Surface Structures of MgO

At pH 5 the rate-determining step was proton diffusion, the rate being proportional to pH. 

At pH 7, the rate-controlling step was OH- adsorption followed by Mg2+ and OH- desorption. These processes are part of the overall dissolution reaction (8.4)  

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Fig. 8.13. A model of the (100) surface of the rocksalt structure of MgO. Large circles are oxygen anions, small circles are Mg cations. A (100) step to another (100) terrace is shown, as also is a missing anion point defect.

Henrich, V. E., G. Dresselhaus, and H. J. Zeiger (1980). Energy-dependent electron-energy-loss spectroscopy Application to the surface and bulk electronic structure of MgO. Phys. Rev. B22, 4764-75. 

Structural defects of various kinds are usually thought to determine the surface reactivity of MgO [87], at variance with the relatively inert regular sites of MgO(OOl) surface considered so far. Among such irregularities, low-coordinated cationic sites on edges (Mg4c) and comers (Mgac) of the... 

Catalyst Preparation The industrial catalyst is prepared by the reduction of iron oxide, Fe304 (94 wt%). It is in the shape of small porous particles with a surface area in the range of 10-15 m /g. Additives that improve its performance include AUO (2.3 wt%), K2O (0.8 wt%), and often CaO (1.7 wt%), MgO (0.5 wt%), and Si02 (0.4 wt%). Al, Mg, Ca, and Si oxides stabilize the pore structure and the surface structure of the iron catalyst K2O, although decreases the iron surface area somewhat still greatly increases the ammonia yield at 613 K from 0.2 mol% to 0.34 mol%. 

Since NiO has the same crystal structure as MgO, with nearly the same lattice constant (4.21 Ain MgO and 4.17 Ain NiO [86]) and is a good insulator as well (the band gap of NiO is about 3.5 eV [87]) it can be expected that the adsorption of small molecules on its (100) surface plane is very similar to that on MgO( 100). However, there are two differences between the electronic structures of MgO and NiO. While the Mg + cations in MgO have a closed shell structure with a fully occupied 2p shell, the NP+ cations have a d configuration with two unpaired electrons and a A2g ground state in the octahedral environment in NiO. Further, the spins at the NP cations are antiferromagnetically coupled. This raises two immediate questions first, is there the possibility of a covalent chemical bond between the partially occupied orbitals at the Ni adsorption site and the adsorbed molecule, and second, are the d-orbitals at the NP+ cations completely localized or do they form delocalized, metal-Hke bands and how does this affect the adsorption properties ... 

The effect of the structure of MgO on its catalytic properties for oxidative coupling of methane was investigated. A series of MgO catalysts were prepared by various methods. XRD, IR, SEM and XPS techniques were adopted to measure the bulk and surface structure. The structural data and the catalytic properties of the catalysts were correlated. The structural factors influencing the catalytic performance for OCM reaction are considered and the difference in the catalytic properties of the samples is discussed in detail. 

Surface structure of alkaline earth oxides was investigated using UV absorption and luminescence spectroscopies. High surface area MgO absorbs UV light and emits luminescence, which is not observed with MgO single crystal. UV absorption, corresponds to the following electron transfer process involving surface ion pairs. 

Figure 5.9 shows the different bulk terminations of MgO in the cubic rock salt structure. The (100) surface is by far the most stable, and MgO particles usually show only (100) facets. Note that there are two different (111) surfaces, namely those terminated by magnesium or by oxygen. Such surfaces possess a net dipole moment and are called polar. The (100) and (110) surfaces of MgO contain equal amounts of Mg and O these are neutral or nonpolar. 

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). 

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

Ir4(CO)i2 reacts with the surface of MgO to generate surface species in which the tetrahedral metal framework is preserved. The structures obtained after decar-bonylation under H2 at 573 K depend on the degree of hydroxylation of the support The iridium cluster nuclearity of 4 was maintained for a low degree of MgO hydroxylation (MgO pretreated at 973 K), but it increased to 6 when the MgO was highly hydroxylated (MgO pretreated at 573 K) [206, 207]. The activity in propane hydrogenolysis of the tailored catalyst is two orders of magnitude less than that of the conventional catalyst at atmospheric pressure and 200 °C. 

Coluccia et al. (5) proposed a model of the MgO surface that shows Mg-O ion pairs of various coordination numbers (Fig. 1). MgO has a highly defective surface structure showing steps, edges, corners, kinks, etc., which provide sites of low... 

It has been reported that surface structures and properties of the VjO, supported catalysts are dependent on support material and the contents of loading [18,19]. In particular, the presence of basic sites on support surface was reported to cause a red shift of the V=0 stretching vibration band of the supported VjOj [20]. VjOjISiOj catalysts with different VjO, contents showed two V=0 stretching vibration bands at 1032-1035 and 927-954 cm, respectively, due to both acidic and basic sites on the surface. However, V O, supported on MgO showed only one band at 922 cm, since MgO is a typical basic oxide. A similar red shift was observed in the VjOj/TiOj system. Hence, it can be concluded from the infrared... 

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