Cluster Structure and Reactions Gaining Insights

Authors: A W Castleman

Publish Date: 2011/08/09

Volume: 141, Issue: 9, Pages: 1243-

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Abstract

To many researchers outside the field of cluster science it may come as a surprise that much can be learned of its relevance to catalysis even restricting the discussion to ionized systems This perspective is largely focused on catalytic oxidation reactions in which oxygen radical centers on transition metal oxides play a dominant role The objective is to present how fundamental insights into reaction mechanisms can be gained through employing alternative approaches that complement rather than supersede more conventional methods in t he field of catalysis In view of the well acknowledged role of defect centers in effecting reactivity and the preponderance of recent papers presenting evidence of the importance of charged sites the need/desire to conduct repetitive experiments is clear Presented herein are approaches using clusters to accomplish this in order to unravel fundamental catalytic reaction mechanisms and to use identified superatoms and the concepts of element mimics to tailor catalysts with desired functionalityConsidering the global need for new sources of energy enhanced methods of pollution abatement advanced approaches to the production of fine chemicals sensors for the detection of harmful materials as well as new methods of evaluating medical and biological issues it is clear that catalysis which impacts all of these is one of the most important subjects for scientific investigation 1 In view of the large diversity of reactions and accompanying physical phenomena involved unraveling the fundamentals of catalytic processes requires a wide range of studies 2 Clearly no single approach is sufficient to elucidate all of the issues which arise in unraveling catalytic processes and advances in the subject benefit from well devised investigations of many types 3 4 5 6One valuable approach employs the use of clusters whose chemistry can be used to follow the course of reactions influenced by various reactive centers/defect sites and the present invited perspective focuses on past developments and future prospects in this area 2 5 7 8 9 10 While acquiring reproducible defect sites for the purpose of elucidating their influence on the course of catalytic reactions is an arduous task it is a relatively straight forward procedure to recreate and conduct repetitive studies on model clusters which mimic specific reaction centers There is growing evidence that sites of differing charge density can greatly influence reactions and the effect of these is identifiable through a study of mass selected cluster ions 7 8 9 10 11 12 Comparison of the reactivity of anionic and cationic clusters can therefore provide insight into the importance of an accumulation or deficiency of electron density on the reactivity of catalytic materialsThere is increasing interest in the field of nanocatalysis which can be easily modeled through the use of clusters both in terms of the operative catalytic reaction mechanisms as well as the production of the catalysts themselves 2 3 4 Moreover gasphase cluster experiments allow the fundamental reactive behavior of catalytic materials to be studied in an environment that enables the influence of such factors as size stoichiometry as well as ionic charge state on cluster reactivity to be determined with atomiclevel precision For nanocatalysis this is particularly relevant since the reactive properties have been known to change dramatically with the addition or removal of even a single atom 7 8 9 10 11 12It is clearly a daunting task to design effective catalysts based solely on fundamental principles and prior to the last few decades much of the earlier successes in this endeavor depended on empirical tests employing materials of known effectiveness for promoting selective classes of reactions The available methods began to change as progress was made in the field of surface science first in the area of UHV and subsequently under more realistic reaction conditions 2 3 In recent years increasing focus has turned to basic studies with various modern techniques such as surface harmonic generation scanning tunneling microscopy ambientpressure Xray photoelectron spectroscopy and Xray absorption spectroscopies for example 4 One of the main problems has been identifying specific reaction sites as these frequently involve defects which are difficult to generate characterize and reproduce

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