Oxidative activation catalyst attributes

MPa O2). The role of the encapsulated [Co(salophen)] complexes is to catalyze the aerobic oxidation of hydroquinone to p-benzoquinone, which in turn oxidizes Pd(0). For the oxidation of 1,3-cyclohexadiene to l,4-diacetoxy-2-cyclohexene, the most active catalyst system involved the encapsulated complex [Co(tetra-tert-butyl-salophen)], which afforded product yields of 85-95% after 3 h at room temperature with greater than 90% trans-selectivity. This complex displayed significantly higher activity than the encapsulated [Co(salophen)] complex (72% yield in 3h) and the analogous homogeneous complex (86% yield in 5h). The increased activity of the t-butyl substituted catalyst was attributed to distortion of the bulky complex by the... 

Baba and Endou 117) reported that CaO is an active catalyst for isomerization of VBH to EBH when it is evacuated at temperatures above 800 K, whereas MgO did not show any activity for this process. However, some discrepancies have been reported by Kabashima et al. 10), who found that MgO, CaO, SrO, and BaO catalyze the isomerization of VBH to EBH, with the order of activity being CaO > MgO > SrO > BaO. This order in activity is attributed to the trends in base strength of oxides (BaO > SrO > CaO > MgO) and the surface area, the latter decreasing in the order MgO > CaO > SrO > BaO (Table II). The activity of the CaO was the highest among these alkaline earth metal oxides, and the activity of the MgO varied with the pre-treatment temperature, reaching a maximum it was 873 K ... 

These results were interpreted in terms of a substantial surface enrichment in Cu, driven by Cu s lower heat of sublimation [23]. The reactivity of these catalysts for CO oxidation, and the clear spectroscopic evidence for surface Pt - CO species indicate that, at least for the heterogeneous systems, particle surface stoichiometries are very sensitive to metal-adsorbate interactions. Similar arguments were presented for the PtAu/silica system, in which monometallic Au particles severely sinter under dendrimer removal conditions. In this case, the retention of small bimetallic particles after activation was attributed to the strength of Pt-silica interactions, which effectively anchored the bimetallic nanoparticles to the support [24],... 

Although there is dispute about the exact oxidation state of titanium in the active species [Ti(III) or Ti(IV)], it was suggested, from the results of ESR measurements, that Ti(III) species form highly active sites for producing syndiotactic polystyrene in styrene polymerisation systems with the TiBz4—[Al(Me)0]x catalyst [50]. The moderately low catalyst activity is attributable to the stability of the benzyl transition metal derivatives towards reduction. 

Indeed, lattice parameters of both the copper and the zinc oxide were found to depend on the catalyst composition. The lattice extension of copper was attributed to alpha brass formation upon partial reduction of zine oxide, and an attempt was made to correlate the lattice constant of copper with the decomposition rate of methanol to methyl formate. Furthermore, the decomposition rate of methanol to carbon monoxide was found to correlate with the changes of lattice constant of zinc oxide. Although such correlations did not establish the cause of the promotion in the absence of surface-area measurements and of correlations of specific activities, the changes of lattice parameters determined by Frolich et al. are real and indicate for the first time that the interaction of catalyst components can result in observable changes of bulk properties of the individual phases. Frolich et al. did not offer an interpretation of the observed changes in lattice parameters of zinc oxide. Yet these changes accompany the formation of an active catalyst, and much of this review will be devoted to the origin, physicochemical nature, and catalytic activity of the active phase in the zinc oxide-copper catalysts. 

Prior to this disclosure, Trifiro (154) proposed that the active catalyst is pure vanadyl pyrophosphate and found that the catalyst was characterized by a slight increase in the vanadium oxidation state after the equilibrium period. The small increase from -1-4.00 to -h4.03 was reproducible and attributed to the formation of isolated V " surface sites. The P/V ratio was proposed to be a key characteristic in the stabilization of V + within the catalyst, as VOPO4 formation becomes very difficult at P/V ratios >2.0. Trifiro had stated that a very high surface P/V ratio is required for an active and selective catalyst, and experimentally he has found surface P/V ratios of 10 1. 

Bej and Rao (186-190) conducted a detailed investigation of molybdenum- and cerium-promoted vanadium phosphate catalysts. They foimd an increase in the selectivities of these catalysts as a result of incorporation of the promoters, albeit with slight decreases in activity. They attributed the improved selectivity to a role of the promoters in preventing overoxidation of the MA to carbon oxides. They also found that the promoted catalysts could withstand more severe reaction conditions than the unpromoted catalyst, and this property was also attributed to the formation of less carbon oxides, which can poison the catalyst. 

Experimental results on pure vanadium phosphate phases and active catalysts suggest that the active catalyst is VPP with domains of on the (100) face [33] The lack of selectivity of side faces found by Inumaru and coworkers [34, 35] is attributed to the difficulty of the re-oxidation to of these planes. Hutchings and... 

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