Preparation of Pt supported on mesoporous Mg–Al oxide catalysts for efficient dehydrogenation of methylcyclohexane

Hydrogen energy, characterizing by high-energy density, non-pollution and renewability, is regarded as an ideal clean green energy, and the chemical hydrogen storage is an optimal strategy to realize its large-scale utilization. In this study, to enhance the hydrogen evolution rate in the dehydrogenation of methylcyclohexane (MCH), Pt supported on Mg–Al oxide catalysts were prepared and the effects of the co-precipitation reaction time during the preparation of Mg–Al hydrotalcite on their structural properties were studied in detail. The results showed that both the pore diameter and Pt dispersion were increased after prolonging the precipitation reaction time. During the dehydrogenation of MCH, these resultant catalysts presented high activity and good stability: hydrogen evolution rate reached up to 1892 mmol·g_Pt^?1 min^?1 at 623 K and the conversion was still held at 92% after 218 h. Of course, a slight decrease on the conversion during the dehydrogenation reaction was also observed, which was mainly attributed to the aggregation of Pt particles at high temperature.     

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.     

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.     

Preparation of Ni–Cu/Mg/Al catalysts from hydrotalcite-like compounds for hydrogen production by steam reforming of biomass tar

Ni–Cu/Mg/Al bimetallic catalysts were prepared by the calcination and reduction of hydrotalcite-like compounds containing Ni²⁺, Cu²⁺, Mg²⁺, and Al³⁺, and tested for the steam reforming of tar derived from the pyrolysis of biomass at low temperature. The characterizations with XRD, STEM-EDX, and H₂ chemisorption confirmed the formation of Ni–Cu alloy particles. The Ni–Cu/Mg/Al bimetallic catalyst with the optimum composition of Cu/Ni = 0.25 exhibited much higher catalytic performance than the corresponding monometallic Ni/Mg/Al and Cu/Mg/Al catalysts in the steam reforming of tar in terms of activity and coke resistance. The catalyst gave almost total conversion of tar even at temperature as low as 823 K. This high performance was related to the higher metal dispersion, larger amount of surface active sites, higher oxygen affinity, and surface modification caused by the formation of small Ni–Cu alloy particles. In addition, the Ni–Cu/Mg/Al catalyst showed better long-term stability than the Ni/Mg/Al catalyst. No obvious aggregation and structural change of the Ni–Cu alloy particles were observed. The coke deposition on the Ni–Cu/Mg/Al catalyst was approximately ten times smaller than that on the Ni/Mg/Al catalyst, indicating good coke-resistance of the Ni–Cu alloy particles.     

Preparation and characterization of bimetallic Ni–Ir/C catalysts for HI decomposition in the thermochemical water-splitting iodine–sulfur process for hydrogen production

A series of bimetallic 10%Ni-xIr/C (x = 0.5, 1.0, 1.5, 2.0 wt%) and monometallic 10%Ni/C and 2%Ir/C catalysts were prepared through the impregnation–reduction method modified by adding the ionic surfactant hexadecyltrimethylammonium bromide (CTAB) as the stabilizing agent during the impregnation. Their catalytic performance was tested by HI decomposition under atmospheric pressure at 400 °C and 500 °C. X-ray diffraction, Brunauer–Emmett–Teller surface area, transmission electron microscopy, and X-ray photoelectron spectroscopy were adopted to characterize the structure, specific surface area, morphology, and surface chemical state, respectively. Results showed that the addition of Ir metal and the use of CTAB played important roles in enhancing the activity and stability of the Ni-based bimetallic catalysts. Among all the catalysts tested, the bimetallic 10%Ni-1.5%Ir/C catalyst presented excellent activity and stability for HI decomposition.     

The stability of Pt–Ir/C bimetallic catalysts in HI decomposition of the iodine–sulfur hydrogen production process

Decomposition of HI is the key reaction of hydrogen production in the iodine–sulfur thermochemical water splitting cycle, so studies about the catalysts for HI decomposition have drawn increasing attention. In this study, a series of monometallic Pt/C((Pt/C-400, Pt/C-500, Pt/C-600, Pt/C-700 and Pt/C-800), Ir/C(Ir/C-400, Ir/C-500, Ir/C-600, Ir/C-700 and Ir/C-800) and bimetallic Pt–Ir catalysts supported on active carbon (Pt–Ir/C-400, Pt–Ir/C-500, Pt–Ir/C-600, Pt–Ir/C-700 and Pt–Ir/C-800 were prepared by the impregnation-reduction-calcination method. Their catalytic activities were evaluated for HI decomposition in a fixed bed reactor at 400 and 500 °C under atmospheric pressure. Their structures, metal particles size and distribution, and specific surface area were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) surface area, respectively. The results showed that the bimetallic Pt–Ir catalyst had the excellent stability in terms of the anti-sintering structure and catalytic activity. Therefore, the bimetallic Pt–Ir catalysts are the good candidates to take the place of the traditional monometallic Pt/C catalyst for catalyzing the HI decomposition.     

The roles of metals and their oxide species in hydrophobic Pt–Ru catalysts for the interphase H/D isotope separation

Liquid phase catalytic exchange is mainly used for separation of hydrogen isotopes from liquid water. Based on the carbon-supported Pt and Pt–Ru catalysts with different metal and oxide species distributions, several hydrophobic catalysts, used in the reaction, were fabricated. The characterization results indicated that alloy and amorphous nanoparticles were formed in the Pt_0.5Ru_0.5/C catalyst using the microwave-irradiated polyol method. After reduction, the content of metallic species increased and that of hydrous Ru oxide species significantly decreased. A Pt_0.5Ru_0.5O₂/C catalyst containing more oxide species was also synthesized by the microwave-irradiated oxidation precipitation method. Performance tests demonstrated that the presence of more metallic Pt species in both the hydrophobic Pt and Pt–Ru catalysts resulted in higher catalytic activity. The addition of Ru, as an alloy or as a hydrous oxide, can improve the catalytic activity of pure Pt. These experimental results were explained by the reaction mechanisms.     

Evaluation of the stability and durability of Pt and Pt–Co/C catalysts for polymer electrolyte membrane fuel cells

Carbon supported Pt and Pt–Co nanoparticles were prepared by reduction of the metal precursors with NaBH₄. The activity for the oxygen reduction reaction (ORR) of the as-prepared Co-containing catalyst was higher than that of pure Pt. 30 h of constant potential operation at 0.8 V, repetitive potential cycling in the range 0.5–1.0 V and thermal treatments were carried out to evaluate their electrochemical stability. Loss of non-alloyed and, to a less extent, alloyed cobalt was observed after the durability tests with the Pt–Co/C catalyst. The loss in ORR activity following durability tests was higher in Pt–Co/C than in Pt/C, i.e. pure Pt showed higher electrochemical stability than the binary catalyst. The lower stability of the Pt–Co catalyst during repetitive potential cycling was not ascribed to Co loss, but to the dissolution–re-deposition of Pt, forming a surface layer of non-alloyed pure Pt. The lower activity of the Pt–Co catalyst than Pt following the thermal treatment, instead, was due to the presence of non-alloyed Co and its oxides on the catalyst surface, hindering the molecular oxygen to reach the Pt sites.     

Enhanced low temperature catalytic activity of Ni/Al–Ce0.8Zr0.2O2 for hydrogen production from ammonia decomposition

Ammonia decomposition is an effective way for high purity hydrogen production, yet the increase of catalytic activity at low temperatures remains a big challenge for this process. In this paper, a CeO₂–ZrO₂ composite with Al as the secondary dopant was synthesized by the co-precipitation method, which was used as the carrier of nickel metal for ammonia decomposition. The experimental results showed that an obvious increase in catalytic activity of the ammonia decomposition at the relatively low temperature range of 450–550 °C was achieved over the nickel catalyst with CeO₂–ZrO₂ composite as the metal carrier. Specifically, the complete decomposition of ammonia was achieved at 580 °C for Ni/Al–Ce_0.8Zr_0.2O₂ catalyst, while only 92% of ammonia was decomposed at 600 °C over the reference Ni/Al₂O₃ catalyst. The characterization results indicated that the introduction of Al as the secondary dopant of ceria not only increases the specific surface area and oxygen defects on the surface, but also enhances the nickel metal dispersion and metal-support interaction, thus enhances the catalytic performance of Ni/Al–Ce_0.8Zr_0.2O₂ catalyst in the ammonia decomposition.     

Development of Fe2O3–TiO2 mixed oxide incorporated Ni–P coating for electrocatalytic hydrogen evolution reaction

Considering the electronic parameters and chemical characteristics, a synergistic catalytic effect of Fe₂O₃ along with TiO₂ could be achieved for electrochemical reactions if both the oxides are produced in a mixed oxide form. The present study explored the mixed oxide composite viz; Fe₂O₃–TiO₂, synthesized via thermal decomposition method, to increase the catalytic efficiency of Ni–P electrodes, the well known catalytic electrodes for hydrogen evolution reaction in alkaline medium. The incorporation of the Fe₂O₃–TiO₂ mixed oxide into Ni–P matrix substantially reduced overpotential during hydrogen evolution reaction (HER) in 32% NaOH solution. A significant improvement on the electrochemical activity of the Ni–P coated electrodes was achieved as evidenced from the results of Tafel and impedance studies. The incorporation of Fe₂O₃–TiO₂ mixed oxide composite into the Ni–P matrix has improved both metallurgical and electrochemical characteristics and hence its amount of incorporation should be optimum. The electrodes exhibited high stability under dynamic experimental conditions. The role of the composite and the possible mechanism are discussed in this paper.     

Improved hydrogen storage kinetics and thermodynamics of RE-Mg-based alloy by co-doping Ce–Y

Aiming at improving hydrogen storage performance of Mg-base alloy, the Mg₉₀Ce₅Y₅ alloy, which has high capacity and high stability, was prepared by vacuum induction melting. The XRD, SEM, TEM, PCI, and DSC were used to characterize the microstructure and phase transformation of alloy as well as hydrogen storage property. The results indicate that the Mg₉₀Ce₅Y₅ alloy consists of multiphase structure, including the CeMg₁₂, Y₅Mg₂₄, Ce₂Mg₁₇ as well as residual Mg phase, besides, part Y dissolved in both Mg and CeMg₁₂/Ce₂Mg₁₇ phase to form solid solutions. After hydrogen absorption, these phases transform into the MgH₂, CeH_2.73 and YH₂ phase, while after hydrogen desorption, the MgH₂ transforms into the Mg phase, but the rare earth hydride phase was not changed. There is another reversible transformation between the CeH_2.73 and CeO₂ phase, which is beneficial for the cyclic stability of the alloy. The alloy has the reversible hydrogen capacity of about 6.0 wt% H₂ as well as the activation energy of 114.3 kJ/mol, and also exhibits enhanced kinetics compared with the pure MgH₂ sample, as a result of the synergistic effect of rare earth hydride phase. Meanwhile, it is also noted that the Mg₉₀Ce₅Y₅ alloy begins to release hydrogen below 250 °C and the rate of hydrogen desorption is mainly dominated by surface controlled.     

Effect of pretreatment on Pt–Co/C cathode catalysts for the oxygen reduction reaction

Carbon supported Pt and Pt–Co electrocatalysts for the oxygen reduction reaction in low temperature fuel cells were prepared by the reduction of the metal salts with sodium borohydride and sodium formate. The effect of surface treatment with nitric acid on the carbon surface and Co on the surface of carbon prior to the deposition of Pt was studied. The catalysts where Pt was deposited on treated carbon the ORR reaction preceded more through the two electron pathway and favored peroxide production, while the fresh carbon catalysts proceeded more through the four electron pathway to complete the oxygen reduction reaction. NaCOOH reduced Pt/C catalysts showed higher activity that NaBH₄ reduced Pt/C catalysts. It was determined that the Co addition has a higher impact on catalyst activity and active surface area when used with NaBH₄ as reducing agent as compared to NaCOOH.     

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.     

Preparation of mesoporous nanocrystalline 10% Ni/Ce1−xMnx O2 catalysts for dry reforming reaction

Methane dry reforming (MDR) was employed for the syngas production over 10% Ni/Ce_1?xMn_xO₂ (x = 0, 0.05, 0.25, 0.50, 0.75 and 1) catalysts. The obtained results indicated that the incorporation of Mn to the cerium oxide improved the BET area. However, there was an optimum content of Mn, in which the composite material exhibited the highest BET area. Addition of Mn more than 50 mol.% declined the BET area and significantly raised the mean pore size. The catalytic results indicated that the incorporation of Mn into the catalyst carrier slightly decreased the catalytic activity. However, the catalytic stability improved upon the addition of Mn. The TPO analysis confirmed the decrease in the amount of deposited carbon with the increase of Mn content. The whisker type carbon was observed on the surface of the spent catalysts. The catalytic results showed that the 10% Ni/Ce_0.95Mn_0.05O₂ exhibited the highest catalytic performance in methane reforming with carbon dioxide.     

Unidirectional sensing characteristics of structured Au–GaN–Pt diodes for differential-pair hydrogen sensors

A new metal-semiconductor-metal (MSM) hydrogen sensor was proposed to avoid (or to reduce) false alarms due to temperature drift when it is used in differential-pair hydrogen-sensing systems. A GaN semiconductor layer together with Pt as catalytic metal and Au as Schottky metal was employed to structure an Au–GaN–Pt MSM sensor. In particular, the structured Au–GaN–Pt MSM sensor can function as an active sensor and a reference sensor, depending on the polarity of applied voltage, in a differential-pair sensing circuit. Possible sensing mechanisms associated with the Au–GaN–Pt MSM sensor were described first to include band diagrams and graphical analysis. Experimental results reveal that an active sensor by forward-biasing the Au–GaN–Pt MSM sensor responses well to hydrogen-containing gases (50, 500, and 5000 ppm H₂/N₂) at various temperatures (25 °C, 50 °C, 70 °C, and 90 °C). High sensing current gains over 10⁴ were obtained. Further, the Au–GaN–Pt MSM sensor can also be reverse-biased to act as a reference sensor which shows negligible responses to hydrogen-containing gases. The differential-pair sensing circuit with the proposed Au–GaN–Pt MSM sensor reduces false alarms due to ambient temperature variation while it provides a short detection time.     

Decomposition of hydrogen iodide over Pt–Ir/C bimetallic catalyst

Decomposition of hydrogen iodide (HI) is one of the key reactions in the sulfur–iodine (S–I) thermochemical water splitting promising for the massive hydrogen production. Much effort has been made to explore the preparation of high performance catalyst for this hydrogen-producing reaction. Although platinum has long been found to be an efficient metallic catalyst, it was prone to agglomerate at elevated temperature resulting in a decrease in the hydrogen yield. A series of bimetallic Pt–Ir/C catalysts were prepared by electroless plating to investigate the effect of Ir/Pt molar ratio on the HI conversion compared with Pt/C and Ir/C catalysts. The physical properties and morphology of the catalysts were characterized by BET, XRD, TEM and ICP-AES. The synergistic effect of platinum and iridium with respect to HI decomposition was confirmed by the fact that the bimetallic Pt–Ir/C-0.77 catalyst with 1 wt% Pt loading and 0.77 wt% Ir loading showed much higher catalytic activity and thermostability compared with Pt/C and Ir/C catalyst. Based on the experimental results obtained, it may be concluded that the bimetallic Pt–Ir/C catalyst was supposed to be a cost-effective and high performance catalyst promising to be employed for the hydrogen production via the S–I thermochemical water splitting cycle.