Pd/C Catalysts Prepared by Controlled Adsorption of Pd(II) Species on SX PLUS Carbon in the Aqueous Phase

Abstract

Pd/C catalysts were prepared on SX PLUS activated carbon by a controlled adsorption method in which the interactions between the Pd precursor and the carbonaceous support in aqueous solution were optimized. This strategy involved (i) determining the surface charge state of the carbon support as a function of the pH, (ii) establishing adsorption curves for Pd on C to identify the pH windows where maximum adsorption occurs, and (iii) determining the distribution of Pd(II) species in solution from the stability constants of the various compounds involving the ligands H2O, OH-, and/or Cl-. It was found that the charge of metal species present in solution, which depends on the pH, influences markedly the adsorption of Pd on carbon. The actual catalysts were then prepared by keeping the pH of the impregnating Pd solution within the optimal adsorption range throughout all synthetic steps, followed by chemical or thermal activation. This optimized preparation method led to highly efficient catalysts for the selective oxidation of glyoxal into glyoxalic acid, with activities superior to that of comparable monometallic materials (prepared by other methods or commercial manufacturing) and equivalent to that of bimetallic (promoted) catalysts. The Pd/C catalysts were characterized by XPS, SEM, TEM, and CO chemisorption, and it was found that the catalytic activity could be correlated to high Pd dispersion in the form of very small particles homogeneously dispersed on the support and to an important degree of Pd reduction.

Molybdenum Oxoanions as Dispersing Agents in the Preparation of Pd/C Catalysts for the Selective Oxidation of Glyoxal

“…The results obtained were by far better than with reference monometallic Pd/C catalysts (Table 1), and compared very well with bimetallic Bi-Pd/C catalysts, whose performance were reported earlier [44][45][46]. The results obtained here are similar to Pd/C catalysts prepared by the optimized adsorption method [28], but obviously not as good (at t = 20 h) as the Au-promoted optimized Pd/C systems [29,30]. A Pd-free catalyst prepared following the same procedure, but introducing only the Mo precursor, was checked to be completely inactive (Y AGLY \ 1%).…”

“…By contrast with previously reported results with Bi-Pd/C catalysts, where it had been identified at 24 h [45], the maximum yield in glyoxalic acid is here not yet attained after 48 h, but it is still in constant augmentation. The yield in glyoxalic acid obtained (after 48 h) is thus superior (or equal) to all former results obtained so far with the best catalysts for this reaction [28][29][30][44][45][46][47][48]. This is particularly obvious when comparing the yields in glyoxalic acid normalized with respect to the amount of active metal (Pd) [28][29][30][44][45][46][47][48], Table 2.…”

“…We have shown that the so-called 'Strong Electrostatic Absorption' (SEA) method developed by Regalbuto et al [27] has its limits for depositing Pd(II) on carbon, because the large pH-window for maximum surface absorption corresponds to the existence of neutral Pd species in solution, thus precluding an electrostatic mechanism from taking place [28][29][30]. The goal of the present study is to take advantage of the charged nature of Mo POMs to interact electrostatically with the surface and serve as intermediate for the adsorption of Pd on carbon, for catalytic application in glyoxal selective oxidation.…”

Influence of Metal Deposition and Activation Method on the Structure and Performance of Carbon Nanotube Supported Palladium Catalysts

“…Various metal deposition methods such as impregnation, deposition precipitation, deposition reduction, ion adsorption, and colloidal routes have been studied. The deposited metal precursor is commonly reduced in the gas phase, and this typically yields well‐distributed Pd nanoparticles .…”

Eco-environmental synthesis of vinyl benzoate through transesterification catalyzed by Pd/C catalyst

“…The preparation of supported palladium catalyst is widely reported [23][24][25][26][27], most of which are concerned with the preparation of Pd/C catalysts. In this work, the catalyst with a content of 5 wt % Pd-loading as reported in the literature [24] was prepared using H 2 PdCl 4 solution for the synthesis of vinyl benzoate. The activated carbon with the particle size of 350 mesh was used as a support.…”