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The nature of molecules adsorbed on catalytic surfaces : pulsed nuclear magnetic resonance and infrared absorbance studies

Citation

Duncan, T. Michael (1980) The nature of molecules adsorbed on catalytic surfaces : pulsed nuclear magnetic resonance and infrared absorbance studies. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-10272006-080057

Abstract

The nature of molecules adsorbed on high surface catalysts have been studied with infrared absorbance and nuclear magnetic resonance (NMR) techniques. The well-separated infrared vibrational spectra have been used mainly to identify the various surface species by comparison with known compounds. The frequency shifts in the molecular stretching modes indicate the strengthening or weakening of the bonds of the molecules upon adsorption. The NMR spectra have been used to quantify the distributions among the surface states and to describe the motional properties of the adsorbed molecules. The NMR data were obtained through the application of several recently developed multiple-pulse techniques. By analyzing the spin-lattice relaxation times, the dipolar interactions, and the chemical shift anisotropies, it has been possible to separate the overlapping NMR spectra and determine the molecular symmetries, bond lengths, and the nature of the adsorption site. The results of these two techniques were combined to describe the adsorbed states of CO on Rh dispersed on Al2O3, and the distribution of formic acid on zeolite surfaces. The infrared spectroscopy confirmed the previous indentification of three surface CO states; linear and bridged sites that form on Rh atoms in clusters on the Al2O3, and dicarbonyl species on isolated Rh atoms. The desorption rate at 295 K is very slow. All three states readily exchange with CO(g) at 295 K, but only the dicarbonyl species exchanges at temperatures below 200 K. H2CO decomposed on the dispersed Rh into adsorbed CO states, with no evidence of any HxCO surface states, though there was further demonstration of isolated Rh atom sites. The 13C NMR results quantified the distribution of the three surface sites, and thus calibrated the infrared absorbances, as well as the local motions on the three sites. The infrared spectra of formic acid on the Y zeolites suggested the presence of unidentate and bidentate formate species on the surface. The bidentate structure was the dominant species on the more catalytically active zeolite. The combination of the results of several multiple-pulse NMR techniques are analyzed to measure the site distribution between the unidentate and bidentate species, as well as to further describe the bidentate formate ion. The bidentate structure is bonded to the Al atoms rather than the Si atoms, has a structure similar to formate salts, and a more acidic carbonyl hydrogen, relative to formic acid.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Weinberg, William Henry (advisor)
  • Chan, Sunney I. (advisor)
Thesis Committee:
  • Unknown, Unknown
Defense Date:25 February 1980
Record Number:CaltechETD:etd-10272006-080057
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-10272006-080057
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4276
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:07 Nov 2006
Last Modified:26 Dec 2012 03:07

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