Note: Descriptions are shown in the official language in which they were submitted.
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CO~n~DUNDS FOR THE SORPl~ON OF GASES
Technical Field
The present invention relates to a new class of
compounds having a gas-fixing activity, to a method for
preparing said compounds, to methods for producing
composite materials on various matrices and thin or thick
5 films deposited on various substrates and containing said
compounds, and to their use, as well as to a method for
eliminating certain gases from a mixture that comprises
them by using said compounds.
Background Art
Various classes of materials with gas-fixing
lo capabilities are currently known. They can be divided into
two categories, depending on whether a) the fixing
properties depend on actual chemical reactions, which
entail the decomposition of the fixing material or b) the
fixing properties depend on the adsorption characteristics
15 at the physical surface of the fixer and, in general, on
the size of the molecules to be fixed. Some typical
examples of type a) mate~ials are compounds capable of
eliminating water vapor from a mixture of gases, for
example calcium sulfate, phosphorus pentoxide, magnesium
20 chloride, or carbon dioxide from a mixture of gases, for
example sodium and potassium hydroxides and calcium,
strontium, and barium oxides. ~lassic examples o~ type b)
fixing materials are materials having an activated surface,
such as activated charcoal or the various types of zeolite
s 25 or some kinds of clay.
The fixing properties of type a) materials are
selective, in that a compound is capable of fixing a single
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type of gas. The range of usa~le spontaneous reactions is
rather limited and does not include gases which are highly
harmful to the health and to the environment, such as
nitrogen monoxide No and carbon monoxide CO. Moreover, the
involved reactions may be irreversible, so that the fixer
loses all activity after a given utilization cycle.
On the other hand, type b~ materials are not selective
and fix gas molecules according to their size, degree of
polarity, and relative molecular mass. These materials are
unable to fix light molecules, such as the combustion
products that are most harmful to the health and to the
environment, such as mixtures of nitrogen oxides NO and N02
and carbon oxides, particularly the monoxide CO.
Disclosure of the Invention
A principal aim of the present invention is to
eliminate the drawbac~s of conventional materials having
gas-fixing capabilities, with particular interest for the
removal of noxious components from combustion products and
more generally from gas mixtures.
This.aim as well as other ob~ects which will become
apparent ~rom the following detailed description of the
invention are achieved by a class of compounds according to
the invention, which is represented by a compound having
the formula A2B306+d, wherein A is an alkaline-earth metal,
an alkaline metal, a lanthanide or a solid solution
thereof; B is a transition metal, an element of group III,
or a solid solution thereof; and d has a value between O
and 1.
Advantageously, A is chosen from the group constituted
by barium, cesium, potassium, strontium, a lanthanide, or
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solid solutions thereo~.
Conveniently, B is chosen from the group constituted
b by copper, nickel, manganese, iron, palladium, titanium,
aluminum, gallium, zinc, cobalt, or solid solutions
thereof.
Examples of compounds according to the invention,
wherein A is a solid solution of the above-mentioned
cations, are those having the formula
(Ba2_xSrx)Cu306 produced with values of x up to 0 75.
Other examples of compounds according to the
invention, wherein B is a solid solution of the above-
mentioned cations, have the formula
Ba2(Cu3_yPdy)Od produced with y up to 0.33;
Ba2(Cu3_yNiy)~d produced with y up to 1Ø
A compound having the formula Ba2Cu305+x has been
identified and partially described in the literature (see
for example w. Wong-Ng and L.P. Cook, Powder Dif~raction, 9
(1994), p. 280-289 and the references listed therein).
However, the researchers who preceded the inventors of the
20 present invention did not realize that they were in the
presence of a new class of compounds having particular
chemical activity characteristics.
In the compounds of the class to which this
description relates, several phenomena which are
25 intermediate between the behavior observed in type a) and
type b) fixing compounds have been observed for the first
time. The fixed gas molecules in fact react with the fixing
material, in that they are truly bonded to the structure of
the solid in the form of anions, as in type a) materials.
30 However, the chemical reaction, within wide limits in terms
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of fixed gas amount, does not produce the decomposition of
the fixing compound, the structural characteristics whereof
vary to a very limited extent and in any case continuously
and reversibly as a function of the amount of gas removed
5 from the reaction atmosphere, similarly to the behavior
observed in type b) fixers. Moreover, the fixing properties
of the compounds according to the invention are not
selective as in type a) materials and occur with a
relatively wide variety of gases, such as gaseous halogens
10 and oxides.
Compounds having the formula A2B306+d according to the
present invention can be prepared by direct reaction
starting from mixtures containing oxide, peroxide, and
nitrate precursors. These compounds lead to a
15 characteris~ic X-ray powder dif~raction profile, a typical
example whereof, related to a sample of Ba2Cu306+d, is
shown in figure 1 (CuK~ radiation was used). The spectrum
in the figure can be indexed on the basis of an
orthorhombic cell with a = 4.316(1), b = 6.889(2), and c =
20 11.442(3) A; the cell parameters, however, undergo
significant variations as a function o~ d.
The formation temperatures of the compounds according
to the present invention are typically within the range of
300 to 950~C. The optimum values of course vary as a
25 function of the cations being used.
The method for preparing the compounds having the
formula A2B306~d according to the present invention
comprises a stage for the heat treatment of mixtures of
oxides, peroxides, or salts of the required cations in
30 highly oxidizing conditions. For example, heating occurs in
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a controlled atmosphere that contains only o~ygen,
nitrogen, and inert gases.
of course, the higher the content of oxidizing
compounds (for example peroxides or nitrates) in the
5 initial mixture, the lower the partial pressure of oxygen
in the atmosphere required to prepare the chosen compound.
Vice versa, the higher the pressure of the o~ygen in the
reaction atmosphere, the lesser the role of the oxidizing
component in the mixture of precursors.
lo The compounds according to the present invention can
be prepared as polycrystalline aggregates, as components in
composite materials having various matrices, and in the
form of thin or thick layers on various substrates.
Advantageously, the compounds according to the present
15 invention can be prepared starting from a mixture
comprising one or more oxygenated compounds of an alkaline
or alkaline-earth metal and one or more oxides of
transition metals or elements of group III.
Moreover, the compounds according to the present
20 invention can be prepared starting from a mixture
comprising one or more nitrates of an alkaline-earth or
alkaline metal and one or more nitrates of transition
metals or of elements of group III.
The following procedures illustrate the methods for
25 preparing the compound A2B306+d.
The following examp7es of a preparation method for the
compounds according to the present invention are given only
by w~y of non-limitative example.
In all the procedures presented hereafter, the
30 cationic molar ratio for producing the indicated solutions
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and mixtures is given. For the sake of greater precision,
the corresponding weight ratios are specified hereafter for
each procedure.
Procedure A
A1) A mixture of fine powders of barium peroxide and
copper oxide in a 2:3 molar ratio [0.704 grams of copper
oxide (CuO) for every gram of ~arium peroxide (BaO2)~ is
produced;
A2) The mixture is homogenized by milling with a
mechanical mill or manually with a mortar and pestle of
agate or with another method for dry mixing or for mixing
in the presence of appropriate liquids;
A3) The homogenized mixture is placed in an inert
refractory container (alumina or the like) and is heated in
a furnace under a stream of oxygen and inert gas (the
partial pressure of the oxygen is typically 2 0.2 bar, 1
cc/sec) at 580.650~C.
A4) The compound is kept at the same temperature for
12 hours.
A5) Steps A2, A3, and A4 are repeated until a compound
is obtained which provides the X-ray diffraction pattern
shown in figure 1, which characterizes the Ba2Cu306+d
phase.
Procedure B
Bl) A mixture of fine powders of barium nitrate and
copper oxide in a 2:3 molar ratio [0.457 grams of copper
oxide (CuO) for every gram of barium nitrate (Ba(N03)2)] is
produced;
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B2) Same as A2.
B3) Same as A3.
B4) The compound is kept at the same temperature until
the solution o~ No2 gas generated by the decomposition of
the nitrate salts is depleted. At the end of the process,
the compound is obtained which gives the X-ray diffraction
pattern shown in figure 1, which characterizes the
Ba2CU3~6+d phase,
B5) With this procedure, the dimensions of the
resulting granules are in the millimeter range and allow
the use of sing~e-crystal characterization techniques.
Procedure C
C1) A mixture of fine powders of barium nitrate and
copper nitrate in a 2:3 molar ratio [1.335 grams of
hemipentahydrate copper nitrate (Cu(N03)2+2.520) or 1.077
grams of anhydrous copper nitrate (Cu(N03)2) for every gram
of barium nitrate] is produced;
C2) Same as A2.
C3) S.ame as A3.
C4) Same as B4.
C5) Same as B5.
Procedure D
D1) A mixture of fine powders of barium oxide and
copper oxide in a 2:3 molar ratio ~0.778 grams of copper
2~ oxide (CuO) for every gram of barium oxide (BaO)] is
produced;
D2) Same as A2;
D3) The homogenized mixture is placed in an inert
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8 _
refractory container ~alumina or the li~e) and is heated in
a pressurized furnace with a partial oxygen pressure in
excess of 1 bar at 580.650~C;
D4) Same as A4;
D5) Steps D2, D3, and D4 are repeated until a compound
is obtained which has the X-ray diffraction pattern shown
in figure 1, which characterizes the Ba2Cu306+d phase.
Procedure E
El) A solution of barium nitrate and copper nitrate in
10 a 2:3 molar ratio is prepared in distilled water up to the
solubility limit for barium nitrate [1.335 grams of
hemipentahydrate copper nitrate (Cu(N03)2+2.520) or 1.077
grams of anhydrous copper nitrate (Cu(N03)2) for every gram
of barium nitrate];
E2) An inert, temperature-resistant, porous medium
(for example neutral activated Brockman alumina) is
impregnated with the solution thus prepared;
E3) The water is eliminated with a drying treatment at
150~C for 2 hours.
E4) Same as B4, but the reflections of the porous
substrate, if crystalline, are also found in the X-ray
diffractogram.
E5) The final product is a composite material wherein
the compound Ba2Cu306+d fills the microcavities o~ the
25 porous substrate.
Procedure F.
Fl) Same as El;
F2) The solution is used to wet the surface of a
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_ g _ :
substrate of temperature-resistant, non-porous, inert
material, constituted by polycrystalline A1203 with a
relative density of 99.9% (other examples of usable non-
porous inert materials are quartz, porcelain, Inconel,
5 oxidation-resistant alloys and metals, etcetera);
F3) The deposited water is quickly evaporated ~y
electrical heating to approximately 230~C;
F4) Steps F2 and F3 are repeated until a thin
deposition of nitrates of the desired thickness, for
lo example approximately 10 ~, is o~tained;
F5) Same as B4.
F6) The final product is a film of Ba2Cu306+d having a
presettable thickness.
In another aspect, the present invention relates to a
15 method for eliminating certain gases from a gaseous mixture
including them. The ability of the compounds according to
the present invention to fix molecules of various gases
directly from the gaseous state has been verified by direct
measurements, such as thermogravimetry, analysis of the
gases in the reaction atmosphere, or by indirect
measurements, such as Raman and infrared spectroscopy. The
compounds according to the present invention are
particularly adapted for fixing oxides and halogens in the
gaseous phase.
Fxamples of such gases are N02, C02, S02, N0, C0, F2,
and C12.
The way of fixing the gas depends on the type of gas
and on the temperature and composition of the atmosphere in
which the reaction occurs.
The inventors of the present invention have observed
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for the first time in the compounds of the class to which
the present invention relates ~ixing phenomena which are
intermediate between the behavior observed in conventional
fixing compounds of types a) and b) described above. The
5 gas molecules are in fact fixed by reacting chemically with
the fixing material, in that they are bonded to the
structure of the solid in the form of anions. However, the
chemical reactions related to the fixing processes,
although continuously modifying the composition of the
lo fixer, do not cause, over a broad range in terms of amount
of fixed gas, the destruction of the structure of the
fixing solid, the structural parameters whereof vary to a
very small extent and in any case continuously as a
function of the amount of gas removed from the reaction
15 atmosphere. If the process is continued beyond these
limits, absorption can continue but it entails the
destruction of the fixer structure and the formation of
oxides or of the salts corresponding to the anions that
form.
The gases fixed by means of the mechanisms that are
active in the first stages of the process, regardless of
their nature, enter the compounds according to the
invention in the form of oxyanions or halide ions. The
structural characteristics of the fixing compounds vary
25 continuously during this first part of the process, in the
same way in which the structural characteristics of a solid
solution vary as one of its components varies. During this
stage, the fi~ing process is reversible for many of the
fixable gases; by varying the partial pressures and the
30 temperature it is possible to d-esorb them fully or
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1 1
partially. Gaseous oxides with a low oxidation state (NO
and CO, for example) can be desorbed in a higher oxidation
state (N02 and C02, for example) if the atmosphere in which
release occurs is sufficiently oxidizing. Once a first
5 limit concentration of oxyanions or halides, which depends
on the type of gas and on the temperature, has been
reached, reversibility is lost except in the case of
nitrogen oxides. In this particular case, reversibility of
the fixing process is complete, since the final product
10 corresponds to the initial material for the production of
the compounds at issue, as described in preparation
procedure B) or C), depending on whether the process occurs
above or below the temperature at which the copper nitrate
decomposes to copper oxide.
The chemical reactions related to the fixing processes
occur by virtue of the presence of an excess of oxygen in
the structure. By way of example, in the case of the
compound in which A = Ba and B = Cu, the oxygen composition
produced by the normal state of oxidation of the cations
20 (Ba2+ and Cu2+) would be 5 atoms per unit of the formula
(Ba2Cu305), whilst accurate determinations of the oxygen
content in samples prepared according to the previously
described procedures show an oxygen content that is
typically in the range of 5.5 to 6.1 atoms per unit of the
25 formula. Accordingly, the fixing processes do not require
r the addition of external oxygen; however, their yield is
increased by the presence o~ oxygen in the gas mixtures
placed in contact with the fixing materials.
The method according to the present invention for
30 eliminating certain gases from a gaseous mixture that
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includes them consists in placing the gas mixture in
contact with a compound having the formula A2B306+d in pure
form or as a component of composite materials, in which A
is an al~aline-earth metal, an alkaline metal, a
5 lanthanide, or a solid solution thereof; B is a transitiOn
metal, an element of group III, or a solid solution
thereof; and d has a value between 0 and 1, at a
temperature between the melting temperature of the compound
to be fixed and 650~C. Preferably, A is chosen from the
10 group constituted by barium, cesium, potassium, strontium,
or solid solutions thereof, and B is chosen from the group
constituted by copper, nickel, manganese, iron, palladium,
titanium, aluminum, gallium, zinc, cobalt, or solid
solutions thereof.
The method is described in greater detail hereinafter
with two examples referring to situations that produce
different yields:
1) fixing of N02 at room temperature from an oxygen-
containing atmosphere;
2) hot fixing o~ N0 ~rom an o~ygen-free atmosphere.
The following examples of gas fi~ing methods using
compounds according to the present invention are given
merely by way of non-limitative example.
Example 1) N02 fixing
3 grams of ~a2Cu306+d produced according to procedure
B) are placed in a U-shaped tube wherein a stream of No2
(50%) and air is made to flow (90 cc/min flow, 20~C
temperature). The gas fixing activity is clearly shown in
figure 2, which shows a first U-shaped tube (1) and a
30 second U-shaped tube (2) which are parallel-connected with
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_ ~3
respect to the stream of N02 and air; a layer of Ba2Cu30~+d
is present in the base of tube (1), whilst a layer of white
material (for example cotton) is present in the base of
tube (2). It is o~served that in tube (1) the gas changes
5 color from orange (represented by stippling) to colorless
as a consequence o~ its passage through the layer of
sa2cu306+d, whilst in tube (2) it remains of the same
orange color after passing through the white layer. In
these conditions, the ~ixing activity persists for
lo approximately 300 minutes. At the end of the process, a
weight increase was found which corresponded, within the
measurement errors (5%), to the total conversion of the
initial compound into copper and barium nitrate salts and
to the fixing of 10 moles of gas per mole of fixer.
15 Example 2) NO fixing
Figure 3 shows the result of an N0 concentration
analysis by mass spectrometry on two gas streams (112
cc/min) originating ~rom a common source o~ a mixture of
He+0.4%N0, one of which passes through a reactor brought to
20 350~C which contains 1 gram of Ba2Cu306+d compound produced
according to procedure A). This measurement points out
different fixing processes with separate kinetics. The
amount of gas fixed at saturation (2 hours) is 0.28 moles
per mole of fixer.
Another aspect of the present invention relates to an
electric sensor for gas concentration, which comprises a
compound having the formula A2B306+d. The inventors have
in fact found that for the compounds according to the
present invention, during the gas fixing process, the value
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of electrical resistivity increases in proportion to the
amount of gas that is incorporated. For example, the
compound Ba2Cu306+d is a semiconductor with typical values
of electrical resistance at room temperature on the order
of 10-100 ohm/cm. In the case of the fixing of nitrogen or
carbon oxides, this value is up to 4 orders of magnitude
higher. Figure 4 plots the electrical resistance of a film
produced according to procedure E) and exposed to a stream
of No2 (50~) and air at room temperature.
Moreover, in another aspect the present invention
relates to optical gas concentration sensors which comprise
a compound having the formula A2B306+d.
The inventors of the present invention have found that
the compounds having the formula A2B3~6+d show
considerable variations in their optical properties during
the gas incorporation process. These variations become
apparent as variations in the intensities of the
characteristic modes in infrared and Raman spectra, with
the appearance of new optical modes caused by the
characteristic vibrations of the incorporated anions and
with the appearance of characteristics which cannot be
ascribed to the initial material or to the guest anions,
such as ~or example a plurality of highly intense
luminescence peaks which appear as a consequence of the
incorporation of small amounts of carbon oxides. In
addition to the variations in the measured spectra,
macroscopic changes in color are observed as a consequence
of the incorporation of small amounts of carbon oxides. For
example, the color of the Ba2Cu306+d changes from the
initial dark blue to black, whilst a color change is
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observed towards greenish pale blue as a consequence of the
incorporation of large amounts of nitrogen oxides below
70~C .
s ~ h ~i
