Phase-separated Ce–Co–O catalysts for CO oxidation

As a major gas pollutant, the control of CO gas emission from chemical industry and vehicle exhausts has aroused global attention in the past years and it is extremely essential to develop efficient, low-cost and environmental friendly catalysts for CO conversion and removal. In this work, we report the facile synthesis of Ce–Co–O catalysts through a simple ultrasonic spray pyrolysis process and investigate their application for low-temperature CO oxidation. The Ce–Co–O catalysts comprising of separated CeO₂ and Co₃O₄ phases show superior CO oxidation capability below 473 K (200 °C) without the assistance of any other co-catalyst or noble metal. With the increase of Co₃O₄ concentration, the CO oxidation temperature of Ce–Co–O catalysts decreases quickly and reaches a complete conversion temperature of 410 K (137 °C) in the case of the optimized Co content. Microstructure analysis using high-resolution transmission electron microscope reveals that the tiny CeO₂ and Co₃O₄ phases assembled into porous particles are well crystallized and show high chemical purity. The porous feature of Ce–Co–O catalysts synthesized from feasible ultrasonic spray pyrolysis makes them more competitive and promising towards gaseous environmental pollution processing including CO oxidation, TWCs, SCR, etc.     

Multifunctional Co–B–O@CoxB catalysts for efficient hydrogen generation

The development of efficient and non-noble catalyst is of great significance to hydrogen generation techniques. Three surface-oxidized cobalt borides of Co–B–O@Co_xB (x = 0.5, 1 and 2) have been synthesized that can functionalize as active catalysts in both alkaline water electrolysis and the hydrolysis of sodium borohydride (NaBH₄) solution. It is discovered that oxidation layer and low boron content favor the oxygen evolution reaction (OER) activity of Co–B–O@Co_xB in alkaline water electrolysis. And surface-oxidized cobalt boride with low boron content is more active toward hydrolysis of NaBH₄ solution. An alkaline electrolyzer fabricated using the optimized electrodes of Co–B–O@CoB₂/Ni as cathode and Co–B–O@Co₂B/Ni as anode can deliver current density of 10 mA cm⁻² at 1.54 V for overall water splitting with satisfactory stability. Meanwhile, Co–B–O@Co₂B affords the highest hydrogen generation rate of 3.85 L min⁻¹ g⁻¹ for hydrolysis of NaBH₄ at 25 °C.     

Platinum–ceria–zirconia catalysts for hydrogen production in sulfur-iodine cycle

In this study, Pt/Ce_1−xZr_x catalysts with different Zr mole concentration (x = 0, 0.2, 0.5, 0.8, 1) have been tested to evaluate their effects on hydrogen iodide (HI) decomposition for hydrogen production in the sulfur-iodine (SI or IS) cycle at various temperatures. The Pt/Ce_1−xZr_x catalysts strongly enhanced the HI conversion to H₂ by comparison with blank test, especially the Pt/Ce_0.8Zr_0.2 catalyst. BET, XRD, TEM, EDS, TPR were performed for catalysts characterization. It was found that, through introducing ZrO₂ into Pt/CeO₂, a synergistic effect between Pt and CeO₂-ZrO₂ solid solution was different from Pt and CeO₂ yield, such as improvement of the thermal stability and increase of Pt-O-Ce reducibility. Among the three samples containing Zr, the one with 20 mol% displayed the best activity for hydrogen production.     

DRIFTS study of Cu–Zr–Ce–O catalysts for selective CO oxidation

The effects of different components in Cu₁Zr₁Ce₉O_δ catalyst and the variations of the feed stream on the catalytic performance of selective CO oxidation were investigated by diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) technique. It is found that the active sites of Cu₁Zr₁Ce₉O_δ catalyst are mainly Cu⁺ species. Formate species is formed through the reaction between CO gas and hydroxyl groups on the reduced cerium surface. CeO₂ in the Cu₁Zr₁Ce₉O_δ catalyst facilitates the formation of Cu⁺ species and improves the amount of CO adsorption whereas it is unfavorable to the deep reduction of Cu⁺ species. ZrO₂ doped into the Cu₁Zr₁Ce₉O_δ catalyst increases the Cu coverage and CO adsorption capacity, while it decreases the adsorption of CO₂ on the catalyst surface. The adsorption capacities of oxygen and CO are associated with the catalytic performance for the selective CO oxidation at lower and higher temperatures, respectively. The presence of CO in the feed stream promotes the reduction of Ce⁴⁺ species and the production of geminal OH group on the reduced ceria surface. Hydrogen in the feed diminishes the CO adsorption ability but stimulates the CO desorption. CO₂ in the feed occupies the active sites and decreases the adsorption of the reactants, thus deteriorates the catalytic performance for the selective CO oxidation.     

Ni–Fe–B catalysts for NaBH4 hydrolysis

A series of Ni–Fe–B catalysts with different Fe/(Fe + Ni) molar ratios, used for the hydrolysis of NaBH₄, were prepared by chemical reduction of NiCl₂ and FeCl₃ mixed solution with NaBH₄. The measurements revealed that the catalysts with the molar ratio of Fe/(Fe + Ni) (30%) exhibited the highest catalytic activity, and the optimal reduction temperature is 348 K. In addition, the effects of the concentration of NaBH₄, NaOH and the hydrolytic temperature of NaBH₄ were discussed in detail. The results show that the reaction rate of hydrolysis first rises up and then goes down subsequently with the increase of NaBH₄ concentration, as well as the concentration of NaOH. The activation energy of the hydrolysis for Ni–Fe–B catalysts is fitted to 57 kJ/mol. The maximum value of hydrogen generation is 2910 ml/(min g) at 298 K.     

Facile synthesis of highly active monolithic water-separated W-doped Ni–Fe–P catalysts

A new type of highly active and cost-effective nanoporous W-doped Ni–Fe–P catalyst on nickel foam (NF) was synthesized by a facile electroless plating method. The W-doped Ni–Fe–P/NF catalysts exhibit extraordinary catalytic activity for hydrogen evolution reaction (HER) in alkaline media, capable of yielding a current density of −10 mA cm⁻² at an overpotential of only 68 mV. Furthermore, the catalysts also show efficient activity towards oxygen evolution reaction (OER) with an overpotential of 210 mV at j = 10 mA cm⁻² as well. The W-doped Ni–Fe–P/NF electrocatalyst exhibits a long-term durability over 13 h test.     

Ni–P and Ni–Cu–P modified carbon catalysts for methanol electro-oxidation in KOH solution

The electrocatalytic oxidation of methanol was studied on Ni–P and Ni–Cu–P supported over commercial carbon electrodes in 0.1 M KOH solution. Cyclic voltammetry and chronoamperometry techniques were employed. Electroless deposition technique was adopted for the preparation of these catalysts. The effect of the electroless deposition parameters on the catalytic activity of the formed samples was examined. They involve the variation of the deposition time, pH and temperature. The scanning electron micrography showed a compact Ni–P surface with a smooth and low porous structure. A decreased amount of nickel and phosphorus was detected by EDX analysis in the formed catalyst after adding copper to the deposition solution. However, an improvement in the catalytic performance of Ni–Cu–P/C samples was noticed. This is attributed to the presence of copper hydroxide/nickel oxyhydroxide species. It suppresses the formation of γ-NiOOH phase and stabilizes β-NiOOH form. Linear dependence of the oxidation current density on the square root of the scan rate reveals the diffusion controlled behaviour.     

Natural gas reforming of carbon dioxide for syngas over Ni–Ce–Al catalysts

A series of Ni–Ce–Al composite oxides with various Ni molar contents were synthesized via the refluxed co-precipitation method and used for natural gas reforming of CO₂ (NGRC) for syngas production. The effect of Ni molar content, reaction temperature, feed gas ratio and gas hourly space velocity (GHSV) on the Ni–Ce–Al catalytic performance was investigated. The Ni₁₀CeAl catalyst was selected to undergo 30 h stability test and the conversion of CH₄ and CO₂ decreased by 2.8% and 2.6%, respectively. The characterization of the reduced and used Ni₁₀CeAl catalyst was performed using BET, H₂-TPR, in-situ XRD, TEM, and TGA-DTG techniques. The in-situ XRD results revealed that Ce₂O₃, CeO₂ and CeAlO₃ coexisted in the Ni₁₀CeAl catalyst after reduction at 850 °C for 2 h. The results of the TEM analysis revealed that the Ni particle size increased after the NGRC reaction, which mainly caused the catalyst deactivation.     

A methanol – Tolerant carbon supported Pt–Sn cathode catalysts

Carbon supported Pt–Sn bimetallic electrocatalysts with a Pt:Sn 90:10 atomic ratio were prepared by impregnation method and then heat treated at 300 and 500 °C under Helium atmosphere. The purpose of this work is to investigate the effect of tin addition to platinum for methanol tolerant oxygen reduction reaction. In this sense, structure and morphological properties of supported bimetallic catalysts were correlated to the catalytic performance. Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterizations confirm the formation of Pt–Sn bimetallic electrocatalysts with a Pt single-phase material alloy and revealed an increase in the average particle size after heat treatment. The electrocatalytic activities of these samples for the oxygen reduction reaction (ORR) were examined in acidic medium using both a rotating disk (RDE) and a rotating ring disk (RRDE) electrodes. Compared with the Pt/C, Pt–Sn/C bimetallic catalysts show superior electrocatalytic activity towards ORR with an approaching four electron pathway leading to water formation. The specific and mass activity for ORR follow the order of Pt–Sn/C-500 ≈ Pt–Sn/C-300 > Pt–Sn/C > Pt/C. Furthermore, it is found that among the three Pt–Sn samples, Pt–Sn/C-500 exhibits the highest methanol tolerance. These experimental observations indicate that the addition of Sn into Pt is favorable to maximize the ORR performances of platinum and further the heat treatment is beneficial to improve the methanol tolerance behavior. On this basis, the novel Pt–Sn catalysts can be considered as potential candidates to be used as cathodes in Direct Methanol Fuel Cells.     

Ag and Ag–Mn nanowire catalysts for alkaline fuel cells

Low loadings of Ag and Ag–Mn nanowire catalysts were applied to the surface of a CNT-base electrode. The catalyzed electrodes had a 60 mV larger onset potential and promoted the ORR via the direct 4 electron pathway. The Ag/CNT, Ag–Mn/CNT, and CNT samples produced a Tafel slope of about 70 mV/decade which confirmed the ORR activation was limited by the migration of oxygen molecules to active surface sites. The catalytic performance of the Ag and Ag–Mn nanowires was also comparable to that of a bulk catalyst but at a much lower loading. Electrochemical test results showed that the Ag and Ag–Mn catalysts exhibited similar performance. The Ag–Mn nanowire catalysts were synthesized using a unique electroless deposition procedure to co-deposit Ag and Mn. ICP confirmed that 2 to 9 at% Mn was present in the nanowires. XPS and XRD analysis showed that the Ag–Mn nanowires were composed of Mn in solid solution with Ag and a thin surface layer containing MnO and MnO₂. The Ag–Mn nanowires were expected to be the most active. The equivalent performance between Ag and Ag–Mn samples was attributed to the presence of inactive MnO and low concentrations of MnO₂ in the nanowires. Although MnO₂ is known to be active towards the ORR, the dominant Mn species in the nanowires was MnO.     

Evaluation of Pt–Ru–Ni and Pt–Sn–Ni catalysts as anodes in direct ethanol fuel cells

In this study, the electrooxidation of ethanol on carbon supported Pt–Ru–Ni and Pt–Sn–Ni catalysts is electrochemically studied through cyclic voltammetry at 50 °C in direct ethanol fuel cells. All electrocatalysts are prepared using the ethylene glycol-reduction process and are chemically characterized by energy-dispersive X-ray analysis (EDX). For fuel cell evaluation, electrodes are prepared by the transfer-decal method. Nickel addition to the anode improves DEFC performance. When Pt₇₅Ru₁₅Ni₁₀/C is used as an anode catalyst, the current density obtained in the fuel cell is greater than that of all other investigated catalysts. Tri-metallic catalytic mixtures have a higher performance relative to bi-metallic catalysts. These results are in agreement with CV results that display greater activity for PtRuNi at higher potentials.     

Spectroscopic characterization of Mn–Co–Ce mixed oxides,active catalysts for COPROX reaction

Mn–Co–Ce mixed oxides are active and selective catalysts for the CO preferential oxidation (COPROX), which is a promising process for the purification of hydrogen streams. In this work, we report a careful spectroscopic characterization of the said system, with the aim of relating its physical chemistry properties to the catalytic behavior. In all the Co–Mn–Ce samples, we detected the formation of partially developed (Mn,Co)₃O₄ mixed spinels. The presence of these species, which can be reduced during the TPR experiments at an intermediate temperature range (300–600 °C), was also suggested by XRD and LRS. XPS results show that in all cases the catalytic surface is enriched in Mn, while the opposite occurs for Co. As regards the catalytic activity, we observed that the best formulations were those containing intermediate Mn/Co ratios (1/4 and 1/1), which can be ascribed to the promoting effect of Mn in improving the redox properties of Co active sites and provoking an increase in surface area. The best catalyst, which has a Mn/Co ratio of 1/4, was very stable after 75 h of time-on-stream with CO₂ included in the feed.     

Co-precipitated Ni–Mg–Al catalysts for hydrogen production by supercritical water gasification of glucose

Supercritical water gasification (SCWG) is a promising process for hydrogen production from biomass. In this study, a series of Ni–Mg–Al catalysts with different Mg/Al molar ratios has been synthesized by a co-precipitation method for hydrogen production by SCWG of glucose. Effects of Mg addition on the catalytic activity, hydrothermal stability and anti-carbon performance of alumina supported nickel catalyst were investigated. The highly dispersed nickel catalysts prepared by co-precipitation could greatly enhance the gasification efficiency of glucose in supercritical water. Among the tested Ni–Mg–Al catalysts, NiMg_0.6Al_1.9 showed the highest catalytic activity with the hydrogen yield of 11.77 mmol/g (912% as that of non-catalytic test). NiMg_0.6Al_1.9 also showed the best hydrothermal stability probably due to the formation of MgAl₂O₄. Mg could efficiently improve the anti-carbon ability of Ni–Al catalyst by inhibiting the formation of graphite carbon. It is also confirmed that MgO supported nickel catalyst is not suitable for SCWG process owing to the difficulty on nickel oxides reduction in the precursors and the phase change of MgO to Mg(OH)₂ under the hydrothermal condition.     

Dehydrogenation of Ammonia Borane with transition metal-doped Co–B alloy catalysts

The catalytic performance of transition metal-doped Co–B ternary alloys were tested for H₂ generation by hydrolysis of Ammonia Borane (AB). Chemical reduction method was used to dope Co–B catalyst with various transition metals, namely Cu, Cr, Mo, and W, using their corresponding metal salts. All transition metals induce significant promoting effects on the Co–B catalyst by increasing the H₂ generation rate by about 3–6 times as compared to the undoped catalyst. The effect of metal dopant concentration on overall catalyst structure, surface morphology, and catalytic efficiency were examined by varying the metal/(Co + metal) molar ratio. Characterizations such as XPS, XRD, SEM, BET surface area measurement, and particle size analysis were carried out to understand the promoting role of each dopant metal during AB hydrolysis. Dopant transition-metals, in either oxidized or/and metallic state, act as an atomic barrier to avoid Co–B particle agglomeration thus preserving the effective surface area. In addition, the oxidized species such as Cr³⁺, Mo⁴⁺, and W⁴⁺, act as Lewis acid sites to enhance the absorption of OH⁻ group to further assist the hydrolysis reaction over alloy catalysts. The promoting nature of transition metal dopants in Co–B alloy powders is demonstrated by the evaluated low activation energy of the rate limiting step and high H₂ generation rate (2460 ml H₂ min⁻¹ (g of catalyst)⁻¹ for Co–Mo–B) in the hydrolysis of AB.     

Sodium borohydride hydrolysis on highly efficient Co–B/Pd catalysts

Cobalt–Boron (Co–B) catalysts are prepared on the nickel foam substrate (NiFS) by in situ reduction of Co²⁺ ions in sodium borohydride (NaBH₄) solution for the catalytic generation of hydrogen from NaBH₄. The formation of Co–B catalysts on the substrate is much faster by using a dip-coating and extended drying (“dry-dip-coating method”) followed by chemical reduction as compared to that prepared by a conventional dip-coating method followed by chemical reduction. The dry treatment results in a significant reduction in the re-dissolution of the dip-coated Co–B catalysts during the following dipping processes. Co–B catalysts on Pd modified NiFS have also been prepared using dry-dip-coating method. The factors affecting the performance of the catalysts such as dipping time, calcination temperature, Co–B loadings, Pd formation time and operating temperature, are studied. The best catalytic activity and stability is obtained on Co–B on Pd modified NiFS.     

Upgrading of cyclohexanone to hydrocarbons by hydrodeoxygenation over nickel–molybdenum catalysts

Cyclohexanone is largely generated in the direct or indirect conversion of lignin-derived bio-oils. Hence, the upgrading of cyclohexanone, i.e. deoxygenation in the presence of hydrogen is of great interest. In this regard, two nickel-molybdenum catalysts on alumina support were investigated in the temperatures up to 400 °C and pressures up to 15 bar. High activity, selectivity, and yield were achieved by utilizing these catalysts at the studied condition. The main products of the upgrading of cyclohexanone were C₆, C₇, and C₁₂ cyclic, aromatic, and bicyclic including cyclohexane, cyclohexene, benzene, and cyclohexylbenzene. The results of the present study imply that these catalysts are beneficial in producing hydrocarbon-rich products from cyclohexanone and lignin-derived bio-oils. Based on the achievements of the present study, the nickel-molybdenum catalyst composed of 1.14 wt% nickel and 14.27 wt% molybdenum showed about 87%, 100%, and 116% conversion of cyclohexanone, total hydrocarbon selectivity, and total hydrocarbon yield, respectively. The optimum condition for obtaining such results was at 400 °C and 8 bar.     

Hydrogen generation from catalytic hydrolysis of sodium borohydride solution using Cobalt–Copper–Boride (Co–Cu–B) catalysts

Co–Cu–B, as a catalyst toward hydrolysis of sodium borohydride solution, has been prepared through chemical reduction of metal salts, CoCl₂·6H₂O and CuCl₂, by an alkaline solution composed of 7.5wt% NaBH₄ and 7.5wt% NaOH. The effects of Co/Cu molar ratio, calcination temperature, NaOH and NaBH₄ concentration and reaction temperature on catalytic activity of Co–Cu–B for hydrogen generation from alkaline NaBH₄ solution have been studied. X-ray diffraction (XRD), scanning electron microscope (SEM) and Nitrogen adsorption–desorption isotherm have been employed to understand the results. The Co–Cu–B catalyst with a Co/Cu molar ratio of 3:1 and calcinated at 400 °C showed the best catalytic activity at ambient temperature. The activation energy of this catalytic reaction is calculated to be 49.6 kJ mol⁻¹.     

Novel microemulsion fuel additive Ce–Ru–O catalysts with algae biofuel on diesel engine testing

Biodiesels derived from microalgae oil promise to be an alternative for the conventional diesel fuel due to their similarity in properties. In this present work, Ce–Ru–O catalysts are used as an additive to the NOME in the form of an emulsion. A single-cylinder, four-stroke direct injection compression ignition engine is made to run on B20+CeO₂, and B20 dozed with different dosage levels of Ce_0.95Ru_0.05O₂, Ce_0.9Ru_0.1O₂, and Ce_0.8Ru_0.2O₂ microemulsions. Ce–Ru–O at different concentrations is used to study the effect of metal oxide on emission characteristics of the fuel. Experimental results show that addition of microemulsion has a positive effect on emission characteristics and also acts as an oxidation catalyst.