Palladium catalyst oxidation-reduction conversion

A sputtered palladium catalyst exhibited low conversion and large deactivation of the catalyst was found initially (60 °C 4 bar) [150]. The corresponding selectivity was also low. A slightly better performance was obtained after an oxidation/reduction cycle. Following a steep initial deactivation, the catalyst activity stabilized at 2 % conversion and at about 60% selectivity. After reactivation, selectivity approached... 

Four preparation procedures of distinctively different nature were tested for the palladium catalyst for the hydrogenation of nitrobenzene [41]. A sputtered palladium catalyst gave only low conversion and low selectivity. Even then, deactivation was pronounced. An oxidation/reduction cycle led to slightly improved performance. After a steep initial deactivation with initial selectivity of nearly 100%, a stable operation at 2-4% conversion and about 60% selectivity followed. 

Platinum is an effective oxidation catalyst for carbon monoxide and the complete oxidation of hydrocarbons. Palladium also promotes the oxidation of carbon monoxide and hydrocarbons but is more sensitive to poisoning than platinum in the exhaust environment. Both platinum and palladium promote the reduction of nitric oxide but are less effective than rhodium. In addition to the noble metals, three-way catalysts contain the base metal cerium and possibly other additives such as lanthanum, nickel or iron. These base metal additives are believed to improve catalyst performance by extending conversion during the rapid air-fuel ratio perturbations and help to stabilize the alumina support against thermal degradation. 

Conversions of aUyl alcohol (S) in PdCU solution can he used as another example of a complex multiroute reaction (Fig. 5.33), where several forms of the metal (palladium) catalyst are taking part in different redox reactions. Besides oxidation products Pi P3 also a reduction product P4 is formed. 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

The comparison (Fig. 1 and 6) between NO oxidation by oxygen in the absence of hydrocarbon and NO reduction in the presence of C10H22 and O2 shows that the oxidation and the reduction of NO are comparable and occur in the same temperature range with platinum or ruthenium catalysts, the conversion rates being very low with ruthenium. With rhodium or iridium and more particularly with palladium or copper NO oxidation is very limited at the temperature where NO reduction occurs. 

Cavallito (66) reports that neither it nor 3-benzoxypyridine was hydrogenated in the presence of Willstatter s palladium sponge catalyst (67). Under low pressure conditions in ether or dioxane Raney nickel and platinum oxide were ineffective. However, other examples show that reduction takes place readily under a variety of conditions. Biel used Raney nickel at 125° and 50 atm (68), excellent yield of 6-propyl-3-hydroxypiperidine resulted from reduction of the pyridine in acetic acid with platinum oxide (69). Ruthenium in the conversion of 3-hydroxypyridine in aqueous solution gave very high yield of the corre-... 

Hydrogenation of nitro compounds is rather straightforward, and palladium, platinum, and nickel catalysts have been used. Palladium is the most common catalyst for both aromatic and aliphatic nitro compounds. The poor results obtained for reduction of aromatic nitriles with hydride reagents (sec. 4.2.C.iii) make catalytic hydrogenation the preferred method. Reduction of 407 involved conversion of the aromatic nitrile moiety to the benzylamine derivative when palladium and a trace of platinum oxide was used. Hydrogenation using platinum oxide converts aromatic nitro compounds to aniline derivatives, even in the presence of other reducible groups. OS... 

This chapter will begin with the three-way-conversion (TWC) catalyst, which simultaneously converts CO, hydrocarbons and nitrogen oxides. TWC catalysts are currently used on all gasoline-powered passenger cars made in the US, as well as in many other world markets. The three milestones cited will be ceria, ceria-zirconia and close-coupled palladium. The first two of these bear on the central theme of gas-composition control, without which three-way conversion cannot be realized. The third milestone allowed the catalyst to become active at a much earlier point in the test cycle, leading to a major reduction in the amount of hydrocarbon emissions. 

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