Supported metals, small particles bonding with support

The principal difficulty with metal only mechanisms is to find a plausible way of activating the oxygen molecule. The problem is discussed in Section 5.2.2 formation of the Oj ion, in which the 0-0 bond is considerably weakened (see Table 5.2), appears possible on small particles,130,131 and the suggestion that the adsorption of oxygen and of carbon monoxide is mutually supportive opens a possible route for a metal-only mechanism. 

The nature of the bonding at the interface between metal species and oxide support is known only very approximately. Ideally, one would like to be able to prepare monodispersed catalyst particles, all with the same and, if possible, very small number of atoms. In fact, the active part of the catalyst is its surface where low-coordinated atoms are responsible for most of the specific chemistry [20]. However, surface atoms form only a small fraction of the total number of atoms of the particle. Because heterogeneous catalysis often deals with precious metals, reducing the size of the particle immediately results in reduced cost due to a better surface-to-volume ratio with a given quantity of material, it is... 

Upon calcination, the Pt is present as a platinum(rV) oxide phase (Cl-free) or a platinum(IV) oxychloride phase (with Cl). Since there are no apparent Pt-Pt bonds, the oxide is present as very small particles or is highly disordered. Reduction of the calcined catalysts leads to small metallic particles in contact with the oxygen ions from the alumina support. In the Cl catalyst, there are no longer any Pt-Cl bonds. Oxidation at 300°C leads to oxidation of the surface, but the Pt particles are not fully oxidized. The Cl-free catalyst still has exposed, reduced Pt atoms at the particle surface, which chemisorb CO, undergo oxygen isotopic exchange, and are active for methane oxidation. These results also suggest that the active site is metallic Pt. 

NPLC stationary phases include metal oxides and moderately or strongly polar chemically bonded phases. Unmodified silica gel and silica-based bonded phases are most frequently used nowadays. Considerable effort in the development of new HPLC column packing materials in the past years has resulted in significant improvement of the column efficiency, reproducibility, and increased stability at elevated temperatures and at higher pH, enabling better compatibility with HPLC/mass spectrometry techniques and rapid analyses. Even though the new column technologies were primarily focused on RPLC separations, normal-phase HPLC also benefits from the improved properties of the support materials with uniform small particles and well-defined pore size. 

Thus, recent experimental evidence seems to support the idea that growing small particles have maximum chemical reactivities, and certain sized/shaped small particles may have the highest reactivities. What size and/or shape varies with the metal in question and the reaction in question This information strongly supports three ideas (1) structure sensitivity in chemical reactions on metal surfaces is very important, (2) more than one atom is necessary to carry out at least some bond breaking processes, and (3) defect sites on growing small particles are extremely reactive (see Fig. 9). It has also been possible by pulsed laser vaporization to produce many types of gas phase metal clusters. Particularly interesting have been reactivity studies of niobium clusters Nb where X = 5-20. A definite cluster size dependence on reactivity was observed. Exposure... 

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