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Mar 30, 2009 - Gamman, J. J.; Jackson, S. D.; McGregor, J.; Gladden, L. F. J. Catal. 2006,. 242, 26–37. (c) Jackson, S. D.; Monaghan, A. Catal. Toda...
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J. Phys. Chem. C 2009, 113, 6610–6615

Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations Daniel Weber, Jonathan Mitchell,* James McGregor, and Lynn F. Gladden Department of Chemical Engineering and Biotechnology, UniVersity of Cambridge, Pembroke Street, Cambridge, CB2 3RA, U.K. ReceiVed: December 19, 2008; ReVised Manuscript ReceiVed: March 2, 2009

Two-dimensional nuclear magnetic resonance (NMR) relaxation time correlation measurements have been used to observe the behavior of liquids inside porous catalyst pellets; in particular, liquids of relevance to the hydrogenation of 2-butanone over a silica-supported ruthenium catalyst (Ru/SiO2). The behavior of 2-butanone is studied and compared to that of water and 2-propanol, which are used as solvents in this hydrogenation reaction. From the ratio of NMR relaxation times, T1/T2, for the liquids confined in the pores, it is possible to infer the relative strengths of the surface interaction for each liquid. Water is seen to have the strongest surface interaction, and 2-butanone has the weakest surface interaction. These results are supported by displacement experiments, in which one liquid replaces the other over time within the pore space of the catalyst. For comparison, the behavior of the same liquids in an alumina-supported palladium catalyst (Pd/ Al2O3) was also studied. The variation in the strengths of surface interactions was more pronounced in the Pd/Al2O3 catalyst than in the Ru/SiO2 catalyst. This work demonstrates the applicability of NMR relaxation time correlation experiments to real catalytic systems containing metallic components. From these measurements, information on the access of reactants to surface adsorption sites can be inferred. Introduction Understanding the interactions between adsorbates and surfaces is a key area of research in the field of heterogeneous catalysis.1-3 Such an understanding is essential to optimize existing processes and to enable the rational design of new catalysts with enhanced properties: higher activity, selectivity, and stability.3 In the present work, we demonstrate the use of two-dimensional nuclear magnetic resonance (NMR) relaxation time correlation measurements in probing adsorbate/adsorbent interactions of relevance to liquid-phase heterogeneous catalysis. Currently, liquid-phase catalysis is receiving increasing attention; in particular, with regard to the production of fuels and chemical commodities from sustainable sources.4 In the majority of reaction systems, multiple components are present in the liquid phase, either as reactant species adsorbing competitively or as a combination of reactants and solvents. Knowledge of the relative strength of interaction of each adsorbate with the catalyst surface and the influence of each component upon the adsorption of the others is a crucial step towards understanding the catalytic reaction system. For instance, solvents can play a key role in catalytic reactions, with catalytic activity depending critically upon the choice of solvent.5 NMR relaxation time measurements have been used extensively to characterize the molecular dynamics of liquids in porous media. Brownstein and Tarr demonstrated that the longitudinal T1 and transverse T2 relaxation times of liquids confined in pores are proportional to the volume-to-surface ratio of the pores.6 A distribution of pore sizes can be obtained by applying a numerical inversion to a Fredholm integral describing the relaxation time data.7 The implementation of a fast algorithm for compressing and inverting two-dimensional data sets with tensor-product structure8 has allowed relaxation time correlation * E-mail: [email protected].

measurements to be used in studies of porous reservoir rocks,9 cements,10 and food products.11 Previously, T1-T2 relaxation correlations have been analyzed using a model describing the frequency (magnetic field strength) dependence of the ratio T1/T2 for liquids in pores.10,12 This model relies on the interaction between 1H spins in the liquid nuclei and electron spins in paramagnetic species on the pore surface. In the catalyst pellets studied here, the pore surfaces are almost devoid of paramagnetic species, so this model is not applicable. However, enhanced surface relaxation will occur for both T1 and T2 due to a change in the molecular mobility of the liquid molecules when on the surface.12 The T2 relaxation times will be further depressed due to the presence of magnetic susceptibility induced internal field gradients.13 If the pores are effectively “small”, such that the effect of the internal gradients is averaged across all molecules in the pore, the T2 relaxation time will still be depressed,14 but the observed T2 will depend on the surface diffusivity. However, if the internal gradients are very large, such that the nuclear spin dephasing distance is small compared to the pore size, then the signal derived from molecules diffusing in the pores will be further attenuated,15 leading to a greater sensitivity in the observation of surface interaction. It has been shown elsewhere that T1 is unaffected by internal gradients.16 The measured ratio T1/T2 is thus expected to increase for liquids with stronger surface interactions. In this work, we are, to the best of our knowledge, the first to apply two-dimensional T1-T2 NMR relaxation correlation measurements to catalysts. As well as using the T1-T2 correlations to estimate relative strengths of surface interactions by determining the ratio T1/T2 for the reactant and solvents of interest, the T1-T2 correlations are also utilized to monitor the displacement of one liquid by another within the pore structure of catalyst pellets. Elsewhere, NMR cryoporometry17 has been used to monitor similar displacement experiments for binary mixtures of miscible and immiscible liquids in silica gels.18,19

10.1021/jp811246j CCC: $40.75  2009 American Chemical Society Published on Web 03/30/2009

Comparing Strengths of Surface Interactions Here, T1-T2 correlations are used to study liquids in two catalytic materials: Ru/SiO2 and Pd/Al2O3. The internal surfaces of these materials exhibit different surface chemistries and so provide a comparison to demonstrate the ability of the T1-T2 correlations to distinguish differing relative strengths of surface interactions between different catalytic systems. The hydrogenation of 2-butanone (methyl ethyl ketone, MEK) over supported-metal catalysts has been studied previously employing a mixture of 2-propanol (isopropyl alcohol, IPA) and water as a solvents; these studies revealed that the solvents has a significant effect on the reaction rate. The liquids studied here are therefore of relevance to this reaction system. Specifically, the rate of hydrogenation was seen to increase as the water mole fraction increased.20 Other workers have demonstrated previously that the presence of water can facilitate the hydrogenation of carbonyl moieties over ruthenium catalysts.21 In contrast, palladium catalysts show much lower activity for carbonyl hydrogenation, a fact exploited in the development of selective hydrogenation catalysts for multifunctional molecules.22 Of course, in addition to the catalytic metal, the supports can also play a key role in influencing hydrogenation reactions. For example, silica-supported Pt catalysts have been shown to be more active than alumina-supported Pt catalysts for the hydrogenation of cinnamaldehyde employing 2-propanol as a solvent.23 In general, hydrogenation reactions represent an important class of catalytic transformations in both the gas2,24 and liquid phases, with solvent effects observed frequently in the latter.21,25 Here, we employ two-dimensional NMR relaxation measurements to further understand the adsorbate-adsorbent interactions; namely, the behavior of 2-butanone, 2-propanol, and water with both of the supported-metal catalyst surfaces. These studies provide information on the ease of access of 2-butanone to the surface sites, an important consideration in understanding catalytic performance. Materials and Methods The catalysts were obtained from Johnson Matthey, in both cases, the metal having been deposited onto the extruded support via incipient wetness impregnation. The catalysts were used without further modification or treatment. The Ru/SiO2 catalyst consisted of cylindrical pellets with an average size of 4 mm × 3 mm (length × diameter), a metal surface area of 0.7 m2 g- 1, a BET surface area of 295 m2 g- 1, and a BJH median pore size of 18 nm. The Pd/Al2O3 catalyst consisted of trilobes 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 difference in the pore size distributions was not considered to have a significant influence on the NMR relaxation measurements, since the pores were large enough to accommodate both surface-adsorbed and free-diffusing molecules for each of the imbibed liquids. Additionally, the pores are of suitably small size for the “fast diffusion” limit to apply6 such that the observed relaxation times will be averaged across the surface-adsorbed and free-diffusing molecules for single liquid components in the pores. The catalysts were soaked in excess liquid for at least 12 h before NMR measurements were conducted. Previous studies have shown that this is sufficient to saturate the pores completely.26 Prior to the NMR experiments, the excess liquid was removed from the external surfaces of the pellets by placing them on presoaked filter paper; the presoaking prevented liquid from being removed from the pores during this process. For the displacement studies, catalyst pellets were left in an excess of

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Figure 1. The T1-T2 pulse sequence. The thin and thick vertical bars represent 90° and 180° radio frequency (rf) pulses, respectively. T1 relaxation is encoded in the variable time td. T2 relaxation is encoded in the train of n echoes, each one occurring at a time 2nτ. A single data point is acquired at the center of each echo.

the displacing liquid for the amount of time specified. The excess liquid was again removed prior to the acquisition of the NMR data. All NMR experiments were conducted on a 2 T (85 MHz for 1H) horizontal imaging magnet controlled via a Bruker AV spectrometer. The T1-T2 pulse sequence9 comprises a 180° inversion pulse, followed by a variable recovery delay, td, to encode T1,27 and then a Carr-Purcell-Meiboom-Gill (CPMG) echo train to encode T2.28 The pulse sequence is shown in Figure 1. The 16 recovery delays varied logarithmically from td ) 1 ms to 5 s. In the CPMG echo train, the intensity of n ) 1024 echoes were acquired in a single shot with an echo spacing of 2τ ) 1 ms, although only the even echoes were used in the data analysis. Each two-dimensional data set was acquired in ∼50 min with 16 scans to accommodate the phase cycle. The T1-T2 data sets were processed using an algorithm based on the code presented by Venkataramanan et al.8 The twodimensional data are first compressed into the form of a linear Fredholm integral, then truncated to the noise floor of the data before being inverted using nonnegative least-squares optimization. The smoothing parameter was chosen by the Butler-ReedsDawson (BRD) protocol.29 Results and Discussion The aim of this paper is to report the use of two-dimensional NMR relaxometry techniques as a method for probing the composition of coadsorbed chemical species on catalyst surfaces; in this case, reactant and solvents. The composition of species at the surface is critical to understanding the access of the reactant to the active surface sites on heterogeneous catalysts. First, experiments are performed on pure liquids in bulk and within the catalysts to characterize the relative strength of surface interaction of these molecules with the pore surface. Second, displacement of one liquid by another is tracked by NMR to investigate the extent to which one liquid-phase species can displace another at the pore surface. To validate the T1-T2 correlations, and to provide a comparison to the measurements of liquids in catalysts, T1-T2 correlations were obtained for each of the three liquids in the absence of an adsorbent; the plots are shown in Figure 2. In all cases, a single T1 and T2 component is observed, as expected, close to the T1 ) T2 diagonal. The relaxation times of the bulk liquids were as follows: for water, T1 ) 2.49 s; for 2-propanol, T1 ) 1.42 s; and for 2-butanone, T1 ) 2.49 s. The T2 relaxation time distributions are seen to be broader than the corresponding T1 distributions for each liquid, and the modal T2 times shorter than T1, due to diffusive attenuation in the CPMG measurement. T1-T2 correlations for the solvents and reactant in the Ru/ SiO2 pellets are shown in Figure 3. The T1-T2 plot for water, Figure 3a, is typical for a liquid in a monodispersed porous medium.30 Narrow distributions of T1 and T2 are observed, as can be seen on the one-dimensional projections, with modal relaxation times of 0.673 and 0.066 s, respectively. Therefore,

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Figure 2. T1-T2 relaxation correlation plots for bulk (a) water, (b) 2-propanol, and (c) 2-butanone. The dotted diagonal lines indicate T1 ) T2. The projections of the one-dimensional T1 and T2 distributions are shown on each plot for clarity.

the average ratio 〈T1/T2〉 ≈ 10.2 for water. This value is indicated by the dashed diagonal line in each plot for comparison. For 2-propanol, Figure 3b, a narrow distribution is observed in the T1 dimension, centered on T1 ) 0.739 s, whereas the T2 distribution is broad with a modal relaxation time of T2 ) 0.087 s so that 〈T1/T2〉 ≈ 8.5. The small peaks visible in the T1-T2 plot for 2-propanol at long T2 relaxation times are due to excess liquid on the outside of the pellets. For 2-butanone, Figure 3c, the modal relaxation times are T1 ) 1.292 s and T2 ) 0.152 s, providing a ratio 〈T1/T2〉 ≈ 8.5. From the 〈T1/T2〉 ratios, it is possible to deduce the relative strengths of surface interaction for the three liquids. Water, with 〈T1/T2〉 ≈ 10.2, has the strongest surface interaction. This is not unexpected, since the water molecules can hydrogen-bond to OH groups usually present on the surface of the SiO2 support. Both the 2-propanol and 2-butanone appear to have similar strengths of surface interaction, with 〈T1/T2〉 ≈ 8.5. The experiments conducted for the liquids in the Ru/SiO2 pellets were repeated with the Pd/Al2O3 trilobes. The corresponding T1-T2 correlation plots are shown in Figure 4 for (a) water, (b) 2-propanol, and (c) 2-butanone. Again, a single high intensity feature is observed in all the one-dimensional T1 and T2 projections that can be associated with the liquids in the pores of the catalyst. Additional minor peaks are also visible at longer T1 and T2 times, corresponding to excess liquid on the external surfaces of the trilobes. The T2 relaxation times of these additional peaks tend toward the solid diagonal line, where T1 ) T2, as would be expected for free liquid. In the case of water, the modal T1 and T2 relaxation times are approximately 0.072 and 0.007 s, respectively, so the ratio 〈T1/T2〉 ≈ 10.2, as indicated by the dashed diagonal line. This ratio is shown in each plot for comparison. For the 2-propanol, modal relaxation times of T1 ) 0.242 s and T2 ) 0.034 give 〈T1/T2〉 ≈ 7.1; for 2-butanone,

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Figure 3. T1-T2 relaxation correlation plots for (a) water, (b) 2-propanol, and (c) 2-butanone in porous Ru/SiO2 pellets. The dotted diagonal lines indicate T1 ) T2. The solid diagonal lines indicate (a) T1 ) 10.2T2, (b) T1 ) 8.5T2, and (c) T1 ) 8.5T2. The dashed diagonal lines in b and c indicate T1 ) 10.2T2 for comparison. The projections of the one-dimensional T1 and T2 distributions are shown on each plot for clarity.

T1 ) 0.464 s and T2 ) 0.152 so that 〈T1/T2〉 ≈ 3.1. There is a significant difference in the T1/T2 ratio for each of the liquids in the Pd/Al2O3 trilobes. Deducing the order of relative strengths of interaction with the pore surface is therefore straightforward: water has the strongest interaction, and 2-butanone, the weakest. It is tempting at this point to draw a direct comparison between the T1/T2 ratios observed for the liquids in the two catalytic systems. For example, the T1/T2 ratio is almost the same for water in the Ru/SiO2 pellets and the Pd/Al2O3 trilobes. However, we do not conclude from this that the strength of interaction is the same for water on both surfaces, since the T1/T2 ratio will be influenced by other factors, such as the magnitude of the internal magnetic field gradients: this topic, discussed briefly above, will be the subject of a future publication. It has been shown elsewhere that T2 relaxation times can vary significantly for the same adsorbates in similar types of porous material,19 and so care should be taken when comparing such experimental results derived from different catalytic systems. As well as providing two independent measurements (i.e., T1 and T2) of the molecule-sorbent interactions, acquisition of the T1-T2 correlation data also enables the identification of individual chemical species within a mixture, where acquisition of the T1 or T2 data alone would not provide adequate discrimination between species. The T1-T2 correlation measurements can therefore be used to examine liquids coadsorbed onto the pore surfaces. As an example, we consider the behavior of the two solvents, water and 2-propanol, in the Ru/SiO2 pellets. The displacement of water by 2-propanol is shown in Figure 5a-d. Initially, in Figure 5a, a single peak associated with water

Comparing Strengths of Surface Interactions

Figure 4. T1-T2 correlation plots for (a) water, (b) 2-propanol, and (c) 2-butanone in porous Pd/Al2O3 trilobes. The dotted diagonal lines indicate T1 ) T2. The solid diagonal lines indicate (a) T1 ) 10.3T2, (b) T1 ) 7.1T2, and (c) T1 ) 3.1T2. The dashed diagonal lines in b and c indicate T1 ) 10.3T2 for comparison. The projections of the onedimensional T1 and T2 distributions are shown on each plot for clarity.

is visible. After the pellets have been soaked in 2-propanol for 2 min, a secondary feature is visible above the main peak (see Figure 5b) that we associate with 2-propanol due to its broad T2 relaxation time distribution. The primary peak, now associated here with water due to its T1/T2 ratio, has shifted to shorter T1 and T2 times, suggesting the water is interacting with the pore surfaces. After soaking for 12 min (Figure 5c), the “2propanol” peak has increased in intensity relative to the broadened “water” peak. After the pellets have been soaking in 2-propanol for 65 min (Figure 5d), two distinct features are visible, attributed to 2-propanol (long T1 and T2) and water (short T1 and T2). This peak assignment implies 2-propanol coadsorbed with water in the pore network exhibits similar relaxation times, as compared to 2-propanol adsorbed as a single component in the Ru/SiO2 pellets. The peak assigned to water is, however, located at significantly shorter values of T1 and T2 than for water adsorbed as a single component in the Ru/SiO2 pellets, although the ratio T1/T2 is the same. It therefore appears that 2-propanol cannot displace water completely from the pores on the time scale of this experiment. In the reverse experiment, for water displacing 2-propanol from the Ru/SiO2 pellets (Figure 5e-h), the water is seen to displace almost all of the 2-propanol within 65 min. As the 2-propanol saturated pellets are exposed to water, a peak assigned to water appears at short relaxation times. After 65 min (Figure 5h), a single peak remains with relaxation times consistent for water adsorbed as a single component in the Ru/ SiO2 pellets. Water is able to displace 2-propanol completely from the pores on the time scale of the experiment, suggesting water has a stronger interaction with the pore surfaces than 2-propanol.

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Figure 5. A selection of T1-T2 correlation plots showing displacement of (a-d) water by 2-propanol, and (e-h) 2-propanol by water in Ru/ SiO2 pellets. The data were acquired after displacement times of (a, e) 0, (b, f) 2, (c, g) 12, and (d, h) 65 min. The diagonal lines indicate T1 ) T2 (dotted), T1 ) 10.2T2 (dashed), and T1 ) 8.5T2 (solid). The boxes provide a guide to the eye, indicating the peaks attributed to water; any other peaks are assumed to originate from the 2-propanol.

Integration of the peaks associated with each of the liquids in the T1-T2 correlations provides the volume fraction of liquid in the pores as a function of displacement time; boxes representing areas of integration for the peaks associated with water have been added to the plots in Figure 5 as a guide to the eye. The integration serves to remove the relaxation weighting associated with the signal intensity of each liquid, and so the resultant integral intensities from the T1-T2 correlation plots can be compared directly within each complete displacement experiment. We present a brief, coherent set of displacement experiments for several reactant/solvent combinations in each of the catalytic materials to demonstrate this process. The displacement of 2-butanone by water (and vice versa) is shown in Figure 6 for the (a) Ru/SiO2 pellets and (c) Pd/Al2O3 trilobes. In both catalysts, the water displaces the 2-butanone rapidly, and complete displacement of the 2-butanone is observed after about 4 × 102 s. In contrast, the 2-butanone takes much longer to displace the water, and complete exchange is not observed in the time frame of the experiment. After soaking the initially water-saturated catalysts in 2-butanone for 105 s, the Ru/SiO2 pellets still contain ∼40% water by volume, and the Pd/Al2O3 trilobes, ∼36% water by volume. The displacement of 2-butanone by 2-propanol (and vice versa) is shown in Figure 6 for (b) Ru/SiO2 pellets and (d) Pd/ Al2O3 trilobes. 2-Propanol displaces 2-butanone almost com-

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Figure 6. Volume fraction of 2-butanone remaining in Ru/SiO2 pellets determined during (a) displacement of 2-butanone by water (1) and water by 2-butanone (2), and (b) displacement of 2-butanone by 2-propanol (9) and 2-propanol by 2-butanone ((). Volume fraction of 2-butanone remaining in Pd/Al2O3 trilobes determined during (c) displacement of 2-butanone by water (1) and water by 2-butanone (2), and (d) displacement of 2-butanone by 2-propanol (9) and 2-propanol by 2-butanone ((). The connecting lines are shown as a guide to the eye.

pletely from the Ru/SiO2 pellets. After the initially 2-butanonesaturated pellets have been soaked in 2-propanol for 1.3 × 103 s, a plateau in the 2-butanone volume fraction of 2% is obtained. Similarly, for the Pd/Al2O3 trilobes, a final volume fraction of 4% 2-butanone is achieved when the 2-butanonesaturated trilobes are soaked in 2-propanol. For the reverse displacement, 2-butanone cannot displace all of the 2-propanol in the Ru/SiO2 pellets, and 13% 2-propanol by volume remains in the pores. The 2-butanone does displace nearly all of the 2-propanol from the Pd/Al2O3 trilobes in the time available, although the data points suggest this process occurs more slowly than in the Ru/SiO2 pellets. The results from the displacement experiments in Figure 6 are in agreement with the relative strengths of surface interaction for the three liquids adsorbed as single components in the pores deduced from the T1-T2 correlation plots in Figures 3 and 4. For example, the strength of the surface interaction for water was seen to be much greater than for 2-butanone: in both catalysts, water displaced 2-butanone rapidly, whereas 2-butanone could not fully displace water in the time available. This is highlighted in the T1-T2 correlation plot shown in Figure 7, recorded after the initially water-saturated Pd/Al2O3 trilobes had been soaked in 2-butanone for 24 h (∼105 s), which contains two peaks of significant intensity. The secondary peak at short relaxation times is associated with water due to its T1/T2 ratio. The origin of the primary peak at long T1 and T2 times is more difficult to determine: the modal T1 ) 2.05 s, close to but less than T1 ) 2.49 s for bulk water and 2-butanone. The T2 distribution associated with this peak is broad, yet monomodal, centered on T2 ) 0.423 s. The most likely origin of this peak is 2-butanone in the pore bodies or large pores (hence, long, bulklike T1 due to an increased volume-to-surface ratio, and short T2 due to diffusion in internal gradients) so that the secondary peak would originate from water on the pore surfaces or in small pores. Further experiments, beyond the scope of this current work, would be required to clarify the distribution of the remaining water. The relative strengths of surface interaction of the three liquids in the Pd/Al2O3 trilobes are significantly different. Water has a much stronger affinity for the pore surface than either 2-propanol

Figure 7. T1-T2 correlation plot of water and 2-butanone in Pd/Al2O3 trilobes after the initially water-saturated trilobes have been exposed to 2-butanone for 24 h. A significant water signal is still observed at short relaxation times, indicating the 2-butanone cannot displace all of the water. The dotted diagonal line indicates T1 ) T2, the dashed diagonal line indicates T1 ) 10.3T2 (adsorbed water), and the solid diagonal line indicates T1 ) 3.1T2 (adsorbed 2-butanone).

or 2-butanone, although 2-propanol still has a stronger interaction than 2-butanone. It is possible to suggest for this material that the affinity of water and 2-propanol for the surface may inhibit the ability of the 2-butanone to access the surface adsorption sites. In contrast, 2-propanol has a strength of surface interaction similar to 2-butanone in the Ru/SiO2 cataylst, where the access of the reactant to the surface adsorption sites is not impeded by the presence of the solvent. However, to determine the role of surface interaction strength in dictating catalytic behavior, it is necessary to study catalysts in their working state (e.g. after activation (reduction)) and for a range of support materials. This is beyond the scope of this current work and is the subject of ongoing study. Here, we conclude only that twodimensional NMR relaxometry techniques, used to characterize multicomponent liquid-phase systems, constitute a powerful new tool to probe the composition of reactants and solvents on the surface of catalytic materials in liquid-phase reactions. This, in turn, provides an indication of the ability of adsorbates to access sites on the catalyst surface. Conclusions In this paper, we have utilized T1-T2 correlations to determine the relative strengths of the surface interaction of three liquids

Comparing Strengths of Surface Interactions relevant to the hydrogenation of 2-butanone in porous catalyst materials for the first time. In Ru/SiO2, water was seen to have a stronger interaction with the surface than 2-propanol or 2-butanone. The 2-propanol and 2-butanone had similar relative strengths of surface interaction. In Pd/Al2O3, water was seen to have the strongest surface interaction; then 2-propanol; and finally, 2-butanone, having the weakest interaction. The relative strengths of surface interaction were deduced directly from the average ratio of relaxation times, 〈T1/T2〉, determined from twodimensional T1-T2 correlation plots. Displacement experiments provided support for the trends observed in the 〈T1/T2〉 measurements; agreement was found in all cases. The fact that the solvent liquids have strong surface interactions in Pd/Al2O3 suggests 2-butanone would have reduced access to adsorption sites on the catalyst surface as compared to the surface composition in Ru/SiO2, where the liquids have similar strengths of interaction. This method provides additional information on the behavior of liquids inside porous catalysts and could allow for improved predictions of catalyst performance. In future applications of this method, we will endeavor to explore the surface interactions for coadsorbed liquids using the recent implementation of a rapid pulse sequence for determining T1-T2 correlations while retaining chemical shift information31 and explore possible relationships between surface liquid composition and catalytic activity. Acknowledgment. We thank Schlumberger Cambridge Research for providing access to the original Fast Laplace Inversion code, Mr. Thusara Chandrasekera for assistance with the data analysis, and Dr. Andy York for helpful discussions. For financial support, the authors thank the CARMAC consortium (EPSRC Grant GR/S43719/01). For additional funding, J. Mitchell thanks Schlumberger Cambridge Research and D. Weber thanks the Cambridge European Trust and the Studienstiftung des deutschen Volkes. References and Notes (1) (a) Moggi, P.; Predieri, G.; Di Silvestri, F.; Ferretti, A. Appl. Catal., A 1999, 182, 257–265. (b) Maroto, A.; Rodriguez-Ramos, I.; GuerreroRuiz, A.; Llorca, J.; de la Piscina, P. R.; Homs, N. Appl. Organomet. Chem. 2000, 14, 783–788. (c) Mantle, M. D.; Steiner, P.; Gladden, L. F. Catal. Today 2006, 114, 412–417. (d) Norskov, J. K.; Scheffler, M.; Toulhoat, H. MRS Bull. 2006, 31, 669–674. (e) Nurunnabi, M.; Murata, K.; Okabe, K.; Inaba, M.; Takahara, I. Catal. Commun. 2007, 8, 1531–1537. (f) Baylet, A.; Royer, S.; Marecot, R.; Tatibouet, J. M.; Duprez, D. Appl. Catal., B 2008, 77, 237–247. (g) Gopinath, R.; Babu, N. S.; Kumar, J. V.; Lingaiah, N.; Prasad, P. S. S. Catal. Lett. 2008, 120, 312–319. (2) McGregor, J.; Gladden, L. F. Appl. Catal., A 2008, 345, 51–57. (3) Christensen, C. H.; Norskov, J. K. J. Chem. Phys. 2008, 128, 182503. (4) (a) Kunkes, E. L.; Simonetti, D. A.; West, R. M.; Serrano-Ruiz, J. C.; Gartner, C. A.; Dumesic, J. A. Science 2008, 322, 417–421. (b) Christensen, C. H.; Rass-Hansen, J.; Marsden, C. C.; Taarning, E.; Egeblad, K. ChemSusChem 2008, 1, 283–289. (5) (a) Asplund, S.; Fornell, C.; Holmgren, A.; Irandoust, S. Catal. Today 1995, 24, 181–187. (b) Jin, G.; Ido, T.; Goto, S. Catal. Today 2003, 79, 471–478. (c) Liu, X.; Wang, X.; Guo, X.; Li, G. Catal. Today 2004, 93-95, 505–509. (6) Brownstein, K. R.; Tarr, C. E. Phys. ReV. A 1979, 19, 2446–2453.

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