Transition-metal nanocluster stabilization for catalysis: A critical review of ranking methods and putative stabilizers

A significant problem in the burgeoning transition-metal nanocluster literature is the myriad of proposed, putative stabilizers. A main objective of the present contribution is to provide a critical review of the methods for, and current rankings of, claimed transition-metal nanocluster stabilizers, With a focus on catalytically active nanoclusters. Following a brief introduction to the literature methods for evaluating colloidal stabilizers (methods which are 41 and 105 years old), the need for modem methods to measure nanocluster stability via agglomeration kinetics is presented. Discussed next is the one presently available method for evaluating additives (i.e., putative stabilizers) for the formation and stabilization of transition-metal nanoclusters, the so-called 5 criteria method; this is followed by a scheme presenting established nanocluster stabilization modes. Next, a table of 48 prototype examples of established and novel nanocluster stabilizers is presented, followed by a discussion of each stabilization mode with selected, representative examples. One conclusion of this review is that it is clear that reliable, quantitative studies ranking claimed nanocluster stabilizers, and understanding of how they work, are just now appearing. A second conclusion is that given the lack of quantitative methods to measure stability and thereby rank stabilizers, it follows that much of the information regarding nanocluster stabilization is not on firm ground. This first review of transition-metal nanocluster stabilizers is just the initial step towards achieving the overall goal of simplifying the dizzying variety of claimed nanocluster stabilizers into a preferred, small set of solvents and stabilizers en route to high stability, high catalytic activity nanoclusters. (c) 2006 Elsevier B.V. All rights reserved.