CO-Induced Segregation of Hydrogen into the Subsurface on Ni(110)

Abstract

Scanning tunneling microscopy measurements on Ni(110) suggest that the segregation of hydrogen into the subsurface (β3) state is mediated by CO. At 375 and 390 K exposure of the saturated added row Ni−H overlayer to 1 × 10-8 Torr of CO(g) induces the dissolution of the Ni−H rows to yield Ni islands of monatomic step height. Structures associated with the adsorption of CO onto the clean surface are not observed, and the surface formed under these conditions is resistant toward hydrogen uptake upon further H2(g) exposure. This reconstruction is activated; it does not occur at temperatures below 370 K at the same pressure of hydrogen. With heating, H2 is evolved from the β3 desorption state from this surface; no other gases are observed to desorb. Cooling the surface to room temperature permits formation of the “normal” Ni−H overlayer upon further H2(g) exposure. These findings suggest that at temperatures above 370 K CO induces a surface reconstruction with a fraction of the hydrogen from the Ni−H overlayer segregating into the subsurface, while the remainder evolves into the gas phase. Under a constant CO background pressure of 6 × 10-8 Torr at 350 K a low-coverage c(4×2)-CO structure appears as unresolved [001]-oriented rows separated by four unit vectors along [11̄0]. The mobility of CO along the [001] azimuth appears to result in a loss of surface registry of CO in that direction.

Evidence of stable high-temperature Dx-CO intermediates on the Ru(0001) surface

“…30 However, it should be noted that DFT calculations indicate that hydrogen occupation of subsurface sites is generally unfavorable relative to the surface sites of bare Ru(0001), although it has been seen in the case of high coverage exposures (θ H > 1). 12,31 Also, in the case of Ni(110) the CO-induced segregation of H to the subsurface was repulsive for the CO. 29 Even though the precise configuration can only be speculated upon, the TPD data do clearly demonstrate that D can remain associated with CO on the Ru(0001) surface at unprecedented high temperatures. Similar desorption features have not been reported either for diffuse dosing of D 2 23, 28 or for dosing with thermal atomic hydrogen.…”

“…This would be the case if, for instance, the stabilization is the result of D atoms being forced subsurface under the influence of CO, such as has been reported for the Ni(110) surface. 29 The possibility of subsurface hydrogen is interesting given the potential role on bulk hydrogen in catalytic hydrogenation. 30 However, it should be noted that DFT calculations indicate that hydrogen occupation of subsurface sites is generally unfavorable relative to the surface sites of bare Ru(0001), although it has been seen in the case of high coverage exposures (θ H > 1).…”

DFT-Based Characterization of the Multiple Adsorption Modes of Nitrogen Oxides on Pt(111)

“…The higher-frequency mode is similar to an asymmetric stretching mode observed for a surface nitrate on Ag(111). 68 The symmetric stretch is red-shifted by 139 cm -1 . Analogous to observations for NO 3 adsorption on basic metal oxides, 69 a metastable NO 3 adsorption mode can also be found in which the adsorbate binds planar and side-on to Pt(111) with local D 3h symmetry (Figure 7c).…”

“…To our knowledge, NO 3 adsorbates have not been explicitly identified on Pt(111). However, NO 3 adsorbates have been observed on Ag(110) 65 and Ag(111) [66][67][68] at high NO 2 exposures and have been suggested to account for some features in the temperature-programmed desorption of NO 2 from Pt(111) at high initial coverages. 26 DFT results indicate that NO 3 does bond to the Pt(111) surface.…”