Work function, and adsorption

Boudart (26) suggests that the presence of the electrical double layer produced by the surface dipoles can account for the observed fall in the heat of adsorption and change in work function as the surface coverage is increased. Furthermore, assuming that the dipole interaction is negligible, as will be the case for small surface coverages, the heat of adsorption and work function changes should be related by the equation... 

The reaction was investigated under atmospheric pressure and at temperatures 500°C to 600°C, where the only product was CO, as Pd, contrary to Rh, does not adsorb C02 dissociatively.59 This difference in reaction pathway is also reflected in the NEMCA behaviour of the system, since in the present case CO formation is enhanced (by up to 600%) not only with decreasing catalyst potential and work function, but also enhanced, although to a minor extent, via catalyst potential increase (Fig. 8.56). Enhancement factor A values up to 150 were measured. The reaction exhibits typical inverted volcano behaviour, which is characteristic of the weak adsorption of the reactants at the elevated temperature of this investigation, and thus of promotional rule G4. 

Bertel, E. and Netzer, F.P. (1980) Adsorption of bromine on the reconstructed Au(lOO) surface LEED, thermal desorption and work function measurements. Surface Science, 97, 409-424. 

We should note that adsorption of acceptor particles on oxide semiconductors of p-type influences their electric conductivity and work function in the opposite way. As for donor particles such as atmns of H, Na, K, Zn, Cd, Pb, Ag, Fe, Ti, Pt, Pd and many others, their adsorption at medium and low temperatures (when there is no notable diffusion of atoms proper into the crystal and, consequently, there is no substitution of atoms created, the latter obeying the Vervey rule) is always accompanied by increase in electric conductivity and decrease in the work function for semiconductor adsorbent of -type, the opposite being valid in case of p-type adsorbent. 

In general, the peculiarities of the surface effects in thin semiconductors, for which application of semi-infinite geometry becomes incorrect were examined in numerous papers. As it has been shown in studies [101, 113, 121 - 123] the thickness of semiconductor adsorbent becomes one of important parameters in this case. Thus, in paper [121] the relationship was deduced for the change in conductivity and work function of a thin semiconductor with weakly ionized dopes when the surface charge was available. Paper [122] examined the effect of the charge on the temperature dependence of the work function and conductivity of substantially thin adsorbents. Papers [101, 123] focused on the dependence of the surface conductivity and value of the surface charge as functions of the thickness of semiconductor and value of the surface band bending caused by adsorption and application of external field. 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

The significance and impact of surface science were now becoming very apparent with studies of single crystals (Ehrlich and Gomer), field emission microscopy (Sachtler and Duell), calorimetric studies (Brennan and Wedler) and work function and photoemission studies (M.W.R.). Distinct adsorption states of nitrogen at tungsten surfaces (Ehrlich), the facile nature of surface reconstruction (Muller) and the defective nature of the chemisorbed oxygen overlayer at nickel surfaces (M.W.R.) were topics discussed. 

Fig. 6-34. Adsorption coverage of hydroxyl radicals on, and work function of, a platinum (111) surface plane observed as functions of coverage of potassium atoms coadsorbed with water molecules adsorption of water vapor takes place on a potassium-adsorbed surface of platimun at 305 K. 6k = coverage of adsorbed potassium atoms 6oh = coverage of hydroxyl radicals adsorbed by partial dissociation of water molecules A

Charge transfer resistance, 1056 Charge transfer overpotential, 1231 Charge transfer, partial. 922. 954 Charges in solution, 882 chemical interactions, 830 Charging current. 1056 Charging time, 1120 Chemical catalysis, 1252 Chemical and electrochemical reactions, differences, 937 Chemical equilibrium, 1459 Chemical kinetics, 1122 Chemical potential, 937, 1058 definition, 830 determination, 832 of ideal gas, 936 interactions, 835 of organic adsorption. 975 and work function, 835... 

According to section I, the electronic interaction between an adsorbing metal surface and adsorbed foreign molecules is best studied by observation of the electronic properties of the catalyst surface before and after adsorption. The work function can be measured directly by photoelectric,... 

Fig. 11. Adsorption energy as a function of the CO coverage and work function change on Pd(Ill) (87).

The previous results show that Rh sites are those with the higher activity for NO dissociation. What do we know about CO adsorption Does CO prefer Rh or Pt sites These questions were addressed by Rutten ei at (145) in an investigation of CO adsorption on a Pto.as- Rho.75 (111) surface with RAIRS, TDS, and work function measurements. It was found that CO shows a pronounced preference for Rh sites at both 100 and 300 K up to a coverage of 6 0.27. Adsorption was found to be predominantly... 

More recently Germer and co-workers " " have developed a different type of apparatus which allows the entire diffraction pattern to be displayed on a fluorescent screen. Both variations of the leed technique are rather difficult to apply to the measurement of adsorption kinetics, which in effect requires a measurement of the change in the intensity of the diffracted beam(s) with time. This intensity change is not necessarily proportional to the rate of adsorption however, detailed data on the adsorption of oxygen and carbon monoxide on nickeF have been obtained by a combination of leed and work function measurements. 

Recently S. D. Parker has studied Li adsorption on Ag(lll) using AES, LEED and work function measurements. The Auger data indicated that initial deposition led to the formation of a monolayer of Li. During the growth of this monolayer some diffusion of Li into the substrate occurred and this became more and more pronounced towards the completion of the layer. The AES... 

Gomer and Bell (35) report that the virgin phase appears to be stable up to about 170°K. This is in close agreement with the 200°K at which Klein (12) observed a reorganization of the pseudo-clean pattern obtained after CO adsorption at 20°K. Edein (12) also reported slight changes in the observed Fowler-Nordheim pre-exponential factor and work function above this temperature. 

A radiotracer study similar to that described by Crowell et al. (85) using i CO adsorbed on Ni(lOO) and Ni(llO) surfaces has recently been reported (211). This work confirms the previous results (151) that little CO is adsorbed on a surface only cleaned by heat treatment. On argon bombardment cleaned crystals, the results indicated a single type of binding state for CO adsorbed at all coverages. A very recent LEED and work function study of the type described on the Pd(lOO) surface (163) (Section E) has been carried out on a Ni(lOO) surface (212). A series of compression structures qualitatively very similar to those seen on the Pd(100) surface were observed. These results appear to support the idea of a continuum of adsorption states, as suggested by the radiotracer study (211). 

The similarity of behavior of NH3 on the W(IOO) and W(112) faces is illustrated by comparing the two sets of LEED experiments. In both of them adsorption of NH3 on the clean surface was followed by thermal breakup resulting from heating. Desorption into a mass spectrometer and work function measurements were made concurrently. 

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