Exploring Surface Interactions in Catalysts Using Low-Field Nuclear

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Exploring Surface Interactions in Catalysts Using Low-Field Nuclear Magnetic Resonance Jonathan Mitchell,*,† Lionel M. Broche,‡ Thusara C. Chandrasekera,† David J. Lurie,‡ and Lynn F. Gladden† †

Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, U.K. Aberdeen Biomedical Imaging Centre, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, Scotland, U.K.

‡

S Supporting Information *

ABSTRACT: Fast field cycling (FFC) nuclear magnetic resonance (NMR) is applied to probe the slow dynamics of liquid molecules imbibed in porous catalysts. The FFC measurements are used to determine surface diffusion correlation and residence times that provide information on the molecular dynamics of surface adsorbed species. The longitudinal relaxation time T1 dispersion curves reveal biphasic diffusion of adsorbed water that we attribute to the presence of “strongly bound” and “weakly bound” molecules. FFC measurements of organic liquids (2butanone, 2-propanol) do not show such behavior. These observations agree with molecular dynamics simulations. The frequency dependence of the relaxation time ratio T1/T2 is also considered; it is demonstrated that T1/T2 remains a valid indicator of adsorption energy regardless of the field strength at which the measurement is taken in the range B0 = 0.1 mT to 0.23 T.

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hydrogenation of carbonyl species,20 reports on the effect of water on catalytic activity, in general, vary. In other hydrogenation reactions, water has been observed to reversibly increase29−31 or decrease activity,31 or result in irreversible deactivation of the catalyst.31−34 The aim of the present work is to capture the extent to which nuclear magnetic resonance (NMR) fast field cycling (FFC) can give insights into the physical environment of different molecular species within the pore structure of the catalyst. Longitudinal T1 relaxation processes are sensitive to molecular dynamics occurring at the 1H Larmor frequency ω0 which is proportional to the magnetic field strength B0. FFC NMR dispersion measurements are analyzed using a surface mediated relaxation model based on the work of Brownstein and Tarr.35,36 Over the frequency range of interest, the bulk liquid relaxation rate 1/T1,bulk is assumed to be invariant, whereas the contribution from the surface influenced liquid layer 1/T1,surf will be a function of the 1H (spin I) Larmor frequency ω0. Enhanced nuclear relaxation occurs for spins in molecules in proximity to strong relaxation sinks at the pore surface. These relaxation sinks take the form of adsorption sites where hindering of molecular reorientations leads to enhanced intra- and intermolecular dipole coupling. The surface relaxation is therefore dictated by the characteristics of twodimensional (2D) molecular diffusion occurring on the catalyst surface.37−40

INTRODUCTION Understanding the interactions between adsorbate and surface is central to studies of heterogeneous catalysis.1−9 Knowledge of surface activity is essential if existing processes are to be optimized and new catalyst design approached in a rational manner.8 Liquid-phase catalysis is an active research topic with regard to the production of fuels and chemical commodities from sustainable sources.10,11 In these catalytic processes, multiple components are present in the liquid phase, either as reactant species adsorbing competitively or as a combination of reactant, intermediate, and product species, as well as any solvent species present. Knowledge of the strength of interaction of each possible adsorbate with the catalyst surface and the influence of each component upon the adsorption of the others is a crucial step toward understanding the catalytic reaction system. Solvents can play a key role in catalyzed reactions with catalytic activity and selectivity depending on the choice of solvent.12−14 In general, hydrogenation reactions represent an important class of catalytic transformations in both gas15−19 and liquid phase, with solvent effects observed in the latter.20−26 In previous work, we have considered the role of adsorption on palladium and ruthenium catalysts in the context of 2-butanone hydrogenation.27,28 The hydrogenation of 2-butanone over the supported metal has been studied previously employing a mixture of 2-propanol and water as solvents. These studies revealed that solvent composition influences the reaction rate. Specifically, the rate of hydrogenation was seen to increase as the water mole fraction increased.28 However, while it is recognized that the presence of water can facilitate the © 2013 American Chemical Society

Received: June 17, 2013 Revised: July 31, 2013 Published: July 31, 2013 17699

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In this work, we present a preliminary investigation of surface interactions in liquid-saturated catalyst pellets using FFC; for convenience, we have chosen to study the interactions of 2butanone, 2-propanol, and water with a Pd/Al2O3 catalyst surface. As FFC requires a specialized instrument capable of generating a variable magnetic field, the technique is not available in the majority of NMR laboratories. Therefore, the application to catalysts has hitherto been limited: Stapf et al. explored coke residues in catalyst pellets,41 and Conte et al. studied water over rutile and anatase titania surfaces.42 However, FFC NMR dispersion measurements have been applied to a wide variety of other porous materials including chromatographic glass, 43−45 granular packings, 46,47 cements,48−54 gypsum plaster,55 sedimentary rocks,56 and biochars.57,58 We extract the relevant theories from these previous works as are applicable to our choice of catalyst systems. The majority of standard NMR experiments on catalytic materials are conducted at high magnetic field strengths where sensitivity to reactants, products, and solvents is available through spectroscopic (chemical shift) identification of different species. Previously, we demonstrated a method for comparing surface interaction strengths of reactants and solvents in heterogeneous catalysts at high-field using 2D NMR relaxation time correlations.27 The ratio of longitudinal to transverse relaxation time T1/T2 is considered to be analogous to the energy of adsorption on a surface.47 This analogy is explained by recognizing that the ratio T1/T2 is sensitive to molecular dynamics, where T1 and T2 depend on rotational and translational motions to differing extents. Molecules adsorbed onto surfaces exhibit modified rotational dynamics59 and slower translational diffusion;60 hence, T1/T2 increases.61 It is not possible with current FFC technology to measure the frequency dependence of T2. Therefore, we use the correlation times obtained from the FFC measurements to predict the relaxation time ratios at high-field and we compare these predictions with measured values. The FFC measurements reveal a biphasic structure of water adsorbed on the catalyst surface, whereas adsorbed 2-butanone and 2-propanol exhibit only a single, weak mode of interaction. We suggest the anomalous surface diffusion of water enables it to promote or inhibit liquid-phase reactions.31 Our predictions of the ratio T1/T2 over a wide range of field strengths agree with experimental results obtained at fixed-field, validating the theoretical field dependence of T1/T2 and thereby allowing us to apply it as an indicator of surface interaction energy at any field strength. This validation is especially significant at lowfield, as some catalyst materials containing paramagnetic species are inappropriate for study at high-field. Surface Relaxation Theory. For the catalytic materials of interest in this work, we interpret the relaxometry data obtained by considering the dipolar interactions of like spins, assuming enhanced relaxation occurs only through restriction of molecular dynamics associated with the adsorption process. The surface motions of an adsorbed molecule are illustrated in Figure 1. The molecule first adsorbs onto the surface from the bulk liquid. It then diffuses across the surface by moving between neighboring adsorption sites (relaxation sinks). This surface motion is represented by a diffusion correlation time τm averaged over all surface adsorbed molecules; τm describes the frequency of transitions between adsorption sites. The molecule will then desorb back to the bulk liquid. This exchange between surface adsorbed molecules and those in the bulk liquid is

Figure 1. Illustration depicting the motion of a molecule adsorbed on a pore surface. A molecule adsorbs onto the surface from the bulk liquid, diffuses across the surface by moving between adsorption sites (at a rate determined by τm), and then desorbs back into the liquid after a time τs.

described by the surface residency time τs. Assuming τs ≫ τm (i.e., a molecule undergoes some diffusion across the surface before desorbing), then the spectral density function is ⎡ ⎢ J(ω0) = τm ln⎢ ⎢ ⎢⎣

⎤ ⎥ 1 + ω0 τm ⎥ 2 τm 2 2⎥ + ω0 τm ⎥ τs ⎦ 2

( )

2

(1)

The derivation of this spectral density function follows the method given by Godefroy et al.,46 except here we have used the theory pertaining to the interaction of like spins (proton− proton), rather than the more common theory describing the behavior of unlike spin interactions (proton−electron). The frequency 1/τs provides an effective cutoff in ω0, below which the NMR experiment is no longer sensitive to surface motion. A useful parameter for probing surface interactions is the ratio of relaxation rates T1/T2,61 as this eliminates the prefactors that are difficult to calculate empirically.62 At lowfield, we make the assumption ω0 ≪ 1/τs, so that T1,2,surf ≪ T1,2,bulk (i.e., surface relaxation times are much shorter than bulk liquid relaxation times). Therefore, we are able to ignore the bulk relaxation and hence, by inclusion of the modified spectral density function, eq 1, in the usual descriptions of T1 and T2 given elsewhere63,64 (see the Supporting Information for details), we obtain the ratio of surface relaxation times T1,surf T1 ≡ T2 T2,surf ⎡ ⎤ ⎡ ⎤ τ 1 + ω0 2τm 2 ⎥ ⎢ 1 + 4ω02τm 2 ⎥ 6 ln τ s + 5 ln⎢⎢ 2 ln + 2 ⎢ τm 2 + 4ω 2τ 2 ⎥ τm 2 2⎥ m 0 m ⎦ ⎣ ( τs ) + ω0 τm ⎦ ⎣ ( τs ) = ⎡ ⎤ ⎡ ⎤ 1 + ω0 2τm 2 ⎥ 1 + 4ω0 2τm 2 ⎥ 2 ln⎢⎢ + 8 ln⎢⎢ 2 2 τm τm 2 2⎥ 2 2⎥ ⎣ ( τs ) + ω0 τm ⎦ ⎣ ( τs ) + 4ω0 τm ⎦

( )

(2)

Now we have a frequency dependent relaxation time ratio that varies only with τm and τs for a given ω0, where T1/T2 is obtained directly from a 2D T1−T2 correlation experiment and is considered equal to the ratio of surface relaxation times. It is trivial to show from eq 2 that, in the limit τm → τs (i.e., bulk 17700

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find 1/T1 ≡ 1/T1,surf for all practicable NMR frequencies such that a separate determination of T1,bulk is unnecessary. Low-field T1−T2 correlations67 were obtained on an Oxford Instruments’ Maran DRX2-HF spectrometer (Oxford Instruments, Abingdon, U.K.) with a B0 = 50 mT (ν0 = 2 MHz) permanent magnet. Intermediate-field T1−T2 correlations had been acquired previously using a B0 = 2 T (ν0 = 85 MHz) superconducting, horizontal bore magnet with a Bruker AV console (Bruker Biospin GmbH, Rheinstetten, Germany).27 High-field data were obtained at B0 = 7 T (ν0 = 300 MHz) using a superconducting, vertical bore magnet with a Bruker DMX console. The pulse sequence is illustrated in the Supporting Information. The delay τ1 used to encode the T1 dimension was varied logarithmically between τ1 = 0.1 ms and 10 s in 32 steps. The CPMG echo trains were acquired with an echo time spacing of te = 1.2 ms; 32 repeat scans were acquired for signal averaging to improve the signal-to-noise ratio (SNR). The recovery delay between scans was 10 s; the total acquisition time was ∼1 h. The same method and parameters were used to measure the bulk liquid relaxation times. The same parameters were used for the T1−T2 measurements at both low-field and high-field. The T1−T2 data were processed using Tikhonov regularization68 with the optimum smoothing parameter chosen by the generalized cross validation (GCV) method.69 A recent review of 2D processing methods has been published elsewhere.70

liquid), the ratio T1/T2 → 1 as expected. As the ratio τs/τm is considered a measure of surface affinity,46 the dependent ratio T1/T2 is comparable to the strength of surface interaction.27,61 At high-field, as ω0τm → 1, the assumption that the bulk relaxation rate is negligible becomes invalid because 1/T1,surf → 0, so eq 2 cannot be applied. The ratio T1/T2 is obtained instead by dividing the sum of bulk and surface relaxation contributions (see the Supporting Information). As the prefactors in the relaxation rate equations no longer cancel, predicting the variation in T1/T2 with ω0 at high-field is not straightforward. Given that the prefactor on the T1 term is independent of frequency,63,64 we use an empirical prefactor for T1,surf obtained from the FFC data and consider this valid over all frequencies of interest.

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MATERIALS AND METHODS The 1 wt % Pd/Al2O3 catalyst was supplied by Johnson Matthey. The metal was deposited onto the support via the incipient wetness impregnation method and prereduced. The catalyst was studied as received. The Pd/Al2O3 catalyst was confirmed to be devoid of paramagnetic species by electron spin resonance (ESR) analysis. The Pd/Al2O3 catalyst was in the form of alumina trilobe pellets with an average size of 7 mm × 1 mm (length × diameter), a metal surface area of 2.4 m2 g−1, a BET surface area of 110 m2 g−1, and a BJH median pore size of 20 nm. The pores were considered large enough to accommodate both surface-adsorbed and free-diffusing molecules for each of the imbibed liquids while satisfying the “fast diffusion” limit.35,36 The catalyst was dried in a vacuum oven at 60 °C for 24 h before exposure to liquids to remove any physisorbed water. The trilobes were then soaked in excess liquid for 24 h before NMR measurements were conducted. Previous studies have shown that this time is sufficient to saturate the pores completely.65 Excess liquid was removed from the external surfaces of the pellets prior to the experiments. The FFC NMR experiments were conducted on a Stelar SMARtracer benchtop spectrometer (Stelar s.r.l., Italy). The T1 relaxation data were acquired with the recovery time τ1 varying logarithmically in 16 steps from τ1 = 0.01 to 0.4 s. The spin ensemble was inverted in the polarizing magnetic field of Bpol = 0.17 T. The magnetic field was then switched rapidly to allow the spins to relax with T1 toward equilibrium in the Brelax field. The switching time was 2 ms. The magnetic field was adjusted in consecutive experiments to obtain T1 as a function of Brelax; 25 field strengths were explored, spaced logarithmically between Brelax = 0.1 mT and 0.23 T. After a time τ1, the magnetic field was returned to Bacq = 0.17 T (corresponding to ν0 = 7.4 MHz) for detection of a free induction decay (FID).66 The pulse sequence is illustrated in the Supporting Information. The pulse sequence was repeated for all combinations of τ1 and Brelax to form a dispersion curve of T1(ω0). The dispersion curves were fitted using 1 = A[J(ω0) + 4J(2ω0)] T1

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RESULTS AND DISCUSSION Bulk liquid relaxation rates, measured at ν0 = 2 MHz but assumed to be independent of frequency, were determined as 1/T1,bulk = 0.5 s−1 (2-butanone), 0.7 s−1 (dodecane), 0.7 s−1 (2propanol), 0.7 s−1 (ethanol), and 0.3 s−1 (water). Dynamics of Adsorbed Molecules. FFC NMR dispersion curves are shown in Figure 2 for (a) 2-butanone, (b) 2propanol, and (c) water imbibed in the Pd/Al2O3 catalyst trilobes. On the basis of our previous studies of this catalyst system,27 we assume that 1/T1 is proportional to the strength of interaction between the adsorbate and adsorbent. It is obvious then, from Figure 2, that 2-butanone has the weakest surface interaction (slowest 1/T1 rate). Water has the strongest interaction (fastest 1/T1 rate), as expected on the basis of the mechanisms of surface adsorption:27 water adsorbs strongly in the liquid phase due to hydrogen bonding between liquid molecules and surface hydroxyl groups. To quantify the surface behavior, we fit the data in Figure 2 using eq 3. The dispersion curve for 2-butanone, Figure 2a, exhibits a weak dependence on field strength, suggesting the motion on the surface is similar to that in the bulk liquid. The fit shown for 2-butanone is obtained with a surface diffusion correlation time of τm = 18 ps, indicating that diffusion on the surface is rapid and only slightly slower than in the bulk liquid. The average residence time of a 2-butanone molecule on the surface is τs = 4.5 μs, corresponding to a frequency of ω0 ≈ 105 rad s−1 below which T1 is independent of frequency. Figure 2b shows that 2-propanol exhibits a higher relaxation rate than 2-butanone, and for 2-propanol, τm = 200 ps. This surface diffusion correlation time is an order of magnitude longer than the correlation time in the bulk liquid (τc ∼ 10 ps), indicating that motion on the surface is hindered. The OH group on the alcohol can hydrogen bond to the alumina surface, providing a strong adsorption mechanism. The dispersion curve for 2-propanol has a weak frequency dependence. The average residence time of a 2-propanol

(3)

with J(ω0) given by eq 1 and A being an empirically determined amplitude scaling. As stated in the Surface Relaxation Theory section, at low-field, we can make the assumption that any contribution to the measured relaxation time from bulk liquid will be negligible, and indeed in our dispersion curves fits, we 17701

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dispersion curve for water in Pd/Al2O3 catalyst trilobes using a single diffusion correlation time. Instead, the data were fitted in two parts: a “low frequency” region (ω0 ≤ 106 rad s−1) and a “high frequency” region (ω0 > 106 rad s−1). In the high frequency region, a surface diffusion correlation time of τm = 1.8 ns was obtained. Although this time is an order of magnitude larger than that for 2-propanol, we associate this high frequency behavior with water weakly bound in a surfaceinfluenced hydrogen bonding network similar to that of the alcohol. In the low frequency regime, τm = 0.26 μs. We associate the low frequency behavior with water strongly bound to the oxide surface. We were unable to reach sufficiently low frequencies to see a plateau in the dispersion curve, indicating that τs ≪ 10 μs for water. Observations of biphasic water in model porous media have been presented elsewhere.71,72 In other work, classical molecular dynamics simulations have been used to determine the structuring of water and 2-propanol on alumina surfaces.73 The pure water simulation predicted a strongly bound surface layer. A secondary water layer exhibited reduced mobility (compared to bulk liquid) with the molecular structuring in the second layer dependent on the structure of the surface layer. At three molecular layers, the water resumed its usual hydrogen bonding network. In comparison, pure 2propanol formed a single surface layer (adsorption occurred through the OH functionality), which presented a hydrophobic interface, through the carbonyl backbone, to the molecules further away from the surface. The second layer of 2-propanol therefore exhibited bulk-like behavior and was not dependent on the structuring in the surface layer. These simulations agree with the biphasic nature of adsorbed water (strongly bound surface layer and weakly bound secondary layer) and the monophasic nature of adsorbed 2-propanol (weakly bound surface layer) seen in the FFC dispersion curves. In related studies, the hydrogenation of 2-butanone over Ru/SiO2 catalyst was found to progress faster (by an order of magnitude) in water than 2-propanol;28 the authors suggested the hydrogen bonding network of the water stabilized the 2-butanone molecules adsorbed on the active catalyst. It has been proposed that this reaction depends on the proton transfer through hydrogen bonding networks in the solvent. Therefore, increasing the water concentration in a solvent mix of water and 2-propanol promotes hydrogenation as the water structuring becomes increasingly regular. The complicated structuring of water over an oxide support is clearly evident in the biphasic FFC dispersion curve. In future work, we will extend these experiments to consider the adsorption dynamics of water in solvent mixtures. Surface Interaction Strength. We now use our FFC data to explore the validity of the ratio T1/T2 as an indicator of surface interaction energy across a range of field strengths. We have shown previously, at intermediate-field (ν0 = 85 MHz), that the ratio of relaxation times T1/T2 is a sensitive probe of surface interaction strength.27 Here we repeat these T1−T2 measurements of 2-butanone, 2-propanol, and water in Pd/ Al2O3 catalyst trilobes at low-field (B0 = 50 mT), intermediatefield (B0 = 2 T), and high-field (B0 = 7 T). The low-field T1−T2 correlation plots, of particular interest here as they overlap with the frequency range explored by FFC, are shown in Figure 3. In these typical T1−T2 plots, a single peak is observed at short T1 and T2 times corresponding to imbibed liquid. The shape of the peaks in the 2D distributions is determined predominantly by the degree of smoothing in the numerical inversion which is determined by the SNR of the raw data. At low-field, SNR is

Figure 2. Frequency dependent relaxation rate of (a) 2-butanone, (b) 2-propanol, and (c) water in Pd/Al2O3 catalyst trilobes. In each case, the horizontal line indicates the bulk liquid 1/T1,bulk relaxation rate; in part c, 1/T1,bulk ≈ 0.3 s−1 is coincident with the horizontal axis. For (a) 2-butanone, the best fit to the data gives τm = 1.8 × 10−11 s and τs = 4.5 × 10−6 s; for (b) 2-propanol, τm = 2.0 × 10−10 s and τs = 1.1 × 10−5 s. For (c) water, two different relaxation regions are observed, with τm = 2.6 × 10−7 s (ω0 ≤ 106 rad s−1) and τm = 1.8 × 10−9 s (ω0 > 106 rad s−1). The dashed lines in part c show the extrapolation of the fits to higher and lower frequencies; the arrow indicates the change from “low frequency” to “high frequency” behavior for water at ω0 ≃ 106 rad s−1.

molecule on the surface is τs = 11 μs, again larger than that for 2-butanone, supporting evidence that stronger adsorption occurs due to hydrogen bonding with the surface. A frequency-independent plateau is observed below ω0 ≈ 7 × 104 rad s−1. The dispersion curve of water, Figure 2c, is notably different from those observed for the organic liquids. This variation is not unexpected, as water is known to undergo anomalous surface diffusion.45 It was not possible to fit the entire 17702

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reflect the differences in surface interaction strength observed in this catalyst system at higher fields. We use our estimates of τm,s and eq 2 to predict the frequency dependence of the ratio T1/T2, as shown in Figure 4

Figure 4. Ratios of T1/T2 for (a) 2-butanone, (b) 2-propanol, and (c) water on Pd/Al2O3 catalyst trilobes, predicted using eq 2 (solid lines) with the τm and τs values obtained in Figure 2. For water (c), the dotted line indicates the prediction from the low frequency range measure of τm. Measured T1/T2 (×) are shown for comparison, corresponding in each case to Larmor frequencies of (left to right) ν0 = 2, 85, and 300 MHz.

Figure 3. T1−T2 correlations of (a) 2-butanone, (b) 2-propanol, and (c) water in Pd/Al2O3 catalyst trilobes, measured at ν0 = 2 MHz (ωI = 1.26 × 107 rad s−1). The diagonal dashed lines indicate the identity T1 = T2. The marginal projections of T1 and T2 are shown for clarity. The contour levels are the same in each plot.

inherently low so the degree of smoothing is high. Therefore, we do not attempt to interpret the peak shapes in terms of sample-to-sample variation. Instead, we use the 2D distributions to determine T1/T2 at the maximum intensity of the main peak. The T1/T2 ratios obtained at low-field are compared to the intermediate- and high-field results in Table 1.

for (a) 2-butanone, (b) 2-propanol, and (c) water in the Pd/ Al2O3 catalyst. The predictions are compared against actual T1/ T2 values determined from the fixed-field measurements. Agreement is seen between the predicted and measured T1/ T2 values for 2-butanone and 2-propanol at low-field, as expected where T1/T2 → 1. At intermediate- to high-field, the prediction underestimates T1/T2. This under-prediction at high-field strengths is reasonable given that eq 2 is not strictly valid at these field strengths. The prediction for water, Figure 4c, shows an overestimate of T1/T2 at high-field. Of more interest is the under-prediction of T1/T2 for water even at low-field. We attribute this deviation to the anomalous dynamics of adsorbed water. Noting that the T1 values measured by FFC and fixed low-field NMR are consistent, the deviation must be due to T2. Clearly, the theoretical description of transverse relaxation on a surface (given in the Supporting Information) is not precisely applicable to biphasic water. Given the many assumptions in the predicted frequency dependence of T1/T2, obtaining predicted values of the right magnitude is sufficient to validate the measurement of T1/T2 over a wide range of field strengths. Overall, we conclude that, for the systems considered, the comparison between experiment and theory is satisfactory.

Table 1. Relaxation Time Ratios T1/T2 for 2-Butanone, 2Propanol, and Water Imbibed in Pd/Al2O3 Catalyst Trilobes, Measured with Different Fixed-Field NMR Spectrometers ν0 (MHz)

2

85

300

2-butanone 2-propanol water

1.4 1.8 5.0

3.1 7.1 10.3

4.2 12.3 18.1

The trends in T1/T2, see Table 1, are the same regardless of field strength. 2-Butanone has the weakest interaction with the pore surface (smallest T1/T2 ratio), and water has the strongest interaction (largest T1/T2 ratio) as expected. The large T1/T2 ratio for water is a consequence of the “strong” surface adsorbed layer seen in the FFC dispersion curve. It is clear that the dynamic range (sensitivity) of T1/T2 is reduced at lower fields. Notwithstanding, the low-field measurements accurately 17703

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Given this result, this analysis demonstrates that any field strength in the range ν0 = 2−300 MHz is appropriate for measurements of T1/T2 to compare the strength of surface interactions in the catalysts studied. Low-field NMR offers the potential of examining metal catalysts such as iron, copper, or cobalt that are inappropriate for analysis with high-field NMR due to the presence of magnetic susceptibility induced “internal gradients”,74 and we shall consider such applications elsewhere. Since our ongoing work9 focuses on implementing NMR techniques which can probe the behavior of multicomponent liquid systems inside porous catalysts, this validation of using T1/T2 to characterize molecule/surface interactions for all likely field strengths to be used is valuable.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Damien Murphy, Cardiff University, for the ESR measurements and Edmund Fordham, Schlumberger Gould Research, for providing access to the low-field magnet. We thank Tegan Roberts for acquiring the high-field data. We also thank Andy York and Johnson Matthey for supplying the catalysts and materials characterization data. J.M. was funded by Schlumberger Gould Research; T.C.C. was funded by the EPSRC. Additional funding was supplied by the CASTech consortium (EPSRC grant EP/G011397/1) and EPSRC grant EP/F047991/1.

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CONCLUSIONS We have considered the use of FFC NMR relaxation time dispersion measurements for the study of supported-metal catalysts. The FFC NMR dispersion data provide quantitative information on the behavior of liquids within the pore structure of supported-metal catalysts. We presented the specific examples of 2-butanone, 2-propanol, and water in 1 wt % Pd/Al2O3. These liquids and catalyst are relevant to the hydrogenation of 2-butanone. The FFC dispersion curves yielded diffusion correlation times for molecules adsorbed on the pore surface, providing information on molecular dynamics that cannot be determined readily by other techniques. It is clear from these preliminary investigations that water forms a much stronger interaction with an oxide surface than organic liquids, including alcohols, in support of our previous observations.27,28 The biphasic behavior of water, coupled with its very strong surface interaction, may explain its ability to promote or inhibit liquid-phase reactions.31 Further work, beyond the scope of this study, will be required to fully explain the anomalous behavior of water on these catalyst surfaces. Overall, we note that the surface correlation times, combined with surface residence times, give a clear indication of the strength of surface adsorption. The FFC measurements provide a more complete picture of the adsorption occurring on catalyst surfaces than can be obtained from fixed-field relaxation measurements. FFC is therefore recommended as a tool for studying systems of relevance to liquid-phase heterogeneous catalysis. In the future, we will expand these preliminary studies to investigate the dynamics of coadsorbed molecules. We have also considered the application of low-field NMR instrumentation for obtaining T1/T2 ratios in liquid-saturated catalysts. The expected ordering of surface interaction strengths is observed over a wide range of field strengths. While the sensitivity of the T1/T2 ratio is reduced at lower Larmor frequencies, we have successfully demonstrated that T1/T2 measurements at low-field are meaningful, an important validation, since catalytic materials containing paramagnetic species, in particular, are inappropriate for study at intermediate- or high-field strengths.

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ASSOCIATED CONTENT

S Supporting Information *

Basic relaxation theory and pulse sequence timing diagrams. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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