(188d) Catalyst/Support Interactions Between Pt Nanoparticles and Amorphous Silica Using Density Functional Theory

Metal nanoparticles (NPs) play a key role in catalysis due to large surface-to-volume ratios and low-coordination active surface sites. These catalysts are typically immobilized on oxide supports to suppress sintering, which leads to catalyst deactivation and degradation of catalyst selectivity. An atomistic understanding of catalyst systems is essential to the rational design of improved catalyst systems. Catalyst activity and selectivity are highly dependent on shape and size of NPs as well as the nature of the oxide support. Despite significant advancement of experimental characterization techniques, a detailed understanding of the catalyst-support interface for very small NPs (~ 1 nm or smaller) is largely precluded by experimental investigations. Computational methods provide one means to overcoming this challenge. In previous studies catalyst supports have generally been either neglected or treated as highly ideal structures, however, oxide supports are often used in an amorphous state, exhibiting a wide range of surface sites.

Due to its thermal stability and tunable porosity, and hence specific surface area, amorphous silica is widely used as a catalyst support, but is still poorly understood at the atomistic level due to limitations in both experimental and computational methods. We have developed and experimentally validated a method for generating realistic atomistic surface models of amorphous silica for a range of temperatures by simulating the process of surface dehydroxylation.

Using these surface models, we have studied the effects of amorphous silica supports on small platinum NPs (13 atom cuboctahedral structures) with regards to their catalytic properties. Specifically, we have studied the impact of catalyst-support interactions on NP morphology and energetics. We find that the morphology of Pt clusters undergo significant restructuring that depends on the local structure at the Pt-Silica interface. This restructuring typically leads to significantly increased exposure of lower-coordination atoms than are present on the unsupported NP. Additionally, we observe a compressive Pt-Pt bond strain that increases with the number of bonds at the Pt-Silica interface.  The energy of Pt-silica interaction depends almost exclusively on the number of oxygen atoms bonding to the Pt cluster, which is in turn a function of surface hydroxylation, and hence, the pretreatment temperature of the silica support. We are currently extending these studies to larger systems (55 and 147 atoms) as well as studying the effects of Pt-Silica interactions on CO binding energies for select systems.