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.     

Recent progress in catalytical CO purification of H2-rich reformate for proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) fueled by hydrogen-rich gas are promising systems to substitute fossil fuel resources. This review evaluates state of the art and perspectives for the catalysts such as supported Ni, noble metals, and base metal oxide catalysts used for catalytical CO purification (SMET, PROX) in H₂-rich reformates for PEMFCs applications. The factors affecting activity like support effect, metal size effect, promoters, and metal-support interaction are assessed and thoroughly discussed. It is remarked that the challenges for their practical applications are to (i) achieve acceptable CO outlet concentration in ppm level with wide temperature window (ii) minimize undesired loss of H₂ and inhibit the occurrence of side reactions (iii) develop high performance catalysts with high resistance to CO₂ and steam under realistic conditions. Developing novel catalysts for catalytical CO purification based on the structure-activity relationship will resolve the challenges required for their practical applications.     

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.     

Bismuth effect on the Co–Mn/Ce0.85Zr0.15O2 nanoparticulate for CO preferential oxidation in simulated syngas

The novel and efficient bismuth modified supported Co–Mn catalysts were prepared and employed to catalyze the preferential oxidation of CO (CO PROX) in simulated syngas. The effects of introducing-methods and loadings of bismuth on both catalytic performance and catalyst nature were investigated. The N₂ adsorption/desorption measurement, X-ray diffraction (XRD) and H₂-temperature programmed reduction (H₂-TPR), and O₂-TPD (O₂-temperature programmed desorption) characterization techniques were performed to reveal the relationship between the catalytic properties and the nature of the catalysts. Results demonstrate that the as-prepared Bi modified supported Co–Mn catalyst exhibits excellent catalytic performance, depending on the introducing method and loadings of Bi. The enhancement of Bi addition into supported Co–Mn catalyst in the catalytic performance for CO PROX reaction is mainly ascribed to the dramatically improved reducibility of the Bi modified sample. Moreover, the decrease in hydrogen transformation over the Bi modified samples can be observed, suggesting the introduction of Bi can compress the catalytic activity for hydrogen oxidation. This study definitely demonstrates the existence of synergistic effect between the added bismuth and Co–Mn/Ce_0.85Zr_0.15O₂ in the Bi modified supported Co–Mn catalyst for CO PROX reaction. The developed Co₃O₄–MnO_x/Ce_0.85Zr_0.15O₂–Bi₂O₃ catalyst with bismuth content of 4.2 wt.% presents the outstanding catalytic activity, selectivity, and durability for CO PROX reaction in the simulated syngas, and it can be considered as a promising candidate for highly efficient CO elimination from H₂-rich stream.     

Mesoporous CexMn1−xO2 composites as novel alternative carriers of supported Co3O4 catalysts for CO preferential oxidation in H2 stream

The mesoporous Co₃O₄ supported catalysts on Ce–M–O (M = Mn, Zr, Sn, Fe and Ti) composites were prepared by surfactant-assisted co-precipitation with subsequent incipient wetness impregnation (SACP–IWI) method. The catalysts were employed to eliminate trace CO from H₂-rich gases through CO preferential oxidation (CO PROX) reaction. Effects of M type in Ce–M–O support, atomic ratio of Ce/(Ce + Mn), Co₃O₄ loading and the presence of H₂O and CO₂ in feed were investigated. Among the studied Ce–M–O composites, the Ce–Mn–O is a superior carrier to the others for supported Co₃O₄ catalysts in CO PROX reaction. Co₃O₄/Ce_0.9Mn_0.1O₂ with 25 wt.% loading exhibits excellent catalytic properties and the 100% CO conversion can be achieved at 125–200 °C. Even with 10 vol.% H₂O and 10 vol.% CO₂ in feed, the complete CO transformation can still be maintained at a wide temperature range of 190–225 °C. Characterization techniques containing N₂ adsorption/desorption, X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR) and scanning electron microscopy (SEM) were employed to reveal the relationship between the nature and catalytic performance of the developed catalysts. Results show that the specific surface area doesn’t obviously affect the catalytic performance of the supported cobalt catalysts, but the right M type in carrier with appropriate amount effectively improves the Co₃O₄ dispersibility and the redox behavior of the catalysts. The large reducible Co³⁺ amount and the high tolerance to reduction atmosphere resulted from the interfacial interaction between Co₃O₄ and Ce–Mn support may significantly contribute to the high catalytic performance for CO PROX reaction, even in the simulated syngas.     

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.     

Tuning activity of Pt/FeOx/TiO2 catalysts synthesized through selective-electrostatic adsorption for hydrogen purification by prox reaction

Hydrogen purification by removing CO traces was studied via the preferential CO oxidation (PROX) reaction using highly dispersed Pt catalysts supported on dual oxide FeO_x/TiO₂. These catalysts were prepared by the strong electrostatic adsorption (SEA) method by varying the pH of synthesis and the calcination temperature. By measuring the point of zero charge (PZC) of the support components, it was possible to determine the pH in which Pt can be selectively deposited onto one of the support components, obtaining Pt dispersion values above 90%. The selective SEA of a Pt precursor onto the co-support (FeO_x) was achieved at a synthesis pH between the PZCs of the support components (i.e., TiO₂ PZC = 5.2 and Fe₂O₃ PZC = 6.9) by using a Pt anionic complex. The catalytic activity for the PROX reaction, expressed in terms of the CO conversion, O₂ selectivity to CO₂, apparent activation energy, and turnover frequency, confirmed that the SEA prepared catalysts were active and selective for the PROX reaction. XPS and TPR results of the Pt/FeO_x/TiO₂ catalysts showed the formation of Pt-FeO_x interfaces, called as (Pt-FeO_x)_i interfacial sites, which enhanced the stability and catalytic activity for the PROX reaction. The concentration of these sites can be controlled by the synthesis conditions used, mainly pH and to a lower extent the calcination temperature.     

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.     

Correlation between CO surface coverage and selectivity/kinetics for the preferential CO oxidation over Pt/γ-Al2O3 and Au/α-Fe2O3: an in-situ DRIFTS study

We present in-situ IR (DRIFTS) measurements on CO adsorption and preferential CO oxidation (PROX) in H₂-rich gas on Pt/γ-Al₂O₃ and Au/α-Fe₂O₃ catalysts at their envisaged operating temperatures of 200°C and 80°C, respectively, which in combination with kinetic data show that the underlying reason for the very different PROX reaction kinetics on these two catalysts is the difference in steady-state CO coverage. Whereas on the platinum catalyst this is always near saturation under reaction conditions, causing a negative reaction order (−0.4) and a p_CO-independent selectivity, the amount of adsorbed CO on the gold particles (indicated by an IR band at ∼2110 cm⁻¹) strongly depends on the CO partial pressure. From the position of the IR band of CO adsorbed on Au/α-Fe₂O₃, the steady-state coverages on the Au surface are shown to be significantly below saturation, with an upper limit of approximately θ_CO=0.2. Low reactant surface concentrations on Au explain the positive reaction order with respect to p_CO (+0.55 at 80°C) as well as the observed decoupling of the CO and H₂ oxidation rates, which results in a loss of selectivity with decreasing p_CO.     

CuO–CeO2 catalysts synthesized in one-step: Characterization and PROX performance

PEM fuel cells seem to be the most affordable and commercially viable hydrogen-based cells, the biggest challenge being to obtain CO-free H₂ (<100 ppm) as the fuel. In this study, the use of CuO–CeO₂ catalysts in preferential oxidation of CO to obtain CO-free H₂ (PROX reaction) was investigated. Ce_1−xCu_xO₂ catalysts, with x (mol%) = 0, 0.01, 0.03, 0.05 and 0.10, were synthesized in one-step by the polymeric precursor method, to obtain a very fine dispersion and strong metal-support interaction, to favor active copper species and a preference for the PROX reaction. The results obtained from catalyzed reactions and characterization of the catalysts by XRD, Rietveld refinement, BET surface area, UV–Vis and TPR, suggest that this one-step synthesis method gives rise to catalysts with copper species selective for the PROX reaction, which reaches a maximum rate on Ce_0.97Cu_0.03O₂ catalyst.     

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.     

Copper manganite as a catalyst for the PROX reaction. Deactivation studies

CuMn₂O₄ nanocatalysts synthesised by silica aquagel confined co-precipitation were analysed for the preferential oxidation of CO at different temperatures and concentration conditions. The catalysts show a higher activity than copper–ceria catalysts synthesised by the same method but, like these, they suffer from slow deactivation during the reaction. Surface analysis (FTIR and XPS) was used to unravel the deactivation mechanisms. Gradual reduction of the catalysts by the carbon monoxide present in the PROX stream was concluded to be the main cause of deactivation.     

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.     

Alumina supported Au/Y-doped ceria catalysts for pure hydrogen production via PROX

Gold catalysts on Y-doped ceria dispersed on high surface area γ-Al₂O₃ were synthesized and tested in preferential CO oxidation in hydrogen rich stream (PROX). The effect of ceria loading (10, 20 or 30 wt%) was studied. The gold catalyst with the lowest ceria amount exhibited the highest PROX activity. The addition of Y₂O₃ (1 wt%) led to improved performance. The most favorable effect was observed in the sample with 20 wt% ceria amount. This gold catalyst showed good PROX activity and stability in the presence of CO₂ and water. Catalysts characterization by XRD, HRTEM/HAADF, XPS and H₂-TPR was used to elucidate the relationship between the chemical composition, state of gold, support features and catalytic properties.     

Model-based analysis of reactor geometrical configuration on CO preferential oxidation performance

Modeling studies have been conducted on Preferential Oxidation (PROX) Reaction, covering chemical kinetics and heat/mass transfer phenomena that occur in a shell and tube system, to be used in a beta 5 kWe hydrogen generator for Polymer Electrolyte Fuel Cells (PEFCs). The critical issue in the PROX reactor design is to achieve temperature control along the catalyst bed, because poor selectivities primarily result from excess reactor temperature. Aim of the model is to investigate the effects of the reactor dimensions on process performance, in order to obtain high CO conversion and high selectivity with respect to the undesired H₂ oxidation. The CO removal from simulated reformate was examined, by evaluating the temperature and the gas concentration profiles along the reactor. The sensitivity analysis showed that the overall performance is strongly dependent upon the geometrical configurations examined.     

Preferential oxidation of CO in H2 stream on Au/ZnO–TiO2 catalysts

A series of gold catalysts supported on ZnO–TiO₂ with various ZnO contents were prepared. ZnO–TiO₂ was prepared by incipient-wetness impregnation using aqueous solution of Zn(NO₃)₂ onto TiO₂. Gold catalysts with nominal gold loading of 1 wt. % were prepared by deposition-precipitation (DP) method. Various preparation parameters, such as pH value and Zn/Ti ratio on the characteristics of the catalysts were investigated. The catalysts were characterized by inductively-coupled plasma–mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The preferential oxidation of CO in H₂ stream (PROX) on these catalysts was carried out in a fixed bed micro-reactor with a feed of CO: O₂: H₂: He = 1: 1: 49: 49 (volume ratios) and a space velocity of 30,000 ml/g h. Limited amount of oxygen was used in the feed. A high gold dispersion and narrow gold particle size distribution was obtained. Au/ZnO–TiO₂ with Zn/Ti atomic ratio of 5/95 showed the highest CO conversion at room temperature. The conversion increased with increasing temperature even in the presence of limited amount of oxygen, showing suppression in H₂ oxidation. Au/ZnO–TiO₂ prepared at pH 6 had a higher CO conversion and higher selectivity of CO oxidation than those prepared at other pH values. The addition of ZnO on TiO₂ resulted in higher dispersion of gold particles and narrow particle size distribution. The stronger the Au–Zn(OH)₂ interaction, the finer the supported Au nanoparticles, and the better the catalytic performance of the catalyst for PROX reaction. Part of Au was in Au⁺ state due to the interaction with Zn(OH)₂ and nano Au size. The oxidation state of gold species played an important role in determining its CO conversion and selectivity of CO oxidation in hydrogen stream. The catalysts were stable at 80 °C for more than 80 h.     

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.     

TiO2 and ZrO2 supported Ru catalysts for CO mitigation following the water-gas shift reaction

CO oxidation and methanation over Ru-TiO₂ and Ru-ZrO₂ catalysts were investigated for CO removal for applications in proton exchange membrane fuel cells. The catalysts were synthesised by the deposition precipitation method at a pH of 7–7.5 for better interactions between the support and the active Ru metal. Various characterization experiments such as TPR, XPS, FTIR-CO, CO chemisorption and HRTEM were conducted to better understand the physio-chemical properties of Ru on the supports. Both catalysts showed excellent activity for the total oxidation of CO, however, with the addition of H₂, the catalysts activity to CO oxidation decreased significantly. Higher temperatures for the preferential oxidation reaction indicated that the Ru catalysts not only oxidize CO, but hydrogenate it as well. Furthermore, H₂ oxidation was favoured over the catalysts. Hydrogenation of CO over these catalysts gave high CO conversion and selectivity towards CH₄. Both the catalysts showed similar activity across the temperature range screened and gave maximum CO conversions of 99.9% from 240 °C onwards, with 99.9% selectivity towards CH₄. The catalysts also showed good stability in the reaction and the similarities in the catalytic activity of these were attributed to the well-dispersed Ru metal over the supports. The Ru catalysts effectively reduced CO concentrations in the reformate gas to less than 10 ppm, as is required for practical applications.     

Preferential CO oxidation on Ru/Al2O3 catalyst: An investigation by considering the simultaneously involved methanation

The CO removal with preferential CO oxidation (PROX) over an industrial 0.5% Ru/Al₂O₃ catalyst from simulated reformates was examined and evaluated through considering its simultaneously involved oxidation and methanation reactions. It was found that the CO removal was fully due to the preferential oxidation of CO until 383 K. Over this temperature, the simultaneous CO methanation was started to make a contribution, which compensated for the decrease in the removal due to the decreased selectivity of PROX at higher temperatures. This consequently kept the effluent CO content as well as the overall selectivity estimated as the ratio of the removed CO amount over the sum of the consumed O₂ and formed CH₄ amounts from apparently increasing with raising reaction temperature from 383 to 443 K when the CO₂ methanation was yet not fully started. At these temperatures the tested catalyst enabled the initial CO content of up to 1.0 vol.% to be removed to several tens of ppm at an overall selectivity of about 0.4 from simulated reformates containing 70 vol.% H₂, 30 vol.% CO₂ and with steam of up to 0.45 (volume) of dry gas. Varying space velocity in less than 9000 h⁻¹ did not much change the stated overall selectivity. From the viewpoint of CO removal the article thus concluded that the methanation activity of the tested Ru/Al₂O₃ greatly extended its working temperatures for PROX, demonstrating actually a feasible way to formulate PROX catalysts that enable broad windows of suitable working temperatures.     

Au/TiO2 catalysts prepared by photo-deposition method for selective CO oxidation in H2 stream

A series of Au/TiO₂ catalysts were prepared by photo-deposition (PD) method. Various preparation parameters, such as pH value, power of UV light and irradiation time on the characteristics of the catalysts were investigated. The catalysts were characterized by inductively-coupled plasma-mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The preferential oxidation of CO in H₂ stream (PROX) on these catalysts was carried out in a fixed-bed micro reactor with a feed of CO: O₂: H₂: He = 1: 1: 49: 49 (volume ratios) and a space velocity of 30,000 ml/g h. Limited amount of O₂ was used to investigate the selectivity of O₂ reacting with CO or H₂. Au/TiO₂ catalysts prepared by PD method showed narrow particle size distribution of gold particles within few nanometers and were found to be 1.5 nm. The particle size of gold nanoparticles deposited on the support depends on irradiation time, UV light source and pH value of preparation. The electronic structure of Au was a function of particle size. The smaller the Au particle size was, the higher the concentration of Au cation was. Using weak power of UV light, appropriate irradiation time and suitable pH value, very fine gold particles on the support could be obtained even in the powder form. The samples prepared with PD method did not need heat treatment to reduce Au cation, UV irradiation could reduce it. Therefore it is easier to have smaller particle size. Au/TiO₂ catalysts prepared by PD method were very active and selective in PROX reaction. In long time test, the catalysts were stable at 80 °C for more than 60 h.