Cooperative effect of Pt and Cu on CeO2 for the CO-PROX reaction under CO2–H2O feed stream

Catalyst improvement for the preferential oxidation of CO (CO-PROX) is essential in developing efficient fuel cell technologies. Here, we investigate the promotion of the Cu/CeO₂ system with Pt, prepared by impregnation and alcohol-reduction methods, in the CO-PROX reaction under ideal and realistic feed compositions. The high Pt dispersion in PtCu/CeO₂ prepared by impregnation led to a CO conversion of 62% and CO₂ selectivity of 83% at 50 °C under a feed stream composed of H₂/CO/O₂, while monometallic Cu/CeO₂ and Pt/CeO₂ showed negligible activity at these conditions. By adding CO₂–H₂O to the feed stream, PtCu/CeO₂ catalysts prepared by both methods presented similar activity. The maximum CO conversion temperature was shifted to 100 °C. Under these conditions, Cu/CeO₂ was inactive, and Pt/CeO₂ showed identical conversion but lower CO₂ selectivity. In-situ XANES revealed that fast oxidation of Cu species at low temperatures is responsible for Cu/CeO₂ deactivation, while preferential adsorption of CO on Pt⁰ sites in PtCu/CeO₂ avoided deactivation. The use of deactivation-resistant Pt sites as complimentary sites for CO activation associated with improved oxygen mobility over Cu–CeO₂ surface proved to be an effective strategy for CO-PROX under H₂O/CO₂ feed stream at low temperatures.     

Comparative study of CeO2/CuO and CuO/CeO2 catalysts on catalytic performance for preferential CO oxidation

The CeO₂/CuO and CuO/CeO₂ catalysts were synthesized by the hydrothermal method and characterized via XRD, SEM, H₂-TPR, HRTEM, XPS and N₂ adsorption–desorption techniques. The study shows that the rod-like structure is self-assembled CeO₂, and both hydrothermal time and Ce/Cu molar ratio are important factors when the particle-like CeO₂ is being self-assembled into the rod-like CeO₂. The CuO is key active component in the CO-PROX reaction, and its reduction has a negative influence on the selective oxidation of CO. The advantage of the inverse CeO₂/CuO catalyst is that it still can provide sufficient CuO for CO oxidation before 200 °C in the hydrogen-rich reductive gasses. The traditional CuO/CeO₂ catalyst shows better activity at lower temperature and the inverse CeO₂/CuO catalysts present higher CO₂ selectivity when the CO conversion reaches 100%. The performance of mixed sample verifies that they might be complementary in the CO-PROX system.     

Comparative study of CuO supported on CeO2, Ce0.8Zr0.2O2 and Ce0.8Al0.2O2 based catalysts in the CO-PROX reaction

CuO supported on CeO_2, Ce_0.8Zr_0.2O₂ and Ce_0.8Al_0.2O₂ based catalysts (6%wt Cu) were synthesized and tested in the preferential oxidation of CO in a H₂-rich stream (CO-PROX). Nanocrystalline supports, CeO₂ and solid solutions of modified CeO₂ with zirconium and aluminum were prepared by a freeze-drying method. CuO was supported by incipient wetness impregnation and calcination at 400 °C. All catalysts exhibit high activity in the CO-PROX reaction and selectivity to CO₂ at low reaction temperature, being the catalyst supported on CeO₂ the more active and stable. The influence of the presence of CO₂ and H₂O was also studied.     

CuO/CeO2 supported on Zr doped SBA-15 as catalysts for preferential CO oxidation (CO-PROX)

The application of the catalytic system CuO/CeO₂ supported on Zr doped SBA-15 mesoporous silica to the preferential oxidation of CO on hydrogen streams (CO-PROX) suitable to be used to feed PEM fuel cells, has been studied. A loading of 20% (wt.) Ce and 6% (wt.) Cu was found optimal for the CO-PROX reaction. The influence of the presence of CO₂ and H₂O in the gas feed was also studied in order to simulate the real operation conditions of a PEMFC feed stream generated by alcohol steam reforming. The catalysts were characterized by XRD, adsorption-desorption of N₂ at −196 °C, TEM, -H₂-TPR and XPS. The system reducibility was found modified by the incorporation of zirconium in the support, with improvement of both the conversion and selectivity of the catalytic system, compared to the same material without Zr.     

One-Step synthesis of PtFe/CeO2 catalyst for the Co-Preferential oxidation reaction at low temperatures

Active Pt-based catalysts at low temperature towards the preferential oxidation of carbon monoxide in hydrogen-rich stream reaction (CO-PROX) are of great importance for H₂-fueled fuel cells, but still remain a challenge. Herein, we propose a simple approach to synthesize a highly active Pt₂₀Fe/CeO₂ catalyst employing the borohydride reduction process. Transmission electronic microscopy revealed monodispersed 2.8 nm-Pt nanoparticles on CeO₂, and the role of Fe species on the activity is discussed. The excellent CO conversion of 99.6% and CO₂ selectivity of 92.3% carried out at ambient temperature meet the CO-PROX requirements for an adequate supply of hydrogen in fuel cell device.     

Enhanced stability for preferential oxidation of CO in H2 under H2O and CO2 atmosphere through interaction between iridium and copper species

Preferential oxidation of CO (CO-PROX) is one of the most investigated methods for reducing residual CO in H₂-rich stream to acceptable level in proton exchange membrane fuel cells. However, development of catalyst with high stability under simulated practical conditions is still challenging. Herein, a series of Cu_xCe_1-xO₂ (x = 0, 0.05, 0.09, 0.17) supported Ir catalysts were prepared and 1 wt%Ir/Cu_0.09Ce_0.91O₂ exhibited full conversion of CO in a wide temperature window (80–180 °C), excellent stability and resistance to CO₂ and H₂O poison. Characterization results reveal that the superior performance was mainly associated with the interaction between Ir and Cu species, which resulted in that the adsorbed H₂O on Ir sites was activated to react with adsorbed CO on Cu sites to form easily decomposable bicarbonates and formate species instead of main intermediate of carbonates for 1 wt% Ir/CeO₂ and Cu_0.09Ce_0.91O₂. This work provides a new sight for developing high-performance heterogeneous catalysts.     

Catalysts for H2 production using the ethanol steam reforming (a review)

This review aims to provide an overview of the main catalytic studies of H₂ production by ethanol steam reforming (ESR). The reaction is endothermic and produces H₂, CO₂, CH₄, CO and coke. The conversion and H₂ selectivity of these products depended greatly of the physicochemical properties of the catalysts, active metal, promoters, temperature, long-term reaction, water/ethanol ratio, space velocity, contact time, and presence of O₂. Initial total conversion has been reported in all catalysts evaluated between 300 and 850 °C. The noble catalysts with high selectivity to H₂ (more than 80%) were: Rh, Ru, Pd and Ir and non-noble metal catalysts were: Ni, Co and Cu. The support materials include CeO₂, ZnO, MgO, Al₂O₃, zeolites-Y, TiO₂, SiO₂, La₂O₂CO₃, CeO₂–ZrO₂ and hydrotalcites. The impregnation method produced the best noble metal catalysts in terms of selectivity and conversion. The decrease of coke was related with the presence of basic sites on the support.     

Combustion synthesis of copper catalysts for selective CO oxidation

Copper catalysts supported on ceria, zirconia and niobia were prepared by combustion method with urea, containing a CuO loading of 6 wt.%, and tested on selective oxidation of CO. The characterization of the samples by X-ray diffraction (XRD) presented the formation of solid solution on CuO–CeO₂ catalyst and a change in crystalline structure of the support with copper insertion on ZrO₂ and Nb₂O₅ catalysts. The analysis of temperature-programmed reduction (TPR) revealed different interaction degrees of copper with the supports, with reduction peaks between 222 and 390 °C. The temperature-programmed desorption of CO (TPD-CO) profiles showed formation of CO₂ and H₂ only for the ceria and zirconia catalysts. In relation to the catalytic tests, the CuO–CeO₂ catalyst presented the best performance, with CO conversion of 95% at 150 °C up to 45 h on stream, and CO₂ selectivity of 55%.     

Selective CO oxidation in H2 streams on CuO/CexZr1−xO2 catalysts: Correlation between activity and low temperature reducibility

Catalysts synthesized by incorporating CuO (7 wt.% of Cu) on six commercial Ce_xZr_1−xO₂ mixed oxides (x = 1, 0.8, 0.68, 0.5, 0.15, 0) have been prepared by conventional wetness impregnation method. These catalysts have been screened for CO oxidation in hydrogen streams (CO-PROX) and characterized by means of XRD, BET, Raman, XPS and H₂-TPR experiments. Activity towards CO oxidation in hydrogen streams has been discussed and correlated with the properties of the catalysts. XRD and Raman analysis of the supports show an increase of oxygen defect as Zr content increase. Below 150 °C the catalysts reducibility measured by H₂-TPR correlates with ceria content in the support, although an increase of Zr content in the support increases considerably the reduction degree of ceria in the 0–600 °C interval. Activity towards CO oxidation in hydrogen streams also correlates with Ce/Cu molar ratio and low temperature reducibility of copper species. Most of the catalysts give complete CO conversion with high selectivity operating with λ = 2. The most active catalysts is CuO supported on pure ceria, which is able to oxidize completely CO in the interval 96–164 °C, with maximum selectivity of 90%. On the other hand, the operation window becomes narrower as Zr content in the supports increases.     

Preferential oxidation of carbon monoxide in simulated reformatted gas over PtAu/CexZnyO2 catalysts

A series of Pt–Au catalysts prepared by co-precipitation (CP) and single step sol-gel (SSG) methods was investigated for selective CO oxidation. The characteristics of the prepared catalysts were determined by XRD, BET surface area, SEM, H₂-TPR, chemisorption analysis, and FTIR. The simulated reformatted gas consisted of 1% CO, 1% O₂, 0% to 10% H₂O, 0–20% CO₂, and 40% H₂ in He balance. The operating temperature range was varied from 50 °C to 190 °C at atmospheric pressure. The experimental results elucidated that the catalytic preparation method had a significant effect on the catalyst characteristics and its activity. The catalytic performance over PtAu/Ce₁Zn₁O₂ prepared by co-precipitation was higher than that of PtAu/CeO₂ and PtAu/ZnO because of the incorporation of Ce⁴⁺ ions and the Zn²⁺ ions in the lattice. To encourage better catalytic performance, the catalysts should be calcined at 500 °C for 5 h and pretreated in a H₂ atmosphere. The CO conversion for the single- and double-stage reaction was reduced when adding water vapor and CO₂ to the feedstream; the water vapor and CO₂ molecules compete for the adsorption with CO on the active sites of the catalysts. During the deactivation test for 60 h, the CO conversion and selectivity are maintained.     

Effect of copper loading on copper-ceria catalysts performance in CO selective oxidation for fuel cell applications

Copper–ceria catalysts with three different Cu loadings (1, 7 and 15 wt%) were prepared by incipient wet impregnation, dried at 120 °C and calcined in air at 500 °C. The as-prepared catalysts were characterized by XRD, BET, Diffuse Reflectance Spectroscopy (DRS–UV–visible), Raman spectroscopy, CO and H₂-TPR, CO-TPR, CO-TPD and Oxygen Storage Capacity (OSC) measurements (with CO and O₂ concentration step-changes). The results indicated a good dispersion of copper for catalysts with 1 and 7 wt% Cu; however, bulk CuO was present for catalyst with 15 wt% Cu loading. Catalyst with 7 wt% Cu was observed to have very high capacity to release lattice oxygen to oxidize CO at low temperature. Activity results for CO oxidation in the absence and in the presence of 60% H₂, demonstrated a very similar performance for catalysts with 7 and 15 wt% Cu (both with T₁₀₀ = 112 °C), and much better than that of catalyst loaded with 1 wt% Cu. Catalyst with 7 wt% of copper shows very high activity (100% in a wide temperature window) and selectivity (higher than 85%), which makes an attractive for its use in purification of hydrogen for fuel cell applications. The presence of a mixture of CO₂ and H₂O inhibited catalyst activity, with CuO/CeO₂ catalyst with 7 wt% Cu exhibiting the best performance in the overall reaction temperature range. This could be attributed to the presence of highly disperse copper, only part of it in deep interaction with ceria. The effect of O₂/CO ratio (λ) and the potential reversibility of the inhibitory effect of CO₂ and H₂O were also investigated.     

Study on water–gas shift reaction performance using Pt-based catalysts at high temperatures

The water–gas shift reaction (WGSR) performance was experimentally studied using Pt-based catalysts for temperature, time factor and steam to carbon (S/C) molar ratio at ranges of 750–850 °C, 10–20 g_cat h/mol_CO, and 1–5, respectively. Al₂O₃ spheres were used as the catalyst support. For the high S/C cases, it was found that CO conversion can be enhanced when Pt/CeO₂/Al₂O₃ catalyst was used as compared with Pt/Al₂O₃. For the low S/C ratio cases, CO conversion enhancement was not significant with the addition of CeO₂. It was also found that CO conversion was not influenced by the CeO₂ amount to a large extent. Using bimetallic Pt–Ni/CeO₂/Al₂O₃ catalyst, it was found that higher CO conversion can be obtained as compared with CO conversions obtained from monometallic catalysts (Pt/Al₂O₃ or Pt/CeO₂/Al₂O₃). The experimental data also indicated that good thermal stability can be obtained for the Pt-based catalysts studied.     

Hydrogen production from low temperature WGS reaction on co-precipitated Cu–CeO2 catalysts: An optimization of Cu loading

Low temperature water–gas shift (WGS) reaction has been carried out at the gas hourly space velocity of 72,152 h⁻¹ over Cu–CeO₂ catalyst prepared by a co-precipitation method. Cu loading was optimized to obtain highly active co-precipitated Cu–CeO₂ catalysts for low temperature WGS. 80 wt% Cu–CeO₂ exhibited the highest CO conversion as well as the most stable activity (X_CO > 46% at 240 °C for 100 h). The excellent catalytic performance is mainly due to a strong metal to support interaction, resulting in the prevention of Cu sintering.     

Catalysts of ceria supported on copper-chromium oxide: Ceria promotion of the CO-PROX activity

A copper-chromium mixed oxide with Cu:Cr 1:2 ratio has been prepared by a microemulsion method and employed as support of CeO₂ in different amounts from 5 to 80 wt%. The catalysts have been characterized by XRD, HRTEM/XEDS, H₂-TPR and XPS. The characterization results have been examined in correlation with their catalytic properties for the CO-PROX process complemented by operando-DRIFTS. The catalysts are formed by composites of CuCr₂O₄ and CuO on which aggregates of CeO₂ nanoparticles are dispersed. Incorporation of CeO₂ onto the CuCr mixed oxide appreciably enhances the CO-PROX performance of the catalyst. This is related to a promotion of the generation and stabilization of active partially reduced copper sites during the course of the interaction of the catalysts with the reactant mixture and which are evidenced to be formed on both CuCr oxide and CeO₂ components of the catalysts.     

Supported mesoporous Cu/CeO2-δ catalyst for CO2 reverse water–gas shift reaction to syngas

The design and development of a high performance hydrogenation catalyst is an important challenge in the utilization of CO₂ as resources. The catalytic performances of the supported catalyst can be effectively improved through the interaction between the active components and the support materials. The obtained results demonstrated that the oxygen vacancies and active Cu⁰ species as active sites can be formed in the Cu/CeO_2-δ catalysts by the H₂ reduction at 400 °C. The synergistic effect of the surface oxygen vacancies and active Cu⁰ species, and Cu⁰–CeO_2-δ interface structure enhanced catalytic activity of the supported xCu/CeO_2-δ catalysts. The electronic effect between Cu and Ce species boosted the adsorption and activation performances of the reactant CO₂ and H₂ molecules on the corresponding Cu/CeO_2-δ catalyst. The Cu/CeO_2-δ catalyst with the Cu loading of 8.0 wt% exhibited the highest CO₂ conversion rate in the RWGS reaction, reaching 1.38 mmol·g_cat⁻¹ min⁻¹ at 400 °C. Its excellent catalytic performance in the RWGS reaction was related to the complete synergistic interaction between the active species via Ce³⁺-□-Cu⁰ (□: oxygen vacancy). The Cu/CeO_2-δ composite material is a superior catalyst for the RWGS reaction because of its high CO₂ conversion and 100% CO selectivity.     

Modulating Cu species in copper–ceria catalyst for boosting CO preferential oxidation: Elucidating the role of ceria crystal plane

Copper–ceria has been regarded as an active catalyst for CO preferential oxidation (CO-PROX). However, modulation of Cu species by regulating the ceria crystal plane has remained largely underdeveloped. Herein, CuO/CeO₂-X catalysts (X stands for treating temperature) were synthesized by the freeze-drying impregnation method using CeO₂-X nanorods with variational termination planes as supports and employed to boost CO-PROX. The results from various characterization techniques reveal that the mass transfer between CuO_x clusters and CeO₂-X weakened, and the sintering of CuO_x enhanced as the stability of CeO₂-X exposed planes increased. It is manifested that the content and the effective diameter of CuO_x clusters increase, and the cerium doping in CuO_x clusters decreases. The CuO/CeO₂-400 catalyst prepared from CeO₂-400 with suitable crystal plane stability induces the most significant amount of Cu⁺–CO during the CO-PROX reaction, displaying optimal reactivity. This work expounds on the relationship between the initially exposed crystal plane of CeO₂ and the structure of Cu species after loading CuO, which is of great significance for designing efficient catalysts for CO-PROX.     

The effect of copper on the selective carbon monoxide oxidation over alumina supported gold catalysts

The selective oxidation of CO in the presence of H₂ was investigated on Au catalysts promoted with different amounts of Cu. Au catalysts were prepared by the deposition–precipitation method and exhibited a satisfactory activity at low temperature with adequate selectivity. A considerable improvement in CO conversion was achieved when the O₂/CO ratio was increased from the value of 0.5–1.0. The addition of Cu to Au/Al₂O₃ catalysts caused an increase in the selectivity to CO oxidation due to an interaction between Au and Cu on the surface of the catalysts. However, this beneficial effect was limited to an optimal content of Cu. The catalysts were characterized by temperature programmed reduction and DRS UV–vis spectroscopy, indicating the formation of small bimetallic Au–Cu particles. The presence of water vapor in the feed stream played a positive effect in the CO conversion and selectivity while the CO₂ presence diminished the CO conversion and selectivity. In the case of a realistic reformate, when both H₂O and CO₂ are present, the positive effect of H₂O was able to compensate the negative effect of CO₂ depending on the temperature of reaction.     

Na-intercalated carbon nanotubes-supported platinum nanoparticles as new highly effective catalysts for preferential CO oxidation in H2-rich stream

Na⁺-intercalated carbon nanotubes (Na-CNTs) were obtained by impregnation of CNTs with sodium acetate followed by annealing at high temperatures under argon. Stable Na-CNTs-supported Pt catalysts (Pt/Na-CNT catalysts) were then prepared for hydrogen purification via preferential CO oxidation in a H₂-rich stream (CO-PROX). Characteristic studies show that the content of Na⁺ species in CNTs is increased with increased annealing temperature and the Pt nanoparticles with an average size of 2–3 nm are uniformly dispersed on the surfaces of Na-CNTs. An optimized Pt/Na-CNT catalyst with 5 wt% Pt loading can completely remove CO from 40 °C to 200 °C. This catalyst also exhibits long-term stability for 1000 h at 100 °C in feed gas containing 1% CO, 1% O₂, 50% H₂, 15% CO₂, and 10% H₂O balanced with N₂. The electron transfer between the Pt nanoparticles and Na⁺ species plays an important role in enhancing the CO-PROX performance of the catalyst.     

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.     

Low-temperature CO preferential oxidation in H2-rich stream over iron modified Pd–Cu/hydroxyapatite catalyst

The effect of FeCl₃ addition on the catalytic property of Pd–Cu/hydroxyapatite (Pd–Cu/HAP) for low-temperature CO preferential oxidation (CO-PROX) under H₂-rich condition has been investigated. It can be found that CO conversion of Pd–Cu/HAP rapidly decreases from 56% to 21% within 2 h at 30 °C in the presence of water, however, the Pd–Cu–Fe/HAP with the Fe/Cu atomic ratio of 1:1 presents a stable CO conversion of 40% and CO₂ selectivity of 100% under the same reaction conditions. The characterization results display that the addition of FeCl₃ to Pd–Cu/HAP causes the formation of Fe₂O₃ species, and the strong interaction presents between Fe₂O₃ species and Pd–Cu/HAP. Thus, the Pd⁰ species generated during CO-PROX over Pd–Cu–Fe/HAP can be more easily oxidized than that over Pd–Cu/HAP, which could avoid H₂ adsorption on Pd⁰ species and maintain CO adsorption and activation.