The Effect of Surface Structure on the Decomposition of Nitric-Oxide and Ammonia on Platinum
Gohndrone, John Michael
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Permalink
https://hdl.handle.net/2142/69789
Description
Title
The Effect of Surface Structure on the Decomposition of Nitric-Oxide and Ammonia on Platinum
Author(s)
Gohndrone, John Michael
Issue Date
1987
Doctoral Committee Chair(s)
Masel, Richard I.
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Engineering, Chemical
Abstract
Temperature programmed desorption (TPD) was used to compare nitric oxide (NO) decomposition on Pt(100), Pt(411), and Pt(211). The three surfaces were predicted to have similar orbital availabilities but different site densities. The measured dissociation fractions were 66, 70, and 66% on Pt(100), Pt(411) and Pt(211) respectively. The NO binding energy on the (100) steps of Pt (211) was unusually strong. The activation energy for N$\sb2$ formation was nearly identical (28 kcal/mol) on Pt(211) and Pt(411). Assuming that NO dissociation occurred at the (100) sites 90% of the NO associated with these sites on Pt(210) dissociated compared to 70% on Pt(411) and 66% on Pt(100). NO dissociation on Pt(100) was inhibited by a (1 x 1) $\to$ (1 x 5) surface reconstruction. Except for some minor effects, the variations in reactivity with changing crystal face were as expected from orbital symmetry and did not scale with site density.
TPD and modulated molecular beam spectroscopy were used to study ammonia (NH$\sb3$) decomposition on Pt(111), Pt(100), Pt(211), and Pt(210). An increase in the NH$\sb3$ sticking coefficient was observed going from Pt(100), Pt(211), to Pt(210). NH$\sb3$ also adsorbed more strongly on highly stepped platinum surfaces at low NH$\sb3$ coverages. No NH$\sb3$ dissociation was observed prior to desorption on any of the surfaces studied. Coadsorption of NH$\sb3$ and NO on Pt(210) resulted in the formation of a NH$\sb3$-NO surface complex that broke up at 200 K, without any direct reaction between NO and NH$\sb3$ on the surface. An increase in NO dissociation was observed as a consequence of coadsorption. Modulated molecular beam studies compared N-H bond breaking on Pt(210) and Pt(111) over the temperature range of 500 to 1200 K. On Pt(111) no dissociation was observed at any temperature. On Pt(210) considerable NH$\sb3$ dissociation was observed between 600 and 800 K, with a maximum at 670 K. A comparison of the data showed Pt(210) to be at least 100 times more active than Pt(111). Comparing the TPD and molecular beam results showed the variation in NH$\sb3$ dissociation with surface structure was strongly influenced by changes in the sticking coefficients and residence times of NH$\sb3$ on platinum surfaces.
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