Zeolites reduction conditions effects

Rhodium catalysts were prepared by hydrogen reduction at atmospheric pressure of a cationic organometallic rhodium complex and anchored onto lamellar and zeolitic products. The effect of the structure and characteristics of the support on metal load and dispersion was studied in the heterogeneous catalysts prepared. The new rhodium catalysts were applied in the hydrogenation of acetone. The reaction was carried out under milder conditions. 

The efficiency of the above catalysts for NO reduction depends definitely on the kind of metals and their loadings onto supports, the type of reductants and the feed gas composition employed as well as on the kinds of supports and structure of the parent zeolite and its historical nature during preparation. In particular, the effect of the presence of H2O and SO2 in the exhaust gas from mobile sources is well documented on the maintenance of time-on-stream deNOx activity of SCR catalysts, and their resistance to these co-existing gases is an essential parameter determining successful applications to engine sources. The durability of the documented catalysts under hydrothermal conditions should also be considered to verify if those were applicable to controlling vehicle NOx... 

Most flue gases from the combustion sources contain, in addition to NO, HjO vapor and SO2, which may cause SCR catalysts to lose activity. Therefore, sulfur and water tolerances of catalysts are needed for the successful commercial application of the catalytic process. Since a dramatic loss of NO removal activity of transition-metal ion-exchanged zeolite catalysts was observed for NO reduction by hydrocarbons at wet conditions [1,4,9, 11-13], severd studies have focused on the cause of activity loss by HjO [14-18]. However, effects of SOj on SCR catalysts employing hydrocarbons as a reductant have received considerably less attention. 

The interaction of CO with Ni -exchanged zeolites has been studied by i.r. spectroscopy. It was concluded that, under special preparative conditions, a fraction of Ni ions is reduced by CO to Ni with subsequent formation of Ni -CO complexes characterized by CO stretching frequency located at 2205 cm. Reduction of Ni ions in hydrated Y- and M-type zeolites occurs also when the catalyst is dosed with NO and the formation of Ni -NO complexes is observed (y = 1895 cm" and characteristic e.s.r. signal).On activated Ni M-zeolites, the CO interaction causes the formation of Ni (CO)2 dicarbonylic complexes, whose structure has been well documented by CO- CO isotopic substitution experiments. The effect of CO is to stabilize Ni ions formed by the reverse disproportionation reaction... 

Palladium II complexes are also known as efficient homogeneous catalysts of ethylene dimerization. Interestingly Pdll zeolites, even though they convert ethylene under mild conditions, showed poor selectivity towards butenes. Reduction of palladium had an adverse effect on the activity and the selectivity, which is consistent with bivalent palladium as the active site for oligomeraition... 

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