Cryophotoclustering techniques for synthesizing very small, naked metallic and bimetallic clusters of known size; the cr-mo system; relevance to metallic and bimetallic cluster catalysts

Abstract

Early experiments concerned with the ultraviolet-visible irradiation of transition metal atomic species, isolated in low temperature inert supports, have unveiled a phenomenon which, at this stage of research, appears to be best described as “cryophotoclustering”. For example, we find that selective excitation of the low energy atomic resonance transitions of Cr and Mo (300 – 400 nm) in CrAr ∼ 1/10 3, MoAr ∼ 1/10 3 and CrMoAr ∼ 1/10 3 mixtures at 10 K provides a controlled synthetic pathway not only to the known species Cr 2, Mo 2 and CrMo (obtained previously by quantitative CrAr, MoAr, and CrMoAr matrix cocondensation techniques), but also significantly to hitherto unknown higher metallic and bimetaltic clusters which we tentatively ascribe to Cr 3, Mo 3, Cr 2Mo and CrMo 2. The concepts of localized atomic excitation, electronic to lattice (phonon) vibrational energy transfer, matrix cage softening, and short-range bulk diffusion, are employed to rationalize our experimental observation of photo-induced bulk diffusion and aggregation of Cr and Mo atoms to small metallic and bimetallic clusters. The optical spectra of these few-atom clusters are discussed briefly and the ramifications of these types of “mini-cluster” experiments on related studies in the field of single and multicomponent “massive” particle catalysts (10 – 1000 Å ) are briefly contemplated.