Process for oxidation of ammonia

Details Process for oxidation of ammonia Process for oxidation of ammonia

1999-06-17

2004-12-07

Langel, Wayne A. (Department: 1754)

Chemistry of inorganic compounds

Nitrogen or compound thereof

Oxygen containing

Reexamination Certificate

active

06827917

ABSTRACT:

This invention relates to ammonia oxidation. Ammonia oxidation is widely employed in the manufacture of nitric acid and hydrogen cyanide. In the manufacture of nitric acid ammonia is oxidised with air to nitric oxide, while in the manufacture of hydrogen cyanide a mixture of ammonia and methane (often as natural gas) is oxidised with air. In both processes, the gas mixture is passed at an elevated temperature over a catalyst to effect the oxidation. Side reactions, such as the formation of nitrogen or nitrous oxide, are undesirable. Consequently, in addition to good activity, the catalyst is required to have a good selectivity.
For many years the catalysts employed have been platinum sometimes alloyed with other precious metals, in the form of meshes or gauzes formed from the metal wire. Such catalysts have good activity and selectivity but suffer from the disadvantage that not only is the catalyst very expensive, but at the temperatures encountered, the metals exhibit an appreciable volatility and so gradually the metal is lost into the gas stream. While it is well known to provide downstream means to trap the volatilized metal so that it may be revered subsequently, because of the continual volatilization, the life of the catalyst is short and frequent replacement is necessary. Furthermore the recovery of the metals from the downstream trap and re-fabrication of the catalyst meshes or gauzes involves a considerable allocation of working capital.
It is therefore desirable to provide a replacement for such precious metal catalysts.
It is well known that cobalt oxide exhibits activity for ammonia oxidation. In order to improve the activity and selectivity there have been numerous proposals to incorporate various promoters such as rare earths into a cobalt oxide catalyst.
For example, it has been proposed in CN-A-86108985 to employ lanthana/ceria/cobalt oxide compositions of the general formula La
1-x
Ce
x
CoO
3
(where x is from 0 to 1) made by a specified co-precipitation route as ammonia oxidation catalysts. Such materials were reported as having good activity and selectivity when tested on a small scale, although there is some suggestion that the activity and/or selectivity is decreased at operating temperatures in the upper end of the temperature range normally employed for ammonia oxidation (800-1000° C.).
We have found that it is important that, in this type of catalyst, the bulk of the cobalt is present as a mixed oxide phase, e.g. as the Perovskite structure RECoO
3
, (RE=rare earth), or a form thereof in which the oxygen is non-stoichiometric, and is not present as free cobalt oxides e.g. cobalto-cobaltic oxide CoO
3
O
4
or cobalt monoxide CoO. We believe that it an appreciable proportion of the cobalt is present as the free oxides; in use at high temperatures, e.g. above about 850° C., the free cobalt oxides are able to catalyse the side reaction oxidations e.g. to nitrogen or nitrous oxide, whereas if the bulk of the cobalt is “locked” into a mixed oxide phase, such as the Perovskite structure, the oxidation capability is more limited to the desired oxidation.
Producing the catalyst simply by co-precipitation of the component oxides (or compounds that readily decompose thereto) or by evaporating a solution of a mixture of thermally decomposable salts, e.g. nitrates, of the desired metals, followed by calcination at moderate temperatures, e.g. 600-900° C., does not necessarily lock the bulk of the cobalt into a mixed oxide phase such as the Perovskite structure even if the components are present in the requisite proportions. Heat treatment of the product is necessary to obtain the desired structure. In the aforesaid CN-A-86108985 the catalysts were calcined at 900° C. for 5 hours prior to use; we believe that such heat treatment is inadequate, and treatment at higher temperatures and/or for longer times is required to minimise the quantity of free cobalt oxide present. However heating at too high a temperature, above about 1150° C., may give rise to decomposition of mixed oxide phases, releasing free cobalt oxides. Alternatively, or additionally, steps may be taken to remove free cobalt oxides from the composition: for example the composition may be washed with an ammoniacal solution or other solution containing a complexing agent for cobalt. Ethylene diamine tetra-acetic acid is an example of such a complexing agent.
Accordingly the present invention provides an oxidation catalyst comprising oxides of (a) at least one element A selected from rare earths and yttrium, and (b) cobalt, said cobalt and element A begin such proportions that the element A to cobalt atomic ratio is in the range 0.8 to 1.2, at least some of said cobalt and element A oxides being present as a mixed oxide phase with less than 30%, preferably less than 25%, of the cobalt (by atoms) being present as free cobalt oxides.
The catalyst thus contains at least one mixed oxide phase containing cobalt and at least one element A. The catalyst may also contain free element A oxides and/or one or more mixed oxide phases containing two or more elements A. The element A to cobalt atomic ratio is 0.8 to 1.2, particularly 1.0 to 1.2. Preferably less than 25% (by atoms) of the cobalt is present as free cobalt oxides, and in particular it is preferred that less than 15% (by atoms) of the cobalt is present as the cobalt monoxide, CoO. The proportion of the various phases may be determined by X-ray diffraction (XRF) or by thermogravimetric analysis (TGA) making use, in the latter case, of the weight loss associated with the characteristic thermal decomposition of Co
3
O
4
which occurs at approximately 930° C. in air. Preferably than 10%, particularly less than 5%, by weight of the composition is free cobalto-cobaltic oxide end less than 2% by weight is free cobalt monoxide.
Preferably at least one element selected from yttrium, cerium, lanthanum, neodymium, and praseodymium is used as part or all of element A. Element A may comprise a mixture of at least one variable valency element Vv selected from cerium and praseodymium and at least one non-variable valency element Vn selected from yttrium and the non-variable valency rare earth elements such as lanthanum or neodymium. In particular it is preferred that the atomic proportions of variable valency element Vv to non-variable valency element Vn is in the range 0 to 1, party 0 to 0.3. It is preferred that most of the cobalt is present as a Perovskite phase ACoO
3
, but where element A comprises two or more elements, e.g. Vv and Vn, it is not necessary that there is a mixed Perovskite phase, e.g. Vv
x
Vn
1-x
CoO
3
where x is between 0 and 1. Thus there may be a Perovskite phase, e.g. VnCoO
3
or VvCoO
3
, mixed with other phases such as Vv
2
O
3
, Vn
2
O
3
, (Vv
x
Vn
1-x
)
2
O
3
or Vv
x
Vn
1-x
O
2
.
As indicated above the catalyst may be in a form wherein the amount of oxygen is non-stoichiometric. This arises from the variable valency of cobalt and also of any variable valency rare earth present as part, or all, of element A.
The catalyst may be formed by heating a composition containing the cobalt and element A oxides, preferably in air, to a temperature in the range 900-1200° C. in order to produce a material in which only a small proportion of the cobalt is present as free oxides.
The compositions may be made by precipitation, e.g. by adding a solution of soluble salts of the relevant metals to a solution of a base, e.g. ammonium carbonate or hydroxide, to precipitate the relevant metals as (basic) carbonates, hydroxides, or oxides followed by calcination to convert the precipitated compounds to the oxides. The use of alkali metal compounds as the base to effect precipitation is less preferred as they inevitably cause some contamination of the product with sodium which could act as a catalyst poison. The precipitation may alternatively, but less preferably, be effected by adding the base to the solution of the mixed salts. Alternatively, the composition may be made by forming a solution of thermally decomposable salts, e.g. nitrates or salts of organic

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