Dissociative Adsorption Of NH3 Research Articles

Surface and subsurface acidity of faujasite-type zeolites in relation to their composition: an XPS and TPD of ammonia study

A combination of temperature-programmed desorption (TPD) of NH3 and X-ray photoelectron spectroscopy (XPS) has been used for the qualitative and quantitative study of the surface acidity of HY zeolites with different Si : Al molar ratios (ca. 2.7, 5.8, 24). The N 1s binding energies of the probe molecule showed two types of sites, Brønsted and Lewis, whose relative proportions varied with pretreatment temperature, as well as dissociative adsorption of NH3 following dehydroxylation. The measurement of the NH3 concentration in zeolites as a function of desorption temperature enabled the site distribution to be determined in terms of strength, for the external surface of crystals and for a subsurface which is partially free of the very slightly acidic extraframework alumina and silica species that cover it.

Interaction of atomic nitrogen with Rh(110)

The behaviour of atomic nitrogen layers on a Rh(110) surface produced by dissociative adsorption of NH3 at 430–450 K or by atomization of N2 on a hot filament was studied by means of thermal programmed desorption (TPD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Combined LEED, TPD and Auger measurements were used to investigate the growth, ordering and stability of the nitrogen layers. Two ordered phases 3 × 1 and 2 × 1 were observed with increasing nitrogen coverage. The transition from a disordered layer at very low coverages, to ordered phases at moderate and high coverages, resulted in apparent higher temperature shift of the N2 TPD traces indicating stabilization of the adlayer. The desorption parameters corresponding to the disordered, 3 × 1 and 2 × 1 phases were evaluated from the desorption data using leading edge lineshape analysis of the TPD traces and desorption at constant temperature. It was found that at moderate and high coverages the desorption is best fitted to a first-order desorption kinetics. The desorption energy increased by ∽ 40 kJmol as a result of ordering of the layer. Comparison of the TPD, Auger and LEED data for various nitrogen layers indicates that a fraction of nitrogen adatoms penetrates beneath the surface and desorbs after liberation of the surface sites in a wide temperature range (550–800 K). Several models including possible N-induced structural changes of the surface are proposed to explain the structure and stability of the nitrogen layers.

Dissociative adsorption of NH3 on cleaved Si(111)

Clean cleaved Si(111) surfaces were exposed to NH3 up to a few langmuirs and studied by low energy electron diffraction, Auger electron spectroscopy, electron energy loss spectroscopy and photoemission yield spectroscopy. The sticking coefficient is unity almost up to saturation which occurs at a N coverage of 14 monolayer. There, the surface structure is 1 × 1 with some disorder, the ionization energy and the work function are decreased by 0.425 and 0.6 eV, respectively, and the clean surface state band is replaced by a narrow band, 0.1 eV below the valence band edge. The results are explained by a high degree of dissociation of the adsorbed NH3 molecules, mainly into N, or NH and H atoms without large loss of hydrogen.

The stepwise dissociation of NH3 on the Si(111)–(7×7) surface: Low-temperature dissociative adsorption and thermal effects

The molecular and dissociative adsorption of NH3 on a clean Si(111)–(7×7) surface has been studied using high-resolution electron-energy-loss spectroscopy (HREELS), Auger electron spectroscopy, and temperature-programmed desorption (TPD). All NHx (3≥x≥1) species have been observed vibrationally under varying experimental conditions. Adsorbed molecular ammonia is observed by both HREELS and TPD below 200 K. The absence of a strong δs(NH3) mode in our vibrational spectra suggests that NH3 is bound as a tilted species to the Si(111)–(7×7) surface. Ammonia exposures above 1.7×1014 NH3/cm2 at 110 K populate a weakly bound NH3(a) state which desorbs upon heating with a peak desorption temperature of 115 K. The NH2(a) species is observed to form at 80 K at all coverages and exhibits an enhanced thermal stability at higher ammonia exposures. The NH(a) species is detected above 200 K by an energy-loss feature at 1100 cm−1, which we assign to the δ(NH) mode. NH(a) is stable to 750 K on high coverage layers. Finally, we observe an increase in the Si–H stretching frequency as the concentration of NH(a) and N(a) species is increased by heating.

Laser Raman spectroscopy of NH3 and ND3 adsorbed on TiO2(anatase)

The adsorption of NH3 and ND3 on the anatase phase of TiO2 has been studied using laser Raman spectroscopy. Peaks are observed due to OH (OD), NH3 (ND3) and NHx. (NDx) species adsorbed on the oxide surface. Analysis of the isotopic shifts observed between adsorbed NH3 and ND3 together withG-matrix calculations, leads to the conclusion that one type of chemisorbed NH3 is present on the surface, in contrast to previous studies suggesting the presence of two distinct types of chemisorbed NH3. Evidence is also presented for adsorbed NHx (x=1 or 2) species formed by the dissociative adsorption of NH3.

A scanning tunneling microscopy study of the reaction of Si(001)-(2×1) with NH3

We have performed a study of a semiconductor surface chemical reaction at the atomic level using a scanning tunneling microscope (STM). The ‘‘topographic’’ structure and electronic density of states are measured before and after reacting Si(001)-(2×1) with NH3. While both clean and NH3-dosed surfaces exhibit a (2×1) symmetry, STM images reveal changes in the spatial distribution of occupied electronic states which allow us to distinguish reacted and unreacted Si(001) dimers. Using tunneling spectroscopy, the occupied and unoccupied surface states of the clean and reacted surface are identified. The results are interpreted in terms of a Si(001)-(2×1)H monohydride resulting from dissociative adsorption of NH3.