Catalysts For Preferential Oxidation Of CO Research Articles

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.

Ru/Ir nanojunctions supported on CNTs as active and ultrastable catalysts for CO preferential oxidation toward hydrogen purification

Hydrogen purification is animportanttechnological step for proton exchange membrane fuel cells (PEMFCs), which can promote the service life of batteries. The preferential oxidation of CO (PROX) in hydrogen is a promising solution for hydrogen purification to avoid poisoning of catalysts by the trace amount of CO present in thehydrogen-richfeedgas. Herein, we demonstrate an interface engineering strategy to prepare Ru/Ir-CNTs nanojunctionswith remarkablecatalyticactivity for CO oxidation. The as-prepared catalyst shows thetotal COconversionin awidetemperaturerange of 50-200℃ under a high feeding gas flow velocity of 72000 mL g-1h−1 and remains stable after 48 h of continuous working. The catalyst also exhibits 100 % CO conversion in temperature from 80 to 100℃ under a much higher feeding gas flow velocity of 108000 mL g-1h−1. Moreover, the CO conversion remains 100 % after intermittent performance testing over a one-week time period. Theoretical calculations reveal that the interface of Ru/Ir can weaken the adsorption of *CO2, thus greatly increasing the temperature range of the complete transformation of CO. Compared to oxide supports, CNTs are more difficult to combine with CO2, which makes the catalyst completely convert CO for a long time within the operating temperature range.

Highly dispersed copper catalysts for CO preferential oxidation in hydrogen-rich atmosphere

CeO2 and ZnO with finely rod shape were used as carriers to support copper. High-resolution transmission electron microscopy demonstrated that copper species were existed mostly in bilayer clusters containing underlying Cu+ and upper Cu0 on CeO2, but in big particles of 5–10 nm consisting of only Cu0 on ZnO. The amounts of underlying Cu+ in bilayers and oxygen vacancies (Ov) at copper-ceria interface reached maximum when copper loading was about 10 wt%. In CO preferential oxidation reaction, the 10-Cu/CeO2 catalyst showed the highest activity suggesting that CO adsorbed on Cu+ in bilayer clusters reacted rapidly with O2 absorbed on adjacent Ov. The near synergistic effect not only reduced the CO reaction temperature but also enhanced the CO conversion. The Cu/ZnO catalyst had weak CO adsorption and long distance of Cu0 and Ov, thus it showed much low activity. By filling the 10-Cu/CeO2 catalysts in a two-stage reactor, 1 % CO in the methanol steam reforming gas could be removed to 2 ppm after two successive CO oxidation processes, which provided a new solution for H2 purification.

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.

Cu-Embedded CeO2 Nanotubes as CO2–Resistant Catalysts for Preferential CO Oxidation in H2-Rich Streams

Cu-Embedded CeO₂ Nanotubes as CO₂–Resistant Catalysts for Preferential CO Oxidation in H₂-Rich Streams

Effect of acid treatment on the catalytic performance of CuO/Cryptomelane catalyst for CO preferential oxidation in H2-rich streams

The effect of acid treatment on the catalytic performance of CuO/Cryptomelane (CuO/CR) for CO preferential oxidation (CO-PROX) in H2-rich streams has been investigated. The CR supports are synthesized via the sol-gel approach. The hydrochloric acid or water is used to treat the CR support, and the corresponding CuO/CR catalysts are prepared by an initial wet impregnation method. Compared with the pristine CuO/CR and water-treated CuO/CRW catalysts, the acid-treated CuO/CRH exhibits the best catalytic activity with almost 100% of CO conversion at 110 °C, which can be maintained at least 100 h. The characterization results show that acid treatment decreases the K+ content in the CuO/CRH catalyst, which is conducive to the formation of more oxygen vacancies, thereby promoting the reducibility of CuO/CRH. This is the main reason for the high catalytic activity of the acid-treated CuO/CRH catalyst. Moreover, the abundant Brönsted acid sites on CuO/CRH are favorable for the desorption of acidic product CO2, which also could result in the significant promotion of the catalytic activity for CO-PROX. This study sheds a light on the importance of acid treatment for cryptomelane and provides an efficient catalyst for hydrogen purification.

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 CuOx clusters and CeO2-X weakened, and the sintering of CuOx enhanced as the stability of CeO2-X exposed planes increased. It is manifested that the content and the effective diameter of CuOx clusters increase, and the cerium doping in CuOx 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.

Promotional Effect of H2 Pretreatment on the CO PROX Performance of Pt1/Co3O4: A First-Principles-Based Microkinetic Analysis.

Atomic Pt studded on cobalt oxide is a promising catalyst for CO preferential oxidation (PROX) dependent on its surface treatment. In this work, the CO PROX reaction mechanism on Co3O4 supported single Pt atom is investigated by a comprehensive first-principles based microkinetic analysis. It is found that as synthesized Pt1/Co3O4 interface is poisoned by CO in a wide low temperature window, leading to its low reactivity. The CO poisoning effect can be effectively mitigated by a H2 prereduction treatment, that exposes Co ∼ Co dimer sites for a noncompetitive Langmuir-Hinshelhood mechanism. In addition, surface H atoms assist O2 dissociation via "twisting" mechanism, avoiding the high barriers associated with direct O2 dissociation path. Microkinetic analysis reveals that the promotion of H-assisted pathway on H2 treated sample helps improve the activity and selectivity at low temperatures.

Ceria-silica mesoporous catalysts for CO preferential oxidation in H2-rich stream: The effect of Ce:Si ratio and copper modification

In this work mesoporous CeO2-SiO2 catalysts with the Ce:Si molar ratios of 1:1 and 4:1 were synthesized by template method and modified with copper to elucidate the role of copper-ceria and ceria-silica interactions in the catalyst efficiency in CO-PROX. The catalysts were characterized by SEM-EDS, AAS, XRD, TPR-H2, TEM, and spectroscopic methods (Raman, EPR and in situ DRIFT of adsorbed CO). The binary catalyst with the equimolar Ce:Si ratio demonstrated better catalytic performance in CO-PROX due to its favorable textural properties and strong ability to anion vacancy formation as confirmed by N2 physisorption and Raman spectroscopy. CuOx/CeSiOy (Ce:Si = 4:1) demonstrated better low-temperature activity, CO2 selectivity and stability than CuOx/CeSiOy (Ce:Si = 1:1) and both binary systems because of its unique structure, comprising fine CeO2 particles with a narrow size distribution of 2–3 nm well dispersed on a large surface area, high concentration of the most active in CO oxidation Cu+ sites, formed on CuOx-CeO2 interfaces, and improved ability to surface reoxidation after reduction with the reagents resulted in the decreased Ce3+ concentration. It was also stable in the presence of CO2 and H2O in the reaction mixture. These properties would be difficult to achieve via template preparation methods without SiO2 addition.

Effect of one-dimensional ceria morphology on CuO/CeO2 catalysts for CO preferential oxidation

The one-dimensional CeO2 nanocrystals (nanorods, nanowires and nanotubes) were synthesized by hydrothermal methods and supported CuO for CO preferential oxidation (CO-PROX). The supports and catalysts were characterized by various techniques. The results show that the morphologies of support and the interaction between copper species and ceria greatly affect the activity of catalysts. Compared with CeO2-W and CeO2-R, CeO2-T presents higher activity at low-temperature. After loading CuO, the strong interaction between CuO and CeO2 nanocrystals makes the activity of CuCeO catalysts much higher than that of CeO2. Among the CuCeO catalysts, CuCeO-T displays the best catalytic activity for CO oxidation. The CO conversion over CuCeO-T can reach to 100% at 100 ​°C with O2 selectivity of 89%. The addition of copper species further enriches the surface defects, Ce3+ concentration and oxygen vacancies, which should be the main reason for the high activity of CuCeO-T catalyst.

Effect of One Dimensional Ceria Morphology on Cuo/Ceo2 Catalysts for Co Preferential Oxidation

Effect of One Dimensional Ceria Morphology on Cuo/Ceo2 Catalysts for Co Preferential Oxidation

Effect of manganese promotion on the activity and selectivity of cobalt catalysts for CO preferential oxidation

The preferential oxidation of CO in H2-rich mixtures (COPrOx) is a major catalytic reaction utilized for hydrogen purification. In the exploration of alternatives to noble metals, cobalt-based catalysts appear to be a very promising choice. The activity and stability of cobalt in the COPrOx reaction can be improved by the addition of transition metals and manganese is maybe the most prominent among them. Yet, the arrangement of the two components in the catalytically active state is largely unknown, which hinders in-depth understanding of the manganese promotion effect. Here, we compare pure and Mn-modified cobalt catalysts and correlate their structural and chemical characteristics with their COPrOx performance. The Mn-promoted cobalt catalyst is significantly more active than pure cobalt especially at intermediate reaction temperatures (around 200 °C). The addition of Mn improves the structural stability of the catalyst and helps to maintain higher specific surface areas. Chemical and microstructural analysis using various operando and in situ techniques revealed that Mn promotes CO conversion by partially stabilizing CoO phase during reaction conditions. It is also suggested that at high temperature, Mn suppress CO methanation reaction but promotes H2 oxidation. Apart of the particular interest in COPrOx reaction, in a general context, this work shows how the spatial distribution of the different catalyst components at nanoscopic level, may affect the surface chemistry and consequently control the reactivity.

Effects of hydrothermal oxidation time of Al on the catalytic performance of Ru/Al@Al2O3 for selective oxidation of CO in H2

The thermal-conducting support is highly desirable for endothermic and exothermic reactions as long as highly dispersed active sites can be maintained. The core-shell Al@Al2O3 supports with different aluminum (Al) contents were prepared from Al particle by controlling the reaction time during hydrothermal surface oxidation and applied as a support to the supported Ru catalysts for preferential CO oxidation in H2 (PROX). The prepared catalysts were characterized by N2-physisorption, X-ray diffraction, CO chemisorption, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), in situ diffuse reflectance infrared Fourier transform spectroscopy after CO adsorption (CO-DRIFTS), and temperature-programmed desorption of ammonia (NH3-TPD) and ethanol (ethanol-TPD). The catalytic activity was dependent on the Al content in the Al@Al2O3 support. The most active Ru/Al@γ-Al2O3 catalyst oxidized CO selectively in H2 over a wide reaction temperature. The surface property of the outermost exterior alumina layer in the Al@γ-Al2O3 support, determined with ethanol TPD, was beneficial for formation of Ru nanoparticles with weak adsorption of CO, probed with CO-DRIFTS, results in the high catalytic performance for PROX.

A PdCu nanoalloy catalyst for preferential CO oxidation in the presence of hydrogen

Alloy PdCu nanoparticles for CO oxidation in the presence and absence of hydrogen.

Insights into the essential roles of tin and chloride species within Cu–CeO2 based catalysts for CO preferential oxidation in H2-rich stream

Sn modified Cu–Ce catalysts have been synthesized by the solvent-free combustion method for preferential oxidation of CO in H2-rich stream (CO-PROX) and characterized by N2 adsorption-desorption (BET), H2 temperature-programmed reduction (H2-TPR), X-ray diffraction (XRD), Raman, in-situ DRIFTS, X-ray photoelectron spectroscopy (XPS), and Oxygen storage capacity (OSC). The results show that the incorporation of Sn species accelerates the redox cycle among Sn, Cu and Ce, thus strengthening the Cu–Ce interaction and promoting the formation of surface and bulk lattice Cu-[Ox]-Ce structures Meanwhile, to balance the charge, more oxygen vacancies are formed and thus facilitate the migration of oxygen species. Combined with the more strong absorbed ability of SnO2 for O2, active Cu-[Ox]-Ce structures promotes the easier conversion of more CO into CO2. However, the existence of Cl ions decreases catalytic performance because the formation of CuCl makes the Cu–Ce interaction weakened, resulting in the formation of fewer Cu-[Ox]-Ce structures and oxygen vacancies. Although CuCl is prone to be in favor of absorbing more CO, there are not enough O species available, hence a decrease in performance is observed.

One-Step Synthesis of AuCu/TiO2 Catalysts for CO Preferential Oxidation

Au/TiO2 (1wt% Au), Cu/TiO2 (1wt% Cu) and AuCu/TiO2 (1wt% AuCu) catalysts with different Au:Cu mass ratios were prepared in one-step synthesis using sodium borohydride as reducing agent. The resulting catalysts were characterized by X-ray diffraction (XRD), X-ray Dispersive Energy (EDX), Transmission Electron Microscopy (TEM) and Temperature Programmed Reduction (TPR) and tested for the preferential oxidation of carbon monoxide (CO-PROX reaction) in H2-rich gases. EDS analysis showed that the Au contents are close to the nominal values whereas for Cu these values are always lower. X-ray diffractograms showed only the peaks of TiO2 phase; no peaks of metallic Au and Cu species or oxides phases were observed. TPR and high-resolution TEM analysis showed that AuCu/TiO2 catalysts exhibited most of Au in the metallic form with particles sizes in the range of 3-5 nm and that Cu was found in the form of oxide in close contact with the Au nanoparticles and well spread over the TiO2 surface. The AuCu/TiO2 catalysts exhibited good performance in the range of 75-100 °C and presented a better catalytic activity when compared to the monometallic ones. A maximum CO conversion of 98.4% with a CO2 selectivity of 47% was obtained for Au0.50Cu0.50/TiO2 catalyst at 100oC.

Heterogeneous Cu-Fe oxide catalysts for preferential CO oxidation (PROX) in H2-rich process streams.

The influence of Fe loading in Cu–Fe phases and its effect on carbon monoxide (CO) oxidation in H2-rich reactant streams were investigated with the catalyst material phases characterized by Field Emission Scanning Electron Microscopy (FESEM), X-ray diffraction (XRD) studies and Mössbauer Spectroscopy (MS). There was no change in the oxidation state of the Fe ions with copper or iron loading. The catalytic activity was examined in the feed consisting of H2, H2O and CO2 for the preferential CO oxidation (PROX) process. These catalysts showed an optimized performance in converting CO in WGS streams in the temperature range of 80–200 °C. In addition to the formation of the CuFe2O4 phase, the Fe and Cu were found to be incorporated into a Cu–Fe supersaturated solid solution which improved CO oxidation activity, with carbon dioxide and water produced selectively with high catalytic activity in depleted hydrogen streams. Relatively high conversion of CO was obtained with high Fe metal loading. In addition to their catalytic efficiency, the employed heterogeneous catalysts are inexpensive to produce and do not contain any critical raw materials such as platinum group metals.

Correlation Between Reactivity and Oxidation State of Cobalt Oxide Catalysts for CO Preferential Oxidation

Catalytic performance is known to be influenced by several factors, with the catalyst surface oxidation state being the most prominent of all. However, in the great majority of industrial heterogen...

Ceria-nano supported copper oxide catalysts for CO preferential oxidation: Importance of oxygen species and metal-support interaction

CeO2 with four different morphologies (sphere, rod, octahedron, cube) are used as the supports to synthesize the CuCe catalysts for the CO preferential oxidation in the H2-rich gas and characterized by various techniques (TEM, HR-TEM, SEM, XRD, N2 physisorption, XPS, Raman spectra, CO-TPD, CO-DRIFTS, O2-TPD, CO pulses, H2-TPR) to obtain an insight into the impact of the CeO2 morphology on the surface structure, composition and redox properties, especially the active species. The four supports expose {111}, {110} and {100} planes, and sphere-shape CeO2 has a higher ratio of {111}, which can form more reductive copper species and active oxygen species. And the catalytic performance followed by this order: spheres (28.5 kJ/mol) > rods (31.0 kJ/mol) > octahedrons (34.1 kJ/mol) > cubes (40.3 kJ/mol). By further analysis of the characterization results, a series of structure-activity relationships are established to confirm that the active oxygen species is the decisive factor of preferential oxidation of CO in the H2-rich stream.

CuO-CeO2 nanocomposite catalysts produced by mechanochemical synthesis

Mechanochemical synthesis based on ball-milling of individual oxides was applied as a one-step preparation technique for CuO-CeO2 catalyst for preferential CO oxidation in H2 excess. The mechanical energy dose transferred to the original powder mixture determines both the catalyst composition and activity. It is found that after 90 min of milling (corresponding to a dose of 372 kJ mol–1), a mixture of 10 wt.% CuO-CeO2 powder exhibits a CO conversion of 97% at 423 K. Four active oxygen states, which are not observed in case of pure CeO2, were detected in the nanocomposite lattice and attributed to the presence of Cu in surface sites as well as in subsurface bulk sites of CeO2, in nearest neighbor and next nearest neighbor positions. Correspondingly, oxidation of CO to CO2 was found to occur in a two-stage process with Tmax = 395/460 K, and oxidation of H2 to H2O likewise in a four-stage process with Tmax = 426/448/468/516 K. The milled powder consists of CeO2 crystallites sized 8–10 nm agglomerated to somewhat larger aggregates, with CuO dispersed on the surface of the CeO2 crystallites, and to a lesser extent present as Cu2O.