^τitle: GERMANIUM COMPOUNDS AS SUPPRESSORS OF H2S

Field of the invention

This invention relates to the use of germanium compounds as suppressors of hydrogen sulphide (H_S) in exhaust systems, and to catalysts for use in the exhaust system o •P 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 (C0~), water (H ₂ 0) and nitrogen (N ₂ ). However, complete combustion rarely occurs inside a vehicle engine and gases such as carbon monoxide (CO), hydrocarbons (H C ) and unreacted oxygen (0_) are therefore also present in the exhaust gases. Nitrogen oxides, of the general formula NO , are also present, having been formed by the reaction between N ₂ and 0 ₂ _

CO, H C and NO are al] atmosoheric pollutants, and it is x y ^" x therefore desirable to reduce their presence in the exhaust gases to an environmentally (and, increasingly,

legally) acceptable level. The content of these pollutants in exhaust gases can be reduced to reasonably low levels by 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" ("T C catalysts"), which contain small σuantities of precious metals, such as platinum and rhodium. When hot exhaust gas contacts these catalysts, CO and HC react with 0_ and NO to form the harmless gases C0„, H„0 and N„.

In practice, the catalysts are mounted in a meta] casing (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 coating on the channel walls. The TWC catalysts are able to reduce the emission of all three pollutants simultaneously, provided that the engine is operated at an optimum air/fuel ratio of 14.7:1.

Often, additives such as cerium dioxide (CeO _? ) are incorporated into the catalysts to enhance the catalytic activity.

Vehicle engine exhaust gases also contain other atmospheric pollutants, however, among them hydrogen sulphide (H ₂ S), which is thought to be formed by the reaction cf culphur contained in the n^l with a TWC catalyst. Again, emissions of this chemical from vehicle engines have to be reduced to acceptable levels. There is

an urgent need to find an efficient way in which to remove H ₂ S from exhaust gases, which conventional TWC catalysts are unable to do.

The current method of H ₂ S removal is to add a nickel (II) oxide (NiO) "suppressor" component to the TWC catalyst in a vehicle exhaust system. Whilst NiO is an effective H„S 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 H»S 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 germanium (IV) oxide (germanium dioxide, Ge0 ₂ ) as such an alternative, and others working in the field are also directing their research into the use of GeO~ as an H ₂ S suppressor. However, at higher operating temperatures (above about 680°C) and under reducing conditions, such as are found in a vehicle engine usually when the vehicle is operated at high speeds, Ge0 ₂ is reduced to gaseous germanium (II) oxide (GeO) . This can lead to significant and unacceptable losses of the GeO„ suppressor.

Yamada et al's work involved investigating the properties of various single metal oxides and their suitability as replacements for NiO in TWC catalysts. Whilst Ge0 ₂ was found to be the most acceptable of the oxides tested, the above shows that it is still h ^v no means ideal. A TWC catalyst additive is still needed which will efficiently

and safely reduce levels of H„S formation by the catalyst.

In order to develop such a catalyst additive, the mechanism of H_S formation in exhaust systems must first be understood. This mechanism is well documented and is thought to consist of two stages, sulphur storage by the catalyst (at high airrfuel ratios) and subsequent sulphur release (at lower airrfuel ratios).

Sulphur dioxide (S0 ₉ ) is formed by combustion of sulphur contained in the fuel: this is stored on the catalyst at high airrfuel ratios (i.e. lean fuel mixtures):

(1) (2) (3)

(Here, MO designates a hypothetical metal (M) oxide, which may be, for instance, CeO_ such as in the commonly used Pt/Rh/Ce0 ₂ TWC catalyst).

At low air:fuel ratios (i.e. rich fuel mixtures) and temperatures of around 500°C, reductive sulphur release:

MSO4. + 4H2 MO + H ₂ S + 3H ₂ 0 (4)

and steady state sulphur release:

S0 ₂ + 3H ₂ >H ₂ S + 2H ₂ 0 (5)

can both occur, and levels of H ₂ S in the exhaust gases therefore increase.

H„S 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 H ₂ S emission by NiO and some other oxides is thought to lie in the "tying up" of the H ₂ S as a metal sulphide, ie. -

H ₂ S + NiO » NiS + H ₂ 0 (6)

Thus, a chemical which is to act as an H~S supressor in place of NiO must have two essential qualities:

1) it must not form a sulphate at low airrfuel ratios;

2) it must be capable of traυpinq H„S as a sulohide at high air:fuel ratios.

The chemical should also be thermally stable in air and under reducing conditions, at temperatures higher than those to which Ge0 ₂ is stable (ie around 680 ^β C) and ideally up to around 1000°C.

It is an aim of the present invention to provide such a chemical, which can be used to supress H~S 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 H~S supressors.

Statement of the invention

In one aspect the present invention provides a germanate for use as a suppressor of H ₂ S in an exhaust system of an internal combustion engine.

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

MGeO , 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

(M. ) (M ) , (M_ ) . . . Ge 0 , where M. , M , M, . . - are rare l a o c y z 1 3 earth metals or yttrium. The germanates of use in the present invention may also include those in which one or more of the metals M.. , M„, M,... is not a rare earth metal or yttrium.

The group of rare earth metals (also known as "lanthanides") comprises the fourteen 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), Ytterbium (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.

Yttrium is often grouped together with the rare earth metals because it shares the chemical properties of the

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 H ₂ S suppressors, i.e. they do not form sulphates under oxidising conditions and they form sulphides in the presence of H_S under reducing conditions. They are also more thermally stable than Ge0 ₂ under reducing conditions, and lack the toxicity problems associated with the use of NiO as an H ₂ S suppressor.

Although the use, as H ₂ S 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 H-S.

The germanate is preferably a lanthanide or yttrium containing germanate, more preferably a lanthanide or yttrium containing digermanate such as La-.Ge.-0_ or Y ₂ Ge ₂ 0 _? .

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 for the purpose of suppressing H„S 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 comprising 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 H„S 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

existing as mixtures of their component oxides, M 0 and GeO_. (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 of the following example and with reference to the accompanying Figures, of which:-

Figures 1, 15 and 21 show schematically apparatus used to investigate 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 H ₂ S suppressors in

exhaust systems. The sixteen sample germanates, B-X, were also compared with germanium dioxide itself (sample A), for which the GeO~ 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 1000°C. (It is known that, for example car catalysts, under exceptional conditions, can operate at as high as 1000°C).

2) They should be thermally stable under reducing conditions at significantly higher temperatures than is GeO (1000°C 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 H_S suppressor for an acceptable period of use).

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

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

Sample preparation

The samples investigated had the following formulae:

A = Germanium dioxide, Ge0 ₂

B = Calcium monogermanate, CaGeO-

C = Calcium diger anate, CaGe ₂ 0-.

D = Dicalcium heptagermanate, Ca ₂ Ge-0 _lfi

E = Aluminium germanate, AlgGe ₂ 0 ₁

F = Yttrium digermanate, Y ₂ Ge ₂ 0 ₇

G = Lanthanum digermanate, La ₂ Ge„0_

H = Gadolinium digermanate, Gd ₂ Ge ₂ 0 ₇

I = Dysprosium digermanate, Dy„Ge--0 ₇

J = Neodymium digermanate, Nd ₂ Ge 0 ₇

K = Praseodymium digermanate, Pr-Ge-O..,

L = Lanthanum aluminium digermanate, LaAlGe-0 ₇

M = Gadolinium aluminium digermanate, GdAlGe ₂ 0 ₇ ϋ = Dysprosium monogermanate Dy.-GeO-

V = Neodymium monogermanate Nd ₂ GeO-. w = Erbium monogermanate Er ₂ GeO--

X = Gadolinium monogermanate Gd ₂ GeO-.

This allowed the comparison of rare earth or yttrium- containing germanates, F-X, with germanates 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 H_S suppressors, and for this reason germanates of aluminium and calcium were experimentally evaluated. Whilst germanates of the heavier transition metals might also be expected to have the necessary stability, no such compounds were tested since toxicity problems can be encountered with transition metals, making them unsuitable for use in exhaust systems. Moreover, some transition metals can act as catalyst poisons.

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 B-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 (CaCO_) used as a reactant.

The samples were placed in platinum crucibles and initially ignited at 900°C for 4-5 hours in order to remove water and C0 ₂ (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 firings were done in air, in electrically- heated muffles. The accuracy of temperature control was +_ (2-4 ^c C) , and the absolute accuracy of the recorded temperature was Hr20°C. All heatings were performed in platinum crucibles. No signs of attack on the crucible were noted.

Sample B, CaGe0 ₃

Reactants: Analar* CaCO-. and Meldform* Ge0 ₂ .

Stage 1 Ignited at 900°C for 11 hours Stage 2 Ignited at 1200°C for 13 hours Stage 3 Ignited at 1200°C 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 a ₀ = 8.07; b„ = 7.46; C ₀ = 7.23 angstroms.

Reference: Jost, Wolf and Thilo, Z. 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 given above, a list of X-ray d-spacings was calculated from the unit cell parameters, which indicated that these extra reflections could in fact be due to monogermanate. The suspicion is therefore that these "extra" reflections were simply overlooked in previous data. The product (Sample B) is therefore essentially single-phase.

* Proprietary names.

Sample C, CaGe.,0-

Reactantsr Analar CaCO, and Meldform GeO„.

Stage 1 Ignited at 900°C for 7 hours Stage 2 Ignited at 1150°C for 12 hours Stage 3 Ignited at 1150°C for 5 hours.

Weight loss during final 5 hour treatment = 0.044%

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

System: Triclinic a _c = 6.860o; bo = 8.787; oCo =• 6.527 angstoroms alpha = 91.02 ; beta = 113.02 ; gamma = 88.18

Referencer Technisch Physische Dienst, Delft, Holland.

The X-ray powder pattern gave reasonable agreement with the above card. A powder X-ray pattern was generated, using single-crystal data from Belov et al's 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 Ca ₂ Ge_-0..,. Thus Sample C probably contains ca 5% of Ca ₂ Ge ₇ 0 _1g .

Sample D, Ca ₂ Ge ₇ 0 _lg

Reactants: Analar CaC0-> and Meldform Ge0 ₂

Stage 1) Ignited in the Ca0.4Ge0 ₂ ratio at 900°C for 6 hours. Sufficient CaC0_ then added to bring

- 15 - the ratio to 2:7.

Stage 2) Ignited at 1125°C for 16 hours + 1125°C for 14 hours, adding excess CaCO- at this stage.

Stage 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- 286.

System: Orthorhombic a ₀ = 11.3456; bo = 11.3436; c> = 4.6409 angstroms.

Reference: Breuer and Eysel, Univ. of Heidelberg.

Very good agreement was obtained.

Note: Various published phase diagrams of the CaO-GeO_ system disclose the existence of a "tetragermanate", Ca0.4Ge0 ₂ . We were unable to make this phase. When pilot batches showed the appearance of Ca ₂ Ge ₇ 0 _1fi , and unequivocal 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, AlgGe ₂ 0 ₁₃

Reactants: Cera* Alumina (A1 0 ₃ ) and Meldform GeO„

Stage 1) Ignited at 900°C for 4 hours.

Stage 2) Ignited at 1125°C for 14 hours, then at 1300°C for 15 hours Stage 3) Ignited at 1300°C for 5 hours.

Weight loss at 1300°C for final 5 hours = 0.035%

The X-ray diffraction pattern was compared to JCPDS card 27-1005.

System: Orthorhombic a _D = 7.655; bo = 7.775; c. = 2.924 angstroms. (Mullite structure).

Reference: Toropov, et al. J. Appl. Chem USSR _4_3_ 2171-2174 (1970).

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

^♦ Proprietary name.

Sample F, Y ₂ Ge ₂ 0.

Reactants: Meldform Y C and Meldform GeO _?

Stage 1) Ignited at 900°C for 5 hours Stage 2) Ignited at 1400°C for 15 hours Stage 3) Ignited at 1400°C for 5 hours.

Weight loss at 1400°C 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 a _c = 6.8040 Co = 12.375 angstroms.

Very good agreement was obtained between observed and calculated patterns: the product was essentially single phase.

Sample G, La _? Ge ₂ 0 ₇

Reactants: Meldform La-O, and Meldform Ge0 ₂ .

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

Weight loss at 1300°C 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 ( 1967).

No unit cell data.

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

Using crystal data from Smolin Yu et al, Doklady Akademie Nauk USSR (1969) an X-ray diffractometer trace was calculated. This agreed very well with the observed X-ray trace for sample G.

However, 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 uncertain - literature characterisation data are inadequate - the sample prepared appeared to consist of a single phase of the La ₂ 0 ₃ .2Ge0 ₂ composition.

Sample H, Gd ₂ Ge ₂ 0 ₇

Reactantsr Laboratory supply of Gd ₂ 0 ₃ (heated at 900°C for 6 hours) and Meldform Ge0

Procedurer Reactants were heated at 1100°C for 20 hours, re-mixed and heated at 1150°C for 18 hours and finally re¬ mixed and heated at 1150°C for 22 hours.

From its X-ray diffraction pattern, the product appeared to be isostructural with Dy ₂ Ge ₂ 0 ₇ (JCPDS Card 38-289).

Sample I, Dy_Ge ₂ 0 ₇

Reactantsr Laboratory supply of Dy ₂ 0 ₃ (heated at 900°C fo 6 hours) and Meldform GeO„.

Procedurer As for Sample H.

X-ray diffraction pattern found to be in agreement with JCPDS Card 38-289 for Dy ₂ 0_.2Ge0 ₂ .

Sample J, ₂ Ge ₂ 0 ₇

Reactantsr Meldform Nd ₂ 0-, and Meldform Ge0 ₂ .

Procedurer Reactants were heated at 1150°C for 18 hours, then re-mixed and heated at 1150°C 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%.

The X-ray diffraction pattern was very similar to that observed for La ₂ Ge ₂ 0 ₇ (Sample G).

Sample K, Pr ₂ Ge ₂ 0 ₇

Reactants: Meldform Pr,0.... and Meldform GeO-

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

^Pr 6°11 ^{to Pr} 2°3-

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 La ₂ Ge ₂ 0 ₇ , which indicates that the praseodymium is probably present in the +III oxidation state.

Sample L, LaAlGe ₂ 0 ₇

Reactantsr Meldform La ₂ 0 ₃ ; Meldform Ge0 ₂ AnalR Alumina

Procedurer Reactants were heated at 1150°C for 24 hours, re-mixed and heated at 1300°C for 3 days and finally re¬ mixed and heated at 1320°C 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 NdAlGe-O- _^ -type structure.

Sample M, GdAlGe ₂ 0 ₇

Reactantsr Laboratory supply of Gd O-, (heated at 950 ^C C for 6 hours); Meldform Ge0 ₂ ; AnalR Alumina.

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

The X-ray diffraction pattern of the product gave reasonably good agreement with that given by Kaminski et al for LaGe ₂ 0 ₇ .

Sample U Dy-GeO..

Reactants Meldform Dy^O.-,, Ge0 ₂

Dy ₂ 0 ₃ heated at 100°C for 5 hours.

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

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

Sample V Nd ₂ Ge0 ₅

Reactants Meldform Nd ₂ 0 ₃ Ge0 ₂

N ₂ 0^ heated at 900°C for 5 hours,

Procedure As for sample U.

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

Sample W Er ₂ Ge0-.

Reactants Meldform Er ₂ 0 ₃ e0 ₂

Er ₂ 0 ₃ heated for 5 hours at 800°C.

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

X-ray powder diffraction pattern agreed with JCPDS Card 38-287.

Sample X Gd ₂ GeO_

Reactants Laboratory supply of Gd ₂ 0 ₃ , M Meellddffoorrmm GGee00 ₂₂ (Gd ₂ 0 ₃ heated for 5 hours at 800°C

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

measured into a small open crucible and placed in a well- ventilated furnace at 1000°C. 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 furnace 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 (GeO_) 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 Ge0 ₂ was within acceptable limits.

b) Stability under Reducing Conditions

The reduction tests were carried out in a Dupont 951 ther obalance with a 9900 controller and data handling facilities. The schematic configuration of the apparatus is shown in Figure 1.

Approximately 35 g 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% H ₂ in N, (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

- 24 - and 100 cubic centimetres per minute.

The furnace temperature (controlled by programmer 3) was raised to 1000°C and then maintained at that level. Thermocouple 35 measured the temperature in the furnace as close to the sample as possible (T ), 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 balance 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 (GeO^) with those of samples B, C and D (Figure 13) and with those of samples E, F and G (Figure 14).

Germanium dioxide itself (Figure 2) showed an initial

. .

- 25 - decrease of 1.731% at 300°C, which is consistent with loss of water of crystallisation. Major weight loss commenced at 680°C, and by the time the temperature had been at 1000°C for 25 minutes 67.15% of the Ge0 ₂ had been lost.

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

Sample C (Figures 4A and 4B) showed an initial weight loss at 750°C 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 700°C 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 1000°C. 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, 900°C. However, a total weight loss of 38.1% was recorded over the run.

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

Sample G (Figure 8) commenced weight loss at 820°C and

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 at 710 C and over a period of 88 minutes an overall weight loss of 10.64% was recorded. For samples I, J and K (Figureso10,

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 790 ^C C respectively. Sample L showed an overall weight loss of 25.69% 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 900 ^C C. Sample U (Figure 12C) commenced weight loss at 970°C and after 80 minutes a weight loss of 3% was recorded.

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

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

The results of the above reduction tests, summarised in Table II, indicate that all 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 for germanium dioxide.

On the basis of these characteristics Samples B, E, F,G and U-X were submitted for further work. Samples 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 Figure 15B. This pan was supported in a specially made support 6 (see Figure 15C) comprising a platinum plate 7 held in a 0.5 mm platinum wire cradle 8.

A stream of a sulphur dioxide (S0 ₂ ) and air mixture

(approximately 17% S0 ₂ 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

₃ of 50 cm / in, and S0 through auxiliary gas inlet 11 at a rate of 10 cm /min. The air/S0 ₂ mixture was passed out of

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 1000°C, at a heating rate of 10 ^c C/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 (B) 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 SO- in the water of crystallisation of the sample, followed by removal of the water of crystallisation. Thereafter the weight of the sample stabilised.

Sample B (Figure 17) showed a gain of 0.5% up to 700°C, a further gain of 1.250% up to 900 ^β C and then a gain of 7.8% after two hours at 1000°C. From the plot the sulphation reaction appears still to be continuing at this stage.

Samples E and F (Figures 18 and 19) 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 S0 ₂ on the sample pan and/or the sample.

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 (H ₂ S) gas.

Because of the toxic and corrosive nature of H^S, 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 14. 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 10°C/min) during each experiment to a final temperature of 1000°C. A mixture of 5% H ₂ S gas in nitrogen was passed through the furnace during experiments. The spring 12 was protected by a thermally insulating, water-cooled 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 16.

"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 from the spring 12, in order to calibrate buoyancy effects in the system. Correction could then be made to subsequent results for any expansion or contraction of the spring due to thermal currents 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 carried 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, which show percentage weight loss of sample as a function of furnace temperature for show- percentage weight loss as a function of time for the period during which the furnace was maintained at a

temperature of 1000 ^C C (i.e. isothermal weight loss).

Sample A (Figures 22 and 26) commenced losing weight at 650°C, 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 1000°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 significant.

The germanium .dioxide was exhibiting characteristics borne out by the literature i.e. sulphide formation in the presence of H_S. Germanium disulphide has a vapour pressure of 2 x 10 -1 mmHg at 650 ^C C 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 1000°C and holding for 60 minutes.

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 500°C. This should alleviate the problem of product volatilisation.

e) Conclusion

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 H S scavenger catalyst, but performs relatively poorly in assessments of thermal stability under reducing conditions. Samples B-X are clearly superior in this regard, Sample W being stable up to 980°C and Sample V up to 983°C.

Of the samples tested for sulphate and sulphide formation,

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

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

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 F) all exhibited characteristics suited to those of an H ₂ S 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 H„S 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 H„S 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 (Ge0 ₂ ). The indices used to determine the effects of grinding were crystallinity (as judged by relative percentage of X-ray diffraction, XRD) and/or surface area (as measured by the method of Brunauer Emmett Teller, BET).

The results are shown in Table IV. This Table shows that dry grinding has an effect on crystallinity and that surfac

2 area reaches a maximum (approximtely 14m per gram) after 1 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 GeO„ .

Similar grinding tests were performed on four monogermanates and two digermanates. 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

- 3.„0m2 per gram.

TABLE 1

Time Weight Loss

Sample (Days ) (% 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

TABLE II

TABLE III

Summary of Results

Thermal Decomposition Stability Temp under Red.

Sam le In Air Conditions (°C)

680 788 750 700 900 880 820 710 810 760 730 875 790 970 983 980 919

TABLE IV

Crystallinity Surface Area

Treatment (XRD relative percentage)

0 High (100)

5 mins Slight decrease (72)

10 mins Further decrease (59)

15 mins Moderately crystalline (48)

5 mins (wet) Slight reduction (67)

TABLE V

Surface Area

Sample Treatment (BET) m ² /g

d ₂ Ge0 ₅ 0.57 8.14

G ₂ Ge0 ₅ 0.35 8.53

7.99

Dy ₂ GeO, 0.33 7.65

Y ₂ Ge ₂ 0 _? 0.345 9.40

La Ge 0. 0.235 7.65

TABLE VI

Surface Area

Sample

Dy ₂ Ge0 ₅

N ₂ Ge0 ₅

Result

Dysprosium Monogermanate

Surface Area

2

Treatment (m /g)

1200°C 18 hrs, 1200°C 5 hrs 0.33

As above ground (15 mins) 7.21

1120°C 5 hrs 0.60

As above ground (15 mins) 8.49

Erbium Monogermanate

Surface Area

2

Treatment (m /g)

1250°C 24 hrs, 1250°C 5 hrs 0.35

As above ground (15 mins) 7.73

1100°C 7 hrs, 1150°C 4 hrs, 0.35 1180°C 2 hrs, 1250°C 4 hrs

As above ground (15 mins) 7.92