THE GROWTH MODES AND STRUCTURES OF NO ON Ni{111}

Abstract

The growth modes of NO on Ni{111} have been determined using a novel LEED technique. Measurement of LEED intensities at the superstructure beam positions as a function of coverage indicate that initial adsorption occurs randomly up to a coverage of 0.25 ML. After this point islanding occurs until the c(4×2) half-monolayer coverage is reached. Diffuse LEED I(V) analysis has also been used to determined the structure of a low coverage phase. This low coverage phase gives rise to an optimum structure consisting of 70% fcc and 30% hcp threefold hollow site occupation, which contains a large buckling of the first nickel layer. This contrasts with the optimum structure for the ordered, 0.5 ML c(4×2) phase where a 50/50 mix of fcc and hcp site occupation is determined and no buckling is found. These site assignments do not agree with assignments made using vibrational spectroscopy. Our results provide the basis for a new interpretation of the vibrational data allowing us to give a complete picture for the adsorption of NO on Ni{111} at low temperatures.

Very Low Temperature Surface Reaction: N2O Formation from NO Dimers at 70 to 90 K on Ag{111}

“…In addition, recent studies on Ni{l 11} have thrown some doubts on band assignments for NO. 26,27 Figure lf,g show the results of heating the (NO)2 multilayer formed at 45 K. The multilayer can be seen to desorb between 57 and 63 K, in complete agreement with the 60 K multilayer desorption temperature reported elsewhere. 4 The band that is left after desorption of the multilayer is at 1863 cm-1.…”

Site Switching and Surface Restructuring Induced by NO Adsorption on Pt{110}

“…For the adsorption of NO on Pt{110} assignments are based on the following band ranges: G1720 cm -1 , on top NO; 1600-1720 cm -1 , bridged NO; < 1600 cm -1 , 3-fold NO, which are fully consistent with all available data. We note that recent structural 15,16 and theoretical 19,20 studies show no evidence for bent NO species.…”

“…There is some controversy about band assignments for NO adsorbed on surfaces, with NO adsorption on Ni{111} being a particularly good example. Here, it was originally concluded from infrared data that NO occupied different adsorption sites as a function of coverage, but photoelectron diffraction 15 and LEED 16 measurements showed that this was not the case. Infrared spectroscopic investigations showed two NO stretching bands: 1507 cm -1 at low coverage and 1581 cm -1 at 0.5 ML coverage.…”

NO Chemisorption and Reactions on Metal Surfaces: A New Perspective

“…The first case demonstrating this was the adsorption of NO on Ni{111}, which was studied using a wide variety of techniques over the years. [29][30][31][32][33][34][43][44][45][46][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] The main controversy arose from the assignments of vibrational bands identified in a RAIRS study by Erley 29 and in an earlier EELS study by Ibach and co-workers. 30 In the RAIRS study, NO adsorption on Ni{111} was studied at 85 K. Previous work 66,69 had shown that, at temperatures below 300 K, NO adsorbed molecularly and a c(4 × 2) LEED pattern was observed at the saturation coverage of 0.5 ML.…”

“…For the particular case of NO adsorption on Ni{111}, vibrational studies 29,30 suggested that NO was adsorbed in a 2-fold bridge site, and both bent and upright species were suggested as a function of coverage. However, diffraction based structural studies [31][32][33][34] showed that NO occupies 3-fold sites over the whole coverage range. Recent density functional theory (DFT) calculations 35,36 have also shown disagreement with conclusions from the earlier vibrational studies for NO adsorption on other surfaces.…”