Propane oxidation on Pt-WO3/gamma -AL2O3 catalytic systems

Propane oxidation on Pt-WO₃/g -AL₂O₃ catalytic systems

M.A.Pereira da Silva^I; R.M.Cardoso^I; M.Schmal^I;II

^IEscola de Química, Universidade Federal do Rio de Janeiro, UFRJ

^IINUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro UFRJ, Cx.P. 68502, 21945-970, Fax [phone], Rio de Janeiro, RJ, Brazil

^{Address to correspondence}

ABSTRACT

The oxidation of propane on was studied with Pt-xWO₃/Al₂O₃ catalysts was studied ,by varying the concentration of tungsten sublayer. Thermal analysis and XRD in situ showed that the enrichment of tungsten at the surface is associated with the formation of H_xWO₃ bronze. FTIR results with C₃H₈ and O₂ indicated that the catalyst surface properties and the interaction between W and Pt were modified. These modified surface complexes prevented the formation of acetates and formate species. The addition of W increased the activity of Pt/Al₂O₃ towards in C₃H₈ oxidation. Tungsten was the main responsible for the stability of the bimetallic catalysts in the presence of water.

Keywords: Platinumplatinum-tungsten catalysts, Automotive automotive catalysts, Propane propane oxidation.

INTRODUCTION

Automotive catalysts eliminate promote simultaneously elimination of CO, NO_x and hydrocarbons from exhaust gases, but their performance depends on the noble metal content, which in turn are very expensive and rare. Several researchers (Regalbuto et al. 1987, Adams and Gandhi 1983) have shown that the association of tungsten and noble metals (Pt or Pd) improves the performance of the catalyst towards in the reduction of NO_x and oxidation of CO and hydrocarbons.

In addition, our previous results for in this system showed the interaction between Pt and tungsten with formation of new active sites characterizing an electronic effect. For high tungsten loading the surface active sites decreased due to the geometric effect, causing a decrease of in the catalytic activity (Silva et al. 1999).

The presence of water in the oxidation of propane on Pd/Al₂O₃ was studied_. It This caused a large decrease in the activity. The mechanism of inhibition of propane oxidation in the presence of water consisted in the dissociate adsorption of water on palladium sites with the possible formation of palladium hydroxide (Pd--OH) at the surface, thereby diminishing the number of active surface sites (Schmal et al. 2000).

The objective of this work was to study the interaction between platinum (1 wt. %) and tungsten oxide when supported on g -Al₂O₃ at different concentrations, aiming at a monolayer surface with high Pt and W dispersions.

The catalysts were characterized by thermal analysis, X-ray diffraction in situ (XRD) and infrared spectroscopy with C₃H₈ and O₂. The activity of the catalysts was evaluated with propane oxidation under in excess of oxygen, and the influence of the water on the stability of the catalysts was studied in a reaction mixture containing 10% H₂O.

EXPERIMENTAL

Preparation of Catalysts

The support was g-Al₂O₃ (AL-3916P) from Engelhard Corp. (213 m²/g and 0.54 cm³/g). The catalysts were prepared using successive wet impregnation with ammonium paratungstate ((NH₄)₁₀W₁₂O₄₁^.5H ₂O) solution at desired concentrations. The samples were dried at 110^oC for 20 h and then calcined in airflow at 500^oC for 2 h. The metal loading of tungsten varied between 5 and 20 wt. %. Platinum was impregnated using a solut to obtain an almost conion of H₂PtCl₆stant value of 1 wt %. For the sake of simplicity the catalysts will be referred to as xPtyW, where x (1%) and y (5-20%) represent the weight percentage of Pt and W, respectively.

Characterization

The thermal analysis was performed in a Rigaku TG 8110 thermobalance. The purge gas was a mixture of 30 ml/min of N₂ and 10 ml/min of H₂, and the temperature was raised at a heating rate of 10^oC/min up to 1200^oC. The reference material was a a-Al₂O₃.

X-ray diffraction patterns were obtained with DMAX/Rigaku equipment using CuKa radiation. Diffraction patterns were obtained for 2q between 20^o and 80°, using a step of 0.05° and a counting time of 1 second per step. The catalysts had been were previously reduced and analyzed at room temperature.

The infrared analysis was performed in a Perkin Elmer System 2000 FTIR equipment and the resolution was 2 cm^-1. Samples were pretreated by flowing He at 150^oC for 1 h (10^oC /min), followed by reduction at 500^oC (10^oC/min) with pure H₂ flux (50 cm³/min) for 2 h and evacuation for 1 h. After cooling to room temperature the IR spectrum was taken. A mixture containing 1 % C₃H₈ and 5 % O₂ balanced by He was adsorbed at room temperature and the spectrum taken. Then, the temperature was raised to 100^oC, 200^oC and 300^oC and new spectra were taken.

The oxidation of propane was performed in a microreactor at atmospheric pressure. The catalyst (25 mg) was diluted in quartz (250 mg), dried in nitrogen flow at 150^oC, and then reduced with pure hydrogen at 500^oC for 2h. After reduction, the reaction was carried out stepwise, varying the temperature between 150 and 500^oC at intervals of 15^oC. The reaction consisted of 6.53 O₂/0.65 C₃H₈/92.82 He (flow rate = 150 mL/min). Exit gases were analyzed with on-line chromatography using a Haysep D column (6m, carrier gas He). Propane conversion was calculated from the molar balance. The effects of the presence of water presence was ere studied under similar conditions by introducing 10 % of water in the reaction mixture at 15 mL/min.

RESULTS AND DISCUSSION

Analyses of TG/DTA

Figure 1 displays the profiles obtained by thermal analyses of different catalysts.

Results in Fig. 1A show a continuous loss of mass loss, due to the reduction of platinum and tungsten oxides. However, it can be seen that there was no apparent loss of mass on the 1Pt20W catalyst from 600 ^oC up to 885 ^oC. It This suggests that for higher tungsten contents, the reduction of WO₃ was easier. According to the literature (Regalbuto et al. 1987) such a reduction is associated and compensated by with the formation of H_xWO₃ species.

Fig.1B displays the DTA curves for the catalysts. With the exception of 1Pt20W, there is an endothermic peak, associated to with water loss and to with reduction of WO₃ and PtO₂. However, the 1Pt20W sample exhibited a broad exothermic peak, which can probably be attributed to the formation of H_xWO₃ species.

X-Ray Diffraction

Figure 2 displays the diffraction patterns for the 1Pt20W and 1%Pt/WO₃ catalysts calcined and reduced in situ. According to Fig. 2A it is rather difficult to identify the peaks of the 1Pt20W sample, since Al₂O₃ diffracts in the same range as WO₃ and the Pt content is so small that it cannot be detected. Peaks at 40, 46 e and 67^o were attributed to Al₂O₃. The diffractogram of this sample after reduction at 500^oC showed that the intensity of the peaks around 36⁰, 40⁰ and 46⁰ increased. According to Hoang-Van and Zegaoui (1995) this can be attributed to the cubic W₃O phase, only observable at this reduction temperature. However, it can be better described by the appearance of a mixed form of W₃O and W₂₀O₅₈. Indeed, in Fig. 2B the calcined 1%Pt/WO₃ catalyst displays only the most intense peaks at 24⁰ and 35⁰, which corresponds to the WO₃ monoclinic phase. On the other hand, Fig.2B also displays the patterns of the 1%Pt/WO₃ catalyst after reduction at different temperatures. It turns out that Tthese patterns are quite different at 100 ^oC, 300 ^oC and 500 ^oC. After reduction at 100^oC, peaks corresponding to tetragonal H_0.33WO₃ species appeared. However, reduction at 300^oC showed only the tetragonal phase H_0.23WO₃, while after reduction at 500^oC only W₂₀O₅₈ monoclinic and metallic Pt was present. The stoichiometries of these bronzes are in agreement with volumetric measurements of H₂ consumption by Hoang-Van and Zegaoui (1995), suggestive of bronze species H_xWO₃ with x between 0.3 and 0.6, depending on the reduction temperature. Regalbuto et al. (1987) observed formation of tungsten bronzes species formation H_0.1WO₃ and H_0.33WO₃ on PtWO₃/SiO₂ with 25% (wt.) WO₃ after reduction at 400^oC.

FTIR Results

The infrared results of C₃H₈ and O₂ adsorption on reduced 1Pt and 1Pt15W at different temperatures are displayed in Figure 3 . The bands between 1449 and 1644cm^-1 are assigned to acetate formation and water. The band at 1644cm^-1 increased with increasing temperature suggesting the presence of water and acetate formation during the reaction. Moreover, at room temperature CO is adsorbed on Pt⁰ at 2068cm^-1, disappearing with increasing temperature but with the formation of CO₂ at 2340cm^-1. These are typical roto-vibrational contours (asymmetric stretching and deformation). On the other hand, there are typical bands at 2900 and 2964cm^-1, which are assigned to C-H stretching species and asymmetric CH₃ species, respectively. With increasing temperature these bands decreased, in particular those of CH -species. Noteworthy is Tthe increasing formation of hydroxyls resulting from the products of the reaction is worthy of notice. According to Ermini et al. (2000), the surface species strongly suggest that CO_x arises from the combustion of the carboxylate species (acetates and formates) formed by adsorbed propane. In agreement with these authors, we did not find any intermediate species of propene. The 1Pt15W exhibited a sharp peak at 1640cm^-1 and a clear band at 2965cm^-1, which according to previous discussion correspond to the water formation and the CH₃ stretching bands. At room temperature the The 2900cm^-1 band is present at room temperature exist and but disappears with increasing temperature. It is evident that the acetate and formate species were not directly observed around 1449cm^-1. Probably in this case the presence of a sublayer of W as sublayer prevented the formation of acetate and formate. These results also suggest that the C-H asymmetric species adsorbed at on the surface facilitate the combustion of C₃H₈, preventing formation of intermediates as observed on alumina support.

Propane Oxidation

The oxidation of C₃H₈ was carried out at different temperatures under oxidant conditions (O₂/C₃H₈=10). By setting the temperature for 50% conversion of propane for comparison of the activity, the catalysts had the following order of activity 1Pt15W @ 1Pt10W >1Pt5W@ 1Pt20W > 1Pt , according to the results shown in the Fig. 4A. The temperature needed for 50% conversion of propane (light-off temperature) is at a minimum for 1Pt10W @ 1Pt15W samples. This type of catalytic behavior is generally explained by a the competitive adsorption of hydrocarbons and oxygen on the same metal sites and by differences in the reactivity of the adsorbed oxygen on the metal surfaces.

Moreover, the oxidation of saturated hydrocarbons was described by a rate limiting step which that involves a surface reaction of an alkane-oxygen complex, whose concentration increases rapidly with the particle size (Marécot et al. 1994, Otto et al. 1991). This optimum corresponds to a metallic dispersion near 20% on platinum. The use of chlorinated precursory salts (H₂PtCl₆) during preparation of catalysts caused loss of activity due to the chemical interaction between residual chlorine with and the catalyst surface or to the formation of platinum oxide-chloride species which are more stable and consequently less reactive than oxygen ones. Therefore, the existence of an importanta significant amount of chlorine in catalysts with a low concentration of tungsten, identified in the analyses of XPS, would decrease their activities.

Otto et al. (1991) suggested that propane oxidation is facilitated by a favorable ensemble of active Pt sites, which are more likely to form on larger crystallites. They observed an increase in the specific activity as the catalyst particle size increased.

In fact, our results on 1Pt catalyst agreed very well with this explanation. However, they are indicate ing that the activity is markedly affected by the presence of interfacial active sites on the 1Pt15W catalyst. These interfacial sites were observed through XRD, due to the formation of different H_xWO₃ species, which that interact with Pt^{d +} favoring their formation. These interfaces are responsible for the increasing number of active sites available for the reaction. According to Burch et al. (1999) the oxidation of propane on Pt/Al₂O₃ depends on the adsorption and breaking of C-H and if whether adsorbed oxygen is available at the neighboring site, resulting in the formation of surface hydroxyls, according to the reaction:

After breaking the C-H bond, quick subsequent reaction occurs with the adsorbed atomic oxygen under prevailing oxidant conditions. Indeed, FTIR results (Fig.3) showed the presence of these C-H asymmetric species at different temperatures on both catalysts. In particular, the increasing availability of OH groups with by raising the temperature for 1Pt15W at 1640cm^-1, reinforces the proposed mechanism.

Figure 4B shows the results of the effect of water on the stability at 300 ^oC for the 1Pt and at 240^oC for the 1Pt15W with TOS during 30 h. After 20h with TOS, 10% of water was introduced in to the composition of the reaction mixture. On 1Pt the activity increased during the first two hours. This fact is generally attributed to the removal of chlorine species at the surface by water generated during the reaction. Such These species interact with metal clusters causing reaction inhibition due to formation of inactive oxychlorides species. Their elimination restored the active sites for the propane oxidation. A remarkable decrease on in the activity was observed with the addition of water to the reaction mixture. This behavior agreed with Cullis and Willatt (1984) and Schmal et al. (2000) for similar reaction on the Pd/CeO₂/Al₂O₃ catalyst. The degree of inhibition increased due to the reaction product. However, the 1Pt15W catalyst remained unchanged with the addition of water, inducing higher stability for this system, due to the presence of a sublayer of tungsten. In fact, FTIR results suggest that a stable sublayer prevents the production of formate or intermediate species, favoring the formation of hydroxyls on specific sites, without affecting the surface stability.

CONCLUSIONS

Catalyst characterization revealed the presence of different bronze species, depending on the temperature of reduction as identified by XRD. FTIR results indicated that the tungsten sublayer modified the surface properties and interaction with Pt. These modified surface complexes prevent the formation of acetates and formate species. This fact explains the better performance and stability of the 1Pt15W catalyst in the presence of water, when compared to the Pt/Al₂O₃ catalyst.

ACKNOWLEDGEMENTS

Mônica Antunes Pereira da Silva and Rodrigo Marques Cardoso express acknowledgement for the financial support received from to FAPERJ.

M.Schmal

E-mail: [e-mail]

Received: March 5, 2002

Accepted: August 30, 2002

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Publication Dates

• Publication in this collection
  19 Mar 2003
• Date of issue
  Mar 2003

History

• Received
  05 Mar 2002
• Accepted
  30 Aug 2002