CA2074666A1 - Germanium compounds - Google Patents

Abstract

ABSTRACT

Title: Germanium Compounds Germanate compounds suitable for removing hydrogen sulphide from exhaust emissions are disclosed. These compounds comprise germanium oxide and the oxide of at least one other metal. Preferably the other metal oxide is the oxide of a rare earth metal or yttrium. The germanates of use in the present invention may contain more than one rare earth metal or yttrium oxide and may additionally contain one or more oxides of a metal which is not a rare earth metal or yttrium.

Description

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Title: Germanium Componds .

Field of the invention !
This invention relates to the use of germanium compounds as suppressors of hydrogen sulphide (H2S) in exhaust systems, and to catalysts for use in the exhaust system of -an internal combustion engine and exhaust systems incorporating such catalysts.

Background to the ~invention ;

The exhaust gases produced by an internal combustion engine should ideally, if complete combustion of the air/fuel mixture has occurred, contain only the fully oxidised products of fuel combustion - carbon dioxide (C02), water ~H20) and nitrogen (N2). However r complete combustion rarely occurs inside a vehicle engine and gases such as carbon monoxide (CO), hydrocarbons (HXCy) and unreacted oxygen (2) are therefore also present in the exhaust gases. Nitrogen oxides, of the general formula NOx, are also present, having been formed by the reaction betwe~n N2 and 2 CO, HXCtl ~nd NOx are all atmospheric pollutants, and it is therefore desirable to reduce their p\resence in the exhaust gases to an environmentally (and, increasingly, . .
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legally) acceptable level. The content of these pollutants in exhaust gases can be reduced to reasonab],y low levels hy the use of suitable catalysts, across which the flow of exhaust gases is directed before being released to the atmosphere. The catalysts conventionally used are so-called "three~way-catalysts" ("TWC
catalysts"), which contain small auantities of precious metals, such as platinum and rhodium. When hot exhaust gas contacts these cata]ysts, CO and HC react with 2 and NOx to form the harmless gases CO2, H20 and N2.

In practice, the catalysts are mounted in a meta] casi,ng ,~
(a "catalytic converter") within the exhaust system. The catalysts consist of a ceramic or metal substrate and a catalytic coating, containing the precious metals, on the substrate. The exhaust gas stream is directed to flow through a large number of channels provided on the substrate and thus contacts the active reaction sites of the catalyst which are located in the catalytic coatinq on the channel walls. The TWC catalysts are able to reduce the emission of all three pollutants simultaneous]y, provided that the engine is operated at an opti~um air/fuel ratio of 14.7:1.

Often, additives sueh as cerium dioxide (CeO2) are incorporated into the eatalysts to enhance the catal,ytic activity.

Vehicle engine exhaust gases also contain other atmospheric pollutants, however, among them hydroaen sulphide (H2S)) which is thought to be formed by the reaction of sulphur eontained in the fuel with a TWC
catalyst. Again, emissions of this chemical from vehicle engines have to be reduced to acceptable levels. There is " . . . ..
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an urgent need to find an efficient way in which to re~ove H2S from exhaust gases, which conventional TWC catalysts are unable to do.

The current method of H2S removal is to add a nicke] (II) oxide (NiO) "suppressor" component to the TWC catalyst in a vehicle exhaust system. Whilst NiO is an effective H2S
suppressor, however, it is not an ideal component for use in vehicle engines, partly because nickel is thought to be a carcinogen and partly due to the fear that nickel carbonyl (an extremely toxic compound) may be formed during the catalytic H2S removal process. Accordingly, a more acceptable alternative to NiO must be sought.

Yamada et al (SAE International Congress and Exposition, Detroit, Michigan,~26 February - 2 March 1990) have investigated the use of aermanium (I~) oxide (germanium dioxide, GeO2) as such an alternative, and others working in the field are also directing their research into the use of GeO2 as an H2S suppressor. However, at hiaher operating temperatures (above about 680C) and under reducing conditions, such as are found in a vehicle enaine usually when the vehicle is operated at high speeds, GeO2 is reduced to gaseous germanium (II) oxide (GeO). This can lead to significant and unacceptable losses of the GeO2 suppressor.

Yamada et al's work involved investigatina the properties of various single metal oxides and their suitability as replacements or NiO in TWC catalysts. Whi]st GeO2 was found to be the most acceptable of the oxides tested, the above shows that it is still by no means ideal. A TWC
catalyst additive is still needed which will efficiently : i . :

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In order to develop such a catalyst additive, the mechanism of H2S formation in exhaust systems m~st first be understood. This mechanism is well documented and is thought to consist of two stages, sulphur storage by the catalyst (at high air:fuel ratios) and subseauent sulphur release (at lower air:fuel ratios).

Sulphur dioxide (SO2) is formed by combustion of sulphur contained in the fuel; this is stored on the catalyst at r high air:fuel ratios (i.e. lean fuel mixtures):

S2 + ~ 2 ~ SO3 (1) MO + S03 - -3 MSO4 (2) MO + S02 + 2 2 ---t MS04 (3) (Here, MO designates a hypothetical metal (M) oxide, which may be, for instance, CeO2 such as in the commonly used Pt/Rh/CeO2 TWC catalyst).

At low air:fuel ratios (i.e. rich fuel mixtures) and temperatures of around 500C, re~uctive su]phur release:

MS 4 2 ~ ~O 2 2 and steady state sulphur release:

S2 + 3H2 ------~H2S + 2H20 (5) can both occur, and levels of H2S in the exhaust gases therefore increase.

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H2S release is thought to occur when the vehicle is accelerated or decelerated, or when its engine is allowed to "idle".

The mechanism of suppression of H2S emission by NiO and some other oxides is thought to lie in the "tying up" of the H2S as a metal sulphide, ie.-H2S ~ NiO 3 NiS + H2O (6).
! Thus, a chemical which is to act as an H2S supressor in place of ~iO must have two essential qualities:
,:
1) it must not form a sulphate at low air:fuel ratios;

2) it must be capable of trappinq H2S as a sulphide at high air:fuel rati~s.

The chemical should also be thermally stable in air and under reducing conditions, at temperatures higher than those to which GeO2 is stable (ie around 680C) and ideally up to around 1000C.

It is an aim of the present invention to provide such a chemical, which can be used to supress H2S emissions from exhaust systems, the use of which in such a manner overcomes or at least mitigates the above described problems encountered with the use of conventional H2S
supressors.

Statement of the invention In one aspect the present invention provides a germanate for use as a suppressor of H2S in an exhaust system of an internal combustion engine.

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A germanate is a compound comprising a germanium oxide and the oxide(s) of another metal or metals, the oxides being combined chemically in an appropriate stoichiometric ratio~

In one particular embodiment the invention provides a germanate, defined above, comprising the oxide of a rare earth metal or yttrium.

The simplest germanates according to the particular embodiment defined above will have the empirical formula MGeOx, where M is a rare earth metal or yttrium. However, the germanate may contain more than one rare earth metal or yttrium oxide, i.e. it may have the formula (M1) (M2)b(M3) . . Ge O , where ~1~ M2, M3.. are rare earth metals or ytt-rium. The germanates of use in the -present invention may also include those in which one or more of the metals M1, M2, M3... is not a rare earth metal or yttrium.

The group of rare earth metals (also known as "lanthanides") comprises the four-teen elements following lanthanum in the periodic table and also, for the purposes of this description, lanthanum itself. The rare earth metals are therefore Lanthanum (La), Cerium ~Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), ~tterbium (Yb) and Lutetium (Lu). Of these, however, promethium is highly radioactive and its compounds, if available, are unlikely to be of use in the in the present invention.

"ttrium is often grouped together with the rare earth metals because it shares the chemical properties of the .. . : . .

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group in many respects. Thus it is found that where a rare earth-containing germanate is of use in the present invention, so too is an yttrium-containing germanate.

Germanates containing rare earth metals or yttrium have been found able to act as H2S suppressors, i.e. they do not form sulphates under oxidising conditions and they form sulphides in the presence of H2S under reducing conditions. They are also more thermally stable than GeO2 under reducing conditions, and lack the toxicity problems associated with the use of NiO as an H2S suppressor.

Although the use, as H2S suppressors, of germanates other than those containing rare earth metals has been investigated, the latter have generally been found to give better results. For- instance, calcium germanates tend to have less thermal stability than their rare earth or yttrium counterparts, and aluminium germanates, though thermally stable, do not react in the requisite manner to form sulphides and not sulphates in the presence of H2S.

The germanate is preferably a lanthanide or yttrium containing germanate, more preferably a lanthanide or yttrium containing digermanate such as La2Ge207 or Y2Ge2 7 The germanate is conveniently used as an additive to a catalyst already present in the exhaust system. This catalyst will typically be a three-way-catalyst (TWC
catalyst) or other conventional exhaust catalyst, preferably a platinum/rhodium/cerium dioxide based TWC
catalyst.

The invention thus additionally provides the use of a germanate preferably containing a rare earth metal or .
, yttrium, as an additive to a catalyst for use in an exhaust system, the additive being present in the catalyst Eor the purpose of suppressing H2S from exhaust gases present in the exhaust system.
.
The invention also provides a catalyst for use in an exhaust system, the catalyst comprising a germanate, preferably a germanate which contains a rare earth metal -or yttrium. Again, the catalyst will preferably be a TWC
catalyst, typically a platinum/rhodium/cerium dioxide based TWC catalyst.
:, The invention further provides a catalytic converter for use in an exhaust system, which converter comprises a catalyst such as is provided by the present invention, the catalyst comprisin~ a germanate, preferably a germanate containing a rare earth metal or yttrium.

Further, the invention provides an exhaust system comprising a catalyst or a catalytic converter such as are provided by the invention, and an exhaust system in which a germanate preferably containing a rare earth metal or yttrium is used as an H2S suppressor.

The catalyst is conveniently applied to a supporting substrate in the vehicle exhaust system by coating the substrate in a suspension containing all the catalyst ingredients, including the germanate. The coated substrate is then fired to leave a solid catalyst coating on the substrate. The germanates used in the method of, or the catalyst or catalytic converter of, the present invention are therefore preferably capable of being made up into a suspension for coating an appropriate catalyst substrate. The germanates preferably remain as the undissociated germanates in such suspension, rather than ~ . .... , " ~ !
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g existing as mixtures of their component oxides, MxOy and GeO2. (The firing temperatures typically used will. not generally be high enough to cause recombination of these component oxides.) According to another aspect the invention provides a method of producing a catalyst including a germanate according to said one aspect.

The invention will now be described by means oE ~he followlng exdmple and -~ith reference to the accompanying Figures, of which:-~igures 1, 15 and 21 show schematically apparatus used toinvestigate the properties of germanates such as are of use in the present-invention;

Figures 2 - 14 show the results of experiments investigating the thermal stability under reducing conditions of germanates such as are of use in the present invention;

Figures 16 - 20 show the results of experiments investigating the sulphation of certain of those germanates under oxidising conditions; and Figures 22 - 29 show the results of experiments investigating the sulphidation of certain of those germanates under reducing conditions.

Detailed description In the following examples, the physical and chemical properties of various germanates were investigated, with a view to assessing their suitability as H2S suppressors in ,: :, :
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-- , o exhaust systems. The sixteen sample germanates, B-~, were also compared with germanium dioxide itself (sample ~), for which the GeO2 was used in its hexagonal crystalline form.

The following criteria were laid down for the evaluation of the samples:-1) They should be thermally stable in air at 1000C. (Itis known that, for example car catalysts, under exceptional conditions, can operate at as high as 1000C).

2) They should be thermally stable under reducing conditions at significantly higher temperatures than is GeO2 (1000C being-the ideal~.

(Thermal stability is required so that a given amount of the compound added to an exhaust catalyst will remain effective as an H2S suppressor for an acceptable period of use).

3) They should not form sulphates under oxidising conditions, i.e. at high air:fuel ratios (the "lean" fuel region).

4) They should form sulphides under reducing conditions, i.e. at low air:fuel ratios (i.e. the fuel-rich region).

Sample preparation The samples investigated had the following formulae:

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A = Germanium dioxide, GeO
B = Calcium monogermanate, CaGeO3 C = Calcium digermanate, CaGe205 D = Dicalcium heptagermanate, Ca2Ge7016 E = Aluminium germanate, A16Ge2013 F = Yttrium digermanate, Y2Ge207 G = Lanthanum digermanate, La2Ge207 H = Gadolinium digermanate, Gd2Ge207 I = Dysprosium digermanate, Dy2Ge207 J = Neodymium digermanate, Nd2Ge207 ! K = Praseodymium digermanate, Pr2Ge207 L = Lanthanum aluminium digermanate, LaAlGe207 M = Gadolinium aluminium digermanate, GdAlGe207 ~ = Dysprosium monogermanate Dy2GeO5 V = Neodymium monogermanate Nd2GeO5 W = Erbium !nonogermanate Er2GeO5 X = Gadolinium monogermanate Gd2GeO

This allowed the comparison of rare earth or yttrium-containing germanates, F-X, with qermanates containing only aluminium and calcium.

Germanates of metals whose oxides have high melting points were thought likely to possess the necessary thermal stability to be of use as H2S suppressors, and for this reason germanates of aluminium and calcium were experimentally evaluated. Whilst germanates of the 'neavier transition metals might also be expected to have the necessary stability, no such compounds were tested since toxici.ty problems can be encountered with transition metals, making them unsuitable for use in exhaust systems.
Moreover, some transition metals can act as catalyst poisons.

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The general procedure for preparation of these samples (excepting Sample A) involved weighing out, to four decimal places, appropriate weights of reactants, which were then transferred to an agate monitor, moistened with ethanol and blended together. In the case of the calcium germanates (Samples ~-D), two batches had to be mixed and then combined in one crucible before ignition. This was due to the volume of the calcium carbonate (CaCO3) used as a reactant.

The samples were placed in platinum crucibles and initially ignited at 900C for 4-5 hours in order to remove water and CO2 (Stage 1).

The samples were then removed from the crucibles, remixed dry and put back into the crucibles to be heated at an appropriate temperature for 14-18 hours (Stage 2).

After this time each sample was remixed dry, crushed to pass through a 300 um sieve, and an X-ray diffraction pattern obtained to confirm that reaction of the reactants had occurred. When reaction was judged complete, one further 4--6 hour sintering was used to ensure complete reaction (Stage 3).

The appropriate sintering temperature for each sample was determined by trial-and-error, using small (1-2g) trial batches. The firi.ngs were done in air, in electrlcally-heated muffles. The accuracy of temperature control was +
(2-4C), and the absolute accuracy of the recorded temperature was +20C. All heatings were performed in platinum crucibles. No signs of attack on the crucible were noted.

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Sample B, CaGeO3 Reactants: Ana]ar* CaCO3 and Meldform* GeO2.

Stage 1 Ignited at 900C for 11 hours Stage 2 Ignited at 1200C for 13 hours Stage 3 Ignited at 1200C for 6 hours Weight loss during final 6 hour treatment = 0.043~

An X-ray diffraction photograph was taken and compared with JCPDS (Joint Committee on Powder Diffraction Standards) Card 21-142.

System: Triclinic-aO = 8.07; bo = 7.4Ç; C~ = 7.23 angstroms.

Reference: Jost, Wolf and Thilo, ~. Anorg. Allgem. Chem.
353 42-47 (1967).

A reasonably good fit was obtained, but with some minor extra reflections. Using call data from the card aiven above, a list of X-ray d-spacings was calcuLated from the unit cell parameters, which indicated that these extra reflections co~ld in fact be due to monogermanate. The suspicion is therefore that these l'extra" reflections were simply overlooked in previous data. The product (Sample B) is therefore essentially single-phase.

* Proprietary names.

Sample C, CaGe2Os ~, :
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Stage 1 Ignited at 900C for 7 hours Stage 2 Ignited at 1150C for 12 hours Stage 3 Ignited at 1150C for 5 hours.
Weight loss during final 5 hour treatment = 0.044~

X-ray diffraction pattern compared with JCPDS Card 23-869.

System: Triclinic aO = 6.860; bo = 8.787; cO = 6.527 angstroms alpha = 91.02 ; beta - 113.02 : gamma = 88.18 Reference: Technisc~ Physische ~ienst, Delft, ~o]land.

The X-ray powder pattern gave reasonable agreement with the above card. A powder X-ray pattern was generated, using single-crystal data from ~elov et alls crystal structure determination. An immediate and very significant improvement was obtained between 'observed' and 'calculated' values. One extra reflection occurred (minor) which was subsequently found to be the strongest reflection of Ca2Ge7O16. Thus Sample C probably contains ca 5% of Ca2Ge716 Sample D, Ca2Ge7O16 Reactants: Analar CaCO3 and Meldform GeO2 tage 1) Ignited in the CaO.4GeO2 ratio at 900C for 6 hours. Sufficient CaCO3 then added to bring . ~ ,, . ,:: ,~ , .

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the ratio to 2:7.
tage 2) Ignited at 1125C for 16 hours + 1125C for 14 hours, adding excess CaCO3 at this stage.
tage 3) Ignited at 1125 for 4 hours.

Weight loss over final 4 hours = 0~013 The X-ray powder pattern was compared with JCPDS Card 34-System: OrthorhombicaO = 11 . 3456; bo = l 1. 3436; cO = 4.6409 angstroms.
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Reference: Breuer and Eysel~ Univ. of ~eidelberg.

Very good agreement was obtained.

Note: Various published phase diagrams of the CaO-GeO2 system disclose the existence of a "tetragermanate", CaO.4GeO2. We were unable to make this phase. When pilot batches showed the appearance of Ca2Ge7O16, and uneauivocal literature data disclosed this to be its composition, the large batch, (whose preparation had commenced) was altered to the 2:7 ratio. It is recommended that future preparations should work to the 2:7 ratio and not, as we did, go through a two-steP
synthesis.

The product obtained was essentially single phase.

Sample E, Al6Ge2O13 ' ~, ~' "

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Reactants: Cera* Alumina (Al2O3) and Meldform GeO2 Stage 1) Ignited at 900C for 4 hours.
Stage 2) Ignited at 1125C for 14 hours, then at 1300C
for 15 hours Stage 3) Ignited at 1300C for 5 hours.

Weight loss at 1300C for final 5 hours = 0.03596 The X-ray diffraction pattern was compared to JCPDS card 27-1005.

System: Orthorhombic aO = 7.655; bo - 7.775, cO = 2.924 angstroms.
(Mullite structure~.

Reference: Toropov, et al. J. Appl. Chem ~SSR 43 2171-2174 (1970).

Very good agreement was obtained. The product was found to be essentially single-phase.

*Proprietary name.

Sample F, Y2Ge2O7 Reactants: Meldform Y2O3 and Meldform GeO2 Stage 1) Ignited at 900C for 5 hours Stage 2) Ignited at 1400C for 15 hours Stage 3) Iynited at 1400C for 5 hours.

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Weight loss at 1400C for final 5 hours = 0.068%

The X-ray powder diffraction pattern was compared to JCPDS
Card 38-288.

Reference: Larson and McCarthy, North Dakota State Univ.
JCPDS. Grant in-aid proiect.

System: Tetragonal aO = 6.8040 cO = 12.375 angstroms.
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Very good agreement was obtained between observed and calculated patterns: the product was essentially single phase.
-Sample G, La Ge O

Reactants: Meldform La2O3 and Me:Ldform GeO2.

Stage 1) Ignited at 900C for 5 hours.Stage 2) Ignited at 1250C for 16 hours.Stage 3) Ignited at 1300C for 6 hours.

Weight loss at 1300C for final 6 hours = 0.072%.

The X-ray diffraction pattern was compared with JCPDS Card 23-313.

Reference: Glushkova et al, Inorg. Materials 3, 96 (l967)-No unit cell data.

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The initial pilot sample of around 2-3 g gave an X-ray pattern with major lines simi]ar to the above data but being otherwise not a very good fit.

Using crystal data from Smolin Yu et al, Doklady Akademie Nauk ~SSR (196g) an X-ray diffractometer trace was calculated. This agreed very well with the observed X-ray trace for sample G.

~owever, on making a larger sample, a somewhat different X-ray pattern was obtained. This was in rough agreement with the JCPDS Card 23-313. Although there was not perfect agreement, the data on the card were not of high quality and it was difficult to be certain where the disagreements lay. As both card pattern and single-crystal data pattern were roughly similar (for major X-ray reflections) it was thought that there may be two polymorphs of this compound.

Fast cooling of a small sample of the large batch did not affect the appearance of -the pattern. This may be due to the (large batch structure) - (small batch structure) reaction being irreversible.

While the polymorphism of this phase is still somewhat ~ncertain - literature characterisation data are inadequate - the sample prepared appeared to consist of a single phase of the La203.2GeO2 composition.

Sample H, Gd2Ge207 Reactants: Laboratory supply of Gd203 (heated at 900C for 6 hours) and Meldform GeO2 : . ' ~: . :, ,~
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Procedure: Reactants were heated at 1100C for 20 hours, re-mixed and heated at 1150C for 18 hours and finally re-mixed and heated at 1150C for 22 hours.

From its X-ray diffraction pattern, the product a~peared to be isostructural with Dy2Ge2O7 (JCPDS Card 38-289).

Sample I, Dy2Ge2O7 Reactants: Laboratory supply of Dy2O3 (heated at 900C fo 6 hours) and Meldform GeO2.

Procedure: As for Sample H.

X-ray diffraction pattern found to be in agreement with JCPDS Card 38 289 for Dy2O3.2GeO2.

Sample J, Nd2Ge2O7 Reac~ants: Meldform Nd2O3 and Meldform GeO2.

Procedure: Reactants were heated at 1150C for 18 hours, then re-mixed and heated at 11 50C fo 22 hours. After the first heating, the sample was observed to be of a lilac colour on the outside and dark lilac on the inside. After the second heating, the sample had a constant colour throughout.

The weight loss of the product during the second heating was 0.0005%-- . , , , ~" , , ., . : . .: . ;' . ! ., . ~ ' ~
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The X-ray diffraction pattern was very similar to that observed for La2Ge2O7 (Sample G).

Sample K, Pr2Ge2O7 Reactants: Meldform Pr6O11 and Meldform GeO2 Procedure: As for Sample J. After the first heating, the -sample had changed colour from very dark brown to light green. This was probably due to the reduction of the Pr6O1l to Pr23 The weight loss of the product during the second heating was 0.0005~.

Again, the X-ray diffraction pattern was very similar to that for La2Ge2O7, which indicates that the praseodymium is probably present in the +III oxidation state.

Sample L, LaAlGe2O

Reactants: Meldform La2O3: Meldform GeO2; AnalR Alumina Procedure: Reactants were heated at 1150C for 24 hours, re-mixed and heated at 1300C for 3 days and finally re-mixed and heated at 1320C for 64 hours.

The X-ray diffraction pattern of the product gave reasonably good agreement with that given by Kaminski et al for this compound. The structure was an NdAlGe2O7-type structure.

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Reactants: Laboratory supply of Gd2O3 (heated at 950C for 6 hours); Meldform GeO2; AnalR Alumina.

Procedure: As for Sample L, with a further re-mixing and heating at 1350C for 5 hours.

The X-ray diffraction pattern of the product gave reasonably good agreement with that given by Kaminski et al for LaGe2O7.

Sample ~ Dy GeO

Reactants Meldform Dy2O3, GeO2 Dy2O3 heated at 100C for 5 hours.

Procedure Reactants heated at 800C for 5 hours, remixed and heated at 1225C for 18 hours and finally remixed and heated at 1225~C for 22 hours.

X-ray powder diffraction pattern aqreed with JCPDS card 38.286.

Sample V Nd2GeO5 Keactants Meldform Nd2O3 GeO2 Nd2O3 heated at 900C for 5 hours.

Procedure As for sample U.

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X-ray powder diffraction pattern agreed with JCPDS Card 38-339.

Sample W Er2GeO

Reactants Meldform Er2O3 GeO2 Er2O3 heated for 5 hours at 800C.

! Procedure Reactants heat at 900C for 4 hours remixed then heated at 1250C for 22 hours and finally remixed and heated :
for 5 hours at 1250C.

X~ray powder diffraction pattern agreed with JCPDS Card 38-287.
' Sample X Gd2GeO5 Reactants Laboratory supply of Gd2O3. :
Meldform GeO2 Gd2O3 heated for 5 hours at 800C

Procedure As for sample W.
X-ray powder diffraction pattern agreed with JCPDS 38.706.

Sample Analysis a) Thermal Stability in Air Approximately 1.5 grams of each sample material A-X was ::.,. ;.. '. : , , , , ::

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measured into a small open crucible and placed in a well-ventilated furnace at 1000C. After 1.5 hours the samples were removed and re-weighed. This was to ensure that no surface moisture etc remained on the samples or crucibles.
This new weight value was taken as the 'true' starting weight.

The samples were then returned to the urnace for a certain number of days, removed and re-weighed in order to determine the weight loss of the samples over that period.

The results are shown in Table I, from which it can be seen that the weight losses for all but Sample A (GeO2) were insignificant over periods of up to 12 days. All samples fared well in this respect, but particularly samples U-X which underwent no detectable weight loss.

The weight loss for GeO2 was within acceptable limits.

b) Stability under Reducing Conditions The reduction tests were carried out in a Dupont 951 thermobalance with a 9900 controller and data handling facilities. The schematic configuration of the apparatus is shown in Figure 1.

Approximately 35 mg of each sample was placed in an alumina crucible 1 inside a furnace tube 30 in furnace 2, in which the atmosphere consisted of 5% H2 in N2 (i.e. a reducing atmosphere). These gases were passed through the furnace tube 30 in the direction shown and allowed to leave through the purge gas outlet 33 (the gas inlet is not shown in Figure 1). The rate of flow was between 50 - , . ; , - : .
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and lOO cubic centimetres per minute.

The furnace temperature (controlled by programmer 3) was raised to 1000C and then maintained at that level.
Thermocouple 35 measured the temperature in the furnace as close to the sample as possible (Ts), and these measurements were fed to recorder 36 to allow the furnace temperature to be accurately controlled.

The samples were not pre-treated in any way.
!

The electromagnetic halance 4 of the thermobalance allowed for the continuous measurement of sample weight and the calculation of percentage weight losses (based on the initial weight of the sample) during the period for which the sample remained-in the furnace. The balance had a photo-sensitive null detector 31 and tare weights 32.
Tare weights 32 allowed for the balance to be "zero-d" at the start of each sample test, so that only changes in the sample weight were detected.

The results of the experiment are shown in Figures 2-12, each of which is a graph showing the weight loss curve (A), the derivative of the weight loss curve (B) (not shown in Figure 12F), and the temperature curve (C) over the period for which the sample remained in the furnace.
Weights are shown as percentages of the original sample weight.

Figures 13 and 14 allow comparison of the weight losses of sample A (GeO2) with those of samples s, C and D (Figure 13) and with those of samples E, F and G (Figure 14).

Germanium dioxide itself (Figure 2) showed an initia]

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decrease of 1.731% at 300C, which is consistent with loss of water of crystallisation. Major weight loss commenced at 680C, and by the time the temperature had been at 1000C for 25 minutes 67.15% of the GeO2 had been lost.

For sample B, weight loss commenced at 788C and 16.76%
weight loss had occurred by the time the temperature had reached 1000C. A further 6.237% loss was recorded after 21 minutes, followed by another loss of 2.177% after 51 minutes. (See Flgures 3A and 3s).

Sample C (Figures 4A and 4B) showed an initial weight loss at 750C and further heating for 30 minutes resulted in a weight loss of 33.81%. A further 5.35% was lost after 17 minutes. The run was extended for 330 minutes (see Figure 4B), resulting in a total weight loss of 58.09%.

Sample D (Figure 5) started to lose weight at 700C and by the time the temperature had reached 950~C, 27.72% weight loss had occurred. A further 5% loss occurred when the temperature reached 1000C. Again, extended heating resulted in a further 4.508% loss.

Sample E (Figure 6) commenced weight loss at the highest temperature of all of the samples, namely, 900C.
However, a total weight loss of 38.1% was recorded over the run.

Sample F (Figure 7) commenced weight loss at 880C, and after 95 minutes a 21.73% loss had been recorded. Over a further 25 minutes 5% more was l.ost, and weight loss was continuing when the run was terminated.

Sample G (Figure 8) commenced weight loss at 820C and ;, , : .: . .
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., after 30 minutes 11.86% had been lost. The run was extended and further losses of 5.455% and 2.911% were recorded after a further approximately 145 and 235 minutes respectively.

O
Sample H (Figure 9) commenced weight loss a-t 710 C and over a period of 88 minutes an overall weight loss of 10.6~% was recorded. For samples I, J and K (Figures 10, 11 and 12), weight loss commenced at 810, 760 and 730 C
respectively. Overall losses of 13.80% in 78 minutes (Sample I), 13.59% in 88 minutes (Sample J) and 14~58% in 88 minutes (Sample K) were recorded.

Samples L and M (Figures 12A and 12B) commenced loss at 875 and 790C respectively. Sample L showed an overall weight loss of 25.-59~ over the following 50 minutes and Sample M a loss of 23.87% over an 80 minute period.

Greater thermal stability under reducing conditions was exhibited by samples U X (Figures 12C-12F). All four samples commenced weight loss only at temperatures considerably in excess of 900C. Sample ~ (Figure 12C) commenced weight loss at 970C and ater 80 minutes a weight loss of 3% was recorded.

The greatest stability was illustrated by Sample V (Figure 12D), which commenced weight loss at 983C; after 75 minutes the recorded weight loss was 1%.

Sample W (Figure 12E) commenced weight loss at 980C with a weight loss of 2.25~ registered after 30 minutes.
Finally, weight loss in Sample X (Figure 12F) commenced at 919C and attained 1.5~ after 90 minutes.

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: . ::, ~ 3 The results of the above reduction tests, summarised in Table II, indicate that a]l of the sample materials B-X
are more thermally stable than germanium dioxide under reducing conditions, since weight loss occurs at higher temperatures, and to a lesser extent, for these materials than or germanium dioxide.

On the basis of these characteristics Samples B, E, F,G
and U-X were submitted for further work. Samp]es C and D
were not included because they have a lower thermal stability under reducing conditions than B and yet are similar in structure to B.

c) Sulphation under Oxidising Conditions The same equipment-was used as in the reduction tests described above, although certain modifications were required due to the use of sulphur dioxide gas. The modified instrumentation is shown schematically in Figure 15.

Approximately 40 mg of each sample to be tested (not pre-treated in any way) was placed in a high density alumina sample pan 5 having the form shown in Fiqure 15B. This pan was supported in a specially made support 6 (~ee Figure 15C) comprising a platinum plate 7 held in a 0.5 mm platinum wire cradle 8.

A stream of a sulphur dioxide (SO2) and air mixture (approximately 17% SO2 in air) was directed to flow through the furnace 9 in which the sample was placed. Air was supplied through the standard gas inlet 10 at a rate of 50 cm3/min, and SO2 through auxiliary gas inlet 11 at a rate of 10 cm3/min. The air/S02 mixture was passed out of ' 7,~ 3 the furnace to a sodium hydroxide absorber at point 34.

The standard thermocouple of the Dupont 951 thermobalance was replaced by a stainless steel-sheathed type "K"
thermocouple.

Once the sample had been placed inside the furnace 9, the furnace was heated up to a final temperature of 1000C, at a heating rate of 10C/min.

The results of this sulphation experiment are shown in Figures 16-20, which are graphs showing percentage weight of the initial weight (A) and furnace temperature (s) as functions of time.

Sample A (Figure 16) showed an initial weight gain followed by a small loss. The gain was assumed to be due to adsorption of SO2 in the water of crystallisation of the sample, followed by removal of the water of crystallisation. Thereafter the weight of the sample s-tabilised.
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Sample B (Figure 17) showed a gain of 0.5% up to 700C, a further gain of 1.250% up to 900C and then a gain of 7.8 after two hours at 1000C. From the plot the sulphation reaction appears still to be continuing at this stage.

Samples E and F (Figures 18 and l9) gave effectively the same result as for sample A, i.e. an essentially flat trace, which may well be simply the buoyancy curve for the instrument under the gas flow conditions used, compounded by possible adsorption and desorption of SO2 on the sample pan and/or the sample.

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., . ", The curve for sample G (Figure 20) again shows a flat trace, with the weight gain being somewhat larger than for samples E and F. The curve includes a vertical displacement (at around 62 minutes) which is considered to be due to an instrument error rather than loss of sample material. The vertical displacement was measured on an expanded trace and no discrete weight changes were observed.

The large gain, relative to that seen for E and F, may well reflect a higher adsorptive capacity for material G.

Thus, overall, only sample B showed any weight gain and this was not significant. The conclusion is that samples A, E, F and G show no significant reactivity to sulphur dioxide, i.e. no significant sulphation, under the oxidising conditions used. In addition, similar experiments conducted on samples H-K and U-X (data not shown) suggest that there is little, if any, sulphation of these materials under test conditions.

d) Sulphide Formation In order to investigate sulphide formation of the samples, it was necessary to heat the samples in the presence of hydrogen sulphide (H2S) gas.

Because of the toxic and corrosive nature of H2S, a conventional thermobalance could not be used. A special apparatus was therefore designed, and this is shown schematically in Figure 21.

The apparatus works on the extension and contraction of a quartz spring 12, due to increases or decreases in weight , , :

of the sample 13 which is suspended from the spring, in a clear silica crucible, by means of a silica rod 1~. The amount of extension or contraction of the spring 12 is measured using a conventional travelling microscope 15.

The sample was suspended in a furnace 16 which was heated (at a rate of 10C/min) during each experiment to a final temperature of 1000C. A mixture of 5~ H2S gas in nitrogen was passed through the furnace during experiments. The spring 12 was protected by a thermally insulating, water-eooled jacket 17, having a window through which spring 12 could be viewed using the microscope 15. Jacket 17 served to isolate, as far as possible, the spring from thermal currents produced from furnace 1~.
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"Blank" experimental runs were carried out prior to experimental analysis of samples, in which measurements were taken using the travelling microscope 15, with no sample suspended rom the spring 12, in order to calibrate buoyancy efects in the system. Correction could then be made to subseauent results for any expansion or contraction of the spring due to thermal eurrents from the furnace. Although the apparatus was not as sensitive as the thermobalance, it was felt that sufficiently accurate results could be obtained using this apparatus.

In each experiment earried out, approximately 200mg of each sample (not pre-treated) were heated in the apparatus of Figure 21. The results of these experiments are shown in Figures 22-25, whieh show pereentage weight loss of sample as a funetion of furnace temperature for show pereentage weight loss as a funetion of time for the period during whieh the furnace was maintained at a - ': : ,':', ' ~' ~, ~. , ~ , . ..
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temperature of 1000C (i.e. isothermal weight loss).

Sample A (Figures 22 and 26) commenced losing weight at 650C, a white vapour appearing in the furnace. Gas-flow through the furnace was increased to stop vapour rising up the apparatus and obscuring the view of the spring 12.
The weight loss of the sample was continuous up to 1 000 C . -~

On disassembly of the apparatus it was found that a large amount of white powder had condensed on the silica support rod 14. Since the rod is a part of the weighing system, the white powder would have been recorded as part of -the sample. This means that the percentage weight losses recorded are not to be taken as meaningful. However, the observations are si~nificant~

The germanium dioxide was exhibiting characteristics borne out by the literature i.e. sulphide formation in the presence of H2S. Germanium disulphide has a vapour pressure of 2 x 10 1 mmHg at 650C and as the temperature was increased one would expect the weight loss of the germanium dioxide sample to increase quite quickly. The initial weight loss was due to the loss of water of crystallisation.

Samples B, F and G (Figures 23-25 and 27-29) showed similar behaviour to sample A, commencing weight loss at between 700 and 800~C. In all cases a white vapour was formed which condensed on the lower part of the apparatus.
Again, percentage weight loss figures are not meaningful.

Sample E did not show any weight change, either gain or loss, on heating to 1000C and holding for 60 minutes.

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There was no deposit formed on the inside of the tube.
Graphs for sample E are therefore not included.

Similar tests (data omitted for the sake of brevity) under the same conditions have been performed on samples H-X, all of which have demonstrated sulphide formation by these materials.

Analytical tests are currently being carried out to determine the nature of the white condensate released from ! Samples B, F and G, and it is expected that results will confirm that germanium disulphide has been formed in each case. Analytical tests are also being carried out to determine whether the original composition of Sample E had changed in any way during the experiment. It should be noted that the test- results indicate that more meaningful results would be obtained by undertaking the experiments at 500C. This should alleviate the problem of product volatilisation.

e) Conclus on Table III contains a summary of the results of the various experiments carried out on Samples A-X.

As expected, germanium dioxide itself (sample A) fulfills three of the four criteria laid down for a suitable H2S
scavenger catalyst, but performs rela-tively poorly in assessments of thermal stability under reducing conditions. Samples B-X are clearly superior in this regard, Sample W being stable up to 980C and Sample V up to 983C.

Of the samples tested for sulphate and sulphide formation, ': .. , , ~ . .,~, ' ~ ' ~ . , " " ' ' :

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Samples B-X are clearly superior in this regard, Sample W
being stable up to 980C and Sample V up to 983C.

Of the samples tested for sulphate and sulphide formation, Samples F-K, W and X were found not to form sulphates under oxidising conditions, but to form sulphides under reducing conditions, i.e. they fulfilled all four o the criteria used to assess suitability as H2S suppressors. In addition, Samples U and V produced only minor amounts of sulphate and fulfilled all the other criteria complete]y.

Thus, samples of germanates containing a light rare earth metal, such as Sample G, samples of germanates containing intermediate rare earth metals (such as Sample X) and samples of germanates containing heavy rare earth metals (such as Sample ~) all exhibited characteristics suited to those of an H2S suppressor in an exhaust catalyst. It is reasonable to assume therefore that germanates containing other elements in the rare earth metal group may be equally useful as H2S suppressors, as it is well known that the chemical and physical properties of compounds of the rare earth metals tend to be similar across the series.

The effectiveness of H2S suppressing compounds may be enhanced by maximising the surface area available for absorption.

A number of experiments were performed to find ways of increasing the surface area of the germanate compounds. The preliminary experiment involved the grinding of germanium dioxide (GeO2). The indices used to determine the eEfects of grinding were crystallinity (as judged by relative percentage of X-ray diffraction, X~D) and/or surface area (as measured by the method of Brunauer Emmett Teller, BET).

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The results are shown in Table IV. This Table shows that dry grinding has an effect on crystallinity and that sur~ace area reaches a maximum (approximtely 14m per gram) after 10 minutes. It is considered -that prolonged grinding is unlikely to produce surface areas significantly greater than this~ Wet grinding was less effective, possibly due to some reaction between the solvent and the GeO2.

Similar grinding tests were performed on four monogermanates and two diyermanates. Essentially similar results were obtained, as shon in Table V. Tests were undertaken on neodymium and dysprosium monogermanates in a different grinding system. The results are shown in Table VI. The different grinding method did not significantly increase the surface area of the final product.

Dysprosium and erbium monogermanates were further investigated using samples which had been synthesised at lower temperatures. It was thought that lowering the synthesis temperature might have an effect on the surface area of the end product. The data are illustrated in Table VII. These data indicate that lowering the synthesis temperature had no significant effect on the surface area of the product following grinding. Ideally, the end product should have a surface area in the range of 10 to 30m per gram.

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Time Weight Loss Sample ~y~ of starting weight) A 12 0.208 A 22 0.330 B 12 0.141 ~ .
C . -12 0.115 D 12 0.084 E 10 0.125 F 10 0.130 G 10 0.111 H 7 0.119 I 7 0.091 J 3 0.000 K 3 0.000 L 5 0.037 M 5 0.037 U 7 0.000 V 7 0.000 W 7 0.000 X 7 0.000 : , . . ,.. ,: . , ': ' ' :: ~ . : :: ,:,: , . . : . , ... . . .. .

~ ~ '7 ~ 3 TABLE II

Temp at which WT % Time over which loss commenced Ge in Wt Loss wt loss occurred Sample _ (C) Sample ~_ _ (mins) A 680 67.7 67 60 B 788 45.18 25.174100 C 750 54.73 5$.09256 D 700 60.18 37.228102 E 900 28.19 38.10145 F 880 33.36 21.7390 G 820 27.13 20.226400 H 710 25.40 10.6488 I 810 ~ 24.94 13.8078 J 760 26.60 13.5988 K 730 26.94 14.5888 L 875 33.07 25.6950 M 790 32.89 23.8780 U 970 15.20 3 80 V 983 16.46 1 75 W 980 14.90 2,25 90 X 919 15.54 1.5 80 :' ' ' , ~ ,; , ,, ;"

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TABrlE III

Summary of Results Thermal Decomposition Sulphate Stability Temp under Red. Formation Sulphide Sample In Air Conditions ( C) up t_ 1000C Formation A Yes 680 No Yes B Yes 788 Minor Yes C Yes 750 Not Tested Not Tested D Yes 700 Not Tested Not Tested E Yes 900 No No : F Yes 880 No Yes G Yes 820 No Yes H Yes 710 No Yes I Yes 810 No Yes J Yes 760 No Yes K Yes 730 No Yes L Yes 875 Not Tested Yes M Yes 790 Not Tested Yes U Yes 970 Very Minor Yes V Yes 983 Minor Yes W Yes 980 No Yes X Yes 919 No Yes ., , : ., . : : :
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TABLE I~.7 Crystallinity Surfase Area Treatment (XRD relative percentage) (BET) m /g 0 High (100) 2.231

5 mins Slight decrease (72) l2.077 10 mins Further decrease (59) 14.015 15 mins Moderately crystalline (48) 13.905 5 mins (wet) Slight reduction (67) 5.287 !
TABLE V ~.
,:
Surface Area Sample Treatment (BET) m /g Nd2Ge5 0'57 8.14 Gd2GeO5 0 0 35 8.53 Er2GeO5 0 35 7.99 Y2 ~ 5 7.65 .:~i.`
Y2Ge207 9.40 La2Ge207 072.65 ,."' '"'': ' ' :

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TABLE VI

Surface Area Sample Treatment (mins) m /g , Dy2GeO5 0 0.334 5.6 7.65 Nd2GeO5 0.569

6.5 6.11 8.31 TABLE VII

Result Dysprosium Monogermanate Surface Area Treatment (m /g) 1200C 18 hrs, 1200C 5 hrs 0.33 As above ground (15 mins) 7.21 1120C 5 hrs 0.60 As above ground (15 mins) 8.49 Rrbium Monogermanate Surface Area Treatment (m /g) 1250C 24 hrs, 1250C 5 hrs 0.35 As above ground (15 mins) 7.73 1100C 7 hrs, 1150C 4 hrs, 0.35 1180C 2 hrs, 1250C 4 hrs As above ground (15 mins) 7.92 ., : ~ : .. . . .....
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Claims (13)

Claims:

1. A germanate for use as a suppressor of H2S in an exhaust system of an internal combustion engine.

2. A germanate according to claim 1 comprising the oxide of a rare earth metal or yttrium.

3. A germanate according to claim 1 or 2, further comprising one or more oxides of metals which are not rare earth metals or yttrium.

4. A germanate according to any of the preceding claims wherein the germanate is a monogermanate.

5. A germanate according to claim 4, wherein the monogermanate is one of the members of the group comprising: dysprosium monogermanate Dy2GeO5, neodymium monogermanate Nd2GeO5, erbium monogermanate Er2GeO5 or gadolinium mongermanate Gd2GeO5 or gadolinium monogermanate Gd2GeO5.

6. A germanate according to any of claims 1 to 3, wherein the germanate is a digermanate.

7. A germanate according to claim 6, wherein the digermanate is one of the members of the group comprising:
yttrium digermanate Y2Ge2O7, lanthanum digermanate La2Ge2P7, gadolinium digermanate Gd2Ge2O7, dysprosium digermanate Dy2Ge2O7, neodymium digermanate Nd2Ge2O7 praseodymium digermanate Pr2Ge2O7, Lanthanum aluminium digermanate LaAlGe2O7 or gadolinium aluminium digermanate GdAlGe2O7.

8. A germanate according to any of the preceding claims having a surface area in the range of 10-30m2 per gram.

9. A catalyst for use in an exhaust system of an internal combustion engine, comprising the germanate of any of the preceding claims.

10. A catalyst according to claim 9 comprising a three-way catalyst.

11. A catalytic convertor for use in an exhaust system of an internal combustion engine, comprising the catalyst of claim 9 or 10.

12. An exhaust system for an internal combustion engine comprising the catalyst of claim 9 or 10.

13. A method of producing the catalyst of claim 9 or 10 comprising the steps of: forming a suspension of catalyst components including the germanate of any of claims 1 to 8, applying the suspension to a supporting substrate and firing the substrate so as to form on it a coating of the catalyst components including the germanate.