Work function alkali metals, effect

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

Photoelectron spectroscopy involves the ejection of electrons from atoms or molecules following bombardment by monochromatic photons. The ejected electrons are called photoelectrons and were mentioned, in the context of the photoelectric effect, in Section 1.2. The effect was observed originally on surfaces of easily ionizable metals, such as the alkali metals. Bombardment of the surface with photons of tunable frequency does not produce any photoelectrons until the threshold frequency is reached (see Figure 1.2). At this frequency, v, the photon energy is just sufficient to overcome the work function

[Pg.289]

Figure 9.10 Up work function of alkali-promoted metals as a function of alkali coverage (see also Table 9.2). Down electrostatic potential around a single alkali atom adsorbed on jellium. The effective local work function at each position is the sum of the substrate work function and the value of the electrostatic potential in the figure (from Lang el at. [39]).

De Boer (130) first drew attention to the contribution of the work-function effect to the heat of chemisorption of alkali atoms on a metal surface. With Cs on W, for example, the heat of adsorption is described by the equation,... 

The only single-phase item involved is the solution heat exchanger, which is intensified by the use of laminar flow in a matrix of fine channels. A sketch of the single-effect Rotex (13) design is shown in Figure 15, where it can be seen that a hermetically sealed rotating disc assembly fulfills the four functions listed. The working fluid consists of a water solution of either mixed alkali metal hydroxides or lithium bromide. 

The transition from the atom to the cluster to the bulk metal can best be understood in the alkali metals. For example, the ionization potential (IP) (and also the electron affinity (EA)) of sodium clusters Na must approach the metallic sodium work function in the limit N - . We previously displayed this (1) by showing these values from the beautiful experiments by Schumacher et al. (36, 37) (also described in this volume 38)) plotted versus N". The electron affinity values also shown are from (39), (40) and (34) for N = 1,2 and 3, respectively. A better plot still is versus the radius R of the N-mer, equivalent to a plot versus as shown in Figure 1. The slopes of the lines labelled "metal sphere" are slightly uncertain those shown are 4/3 times the slope of Wood ( j ) and assume a simple cubic lattice relation of R and N. It is clear that reasonably accurate interpolation between the bulk work function and the IP and EA values for small clusters is now possible. There are, of course, important quantum and statistical effects for small N, e.g. the trimer has an anomalously low IP and high EA, which can be readily understood in terms of molecular orbital theory (, ). The positive trimer ions may in fact be "ionization sinks" in alkali vapor discharges a possible explanation for the "violet bands" seen in sodium vapor (20) is the radiative recombination of Na. Csj may be the hypothetical negative ion corresponding to EA == 1.2 eV... 

The other major body of work relating to catalytic hydrogenation in the liquid phase is due to Sokol skii and co-authors. In a succession of papers he reports studies of the hydrogenation of 3-butyn-l-ol on Pd at open-circuit and controlled potentials, a number of alkynes" such as hexyne and octyne, the effect of alkali-metal cations, the hydrogenation of DMVC on Pd as a function of composition open circuit and driven hydrogenation of furfural and furfuryl alcohol at a Pd-Pt surface and most recently Ni catalysed reduction of pyridine. In the more recent of these papers the use of product analysis methods affords a further insight into the process and... 

To function as an effective polyelectrolyte, the polymer should have a low glass transition temperature to allow the freedom of molecular movement necessary for ion transport. While the advantages of PEO-alkali metal complexes are their good electrochemical stability as well as faster kinetics at the ion transfer between electrode and electrolyte, the disadvantage is the low conductivity of the polymeric electrolyte at normal operating temperatures. Thus, in order to obtain high-power densities, it is necessary to work... 

---