Structure-Property Relationships of BaCeO Perovskites for the Oxidative Dehydrogenation of Alkanes
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favors complete oxidation of the alkane. To combat this thermodynamic trend, a catalyst needs to have good selectivity. For selective catalysts the reaction usually involves the lattice oxygen of the material, as shown in the following general formulas: (L = lattice, V = vacancies) (1)CnH 2n+ OL - CH2nl + OH; (2) CH 2 ,+l + OH - CH2 + 120 + Ov; (3)02 + 2 0 v - 2 0 L. According to Mars-van Krevelen mechanism2 , oxygen does not necessarily readsorb at the same site that the hydrocarbon reacts. The active catalytic site of the metal oxide is reduced as the alkane is oxidized. This site is then reoxidized by oxygen from a second site in the metal oxide lattice, usually associated with a different type of metal atom. The second site is then reoxidized by oxygen (introduced from the gas phase) and transported through the lattice. Literature has shown that rare earth metal oxides are of considerable interest for the oxidative dehydrogenation of alkanes. 3 As early as 1978, MoN/Nb/O systems 4 have been reported to be very active for ethane oxidative dehydrogenation. However, the catalytically active site is still not well characterized. Perovskites containing rare earth metal oxides possess the unique capability of sustaining nonstoichiometric compositions without affecting their structural integrity. Oxygen-deficient perovskites exhibit ionic conductivity and catalytic activity due to the facile loss and gain of oxygen. 5 -3 Oxygen vacancies can be introduced into a rare earth containing perovskite, such as BaCeO 3 , by doping in metal oxides, so that the new phase exhibits mixed conduction of mobile oxygen ions and electron holes.' 1-13 This mobility led researchers to studies which show SrCel-xYbxO 3., (x = 0 - 0.5) as an active catalyst with respect to oxidative dehydrogenation of ethane.14 Over 400'C, La1 .×Sr.FeO 3 6_ is another good perovskite-like 5 ODH catalysts for ethane.1 We have chosen to investigate crystalline perovskite-like catalysts with the general formulas BaCeO 3-, BaMxCel.xO3.,, and Bal-xMxCeO 3-.. Perovskites can be formed from reducible oxides, which are required to provide lattice oxygen. Furthermore, they allow for a wide range of stoichiometric substitutions without loss of crystallographic structure, which is easily monitored. These catalysts are being extensively characterized by powder X-ray diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) and were tested for catalytic activity (as monitored by gas chromatography (GC)) with respect to the oxidative dehydrogenation of ethane to ethylene. EXPERIMENT Catalyst Preparation: Our catalysts were the following target perovskite-like compounds: BaCeO 3.a, BaMCel-xO 3 -,, Ba 1 .xMxCeOj,•, with x = 0.1-0.5. The starting materials (all from Sigma Chemicals) were Ba(C0 3)2 (99.999%), CeO 2 (99.999%) and dopant metals (La 20 3, 99.999%; Y2 0 3, 99.999%; Nb 20 3, 99.999%; MgO, 99.99%; Ca(C0 3)2 , 99.999%; Sr(C0 3)2, 99.995%). They were mixed for three hours in an agate ball mill. Each mixture was heated (300°C/hour ramp rate) in air at 12501C for
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