Selective XAFS measurements of 0.6-3.0 wt% V sites on/in TiO 2 were enabled utilizing a fluorescence spectrometer. The V sites were V IV species in the case of high-surface-area V-TiO 2 in contrast to V V species in the case of conventional V/TiO 2 catalysts, V supported on HSA-TiO 2 , and V-TiO 2 prepared in the absence of dodecylamine (general sol-gel method).Vanadium catalysts are used for the reduction of nitric oxide with ammonia, 1 the oxidation of naphthalene/o-xylene to phthalic anhydride, and the oxidation of butane to maleic anhydride. 2 The optimal catalytic performance in these applications has been reported to correspond to concentrations of V of less than a monolayer, dispersed on TiO 2 . When the concentration of V corresponds to monolayer levels, major V species has been proposed to be a monomeric monooxo 3 or dioxo vanadate, 4 polyvanadates 3 such as decavanadate ([V 10 O 28 ] 6À ), 5 or to an epitaxial V 2 O 5 (010) layer over TiO 2 6 by Raman, 51 V NMR, UVvisible, etc.XAFS directly determines the local structure of non-crystalline materials. However, exceptionally it is very difficult to measure XAFS data for low concentrations of V in the TiO 2 matrix. When 0.6 wt% of V (1.1 wt% V 2 O 5 ) is mixed with TiO 2 , the V K-edge jump is only 0.037 compared to a total absorption of 4 in transmission mode. The photon number ratio of V K 1 /Ti K 1 is only 0.012 in fluorescence mode. When a solid-state detector (SSD; ÁE % 100 eV) is used, the V K 1 (4952.2 eV) peak can be separated from the Ti K 1 (4510.8 eV) peak. However, the Ti K 1;3 (4931.8 eV) still overlaps the V K 1 . The ratio of V K 1 /Ti K 1;3 is 0.058. The selective detection of V K 1;3 (5427.3 eV) is difficult using the SSD because scattered X-rays overlap. Auger or secondary photoelectrons derived from V are selectively monitored using an electron energy analyzer. However, these experiments need ultra-high vacuum, and in-situ measurements are impossible. In this Letter, XAFS combined with fluorescence spectrometry 7;8 was applied to selectively monitor low concentrations of V on/in TiO 2 .TiO 2 (P25, 60 m 2 g À1 ) was impregnated with V triisopropoxide oxide (1) in isopropanol solution (V/TiO 2 ). A high surface area (HSA; 1200 m 2 g À1 ) V-TiO 2 was prepared from compound 1, Ti tetraisopropoxide (2), and dodecylamine. 9 An aqueous solution was kept at 333 K for six days, and filtered. The obtained powder was heated at 453 K for ten days, and then washed by p-toluenesulfonic acid/ethanol. HSA-TiO 2 prepared in a similar procedure was impregnated with compound 1 (V/HSA-TiO 2 ). V-TiO 2 was prepared from compounds 1 and 2 in the absence of docecylamine in a similar manner to the case of HSA V-TiO 2 . In all cases, the dried powder was heated in air at 473 K and pressed into a disk in ambient air.The XAFS spectra were measured at KEK-PF 7C. The storage ring energy was 2.5 GeV, and the current was 390-270 mA. A Si(111) double crystal monochromator was used. The beam was focused and fully tuned. The X-ray fluorescence from the sample was analyze...