is formed. Slowly a t 240 K and more rapidly at higher temperatures the a C-H bond is cleaved and the pyridine is converted to an a pyridyl state characterized by the ring becoming perpendicular to the surface. This transformation may be reversible to an extent of approximately (26) It is interesting to note that for high coverages a pyridine desorption is evident at higher temperatures (-470 K) and thus disproportionation of the pyridine might be invoked to provide for a small degree of reversibility of the formation of the a pyridyl. More detailed thermal desorption studies and EELS results will be presented by Grassian, V. and Muetterties, E. L. (in preparation).(27) The gas-phase Is binding energy relative to the vacuum level of carbon in pyridine is not known but should be between 290.2 eV (C 1s for C6Htna) and 293.5 eV (C 1s for HCNZgb); the nitrogen 1s binding energy in pyridine is 404. Analysis of in situ EXAFS measurements on a 2.4 wt % Rh/A120, catalyst, reduced a t 473 K after calcination a t 623 K, shows the presence of two different rhodium-oxygen bonds (viz. 2.05 and 2.68 A). The oxygen neighbors of rhodium a t a distance of 2.05 A disappear after reduction a t 673 K. The coordination distance 2.05 A is the same a s found for R h 2 0 3 and indicates that this rhodium-oxygen bond is present due to incomplete reduction of the catalyst a t 473 K. Rhodium-oxygen bonds with a coordination distance of 2.68 A are present a t both reduction temperatures. These bonds arise from a coordination of the interfacial rhodium atoms with the oxygen ions of the support. Assuming a half-spherical shape for the rhodium metal crystallites, numerical values for the average interfacial rhodium-oxygen coordination number (N = 2.5) and the crystallite diameter (d = 11 A) a r e derived from the experimental E X A F S parameters.
IntroductionThe interaction between metal crystallites and the support can have a significant influence on the catalytic behavior of a suppofid
Electrolysis of a 22 wt % NaOH solution has been carried out in a vertical tall rectangular cell with two segmented electrodes. The ohmic resistance of the solution between a segment pair has been determined as a function of a number of parameters, such as, current density and volumetric rate of liquid flow. It has been found that the ohmic resistance of the solution during the electrolysis increases almost linearly with increasing height in the cell. Moreover, a relation has been presented describing the voidage in the solution as a function of the distance from the electrodes and the height in the cell.
Notation
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