NiO-MgO solid solutions

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

From the above results, one can conclude that different NiO/MgO solid-solution catalysts can have very different catalytic performances. For example, Fujimoto et aV s Nio.03Mgo.97O solid-solution catalyst exhibited relatively low activities. To reach about 82% conversion of CH4 in the presence of this Nio.03Mgo.97O catalyst, the space velocity had to be reduced to 18,670 mL (g catalyst)-1 h-1 at 1123 K (Fig. 15) (238). In contrast, Ruckenstein and Hu s NiO/MgO catalysts have very high activities (>91% conversion of CH4 and >95% selectivities of CO and H2 at the space velocity of 60,000 mL (g catalyst)-1 h-1 at 1063 K) (Fig. 14) (239). Hu and Ruckenstein (239,257,259) noted that the properties of the MgO, such as its surface area, pore size distribution, and crystal structure, have important effects on the NiO/MgO solid-solution catalysts. They found that the MgO supplied by Aldrich, which has... 

In summary, the basicity and the strong NiO-MgO interactions in binary NiO/MgO solid solution catalysts, which inhibit carbon deposition and catalyst sintering, result in an excellent catalytic performance for C02 reforming. The characteristics of MgO play an important role in the performance of a highly efficient NiO/MgO solid-solution catalyst. Moreover, the NiO/MgO catalyst performance is sensitive to the NiO content a too-small amount of NiO in the solid solution leads to a low activity, and a too-high amount of NiO to a low stability. CoO/MgO solid solutions have catalytic performances similar to those of NiO/MgO solid solutions, but require higher reaction temperatures. So far, no experimental information is available regarding the use of a FeO/MgO solid solution for CH4 conversion to synthesis gas. 

NiO-MgO solid solutions. The interaction of NO with Mg2+ ions exposed on the (001) faces of MgO and NiO-MgO solid solutions remains weak since only polarization forces are involved (because of the absence of //-orbitals of suitable energy centered at the adsorbing Mg2+ ion). It is therefore expected that a probe molecule with greater d-n acceptor ability, such as NO, could provide further information about the propensity of the //-orbitals to participate in bond formation at the transition metal centers (because it is able to amplify the d-n effects). The experimental results confirm this view (i) On MgO and on Ni2+-free portions of the NiO-MgO solid solution, the NO is so weakly bonded to the surface centers that (at 77 K) it gives preferentially lateral interaction products (dimerization with formation of ds-N202 species) (ii) on Ni2+ ions, in contrast, Ni2+ NO complexes are stable at room temperature. The stability of this complex, together with its... 

The disproportionation of NO into N2O and surface nitrates was investigated for CoO-MgO and NiO-MgO solid solutions by IR and EPR techniques (379). The heterolytic dissociation of H2 on Co2+ and O2 pairs present at edges and steps of CoO-MgO cubes to generate hydride and hydroxyl species has been shown by IR spectroscopy (380). The decomposition of N2O was also investigated (356). 

Nurunnabi, M., Fujimoto, K.-I., Suzuki, K., Li, B., Kado, S., Kunimori, K., and Tomishige, K. Promoting effect of noble metals addition on activity and resistance to carbon deposition in oxidative steam reforming of methane over NiO-MgO solid solution. Catalysis Communications, 2006, 7 (2), 73. 

Performance and Characterization of NiO-MgO Solid Solution Modified with Noble Metals in Oxidative Steam Reforming of Methane... 

The additive effect of noble metals to NiO-MgO solid solution on the activity and on the resistance to the carbon deposition in oxidative steam reforming of methane under pressurized condition (I.O MPa) was investigated. It was found that the addition of small amount of Rh or Pd was effective to enhance the activity and to suppress the carbon deposition. The Pd and Rh K-edge EXAFS analysis indicated that M-Ni alloy phase (M=Rh or Pd) was formed, and FTIR spectra of CO adsorption suggests that the alloy could be formed on the surfece of bimetallic particles. 

CH4 reactions with CO2 or H2O on group VIII or noble metals (Ru, Rh, Pd, Ir, Pt) [1] form synthesis gas which is the precursor to valuable fuels and chemical compounds, as lirst shown by Fischer and Tropsch [2]. Due to the cost and availability of the nickel, compared to noble metals, Ni catalysts are used industrially. However, Ni-based catalysts tend to form inactive carbon residues that bloek the pores as well as the active sites of catalyst, and whose main activity is die formation of carbon filaments [3]. Therefore, the industrial methane steam reaction is usually performed under an excess of water to maintain the catalyst activity. Another alternative is the modification of the composition of the catalyst (generally Ni/Al203) by addition of a basic compound like MgO [4]. It is well known that the formation of NiO-MgO solid solution is easily favoured by calcining the mixed oxide at high temperatures [5] and much attention was devoted to its specific properties [6]. Parmaliana and al. 

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