An electron spectroscopic study of the interaction of C2H2 on W(100) and W(110) single crystal surfaces
An electron spectroscopic study of the interaction of C2H2 on W(100) and W(110) single crystal surfaces
AES supported by ELS, LEED, thermal desorption and work function measurements has been used to study the interaction of C2H2 with W(100) and W(110). On W(100) at 300°K, acetylene is found to adsorb into two binding states. In the first of these the molecule dissociates completely into strongly adsorbed carbon atoms and hydrogen adatoms. From n. 0.51, in parallel to the first form, an associative state, a-C2H2, starts populating. Saturation is reached after ' 4L with coverages: It, 1 x 1015 C atoms/cm , plus t. 0.25 x 1015 mol./cm2 of a-C2H2, plus 0.1 - n.2 x 1015 H atoms/cm2. After heating to 1000°K, a-C2H2 desorbs and a monolayer of carbon atoms is left behind. Acetylene adsorbs on the carburised surface in the form of a-C2H2 which desorbs on heating. A random and immobile adsorption model is proposed that explains various observed features and in particular the role of adsorbed hydrogen. On W(100) at 80°K, acetylene adsorbs in much the same way as at 3000 K. The main difference is that the relative rate of molecular to dissociative adsorption shifts in favour of the former. On W(110) at 300°K, acetylene adsorbs in one form up to saturation which is probably a C 2 H radical. The adsorption is slow, reaching saturar4on at '- IDOL, and mobile as LEED patterns indicate. Up to'~, 0.51, the C 2 H units adsorb in a p(2 x 2) structure which is gradually replaced by a c(2 x 2) structure at higher exposures. The coverage after 100L is n. 0.65 x 1015 mol./cm2. Annealing on this surface leads to C-H bond scission at,,. 500°K and to CC bond scission from n. 650°K. A reduction of coverage above n. 900°K is attributed to carbon dissolution. As for W(100), acetylene adsorbs molecularly on carburised W(110). Heating of that leads to decomposition and subsequent carbon dissolution rather than desorption. Dissociation of the C-H bond was observed by electron bombardment for both adsorbates C2H and C2H2.Segregation of carbon on W(100) and W(110) was studied. A Langmuir type model yielded segregation enthalpies of 56 kcal/mole for W(100), 57 kcal/mole for W(110) below 1660°K and 45 kcal/mole for W(110) above 1815°K. A surface reconstruction to a dilute surface carbide is deduced for C + W(110) above 1815°K.The deviation from linearity of the 'ptph vs. peak area' graph is used to study the adsorption of NO on W(110).
University of Southampton
Foulias, Stylianos Demetrios
1981
Foulias, Stylianos Demetrios
Foulias, Stylianos Demetrios
(1981)
An electron spectroscopic study of the interaction of C2H2 on W(100) and W(110) single crystal surfaces.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
AES supported by ELS, LEED, thermal desorption and work function measurements has been used to study the interaction of C2H2 with W(100) and W(110). On W(100) at 300°K, acetylene is found to adsorb into two binding states. In the first of these the molecule dissociates completely into strongly adsorbed carbon atoms and hydrogen adatoms. From n. 0.51, in parallel to the first form, an associative state, a-C2H2, starts populating. Saturation is reached after ' 4L with coverages: It, 1 x 1015 C atoms/cm , plus t. 0.25 x 1015 mol./cm2 of a-C2H2, plus 0.1 - n.2 x 1015 H atoms/cm2. After heating to 1000°K, a-C2H2 desorbs and a monolayer of carbon atoms is left behind. Acetylene adsorbs on the carburised surface in the form of a-C2H2 which desorbs on heating. A random and immobile adsorption model is proposed that explains various observed features and in particular the role of adsorbed hydrogen. On W(100) at 80°K, acetylene adsorbs in much the same way as at 3000 K. The main difference is that the relative rate of molecular to dissociative adsorption shifts in favour of the former. On W(110) at 300°K, acetylene adsorbs in one form up to saturation which is probably a C 2 H radical. The adsorption is slow, reaching saturar4on at '- IDOL, and mobile as LEED patterns indicate. Up to'~, 0.51, the C 2 H units adsorb in a p(2 x 2) structure which is gradually replaced by a c(2 x 2) structure at higher exposures. The coverage after 100L is n. 0.65 x 1015 mol./cm2. Annealing on this surface leads to C-H bond scission at,,. 500°K and to CC bond scission from n. 650°K. A reduction of coverage above n. 900°K is attributed to carbon dissolution. As for W(100), acetylene adsorbs molecularly on carburised W(110). Heating of that leads to decomposition and subsequent carbon dissolution rather than desorption. Dissociation of the C-H bond was observed by electron bombardment for both adsorbates C2H and C2H2.Segregation of carbon on W(100) and W(110) was studied. A Langmuir type model yielded segregation enthalpies of 56 kcal/mole for W(100), 57 kcal/mole for W(110) below 1660°K and 45 kcal/mole for W(110) above 1815°K. A surface reconstruction to a dilute surface carbide is deduced for C + W(110) above 1815°K.The deviation from linearity of the 'ptph vs. peak area' graph is used to study the adsorption of NO on W(110).
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Published date: 1981
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Local EPrints ID: 459840
URI: http://eprints.soton.ac.uk/id/eprint/459840
PURE UUID: 69efa001-d3f4-4d1a-9284-0cc3151be057
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Date deposited: 04 Jul 2022 17:19
Last modified: 04 Jul 2022 17:19
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Author:
Stylianos Demetrios Foulias
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