Small Platinum Clusters in Zeolites:  A Density Functional Study of CO Adsorption on Electronically Modified Models

Abstract

Very small transition metal particles can be stabilized inside zeolite cavities. Both electron-enriched and electron-deficient encapsulated metal species have been proposed on the basis of experimental data. In this work, structure and adsorption properties of the cluster Pt 4 , in both neutral and electronically modified forms, have been studied computationally with the help of a scalar-relativistic density functional method. The species Pt 4 + has been chosen to represent the case of a metal particle interacting with an electron attracting zeolite host; likewise, Pt 4 -has been taken to mimic the effect of an electron-donating host. Adsorption of CO probe molecules at on-top, bridge, and 3-fold hollow sites of the moieties Pt 4 , Pt 4 + , and Pt 4 -has been investigated to determine a relationship between the cluster charge and the C-O vibrational frequency shift ∆ω(CO). The chemical effect of electron-donor and electron-acceptor species on the electronic structure of the Pt 4 clusters and on the properties of adsorbed CO probes has been also explicitly taken into account by employing various models XPt 4 CO (X ) Na, Na + , NH 3 ). Properties of adsorbed CO probe molecules were calculated to be rather sensitive to the electronic state and the adsorption site of the Pt 4 particles, in line with experimental findings. A linear correlation between the effective charge of the metal cluster and the adsorption-induced vibrational frequency shift ∆ω(CO) has been found for CO adsorbed at on-top positions.

Nanocatalysis: Mature Science Revisited or Something Really New?

“…As catalytic reactions occur usually at elevated temperatures, the band-gap effect is of significant influence only for very small objects [23] of about 1 nm in size. Such objects are in a non-encapsulated form, [24,25] which are not very stable under reaction conditions and tend to sinter. Thus an average ground-state electronic structure modification that is well known to exist in small clusters [23,26] is in most cases not responsible for beneficial "nanoeffects" in catalysis.…”

Impurity Atoms on Small Transition Metal Clusters. Insights from Density Functional Model Studies

“…In particular, we modeled clusters containing up to 6 nickel atoms [35][36][37]45] as well as tetra-atomic clusters of palladium [39], iridium [33,40,41,47,48], and platinum [34,42], and hexa-atomic rhodium [38]. All these clusters were considered to interact with hydrogen atoms, while on some clusters we also modeled the coordination of C and O.…”

“…We investigated the stability as well as electronic and spectral characteristics of complexes obtained by CO adsorption on bare nickel [50] and platinum [34] tetramers as well as on Ni 4 X clusters (X = H, C, or O) [50]. Table 2 [50].…”

“…Here, computational modeling can make important contributions. Stimulated by the success of Gates and co-workers in preparing and exploring such cluster systems, we joined the efforts to understand the behavior of (small) metal clusters, by applying density functional methods [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. These methods have become essential for investigating various chemical systems, in particular transition metal clusters [51][52][53][54][55][56][57][58][59][60].…”

“…Since in all cases the values derived from EXAFS were notably longer, up to 20 pm, than the calculated ones and the computational method was confirmed to perform accurately for similar systems (see below and [33,38,47]), we considered the possible presence of impurity atoms on the experimentally prepared metal clusters. In this context we modeled the adsorption of single hydrogen, carbon, or oxygen atoms on nickel [34][35][36][37], rhodium [38], palladium [39], iridium [33,40,41,47] and platinum [34,42] clusters. We also studied the dissociative adsorption of up to 6 H 2 molecules on different metal tetramers [43,44,48].…”