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Title: Thermal and electron stimulated chemistry of complex adsorbates on metal surfaces
Author: Fleming, Christopher
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2008
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Due to intrinsic limitations of conventional silicon based devices the trend of miniaturisation cannot continue indefinitely, thus molecular devices are being used to develop smaller, faster and higher storage density memory devices. We present a thermally activated, switchable hetero-polyoxometalate (HPOM) cluster immobilised on a highly polarisable gold surface which has potential as such a device. This cluster consists of a nanometre sized Mo(IV) oxide “shell” which encapsulates two electronically active pyramidal sulfite (SIVO32-) groups, and has the ability to reversibly interconvert between two electronic states. In the passive state, at cryogenic temperatures (77 K), the two SO32- groups are non-bonding with respect to the sulfur centres, however upon thermal activation, i.e. when the temperature is increased to room (298 K), two electrons are ejected from the active sulfite anions and delocalised over the metal oxide cluster cage. This has the effect of switching it from a fully oxidised to a two-electron reduced state, along with the concomitant formation of an S-S bonding interaction between the two sulfur centres inside the cluster shell. This process does not occur in the crystalline state and to proceed requires the stabilising effects provided by an image charge, generated as a consequence of being adsorbed onto a metal surface. The prototypical enantio-selective heterogeneously catalysed reaction involves the hydrogenation of the α-ketoester, methyl pyruvate on Pt. Using TPD, XPS and UPS we have investigated this compound’s behaviour on a model Cu(111) single crystal surface. Monolayers of methyl pyruvate at 180 K consist predominately (ca. 66%) of a chemisorbed methyl pyruvate moiety, with its keto-carbonyl bonded to the surface in a η2configuration, this moiety desorbs intact at 364 K. The rest of the monolayer contains weakly adsorbed methyl pyruvate, which desorbs at 234 K, and interacts with the surface through the lone pair electrons of the oxygen atoms of the C=O groups, adopting a η1 configuration. The observation of a strongly chemisorbed moiety in the present study is attributed to the activation of the keto-carbonyl by the electron withdrawing ester group, and is consistent with the homogeneous inorganic chemistry of ketones. It is widely assumed that the α-ketoester needs to be π-bonded to the surface for the enantio-selective hydrogenation to proceed, consequently, given both the formation of a η2 bonded methyl pyruvate moiety on Cu(l11) and the known activity of Cu as a selective hydrogenation catalyst, it is suggested that it is maybe worthwhile considering the possibility of testing the effectiveness of chirally modified supported Cu as an enantio-selective catalyst. The thermal and electron induced chemistry of (S)- and (R)-methyl lactate (MLac) on Cu(111) was investigated; both enantiomers exhibited similar behaviour. MLac adopts one of two adsorption modes on the terraces of a Cu(111) crystal, which desorb molecularly at 209 K and 220 K. Concerning the molecules adsorbed at defect sites, as the temperature is increased over the range 250 – 300 K, a fraction desorb intact, while the majority lose a hydrogen atom to form the more strongly bound alkoxy species on the surface. Of these, some recombine with the hydrogen and proceed to desorb as MLac at 360 K, while a larger proportion are dehydrogenated further and methyl pyruvate and hydrogen are ejected from the surface at 380 K. When a monolayer of MLac is irradiated with a low energy electron beam, the molecules at the terrace sites are electronically excited and desorb as intact molecules, while those at the defect sites undergo electron induced hydroxyl O-H bond cleavage. Subsequent to electron bombardment there is consequently a decrease in molecularly adsorbed MLac and an increase in the number of strongly bound alkoxy species on the surface, entities which are not susceptible to ESD. We believe the ESD excitation mechanism is dissociative electron attachment. Low energy electrons of <1 eV are prevalent in the secondary electron background and can excite the hydroxyl O-H stretch, facilitating its cleavage at a threshold of 1.4 + 0.7 eV. The cross sections for the electron induced processes are high, 3.0 + 0.4 x 10-16 cm2 for 50 eV electrons, thus MLac is extremely susceptible to electron stimulated desorption. The enantio-specific adsorption of both the (S)- and (R)- enantiomers of methyl lactate on the chiral Cu(643)R surface has been investigated. The results from the (111) surface enabled us to assign the features in the TPD profiles. The peaks arising from molecular desorption at terrace and step sites occurred at the same temperature for both enantiomers, however, those attributed to desorption from the kink sites differed by 13 K, representing an enantio-specific difference in desorption energies of 0.94 kcal mol-1. This value is significantly larger than those observed in previous experimental work, although it is consistent with theoretical studies. Furthermore, we also observed enantio-specific surface reactions. It was found that there was a greater tendency for the (R)- enantiomer to undergo both the alkoxide recombination reaction and further dehydrogenation to methyl pyruvate, while the (S)-enantiomer had a greater proclivity to undergo total decomposition. We have discovered, to the best of our knowledge, the first example of enantio-specific surface chemistry initiated by a beam of non-chiral low energy electrons. When (S)- and (R)-methyl lactate molecularly adsorbed at the chiral kink sites of a Cu(643)R substrate is irradiated with 50 eV electrons, it has been found that (R)-methyl lactate is more receptive to both electron induced desorption of the parent molecule and electron induced cleavage of the hydroxyl O-H bond. This behaviour has been attributed to the (S)-enantiomer forming a more intimate bond with the kink site than the (R)-enantiomer, as evidenced by its higher desorption temperature. Consequently the substrate is more effective at providing relaxation channels to the electronically excited adsorbate, which reduces the probability of ESD occurring. Starting with a racemic mixture, we have demonstrated a 20% enantiomeric enrichment in the molecular adsorbates at the chiral kink sites, after only 30% depletion of the initial population. As a control, the initial rates of desorption from terrace and step sites were found to be unaffected by enantiomeric identity, which was to be expected because these sites are achiral, and as such both enantiomers interact to a similar degree with each. When the monolayer is considered as a whole, it was found that electron irradiation drives desorption more completely with an (R)-MLac covered surface than with (S). It has been suggested that this property of the system could be exploited in the laboratory as a method for separating racemic mixtures, and that in an astrochemical context, it could provide insight into the origins of biohomochirality.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QD Chemistry ; Q Science (General)