Silica Support Affects the Catalytic Hydrogen Evolution by Silver

Exploring the Adsorption and Reactions of Methyl Radicals on M(111) Surfaces (M=Cu, Ag, Au): A DFT Study

It was reported that adsorbed methyl radicals produce ethane with Ag0- and Au0-nanoparticles in aqueous media, whereas on Cu0-powders, the product is methanol. The source of these differences was explored computationally, using the DFT method. The results indicate that up to six radicals can be adsorbed on Ag(111) and Au(111), (top site), while only four can be adsorbed on Cu(111) (fcc site), each surface containing eight atoms. The diffusion of the radicals on the surface is very easy on silver and copper, as this is achieved with a very low barrier (0.06 eV and 0.15 eV for Ag(111) and Cu(111), respectively), while on Au(111), the barrier is higher, 0.51 eV. The formation of ethane via a reaction of two adsorbed radicals is thermodynamically plausible for all studied coverage ratios on the three surfaces, but kinetically, it is plausible at room temperature only on Au(111) and Ag(111) at full coverage. Ethane can also be produced on Au(111) and Ag(111) by a collision of a solvated radical and an adsorbed radical. This is a barrierless process. On Cu(111), the yield of such a process is CH4(aq), and an adsorbed CH2 which reacts further with a non-adsorbed water molecule to produce adsorbed CH3OH.

Effect of Sol-Gel Silica Matrices on the Chemical Properties of Adsorbed/Entrapped Compounds

The sol-gel process enables the preparation of silica-based matrices with tailored composition and properties that can be used in a variety of applications, including catalysis, controlled release, sensors, separation, etc. Commonly, it is assumed that silica matrices prepared via the sol-gel synthesis route are "inert" and, therefore, do not affect the properties of the substrate or the catalyst. This short review points out that porous silica affects the properties of adsorbed/entrapped species and, in some cases, takes an active part in the reactions. The charged matrix affects the diffusion of ions, thus affecting catalytic and adsorption processes. Furthermore, recent results point out that ≡Si-O. radicals are long-lived and participate in redox processes. Thus, clearly, porous silica is not an inert matrix as commonly considered.

Hydrogen adsorption on various transition metal (111) surfaces in water: a DFT forecast

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.

DFT Study of the BH4- Hydrolysis on Au(111) Surface

The mechanism of the catalytic hydrolysis of BH₄⁻ on Au(111) as studied by DFT is reported. The results are compared to the analogous process on Ag(111) that was recently reported. It is found that the borohydride species are adsorbed stronger on the Au⁰‐NP surface than on the Ag⁰‐NP surface. The electron affinity of the Au is larger than that of Ag. The results indicate that only two steps of hydrolysis are happening on the Au(111) surface and the reaction mechanism differs significantly from that on the Ag(111) surface. These remarkable results were experimentally verified. Upon hydrolysis, only three hydrogens of BH₄⁻ are transferred to the Au surface, not all four, and H₂ generation is enhanced in the presence of surface H atoms. Thus, it is proposed that the BH₄⁻ hydrolysis and reduction mechanisms catalyzed by M⁰‐NPs depend considerably on the nature of the metal.

Calculating the adsorption energy of a charged adsorbent in a periodic metallic system - the case of BH4- hydrolysis on the Ag(111) surface

The hydrolysis of borohydride on the Ag(111) surface is explored theoretically to obtain the in-depth reaction mechanism. Many heterogeneously catalyzed reactions like this involve the adsorption of charged species on metals. DFT calculations of charged systems, with periodic boundaries, face serious problems, concerning convergence and reliability of the results. To study the heterogeneously catalyzed reactions, a simple method to calculate the adsorption energy of charged systems in metallic periodic cells is proposed. In this method, a counter ion is placed at a non-interactive distance, in an aqueous medium, so that the calculated system is neutral. Bader analysis is used to validate that the calculated couple is charged correctly. Adsorption energies of F^-, Cl^-, Br^-, OH^-, BH₄^-, ClO₄^- and H^- ions on the Ag(111) surface in an aqueous medium were determined using Na⁺ and K⁺ as counter ions, to evaluate the performance of this method. The adsorption of the divalent ions S^2-, Se^2- and SO₄^2- on different surfaces was studied as well. Then this method was used to explore the hydrolysis of BH₄^- ions, which have a high theoretical hydrogen storage capacity, on the Ag(111) surface. The results point out that during the catalytic hydrolysis only one hydrogen atom from borohydride is transferred to the surface. In the first step one hydrogen atom from BH₄^- is transferred to the silver surface; this H atom reacts with a hydrogen atom that is released from an adsorbed water molecule; in addition, a hydrogen molecule is released in the second step (one atom from *BH₄^- and one from *H₂O). Thus, the mechanisms of the catalyzed reductions by BH₄^- and the hydrogen evolution reactions must be reconsidered.