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ACTIVATION OF THE C-H BOND ON SURFACES

Objective


Work has concentrated on the question of orientation of aromatic hydrocarbon species, adsorbed at various metal surfaces, before and after hydrogen loss. In the case of the toluene benzyl conversion, there is little geometry change and it has proved very difficult to detect this transition.

A new ultraviolet photoelectron spectroscopy (UPS) source was developed for studies using the electron spectroscopy for chemical analysis (ESCA) instrument. The source is based on a simplified design with 2 of the collimating capillaries constructed as a unit. The discharge capillary is somewhat wider than usual, 1.6 mm instead of 1 mm, which also simplifies alignment, since the collimating capillaries are 1 mm in diameter. The wider capillary does not appear to cause much loss of intensity. Under simplified conditions, with 50-70 mA discharge current, target currents of 1-2 nA are obtained, comparable with most commercial sources. The lamp is particularly suitable for helium II work. The majority of commercial sources can be tuned, with some difficulty, to give helium II signals within the range 5% - 20% of the helium I signal. In contrast the new lamp gives helium II signals similar to the helium I count rate, and on clean platinum(110) the normal emission d band intensity is actually more intense in the IIa than in the la. Tuning is much easier than with most commercial sources since, with clean helium the lamp does not extinguish until after a maximum absolute helium II intensity has been passed.

The new lamp has been mounted on the ESCA system, in the manipulator rotation circle, at an angle of 45 degrees to the analyser axis. Studies of the adsorption of benzene, toluene and pyridine on ruthenium(001), and of benzene on platinum(110) have been made.

The toluene ruthenium(001) system has been studied by high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED) and thermal desorption spectroscopy (IDS). From TDS it is possible to separate out a thermal desorption signal for the first hydrogen loss from the toluene molecule by coadsorbing oxygen, and this is assigned to the breaking of 1 methyl carbon hydrogen bond producing a coordinated benzyl. However, in the absence of coadsorbates the 3 methyl hydrogen desorptions show only a single TDS peak.

Surprisingly little work function change occurs during the first hydrogen loss, though later dehydrogenation produces large increases. In benzene similar large changes occur, so it seems likely that the later stages of dehydrogenation of the methyl and benzyl hydrogens overlap. This is supported by isotopic labelling experiments, which indicate some scrambling during the desorption. Before the dehydrogenation occurs, a (3 x 3) LEED structure can be observed, which disorders as the benzyl is produced. The (3 x 3) structure was accompanied by the presence of a new band in the HREELS spectrum, at about 1650 cm{-1}, and at first it seemed likely that this was due to toluene, which would suggest a nonparallel coordination. However, coadsorption of carbon monoxide was tried and this increased the intensity substantially, so it seems most probable that the band is due to low frequency coordinated carbon monoxide.

However, this is a quite remarkable observation. Carbon monoxide will coordinate to clean palladium(111) in both on top and bridge sites, with clearly distinguishable carbon oxygen stretching frequencies, but on clean rhodium(111) carbon monoxide can be forced into a bridge position, as shown by LEED and by the presence of a low frequency band. In marked contrast, on ruthenium(001) so far carbon monoxide has only been observed at low frequencies in the coadsorption systems with alkali atoms and benzene will not force the t ransition to a bridge site. The most plausible interpretation of this observation is that the presence of a methyl group forces a rotation of the ring away from the azimuthal orientation favoured by benzene, and that this limits the carbon monoxide coordination to bridge sites. Probably this is a combination of an electronic and a steric effect. The interest in this coadsorption system arises because it allows detection of significant differences between the vibrational spectra of adsorbed toluene and benzyl, so that the disappearance of the low frequency carbon monoxide gives a useful means of following the progress of the reaction.
In many reactions of significance in the industrial chemistry of hydrocarbons, the fundamental process involves the breaking of a C-H bond by a catalytic surface, to give an intermediate species on the surface which can then undergo further reactions to give more useful products; such reactions are usually referred to as activation. We propose to investigate this process, and specifically the nature of the intermediates, to obtain greater understanding of the factors which are responsible for producing this activation. A better understanding will lead to better catalysts; a distant but very important goal in such work is to be able to activate methane and so make use of this as a feedstock instead of a fuel. Complementary studies of the activation process will be undertaken in the two laboratories. In the Chemistry Department at Trinity College, Dublin, survey work will be carried out looking for likely examples of reactions producing such intermediates using fixed-angle UPS, thermal and chemical desorption, XPS; examples found will be better characterised using angle-resolved UPS, and molecules containing similar species to chose on the surfaces will be synthesised for comparative spectroscopic study. In Munich more detailed studies will be carried out using the wide array of techniques available in the surface physics group of Professor Menzel at the Technical University so that the surface species may be more fully characterised with regard to their structure, orientation and bonding to the surface.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

THE PROVOST, FELLOWS AND SCHOLARS OF THE COLLEGE OF THE HOLY AND UNDIVIDED TRINITY OF QUEEN ELIZABETH NEAR DUBLIN HEREINAFTER TRINITY COLLEGE DUBLIN
Address
Trinity College
2 Dublin
Ireland

Participants (1)

TECHNICAL UNIVERSITY OF MUNICH
Germany
Address
21,Alte Akademie 16
85350 Freising