Effects of the second metals on the active carbon supported Pt catalysts for HI decomposition in the iodine–sulfur cycle

The active carbon supported monometallic Pt and Pt-based bimetallic catalysts (including Pt–Ni, Pt–Pd and Pt–Rh) were prepared by the impregnation-reduction method. Their catalytic activities were compared for HI decomposition in a fixed bed reactor. The fresh and used monometallic Pt and Pt-based bimetallic catalysts were characterized by BET, XRD and TEM in order to investigate their changes in surface area, structure, and morphology, respectively. The results showed that the Pt-based bimetallic catalysts had better activity and higher stability than the monometallic Pt catalyst in HI decomposition.     

On the possibility of replacement of Pt by Pd in a hydrogen electrode of PEM fuel cells

Nanostructured Pt- and Pd-based electrocatalysts for hydrogen oxidation in proton exchange membrane (PEM) fuel cells have been synthesized and characterized. Electro-active metallic particles were obtained by chemical reduction of precursor salts on Vulcan XC-72 carbon carrier using ethylene glycol with the addition of formaldehyde. Using the synthesized Pd- and Pt-based catalysts membrane–electrode assemblies (MEAs) have been prepared and successfully tested in single fuel cells. Comparison of MEA performances demonstrates the principal possibility of replacement of the Pt by the Pd on the hydrogen electrode of PEM fuel cells.     

Hydrogen production through the aqueous phase reforming of ethylene glycol over supported Pt-based bimetallic catalysts

The catalytic activities of supported Pt-based bimetallic catalysts (Pt-M) were studied for hydrogen production via aqueous phase reforming (APR) using a 10 wt% ethylene glycol solution. The catalysts and supports used were characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption-desorption, CO chemisorption, and temperature programmed reduction (TPR) techniques. It was found that the Pt–Mn (Pt:Mn = 1:1, molar ratio) bimetallic catalyst significantly enhanced the catalytic performances such as the hydrogen yield and hydrogen production when compared with monometallic catalysts and other bimetallic catalysts that were examined. The XRD and TPR studies confirmed the interaction between the Pt and Mn species, leading to the Pt–Mn alloys supported on CMK-3. Related to the effect of the type of support, the CMK-3 support demonstrated better performance than the commercial activated carbon and alumina. Accordingly, it can be understood that the better catalytic performance of the APR reaction over Pt–Mn/CMK-3 catalyst is dependent on the alloy effect as well as the structural properties and nature of support given by the addition of the second metal.     

Catalytic partial oxidation of methyl acetate as a model to investigate the conversion of methyl esters to hydrogen

Rhodium and rhodium–ceria catalysts were investigated in the catalytic partial oxidation of methyl acetate, the simplest methyl ester, to better understand the conversion of biodiesel to hydrogen. Both catalysts were active in producing carbon monoxide, carbon dioxide, water, and hydrogen, although the rhodium–ceria catalyst demonstrates the higher conversion and the higher hydrogen selectivities at high gas hourly space velocity (GHSV). A low methyl acetate to oxygen feed ratio (C/O) favored the methyl acetate conversion, the hydrogen selectivity, and the carbon monoxide selectivity. Also, a high GHSV also improved the reactor performance. The experimental data show evidence that the oxidation of methyl acetate produces carbon monoxide and water as a primary step, instead of carbon dioxide and water. Overall, methyl acetate did not yield as much synthesis gas as biodiesel. The methyl ester functional group of biodiesel will limit the yield of hydrogen, so new catalysts targeting the decomposition of methyl esters to hydrogen should be investigated.     

Pt-based electrocatalysts for polymer electrolyte membrane fuel cells prepared by supercritical deposition technique

Pt-based electrocatalysts were prepared on different carbon supports which are multiwall carbon nanotubes (MWCNTs), Vulcan XC 72R (VXR) and black pearl 2000 (BP2000) using a supercritical carbon dioxide (scCO₂) deposition technique. These catalysts were characterized by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and cyclic voltammetry (CV). XRD and HRTEM results demonstrated that the scCO₂ deposition technique enables a high surface area metal phase to be deposited, with the size of the Pt particles ranging from 1 to 2 nm. The electrochemical surface areas (ESAs) of the prepared electrocatalysts were compared to the surface areas of commercial ETEK Pt/C (10 wt% Pt) and Tanaka Pt/C (46.5 wt% Pt) catalysts. The CV data indicate that the ESAs of the prepared Pt/VXR and Pt/MWCNT catalysts are about three times larger than that of the commercial ETEK catalyst for similar (10 wt% Pt) loadings. Oxygen reduction activity was investigated by hydrodynamic voltammetry. From the slope of Koutecky–Levich plots, the average number of electrons transferred in the oxygen reduction reaction (ORR) was 3.5, 3.6 and 3.7 for Pt/BP2000, Pt/VXR and Pt/MWCNT, correspondingly, which indicated almost complete reduction of oxygen to water.     

Dry reforming of ethanol for hydrogen production: Thermodynamic investigation

Thermodynamics of ethanol reforming with carbon dioxide for hydrogen production has been studied by Gibbs free energy minimization method. The optimum conditions for hydrogen production are identified: reaction temperatures between 1200 and 1300 K and carbon dioxide-to-ethanol molar ratios of 1.2–1.3 at 0.1 MPa. Under the optimal conditions, complete conversion of ethanol, 94.75–94.86% yield of hydrogen and 96.77–97.04% yield of carbon monoxide can be achieved in the absence of carbon formation. The ethanol reforming with carbon dioxide is suitable for providing hydrogen-rich fuels for molten carbonate fuel cell and solid oxide fuel cell. The carbon-formed and carbon-free regions are found, which are useful in guiding the search for suitable catalysts for the reaction. Inert gases have a positive effect on the hydrogen and carbon monoxide yields.     

Effect of gas composition on Ru dissolution and crossover in polymer-electrolyte membrane fuel cells

Pt-Ru-based anodes are commonly used in polymer-electrolyte membrane fuel cells (PEMFCs) to provide improved CO tolerance for reformate fuel applications. However, Ru crossover from the anode to the cathode has been identified as a critical durability problem that has severe performance implications. In the present study, an anode accelerated stress test (AST) was used to simulate potential spikes that occur during fuel cell start-ups and shutdowns to induce Ru crossover. The effects of fuel gas composition, namely hydrogen and carbon dioxide concentrations, on Ru dissolution and crossover were investigated. The cell performance losses were correlated with the degree of Ru crossover as determined by the changes in cathode cyclic voltammetry (CV) characteristics and neutron activation analysis (NAA). It was found that higher hydrogen concentration in the fuel accelerated Ru crossover and that the presence of carbon dioxide hindered Ru crossover. In particular, the injection of 20 vol.% carbon dioxide during potential cycling resulted in very minor Ru crossover, which showed essentially identical performance losses and CV characteristic changes as a fuel cell composed of a Ru-free anode. The experimental results suggest that the Ru species in our Pt-Ru metal oxide catalysts need to go through a reduction step by hydrogen before dissolution. The presence of carbon dioxide may play a role in hindering the reduction step.     

Liquid phase reforming of woody biomass to hydrogen

This work concentrates on the production of H₂ directly from raw biomass through liquid phase reforming in the presence of a liquid base and a solid catalyst. Both precious metal and base metal catalysts were found to be active for the liquid phase hydrolysis and reforming of wood. Pt-based catalysts, particularly Pt–Re, were shown by atomistic modeling to be more selective toward breaking C–C bonds, resulting in a higher selectivity to hydrogen versus methane. Ni-based catalysts were found to prefer breaking C–O bonds, favoring the production of methane. The results showed that at a constant wood concentration, increasing the concentration of base (base to wood ratio) in the presence of Raney Ni catalysts resulted in greater selectivity toward hydrogen. The amount of wood converted to gas was lower due to increased production of undesirable organic acids from the wood at higher base concentrations. It was shown that by modifying Ni-based catalysts with dopants, it was possible to reduce the base concentration while maintaining the selectivity toward hydrogen and increasing wood conversion to gas versus organic acids.     

Carbons as low-platinum catalyst supports and non-noble catalysts for polymer electrolyte fuel cells

Polymer electrolyte fuel cells, including acidic proton exchange membrane fuel cells (PEMFCs) and alkaline anion exchange membrane fuel cells (AEMFCs), are the types of the most promising high-efficiency techniques for conversion hydrogen energy to electricity energy. However, the catalysts’ insufficient activity and stability toward oxygen reduction reaction (ORR) at the cathodes of these devices are still the important constraints to their performance. So far, carbon black supported platinum (Pt/C) and its alloys are still the most practical and best-performing type of catalysts. However, the scarcity of Pt is highly challenging and the high price of commercial catalyst will continue to drive up the cost of both PEMFCs and AEMFCs. Moreover, the traditional carbon black support is susceptible to corrosion especially under electrochemical operation, itself inactive for ORR and weakly binding with Pt-based nanoparticles. In this review, the advanced carbons synthesized by various template methods, including hard-template, soft-template, self-template and combined-template, are systematically evaluated as low-Pt catalyst supports and non-noble catalysts. For the templates-induced carbon-based catalysts, this review presents a comprehensive overview on the carbon supported low-Pt catalysts from aspect of composition, size and shape control as well as the non-noble carbon catalysts such as transition metal-nitrogen-carbons, metal-free carbons and defective carbons. Furthermore, this review also summarizes the applications of low/non-Pt carbon-based catalysts base on the template-induce advanced carbons at the cathodes of PEMFCs and AEMFCs. Overall, the templates-induced carbons can show some perfect attributes including ordered morphology, reasonable pore structure, high conductivity and surface area, good corrosion resistance and mechanical property, as well as strong metal–support interaction. All of these features are of particular importance for the construction of high-performance carbon-based ORR catalysts. However, some drawbacks mainly involve the removal of templates, maintenance of morphological structure, and demetalation. To address these issues, this review also summarizes some effective strategies, such as employing the easily removed hard/soft-templates, developing the advantageous self-templates, enhancing the metal–support interaction by formation of chemical binds, etc. In conclusion, this review provides an effective guide for the construction of template-induced advanced carbons and carbon-based low/non-Pt catalysts with analysis of technical challenges in the development of ORR electrocatalysts for both PEMFCs and AEMFCs, and also proposes several future research directions for overcoming the challenges towards practical applications.     

Effects of the composition on the active carbon supported Pd–Pt bimetallic catalysts for HI decomposition in the iodine–sulfur cycle

A series of binary Pd–Pt catalysts supported on active carbon were prepared by the co-impregnation and reduction method. For comparison, active carbon supported monometallic Pt and Pd catalysts were also prepared by the impregnation–reduction method. Their structure, morphology and surface area were investigated by means of X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) surface area, respectively. Their catalytic activities were evaluated for the decomposition of hydrogen iodide (HI). Furthermore, their thermal stabilities were also investigated. The results of activity tests showed that the composition of Pd–Pt binary catalysts played the important role in dictating the catalyst activity. Among the Pt, Pd and binary Pd–Pt catalysts, the 2.5%Pd–2.5%Pt/C showed the best catalytic performance for the decomposition of HI. The results of thermal stability tests showed that the binary Pd–Pt catalyst had the higher stability than the monometallic Pt and Pd catalysts.     

Supported Pt nanoparticles for the hydrogen mitigation application

Accidents at TMI, USA and the recent one at Fukushima, JAPAN has emphasized the need for mitigation of hydrogen generated in nuclear reactor containment during accidental conditions. Passive Autocatalytic Recombiner (PAR) is one of the best solutions to deal with such situations during loss of coolant accidents. A novel breed of catalysts based on cordierite supported Pt nanoparticles has been prepared by chemical reduction route for mitigating the hydrogen generated during such accidents. These catalysts have been characterized for their phase purity, surface composition, surface area, particle size, morphology and elemental loading of the noble metal. The supported noble metal particles are found to be of 7 nm–11 nm dimensions, depending on the extent of noble metal loading. The catalytic activity of these catalysts has been evaluated in presence and absence of various prospective contaminants like carbon monoxide, carbon dioxide, methane, humidity, and condensed water in a laboratory scale reactor. Developed catalysts have been found to be catalytically active for H₂–O₂ recombination reaction in presence of 100% relative humidity and 2000 ppm of carbon monoxide.     

Nanocrystalline MgO supported nickel-based bimetallic catalysts for carbon dioxide reforming of methane

Nanocrystalline magnesium oxide with high surface area and plate-like shape was employed as catalyst support for preparation of nickel-based bimetallic catalysts in methane reforming with carbon dioxide. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption (BET), Temperature programmed oxidation and desorption (TPO–TPD), Thermal gravimetric and differential thermal gravimetric (TGA–DTG), H₂ chemisorption and Transmission and electron microscopies (TEM and SEM) analyses. CO₂–TPD data showed the high CO₂ adsorption capacity of catalysts which improves the resistance of catalysts against the carbon formation. The H₂ chemisorption results also indicated that the addition of Pt to nickel catalyst improved the nickel dispersion. The obtained results revealed that the prepared catalysts showed a high activity and stability during the reaction with a low amount of deposited carbon. Addition of Pt to nickel catalyst improved both the activity and resistivity against carbon formation.     

Lattice-strained Pt nanoparticles anchored on petroleum vacuum residue derived N-doped porous carbon as highly active and durable cathode catalysts for PEMFCs

High cost and poor durability of Pt-based cathode catalysts for oxygen reduction reaction (ORR) severely hamper the popularization of proton exchange membrane fuel cells (PEMFCs). Tailoring carbon support is one of effective strategies for improving the performance of Pt-based catalysts. Herein, petroleum vacuum residue was used as carbon source, and nitrogen-doped porous carbon (N-PPC) was synthesized using a simple template-assisted and secondary calcination method. Small Pt nanoparticles (Pt NPs) with an average particles size of 1.8 nm were in-situ prepared and spread evenly on the N-PPC. Interestingly, the lattice compression (1.08%) of Pt NPs on the N-PPC (Pt/N-PPC) was clearly observed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), which was also verified by the shift of (111) crystal plane of Pt on N-PPC to higher angles. The X-ray photoelectron spectroscopy (XPS) results suggest that the N-PPC support had a strong effect on anchoring Pt NPs and endowing surface Pt NPs with lowered d band center. Thus, the Pt/N-PPC as a catalyst simultaneously boosted the ORR activity and durability. The specific activity (SA) and mass activity (MA) of the Pt/N-PPC at 0.9 V reached 0.83 mA cm⁻² and 0.37 A mg_Pt⁻¹, respectively, much higher than those of the commercial Pt/C (0.21 mA cm⁻² and 0.11 A mg_Pt⁻¹) in 0.1 M HClO₄. The half-wave potential (E_1/2) of Pt/N-PPC exhibited only a minimal negative shift of 7 mV after 30,000 accelerated durability tests (ADT) cycles. More importantly, an H₂–O₂ fuel cell with a Pt/N-PPC cathode achieved a power density of 866 mW cm⁻², demonstrating that the prepared catalyst has a promising application potential in working environment of PEMFCs.     

Reduction time effect on structure and performance of Ni–Co/MgO catalyst for carbon dioxide reforming of methane

Reducing catalysts with hydrogen is an important process for carbon dioxide reforming methane since metallic active sites are exposed and dispersed during the reduction process. In this work, Ni–Co/MgO catalysts were prepared for syngas production by using a multiple-impregnation method with a carbon dioxide reforming methane reaction. Activity evaluation showed that catalysts that reduced for 1 h exhibited superior catalytic activity with methane and carbon dioxide conversion at 92% and 97%, respectively, and the syngas ratio close to unity (0.98). The high activity is ascribed to the better metal dispersion (10.5%) and smaller active metal particle size (10 nm). Raman spectra analysis indicated that catalysts that reduced for a longer time possessed larger active metal particle size, and were more susceptible to the formation of graphite-like carbon deposits, which were difficult to be removed by the active oxygen species derived from carbon dioxide dissociation.     

Hydrogen production from ethanol steam reforming over Ni/SiO2 catalysts: A comparative study of traditional preparation and microwave modification methods

Four silica‐supported nickel catalysts with Ni content of 10 wt% were prepared by impregnation and coprecipitation methods with or without microwave‐assisted calcination. The prepared catalysts were characterized by some techniques (BET, XRD, TEM, XPS, H₂‐TPR, etc.) and evaluated with respect to steam reforming of ethanol (SRE) for hydrogen production. The results show that the prepared Ni/SiO₂ catalysts are all very active and selective for SRE. The high activity of the four catalysts may benefit from their high specific areas and the good dispersion of active components on the carrier. The rate of carbon deposition decreases with reaction temperature especially below 450 °C. The maximum hydrogen yield of 4.54 mol H₂/mol EtOH‐reacted can be obtained over the Ni/SiO₂ catalyst by the microwave‐assisted coprecipitation method at a reaction temperature of 600 °C, EtOH/H₂O molar ratio of 1:12, liquid hourly space velocity of 11.54 h^?1 and time on stream within 600 min. The Ni/SiO₂ catalysts with microwave modification exhibits better performances of hydrogen production, stability and resistance to carbon deposition than that without microwave modification preparation, which is mainly attributed to that the microwave‐assisted treatment can decrease the catalyst acidity and enhance the interaction between metal support. Copyright © 2013 John Wiley & Sons, Ltd.     

Diameter-controlled carbon nanotubes and hydrogen production

Simultaneous production of hydrogen and carbon nanomaterials over Ni-loaded ZSM-5 catalysts via catalytic decomposition of methane was investigated. The effects of nickel particle size and reaction temperature on the hydrogen production, catalyst deactivation and the morphologies of the carbon nanotubes were investigated. Two catalyst were prepared: Ni/ZSM-5(300) – predominant size of the Ni particles 30–60 nm and nNi/ZSM-5(300) predominant size of the Ni particles 10–20 nm.     

Properties of Pt-based electrocatalysts containing selectively deposited Sn as the anode for polymer electrolyte membrane fuel cells

Sn-promoted Pt-based catalysts were prepared by the chemical vapor deposition (CVD) of Sn on commercial Pt/C and PtRu/C catalysts using Sn(CH₃)₄ as an Sn precursor. The prepared catalysts showed higher CO tolerance than those prepared by adding Sn using an impregnation (IMP) method. This result was obtained because Sn added by CVD was selectively deposited on the Pt and Ru surfaces, instead of on a carbon support, such that the interfacial contact between Pt and Sn was greater in the Sn-CVD catalyst than in the others, as confirmed by in-situ infrared and X-ray photoelectron spectroscopic observations of the catalysts.     

Hydrogen and multiwall carbon nanotubes production by catalytic decomposition of methane: Thermogravimetric analysis and scaling-up of Fe–Mo catalysts

Fe-based catalysts doped with Mo were prepared and tested in the catalytic decomposition of methane (CDM), which aims for the co-production of CO₂-free hydrogen and carbon filaments (CFs). Catalysts performance were tested in a thermobalance operating either at isothermal or temperature programmed mode by monitoring the weight changes with time or temperature, respectively, as a result of CF growth on the metal particles. Maximum performance of Fe–Mo catalysts was found at the temperature range of 700–900 °C. The addition of Mo as dopant resulted in an increase in the rate and amount of deposited carbon, reaching an optimum in the range 1.7–5.1% (mol) of Mo for Fe–Mo/Al₂O₃ catalysts, whereas for Fe–Mo/MgO catalyst an optimum at 5.1% Mo loading was obtained. XRD study revealed the effect of the Mo addition on the Fe₂O₃/Fe crystal domain size in the fresh and reduced catalysts. Tubular carbon nanostructures with high structural order were obtained using Fe–Mo catalysts, mainly as multiwall carbon nanotubes (MWCNTs) and bamboo carbon nanotubes. Fe–Mo catalysts showing best results in thermobalance were tested in a rotary bed reactor leading to high conversions of methane (70%) and formation of MWCNTs (5.3 g/h).     

Role of two carbon phases in oxygen reduction reaction on the Co–PPy–C catalyst

In spite of a significant progress in their performance in recent years, the heat-treated metal-nitrogen–carbon (M-N–C) non-precious metal catalysts for oxygen reduction reaction (ORR) at the cathode of polymer electrolyte fuel cells (PEFCs) are in need of further improvement to match the activity and, especially, the stability of Pt-based nanoparticle catalysts of oxygen reduction. A better understanding of the role of individual components in M-N–C catalysts is vital for the development of more advanced formulations. In this work, using a cobalt-polypyrrole-carbon catalyst system as an example, we demonstrate that carbon originating from the organic nitrogen precursor (ONP) has different properties than the carbon support. Unlike the carbon originating from polypyrrole, the support carbon helps to enhance ORR performance but negatively impacts the stability. To the best of our knowledge, this may be the first time that the properties of the ONP-derived carbon are being differentiated from the properties of carbon in the carbon support, emphasizing the potential importance of carbon phases in ORR electrocatalysis on heat-treated M-N–C catalysts.     

Durability of PEM fuel cell electrocatalysts prepared by microwave irradiation technique

In this study, the durability of the PEM fuel cell electrocatalysts was investigated by using cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. Accelerated degradation tests (ADTs) were applied to the commercial catalysts and the electrocatalysts prepared by microwave irradiation technique in order to determine the platinum dissolution/agglomeration and carbon corrosion characteristics. The parameters examined for the commercial catalysts were carbon to Nafion (C/N) ratio in the catalyst ink and Pt loading over the carbon support. The parameters examined for the home-made catalysts were the conditions altered in the microwave environment including the base concentration and microwave duration. The hydrogen oxidation reaction (HOR) and oxygen reduction reactions (ORR) were examined before and after ADTs. The results showed that the catalyst properties differently affect the HOR and ORR activities of the catalysts before and after ADTs.