Oxygen, addition effect activation

The above-mentioned results indicate the additive effect of protons. Actually, a catalytic process is formed by protonation of the metal-oxygen bond instead of silylation. 2,6-Lutidine hydrochloride or 2,4,6-collidine hydrochloride serves as a proton source in the Cp2TiCl2-catalyzed pinacol coupling of aromatic aldehydes in the presence of Mn as the stoichiometric reduc-tant [30]. Considering the pKa values, pyridinium hydrochlorides are likely to be an appropriate proton source. Protonation of the titanium-bound oxygen atom permits regeneration of the active catalyst. High diastereoselectivity is attained by this fast protonation. Furthermore, pyridine derivatives can be recovered simply by acid-base extraction or distillation. 

Compared to Pt or Rh, Pd presents specific properties, such as a good thermal stability and an ability to keep a good activity under an excess of oxygen Nevertheless, the activity of Pd for the reduction of NO by CO is lower than that of Rh and attention paid to increase its activity for the NO reduction by additive effects. 

Then they investigated the effect of oxygen addition on the WGS activity of 0.5% Pd/Ti02 catalyst. As shown in Figure 8.6 the WGS activity decreases when they injected 0.2 mL of O2 into the reaction chamber. 

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