CO-PROX Reaction Research Articles

Recent Advances in Regulating Ceramic Monolithic Catalyst Structure for Preferential Oxidation of CO in H2.

Preferential oxidation of CO (CO-PROX) has tremendous significance in purifying hydrogen for fuel cells to avoid catalyst poisoning by CO molecules. Traditional powder catalysts face numerous challenges, including high pressure drop, aggregation tendency, hotspot formation, poor mass and heat transfer efficiency, and inadequate thermal stability. Accordingly, ceramic monolithic catalysts, known as their excellent thermal stability, high surface area, and superior mass and heat transfer characteristics, are gaining increasing research attention. This review examines recent studies on ceramic monolithic catalysts in CO-PROX, placing emphasis on the regulation of active sites (e.g., precious metals like Pt and Au, and non-precious metals like CuO and CeO2), monolith structures, and coating strategies. In addition, the structure-catalytic performance relationships, as well as the potential and limitations of different ceramic monolithic catalysts in practical application, are discussed. Finally, the challenges of monolithic catalysts and future research prospects in CO-PROX reactions are highlighted.

MOF-derived high oxygen vacancies CuO/CeO2 catalysts for low-temperature CO preferential oxidation

The CO preferential oxidation reaction (CO-PROX) is an effective strategy to remove residual poisonous CO in proton exchange membrane fuel cells, in which oxygen vacancies play a critical role in CO adsorption and activation. Herein, a series of CuO/CeO2 catalysts derived from Ce-MOFs precursors were synthesized using different organic ligands via the hydrothermal method and the CO-PROX performance was investigated. The CuO/CeO2-135 catalyst derived from homophthalic tricarboxylic acid (1,3,5-H3BTC) exhibited superior catalytic performance with 100 % CO conversion at a relatively low temperature (T100% = 100 °C), with a wide reaction temperature range and excellent stability. The superior catalytic properties were attributed to the structural improvements provided by the 1,3,5-H3BTC precursors and the promotional effects of oxygen vacancies. Additionally, in-situ Raman spectroscopy was performed to verify the dynamic roles of oxygen vacancies for CO adsorption and activation, while in-situ DRIFTS analysis revealed key intermediates in the CO-PROX reaction, shedding light on the mechanistic aspects of the catalytic process. This work not only demonstrates insights into the effective CuO/CeO2 catalysts for CO preferential oxidation, but also provides a feasible way to synthesize MOF-derived catalysts.

Synergistic hydroxyl mechanism on halloysite-confined PtFe alloy boosting low-temperature CO-PROX performance

Both structural and adsorbed surface hydroxyl groups in solid materials are important carriers for oxygen species transport. The role of identifiable structural hydroxyl groups in catalytic reactions, however, is often underestimated. Here, we propose a synergistic hydroxyl reaction mechanism on Pt1Fe0.33/HNTs catalyst in CO-PROX reaction, which is critical to reveal the new way of oxygen transport in the oxidation reaction. Particularly, the Pt1Fe0.33 nanoparticles (NPs) have been confined in the cavity of halloysite nanotubes (HNTs) to activate structural hydroxyl groups linked to the AlVI via a two-step reduction treatment. The activated structural hydroxyl groups exhibit sensitivity to CO, as revealed by in situ DRIFTS and MS data. The experimental and DFT data demonstrate that the cooperation of Pt, Fe, and hydroxyl sites in the active units on Pt1Fe0.33/HNTs can induce a synergistic participation of structural and adsorbed hydroxyl groups in CO oxidation through the formate pathway. In such catalytic process, the oxygen mobility is dominated by oxygen-containing hydroxyl species rather than reactive oxygen species. This work contributes to a deeper understanding of the role of hydroxyl groups in catalysis and lays the foundation for establishing a comprehensive oxidation reaction mechanism in the future.

Effect of silica on generation and stability of Cu+ cations in CuO/CeO2 catalysts for CO oxidation in H2-rich atmosphere

Developing efficient CuO/CeO2 catalysts for CO preferential oxidation in H2-rich atmosphere (CO PROX) is essential for removing residual CO in the reformate gas. However, achieving robust performance in a wide working temperature window is still a great challenge. Herein, SiO2 was introduced to modify the CuO/CeO2 catalysts and the best catalyst exhibited 100 % of CO conversion and >50 % of CO2 selectivity from 120 to 220 °C over CO PROX reaction. Combined characterization techniques demonstrate the enhanced interaction between copper species and CeO2 support after deposition of SiO2, promoting the generation of active Cu+ sites as well as the reactivity of lattice oxygen in CeO2 for CO conversion. Besides, in presence of SiO2, the main intermediates changed from carbonates to formate species. More importantly, the active Cu+ sites were stabilized by SiO2 due to formation of Cu+-O-SiOx interface, retarding the competitive H2 oxidation at high temperatures.

Improved CO-PROX selectivity of CuO/CeO2 catalysts by decorating with lanthanum via surface Cuξ+ redox site

Hydrogen purification is essential to satisfy the various purity requirements for different applications. Preferential oxidation of carbon monoxide (CO-PROX) is a promising strategy for removing CO from H2-rich reformate in proton exchange membrane fuel cells (PEMFCs). Nonetheless, the large proportion of H2 production via methanol steam reforming (MSR) will intensify oxidation competition between H2 and CO, and the CO conversion and CO2 selectivity decrease considerably with increasing temperature. The tolerance of the PEMFCs against the CO species absorbed upon the surface presents the biggest dilemma. This work modified the conventional CuO/CeO2 catalyst in the CO-PROX by doping the support with lanthanum (La). CuO/La-CeO2 catalyst can effectively eliminate CO (<10 ppm) in 70 vol% H2 and exhibit superior stability over 50 % selectivity in CO-PROX reaction. The surface properties of the catalyst before and after modification are characterized by XRD, N2 isothermal adsorption–desorption, H2-TPR, XPS, (HR)TEM Raman and Operando FT-IR, demonstrating that the La-CeO2 anchors CuO particle provides stable low-valence Cu sites (Cuξ+) with enhanced catalytic selectivity.

Spectroscopic insights into gold-cerium oxidation states influencing catalytic activity and selectivity in CeO2-supported gold during CO-PROX reaction

Here we present an investigation into the evolution of gold-cerium oxidation states in Au/CeO2 catalysts concerning Au content and its relationship between the influence of these oxidation states on catalytic activity and selectivity during the CO-PROX reaction. The Au-loading notably influences the electronic states of gold and cerium species. As gold content increases, metallic gold particles form, and their stability under reaction conditions decreases. Oxygen vacancies and dynamic oxygen storage capacity (OSC) increase at low gold loadings (0.32 wt.%). Catalytic activity depends on Au content, exhibiting higher CO2 selectivity at lower Au content (0.32 wt.%) under 120 °C. CO oxidation activity heavily depends on dynamic OSC, which is associated with oxygen vacancies. Selectivity depends on the specific electronic properties of catalysts. CO2 selectivity depends on the presence of cationic Au species. Meanwhile, H2 oxidation occurs on surfaces where a high content of metallic Au species is exposed on the catalyst surface.

Highly homogeneous multicomponent mesoporous catalysts: Defective amorphous vs. nanocrystalline CeO2 structure towards CO-PROX reaction

Mesoporous multicomponent materials with uniform pore sizes and well-defined network geometries are viewed as highly attractive candidates in the catalysis field. However, their synthesis at a high homogeneity level is considered quite challenging. Herein, alumina based mixed oxides (Al/Ce/Cu or Fe) are produced through a facile evaporation-induced-self-assembly route and tested towards preferential oxidation of CO in H2-rich gas. The effect of several synthetic parameters is investigated, with citric acid addition identified as a key factor in view of obtaining high mesoscopic order and notable homogeneity, particularly at high dopant amounts. Following thermal aging at 900 °C, metal oxides distribution and nanoporous nature are well-preserved with a parallel nucleation of ceria nanoparticles into the semi-crystalline inorganic framework. CO-PROX assessment of the aged samples reveals a drastic enhancement in catalytic activity, especially for the ternary Cu–Ce–Al system, associated with material's structural reconstruction strongly affecting metal–support interaction.

Mechanochemically activated Au/CeO2 for enhanced CO oxidation and COPrOx reaction

COPrOx reaction is one of the particularly appealing and cost-effective solutions for the selective oxidation of CO in hydrogen-rich gas streams to meet the stringent requirement of the current electrocatalysts employed in low-temperature fuel cells. Herein, we synthesized ultrasmall Au clusters supported on ceria by one-step solvent-free mechanochemical method. These catalysts exhibited higher activity for COPrOx and CO oxidation compared to the sample prepared by conventional incipient wetness impregnation. The unique Au-Ce interaction caused by impact and friction between the ball, vessel, and powders, greatly promotes the generation of positive charged Auδ+ active sites and, at the same time, the reducibility of the catalyst. Interestingly, an aging treatment of the ball-milled samples resulted in a significant superior performance of the catalytic activity. This enhancement has been attributed to a change in the oxidation state of Au between the fresh and the aged catalysts prepared by ball milling.

An improved preparation method for a CuO/CeO2-coated monolith for the CO–PrOx reaction

In this study, we present a method for directly coating monoliths with a CeO2/CuO catalyst using the urea-nitrate combustion method. The catalyst was characterized by means of XRD, SEM/EDX, and EPR measurements. Experimental results are described, when this catalyst was used for the preferential oxidation of CO. The catalytic activity for the CO–PrOx-reaction was measured by recording CO conversion as a function of the reaction temperature in a hydrogen-rich gas mixture in the presence and absence of water vapor. In a long-term test of over 310 h, the catalyst’s long-term stability was demonstrated. Direct coating is shown to be a promising approach by which a larger amount of catalyst can be deposited onto the monolith in a single step than would be possible with washcoats.

Bare and Pt-loaded LaCo1-xFexO3 perovskites as catalysts for CO-PROX reaction

Perovskites based on cobalt and iron LaCo1-xFexO3 (x = 0, 0.1 and 1) and their behavior in the presence of platinum yPt/LaCo1-xFexO3 (y = 0.1, 0.5 and 2.5% wt.) were evaluated as catalysts in the preferential oxidation of carbon monoxide (CO-PROX). Perovskites (general formula ABO3) are alternative materials to conventional transition metal oxides, expanding the tunable redox properties according to the combination of elements. In CO-PROX, Co3O4 and Pt/Fe3O4 are well-studied systems. The superficial Co3+/Co2+ redox pair is highly active. Still, the Co3O4 is easily reduced at low temperatures under the reducing conditions of CO-PROX, forming less active phases enriched in Co2+ and Co0. On the other hand, Fe3O4 is not active but becomes highly reactive when associated with Pt through the redox mechanism intermediated by the Fe3+/Fe2+ pair created at the metal interface. Supported by in situ characterization under reducing conditions and complementary ex situ analyses, we found that the LaCoO3 resembles the Co3O4, and the addition of Fe forming LaCo0·9Fe0·1O3 induced higher stability of the perovskite structure under reducing conditions. The presence of Pt significantly impacted the stabilization of the active sites under CO-PROX conditions depending on the perovskite composition. Finally, the Pt/LaFeO3 catalyst combined the high activity of the Pt/Fe3O4 system with the thermal stability of the perovskite structure.

Highly Effective Pt-Co/ZSM-5 Catalysts with Low Pt Loading for Preferential CO Oxidation in H2-Rich Mixture

New Pt-Co catalysts of hydrogen purification from CO impurities for fuel cells were fabricated via the deposition of monodispersed 1.7 nm Pt nanoparticles using laser electrodispersion on Co-modified ZSM-5 prepared by the Co(CH3COO)2 impregnation. The structure of prepared Pt-Co zeolites was studied by low-temperature N2 sorption, TEM, EDX, and XPS methods. The comparative analysis of samples with different Pt (0.01–0.05 wt.%) and Co (2.5–4.5 wt.%) contents on zeolites with the ratio of Si/Al = 15, 28, and 40 was performed in the CO-PROX reaction in H2-rich mixture (1%CO + 1%O2 + 49%H2 + 49%He). The synergistic catalytic action of Pt and Co on zeolite surface makes it possible to completely remove CO from a mixture with hydrogen in a wide temperature range from 50 to 150 °C; the high efficiency of designed composites with low Pt loading is maintained for a long time. The enhancement of PROX performance originates from the formation of new active sites for the CO oxidation at the Pt-Co interfaces within zeolite channels and at the surface. In terms of their activity, stability, and selectivity, such composites are significantly superior to known supported Pt-Co catalysts.

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.

Photothermal CO-PROX reaction over ternary CuCoMnOx spinel oxide catalysts: the effect of the copper dopant and thermal treatment.

The purification of carbon monoxide in H2-rich streams is an urgent problem for the practical application of fuel cells, and requires the development of efficient and economical catalysts for the preferential oxidation of CO (CO-PROX). In the present work, a facile solid phase synthesis method followed by an impregnation method were adopted to prepare a ternary CuCoMnOx spinel oxide, which shows superior catalytic performance with CO conversion of 90% for photothermal CO-PROX at 250 mW cm-2. The dopant of copper species leads to the incorporation of Cu ions into the CoMnOx spinel lattice forming a ternary CuCoMnOx spinel oxide. The appropriate calcination temperature (300 °C) contributes to the generation of abundant oxygen vacancies and strong synergetic Cu-Co-Mn interactions, which are conducive to the mobility of oxygen species to participate in CO oxidation reactions. On the other hand, the highest photocurrent response of CuCoMnOx-300 also promotes the photo-oxidation activity of CO due to the high carrier concentration and efficient carrier separation. In addition, the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirmed that doping copper species could enhance the CO adsorption capacity of the catalyst due to the generation of Cu+ species, which significantly increased the CO oxidation activity of the CuCoMnOx spinel oxide. The present work provides a promising and eco-friendly solution to remove the trace CO in H2-rich gas over CuCoMnOx ternary spinel oxide with solar light as the only energy source.

Highly porous CuZnAl layered double hydroxides prepared by biochar-templated co-precipitation method as catalysts for the preferential oxidation of CO reaction

This study evaluates the effect of the porosity of Layered Double Hydroxide (LDH) containing copper (Cu), zinc (Zn), and aluminum (Al) synthesized by a co-precipitation method with biochar as a hard template on the preferential oxidation of CO (CO-PROX) reaction. Experimental data showed biochar as a hard template promoted structural changes in the LDH, such as 9 to 42 nm increase in pore size, decrease in the copper oxide crystallite size, and increase in metallic area, and enhanced its activity in the CO-PROX reaction. In situ DRIFTS analyses revealed CO was adsorbed on both Cu+ and Cu0 sites and Cu+ was the main species during the reaction for all catalysts.

Metal-oxide catalysts for CO TOX and PROX processes in the Pt–Cr/Mo/W systems

The properties of Pt-based catalysts could be modified by the addition of second metal and formation of an alloy or metal-oxide interface. In the present work we studied the effect of Cr, Mo and W doping on the Pt performance in CO TOX and PROX reactions. [Pt(NH3)4]MO4 (M = Mo, W) salts and solid solutions [Pt(NH3)4](M’O4)x(M″O4)1-x (M’ = Cr,Mo,W; M’‘ = Cr,Mo,W) were used as a single-source catalyst precursors. They were synthesized and characterized by a number of physicochemical methods of analysis. The thermal properties of obtained salts were studied in an oxidative atmosphere. The process of the decomposition is multistage and ends with a formation of platinum and the corresponding oxides of base metals (Cr/Mo/W).

Understanding the role of K on PtCo/Al2O3 for preferential oxidation of CO in H2

CO preferential oxidation reaction (CO-PROX) can effectively eliminate CO in H2 rich atmosphere to avoid CO poison the Pt anode of Proton Exchange Membrane Fuel Cell (PEMFC). To match the operation temperature window for PEMFC, PtCo nanoparticles supported on K modified Al2O3 (PtCo/K–Al2O3) were prepared to promote CO-PROX activity. The addition of K species weakened the interaction between PtCo nanoparticle and support, which improved the dispersion of Pt particles and redox property of PtCo/Al2O3. It also facilitated the formation of Pt3Co species and active surface −OH groups, which were involved in CO-PROX reaction. According to in situ DRIFTS spectra, HCO3− and HCOO− were intermediates of PtCo/K–Al2O3 catalyzed CO-PROX at low temperature and high temperature, respectively. Thus, the addition of 1 wt% K to PtCo/Al2O3 (PtCo/1K–Al2O3) could completely oxidize CO in the temperature range of 127–230 °C with O2 selectivity at 50%. The 100% CO conversion temperature window of PtCo/1K–Al2O3 is expanded by 100 °C in comparison of PtCo/Al2O3.

Promoter Effect of MAl2O4 (M = Co and Ni) Aluminates in the Performance of Pt/Al2O3 Catalyst for CO-PROX Reaction

Promoter Effect of MAl2O4 (M = Co and Ni) Aluminates in the Performance of Pt/Al2O3 Catalyst for CO-PROX Reaction

Simulated solar light-driven photothermal preferential oxidation of carbon monoxide in H2-rich streams over fast-synthesized CuCeO2–x nanorods

Aiming for purification the trace amount of CO in H2-rich streams with reduced energy consumption and low cost, solar-driven photothermal preferential oxidation of carbon monoxide on the non-precious metal oxide catalyst is proposed in this work. Cu doped CuCeO2−x nanorods catalysts were synthesized with a fast and simple coprecipitation method at the room temperature, which shows high CO oxidation activity in photothermal preferential oxidation of CO (CO-PROX) under UV-Vis-IR light irradiation. Rely on the various characterization methods such as UV-Vis-IR diffuse reflection spectrum (UV-Vis-IR DRS), photoluminescence spectrum (PL), transient photocurrent test, HRTEM, XRD, XPS, UV-Raman and H2-TPR, the optical and chemical properties of the CuCeO2−x nanorods catalysts were uncovered. The photothermal catalytic activity of CuCeO2−x nanorods doped with 10 wt% Cu reaches to 90% CO conversion under Xe lamp illumination (2.5 suns), and the solar driven photothermal CO-PROX reaction on CuCeO2−x nanorods were proposed to be proceeded by the light-to-thermal conversion and subsequently to drive a thermal catalytic process. The catalytic performance of CuCeO2−x nanorods in photothermal CO-PROX is closely related to the photo-to-thermal conversion efficiency and Cu-Ce synergetic interaction of CuCeO2−x nanorods catalyst. The introduction of CuOx greatly broaden the optical absorption range and promotes the light absorption capacity of ceria nanorods, which induces the catalyst with high photo-to-thermal conversion capability. Moreover, the optimal copper dopant benefits to enhance the Cu-Ce synergetic interaction and accelerate the oxidation reaction taking place at low temperature. CuCeO2−x nanorods catalyst shows promising competitive activity and ultra-low cost compared with the noble-based catalyst for the purification of hydrogen streams by the clean and eco-friendly sunlight sources.

Sponge-like carbon monoliths: Porosity control of 3D-printed carbon supports and its influence on the catalytic performance

Sponge-like carbon monoliths with tailored channel architecture and porosity were prepared by combining sol–gel polymerization and 3D printing technology. The pore size distribution (PSD) and macropore volume were controlled by varying the water concentration used in the synthesis. The size and interconnection degree of primary particles, and consequently the pore width and macropores volume, increases by increasing the water concentration. However, a more heterogeneous PSD was detected at high water concentration, due to the better-defined spheres-like morphology of primary particles which leaves voids and corners between fused spheres together with bigger macropores leaves by the coral-like structure. The role of this porosity control on the CuO/CeO2 catalytic performance was pointed out in the CO-PrOx reaction. The CuO/CeO2 dispersion and distribution along the carbon network increases by increasing the water concentration, i.e. the pore width and macropore volume, enhancing the catalytic activity. However, this improvement is not observed at high water concentration in which preferential flow pathways are created favored by the heterogeneous PSD. This manifest that the porosity control plays an important role in the catalytic performance of monolithic catalysts and thus, the monolithic support must be specifically designed to optimize the catalytic performance of active phases for each application.

High jolt-resistance monolithic CuO–CeO2/AlOOH/Al-fiber catalyst for CO-PROX: Influence of AlOOH/Al-fiber calcination on Cu–Ce interaction

Micro-reactors for the preferential oxidation of CO in H 2 -rich stream (CO-PROX) are attractive for PEMFCs employed in portable electronic devices and automobiles, but the jolt is inevitable, which makes micro-reactors necessitate high jolt resistance. The monolithic structured catalyst could effectively resolve these problems. Herein, we employed the thin-felt monolithic Al-fiber substrate to fabricate the CuO–CeO 2 /AlOOH/Al-fiber catalyst for the CO-PROX reaction. This catalyst was prepared via first growing AlOOH nanosheets onto the Al-fiber surface by steam oxidation method, followed by depositing CuO–CeO 2 onto the AlOOH/Al-fiber support. The preferred catalyst delivered 100% CO conversion and 81% O 2 selectivity at 140 °C with a gas hourly space velocity of 12,000 mL g −1 h −1 , and particularly, performed stably for 120 h at the changeable temperatures of 120–160 °C. This work provides a strategy to tailor a qualified monolithic catalyst that couples the promising jolt resistance and catalytic performance at 120–160 °C. • The monolithic CuO–CeO 2 /AlOOH/Al-fiber catalyst was fabricated for CO-PROX. • This catalyst exhibits both good performance and high jolt resistance. • As-grown AlOOH nanosheets on Al-fiber enable high Cu–Ce dispersion and interaction. • The calcination of AlOOH/Al-fiber leads to decreased activity and hydroxyl amount.