Reaction of Carbon Dioxide with Gaseous Niobium and Niobium Oxide Clusters.

摘要

Small niobium and niobium oxide clusters (Nb(x) and Nb(x)O(y); X = 1-13, y = 1 -4) were synthesized by laser vaporization in a supersonic molecular beam system and reacted with carbon dioxide and its oxygen isotopically labeled derivative (CO*2). The reaction between Nb(x) with CO2 gave Nb(x)O(+) mass peaks for all x values observed: Nb(x)CO2(+) for x > or = 5, Nb(x)(CO2) 2(+) for x > or = 7, and Nb(x)(CO2)3(+) for x > or = 9. The relative apparent rate constant for the total reaction of Nb(x) with CO2 seems to increase almost smoothly with increasing cluster size. However, the relative yields of the oxide formation initially increased but later decreased for cluster sizes where the CO2 addition becomes observed. These results are described in terms of a mechanism involving complex formation between Nb(x) and CO2 leading to the formation of Nb(x)O + CO. Due to the exothermicity of the formation of the Nb-O bond and CO, small clusters seem to boil off the CO molecule formed. As the size of the clusters increases, its internal degrees of freedom increase and its ability to retain and bind the resulting CO molecule(s) increases. This, together with the increasing ability of Nb(x) for large x to form Nb(x)(CO2)(n) (or Nb(x)O(y) to form Nb(x)O) by multiple collisions, opens up more channels by which Nb(x) (or Nb(x)O) can form products and leads to the observed increase in the relative overall reaction rate constant with increasing cluster size. Reprints. (aw)