ARTICLE pubs.acs.org/JPCC
Au/Rutile Catalysts: Effect of the Activation Atmosphere on the GoldSupport Interaction ngeles-Chavez†,§ Xim Bokhimi,*,† Rodolfo Zanella,‡ Antonio Morales,† Viridiana Maturano,‡ and Carlos A †
Instituto de Física, Universidad Nacional Autonoma de Mexico, A. P. 20-364, 01000 Mexico D. F., Mexico Centro de Ciencias Aplicadas y Desarrollo Tecnologico, Universidad Nacional Autonoma de Mexico, A. P. 70-186, 04510 Mexico D. F., Mexico § Instituto Mexicano del Petroleo, Eje Central No. 152, 07730 Mexico D. F., Mexico ‡
ABSTRACT: Au/rutile catalysts were activated in hydrogen and in air, and their catalytic activity for the oxidation of CO was analyzed as a function of time. The catalysts were characterized with X-ray powder diffraction, the refinement of the crystalline structures, transmission electron microscopy, and infrared spectroscopy. The activation atmosphere determined the interaction between the gold atoms and the support, which in turn determined the mobility of the gold particles with time and, with that, the gold particle dimensions and, consequently, their catalytic activity for the oxidation of CO. This catalytic activity was also affected by the adsorption, with time, of carbonate species on the active sites, species that were eliminated by heating the aged catalysts in air at 200 °C, recovering the catalytic activity.
’ INTRODUCTION Supported gold catalysts are excellent for catalyzing the oxidation of CO even at 200 K.1 Their catalytic activity depends on the dimensions of the gold particles,2-4 the material used as support, the synthesis method, and the catalyst activation procedure.5-8 For most of the synthesis methods of these catalysts, the gold in the catalyst precursor phase is in the state AuIII,8,9 which is reduced to Au0 with a thermal treatment in flowing hydrogen or in flowing air to produce the active catalyst. During the catalyst activation in air, the gold reduction occurs through the reaction of the oxygen atoms with the gold complex without forming the Au2O3 because it has a positive enthalpy of formation of 19.3 K Joule/mol.10 In most of the publications on supported gold catalysts, the catalyst activation is performed in air at temperatures higher than 100 °C.8,11 The parameters of the thermal treatment to activate the gold catalysts strongly influence the dimensions and the gold particle and, with that, their catalytic activity for the oxidation of CO. Examples of these parameters are the flux and the type of the gas used in the activation.3,12,13 The literature shows that the catalyst activation in hydrogen produces a higher catalytic activity than that in oxygen,12,14 which contrasts with the reports of Okumura et al.15 and Haruta,16 who mention that the pretreatment of the support with oxygen gives rise to a higher catalytic activity. The dimensions of the gold particles in these catalysts also depend on the interaction between the gold atoms and the support. If this interaction is weak, during the catalytic reaction, the gold particles move on the surface of the support particles through Ostwald ripening, producing larger gold particles and losing their catalytic activity for the oxidation of CO.17,18 This r 2011 American Chemical Society
catalytic activity is also decreased by the adsorption of carbonates on the reactive sites during the reaction.19,20 When titania is used as a support of the gold catalysts, the catalyst activation in air strengths the interaction between the gold atoms and the support. For example, Tsubota et al.21 report that a strong metal-support interaction is responsible for the improved catalyst activity at high calcination temperatures in air. Other authors also propose that the catalyst activation in air not only reduces the gold atoms but also rids the surface of the catalyst particles from the carbonaceous contaminants generated during the synthesis.22 When the catalyst support is titania, the activation of the gold catalysts in hydrogen also reduces the support, producing oxygen vacancies on the surface of the support particles.22-24 For example, the reduction of a clean rutile-TiO2(110) surface with hydrogen produces concentrations of oxygen vacancies varying between 5 and 10%.24-26 These vacancies generate crystalline defects that work as pinning centers for the gold particles and can be indirectly detected with scanning tunneling microscopy. For example, Chen et al.27 observed a significant decrease in the intensity of the Ti3þ signal in a reduced support when a gold/titania catalyst was activated in hydrogen, suggesting that the gold atoms binds the Ti3þ ions. The chloride used during the synthesis of gold catalysts can also promote the sintering of the gold particles during the activation of the catalyst or during the catalytic reaction itself28 Received: December 2, 2010 Revised: February 21, 2011 Published: March 11, 2011 5856
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The Journal of Physical Chemistry C because the chlorine atoms form bridges between the gold particles.28 The presence of chlorine atoms in the catalyst can also block out the catalytic active sites.29 Therefore, to produce an active catalyst, it is essential to remove these chlorine atoms from the catalysts after their synthesis. For the gold catalysts, this can be done through their activation in hydrogen because it reacts with the chlorite to produce HCl, which leaves the catalyst easily. It is worth mentioning that the steps on the surface of the support particles are also defects that could work as pinning centers for the gold particles.30 For Au/titania catalysts, theoretical calculations have demonstrated that the gold particles bind stronger to a defect-rich surface than to a defect-deficient surface of the titania particles and, that a significant charge transfer from the titania to the Au particles occurs,31-34 which could explain, in these catalysts, the catalytic activity of the Au particles for the oxidation of CO. In the present work, we analyze the time evolution of gold catalysts when they are activated in oxygen and in hydrogen. In our case, the heat treatment of the catalysts was done after the gold precursor was deposited on the support, which is different from the reported heat treatment in these gases of the support before the gold precursor was deposited.15,16 This explains the different catalytic behavior of the catalysts with the gas used in the treatment; for example, in the present work, the catalytic activity for the oxidation of CO was higher when the catalyst was treated in hydrogen, whereas in the report for the pretreated support,15,16 it was higher when the treatment was done in oxygen.
’ EXPERIMENTAL SECTION Catalyst Preparation. Details about the synthesis of the support (rutile) are reported elsewhere.3,13,35,36 The rutile was annealed in air at 500 °C for 24 h before the catalyst synthesis. HAuCl4 3 3H2O (Aldrich) was used as a gold precursor. The synthesis of the Au/rutile catalysts (4 wt % Au) was done in darkness, to avoid the transformation of the gold precursor, using the deposition-precipitation method with urea.9,12,37 The catalyst activation was done at 200 °C, at a heating rate of 2 °C/min in a U reactor with a fritted plate 1.5 cm in diameter, by flowing hydrogen or air for 2 h on the solid catalyst precursor at 1 mL min-1 mg-1. After the activation, a portion of the catalyst was used to catalyze the oxidation of CO in the temperature range between 0 and 200 °C, and the rest of the catalyst was used to measure its X-ray diffraction pattern at room temperature for 2 h. Thereafter, the catalyst was kept in a desiccator in darkness until the next set of catalytic activity and X-ray diffraction experiments was done; this was repeated for more than 2 months. Characterization Techniques. Catalytic Activity. The CO þ O2 reaction was studied in a RIG 150 LT microreactor from 0 to 200 °C at atmospheric pressure. During the reaction, the reactant gas contained 1% CO, 1% O2, and 98% N2 and flowed through the catalyst at 100 mL/min. The exit gases were analyzed online with a gas chromatograph from Agilent Technologies, model 6890N, with detectors of flame ionization and thermal conductivity. X-ray Powder Diffraction. The X-ray powder diffraction patterns of the catalysts were measured in air at room temperature with a Bruker D-8 Advance diffractometer with the Bragg-Brentano θθ geometry, Cu KR radiation, a Ni 0.5% Cu-Kβ-filter in the secondary beam, and a one-dimensional position-sensitive silicon
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Figure 1. X-ray diffraction patterns of the catalysts activated in hydrogen and aged at room temperature in atmospheric air. The indices in the upper curve correspond to rutile; those in the lower curve correspond to metallic gold.
strip detector (Bruker, Lynxeye).38 The diffraction intensity as a function of 2θ angles was measured between 20 and 130°, with a 2θ step of 0.019447°, for 264 s per point. The crystalline structures were refined using the Rietveld method implemented in the TOPAS code, academic version 4.1.4.39 The crystallite size and morphology were modeled in reciprocal space with a symmetrized harmonics expansion.40 The lattice deformations were assumed to be anisotropic and modeled with a multidimensional distribution of lattice metrics.41 The background model was a polynomial function that, in addition to the constant, linear, quadratic, and cubic terms in 2θ, also included the terms (1/2θ) and (1/2θ)2. The standard deviations, given in parentheses in the text, show the variation in the last digit of a number; when they correspond to Rietveld refined parameters, the values are not estimates of the probable error in the analysis as a whole but only of the minimum possible probable errors based on a normal distribution.42 Transmission Electron Microscopy. The catalysts were analyzed with an analytical transmission electron microscope from Jeol, model JEM 2010F FASTEM, equipped with a Z-contrast annular detector. Digital image processing (including Fourier transform) was accomplished using the DigitalMicrograph code, version 3.7.0, from Gatan Inc. Chemical Analysis. The average Au weight concentration in the catalysts after their activation was determined with energydispersive X-ray spectroscopy (EDS) using an Oxford-ISIS detector coupled to a scanning electron microscope (JEOL JSM-5900-LV). Infrared Spectroscopy. The experiments were carried out in a Nicolet Nexus 670 spectrophotometer. In each experiment, about 0.005 g of the catalyst was mixed with 0.1 g of dried KBr. The mixture was pressed in a pellet and analyzed in IR transmission mode. The spectra were collected in air at room temperature from 128 scans with a resolution of 4 cm-1. 5857
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Figure 3. Rietveld refinement plot of a Au/rutile catalyst activated in hydrogen and aged in atmospheric air for 50 days. The upper marks correspond to gold, whereas the lower ones correspond to rutile.
Figure 2. TEM high-resolution micrograph of a Au/rutile catalyst taken along the [010] zone axis of the rutile crystallite (A) and along the [110] zone axis of the metallic gold crystallite (B).
’ RESULTS AND DISCUSSION The titania support used to prepare the gold catalysts contained only the polymorph rutile, which was synthesized at 90 °C and had an average crystallite size of 8 nm. The fresh rutile particles had a large amount of hydroxyls that were eliminated by annealing the support at 500 °C because the main interest of the present study was to analyze the effect of the activation atmosphere on the catalytic properties of the gold catalysts. This annealing also increased the dimensions of the rutile crystallites to 20 nm and eliminated the microstrains produced during their synthesis.43 The control of the microstrains in the support particles was essential because they are important for the formation of
defects in the rutile crystallites, which eventually control the mobility of the deposited gold atoms on the surface of the rutile particles. Regardless of the atmosphere used to activate the catalysts, they always had two crystalline phases, metallic gold and rutile; both are identified in the X-ray powder diffraction patterns (Figures 1) and in the high-resolution micrographs generated with the transmission electron microscope (Figure 2). The EDS analysis showed that the actual gold loading in the catalyst was 3.9 wt %, which approximated to the nominal gold loading (4 wt %). The diffraction patterns of the aged catalysts activated in air were similar to those of the aged catalysts activated in hydrogen (Figure 1); therefore, we only present the series of the diffraction patterns of the aged catalysts activated in hydrogen. The difference between the diffraction patterns can only be obtained through the Rietveld refinement. To get a quantitative analysis of the X-ray diffraction patterns, they were modeled using the Rietveld method (Figure 3). The crystalline structure of the gold phase was modeled with a cubic unit cell where the symmetry of the atom distribution was given by the space group Fm3m, a basis with a gold atom at the origin of the unit cell, and a fixed lattice parameter of 0.408 nm. The rutile crystalline structure was modeled with a tetragonal unit cell with an atom distribution described by the space group P42/mnm and a basis with one titanium atom at the origin of the unit cell and one oxygen atom at the relative coordinates (x,x,0), with x values around 0.30. The refined lattice parameters of this phase had values approximated to a = 0.460 nm and c = 0.296 nm. The anisotropy of the crystallite morphology of the rutile phase was modeled using an expansion of symmetrized harmonics. Because the gold concentration was low, the gold crystallites were modeled using only the first symmetrized harmonic, that is, it was assumed that they had an isotropic form. The average crystallite dimension of the rutile crystallites was around 21 nm, whereas that of the gold crystallites was between 2.5 and 3.2 nm depending on the aging time and the activation atmosphere. In the fresh activated catalyst, the size of the gold crystallites was almost independent of the activation atmosphere; the diameter of the average gold crystallite for the catalyst activated in hydrogen was 2.5(1) nm (Figure 4A), whereas for the catalyst activated in air, it was 2.8(1) nm. The time evolution of these crystallite dimensions in atmospheric air, however, depended on the atmosphere used for 5858
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Figure 4. Z-contrast TEM micrographs; (A) catalyst activated in hydrogen and (B) catalyst activated in air.
Table 1. Catalyst Activated in Hydrogen: Average Gold Crystallite Size As a Function of Aging Time aging time (days)
average gold crystallite size (nm)
0
2.5(3)
1
2.6(1)
2 3
2.5(1) 2.5(1)
4
2.5(1)
6
2.6(1)
8
2.6(1)
10
2.6(1)
13
2.5(1)
15
2.5(1)
17 23
2.3(1) 2.4(1)
28
2.6(1)
38
2.7(1)
50
2.4(1)
activating the catalyst. When the activation atmosphere was hydrogen, the diameter of the average crystallite did not change with time (Table 1, Figure 5) even after aging the sample for 50 days. When the activation was done in air, the diameter of the gold crystallites increased from 2.8(1) nm in the fresh sample to 3.2(2) nm after aging the sample in air for 62 days (Table 2). When the catalysts were activated in hydrogen, the surface of the support particles was partially reduced, generating a lot of defects through the elimination of many oxygen atoms. This reduction was particularly strong at the edges on the surface of the support particles because at these positions, the oxygen atoms were weakly bounded to the surface. At this point, it is important to notice that the rutile particles had a porous morphology (Figure 6A), which produced many edges that served as pinning centers for the growing of the gold particles (Figure 6B). During the catalyst activation, the defects created by reducing the local environments trapped the initial gold atoms that worked as seeds for the formation of the gold particles; this interaction was more favorable than the interaction between the gold atoms and a local environment rich in oxygen. The affinity of the gold atoms with the reduced local environment on the titania surface produced a strong interaction between them, which stabilized the gold particles. This strong interaction maintained, in time, the spatial distribution of the gold particles in the atmospheric air at
Figure 5. Contribution of gold to the diffraction patterns of the catalysts activated in hydrogen and aged in atmospheric air for several days. This information was extracted from the refinement.
Table 2. Catalyst Activated in Air: Average Gold Crystallite Size As a Function of Aging Time aging time (days)
average gold crystallite size (nm)
0
2.8(1)
1
2.9(1)
2
2.8(1)
3 4
2.9(1) 3.0(1)
7
3.1(1)
9
3.1(1)
11
3.1(1)
22
3.4(2)
31
3.1(1)
41
3.2(1)
62
3.2(1)
room temperature (Figure 7A). Another possible explanation of this behavior is to assume that the activation of the catalyst in hydrogen removed the residual chlorine ions in the catalyst produced during their synthesis, with that hindering the formation of bridges of chlorine ions between the gold atoms upon heating28 and the poisoning of the active sites with chlorine atoms during the reaction of CO oxidation.29 When the gold catalysts were activated in air, the edges on the surface of the support particles were also pinning centers for the gold particles (Figure 8), but the covering of the support surface with these particles was not so rich as that in the catalysts activated with hydrogen (Figure 6B). This means that during the activation in air, most of the gold particles grew at the defects 5859
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Figure 6. Z-contrast TEM micrograph of a catalyst activated in hydrogen. Figure 9. CO conversion of the catalysts activated in hydrogen and aged in atmospheric air for several days.
Figure 7. Z-contrast TEM micrographs; (A) catalyst activated in hydrogen and aged in atmospheric air for 89 days and (B) catalyst activated in air and aged in atmospheric air for 62 days.
Figure 10. CO conversion of the aged catalysts after their heating at 200 °C, first in air and then in hydrogen. The catalyst activated in air was aged for 70 days, whereas the one activated in hydrogen was aged for 98 days.
Figure 8. Z-contrast TEM micrograph of a fresh catalyst activated in air.
on the surface that were different from those associated with the edges. This suggests that, in this case, the interaction between the gold particles and the titanium and the oxygen atoms at the rutile surface was weak, which explains why the gold particles moved easily with time on the surface at room temperature, causing Ostwald ripening and increasing with that the average dimensions of the gold particles. The micrographs of the aged catalysts activated in air (Figure 7B) show that the larger gold particles stayed at the edges on the surface of the support particles. This indicates that the gold particles that moved during the Ostwald ripening process were those pinned outside of the edges, which were weakly linked to the support. This fact also explains the observed changes with time of the spatial distribution of the gold particles on the rutile surface (Figures 7B and 8).
The above-described changes in the dimensions of the gold particles with time and activation atmosphere affected the catalytic activity of the gold catalysts to oxidize CO. For example, when the catalyst was activated in hydrogen and aged in environmental air at room temperature, its activity decreased with time (Figure 9). The decrease occurred during the first 16 days of aging; thereafter, the activity was almost constant. The aged catalysts were partially reactivated when they were annealed at 200 °C in air and then in hydrogen at the same temperature (Figure 10). After the reactivation, about 50% of the initial catalytic activity was recovered to its original activity. This result suggests that the loss of activity of the corresponding gold particles was produced by molecules (most probably carbonates) adsorbed on the active sites; these molecules were removed with the heat treatment. As mentioned in the Introduction section, the adsorption of carbonates on the catalytic active sites has been evoked as a cause of deactivation of the gold catalysts.19,20 In order to know if the observed deactivation of the aged catalysts was related to the adsorption of carbonates, both the fresh and the aged catalysts activated in hydrogen were analyzed with infrared spectroscopy in the transmission mode. The absorption spectrum of the fresh catalyst presented some lowintensity bands in the carboxylate region, 1400-1800 cm-1 5860
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Figure 11. FTIR spectra of the aged samples activated in hydrogen. The abbreviation “Reg.” refers to the aged catalyst after its regeneration by its heating at 200 °C in air and then in hydrogen.
(Figure 11), that are characteristic of adsorbed carbonates.44,45 As the catalysts were aged, the intensity of some bands increased, mainly those appearing at 1710 cm-1, which corresponds to an asymmetric stretching of a side-on bend of CO2-carboxylate species adsorbed on Ti3þ sites in close contact with the gold particles.46 These species are known to be intermediate phases during the formation of carbonates and bicarbonates on a catalyst surface.47 The presence of these carbonates in the gold catalysts, however, has been related not only to unreactive species that block the active sites16,18,48,49 but also to intermediate products that might represent a key step in the catalytic oxidation of carbon monoxide.50-52 Because during the FTIR analysis the catalysts were not under the reaction conditions for the oxidation of CO, the carbonate species observed on the catalysts during their analysis must be related to species that blocked the active sites. This assumption was confirmed when the aged catalysts were heated at 200 °C in air to eliminate the carbonates and then in hydrogen at the same temperature to recover the surface of the catalyst to its condition when the catalyst was fresh (Figure 11). From the infrared spectra of the reactivated catalysts, it is observed that the intensity of the bands associated with the carbonate species decreased but did not disappear completely. With this heat treatment, the catalysts did not recover totally their original catalytic activity because not all of the carbonates on the catalysts surface were removed and also because some of the gold particles became dimensions larger than those in the fresh catalyst. These gold particles grew through Oswald ripening when the weakest pinned gold particles moved on the surface of the support. The smaller catalytic activity of these gold particles was intrinsically associated with their larger dimensions. The observed changes, with time, in the catalytic activity of the gold catalysts activated in air followed a behavior similar to that reported for the catalysts activated in hydrogen (Figure 12). Therefore, the explanations of the origin of this behavior are similar to those reported above for the gold catalysts activated in hydrogen. In the catalysts activated in air, however, the growth, with time, of the gold particles was larger, and consequently, the catalytic activity decreased more rapidly. The fresh and aged Au/rutile catalysts activated in air were also analyzed by infrared spectroscopy in the transmission mode (Figure 13). The fresh catalyst did not present bands in the
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Figure 12. CO conversion of the catalysts activated in air and aged in atmospheric air for several days.
Figure 13. FTIR spectra of the aged samples activated in air. The abbreviation “Reg.” refers to the aged catalyst after its regeneration by its heating at 200 °C in air and then in hydrogen.
carboxylate region between 1400 and 1800 cm-1; the aged catalysts, however, presented a band at 1710 cm-1 characteristic of adsorbed carbonates, as in the catalyst activated in hydrogen. When the aged catalysts were heated in air at 200 °C, the intensity of this band decreased, which explains the partial recovery of the catalytic activity (Figure 10). This recovery was lower than that in the case of the aged catalysts activated in hydrogen because the gold particles grew more. This result is in agreement with the proposition of Maciejewski et al.22 that suggests that the calcination of the catalyst in air rids the surface of any carbonaceous contaminant from the support or precursor materials.
’ CONCLUSIONS When Au/rutile catalysts were activated at 200 °C in hydrogen or in air, the time evolution of the dimensions of the gold particles depended on the activation atmosphere and, consequently, their catalytic activity for the oxidation of CO. The different evolution of the gold catalysts for the different activation atmospheres depended on the interaction between the gold 5861
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The Journal of Physical Chemistry C atoms and the support. This interaction was stronger in the catalysts activated in hydrogen because this gas reduced locally the support surface, generating defects poor in oxygen that interacted strongly with the gold atoms that functioned as seeds of the gold particles. This caused that the gold particles supported on the surface regions far away from the surface edges to move easily when the activation was in air, producing larger gold particles and consequently less active catalysts. Both the catalysts activated in hydrogen and those in air adsorbed, with time, carbonate species that poisoned the catalytic active sites, generating an additional loss of catalytic activity, which was recovered by eliminating the poisoning molecules by heating the catalysts in air at 200 °C.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ525556225079. Fax: þ525556225008. E-mail: bokhimi@ fisica.unam.mx.
’ ACKNOWLEDGMENT We thank M. Sc. Manuel Aguilar’s technical support and Laboratorio de Refinamiento de Estructuras Crystalinas (LAREC) of Instituto de Física, Universidad Nacional Autonoma de Mexico, Mexico, and Proyecto Universitario de Nanotecnología Ambiental (PUNTA) for finantial support. R.Z. also acknowledges the finantial support given by the Projects CONACyT 55154, Mexico, and PAPIIT 108310, UNAM, Mexico. ’ REFERENCES (1) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Lu, P.; Akita, T.; Ichikawa, S.; Haruta, M. J. Catal. 2001, 202, 256. (2) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (3) Bokhimi, X.; Zanella, R.; Morales, A. J. Phys. Chem. C 2007, 111, 15210. (4) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (5) Haruta, M. Catal. Today 1997, 36, 153. (6) Bond, G. C.; Thompson, D. T. Catal. Rev.-Sci. Eng. 1999, 41, 319. (7) Grunwaldt, J.-D.; Baiker, A. J. Catal. 1999, 181, 223. (8) Zanella, R.; Giorgio, S.; Shin, C. H.; Henry, C. R.; Louis, C. J. Catal. 2004, 222, 357. (9) Zanella, R.; Delannoy, L.; Louis, C. Appl. Catal., A 2005, 291, 62. (10) Bond, G. C. Gold Bull. 2001, 34, 117. (11) Zwijnenburg, A.; Goossens, A.; Sloof, W. G.; Craje, M. W. J.; Kraan, A. M. v. d.; Jongh, L. J. d.; Makkee, M.; Moulijn, J. A. J. Phys. Chem. B 2002, 106, 9853. (12) Zanella, R.; Louis, C. Catal. Today 2005, 107, 768. (13) Bokhimi, X.; Zanella, R.; Morales, A. J. Phys. Chem. C 2008, 112, 12463. (14) Tsubota, S.; Cunningham, D. A. H.; Bando, Y.; Haruta, M. Stud. Surf. Sci. Catal. 1995, 91, 227. (15) Mitsutaka Okumura, M.; Koji Tanaka, K.; Atsushi Ueda, A.; Masatake Haruta, M. Solid State Ionics 1997, 95, 143. (16) Haruta, M. Cattech 2002, 6, 102. (17) Andreeva, D. Gold Bull. 2002, 35, 82. (18) Konova, P.; Naydenov, A.; Venkov, C.; Mehandjiev, D.; Andreeva, D.; Tabakova, T. J. Mol. Catal. A 2004, 213, 235. (19) Schubert, M. M.; Venugopal, A.; Kahlich, M. J.; Plzak, V.; Behm, R. J. J. Catal. 2004, 222, 32. (20) Wang, G. Y.; Lian, H. L.; Zhang, W. X.; Jiang, D. Z.; Wu, T. H. Kinet. Catal. 2002, 43, 433. (21) Tsubota, S.; Nakamura, T.; Tanaka, K.; Haruta, M. Catal. Lett. 1998, 56, 131.
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