Surfactant-free Pd–Fe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation

Herein, a novel surfactant-free nanocatalyst of Pd–Fe bimetallic nanoparticles (NPs) supported on the reduced graphene oxide (Pd–Fe/RGO) were synthesized using a two-step reduction in aqueous phase. Electrochemical studies demonstrate that the nanocatalyst exhibits superior catalytic activity towards the formic acid oxidation with high stability due to the synergic effect of Pd–Fe and RGO. The optimized Pd–Fe/RGO (Pd:Fe = 1:5) nanocatalyst possess an specific activity of 2.72 mA cm^?2 and an mass activity of 1.0 A mg^?1_(Pd), which are significantly higher than those of Pd/RGO and commercial Pd/C catalysts.     

Synthesis of ultrafine low loading Pd–Cu alloy catalysts supported on graphene with excellent electrocatalytic performance for formic acid oxidation

Here, surfactant free composite catalysts (Pd–Cu/rGO) with Pd–Cu alloy nanoparticles uniformly distributed on graphene sheets are successfully prepared via a facile hydrothermal approach. Compared with pure Pd/rGO catalyst, the introduction of copper could dramatically enhance the performance of the catalyst in the electrocatalytic formic acid oxidation (FAO) due to the strain effect and the ligand effect. With the optimized atomic ratio of 3:1 between palladium and copper, the alloy nanoparticle shows the smallest size of 2.12 nm, thus endowing the composite catalyst with highest catalytic efficiency. With Pd load as low as 14.5%, a maximum mass current density of 1580 mA mg_Pd⁻¹, and residual current of 69.93 mA mg_Pd⁻¹ at 3000 s was achieved with our Pd₃Cu₁/rGO catalyst in the electrocatalytic FAO process.     

Formic acid–Formate blended solution: A new fuel system with high oxidation activity

Formic acid and formate have been proven to be among the most promising fuels for direct liquid fuel cells. To take advantage of both formic acid and formate, the oxidation activity of a formic acid–formate blended solution was studied using cyclic voltammetry, Tafel polarization measurements, and chromoamperometry. With increased concentration of formate, the oxidation potential of formic acid shifted to the negative direction, and the peak current significantly increased, suggesting a greatly enhanced oxidation activity. The transition of formic acid from an indirect to a direct oxidation route was also observed. The enhanced oxidation activity indicated that a formic acid–formate blended solution is a promising fuel system for direct liquid fuel cells. Hence, a new concept of fuel cell, i.e., direct formic acid–formate blended fuel cell, was proposed.     

Electrocatalytic oxidation of formic acid on Pt–Pd decorated polyfluorenes with hydroxyl and carboxyl substitution

A series of novel Pt–Pd/polyfluorenes (PFs) composite catalysts were facilely prepared based on Pt/Pd precursor and PFs with hydroxyl and carboxyl substitution at the C-9 position by electrochemical method and their electrocatalytic performance toward formic acid oxidation were studied. Electrocatalytic experiments demonstrate that the Pt–Pd nanoparticles immobilized on poly(9-fluorenecarboxylic acid) (PFCA)-decorated glassy carbon (GC) electrode (Pt–Pd/PFCA/GC) show larger electrochemical active surface area, higher catalytic activity and stability toward formic acid oxidation than that of other Pt–Pd/PFs/GC, Pt–Pd/GC, as well as the commercial JM 20% Pt/C/GC electrodes, which can be attributed to the small-sized and well-dispersed Pt–Pd nanoparticles on PFCA matrix and the special electronic interaction between the metal nanoparticles and the polymer substrate. Moreover, the electron-withdrawing carboxyl substitution rather than the electron-donating hydroxyl on the polymer main chain is of great benefit to the removal of poison CO as well as the enhancement of catalytic activity of Pt–Pd toward formic acid oxidation.     

Pd/C nanocatalyst with high turnover frequency for hydrogen generation from the formic acid–formate mixtures

Pd/C nanocatalyst with high turnover frequency (TOF) for hydrogen generation from the formic acid (FA)–sodium formate (SF) mixtures was prepared via an ex situ reduction of PdCl₂ used formate in the presence of citric acid. The morphology and property of the Pd/C catalyst before and after decomposition of FA–SF mixture were characterised using transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive spectroscopy, X-ray diffractometer and Fourier transform infrared spectrometer. Over this Pd/C catalyst, a TOF of 228.3 h⁻¹ was observed for a FA–SF mixture with a FA/SF ratio of 1:9. The observed TOF was the highest ever reported for heterogeneous Pd/C catalysts. The deactivation of the Pd/C catalyst was attributed to desorption of citric acid, reduction of Pd^II content and adsorption of CO. Washing and drying could partially recover the activity of the Pd/C catalyst.     

Influence of the carbon support on the Pt–Sn anodic catalyst for the electrochemical reforming of ethanol

Several anodic catalysts based on Pt–Sn (3:1 mass ratio) and 20% total metal loading were prepared on different carbonaceous supports (functionalized and non-functionalized low-density nanofibers, graphite oxide, expanded graphite, graphene flakes and β-SiC), to identify an alternative for the traditional Carbon Vulcan XC-72 support for the electrochemical reforming of ethanol. Of the materials tested, Pt–Sn supported on non-functionalized low-density nanofibers (CNF LS) showed the highest electro-catalytic activity vs. the traditional support. This result was attributed to the combination of different properties such as high surface area and dispersion of the Pt–Sn nanoparticles, high electrochemical active surface area and high basicity. This anodic catalyst was chosen for the development of a Membrane Electrode Assembly (MEA) and tested for the electrochemical reforming of ethanol. A high activity was obtained (120 mA cm⁻² at 1.4 V and 80 °C) for hydrogen production. In addition, the stability of the system and its subsequent regeneration were studied in view of its practical application.     

Effect of ZSM-5‒carbon black composites as supports on the activity of Pd nanoparticles for propan-1-ol oxidation

The impact of ZSM-5 zeolite as a support material on the electrocatalytic activity of Pd nanoparticles for the oxidation of propan-1-ol and propan-2-ol has been investigated. ZSM-5‒carbon black composites with different ZSM-5/C mass ratios, carbon black, and ZSM-5 are used as supports for the Pd nanoparticles. The structures of catalysts are characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analyses. The ZSM-5‒C composites significantly improve the catalytic activity of the Pd nanoparticles for the oxidation of propan-1-ol. In a wide range of ZSM-5/C mass ratios, the Pd/ZSM-5‒C catalysts exhibit considerably higher activities than Pd/C and Pd/ZSM-5, indicating that the ZSM-5‒C composites are superior to carbon black and ZSM-5 as supports for the Pd nanoparticles. The result reveals that efficient electrocatalysts could be fabricated using zeolites as support materials. In contrast, Pd/ZSM-5‒C shows no advantages over Pd/C in the oxidation of propan-2-ol in catalytic activity, suggesting the importance of matching catalyst with reaction to obtain good results.     

Study on the preparation of Ni–La–Ce oxide catalyst for steam reforming of ethanol

Two schemes for design and preparation of Ni–La–Ce oxide catalysts for steam reforming of ethanol were proposed in this work. The one via citrate complexing method was designed as NiO supported on ceria-lanthanum oxide (CL) solid solution, in which the strong interaction between NiO and CL solid solution was beneficial to inhibit the aggregation of NiO particles, and the abundant of oxygen vacancies existed in CL solid solution was in favor of carbon elimination from catalyst surface. The other was schemed as LaNiO₃ with perovskite structure loaded on CeO₂ support by using impregnation method, in which the particles of metal Ni derived from reduction of LaNiO₃ were highly dispersed, and the formation of La₂O₂CO₃ in the reaction process could act as the carbon scavenger. Both of the catalysts exhibited very good performance for steam reforming of ethanol (SRE), complete C₂H₅OH conversion was obtained with 70.3% of H₂ selectivity at 400 °C over the catalyst obtained from former method and complete C₂H₅OH conversion was achieved at 450 °C with 67% of H₂ selectivity over the catalyst from latter method. The catalyst made according to the citrate complexing method was more active for SRE and more selective for H₂ production. Both of the catalysts displayed very good anti-sintering ability which was tested at 650 °C and at a high space velocity of 180,000 ml g_cat⁻¹ h⁻¹ with reaction mixture of H₂O/C₂H₅OH = 3 in mole ratio. The results indicated that both of oxygen vacancy and La₂O₂CO₃ possessed the ability to remove the deposited carbon, and compared with La₂O₂CO₃ the oxygen vacancy could reduce one third more of the carbon deposited according to TG tests.     

A straightforward one-pot synthesis of Pd–Ag supported on activated carbon as a robust catalyst toward ethanol electrooxidation

Direct Ethanol Fuel Cells (DEFCs) have fascinated remarkable attention on account of their high current density and being environmentally friendly. Developing efficient and durable catalysts with a simple and fast method is a great challenge in the practical applications of DEFCs. To this end, the bimetallic Pd–Ag with adjustable Pd:Ag ratios were synthesized via a simple and one-pot strategy on activated carbon as a support in this study. The Pd–Ag/C catalysts with different molar ratios were synthesized by simultaneous reduction of Pd and Ag ions in the presence of the ethanolic sodium hydroxide as a green reducing agent for the first time. Several different methods, including FE-SEM, HR-TEM, XRD, XPS EDX, ICP-OES, and BET were used to confirm the structure and morphology of the catalysts. The performance of catalysts was also examined in ethanol oxidation. Obtained results of electrochemical experiments revealed that the Pd₃–Ag₁/C catalyst had superior catalytic activity (2911.98 mAmg^?1_Pd), durability, and long-stability compared to the other catalysts. The excellent catalytic characteristic can be attributed to the synergistic effect between Pd and Ag. We presume that our simple method have the chance to be utilized as a proper method for the synthesis of fuel cell catalysts.     

Synergistic catalysis of Pd–Ni(OH)2 hybrid anchored on porous carbon for hydrogen evolution from the dehydrogenation of formic acid

The development of a facile yet efficient strategy to boost the catalytic performance of supported Pd nanoparticles (NPs) toward the dehydrogenation of formic acid (FA) is essential but remains challenging. Here, a novel hybrid nanocatalyst comprising Pd and Ni(OH)₂ supported on porous carbon (PC) is developed. The obtained Pd–Ni(OH)₂/PC nanocatalyst exhibits an excellent catalytic performance for FA dehydrogenation to produce hydrogen. The introduction of Ni(OH)₂ in PC support can significantly promote the catalytic activity of Pd NPs toward FA dehydrogenation. Additionally, the catalytic property of Pd–Ni(OH)₂/PC is correlated with the Pd/Ni ratio. The ₂Pd–₁Ni(OH)₂/PC with the optimum Pd/Ni ratio of 2/1 exhibits the maximum turnover frequency (TOF) of 3409 h⁻¹ at 60 °C for FA dehydrogenation. The highly dispersed ultrafine Pd–Ni(OH)₂ hybrid NPs with numerous accessible active sites and Ni(OH)₂−induced positive synergetic effects with Pd NPs considerably boost the catalytic performance for FA dehydrogenation.     

In-situ investigation on the CO tolerance of carbon supported Pd–Pt electrocatalysts with low Pt content by electrochemical impedance spectroscopy

Carbon supported Pd–Pt electrocatalysts (Pd–Pt/C) with low Pt content were investigated in proton exchange membrane fuel cells (PEMFCs) with pure H₂ and CO/H₂ as the feeding fuels, respectively. The Pd–Pt/C catalysts showed high activity for hydrogen oxidation reaction (HOR) and improved CO tolerance. Electrochemical impedance spectroscopy (EIS) was employed to probe the in-situ information of the improved CO tolerance. The dependence of Nyquist plots and Bode plots on current density and feeding gas was investigated in low polarization region. The results of EIS analysis indicated that the improved CO tolerance of Pd–Pt/C catalysts can be attributed to the lower coverage of CO on the Pd–Pt bimetal than that on the pure Pt.     

Three dimensional assembly of electrocatalytic platinum nanostructures on reduced graphene oxide – An electrochemical approach for high performance catalyst for methanol oxidation

By means of co-electrodeposition, we fabricated 3D assembly of Pt nanostructures with dominant (100) plane on reduced graphene oxide (rGO) modified graphite electrode. The strong metal-support interaction at the atomic level makes the nanostructure highly durable and this modified electrode exhibited high electrocatalytic activity towards methanol oxidation. It has been found that the morphology, active site and the electrochemical activity of Pt are highly dependent on the substrate and the number of electrochemical cycling used for the deposition. rGO-Pt composite deposited using one cycle showed a high mass activity of 2.54 A/mg at 0.67 V for methanol oxidation in acidic condition and 1.84 A/mg at ?0.03 V in alkaline medium. This simple and single step approach using electrodeposition to grow the morphology controlled Pt nanostructure on rGO, will aid in the development of active and stable catalyst for fuel cell 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.     

A review on the development of supported non-noble metal catalysts for the endothermic high temperature sulfuric acid decomposition step in the Iodine–Sulfur cycle for hydrogen production

The decomposition of sulfuric acid to sulfur dioxide is an important reaction section in both the thermochemical Iodine–Sulfur (IS) cycle and the Hybrid (Hybs) cycle for hydrogen production. This decomposition reaction is a high temperature, highly endothermic reaction whose activation barrier needs to be reduced by a highly active catalyst. The catalysts reported in this reaction are either exorbitantly expensive, synthesized using noble metals, or exhibiting low activity and stability. Hence, the development of non-noble active and stable catalysts in this reaction is a significant challenge. This review comprehensively discusses the recent developments and activity trends of supported non-noble catalysts including their synthesis, activity & stability trends, mechanistic and kinetic aspects. The catalytic activity of nano-catalysts in sulfuric acid decomposition is largely affected by the size of the metal nanoparticles, dispersion, oxygen vacancies, and metal-support interaction. Herein, we report an in-depth theoretical and experimental understanding of the catalytic challenges and solutions that leads to designing a cost-effective, efficient and stable catalyst in this reaction. The role of catalyst support modification for long run stability is also discussed. Further, literature considering reaction kinetics over various catalysts is also reviewed.     

Electrocatalytic activity of nanostructured Ni and Pd–Ni on Vulcan XC-72R carbon black for methanol oxidation in alkaline medium

Ni and Pd–Ni nanoparticles were chemically deposited on Vulcan XC-72R carbon black by impregnation method using NaBH₄ as a reducing agent. The prepared electrocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The electrocatalytic activity of Ni/C and Pd–Ni/C electrocatalysts towards methanol oxidation in 0.5 M KOH solution was examined using cyclic voltammetry and chronoamperometry. Two methanol oxidation peaks were observed on the Pd–Ni/C at 0 and +860 mV. Their current density values are higher than those at Pd/C and Ni/C electrocatalysts by 1.92 and 1.68 times, respectively. The catalytic rate constant of methanol oxidation reaction at Ni/C and Pd–Ni/C electrocatalysts in (0.2 M MeOH + 0.5 M KOH) solution was estimated using double-step chronoamperometry as 5.64 × 10³ and 6.25 × 10³ cm³ mol⁻¹ s⁻¹, respectively. Pd–Ni/C is more stable than Pd/C and Ni/C electrocatalysts. Therefore, Pd–Ni/C is a suitable as a less expensive electrocatalyst for methanol oxidation in alkaline medium.     

Study on the role of Pt and Pd in Pt–Pd/TiO2 bimetallic catalyst for H2 oxidation at room temperature

The hydrogen safety issue is spotlighted as the hydrogen process is extended. For this reason, we studied catalysts for H₂ oxidation at room temperature to ensure hydrogen safety. Catalysts were prepared by different preparation methods and compared to evaluate the role of Pt and Pd in Pt–Pd/TiO₂ catalysts. The catalytic activity was significantly enhanced when activity metal size was small and it was exposed to catalyst surface to a high Pd ratio. For the 0.1%Pt-0.9%Pd/TiO₂ catalyst, high hydrogen conversion of 90% was obtained under the condition of 0.5% hydrogen injection. To understand the correlation between activity and characteristics of catalyst, the physicochemical characteristics of the various catalysts were investigated by X-ray photoelectron spectroscopy (XPS), temperature-programmed oxidation and reduction (TPOR) and Field Emission-Transmission Electron Microscope (FE-TEM) analysis. From these analysis, it was found that Pt served the role of highly dispersion of active metal (Pt–Pd) and as with increasing Pd ratio of active metal, hydrogen activity was increased, which indicates that hydrogen oxidation had proceeded on the Pd site. Finally, the valence state of the Pd influenced hydrogen oxidation activity of Pt–Pd/TiO₂, which increased with increasing ratio of Pd⁰/Pd_Total.     

A comparative study on the performance of mesoporous SBA-15 supported Pd–Zn catalysts in partial oxidation and steam reforming of methanol for hydrogen production

Using mesoporous SBA-15 (Santa Barbara Amorphous No. 15, a mesoporous material) as support, Pd–Zn nanocatalysts with varying Pd and Zn content were tested for hydrogen production from methanol by partial oxidation and steam reforming reactions. The physico-chemical characteristics of the synthesized SBA-15 support were confirmed by XRD, N₂ adsorption, SEM and TEM analyses. The PdZn alloy formation during the reduction of Pd–Zn/SBA-15 was revealed by XRD and DRIFT study of adsorbed CO. Also, the correlation between Pd and Zn loadings and PdZn alloy formation was studied by XRD and TPR analyses. The metallic Pd surface area and total uptakes of CO and H₂ were measured by chemisorption at 35 °C. The metallic Pd surface area values are in linear proportion with the Pd loading. The formation of PdZn alloy during high temperature reduction was confirmed by a shift in absorption frequency of CO on Pd sites to lower frequency due to higher electron density at metal particles resulted from back-donation. The reduced Pd–Zn/SBA-15 catalysts were tested for partial oxidation of methanol at different temperatures and found that catalyst with 4.5 wt% Pd and 6.75 wt% Zn on SBA-15 showed better H₂ selectivity with suppressed CO formation due to the enhanced Pd dispersion as well as larger Pd metallic surface area. The O₂/CH₃OH ratio is found to play a significant role in CH₃OH conversion and H₂ selectivity. The performance of 4.5 wt% Pd–6.75 wt% Zn/SBA-15 catalyst in steam reforming of methanol was also tested. Comparatively, the H₂ selectivity is significantly higher than that in partial oxidation, even though the CH₃OH conversion is less. Finally, the long term stability of the catalyst was tested and the nature of PdZn alloy after the reactions was found to be stable as revealed from the XRD pattern of the spent catalysts.     

Opposite effects of self-growth amorphous carbon and carbon nanotubes on the reforming of toluene with Ni/α-Al2O3 for hydrogen production

Ni-based catalysts are prone to be deactivated by carbon deposition. This study aims to investigate the influence mechanism of different types of carbon deposition on the activity of Ni/α-Al₂O₃ catalyst at various steam-to-carbon (S/C) ratios during steam reforming of toluene for hydrogen production. At a low S/C ratio of 1, the catalytic activity of Ni/α-Al₂O₃ was inhibited due to the covering and blocking of Ni active sites by the formation of amorphous carbon on the Ni surface. While at a high S/C ratio of 3, more than 80 wt% of carbon deposition was found to be self-growth carbon nanotubes (CNTs) with an average diameter of around 15 nm. The activity of Ni/α-Al₂O₃ in steam reforming of toluene was unusually promoted, which can be attributed to the tip-growth mechanism of CNTs, whereby the Ni particles migrated to the tip or the surface of CNTs, resulting in the improved active site dispersion.     

Influence of polarization on the electrochemical activity of La2–xCaxNiO4+δ electrodes in contact with Ce0.8Sm0.2O1.9 electrolyte

The effect of electrode polarization on the electrochemical activity of La₂NiO_4+δ and La_1.9Ca_0.1NiO_4+δ electrodes in contact with the Ce_0.8Sm_0.2O_1.9 electrolyte is studied by impedance spectroscopy. It is found that anodic polarization facilitates electrode reaction for both electrodes leading to significant decrease in the polarization resistance. The effect of cathodic polarization differs between the electrodes: the polarization resistance of La₂NiO_4+δ electrode slightly increases, while the polarization resistance of La_1.9Ca_0.1NiO_4+δ electrode strongly decreases with the increase in the applied potential. It is established that in all cases the polarization mostly affects the low-frequency stage of the electrode reaction, connected with oxygen surface exchange and diffusion. The surface state of the samples after exposure under polarization is studied by X-ray photoelectron spectroscopy. Correlations between electrochemical activity of the electrodes and the changes in their surface composition under polarization are discussed.     

A more economical choice for the cathode material of Ni-MH batteries with high electrochemical performances: 3D flower-like Ni–Fe LDHs

A series of 3D flower-like Ni–Fe layered double hydroxides (LDHs) were synthesized successfully and used as the cathode materials for nickel-metal hydride battery (Ni-MH battery). The 4Ni–Fe LDH electrode (Ni/Fe molar ratio = 4:1) displays the highest high-rate discharge property and the most excellent cycling performance. The discharge capacity of the 4Ni–Fe LDH electrode can reach up to 291.3 mAh/g at a discharge rate of 200 mA/g and which delivers a high capacity retention of 98.9% over 200 cycles. In contrast, the pure Ni(OH)₂ electrode only has a capacity of 243 mAh/g, and after 100 cycles the capacity retention is just 73.4%. The above improvement can be ascribed to the formation of Ni–Fe LDHs which can consolidate the stability of α-Ni(OH)₂ in the KOH solution. In addition, the unique flower-like morphology and the enlarged interlayer spacing also paly important role to promote ion transmission and charge transfer. Considering the competitive price of Fe, 3D flower-like Ni–Fe LDH may be a more economical choice for the cathode material of Ni-MH batteries.