Oxidative activation catalyst structure-activity correlation

Another contribution to variations of intrinsic activity is the different number of defects and amount of disorder in the metallic Cu phase. This disorder can manifest itself in the form of lattice strain detectable, for example, by line profile analysis of X-ray diffraction (XRD) peaks [73], 63Cu nuclear magnetic resonance lines [74], or as an increased disorder parameter (Debye-Waller factor) derived from extended X-ray absorption fine structure spectroscopy [75], Strained copper has been shown theoretically [76] and experimentally [77] to have different adsorptive properties compared to unstrained surfaces. Strain (i.e. local variation in the lattice parameter) is known to shift the center of the d-band and alter the interactions of metal surface and absorbate [78]. The origin of strain and defects in Cu/ZnO is probably related to the crystallization of kinetically trapped nonideal Cu in close interfacial contact to the oxide during catalyst activation at mild conditions. A correlation of the concentration of planar defects in the Cu particles with the catalytic activity in methanol synthesis was observed in a series of industrial Cu/Zn0/Al203 catalysts by Kasatkin et al. [57]. Planar defects like stacking faults and twin boundaries can also be observed by HRTEM and are marked with arrows in Figure 5.3.8C [58],... 

Apparently, epitaxial thin-film model catalysts provide a well-defined initial state for a systematic study of microstructural changes and structure-activity correlations. Model catalysts were prepared for various noble metal-oxide combinations, including Pt, Rh, Ir, Pd, Re supported by Al Oj, SiO, TiO, CeO, VO, Ga Oj, etc. The number density of the metal particles (island density particles per cm ) and their size can be controlled via the NaCl(OOl) substrate temperature during evaporation and the amount of metal deposited (as measured by a quartz microbalance), respectively (Pig. 15.4). 

Rupprechter G, Seeber G, Goller H, Hayek K (1999). Structure-activity correlations on Rh/ Al Oj and Rh/TiO thin film model catalysts after oxidation and reduction. J Catal, 186, 201... 

Zinc oxides have diverse applications in various industrial [3] and other types of processes[4-5]. It is essentially a dhydrogenating catalyst on which dehydration can also occur. The catalytic activity of zinc oxide is shown to be correlated to catalyst structure [6], although it is also reported that ZnO catalyst is structure insensitive [2], implying that every surface of exposed zinc oxide is equally active. The ZnO structure is shown to correspond to expanded hexagonal close packing with zinc ions filling half the tetrahedral holes[l], Akhter et al. [7] have identified three natural faces of zinc oxide crystals. 

In order to correlate the reactivity with catalyst structure, the BET surface areas were determined. Thus, the poor activity of Z3 is understandable considering its very low surface area. Similarly, sample Zi with highest activity also has the highest surface area. However, the poor activity of sample Z5, inspite of having appreciably large surface area raises questions about the nature of catalytically active centers vis-a-vis finer structural features of the zinc oxide catalyst. 

An extensive literature exists on the characterization and structure—activity correlation of industrial copper-alumina oxychlorination catalysts [95-120]. At least two different major copper species have been identified. At low concentrations of copper (below ca 5 %), a well-dispersed copper species in intimate interaction with the alumina surface is formed. This species has a very low oxychlorination activity. At higher concentrations, a second species, probably formed by the de-position/precipitation of the copper chloro complexes, is observed. The latter gives rise to the active sites during the oxychlorination reaction. On the basis of an FTIR study of the oxychlorination reaction Finocchio et al. [42] postulated the formation of surface copper chloride-ethylene r-complex intermediates (which lead eventually to EDC) and weakly adsorbed HCl during oxychlorination. Formate species associated with copper and probable precursors for formation of the oxides of carbon by combustion were also identified. 

Ahmed et al. [116] carried ont a detailed stndy with the objective of identifying the properties of activated carbons that are important for the SCR of NO they concluded that chemical properties such as surface oxides and mineral matter play a more important role than their physical properties, such as surface area and pore structure. In effect, they found that the catalyst activity correlated directly with the oxygen content of the carbon samples and inversely with their pH. These results indicate that the NO conversion is favored on more acidic carbons. They also reported that NO reduction by ammonia was negligible in the absence of oxygen. Indeed, it has been shown [117] that oxygen enhances the C-NO reaction through the formation of surface oxygen complexes, which are essential for the C-NO reaction to proceed. 

The use of soluble organotin compotinds [85-87], metal oxides other than ZnO, e.g., CaO, MgO, Zr02. Pb02 [80, 88, 89], ionic liquids [90, 91], and mixed oxides [92-95] has been reported. Dibutyltin(IV) soluble precursors give interesting results. Correlations between catalyst structure and activity have been demonstrated for tin catalysts [96]. A series of di-n-butyl tin(IV) compounds have been synthesized, characterized by NMR and IR spectroscopies, and screened for methyl carbamate methanolysis at 463 K. The key reactions proposed are depicted in Scheme 6.13. 

The catalytic activity per surface active site (TOP) toward a specific reaction is the right parameter to obtain reliable surface structure-activity correlations. The knowledge of the TOFs values dismissed the believes that the catalytic activity is influenced by bulk properties and that monolayer supported oxide catalysts are more active than bulk oxide catalysts. There is no doubt that the specific activity would contribute to design more active and selective catalytic materials at a molecular level. 

The porous structure of the catalysts was characterized by mercury penetration and nitrogen physisorption, and die morphology of the surfiice die catalyst spheres was assessed by scanning dectron microscopy (SEM). The oxidation of carbon monoxide [10,11] was used to investigate the effects of transport limitations on the activity. The oxidation activity of the catalysts was correlated widi the textural data. 

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