Kolen’ko, Y.

Kröhnert, J.

Schlögl, R.

Trunschke, A.

Wrabetz, S.

Zhang, W.

English

MPG.PuRe

   Preprint of the Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac)      43. Jahrestreffen Deutscher Katalytiker (2010) Weimar   Surface Study of MoVTeNb oxide Catalyst for Selective Oxidation of Propane by Microcalorimetry and FTIR Spectroscopy using Reactant- and Standard Probe Molecules  S. Wrabetz, Yu.V. Kolen’ko, J. Kröhnert, Wei Zhang, A. Trunschke, R. Schlögl Fritz-Haber-Institut of the Max Planck Society, Department of Inorganic Chemistry, 14195 Berlin, Germany, wrabetz@fhi-berlin.mpg.de  Polycrystalline, mixed MoVTeNb oxides have been reported to show high selectivity and activity in direct oxidation of propane to acrylic acid. The most relevant crystal structure in C-H activation of propane is the orthorhombic, bronze-like Mo7.8V1.2Te0.937NbO28.9 M1 phase [1]. In the present work, the adsorption of reactants (propane, propene) and standard probe molecules (NH3, CO, CO2) on the surface of M1 has been studied by microcalorimetry and FTIR. For this purpose, a series of highly crystalline, phase-pure M1 catalysts has been differently synthesized to generate M1 with different particle size and morphology. The binary oxides of Mo, V, Te, and Nb have been studied as references. The aim is a deeper understanding of the surface characteris-tics and mechanism of propane activation. M1 (Tab. 1) was prepared via three different methods including hydrothermal (M1-a), precipitation (M1-b) [2] and superheated-water vapor treatment synthesis (M1-c) [3]. Microcalo-rimetry was performed on SETARAM Calvet calorimeters combined with a vacuum-gas-dosing apparatus [4]. The IR experiments were carried out with a PerkinElmer 100 FTIR spectrometer. Probe molecules were adsorbed, desorbed and subsequently re-adsorbed at 313 K. The particle size of single phase and highly crystalline M1 can be tailored by the prepara-tion method. The characteristics of three M1 catalysts that show significant differences in the particle morphology, especially in the ratio of the length and width of the cylindrical catalyst particles, are summarized in Table 1. The chemical composition of the three crystalline M1 cata-lysts is very similar. Electron microscopy revealed differences in the microstructure.   Table 1. Properties of highly crystalline, phase-pure M1 catalysts Catalyst  BET N2,77K surface area [m2g-1] Spropane  Apropane~39 Å2 [m2g-1] Mo:V:Te:Nb [at-%] as measured by EDX Particle size  Ø length / Ø width [nm] M1-a  5.9 8.2 66 : 16 : 7 : 11 421 / 213  quasi monomodal M1-b  8.8 11.3 62 : 17 : 7 : 14 199 / 115 quasi monomodal M1-c  13.3 9.9 64 : 15 : 6 : 15 180 / 59 quasi bimodal  Microcalorimetry was applied to follow mainly the interaction of propane with the M1 surface at 313 K. Fig. 1 shows the differential heat of adsorption as a function of coverage. The    Preprint of the Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac) differential heat of propane markedly increases for M1 catalysts compared to the binary oxides. Interestingly, the measured apparent activation energy of propane transformation for M1-b (Ea=56kJ/mol) is similar to the differential heat of adsorbed propane (Hads=56±2 kJ/mol). Re-garding the mixed oxides, the catalyst M1-a with the largest primary particles in the sub-micrometer range shows the highest initial differential heat. Pure TeO2 exhibits the highest initial differential heat (Hads=90±10kJ/mol) of all materials studied. Per surface area and in the region of Langmuir adsorption the number of propane adsorp-tion sites increases significantly for M1 catalysts compared to the binary oxides (Fig. 2). The iso-therms were analyzed with the higher order Langmuir model, which fits very well the experimental adsorption profile [5]. The Langmuir surface area, calculated on the basis of pro-pane adsorption isotherms is comparable to the N2 BET surface area indicating a homogeneous distribution of adsorption sites for propane on the surface of phase-pure M1. Such a good agree-ment was not observed in case of vanadia supported on mesoporous silica (SBA-15) [6,7]. Fur-thermore, M1 catalysts show a propane adsorption order “n” higher than 1, reflecting a slightly activated adsorption process, while the binary oxides were established to be characterized by a propane adsorption order of n = 1.       Figure 1: Differential heats of adsorbed propane on M1 catalysts with different morphology and on binary oxides at 313 K.  Figure 2: Isotherms of adsorbed propane on M1 catalysts  with different morphology and on binary oxides at 313 K.   Conclusions The present study has shown that compositional and structural complexity of M1 ensures an in-creased density of adsorption sites for propane on the surface of the catalyst compared to the bi-nary oxides. The different morphology of M1 is reflected in different distribution of the strength of adsorption sites demonstrating the sensitivity of microcalorimetry in probing the anisotropy of the M1 surface, which is determined by the crystal structure, the morphology of the primary par-ticles and the termination of the mixed oxide.  References: [1]  M. Baca, A. Pigamo, J. L. Dubois, J. M. M.Millet, Topics in Catalysis 23 (2003) 39-46. [2]  P. Beato, A. Blume, F. Girgsdies, R. E. Jentoft, R. Schlögl, O. Timpe, A. Trunschke, G. Weinberg,        Q.Basher,  F. A. Hamid, S. B. A. Hamid, E. Omar, L. Mohd Salim, Appl. Catal. A 307 (2006) 137.    Preprint of the Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac) [3]  A. Celaya Sanfiz, T.W. Hansen, A. Sakthivel, A. Trunschke, R. Schlögl, A. Knoester, H.H.        Brongersma, M.H. Looi, S.B.A. Hamid, Journal of Catalysis, 258 (2008) 35-43. [4]  L.C. Jozefowicz; H.G. Karge, E.N. Coker, J. Phys. Chem. 98 (1994) 8053 – 8060. [5]  I. Langmuir, J. Am. Chem. Soc., 38 (1916) 2221. [6]  A. Dinse, S. Khennache, B. Frank, Ch. Hess, R. Herbert, S. Wrabetz, R. Schlögl, R. Schomäcker,       Journal of Molecular Catalysis A: Chemical 307, 1-2 (2009) 43-50. [7] S. Wrabetz et.al., in preparation  

Surface Study of MoVTeNb oxide Catalyst for Selective Oxidation of Propane by Microcalorimetry and FTIR Spectroscopy using Reactant- and Standard Probe Molecules

Surface Study of MoVTeNb oxide Catalyst for Selective Oxidation of Propane by Microcalorimetry and FTIR Spectroscopy using Reactant- and Standard Probe Molecules

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