Structural and textural study of Ni and / or Co in a common molybdate lattice as catalysts

This work deals with the search for new molybdate catalyst formulations, which are known to be active in light alkane oxidative dehydrogenation, a process which could be replace in the near future the common steam cracking and pure dehydrogenation processes currently used for the production of alkenes. Co, Ni and mixed Ni-Co molybdates of various compositions are prepared by a modified coprecipitation procedure from metal nitrates and ammonium heptamolybdate. Their structural and textural properties were studied by XRD, Raman, B.E.T and XPS. Textural and structural properties of the materials are correlated to the composition.


INTRODUCTION
Transition metal molybdates are important families of inorganic materials which have been a subject of intensive research due to their many applications such as photoluminescence, magnetic properties, electronic properties [1][2][3][4][5][6] and catalysts in a large variety of industrial processes that involve the activation or conversion of hydrocarbons [7][8][9][10][11].For example, cobalt and nickel molybdates are important components of industrial catalysts for the partial oxidation of hydrocarbons and precursors in the synthesis of sulfide, nitride, and carbide materials [12].The MMoO 4 compounds are ideal for exploring possible correlations among the structural, electronic and chemical properties of mixed-metal oxide catalysts, a subject that is receiving a lot of attention.The interest for these compounds is furthermore enhanced by the perspectives of synthesizing new molybdate MMoO 4 and mixed Ni-Co molybdates with improved catalytic properties.It initiated that the investigations on a series of binary nickel-cobalt molybdates displaying improved catalytic properties, whose XRD patterns let conclude that the structures of these mixed compounds are solid solutions of NiMoO 4 and CoMoO 4 .In the present work, MMoO 4 (M = Ni or Co) oxides were synthesized by coprecipitation method and characterized by different techniques.The purpose is to evaluate the effect of Ni or Co on the structural and textural properties of nanomaterials MMoO 4 (M = Ni or Co) and the mixed oxide Ni-Co-Mo-O.

Catalysts preparation
The catalysts (M-Mo-O with M = Ni, Co) were prepared by coprecipitation method as described in references [13].The adequate amounts of M (NO 3 ) 2 ,6H  obtain aqueous solutions (0.25M) with the molybdate composition M/Mo = 1.0.The obtained solution was mixed at 85 • C; and introduced drop wise in to vigorously stirred ammonia solution (2M).

Physicochemical analyses
A number of physicochemical methods were used for characterization of solids before and after calcination, the details of these methods are reported early in [13,14].

RESULTS AND DISCUSSION
Structural properties of catalysts were studied by XRD and Raman techniques at room temperature, the obtained results are showed in Figures 1 and 2. The results of both techniques are in the same direction.The XRD analysis (Figure 1) showed for the Ni-Mo-O system after calcination at 550 • C, the presence of α-NiMoO 4 phase (file 33-0948) with octahedral Mo coordination is through its well-defined pattern displaying peaks at d-spacing values of 3.095, 3.513 and 2.746 Å and the  Raman spectra (Figure 2) showed that in all cases, the spectra characteristic of the solid solutions exhibit bands corresponding essentially to the α-phase.According to the literature sampling of α-NiMoO 4 is assumed to result in the formation of the so-called α'-phase, which is a distorted form of α-NiMoO 4 , and this has been confirmed in our case.In all the other cases, the presence of the β-phase is inferred from the occurrence of two neighbor bands in the range 935-945 cm −1 , resulting in a broad band without clear resolution.The Raman spectra of catalysts whose XRD pattern revealed the presence of both αand β-phases only displayed the bands corresponding essentially to the β-phase.
Textural characteristics of catalysts were studied by their specific surface area by B.E.T and XPS methods.The specific surface areas (S BET ) largely depend on the element M (Ni or Co).S BET is not very high for all solids (<40 m 2 /g), the best surface is observed for NiMoO 4 (37 m 2 /g), the system CoMoO 4 and the mixed oxide Ni 0.5 Co 0.5 MoO 4 present S BET almost similar (14 m 2 /g for CoMoO 4 and 17 m 2 /g for the mixed oxide Ni 0.5 Co 0.5 MoO 4 ).The different results of surface composition of the solids determined by XPS analysis are gathered in the Table 1.

CONCLUSIONS
Nanocrystalline Ni 1−x Co x MoO 4 (x = 0, 0.5 and 1) powders have been successfully synthesized by a coprecipitation method.It led to the formation of crystalline phases at moderate temperature (550 • C).In the case of M = Ni or Co, a single phase of α-NiMoO 4 or β-CoMoO 4 is obtained, while in the mixed oxide Ni 0.5 Co 0.5 MoO 4 , only the category α is observed.The higher specific area is obtained for NiMoO 4 , this later present a surface rich on Ni species (XPS results).These catalysts will be tested in oxireforming of ethane to produce ethylene.

Table 1 .
XPS results of NiMoO 4 and CoMoO 4 .Co (+II) and Mo (+IV) are detected.Both systems present a different surface composition, NiMoO 4 showed a surface rich on Ni species while CoMoO 4 a surface rich on Mo species.