The Influence of the Support Nature and the Metal Precursor in the Activity of Pd‐based Catalysts for the Bromate Reduction Reaction

Abstract

Palladium catalysts supported on different materials (alumina, activated carbon and mixed oxide derived from hydrotalcite) and prepared with different metal precursors (nitrate, chloride and acetate) have been characterized and tested for the bromate reduction reaction. The catalytic behavior depends on the support nature and on the metallic precursor used for the catalyst preparation. Pd catalyst supported on a mixed oxide has a low activity due to the high affinity of the reconstructed support for the Br− formed, preventing the reactants to approximate the active Pd sites. Pd catalyst supported on activated carbon has a surface negative charge and a microporous structure, making difficult the interaction of the active sites with the reactants. The best results are obtained with the catalyst supported on alumina due to its physical‐chemical properties, i. e. mesoporosity, positive surface charge and reversible adsorption of reactants and products. These characteristics make easy bromate and H2 adsorption on the active sites and subsequent reaction, thus resulting in a better activity. The Pd precursor salt also influences the catalytic activity as it has an effect on the Pd nanocrystal size. The best results are obtained with the metal precursor that produces homogeneous and large Pd metallic crystallites.

A Review on the Catalytic Hydrogenation of Bromate in Water Phase

“…The patterns of the catalysts only showed new peaks related to the reduced metal after activation. These peaks were easily observed in catalysts with high metallic content [77] and low metal dispersion.…”

“…The different discussions proved that the activity of the metal sites was related to the metallic crystallite size, being the activity of the large crystals higher than that of small metallic crystals. It is worth mentioning that high crystallite size results in low metal dispersion and then in a large portion of non-accessible metal, for that reason an equilibrium between very active high crystallite size and good metal dispersion is necessary in order to design a commercial catalyst [77].…”

“…In general, most of the reviewed articles presented diverse catalytic performances using catalysts with similar metallic loading, but supported on different materials. These differences are mainly related to three properties of the support, i.e., the surface charge (related with the potential zeta or point of zero charge (PZC)), the porous structure, and the interaction of the metal with the support [77]. Most of the studies concluded that a positive surface charge is necessary for a proper adsorption of the bromate reactant on the catalyst surface, in this sense, a support with a high PZC is the most convenient.…”

“…These authors claimed [79] that the Pd dispersion and therefore, the nanoparticle size depend on the support, obtaining smaller Pd particles with an activated carbon support (2.9 nm) than with TiO 2 support (12 nm). Recently, it was shown that the metallic precursor used for the preparation of the catalyst also influences the final size of the metallic crystallite [77] and the metal distribution on the support surface (see Figure 6).…”

“…The XPS study carried out by Gao and collaborators demonstrated that the Pd content on the catalyst surface mainly depends on the method used for the catalyst preparation [72]. In general, most of the active catalysts exhibited Pd(0) as the main Pd-specie [77], although some Pd n+ species were also formed [78,81]. The oxidated species can be related to the formation of uncoordinated sites due to the size and morphology of the nanoparticles over the support and their presence was representative of strong metal-support interactions [74].…”