High-temperature reduction , effects

High-temperature reactions with vacuum microbalance, 5 119 High-temperature reduction, 34 19 effects on titania-supported metals, 36 176-177, 180... 

It is perhaps worthwhile noting that the promoter patches on the metal can be either created during the wet steps of the catalyst preparation, or when a transition metal oxide is used as a support, they can be created by migration of the support material on the metal upon high temperature reduction. The metal surface can be kept almost completely covered in vacuo (SMSI effect) [52], but in the presence of CO or of the reaction mixture, the layer of oxide recrystallizes and the metal surface becomes accessible again from the gas phase [33]. 

Ceria/noble metal (such as Ru, Rh, and Pd) catalysts are composed of noble metal species such as nanoparticles and clusters dispersed on the ceria supports. The catalysts show typical strong metal-support interactions (SMSI) (Bernal et al., 1999), that is, the catalysts exhibit a number of features for SMSI effects including (1) reducible supports (2) "high temperature" reduction treatments (3) heavily disturbed chemical properties and significant changes in catalytic behavior of the dispersed metal phase (4) reversible for recovering the conventional behavior of the supported metal phase. In these cases, the reducibility of ceria NPs is greatly enhanced by the noble metal species and the catalytic activities of the noble metals are enhanced by ceria NPs. 

Effect of the pretreatment Nuhez and Rouco (ref. 9) found a pronounced decrease in H2 adsorption on Pd/Si02 catalysts after a high temperature reduction and attributed this effect to the presence of strongly bound... 

This promotion of C=0 activation in these systems cannot be accounted neither for a geometric effect nor for an electronic effect in an SMSI-like state. Actually, this is better explained by the creation of new catalytic sites, mixed sites, very active for C=0 hydrogenation (Figure 14). However, electronic effects were also considered as operating at a second level, in Pt/ZnO catalysts, for instance [122]. Upon high-temperature reduction, Pt -Zn entities form an electron enrichment of Pt would occur, which repels the C C bond farther from the Pt surface. 

Similar effects on reduction/oxidation have been observed in other studies but not necessarily explained in the same way. One obvious area of explanation involves sintering and then re-dispersion of the platinum particles under oxidizing conditions possibly by molecular migration. It has also been suggested that apparent loss of dispersion results from decomposition of platinum particles or clusters into an atomic form, which becomes incorporated into the alumina support.High-temperature reduction also decreased hydrogenolysis activity (restored by oxidation at 773 K, followed by reduction at 673 K) but the effect appeared to be specific to platinum itself because similar results could be obtained with unsupported platinum black. ... 

Thus, it is quite conceivable that the metal-titania interaction Influences the CO-H2 synthesis reaction through effects that are concentrated at the contact perimeter and that, since reduction begins here, high temperature reduction is not required for changes in CO-H2 synthesis properties to be observed. On the other hand, major suppression of H2 (or CO) chemisorption, or of hydrogenolysis activity, requires that nearly the whole metal particle be affected. Even if suppression does not require... 

Three main properties have come to characterize the SMSI effect (1). The first of these is a diminished activity toward the chemisorption of H2 and CO induced by the high temperature reduction of a supported metal catalyst (low temperature reductions are ineffective). The second is a significant alteration of the catalytic... 

The reversibility of this phenomenon indicates that during high temperature reduction a reduced sulfur species is formed which migrates to the Pt surface and poisons the Pt surface. Subsequent oxygen treatment oxidizes this sulfur species and the oxidized species migrates back to the support. The oxidized species do not leave the catalyst since the reduction - oxidation sequence produces the same effects over several cycles (see Runs 6 to 9, Table III). 

The effect of high-temperature reduction on polymerization activity is shown in Table 2 for one experiment. Cr/silica was activated in air at... 

The present work reports a promoting effect of the CeQ gZro 4O2 mixed oxide on the catalytic reduction of NO by CO at moderate temperatures. This effect is attributed to the Ce4+/Ce3+ redox couple which efficiently reduces NO. The interaction of the Pd/Ceo.gZro 4O2-AI2O3 catalysts with H2 reveals that considerable metal particle encapsulation occurs after a high temperature reduction. This suggests that the choice of an appropriate metal particle size may be an important factor to avoid the catalyst deactivation. Finally, a methodology for the determination of the H/Pd ratios is discussed. 

---