Properties of Pure and Sulfided NiMoO 4 and CoMoO 4 Catalysts: Tpr, Xanes and Time-Resolved XRD Studdzs
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S. CHATURVEDI, J.A. RODRIGUEZ*('), J.C. HANSON, A. ALBORNOZ", AND J.L. BRITO**()
"ChemistryDepartment,
Brookhaven National Laboratory, Upton, NY 11973, USA Centro de Quimica, IVIC, Caracas 1020-A, Venezuela
ABSTRACT X-ray absorption near-edge spectroscopy (XANES) was used to characterize the structural and electronic properties of a series of cobalt- and nickel-molybdate catalysts (AMoO 4.nH 2 0, aAMoO 4, P-AMoO 4; A= Co or Ni). The results of XANES indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the a-phases to tetrahedral in the P-phases and hydrate. Time-resolved x-ray diffraction shows a direct transformation of the hydrates into the P-AMoO 4 compounds (following a kinetics of first order) at temperatures between 200 and 350 °C. This is facilitated by the similarities that the AMoO 4.nH 20 and P-AMoO 4 compounds have in their structural and electronic properties. The molybdates react with H 2 at temperatures between 400 and 600 °C, forming gaseous water and oxides in which the oxidation state of Co and Ni remains +2 while that of Mo is reduced to +5 or +4. After exposing a-NiMoO 4 and P-N'MoO 4 to H2S, both metals get sulfided and a NiMoSx phase is formed. For the 03phase ofNiMoO 4 the sulfidation of Mo is more extensive than for the a phase, making the former a better precursor for catalysts of hydrodesulfurization reactions. INTRODUCTION
In this article, we describe recent studies that explore possible correlations among the structural, electronic, and catalytic properties of cobalt and nickel molybdates. Under atmospheric pressure, these compounds can exist as hydrates (AMoO 4.nH20, A= Co or Ni) or form the so-called a-AMoO 4 and P-AMoO 4 isomorphs. The catalytic properties of the cobalt and nickel molybdates are closely related to their structure [1-3]. The P-phase on NiMoO 4 is almost twice as selective for the dehydrogenation of propane to propene than the a-phase [1]. In a series of studies [2,3], the sulfided NiMoO 4 .nH 2O and CoMoO 4 .nH 2 0 compounds were found to be much better catalysts for the hydrodesulfurization (HDS) of thiophene than the corresponding sulfided a and P-isomorphs. The HDS activity of these systems increased in the following order: a < P < hydrate [2,3]. Synchrotron-based x-ray absorption near-edge spectroscopy (XANES) has emerged as a powerful tool for characterizing the electronic and structural properties of catalytic materials [4-6]. The L,,, edges of Co, Ni and Mo can provide information about the local site symmetry and oxidation state of these metals in oxides and sulfides [4,5,7]. Investigations at Brookhaven National Laboratory have recently established the feasibility of conducting sub-minute, time-resolved x-ray diffration (XRD) experiments under a wide variety of temperature and pressure conditions (-190 'C < T < 900 °C; P , 45 atm) [8-10]. This important advance results from combining the high intensity of synchrotron (1) Authors to whom correspondence should be addressed
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