For a variety of reasons, silica and alumina structures are important frameworks for the subject of heterogeneous catalysis. Aluminas, and perhaps to a lesser extent silicas, are employed directly as cracking catalysts or as substrates for assorted catalytic systems. From the point of view of catalysis, structures at both the surface and in the interior (bulk properties) are important. This chapter focuses on surfaces. Because of nuclear magnetic resonance's (NMR) sensitivity to local structure and its forgiveness of long-range disorder, solid-state NMR is one of the methods of choice for studying the structures of these materials. The chapter emphasizes the surface characterization of silica and alumina systems and silica aluminas by NMR methods. Technical aspects highly relevant to surface characterization are explicitly discussed. The chapter is also concerned with dynamics at the surface.

The surface structure and adjacent interior of commercially available silicon nanopowder (np-Si) was studied using multinuclear, solid-state NMR spectroscopy. The results are consistent with an overall picture in which the bulk of the np-Si interior consists of highly ordered (“crystalline”) silicon atoms, each bound tetrahedrally to four other silicon atoms. From a combination of 1H, 29Si and 2H magic-angle-spinning (MAS) NMR results and quantum mechanical 29Si chemical shift calculations, silicon atoms on the surface of “as-received” np-Si were found to exist in a variety of chemical structures, with apparent populations in the order (a) (Si–O–)3Si–H > (b) (Si–O–)3SiOH > (c) (HO–)nSi(Si)m(–OSi)4−m−n ≈ (d) (Si–O–)2Si(H)OH > (e) (Si–O–)2Si(–OH)2 > (f) (Si–O–)4Si, where Si stands for a surface silicon atom and Si represents another silicon atom that is attached to Si by either a Si–Si bond or a Si–O–Si linkage. The relative populations of each of these structures can be modified by chemical treatment, including with O2 gas at elevated temperature. A deliberately oxidized sample displays an increased population of (Si–O–)3Si–H, as well as (Si–O–)3SiOH sites. Considerable heterogeneity of some surface structures was observed. A combination of 1H and 2H MAS experiments provide evidence for a substantial population of silanol (Si–OH) moieties, some of which are not readily H-exchangeable, along with the dominant Si–H sites, on the surface of “as-received” np-Si; the silanol moieties are enhanced by deliberate oxidation. An extension of the DEPTH background suppression method is also demonstrated that permits measurement of the T2 relaxation parameter simultaneously with background suppression.

In situ C-13 NMR measurements are reported on C-13-enriched powdered poplar wood that is subjected to pretreatment with 0.5 M sulfuric acid as a function of time and at two temperatures. C-13 MAS (magic-angle spinning) spectra were obtained in both the DP (direct polarization) and CP (cross-polarization) modes, the contrasts in this combination yielding valuable qualitative information on the effect of pretreatment on local molecular mobilities. T-1 values for C-13 and for H-1, as well as T-CH and T-1 rho H, were measured at various stages of treatment with 0.5 M H2SO4 for lignin peaks and for cellulose peaks in the C-13 NMR spectra, as quantitative indicators of the degree of molecular motion for those two structural entities. The results show that a substantial fraction of the solid/semisolid biomass is converted at elevated temperatures to (a) chemically different and more mobile structures and (b) locally similar structures with enhanced atomic-level mobilities and that some fraction of this "mobilized" biomass does not return to the original level of immobility upon cooling the biomass back to room temperature. Analysis of the T-1 results by a rather simple model indicates that, for poplar wood in 0.5% H2SO4, the estimated ("global") motional correlation time (at the multiatom level), tau(c), is in the range of about 0.7-1.5 ns at various stages and temperatures of the treatment.

Ex situ room-temperature C-13 nuclear magnetic resonance (NMR) measurements are reported on powdered poplar wood that has been pretreated with dilute sulfuric acid (concentrations up to 1 wt %) for times ranging up to 20 mm and at temperatures of 120, 130, 140, and 150 degrees C. There are significant, albeit not dramatic, changes in the measured NMR spectra of the biomass as result of dilute sulfuric acid treatment. Values of T-1 for C-13 and as well as T-CH and T-1 rho H, were measured for lignin peaks and cellulose peaks in the C-13 NMR spectra, as potential indicators of the degree of atomic-level motion. For lignin components, one finds a trend to larger T-CH values as the treatment time or H2SO4 concentration is increased for treatment temperatures of 120 and 130 degrees C; however, for treatment temperatures of 140 and 150 degrees C, T-CH apparently decreases as the treatment time is increased. This higher temperature T-CH behavior implies that the lignin may actually become more rigid at later stages of treatment at temperatures >= 140 degrees C, which can be explained by cleavages of ether linkages of lignin and subsequent formation of new linkages, i.e., lignin recondensation. T-1C and T-1H measurements are consistent with this interpretation. The relationships between atomic-level mobility of lignin in biomass and treatment temperature is consistent with published relationships between the sugar yield and treatment temperature. The key role of acid treatment as a pretreatment for enzymatic digestion is evident in NMR measurements, including relaxation measurements, even after the treatment.

13C cross-polarization (CP) and direct-polarization (DP) spectra of an 83 mg sample of a chemical vapor deposited (CVD) diamond film (combined from 12 separate depositions) have been obtained via dynamic nuclear polarization (DNP) combined with magic-angle spinning (MAS). With DNP, the presence of unpaired electron spins in the sample, measured to be 2 × 1018 spins/g, provides a way to enhance the 13C or the “residual” 1H signal by irradiating the sample with microwaves at or near the electron spin resonance (ESR) Larmor frequency; the interactions between the unpaired electrons and protons or 13C spins lead to a transfer of polarization from the electron spin system to the 1H and/or 13C spin systems. No signal for sp2 hybridized carbons could be observed. The DNP-CP-MAS spectrum, obtained in an experiment in which the DNP-enhanced proton polarization is in turn transferred via CP to the 13C spin system, differs significantly from the DNP-DP-MAS spectrum, in which the 13C spins are directly enhanced.

A magnetization storage sequence, ALT-1 (alternating longitudinal and transverse components), is reported. The ALT-1 sequence is a hybrid of two types of storage sequences, the Carr-Purcell type and store-and-restore sequences. During incremental storage periods within the ALT-1 sequence, essentially half of the initially transverse magnetization is stored along the z-axis and the other half is prolonged by an echo-generating pulse. The portions of initial magnetization that are stored as longitudinal components or transverse components are alternated by a pi/2 pulse during the cycle. Both transverse components of the initial magnetization are treated the same in the ALT-1 sequence and orientational (phase) information of the initial magnetization is kept during the storage period. The ALT-1 sequence can preserve magnetization more effectively than a published class of modified Carr-Purcell type sequences, because essentially half of the magnetization during incremental storage periods is not subjected to relaxation from T2 effects.

The molecular dynamics of the silanols of high surface-area silica gel was studied using solid-state deuterium NMR spectroscopy. Deuterated silica gel samples were prepared by replacement of the exchangable silanol protons upon exposure to liquid (2)H(2)O and subsequent dehydration under vacuum at various temperatures that were selected to provide samples with varying populations of specific surface features, including (a) "isolated" silanols, that is, those that are not hydrogen-bonded (as demonstrated by the (1)H NMR chemical shift), (b) hydrogen-bonded silanols with a wide range of hydrogen-bonding strengths (as shown by the broad (1)H NMR peak), and (c) a hydrogen-bonded network of physisorbed, water. On a highly dehydrated surface obtained by dehydration at 500 degrees C and containing only isolated. silanols, analysis of the deuterium line shapes indicates that the "isolated" silanols exhibit a broad, inhomogeneous distribution of librational amplitudes of O-H entities about the internuclear Si-O vector, all in the fast-exchange limit (with reference to a 5 x 10(-6) s time scale defined by deuterium quadrupole interactions). The librational amplitudes depend on the observation temperature: increasing amplitudes with increasing temperature and vice versa. At the lowest observation temperature of 78 K, a small part of the silanols is essentially immobilized, but a sizable fraction remains mobiles At dehydration temperatures of 150 and 300 degrees C, the silica surface contains silanols with a distribution of hydrogen-bond strengths. At each observation temperature, the heterogeneous population of hydrogen-bonded silanols exhibits a broader distribution of librational amplitudes than those exh ibited by the "isolated" silanols at the same observation temperature. Less-aggressive dehydration conditions, for example, dehydration temperatures of 25 and 75 degrees C, creates samples in which, in addition to the silanols, a significant amount of physisorbed (deuterated) water is present. At lower observation temperatures, the effect of this physisorbed water is to narrow the librational amplitude for all of the silanols and water molecules. At the lowest observation temperature of 78 K, most of the surface silanols and water molecules of silica gel dehydrated at 25 and 75 degrees C are essentially immobilized. For silica gel dehydrated at 25 and 75 degrees C, at observation temperatures of 200 K and above, chemical exchange among a fraction of the surface silanols becomes significant, resulting in highly narrowed line shapes, while a substantial fraction of the surface silanols continue to execute the librational motions characteristic of more dehydrated surfaces. Line-shape analysis of the librational motion of the silanols permits the estimation of 121 +/- 1 degrees as the Si-O-(2)H bond angle. The deuterium quadrupole coupling constant of the silanol hydrogens is found to vary with the dehydration conditions and is interpreted as reflecting variations of the ensemble-averaged O-O distance between silanol oxygens and their various hydrogen-bonded partners. These data are interpreted in terms of a distribution of hydrogen-bonding arrangements on the heterogeneous silica surface, as witnessed by the silanols. This work provides an indirect indication that the popular view of a hydrogen bond (e.g., one with an O-O distance of 0.33 nm or less and with a nearly linear O-H-O arrangement) can be extended to admit a class of "weak" hydrogen bonds, for which the distance and geometry of the O-H-O triple are outside the typical view; but at such O-H-O configurations there are still significant interaction energies and molecular dynamics is affected.

The molecular dynamics of pyridine adsorbed onto dry high-surface-area silica gel was studied using solid-state deuterium NMR spectroscopy. Selectively and totally deuterated pyridine samples were used to identify the spectral signatures associated with each ring location and to identify the dynamical modes present in the surface adsorbed species. Loading levels corresponding to 0.8 and 0.2 monolayer coverage were studied. At 174 K and 0.8 monolayer, the adsorbed pyridine molecules are found in three motional modes:  a fraction is found to be static, a larger fraction is found executing fast discontinuous rotation (180° ring flips) about the pyridine Cγ−N axis, and a very small fraction is found in fast continuous rotation (diffusion) about the pyridine Cγ−N axis, all referred to a 5 × 10-6 s time scale. No evidence of O−H−N rotation about the Si−O bond (“swinging arm” motion) is observed. The pyridine flips and rotations permit measurement of the angle between the Cγ−N axis and each of the Cα−D and Cβ−D ...

Chemically and physically fractionated samples extracted from the surface horizon of a soil developed under a mix of coniferous and deciduous vegetation in southwestern Colorado were studied. 13C NMR data on this soil's organic matter and its HF(aq)-washed residue, as well as the classic acid/base-separated humic fractions (humic acid, fulvic acid, humin), were examined for chemical–structural detail, e.g., the various structural functionalities present (especially lipids, carbohydrates, aromatics, polypeptides and carbonyl/carboxyls). Among the humic fractions, it was found that the lipid concentrations are in the order humic acid>fulvic acid= humin; for carbohydrates the order is fulvic acid>humin>humic acid; for aromatic carbons the order is humic acid>humin>fulvic acid; for polypeptides it is humic acid>fulvic acid>humin and for carbonyl/carboxyl species it is humin>humic acid>fulvic acid, but the differences are small. 13C spin–lattice relaxation times indicate that at least two types of "domains" exist in each, corresponding to "higher" and "lower" concentrations of paramagnetic centers, e.g., Fe3+.

The reaction of trimethylaluminum and dry, high-surface-area (500 m2/g) silica gel in a mixed slurry was studied using multinuclear, solid-state NMR spectroscopy. The products of the initial reaction were characterized, and their progress through subsequent washing with diethyl ether and reactions with measured amounts of water was followed. The quantitative distribution of different chemical forms of carbon deposited on the silica surface by the initial reaction was measured. The products of the initial reaction are dominated by methyl species of the types Al(CH3)n (with Si-O-Al linkages), Si-O-CH3, and (Si-O)4-nSi(CH3)n; aluminum is seen to exist predominantly as a five-coordinate species. Subsequent treatment with diethyl ether fails to remove any surface species, but instead the ether becomes strongly associated with the surface and highly resistant to removal. Stepwise additions of water hydrolyze the Al-CH3 and Si-O-CH3 moieties, leading to conversion of five-coordinate aluminum to four- and six-coordinate aluminum, and affect the partial release of the surface-associated diethyl ether; Si-CH3 moieties remain. The effect of aromatic and saturated solvents on the initial reaction was examined and found to cause a small but significant change in the distribution of products. Structures of aluminum-centered species on the silica surface consistent with the spectroscopic data are proposed.

Solid-state 31P and 29Si NMR experiments, with Magic-Angle Spinning (MAS), were used to elucidate the chemistry that occurs when silica gel is treated with phosphorus pentachloride. A low-loading regime (in which the molar ratio of initial PCl5 to surface silanols sites is <<1) and a high-loading regime (in which this ratio is approximately 1) were examined. For each regime, the results for limited and intentional exposure to moisture are presented. The occurrence of phosphorus bridging between two adjacent silanols sites is observed. Bridging structures based on Si-O-P-O-P-O-Si linkages are also indicated.

The decomposition of methyl parathion (an organothio-phosphate pesticide) sorbed on partially hydrated kaolin and montmorillonite clays (in Ca2+, Cu2+, Zn2+, and Al3+ forms) at high concentration (typically 1-10 wt %) has been examined by nuclear magnetic resonance (NMR), using solid-state P-31 NMR (based on magic-angle spinning and cross polarization or direct polarization) and liquid-state P-31 NMR of DMSO and acetone extracts. The results indicate that methyl parathion is initially physisorbed, appearing by solid-state P-31 NMR to exhibit substantial molecular-level motion. The signals due to unreacted methyl parathion diminish and are replaced by ne W P-31 NMR peaks resulting from hydrolysis, isomerization, and oxidation reactions over periods ranging from hours to years. P-31 NMR characteristics indicate that these decomposition products are much more tightly bound to the clay than is methyl parathion. Methyl parathion decomposition is most effectively catalyzed by partially hydrated Cu(II)- and Al-montmorillonites (but with different product distributions); Ca-montmorillonite and kaolin were least effective.

In situ high-temperature electron paramagnetic resonance (EPR) has been used to study the pyrolysis (under a helium atmosphere) of cellulose and cellulose/Na2CO3 mixtures, as well as changes in the resulting chars under exposure to O-2 and H2O after quenching to room temperature. It has been found that the addition of Na2CO3 not only dramatically changes the pyrolysis behavior, but also has substantial effects on the subsequent exposure behaviors of the resulting chars to O-2 and H2O. The presence of Na2CO3 substantially narrows the EPR lines of cellulose chars at all charring temperatures used in this study. For low-temperature (<= 350 degrees C) pure-cellulose chars and for cellulose/Na2CO3 mixture chars prepared at all but the highest temperature (550 degrees C) of this study, short-term (<= 120 min) exposure to O-2 has no obvious effect on the spin concentration and the line width; however, decreases in the spin concentration and increases in the line width are observed for high-temperature (>= 400 degrees C) pure-cellulose chars. Effects of H2O introduction into the He/O-2 gas stream are most dramatic for lower-temperature cellulose/Na2CO3 chars and for higher-temperature chars of pure cellulose. The results are discussed in terms of a complex array of possible free-radical reactions.

In situ high-temperature EPR has been used to study the 350 and 500 °C pyrolysis (under a He atmosphere) of tobacco, as well as the EPR-signaled changes in the resulting char under exposure to O2 and H2O after quenching to room temperature. Exposure of a char to O2 leads to a large increase in free radical concentration, especially for the 350 °C char. Exposure of the char to both O2 and H2O yields an additional increase in free radical content (followed by a decrease after a maximum is reached), the magnitude of this increase being roughly half that of the initial O2-generated increase. The observed patterns result from a complicated chemical interplay involving the creation and destruction of carbon-centered and oxygen-centered free radicals.

Phosphorus-31 nuclear magnetic resonance (31P NMR) was used to follow the decomposition of chlorpyrifos (an organothiophosphate pesticide) adsorbed on soil, humic acid, partially hydrated kaolin clay, and partially hydrated montmorillonite clay at high concentration (typically 2-10 wt %). Solid-state 31P NMR (using magic-angle spinning and cross polarization or direct polarization) and liquid-solution 31P NMR of DMSO and acetone extracts indicate that chlorpyrifos is initially physisorbed, appearing by solid-state 31P NMR to exhibit significant motion on the molecular level, which results in almost liquidlike solid-state spectra. Over periods ranging from hours to years, the signals due to unreacted chlorpyrifos sorbed on the clays diminish and are replaced by new 31P NMR peaks resulting from hydrolysis, isomerization, mineralization, and oxidation reactions. The 31P NMR signal characteristics indicate that these decomposition products are much more tightly bound to the clay than is chlorpyrifos. Solid-state 13C and 27AI NMR spectra were less useful for following the decomposition of chlorpyrifos than those obtained by 31P NMR. Solid-state 31P NMR results indicate that a chlorpyrifos loading level of 10% by weight, used in some of the samples to facilitate 31P NMR detection of less-than-dominant decomposition products, exceeds the adsorption capacity of the soil, humic acid, and kaolinite tested, but not Ca2+ -exchanged montmorillonite. This pattern is consistent with intercalation into the montmorillonite, but only surface adsorption on kaolinite.

Phosphorus-31 nuclear magnetic resonance (P-31 NMR) was used to explore the decomposition of chlorpyrifos (an organothiophosphate pesticide) sorbed at high concentration (typically 2-10 wt%)on partially hydrated montmorillonite clays in four different cation-exchanged forms (Ca2+, Cu2+, Zn2+, and Al3+). Solid-state P-31 NMR (using magic-angle spinning and cross polarization or direct polarization) and liquid-solution P-31 NMR of DMSO and acetone extracts indicate that chlorpyrifos is initially physisorbed, appearing by solid-state P-31 NMR to exhibit significant motion on the molecular level, which results in almost liquidlike solidstate spectra. Over periods ranging from hours to years, the signals due to unreacted chlorpyrifos diminish and are replaced by new P-31 NMR peaks resulting from hydrolysis, isomerization, mineralization, and oxidation reactions. The P-31 NMR signal characteristics indicate that these decomposition products are much more tightly bound to the clay than is chlorpyrifos. Partially hydrated Cu(II)- and Al-montmorillonites most effectively catalyzed chlorpyrifos decomposition (but with different product distributions); Ca-montmorillonites (and, as previously shown, kaolin) were least effective. Solid-state(13)C and 27Al NMR spectra were less useful for following the decomposition of chlorpyrifos than those obtained by P-31 NMR. Pesticide loading levels (1-10% w/w) that are very much higher than those typically found in the environment were used to facilitate P-31 NMR detection of less-than-dominant decomposition species.

EPR (electron paramagnetic resonance) behaviors of cellulose chars during exposure to air at room temperature strongly depend on the identities of any inorganic additives and on the charring temperature. The air exposure of low-temperature (300-400 degreesC) cellulose/alkali carbonate chars or alkali bicarbonate chars appears to involve two different chemical processes, the production and annihilation of free radicals. The "new" radicals formed during air exposure have larger g-values than those generated by pyrolysis. In contrast, the corresponding low-temperature chars of pure cellulose and cellulose/NaCl show only an apparently small, slow decrease in radical concentration upon air exposure. For the high-temperature (>400 degreesC) chars of cellulose and cellulose/additive mixtures used in this study, air exposure is accompanied by a large reduction in apparent radical concentration. However, this decrease is found to depend dramatically on the presence of water in the exposure environment. Adding Na2Co3 into the cellulose not only causes a higher aliphatic-to-aromatic ratio of carbons in the char (assessed by C-13 NMR), but also changes the distribution of functional groups. Carboxyl groups appear in the cellulose/Na2CO3 char prepared at 350 degreesC for 1 h, while in the corresponding pure-cellulose char and cellulose/ NaCl char they were not detected.