X-ray photo-electron spectroscopy combined with Ar+-etching showed that H2 reduction of Ni/TiO2 precursors (773K, 1h) led to small metallic clusters of nickel which diffused towards the bulk of the reduced TiO2 support. Back diffusion of the metallic phase toward the TiO2 surface was achieved by re-oxidation with O2 up to 673K. This latter process was accompanied by re-oxidation of the nickel to its initial conditions. The results were explained in terms of the well known SMSI state in this type of catalysts.

Diffusion of Nickel and Surface Reconstruction in Ni/TiO2 Catalysts Promoted by H2 and O2 Treatments. Espinós, J.P., González-Elipe, A.R., Munuera, G.: Solid State Ionics, 1993, 63-65, 748-54

Table 4

Self-diffusion of 18O in TiO2 single crystals at 806C

Specimen

Condition

Orientation

D (cm2/s)

slab

stoichiometric

perpendicular to c-axis

1.7 x 10-15

slab

stoichiometric

parallel to c-axis

3.2 x 10-16

slab

vacuum-reduced, 1h

perpendicular to c-axis

6.7 x 10-15

slab

vacuum-reduced, 5h

perpendicular to c-axis

6.2 x 10-15

slab

vacuum-reduced, 10h

perpendicular to c-axis

6.6 x 10-15

slab

air-reduced, 1h

perpendicular to c-axis

6.1 x 10-15

slab

air-reduced, 5h

perpendicular to c-axis

5.9 x 10-15

slab

air-reduced, 10h

perpendicular to c-axis

6.3 x 10-15

grain

stoichiometric

-

2.1 x 10-15

grain

vacuum-reduced, 1h

-

7.3 x 10-15

grain

air-reduced, 1h

perpendicular to c-axis

6.8 x 10-15