Pd nanoparticles supported on carbon-modified rutile TiO2 as a highly efficient catalyst for formic acid electrooxidation

In this article, Pd nanoparticles supported on carbon-modified rutile TiO₂ (CMRT) as a highly efficient catalyst for formic acid electrooxidation were investigated. Pd/CMRT catalyst was synthesized by using liquid phase reduction method in which Pd nanoparticles was loaded on the surface of CMRT obtained through a chemical vapor deposition (CVD) process. Pd/CMRT shows three times the catalytic activity of Pd/C, as well as better catalytic stability towards formic acid electrooxidation. The enhanced catalytic property of Pd/CMRT mainly arises from the improved electronic conductivity of carbon-modified rutile TiO₂, the dilated lattice constant of Pd nanoparticles, an increasing of surface steps and kinks in the microstructure of Pd nanoparticles and slightly better tolerance to the adsorption of poisonous intermediates.     

ZrC–C and ZrO2–C as Novel Supports of Pd Catalysts for Formic Acid Electrooxidation

The Pd/ZrC–C and Pd/ZrO₂–C catalysts with zirconium compounds ZrC or ZrO₂ and carbon hybrids as novel supports for direct formic acid fuel cell (DFAFC) have been synthesized by microwave‐assisted polyol process. The Pd/ZrC–C and Pd/ZrO₂–C catalysts have been characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), energy dispersive analysis of X‐ray (EDAX), transmission electron microscopy (TEM), and electrochemical measurements. The physical characteristics present that the zirconium compounds ZrC and ZrO₂ may promote the dispersion of Pd nanoparticles. The results of electrochemical tests show that the activity and stability of Pd/ZrC–C and Pd/ZrO₂–C catalysts show higher than that of Pd/C catalyst for formic acid electrooxidation due to anti‐corrosion property of zirconium compounds ZrC, ZrO₂, and metal–support interaction between Pd nanoparticles and ZrC, ZrO₂. The Pd/ZrC–C catalyst displays the best performance among the three catalysts. The peak current density of formic acid electrooxidation on Pd/ZrC–C electrode is nearly 1.63 times of that on Pd/C. The optimal mass ratio of ZrC to XC‐72 carbon is 1:1 in Pd/ZrC–C catalyst with narrower particle size distribution and better dispersion on surface of the mixture support, which exhibits the best activity and stability for formic acid electrooxidation among all the samples.     

Effect of pH Value and Temperatures on Performances of Pd/C Catalysts Prepared by Modified Polyol Process for Formic Acid Electrooxidation

The highly active Pd/C catalysts for formic acid electrooxidation have been prepared by a modified polyol process at different pH values of reaction solutions and different reducing temperatures, respectively. Their physical properties have been characterised by energy dispersive analysis of X‐ray, X‐ray diffraction and transmission electron microscopy. Their electrochemical performances for formic acid electrooxidation have been tested by cyclic voltammetry and amperometric i–t curves. The results of physical characterisations show that all the Pd/C catalysts present an excellent face centered cubic crystalline structure. Their particle sizes are decreasing firstly and then increasing with the increasing of the pH values of reaction solutions. The reducing temperatures also markedly affect the Pd particle sizes. And their nanoparticles have narrow size distributions and are highly dispersed on the surface of carbon support, and Pd metal loading in Pd/C catalyst is similar to the theoretical value of 20 wt.%. The results of electrochemical measurements present that the Pd/C catalyst prepared by waterless polyol process at the pH value of 10 and the reducing temperature of 120 °C has the smallest particle size of about 5.6 nm, and exhibits the highest catalytic activity (1172.0 A · g_Pd<?h‐2.85>^–1<?h.8>) and stability for formic acid electrooxidation.     

Ultrasonic-assisted synthesis of Pd–Ni alloy catalysts supported on multi-walled carbon nanotubes for formic acid electrooxidation

Pd–Ni alloys with different compositions (i.e. Pd₂Ni, PdNi, PdNi₂) dispersed on multi-walled carbon nanotubes (MWCNTs) are prepared by ultrasonic-assisted chemical reduction. The X-ray diffraction (XRD) patterns indicate that all Pd and Pd–Ni nanoparticles exist as Pd face-centered cubic structure, while Ni alloys with Pd. The transmission electron microscopy (TEM) images show the addition of nickel decreases the particle size and improves the dispersion. The X-ray photoelectron spectroscopy (XPS) spectra demonstrate the electronic modification of Pd by nickel doping. The electrochemical measurements reveal that the PdNi catalysts have better catalytic activity and stability for formic acid electrooxidation, among them PdNi/MWCNTs is the best. The performance enhancement is ascribed to the increase of electroactive surface area (EASA) and nickel doping effect which might modify the electronic structure.     

Kinetic Study of Formic Acid Oxidation on Highly Dispersed Carbon Supported Pd–TiO2 Electrocatalyst

The highly dispersed carbon supported Pd–TiO₂ catalyst was prepared by a liquid phase reduction method with intermittent microwave irradiation. The kinetic parameters, such as the charge transfer parameter (α) and the apparent diffusion coefficient (D) of formic acid electrooxidation on a carbon supported Palladium Titanium dioxide (Pd–TiO₂/C) electrode were obtained under the quasi steady-state conditions. The dependence on temperature of the formic oxidation at a Pd–TiO₂/C electrode was also investigated and the activation energy (E _a) at different potentials was obtained.     

Novel catalyst-support interaction for direct formic acid fuel cell anodes: Pd electrodeposition on surface-modified graphite felt

The electrodeposition of Pd on graphite felt (GF, thickness ~3 mm in uncompressed state) was studied and the resulting catalyst was compared with Pt-Ru/GF for the electro-oxidation of formic acid. A micellar solution composed of the non-ionic surfactant Triton X-102 and an aqueous phase containing PdCl₂ were utilized for the galvanostatic electrodeposition of Pd nanoparticles. The presence of the surfactant during electrodeposition coupled with pretreatment of the GF surface by a Shipley-type method (PdCl₂ + SnCl₂ solution) creating nucleation sites had a major impact on the Pd catalyst morphology and penetration throughout the electrode thickness, affecting, therefore, the electrocatalytic activity toward formic acid oxidation. It was found that large (~1,000 nm) Pd particles with smooth surface favored the indirect CO_ad pathway, while Pd nanoparticles (diameter <40 nm) with rough surface, formed with surfactant and pretreatment, were much more active leading to the direct non-CO_ad pathway. Due to pretreatment the GF surface has been modified and the effective catalytic system could be described as Pd/SnO₂–Pd(PdO)/GF with possible electronic interaction between support and catalyst. In direct formic acid fuel cell (DFAFC) experiments at 333 K and 1 M HCOOH, the peak power density using the Pd/GF anode reached 852 W m^?2 (57 g m^?2 Pd) compared to 392 W m^?2 (40 g m^?2 Pd) with a commercial Pd catalyst-coated membrane (CCM). The long-term stability of Pd-based anodes was poor and inferior to Pt–Ru (4:1 at. ratio) prepared and tested under identical conditions.     

Synthesis, characterization and catalytic activity of an ultrafine Pd/C catalyst for formic acid electrooxidation

An ultrafine Pd/C catalyst with a uniformly sized and highly dispersed nanostructure was synthesized by an improved liquid phase reduction method; in this process, a complexone (trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid, C_yDTA) was used as an alternative stabilizer for the first time. Physicochemical characterizations indicated that the resulting Pd nanoparticles possessed ideal structural characteristics, including an average diameter of 2.1 nm, narrow size distribution ranging from 0.5 to 4.0 nm, no visible agglomerations, and no residual C_yDTA. Electrochemical tests showed that the catalytic activity of the obtained Pd/C catalyst for formic acid electrooxidation was 2.2 times greater than that of Pd/C catalyst prepared in the absence of C_yDTA. This improvement in the electrocatalytic performance was attributed to the uniformly sized and highly dispersed nanostructure, which provided a larger overall electrochemical active surface area.     

One-pot synthesis of PdBi/reduced graphene oxide catalyst under microwave irradiation used for formic acid electrooxidation

The Pd and PdBi nanoparticles dispersed on the reduced graphene oxide (Pd/rGO and PdBi/rGO) have been synthesized through one-pot reaction under the irradiation of microwave and the obtained composites have been characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, and their electrocatalytic activities have also been evaluated. It is found that the PdBi_0.05/rGO catalyst exhibits higher activity and better stability toward formic acid electrooxidation compared with Pd/C and Pd/rGO. The excellent electrocatalytic performance indicates that the addition of appropriate amount of Bi can greatly enhance the activity and stability of Pd catalysts for the formic acid oxidation.     

Synthesis of Pd/C Catalyst by Modified Polyol Process for Formic Acid Electrooxidation

Pd catalyst supported on Vulcan XC‐72 carbon black was prepared by a modified polyol process. Its performance was compared with that of Pd/C catalyst prepared by impregnation reduction method by using NaBH₄ as a reducing agent for formic acid electrooxidation. Their physical characterisations were tested by means of energy dispersive analysis of X‐ray, X‐ray diffraction and transmission electron micrographs. Their activities were presented by cyclic voltammetry and chronoamperometry. The results show that the particle sizes of Pd/C catalysts prepared by modified polyol process and impregnation reduction method are 3.9 and 7.9 nm, respectively. The size dispersion of the former is narrower and more homogeneous than that of the latter. However, both of Pd/C catalysts display the characteristic diffraction peaks of a Pd face‐centred cubic (f.c.c.) crystal structure. The results of electrochemical measurements present that the Pd/C catalyst prepared by modified polyol process has the higher electrocatalytic activity and stability for formic acid electrooxidation in comparison to the Pd/C one by impregnation reduction method due to the particle size effect, and its peak current density of CV and the current of chronoamperometric curve at 1,000 s reach 33.2 and 11.2 mA cm^–2, respectively.     

Effect of Ag addition on the properties of Pd–Ag/TiO2 catalysts containing different TiO2 crystalline phases

Anatase and rutile TiO₂ were used for preparation of the TiO₂ supported Pd and Pd–Ag catalysts for selective hydrogenation of acetylene. It was found that Pd/TiO₂-anatase exhibited higher acetylene conversion and ethylene selectivity than rutile TiO₂ supported ones. However, addition of Ag to Pd/TiO₂-anatase catalyst resulted in lower ethylene selectivity while that of Pd/TiO₂-rutile increased. It is suggested that Ag addition suppressed the beneficial effect of the Ti³⁺ sites presented on the anatase TiO₂ during selective acetylene hydrogenation whereas without Ti³⁺, Ag promoted ethylene selectivity by blocking sites for over-hydrogenation of ethylene to ethane.     

Highly Active Carbon‐supported PdSn Catalysts for Formic Acid Electrooxidation

PdSn/C catalysts with different atomic ratios of Pd to Sn were synthesised by a NaBH₄ reduction method. Electrochemical tests show that the alloy catalysts exhibit significantly higher catalytic activity and stability for formic acid electrooxidation (FAEO) than the Pd/C catalyst prepared with the same method. XRD and TEM indicate that a particle‐size effect is not the main cause for the high performance. XPS confirms that Pd is modified by Sn through an electronic effect which can decrease the adsorption strength of poisonous intermediates on Pd and thus promote the FAEO greatly.     

The enhancement effect of MoOx on Pd/C catalyst for the electrooxidation of formic acid

Molybdenum oxide (MoO_x) was added to a Pd/C catalyst using a novel two-step procedure. The enhancement effect of MoO_x on Pd/C catalyst for the electrooxidation of formic acid was verified by electrochemical experiments. Compared to the Pd/C catalyst, the experimental results showed that the addition of MoO_x could significantly enhance the electrocatalytic performances for the electrooxidation of formic acid. Significant improvements in electrocatalytic activity and stability were primarily ascribed to the effect of MoO_x on the Pd catalyst. In addition to the large specific surface area, the hydrogen spillover effect is speculated to have accelerated the electrooxidation rate of formic acid in the direct pathway.     

Preparation, crystal structure, and photocatalytic activity of TiO2 films by chemical vapor deposition

Photocatalytic activities of TiO₂ films were experimentally studied. TiO₂ films with different crystal structures (amorphous, anatase, rutile) were prepared by a Low Pressure Metal Organic Chemical Vapor Deposition (LPMOCVD) at different reaction temperatures and also by a Sol-Gel method using TTIP (Titanium Tetra Iso-Pro-poxyde). The Effect of CVD preparation method, CVD reaction conditions, crystal structure and wave-length of UV light on the photocatalytic decomposition rate of methylene blue in aqueous solution were studied. First, the characteristics of CVD preparation of TiO₂ films, such as the CVD film growth rate, crystal structure and morphology of the grown TiO₂ films, were experimentally studied as a function of CVD reaction temperature. Secondly, photocatalytic activities of TiO₂ films were evaluated by using two types of photo-reactors. The results indicated that TiO₂ films prepared by CVD exhibit higher photocatalytic activity than a catalyst prepared by the Sol-Gel method. Among the CVD grown TiO₂ films, anatase and rutile showed high photocatalytic activities. However, amorphous TiO₂ films showed lower activities. The activity of the photocatalysts of anatase films was excellent under all types of UV-lamps. The activity of CVD-prepared anatase films was four to seven times higher than that of photocatalyst films prepared by the Sol-Gel method.     

A Novel Synthesis Route of Butyric Acid from Hydrogenation of Maleic Anhydride over Pd/TiO2 Catalysts

A highly efficient Pd/TiO₂ catalyst for the liquid phase hydrogenation of maleic anhydride has been prepared by sol–gel method, and super critical fluid of ethanol drying (SCFE) was applied. The catalyst exhibited excellent activity and high yield to butyric acid. The structural properties of TiO₂ supported Pd catalyst were investigated by BET, TEM, XRD, XPS and TPR techniques with the aim of finding a correlation between the structure parameters and the catalytic activities.     

Synthesis of phase- and shape-controlled TiO2 nanoparticles via hydrothermal process

The TiO₂ nanoparticles with anatase (5.7–12.7 nm), rutile (5.4–8.8 nm), mixed (4.4–8.6 nm) phase were individually prepared using the hydrothermal method. The structure and shape of the particles could be controlled by careful alterations of the hydrothermal conditions. Herein, the TiO₂ nanoparticles were successfully synthesized by employing Ti-isopropoxide as the titanium source into hydrochloric acid solution at mild conditions. The crystal structures such as anatase, rutile and mixed phase of TiO₂ nanoparticles were determined by means of concentration of hydrochloride. Especially, we observed that the rutile TiO₂ crystallites were grown into one-dimensional nanostructures, especially, nanowires, with increasing reaction time. The mechanism of the crystallization of the nanoparticles and the growth habit of TiO₂-rutile structure were discussed.     

Influence of flame conditions on the dispersion of Pd on the flame spray-derived Pd/TiO2 nanoparticles

The Pd/TiO₂ nanoparticles containing 5 wt.% Pd were synthesized by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) results, the average particle sizes of Pd/TiO₂ were increased from 9.7 to 24.6 nm with increasing the precursor concentration and the feed flow rate as well as reduction of the O₂ dispersing gas during FSP synthesis. Although the BET surface area and %anatase phase content were decreased with increasing Pd/TiO₂ particle size, %Pd dispersion as determined from the amounts of CO chemisorption were higher on the larger size FSP-made Pd/TiO₂ nanoparticles. It is suggested that the shorter residence time in flame and/or the lower combustion energy (enthalpy density) resulted in more coverage of Pd surface by the formation of Ti-O groups, rendering lower CO chemisorption ability of the smaller size Pd/TiO₂.     

Pd/Pt on Ti-containing Mixed Oxides as Dearomatization Catalysts: Physico-chemical Characterization and Activity

Pd/Pt supported on pure and doped TiO₂ (TiO₂–WO₃ and TiO₂–WO₃–SiO₂) were prepared and characterized by different techniques (XPS, TEM, XRD, H₂-TPR and TPD of ammonia). These catalysts were investigated in the hydrogenation of tetralin at 6.0 MPa, checking also their thio-tolerance by feeding increasing amounts of dibenzothiophene (DBT, 300 and 1000 wt ppm). The catalytic activity followed the order: Pd/Pt–TiO₂ > Pd/Pt–TiO₂–WO₃–SiO₂ > Pd/Pt–TiO₂–WO₃, evidencing a negative role of a second oxide inside TiO₂. The Pd/Pt–TiO₂ catalyst showed high activity regardless of reaction conditions (temperature, contact time, H₂/tetralin ratio) together with a good thio-tolerance up to 300 wt ppm of DBT.     

Preparation of highly dispersed palladium–phosphorus nanoparticles and its electrocatalytic performance for formic acid electrooxidation

Highly dispersed and ultrafine palladium–phosphorus (Pd–P) nanoparticles (NPs) are prepared with a novel phosphorus reduction method. The structural and electronic properties of Pd–P NPs are characterized using Fourier transform infrared (FT-IR), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The electrooxidation of formic acid on Pd–P NPs are investigated by using cyclic voltammetry, chronoamperometry and CO-stripping measurements. The physical characterizations indicate the doped P element can enhance the content of Pd⁰ species in Pd NPs, decrease the particle size and improve the dispersion of Pd–P NPs. The electrochemical measurements show the Pd–P NPs have a better catalytic performance for formic acid electrooxidation than Pd NPs.     

NiPd/TiO2催化剂的制备及其催化甲酸分解制氢性能

高效、清洁且无毒无害的催化剂是实现以甲酸(HCOOH)为化学储氢材料分解制氢的重点。本文采用水热法在453K的条件下制备TiO₂载体,再通过浸渍法向其中加入总量为0.1 mmol的NiCl₂.6H₂O和K₂PdCl₄金属溶液,将活性组分Ni、Pd负载到TiO₂载体上合成NiPd/TiO₂催化剂,并探究其对催化甲酸分解制氢的性能的影响。探究结果表明,在光照条件下,NiPd/TiO₂催化剂中,当金属Ni:Pd比例为2:8时,催化剂的反应转化频率(TOF)值最大,此时催化剂的 TOF 为3528 h_-1^,且该催化剂上甲酸分解的活化能(E_a)为53.9 kJ/mol。关键词：镍钯催化剂;甲酸;分解制氢;二氧化钛;光照中图分类号：TQ630 文献标识码： A 文章编号：     

Transformation of Crystal Phase of Micron-sized Rutile TiO2 and Investigation on its Sonocatalytic Activity

The partial transformation of crystal phase of micron-sized TiO₂ powder from rutile to anatase was realized utilizing microwave irradiation in hydrogen peroxide solution. Afterwards, the ultrasound of low power was used as an irradiation source to induce the transition crystal TiO₂ powder to perform the sonocatalytic activity through the degradation of azo fuchsine in aqueous solution. The results show that the sonocatalytic activity of the transition crystal TiO₂ powder is obviously higher than that of pure micron-sized rutile and anatase TiO₂ powders. The degradation ratio of azo fuchsine in the presence of the transition crystal TiO₂ powder attains nearly 80% within 80 min ultrasonic irradiation.