Catalytic performance of Cu/hydroxyapatite catalysts in CO preferential oxidation in H2-rich stream

A series of Cu/HAP catalysts was prepared by impregnation and characterised by TEM, BET, DRS, XPS, H₂-TPR, XRD, TEM and FTIR techniques. H₂-TPR and DRS data revealed that at low concentration (<1%) large fraction of Cu incorporated the HAP framework. The increase of the Cu loading significantly increased the density of the CuO particles spread over the carrier. Moreover, as shown by XPS, these species suffered from segregation when the Cu loading was close to that required for the formation of a theoretical monolayer. The structural characterisation of the reduced samples showed a significant increase in the lattice strain with increasing Cu content, indicating an increase in the concentration of Cu lattice imperfections.The activity of the reduced samples in COTOX and COPROX processes revealed remarkable differences when the Cu content was increased from 0.8 to 14.8%. For instance, analysing the TOF values calculated for the two processes and the characterisation data, it was concluded that the catalysts with low Cu loadings (0.8% and 3.6%) were the most active ones and their sites involved in the CO oxidation reaction were almost similar. The addition of CO₂ and water seemed to affect the performances of the two catalysts via different pathways. Indeed, in contrast to the Cu(4)/HAP, the Cu(1)/HAP catalyst became more selective in the presence of CO₂ and water. Moreover, under these realistic COPROX conditions, the latter was 6.6 times more active than the former. A correlation with the characterisation data showed that the most efficient species consisted of those presenting strong interaction with the HAP support. By contrast, the sites exhibiting high concentration of Cu lattice imperfections showed a relatively low activity.     

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/CR_W catalysts, the acid-treated CuO/CR_H 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/CR_H catalyst, which is conducive to the formation of more oxygen vacancies, thereby promoting the reducibility of CuO/CR_H. This is the main reason for the high catalytic activity of the acid-treated CuO/CR_H catalyst. Moreover, the abundant Brönsted acid sites on CuO/CR_H 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.     

CO preferential oxidation in H2-rich stream over the CuO-MnOx mixed oxide catalysts prepared by a facile mechanochemical preparation method

The CuMn samples with various CuO weight percentages were synthesized by the mechanochemical route. The catalytic activity of the prepared samples was determined in the preferential oxidation of CO process (CO-PROX) in the temperature range of 40–250 °C. According to the XRD results, the 5CuMn and 10CuMn samples exhibited the spinel phase in their structures. The spinel phase formation enhanced the CO adsorption active sites and modified the redox properties. The results indicated that the copper incorporation into the manganese oxide modified the catalytic activity and structural properties. The 5CuMn catalyst with the highest BET area (72.1 m² g⁻¹) possessed 96% CO conversion and 50% CO₂ selectivity at 70 °C (GHSV = 30,000 ml/h.g_cat) and significant resistance in the presence of water due to the formation of hydroxyl groups over the catalyst surface. The catalyst activity remained stable for 14 h at 130 °C. Furthermore, the influence of calcination temperature, feed composition, and GHSV value on the catalytic performance of the 5CuMn catalyst was studied.     

Copper catalysts prepared via microwave-heated polyol process for preferential oxidation of CO in H2-rich streams

The purposes of this study were to prepare a copper catalyst by the microwave-heated polyol (MP) process and subsequently to evaluate the feasibility of the preferential oxidation of CO (CO-PROX) in excess H₂. A CeO₂-TD support was firstly prepared by the thermal decomposition from Ce(NO₃)₃·6H₂O precursor. For comparison, commercial ceria (CeO₂-C) and activated carbon (AC) selected as support materials. Experimental results of CO-PROX indicated that the highest catalytic activity is achieved when the Cu/CeO₂-TD used as catalysts. Correlating to the characteristic results, it is found that the CeO₂-TD support prepared by the thermal decomposition has a large surface area and high mesoporosity; these properties contribute to the easy adsorption of pollutants and the effective dispersion of metal particles. Further investigation of feed composition found that Cu/CeO₂-TD catalysts possess 100% CO conversion even existence of CO₂ and H₂O in H₂-rich streams at 150 °C. Besides, a decrease in CO conversion was clearly observed above 175 °C for Cu/CeO₂-TD catalysts due to the reverse water gas shift reaction tending to reform CO from CO₂ and H₂.     

Catalytic processes during preferential oxidation of CO in H2-rich streams over catalysts based on copper–ceria

Nanostructured catalysts based on combinations between oxidised copper and cerium entities prepared by two different methods (impregnation of ceria and coprecipitation of the two components within reverse microemulsions) have been examined with respect to their catalytic performance for preferential oxidation of CO in a H₂-rich stream (CO-PROX). Correlations between their catalytic and redox properties are established on the basis of parallel analyses of temperature programmed reduction results employing both H₂ and CO as reactants as well as by XPS. Although general catalytic trends can be directly correlated with the redox properties observed upon separate interactions with each of the two reductants (CO and H₂), the existence of interferences between both reductants must be considered to complete details for such activity/redox correlation. Differences in the nature of the active oxidised copper–cerium contacts present in each case determine the catalytic properties of these systems for the CO-PROX process.     

CuO/CeO2 catalysts synthesized from Ce-UiO-66 metal-organic framework for preferential CO oxidation

We synthesized a CuO/CeO₂ catalyst using a copper ions encapsulated ceria metal-organic framework (MOF) Ce-UiO-66 as the precursor. The CuO/CeO₂ catalysts derived by calcining the MOF precursor (the x-CuCe catalysts) showed the better activity and selectivity for the preferential CO oxidation in the H₂-rich stream than the CuO/CeO₂ catalyst prepared by wetness impregnation (CuCe-im). A temperature window to match the CO conversion and O₂ to CO₂ selectivity higher than 99.5% at the same time appeared using the x-CuCe catalysts as the catalyst. Raman and XPS results indicated that more oxygen vacancies were formed in the bulk of ceria in the x-CuCe catalysts than that in the CuCe-im catalyst, which could promote the mobility of oxygen. Our results indicated that the surface lattice oxygen and the oxygen vacancies in the bulk of ceria could enhance the catalytic performance of the CuO/CeO₂ catalysts.     

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.     

Enhanced water-gas shift reaction performance of MOF-derived Cu/CeO2 catalysts for hydrogen purification

The water-gas shift (WGS) reaction has received renewed interest because it is one of the key reactions for producing hydrogen and renewable energy in contemporary technologies like fuel cells and bio-refineries. Catalysts play an important role in WGS reaction for achieving high CO conversion and hydrogen generation activity. Thus, the performance and stability of catalysts are vital for the WGS reaction. In the present work, the CuCe metal-organic framework (MOF) is used as a template to derive the nanostructured Cu/CeO₂ catalyst. The influence of CuCe-MOF templated approach on the WGS activity of Cu/CeO₂ has been established. Different Cu doping levels had a significant impact on WGS activity. Amongst, the Ce_0.8Cu_0.2O₂ (Cu2Ce) catalyst had a highest CO conversion (96%). The long-term stability tests further prove that the Cu2Ce catalyst had maintained high CO conversion over 100 h reaction time. XRD and TEM results suggest that different loadings of Cu content have a distinct impact on the dispersion of Cu and the catalytic properties. N₂O chemisorption results suggest that 20 wt.% of Cu loading resulted in high Cu dispersion (52%) compared to other loadings. The H₂-temperature programmed reduction (TPR) revealed that the superior catalytic activity of Cu2Ce catalyst could be attributed to the strong reducibility (i.e. lower redox temperature) derived from CuCe-MOF template. It further suggests well-dispersed copper oxide species at low Cu loadings and crystalline copper oxide species at high Cu loadings. This work emphasizes the significance of Cu/CeO₂ catalysts with exceptional catalytic activity and stability for the WGS process with MOF-precursor.     

Design of efficient noble metal single-atom and cluster catalysts toward low-temperature preferential oxidation of CO in H2

The preferential oxidation of CO in H₂ is attractive for the removal of trace amounts of CO to meet the requirement of proton-exchange membrane fuel cells (PEMFCs) application. The key is to design highly effective catalysts that work well in a wide range of low temperatures. Here, the recent progress in Au and Pt group metal catalysts for the PROX reaction is summarized, covering those with single-atom and cluster dispersed metal species with remarkable performance. Firstly, the progress of some representative catalysts is overviewed, with an emphasis on the strategies for improving low-temperature activity, selectivity, and stability. Then, special attention is focused on the key parameters affecting performance in the PROX reaction. Moreover, the reaction mechanisms in terms of adsorption and activation of reactants are discussed. Finally, the challenges and opportunities are offered for guiding the design of advanced noble metal catalysts toward the PROX process.     

Carbon nanotube supported Pt-Ni catalysts for preferential oxidation of CO in hydrogen-rich gases

A series of carbon nano-tubes supported platinum-nickel catalysts were prepared and used for CO preferential oxidation in H₂-rich streams. The catalysts were characterized by using N₂-adsorption, XRD, HRTEM, H₂-TPD and H₂-TPR techniques. Effects of platinum and nickel loading amount, CO₂ and H₂O in the feed stream on the activity and selectivity over the catalysts were investigated. The results of catalytic performance tests show that the carbon nano-tubes supported Pt-Ni catalysts are very active and highly selective at low temperature for CO preferential oxidation in 1 vol. % CO, 1 vol. %O₂, 50 vol. % H₂ and N₂ gases. Adding 12.5 vol. % of CO₂ into the feed gases has slight negative influence on CO conversion. Adding 15 vol. % of H₂O leads to a little decrease of CO conversion at the temperature range of 100-120 °C, which is proposed to be caused by capillary wetting of water in the micro-pores of carbon nano-tubes. As the reaction temperature is higher, adding water can improve CO conversion. The characterization results indicate that platinum species are in nano-particles uniformly dispersed on the carbon nano-tubes surface. There are two kinds of nickel species, one is interacted with platinum and likely to form Pt-Ni alloy in reduction process, the other is much highly dispersed on carbon nano-tubes and strongly interacted with the supports. The high activity of the catalysts is attributed to the interaction between Pt and Ni with the formation of Pt-Ni alloy.     

A comparative study of differently prepared rare earths-modified ceria-supported gold catalysts for preferential oxidation of CO

The preferential oxidation of CO in H₂-rich gas was studied over gold catalysts supported on ceria modified by rare earths (RE = La, Sm, Gd and Y). The ceria supports were prepared by mechanochemical activation or co-precipitation. The amount of RE₂O₃ was 10 wt%. Gold (2 wt%) was added by the deposition-precipitation method. The samples were characterized using XRD, HRTEM, HAADF, TPR, and Raman spectroscopy. It was established that catalysts prepared by co-precipitation were more active than samples made by mechanochemical activation. A gold catalyst on yttrium-modified ceria, prepared by co-precipitation, exhibited the highest catalytic activity and selectivity, and high stability. No substantial differences in the size distribution and average size of the nanogold particles in the studied catalysts were observed. The main reason for the differences in PROX activity of these gold catalysts was searched into the role of the ceria supports, depending on the preparation method, and the nature of the modifier.     

Activity of Au/ZnO catalysts prepared by photo-deposition for the preferential CO oxidation in a H2-rich gas

In this work, Au supported over ZnO prepared by photodeposition was applied to prepare nano-size Au catalysts by utilizing UV light for the preferential oxidation (PROX) of CO. The results demonstrated that Au can be dispersed homogeneously over ZnO in the size range of 1–2 nm with a narrow size distribution. It was clearly seen that the preparation parameters (i.e. irradiation time, precipitant concentration, calcination, and storage condition) had a significant effect on the catalytic activity. Among the variables studied, low concentrations of precipitant and long irradiation time were by far the most influential on the catalytic activity.     

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.     

Effect of synthesis pH and Au loading on the CO preferential oxidation performance of Au/MnOx-CeO2 catalysts prepared with ultrasonic assistance

As a novel and rather convenient method, ultrasonic pretreatment was employed for the preparation of nanostructured Au/MnO_x-CeO₂ (Mn/Ce = 1:1) catalysts which were used for CO preferential oxidation. The effects of synthesis pH (7.0-11.0) and Au loading (0.5-5.0 wt.%) on the performance of these catalysts were systematically investigated. It is found that the Au(1.0)/MnO_x-CeO₂-10.0 with 1.0 wt.% Au prepared at pH = 10.0 exhibits the best catalytic performance, giving not only the highest CO conversion of 90.9% but also the highest oxygen to CO₂ selectivity of 47.8% at 120 °C. The results of XRD, HR-TEM and XPS indicate that this catalyst possesses the highest dispersion of Au species and the largest amount of surface adsorbed oxygen species, which facilitates CO oxidation. The H₂-TPR results reveal that the selectivity of oxygen to CO₂ is mainly determined by the reducibility of Au species in the catalysts. The strong interaction between Au species and the supports in the catalyst Au(1.0)/MnO_x-CeO₂-10.0 decreases its capability for H₂ dissociation, effectively inhibiting the hydrogen spillover, as a result, the selectivity of oxygen to CO₂ is remarkably increased.     

CO oxidation over the Cu2O/CuO hollow sphere heterojunction catalysts with enhanced low-temperature activities

In this work, a series of the Cu₂O/CuO hollow sphere heterojunction catalysts with various Cu⁺/Cu²⁺ ratios were designed and synthesized to improve the low-temperature catalytic activity of CO oxidation. The physicochemical properties of all the catalysts were characterized by the SEM, TGA, N₂-physisorption, XRD, FT-IR, XPS, and H₂-TPR. The results revealed that the Cu₂O hollow sphere catalyst had the large specific surface area due to the formation of the hollow structure, which could provid more accessible active sites for the gaseous reactants. Meanwhile, the Cu₂O/CuO hollow sphere heterojunction catalysts possessed high concentration of the surface oxygen vacancies and further promoted the adsorption and activation of O₂. As a result, the apparent activation energy of the CO oxidation was decreased and thus low-temperature activity of the CO oxidation was finally promoted. Therefore, the Cu₂O/CuO hollow sphere heterojunction catalysts were considered as a series of high-efficient catalyst candidates for CO oxidation with promising low-temperature performance.     

Effect of the preparation method on the performance of CuO-MnOx-CeO2 catalysts for selective oxidation of CO in H2-rich streams

Selective oxidation of CO in H₂-rich streams is performed over a series of CuO-MnO_x-CeO₂ catalysts prepared by hydrothermal (CuMC-HY), co-precipitation (CuMC-CP), impregnation (CuMC-IM) and citrate sol-gel (CuMC-SG) methods. The catalysts are characterized by N₂ adsorption/desorption, XRD, SEM, HR-TEM, TPR and XPS techniques. The results show that the catalyst prepared by a hydrothermal method exhibits the best catalytic activity, especially at low temperatures. The temperature of 50% CO conversion (T₅₀) is only 74 °C and the temperature window of CO conversions up to 99.0% is about 40 °C wide, from 110 to 140 °C. Moreover, the temperature window is still maintained 20 °C wide even at lower temperatures when there are 15% CO₂ and 7.5% H₂O in the reaction gas. The superior catalytic performance of CuMC-HY is attributed to the formation of Mn-Cu-Ce-O solid solution, the unique pore structure and the existence of more Cu⁺ and Mn⁴⁺ species as well as oxygen vacancies. The sequence of catalytic activity is as follows: CuMC-HY > CuMC-SG > CuMC-IM > CuMC-CP. The worst catalytic activity, obtained from the catalyst prepared by the co-precipitation method, is possibly related to the existence of independent CuO_x and MnO_x oxides, which weakly interact with ceria in the catalyst.     

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.     

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.     

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.     

Hydrogen production via steam reforming of methanol over Cu/(Ce,Gd)O2−x catalysts

Hydrogen production via steam reforming of methanol is carried out over Cu/(Ce,Gd)O_2−x catalysts at 210–600 °C. The CO content in reformate is about 1% at 210–270 °C, which are the typical temperature for hydrogen production via steam reforming of methanol. Largest H₂ yield and CO₂ selectivity and smallest CO content are obtained at 240 °C. The formation rate of CO increases with increasing temperature. The average formation rate of CO becomes larger than that of CO₂ at about 450 °C. The H₂ yield, the CO₂ selectivity and the CO content become constant at about 550 °C. At 240 °C, the smallest CO content is obtained with a catalyst weight of 0.5 g and a Cu content of 3 wt%. The H₂ yield, defined as H₂/(CO + CO₂) in formation rates, at 240 °C is always 3 and not affected by the variations of either the catalyst weight or the Cu content.