Electrocatalysis of carbon black- or chitosan-functionalized activated carbon nanotubes-supported Pd with a small amount of La2O3 towards methanol oxidation in alkaline media

The Pd/C catalysts with and without a small amount of La₂O₃ were synthesized by a simple reduction reaction with sodium borohydride in aqueous solution. The structure and morphology of these catalysts were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy. The electrocatalytic performance of these catalysts for methanol oxidation in alkaline media was investigated using cyclic voltammetry, chronoamperometry and CO stripping experiments. The results show that the Pd–La₂O₃/C catalyst has a higher catalytic activity than the Pd/C catalyst, but the effect of La₂O₃ cannot be explained by a bi-functional mechanism. X-Ray photoelectron spectroscopy analyses suggest that the higher content of metallic Pd caused by the addition of La₂O₃ contributes to the better catalytic activity of Pd–La₂O₃/C. Based on the good electrocatalytic performance of Pd–La₂O₃/C, the Pd–La₂O₃ catalyst supported on chitosan (CS)-functionalized activated carbon nanotubes was prepared, and it exhibited a better catalytic activity. The improvement is attributed to the good dispersion status of metal particles and the further increase of metallic Pd due to the presence of CS.     

MnO2 modified multi-walled carbon nanotubes supported Pd nanoparticles for methanol electro-oxidation in alkaline media

A novel method to prepare MnO₂ modified multi-walled carbon nanotubes (MnO₂/MWCNTs) supported Pd (Pd-MnO₂/MWCNTs) electrocatalyst is reported. The morphology, component and crystallinity of the catalyst were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The activity of Pd-MnO₂/MWCNTs was tested using methanol electro-oxidation in alkaline media. The results showed that the Pd-MnO₂/MWCNTs exhibited higher electrocatalytic activity and stability than Pd/MWCNTs and Pd/Vulcan (Pd/commercial Vulcan XC-72 carbon black).     

Preparation and characterization of Pd/C catalyst obtained in NH3-mediated polyol process

Vulcan XC-72R carbon supported Pd nanoparticles was obtained in a NH₃-mediated polyol process without any protective agent and characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques. The added NH₃ species is found to have a strong complex ability to Pd, which not only avoids the formation of Pd hydroxide precipitate resulted from Pd hydrolysis, but also restrains the further complete reduction of Pd. Temperature-programmed reduction equipped with a mass spectrometry (TPR-MS) is employed to study the reductive behavior of unreduced Pd complex in Pd/C catalyst and the results show that the post-treatment in a reductive atmosphere at higher temperature is needed to ensure the complete reduction of Pd. XRD patterns show the heat-treated Pd/C sample in a reductive atmosphere has a face centered cubic crystal structure and TEM image indicates that the dispersion of Pd nanoparticles on the carbon support is uniform and in a narrow particle size range. Thermodynamic data analysis is carried out to elucidate the possible reaction pathway for the preparation of Pd/C catalyst in this process. The obtained Pd/C catalyst shows high activity to formic acid oxidation and high selectivity to oxygen reduction reaction (ORR) with the presence of methanol.     

Synthesis of highly active and stable Pd/C catalysts toward HCOOH dehydrogenation by a simple NH3 adsorption method

Pd catalysts supported on activated carbon (Pd/C–NH₃) toward HCOOH dehydrogenation were prepared by a simple adsorption method using ammonia (NH₃) and Ar as the working gas. The results show that the TOF_initial of Pd/C–NH₃ was 459.8 h⁻¹ at 50 °C. When the reaction was carried out for 4 h, the HCOOH dehydrogenation ratio over Pd/C–NH₃ was about 81.2%, which was 1.15 and 1.13 times, respectively, as that of the as-prepared Pd/C catalyst without any treatment (Pd/C–As) and the Pd/C catalyst purchased from Sigma-Aldrich (Pd/C-CM). The total amount of H₂ and CO₂ produced by using Pd/C–NH₃ to decompose HCOOH in the third cycle was 99.4% of the gas produced by the first reaction cycle, and 1.80 and 12.60 times, respectively, as that of Pd/C–As and Pd/C-CM. The characterization results indicated that the Pd active species in Pd/C–NH₃ migrated to the outer surface of the carbon support during the reaction, and the pore volume of the carbon support became larger, which were beneficial to the reaction. These factors made Pd/C–NH₃ exhibit excellent HCOOH dehydrogenation activity and stability. NH₃ adsorption is a simple and effective method for preparing high-performance Pd/C HCOOH dehydrogenation catalysts, and has important guiding significance for the preparation of other carbon supported noble metal catalysts.     

Pd nanoparticles supported on HPMo-PDDA-MWCNT and their activity for formic acid oxidation reaction of fuel cells

A novel Pd electrocatalyst is developed by self-assembly of Pd nanopartilces on phosphomolybdic acid (HPMo)-poly(diallyldimethylammonium chloride) (PDDA)-functionalized multiwalled carbon nanotubes supports (Pd/HPMo-PDDA-MWCNTs). The as-synthesized Pd/HPMo-PDDA-MWCNTs were characterized by TEM, EDS mapping, Raman spectra, X-ray photoelectron spectroscopy, electrochmeical CO stripping and cyclic voltammetry techniques. Pd nnaoparticles deposited on HPMo-PDDA-MWCNTs are in the range of 3.1 nm with uniform distributon. Pd/HPMo-PDDA-MWCNT catalysts have lower overpotential for CO_ad oxidation manifested as lower peak and onset potentials as compared to acid-treated MWCNTs supported Pd (Pd/AO-MWCNTs) and carbon supported Pd catalysts (Pd/C). Pd/HPMo-PDDA-MWCNTs catalysts also exhibit a much higher electrocatalytic activity and stability for formic acid oxidation reaction as compared to that on Pd/AO-MWCNTs and Pd/C. The high electrocatalytic activities of Pd/HPMo-PDDA-MWCNTs catalysts are most likely related to highly dispersed and fine Pd nanoparticles as well as synergistic effects between Pd and HPMo immobilized on PDDA-functionalized MWCNTs.     

Monolayer palladium supported on molybdenum and tungsten carbide substrates as low-cost hydrogen evolution reaction (HER) electrocatalysts

Active and low-cost hydrogen evolution reaction (HER) electrocatalysts are needed to minimize capital costs associated with large-scale hydrogen production from water electrolysis. Catalysts based on monolayer (ML) amounts of precious metals supported on carbides are a promising concept for this purpose. In the current study Pd supported on tungsten carbide (WC) and molybdenum carbide (Mo₂C) were evaluated for HER activity. Carbide foils were synthesized using temperature programmed reaction of W or Mo in a CH₄/H₂ atmosphere. Physical vapor deposition was used to deposit Pd on WC or Mo₂C while X-ray Photoelectron Spectroscopy (XPS) was used to determine the Pd surface coverage. Linear sweep voltammetry and chronopotentiometry were used to evaluate the HER activity and electrochemical stability of the catalysts, demonstrating the possibility of using ML Pd on either WC or Mo₂C as active, stable and lower-cost HER catalysts.     

Cyclic voltammetry and X-ray photoelectron spectroscopy studies of electrochemical stability of clean and Pt-modified tungsten and molybdenum carbide (WC and Mo2C) electrocatalysts

The electrochemical stability of tungsten carbide (WC), Pt-modified WC, molybdenum carbide (Mo₂C), and Pt-modified Mo₂C has been examined using an in situ electrochemical half-cell in combination with X-ray photoelectron spectroscopy (XPS). The WC surface, created via the carburization of a tungsten foil, was electrochemically stable to ∼0.8 V with respect to the normal hydrogen electrode (NHE) when exposed to dilute sulfuric acid. At higher potentials, XPS confirmed the surface oxidation of WC to form W_xO_y species. The deposition of submonolayer coverage of Pt on the WC surface increased the region of stability of WC, extending the onset of catalyst oxidation to ∼1.0 V (NHE). These results suggest that both WC and Pt/WC have the potential to be used as anode electrocatalysts. In contrast, both Mo₂C and Pt-modified Mo₂C underwent oxidation at ∼0.4 V (NHE), indicating that molybdenum carbides are not stable enough for applications as anode electrocatalysts.     

Solvothermal synthesis of Mg-Ni/C nanocomposite for hydrogen storage using vitamin C as carbon source

Metal alloys such as Mg₂Ni and TiFe can absorb and store H₂ to form metal hydrides. The alloys are frequently prepared by grinding the metal ingredients together. In this work, a solvothermal synthesis was taken to prepare the Mg-Ni/C nanocomposite from vitamin C, magnesium acetate, and nickel acetate, using ethylene glycol as solvent and reductant. Without adding Pd as nucleating agent, it is proposed to obtain Mg-Ni alloys by reduction and carbon nanoparticles by carbonization. XRD analysis shows that Mg₂Ni and Mg-Ni₂ have been formed as nanocomposite with carbon. After calcination in vacuum at 500 °C for 1 h, the Mg-Ni/C nanocomposite showed enhanced magnetism. XRD results give peaks of Ni and MgO, indicative of phase separation and Mg oxidation. In conclusion, it is feasible to synthesize Mg-Ni/C or other carbon-containing hydrogen storage composites using the solvothermal method.     

Carbon supported PtPdCr ternary alloy nanoparticles with enhanced electrocatalytic activity and durability for methanol oxidation reaction

In this work, the trimetallic PtPdCr nanoparticles with low platinum loading (~5 wt%) supported on Vulcan carbon (PtPdCr/C) were synthesized through a facile two-step co-reduction method and showed superior methanol oxidation activity. The particle size distribution, morphology and elemental composition of the PtPdCr/C were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. The electrochemical performance for methanol oxidation of the PtPdCr/C was found to be higher with the mass activity of 969 mA·mg⁻¹_Pt compared to Pt/C (581 mA·mg⁻¹ _Pt) and PtRu/C (725 mA·mg⁻¹ _Pt). Moreover, the stability studies confirmed the enhanced durability of PtPdCr/C over Pt/C and PtRu/C catalysts after the accelerated durability test (ADT) and chronoamperometry (CA) analysis. The increased methanol oxidation activity and durability of the trimetallic PtPdCr/C in acid medium can be attributed to the change in binding energy of Pt and the induced synergistic effect from Pd and Cr atoms to Pt, which demonstrated a promising strategy for the preparation and utilization of ternary alloy catalysts towards methanol electrooxidation.     

High performance Pd-based catalysts for oxidation of formic acid

Two novel catalysts for anode oxidation of formic acid, Pd₂Co/C and Pd₄Co₂Ir/C, were prepared by an organic colloid method with sodium citrate as a complexing agent. These two catalysts showed better performance towards the anodic oxidation of formic acid than Pd/C catalyst and commercial Pt/C catalyst. Compared with Pd/C catalyst, potentials of the anodic peak of formic acid at the Pd₂Co/C and Pd₄Co₂Ir/C catalyst electrodes shifted towards negative value by 140 and 50 mV, respectively, meanwhile showed higher current densities. At potential of 0.05 V (vs. SCE), the current density for Pd₄Co₂Ir/C catalyst is as high as up to 13.7 mA cm⁻², which is twice of that for Pd/C catalyst, and six times of that for commercial Pt/C catalyst. The alloy catalysts were nanostructured with a diameter of ca. 3–5 nm and well dispersed on carbon according to X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The composition of alloy catalysts was analyzed by energy dispersive X-ray analysis (EDX). Pd₄Co₂Ir/C catalyst showed the highest activity and best stability making it the best potential candidate for application in a direct formic acid fuel cell (DFAFC).     

Bis (dibenzylidene acetone) palladium (0) catalyst for glycerol oxidation in half cell and in alkaline direct glycerol fuel cell

In this study, catalytic activity and performance of bis (dibenzylidene acetone) palladium (0) catalyst, Pd (DBA)₂, was evaluated toward glycerol oxidation reaction (GOR) in alkaline half cell and alkaline direct glycerol fuel cell (DGFC). The electrooxidation of glycerol on Pd (DBA)₂ was characterized in half cell by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) techniques. Obtained results have highlighted the excellent electrocatalyst activity of Pd (DBA)₂ in terms of specific peak current density and onset potential compared to the results obtained by conventional Pd base catalysts. CVs results also demonstrate that Pd (DBA)₂ is still active even after 200 cycles.     

Investigation of tungsten carbide supported Pd or Pt as anode catalysts for PEM fuel cells

In this contribution, we present results of electrochemical characterization of prepared tungsten carbide supported palladium and platinum and Vulcan XC-72 supported palladium. These catalysts were employed as anode catalysts in PEMFC and results are compared to commercial platinum catalyst. Platinum seems to be irreplaceable as a proton exchange membrane fuel cell (PEMFC) catalyst for both the anode and the cathode, yet the high price and limited natural resources are holding back the commercialization of the PEMFCs. Tungsten carbide is recognized as promising catalyst support having the best conductivity among interstitial carbides. Higher natural resources and significantly lower price make palladium good candidate for replacement of the platinum catalyst. The presented results show that all prepared catalysts are very active for the hydrogen oxidation reaction. Linear sweep voltammetry curves of Pd/C and Pd/WC show existence of peaks at 0.07 V vs. RHE, which is assigned to absorbed hydrogen. H₂|Pd/WC|Nafion117|Pt/C|O₂ fuel cell has almost the same efficiency and similar power output as commercial platinum catalyst.     

Carbon-supported PdSn–SnO2 catalyst for ethanol electro-oxidation in alkaline media

Carbon-supported PdSn–SnO₂ with high electrical catalytic activity for ethanol oxidation in alkaline solution was synthesized using an impregnation reduction method. XRD analysis of the as-prepared PdSn–SnO₂/C revealed that the Pd diffraction peaks shifted to lower 2θ values with respect to the corresponding peaks of the Pd/C catalyst, indicating that Sn doping could shrink the Pd crystalline lattice. XPS measurements confirmed the existence of Sn and SnO₂ in the PdSn–SnO₂/C catalysts. The prepared PdSn–SnO₂/C catalysts presented a remarkably higher electrocatalytic activity than that of the Pd–Sn/C and Pd/C catalysts. This was mainly because the easy adsorption-dissociation of OH_ads over the SnO₂ surface changed the electronic effect and accelerated the adsorption of ethanol on the surface of Pd, thus enhancing the overall ethanol oxidation kinetics and contributing to a significant improvement in the catalytic activity.     

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.     

Unique superstructure of Pd–Ir aerogel as a robust three-dimensional electrocatalyst

Herein, a novel, facile, fast, and one-step strategy is introduced to synthesize a three-dimensional (3D) Pd–Ir aerogel. The Pd–Ir aerogel is synthesized by reducing metallic ions in the presence of sodium carbonate and formaldehyde, followed by CO₂ supercritical drying. Controlling the type of generated nanostructures and the reaction kinetics is adjusted by the sodium carbonate concentration. The change of sodium carbonate concentration is employed for creating an efficient anisotropic condition to create a Pd–Ir aerogel. The aerogel depicts a 3D architecture with a large porosity and an ultra-low density (0.022 g cm^?3). This unique architecture illustrates the exceptional electrocatalytic activity and durability owing to the following vital reasons. The 3D architecture with large open pores not only significantly facilitates the accessibility of ethanol molecules to inner active sites but also guarantees the interaction of ethanol molecules with the surface of aerogel. Furthermore, the downfall of durability observed in the Pd/C owing to corrosion can eliminate by the self-supported nature of assembled aerogel. Moreover, the presence of Ir in the structure of aerogel leads to an alteration in the electronic structure of palladium, which facilitates the electrooxidation of ethanol at a high pH environment.     

Synthesis of PdNi catalysts for the oxidation of ethanol in alkaline direct ethanol fuel cells

Carbon-supported PdNi catalysts for the ethanol oxidation reaction in alkaline direct ethanol fuel cells are successfully synthesized by the simultaneous reduction method using NaBH₄ as reductant. X-ray diffraction characterization confirms the formation of the face-centered cubic crystalline Pd and Ni(OH)₂ on the carbon powder for the PdNi/C catalysts. Transmission electron microscopy images show that the metal particles are well-dispersed on the carbon powder, while energy-dispersive X-ray spectrometer results indicate the uniform distribution of Ni around Pd. X-ray photoelectron spectroscopy analyses reveal the chemical states of Ni, including metallic Ni, NiO, Ni(OH)₂ and NiOOH. Cyclic voltammetry and chronopotentiometry tests demonstrate that the Pd₂Ni₃/C catalyst exhibits higher activity and stability for the ethanol oxidation reaction in an alkaline medium than does the Pd/C catalyst. Fuel cell performance tests show that the application of Pd₂Ni₃/C as the anode catalyst of an alkaline direct ethanol fuel cell with an anion-exchange membrane can yield a maximum power density of 90 mW cm⁻² at 60 °C.     

Electrocatalytic oxidation of formaldehyde on palladium nanoparticles supported on multi-walled carbon nanotubes

Palladium (Pd) nanoparticles were dispersed on iodinated multi-walled carbon nanotubes (I-MWNTs) by the aqueous solution reduction of Pd(NO₃)₂ with formaldehyde. The structure and nature of the resulting Pd-MWNT composites were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The electrocatalytic properties of the Pd-MWNT modified glassy carbon electrode (Pd-MWNT/GCE) for formaldehyde oxidation have been investigated by cyclic voltammetry; high electrocatalytic activity of the Pd-MWNT/GCE can be observed. This may be attributed to the high dispersion of palladium catalysts and the particular properties of MWNT supports. The results imply that the Pd-MWNT composite has good potential applications in fuel cells.     

Fabrication of highly dispersed Pd nanoparticles supported on reduced graphene oxide for solid phase catalytic hydrogenation of 1,4-bis(phenylethynyl) benzene

In this work, nanopalladium catalysts supported on the surface of reduced graphene oxide (rGO/Pd) with different palladium loadings have been prepared by one-step reduction in aqueous phase. They were mixed with 1,4-bis(phenylethynyl)benzene (DEB) to form rGO/Pd-DEB composites according to a mass ratio of 1:3. It was shown that nanopalladium particles with particle size of about 2–6 nm were disperse uniformly on the surface of rGO when the Pd loadings were in the range of 3.97–10.60 wt%. The maximum hydrogen uptake capacity of rGO/Pd-DEB composites at 25 °C determined according to PCT method was about 182.5 ml/g after reacted with hydrogen for about 20 h, which was some lower than that of the common Pd/C-DEB pallet getter (216 ml/g) but significantly higher than alkynyl modified polyvinyl alcohol supported palladium hydrogen absorbing materials (0.32 ml/g), indicating that rGO/Pd could be used in solid phase catalytic hydrogenation due to the high dispersion of palladium nanoparticles and the physical proximity of rGO/Pd catalyst with DEB organic molecules. This provides a good potential technical way for perparing the moldable carbon aerogel hydrogen absorption materials.     

Analysis and improvement of cyclic stability of H2 sensing properties of Pd/Mg-Ni films

The nominal electrical resistivity of palladium coated magnesium-nickel (Pd/Mg-Ni) films was measured by exposing the films iteratively to hydrogen (H₂) at a concentration of CH₂ for hydrogenation and air for dehydrogenation. When a low CH₂ was used, the film remained in an amorphous α-phase. H atoms interacted “interstitially” with the atomic network, and the H₂ detection sensitivity S was relatively stable in the cyclic test. If higher CH₂ values were used, the film was partially or completely transformed to an amorphous β-phase. Significant volumetric breathing occurred in the course, leading to severe roughening of the film and oxidation of the Mg-Ni layer. S became unstable. These suggestions are supported by the results of film thickness measurements, atomic force microscopy and X-ray photoelectron spectroscopy analyses. Stability of S of a Pd/Mg-Ni film can be greatly improved by either (i) operating the film in a low CH₂ environment to prevent substantial volume breathing, or (ii) choosing an appropriate thickness of the Pd layer to optimize both oxidation resistance and sensing response of the film sensor.