Carbon monoxide, catalytic oxidation

Houdry s solution to the problem was the first catalytic converter ever designed for an automotive vehicle. The catalytic converters found on almost all cars and trucks in use today are still strikingly similar to his invention. Exhaust gases passed into the converter and over a bed of platinum catalyst, then exited with a greatly reduced concentration of carbon monoxide, nitrogen oxides, and unburned hydrocarbons. Houdry obtained a patent for his device in 1956 and founded a company, Oxy-Catalyst, to manufacture and sell the new product. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

When the reaction is started with a mixture of CO + 02 there is. first observed a rapid adsorption of oxygen. During this initial stage the oxygen covers the active surface of the catalyst. After this, the process proceeds at a rate equal to the rate of adsorption of CO. The removal of 02 from the surface in the form of 2C02 molecules then becomes equal to the amount which enters from the gas phase. Therefore in the stationary state, the catalytic oxidation can be subdivided into two successive stages (a) the reduction of the surface by carbon monoxide, (5) oxidation of the surface by oxygen (15). 

All results presented in the various sections of this article concern the adsorption and the catalytic reaction of simple gases (oxygen, carbon monoxide, nitrous oxide) on the surface of divided nickel oxides. It may seem that we have limited our investigations to a very minute... 

Of particular interest are the block honeycomb-structure SHS catalysts. In these catalytic systems, the gas-dynamic resistance is much lower than in conventional ones, the catalytic layer is immobilized, and the active surface is used more efficiently. The data on the oxidation of carbon monoxide and propane in the block oxynitride SHS catalyst (1.5% CO, 1.5% CsHg, 10% O2 W=7010 h ) are presented in Fig. 4. Note, that at high flow rates, the conversion degree for carbon monoxide and propane attains 100% at 450-500 C. The temperature of complete oxidation can be lowered upon immobilization of the "id transition metals (Co, Ni, Cr, and Fe) oxides on the catalyst surface. Efficiency of the catalysts with immobilized Co and Ni oxides (0.2%) for the oxidation of carbon monoxide and propane is shown in Fig. 5. In this case, carbon monoxide is oxidized at 400-450"C while propane is oxidized at 125-175°C. 

Transition metal carbides (mainly of W and Mo) have been shown to be effective catalysts in some chemical reactions that are usually catalyzed by noble metals such as Pt and Pd (ref.1). Their remarkable physical properties added to lower cost and better availability could make them good candidates for substitute materials to noble metals in automobile exhaust catalysis. Hence, for this purpose, we have prepared several catalysts of tungsten carbide and W,Mo mixed carbides supported on y alumina with different Mo/W atom ratios. The surface composition has been studied by XPS while the quantitative determination of catalytic sites has been obtained by selective chemisorption of hydrogen and of carbon monoxide. The catalytic performances of these catalysts have been evaluated in the simultaneous conversion of carbon monoxide, nitric oxide and propane from a synthetic exhaust gas. 

One important use of catalysts is in catalytic converters on automobiles, which reduce the emission of exhaust gas pollutants such as carbon monoxide (CO), nitrogen oxides (NO., where x is variable), and unburned hydrocarbons. (See the chapter-opening diagrams and photograph for this chapter.) Air is introduced into the exhaust emissions from the automobile engine this mixture of gases then passes over the catalyst, which on its surface adsorbs molecules of CO, NO , and O2. The NO dissociates into N and O atoms, whereas the O2 dissociates into its atomic species. Pairs of nitrogen atoms combine to form N2 molecules, and carbon monoxide is oxidized to form... 

However, such a level can still be considered too high for vehicles having 3-way catalytic converters. In fact, results observed in the United States (Benson et al., 1991) and given in Figure 5.20 show that exhaust pollutant emissions, carbon monoxide, hydrocarbons and nitrogen oxides, increase from 10 to 15% when the sulfur level passes from 50 ppm to about 450 ppm. This is explained by an inhibiting action of sulfur on the catalyst though... 

The stoichiometric and the catalytic reactions occur simultaneously, but the catalytic reaction predominates. The process is started with stoichiometric amounts, but afterward, carbon monoxide, acetylene, and excess alcohol give most of the acrylate ester by the catalytic reaction. The nickel chloride is recovered and recycled to the nickel carbonyl synthesis step. The main by-product is ethyl propionate, which is difficult to separate from ethyl acrylate. However, by proper control of the feeds and reaction conditions, it is possible to keep the ethyl propionate content below 1%. Even so, this is significantly higher than the propionate content of the esters from the propylene oxidation route. 

Synthesis Gas Chemicals. Hydrocarbons are used to generate synthesis gas, a mixture of carbon monoxide and hydrogen, for conversion to other chemicals. The primary chemical made from synthesis gas is methanol, though acetic acid and acetic anhydride are also made by this route. Carbon monoxide (qv) is produced by partial oxidation of hydrocarbons or by the catalytic steam reforming of natural gas. About 96% of synthesis gas is made by steam reforming, followed by the water gas shift reaction to give the desired H2 /CO ratio. 

PGM catalyst technology can also be appHed to the control of emissions from stationary internal combustion engines and gas turbines. Catalysts have been designed to treat carbon monoxide, unbumed hydrocarbons, and nitrogen oxides in the exhaust, which arise as a result of incomplete combustion. To reduce or prevent the formation of NO in the first place, catalytic combustion technology based on platinum or palladium has been developed, which is particularly suitable for appHcation in gas turbines. Environmental legislation enacted in many parts of the world has promoted, and is expected to continue to promote, the use of PGMs in these appHcations. 

Other important uses of stannic oxide are as a putty powder for polishing marble, granite, glass, and plastic lenses and as a catalyst. The most widely used heterogeneous tin catalysts are those based on binary oxide systems with stannic oxide for use in organic oxidation reactions. The tin—antimony oxide system is particularly selective in the oxidation and ammoxidation of propylene to acrolein, acryHc acid, and acrylonitrile. Research has been conducted for many years on the catalytic properties of stannic oxide and its effectiveness in catalyzing the oxidation of carbon monoxide at below 150°C has been described (25). 

In the second stage, a more active 2inc oxide—copper oxide catalyst is used. This higher catalytic activity permits operation at lower exit temperatures than the first-stage reactor, and the resulting product has as low as 0.2% carbon monoxide. For space velocities of 2000-4000 h , exit carbon monoxide... 

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