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Nanosize gold catalysts promoted by vanadium oxide supported on titania and zirconia for complete benzene oxidation
Applied Catalysis A: General 209 (2001) 291–300
Nanosize gold catalysts promoted by vanadium oxide supported on titania and zirconia for complete benzene oxidation D. Andreeva a,∗ , T. Tabakova a , L. Ilieva a , A. Naydenov b , D. Mehanjiev b , M.V. Abrashev c b
a Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.11, 1113 Sofia, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.11, 1113 Sofia, Bulgaria c Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria
Received 18 April 2000; received in revised form 15 August 2000; accepted 17 August 2000
Abstract The complete benzene oxidation reaction was used to test the reactivity of the Au-V2 O5 /TiO2 and Au-V2 O5 /ZrO2 catalytic systems. A strong synergistic effect between gold and vanadia was established when molecular oxygen was used as an oxidizing agent. This effect was more pronounced for titania than for the zirconia support. In the presence of gold, predominantly polyvanadate structures are formed on the surface which are more active in the reaction of complete benzene oxidation in comparison with monovanadate species and bulk V2 O5 . The deposition of gold leads to a relative lengthening of the V=O bond and to a higher electron delocalization. The B and C parameters calculated from ESR spectra showed no differences for the fresh and spent gold-containing samples, i.e. the catalytic systems seemed to be “stabilized” under the working conditions. The effect of gold on the vanadium oxide reducibility, which could be related to the strength of the V–O-support bond, results in a considerable lowering of the temperature of the V 5+ → V 3+ transition. With ozone as the oxidizing agent, an additional lowering of the reaction temperature was achieved and very close values of the catalytic activity on all investigated samples were registered. Upon oxidation by molecular oxygen the oxidant activation takes place on the nanosize gold particles while the vanadium oxide surface species are responsible for the activation of benzene. This is in agreement with synergistic effect between the finely dispersed gold and the surface vanadium structures. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Vanadia-promoted gold catalysts; Preparation; Characterization by TPR; ESR and Raman spectroscopy; Complete benzene oxidation
1. Introduction The design of catalytic systems for complete oxidation of hydrocarbons is an important problem of environmental catalysis. There are data in the literature on the promoting effect of Pd and Ag over supported vanadium oxide for complete benzene oxidation. The effect has been related to the activation of oxygen on ∗ Corresponding author. Fax: +35-92-756116. E-mail address: [email protected] (D. Andreeva).
the metal particles which enables the reverse oxidation of V4+ and leads to an equilibrium in the redox process [1,2]. For the first time, the combination of supported gold and vanadia in complete benzene oxidation has been demonstrated by some of us [3] and a synergistic effect of this combination on the catalytic activity towards complete benzene oxidation has been established. Gold-based catalysts have received particular attention in connection with the environmental protection. The interest in these catalytic systems may be attributed to their high catalytic activity at low
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temperatures, the simple preparation methods and the lower price of gold in comparison with other noble metals. Various metal hydroxides, oxides or zeolites as supports of nanosize gold have been utilized in a series of important reactions [4–10]. Many studies have revealed the high catalytic activity of vanadium supported on different oxides in a number of oxidation reactions [11–15]. Titania and zirconia have been found as the most promising supports for deposited vanadium catalysts [8,14,16]. These oxides have been successfully used as supports for gold catalysts as well [17]. This is the motivation to use titania and zirconia as supports in the present study. Several crucial questions concerning the supported vanadium catalysts are in active discussion in the literature — the role of the support, the vanadium oxide loading, e.g. the type of surface layer on the supports (mono- or double-layer), the strength of V=O bond, the reducibility of the V–O-support bond, etc. The opinion predominates that the active vanadium-based catalysts possess a molecularly dispersed vanadium oxide phase which is typically more active than the bulk V2 O5 . Moreover, Khodakov et al. have suggested that polyvanadate domains are more active than monovanadate species in oxidative dehydrogenation reactions [14]. The strength of the terminal V=O bond has been suggested as a parameter controlling the reactivity of vanadium oxide catalysts [18,19]. Based on the ESR spectra of V2 O5 on different supports, Baiker et al. have proposed the shortening of the V=O bond, or lengthening of the distance to the oxygen ligands in the basal plane, respectively, as a criterion for the catalytic properties [20]. Studying the reactivity in partial oxidation of methanol, Deo and Wachs have suggested the V–O-support bridging bond as a factor controlling both the reactivity and reducibility of supported vanadium oxide catalysts [12]. The same authors have proposed the strength of V–O-support as a fundamental criterion of the oxidation activity of supported metal oxide catalysts. In connection with the evaluation of the catalytic activity and the mechanism of the oxidative reaction, it would be of interest to follow the influence of the oxidizing agent. One of the prominent methods for this purpose is the ozone-catalytic oxidation (OZCO) where heterogeneous catalytic decomposition of ozone is used to produce highly reactive atomic oxygen able to oxidize harmful organic compounds at
low temperatures including room temperature [21]. It has been former shown [22–25] that the simple oxides of transition metals manifest high activity in the decomposition of ozone and in the OZCO reaction of organic compounds. The present paper focuses on the explanation of the synergistic effect between nanosize gold and vanadium oxide species expressed as a significant decrease in the reaction temperature of complete benzene oxidation in the presence of oxygen or ozone. By applying appropriate chemical and physical methods we try to clarify the effect of gold on the structure of VOx surface species and on the strength of both V=O and V–O-support bonds. 2. Experimental 2.1. Sample preparation Six catalyst samples were studied, Au/TiO2 (AT), V2 O5 /TiO2 (VT), Au/V2 O5 /TiO2 (AVT), Au/ZrO2 (AZ), V2 O5 /ZrO2 (VZ), and Au/V2 O5 /ZrO2 (AVZ). The gold-containing samples were prepared in a “Contalab” laboratory reactor (Contraves AG, Switzerland) under complete control of all parameters, temperature, pH, stirrer speed, reactant feed flow rates using a deposition–precipitation method. This involves the deposition of gold onto the support through a chemical interaction of HAuCl4 ·3H2 O (“Merck”) and Na2 CO3 in aqueous solution. As supports, well crystallized TiO2 (pure anatase) and commercial zirconia (“Reachim”) were used. The precipitates were aged for 1 h at 60◦ C, filtered and washed carefully until an absence of Cl− ions. Then, the samples were dried under vacuum at 80◦ C and calcined in air at 400◦ C for 2 h. Vanadium was introduced via impregnation with (NH4 )2 [VO(C2 O4 )2 ]. The precursors were dried again under vacuum and calcinated at 400◦ C for 2 h. 2.2. Sample characterization The BET area of the samples was determined on a ‘Flow Sorb II-2300’ device. Samples for X-ray diffraction studies were prepared by means of Synocryl 9122X in a diluted toluene solution and exposed for 6 h in a Guinier-De Wolff-Nonius camera Mark IV, using Cu K␣1 radiation.
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Transmission electron microscopy (TEM) characterization was performed in a Hitachi-H-600-2 electron microscope. The samples were dispersed in bi-distilled water by ultrasound. A drop was then transferred to a carbon-coated bronze mesh and dried. The TPR measurements were carried out by means of an apparatus described elsewhere [26]. A cooling trap (−40◦ C) for removing water formed during reduction was mounted in the gas line ahead of the thermal conductivity detector. A hydrogen–argon mixture (10% H2 ), passed over a molecular sieve 5A (−40◦ C), was used to reduce the samples at a flow rate of 24 ml min−1 . The temperature was linearly raised at a rate of 15◦ C min−1 . A quartz reactor loaded with 0.15 g of the investigated samples was utilized. The optimal amount of loaded samples was selected by the criterion proposed by Monti and Baiker [27]. Raman spectra were recorded using a SPEX 1403 double spectrometer, equipped with a photomultiplier, working in the photon counting mode. The 488 nm line of an Ar+ ion laser was used for excitation. The laser power on the samples was 60 mW. The spectral slit width was 4 cm−1 . ESR spectra were recorded on a Brucker 200 D instrument. In some cases, samples were analyzed before and after catalytic work. 2.3. Catalytic activity The catalytic activity of the samples for complete benzene oxidation was evaluated in a gradientless reactor with external circulation under the conditions, catalyst volume, 0.2 cm3 (particle size 0.25–0.50 mm), inlet benzene concentration, 0.019 mol m−3 in oxygen, temperature range, 150–300◦ C. The pretreatment of the samples consisted of a 20 min heating at 300◦ C in an oxygen flow.
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The catalytic activity of the samples was also evaluated using ozone as the oxidant in the temperature range 20–60◦ C. Ozone was synthesized in a dried oxygen flow (gas flow rate 4 l h−1 ), the initial concentration of ozone ranging from 22 to 24.5 g m−3 . The ozone analysis was performed by an Ozomat GM (Germany) ozone analyzer. The residual ozone was decomposed in a catalytic reactor filled with an OCA-1 (Bulgaria) catalyst. The experiments were carried out with a circulation ratio of 1:70. Benzene was dosed by an Ismatex MS/6 pump. The initial concentrations of benzene were varied within 0.01–0.03 vol.%. The carrier gas was oxygen. The CO2 formed was measured by Infralyt 2106. The catalytic activity in complete benzene oxidation was expressed by the degree of benzene conversion to CO2 .
3. Results 3.1. Sample characterization The chemical composition of the samples, the BET surface area and the calculated VOx surface densities (ns )are shown in Table 1. The BET surface area of the initial supports is 96.0 m2 g−1 for anatase and 26 m2 g−1 for zirconia, respectively. The loading of gold and vanadia had only the modest effect on the surface area of the support. The VOx surface densities (ns ) were calculated from the V2 O5 content and the BET surface area. It is defined as the number of vanadium atoms per square nanometer of the catalyst. The TEM micrographs of the samples AT and AZ are shown in Fig. 1. The X-ray and TEM data pointed to an average gold particle size below 5 nm and a uniform deposition of gold on the supports. In the case
Table 1 Chemical composition, BET surface area and VOx density of the samples Catalyst
Au content (wt.%)
V2 O5 content (wt.%)
BET surface area (m2 g−1 )
VOx surface density (nm−2 )
AT VT AVT AZ VZ AVZ
3.0 – 3.0 3.0 – 3.0
– 4.0 4.0 – 4.0 4.0
97.0 95.0 86.0 21.0 19.5 22.4
– 2.8 3.1 – 13.7 12.0
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Fig. 2. TPR spectra of the samples.
of AT the support is well crystallized anatase and in the case of AZ, it is crystallized zirconia.
Fig. 1. TEM photographs of calcined samples (a) Au/TiO2 ; (b) Au/ZrO2 .
3.1.1. TPR measurements The TPR spectra of the samples are presented in Fig. 2. In the investigated temperature interval (up to 600◦ C) TPR peaks of the pure supports were not registrated. A low temperature peak (Tmax about 60◦ C) in the TPR spectra of the AT sample is observed, which we assigned to the presence of oxidic species of gold particles. For the VT sample a single TPR peak is observed (T max = 455◦ C). In accordance with Bond [28,29] we assigned this TPR peak to the V 5+ → V 3+ single step transition of the VOx monolayer species. The amount of vanadium pentoxide involved in our case is lower than the “theoretical monolayer equivalent” (0.145 wt.% V2 O5 m−2 of anatase surface) according to the same authors. The ESR spectroscopic measurements of the sample conducted after the end of the TPR peak indicated no presence of the V4+ ions. The presence of gold leads to a considerable lowering of the reduction temperature of the available vanadia species. The peak in the TPR spectra of the AVT sample is narrow with Tmax of only 120◦ C. The same holds true for the AT sample. The low temperature peak (Tmax about 60◦ C) in the TPR spectra of the AZ sample was also assigned to the presence of oxidic gold species.
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A double TPR peak was registered for the VZ sample which may be explained in terms of (i) more than three times lower BET surface area of zirconia in comparison with titania with the same vanadium oxide content and (ii) the nature of the support resulting in a specific vanadium oxide-support interaction which will be discussed later. The TPR profile of AVZ sample shows a very broad double peak in the temperature interval 80–280◦ C. The effect of gold deposition on the zirconia support is not so pronounced as that on the titania support. In both cases there is a substantial decrease in the reduction temperature. 3.1.2. Raman spectroscopy data The structure of the VOx species detected by Raman spectroscopy depends on both the type of support and the VOx surface density. It is noteworthy that Raman spectroscopy gives evidence for V2 O5 crystallites at much lower VOx surface densities on supports such as Al2 O3 , SiO2 , TiO2 , HfO2 and ZrO2 than
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does XRD, indicating greater sensitivity to the presence of crystalline V2 O5 . The Raman spectra of the vanadia-containing samples are presented in Fig. 3. The small bulk V2 O5 can be identified in the VT sample by the bands at 147 and 994 cm−1 and for the VZ sample by those at 144, 288, 406, 526, 698, and 996 cm−1 . The broad bands in the 1000–600 cm−1 region can be ascribed to V=O (1000–800 cm−1 ) and V–O–V (800–600 cm−1 ) stretching modes in polyvanadate domains [11,30]. In the absence of gold, in the samples VT and VZ monovanadate species can be detected, as well (low intensity bands at 1025, 1030 cm−1 ). A small amount of bulk V2 O5 is observed in the AVZ sample (weak band at 995 cm−1 ), but not in the AVT sample. There is no indication for support-vanadia compounds (the bands located at 772, 977 cm−1 for the bulk ZrV2 O7 ) [14,30]. Comparing the Raman spectra of the initial supports, titania and zirconia, with those of the gold and vanadia-containing samples, a modification of the supports, especially in the case of zirconia, was established.
Fig. 3. Raman spectra of vanadium-containing samples (a) on titania; (b) on zirconia.
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Fig. 4. ESR spectra of vanadium-containing samples VT: 1 — fresh and 2 — spent; AVT: 1 — fresh and 2 — spent; VZ: 1 — fresh and 2 — spent; AVZ: 1 — fresh and 2 — spent.
3.1.3. ESR spectroscopy data The surrounding of the V4+ ions in the lattice of the samples was studied by ESR spectroscopy. The ESR spectra of the studied samples are shown in Fig. 4. The spectra features of the paramagnetic centre 51 V4+ (3d1 ) with a nuclear magnetic moment of I = 7/2 are well documented in the literature [20,31]. From the experimental spectra the values of the g|| , g⊥ , A|| and A⊥ parameters were determined. Using these values, B, β 2 ∗2 and C = 1 − [β2∗2 ] parameters
were calculated [20]. It was assumed that the V4+ ions occur in an oxygen environment which is very closed to that of the vanadyl complex VO2+ . The quantity B = 1g|| /1g⊥ reflects the changes in the tetragonal distorsion and is an estimate of the V=O bond strength. β 2 ∗2 is a coeffiecient accounting for the participation of the dxy atomic orbital in the ψ ∗ (b2 ) antibonding orbital centered mainly on the cation of the vanadyl complex. This coefficient is associated with the electron delocalization on the cation. β 2 ∗2
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Table 2 The parameters calculated from the ESR of V2 O5 -containing samplesa Catalyst
B
β 2 ∗2
C
VT fresh VT spent AVT fresh AVT spent AVZ fresh AVZ spent
2.1 0.5 1.0 0.7 0.8 0.7
0.61 0.65 0.59 0.60 0.67 0.68
0.39 0.35 0.41 0.40 0.33 0.32
a The corresponding parameters for the VZ sample are not calculated.
or C = 1−[β 2 ∗2 ], respectively, corresponds to the fraction of unpaired d-electrons delocalized over the oxygen ligands. The parameters calculated from the ESR spectra are shown in Table 2. 3.2. Catalytic activity tests The complete benzene conversion (CBC) at 250◦ C with oxygen is presented in Fig. 5. Depending on the type of the support, the catalytic activity of the samples follows a different order, AVT > AT > VT for the catalysts on titania and AVZ > VZ > AZ for those on zirconia. The comparison between the same type of catalysts over the two different supports yields the following couples in activity VAT > VAZ,
AT ≥ AZ,
VT > VZ
Fig. 6. Catalytic activity in complete benzene conversion at 40◦ C with ozone: 1 — vanadia suported samples; 2 — gold supported samples; 3 — gold-vanadia supported samples.
Evidently, the titania catalysts are more active than the zirconia catalysts. The AVT and AVZ samples where gold and vanadium are simultaneously present manifest the highest catalytic activities. A stronger synergistic effect is observed for the titania sample in comparison with the zirconia sample. The CBC at 40◦ C with ozone as the oxidant is illustrated in Fig. 6. It is seen that, in this case the temperature of the CBC is additionally lowered by about 200◦ . The activity order in this case is VAT > VAZ, AT ≈ AZ, VT ≈ VZ The results presented in Fig. 6 reveal the similar activity values of CBC obtained with ozone over the samples investigated.
4. Discussion
Fig. 5. Catalytic activity in complete benzene conversion at 250◦ C with molecular oxygen: 1 — vanadia suported samples; 2 — gold supported samples; 3 — gold–vanadia supported samples.
In the interpretation of the catalytic activity data obtained in the present investigation, first we shall discuss the relationship between the structure and the reactivity of the studied samples. It follows from the Raman data analysis that a small amount of bulk V2 O5 is present in the non-promoted gold samples of VT and VZ. A small amount of monovanadate species coexists on the surface of the VT sample. Polyvanadate domains are predominantly observed in the vanadium-containing samples on both supports. In the case of a vanadia–zirconia sample a slight modification of the support was detected, which was not visible on titania.
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After gold deposition on the AVT sample only polyvanadate species are observed in the Raman spectra. In the corresponding sample on zirconia (AVZ) polyvanadate domains are also prevailing, but some V2 O5 crystallites are detected as well. Comparing the Raman spectra of the samples non-promoted and promoted by gold, it is interesting to note that the deposition of gold strongly influences the state of the support. In the AVT and AVZ samples titania and zirconia are modified, especially the zirconia support which is strongly disordered. Obviously, the interaction between gold and vanadia is stronger on zirconia than on titania, nevertheless no bulk ZrV2 O7 is observed. The ESR spectroscopy data were used to estimate the influence of the supports and of gold on the vanadyl bond strength and the delocalization of the 3d-electrons on the coordinatively bound oxygen ligands. The parameter B calculated by the ESR data is an estimate of the V=O bond length, i.e. of the V=O bond strength. The calculated parameter C, corresponding to the fraction of unpaired d-electrons may be directly connected with the catalytic properties. The value of the B parameter calculated for the VT sample is higher than that for the AVT sample (Table 2). This indicates a relative lengthening of V=O bond in the presence of gold. Comparing the values of the parameter C for the same samples one sees that the deposition of gold leads to a higher delocalization of 3d-electrons. The values obtained for the spent samples (after catalytic tests in complete benzene oxidation) do not differ significantly from those for the fresh ones. The values of the B parameters for gold–vanadia deposited on zirconia (fresh and spent) are very closed to those for the corresponding samples on titania. However, a comparison between the C parameters of the same samples (AVT and AVZ, fresh and spent) reveals considerably lower values when zirconia is used as a support. These results give grounds to assume a higher electron delocalization in the AVT sample in comparison with the AVZ sample, which is in agreement with the catalytic activity data. The almost equal values of both calculated parameters B and C for the fresh and spent AVT and AVZ samples could be an indication for the “stabilization” of the catalytic system under the working conditions in the presence of gold. In addition, the results obtained by the TPR measurements could be used to estimate
the effect of gold on the vanadium oxide reducibility, which controls the V–O-support strength bond [12]. The low temperature peak registered in the TPR spectra of the AT and AZ samples, assigned to the presence of oxidic gold clusters could be interpreted in terms of the existence of extremely small gold particles (below 4 nm) displaying unusual properties, as postulated by Burke [32]. We did not observe such a behavior in the case of gold promoted Fe2 O3 and Co3 O4 samples, where the average size of the gold particles was about 4 nm or 7–8 nm, respectively, [33,34]. In the TPR spectra of the VT sample a single narrow peak is observed, while the corresponding TPR peak for the VZ sample is broad and double. In our opinion, this difference is due to the higher concentration of VOx species on zirconia in comparison with titania (Table 1). As postulated by Bond et al. [28,29], upon increasing the V2 O5 loading on titania, layers of “disordered” V2 O5 and “paracrystalline” V2 O5 phases are formed on limited areas of the surface. These new V2 O5 species are the cause of the second TPR peak. By analogy with titania we assigned this second TPR peak to the presence of these new vanadia species along with the VOx monolayer species. A small amount of bulk V2 O5 crystallites in the VZ sample was detected by means of the Raman spectra. This satisfactorily explains the appearance of the second peak in the TPR spectra. The specific interaction between vanadia and the support is also of great importance. The Raman spectra show that the deposition of vanadia on zirconia modifies the support as a result of the strong vanadia–zirconia interaction. It may be hence assumed that the reduction V 5+ → V 3+ in the VZ sample proceeds in two steps in a wide temperature interval. The reduction behavior of gold promoted vanadium samples (AVT and AVZ) is quite different. The presence of gold leads to a considerable lowering of the reduction temperature of the available vanadia species for both samples. The registered peak in the TPR spectra of the AVT sample is narrow like the corresponding one in the vanadia–titania sample, Tmax being only 120◦ C, i.e. the decrease in the reduction temperature is about 330◦ . As regards the non-promoted and gold promoted samples on zirconia (VZ and AVZ), a very broad double TPR peak is registered for the AVZ sample and the temperature of the reduction is also decreased. The Raman spectrum
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of AVZ sample gives evidence of a small part of bulk V2 O5 and of support modification. This is probably the reason for the appearance of the very broad double peak in the TPR spectra of the AVZ sample similarly to the non-promoted by gold VZ sample. A strong decrease in the reduction temperature in the presence of gold has been former observed by us for the Au/Fe2 O3 and Au/Co3 O4 catalytic systems (Fe2 O3 → Fe3 O4 [33] and Co3 O4 → Co transitions [34], respectively). In our opinion, the explanation could be based on (i) the increasing number of potential nucleation sites for metal oxide reduction on the interfacial perimeter resulting from the high gold dispersion and (ii) the additional enhancement of the reduction process by a high concentration of active hydrogen. As it was very recently evidenced for the Au/TiO2 catalytic system, hydrogen is dissociated on nanosize gold particles even at room temperature [35]. As it was shown above, the catalytic activity of the studied samples in the CBC reaction depends on the oxidizing agent used. First we shall discuss the behavior of the samples in the catalytic oxidation by molecular oxygen (Fig. 5). Of interest is the catalytic order for the two types of catalysts depending on their support, presented in “Results”. The catalysts promoted only by gold (AT and AZ) show very closed values of catalytic activity. It is known that the particle size is of decisive importance for the high catalytic activity. The gold dispersity of all samples under investigation is of the same order (below 5 nm), which is in agreement with the CBC values found in the experiments. A different picture is observed for the samples promoted only by V2 O5 (VT and VZ). The titania-based catalyst is more active than the zirconia-based one. Such difference could be explained by the higher content of bulk V2 O5 in the zirconia sample. According to the Raman spectroscopy and TPR data, no bulk V2 O5 is present on titania. The prevailing coverage of titania with polyvanadate structures, evidenced by Raman spectroscopy, also agrees with the higher catalytic activity of the VT sample. As was shown, the presence of gold leads to a considerable enhancement of the catalytic activity of vanadium oxide supported on both titania and zirconia for complete benzene oxidation by oxygen. A stronger synergistic effect between gold and vanadia was registered with the titania support. In that case, the temperature of benzene conversion with oxygen was lower
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by about 150◦ than that in the presence of palladium or silver promoted vanadium oxide catalysts [1,2]. The highest catalytic activity of the AVT sample is in agreement with the results of the physicochemical studies. As we have already shown by Raman spectroscopy data, only polyvanadate domains are present in the AVT sample. Khodakov et al. [14] have established that polyvanadate species are more active than monovanadate ones and V2 O5 bulk phase in the oxidative dehydrogenation of propane on vanadium supported catalysts. The higher d-electron delocalization and the weaker V=O bond of the AVT sample in comparison with the other investigated samples (ESR data) agree with its highest catalytic activity. The enhanced reducibility of the vanadium oxide species in both AVT and AVZ samples also agrees with the observed synergistic effect between gold and vanadia. Complete benzene oxidation by ozone displays a different behavior in comparison with molecular oxygen. As it is evident from the activity order shown above, similar values are obtained. Benzene oxidation with ozone takes place at lower temperatures than with molecular oxygen. Our recent results [36] on ozone decomposition over the same samples showed the same activity order as for the benzene oxidation. Evidently, there are different oxidation mechanisms with the two types of oxidants. It may be assumed that ozone decomposition yields highly active oxygen species on the catalyst surface which are able to oxidize benzene even at room temperature. This assumption is in accordance with the reaction mechanism, proposed by Naydenov et al. in a former paper [22]. The synergistic effect manifested in the reaction of complete benzene oxidation by molecular oxygen on supported gold catalysts promoted by vanadia is in accordance with the mechanism, proposed by Andreev et al. [1,2] for the reaction on Pd-V2 O5 /Al2 O3 and Ag-V2 O5 /Al2 O3 catalytic systems. Most probably, the oxygen activation proceeds on the nanosize gold particles, while the vanadium oxide surface species are responsible for the activation of benzene. 5. Conclusions • It was found that polyvanadate structures predominantly exist on the catalyst surface in the presence of gold.
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• The deposition of gold leads to the relatively lengthening of V=O bond and a higher electron delocalization, which “stabilizes” the catalytic system under the working conditions. • The gold-containing samples on both studied supports provide a considerably lower temperature of the V 5+ → V 3+ transition. • The catalytic activity of the promoted by vanadia gold supported on titania and zirconia catalytic systems in complete benzene oxidation strongly depends on the oxidizing agent. A synergistic effect between gold and vanadia was established on both supports in the presence of molecular oxygen at the temperature of 250◦ C. With ozone as oxidizing agent a significantly lower reaction temperature and very close values of the catalytic activity on all investigated samples were found. Obviously, different reaction mechanism may be assumed in these two cases.
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Nanosize gold catalysts promoted by vanadium oxide supported on titania and zirconia for complete benzene oxidation D. Andreeva a,∗ , T. Tabakova a , L. Ilieva a , A. Naydenov b , D. Mehanjiev b , M.V. Abrashev c b
a Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.11, 1113 Sofia, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.11, 1113 Sofia, Bulgaria c Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria
Received 18 April 2000; received in revised form 15 August 2000; accepted 17 August 2000
Abstract The complete benzene oxidation reaction was used to test the reactivity of the Au-V2 O5 /TiO2 and Au-V2 O5 /ZrO2 catalytic systems. A strong synergistic effect between gold and vanadia was established when molecular oxygen was used as an oxidizing agent. This effect was more pronounced for titania than for the zirconia support. In the presence of gold, predominantly polyvanadate structures are formed on the surface which are more active in the reaction of complete benzene oxidation in comparison with monovanadate species and bulk V2 O5 . The deposition of gold leads to a relative lengthening of the V=O bond and to a higher electron delocalization. The B and C parameters calculated from ESR spectra showed no differences for the fresh and spent gold-containing samples, i.e. the catalytic systems seemed to be “stabilized” under the working conditions. The effect of gold on the vanadium oxide reducibility, which could be related to the strength of the V–O-support bond, results in a considerable lowering of the temperature of the V 5+ → V 3+ transition. With ozone as the oxidizing agent, an additional lowering of the reaction temperature was achieved and very close values of the catalytic activity on all investigated samples were registered. Upon oxidation by molecular oxygen the oxidant activation takes place on the nanosize gold particles while the vanadium oxide surface species are responsible for the activation of benzene. This is in agreement with synergistic effect between the finely dispersed gold and the surface vanadium structures. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Vanadia-promoted gold catalysts; Preparation; Characterization by TPR; ESR and Raman spectroscopy; Complete benzene oxidation
1. Introduction The design of catalytic systems for complete oxidation of hydrocarbons is an important problem of environmental catalysis. There are data in the literature on the promoting effect of Pd and Ag over supported vanadium oxide for complete benzene oxidation. The effect has been related to the activation of oxygen on ∗ Corresponding author. Fax: +35-92-756116. E-mail address: [email protected] (D. Andreeva).
the metal particles which enables the reverse oxidation of V4+ and leads to an equilibrium in the redox process [1,2]. For the first time, the combination of supported gold and vanadia in complete benzene oxidation has been demonstrated by some of us [3] and a synergistic effect of this combination on the catalytic activity towards complete benzene oxidation has been established. Gold-based catalysts have received particular attention in connection with the environmental protection. The interest in these catalytic systems may be attributed to their high catalytic activity at low
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temperatures, the simple preparation methods and the lower price of gold in comparison with other noble metals. Various metal hydroxides, oxides or zeolites as supports of nanosize gold have been utilized in a series of important reactions [4–10]. Many studies have revealed the high catalytic activity of vanadium supported on different oxides in a number of oxidation reactions [11–15]. Titania and zirconia have been found as the most promising supports for deposited vanadium catalysts [8,14,16]. These oxides have been successfully used as supports for gold catalysts as well [17]. This is the motivation to use titania and zirconia as supports in the present study. Several crucial questions concerning the supported vanadium catalysts are in active discussion in the literature — the role of the support, the vanadium oxide loading, e.g. the type of surface layer on the supports (mono- or double-layer), the strength of V=O bond, the reducibility of the V–O-support bond, etc. The opinion predominates that the active vanadium-based catalysts possess a molecularly dispersed vanadium oxide phase which is typically more active than the bulk V2 O5 . Moreover, Khodakov et al. have suggested that polyvanadate domains are more active than monovanadate species in oxidative dehydrogenation reactions [14]. The strength of the terminal V=O bond has been suggested as a parameter controlling the reactivity of vanadium oxide catalysts [18,19]. Based on the ESR spectra of V2 O5 on different supports, Baiker et al. have proposed the shortening of the V=O bond, or lengthening of the distance to the oxygen ligands in the basal plane, respectively, as a criterion for the catalytic properties [20]. Studying the reactivity in partial oxidation of methanol, Deo and Wachs have suggested the V–O-support bridging bond as a factor controlling both the reactivity and reducibility of supported vanadium oxide catalysts [12]. The same authors have proposed the strength of V–O-support as a fundamental criterion of the oxidation activity of supported metal oxide catalysts. In connection with the evaluation of the catalytic activity and the mechanism of the oxidative reaction, it would be of interest to follow the influence of the oxidizing agent. One of the prominent methods for this purpose is the ozone-catalytic oxidation (OZCO) where heterogeneous catalytic decomposition of ozone is used to produce highly reactive atomic oxygen able to oxidize harmful organic compounds at
low temperatures including room temperature [21]. It has been former shown [22–25] that the simple oxides of transition metals manifest high activity in the decomposition of ozone and in the OZCO reaction of organic compounds. The present paper focuses on the explanation of the synergistic effect between nanosize gold and vanadium oxide species expressed as a significant decrease in the reaction temperature of complete benzene oxidation in the presence of oxygen or ozone. By applying appropriate chemical and physical methods we try to clarify the effect of gold on the structure of VOx surface species and on the strength of both V=O and V–O-support bonds. 2. Experimental 2.1. Sample preparation Six catalyst samples were studied, Au/TiO2 (AT), V2 O5 /TiO2 (VT), Au/V2 O5 /TiO2 (AVT), Au/ZrO2 (AZ), V2 O5 /ZrO2 (VZ), and Au/V2 O5 /ZrO2 (AVZ). The gold-containing samples were prepared in a “Contalab” laboratory reactor (Contraves AG, Switzerland) under complete control of all parameters, temperature, pH, stirrer speed, reactant feed flow rates using a deposition–precipitation method. This involves the deposition of gold onto the support through a chemical interaction of HAuCl4 ·3H2 O (“Merck”) and Na2 CO3 in aqueous solution. As supports, well crystallized TiO2 (pure anatase) and commercial zirconia (“Reachim”) were used. The precipitates were aged for 1 h at 60◦ C, filtered and washed carefully until an absence of Cl− ions. Then, the samples were dried under vacuum at 80◦ C and calcined in air at 400◦ C for 2 h. Vanadium was introduced via impregnation with (NH4 )2 [VO(C2 O4 )2 ]. The precursors were dried again under vacuum and calcinated at 400◦ C for 2 h. 2.2. Sample characterization The BET area of the samples was determined on a ‘Flow Sorb II-2300’ device. Samples for X-ray diffraction studies were prepared by means of Synocryl 9122X in a diluted toluene solution and exposed for 6 h in a Guinier-De Wolff-Nonius camera Mark IV, using Cu K␣1 radiation.
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Transmission electron microscopy (TEM) characterization was performed in a Hitachi-H-600-2 electron microscope. The samples were dispersed in bi-distilled water by ultrasound. A drop was then transferred to a carbon-coated bronze mesh and dried. The TPR measurements were carried out by means of an apparatus described elsewhere [26]. A cooling trap (−40◦ C) for removing water formed during reduction was mounted in the gas line ahead of the thermal conductivity detector. A hydrogen–argon mixture (10% H2 ), passed over a molecular sieve 5A (−40◦ C), was used to reduce the samples at a flow rate of 24 ml min−1 . The temperature was linearly raised at a rate of 15◦ C min−1 . A quartz reactor loaded with 0.15 g of the investigated samples was utilized. The optimal amount of loaded samples was selected by the criterion proposed by Monti and Baiker [27]. Raman spectra were recorded using a SPEX 1403 double spectrometer, equipped with a photomultiplier, working in the photon counting mode. The 488 nm line of an Ar+ ion laser was used for excitation. The laser power on the samples was 60 mW. The spectral slit width was 4 cm−1 . ESR spectra were recorded on a Brucker 200 D instrument. In some cases, samples were analyzed before and after catalytic work. 2.3. Catalytic activity The catalytic activity of the samples for complete benzene oxidation was evaluated in a gradientless reactor with external circulation under the conditions, catalyst volume, 0.2 cm3 (particle size 0.25–0.50 mm), inlet benzene concentration, 0.019 mol m−3 in oxygen, temperature range, 150–300◦ C. The pretreatment of the samples consisted of a 20 min heating at 300◦ C in an oxygen flow.
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The catalytic activity of the samples was also evaluated using ozone as the oxidant in the temperature range 20–60◦ C. Ozone was synthesized in a dried oxygen flow (gas flow rate 4 l h−1 ), the initial concentration of ozone ranging from 22 to 24.5 g m−3 . The ozone analysis was performed by an Ozomat GM (Germany) ozone analyzer. The residual ozone was decomposed in a catalytic reactor filled with an OCA-1 (Bulgaria) catalyst. The experiments were carried out with a circulation ratio of 1:70. Benzene was dosed by an Ismatex MS/6 pump. The initial concentrations of benzene were varied within 0.01–0.03 vol.%. The carrier gas was oxygen. The CO2 formed was measured by Infralyt 2106. The catalytic activity in complete benzene oxidation was expressed by the degree of benzene conversion to CO2 .
3. Results 3.1. Sample characterization The chemical composition of the samples, the BET surface area and the calculated VOx surface densities (ns )are shown in Table 1. The BET surface area of the initial supports is 96.0 m2 g−1 for anatase and 26 m2 g−1 for zirconia, respectively. The loading of gold and vanadia had only the modest effect on the surface area of the support. The VOx surface densities (ns ) were calculated from the V2 O5 content and the BET surface area. It is defined as the number of vanadium atoms per square nanometer of the catalyst. The TEM micrographs of the samples AT and AZ are shown in Fig. 1. The X-ray and TEM data pointed to an average gold particle size below 5 nm and a uniform deposition of gold on the supports. In the case
Table 1 Chemical composition, BET surface area and VOx density of the samples Catalyst
Au content (wt.%)
V2 O5 content (wt.%)
BET surface area (m2 g−1 )
VOx surface density (nm−2 )
AT VT AVT AZ VZ AVZ
3.0 – 3.0 3.0 – 3.0
– 4.0 4.0 – 4.0 4.0
97.0 95.0 86.0 21.0 19.5 22.4
– 2.8 3.1 – 13.7 12.0
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Fig. 2. TPR spectra of the samples.
of AT the support is well crystallized anatase and in the case of AZ, it is crystallized zirconia.
Fig. 1. TEM photographs of calcined samples (a) Au/TiO2 ; (b) Au/ZrO2 .
3.1.1. TPR measurements The TPR spectra of the samples are presented in Fig. 2. In the investigated temperature interval (up to 600◦ C) TPR peaks of the pure supports were not registrated. A low temperature peak (Tmax about 60◦ C) in the TPR spectra of the AT sample is observed, which we assigned to the presence of oxidic species of gold particles. For the VT sample a single TPR peak is observed (T max = 455◦ C). In accordance with Bond [28,29] we assigned this TPR peak to the V 5+ → V 3+ single step transition of the VOx monolayer species. The amount of vanadium pentoxide involved in our case is lower than the “theoretical monolayer equivalent” (0.145 wt.% V2 O5 m−2 of anatase surface) according to the same authors. The ESR spectroscopic measurements of the sample conducted after the end of the TPR peak indicated no presence of the V4+ ions. The presence of gold leads to a considerable lowering of the reduction temperature of the available vanadia species. The peak in the TPR spectra of the AVT sample is narrow with Tmax of only 120◦ C. The same holds true for the AT sample. The low temperature peak (Tmax about 60◦ C) in the TPR spectra of the AZ sample was also assigned to the presence of oxidic gold species.
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A double TPR peak was registered for the VZ sample which may be explained in terms of (i) more than three times lower BET surface area of zirconia in comparison with titania with the same vanadium oxide content and (ii) the nature of the support resulting in a specific vanadium oxide-support interaction which will be discussed later. The TPR profile of AVZ sample shows a very broad double peak in the temperature interval 80–280◦ C. The effect of gold deposition on the zirconia support is not so pronounced as that on the titania support. In both cases there is a substantial decrease in the reduction temperature. 3.1.2. Raman spectroscopy data The structure of the VOx species detected by Raman spectroscopy depends on both the type of support and the VOx surface density. It is noteworthy that Raman spectroscopy gives evidence for V2 O5 crystallites at much lower VOx surface densities on supports such as Al2 O3 , SiO2 , TiO2 , HfO2 and ZrO2 than
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does XRD, indicating greater sensitivity to the presence of crystalline V2 O5 . The Raman spectra of the vanadia-containing samples are presented in Fig. 3. The small bulk V2 O5 can be identified in the VT sample by the bands at 147 and 994 cm−1 and for the VZ sample by those at 144, 288, 406, 526, 698, and 996 cm−1 . The broad bands in the 1000–600 cm−1 region can be ascribed to V=O (1000–800 cm−1 ) and V–O–V (800–600 cm−1 ) stretching modes in polyvanadate domains [11,30]. In the absence of gold, in the samples VT and VZ monovanadate species can be detected, as well (low intensity bands at 1025, 1030 cm−1 ). A small amount of bulk V2 O5 is observed in the AVZ sample (weak band at 995 cm−1 ), but not in the AVT sample. There is no indication for support-vanadia compounds (the bands located at 772, 977 cm−1 for the bulk ZrV2 O7 ) [14,30]. Comparing the Raman spectra of the initial supports, titania and zirconia, with those of the gold and vanadia-containing samples, a modification of the supports, especially in the case of zirconia, was established.
Fig. 3. Raman spectra of vanadium-containing samples (a) on titania; (b) on zirconia.
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Fig. 4. ESR spectra of vanadium-containing samples VT: 1 — fresh and 2 — spent; AVT: 1 — fresh and 2 — spent; VZ: 1 — fresh and 2 — spent; AVZ: 1 — fresh and 2 — spent.
3.1.3. ESR spectroscopy data The surrounding of the V4+ ions in the lattice of the samples was studied by ESR spectroscopy. The ESR spectra of the studied samples are shown in Fig. 4. The spectra features of the paramagnetic centre 51 V4+ (3d1 ) with a nuclear magnetic moment of I = 7/2 are well documented in the literature [20,31]. From the experimental spectra the values of the g|| , g⊥ , A|| and A⊥ parameters were determined. Using these values, B, β 2 ∗2 and C = 1 − [β2∗2 ] parameters
were calculated [20]. It was assumed that the V4+ ions occur in an oxygen environment which is very closed to that of the vanadyl complex VO2+ . The quantity B = 1g|| /1g⊥ reflects the changes in the tetragonal distorsion and is an estimate of the V=O bond strength. β 2 ∗2 is a coeffiecient accounting for the participation of the dxy atomic orbital in the ψ ∗ (b2 ) antibonding orbital centered mainly on the cation of the vanadyl complex. This coefficient is associated with the electron delocalization on the cation. β 2 ∗2
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Table 2 The parameters calculated from the ESR of V2 O5 -containing samplesa Catalyst
B
β 2 ∗2
C
VT fresh VT spent AVT fresh AVT spent AVZ fresh AVZ spent
2.1 0.5 1.0 0.7 0.8 0.7
0.61 0.65 0.59 0.60 0.67 0.68
0.39 0.35 0.41 0.40 0.33 0.32
a The corresponding parameters for the VZ sample are not calculated.
or C = 1−[β 2 ∗2 ], respectively, corresponds to the fraction of unpaired d-electrons delocalized over the oxygen ligands. The parameters calculated from the ESR spectra are shown in Table 2. 3.2. Catalytic activity tests The complete benzene conversion (CBC) at 250◦ C with oxygen is presented in Fig. 5. Depending on the type of the support, the catalytic activity of the samples follows a different order, AVT > AT > VT for the catalysts on titania and AVZ > VZ > AZ for those on zirconia. The comparison between the same type of catalysts over the two different supports yields the following couples in activity VAT > VAZ,
AT ≥ AZ,
VT > VZ
Fig. 6. Catalytic activity in complete benzene conversion at 40◦ C with ozone: 1 — vanadia suported samples; 2 — gold supported samples; 3 — gold-vanadia supported samples.
Evidently, the titania catalysts are more active than the zirconia catalysts. The AVT and AVZ samples where gold and vanadium are simultaneously present manifest the highest catalytic activities. A stronger synergistic effect is observed for the titania sample in comparison with the zirconia sample. The CBC at 40◦ C with ozone as the oxidant is illustrated in Fig. 6. It is seen that, in this case the temperature of the CBC is additionally lowered by about 200◦ . The activity order in this case is VAT > VAZ, AT ≈ AZ, VT ≈ VZ The results presented in Fig. 6 reveal the similar activity values of CBC obtained with ozone over the samples investigated.
4. Discussion
Fig. 5. Catalytic activity in complete benzene conversion at 250◦ C with molecular oxygen: 1 — vanadia suported samples; 2 — gold supported samples; 3 — gold–vanadia supported samples.
In the interpretation of the catalytic activity data obtained in the present investigation, first we shall discuss the relationship between the structure and the reactivity of the studied samples. It follows from the Raman data analysis that a small amount of bulk V2 O5 is present in the non-promoted gold samples of VT and VZ. A small amount of monovanadate species coexists on the surface of the VT sample. Polyvanadate domains are predominantly observed in the vanadium-containing samples on both supports. In the case of a vanadia–zirconia sample a slight modification of the support was detected, which was not visible on titania.
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After gold deposition on the AVT sample only polyvanadate species are observed in the Raman spectra. In the corresponding sample on zirconia (AVZ) polyvanadate domains are also prevailing, but some V2 O5 crystallites are detected as well. Comparing the Raman spectra of the samples non-promoted and promoted by gold, it is interesting to note that the deposition of gold strongly influences the state of the support. In the AVT and AVZ samples titania and zirconia are modified, especially the zirconia support which is strongly disordered. Obviously, the interaction between gold and vanadia is stronger on zirconia than on titania, nevertheless no bulk ZrV2 O7 is observed. The ESR spectroscopy data were used to estimate the influence of the supports and of gold on the vanadyl bond strength and the delocalization of the 3d-electrons on the coordinatively bound oxygen ligands. The parameter B calculated by the ESR data is an estimate of the V=O bond length, i.e. of the V=O bond strength. The calculated parameter C, corresponding to the fraction of unpaired d-electrons may be directly connected with the catalytic properties. The value of the B parameter calculated for the VT sample is higher than that for the AVT sample (Table 2). This indicates a relative lengthening of V=O bond in the presence of gold. Comparing the values of the parameter C for the same samples one sees that the deposition of gold leads to a higher delocalization of 3d-electrons. The values obtained for the spent samples (after catalytic tests in complete benzene oxidation) do not differ significantly from those for the fresh ones. The values of the B parameters for gold–vanadia deposited on zirconia (fresh and spent) are very closed to those for the corresponding samples on titania. However, a comparison between the C parameters of the same samples (AVT and AVZ, fresh and spent) reveals considerably lower values when zirconia is used as a support. These results give grounds to assume a higher electron delocalization in the AVT sample in comparison with the AVZ sample, which is in agreement with the catalytic activity data. The almost equal values of both calculated parameters B and C for the fresh and spent AVT and AVZ samples could be an indication for the “stabilization” of the catalytic system under the working conditions in the presence of gold. In addition, the results obtained by the TPR measurements could be used to estimate
the effect of gold on the vanadium oxide reducibility, which controls the V–O-support strength bond [12]. The low temperature peak registered in the TPR spectra of the AT and AZ samples, assigned to the presence of oxidic gold clusters could be interpreted in terms of the existence of extremely small gold particles (below 4 nm) displaying unusual properties, as postulated by Burke [32]. We did not observe such a behavior in the case of gold promoted Fe2 O3 and Co3 O4 samples, where the average size of the gold particles was about 4 nm or 7–8 nm, respectively, [33,34]. In the TPR spectra of the VT sample a single narrow peak is observed, while the corresponding TPR peak for the VZ sample is broad and double. In our opinion, this difference is due to the higher concentration of VOx species on zirconia in comparison with titania (Table 1). As postulated by Bond et al. [28,29], upon increasing the V2 O5 loading on titania, layers of “disordered” V2 O5 and “paracrystalline” V2 O5 phases are formed on limited areas of the surface. These new V2 O5 species are the cause of the second TPR peak. By analogy with titania we assigned this second TPR peak to the presence of these new vanadia species along with the VOx monolayer species. A small amount of bulk V2 O5 crystallites in the VZ sample was detected by means of the Raman spectra. This satisfactorily explains the appearance of the second peak in the TPR spectra. The specific interaction between vanadia and the support is also of great importance. The Raman spectra show that the deposition of vanadia on zirconia modifies the support as a result of the strong vanadia–zirconia interaction. It may be hence assumed that the reduction V 5+ → V 3+ in the VZ sample proceeds in two steps in a wide temperature interval. The reduction behavior of gold promoted vanadium samples (AVT and AVZ) is quite different. The presence of gold leads to a considerable lowering of the reduction temperature of the available vanadia species for both samples. The registered peak in the TPR spectra of the AVT sample is narrow like the corresponding one in the vanadia–titania sample, Tmax being only 120◦ C, i.e. the decrease in the reduction temperature is about 330◦ . As regards the non-promoted and gold promoted samples on zirconia (VZ and AVZ), a very broad double TPR peak is registered for the AVZ sample and the temperature of the reduction is also decreased. The Raman spectrum
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of AVZ sample gives evidence of a small part of bulk V2 O5 and of support modification. This is probably the reason for the appearance of the very broad double peak in the TPR spectra of the AVZ sample similarly to the non-promoted by gold VZ sample. A strong decrease in the reduction temperature in the presence of gold has been former observed by us for the Au/Fe2 O3 and Au/Co3 O4 catalytic systems (Fe2 O3 → Fe3 O4 [33] and Co3 O4 → Co transitions [34], respectively). In our opinion, the explanation could be based on (i) the increasing number of potential nucleation sites for metal oxide reduction on the interfacial perimeter resulting from the high gold dispersion and (ii) the additional enhancement of the reduction process by a high concentration of active hydrogen. As it was very recently evidenced for the Au/TiO2 catalytic system, hydrogen is dissociated on nanosize gold particles even at room temperature [35]. As it was shown above, the catalytic activity of the studied samples in the CBC reaction depends on the oxidizing agent used. First we shall discuss the behavior of the samples in the catalytic oxidation by molecular oxygen (Fig. 5). Of interest is the catalytic order for the two types of catalysts depending on their support, presented in “Results”. The catalysts promoted only by gold (AT and AZ) show very closed values of catalytic activity. It is known that the particle size is of decisive importance for the high catalytic activity. The gold dispersity of all samples under investigation is of the same order (below 5 nm), which is in agreement with the CBC values found in the experiments. A different picture is observed for the samples promoted only by V2 O5 (VT and VZ). The titania-based catalyst is more active than the zirconia-based one. Such difference could be explained by the higher content of bulk V2 O5 in the zirconia sample. According to the Raman spectroscopy and TPR data, no bulk V2 O5 is present on titania. The prevailing coverage of titania with polyvanadate structures, evidenced by Raman spectroscopy, also agrees with the higher catalytic activity of the VT sample. As was shown, the presence of gold leads to a considerable enhancement of the catalytic activity of vanadium oxide supported on both titania and zirconia for complete benzene oxidation by oxygen. A stronger synergistic effect between gold and vanadia was registered with the titania support. In that case, the temperature of benzene conversion with oxygen was lower
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by about 150◦ than that in the presence of palladium or silver promoted vanadium oxide catalysts [1,2]. The highest catalytic activity of the AVT sample is in agreement with the results of the physicochemical studies. As we have already shown by Raman spectroscopy data, only polyvanadate domains are present in the AVT sample. Khodakov et al. [14] have established that polyvanadate species are more active than monovanadate ones and V2 O5 bulk phase in the oxidative dehydrogenation of propane on vanadium supported catalysts. The higher d-electron delocalization and the weaker V=O bond of the AVT sample in comparison with the other investigated samples (ESR data) agree with its highest catalytic activity. The enhanced reducibility of the vanadium oxide species in both AVT and AVZ samples also agrees with the observed synergistic effect between gold and vanadia. Complete benzene oxidation by ozone displays a different behavior in comparison with molecular oxygen. As it is evident from the activity order shown above, similar values are obtained. Benzene oxidation with ozone takes place at lower temperatures than with molecular oxygen. Our recent results [36] on ozone decomposition over the same samples showed the same activity order as for the benzene oxidation. Evidently, there are different oxidation mechanisms with the two types of oxidants. It may be assumed that ozone decomposition yields highly active oxygen species on the catalyst surface which are able to oxidize benzene even at room temperature. This assumption is in accordance with the reaction mechanism, proposed by Naydenov et al. in a former paper [22]. The synergistic effect manifested in the reaction of complete benzene oxidation by molecular oxygen on supported gold catalysts promoted by vanadia is in accordance with the mechanism, proposed by Andreev et al. [1,2] for the reaction on Pd-V2 O5 /Al2 O3 and Ag-V2 O5 /Al2 O3 catalytic systems. Most probably, the oxygen activation proceeds on the nanosize gold particles, while the vanadium oxide surface species are responsible for the activation of benzene. 5. Conclusions • It was found that polyvanadate structures predominantly exist on the catalyst surface in the presence of gold.
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• The deposition of gold leads to the relatively lengthening of V=O bond and a higher electron delocalization, which “stabilizes” the catalytic system under the working conditions. • The gold-containing samples on both studied supports provide a considerably lower temperature of the V 5+ → V 3+ transition. • The catalytic activity of the promoted by vanadia gold supported on titania and zirconia catalytic systems in complete benzene oxidation strongly depends on the oxidizing agent. A synergistic effect between gold and vanadia was established on both supports in the presence of molecular oxygen at the temperature of 250◦ C. With ozone as oxidizing agent a significantly lower reaction temperature and very close values of the catalytic activity on all investigated samples were found. Obviously, different reaction mechanism may be assumed in these two cases.
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