中央研究院 原子與分子科學研究所

Solid acid materials with tunable structural and acidic properties are promising heterogeneous catalysts for manipulating and/or emulating the activity and selectivity of industrially important catalytic reactions. The performances of acid-catalyzed reactions are mostly dictated by the acidic features, viz. type (Brønsted vs Lewis acidity), amount, strength, and local environments of acid sites. The latter is relevant to their location (intra- vs extra-crystalline), and possible confinement and Brønsted-Lewis acid synergy effects that may strongly affect the host-guest interactions, reaction mechanism, and shape selectivity of the catalytic system. Based on our experimental as well as theoretical studies in the past decade, this account aims to highlight important applications of advanced solid-state NMR (SSNMR) techniques in conjunction with density functional theory (DFT) calculations for exploring the structural-activity correlation of the catalytic system at a microscopic scale. Recent developments and applications of ex-situ and/or in-situ SSNMR techniques, such as two-dimensional (2D) double-quantum magic-angle spinning (DQ MAS) homonuclear correlation spectroscopy for investigations of structure and acidic property of solid acids are introduced. Moreover, the energies and electronic structures of the catalyst and detailed reaction processes, including identification of reaction species and elucidation of reaction mechanism are also discussed.

A facile synthesis routefor preparation of highly N-dopedordered mesoporous carbons(NOMCs) containing well-dispersed,highly stable Pt nanoparticles(NPs) has been developed. These mesostructured Pt-NOMC materials invokes pyrolysis of co-fed carbon sources and Pt precursorsin amine-functionalized mesoporous SBA-15 silicas, which served simultaneously as the N source and hard template. It was found that the dispersion of Pt NPs increaseswith increasing N content in the Pt-NOMC electrocatalysts.The superior electrocatalytic activities during oxygen reduction reaction (ORR) and methanol-tolerant stabilitiesobserved for these novel Pt-NOMCmaterialshave been attributed to the dispersion and unique nanostructure of Pt NPs particularly in the presence of pyridinic-Natoms in the mesoporous carbon supports. The Pt-NOMC nanocompositesso fabricated should render future practical and cost-effective applications in proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs).

We have shown that ³¹P solid-state nuclear magnetic resonance (SS-NMR) of adsorbed trialkylphosphine oxides (R₃PO) is capable of providing important acid properties, viz. types (Brønsted vs. Lewis acids), location, strength, and concentration, of acid sitesin solid acids. In continuing of such a R&D endeavor, we have also adopted density functional theory (DFT) calculations to confirm that ³¹PNMRchemical shifts of adsorbed R₃POprobe molecules increase linearly with the acidic strength (J. Phys. Chem. B 112, 4496, 2008；J. Phys. Chem. A 112, 7349, 2008), as been demonstrated by a variety of different solid acid systems, such as zeolites, metal oxides, and heteropolyacids(HPAs；J. Phys. Chem. C 114, 15464, 2010). It has been shown that such acidity characterization technique not only capable of identifying solid acids with a wide variety of acidic strengths covering from weak, medium, strong, to super acids, but also applicable for liquid acids. The ³¹P chemical shifts of the adsorbed R₃POtherefore offer a reliable and revolutionary acidity scale for both solid as well as liquid acids; more details may be found in recent review articles (Chin. J. Magn. Reson. 27, 485, 2010; Phys. Chem. Chem. Phys. 13, 14889, 2011).