher rate of methanation is accompanied by a shift in selectivity compared to a steady state operation due to an enhanced disproportionation of CO.     

Surface and bulk changes of a Pt 1%/Ce0.6Zr0.4O2 catalyst during CO oxidation in the absence of O2

Topics in Catalysis - The reduction of a Pt 1%/Ce0.6Zr0.4O2 catalyst by CO in the absence of gaseous oxygen was studied by transient reactivity tests, temperature programmed surface reaction with...     

CO Oxidation Kinetics over a Nanostructured Cu0.1Ce0.9O2?y Catalyst: A CO/O2 Concentration Cycling Study

Feed composition cycling as a transient kinetic technique provides detailed information about elementary steps in CO oxidation over a nanostructured Cu_0.1Ce_0.9O_2–y catalyst. This catalyst has a great potential as a future PROX reactor catalyst since it has great oxygen storage capacity as well as high reoxidation rate under oxygen rich conditions.     

Pulse-MS study of the partial oxidation of methane over Ni/La2O3 catalyst

The CH₄ direct oxidation reaction was studied at 600°C by the pulse-MS transient method over the Ni/La₂O₃ catalyst. Over the freshly prepared catalyst (which contains NiO), the CO selectivity and CH₄ conversion increased and attained constant values as the number of CH₄/O₂ pulses increased. Over the reduced catalyst (containing Ni), as the number of CH₄/O₂ pulses increased, the CO selectivity and CH₄ conversion decreased before they reached the same constant values as over the fresh catalyst. The CO selectivity increased as the residence time of the reactants shortened, implying that CO was directly generated without the preformation of CO₂. The activation energies of CH₄ dehydrogenation in the presence and absence of oxygen have been calculated using the bond-order conservation Morse-potential approach. The results indicate (1) the direct dehydrogenation steps are more likely to occur; (2) the transient oxygen species adsorbed on-top of the metal atoms promote dehydrogenation; (3) the oxygen species adsorbed on bridge or hollow sites do not promote dehydrogenation.     

Reaction and Surface Characterization Studies of Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts for CO Preferential Oxidation in Reformed Gas

In order to investigate the reasons the activation of a Ru/Al₂O₃ catalyst by heating in a H₂/N₂ mixed gas improves the CO preferential oxidation (PROX) activity, the oxidation state of the Ru on the catalyst surface was studied by using ESCA. As the ratio of Ru(0) to total Ru on the surface was increased, the temperature window of the Ru catalyst, where CO was reduced to below 10 ppm, was expanded to the lower temperature side. The activity of CO oxidation by O₂ of the Ru catalyst at lower temperatures was improved by increasing the ratio of Ru(0). However, the selectivity for CO oxidation hardly varied with the change in the surface Ru(0) ratio at these low temperatures. It is considered that O₂ activation on Ru(0) plays an essential role in CO PROX activity on the Ru catalyst at low temperatures.     

合成甲醇基元过程瞬态动力学的模型化(Ⅰ)——基元过程序列结构

采用动态原位红外测试技术和TPD、TPSR方法,系统地研究了合成甲醇有关物种H_2、CO、CO_2和CH_3OH在铜基催化剂上的吸附与反应特性,判识了反应过程中在催化剂活性表面上的吸附态中间物的类型。在实验信息的基础上,提出了在催化剂活性表面上可能的基元过程序列结构。     

MODELING OF ELEMENTARY PROCESS KINETICS FOR METHANOL SYNTHESIS ( Ⅰ )IDENTIFICATION OF STRUCTURE OF ELEMENTARY STEP SEQUENCE

采用动态原位红外测试技术和TPD、TPSR方法,系统地研究了合成甲醇有关物种H_2、CO、CO_2和CH_3OH在铜基催化剂上的吸附与反应特性,判识了反应过程中在催化剂活性表面上的吸附态中间物的类型。在实验信息的基础上,提出了在催化剂活性表面上可能的基元过程序列结构。