INDEX reactions, structure sensitivity

Much experience concerning the hydrogenation of conjugated dienes was obtained with butadiene hydrogenation. On Pt single crystals the reaction was found to be structure sensitive the activity sequence of different planes (marked with Miller s index) is... 

In a series of studies, the dehydrogenation and hydrogenolysis of cyclohexane was studied on both the stepped and low Miller index (111) crystal faces of platinum at a surface temperature of 300°C and a hydrogen to cyclohexane ratio of 20 1. While the rates on the stepped and low Miller index surfaces were not very different for the formation of benzene and hexane, the formation of cyclohexene was very structure sensitive, its rate being 100 times greater on the stepped surface than on the (111) crystal face. In Table III mrnnare the initial turnover numbers for the various reactions at low... 

The rate of a structure sensitive reaction catalyzed by a metal single crystal is also a function of the exposed plane. As illustrated in Figure 5.1.3, the low-index planes of common crystal structures have different arratigctncnls of surface atoms. Thus, caution must be exercised when the rates of structure sensitive reactions measured on single crystals are compared to those reported for supported metal particles. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

Re preferentially occupies sites on low index Pt, so reducing the contribution from structure-sensitive reactions.110... 

In a later paper Lee et al. have confirmed the antipathic structure sensitivity of methanation over Ni/Si02 (337). They associate this result with the previous finding (335, 336) that the high-index planes of Ni tend to accumulate carbon that can transform to graphite, a poison for the CO/H2 reaction. Similar results have been found by Doering et al. (338) for Pd/mica. On Pt/mica, there is no CO dissociation (339). 

The development of mass spectroscopic techniques such as matrix assisted laser desorption (MALDI) and electrospray mass spectrometry has allowed the absolute determination of dendrimer perfection [7,8], For divergent dendrimers such as PAMAM and PPI, single flaws in the chemical structure can be measured as a function of generation to genealogically define an unreacted site of or a side reaction producing a loop at a particular generation level. Mass spectromet-ric results on dendrimers, not only demonstrate the extreme sensitivity of the technique, but also demonstrate the uniformity of the molecular mass. The polydispersity index of Mw/Mn for a G6 PAMAM dendrimer can be 1.0006 which is substantially narrower than that of living polymers of the same molecular mass [7],... 

The combustion is a reaction between the combustible and oxygen. Oxygen consumption is one of the indicators of the amount of generated heat The limiting oxygen concentration is a convenient material flammability index. Practice has shown that the standard LOI procedure (ASTM D 2863-76) is the most reproducible test, and most sensitive to any compositional or structural variations in the material. 

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