Reactions clean surfaces

Chemical reactions of aimnonia with the silicon surface have also been clearly observed using STS [21], where the disappearance of the it and it states characteristic of the clean surface coincides with the fomiation of Si-H antibonding states corresponding to the dissociation of the ammonia on the Si surface. 

Retaining the clean surface until coating takes place and assisting reaction... 

The surface after the reaction was found, using AES, to be clean with the exception of the presence of strongly adsorbed nitrogen and some small amounts of sulfur that segregated from the crystal bulk during the course of the reaction. Exposure of this surface to reaction conditions resulted in a reaction rate similar to that over the clean surface. A layer of PeN was prepared by exposing the surface to 5x10 Torr of ammonia at 673 K for 10 hrs. ( ). ... 

This surface had an Auger spectrum similar to that of the surface produced during the reaction and exhibited a catalytic activity similar to that of the clean surface. This suggests that during the reaction the amount of nitrogen deposited on the surface is close to that present in the compound PeN and possibly that this is actually present at the surface during the reaction (6). 

Temperature programmed reaction spectra depicting the reaction of H2O and OH groups with oxygen on Pd(lOO) are shown in fig. 3. Curve (a) was obtained for H2O adsorption on the clean surface and contains two peaks, the state at 167 K of multilayer H2O, and the <>2 state at 182 K due to H2O bound directly to the surface /7/. An additional state at 255 K, labelled y, is observed following coadsorption of 1 0 and 0 (fig. 3b). This state represents the reaction of OH groups III... 

Figure 1 shows two reactor configurations we have used to measure reaction rates on clean surfaces. In Figure 1(a) is shown a high pressure cell inside the UHV system ( ) with the sample mounted on a bellows so it can be moved between the reaction cell and the position used for AES analysis. In Figure 1(h) is shown a reaction cell outside the analysis system with the sample translated between heating leads in the reactor and in the UHV analysis system ( ). 

The mechanism Implied by these expressions Is that of a submonolayer of surface carbon which blocks n sites for NH adsorption. This suppresses NH decomposition which would occur on the clean surface (Figure 3(a)) and allows NH fragments to react with carbon to form HCN. Selectlvltles of HCN production exceed 90% at 1 Torr, and reaction probabilities (fraction of CH flux yielding HCN) approach 0.01. 

H2S adsorption on the (2x2)-S covered Pt(lll) surface at IlOK contrasts with adsorption on the clean surface. On the (2x2)-S surface no complete dissociation Is observed at low temperature Instead, H2S partially dissociates to form an adsorbed SH Intermediate with a characteristic bend vibration at 585 cm . Heating adsorbed SH on the (2x2)-S covered surface leads to a SH+H recombination reaction not observed on clean Ft. The recombination process removes the excess SH so that the stable, high coverage (/3 X /3)R30 -S lattice can be formed. 

Transient kinetic experiments have also been carried out to complement the information deduced from the steady-state measurements [33], Systematic variations were observed during the transition from the clean surface to the steady-state catalytic regime that correlate well with the overall reaction rates in the latter. Specifically, there is a time delay in the production of molecular nitrogen because of the need to buildup a threshold of atomic nitrogen coverage on the surface. This atomic nitrogen coverage, which could... 

Such a possibility has been recognized by early workers,9 but in spite of this intriguing possibility, only recently has such a metal surface been created. Chiral kink sites were created on Ag single crystal surfaces to produce the enantiomeric surfaces Ag(643)s and Ag(643)R however, no differences between (R)- and (S)-2-butanol were observed for either the temperature-programmed desorption from the clean surfaces or the dehydrogenation (to 2-butanone) from preoxidized surfaces.10 Unfortunately, Ag exhibits few catalytic properties, so only a limited array of test reactions is available to probe enantioselectivity over this metal. It would be good if this technique were applied to a more catalytically active metal such as Pt. 

In organic chemistry this is called a retro-pericyclic reaction and it should indeed have a low barrier of activation. Under the high vacuum techniques applied, on clean surfaces, the reaction proceeds already below 140 K (the temperature at which desorption of the weakly co-ordinated butadiene is observed). 

Clean single-crystal surface, 15 31 Clean surfaces, alcohol reactions, 29 37-38 Clear Air Act of 1970, 24 59, 62 Cleavage surfaces, dislocations on, 19 331-333 ClOj, ESR of, 22 309... 

Reactions alcohols, 29 36-49 adsorption, 29 36-37 clean surfaces, 29 37-38 ethanol oxidation, 29 44—48 methanol oxidation, 29 38-44 oxidation on copper and silver, 29 38-48 oxidation reaction, silver, 29 48-49 base-catalyzed, of hydrocarbons, 12 117 free radical mechanism in, of hydrogen peroxide, 4 343... 

The metal surfaces are always covered with a monolayer of CO upon evacuation of the reactor and transfer to the UHV system. On both Pd and Ir the CO, which desorbs as CO2 when reacted with the oxide species, desorbs at a much higher temperature than CO from the clean surface. This result implies that the oxide species forms an inactive complex with CO upon adsorption of CO under reaction conditions. While the presence of the oxide species reduces the overall rate of reaction, the activation energy is unchanged, suggesting that oxygen serves as a simple site blocker on the surface. 

---