Adsorption of ammonia on the rhodium (111), (100), and stepped (100) surfaces: An ab initio and experimental study

Abstract

The adsorption of ammonia on the two low index ͑111͒ and ͑100͒ surfaces of rhodium has been studied by periodic calculations with density functional theory and compared to experimental results. The geometries of the adsorbates and the surfaces are completely optimized. For both surfaces the top site is found to be the most stable while the adsorption energy of ammonia is 8-10 kJ•mol Ϫ1 larger on the ͑100͒ surface. The presence of steps on the ͑100͒ surface has a minor effect on the heat of adsorption. The theoretical predictions of the adsorption energies and the changes in work function by NH 3 are in good agreement with experimental data. Moreover the prediction of the ontop adsorption as well as the weak interactions between the adsorbates is confirmed. The broadening of the temperature programmed desorption spectra and the two desorption peaks for the first adlayer are mainly due to an entropy effect which affects the preexponential factor of the desorption rate constant.

Bridging the structure gap: Chemistry of nanostructured surfaces at well-defined defects

“…It was moreover concluded that the adsorption site on top of Rh atoms at the bottom of the step is unstable; NH 3 molecules present there experience a significant repulsion from the step itself and migrate to the top sites in the middle of the terrace (E ads = 93 kJ/mol) or to the tilted site at the step (E ads = 98 kJ/mol). Such E ads values compare well with those of 82 kJ/mol and 91 kJ/mol predicted theoretically for Rh (111) and Rh(100), respectively, and with the experimental values estimated from the analysis of TPD spectra [382].…”

“…With increasing coverage low-temperature shoulders extending down to 150 K appear in the TPD spectra of both surfaces. There is no general agreement on the interpretation of these features; they are explained either as due to a decrease in the heat of adsorption caused by lateral interactions [383] or as an entropy effect affecting the pre-exponential factor at increasing coverage [382]. It seems, however, established that the presence of steps on the (100) surface marginally affects the heat of adsorption, in agreement with the fact that the bond strength changes only by 10% between Rh(111) and Rh (100).…”

“…Fig. 74 shows TPD spectra [382] following adsorption of ammonia at Rh (111) and Rh(100). In the low coverage limit the desorption peak is observed at 360 K on Rh(100) and at 320 K on Rh (111) and the heat of adsorption (as estimated by the Readhead method) is thus 91 kJ/mol and 82 kJ/mol (with uncertainties of 3-4 kJ/mol) for the two surfaces, respectively.…”

Theoretical study of adsorption and dissociation of NH3 on the Ir{110}(1×2) surface

“…This is due to the interaction between the nonbonding lone pair of NH 3 and the substrate involved in the NH 3 -substrate bond [8] . We further found that rotation of the molecule results in only small difference in the adsorption energy, which implies that the barrier of rotation is negligible, similar to the ammonia adsorption on Pt{100} [9] , Pt{111} [9] , Rh{100} [8] , and Rh{111} [8] .…”

“…Rh{100} [8] . This is due to the interaction between the nonbonding lone pair of NH 3 and the substrate involved in the NH 3 -substrate bond [8] .…”