Copper oxide combustion catalyst

Reforming is completed in a secondary reformer, where air is added both to elevate the temperature by partial combustion of the gas stream and to produce the 3 1 H2 N2 ratio downstream of the shift converter as is required for ammonia synthesis. The water gas shift converter then produces more H2 from carbon monoxide and water. A low temperature shift process using a zinc—chromium—copper oxide catalyst has replaced the earlier iron oxide-catalyzed high temperature system. The majority of the CO2 is then removed. 

Cupric oxide reacts with CO at temperatures which are substantially higher than those at which manganese dioxide is effective. The use of copper oxide in the combustion of CO in gas analysis is well known. By itself, it is not an effective catalyst for the oxidation of CO at ordinary temperatures. It may, however, greatly increase the activity of other oxides when used in mixtures of the proper proportion. The early work of Hofmann (23) indicated that a previously prepared surface of CuO will oxidize a mixture of CO and air, but that the rate can be increased by a factor of 3 if the copper is moistened with a little alkali. The catalytic activity is further increased if a little iridium is incorporated in the oxide. It was thought that the oxidation of CO depended upon the formation of an unstable copper peroxide of the composition, Cu203 or Cu02, which reacted with CO to form C02 and CuO. The principle was proposed for a gas generator cell of the type O/Cu/alkali/Cu/CO. The reaction was, however, too slow to be of practical importance. 

The noble metals and their oxides, especially palladium, are used chiefly as promoters of other catalysts. Small amounts of palladium, less than 1%, greatly increase the catalytic action of copper oxide for the combustion of CO near room temperature (58). Hurst and Rideal (59) found that a copper catalyst activated by Pd showed increased adsorption of CO and also increased the ratio of oxidized CO to oxidized H2 when the combustion was carried out in a mixture of these gases containing 02. 

The first row transition metal oxides are also surprisingly volatile (Figure 6). Iron (primarily relying on estimates based on the relatively limited data for halides), appears to possess the most stable oxide with acceptable volatility to temperatures as high as 1000°C. Ruthenium and copper oxides are extremely volatile and caimot be tecoimnended as active components in combustion catalysts. The least volatile transition metal oxide is iron (based primarily on estimated enthalpies of formation). A trend is observed for the more active transition metal oxides of increasing volatil-... 

For both copper and silver catalysts, the complete oxidation mechanism involves atomic oxygen. On copper, this involves the irreversible adsorption of the alkene to give adsorbed carboxylate species after cleavage of the C=C double bond. These species are subsequently oxidized to carbon dioxide. The mechanism on silver is different and a surface glycoxide species is probably formed. Abstraction of hydrogen from this species by atomic oxygen is likely to be the rate-determining step for the combustion reaction on silver. 

The presence of sulphur in diesel exhaust gases or particles has to be considered as a poisoning agent for the catalysts used in soot combustion reactions. Copper oxide has been reported to be sensitive towards sulphur dioxide (7) which implies a deactivation of the solid and then eventual modifications of its sinface properties. In this way, lCulCel073 sample was treated in a microflow reactor under SO2 flow (2L.h ) at room temperature for 30 minutes. 

The catalyst system mast be rid periodically of the deposits formed by controlled combustion with air, between 250 and 350°C. This operation, which lasts about 1 1/2 to 2 days, is preceded and Mowed by a nitrogen purge of the equipment Before restartup, it also requires reduction by hydrogen, at about 250°C,.of the copper oxide that may have been formed during the oxidizing treatment... 

An extensive literature exists on the characterization and structure—activity correlation of industrial copper-alumina oxychlorination catalysts [95-120]. At least two different major copper species have been identified. At low concentrations of copper (below ca 5 %), a well-dispersed copper species in intimate interaction with the alumina surface is formed. This species has a very low oxychlorination activity. At higher concentrations, a second species, probably formed by the de-position/precipitation of the copper chloro complexes, is observed. The latter gives rise to the active sites during the oxychlorination reaction. On the basis of an FTIR study of the oxychlorination reaction Finocchio et al. [42] postulated the formation of surface copper chloride-ethylene r-complex intermediates (which lead eventually to EDC) and weakly adsorbed HCl during oxychlorination. Formate species associated with copper and probable precursors for formation of the oxides of carbon by combustion were also identified. 

More recently, Cu/CeOg catalysts have been prepared in a different way using electrospinning and have been evaluated for methane combustion by Ontelli et A high temperature was reached during combustion, which modified the surface area of the catalysts owing to sintering. However, the reduction in the surface area did not affect the nature of the active sites. They proposed that the best results are due to the introduction of copper oxide, which increases the number of defects in the crystal lattice of the catalyst, and thereby improves catalyst performance. 

The most popular method for determining hydrocarbons in liquid oxygen is probably combustion over hot copper oxide followed by infrared determination of the resultant carbon dioxide. However a rapid, more simple and economical method which is sufficiently accurate for production control and general checking is available. As shown (Fig. 1) it involves vaporization of oxygen in a simple glass system, combustion over Burrell catalyst, and indication of carbon dioxide with a bicarbonate solution of known strength. The pH varies inversely with the carbon... 

Silver-containing catalysts are used exclusively in all commercial ethylene oxide units, although the catalyst composition may vary considerably (129). Nonsdver-based catalysts such as platinum, palladium, chromium, nickel, cobalt, copper ketenide, gold, thorium, and antimony have been investigated, but are only of academic interest (98,130—135). Catalysts using any of the above metals either have very poor selectivities for ethylene oxide production at the conversion levels required for commercial operation, or combust ethylene completely at useful operating temperatures. 

Metals in the platinum family are recognized for their ability to promote combustion at lowtemperatures. Other catalysts include various oxides of copper, chromium, vanadium, nickel, and cobalt. These catalysts are subject to poisoning, particularly from halogens, halogen and sulfur compounds, zinc, arsenic, lead, mercury, and particulates. It is therefore important that catalyst surfaces be clean and active to ensure optimum performance. 

This type of catalyst is not limited to nickel other examples are Raney-cobalt, Raney-copper and Raney-ruthenium. When dry, these catalysts are pyrophoric upon contact with air. Usually they are stored under water, which enables their use without risk. The pyrophoric character is due to the fact that the metal is highly dispersed, so in contact with oxygen fast oxidation takes place. Moreover, the metal contains hydrogen atoms and this adds to the pyrophoric nature. Besides the combustion of the metal also ignition of organic vapours present in the atmosphere can occur. Before start of the reaction it is a standard procedure to replace the water by organic solvents but care should be taken to exclude oxygen. Often alcohol is used. The water is decanted and the wet catalyst is washed repeatedly with alcohol. After several washes with absolute alcohol the last traces of water are removed. 

Cuprous oxide is also reduced violently by electropositive metals as discovered in an accident which occurred with aluminium. With chromium (III) oxide the reaction enables one to make copper chromite, which is a very common catalyst. The activity of copper chromite is such that it frequently combusts at the end of the reaction. 

Solymosi, F. et al., Proc. 14th Combust. Symp., 1309-1316, 1973 The mixed oxide (copper chromite) was the most effective of several catalysts for the vapour-phase decomposition of perchloric acid, decomposition occurring above 120°C. 

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