Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-02-25
2002-11-05
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S326000, C502S327000, C502S328000, C502S332000, C502S333000, C502S334000, C502S339000
Reexamination Certificate
active
06475951
ABSTRACT:
FIELD OF THE INVENTION
The invention provides a powdered catalyst material based on aluminum oxide, which contains at least one basic metal oxide and at least one noble metal from the platinum group of the Periodic Table of Elements, as well as aluminum oxide. The catalyst according to the invention has outstanding thermal stability with a high surface area and a high, aging-stable dispersion of the catalytically active components. It is particularly suitable for the production of catalysts for the treatment of exhaust gases from internal combustion engines.
BACKGROUND OF THE INVENTION
Aluminum oxide is frequently used as a support material for the catalytically active elements from the platinum group. It is obtainable in so-called high surface area modifications on whose surface the noble metals from the platinum group can be deposited in a high dispersion.
High surface area materials in the context of this invention are materials with a specific surface area of more than 10 m
2
/g, determined by evaluating nitrogen adsorption isotherms in accordance with DIN 66132 . Aluminum oxides which satisfy this condition are called active aluminum oxides. These include chi, kappa, gamma, delta, theta and eta aluminum oxide (see Ullmann's Encyclopedia of Industrial Chemistry, vol. A1, 561-562, 1985, which is incorporated herein by reference in its entirety).
For optimum use of the catalytic activity of the expensive platinum group metals, they have to be deposited in a very high dispersion on the support material. Efforts are made to produce crystallite sizes for the noble metals of between 1 and 10 nm on the surface of the support material. The noble metals are deposited, for example, by impregnating the support material with aqueous solutions of precursor compounds of the noble metals. Then the impregnated material is dried and calcined to decompose the noble metal compounds, optionally under reducing conditions.
Four properties of the catalyst material obtained in this way are important for later use in catalytic processes:
a) the surface area of the material, measured as the specific BET surface area according to DIN 66132;
b) the resistance of the crystallographic structure and surface area of the support material to stresses which occur during the catalytic process, in particular to high temperatures;
c) the dispersion of the catalytically active components on the surface area of the support material;
d) the resistance of the dispersion of catalytically active components to stresses which occur during the catalytic process, in particular to high temperatures.
In order to stabilize the crystallographic structure and the surface area of the aluminum oxide used as support material, this is frequently doped with basic metal oxides such as, for example, barium oxide and lanthanum oxide, cerium oxide, or other rare earth oxides or mixtures of these oxides. This results in a slowing of the conversion into thermally stable, but low surface area, alpha aluminum oxide. The amount of doping components required for this purpose is 1 to 10 wt. %, with respect to the total weight of doped aluminum oxide.
U.S. Pat. No. 3,867,312 describes the preparation of a support material based on aluminum oxide which contains oxides of the lanthanides uniformly distributed in the support material. This slows down phase conversion of the aluminum oxide. The lanthanide oxides may be present in the support material in concentrations of 1 to 45 wt. %, with respect to the aluminum oxide. The support material is obtained, for example, by melting lanthanum acetate and aluminum nitrate together by heating and by converting these into the oxides by increasing the temperature further to 600° C. This patent is incorporated herein by reference in its entirety.
U.S. Pat. No. 4,170,573 describes a catalyst material in the form of a support material consisting of cerium oxide, lanthanum oxide-and aluminum oxide, onto which platinum group metals are deposited. To prepare the support material, active aluminum oxide is impregnated with a solution of lanthanum nitrate, dried and calcined for one hour at a bed temperature between 1223 and 1253° C. Then the material is impregnated with an aqueous solution of cerium nitrate in a similar way, dried and calcined. The catalytically active noble metals are deposited onto this support material using ammonium/sulfito complexes of these metals. The surface area of the materials prepared in this way is less than 50 m
2
/g. This patent is incorporated herein by reference in its entirety.
EP 0 170 841 A1 describes a catalyst which has 1 to 10 wt. % of lanthanum oxide as stabilizer, 1 to 20 wt. % of cerium oxide as promoter, at least 0.5 to 5 wt. % of an alkali metal oxide as promoter, and one or more platinum group metals on an aluminum oxide support material. Lanthanum oxide and the promoters are incorporated in the support material which is present in the form of pellets by impregnation. After impregnating the pellets with a salt of lanthanum, the support is calcined at temperatures between 800 and 1100° C. in order to convert the salt into lanthanum oxide and for thermal stabilization purposes. This patent is incorporated herein by reference in its entirety.
EP 0 171 640 A2 describes a catalyst which contains a composite material consisting of aluminum oxide, lanthanum oxide, cerium oxide and at least one platinum group metal. Lanthanum and cerium are introduced into the aluminum oxide in sequence by impregnating with lanthanum nitrate and cerium nitrate, and are then converted into the oxides by calcining at a temperature of at least 983° C. The resulting material has a surface area of less than 50 m
2
/g. This patent is incorporated herein by reference in its entirety.
Another process for preparing a thermally stable support material based on aluminum oxide is the solgel process. This process provides a homogeneous distribution of aluminum and rare earths, at the atomic level, by the co-precipitation of oxidic aerogels of aluminum and rare earths. These materials have a constant ratio by weight of aluminum oxide to rare earth oxide over the entire volume of the solid material. The highly dispersed composite material obtained is then stabilized by calcination. The surface areas which can be produced using this process, with good thermal stability, are substantially higher than those achieved by the previously described impregnation method. Typical values are 100 to 300 m
2
/g.
The properties of the known processes for preparing a stabilized support material based on aluminum oxide are thus characterized as follows:
In order to stabilize aluminum oxide by impregnation with, for example, lanthanum oxide, the impregnated material has to be calcined at temperatures of more than 800° C. in order to enable diffusion of lanthanum into the inner depths of the particles of aluminum oxide, and incorporation into the crystal lattice of aluminum oxide. The resulting material generally has a surface area of less than 50 m
2
/g and a substantially homogeneous distribution of doping element over the cross section of the aluminum oxide particles.
Preparing a stabilized support material based on aluminum oxide by co-precipitation provides a support material with a substantially higher surface area than when using the impregnation methods. The doping element is distributed very homogeneously over the cross section of the support particles.
The catalytically active components are mostly applied to these stabilized support materials by impregnation. It is important here to produce a high dispersion of the catalytically active components, which are very stable even under high thermal stresses. This is not always guaranteed with known support materials. In particular, grain enlargement due to diffusion,of the particles to the surface, and aggregation of these, is frequently observed, so the catalytic activity of these materials is reduced by high temperatures.
Thus, the object of the present invention is to provide a catalyst material based on aluminum oxide which has a high surface area and a high d
Domesle Rainer
Göbel Ulrich
Kreuzer Thomas
Lox Egbert
Mussmann Lothar
Degussa-Huls Aktiengessellschaft
Kalow & Springut LLP
Nguyen Cam N.
Silverman Stanley S.
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