260
Acc. Chem. Res. 1992,25, 260-265
Preparation and Reactivity of Alkyl Groups Adsorbed on Metal Surfaces FRANCISCO ZAERA Department of Chemistry, University of California, Riverside, California 92521 Received July 18, 1991 (Resubmitted Manuscript Received December 3,1991)
Heterogeneously catalyzed reactions constitute the basis of most chemical and fuel technologies. The conversion of hydrocarbons in particular is at the heart of the refining of crude oil, the modification of fat in food industries, and the synthesis of pharmaceutical products and fine chemicals. Unfortunately, since catalytic processes are generally quite complex, little is known about the individual steps that take place at the molecular level in such systems. It is only recently that the advent of a variety of modern surface-sensitive instrumentation has made those studies feasib1e;l the power of using such techniques in elucidating reaction mechanisms has been shown already in several key case^.^,^
In spite of the recent technological advances mentioned above, the details of the processes involved in the conversion of saturated hydrocarbons are still not well understood. The activation of those compounds is believed to occur through an initial C-H bond scission step that results in the formation of alkyl groups on the surface. However, since that reaction often displays quite low probabilities, catalytic processes need to be performed under fairly severe prmsure and temperature conditions in order to reach reasonable conversion rates. High pressures are not easy to reproduce in the vacuum environments normally used in surface science studies, and in addition, they tend to accelerate the conversion of alkyl groups into other species, making the steadystate concentration of those intermediates on the catalyst surface too low to be detected. Several approaches have been taken in order to overcome the difficulties mentioned above. One way to make C-H bond breaking reactions easier under vacuum is by using open surfaces with low-coordination metal atoms, because those sites tend to display higher rea~tivity.~Alternatively, supersonic beams can be used to deliver gas molecules having the high energies needed for their a ~ t i v a t i o n . ~However, ?~ even though studies based on these ideas have yielded very exciting results, they have proven less than general for investigating the surface chemistry of hydrocarbons on surfaces. Recent studies in my laboratory and a few others have shown the feasibility of yet another method for preparing chemisorbed alkyl groups, namely, by dissoFrancisco Zaera was born in Caracas, Venezuela, on May 11. 1958. He received a Licemiate degree in chemistry from the SI* M v a r University in 1979, and after working there for an additional year as an Instructlng Professor, he went to the University of Callomla. Berkeley, where he earned his Ph.D. degree in 1984. He then wwked at the Natlonai Synchrotron Llght Source of Brookhaven National Laboratorles as an asslstant scientist in a collaborative projact with a team from the Exxon Research and Engineering Company. He joined the faculty of the Unkersity of California, Rkerslde, In 1987, where he is now an associate professor in the Chemistry Department. His research interests include the study of the dynamics of surface reactions and the manufacturing and characterization of surface films.
0001-4842/92/0125-0260$03.00/0
Table I Literature Survey o f the Surface Characterization Studies Done on the Chemisorption and Surface Activation o f A l k y l Halides
surface W(100)
molecule CH3C1,CH3Br,CHJ Fe(100) CH&1 Ru(100) CHJ polycrystalline Co CHiC1, CHZClz Ni(100) CH,Cl, CH3Br,CHJ, C2H51,
activation thermal thermal thermal thermal thermal
ref 9 10 11 ~~
12 13-15
CH.1
Ni(ll1)
C
