t o I Ti 0 2 \ \ \ semiconductor Flgurb 5. Schematic description of proposed rate-controlling step in the watergas shift reaction.Since neither loadings greater than 2 wt % Pt nor the presence/absence of SMSI Pt has a significant influence on the rate, it is not surprising that catalysts prepared by the photodecomposition and impregnation methods have almost the same activity. The latter is preferred operationally just because the former is much more time-consuming and more ~omplicated.~ As noted earlier, adding NaOH gives the greatest improvement in the rate, the 3.5 wt % NaOH (sample 10) having 16 times the rate of the corresponding 0 wt % material (sample 14). Earlier work from this laboratory gave a factor of 2.5 rate increase for a 4 wt % NaOH loading. We attribute this difference to larger surface areas intercepting radiation in the present experiments since the materials used here were ground in an agate mortar after platinization but before adding NaOH. Significant enhancement by OH-of the photoassisted water decomposition reaction on the SrTi03 system has also been reported by Wagner et al." (11) (a) Wagner, F. T.; Somorjai, G. A. Nature (London) 1980, 285, 559. (b) Wagner, F. T.; Somorjai, G. A. J. Am. Chem. SOC. 1980, 102,
5494.The light-intensity study shows that for our conditions there is no evidence for rate control involving reactions between more than one photogenerated species. Since the rate is first order in the light intensity, we do expect that one photoderived species will participate. Moreover, the quantum efficiency is no more than a few percent so the process is noncatalytic in photons. Since electrons are the majority carriers in our n-type titania and since the reaction requires band-gap radiation, we propose that holes, the minority carriers, are involved in the rate-determining step of the mechanism previously proposed3 which is also probably the rate-determining step in the photoassisted liquid water decomposition reaction over NaOH/Pt/Ti02 ~atalysts.~ This kind of rate-controlling process is shown schematically in Figure 5.
ConclusionsThe present study establishes several important features concerning the performance of NaOH/Pt/Ti02 substrates in the photoassisted water-gas shift reaction and suggests that the process in which hydroxide ion donates its electron to the surface hole is rate determining. Those parameters which increase the surface hole population, like reduction in H,, higher light intensity, and higher NaOH loading, suggest that improved hole generation rates, electron-hole separation, and hole utilization at the surface are useful routes to improve performance.
Acknowledgment.We thank Professor H. Steinfink for the use of an XRD-5 diffractometer in his lab. S.M.F. thanks Professor S. Sat0 for considerable assistance in learning the techniques employed here.The kinetics of the reaction of SO, with SO2 to form the free radical complex S204-has been studied in solutions of tetraethylammonium perchlorate in N,N-dimethylformamide with simultaneous electrochemical-electro...