Preparation of the Pd/C catalysts: A molecular-level study of active site formation

This review summarizes the results of molecular-level studies on the mechanism of Pd/C catalyst formation from the PdCl₂ precursor. Two processes occur in acidic media during the contact of H₂PdCl₄ with carbon: (a) adsorption of palladium chloride to form surface complexes and (b) redox interaction between PdCl₂ and carbon with the formation of palladium metal particles. The ratio between these adsorbed palladium species depends on the conditions of adsorption and especially on the size of carbon support grains and the oxidative atmosphere. The observations are explained by the fact that carbon support exhibits electrochemical and ligand properties. X-ray diffraction, X-ray scattering, XPS, and high-resolution electron microscopy revealed that the nanostructure of carbon materials, in particular the extent of their three-dimensional ordering, is crucial for the ligand properties. The presence of two forms, metallic and ionic, of sorbed palladium determines the bimodal size distribution of the metal. After the reduction of ionic species, metal particles are “blocked” with support. The nature of the ionic forms of palladium (mostly (PdCl₂)_n) clusters chemically and epitaxially bound to the carbon surface suggests the mechanisms of the bimodal distribution of the supported metal particles on the surface and the methods for the control of the ratio between “blocked,” low-dispersed, and highly-dispersed particles in the catalyst. One of these methods is the use of palladium polynuclear hydroxo complexes (PHCs) with low oxidation potentials as starting compounds for catalysts preparation. The data on the PHC structure in a solution and its change upon the adsorption of PHC on the surface of the carbon material obtained by the¹⁷O,²³Na,¹³³Cs, and³⁵Cl NMR techniques are discussed. PHCs are shown to be a clew of the [Pd(OH)₂]_n polymeric filament, whose fractions are bound with alkali metal ions. When PHC is adsorbed on the surface of the carbon support and then dried, palladium oxide is formed from which highly dispersed metal particles are formed during reduction. The nature of alkali metal ions in PHC affects the activity of the Pd/C catalyst. An important role of the ligand, electrochemical, and lyophilic properties of carbon material during the formation of the species of the active catalyst component is discussed.