An Atomic‐Scale View of Palladium Alloys and their Ability to Dissociate Molecular Hydrogen

Abstract

Palladium and its alloys play a central role in a wide variety of industrially important applications such as hydrogen reactions, separations, storage devices, and fuel‐cell components. Alloy compositions are complex and often heterogeneous at the atomic‐scale and the exact mechanisms by which many of these processes operate have yet to be discovered. Herein, scanning tunneling microscopy (STM) has been used to investigate the atomic‐scale structure of Pd–Au and Pd–Cu bimetallics created by depositing Pd on both Au(111) and Cu(111) single crystals at a variety of surface temperatures. We demonstrated that individual, isolated Pd atoms in an inert Cu matrix are active for the dissociation of hydrogen and subsequent spillover onto Cu sites. Our results indicated that H spillover was facile on Pd–Cu at 420 K but that no H was found under the same H2 flux on a Pd–Au sample with identical atomic composition and geometry. In the case of Au, significant H uptake was only observed when larger ensembles of Pd were present in the form of nanoparticles. We report experimental evidence for hydrogen’s ability to reverse the tendency of Pd to segregate into the Au surface at catalytically relevant temperatures and our STM images reveal a novel H‐induced striped structure in which Pd atoms aggregated on top of the surface in regularly spaced rows. These results demonstrate the powerful influence an inert substrate has on the catalytic activity of Pd atoms supported in or on its surface and reveal how the atomic‐scale geometry of Pd–Au alloys is greatly affected by the presence of hydrogen.

Direct synthesis of hydrogen peroxide using concentrated H2 and O2 mixtures in a wall-coated microchannel – kinetic study

“…As established by the kinetic model, direct formation of hydrogen peroxide from hydrogen and oxygen seems to involve hydrogen adsorption, dissociation, but not spill-over of H-species over the AuPd surface. It is most likely that reaction occurs between non-dissociated oxygen and hydrogen species adsorbed pairwise as suggested by several studies on hydrogen adsorption over AuPd surfaces [53,54] .…”

“…Pd surface H 2 spill-over is impossible [53] . The formation of Pd ensembles is crucial for dissociative adsorption of hydrogen, considering that H ad is the most stable on Pd 3 threefold hollow sites, less stable on Pd 2 bridge sites and least stable on Pd monomers [54] .…”

Determination of mass transfer resistances of fast reactions in three‐phase mechanically agitated slurry reactors

“…12,19,20 1. Dissociative and molecular chemisorption onto the catalyst surface A. Hydrogen dissociative chemisorption [21][22][23][24][25] At the equilibrium,…”

Carbon Monoxide Poisoning Resistance and Structural Stability of Single Atom Alloys

“…In several cases it has been shown that by doping the coinage metals with PGMs at very low molar fractions, such that these more reactive metals disperse as isolated single atoms in the surface layer of the host material, the activity of the coinage metal surface can be dramatically enhanced whilst retaining excellent reaction selectivity [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. These single atom alloys (SAAs) of Sykes and co-workers exhibit tolerance to CO [18] and have been employed to catalyse hydrogenation [16][17][18][19][20], dehydrogenation [22,23], C-H activation and hydrosilylation [25] reactions with high activity and selectivity, as extended model surfaces and/or as real catalyst nanoparticles.…”