Ammonia adsorption and decomposition on several faces of platinum

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Abstract
This paper is part of an ongoing study of the structure sensitivity of ammonia adsorption on platinum. In the work here, the adsorption of ammonia on Pt(111) and Pt(210) are compared using temperature programmed desorption (TPD), single-reflection infrared (IR), electron energy-loss spectroscopy (EELS), and molecular-beam techniques. In addition, the adsorption of ammonia on Pt(111), (5×20)Pt(100), (2×1)Pt(110), Pt(210), and Pt(211) are compared with TPD. It is found that the TPD spectrum of ammonia is similar on Pt(111), Pt(100), Pt(110), Pt(210), and Pt(211). There was always a molecular peak between 310 and 370 K, another peak at 170 K, and a multilayer peak at 120 K. The sticking probability was found to vary by a factor of 10 from Pt(210) to Pt(111). However, the infrared spectrum of ammonia adsorbed on Pt(111) is virtually identical to the EELS of ammonia on Pt(210) after accounting for a difference in the sensitivity of IR and EELS. Analysis of the TPD data indicates that the surface concentration of ammonia is a factor of 100–200 higher on Pt(210) than on Pt(111) at 670 K. By comparison, Pt(210) was found to be at least a factor of 100 more active than Pt(111) for ammonia dissociation at 670 K. There is more work to be done. However, the results so far indicate that the vaariations in sticking probability and residence time play an important role in the variations in rate.

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