Porous silica supported Ag core-Pd shell composite: Seed-mediated stepwise reduction and tunable dispersion for boosting catalytic hydrogen evolution

Abstract

For developing efficient and practical catalysts to achieve controllable hydrogen evolution from chemical hydrogen storage material, we herein demonstrate a seed-mediated stepwise reduction and tunable dispersion protocol to fabricate porous silica supported Ag core - Pd shell composites (Pd@Ag/SiO2) with serial metal molar ratios. By reasonable regulation for reduction sequence and growing conditions, the Pd@Ag bimetal layered nanostructure could be piloted to take orderly shape; via adjusting addition time and dosage for the reactants and additives, the resultant Pd@Ag nanoagents can be firmly attached on porous silica. The little by little assembled nanoagents with mean size of 10 nm on the silica microspheres construct a loose and porous secondary microstructure, leading to increment of specific surface area to some degrees. The generation of electron synergistic effect among the metals and carrier, verified by the XPS analysis, contributes to boosting the catalytic ability. The catalytic activity of the bimetal composites all exceed that of mono-metal composites, of which the bimetal composite Pd0.75@Ag0.25/SiO2 holds the top catalytic ability, even beyond that of Pd/SiO2 or Pd-NPs, followed by Pd0.5@Ag0.5/SiO2 and Pd0.25@Ag0.75/SiO2. The AB hydrolysis catalyzed by Pd0.75@Ag0.25/SiO2 at the given temperatures (293 to 313 K) is validated to be a first-order reaction, with apparent activation energy of 42.46 kJ·mol−1 and TOF value of 109.99 min−1. The catalyst Pd0.75@Ag0.25/SiO2 presents satisfactory stability, still retaining 83.2 % initial catalytic activity after five repeated use.