Investigation of the Platinum Cluster Size and Location on Zeolite KL with 129Xe NMR, XAFS, and Xenon Adsorption

Nature of the Metal−Support Interaction in Bifunctional Catalytic Pt/H-ZSM-5 Zeolite

The metal-support interaction of a dispersed Pt atom on H-ZSM-5 zeolite has been investigated by using an embedded cluster and cluster models with the density functional theory/B3LYP functional method. We found that the Pt atom interacts with a Brønsted proton and a nearby framework oxygen. Interaction with the framework oxygen causes electron transfer from the zeolite to the Pt atom. Concurrently, a Brønsted proton stabilizes the Pt atom on the zeolite surface by withdrawing excess electron density from the Pt atom. These charge transfers result in a zero net charge on the Pt atom while changing its orbital occupation. The binding energy of Pt on the Brønsted acid was 15 kcal/mol. Inclusion of the Madelung potential by Surface Charge Representation of the Electrostatic Embedded Potential method (SCREEP) had small effects on structure and charge density of Pt/H-ZSM-5 but it shifted the stretching mode of CO toward a higher frequency by almost 40 cm(-1). The frequency shift of absorbed CO calculated with embedded cluster models was from 8 to 11 cm(-1) red shift, compared to 20 cm(-1) red shift from experiment. This implies that not only the electronic state of the Pt atom but also the Madelung potential of the support is responsible for the observed small red shift of CO on the Pt-H-ZSM-5.

Nuclear Magnetic Resonance Studies of Resorcinol−Formaldehyde Aerogels

In this article, we report a detailed study of resorcinol-formaldehyde (RF) aerogels prepared under different processing conditions, [resorcinol]/[catalyst] (R/C) ratios in the starting sol-gel solutions, using continuous flow hyperpolarized (129)Xe NMR in combination with solid-state (13)C and two-dimensional wide-line separation (2D-WISE) NMR techniques. The degree of polymerization and the mobility of the cross-linking functional groups in RF aerogels are examined and correlated with the R/C ratios. The origin of different adsorption regions is evaluated using both coadsorption of chloroform and 2D EXSY (129)Xe NMR. A hierarchical set of Xe exchange processes in RF aerogels is found using 2D EXSY (129)Xe NMR. The exchange of Xe gas follows the sequence (from fastest to slowest): mesopores with free gas, gas in meso- and micropores, free gas with micropores, and, finally, among micropore sites. The volume-to-surface-area (V(g)/S) ratios for aerogels are measured for the first time without the use of geometric models. The V(g)/S parameter, which is related both to the geometry and the interconnectivity of the pore space, has been found to correlate strongly with the R/C ratio and exhibits an unusually large span: an increase in the R/C ratio from 50 to 500 results in about a 5-fold rise in V(g)/S.

Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles

Nanostructured carbon materials are potentially of great technological interest for the development of electronic, catalytic and hydrogen-storage systems. Here we describe a general strategy for the synthesis of highly ordered, rigid arrays of nanoporous carbon having uniform but tunable diameters (typically 6 nanometres inside and 9 nanometres outside). These structures are formed by using ordered mesoporous silicas as templates, the removal of which leaves a partially ordered graphitic framework. The resulting material supports a high dispersion of platinum nanoparticles, exceeding that of other common microporous carbon materials (such as carbon black, charcoal and activated carbon fibres). The platinum cluster diameter can be controlled to below 3 nanometres, and the high dispersion of these metal clusters gives rise to promising electrocatalytic activity for oxygen reduction, which could prove to be practically relevant for fuel-cell technologies. These nanomaterials can also be prepared in the form of free-standing films by using ordered silica films as the templates.