Selective CO oxidation over a commercial PROX monolith catalyst for hydrogen fuel cell applications

Preferential oxidation (PROX) of CO over noble-metal-containing monolith catalysts is one of the most promising approaches for removing CO to generate low temperature fuel cell quality H₂. The monolith-supported washcoated catalyst comprising Cu and Fe promoted with Pt is highly effective in reducing the CO in practical reformates to less than 10 ppm over a broad range of feed compositions, inlet temperatures and turn down ratios. It is speculated that Pt dissociates the H₂ which then reduces the CuO to its active state. Pt may also act as a cocatalyst for CO adsorption with metal oxides supplying oxygen for PROX reaction. The catalytic system is operated adiabatically with an inlet temperature between roughly 65–120 °C reaching an exit temperature close to 150 °C with no evidence of reverse water gas shift or methanation. The goal was to find the proper operating conditions to achieve <10 ppm CO. Turn down ratios (varying space velocities) at a factor of 4–5 are routinely achieved up to at least 34,000 h⁻¹ with high steam levels of up to 45%. The wide operating window simplifies the control of the PROX reactor and improves the fuel processor’s performance for fast startup and shutdown and responses to transient loads. The catalyst also retains its performance after multiple start and stops modes of operation in reformate.     

Preferential CO oxidation over CuO–CeO2 in excess hydrogen: Effectiveness factors of catalyst particles and temperature window for CO removal

In the preferential oxidation of CO in hydrogen mixtures (PROX), CO and H₂ oxidation occur in parallel on the surface in a porous catalyst. The diffusion of the reactants into the pore structure of the catalyst can affect the catalyst performance significantly, and its effect can be accounted for in terms of the effectiveness factor. Conventional methods for estimating the effectiveness factor are not directly applicable because they have been developed for a single reaction in a catalyst particle. A novel method for a simultaneous estimation of the effectiveness factors of the two reactions was developed in this study. This method is based on the PROX kinetics over a CuO–CeO₂ catalyst and is applicable to the cases where the CO oxidation can be approximated by a first-order reaction and both oxidations are zero-order reactions with respect to the O₂ partial pressure. With the method, the performance of an isothermal PROX reactor was simulated to determine the effects of the feed flow rate, feed composition, reactor temperature and catalyst size on the CO clean-up.     

A combined methanol autothermal steam reforming and PEM fuel cell pilot plant unit: Experimental and simulation studies

An integrated system for hydrogen production via autothermal steam reforming of methanol and consequent power generation in a polymer electrolyte membrane (PEM) fuel cell has been developed and operated at C.P.E.R.I. The pilot plant comprises an autothermal reforming reactor to produce hydrogen, a preferential oxidation reactor (PROX) to reduce CO concentration below 50 ppm and a PEM fuel cell for power generation.The present paper deals with the study of this system, both from an experimental and a theoretical point of view. The experimental work aims to: (a) examine the effect of the reforming temperature on methanol conversion and on the effluent stream concentration, (b) investigate the effect of reaction temperature and O₂/CO ratio on the performance of the PROX reactor, and (c) evaluate the operation of a 10-cell PEM fuel cell, using pure hydrogen and air at three temperature levels. The experimental data are subsequently utilized for the validation of one-dimensional pseudo-homogeneous models that have been developed for the two reactors and also for the identification of the voltage–current characteristic curve of the PEM fuel cell. The validated models are then used to investigate the behavior and explore the interactions, both static and dynamic, among the various process subsystems.     

A complete miniaturized microstructured methanol fuel processor/fuel cell system for low power applications

A complete miniaturized methanol fuel processor/fuel cell system was developed and put into operation as compact hydrogen supplier for low power application. The whole system consisting of a micro-structured evaporator, a micro-structured reformer and two stages of preferential oxidation of CO (PROX) reactor, micro-structured catalytic burner, and fuel cell was operated to evaluate the performance of the whole production line from methanol to electricity. The performance of micro methanol steam reformer and PROX reactor was systematically investigated. The effect of reaction temperature, steam to carbon ratio, and contact time on the methanol steam reformer performance is presented in terms of catalytic activity, selectivity, and reformate yield. The performance of PROX reactor fed with the reformate produced by the reformer reactor was evaluated by the variation of reaction temperature and oxygen to CO ratio. The results demonstrate that micro-structured device may be an attractive power source candidate for low power application.     

Model-based investigation of a CO preferential oxidation reactor for polymer electrolyte fuel cell systems

This paper deals with a two-dimensional model of a preferential oxidation (PROX) reactor to be used in a beta 5 kWe hydrogen generator, named HYGen II, to integrate with polymer electrolyte fuel cells (PEFCs) for residential applications. The reactor geometrical configuration developed is a single-stage multi-tube configuration, in which a cocurrent air flow in the interspace is aimed at improving heat transfer and consequently controlling the temperature of the reactor. The aim of the model is to investigate the process performance of the reactor in order to enhance optimization and control of the PROX unit. The model concerns chemical kinetics and heat and mass transfer phenomena in the reactor. The model plays a key role in overcoming the issues of system design, by evaluating the temperature and the gas concentration profiles in the reactor. The CO removal from simulated reformate was examined with varying inlet temperature. Simulation results showed the strong dependence of the overall performance upon the reactor geometrical configuration.     

CeO2-supported Pt–Cu alloy nanoparticles synthesized by radiolytic process for highly selective CO oxidation

CeO₂-supported Pt–Cu bimetallic catalysts were synthesized by radiolytic process and their PROX activities were evaluated in relation to structural properties of the catalysts. Irradiating the aqueous precursor solution yielded Pt–Cu alloy nanoparticles and amorphous-like CuO on CeO₂ which are thermodynamically stable products formed from reduced Pt and Cu. Addition of Cu to Pt significantly improved CO selectivity in PROX reaction. The Pt–Cu catalysts had wide temperature window for 100% CO conversion in contrast to very narrow window for monometallic Pt and Cu catalysts. Much lower light-off temperature for Pt–Cu catalysts than Cu catalyst revealed that Pt-Cu alloy surface is the active center. Regardless of the amount of CuO phase, the bimetallic catalyst exhibited high catalytic performance, which further revealed that Cu in close contact with Pt is responsible for the improved selectivity. The CuO phase was suggested to promote oxygen supply to CO chemisorbed on Pt–Cu alloy surface.     

Improving temperature uniformity and performance of CO preferential oxidation for hydrogen-rich reformate with a heat pipe

Preferential oxidation (PROX) is an effective, but highly temperature-sensitive, method of CO removal for hydrogen-rich reformates. In a packed-bed catalytic reactor, oxidation is strongest at the inlet side and the local catalyst pellets become over-heated with poor heat conduction. As a result, the enhanced parasitic H₂ oxidation consumes oxygen and suppresses CO conversion. This study applies a heat pipe to improve the temperature uniformity in a tubular one-stage packed-bed reactor by transporting heat downstream and thereby improve CO removal. In the experiments, the fuel mixture containing 2% of CO, 75% of H₂, and 23% of CO₂, further mixed with air at O₂/CO = 0.75, 1.0 or 1.25, is supplied with stepwise increase of feeding rate under a fixed environmental temperature of 99 ± 1 °C. The proposed simple method is found to significantly improve temperature uniformity and CO removal for the present test conditions with O₂/CO = 1.0 and 1.25.     

Final step for CO syngas clean-up: Comparison between CO-PROX and CO-SMET processes

The performance of the CO preferential oxidation (PROX) process was compared with the CO selective methanation (SMET) one, both applied as the last clean-up process step of a fuel processor unit (FPU) to remove CO from syngas. The FPU was completed with the reformer (autothermal reformer ATR or steam reformer SR) and a non-isothermal water gas shift (NI-WGS) reactor. Furthermore, the reforming of different hydrocarbon fuels, among those most commonly found in service stations (gasoline, light diesel oil and compressed natural gas) was examined. The comparison, in terms of different FPU configurations and fuels, was carried out by a series of steady-state system simulations in Aspen Plus^®. From the obtained data, the performance of CO-PROX was not very different from that of CO-SMET, making it complex to give a definitive answer on the best FPU scheme. The most promising fuel processor with respect to performance is a chain of ATR, NI-WGS and CO-SMET. However, maintaining the same chain of clean-up reactors, the FPU with SR instead of ATR could also be a satisfactory choice. Even if there are lower efficiencies and H₂ specific production compared to the ATR-based FPU, the SR-based one does not produce a syngas with the high N₂ concentration typical of the ATR-based FPU. The syngas dilution by nitrogen is somehow detrimental for the stack efficiency, when syngas feeds PEM-FCs, since it lowers the polarization curve.     

Design of efficient noble metal single-atom and cluster catalysts toward low-temperature preferential oxidation of CO in H2

The preferential oxidation of CO in H₂ is attractive for the removal of trace amounts of CO to meet the requirement of proton-exchange membrane fuel cells (PEMFCs) application. The key is to design highly effective catalysts that work well in a wide range of low temperatures. Here, the recent progress in Au and Pt group metal catalysts for the PROX reaction is summarized, covering those with single-atom and cluster dispersed metal species with remarkable performance. Firstly, the progress of some representative catalysts is overviewed, with an emphasis on the strategies for improving low-temperature activity, selectivity, and stability. Then, special attention is focused on the key parameters affecting performance in the PROX reaction. Moreover, the reaction mechanisms in terms of adsorption and activation of reactants are discussed. Finally, the challenges and opportunities are offered for guiding the design of advanced noble metal catalysts toward the PROX process.     

Integrated methanol reforming and oxidation in wash-coated microreactors: A three-dimensional simulation

A microreactor consisting of two parallel channels is numerically simulated where methanol steam reforming takes place in one channel, and the required heat is supplied by methanol oxidation in the other channel. Effects of different parameters on methanol conversion, hydrogen yield and CO concentration are examined. Results from the parametric study are then used to propose conditions for high methanol conversion and hydrogen yield. A microreactor with enhanced output conditions is thus designed which is capable of producing a gas stream consisting of 74% hydrogen (dry). CO concentration in the generated synthesis gas stream is low enough to require only a PROX reactor for CO clean-up, eliminating the need for a bulky water–gas shift reactor. The produced hydrogen from an assembly of such microreactors can feed a low-power PEM fuel cell. A cluster of these microreactors would take a volume of about 91 cm³ to feed a typical 30-watt PEM fuel cell.     

Thermally integrated fuel processor design for fuel cell applications

Effective thermal integration could enable the use of compact fuel processors with PEM fuel cell-based power systems. These systems have potential for deployment in distributed, stationary electricity generation using natural gas. This paper describes a concept wherein the latent heat of vaporization of H₂O is used to control the axial temperature gradient of a fuel processor consisting of an autothermal reformer (ATR) with water gas shift (WGS) and preferential oxidation (PROX) reactors to manage the CO exhaust concentration. A prototype was experimentally evaluated using methane fuel over a range of external heat addition and thermal inputs. The experiments confirmed that the axial temperature profile of the fuel processor can be controlled by managing only the vapor fraction of the premixed reactant stream. The optimal temperature profile is shown to result in high thermal efficiency and a CO concentration less than 40 ppm at the exit of the PROX reactor.     

Development of 10-kWe preferential oxidation system for fuel cell vehicles

A preferential oxidation (PROX) reactor for a 10-kWe polymer electrolyte membrane fuel cell (PEMFC) system is developed. Pt-Ru/Al₂O₃ catalyst powder, with a size of 300–600 μm is applied for the PROX reaction. To minimize pressure drop and to avoid hot spots in the catalyst bed, the reactor is designed as a dual-staged, multi-tube system. The performance of the 10-kWe PROX unit is evaluated by feeding simulated gasoline reformate which contains 1.2 wt.% carbon monoxide (CO). The CO concentration of the treated reformate is lower than 20 ppm in the steady-state and is under 30 ppm at 65% load change. Hydrogen loss in the steady-state is about 1.5% and the pressure drop across the reactor is 4 psi. Start-up characteristics of the 10-kWe PROX system are also investigated. It takes 3 min to reduce the CO concentration to below 20 ppm. Several controllable factors are found to shorten the start-up time.     

Microfibrous structured catalytic packings for miniature methanol fuel processor: Methanol steam reforming and CO preferential oxidation

The microfibrous structured catalytic packings for miniature fuel processor consisting of a methanol steam reformer and a subsequent CO cleanup train has been investigated experimentally. A highly void and tailorable sinter-locked microfibrous carrier consisting of 3.5 vol% 8 μm diameter Ni-fibers is used to entrap 35 vol% 150-250 μm catalyst particulates for both methanol steam reforming (MSR) and CO preferential oxidation (PROX). We demonstrate a microfibrous entrapped Pd-ZnO/Al₂O₃ catalyst packings for high efficiency hydrogen production by the MSR reaction. The use of microfibrous entrapment technology significantly enhances the catalyst utilization efficiency by a 4-fold improvement of the weight hourly space velocity (WHSV), compared to the single Pd-ZnO/Al₂O₃ particulates as keeping the methanol conversion at >98%. The microfibrous entrapped Pt-Co/Al₂O₃ catalyst packings can drive the CO from 2% down to <50 ppm at 150 °C with O₂/CO ratio of 1 using a gas hourly space velocity (GHSV) of 32,000 h⁻¹. Finally, a prototype fuel processor system integrating MSR reformer and CO PROX train is demonstrated as three reactors in series. Such test rig is capable of producing roughly 1700 standard cubic centimeter per minute (sccm) PEMFC-grade H₂ (equivalent to ∼163 W of electric power) in a longer-term test, in which the MSR reactor is operated at 300 °C using a methanol/water (1/1.1, mole) mixture WHSV of 9 h⁻¹ and CO PROX reactors at 150 °C using an O₂/CO molar ratio of 1.3, respectively. In the test of this prototype system, MSR reactor delivers >97% methanol conversion throughout the entire 1200-h test; the CO cleanup train placed in line after 800-h MSR illustrates the capability to decrease the CO concentration from ∼3.5% to ∼1% at PROX-1 and then to less than 20 ppm at PROX-2 until to the end of test.     

Three-dimensional simulation and optimization of an isothermal PROX microreactor for fuel cell applications

Three-dimensional numerical simulations of the reacting flow in rectangular micro-channel PROX reactors are performed. To solve the set of governing equations, a finite volume method is applied using an improved SIMPLE algorithm. A three-step surface kinetics for the chemical reactions is utilized that includes hydrogen oxidation, carbon monoxide oxidation, and water–gas shift reaction. The kinetics chosen are for a Pt–Fe/γ-Al₂O₃ catalyst and operating temperatures of about 100 °C. The PROX reactor is expected to remove the carbon monoxide content in a hydrogen-rich stream from about 2% to less than 10 ppm. Effects of the inlet steam content, oxygen to carbon monoxide ratio, reactor wall temperature, aspect ratio of the channel cross section, and the channel hydraulic diameter are investigated. It is found that increasing the steam content, oxygen to carbon monoxide ratio, or wall temperature may improve the performance of the microreactor. It is also shown that the rate of water–gas shift reaction or its reverse is much lower than the oxidation reactions. Finally, it is revealed that based on a modified CO yield definition, the optimum channel geometry is a square shape.     

Kinetics,simulation and insights for CO selective oxidation in fuel cell applications

The kinetics of CO preferential oxidation (PROX) was studied to evaluate various rate expressions and to simulate the performance the CO oxidation step of a methanol fuel processor for fuel cell applications. The reaction was carried out in a micro reactor testing unit using a commercial Engelhard Selectoxo (Pt–Fe/γ-alumina) catalyst and three self-prepared catalysts. Temperature was varied between 100 and 300 °C, and a of range feed rates and compositions were tested. A reaction model in which three reactions (CO oxidation, H₂ oxidation and the water gas shift reaction) occur simultaneously was chosen to predict the reactor performance. Using non-linear least squares, empirical power-law type rate expressions were found to fit the experimental data. It was critical to include all three reactions to determine good fitting results. In particular, the reverse water gas shift reaction had an important role when fitting the experimental data precisely and explained the selectivity decrease at higher reaction temperatures. Using this three reaction model, several simulation studies for a commercial PROX reactor were performed. In these simulations, the effect of O₂/CO ratio, the effect of water addition, and various non-isothermal modes of operation were evaluated. The results of the simulation were compared with corresponding experimental data and shows good agreement.     

Synthesis of highly-dispersed CuO–CeO2 catalyst through a chemisorption-hydrolysis route for CO preferential oxidation in H2-rich stream

The CuO–CeO₂ catalyst (CuO loading: 15 wt%) was prepared by a novel chemisorption-hydrolysis method, and employed for the preferential oxidation of CO (CO PROX) in H₂-rich stream. For comparison, several other conventional methods such as impregnation, co-precipitation and deposition-precipitation were also used to prepare the catalyst. It is found that the CuO–CeO₂ catalyst prepared by chemisorption-hydrolysis method exhibits the best catalytic performance, giving not only the widest temperature window (120–170 °C) for CO complete conversion, but also the highest oxygen to CO₂ selectivity of 99.9% at 120 °C. The results of XRD, N₂O chemisorption and in-situ FT-IR conformably indicate that this catalyst possesses the highest dispersion of Cu species, which facilitates the formation of Cu⁺ carbonyl species, and simultaneously prevents the adsorption and oxidation of H₂. With the increase of reaction temperature, Cu⁺ is gradually reduced to Cu⁰, enhancing the adsorption and oxidation of H₂, as a result, the selectivity of oxygen towards CO₂ is lowered obviously. The presence of CO₂ and H₂O exhibits negative effects on the catalytic performance, shortening the activity window to 150–170 °C region and decreasing the CO₂ selectivity to 87% at the temperature for the initial 100% conversion of CO. Based on the above study, a potential reaction pathway for CO PROX over the CuO–CeO₂ catalyst is proposed.     

Novel highly efficient alumina-supported cobalt nitride catalyst for preferential CO oxidation at high temperatures

Novel alumina-supported cobalt nitride catalysts with Co loading ranging from 1 to 10 wt% prepared by NH₃-temperature-programmed reaction were investigated as potential catalysts for preferential CO oxidation (PROX) in excess H₂ at high temperatures. The formation of the Co₄N phase was confirmed by a combination of XRD and XPS, and the Co 2p binding energies of Co₄N reported previously were corrected to 798.2 ± 0.2 and 782.5 ± 0.2 eV. We observed that the catalytic activities of these nitrided Co/γ-Al₂O₃ catalysts were greatly related to their Co loadings. The nitrided 3 wt% Co/γ-Al₂O₃ catalyst showed the best PROX performance in temperature range of 200-220 °C, which was quite different from Co oxide precursor but was similar to Pt-group metals.     

Preferential catalytic oxidation of carbon monoxide in presence of hydrogen over bimetallic AuPt supported on zeolite catalysts

A series of AuPt/A zeolite and Pt/A zeolite catalysts prepared by incipient wetness impregnation are investigated for the preferential oxidation (PROX) of carbon monoxide in the presence of hydrogen over the temperature range of 50–310 °C under atmospheric pressure. The results indicate that when a small amount of gold is added to the Pt/A zeolite catalyst, the CO selectivity is improved at low temperatures, and 1% AuPt/A zeolite (at a weight ratio of Au:Pt = 1:2) gives the best performance, which provides a high CO conversion in combination with a high CO selectivity. In more realistic simulated reformate gases containing 10% CO₂ and 10% H₂O, there is not much difference between those in the presence and the absence of CO₂ and H₂O. Transmission electron microscopic and X-ray diffraction studies show that the two metals, Au and Pt, appear to be severely phase separated, which is confirmed by energy dispersive investigations.     

Catalytic performance of a copper–promoted CeO2 catalyst in the CO oxidation: Influence of the operating variables and kinetic study

A study of the CO oxidation reaction was conducted on a CuO–CeO₂ catalyst, with a copper content of 20 at%. The stability, activity and selectivity of this sample were evaluated in the absence of H₂ and also under PROX conditions (i.e. in great H₂ excess). The influence of the reaction temperature and the feed composition was also analyzed. It was found that water and CO₂ have a negative effect on the catalytic activity. Except for the undesired oxidation of hydrogen, the occurrence of other lateral reactions that could be present in the PROX reactor can be discarded. A kinetic study was carried out in order to fit a classical power-law expression for CO oxidation rate. Although this mathematical expression gave a good fit in limited concentration ranges, it was found that the partial reaction orders depend on the reactant molar fractions.     

The cleanup of CO in hydrogen for PEMFC applications using Pt,Ru, Co,and Fe in PROX reaction

An experimental investigation is performed into the cleanup of CO in hydrogen for proton exchange membrane fuel cell (PEMFC) using Pt/Al₂O₃ and Ru/Al₂O₃ catalysts. Additionally, the effects of adding the transition metals Co and Fe to a Ru/Al₂O₃ catalyst are examined. The results show that as the level of Pt addition is increased, the maximum CO conversion rate is achieved at a lower temperature. With Ru/Al₂O₃ catalysts, the CO conversion rate increases significantly with increasing Ru addition at temperatures lower than 80 °C For both catalysts, the methane yield increases with increasing temperature and increasing noble metal addition. At temperatures in the range of 100–140 °C, the CO conversion rate and methane yield of the Pt- and Ru-based preferential oxidation (PROX) reactions are both insensitive to the density of the honeycomb carrier. The CO conversion rate is significantly improved by the addition of Fe at temperatures lower than 160 °C and by the addition of Co at temperatures higher than 200 °C. Of the two metals, Fe results in a greater reduction of the methane yield at high temperatures. Finally, both catalysts achieve a stable cleanup performance over the course of a 12-h stability test and suppress the CO concentration to an acceptable level for PEMFC applications.