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Structure Sensitivity in L-Arabinose Oxidation over Au/Al2O3 Catalysts† Olga A. Simakova,‡ Bright T. Kusema,‡ Betiana C. Campo,‡ Anne-Riikka Leino,§ Krisztia´n Korda´s,§ Veronique Pitchon,| Pa¨ivi Ma¨ki-Arvela,‡ and Dmitry Yu. Murzin*,‡ Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi UniVersity, FIN-20500 Åbo/Turku, Finland, Microelectronics and Materials Physics Laboratories, EMPART Research Group of Infotech Oulu, UniVersity of Oulu, FIN-90014 Oulu, Finland, and LMSPC, Laboratoire de Mate´riaux, Surfaces et Proce´de´s pour la Catalyse, UMR 7515 du CNRS-ECPM-ULP, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France ReceiVed: June 15, 2010; ReVised Manuscript ReceiVed: August 27, 2010
Several Au/Al2O3 catalysts were prepared and evaluated in the oxidation of L-arabinose. Metal cluster size was varied by application of different preparation methods (DPU and DIE), washing agents (ammonia or water), calcination temperatures, and concentrations of the initial precursor solutions. The structure sensitivity of the reaction over these catalysts was proven. A volcano relationship of activity with the mean particle size was observed, the catalyst prepared by DPU being the most active one. The catalyst electric potential was correlated with the particle size by an inverse volcano shape. At 30% of conversion, the DPU sample exhibited the lowest potential. 1. Introduction From the point of view of biosustainable chemical processes, woody biomass offers a wide range of possibilities. It can be transformed not only into fuels and basic chemicals but also into several high value-added products used in the pharmaceutical and food industries. Catalysis plays an important role in valorization of wood components by, for example, hydrogenation, oxidation, and isomerization to name a few. Particular interest has been paid to the oxidation of carbohydrates to produce carboxylic acids. Several studies were performed, one of the main achievements being the understanding of the reaction mechanism that follows an oxidative dehydrogenation route.1-3 A detailed account on various aspects of oxidation of sugars was provided by Mallat and Baiker.3 As a consequence, alkaline pH is required to assist in the deprotonation of the substrate, allowing the fast desorption of the reaction product.4,5 Several metals have been used as catalysts (Ru,6,7 Cu,6 Ag,6 Pt,8 Rh,8 Pd,9 and Au4) with different degrees of success. For example, palladium is highly active but its main disadvantage is fast deactivation of the active surface due to overoxidation. On the contrary, gold is also active and stable under reaction conditions. Moreover, it has an additional advantage of high selectivity toward the monoxidation, therefore being a suitable catalyst for sugar oxidation. The use of wood as a raw material in the synthesis of valuable chemicals implies the need to study the oxidation of different monosaccharides and disaccharides. It is well-known that 55-75% of wood’s carbohydrates are glucose; the remaining carbohydrates are arabinose, galactose, xylose, and mannose, among others. Because of its abundance, glucose oxidation has been widely studied. On the other hand, only preliminary research has been performed with other monosaccharides. These initial studies showed that the activity of Pt,8 Au, and Pd9 †
Part of the “Alfons Baiker Festschrift”. * Corresponding author. ‡ Åbo Akademi University. § University of Oulu. | UMR 7515 du CNRS-ECPM-ULP.
catalysts is influenced by the reactant. However, the influence of reaction parameters and catalyst characteristics on the oxidation is still uncertain. The application of gold catalysts involves the possibility that the studied reaction exhibits structure sensitivity. This behavior for gold was observed in hydrogenation of 1,3-butadiene,10 acrolein,11 and crotonaldehyde;12 oxidation of CO13 and glycerol;14 and oxidative dehydrogenation of ethanol.15 In the field of sugar oxidation, Claus and co-workers4 and Haruta and coworkers16 showed that the activity of carbon-supported gold catalysts in the oxidation of glucose is altered by the particle size. These studies described a decreasing trend in the activity with the increasing particle diameter. The same reaction was carried out over gold supported on different oxides (Al2O3, TiO2, CeO2, ZrO2), showing a dependence of activity not only on the cluster size but also on the support type.17 It has been reported that some structure sensitive reactions exhibited a maximum in activity. Depending on the reaction, the average gold particle size giving the highest reaction rate can be different; in CO oxidation over Au/TiO2(001)/Mo(100), the maximum is around 3 nm,12 while in gas phase crotonaldehyde hydrogenation over Au/TiO2 the optimal size is around 2 nm.12 In order to study if sugar oxidation could also follow this tendency, several Au/ Al2O3 samples were prepared applying different preparation methods and treatments, allowing a wide range of particle diameters (1-20 nm). The oxidation of pentose, L-arabinose, over gold catalysts into arabinolactone or arabinonic acid, the last one being the desired product, has been studied in this work (see Figure 1). Arabinose is a monomer obtained by hydrolysis of arabinogalactans, hemicelluloses which appear in large quantities 15% of heartwood in larch species.18 Following a general mechanism described by Mallat and Baiker,3 it can be suggested that the reaction starts with the dehydrogenation of the adsorbed arabinose, leading to the formation of a gold hydride and desorption of the arabinolactone. The hydrolysis of this molecule gives arabinonic acid as a product. The gold hydride formed
10.1021/jp105509k 2011 American Chemical Society Published on Web 09/23/2010
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Figure 1. Reaction pathway of arabinose oxidation.
during the dehydrogenation reacts with activated oxygen, resulting in a molecule of water. 2. Experimental Section 2.1. Catalyst Preparation. Catalysts were prepared using Al2O3 (UOP, A-201, SBET ) 200 m2/g) as a support. Aqueous solutions of HAuCl4 (99.9% Sigma Aldrich) were used as a gold precursor. The samples were prepared by direct ion exchange (DIE)19 and deposition-precipitation with urea (DPU)20 as described below. Direct Ion Exchange (DIE). To achieve different particle sizes, the initial concentration of precursor species, the washing procedure, and the calcination temperature were varied. Initial Concentration. HAuCl4 aqueous solutions of concentrations of 5 × 10-3, 10-3, and 5 × 10-4 M, respectively, were prepared in amounts corresponding to the final Au loading of 2 wt %. Solutions were heated up to 70 °C; thereafter, Al2O3 powder (45-63 µm) was added. The slurries were mixed for 1 h, washed with ammonia (4 M) by adding ammonia aqueous solution to the slurry, and stirred for 1 h. The obtained sample was filtered, washed with water, dried overnight at 80 °C, and calcined in air at 300 °C for 4 h. The samples were labeled as DIE-300-1, DIE-300-2, and DIE-300-3, corresponding to solution concentrations of 5 × 10-4, 10-3, and 5 × 10-3 M, respectively. Washing Procedures. The suspension was prepared as described above with HAuCl4 aqueous solution of a concentration of 5 × 10-4 M; then, deionized water was applied as a washing agent (DIE-300-W). Thereafter, the sample was dried overnight at 80 °C and calcined in air at 300 °C for 4 h. Calcination Temperature. Catalysts prepared using the initial HAuCl4 aqueous solution with the concentration 5 × 10-4 M and ammonia as a washing agent were dried overnight at 80 °C and calcined at 400 °C (DIE-400), 500 °C (DIE-500), and 600 °C (DIE-600). Deposition-Precipitation with Urea. The aqueous solution of HAuCl4 with a concentration of 5 × 10-4 M corresponding to the final Au loading of 2 wt % was mixed with urea (SigmaAldrich). The amount of urea was calculated to achieve the concentration of 0.21 M. The solution was heated up to 81 °C, and then, Al2O3 powder (45-63 µm) was added. The suspension was mixed for 24 h at 81 °C, then filtered, washed with water, dried overnight at 80 °C, and calcined at 300 °C. This catalyst was marked as DPU-300. 2.2. Catalyst Characterization. The metal loading was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) in a PerkinElmer, Optima 5300 DV spectrometer. For that purpose, the samples (50 mg) were treated with 5 mL of aqua regia, digested in a microwave oven, diluted to 100 mL, and analyzed in the spectrometer.
catalyst
metal loading (%)
particle size (nm)
DIE-300-1 DIE-300-2 DIE-300-3 DIE-400 DIE-500 DIE-600 DIE-300-W
1.5 1.6 2.0 2.2 2.2 2.2 2.1
DPU
2.0
1 1.2 1.3 2.5 3.4 3.7 2.9 and 28.1 2.3
dispersion (%) 77 72 70 46 36 33 n.d. 49
Cl (atomic %)
initial rate [mmol/ (L*gAu*s)]
0.4 0.3 0.2 0.9
3.74 3.29 3.81 2.62 1.82 1.65 0.30 6.35
Electron microphotographs of the samples were taken by a LEO 912 OMEGA energy-filtered transmission electron microscope using a 120 kV acceleration voltage. Histograms of particle size distributions were obtained by counting at least 100 particles on the micrographs for each sample. XPS measurements were carried out in a PHI Quantum 2000 Scanning ESCA Microprobe spectrometer with Al anode. The survey and multispectra were collected at 187.75 and 11.75 eV pass energy, respectively. Spectra were processed by means of XPS Peak 4.1 software. The peak areas were computed by fitting the experimental spectra to Gaussian/Lorentzian curves after removing the background (using the Shirley function). 2.3. Arabinose Oxidation. The catalytic evaluation was carried out in a semibatch reactor. The catalysts were sieved for particle sizes of 45-63 µm to facilitate the absence of mass transfer limitations. The catalyst (200 mg), suspended in deionized water, was prereduced in situ by hydrogen (AGA, 99.999%) at 60 °C for 10 min. An aqueous solution of L-arabinose (Danisco, 99.9%) was introduced in the reactor, leading to 100 mL of a 0.1 M solution of arabinose. The oxygen flow rate through the reactor was 2.5 mL/min diluted in 17.5 mL/min of nitrogen. The pH was kept at 8, by the addition of NaOH solution (2.5 M) using an automatic titration device (Metrohm Titrino 751). The stainless steel reactor walls were utilized as an electrode collector for measurements of the catalyst potential. Application of catalyst potential measurements are in particular advantageous in oxidation reactions over metals, as pointed out by Mallat and Baiker.3 The potential measurements were performed by means of an Ag/AgCl/3 M KCl electrode. Aliquots of the reaction mixture were taken at determined time intervals and analyzed by HPLC, equipped with a Bio-Rad Aminex HPX-87C carbohydrate column. 3. Results and Discussion 3.1. Catalyst Characterization. The metal loading, average particle size, and dispersion of the different catalysts are summarized in Table 1. Since it is not possible to determine metal dispersions by the traditional chemisorption methods, these values were calculated from the mean diameters determined by TEM. Calculations were performed under the assumption of cuboctahedral (fcc) particles with one face in contact with the support. As a calculation model, the one proposed by Poliset21 was used. This methodology has been applied previously by Ivanova et al.19 to estimate the dispersion of Au/Al2O3 catalysts prepared by direct ion exchange. TEM microphotographs of samples DIE-300-1, DIE-300-2, and DIE-300-3 are presented in Figure 2. A slight increase of the particle size with an increase on the concentration of the precursor solution was observed. However, it must be noted that the differences are within the range of the experimental error.
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Figure 2. TEM microphotographs of (a) DIE-300-1, (b) DIE-300-2, and (c) DIE-300-3.
The effect of the calcination temperature on the particle size is seen in Figure 3. The particle diameter increased from 2.2 to 3.7 nm with the increasing treatment temperature. Compared with the DIE-300 sample, the size of gold particles was slightly increased by calcination at 400 °C. A more noticeable sintering effect was achieved when samples were calcined at 500 and 600 °C. Nevertheless, the particle sizes of these last samples were quite similar. One of the main disadvantages of gold is its low melting point (1063 °C) compared with other metals such as Pd (1552 °C) and Pt (1769 °C). As a consequence, it is susceptible to particle sintering when the treatment and/or reaction temperatures are high. Thus, the particle size of gold can be tuned by changing the catalyst calcination temperature. As it was shown by the TEM measurements, the particle sizes increased with the calcination temperature. The increase is also related to the influence of chlorine species. The atomic percentages presented in Table 1 demonstrate that the amount of these undesired species decreased with the increasing temperature, but even after calcination at 600 °C, they were not completely removed. It is remarkable that the noticeable change in the particle size was obtained after calcination at 500 °C. The reason could be that at this temperature gold nanoparticles can melt. The dependency of the melting point of gold nanoparticles encapsulated on silica on their particle sizes was studied by Dick et al.22 It was found that smaller particles melt at lower temperatures, much lower than the melting point of bulk gold. For particles with a diameter around 2.3 nm, the melting temperature was reported to be ca. 405 °C, whereas the theoretical one was about 490 °C.
When catalysts are prepared by direct ion exchange, the nature of the washing agent can modify the particle size of gold.19 The ion exchange (anionic in the present case due to the nature of the precursor) takes place during the time when the suspension of alumina in HAuCl4 solution is stirred and kept at 70 °C. The use of ammonia leads to the partial or total elimination of chlorine species, being responsible for sintering. Therefore, smaller particles are expected for DIE-300-3 compared with DIE-300-W; this can be observed in Figure 4. The catalyst DIE-300-W exhibited a bimodal distribution as follows. The average size for small particles (between 1 and 5 nm) was 2.9 and 28.1 nm, respectively. The preparation of gold catalysts by different methods affected the gold dispersion. The DPU method is a suitable method to prepare active gold catalysts but leads to particles larger than the ones prepared by direct ion exchange (DIE). In this case, the particle size achieved by the former method was 2.3 nm, whereas a mean diameter of 1-1.3 nm was obtained by the second one. The origin of this difference is in the precipitation step where the gold particles agglomerate. The particle size is, however, in the required range to possess catalytic activity. Since the impregnation is the simplest method of catalyst preparation, this way was attempted for preparation of gold catalysts.23,24 However, this approach has a number of disadvantages. First of all, impregnation often does not allow supporting the desired amount of the metal. Second, when gold chloride precursors are applied (HAuCl4 or AuCl3), the chloride presence results in particle agglomeration during the calcination
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Figure 3. TEM microphotographs of (a) DIE-400, (b) DIE-500, and (c) DIE-600.
Figure 4. TEM microphotographs of (a) DIE-300-W and (b) DPU.
step.25 Thus, impregnation leads to lower metal loading and the formation of large gold particles (up to 10-35 nm),26 while the DPU and DIE techniques are more suitable for the preparation of active gold catalysts.
The presence of chlorine on the catalyst surface and the oxidation state of gold before and after the reaction were analyzed by XPS. Chlorine species were detected, as was expected, in DIE-300-W. They were also identified in DIE-400,
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DIE-500, and DIE-600, showing that ammonia washing was not efficient enough to remove them. As it can be expected, the chlorine content decreased with increasing calcination temperature (see Table 1). The most frequently used precursor, HAuCl4, has as the main disadvantage the presence of chlorine. These species are able to form Au-Cl-Au bridges that allow the particles to sinter during the calcination step. To avoid this undesired agglomeration, two methods were applied in the current work: preparation of catalysts using different concentrations of the gold precursor and washing with ammonia. The hydrolysis of gold species using HAuCl4 as a gold precursor with different concentrations has been reported by Ivanova et al.27 It was reported that decreasing the concentration of the precursor leads to an increased hydrolysis of gold species. Therefore, it is possible to decrease the amount of chlorine linked to gold atoms, providing a high stability of the nanosize of gold particles during calcination. As a consequence, the increase in the concentration of precursor solution leads to large particles. When the catalysts are washed with ammonia, the procedure is successful when less than 150 ppm of chlorine is detected in the elemental analysis of the catalysts.19 This is the case with DIE-300-1, DIE-300-2, and DIE-300-3 samples, which were submitted to calcination at 300 °C, affording particles with a size of less than 1.5 nm. The relationship between chlorine remaining on the surface and the particle size reached after calcinations can be inferred from the values reported for DIE300-3 and DIE-300-W catalysts, showing that the average gold particle size increased by a factor of 5 when increasing the remaining chlorine content in the catalysts only by a factor of 2.5 (Table 1, entries 3 and 7), clearly showing a possibility to change particle sizes by washing with ammonia. Although it is known that the presence of chlorine can inhibit adsorption of sugars when they are present in significant amounts,28 Table 1 demonstrates that the quantity of Cl is on the very low level and thus its influence on structure sensitivity in the present study can be neglected. The evaluation of the oxidation state of gold was hindered for the inclusion of gold particles into the pores of the support, resulting in low availability of gold to the X-ray beam. Consequently, the recorded spectra were broad with low intensity. The catalysts IMP and DIE-300-W displayed narrow peaks (fwhm 1.4 eV) centered at 83.9 eV, which corresponds to metallic gold. The catalysts prepared by DIE exhibited quite broad peaks. Due to the low intensity of these doublets, deconvolution of the spectra proposing more than one oxidation state would be too speculative. However, according to the binding energies of these peaks (83.4-83.7 eV), the presence of metallic gold and Auδ- species due to gold particles with sizes lower than 2 nm can be postulated. The quality of the spectra from the spent catalysts did not allow an in-depth study about the changes in the oxidation state of gold brought by the reaction. However, the binding energies and fwhm demonstrated minor deviations that are within the range of error of the technique. Therefore, it can be supposed that the surface is not modified under the reaction conditions. 3.2. Arabinose Oxidation. The reaction was carried out at 60 °C and at a pH of 8, and thus, the undesired reactions, such as condensation and isomerization, can be avoided. The HPLC analysis showed only arabinolactone and arabinonic acid as reaction products. Experimental data are collected in Figures 5-7.
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Figure 5. Arabinose oxidation. Effect of the initial gold concentration over (a) conversion, (b) selectivity to arabinonic acid, and (c) catalyst potential.
3.2.1. ActiWity-Structure SensitiWity. The different particle sizes presented for the studied catalyst allowed a systematic evaluation of the influence of the average particle diameter on the catalytic activity. An optimal particle size giving the highest activity was achieved over the average gold particles of 2.3 nm (Figure 8). In the liquid phase reactions, the activity of supported gold catalyst could depend on the particle size (structure sensitivity) and the nature of the support.17 When gold is supported on an inert support such as carbon, the catalytic activity achieved is only related to the properties of gold. When the support is a metal oxide, the scenario is different, because of the interactions between the support and the noble metal, which cannot be neglected even for such a support as alumina. Analysis of structure sensitivity in the present work follows a classical approach based on the average values, obtained by HRTEM. Activity values are not related to particle morphologies or particle size distribution. The calculations of the gold dispersions were based on the assumption that gold particles are cubooctahedral, even though particles around 1 nm have also icosahedral structures and above 1 nm decahedral crystals appear.
Structure Sensitivity in L-Arabinose Oxidation
Figure 6. Arabinose oxidation. Effect of the calcination temperature over (a) conversion, (b) selectivity to arabinonic acid, and (c) catalyst potential.
By analogy with the mechanism proposed by Claus for the oxidation of glucose, it is reasonable to assume that the L-arabinose is adsorbed on the gold atoms with low coordination number (corner, edge, steps) via the carbonyl group. The number of low coordinated atoms has a maximum around 2 nm.29 Below this size, the particles are monolayers and all the atoms are in contact with the support, whereas for the particles above 2 nm the number of these sites decreases. Then, it can be assumed that the catalysts prepared by DPU with an average diameter of 2.3 nm had the maximum number of actives sites for the adsorption of arabinose. The better performance of catalyst with a higher number of low coordinated atoms could also be associated with the fact that these sites might be responsible for the O2 activation.30 Moreover, in the study of ethanol dehydrogenation over gold supported on mesoporous silica, it was proposed that the step sites are the active ones for the removal of the R-hydrogen that in this case is the hydrogen attached to the anomeric carbon.31 Therefore, the volcano-type relationship (Figure 8) between the activity and the gold particle size can be explained by the population of the active sites for the adsorption of arabinose; dehydrogenation step and oxygen activation.
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Figure 7. Arabinose oxidation. Effect of the preparation method on (a) conversion, (b) selectivity to arabinonic acid, and (c) catalyst potential.
Figure 8. Dependence of initial rates in arabinose oxidation on the particle size of Au/Al3O2 catalysts.
Such differences in population could be related to the chemical potential, which in essence is the amount by which the Gibbs energy of the system would change if an additional
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adsorbed molecule is introduced, with the pressure and temperature fixed. A quantitative thermodynamic approach of the cluster size effect was considered in refs 33 and 34 with the aim to describe the size dependent Langmuir-Hinshelwood mechanism and the two-step catalytic cycle. The general treatment took into account surface energy excess due to an intrinsic increase in chemical potential with size decrease, as discussed by Parmon,34 as well as the changes in chemical potential upon adsorption. An expression for the Langmuir-Hinshelwood mechanism was obtained in ref 32, which could be written for arabinose oxidation in the following way
V(r) )
Figure 9. Dependence of potential on the cluster size.
k'KACAKO2CO2e2R∆δ/RTr (1 + KACAe∆δ/RTr
1 ∆δ/RTr 2 r + KO2CO2e )
(1)
where V is the catalyst activity, CA, etc., is the concentration of arabinose and dissolved oxygen, k is the rate constant, KA and KO2 are the adsorption coefficients for arabinose and oxygen, respectively, ∆δ is a proportionality coefficient between the chemical potential increment for nanoclusters compared to extended surfaces and the cluster radius r, and R is the Polanyi parameter (typically close to 0.5). A kinetic equation somewhat similar to eq 1 was utilized in ref 35 for a quantitative description of lactose oxidation on gold catalysts. Equation 1 could be simplified, leading to a fourparameter dependence
V(r) )
p1e2p4p3/d (1 + p2e
1
(2)
p3/d 2 d
)
where
p1 ) 2k'KACAKO2CO2,
p2 ) KACA + KO2CO2, p3 ) 2∆δ/RT,
p4 ) R (3)
and d is the diameter of clusters. Results of the calculations along with the values of parameters are shown in Figure 8, confirming a good correspondence between the calculations and the model. The selectivity toward arabinolactone and arabinonic acid only showed dependency with the conversion. The selectivity was not influenced by the particle size most probably because both of the reactions take place over the same kind of sites. Increase of the availability of those sites results in an increase of both reaction rates. 3.2.2. Oxidation Rates and ConWersion after Prolonged Reaction Times. From the viewpoint of the catalyst activity, the decreasing concentration of the precursor solution exhibited a beneficial effect, leading to more active catalysts (see Figure 5). The effect of calcination temperature on the catalyst activity is shown in Figure 6, demonstrating lower reaction rate values by increasing treatment temperatures, causing sintering of gold (see section 3.1). It must be noted that the kinetic curves for DIE-300 and DIE-500 almost overlapped with those for DIE400 and DIE-600, respectively, as a result of the similarities between the samples. DIE-300-W exhibited very low oxidation activities (Figure 7). The former one has a low activity, but the conversion
increased all along the reaction time up to 19% after 200 min. On the other hand, the DIE-300-W sample reached a plateau around 5% of conversion. Since a bimodal distribution was obtained for this catalyst exhibiting low activity, it was excluded in the quantitative evaluation of the cluster size effect. The catalyst prepared by DPU exhibited the highest activity, but nearly equally good results were achieved with DIE samples. 3.2.3. Catalyst Electric Potential. The catalyst potential was measured and plotted as a function of conversion (Figures 5c, 6c, and 7c). Once oxygen was introduced into the reactor (time ) 0), the expected potential increase due to the oxygen adsorption was observed up to 10% of conversion. At this point, the active catalysts reached a region of linear dependence with the conversion. Usually, this behavior could be observed when the reaction is performed under limited oxygen transfer.36 In this study, the reaction conditions and catalyst granulometry minimized the oxygen transport limitation. However, along the reaction run, the system reached a state of oxygen feed control given by the decrease in oxygen surface concentration. This depletion could be due to the reaction of activated oxygen with the gold hydride formed during the arabinose dehydrogenation step and/or the unavailability of active sites due to the adsorption of the carbohydrate and the reaction products. When the conversion reached values higher than 60%, the potential increased again until the total conversion was reached. At this point, the reaction rate on the surface is slow enough to allow the adsorption of oxygen on the active sites, provoking the potential increase toward the oxygen region. As mentioned above, Murzin et al.35 studied the oxidation of lactose over gold supported catalysts. They suggested that the initial adsorption of oxygen takes place on the low coordinated atoms and then the oxygen migrates toward the catalytically inactive facets. According to the authors, the gold atoms placed on the facets are responsible for the potential variation. The formation of gold hydride during the dehydrogenation step of the reaction could also contribute to the catalyst potential. Under the light of these two points, the potential could be related to the particle size. For that purpose, the potential at 30% of conversion was plotted as a function of the mean diameter of gold clusters. At this conversion value, the linear dependency of catalyst potential with the conversion was already reached. In Figure 9, an inverse volcano relationship is observed, reaching a minimum at 2.3 nm of DPU catalyst. It was previously established that this sample presented the highest activity because it has the higher amount of active sites, i.e., low coordinated atoms. Therefore, it is possible that this sample exhibited a more reduced state due to the highest activity and consequently a major presence of gold hydride. Additionally, this catalyst also has the lowest amount of surface atoms placed
Structure Sensitivity in L-Arabinose Oxidation in the facets and consequently the amount of oxygen adsorbed over these sites is lower. 3.2.4. Product Distribution and SelectiWity. Selectivity profiles of arabinonic acid are shown in Figures 5b, 6b, and 7b. During the initial stage, the selectivity toward arabinonic acid was low. However, once 5-10% of conversion was reached, the values started to increase. In those cases where the total conversion was reached, only arabinonic acid was identified in the final sample. A special consideration must be performed for DIE-300-W. This sample had the same selectivity behavior, but the increase in selectivity was registered at lower conversion compared to the other catalysts. According to the reaction mechanism proposed by De Wit et al.,37 oxidation of sugars proceeds through an oxidative dehydrogenation where the arabinolactone is the intermediary between arabinose and arabinonic acid. 4. Conclusions Preparation of Au/Al2O3 catalysts to study the structure sensitivity in arabinose oxidation required an application of different preparation methods and conditions. An increase of the particle size was observed with an increase in the concentration of the initial HAuCl4 solution as well as calcination temperature. The ability of ammonia washing to influence the particle size was demonstrated. It was proven that the reaction is structure sensitive. Activity has a volcano relationship with a maximum at the cluster size 2.3 nm. This relationship was explained by the availability of low coordinated atoms, which are the active sites for the oxygen activation, the arabinose adsorption, and the dehydrogenation of the anomeric carbon. A kinetic model based on the thermodynamic approach was utilized to obtain a quantitative description of the data. Catalyst electric potential was also related to the particle size by an inverse volcano curve. The higher amount of oxygen on the surface of the particles with sizes above and below 2.3 nm was attributed to the higher availability of atoms in facets. These are the sites where the oxygen migrates after the initial adsorption in low coordinated atoms. Acknowledgment. This work is a part of activities at the Åbo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Programmes (2000-2011) by the Academy of Finland. Sten Lindholm (Åbo Akademi University) is gratefully acknowledged for his help in the catalyst characterization. References and Notes (1) Wieland, H. Ber. 1921, 54, 2353–2376.
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