Catalytic decomposition of hydrogen iodide over pre-treated Ni/CeO2 catalysts for hydrogen production in the sulfur–iodine cycle

The Ni/CeO₂ catalysts with successive oxidization, reduction and re-oxidization have been tested for hydrogen production in sulfur–iodine (SI or IS) cycle. The samples were characterized by BET, XRD, TEM, TPR and XPS. The oxidative/reductive atmosphere affected the structure and performance of the catalysts. It was suggested that a migration of Ce⁴⁺ from the bulk to the surface occurred during the reductive treatment. The diffusion process was reversed when the atmosphere was changed to an oxidative one. The reduced and re-oxidized samples seemed to be similar all the time and showed better catalytic activity in comparison with the as-received and oxidized samples. For the re-oxidized sample, the strongest interaction compared with other samples occurred between Ni and CeO₂ and oxygen vacancies transferred from bulk to surface, which led to form more surface sites and oxygen vacancies. The metal Ni was found only on the surface of the reduced sample. The active site of metal Ni besides the surface site and oxygen vacancy were assumed to play an important role for hydrogen production.     

Studies on cerium (Ce/Ce)–vanadium(V/V) redox flow cell—cyclic voltammogram response of Ce/Ce redox couple in H2SO4 solution

A novel Ce⁴⁺/Ce³⁺–V²⁺/V³⁺ redox flow cell has been designed. The electrochemical responses of higher concentration Ce⁴⁺/Ce³⁺ couple in H₂SO₄ solution were investigated via cyclic voltammetry. The normal potential and the kinetic parameters for anodic oxidation of Ce³⁺ and cathodic reduction of Ce⁴⁺ were measured. The results showed the surface of platinum electrode was fully covered with type I oxide that inhibited the reduction of Ce⁴⁺. The reversibility of the Ce⁴⁺/Ce³⁺ couple improved with the increase of H₂SO₄ concentration. Different electrochemically active substances existed at various state of charge (SOC) and the reversibility of the Ce⁴⁺/Ce³⁺ couple at the carbon electrode was superior to platinum.     

Development of nanophase CeO2-Pt/C cathode catalyst for direct methanol fuel cell

Incorporation of nanophase ceria (CeO₂) into the cathode catalyst Pt/C increased the local oxygen concentration in an air atmosphere, leading to enhanced single-cell performance of direct methanol fuel cell (DMFC). Ceria doped catalysts were effective at low oxygen partial pressure (≤0.6 atm) conditions and 1 wt.% CeO₂ doped Pt/C exhibited the highest performance. The effect of ceria was more prominent with air as the cathode reactant and the ceria acted as a mere impurity in a pure oxygen atmosphere, decreasing the DMFC performance. Impedance spectra showed a decrease in polarization resistance with the ceria addition to the cathode catalyst in low-potential regions confirming the facile mass transfer of the reactant oxygen molecules to catalytic sites. Transmission electron microscopy (TEM) pictures showed a uniform distribution of CeO₂ around platinum sites.     

Codeposited PtSb/C catalysts for direct formic acid fuel cells

Carbon supported PtSb catalysts were synthesized by codeposition of platinum and antimony on Vulcan^® carbon black. X-ray diffraction (XRD) analysis revealed that the Sb was alloyed with the Pt while XPS indicated that a large fraction of the Sb was in an oxidized state, with only partial alloying. The performances of catalysts with a range of compositions were compared in a multi-anode direct formic acid fuel cell (DFAFC). A 0.29 mol fraction of Sb was found to provide the best performance with a maximum specific power output of 280 W g⁻¹ Pt. CO stripping results indicated that the addition of Sb at this optimum level greatly suppressed both CO adsorption and H adsorption/desorption, as well as promoting oxidative stripping of CO. The results are compared with those for previously studied catalysts prepared by the reductive deposition of Sb on a carbon supported Pt catalyst.     

Optimization of a water–gas shift reactor over a Pt/ceria/alumina monolith

The water–gas shift (WGS) reaction is an important step in the purification of hydrogen for fuel cells. It lowers the carbon monoxide content and produces extra hydrogen. The constraints of automotive applications render the commercial WGS catalysts unsuitable. Pt/ceria catalysts are cited as promising catalysts for onboard applications as they are highly active and non-pyrophoric. This paper reports on a power law rate expression for a Pt/CeO₂/Al₂O₃ catalyst. This rate equation is used to compare different reactor configurations for an onboard water–gas shift reactor. A one-dimensional heterogeneous model that accounts for the interfacial and intraparticle gradients has been used to optimize a dual stage adiabatic monolith reactor.     

Pt supported on doped CeO2/Al2O3 as catalyst for dry reforming of methane

This work investigated the effect of the nature of dopant (Pr, Nb and Zr) on the performance of Pt supported on cerium-based oxides deposited on alumina for dry reforming of methane. in situ XRD and XANES analyses showed that the sample doped with Pr exhibited the highest redutibility of ceria (23%). Furthermore, the cyclohexane dehydrogenation reaction revealed that the addition of Pr improved the resistance to metal sintering during the dry reforming reaction. In the absence of doped-ceria oxide, a strong deactivation took place on Pt/Al₂O₃ catalyst during reaction, which was due to the absence of support reducibility and the highest Pt sintering. Among the doped-ceria samples, Pt/CePr/Al₂O₃ exhibited the highest activity and stability. These results were attributed to: (i) the oxygen mobility of the supports containing ceria, mainly for the sample doped with Pr, which favors the carbon removal mechanism; and (ii) the absence of Pt sintering during the reaction.     

Defect configuration of ceria for Pt anchoring toward efficient methanol oxidation

As a potential next-generation power source for portable electronic devices, commercialization process of direct methanol fuel cell (DMFC) technology is hindered by the high dependence of anode methanol oxidation reaction (MOR) on precious Pt catalyst. In order to improve the efficiency of Pt toward MOR catalysis, a Ni doping strategy is proposed for defect engineering on ceria substrate to achieve uniform dispersion of Pt nanoparticles. Besides, Ni could also act as electron donor for Pt and hence favor the removal of CO intermediate on Pt and act as a co-catalyst toward MOR. Superior MOR activity and great stability is therefore achieved for the as-prepared Pt/CeO₂@Ni catalyst with 3 times higher peak MOR current density compared with Pt/C catalyst. Due to the evenly anchored Pt and enhanced CO oxidation ability caused from Ni doped ceria substrate, Pt utilization of the Pt/CeO₂@Ni catalyst is calculated to be 3.24 times higher than that of the commercial Pt/C catalyst. By considering the significantly improved stability, the Pt/CeO₂@Ni catalyst has the potential for application in DMFC devices.     

Photo induced hydrogen evolution from water in the presence of EDTA and a Pt/TiO2 supported catalyst

A flow reaction system was developed to study the steady state kinetics of hydrogen evolution in the presence of Ru(bipy)²⁺, MV⁺², EDTA and a heterogeneous supported catalyst. Under the conditions used, steady state rates similar to those previously reported with colloidal Pt particles were attained. Furthermore, over much of the range of concentrations used, the reaction was zero order and it did not exhibit an Arrhenius behavior. Examination of the various rate determining steps involved led us to conclude that surface processes such as the recombination and desorption of hydrogen from the Pt surface might be rate determining.     

Effect of the CeO2 synthesis method on the behaviour of Pt/CeO2 catalysis for the water-gas shift reaction

A comparative study of three different ceria synthesis procedures (template- and MW- assisted hydrothermal synthesis and urea homogeneous precipitation) is reported in this paper. The obtained materials were employed as supports for Pt nanoparticles, and the Pt/CeO₂ catalysts were evaluated in the WGS reaction under model and realistic conditions. The influence of the support, e.g., its morphology and electronic properties, has been studied in detail by means of XRD, H₂-TPR, XPS, UV–Vis spectroscopy and toluene hydrogenation (for metal dispersion assessment). The catalytic performance of the samples is directly correlated with the modification of the electronic properties, as a result of the preparation method used. The conventional homogeneous precipitation method with urea resulted to be the best option, leading to enhanced ceria reducibility and adequate Pt dispersion, which in turns resulted in a very efficient WGS catalyst.     

Glycol/air fuel cells

The development of glycol/air fuel cells is presented, and the key technical problems solved are described. The electrode catalyst, which was composed of Pt:Pd:Bi = 3:2:1 with active carbon as the carrier, has been shown to be very effective for the electrochemical oxidation of glycol in KOH solution at room temperature. The amount of Pt and Pd used can be greatly reduced (the loading of Pt-Pd in the electrodes can be as low as 1 mg cm⁻²). A study of how the fuel-electrolyte solution in a cell behaves has been made. The optimum composition of the fuel-electrolyte solution has been determined. A 2 W battery consisting of 10 cells with a capacity of 1000 Ah has been made.     

A polyoxometalate-deposited Pt/CNT electrocatalyst via chemical synthesis for methanol electrooxidation

Polyoxometalate anion PMo₁₂O₄₀³⁻ (POM) is chemically impregnated into a Pt-supported carbon nanotubes (Pt/CNTs) catalyst that is prepared via a colloidal method. The POM-impregnated Pt/CNTs catalyst system (Pt/CNTs-POM) shows at least 50% higher catalytic mass activity with improved stability for the electrooxidation of methanol than Pt/CNTs or POM-impregnated Pt/C (Pt/C-POM) catalyst systems. The enhancement in electrochemical performance of the Pt/CNTs-POM catalyst system can be attributed to the combined beneficial effects of improved electrical conductivity due to the CNTs support, highly dispersed Pt nanoparticles on the CNTs, and increased oxidation power of the polyoxometalate that can assist oxidative removal of reaction intermediates adsorbed on the Pt catalyst surface.     

TiO2纳米纺锤体负载Pt在氧还原反应中的应用

燃料电池中催化剂的稳定性是影响其实际应用的关键问题之一。本研究合成了锐钛矿型纺锤状TiO₂纳米材料,并负载纳米Pt制备了TiO₂-Pt双组分复合催化材料。将其制作成电极材料后,进行了TEM、XRD、拉曼光谱、电化学特性分析。结果表明：TiO₂-Pt材料中Pt纳米颗粒的TEM形貌与TiO₂的表面亲和力有关;该双组分催化剂呈现出两个单独的氧还原反应（ORR）峰;在负载Pt后,材料电荷传输电阻明显减小,使得TiO₂-Pt中TiO₂纺锤体组分上的ORR性能明显增强;紫外光可同时促进TiO₂-Pt中两组分的ORR性能;TiO₂-Pt比炭黑负载Pt具有更好的稳定性。     

Oxygen-enhanced water gas shift over ceria-supported Au–Cu bimetallic catalysts prepared by wet impregnation and deposition–precipitation

Au–Cu/ceria bimetallic catalysts were prepared incorporating Au by incipient wetness impregnation (IWI) and deposition-precipitation (DP) methods (with loadings of 1 wt.% and 7 wt.% of Au and Cu, respectively). The as-prepared catalysts were characterized by techniques such as BET, XRD, Raman, XPS, H₂-TPR, CO-TPD and Oxygen Storage Capacity (OSC) measurements. The results indicated a good dispersion of gold and copper for copper ceria catalyst and Au–Cu bimetallic catalysts. Addition of Au to CuO/CeO₂ increases highly the capacity to release lattice oxygen to oxidized CO at low temperatures compared to pure CuO/CeO₂. Au/CeO₂ and Au–CuO/CeO₂ catalyst prepared by DP show higher OSC value than counterparts prepared by IWI, either at 120 and 250 °C. Also, gold-containing catalysts prepared by DP show lower temperature of reduction that the samples prepared by IWI as a consequence of the higher dispersion of gold in the former samples. The presence of gold at different oxidation states was observed by XPS analysis. Preparation method strongly affects to the atom ratio of Au and Au + Cu with respect to surface ceria. The gold incorporation method was a key factor that enhances the redox properties and activity in both WGS and OWGS reactions. The present study shows the gas phase oxygen enhanced the activity of monometallic CuO/ceria and bimetallic Au–Cu/ceria prepared by IWI and DP methods in both WGS and OWGS reactions. AuCC catalyst prepared by DP shows higher hydrogen yield and also higher CO conversion than other prepared by IWI during OWGS reaction.     

Impedance measurements of air-oxidized and H2-annealed Raney-Ni catalysts for alkaline fuel cells

The catalytic properties and the long-term performance of Raney-NiTi2 catalysts being used in H₂ electrodes of alkaline H₂---O₂ fuel cells can be significantly improved by slow air oxidation and subsequent annealing in an H₂ atmosphere at 300°C. Individual reaction steps are investigated by means of impedance measurements. Theoretical estimations, on the basis of a simple equivalent circuit of a supported electrode, result in a frequency-response relationship which is in very good agreement with the experimental data referring to the relevant frequency range of 10⁻³−10⁻¹ Hz. A method to evaluate the impedance spectra is described in some detail. Calculated and measured impedance data are in good agreement, thus indicating the validity of the charge transfer resistance, the diffusion resistance, as well as the chemisorption capacity and the double-layer capacity. Experiments on the influence of catalyst annealing in an H₂ atmosphere at 350°C show a strong increase in the charge transfer resistance and an obvious decrease in the diffusion resistance, depending on the annealing time. A similar influence on the chemisorption capacity and the double-layer capacity is not observed.     

Performance optimisation of PEM fuel cell during MEA fabrication

We have analysed membrane electrode assemblies (MEAs) involving fabricated and commercially available electrodes using a scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) and developed simple mathematical models to simulate the best performance and design conditions. The analysis showed that a MEA surface with the catalyst layer consisting of 10 wt% Pt/C and 30 wt% Teflon^® (PTFE, designated E2) loaded with 0.38 mg Pt/cm² showed good localisation of the platinum particles. The SEM image of the E2 electrode showed the existence of a diffusion layer, while the cross-section of electrode E3 (without diffusion layer) showed only the backing layer of the carbon cloth. It was seen that good adhesion of the catalyst on the membrane was obtained as a result of the hot press used in fabrication. XPS analysis showed that the electrode surfaces consisted of C, O, F, Si and Pt, whose binding energies for the PTFE/C layer were C 1s, O 1s, F 1s and Si 2p states and were 285.0, 532.7, 689.5 and 103.0 eV, respectively. While for the catalyst layer, the binding energies for the elements, C 1s, O 1s, F 1s, Si 2p and Pt 4f states, were 284.3, 532.4, 689.3, 102.9 and 74.1 eV, respectively. Similar observations were made for a commercial E-TEK electrode. The mathematical and simulation investigations supported the hypothesis made in an earlier study in terms of optimum PEM fuel cell performance determination and design simulation. The calculated values of the voltage operational limit V_{opl cal.} agreed quite well with the experimental data V_{opl exp.} reported earlier. Other works from the open literature were also correlated using the mathematical model, and it was found that the V_opl values were comparable. Hydrogen usage thus calculated was best with the E2 electrode compared to E1, E3 and the commercially available E-TEK electrode.     

Sulfuric acid decomposition on the Pt/n-SiC catalyst for SI cycle to produce hydrogen

The sulfuric acid decomposition should be performed in the wide temperature ranges from 550 °C to 950 °C to absorb the sensible heat of He in SI process. Therefore, the catalysts for the reaction should be stable even in the very corrosive reaction condition of 650 °C. Here, the Pt/n-SiC catalyst was prepared for the purpose and compared with the Pt/SiC catalyst. The both catalysts showed the high stability in the temperature ranges from 650 to 850 °C. The n-SiC with the surface area of 187.1 m²/g was prepared using nano-sized SiO₂, which resulted in amorphous SiC phase. The SiC support with the surface area of 19.2 m²/g for the comparison showed the well crystalline structure. In spite of the large surface area differences between the n-SiC and SiC support, the Pt particle sizes of the Pt/n-SiC (average Pt size: 26.4 nm) catalyst were not so much different from those of the Pt/SiC (average Pt size: 26.1 nm) catalyst after the calcination at 1000 °C for 3 h. However, the catalytic activity of the Pt/n-SiC was much higher than that of the Pt/SiC. XRD analysis indicated that the Pt particles on the Pt/n-SiC was more stable than those of the Pt/SiC in the sulfuric acid decomposition and XPS analysis showed that the Pt valence state on the Pt/n-SiC was higher than that on the Pt/SiC. The surface analysis showed that the surface of the n-SiC particles was covered by SiO₂ and Si₄C_4−xO₄. These experimental results indicate that the Pt metal particles on n-SiC were stabilized on the oxidized Si surface. Therefore, it is suggested that the Pt particles stabilized on the oxidized Si surface can be a reason for the higher activity of the Pt/n-SiC catalyst as compared with the Pt/SiC catalyst.     

Energy-efficient conversion of propane to propylene and ethylene over a V2O5/CeO2/SA5205 catalyst in the presence of limited oxygen

Oxidative conversion of propane to propylene and ethylene over a V₂O₅/CeO₂/SA5205 (V:Ce=1:1) catalyst, with or without steam and limited O₂, has been studied at different temperatures (700–850 °C), C₃H₈/O₂ ratio (4.0), H₂O/C₃H₈ ratio (0.5) and space velocity (3000 cm³ g⁻¹ h⁻¹). The propane conversion, selectivity for propylene and net heat of reaction (ΔHr) are strongly influenced by the reaction temperature and presence of steam in the reactant feed. In the presence of steam and limited O₂, the process involves a coupling of endothermic thermal cracking and exothermic oxidative conversion reactions of propane which occur simultaneously. Because of the coupling of exothermic and endothermic reactions, the process operates in an energy-efficient and safe manner. The net heat of reaction can be controlled by the reaction temperature and concentration of O₂. The process exothermicity is found to be reduced drastically with increasing temperature. Due to the addition of steam in the feed, no coke formation was observed in the process.     

Use of electropolymerized films of macrocyclic compounds in direct methanol fuel cell components

Electropolymerized Co(III) and Ru(II)(CO)(2-aminophenyl)porphyrins (poly[Co(III)] and poly[Ru(II)]) were used as catalysts in a direct methanol fuel cell for the reduction of oxygen at the cathode and the oxidation of methanol at the anode, respectively. Although the half-wave potentials for oxygen reduction are +0.3 and +0.55 V when using poly[Co(III)]/C and Pt/C, respectively, as catalysts, higher limiting currents can be obtained with the non-noble metal catalyst. Moreover, the macrocyclic catalyst is 10-fold less prone to methanol poisoning than the one based on Pt. The H₂O₂ yields obtained during oxygen reduction, as measured by the RRDE technique, were 1.9, 4.1 and 2.3% for poly[Co(III)]/C, Pt/C and for a commercial heat-treated Co(III)porphyrin. Methanol oxidation with a catalyst consisting of Pt and poly[Ru(II)] was characterized by a higher limiting current (i_L=13 mA/cm², E_1/2=+0.6 V) than that obtained with a commercial Pt-Ru catalyst (i_L=4 mA/cm², E_1/2=+0.5 V) although the same Pt content was used in the two cases (1 mg/cm²). Experiments conducted in a fuel cell configuration confirmed the half-cell results and indicated that better distribution of the catalysts in the porous structure of the electrodes and reduction of methanol crossover through the membrane are necessary in order to improve the performance of the cell.     

Thermal annealing synthesis of double-shell truncated octahedral Pt-Ni alloys for oxygen reduction reaction of polymer electrolyte membrane fuel cells

Shape-controlled Pt-Ni alloys usually offer an exceptional electrocatalytic activity toward the oxygen reduction reaction (ORR) of polymer electrolyte membrane fuel cells (PEMFCs), whose tricks lie in well-designed structures and surface morphologies. In this paper, a novel synthesis of truncated octahedral PtNi_3.5 alloy catalysts that consist of homogeneous Pt-Ni alloy cores enclosed by NiO-Pt double shells through thermally annealing defective heterogeneous PtNi_3.5 alloys is reported. By tracking the evolution of both compositions and morphologies, the outward segregation of both PtO_x and NiO are first observed in Pt-Ni alloys. It is speculated that the diffusion of low-coordination atoms results in the formation of an energetically favorable truncated octahedron while the outward segregation of oxides leads to the formation of NiO-Pt double shells. It is very attractive that after gently removing the NiO outer shell, the dealloyed truncated octahedral core-shell structure demonstrates a greatly enhanced ORR activity. The as-obtained truncated octahedral Pt_2.1Ni core-shell alloy presents a 3.4-folds mass-specific activity of that for unannealed sample, and its activity preserves 45.4% after 30000 potential cycles of accelerated degradation test (ADT). The peak power density of the dealloyed truncated octahedral Pt_2.1Ni core-shell alloy catalyst based membrane electrolyte assembly (MEA) reaches 679.8 mW/cm², increased by 138.4 mW/cm² relative to that based on commercial Pt/C.     

Pt electrodeposited on CeZrO4/MCNT as a new alternative catalyst for enhancement of ethanol oxidation

Electrocatalytic preparation of Pt-based nanocomposites has been investigated for improvement of direct ethanol fuel cells (DEFCs). In this study, new alternative catalysts of Pt-decorated cerium zirconium oxide-modified multiwalled carbon nanotubes (Pt/CeZrO₄/MCNT) were successively prepared to improve the activity of the ethanol oxidation reaction (EOR). The prepared CeZrO₄ with a face-centered cubic (fcc) structure compatibly dispersed onto MCNT provides abundant active Pt sites for highly active catalysts. The fcc-structured Pt was also satisfactorily decorated onto CeZrO₄/MCNT, resulting in highly active Pt. The Ce⁴⁺/Ce³⁺ redox property can promote oxygen vacancies to improve the electrochemical activity for oxidation of carbonaceous species. An increase in roughness and a stabilized catalyst structure can also be produced by inserting Zr⁴⁺ into the ceria metal oxide. The prepared Pt/20%CeZrO₄/MCNT catalysts present excellent electrochemical active surface area, mass activity, CO tolerance and high electron kinetic transfer with low resistance and high stability over commercial PtRu/C toward EOR. This promising catalyst material could be introduced to enhance the anodic oxidation reaction in DEFCs.