Direct observation of subsurface oxygen on Rh(111)

Wider, J.; Greber, Thomas; Wetli, E.; Kreutz, T. J.; Schwaller, P.; Osterwalder, Jürg

doi:10.1016/s0039-6028(98)00674-8

Cited by 64 publications

(70 citation statements)

References 22 publications

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“…We exclude ͑partial͒ migration of O ads into subsurface sites on Rh͑111͒ as the cause for this lowered selectivity to CO 2 . A recent X-ray Photoemission Diffraction ͑XPD͒ study by Wider et al 48 shows that subsurface sites only become occupied at elevated temperature and high O 2 exposures, i.e., when O exceeds 0.50 ML.…”

Section: A Structure Sensitivity Of the Co Oxidation On Rhodiummentioning

confidence: 99%

Structure sensitivity in the CO oxidation on rhodium: Effect of adsorbate coverages on oxidation kinetics on Rh(100) and Rh(111)

Hopstaken

Niemantsverdriet

2000

The Journal of Chemical Physics

112

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Temperature-programmed reaction spectroscopy has been used to study the surface reaction between CO and O-atoms on Rh͑100͒ and Rh͑111͒ at a range of different adsorbate coverages. Comparison of the reaction on both surfaces in the low coverage regime, where the kinetics can be described by a straightforward Langmuir-Hinshelwood mechanism reveals that the CO oxidation is structure sensitive, with the rate constant being an order of magnitude higher on the Rh͑100͒ than on the Rh͑111͒ surface. As a consequence, the selectivity of the COϩO reaction to CO 2 is about 100% on Rh͑100͒, whereas on Rh͑111͒ the oxidation reaction competes with CO desorption. At low CO coverage, CO oxidation is an elementary step on Rh͑100͒ for a broad range of oxygen coverages. We report kinetic parameters E a ϭ103Ϯ5 kJ/mol and ϭ10 12.7Ϯ0.7 for O ϭ CO →0 on Rh͑100͒. The activation energy for CO oxidation on Rh͑100͒ decreases continuously with increasing O-coverage. At low coverage ( O Ͻ0.25 ML͒ we attribute this to destabilization of CO, leading to an increase in the CO 2 formation rate. At higher coverage ( O Ͼ0.25 ML͒ O-atoms become destabilized as well, as lateral interactions between O-atoms come into play at these coverages. The interactions result in a greatly enhanced rate of reaction at higher coverages.

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Section: A Structure Sensitivity Of the Co Oxidation On Rhodiummentioning

confidence: 99%

Structure sensitivity in the CO oxidation on rhodium: Effect of adsorbate coverages on oxidation kinetics on Rh(100) and Rh(111)

Hopstaken

Niemantsverdriet

2000

The Journal of Chemical Physics

112

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“…the chemisorption of oxygen on Rh single crystal surfaces, has been the subject of intensive experimental and theoretical research, [7][8][9][10] but only few experimental studies have addressed the ensuing formation of sub-surface oxygen, [11][12][13][14][15][16][17] . In one of these studies it could recently be shown 16 that exposure of the Rh(111) surface to O 2 at moderatly elevated temperatures (∼ 470 K) leads to the formation of sub-surface oxygen species, and there is evidence that at least ∼ 0.9 monolayer (ML) of O is adsorbed on the surface before sub-surface sites are occupied. Interestingly, the analysis of the X-ray photoelectron diffraction (XPD) data of the corresponding study suggested that the sub-surface species (∼ 0.1 ML) occupies octahedral sites between the first and second Rh outermost layers, which lie just underneath the fcc onsurface adsorption sites, while the neighboring on-surface oxygen has switched from its normal fcc to the hcp sites.…”

Section: Introductionmentioning

confidence: 99%

“…This barrier can also be overcome by enhanced O 2 dosage at moderately elevated temperatures 13,16,25 or by exposure to more oxidizing carrier gases like e.g. NO 2 .…”

mentioning

confidence: 99%

Stability of subsurface oxygen at Rh(111)

2002

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Using density-functional theory (DFT) we investigate the incorporation of oxygen directly below the Rh(111) surface. We show that oxygen incorporation will only commence after nearly completion of a dense O adlayer (θtot ≈ 1.0 monolayer) with O in the fcc on-surface sites. The experimentally suggested octahedral sub-surface site occupancy, inducing a site-switch of the onsurface species from fcc to hcp sites, is indeed found to be a rather low energy structure. Our results indicate that at even higher coverages oxygen incorporation is followed by oxygen agglomeration in two-dimensional sub-surface islands directly below the first metal layer. Inside these islands, the metastable hcp/octahedral (on-surface/sub-surface) site combination will undergo a barrierless displacement, introducing a stacking fault of the first metal layer with respect to the underlying substrate and leading to a stable fcc/tetrahedral site occupation. We suggest that these elementary steps, namely, oxygen incorporation, aggregation into sub-surface islands and destabilization of the metal surface may be more general and precede the formation of a surface oxide at close-packed late transition metal surfaces.

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“…Since our intention was rather to compare the reactive behaviour of µm-and nm-sized stepped Rh surfaces, conditions were chosen, at which the formation of subsurface oxygen is not observed, i.e. in the oxygen pressure range of 10 −7 -10 −6 mbar and in the temperature range 434-500 K [23].…”

Section: Peem Studiesmentioning

confidence: 99%