Pulsed-Field Gradient NMR Spectroscopic Studies of Alcohols in

Oct 4, 2010 - ... Dan I. Enache, Ewa Nowicka, Scott P. Davies, Jennifer K. Edwards, Carmine D'Agostino, Darren P. Mascarenhas, ... F. Lee , and Rob Ev...
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J. Phys. Chem. C 2011, 115, 1073–1079

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Pulsed-Field Gradient NMR Spectroscopic Studies of Alcohols in Supported Gold Catalysts† Mick D. Mantle,‡ Dan I. Enache,§ Ewa Nowicka,§ Scott P. Davies,§ Jennifer K. Edwards,§ Carmine D’Agostino,‡ Darren P. Mascarenhas,‡ Lorraine Durham,‡ Meenakshisundaram Sankar,§ David W. Knight,§ Lynn F. Gladden,‡,§ Stuart H. Taylor, and Graham J. Hutchings*,§ Department of Chemical Engineering and Biotechnology, UniVersity of Cambridge, Pembroke Street, Cambridge, CB2 3RA, and Cardiff Catalysis Institute, School of Chemistry, Cardiff UniVersity, Cardiff, CF10 3AT, U.K. ReceiVed: June 28, 2010; ReVised Manuscript ReceiVed: September 15, 2010

We report a pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopic study of the effective diffusivity of alcohols in catalysts comprising gold supported on silica, titania and ceria and gold-palladium alloy nanoparticles supported on titania. These catalysts are shown to be highly active for the selective oxidation of alcohols. However, we observe that molecules possessing hydroxyl functional groups in the 2-position exhibit very low reactivities. To help understand the nature of conversion and selectivity, we observe from traditional catalytic measurements involving gas chromatography of the reaction mixtures, we have studied the effective self-diffusivities, Deff, of 1-, 2-, and 3-octanols and 1,2- and 1,4-butanediols in Au-ceria, Au-silica, Au-titania, and Au-Pd-titania using PFG-NMR spectroscopy. The results show that the octanols diffuse approximately 35% slower on silica supports than on titania. In addition, a marked two-component diffusive behavior is seen for ceria-supported catalysts with the dominant component, for 1-, 2-, and 3-octanols, being close to that of the free bulk liquid, and the slower component being an order of magnitude slower. The values of the 1,2- and 1,4-butanediol self-diffusion coefficients for silica-based gold catalysts are closer to those of the bulk liquid 1,2- and 1,4-butanediols. Au-Pd-titania also showed reduced self-diffusivities when compared with the bulk liquids but were similar to their monometallic counterparts. A new parameter, ξ, the PFG-NMR interaction parameter, is introduced and is defined as the ratio of free liquid diffusivity to effective liquid diffusivity within the porous medium and accounts, collectively, for the functional group interaction of the probe molecule with itself and the porous medium. This parameter, along with reference tortuosity values determined by PFG-NMR gives new insight into the dynamics of hydrogen-bonded networks of different functional groups that exist within the porous catalyst matrix. The inhibition effect observed from traditional catalytic activity studies for the oxidation of 2-octanol is considered to result from competitive adsorption of the ketone product. Introduction Liquid-phase heterogeneous catalysis is widely used in the commercial manufacture of intermediates and finished products. It is widely used, for example, in hydrogenation, dehydrogenation, isomerization, and oxidation reactions. The performance of the catalyst in terms of its conversion and selectivity is linked closely with the transport of reactants and products to and from active surface sites. These processes are controlled by molecular diffusion of reactants and solvents in the liquid phase.1,2 A detailed understanding of the behavior of liquids within the pore space of the catalyst matrix and hence the degree of access to catalytically active sites is important in the design and optimization of new catalysts with improved performance including higher activity and reactivity. Nuclear magnetic resonance (NMR) is one of the few methods that can probe directly the self-diffusion coefficients of liquids in porous media3 as well as the interactions between adsorbates and adsorbents by using relaxation measurements.4 Recently,5 the ability to probe the strength of surface interaction for reactants and solvents in porous heterogeneous catalysts using †

Part of the “Alfons Baiker Festschrift”. University of Cambridge. § Cardiff University. ‡

correlations of longitudinal T1 and transverse T2 NMR relaxation times was demonstrated. Field cycling NMR relaxometry has also been used to probe surface dynamics in saturated porous media.6–8 This method gives estimates of surface diffusion correlation times and surface residence times, from which diffusion coefficients can be calculated. However, the analysis assumes that probe molecules are jumping between paramagnetic relaxation centers on the pore surface, which is unlikely for the porous catalytic materials studied here due to the lack of paramagnetic species. The study of liquid diffusion in porous media using pulsed field gradient nuclear magnetic resonance (PFG-NMR) methods3 has long been established as a useful tool to probe the behavior of liquid molecules held within a porous matrix.9,10 More recently, Weber et al.11 showed that PFG-NMR measurements could be used to probe both bulk liquid pore-diffusion and the diffusion of a liquid surface in a Pd/alumina catalyst matrix. The results presented here do not include surface diffusion coefficients and the measurements are for bulk pore liquids and give the effectiVe self-diffusion coefficient, Deff, in accordance with the terminology of Hoogschaden.12 Selective oxidation is an important process for the chemical industry.13 In many cases the reaction is carried out using a three-phase reaction mixture (solid catalyst, liquid-

10.1021/jp105946q  2011 American Chemical Society Published on Web 10/04/2010

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phase reactant, and molecular oxygen). Recently, we have shown that gold-palladium nanoparticles supported on titania are exceptionally effective for the oxidation of primary alcohols.14 Primary alcohols are particularly difficult substrates for selective oxidation, but we found that this catalyst was effective for the oxidation of straight chain alcohols, such as 1-octanol. However, although 1-octanol was reactive, 2-octanol was found to be unreactive in contrast to the general trend of reactivities where secondary alcohols are typically more reactive than primary alcohols. In this paper, we study this effect in more detail and, in particular, use PFG-NMR to help determine if the origin of this effect is due to diffusion and physical interaction of the reactants and products with the porous catalyst matrix.

TABLE 1: PFG-NMR Parameters Used for Both Pure Liquids and Liquids in Porous Media

Experimental Methods

both pure liquids and liquids imbibed in porous media. The two different sets of PFG-NMR parameters given in Table 1 were optimized according to whether the diffusion of pure liquids or liquids in porous media were being measured. Following excitation of the nuclear spin system (here the 1H’s associated with alcohols, ketones, and diols), a magnetic field gradient g1 is applied for a time δ, which imparts a phase shift φ1(r) on the nuclear spin system. Here the dependence on r indicated that this phase shift is spatially dependent. The spin system is then allowed to evolve (diffuse) over an observation time ∆ before another gradient pulse-g1 of equal magnitude but opposite polarity is applied. If the spin system is static (i.e., no diffusion), then the phase shift imparted by the first is completely refocused and the acquired signal will be the same whether the gradient pulse is applied or not. However, if molecular diffusion translates the nuclear spins to a different spatial region within the magnetic field gradient, the phase shift φ1(r) will not be refocused and a reduction in the overall signal intensity will occur. By repeating this basic experiment for different gradient magnitudes g, one then obtains a data set that can be used to extract the value of the self-diffusion coefficient D as follows. In this work, we employed the 13-interval of Cotts et al.15 (alternating pulsed gradient stimulated echo sequence (Figure 1) (APGSTE sequence) to minimize signal loss due to diffusive motion in “background magnetic field gradients” that exist within the sample as a result of differences in material magnetic susceptibility. The acquired signal of a PFG-NMR experiment as a function of gradient strength S(g) is defined by

Selective Oxidation of Alcohols. Catalyst Preparation. Catalysts comprising 2.5 wt % Au/2.5 wt % Pd/TiO2 were prepared using the following standard method (all quantities stated are per gram of finished catalyst). PdCl2 (0.042 g, Johnson Matthey) was added to a HAuCl4 · 3H2O solution (Johnson Matthey, 2.5 mL, 5 g in 250 mL) and stirred at 80 °C until the Pd dissolved completely. The titania support (0.95 g; TiO2 (P25, Degussa)) was then added to the solution, and the resulting mixture was stirred to form a paste. The paste was dried (110 °C, 16 h) before calcination (400 °C, 3 h). This material has been characterized and is known to comprise Au-Pd nanoparticles with a Au-rich core and Pd-rich surface.13 Monometallic catalysts were prepared using a similar procedure. In addition, ceria (Aldrich) and silica (Aldrich) were used as supports. All supports contained nanopores with pre diameters of e20 nm. Alcohol Oxidation. The oxidation of alcohols was carried out in a stirred reactor (100 mL, Autoclave Engineers Inline MagneDrive III). Samples from the reactor were taken periodically, via a sampling system. For the analysis of the products a GC-MS and GC (a Varian star 3400 cx with a 30 m CP-Wax 52 CB column) were employed. The products were identified by comparison with known standard samples. For the quantification of the amounts of reactants consumed and products generated, an external standard was used. NMR Studies. NMR Sample Preparation. n-Octane, 1-octanol, 2-octanol, 3-octanol, 2-octanone, 1,2-butanediol, and 1,4butanediol were obtained from Sigma Aldrich and used as received. For PFG-NMR measurements, a small amount of each catalyst was completely immersed in the liquid under investigation and left for at least 24 h to equilibrate. The catalyst was then removed and gently rolled on filter paper (which was presoaked in the diffusing liquid) to remove any excess liquid from the external surface. This was to ensure the PFG-NMR measurement was for the liquid inside the catalyst. The pore space of each sample is assumed to be saturated with liquid during the course of the PFG experiment. To minimize errors associated with subsequent evaporation of the relevant liquids from within the liquid filled pore space, a small amount of pure liquid was placed onto absorbent paper, which was subsequently folded and placed tightly into the cap of the 5 mm NMR sample tube to ensure a saturated atmosphere above the sample. PFG-NMR Experimental Method. All experiments were conducted using a Bruker Biospin DMX-300 NMR spectrometer with a 7.07 T vertical magnet, equipped with a shielded gradient coil operating at a 1H frequency of 300.13 MHz and a temperature of 293 K. For proton PFG NMR, a 5.0 mm saddle coil was used to excite and detect the NMR signal. The 13interval APGSTE sequence of Cotts et al.15 was used for all experiments. Table 1 shows the PFG-NMR parameters used for

1

H

liquids adsorbed pure liquids in catalyst max. gradient value (G/cm) gradient start value (G/cm) no. of gradient steps gradient pulse duration, δ (ms) diffusion time, ∆ (ms) gradient ramp time (ms) gradient pulse stabilization time (ms) no. of scans

100 0.5 16 2 75 0.1 5 16

S(g) δ ) exp -γ2δ2g2D ∆ S0 3

[

(

500 0.5 16 1.5 50 0.1 0.5 16

)]

(1)

where S(g) is the acquired signal, γ is the gyromagnetic ratio of the nucleus under investigation, δ is the length of the applied magnetic field gradient pulse, g is the strength of the applied magnetic field gradient pulse, D is the diffusion coefficient, and ∆ the observation time. For free diffusion a plot of log(S(g)/S0)

Figure 1. Schematic of the 13-Interval APGSTE pulse sequence used in this work.

Alcohols in Supported Gold Catalysts

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TABLE 2: Oxidation of Alcohols Using Supported Au Catalystsa catalyst

temperature (°C)

alcohol

TOF (s-1)b

0.7% Au/SiO2 2% Au/TiO2 2%Au/CeO2 5%Au/TiO2 5%Au/TiO2 5%Au/TiO2 0.7% Au/SiO2 2% Au/TiO2 2%Au/CeO2

100 100 100 160 160 160 100 100 100

1-octanol 1-octanol 1-octanol 1-octanol 1,2-butanediol 1,4butanediol benzyl alcohol benzyl alcohol benzyl alcohol

0 0 4.4 7.6 5.2 10.0 5.0 5.9 30.9

a Reaction conditions: alcohol (20 mL), catalyst (100 mg), O2, 2 bar pressure, stirring 1500 rpm. b Turnover frequency determined at 3 h reaction time.

TABLE 3: Oxidation of Alcohols Using Au-Pd/TiO2 as a Catalysta alcohol

TOF (s-1)b

1-octanol 2-octanol 3-octanol 1-octanol + 2-octanolc 1,2-butandiol 1,4-butandiol benzyl alcohol

0.6 0 29.6 0 0.4 28.9 24.0

a

Reaction conditions: alcohol (40 mL), Au-Pd/TiO2 catalyst (200 mg), 160 °C, O2, 1 bar pressure, stirring 1500 rpm. b Turnover frequency determined at 30 min reaction time. c 1:1 mol ratio.

against γ2δ2g2(∆ - δ/3) (known as a “log-attenuation plot”) produces a straight line where the slope represents the diffusion coefficient D. In certain cases a curve may result and often there are two distinguishable gradients. There can be one or more reasons16 for this curvature in the log-attenuation plot that may result from a number of different contributions: (1) the combination of bulk and surface diffusing species;11 (2) approximation of the average pore diameter by the root mean squared displacement of the diffusing molecule;3 (3) multiple components, i.e., different diffusing species; (4) instrumental artifacts, like eddy currents, internal magnetic field gradients, or incomplete phase cycling causing nonlinearity in the data obtained as a result of incoherent magnetization being present during the acquisition of the signal S(g). All individual diffusion coefficients are quoted to be accurate within 2% and therefore the error in the quoted percentage differences between two different samples is approximately 3%. Results Selective Oxidation of Alcohols. Initial trials using monometallic supported gold catalysts and primary alcohols showed little promise (Table 2) when low concentrations of Au were used. Increasing the Au concentration and the reaction temperature did enhance the activity, and we were able to observe reactivity with 1-octanol, 1,2-butanediol, and 1,4-butanediol. We also used benzyl alcohol as a substrate, and we found that with low concentrations of gold, the TiO2-supported catalyst gave the highest activity. We then turned to the use of Au-Pd/TiO2 as we had previously found this to be very active.14 The oxidation of a range of alcohols was carried out using standard reaction conditions using the Au-Pd/TiO2 catalyst, and the results are shown in Table 3. The results are expressed as turnover frequencies (TOF, moles of alcohol converted per moles of metal per second) determined at 30 min reaction time.

Figure 2. Time online study of the oxidation of 1-octanol and 3-octanol with 5%(Au-Pd)/TiO2. Reaction conditions: alcohol (20 mL), catalyst (3.5 mg), 160 °C, O2, 2 bar pressure, stirring 1500 rpm. Key: 9, conversion of 3-octanol; b, conversion of 1-octanol. The selectivity was 100% to the aldehyde.

1-Octanol reacts slowly and with total selectivity to octanal. In contrast, 3-octanol is highly reactive, giving 3-octanone. This is in agreement with the general trend that secondary hydroxyl groups are more readily oxidized compared to primary alcohols. Interestingly, although 3-octanol is initially very reactive, the oxidation does not continue to completion and hence we observe inhibition with this system at longer reaction times (Figure 2). This was also observed with 1-octanol and it is interesting that the initial rates of reaction for 1- and 3-octanol appear to be very similar when investigated in this way. However, in contrast, 2-octanol is unreactive and subsequent experiments show that the addition of 2-octanone to 1-octanol switches off the oxidation of the 1-octanol totally, and this can be observed if only 2% of the ketone is added. The oxidation of 1,2- and 1,4-butanediol also shows a large contrast in their relative rates. 1,4-Butanediol is very reactive under these conditions, and one alcohol group is oxidized selectively to the aldehyde. Consistent with the foregoing findings, the 1,2-butanediol isomer reacted much more slowly. Benzyl alcohol is slightly less reactive, possibly due to steric hindrance due to the phenyl group. To gain an understanding of these differences in rate, we have carried out a detailed PFG-NMR spectroscopic study to determine if diffusion plays a role in the oxidation of these alcohols and this is discussed in a subsequent section. PFG-NMR Studies. Figure 3a-c shows the log-attenuation plots for 1-octanol, 2-octanol, 3-octanol, 1,2-butanediol, and 1,4butanediol imbibed within gold-silica (Au/SiO2), gold-ceria (Au/CeO2), and gold-titania (Au/TiO2) catalysts, respectively. The fitted exponential function given by eq 1 is represented on the semilog plot using a one-component fit whose gradient gives the value of the self-diffusion coefficient, D. Octanols diffusing in Au-CeO2 catalysts were fitted to two components, i.e., the sum of two exponential functions. Figure 3d shows the log-attenuation plot for 1-octanol, 2-octanol, 3-octanol, 2-octanone, 1,2-butanediol, and 1,4-butanediol imbibed within the Au-Pd/titania catalyst. The diffusion coefficients calculated from the fits to eq 1 of all samples are summarized in Table 4. Table 4 shows that for the Au-SiO2 catalyst the octanols selfdiffusion coefficient is approximately 50% slower than those of the pure liquids; however, for Au-TiO2 and Au-Pd/TiO2 the value of the self-diffusion coefficients for the octanols within the porous catalyst is only some 23% lower than those of the pure octanols. A noticeable two-component behavior exists for

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Figure 3. Log-attenuation plot of NMR signal intensity S(g) vs γ2g2δ2(∆ - δ/3) for liquids imbibed within the various Au and Au-Pd catalysts. The fitted lines give the value of the diffusion coefficient D.

TABLE 4: Diffusion Coefficients Obtained from the Gradient of the Straight Line Fits Shown in Figure 3 diffusion coefficient (10-10 m2/s) chemical

pure liquid

1-octanol 2-octanol 3-octanol 2-octanone 1,2-butanediol 1,4-butanediol n-octane

1.23 1.43 1.44 12.1 0.28 0.20 22.2

Au/SiO2

Au/CeO2

Au/TiO2

0.66 0.68 0.66

(0.83)/(0.11) (1.32)/(0.13) (1.30)/(0.12)

1.00 1.03 0.99

0.20 0.14 12.7

0.29 0.27

0.24 0.18 14.1

Au-Pd/ TiO2 0.93 1.02 0.88 6.20 0.24 0.18 12.0

gold-based ceria catalysts for octanol diffusion. The faster component resembles more closely the self-diffusion coefficient of the pure liquid and the slower component is an order of magnitude slower than the pure liquid value. The values of 1,2- and 1,4-butanediol self-diffusion coefficients for Au/SiO2 are approximately 30% lower than those of the bulk liquid butanediols, but the values of the self-diffusion coefficient for the Au/TiO2 and Au-Pd/TiO2 are only 15% lower than the pure liquid self-diffusivities of the butanediols. The log-attenuation plots for 1,2- and 1,4-butanediol selfdiffusion in Au/CeO2 are closer to a one-component behavior than two-component behavior and were thus fitted as a single component. The value of the self-diffusivity of the 1,2- and 1,4butanediols in Au/CeO2 is similar to that of the pure liquid for 1,2-butanediol but is actually enhanced for the 1,4-butanediol Au/CeO2 system relative to the pure 1,4-butanediol liquid.

Au-Pd/TiO2 showed decreased self-diffusivity coefficients for 1,2- and 1,4-butanediol when compared with those of the bulk liquids. The diffusivity of the ketone, 2-octanone, was 50% smaller than that of the pure liquid ketone. A discussion of these results in terms of degrees of hydrogen bonding is presented. Discussion Self-Diffusion of Octanols. The diffusion data shown in Figure 3a-d for the three octanols show an essentially linear behavior except for the Au/CeO2 catalyst. With the exception of the ceria catalyst the lower value of the self-diffusivity for all of the octanols, when compared with that of the pure liquid, is indicative of restricted diffusion in these nanoporous materials. The apparent lack of curvature in Figure 3 (except for Au/CeO2) also shows that restricted diffusion of the probe molecule is occurring inside the pore space of the catalyst matrix over a length scale that is much smaller than the root mean squared displacement of the free pure liquid; for ∆ ) 50 ms and D(1,4butanediol) ) 0.20 × 10-10 m2 s-1 the root mean squared displacement is given by rms ) (2D∆)1/2 ≈ 1.4 µm. Thus, during the ∆ ) 50 ms observation time, there are many collisions of even the slowest diffusing liquid molecules with the pore walls. As the samples are saturated with liquid, the system is likely to be in the region of bulk liquid pore diffusion as described by Hoogschaden.12 Using the values presented in Table 4, we observe that for 1-octanol, 2-octanol, and 3-octanol imbibed in Au/SiO2, the value of the self-diffusivity is some 46%, 52%,

Alcohols in Supported Gold Catalysts and 54% slower, respectively, than their pure liquid counterparts. The fact that the value is lower for Au/SiO2 when compared to that for Au/TiO2, which for 1-octanol, 2-octanol, and 3-octanol are 19%, 29%, and 39% slower, respectively, implies that there is a stronger physical interaction of the liquid with SiO2 than with TiO2. This is discussed in more detail subsequently in this paper. The data for the Au/CeO2 sample are intriguing. There is a definite two-component behavior that may be due to a distinct bimodal pore size distribution being present in the catalyst. The observation that the fastest component is close to that of the pure alcohol suggests that there may be pores/a porous network within the catalyst that is of the order of several tens of micrometers in diameter and thus we would expect unrestricted diffusion if this were the case. Further physical characterization will be necessary to confirm this theory. The diffusion of the octanols in the alloyed Au-Pd/TiO2 catalyst show values that are 24%, 29%, and 39% smaller than their pure liquid counterparts for 1-octanol, 2-octanol, and 3-octanol diffusion, respectively. However, these values are similar to the values of the monometallic Au/TiO2 counterpart. It is unclear why 3-octanol is so significantly slower when compared with 1-octanol and 2-octanaol. Self-Diffusion of Butanediols. The values of the selfdiffusion coefficients for 1,2- and 1,4-butanediol for Au/SiO2 are approximately 30% lower than those of the bulk pure liquid butanediols, which indicates again, along with the linearity of the fit, that the butanediols undergo restricted diffusion within this catalyst. Even though the self-diffusivity of the pure liquid is smaller for butanediols than with octanols, the root mean squared displacement of the pure liquid butanediols is still large at approximately 1.5 µm compared to displacements of the nanoporous materials used here. The log-attenuation plots for the effective self-diffusivity of 1,2- and 1,4-butanediol in Au/ CeO2 are closer to a one-component behavior than twocomponent behavior and were thus fitted as single components. As noted earlier, the value of the self-diffusivity of the butanediols in Au/CeO2 is similar to that of the pure liquid for 1,2-butanediol but is enhanced for the 1,4-butanediol/Au-CeO2 system relative to the pure 1,4-butanediol liquid. However, inspection of Figure 3b shows that the linearity of the data for the diffusion of the butanediols in Au/CeO2 is not good, and thus we are unable to draw a definitive conclusion for this sample. The values of the self-diffusivities for butanediols imbibed in the Au/TiO2 and the Au-Pd/TiO2 are only 14% and 10% slower than the self-diffusivities of the pure liquids, which presents us with an interesting question: why should the diffusion coefficients of the butanediols be much closer to the free liquid values than the octanols, given that the physical structure of the porous media is the same? Before we begin to attempt to answer the above question, we finally comment upon two other self-diffusivities. To introduce the concept of how the probe molecule interacts physically with the porous medium, we performed four additional measurements, namely, the diffusion of n-octane in Au/ SiO2, Au/TiO2, and Au-Pd/TiO2 and 2-octanone in Au-Pd/ TiO2. The aim here was to examine how the organic molecule functional group affects effective diffusivities. It is useful at this stage to introduce a dimensionless variable, ξ, representative of a combined parameter that reflects simultaneously the strength of physical interaction of the probe molecule with the porous catalyst coupled with the intermolecular and intramolecular probe molecule interactions and the collisions of the probe molecule with the wall structure of the porous medium. We

J. Phys. Chem. C, Vol. 115, No. 4, 2011 1077 TABLE 5: PFG-NMR Interaction Parameter, ξ, and Reference Tortuosity, τ, of Supported Gold Catalysts chemical

Au/SiO2

Au/TiO2

Au-Pd/TiO2

1-octanol 2-octanol 3-octanol 2-octanone 1,2-butanediol 1,4-butanediol

1.86 2.10 2.18

1.23 1.39 1.45

1.40 1.43

1.17 1.11

1.32 1.40 1.64 1.95 1.17 1.11

n-octane

1.74

1.57

1.85

ξ

R τPFG-NMR

define ξ (the combined PFG-NMR interaction parameter), as the ratio of the free bulk liquid diffusion D0 to the effective diffusivity Deff of a probe molecule within the porous medium.

ξ)

D0 Deff

(2)

We do note that a similar ratio17 of D/Deff has also been referred to in the literature as being representative of the tortuosity, τ, of a porous medium. Tortuosity, in terms of molecules diffusing in porous media, is essentially a measurement of how twisted or convoluted the path of a molecule is between two points A and B relative to a direct/unhindered path, i.e., a straight line between those two points. Tortuosity is commonly calculated from PFG-NMR experiments17 as

τ)

D0 Deff

(3)

Inspection of the two equations above shows that they are essentially identical, but a true tortuosity is a structural parameter of the porous medium alone and can only be estimated from a PFG-NMR experiment when there is no chemical interaction of the probe molecule with the porous medium; i.e., the probe molecule must only undergo noninteracting wall collisions. We now assume that the pure n-octane hydrocarbon interacts least with the surface of the porous medium and thus satisfies the condition that allows us to use the ratio of diffusivities of saturated alkanes to give a true measure of the tortuosity. The self-diffusivity of pure n-octane liquid was found to be 2.22 × 10-9 m2 s-1, and n-octane liquid imbibed into Au-Pd/TiO2, which was found to give a straight line fit to the data (see Supporting Information), gave a value of the diffusion effective coefficient of 1.20 × 10-9 m2s -1. We will refer to these n-octane derived tortuosities as the PFG-NMR reference tortuosities, τRPFG-NMR. Table 5 gives the results for the reference tortuosities, R , and PFG-NMR interaction parameter, ξ, for the τPFG-NMR systems studied here. R reference values for the n-octane For example, τPFG-NMR Au-SiO2, Au-TiO2, and Au-Pd-TiO2 systems are 1.74, 1.57, and 1.85, respectively, and thus we would expect the self-diffusion coefficient of any noninteracting liquid to be lower by a factor of 1.74, 1.57, and 1.85 for the Au-SiO2, Au-TiO2, and Au-Pd/TiO2 systems, respectively, because of the inherent tortuosity of the system. Let us apply this theory to the diffusion and tortuosity results for the Au-Pd/ TiO2 catalyst. Table 5 shows that the 1-, 2-, and 3-octanol R have consistently lower values of tortuosity than τPFG-NMR ) 1.85. This implies that there is actually an enhanced self-

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diffusivity of the octanols within the porous catalyst. Theoretically, we would expect the self-diffusivities of the 1-, 2- and 3-octanols imbibed in the Au-Pd/TiO2 catalyst to be equal to 0.66 × 10-10, 0.77 × 10-10, and 0.78 × 10-10 m2 s-1, respectively (based on tortuosity scaling alone). The fact that the measured self-diffusivities of the pure liquid octanols are, in general, an order of magnitude slower than both n-octane and 2-octanone is indicative of the (known) existence of extensive, dynamic, hydrogen-bonding networks in these liquids. If the hydrogen-bonded networks of the octanols were to remain essentially identical when imbibed within the porous medium, then we would expect a selfdiffusivity for the octanols to be 1.85 times slower than their pure liquid counterparts. This is not observed for the Au-Pd/ TiO2 samples and the factor is only around 1.35 times as slow, i.e., it is enhanced relative to our theoretical value. Thus it is plausible that the dynamic hydrogen-bonding network of the octanols is disrupted or broken down to some extent by the porous medium, which results in an enhanced self-diffusion coefficient relative to one in which a hydrogenbonded network remained intact inside the porous medium. This theory is supported further by examining the values in Tables 4 and 5 for the butanediols in Au-Pd/TiO2. Using our value of τRPFG-NMR ) 1.85 from n-octane, we would expect the theoretical values of the 1,2- and 1,4-butanediol selfdiffusion coefficients imbibed in the porous media to be 0.15 × 10-10 and 0.11 × 10-10 m2 s-1 as opposed to the measured (enhanced) values of 0.24 × 10-10 and 0.18× 10-10 m2 s-1, respectively. More evidence supporting this argument is gained by examining the data from the 2-octanone diffusivity experiment. Considering the pure liquid, it is seen that the self-diffusivity is approximately an order of magnitude greater than, for example, its alcohol counterpart, 2-octanol. This difference has to be related to the fact that there is little if any hydrogen bonding present in 2-octanone. Without even measuring the effective self-diffusivity of 2-octanone in the Au-Pd/TiO2 catalyst, we could calculate it (from tortuosity scaling alone) to be 1.85 times smaller and thus expect a value of approximately 6.54 × 10-10 m2 s-1. This value is very close to what we do measure (6.2 × 10-10 m2 s-1) and the interaction parameter is close in value to that of the reference tortuosity. The data given in Tables 4 and 5 show that the same argument holds for the Au/TiO2 system studied here. The reference tortuosity of the Au/TiO2 catalyst, calculated from the n-octane diffusion experiment (see R ) 1.57. All values of the ξ supplementary data), is τPFG-NMR in Table 5 for the alcohols (both mono-ols and diols) are lower than that from the n-octane experiment and thus we expect an enhanced diffusion of these compounds in the Au/ TiO2 relative to what we would expect on the basis of tortuosity scaling alone. Turning now to the data for Au/ SiO2 we observe that the values of ξ in Table 5 for 1-, 2-, and 3-octanol imbibed in the Au/SiO2 catalyst are all greater R . This implies that any disruption to hydrogen than τPFG-NMR bonding may now been overtaken by physical interactions with the porous medium itself. We consider this to be due to the relative surface acidity of the support as determined by its isoelectric point (IEP).18 Of the supports studied, silica is the most acidic (IEP ) 2) compared with TiO2 (IEP ) 6.5) and CeO2 (IEP ) 7). The enhanced acidity of SiO2 therefore significantly affects the diffusion of the alcohols within the catalyst structure, as would be expected. In terms of correlating the PFG-NMR results with the reaction data we observe that, when taken collectively, the

Mantle et al. PFG results show that the slower diffusing diol correlates with the greater turnover frequency (TOF), which implies that the oxidation reaction is unlikely to be mass transfer limited. Moreover, within each family of alcohols studied, there appears to be evidence for an inverse correlation between the values of the diffusion coefficient determined from the PFG data and the TOF for the oxidation reaction. Thus the faster moving alcohols have a smaller TOF. We can relate this behavior to our discussion regarding hydrogenbonding networks and tortuosity. Hence for 3-octanol we observe that the greatest TOF correlates inversely with the slowest effective self-diffusivity, which in turn represents the least disruption to the hydrogen-bonding network by the porous matrix. Another intriguing observation was that the addition of small amounts of 2-octanone completely inhibited the oxidation of 1-octanol over Au-Pd/TiO2. 1H PFG measurements for the self-diffusivity of 2-octanone in the catalysts showed a 6-fold enhancement compared to the 2-octanol alone. In addition, we observed that ξ in Table 5 calculations indicated that 2-octanone could interact more strongly with the porous matrix than 2-octanol or other alcohols. Thus it is likely that the enhanced diffusion (relative to that of the octanols) of 2-octanone along with a greater affinity for the pore surface may be a contributing factor to the halting of the catalytic reaction for the oxidation of 2-octanol. The results suggest that 2-octanone once formed should diffuse rapidly within the catalyst but may interact more strongly with the pore surface. As no products are observed from the interaction of 2-octanol with the catalyst we consider that the observed effects are due to competitive adsorption of the products when the alcohol is substituted in the 2-position. Additionally, it is interesting to note that no appreciable oxidation of 1-octanol (Table 2) is observed for Au/SiO2 and Au/TiO2. The PFG-NMR results along with the discussion of the interaction parameters ξ and reference tortuosities in Table 5 show that the 1-octanol interacts more strongly with the silica porous matrix and as the interaction between the alcohol and the catalyst surface is very strong this leads to the limited reactivity observed. Taken overall, the data we present in this study demonstrate the value of using PFG-NMR spectroscopy to unravel complex catalytic data and also give some insight into the nature of the active site since, based on the reactivity of 2-octanol and 1,2-butanediol, the catalyst is clearly able to adsorb the products from these reactions preferentially. Acknowledgment. We thank Johnson Matthey and the EPSRC (Project ATHENA, GR/R54590/01) for financial support. M.D.M. thanks Dr. Andrew Sederman for useful discussions. In addition, we thank the EPSRC Green Oxidation Catalysis GR/S41906/01 and the Leverhulme Trust F00407AQ for financial support. Supporting Information Available: Log-attenuation plot of n-octane diffusion in Au/SiO2, Au/TiO2, and Au-Pd/TiO2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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