ROLE OF CL- ADSORBED ON SILVER-LOADED ZIRCONIUM-PHOSPHATE FOR THE PHOTOOXIDATION OF OH- TO OH-CENTER-DOT

When a silver-loaded zirconium phosphate [Ag1-xHxZr2(PO4)(3)] suspensi on was irradiated with a 500 W Xe lamp, the generation of OH. was obse rved as the DMPO-OH using an ESR spin-trapping technique. The generati on rate of DMPO-OH changed by adding various ions to the suspensions. The addition of Fe3+, which has a redox potential of +0.771 V vs. NHE, increased the generation rate of DMPO-OH, indicating that the ion act ed as an electron acceptor. In the presence of I-, the generation rate of DMPO-OH decreased by oxidizing I- with OH.. However, in the presen ce of Cl-, the generation rate remarkably increased with the increase in the concentration of Cl-. The quantum yield of the generation rate of DMPO-OH was about 10 times larger than that in the absence of Cl-; at 400 nm from 0.36% to 2.7%, and at 500 nm from 0.037% to 0.41%. Such an effect of Cl- could not be observed in the absence of Ag1-xHxZr2(P O4)(3). Furthermore, the generation rate of DMPO-OH during loading of Cl- into the suspension showed a pH dependence with the maximal value at pH ca. 5.8. From a Lineweaver-Burk plot, K-m and Vmax(DMPO-OH) were determined to be 83.3 mu mol dm(-3) and 0.44 mu mol dm(-3) min(-1), r espectively. These results suggested that the adsorption of Cl- on the surface lead to the formation of an active site like a catalysis cent er in the enzyme. Cl- adsorbed on a Zr atom not on a Ag atom in the su rface region, because of the ca. 1 eV shift in binding energy at the p eak of only Zr-3s1/2 in the XPS spectra of the Cl--adsorbed Ag1-xHxZr2 (PO4)(3). Consequently, the Cl--adsorbed Zr atom on the surface was su ggested to act as an electron pool in the photo-induced charge separat ion. (C) 1998 Elsevier Science S.A.