Activity of Pd/C for hydrogen generation in aqueous formic acid solution

A commercial Pd/C catalyst was found to exhibit high activity for formic acid (HCOOH) decomposition into CO₂ and H₂ in aqueous solution at near ambient temperatures. The performance of the catalyst toward HCOOH decomposition in aqueous solution was investigated in a batch reactor at temperatures between 21 and 60 °C and HCOOH concentrations between 1.33 and 5.33 M. The apparent activation energy of the overall reaction for the production of H₂ from aqueous HCOOH was determined to be 53.7 kJ/mol on the heterogeneous Pd/C catalyst. This is in good agreement with the previously reported theoretical energy barrier (∼52 kJ/mol) for H₂ evolution on a Pd surface. Under the present experimental conditions, the catalyst lost activity continuously over time and the apparent deactivation energy was estimated to be 39.2 kJ/mol. Furthermore, the deactivated and spent catalyst was studied by temperature-programmed desorption experiments to reveal the possible species that caused the loss of the activity. Combining the results of our previous DFT calculations and the present experimental results, elementary steps of HCOOH decomposition on Pd in aqueous solution were proposed and discussed.     

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.     

WO3/C supported Pd catalysts for formic acid electro-oxidation activity

Pd-WO₃/C ternary hybrid was designed as a high-efficient catalyst towards formic acid electrooxidation. WO₃/C hybrids were first prepared with two different synthesis order, and then used as the supports to synthesize two kinds of Pd-WO₃/C catalysts by a quick and facile microwave-assisted ethylene glycol method. Compared with Pd/C, the catalytic performances of two Pd-WO₃/C catalysts towards formic acid electrooxidation are significantly enhanced. We elected the better synthesis order and optimized the best proportion of WO₃ and C in the hybrid catalyst. When the mass content of WO₃ is 20% of the mass of the support, Pd nanoparticles with narrower particle size distribution are more uniformly dispersed on the surface of WO₃/C support than the other counterparts, resulting in the highest performance in terms of activity and stability towards formic acid oxidation among all the samples. The reasons for the performance improvement may be: first, Pd nanoparticles in Pd-WO₃/C catalysts are of small size and evenly distributed; second, there may be the catalyst-support interaction between Pd and WO₃, substantially improving the catalytic capability of Pd-WO₃/C catalysts; finally, the hydrogen spillover effect produced by WO₃ significantly expedites the dehydrogenation of formic acid on the surface of Pd-WO₃/C.     

Pd nanoparticles immobilized on aniline-functionalized MXene as an effective catalyst for hydrogen production from formic acid

A simple wet chemical method was used to prepare two-dimensional transition metal carbides (MXene); PDA-MXene was prepared by alkalization of p-phenylenediamine (PDA) on MXene. And further, Pd metal nanoparticles (NPs) were conveniently loaded on the surface to catalyze the dehydrogenation of formic acid. The as-prepared Pd/PDA-MXene catalyst for the formic acid dehydrogenation was characterized by XRD, IR, TEM, and XPS. Pd-NPs with a size of about 4 nm were formed upon the PDA-MXene support surface and were well dispersed. The Pd/PDA-MXene exhibited good catalytic activity in the formic acid decomposition process without any additives, and the turnover frequency value at 50 °C was 924.4 h⁻¹, which is comparable to most of the reported noble metal catalysts under similar conditions. It is worth mentioning that the prepared catalyst maintained good catalytic activity in five consecutive catalytic cycles of the formic acid dehydrogenation experiment.     

In situ reduction of PdO encapsulated in MCM-41 to Pd(0) for dehydrogenation of formic acid

A series of PdO@MCM-41-x was in situ synthesized by a facile one-pot method, and their catalytic performance for dehydrogenation of formic acid was investigated. The PdO nanoparticles were immobilized in MCM-41 channel with small size of 2.2–2.5 nm due to the confinement of mesoporous silica. The initial turnover frequency (TOF) of as-prepared catalyst can reach 1096 h^?1 at 323 K with 100% hydrogen selectivity. The hydrogen production activity was better than that of catalyst priorly reduced by hydrogen because of the gradual reduction of PdO nanoparticles and generation of fresh Pd(0) during the formic acid dehydrogenation process, which was confirmed by XPS characterization and catalytic system monitoring. It provides a new way for the preparation of simple and efficient catalysts for hydrogen production, which has great application potential.     

Rapid fabrication of Cu/Pd nano/micro-particles porous-structured catalyst using hydrogen bubbles dynamic template and their enhanced catalytic performance for formic acid electrooxidation

In this work, a self-supporting Pd–Cu bimetallic film with 3D porous structure was electrodeposited at the surface of glassy carbon electrode (GCE) using a facile double-template fabrication process, including hydrogen bubble templating method and galvanic replacement reaction, and its performance investigated as a catalyst for formic acid oxidation (FAO). The structure of the Cu/Pd porous film was characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The electrocatalytic activity of the as-prepared catalysts with high surface areas were evaluated in sulfuric acid solution containing 1 M formic acid using cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry and electrochemical impedance spectroscopy (EIS). The Cu/Pd porous structure exhibited significantly high current densities of formic acid oxidation compared to the Cu/Pd particles film catalyst. The effects of galvanic replacement time and concentration of formic acid on the catalytic activity of as-prepared electrode for FAO were comparatively investigated.     

Two-dimensional engineering of Pd nanosheets as advanced electrocatalysts toward formic acid oxidation

Two-dimensional self-assembled nanostructures of palladium nanosheets are created during one-step strategies at room temperature. Palladium nanosheets are synthesized in the absence and presence of surfactant (CTAB) agent with the aim of considering the surfactant effect on the morphology and electrocatalytic activity of palladium nanosheets. In both reactions, carbon monoxide and acetic acid act as reducing agent and solvent, respectively. Both palladium nanosheets serve as two-dimensional advanced supportless electrocatalysts for oxidation of formic acid, and clarify higher mass activity and durability in comparison to the palladium anchored on carbon. The exceptional performance of both palladium nanosheets is ascribed to their self-support and huge surface area characteristics. Moreover, the paper well proves that the morphology and electrocatalytic efficiency of palladium nanosheets were seriously affected by the presence of surfactant. Palladium nanosheets synthesized in the absence and presence of surfactant display the flat and stack nanosheets with thicknesses of 3.48 and 4.22 nm, respectively. In addition, comparison of both palladium nanosheets demonstrates that palladium nanosheets synthesized in the absence of surfactant reflect better catalytic efficiency and durability. The presence and bonding of surfactant to the surface of palladium nanosheets lead to the occupancy of active sites and degradation of palladium nanosheets performance. We believe that these palladium nanosheets can be applied as advanced electrocatalysts for diverse applications, especially direct formic acid fuel cells.     

An ambient aqueous synthesis for highly dispersed and active Pd/C catalyst for formic acid electro-oxidation

An experimentally simple process is reported in aqueous solution and under ambient conditions to prepare highly dispersed and active Pd/C catalyst without the use of a stabilizing agent. The [Pd(NH₃)₄]²⁺ ion is synthesized with gentle heating in aqueous ammonia solution without formation of Pd(OH)_x complex intermediates. The adsorbed [Pd(NH₃)₄]²⁺ on the surface of carbon (Vulcan XC-72) is reduced in situ to Pd nanoparticles by NaBH₄. The Pd/C catalyst obtained is characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that highly dispersed Pd/C catalyst with 20 wt.% Pd content and with an average Pd nanoparticle diameter of 4.3-4.7 nm could be obtained. The electrochemical measurements show that the Pd/C catalyst without stabilizer has a higher electro-oxidation activity for formic acid compared to that of a Pd/C catalyst prepared in a traditional high temperature polyol process in ethylene glycol.     

Conversion of hydrogen/carbon dioxide into formic acid and methanol over Cu/CuCr2O4 catalyst

Cu/CuCr₂O₄ catalysts were prepared by impregnation method at various calcination temperatures (300, 400, and 500 °C) and then reduced in H₂ stream. The aggregated particles and decreasing surface area/pore volumes of the deactivated catalysts during HCOOH and CH₃OH formations were also observed. Particularly, the EXAFS data showed that first shells of Cu atoms transforms from Cu–O to Cu–Cu after catalytic reactions, their bond distances and coordination numbers are quite different, respectively. It revealed that metallic Cu atoms are one of the important active species over catalyst surface at different reaction temperatures having many unoccupied binding sites for HCOOH and CH₃OH formations. Additionally, the optimal calcination temperature for Cu/CuCr₂O₄ catalysts was demonstrated at 400 °C that attributed to its strongest acidity and basicity. The catalytic reactions in the duration of HCOOH and CH₃OH preparation were proposed that were composed of HCOOH formation, CH₃OH formation, and CH₃OH decomposition happening at CuCr₂O₄, Cu, and CuO active sites, respectively. The highest CO₂ conversion (14.6%), HCOOH selectivity/yield (87.8/12.8%), and TON/TOF values (4.19/0.84) were obtained at 140 °C and 30 bar in 5 h, respectively. Optimal rate constant (2.57 × 10^?2 min^?1) and activation energy (16.24 kJ mol^?1) of HCOOH formation were evaluated by pseudo first-order model and Arrhenius equation, respectively.     

Performance increase of microfluidic formic acid fuel cell using Pd/MWCNTs as catalyst

This paper shows that the combination of an O₂ saturated acidic fluid setup (O₂-setup) and a composite of Pd nanoparticles supported on multiwalled-carbon nanotubes (Pd/MWCNTs) as anode catalyst material, results in the improvement of microfluidic fuel cell performance. Microfluidic fuel cells were constructed and evaluated at low HCOOH concentrations (0.1 and 0.5 M) using Pd/V XC-72 and Pd/MWCNTs as anode and Pt/V XC-72 as cathode electrode materials, respectively. The results show a higher power density (2.9 mW cm⁻²) for this cell when compared to the value reported in the literature that considers a commercial Pd/V XC-72 and 3.3 mW cm⁻² using a Pd/MWCNTs with a 50% less Pd loading than that commercial Pd/V XC-72.     

An effective Pd@Ni-B/C anode catalyst for electro-oxidation of formic acid

The Pd@Ni-B/C catalysts were prepared by using Ni-B/C with different amounts of Ni-B alloys as supports. The structure, morphology and element valence state of these catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The electro-catalytic performance of these catalysts for formic acid oxidation was investigated using cyclic voltammetry (CV), chronoamperometry (CA) and CO stripping experiments. It was found that an appropriate amount of Ni-B alloys plays an important role in enhancing catalytic activity and resistance to CO poisoning and improving stability of Pd based catalysts, all of which imply that Pd@Ni_1.61B_0.0199/C is promising for probable applications in direct formic acid fuel cell field.     

Engineering of the N-doped carbon support for improved performance of supported Pd catalysts in hydrogen production from gas-phase formic acid

Development of N-doped Pd/C catalysts for hydrogen production from gas-phase formic acid is a challenge. To elucidate the efficient routes of nitrogen insertion on the surface of a mesoporous carbon support, the latter was treated with melamine (Mel), dicyandiamide or NH₃ at 673 and 823 K. Pyrolysis of the melamine/carbon mixture taken in a 1:2 ratio provides an increase in the reaction rate by a factor of 5. The inserted N-sites strongly interact with Pd leading to the formation of highly dispersed Pd nanoparticles (∼1.6 nm) and active atomically dispersed Pd²⁺ species. With a further increase of the Mel/C ratio, the number of surface N-sites decreases due to occupation of carbon support pores with a g–C₃N₄–type residue. This provides a decrease in the Pd dispersion leading to lower reaction rates. Therefore, melamine is an efficient N precursor. The considered synthesis of N-doped catalysts could be scaled.     

Highly active carbon nanotube-supported Pd electrocatalyst for oxidation of formic acid prepared by etching copper template method

In this work, a carbon nanotube-supported Pd nano-catalyst (Pd/MCNTs) is prepared by the etching copper template strategy. Cu nanoparticles (NPs) are formed onto MCNTs first as the template and Pd NPs are then obtained through a galvanic displacement reaction between Pd ions and Cu. TEM, XRD, and XPS characterizations show the crystalline of Pd NPs with a typical diameter of 2–5 nm is homogeneously decorated onto MCNTs without aggregation. Electrochemical characterizations reveal that the Pd/MCNTs materials exhibit much higher catalytic activity for the formic acid oxidation than both conventional Pd/MCNTs and commercial Pd/Vulcan catalysts do. The improved activity is mainly attributed the fact that no surfactant is required in synthesis of the catalyst, eliminating the possible passivation of catalytic sites associated with the use of surfactant in conventional synthesis methods. In addition, the narrower distribution and better dispersion of catalyst particles, as well as no defects of MCNTs are also beneficial for the improvement in the catalytic activity. Another feature of the present synthesis method is the loading of Pd can be adjusted by varying the amount of Cu ions.     

Enhancement of anodic oxidation of formic acid on palladium decorated Pt/C catalyst

A palladium decorated Pt/C catalyst, Pt@Pd/C, is prepared by a colloidal approach with a small amount of platinum as core. It is found that the catalyst shows excellent activity towards anodic oxidation of formic acid at room temperature and its activity is 60% higher than that of Pd/C. Decoration of palladium shell on the platinum core is supported by XPS results. Due to the use of platinum as core, active components are dispersed very well and the particle sizes are smaller than those of Pd/C. The cyclic voltammetry measurement clearly shows synthetic electro-oxidation effects of formic acid on Pt@Pd/C. It is speculated that the high performance of Pt@Pd/C may result from the unique core-shell structure and synergistic effect of Pt and Pd at the interface. The preparation method for Pt@Pd/C reported in this work will provide additional options for the design of catalysts for direct formic acid fuel cell (DFAFC).     

Electrocatalytic properties of PdCeOx/C anodic catalyst for formic acid electrooxidation

The PdCeO_x/C catalyst was prepared and studied as anodic catalyst for formic acid electrooxidation. The morphology, composition and electrocatalytic properties were investigated by transmission electronmicroscopy, energy dispersive X-ray analysis, X-ray diffraction, X-ray photoelectron spectroscopy, the linear sweep voltammetry and chronoamperometry, respectively. The average particle size of Pd nanoparticles for PdCeO_x/C catalyst was about 3.4 nm with a narrow size distribution. The electrochemical surface area (ESA) of PdCeO_x/C catalyst was larger than the Pd/C catalyst. According to the Tafel plots, the presence of CeO_x in the Pd/C catalyst can accelerate the rate of formic acid electrooxidation. The electrochemical measurements showed that the PdCeO_x/C catalyst exhibited excellent catalytic activity and stability. For example, the peak current of PdCeO_x/C catalyst for formic acid oxidation was about 1.67 times of the Pd/C catalyst and the stable current at 3600 s of PdCeO_x/C catalyst was about 7 times of the Pd/C catalyst. The promotion effect should be attributed to the larger ESA, the electronic effect and more oxygen-containing species provided by the CeO_x and this catalyst may be used as an effective catalyst for direct formic acid fuel cell.     

Fast hydrogen evolution by formic acid decomposition over AuPd/TiO2-NC with enhanced stability

Well-dispersed AuPd nanoparticles were immobilized on TiO₂-NC supports derived from NH₂-MIL-125(Ti) and used as highly active, stable catalysts for hydrogen production from formic acid under mild conditions. The highest total turnover frequency, i.e., 3207 h⁻¹, for formic acid dehydrogenation was achieved with Au₂Pd₈/TiO₂-NC-800 as the catalyst at 60 °C; this is 1.4 times that achieved with Au₂Pd₈/TiO₂–C-800 under the same conditions. The excellent performance of the Au₂Pd₈/TiO₂-NC-800 catalyst originates from the high anatase TiO₂ content, pyridinic N and oxygen vacancies in the support, the small size and alloying effect of the AuPd nanoparticles, and the metal–support synergistic effect. Doping the support with N improves the catalyst stability because N prevents metal particle aggregation to some extent. These results provide guidelines for the future development and applications of catalysts based on TiO₂ and metal–organic-framework-derived carbon-based materials.     

Porous carbon supported Pd as catalysts for boosting formic acid dehydrogenation

The design of efficient catalysts is the essential to realize formic acid (FA) as a hydrogen carrier. However, it remains a challenging task. Herein, the porous carbon was prepared using ZnCo-containing zeolitic imidazole frameworks (ZIFs) as a precursor, which supported Pd as an effective catalyst for FA dehydrogenation. Porous carbon containing Co and N was synthesized by one-step method, and the Co and N promoted the activity of Pd by modifying its electron state. The catalytic performance was further improved by doping Zn into the predesigned bimetallic ZnCo-ZIFs. The addition of Zn increased the dispersion of PdCo nanoparticles, N content and specific surface area of the catalysts. When Zn/Co molar ratio was 2, the prepared catalyst (Pd/Co@CN-2) with an average diameter of PdCo about 2.6 nm exhibited the best catalytic activity, showing an initial turnover frequency value (TOF) as high as 2302 h⁻¹ even at 30 °C.     

Facile synthesis of well dispersed Pd nanoparticles on reduced graphene oxide for electrocatalytic oxidation of formic acid

In present study, we report a facile synthesis of crystalline, small size Pd nanoparticles (NPs) on reduced graphene oxide (RGO) abbreviated as Pd/RGO for electrocatalytic oxidation of formic acid (FA). Here, first graphene oxide (GO) was reduced by the green method using l-ascorbic acid and citric acid and further Pd NPs were decorated on RGO by a facile method without using any reducing agents. The reduction of GO to RGO and synthesis of Pd NPs was confirmed by the X-ray diffraction (XRD) and X-ray photoelectrons (XPS) techniques. Surface morphology of Pd/RGO nanocomposite was evaluated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The electrocatalytic behavior of Pd/RGO nanocomposite was tested by using of cyclic voltammetric (CV) technique for electro-oxidation of FA in mixed solution of 0.5 M HCOOH + 0.5 H₂SO₄ at RT. Results shows that the higher electrocatalytic activity of Pd/RGO nanocomposite compare to Pd NPs.     

Electrochemical impedance studies of electrooxidation of methanol and formic acid on Pt/C catalyst in acid medium

Cyclic voltammetry (CV), amperometric i − t experiments, and electrochemical impedance spectroscopy (EIS) measurements were carried out by using glassy carbon disk electrode covered with the Pt/C catalyst powder in solutions of 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ CH₃OH and 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ HCOOH at 25 °C, respectively. Electrochemical measurements show that the activity of Pt/C for formic acid electrooxidation is prominently higher than for methanol electrooxidation. EIS information also discloses that the electrooxidation of methanol and formic acid on the Pt/C catalyst at various polarization potentials show different impedance behaviors. The mechanisms and the rate-determining steps of formic acid electrooxidation are also changed with the increase of the potential. Simultaneously, the effects of the electrode potentials on the impedance patterns were revealed.     

Effect of MoO3 on Pd nanoparticles for efficient formic acid electrooxidation

Palladium is a promising formic acid electro-oxidation (FAO) catalyst due to its higher initial activity than platinum. However, suffering from the adsorption of hydrogen and CO-like species, the activity and stability of Pd are still unsatisfied. Herein, palladium nanoparticles deposited on carbon supported molybdenum trioxide (Pd–MoO₃/C) is prepared with MoO₃ as the promoter for FAO. X-ray photoelectron spectroscopy analysis proves the close contact between Pd and MoO₃, which generates the hydrogen spillover effect and forms the Pd–Mo structure. The hydrogen spillover effect enhances the desorption of hydrogen from Pd and facilitates the FAO activity. Both the spillover effect and Pd–Mo structure contribute to the removal of CO_ad and facilitate the durability and anti-CO poisoning ability of the catalyst. With the optimized ratio of MoO₃ to carbon black, the Pd–MoO₃/C-20 catalyst owns the best specific activity of 5.86 mA cm⁻², which is 1.86 times of Pd/C.