Selective Growth of Al2O3 on Size-Selected Platinum Clusters by Atomic Layer Deposition

Abstract

In heterogeneous catalysis, atomic layer deposition (ALD) has been developed as a tool to stabilize and reduce carbon deposition on supported nanoparticles. Here, we discuss use of high vacuum ALD to deposit alumina films on size-selected, sub-nanometer Pt/SiO2 model catalysts. Mass-selected Pt24 clusters were deposited on oxidized Si(100), to form model Pt24/SiO2 catalysts with particles shown to be just under 1 nm, with multilayer three dimensional structure. Alternating exposures to trimethylaluminum and water vapor in an ultra-high vacuum chamber were used to grow alumina on the samples without exposing them to air. The samples were probed in situ using X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (ISS), and CO temperature-programmed desorption (TPD). Additional samples were prepared for ex situ experiments using grazing incidence small angle x-ray scattering spectroscopy (GISAXS). Alumina growth is found to initiate at least 60 times more efficiently at the Pt24 cluster sites, compared to bare SiO2/Si, with a single ALD cycle depositing a full alumina layer on top of the clusters, with substantial additional alumina growth initiating on SiO2 sites surrounding the clusters. As a result, the clusters were completely passivated, with no exposed Pt binding sites.

The interaction of size-selected Ru3 clusters with RF-deposited TiO2: probing Ru–CO binding sites with CO-temperature programmed desorption

“…CS depositions were performed by a laser ablation cluster source which has been used and described in several previous publications. 58,60,62 For cluster source-deposition of bare Ru 3 clusters, the source was a 99.9% pure target of Ru.…”

“…CO-TPD is used to probe the available CO adsorption sites on the metal clusters, as well as for probing the removal of ligands with heating in the case of CO-stabilized clusters. 19,20,[56][57][58][59][60] XPS is used to analyse the composition of the surface and to determine concentration depth proles.…”

Alloying with Sn Suppresses Sintering of Size-Selected Subnano Pt Clusters on SiO2 with and without Adsorbates

“…To probe this, we performed a series of 13 CO TPD experiments. CO is not just a relevant carbonaceous adsorbate, but also is a useful TPD probe that binds strongly to Pt clusters, 10,34 and reports on the number and energetics of reactant- Multiple TPD runs, each with 180 K CO exposure, were run to assess changes in the CO binding site distribution driven by the TPD process (Fig. 3a).…”

“…There are many ways that have been considered for sintering prevention. 3,[7][8][9][10] One way is by doping or co-alloying of precious metals with other elements to favorably modify its electronic structure. It has been suggested that, when used at a proper ratio, Pd can be used as a dopant in order to decrease Pt clusters sintering on TiO 2 (110).…”

“…25 In essence, size-selected Pt n deposited on SiO 2 are used to seed selective addition of Sn, by exposing the samples to H 2 , then SnCl 4 , which binds preferentially to the hydrogenated Pt sites, then again to H 2 to remove Cl as HCl. Sn deposition is selective and self-limiting, iCO binds strongly to Pt clusters,10,34 thus CO TPD is a useful probe of the number and energetics of reactant-accessible Pt binding sites, and their evolution in multiple TPD runs. For each TPD run, the sample was cooled to 180 K, exposed to 10 L of13 CO, then the temperature was ramped at 3…”