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Abstract: 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. [Copyright &y& Elsevier]

Abstract: In this study, we present a simple process to obtain highly dispersed palladium nanoparticles on Vulcan XC-72R carbon support without any protective agent. To obtain high metal loading Pd/C catalyst without any surfactant, we modified the polyol process by employing NH3 species as a mediation to control the reaction pathway to avoid the precipitation of Pd(OH)2, and hence the agglomeration of Pd nanoparticles. The obtained Pd/C sample was characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques. The results show that highly dispersed Pd/C catalyst with an average diameter of 3.0nm could be obtained in this novel process. The activity of formic acid oxidation on this Pd/C catalyst was examined via cyclic voltammetry technique and it is found that the catalytic activity is greatly enhanced due to the reduced particle size and the improved dispersion of palladium nanoparticles on the carbon surface. [Copyright &y& Elsevier]

A series of 40 wt.% Pt/C electrocatalysts were prepared by a modified polyol process and an improved aqueous impregnation method with different impregnation time. The characterization results of TEM and XRD identically reveal that the size and distribution of Pt nanoparticles on carbon are controllable by modifying impregnation time in both routes. High dispersion of Pt nanoparticles on carbon is achieved by both methods with 15 min impregnation, while aggregation of Pt nanoparticles takes place with prolonging the impregnation time to 36 h, especially in the aqueous impregnation procedure. UV–vis spectroscopy measurements verified that the redox reaction between PtCl62- and formaldehyde could take place at a slow rate under ambient conditions via a two-step reaction path, where PtCl42- serves as an intermediate. On the other hand, PtCl62- anion can be directly reduced to Pt0 above 333 K in the modified polyol process through an autocatalytic pathway. The short-time-impregnated 40 wt.% Pt/C as cathode electrocatalysts in direct methanol fuel cell performs better than that of long-time-impregnated electrocatalysts. Experimental evidence provides clues for fundamental understanding of elementary steps of the redox reactions, which helps guiding the design and preparation of highly dispersed Pt catalyst with high metal loadings for fuel cells. [Copyright &y& Elsevier]