ABSTRACT
The hydrogen adsorption and hydrogen evolution at the M(111), (M = Ag, Au Cu, Pt, Pd, Ni & Co) surfaces of various transition metals in aqueous suspensions were studied computationally using the DFT methods. The hydrogens are adsorbed dissociatively on all surfaces except on Ag(111) and Au(111) surfaces. The results are validated by reported experimental and computational studies. Hydrogen atoms have large mobility on M(111) surfaces due to the small energy barriers for diffusion on the surface. The hydrogen evolution via the Tafel mechanism is considered at different surface coverage ratios of hydrogen atoms and is used as a descriptor for the hydrogen adsorption capacity on M(111) surfaces. All calculations are performed without considering how the hydrogen atoms are formed on the surface. The hydrogen adsorption energies decrease with the increase in the surface coverage of hydrogen atoms. The surface coverage for the H2 evolution depends on each M(111) surface. Among the considered M(111) surfaces, Au(111) has the least hydrogen adsorption capacity and Ni, Co and Pd have the highest. Furthermore, experiments proving that after the H2 evolution reaction (HER) on Au0-NPs, and Ag0-NPs surfaces some reducing capacity remains on the M0-NPs is presented.
ABSTRACT
The mechanism of reduction of AuCl4-/AuCl3OH- by BH4- was analyzed by density functional theory (DFT). The results point out that Auatoms0 are not intermediates in the process. The derived mechanism differs considerably from that reported for the analogous process involving the reduction of Ag(H2O)2+ by BH4-. Thus, though both processes follow the Creighton procedure, the detailed mechanism differs significantly. For Au, the agglomeration starts with AuH2-, whereas for Ag, it starts with (H2O)AgH. Stopped-flow measurements support the complicated mechanism.
ABSTRACT
The redox potentials of M(H2O)mn+/M0(atom) couples are often far too negative to enable the formation of M0(atom) by most reducing agents. Therefore, one has to reconsider the mechanism of formation of M0-NPs by the bottom-up procedure. A deep and detailed theoretical analysis of the reduction of Ag(H2O)2+ by BH4- points out that silver cations act mainly as catalysts of the reactions BH4- + 4H2O â B(OH)4- + 4H2. However, the transition states of the catalyzed process differ from those of the un-catalyzed process. The formation of (H2O)Ag-H, which is the starting stage for the formation of intermediates with Ag-Ag bonds, is only a side reaction in the process. Experimental evidence of the complexity of the process is presented, by stopped-flow; at least four processes are observed prior to the formation of Ag0-NPs. The spectra of these intermediates differ from those of Ag0atom and Ag2+aq. Though DFT calculations were performed only for silver cations, it is believed that analogous mechanisms are involved in the reductions of other cations.
ABSTRACT
The nature of H-atoms adsorbed on M0 -nanoparticles is of major importance in many catalyzed reduction processes. Using isotope labeling, we determined that hydrogen evolution from transient {(M0 -NP)-Hn }n- proceeds mainly via the Heyrovsky mechanism when n is large (i.e., the hydrogens behave as hydrides) but mainly via the Tafel mechanism when n is small (i.e., the hydrogens behave as atoms). Additionally, the relative contributions of the two mechanisms differ considerably for M=Au and Ag. The results are analogous to those recently reported for the M0 -NP-catalyzed de-halogenation processes.
