Anatase surfaces

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

H2 adsorption is weak on the anatase surfaces [8], No dissociative adsorption of H2 takes place over the smooth surfaces of Au at temperatures below 473 K [9,10]. On small Au particles, adsorption is possible at low temperature. Dissociative adsorption of H2 can be accelerated by the negatively charged molecular oxygen species at steps, edges, comers of Au particles [5]. 

The authors assume that the activation of Mg(OH)2+Ti02 (anatase) mixture involves the transition of proton from OH groups on anatase surface to the hydroxyl group of Mg(OH)2 which results in the formation of water and Mg-O-Ti bond, the latter being a nucleus for the formation of MgTiOj phase during consequent calcination and agglomeration. 

The essential point arising from the above discussion is that the saturation coverage of titanium dioxide with the surface-bonded peroxo species, photogenerated in alkaline or neutral solutions, ranges most likely from 4 to 5 peroxo groups per nm . The latter value is close to the total number of active OH groups present initially on each nm of fully hydroxylated (and unilluminated) anatase surface. 

Bonapasta AA, Friippone F (2005) Photocatalytic reduction of oxygen molecules at the (100) HO anatase surface. Surf Sci 577 59... 

Haase et al. observed spreading of V2O5 on the anatase surface at 873 They also showed that the spreading was enhanced in an air atmosphere containing HCl gas, under which highly mobile oxychlorides are formed. Del Arco et al. reported the... 

The proposed mechanism also explains why the slopes of the H2 chemisorption versus In t curve for Rh on anatase were smaller than those for Rh on rutile. For the Ti + cations at the rutile surface are predicted to be more easily reduced than those at the anatase surface (30). 

Figure 3. The anatase surface shows zigzag chains of Ti surface octahcdra. Each Ti site carries a =TiOH ligand that may bind adsorbed metal centers. The two surface groups to the right represent a bidentate (above) and a tridentate surface complex (below).

The results were different in one important respect from the earlier ones for photolysis of Fe(III) complexes in solution. The Fe(II) yield here showed very little dependence on t-butanol concentration and indicated a primary quantum yield of 0.170, approximately half the extrapolated intercept for methanol of 0.330. In ferric perchlorate photolysis studied earlier, both alcohols gave a common extrapolated yield equal to the independently determined primary yield for hydroxyl radical production. In other words, at the TiOa surface we appear to generate more "OH radical for reaction with CHaOH than "OH radical" to react with t-butanol. From these results, we infer that a hole reaching the anatase surface may produce one of two distinct oxidants in approximately equal quantity. These are a species capable of abstracting hydrogen (e.g. the OH radical) and a second less reactive oxidant. Preliminary results from parallel experiments... 

Donors of appropriate redox potential to react with holes at the anatase surface include organic acids, carbohydrates, fats, CN, and halides 2 ). (The cyanide reaction has been studied for its utility in treatment of the waste streams from Hold mininK operations in the Canadian Northwest Territories.) More immediately releyant to natural water is the observation that an anatase slurry could effect the decoloration of a chlorinated bleach plant effluent. A sample of amber colour, pH = 1.8, and low residual chlorine was irradiated in the presence of 0.5% (wt) anatase with li((ht of 350 nm for periods up to 18 hr. The optical absorbance decreased by half in 1080 min. Small amounts of chloride and formaldehyde were detected ( ). This reaction may provide a precedent for observation of a relation between photobleachinK of humics in water and metal ions. If so, we are brouj ht to the question of the reactivity of colloidal iron oxides. 

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