Potassium promoted Ru/meso-macroporous SiO2 catalyst for the preferential oxidation of CO in H2-rich gases

A series of potassium promoted Ru/meso-macroporous SiO₂ catalysts were prepared and used for the preferential oxidation of CO (CO-PROX) in H₂-rich gases. The catalysts were characterized by using techniques of TEM, SEM TPR, XPS, and N₂ adsorption/desorption. The catalytic activity of Ru/meso-macroporous SiO₂ was markedly improved by the introduction of potassium. The catalyst of K-5 wt.% Ru/meso-macroporous SiO₂ with molar ratio of K:Ru = 5:7 exhibited relatively high activity and selectivity for CO-PROX. Nanoparticles of ruthenium species can be highly dispersed on the meso-macroporous SiO₂ support by the simple impregnation method. The addition of potassium weakened the interaction between metallic Ru and the silica support. Lowering the reduction temperature of ruthenium ions could keep ruthenium in the state of metallic Ru, and it was proposed that potassium acted as an electron donating agent. The electron donating effect of potassium improved the low temperature activity for CO oxidation and increased the selectivity of O₂ for CO oxidation, thus K-modified Ru/meso-macroporous SiO₂ catalyst showed obviously a wide temperature window for CO elimination from H₂-rich gases, meanwhile the related mechanism was discussed.     

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.     

Methanation of CO on Ru/graphitized-carbon catalysts: Effects of the preparation method and the carbon support structure

Two kinds of Ru/C catalysts prepared by two different methods and supported on two graphitized carbons differing in their surface area were studied in CO methanation in the H₂-rich gas. The textural parameters of the support materials were characterized by means of N₂ physisorption. XRPD, XPS, TEM and CO- chemisorption studies indicate that the application of wet impregnation leads to more homogeneous composition of the Ru/carbon system and higher Ru dispersion than dry impregnation for both supports. The activity of the Ru/carbon samples in CO methanation in a H₂-rich gas stream depends on the structure and average size of the active phase crystallites. The combination of wet impregnation and the use of graphitized carbon of appropriate structure in the preparation of the Ru/C catalyst lead to a complete conversion of CO at 240 °C.     

Highly dispersed Ru on K-doped meso–macroporous SiO2 for the preferential oxidation of CO in H2-rich gases

In this work, highly dispersed Ru nanoparticles which had a uniform small nanoparticle size were supported on K-promoted meso–macroporous SiO₂ by using the simple impregnation method. The effect of the size of Ru nanoparticle on the catalytic performance for the preferential oxidation of CO (CO-PROX) in H₂-rich gases was investigated. Meanwhile, the related mechanism on size effect was discussed. The catalysts were characterized by using techniques of transmission electron microscopy, temperature-programmed reduction and CO-chemisorption. The results indicate that the K-promoted Ru/SiO₂ catalyst with the size of metal Ru particles at about 7 nm showed obviously higher turnover frequency (TOF) than that of K-Ru/SiO₂ with smaller size of Ru particles of around 2 nm. As for oxidizing CO to CO₂ on specific weight of ruthenium, the catalyst with the smaller size of metal Ru exhibited better performance owing to its much higher specific surface area of metal Ru. The catalyst with the smaller size of Ru nanoparticles showed much better methanation formation resistance for CO and CO₂. The K-promoted and highly dispersed Ru on SiO₂ exhibited excellent activity and selectivity for the CO-PROX reaction.     

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.     

Effect of magnesium on preferential oxidation of carbon monoxide on platinum catalyst in hydrogen-rich stream

The effects of magnesium on platinum catalyst used for the preferential oxidation of carbon monoxide for polymer electrolyte membrane fuel cell applications are investigated. The CO conversion and selectivity on Pt–Mg/Al₂O₃ for a H₂-rich stream are 93.1 and 62.0%, respectively, but only 70.2 and 46.89% on Pt/Al₂O₃. The superior activity of Pt–Mg/Al₂O₃ for the preferential oxidation of CO is due to an increase in the hydroxyl groups that results from an increase in basicity with the addition of Mg, as well as to an increase in the electron density on the surface of the Pt catalyst. Moreover, the content of hydroxyl groups on the Pt catalysts is promoted by water vapour.     

Mn-induced Cu/Ce catalysts with improved performance for CO preferential oxidation in H2/CO2-rich streams

A series of xMnCu/Ce catalysts with constant low Cu loading of 1 wt% were prepared by the simple impregnation method. The obtained catalysts were characterized by XRD, BET, H₂-TPR and XPS, and the preferential oxidation of CO was evaluated in CO₂/H₂-rich atmospheres. It was shown that partial Mn and Cu could be incorporated into the Ceria lattice, forming surface ternary Cu–Mn–Ce oxide solid solutions. At Mn/Cu = 0.6, the catalyst presented strong interaction among Cu, Mn and Ce, had more Ce³⁺ and Mn⁴⁺ at the surface and showed the best catalytic performance, making CO conversion increase of 23.57% at 90 °C as compared with the Cu/Ce catalyst. For CO-Prox, the highest CO conversion was 94.7% with an oxidation selectivity of 78.9% at 125 °C. At this temperature, the catalyst revealed stable catalytic performance for a total TOS of 205 h. In addition, with CO/Ar as feed gas, CO conversion was 100%, confirming the negative effects of CO₂/H₂.     

Enhanced catalytic performance of Au/CuO–ZnO catalysts containing low CuO content for preferential oxidation of carbon monoxide in hydrogen-rich streams for PEMFC

In this study, effects of Au and CuO loadings in Au/CuO–ZnO nanocatalysts for preferential oxidation of carbon monoxide in H₂-rich streams (PROX) are investigated. CuO–ZnO supports were synthesized by a co-precipitation method. Au was also incorporated into the catalysts by a deposition-precipitation procedure. The catalysts were characterized by XRD, BET surface area, FESEM, HRTEM, H₂-TPR, FTIR, and CO-TPD. 2–10 nm Au nanoparticles are dispersed on CuO–ZnO support and significantly enhance the reducibility of CuO. The Au/CuO–ZnO catalysts containing low amount of CuO were found to be more active for PROX compared to the Au/ZnO catalyst. Moreover, as more CuO is added to Au/ZnO, the CO₂ selectivity increases in the whole PROX temperature range. The catalyst containing 2 wt% Au and 1 wt% CuO on ZnO exhibited the highest activity and selectivity in the operating temperature range of PEM fuel cells. The activity of this catalyst also remained almost intact during 900 min of PROX time on stream at 80 °C.     

The promoted effect of UV irradiation on preferential oxidation of CO in an H2-rich stream over Au/TiO2

The catalytic oxidation of CO over Au/TiO₂ in an H₂-rich stream was performed under UV irradiation. It is found that UV irradiation over Au/TiO₂ promotes the preferential oxidation of CO in the H₂-rich stream. The respective chemisorption of CO, H₂ and O₂ at Au/TiO₂ can be described as a process of forming –OH or H₂O species. UV irradiation over Au/TiO₂ enhances the chemisorption of CO but suppresses the chemisorption of H₂ both at TiO₂ and Au surface. It is proposed that the photogenerated electrons from TiO₂ will cause the change of the chemisorption of CO, H₂ and O₂ at Au/TiO₂, which promotes the preferential oxidation of CO in an H₂-rich stream.     

Ni/Ru–Mn/Al2O3 catalysts for steam reforming of toluene as model biomass tar

The catalytic steam reforming of the major biomass tar component, toluene, was studied over two commercial Ni-based catalysts and two prepared Ru–Mn-promoted Ni-base catalysts, in the temperatures range 673–1073 K. Generally, the conversion of toluene and the H₂ content in the product gas increased with temperature. A H₂-rich gas was generated by the steam reforming of toluene, and the CO and CO₂ contents in the product gas were reduced by the reverse Boudouard reaction. A naphtha-reforming catalyst (46-5Q) exhibited better performance in the steam reforming of toluene at temperatures over 873 K than a methane-reforming catalyst (Reformax 330). Ni/Ru–Mn/Al₂O₃ catalysts showed high toluene reforming performance at temperatures over 873 K. The results indicate that the observed high stability and coking resistance may be attributed to the promotional effects of Mn on the Ni/Ru–Mn/Al₂O₃ catalyst.     

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

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

Preferential oxidation of CO in H2 stream on Au/CeO2–TiO2 catalysts

A series of Au catalysts supported on CeO₂–TiO₂ with various CeO₂ contents were prepared. CeO₂–TiO₂ was prepared by incipient-wetness impregnation with aqueous solution of Ce(NO₃)₃ on TiO₂. Gold catalysts were prepared by deposition–precipitation method at pH 7 and 65 °C. The catalysts were characterized by XRD, TEM and XPS. The preferential oxidation of CO in hydrogen stream was carried out in a fixed bed reactor. The catalyst mainly had metallic gold species and small amount of oxidic Au species. The average gold particle size was 2.5 nm. Adding suitable amount of CeO₂ on Au/TiO₂ catalyst could enhance CO oxidation and suppress H₂ oxidation at high reaction temperature (>50 °C). Additives such as La₂O₃, Co₃O₄ and CuO were added to Au/CeO₂–TiO₂ catalyst and tested for the preferential oxidation of CO in hydrogen stream. The addition of CuO on Au/CeO₂–TiO₂ catalyst increased the CO conversion and CO selectivity effectively. Au/CuO–CeO₂–TiO₂ with molar ratio of Cu:Ce:Ti = 0.5:1:9 demonstrated very high CO conversion when the temperature was higher than 65 °C and the CO selectivity also improved substantially. Thus the additive CuO along with the promoter and amorphous oxide ceria and titania not only enhances the electronic interaction, but also stabilizes the nanosize gold particles and thereby enhancing the catalytic activity for PROX reaction to a greater extent.     

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 H₂-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–CeO₂/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–CeO₂ onto the AlOOH/Al-fiber support. The preferred catalyst delivered 100% CO conversion and 81% O₂ 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.     

Experimental optimization analysis on operating conditions of CO removal process from hydrogen-rich reformate

The catalytic effects of CO preferential oxidation and methanation catalysts for deep CO removal under different operating conditions (temperature, space velocity, water content, etc.) are systematically studied from the aspects of CO content, CO selectivity, and hydrogen loss index. Results indicate that the 3 wt% Ru/Al₂O₃ preferential oxidation catalysts reduce CO content to below 10 ppm with a high hydrogen consumption of 11.6–15.7%. And methanation catalysts with 0.7 wt% Ru/Al₂O₃ also exhibit excellent CO removal performance at 220–240 °C without hydrogen loss. Besides, NiCl_x/CeO₂ methanation catalysts possess the characteristics of high space velocity, high activity, and high water-gas resistance, and can maintain the CO content at close to 20 ppm. Based on these experimental results, the coupling scheme of combining NiCl_x/CeO₂ methanation catalysts (low cost and high reaction space velocity) with 0.7 wt% Ru/Al₂O₃ methanation catalysts (high activity) to reduce CO content to below10 ppm is proposed.     

Preparation of mesoporous nanocrystalline CuO–ZnO–Al2O3 catalysts for the H2 purification using catalytic preferential oxidation of CO (CO-PROX)

In this article, CuO–ZnO–Al₂O₃ catalysts with various copper contents were synthesized by a co-precipitation method and employed for the elimination of carbon monoxide from a mixture of 97% H₂, 1% CO and 2% O₂ at atmospheric pressure via carbon monoxide preferential oxidation (CO-PROX). The influence of the copper and zinc contents on the physicochemical characteristics and catalytic performance was investigated. The prepared samples were characterized using the N₂ adsorption-desorption (BET), X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM) and temperature programmed reduction (TPR) techniques. The increment in CuO loading improved the activity of CuO–ZnO–Al₂O₃ catalysts for CO oxidation reaction. Among the prepared catalysts, the 50%CuO-3% ZnO-47% Al₂O₃ catalyst calcined at 400 °C with a BET area of 82.3 m²/g exhibited the best activity with a CO conversion of 88.9% at 125 °C. The effects of the presence of CO₂ and H₂O in the reaction feed stream and gas hourly space velocity (GHSV) were also 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.     

Ru–Ni/GA-MMO composites as highly active catalysts for CO selective methanation in H2-rich gases

CO selective methanation (CO-SMET) is as an ideal H₂-rich gases purification measurement for proton exchange membrane fuel cell system. Herein, the graphene aerogel-mixed metal oxide (GA-MMO) supported Ru–Ni bimetallic catalysts are exploited for CO-SMET in H₂-rich gases. The results reveal that a three-dimensional network structure GA-MMO aerogel with higher specific surface area, better thermal stability and more defects or structural disorders is formed when MMO:GO mass ratio is in the range of 1–4. After loading of Ru, more NiO are reduced to metallic Ni by hydrogen spillover effect, and thus obviously enhances the reactivity. The GA-MMO supported Ru–Ni catalyst exhibits more excellent metal dispersion, reducibility, stronger CO adsorption and activation than the MMO supported Ru–Ni catalyst, thereby resulting in better catalytic performance and stability. This work offers new insights into the construction of highly active catalyst for the efficient generation of high-quality H₂ from H₂-rich gases.     

Lowering risk of methanation of carbon support in Ru/carbon catalysts for CO methanation by adding lanthanum

Two series of Ru/C catalysts doped with lanthanum ions are prepared and studied in CO methanation in the H₂-rich gas. The samples are characterized by N₂ physisorption, TG-MS studies, XRD, XPS, TEM/STEM and CO chemisorption. Two graphitized carbons differing in surface area (115 and 80.6 m²/g) are used as supports. The average sizes of ruthenium crystallites deposited on their surfaces are 4.33 and 5.95 nm, respectively. The addition of the proper amount of La to the Ru/carbon catalysts leads to an above 20% increase in the catalytic activity along with stable CH₄ selectivity higher than 99% at all temperatures. Simultaneously, lanthanum acts as the inhibitor of methanation of the carbon support under conditions of high temperature and hydrogen atmosphere. Such positive effects are achieved at a very low concentration of La in the prepared samples, a maximum 0.04 La/Ru (molar ratio). 0.01 mmol La introduced to the Ru/C system leads to 98% CO conversion at 270 °C.     

Preferential CO oxidation over CuOCeO2 catalyst synthesized from MOF with nitrogen-containing ligand as precursor

A set of highly dispersed copper ceria catalysts were synthesized by using the CeMOF precursor featured rich nitrogen-containing ligand. Owing to the existence of coordination interactions between metal ions and nitrogen atoms, the copper ions could be adsorbed into the pore of Ce-MOF and stabilized by the ordered nitrogen atom on the pore wall. After calcinations, the generated CuO/CeO₂ catalyst featured more well-dispersed active sites, which was evidenced by varieties of characterizations such as FT-IR, UV-vis spectroscopy, PXRD, TEM, H₂-TPR, Raman spectroscopy and XPS. The as-synthesized CuO/CeO₂ catalysts displayed outstanding catalytic activities and stabilities for preferential carbon monoxide oxidation in H₂-rich stream.