Titania surface reduction

Lanthanides in combination with transition metals have been shown to have a positive effect in promoting heterogeneous catalytic reactions. The bimetallic Yb—Pd catalyst obtained from the precursor (pMF)i0Yb2 Pd(CN)4]3 K on a titania surface offers improved performance over a palladium-only catalyst for the reduction of NO by CH4 in the presence of 02.99 100 The structure, shown in Figure 6, consists of two inverted parallel zigzag chains that are connected through the lanthanide atoms by trans-bridging [Pd(CN)4]2- anions.101... 

To see that this is true, qualitative models of the titania surface produced following each of the three pretreatments used in this study (i.e., evacuation at 600 K, evacuation at 720 K, and hydrogen reduction at 720 K) are developed below on the basis of earlier studies of Ti02 surfaces (5-6,10-22). Surface Type I Following outgassing at about 600 K the Ti02 surface should be almost entirely free of molecular water (except on the rutile fraction), but about one half of the surface should be covered... 

An appreciable amount of Ti + may also exist in such samples. Indeed, materials treated at this temperature are gray, suggesting the presence of reduced forms of titania. Surface Type III Hydrogen reduction at 720 K probably produces a surface in which there is no molecular water and only a small number of hydroxyl groups. Furthermore, the surface following this treatment may have a high concentration of Ti + species. Materials treated in this manner were found to be pastel blue in color. The surface "type" of each sample is given in Table I and II. 

The solid-state Si SPE NMR spectra of SBA-15 and the titania surface-coated SBA-15 (Ti-SBA-15) are in accord with this expectation. The spectrum of SBA-15 displays a broad as)mimetric peak at 109 ppm (Q" sites) with shoulders at —101 ppm (Q sites) and 90 ppm(Q sites) in the area ratio 79 19 2. The NMR spectrum of Ti-SBA-15 (one layer) shows a reduction of the band intensity relative to the intensity. The normalized Q Q Q site populations become 85 13 2. No asymmetry is observed in the Q site band. Repetition of the monolayer deposition to form a double layer of titania on silica yields a material whose Si NMR spectrum is indistinguishable from that of the Ti-SBA-15 with a monolayer coverage. As expected, the titania-insulated silica resonances are unperturbed by the second titania layer. 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

It appears, however, that support surfaces are not always as refractory to reduction as chemical intuition would dictate. An important new finding is that lanthanum oxide undergoes reduction, in the presence of a supported metal, to "LaO", and the properties of Pd/lanthana are similar in several respects to those of metal/titania (43,44). Even with alumina supports, surface reduction has been found in some instances (45-471. The reason for this anomalous behavior is not fully understood, although sulfur has been found capable of promoting the reduction (46). A recent report has described suppressed H2 chemisorption on Rh/zirconia (4 ) although this was not found in an earlier study of Ir/zirconia (2). One may suspect differences in surface reducibility between the supports used in the two cases. 

The surfaces of the as-received materials are covered by O and/or OH species, as indicated by the peak at 1.098 GkHz Remarkably, these surfaces were effectively cleaned simply by holding the electrode potential within the electrochemical double-layer region. This mild surface-reduction procedure should be contrasted to the rigorous and often technically demanding methods employed at the solid-gas interface, which usually involve several cycles of high-temperature calcination and reduction, limiting the choice of catalyst support to, typically, non-conductive oxides such as alumina, titania, and silica. 

The present study revealed effects of various rutile/anatase ratios in titania on the reduction behaviors of titania-supported cobalt catalysts. It was found that the presence of rutile phase in titania could facilitate the reduction process of the orbalt catalyst. As a matter of fact, the number of reduced cobalt metal surface atoms, which is related to the overall activity during CO hydrogenation increased. 

The present research showed a dependence of various ratios of rutile anatase in titania as a catalyst support for Co/Ti02 on characteristics, especially the reduction behaviors of this catalyst. The study revealed that the presence of 19% rutile phase in titania for CoATi02 (C0/RI9) exhibited the highest number of reduced Co metal surface atoms which is related the number of active sites present. It appeared that the increase in the number of active sites was due to two reasons i) the presence of ratile phase in titania can fadlitrate the reduction process of cobalt oxide species into reduced cobalt metal, and ii) the presence of rutile phase resulted in a larger number of reduced cobalt metal surface atoms. No phase transformation of the supports further occurred during calcination of catalyst samples. However, if the ratios of rutile anatase were over 19%, the number of active sites dramatically decreased. 

The reduction of the catalyst precursor with sodium formate resulted in a lower Pd dispersion than the catalyst prepared by hydrogen reduction, the particle size is much larger in the former catalyst. The mesoporous carbon supported Pd catalysts are near to those of Pd on titania with respect to their enantiodifferentiating ability. Besides the metal dispersion, the availability of the Pd surface in the pores for the large modifier molecules seems to be the determining factor of the enantioselectivity. 

Figure 9.7 Transmission electron microscopy of rhodium particles on a model titania support after reduction in H2 at 200 °C (top) and the same catalyst in the SMSI state after reduction at 500 °C (bottom). An amorphous overlayer on the surface of the SMSI catalyst is clearly discerned (from Logan etal. [25]).

---