| Summary: | 碩士 === 國立中央大學 === 物理學系 === 102 === Methanol decomposition on Rh nanoclusters supported by Al2O3/NiAl(100) as a model system is studied by IRAS, TPD and PES. The study contained two parts: surface structures of Rh clusters probe with CO and methanol decomposition on Rh nanoclusters. The Rh nanoclusters are grown from vapor deposition.
The IRAS spectra with CO as a probe show that the CO adsorbed on on-top site of Rh nanoclusters; no other site such as bridge or hollow site have been detected. The CO TPD spectra show that CO desorbed with two distinct peaks, one at 430 K, which is observed for CO on Rh single crystal results and the other at about 360 K, which is observed for CO on small Rh clusters. The PES spectra show that the CO dissociation rate ranges between 21 and 55 %, depending on the coverage and hence the size of the clusters. For the clusters annealed to 700 K, the IRAS spectra show the same CO absorption band but the CO TPD spectra show the proportion of low temperature desorption increases. We argue that the desorption temperature depends on the cluster size.
Adsorbed methanol was dehydrogenated on the nanoclusters. The dehydrogenation to CO began below 200 K, and some of the CO formed from the dehydrogenated methanol dissociated into atomic carbon. Our PES results confirm that about 21 - 55 % of molecularly adsorbed CO dissociates into atomic carbon, depending on the size of the clusters. The produced CO desorbed above 300 K and D2 from methanol-d4 above 200 K; no intermediate species were detected in the dehydrogenation process. The PES results show the ratio of methanol dissociating into atomic C is about 43.1 ± 2.0 %. This result suggests that the probability of methanol decomposed into CO is about 86.9 ± 4.5 %.
Annealing the sample to 700 K does not result in oxidation of the Rh clusters. The ratio of CO TPD desorption intensities from dehydrogenated methanol and molecularly adsorbed CO on the annealed Rh clusters is greater than that on the pristine Rh clusters. For 1 ML Rh, the ratio increases from 47.2 ± 1 % to 54.4 ± 10 %, and for 4 ML Rh, the ratio increases from 39.9 ± 10 % to 77.9 ± 10 %. The fraction of monolayer methanol undergoing dehydrogenation on the annealed Rh clusters is also greater than that on the pristine Rh clusters. On the annealed 4 ML Rh, the fraction of methanol undergoing dehydrogenation increased to 83.9 % (from 64.9 %), consistent with the result of CO ratio.
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