Decomposition of Ammonia and Hydrogen on Ir Surfaces:  Structure Sensitivity and Nanometer-Scale Size Effects

Abstract

The adsorption and decomposition of ammonia and hydrogen have been studied on surfaces of clean planar Ir(210) and clean nanoscale-faceted Ir(210), which are prepared from the same crystal in situ. We find evidence for structure sensitivity in recombination and desorption of H2 and in thermal decomposition of NH3 on clean planar Ir(210) versus clean faceted Ir(210). Moreover, the decomposition kinetics of NH3 on faceted Ir(210) exhibit size effects on the nanometer scale, which is the first observation of size effects in surface chemistry on an unsupported monometallic catalyst with controlled and well-defined structure and size.

Growth of oxygen-induced nanoscale-pyramidal facets on Rh(210) surface

“…The average facet size ͑͗l͒͘ of these nanopyramids can be estimated via the average pyramid number density ͗n͘ as ͗l͘ = ͗n͘ −1/2 , i.e., inverse of square root of total number of pyramids per unit area. 18,26 To determine the average pyramid size ͗l͘, we consider distinct pyramids that do not significantly deviate from the typical range in size or distance from neighboring pyramids. The calculated average pyramid size for the above surface is found to be ͗l͘ 850 K ϳ 18 nm.…”

“…A suitable candidate for faceting studies is an atomically rough metal surface that has higher specific surface free energy than a close-packed surface of the same material and is less stable against the adsorbate-induced faceting. [3][4][5][6][7][8][9][10][11][12][13][14][15] The potential application of these faceted surfaces could be as model systems to study structure sensitivity and size effects in catalytic reactions [16][17][18][19][20] and as templates to grow metallic nanoclusters. 15,21 Many studies have been performed on adsorbate-induced faceting of atomically rough surfaces, e.g., bcc W͑111͒ ͑Refs.…”

“…11͒ shows the formation of pyramidal-type facets with ͕311͖ and ͑110͒ faces when annealed in oxygen at and above 600 K. The faceted Ir͑210͒ surface has been used to study structure sensitivity and size effects in chemical reactions. [18][19][20] In this work, we study morphological changes of Rh͑210͒ induced by oxygen using low energy electron diffraction ͑LEED͒, Auger electron spectroscopy ͑AES͒, and scanning tunneling microscopy ͑STM͒, which is mainly motivated by the two considerations. First, Rh͑210͒ has the same fcc structure as Ir͑210͒, which enables us to compare with oxygeninduced faceting of Ir͑210͒ leading to better understanding of the physical insight into the morphological changes in the facets with the nature of metal.…”

“…11͒ and faceted Ir͑210͒ has been successfully used to explore structural and size effects in a series of important chemical reactions. [18][19][20] It is expected that faceted Rh͑210͒ will be also used to probe for structure sensitivity and size effects in chemical reactions. Second, the study of Rh is of technological importance due to the fact that Rh is a catalyst component in current commercial three-way catalysts for reducing air pollution from automobile exhaust gases.…”

Thoughts on Starting the Hydrogen Economy

“…1 The subject remains vibrant today; new mechanisms for the release of hydrogen from ammonia over catalysts at acceptable temperatures are continuing topics of research. 2 A major challenge is toxicity, as Feibelman points out, but all hydrogen storage proposals come with safety issues.…”