CO Preferential Oxidation Reaction Research Articles

Synergistic metal-support interaction promoting the formation of oxygen vacancies and their role in CO-PROX over CuO/NiO-CeO2 catalyst

A series of x/yCuO/NiO-CeO2 catalysts with a constant Cu+Ni molar content and low copper loading of 1 wt% had been prepared by the step-by-step impregnation method and characterized by various techniques (XRD, N2-adsorption/desorption, HR-TEM, H2-TPR, Raman and XPS). CO preferential oxidation (CO-PROX) under H2/CO2-rich conditions had been performed. Experimental results indicated that the catalytic activity of CuO/CeO2 could be enhanced by substituting partial Cu with Ni, and with a Cu/Ni molar ratio of 1/2, the highest catalytic performance was observed. Under the conditions of temperature 130 ℃, space velocity 20266 mL·gcat−1·h−1 and oxygen excess coefficient 1.2, the 1/2CuO/NiO-CeO2 catalyst gave rise to a maximum CO conversion of about 94.8%. The 1/2CuO/NiO-CeO2 catalyst also exhibited good catalytic stability up to 100 h, showing that this catalyst system could be further developed for potential commercial application. Combined with the detailed characterization data, it had been proposed that there was a synergistic interaction among Cu, Ni and CeO2 in the catalyst, generating more oxygen vacancies than the binary CuO/CeO2 and NiO/CeO2 catalysts. With a Cu/Ni molar ratio of 1/2, the formed ternary CuO/NiO-CeO2 catalyst had the highest content of oxygen vacancies, which was believed to play a crucial role in the CO-PROX reaction.

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.

CeO2 crystal plane engineering to modulate CuO-CeO2 interactions and modify the CO-PROX reaction pathway

Adjustment of the CeO2 exposed planes is an effective tool for modifying the metal-support interaction, which is the basis for the excellent CO preferential oxidation (CO-PROX) performance of CuO/CeO2 catalysts. However, the relationship between the CeO2 exposed planes, CuO-CeO2 interaction and reaction performance is still unclear. Herein, we investigated the effect of interactions on the structure of CuO/CeO2 catalysts and the CO-PROX reaction by modulating the CeO2 exposed planes (CuCe-NC (nanocubes) exposed {100} planes and CuCe-NP (nanoparticles) exposed {111} planes). The results indicate that the {100} plane exhibits greater CuO dispersal capabilities due to electron transfer caused by stronger interactions produces an anchoring effect on CuO. This promotes increased contact between CO and CuO during the reaction, creating more CO adsorption sites and improving low temperature activity. Furthermore, effective electron transfer between CuO and {100} planes during reaction maintains Cu+ stability while widening the temperature window. In-situ spectroscopy and DFT calculations showed that the Cu+–CO bond strength is weakened by the enhanced interaction, which together with the excellent surface structure of CeO2-{100}, favours the direct desorption of CO2. However, forming carbonate on CuCe-NP seems to be preferable. Various CO-PROX reaction pathways are suggested for different exposed crystal planes. This insight into crystal plane effects on CO-PROX may guide the design of high-performance catalysts.

A methanol autothermal reforming system for the enhanced hydrogen production: Process simulation and thermodynamic optimization

Methanol autothermal reforming is a potential way to produce hydrogen that can be used for vehicle power batteries like PEMFC. Combining a reformer with a combustor to produce substantial hydrogen is promising, but the challenge of heat transfer efficiency between the reformer and combustor must be considered. Furthermore, the complexity of the system structure is not conducive to its large-scale operation level. In this paper, a novel methanol autothermal reforming hydrogen production system without catalytic combustion was built and developed, aiming to produce hydrogen-rich gas with low CO concentration. Process simulation and thermodynamic optimization on the target system were detailedly performed using Aspen Plus software and parameter sensitivity analysis methods. In addition, a methanol autothermal reforming hydrogen production system using catalytic combustion was taken as the reference system. The results indicated that the novel system could achieve a self-sustaining operation by the coupled methanol partial oxidation and steam reforming. And the product gas contained very low CO concentration (<10 ppm) due to the combined effects of water-gas shifting and CO preferential oxidation reactions. It was verified that under the maximal exergy efficiency condition, the exergy efficiency of the novel system is not significantly improved compared with the reference system, but the hydrogen yield is increased by about 27.65%, the thermal efficiency is increased by about 17.51%, and the exergy loss when generating unit molar H2 is reduced by 20.53 kJ/mol; Under the condition of maximum hydrogen yield, the indicators of the novel system also perform better. Notably, the reformer is the main exergy loss source in the novel system, which provides a theoretical basis for further optimization of parameter configuration. This work will be beneficial to researchers who study the miniaturization design of the integrated system of methanol hydrogen production coupled vehicle power battery.

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.

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.

Impact of Hydrothermally Prepared Support on the Catalytic Properties of CuCe Oxide for Preferential CO Oxidation Reaction

CuCe mixed oxide is one of the most studied catalytic systems for preferential CO oxidation (CO-PrOx) for the purification of hydrogen-rich gas stream. In this study, a series of ceria supports were prepared via a citrates-hydrothermal route by altering the synthesis parameters (concentration and temperature). The resulting supports were used for the preparation of CuCe mixed-oxide catalysts via wet impregnation. Various physicochemical techniques were utilized for the characterization of the resulting materials, whereas the CuCe oxide catalysts were assessed in CO-PrOx reaction. Through the proper modification of the hydrothermal parameters, CeO2 supports with tunable properties can be formed, thus targeting the formation of highly active and selective catalysts. The nature of the reduced copper species and the optimum content in oxygen vacancies seems to be the key factors behind the remarkable catalytic performance of a CO-PrOx reaction.

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.

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 FeOx/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 (FeOx) 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/FeOx/TiO2 catalysts showed the formation of Pt-FeOx interfaces, called as (Pt-FeOx)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.

Viability of Au/La2O3/HAP catalysts for the CO preferential oxidation reaction under reformate gas conditions

The viability of gold supported on lanthanum-modified HAP catalysts is investigated for CO preferential oxidation (PROX) in H2-rich stream. All samples comprise small nanoparticles (NPs) of Au (< 4 nm). Addition of La enhances the chemisorption of CO, whereas it lowers that of H2 and H2O. Moreover, lanthanum improves the reducibility of the catalysts and the mobility of oxygen. FTIR studies show that under CO oxidation conditions Au exists in two distinct forms on La-promoted samples, namely Auσ+ and Au+ species. The catalytic tests under different PROX conditions show an improvement of the performance with lanthanum addition. The observed improvement is linked to suitable chemical properties, whereas efficiency dependence on Au NP sizes is rather secondary. At 80 °C, La-rich catalysts completely eliminate CO and prove to be selective (66%) under realistic PROX conditions (in the presence of H2O and CO2). Moreover, they are exceptionally stable during an extended period (240 h).

CO removal for hydrogen purification via Water Gas Shift and COPROX reactions with monolithic catalysts

A combination of Water Gas Shift (WGS) and CO preferential oxidation (COPROX) reactions is considered a promising approach for CO removal in an H2 feed employed for PEM fuel cells. Honeycomb monolithic catalysts were studied for this purpose, firstly coated with Ce or Ce-Pr, then with Cu or Cu-Ni, (mass gained ∼35%). Computational Fluid Dynamics simulation showed that outer channels (close to reactor wall) presented a slightly lower flow in comparison with flow in inner ones. The Pr content in WGS monoliths modulated the activity-selectivity ratio showing an optimal value for 15 at.% in case of Ni-containing samples due to a significant selectivity improvement towards WGS (hindering methanation). A Cu/ceria monolithic sample was tested as COPROX catalyst. The CO conversion presented a maximum value with the increase of the operation temperature. Besides, a larger contact time (achieved by modifying monolith length) also enhanced the undesired H2 oxidation reaction as same as a larger O2/CO ratio. As a consequence, a temperature window around to 130 °C, a contact time close to 0.1 g.s/cm3, and a O2/CO = 1 M ratio was determined as the most adequate parameters set.

Catalytic performance of copper oxide supported α-MnO2 nanowires for the CO preferential oxidation in H2-rich stream

In this article, the catalytic activity of the copper catalysts with different loadings of copper oxides supported on α-MnO2 nanowires prepared by the simple hydrothermal route was studied in the CO preferential oxidation reaction. The x-ray diffraction (XRD), N2 adsorption/desorption, Fourier transform infrared (FT-IR) spectroscopy, H2-temperature-programmed reduction (H2-TPR), and scanning electron microscopy (SEM) analyses were used to characterize the synthesized samples. The obtained results showed that the 10 wt% CuO/MnO2 catalyst with the highest specific surface area and the lowest particle size demonstrated the highest CO conversion and CO2 selectivity in this reaction. The good catalytic performance of this catalyst was due to the beneficial synergistic interaction between copper and manganese. However, adding CO2 and/or H2O to the feedstock decreased the conversion and selectivity. Moreover, the effect of different parameters such as calcination temperature, O2/CO ratio, gas hourly space velocity (GHSV), and pretreatment conditions were investigated.

Mathematical Modeling of Preferential CO Oxidation Reactions under Advection–Diffusion Conditions in a 3D-Printed Reactive Monolith

Mathematical Modeling of Preferential CO Oxidation Reactions under Advection–Diffusion Conditions in a 3D-Printed Reactive Monolith

Unraveling the Change in Multiple Cu Species Present in CuO/CeO2 over the Preferential CO Oxidation Reaction

CuO/CeO2 with strong metal–support interaction was considered to be a highly efficient catalyst in the PROX (preferential CO oxidation) reaction. Multiple Cu species from this interaction were believed to play a key role in catalytic activity. However, due to the complex chemical properties of copper, there was still controversy about the nature of active sites in the PROX reaction. In this work, the types and proportion of Cu species (Cu–[O]x–Ce, amorphous CuOx, and aggregated CuOx) could be well-controlled since CuO was loaded on surface-modified CeO2. Their properties were studied in detail through a series of characterizations. The Cu–[O]x–Ce structure was found to be the most favorable for preferential CO oxidation. While the amorphous CuOx cluster could facilitate CO oxidation, it would greatly reduce the selectivity. The aggregated CuOx caused by excessive CuO only had activity at high temperature, resulting in an upper limit of the CuO loading amount. The proportion of these Cu species varied with the CuO loading amount, and the catalytic performance was the synergetic result of all Cu species.

Study of CuO–CeO2 catalysts supported on ordered porous silica with different mesostructure and morphology. Influence on CO preferential oxidation

In the present work, SBA-15 and SBA-16 ordered mesoporous silicas were synthesized with different morphologies such as fibers, spheres, rods and irregular shapes. Copper and cerium oxides active phase was introduced by two methods, incipient wetness impregnation and solid-state impregnation by using a ball mill. The obtained catalysts were evaluated in the CO preferential oxidation reaction (COPrOx). By means of SAXS, XRD, SEM and TEM microscopies, the mesostructures and morphologies were characterized, as well as the preservation of the porous structure and the presence of oxides nanoparticles. In addition, XPS spectroscopy was used to analyze the chemical state and relative surface abundance of the elements present in the catalysts. From the catalytic results, it could be seen that the incipient wetness impregnation method was the most favorable. The unidirectional porous structure SBA-15 with fiber shape and incipient wetness impregnation achieved the highest conversion of CO and selectivity to CO2 (CO conversion 98% at 175 °C). SBA-16 based catalysts were found less active. However, it is noteworthy that the best material was prepared by using an irregular shaped SBA-16 impregnated by incipient wetness procedure. This catalyst exhibited a maximum CO conversion of 80% at 175 °C. Finally, the SBA-15 mesoporous structure and the incipient wetness impregnation method had a greater influence on the better catalytic performance.

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.

Improvement the catalytic activity of activated Au/GO–CeO2 nanocatalysts by facile ball milling–hydrothermal methods for PROX operation

Gold-based nanocatalysts have proved to be an appropriate candidate for the selective preferential CO oxidation (PROX) in H2-containing atmosphere. Effectiveness of small gold nanoparticles in the presence of immobilized on GO–CeO2 nanocomposites was investigated by a range of ball milling conditions for the PROX reaction. First, GO–CeO2 nanocomposites were fabricated using the combination of ball milling and hydrothermal methods, and then, the gold nanocomposites were activated at 400 ± 2 °C for 4 h in Ar atmosphere. Next, Au/GO–CeO2 catalysts were synthesized, and after that, the gold nanocontents calcined at 400 ± 2 °C in Ar gas during 4 h. The catalytic activity and selectivity of activated gold catalysts (Au/GO–CeO2) were studied for the preferential CO oxidation at optimized conditions from the ambient temperature to 175 ± 2 °C. The gas and vapor phase components were analyzed by a Fourier Transform Infrared Spectroscopy. The effects of ball mill’s parameters, such as milling time, bead amounts, loading percentage, and milling temperatures, on the catalytic activity and selectivity of gold catalysts were investigated. Under relatively optimized conditions, the results showed the high conversion (C%: 97) and selectivity (S%: 92) for the gold nanocatalysts at 150 °C reaction time towards CO2 as the main product when the Au/GO–CeO2 nanocatalysts were synthesized by the use of 12% of GO solution with 90 zirconia oxide balls at 70 °C for 8 h of the ball milling condition.

Insights into the Oxygen Vacancy Filling Mechanism in CuO/CeO2 Catalysts: A Key Step Toward High Selectivity in Preferential CO Oxidation

The preferential CO oxidation (CO-PROX) reaction is paramount for the purification of reformate H2-rich streams, where CuO/CeO2 catalysts show promising opportunities. This work sheds light on the ...

Selective CO oxidation in the hydrogen stream over Ru/Al@Al2O3 catalysts

Abstract The Al@γ-Al2O3 core-shell microstructures prepared from aluminum particles by hydrothermal surface oxidation were superior to the conventional alumina support owing to the facile heat flux through the metal-ceramic composite structures. The Ru catalysts supported on Al@γ-Al2O3 were used for the preferential CO oxidation (PROX) reaction, which is a highly exothermic reaction requiring high heat flux. For comparison, commercially available α-Al2O3 and γ-Al2O3 were also used as catalyst supports. The catalysts were characterized by N2-physisorption, X-ray diffraction, CO chemisorption, transmission electron microscopy (TEM), temperature-programmed desorption of ethanol (ethanol-TPD) and ammonia (NH3-TPD), and temperature-programmed oxidation (TPO). The Ru/α-Al2O3 catalyst with the lowest Ru dispersion showed the best PROX performance among the tested catalysts in the absence of CO2 and H2O. Even though Ru/Al@γ-Al2O3 and Ru/γ-Al2O3 had similar Ru dispersions, the former catalyst exhibited a much better PROX performance than the latter. This could be explained by the fact that the Ru metal deposited on Al@γ-Al2O3 was more resistant to surface oxidation as compared to that on γ-Al2O3, based on the TPO results. A lower desorption temperature of ethanol was observed over Al@γ-Al2O3 than over γ-Al2O3 during ethanol-TPD experiments, even though both supports had similar amounts of acid sites, as determined by NH3-TPD. Among the tested catalysts, Ru/Al@γ-Al2O3 also showed the best PROX performance in the presence of CO2 and H2O. A further enhancement in the PROX performance was achieved by increasing the Ru metal particle size from 1.4 to 2.6 nm by pretreating the Ru/Al@γ-Al2O3 catalyst with 1 mol% O2 in He at 150 ℃.

Interaction of atomically dispersed gold with hydrothermally prepared copper-cerium oxide for preferential CO oxidation reaction

Gold nanoclusters were highly dispersed on hydrothermally prepared copper-ceria nanocatalysts and resulted in enhanced catalytic performance for preferential CO oxidation reaction, highlighting the direct interaction between Cu and Au through a Cu–Au interface. The presence of gold nanoclusters promoted the reducibility and oxygen mobility of ceria and copper-ceria catalysts, while a reduction pretreatment before gold deposition resulted in re-dispersion of copper species and further enrichment of ceria surface with these active species, thus enhanced interactions with atomically dispersed gold nanoclusters. The catalytic performance in preferential CO oxidation reaction depicted the promoting role of gold nanoclusters and reductive pretreatment in terms of activity, selectivity and CO2/H2O tolerance.