Precursor mobilities, metal

Determination of Metal Precursor Mobilities During Pretreatment. Relative precursor mobilities were obtained by premixing the sllica-or alumina-supported metal catalysts with pure silica (Cab-O-Sll, grade M-5, Cabot Corp.) or pure alumina (Alon C, Cabot Corp.) In a 1 2 ratio prior to pretreatment. The catalyst and silica were ground together using a mortar and pestle for at least 0.5 hr. before they were placed in the Pyrex microreactor for pretreatment. 

Reduction of Pd(NH3)4 in NaY leads to low dispersion 71), in part due to the high mobility of the metal precursor through the zeolite channel system and in part due to the absence of chemical anchors. Higher H/Pd ratios are obtained for the reduction at identical temperatures of partially deamminated or naked Pd divalent ions, which are much less mobile in the same zeolite. 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

The results of this study suggest that the dynamics of the nucleatlon process are of the utmost Importance In determining the structure and the surface composition of supported bimetallic clusters. Because the surface mobility of the metal phase during pretreatment is strongly influenced by the nature of the precursor-support Interaction, it is useful to consider this Interaction in some detail. 

When the halogen in the precursor is exceptionally mobile as an anion, even chloro compounds may give poor yields due to extensive self-destruction. For example, chloromethyl methyl ether can be expediently converted into methoxymethyllithium only if sodium/lithium alloy is used and a carefully elaborated protocol is meticulously followed . In the case of 7-chloronorbomadiene, the lithium/4,4 -di-ferf-butylbiphenyl radical anion has to be employed to further reduce the contact time between 7-norbornadienyllithium and its labile precursor. Many reductive metal insertions into... 

The adsorption of ions on iron oxides regulates the mobility of species in various parts of the ecosystem (biota, soils, rivers, lakes, oceans) and thereby their transport betv een these parts. Examples are the uptake of plant nutrients from soil and the movement of pesticides and other pollutants from soils into aquatic systems. In such environments various ions often compete with each other for adsorption sites. Adsorption is the essential precursor of metal substitution (see Chap. 3), dissolution reactions (see Chap. 12) and many interconversions (see Chap. 14). It also has a role in the synthesis of iron oxides and in crystal growth. In industry, adsorption on iron oxides is of relevance to flotation processes, water pollution control and waste and anticorrosion treatments. 

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