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Materials and Solid State, Chemical Analysis, Catalysis and Mechanism
Our group develops and applies transient techniques in solid-state nuclear magnetic resonance (NMR) to probe the chemical and physical properties of materials involved in heterogeneous catalysis, surface science and materials science. The work on catalysts, which in recent years constituted most of the research effort, focuses on studying the properties of surfaces, as well as the molecular structure, dynamics and reactions of the adsorbed species. Development of new solid-state NMR methods for these studies is the second major research area. Current efforts include the development of techniques based on multiple-quantum magic angle spinning (MQMAS) NMR, homo- and hetero-nuclear correlation experiments for spin-1/2 and quadrupolar nuclei, methods utilizing ultrafast MAS and methods for measuring internuclear distances in solids.
1. High-resolution solid-state NMR methods
1.1Half-integer quadrupolar nuclei. In the past several years, our group has developed and exploited high-resolution methods for studying half-integer quadrupolar spins (70% of all NMR-active nuclei). Quadrupolar broadening contains higher order orientational terms of significant magnitude and isotropic resolution can only be achieved by applying complex motions of the sample (double rotation, dynamic angle spinning) or by combining magic angle spinning and multiple-quantum NMR in an MQMAS experiment. Several new techniques have been developed from this effort, including CP-MQMAS (MQMAS with Cross Polarization), MQ-REDOR (MQ Rotational Echo Double Resonance) and MQ-HETCOR (MQ HETeronuclear CORrelation spectroscopy). These techniques extended the capabilities of NMR to probing the intermediate range order (0.5-1 nm) in inorganic solids under high-resolution conditions.
Applications of these methods included measurements of through-space and through-bond interatomic connectivities and distances between spin-1/2 (e.g. 1H, 19F, 29Si and 31P) and quadrupolar (e.g. 11B, 23Na and 27Al) nuclei in several types of molecular sieves, glasses and other materials. Examples of MQ-HETCOR spectra revealing through-bond (via refocused INEPT) and through-space (via dipolar cross polarization) connectivities are shown in Figures 1a and 1b, respectively. We also conducted experimental and theoretical studies directed toward better understanding of the spin dynamics involved in MQMAS-based experiments on quadrupolar nuclei.
1.2. Ultrafast MAS. Our recent interests include exploitation of state-of-the-art fast MAS in homo- and heteronuclear NMR experiments involving pairs of spin-1/2 nuclei (e.g. 1H-1H, 1H-13C, 1H-29Si). This approach proved useful in the studies of functionalized silica surfaces, where MAS at rates ≥ 40 kHz can provide adequate 1H-1H decoupling. Additional advantages of using fast MAS in HETCOR NMR include easy setup, lack of scaling factors, and ease of acquisition of sideband-free spectra. The loss of sensitivity due to small rotor volume (less than 10 µL), is offset by low requirements for the RF magnetic field homogeneity within the sample.
Utilizing fast MAS, highly resolved 1H-13C HETCOR NMR spectra of various organic groups covalently bound to the silica surface were obtained without 13C enrichment (see Figure 2a). In addition, the sensitivity of 1H-29Si HETCOR NMR was significantly increased by applying a CPMG train of refocusing π pulses to the 29Si spins. The experiments performed thus far in our laboratory indicated that sensitivity gains exceeding one order of magnitude, or two orders of magnitude in the total acquisition time, are common on the silica surfaces. These experiments require unusually long acquisition periods for each free induction decay, which can exceed the duration of the delay between consecutive scans. Under such circumstances, fast MAS offers an additional advantage in that it can be used with low power heteronuclear decoupling. The acquisition of 1H-29Si HETCOR spectrum shown in Figure 2b would not be possible without using the CPMG refocusing. The sensitivity gain can be also exploited in 29Si-29Si double-quantum (DQ) (Figure 2c) and in 27Al-29Si correlation spectroscopy of catalytic materials, especially aluminosilicates and zeolites.
1.3. Other methods. We also explore new methods for studying mesoporous mixed oxides. These oxides often include spin-1/2 (e.g. 111/113Cd, 117Sn) or half-integer quadrupolar (e.g. 17O, 25Mg, 43Ca, 47/49Ti, 78Sr and 95Mo) nuclei, which are difficult to observe because of low magnetogyric ratio and/or low natural abundance. However, acquisition of solid-state NMR spectra of these 'challenging' nuclei is becoming possible, if not routine, using more sophisticated instrumentation (high-field magnets and more sensitive probes), isotope enrichment, and improved protocols for excitation/detection.
Since some of the systems and reactions of interest to us involve liquid-solid interfaces, we are also interested in the use of NMR for studying structures and molecular dynamics in suspensions, where tethered and reacting species exhibit a wide spectrum of mobilities. In the long run, these studies will allow correlating the mobility of surface species with catalytic activity.
2. Studies of heterogeneous catalysts and other complex materials by solid-state NMR
2.1. The use and development of new techniques in solid-state NMR is a vital part of the catalysis program at Ames Laboratory. The main area of current interest involves characterization of functionalized silica- and/or alumina-based, single-site mesoporous catalysts with controlled, nanostructured surface and particle morphology. One- and two-dimensional (1D and 2D) 1H, 29Si and 13C NMR methods are being used to (1) detail the structures of non-functionalized MCM-41 mesoporous silica nanoparticles, (2) study the structures and absolute/relative concentrations of various moieties inside the mesopores, (3) determine their spatial distribution, orientation with respect to the surface and dynamic behavior, and (4) monitor template removal and the catalysts' stability under reaction conditions. Several classes of bifunctional materials are thoroughly characterized, including catalysts for hydrogenation, polymerization and oxidation reactions, in which reaction selectivity is controlled using the novel concept of gatekeepers. Recent developments in solid-state NMR will be further exploited to characterize the mesoporous scaffolds composed of other metal oxides, including Cr2O3, MnO, ZnO, CdO and CaO.
2.2. We also work on (i) characterization of combined homogeneous-heterogeneous, silica-based tethered catalysts for enantioselective hydrogenation and oxidation reactions, by using a suite of 1D and 2D measurements involving 1H, 11B, 13C, 19F, 29Si and 31P nuclei; (ii) 27Al, 7Li, 23Na and 1H NMR studies of novel materials for hydrogen storage - non-transition metal-based complex hydrides, where the metal, metalloid or nonmetal atom is coordinated by two to six hydrogen atoms, and which are synthesized by means of a solvent-free technique (mechanochemistry) or by chemical approaches (self-assembled micelles); and (iii) studies of industrial catalysts.