Size-Dependent Subnanometer Pd Cluster (Pd₄, Pd₆, and Pd₁₇) Water Oxidation Electrocatalysis

Abstract

Water oxidation is a key catalytic step for electrical fuel generation. Recently, significant progress has been made in synthesizing electrocatalytic materials with reduced overpotentials and increased turnover rates, both key parameters enabling commercial use in electrolysis or solar to fuels applications. The complexity of both the catalytic materials and the water oxidation reaction makes understanding the catalytic site critical to improving the process. Here we study water oxidation in alkaline conditions using size-selected clusters of Pd to probe the relationship between cluster size and the water oxidation reaction. We find that Pd4 shows no reaction, while Pd6 and Pd17 deposited clusters are among the most active (in terms of turnover rate per Pd atom) catalysts known. Theoretical calculations suggest that this striking difference may be a demonstration that bridging Pd-Pd sites (which are only present in three-dimensional clusters) are active for the oxygen evolution reaction in Pd6O6. The ability to experimentally synthesize size-specific clusters allows direct comparison to this theory. The support electrode for these investigations is ultrananocrystalline diamond (UNCD). This material is thin enough to be electrically conducting and is chemically/electrochemically very stable. Even under the harsh experimental conditions (basic, high potential) typically employed for water oxidation catalysts, UNCD demonstrates a very wide potential electrochemical working window and shows only minor evidence of reaction. The system (soft-landed Pd4, Pd6, or Pd17 clusters on a UNCD Si-coated electrode) shows stable electrochemical potentials over several cycles, and synchrotron studies of the electrodes show no evidence for evolution or dissolution of either the electrode material or the clusters.

Effect of the size-selective silver clusters on lithium peroxide morphology in lithium–oxygen batteries

“…Such surprising subnanometre size effects have been found recently for other types of electrocatalytic reactions 23,42 as well as heterogeneous catalytic reactions [20][21][22] . However, this is the first time that sizeselected subnanometre clusters have been studied in Li-O 2 cells.…”

“…Size-selected Ag n (n ¼ 3, 9, 15) clusters were soft landed on the alumina-coated GC under identical deposition and coverage conditions (corresponding to 12.35% atomic monolayer equivalent, ML, of an ideal smooth surface). The principle deposition technique of size-selected cluster is described elsewhere [20][21][22][23] (also see Methods).…”

“…20) and selective oxidation of propylene (Ag 3 , Au [5][6][7][8] ) (refs 21,22). In addition, our recent work has shown that Pd 6 and Pd 17 clusters are stable and highly active for electrocatalytic water splitting, while Pd 4 clusters show no activity at all 23 . Thus, it is of interest to use cathode materials on the basis of supported sizeselected metal clusters in Li-O 2 cells to determine whether surface species at the subnanometre size level play a role in the discharge and charge process.…”

Catalytic oxidation of cyclohexane by size-selected palladium clusters pinned on graphite

“…This, somewhat counter-intuitive behaviour, may be caused by competitive absorption of the reactants and desorption of the products; however, the influence of a potentially different oxidation state of the Pd clusters, for various cluster sizes and reactant ratios, cannot be excluded, nor a combined effect. Recent publications show that in some cases partly or fully oxidised small Pd clusters may possess a very stable composition and assumed to be the catalytic active site in CO oxidation or water splitting, [57,58] while they can also readily undergo oxidation and reduction for example in a CO/O 2 environment. [59] The apparent role of undercoordinated Pd perimeter atoms in proximity to the graphite support might either arise from a distinctive electronic structure [60] associated with the local Pd-graphite interaction or to a preferred binding site of cyclohexane at the Pdgraphite border, or of course both.…”

Ligand/cluster/support catalytic complexes in heterogeneous ultrananocatalysis: NO oxidation on Ag3/MgO(100)

“…In this perspective, heterogeneous catalysis by subnanometer metal clusters (or heterogeneous ultrananocatalysis) is an emerging subject [2][3][4] including subnanometer (or ultra-nano) metal oxide clusters. 5 Here, attention is focused on the catalytic activity of very small aggregates (up to 10-20 metal atoms) deposited on a stabilizing substrate, often (but not exclusively) a metal oxide support as will be considered in the present work. A number of concomitant reasons justify the interest of both fundamental and applied research in this field, among which is the fact that larger nanoparticles have been explored extensively so that a trial-and-error optimization approach has been exhausted (whereas ultrananoclusters may show novel energetics and thus a different reaction landscape), and there is an atom-economic use of chemical elements, especially precious metals.…”