Alkali chemisorption

It is important to notice that the work function, , of a given solid surface changes significantly with chemisorption. Thus oxygen chemisorption on transition metal surfaces causes up to 1 eV increase in while alkali chemisorption on transition metal surfaces causes up to 3 eV decrease in . In general electronegative, i.e. electron acceptor adsorbates cause an increase in 0 while electropositive, i.e. electron donor adsorbates cause a decrease in 0. Note that in the former case the dipole vector P formed by the adsorbate and the surface points to the vacuum while in the latter case P points to the surface (Fig. 4.20). 

Alkali Chemisorption on Nickel Surfaces Energy and Geometry... 

It is worth noting that no overbinding seems to occur in the local density description of alkali chemisorption on nickel clusters, in contrast to findings for carbon transition metal bonds (23,27,28). At present, it would be premature to correlate this difference with the character of the various bonds (covalent vs. ionic). Clearly, density gradient corrections to the energy functional (31) would be highly useful in deciding this question. 

NakatsujI H, Kuwano R, Merita H and Nakal H 1993 Dipped adcluster model and SAC-CI method applied to harpooning, chemical luminescence and electron emission in halogen chemisorption on alkali metal surface J. Mol. Catal. 82 211-28... 

Regardless of the exact extent (shorter or longer range) of the interaction of each alkali adatom on a metal surface, there is one important feature of Fig 2.6 which has not attracted attention in the past. This feature is depicted in Fig. 2.6c, obtained by crossploting the data in ref. 26 which shows that the activation energy of desorption, Ed, of the alkali atoms decreases linearly with decreasing work function . For non-activated adsorption this implies a linear decrease in the heat of chemisorption of the alkali atoms AHad (=Ed) with decreasing > ... 

The molecular chemisorption of CO on various alkali-modified metal surfaces has been studied extensively in the literature. It is well established that alkali modification of the metal surface enhances both the strength of molecular chemisorption and the tendency towards dissociative chemisorption. This effect can be attributed to the strongly electropositive character of the alkali, which results in donation of electron density from the alkali to the metal and then to the adsorbed CO, via increased backdonation into the... 

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

The last point is confirmed by measuring the work function changes upon CO chemisorption on clean and alkali-promoted metal surfaces. Figures 2.16 and 2.17 show the work function changes induced by CO adsorption on a K/Pt(lll) and on a Na/Ru(1010) surface respectively, for various alkali... 

The presence of alkali promoters on the substrate surface can affect both the rate of chemisorption, (e.g. on K/Rh(100))55 and the adsorptive capacity... 

The effect of alkali additives on N2 chemisorption has important implications for ammonia synthesis on iron, where alkali promoters (in the form of K or K20) are used in order to increase the activity of the iron catalyst. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The Ru metal area was determined by volumetric H2 chemisorption in the quartz U-tube of an Autosorb 1-C set-up (Quantachrome) following the procedure described in ref. [16]. Prior to chemisorption, the catalysts were activated by passing 80 Nml/min high-purity synthesis gas (Pnj / Phj -1/3) from a connected feed system through the U-tube and heating to 673 K for alkali-promoted catalysts or to 773 K for alkali-free catalysts with a heating rate of 1 K/min. The BET area was measured by static N2 physisorption in the same set-up. 

Results of the H2 chemisorption measurements after NH3 synthesis based on H/Ru = 1/1. NHs synthesis was run at 773 K with Ru/MgO and RU/AI2OS, and at 673 K with all alkali-promoted catalysts. The mean particle size was calculated assunung spherical particles. 

Studying the kinetics of the interaction of N2 with the Ru catalysts revealed that the Cs promoter enhances both the rate of dissociative chemisorption and the rate of recombinative desorption. Ru catalysts were found to be rather inactive for NH3 synthesis without alkali... 

The resonant level model readily explains the change in work function associated with chemisorption. It is well known that alkali atoms such as potassium lower the work function of the substrate, whereas electronegative atoms such as chlorine increase the work function [2,8,19]. Figure A. 10 indicates that potassium charges positively and chlorine negatively when adsorbed on jellium. Remember that the surface contribution to the work function is caused by... 

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