MgO surface

IR Evidence for strong adsorption between cluster precursor and MgO surface... 

TEM-EDS and XPS analyses were conducted on Co/MgO catalysts. The results of surface analyses showed that Co metal is not supported on the MgO as particles, but covers MgO surface in the case of 12 wt.% Co/MgO calcined at 873 K followed by reduction. After the reduction of catalyst at 1173 K, both cobalt oxide and CoO-MgO solid solution are observed on the surface of catalyst. In the steam reforming of naphthalene, two types of coke deposited on the surface of catalyst are observed. These are assigned to film-like and graphite type carbon by TPO analysis. 

Figure S.1S. Upper Equilibrium polyhedrons of metal particles on a support for different combinations of free energies of surfaces and interfaces. Lower Transmission electron microscopy of structures obtained by depositing 5-10 ML of Pd on a MgO surface. [Adapted from www.fysik.dtu.dk/CAMP, and C.R. Henry, Surf. Sci. Rep. 21 (1998) l.j...

The concept of color centers has been extended to surfaces to explain a number of puzzling aspects of surface reactivity. For example, in oxides such as MgO an anion vacancy carries two effective charges, V(2. These vacancies can trap two electrons to form an F center or one electron to form an F+ center. When the vacancy is located at a surface, the centers are given a subscript s, that is, Fs+ represents a single electron trapped at an anion vacancy on an MgO surface. As the trapping energy for the electrons in such centers is weak, they are available to enhance surface reactions. 

The ESR signal due to 02 demonstrates that the heterolytic activation of R-H has occurred. An interesting feature of the H-D exchange reactions over the MgO surface is the low activation energy, i.e., Ea 2 kcal/mol. This is lower than the gas phase for the reaction H + D2 —> 2 D + HD. The high activity of the ionic oxides has been attributed to the presence of basic sites and in particular to defect sites that are formed during the oxide preparation that persist even at elevated temperatures.41 42... 

H3Re3(CO)i2] is deprotonated on MgOyoo to give [H2Re3(CO)i2] [67]. In addition, when a slurry of [Re2(CO)io], hexane and hydroxylated MgO is stirred at temperatures higher than -50°C, [Re2(CO)9] " is formed (Scheme 16.1) [68]. In this surface-mediated synthesis, it is necessary to use an hydroxylated MgO surface because. 

Rhe(CO)ie] on the Sur ce of MgO When RhCl3 H20 or [Rh(CO)2Cl]2 supported on MgO is treated with CO (latm) at room temperature, [Rh,5(CO)i,5] is formed and can be extracted with CH2CI2 (yields not reported) (Scheme 16.10). The unusual stabihty of a neutral cluster on the highly basic MgO surface was attributed to the formahon of surface chloride ions, which modify the strong basic character of surface sites close to the Rh atoms through formation of acidic Mg-Cl sites [104]. 

On sihca added with an excess of K2CO3, such as in strongly basic solution [58] or on the MgO surface [111], the initially formed silica-supported [Ir4(CO)i2] gives sequentially [Ir8(CO)22] and [Ir6(CO)i5] Y By analogy with the Ir chemistry occurring in basic solution [58] or on the MgO surface [111], the first anionic Ir cluster formed on the silica surface with added alkah carbonates is probably [HIr4(CO)ijY... 

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... 

An investigation of the influence of the pre-treatment temperature of the MgO and CaO catalysts showed that MgO exhibited low activity at any pre-treatment temperature, whereas CaO exhibited excellent activities when pre-treated at 673 and 873 K. However, both catalysts exhibited no activity when pre-treated at temperatures below 573 K, which indicates that the active basic sites for this transformation are not OH groups, but rather 0 ions on the MgO surface. 

The identity of a number of different oxygen species has been discussed during the course of the two reviews. In general, the main body of direct evidence on their nature has come from experiments which have been designed to stabilize the various species in a well-defined environment for example, at 77 K on an MgO surface. This is far removed from the situation in many catalytic reactions which occur above 300 K on complex oxides. However, oxygen species have also been identified under conditions closer to the real situations. 

No similar comparative studies have been carried out using other supports and so the reaction behavior cannot be assumed to be general and probably, in part, it is controlled by the specific properties of the MgO surface. 

Z3. PMDA-ODA on MgO. PMDA-ODA peel force data shown in Fig. 7 exhibit a very interesting phenomenon as a function of T H exposure. The peel force is significantly increased as the time in T H is increased. This is somewhat unusual, but apparently repeatable. The exposure to APS has not made much difference in the results, which is understandable from the initial surface analyses after IPA cleaning and APS exposure. The XPS data show no detectable amount of APS on the thus exposed MgO surface. The reasons for the peel force increase as a function of T H exposure are not clear at this time. This is, however, due to increased interfacial strength, and not due to the polyimide mechanical properties (Young s modulus and yield stress) changes. If the latter were the case, then we should see similar effects also in the first two cases, which is not seen. However, more detailed analysis is essential to clarify the exact mechanism and this observation merits further study. 

APS is retained only on Si02 surface cleaned with IPA, but not on A1203, or on MgO surfaces thus cleaned. 

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