Oxygen surface chemistry

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Oxygen surface chemistry:

3. CO oxidation Oxygen undergoes a remarkable variety of interactions with the (110)planes of the fcc metals nickel, copper and, in particular, silver. In the temperature range 200-400 K, 'added row' reconstructions are formed which consist, in effect, of parallel - O - M - O - M - chains located on top of the original surface. Confirmation of the structural details and a direct view of the mechanism of chain formation was one of the early successes of the application of Scanning Tunneling Microscopy (STM) in surface chemistry. On the copper and silver surfaces below the reconstruction temperatures, a surprising number of other oxygen species have also been identified (see the first two articles in this series, Appl. Catal. A, 148 (1996) N8 and Appl. Catal. A, 149 (1997) N1) and CO oxidation has been used extensively as a means of gauging their activities. On Cu(110) at 100 K, oxygen dissociates to give highly reactive atoms; exposure to CO followed by heating produces CO2(g) below 200 K [T. Sueyoshi, et al., Surf. Sci., 343 (1995) 1]. If the oxygen layer is annealed in the absence of CO, short - O - C u - O - chains begin to form at 200 K and then grow in length, aggregating into islands, the process reaching completion at 300 K. Once O atoms become tied up in chains, even short ones, the amount of CO2 generated is reduced greatly and the reactivity after 300 K annealing is some 25 times lower than that of O(ad) at 100 K. The detailed mechanism of CO attack on added-row islands at 400 K has been examined by STM [W.W. Crew and RJ. Madix, Surf. Sci., 349 (1996) 275]. The first step is detachment of an O atom from the

applied catalysis A: General

periphery of an island, estimated to be 500-1000 times more efficient when the oxygen is at the end of a chain than when it lies along the chain length. Once released, the atoms diffuse over the surface, meeting and reacting with adsorbed CO. On Cu(110), production of CO2 below 200 K occurs even when the combined coverage of CO and O is low. On those metals considered the most reactive for CO oxidation --Pt, Pd and Rh-- with equivalent small coverages, the reaction proceeds only above 300 K, while below this temperature, total coverages in excess of half a monolayer are necessary. The high reactivity of Cu(110) may be traced, in part, to the low heat of adsorption of CO on copper compared with these other metals. Unlike its copper analogue, Ag(110) at 100 K chemisorbs oxygen not only dissociatively but also in a molecular form. Moreover, the molecules react readily with CO below 130 K even though previous attempts to achieve the same reaction on other metals have been unsuccessful. The experimental data indicate a stabilisation of CO(ad) by O2(ad) which can be taken into account, formally at least, by including an intermediate CO/O2 complex in the kinetic scheme [U. Burghaus and H. Conrad, Surf. Sci., 364 (1996) 109]: CO(ad) + 02(ad) ~ CO02(ad) CO02(ad) ~ C02(g) + O(ad) CO(ad) + O(acl) --, C02(g) On Ag(110), atomic oxygen is present in the range 100-190 K (coexisting with O2(ad) up to 140 K where the latter both dissociates and desorbs). Two kinetically distinct forms can be distinguished, the more reactive having a rate constant for CO2 production greater than that of mole-

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cular oxygen and about 10 times greater than that of the second atomic form. Unexpectedly, the reactions involving the two atomic species have the same activation energy and differ only in their frequency factors. One possibility is that the lower reactivity is associated with terminal O atoms in very short - O - A g - O - chains which are present as precursors to reconstruction of the surface. Oxygen associated with silver atoms in this way is expected to have a considerably smaller reaction probability than that of mobile O(ad) atoms [U. Burghaus and H. Conrad, Surf. Sci., 338 (1995) L869; U. Burghaus and H. Conrad, Surf. Sci., in press]. With the onset of full reconstruction of the Ag(110)-oxygen surface above 190 K, the CO oxidation rate begins to fall as adsorbed O atoms are incorporated into long chains and the fraction of oxygen passivated by this means grows with increasing surface temperature up to ca. 240 K. Beyond that point, however, the thermal energy becomes sufficient for the embedded species to participate in the reaction and the rate recovers, until at 300 K all the oxygen can be removed by CO. At these higher temperatures, the primary step is thought again to involve O atoms which are not bound in chains. In addition, the reconstruction is sufficiently unstable for it to act as a buffer for the active species, with reduction of the O(ad) concentration by CO2 generation leading to breakdown of the chains [U. Burghaus and H. Conrad, Surf. Sci., 352-354 (1996) 201]: CO(ad) + O(ad) --, CO2(g) O(reconstr) ~-O(ad) An interesting complication emerges when the behaviour of the silver(110) plane

applied catalysis A: General

is compared with that of Ag(100). Under what are considered to be clean working conditions, the probability of CO oxidation by both O2(ad) at 100 K and O(ad) at 300 K on the (100) surface is at least two orders of magnitude lower than on Ag(110). Removal of a cold trap from the line dosing oxygen to the (100) surface, however, is found to have a dramatic effect. Trace adsorbed impurities, unidentified but likely to include water, increase the CO oxidation rate at 100 K by a factor of 15 and at 300 K by a factor of at least two [U. Burghaus, L. Vattuone and M. Rocca, Surf. Sci., to be published]. C.S. McKee Science and Engineering Indicators - 1996

Individuals who have need for at-hand data to tell where science and engineering currently stands in the U.S. should obtain a copy of a document published by the National Science Foundation (NSF) that is entitled 'Science and Engineering Indicators - - 1996.' The current issue of this document provides more than two decades of coverage. The document can serve the needs of a wide audience that includes decision-makers in government, industry, academia, nonprofit organizations, and professional societies. An objective of the Board of the NSF is to provide a document that is relevant to a broad audience in the United States and abroad who need to utilize comprehensive and objective indicators to fulfill their responsibilities. To this end, the NSF has produced a document that both 'sells' science and engineering and provides a vast assortment of data in numerous tables. The coverage has been expanded to include Volume 150 No. 1 - - 27 February 1997