Carbon-supported Pd Research Articles

Bismuth−Palladium Heterometallic Carboxylate as a Single-Source Precursor for the Carbon-Supported Pd−Bi/C Catalysts

The heterometallic complex [Bi(2)Pd(2)(O(2)CCF(3))(10)(HO(2)CCF(3))(2)] (1) was obtained by the solid state reaction of Bi(III) trifluoroacetate/trifluoroacetic acid adduct with unsolvated trinuclear Pd(II) trifluoroacetate. The crystal structure of 1 consists of discrete tetranuclear molecules, in which two paddlewheel [BiPd(O(2)CCF(3))(4)] units are connected by two chelating-bridging trifluoroacetate ligands through bismuth ends. There are no metal-metal bonds in the tetrameric structure of 1, since both Bi...Pd (3.0843(4) A) and Bi...Bi (4.5074(4) A) distances are too long to be considered as bonding interactions. A study of the solution behavior revealed that not only the coordinated trifluoroacetic acid in 1 can be effectively replaced by other donor solvent molecules but also the tetranuclear complex can be cleaved in solution into discrete dinuclear Bi-Pd species. Complex 1 was used as precursor for the preparation of a bimetallic Pd-Bi carbon-supported catalyst. The preparation procedure included the modification of the carbon support to increase the number of oxygenated functions at its surface followed by grafting complex 1 via ligand exchange for surface carboxylates and activating thermally. The resulting catalyst, consisting of small supported metallic particles, was found to be more active than the reference materials prepared from multisource homometallic Pd and Bi precursors.

Hydrogenation of chiral nitrile on highly ordered mesoporous carbon-supported Pd catalysts

A highly ordered mesoporous carbon (C-SBA-15 and C-SBA-16) was synthesized by nanocasting method using its corresponding mesoporous silica (SBA-15 and SBA-16) as a template. The obtained porous carbons have high surface areas, large pore volume and a narrow pore size distribution. The N 2-adsorption data for C-SBA-15 have provided the BET area of 2029 m 2 g −1 and the total pore volume of 1.7 cm 3 g −1, and 1637 m 2 g −1 and 1.1 cm 3 g −1 for C-SBA-16, respectively. The palladium metal impregnated carbon catalysts were employed in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained by using 1 wt% Pd/C-SBA-16. It was investigated that the well-dispersed Pd metals in the confined mesopores are efficient for the hydrogenation of cyano groups to amine.

Kinetics of 4-Chlorophenol Hydrodechlorination with Alumina and Activated Carbon-Supported Pd and Rh Catalysts

The hydrodechlorination of 4-chlorophenol promoted by different commercial catalysts based on Pd and Rh on γ-alumina and activated carbon has been investigated in a continuous stirred basket reactor (20−40 °C and 1 bar) in order to analyze the kinetics of the reaction. External and internal mass transfer effects were found to be negligible from the values of the Carberry and Weisz−Prater numbers. The reaction rate showed a first-order dependence with respect to 4-chlorophenol. All catalysts were effective in the removal of 4-chlorophenol, with phenol, cyclohexanone, and cyclohexanol identified as reaction products, although the last one was only detected with the Rh/AC catalyst. The highest apparent kinetic constant values correspond to the hydrogenation of 4-chlorophenol to phenol. These values were in the range 146−478 L/kgcat·h for Pd/Al2O3, 179−713 L/kgcat·h for Pd/AC, and 44−172 L/kgcat·h for Rh/AC. The values of the apparent activation energy for 4-chlorophenol disappearance ranged within 47−58 kJ/mol.

Oxygen Reduction Reaction on Carbon Supported Pt and Pd in Alkaline Solutions

Carbon-supported Pt and Pd ( metal) were employed as oxygen reduction catalysts in NaOH solutions. Results from X-ray diffraction analysis show that the metals are in the form of Pt face-centered cubic (fcc) and Pd (fcc) crystal only. The metal particles were observed to distribute uniformly on carbon supports, and the average particle size of the Pd and the Pt was 3.1 and , respectively, from the transmission electron microscopy image analysis. The catalyst showed high activity toward the oxygen reduction reaction (ORR) both at room and elevated temperatures (up to ) compared with the . The activation energies for the ORR on the are slightly lower than that on the (39 vs at the overpotential of ). The four-electron path is dominant on both the and the . The yields on both catalysts are analyzed in detail. As a cathode fuel cell catalyst, the catalyst is less sensitive to alcohol contamination as compared to . The challenges for using as an ORR catalyst in alkaline solutions are also discussed in this paper.

Controlled synthesis and characterization of carbon-supported Pd4Co nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells

Carbon-supported Pd4Co nanoalloy samples (denoted as Pd4Co/C) have been synthesized by a modified polyol reduction process, involving a mixture of ethylene glycol (a polyol) and a small amount of sodium borohydride as reducing agents, and evaluated as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. The synthesis process offers Pd4Co nanoparticles with an average particle size of 4.6 nm and a narrow size distribution, which increases to 7.3 and 11.0 nm on annealing, respectively, at 350 and 500 °C in 10% H2–90% Ar. Although the as-prepared Pd4Co/C sample exhibits instability during ORR, annealing at 350 and 500 °C enhances the stability significantly due to an increasing degree of alloying between Pd and Co and increasing particle size. While both Pd/C and Pt/C exhibit a drastic decline in catalytic activity on annealing at 350 and 500 °C due to a significant increase in particle size, the Pd4Co/C sample maintains high catalytic activity even after annealing at 350 °C due to an increase in the degree of alloying. The Pd4Co/C catalyst offers an added advantage of excellent tolerance to methanol poisoning and lower cost compared to the conventional Pt/C catalyst.

Enhanced catalytic dehydrogenation of LiBH4 by carbon-supported Pd nanoparticles

The investigation of thermally induced dehydrogenation of LiBH(4) catalyzed by carbon-supported Pd nanoparticles (Pd/C) reveals that LiBH(4) doped with the Pd/C catalysts shows superior dehydrogenation performance, compared to dehydrogenation in the absence of catalyst, even at very low catalyst content. We have found that smaller Pd nanoparticles result in greater enhanced catalytic dehydrogenation of LiBH(4) than do larger Pd nanoparticles. For doping with 10 wt.% catalyst of an average Pd particle diameter of ca. 5.3 nm, thermal dehydrogenation of LiBH(4) is found to start at ca. 280 degrees C with a total weight loss of 15.8 wt.% for the full temperature range. With increased loading of catalyst within a LiBH(4) sample, the onset dehydrogenation temperature and the two main desorption peaks from LiBH(4) are found to decrease while the hydrogen release amount is found to increase. All the hydrogen of ca. 18.5 wt.% can be released from LiBH(4) below 580 degrees C at a catalyst doped level of 50 wt.%. Importantly, we observed a reversible hydrogenation/dehydrogenation with a capacity of approximately 4.3 wt.%. Pd nanoparticles are found to play an important role in the catalyzed dehydrogenation/rehydrogenation of LiBH(4).

Synthesis of carbon-supported Pd-Sn catalyst by ultrasonic irradiation for oxygen reduction reaction

Carbon-supported Pd-Sn (Pd-Sn/C) catalyst was prepared under ultrasonic irradiation, and its electrocatalytic activity for oxygen reduction reaction (ORR) was evaluated in 0.5 M KOH. TEM images showed that the prepared Pd-Sn/C catalyst particles are smaller in average size than carbon-supported Pd (Pd/C) catalyst particles. XRD and XPS results indicated the small particle size and the electronic interaction between Pd and Sn for the Pd-Sn/C catalyst. The Pd-Sn/C catalyst has a higher ORR activity than the Pd/C catalyst in alkaline media. In addition, the Pd-Sn/C catalyst showed a lower Tafel slope and a larger number of electrons transferred for ORR, compared with those of the Pd/C catalyst. These results indicate that Sn influences both the kinetics and the mechanism of ORR. Based on these results, the Pd-Sn/C catalyst prepared by using ultrasonic irradiation can be expected as a promising ORR catalyst in alkaline media.

Carbon-supported PdM (M=Au and Sn) nanocatalysts for the electrooxidation of ethanol in high pH media

Carbon-supported Pd 4 Au- and Pd 2.5 Sn-alloyed nanoparticles were prepared by a chemical reduction method, and characterized by a wide array of experimental techniques including mass spectrometry, transmission electron microscopy, and X-ray diffraction spectroscopy. Ethanol electrooxidation on the as-synthesized catalysts and commercial Pt/C was then investigated and compared in alkaline media by cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy studies at room temperature. Voltammetric and chronoamperometric measurements showed higher current density and longer term stability in ethanol oxidation with the palladium alloy nanocatalysts than with the commercial one. Electrochemical impedance spectroscopy and Tafel plots were employed to examine the charge-transfer kinetics of ethanol electrooxidation. The results suggest that whereas the reaction kinetics might be somewhat more sluggish on the Pd-based alloy catalysts than on commercial Pt/C, the former appeared to have a higher tolerance to surface poisoning. Overall, the Pd-based alloy catalysts represent promising candidates for the electrocatalytic oxidation of ethanol, and Pd 4 Au/C displays the best catalytic activity among the series for the ethanol oxidation in alkaline media.

Performance of Pd/C Cathode Electro-Catalysts for the ORR in Sulfuric Acid Containing Small Organic Molecules

We studied the performance of carbon-supported Pd cathode electrocatalysts in O2-saturated acid solution containing ethanol. The Pd/C electrocatalysts showed an important selectivity toward the Oxygen Reduction Reaction (ORR), and a very high degree of tolerance to poisoning due to the presence of the alcohol. A negative shift in onset potential for the ORR of only 10 mV for a 20% Pd/C electrocatalyst in 0.5M H2SO4 + 0.125M C2H5OH electrolyte was measured, whereas the shift for a 20% Pt/C electrocatalyst was of 398 mV in the same solution. Similar high tolerances were established at increasing C2H5OH concentrations for Pd/C.

Hydrogen uptake of DPB getter pellets

The physical and chemical properties of 1,4-diphenylbutadiyne (DPB) blended with carbon-supported Pd (DPB–Pd/C) in the form of pellets during hydrogenation were investigated. A thermogravimetric analyzer (TGA) was employed to measure the kinetics of the hydrogen uptake by the DPB getter pellets. The kinetics obtained were then used to develop a semi-empirical model, based on gas diffusion into solids, to predict the performance of the getter pellets under various conditions. The accuracy of the prediction model was established by comparing the prediction models with independent experimental data on hydrogen pressure buildup in sealed systems containing DPB getter pellets and subjected to known rates of hydrogen input. The volatility of the hydrogenated DPB products and its effects on the hydrogen uptake kinetics were also analyzed.

One-step synthesis of carbon-supported Pd–Pt alloy electrocatalysts for methanol tolerant oxygen reduction

A facile, one-step reduction route was developed to synthesize Pd-rich carbon-supported Pd–Pt alloy electrocatalysts of different Pd/Pt atomic ratios. As-prepared Pd–Pt/C catalysts exhibit a single phase fcc structure and an expansion lattice parameter. Comparison of the oxygen reduction reaction (ORR) on the Pd–Pt/C alloy catalysts indicates that the Pd 3Pt 1/C bimetallic catalyst exhibits the highest ORR activity among all the Pd–Pt alloy catalysts and shows a comparative ORR activity with the commercial Pt/C catalyst. Moreover, all the Pd–Pt alloy catalysts exhibited much higher methanol tolerance during the ORR than the commercial Pt/C catalyst. High methanol tolerance of the Pd–Pt alloy catalysts could be attributed to the weak adsorption of methanol induced by the composition effect, to the presence of Pd atoms and to the formation of Pd-based alloys.

Facile preparation of carbon-supported Pd nanoparticles for electrocatalytic oxidation of formic acid

A low-temperature approach via the complexing of PdCl 2 with EDTA followed by NaBH 4 reduction has been used to prepare Vulcan XC-72 carbon-supported Pd nanoparticles (Pd/C). The mean particle size of the Pd/C catalysts is found to increase from 3.3 to 9.2 nm with heat-treated temperature. TEM images demonstrated that the Pd nanoparticles are well dispersed on the support, with a relatively narrow particle size distribution. A correlation between the electrocatalytic activity of formic acid oxidation and the particle size of the Pd/C catalysts indicates that the highest activity of formic acid oxidation is found with a Pd mean particle size of ca. 4.7 nm. The preparation method used here is cost-effective and should be easily scaled for industrial production.

Bimetallic Pd–Au catalysts for benzaldehyde hydrogenation: Effects of preparation and of sulfur poisoning

Abstract The sulfur poisoning of carbon-supported bimetallic Pd–Au systems was investigated. Catalytic activity results for benzaldehyde hydrogenation clearly show that gold addition improves catalyst resistance to sulfur poisoning with respect to the monometallic Pd-based sample. Samples were characterized by pulse-flow CO chemisorption and XRD in order to correlate the structural properties with the resistance to sulfur poisoning. Gold addition prevents the progressive transformation of Pd in Pd4S in the presence of sulfur compounds by hindering the availability of ensembles of four Pd atoms in the correct geometrical configuration.

Bimetallic Pd–Cu Oxygen Reduction Electrocatalysts

A series of Vulcan carbon-supported Pd–Cu catalysts with various molar ratios of Pd to Cu was prepared by co-impregnation followed by a reduction in a hydrogen atmosphere at three different temperatures. The degree of alloying between the two metals, alloy composition, and particle size and size distribution were characterized by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The electrocatalytic activity for the oxygen reduction reaction (ORR) for these various compositions was determined using the thin-film rotating disk electrode technique. Our study reveals that the Pd–Cu bimetallic electrocatalysts, with a suitable degree of alloying, offer a greatly enhanced ORR activity compared to the Pd monometallic electrocatalyst. The best electrocatalytic activities were observed for the bimetallic catalysts that showed alloy nanoparticles with a Pd–Cu molar ratio of approximately 1:1.

Low temperature preparation of carbon-supported PdCo alloy electrocatalysts for methanol-tolerant oxygen reduction reaction

Abstract Carbon-supported Pd Co alloy electrocatalysts of different Pd/Co atomic ratios were simply prepared in an aqueous solution at room temperature with NH 4 F as a complexing agent and H 3 BO 3 as a buffer, followed by NaBH 4 reduction. As-prepared Pd Co bimetallic nanoparticles show a single-phase face-centered-cubic (fcc) disordered structure, and the mean particle size is found to decrease with increase in Co content. TEM images demonstrated that the as-prepared Pd Co alloy nanoparticles are well dispersed on the surface of the carbon support with a small particle size and a relatively narrow particle size distribution. For example, the average particle size of a Pd 2 Co 1 /C catalyst is ca. 3.0 nm, which is much smaller than that of the Pd Co/C bimetallic nanoparticles reported by others. An activity evaluation of the oxygen reduction reaction (ORR) on as-prepared Pd Co/C catalysts with a rotating disk electrode (RDE) technique indicated that the maximum ORR mass activity was observed for a Pd:Co atomic ratio of 4:1, but the highest specific activity was found on a Pd:Co atomic ratio of 2:1. Kinetic analysis reveals that the ORR on Pd Co/C catalysts follows a four-electron process leading to water. Moreover, the Pd Co/C catalyst exhibited much higher methanol tolerance during the ORR than the Pt/C catalyst, assessing that it may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell (DMFC).

Carbon-supported Pd nanocatalyst modified by non-metal phosphorus for the oxygen reduction reaction

A carbon-supported palladium catalyst modified by non-metal phosphorus (PdP/C) has been developed as an oxygen reduction catalyst for direct methanol fuel cells. The PdP/C catalyst was prepared by the sodium hypophosphite reduction method. The as-prepared Pd nanoparticles have a narrow size distribution with an average diameter of 2 nm. Energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) results indicate that P enters into the crystal lattice of Pd and forms an alloy. The PdP/C catalyst has an oxygen reduction reaction (ORR) activity comparable to the commercial Pt/C catalyst and a higher activity than the Pd/C catalyst synthesized by the conventional NaBH 4 reduction method. Its high catalytic activity can be attributed to its small size, lower relative crystallinity, and the formation of PdP alloy.

Hydrodechlorination of 4-chlorophenol in water with formic acid using a Pd/activated carbon catalyst

This work reports on the feasibility of hydrodechlorination as a treatment technique for chlorophenols-bearing wastewaters using formic acid as a hydrogen source. 4-Chlorophenol (4-CPhOH) has been used as target compound and the experiments were carried out in batch and continuous mode with a commercial activated carbon-supported Pd (0.5 wt.%) catalyst. The variables studied in the batch runs were HCOOH/4-CPhOH molar ratio (10–1000), temperature (25–75 °C) and catalyst concentration (250–1000 mg/L). The continuous experiments were performed in a fixed bed reactor where aqueous solutions of formic acid and 4-CPhOH with molar ratios between 50 and 100 were continuously fed to the reactor, at different space–time values in the range of 10.7–42.8 kg cat h/mol. Reaction temperatures from 35 to 100 °C were tested and the pressure was fixed at 2.5 bar. Conversion values above 99% for 4-CPhOH were obtained in batch experiments, but using a HCOOH/4-CPhOH molar ratio as high as 500. Moreover, most of the phenol produced was adsorbed on the catalyst. Continuous runs were performed to evaluate the efficiency of the catalyst under lower HCOOH/4-CPhOH ratios and to explore the possibility of converting phenol to more hydrogenated products. The results indicated that the HCOOH/4-CPhOH molar ratios needed were an order of magnitude lower than those required in batch runs to achieve conversions of 4-CPhOH close to 95%. Besides, phenol was not the only reaction product formed, since a more hydrogenated product such as cyclohexanone was detected in the effluent, which indicates additional hydrogenation of phenol in contrast to the behaviour observed in batch experiments. A loss of activity was observed in the continuous runs after 20–30 h on stream.

A facile preparation of carbon-supported Pd nanoparticles for electrocatalytic oxidation of formic acid

A low temperature approach via the complexing of PdCl 2 with EDTA followed by NaBH 4 reduction has been used to prepare Vulcan XC-72 carbon-supported Pd nanoparticles (Pd/C). The mean particle size of the Pd/C catalysts is found to increase from 3.3 to 9.2 nm with heat-treated temperature. TEM images demonstrated that the Pd nanoparticles are well dispersed on the support with a relatively narrow particle size distribution. A correlation between the electrocatalytic activity of formic acid oxidation and particle size of the Pd/C catalysts indicates that the highest activity of formic acid oxidation is found with a Pd mean particle size of ca. 4.7 nm. The preparation method used here is cost-effective and should be easily scaled for industrial production.

Tuning the electrocatalytic activity and durability of low cost Pd 70Co 30 nanoalloy for oxygen reduction reaction in fuel cells

The factors controlling the electrocatalytic activity for the oxygen reduction reaction (ORR) in fuel cells have been tuned and investigated systematically with the low cost carbon-supported Pd 70Co 30 nanoalloy. The catalytic activity decreases with increasing annealing temperature due to an increasing degree of alloying (Co content in the Pd lattice) and crystallite size. With a controlled crystallite size of ∼8 nm, the activity is found to decrease with increasing degree of alloying from 18 to 30 at.% Co. However, the catalyst durability increases considerably with annealing temperature due to the alloying of Pd with Co and an increase in crystallite size.

Particle Size Effects in Pd-Catalyzed Electrooxidation of Formic Acid

Small carbon-supported Pd nanoparticles with controllable size (2.7−9.0 nm) were prepared by a solution chemistry method, characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopies, and evaluated as anode catalysts for the electrooxidation of formic acid at room temperature. Measurements of catalytic activity by electrochemical methods (cyclic voltammetry and chronoamperometry) revealed a strong potential-dependent particle size effect. Classical volcano plots of catalytic activity vs particle size were obtained from which the optimal Pd particle size for the respective potential region was determined.