120 Gold Bulletin 2001, 34(4) The Role of Nanosized Gold Particles in Adsorption and Oxidation of Carbon Monoxide over Au/Fe 2 O 3 Catalyst Narendra M Gupta* and Arvind K Tripathi Applied Chemistry Division Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India *E-mail: nmgupta@magnum.barc.ernet.in Received: 20 August 2001 The presence of gold is found to promote the development of weakly bonded (CO) ad species over the surface of Au/Fe 2 O 3 catalyst during interaction with carbon monoxide (CO) or a mixture of carbon monoxide and oxygen. The concentration of these species and the nature of the bonding depend on the gold particle size. No such species are formed for gold particles larger than ~11 nm or over gold-free iron oxide. The bulk carbonate-like species, formed in the process with the involvement of the hydroxy groups of the support, are merely side products not responsible for the low temperature activity of this catalyst. Thermochemical measurements reveal that the oxidation of carbon monoxide on both Fe 2 O 3 and Au/Fe 2 O 3 occurs via similar redox mechanisms, involving the abstraction and replenishment of lattice oxygen, where the presence of nanosize gold particles promotes these processes. This is attributed to their capacity to adsorb carbon monoxide because of their inherent defective structural sites. It is suggested that the energy that evolves during chemisorption of CO is responsible for the surge in temperature at the Au-Fe 2 O 3 interfaces, which in turn serve as sites for the accelerated reaction between CO and the support. The role of gold particle size is discussed in terms of the effect of geometry of surface metal atoms in the nanosize clusters. The discovery by Haruta et al of the unexpected low temperature CO oxidation activity of supported gold (1 - 4) has opened up a new dimension in the understanding of the basics of catalysis, since gold appears to be an exception in disregarding the requirement of unfilled d- orbitals for a metal catalysed reaction. Gold, in this new incarnation as a catalyst, has already found application in a number of chemical reactions, such as oxidation (of CO, CH 4 , CH 3 OH, C 6 H 6 , o-hydroxybenzyl alcohol), epoxidation (of propylene), reduction/hydrogenation (of NO, CO, acetylene, butylene), hydrocracking, water gas shift, and isotopic exchange, etc (See recent reviews and publications 5 - 13). Some practical applications of supported gold include: gas sensors (14), regeneration of CO 2 in sealed-off cw CO 2 lasers (15, 16) and purification of air (removal of CO and VOC s ) (5). In our laboratory, we have developed a compact low-power long-life sealed-off cw CO 2 laser, where an outer jacket coated with Au/Fe 2 O 3 helped in the regeneration of CO 2 from the dissociation products CO and O 2 formed during the laser discharge (15, 16). The performance of this gold catalyst was found to be superior to that of the other noble metals dispersed on reducible oxide supports (such as Pt/SnO 2 and Pd/SnO 2 ), advocated earlier for this purpose (17). In most of the above-cited applications, the importance of the size of gold crystallites and the nature of the support is generally emphasized. Various issues pertaining to supported gold catalysts, however, still remain unresolved. For instance: what is the role of the gold particle size and what is the optimum size for a particular application? What is the best method of preparing supported gold? What oxidation state of gold is vital to its activity? Does an electronic bonding of the reactants occur at gold sites? What is the nature of the transient species formed in the absence and in presence of gold? Unequivocal answers to these questions are yet to be found. While the requirement of nanosized gold particles is widely accepted, the actual role of gold as a catalyst and also that of its support in the overall performance have, however, raised many divergent views. Various factors contributing to the high activity of these catalysts are