CA1131197A

CA1131197A - Denitrating catalysts having porous coating and process for producing same

Info

Publication number: CA1131197A
Application number: CA331,302A
Authority: CA
Inventors: Hideya Inaba; Kenichi Nagai; Masami Suzuki; Nobuo Matsumoto; Yasumi Kamino; Masayoshi Ichiki; Hisao Ogiyama
Original assignee: Hitachi Zosen Corp
Current assignee: Hitachi Zosen Corp
Priority date: 1978-07-06
Filing date: 1979-07-06
Publication date: 1982-09-07
Also published as: GB2024640A; JPS5926341B2; NL7904504A; FR2430260A1; NL179552B; JPS558875A; NL179552C; BE877182A; IT7949522A0; DE2926894A1; DE2926894C2; GB2024640B; FR2430260B1; IT1119768B

Abstract

ABSTRACT OF THE DISCLOSURE
A denitrating catalyst having a porous coating is produced by immersing a catalyst base of porous metal in a silica-containing coating bath, drying the catalyst base to form a carrier having a porous silica coating, immersing the carrier in a solution containing an active component and dry-ing the resulting carrier to cause the carrier to support the active component thereon. These catalysts exhibit both improved porosity and strength charac-teristics.

Description

11~1197 This invention relates to denitrating catalysts for use in a reaction in which nitrogen oxides (NOx) in exhaust gases are sel-ectively catalytically reduced with NH3, and to a process for pro-ducing the catalysts.
Many processes for treating NOx in exhaust gases have al-ready been proposed. Among these processes, the denitration pro-cess is most feasible in which nitrogen oxides are selectively cat-alytically reduced to harmless N2 at a specified temperature with NH3 in the presence of a catalyst. Although many catalysts have also been proposed for use in this process, they still remain to be improved. We prepared a catalyst from the porous metal material disclosed in United States Patent No. 4,040,981 by immersing a catalyst base of the metal material in a silica-containing bath and drying the catalyst base to form a porous silica coating thereon.
We found that the catalyst has high mechanical strength and retains sustained stable activity free of poisoning as by KCl. This inven-tion relates to improvements in this catalyst.
Thus this invention seeks to provide silica coatings having improved porosity and therefore enhanced permeability to reaction gases. This invention further seeks to provide denitrating catalysts having improved activity and satisfactory strength.
The denitrating catalysts of this invention having a porous coating are produced by treating a metal material having an aluminum alloy surface layer with an aluminum-dissolving solution to dissolve out aluminum; coating the porous catalyst base in a silica containing coating bath; drying the coated base; immersing the thus obtained carrier in a solution containing the active com-ponent, and drying the resulting carrier having an active component - 1 - ~
~3 -11~1197 thereon.

These and other feature.s of this invention will become more apparent from the following detailed description given by way of example only with reference to the accompanying drawings, in which:

la -A~;

~131~9 ~7 Figure 1 is a graph showing the relation between the pore siæe and the total pore volume;
Figure 2 is a graph showing the relation between the treating time and the average wear thickness;
Figures 3 and 4 are graphs each showing the relation between the reaction temperature and the denitrating efficiency;
Figure 5 is a graph showing the relation between the coating thick-ness and the denitration efficiency;
Figure 6 is a graph showing the relation between the concentration of V compound and the amount of V supported on the carrier per unit amount of the adsorbed substances; and Figure 7 is a graph showing the relation between the denitrating efficiency and the amount of V supported on the carrier per unit amount of the adsorbed substances.
Examples of catalyst bases useful in this invention are those pre-pared from metal materials having an aluminium alloy surface layer by treat-ing the materials with an aluminium dissolving solution to dissolve out alumin-ium and those obtained by etching the surfaces of metal materials to form porous rough surfaces thereon. Examples of useful metal materials are pure iron, iron-base alloys, steel, nickel, nickel-base alloys and copper-base alloys. Metal materials having an aluminium alloy surface layer are prepared forexample by subjecting an aluminium-coated metal mateTial to heat treatment.
Aluminium can be dissolved out from metal materials having an aluminium alloy surface layer by immersing such a material in an aluminium dissolving solution or by spraying the solution onto the material. Exemplary of useful aluminium dissolving solutions are aqueous solutions of alkali metal hydroxides such as NaOH, alkali metal carbonates, alkaline earth metal hydroxides and mineral acids. The aluminium dissolving treatment forms a porous surface layer 113~19\7 on the metal material.
Preferably the porous metal material is subjected to oxidizing treatment and/or S02 treatment. Although the treating temperature, treating time and oxygen concentration for the oxidizing treatment are not particularly limited, it is preferable to treat the metal material in an atmosphere con-taining 0.1 to 20.8 vol. % of oxygen at room temperature to 400C for 0.1 to 20 hours. The S02 treatment is carried out preferably in an atmosphere con-taining at least lOOppm of S02 at room temperature to 400C for 0.1 to 20 hours.
Colloidal silicas include alkaline colloids containing a large amount of alkali metal and acid colloids containing a smaller amount of alkali metal, of which the latter are preferred. Useful acid colloids have a pH of 3 to 4. The concentration and temperature of the coating bath, the time for which the catalyst base is immersed in the bath and the number of immersing procedures repeated are suitably determined so that the resulting coating will have the desired thickness. The immersed catalyst base is dried at a temperatureof 50 to 150C. Preferably the coating step is carried out with use of a colloidal silica containing 10 to 30 wt.% of SiO2 as the coat-ing bath by immersing the catalyst base in the bath at room temperature for about 10 minutes, withdrawing the base therefrom, drying the base at about 90 C for one hour and repeating the immersing and drying procedures once to 6 times to obtain a coating of 7-20 ~ thickness.
When the coating step is conducted with use of a coating bath comprising a mixture of collodial silica and an emulsion of high molecular weight substance by immersing the catalyst base in the bath, drying the wet base to form a coating thereon and baking the coatod base, the resulting coating will have a greatly improved porosity. Examples of preferable high molecular weight substances are acrylic compounds which will not give any harmful gas during baking. The concentration of the high molecular weight ., ' 1~31~97 substance in the coating bath is suitably determined in accordance with the desired mechanical strength, the component and particle size of the dust con-tained in the reaction gas. Preferably 10 to 50 parts by weight of the high molecular weight substance is used per 100 parts by weight of SiO2 in the col-loid. The coated base is baked in air at a temperature of 450 to 700C, pre-ferably 500 to 650C, for 1 to 5 hours. The high molecular weight organic com-ponent is removed from the coated base by the baking step.
When a mixture of a colloidal silica, an emulsion of high molecular weight substance and a titanium compound is used as the coating bath, the re-sulting carrier will have improved properties, giving a catalyst of outstand-ng activity. Further a coating bath comprising a mixture of a colloidal silica, an emulsion of high molecular weight substance, a titanium compo~nd and a tin compound, when used, gives a catalyst having high resistance to sulfuric acid.
Examples of useful titanium compounds are water-soluble organic titanium com-pounds such as ammonium salt of Ti(OH)2- [OCH(CH3) COOH~2 and Ti[OC2(NH4)2O3]4.
Examples of ~useful tin compounds are organic tin compounds such as dibutyltin laurate. Such titanium compound and tin compound undergo thermal decomposition when baked, giving TiO2 and SnO2 respectlvely. Preferably the resulting coating contains 40 to 100 parts by weight of TiO2 per 100 parts by welght of SiO2 and 30 to 70 parts by weight of SnO2 per 100 parts by weight of SiO2.
Examples of solutions containing an active component useful for the immersion treatment of the carrier are solutions containing a vanadium compound such as vanadyl sulfate, vanadyl oxalate, ammonium metavanadate or the like;
solutions containing a hydrolyzable titanate such as tetraisopropyl titanate;
and solutions of a sulfate or halide of copper, iron or antimony, tungstate or ; chromate. When the carrier is immersed in such a solution, a V compound, Ti compound, Fe compound, Cu compound, Sb compound, W compound and/or Cr compound will be supported on the carrier.

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1131~9'~

The concentration and temperature of the solution, immersion time and like conditions are dependent on the amounts of active co~,ponents to be supported on the carrier. Preferred amounts by weight to be supported on the carrier are 0.15 to 1.5% for V, 0.15 to 1~5% for Ti, 0.16 to 1.6% for Fe, 0.17 to 1.7% for Cu, 0.1 to 3.0% for Sb, 0.15 to 1.5% for W and 0.2 to 2.0% for Cr.
The catalysts thus obtained all have high denitrating activity.
Especially the catalysts with a V compound and Ti compound supported thereon have still higher activity and in addition high resistance to sulfuric acid.
Reference Example 1 Measurement of the porosity of coatings A colloidal silica (pH: 3.5) containing 22 wt. % of SiO2 and three kinds of emulsions a, b and c of acrylic high molecular weight substance shown in Table 1 were mixed together in varying proportions to prepare 11 kinds of coating baths A, B, ..., K. The bath A was free from any emulsion. The 11 baths were placed respectively into 11 stainless steel dishes of 5 cm in inside diameter to a depth of about 4 mm. Each of the baths was heated at 90C for one hour to remove the water, and the resulting solid was baked in air at 500C
for one hour. The coating formed on the dish was separated therefrom, and the porosity of the coating was measured by a high-pressure mercury porosimeter.
Table 2 shows the results.

Table 1 Emulsion Glass transitionParticle size temperature ( C~ (A) a -45 2000 _ +25 400 .

.

11311'37 Table 2 BathComposition of bath ~parts by wt.) Total pore High moleuclar weight substance volume SiO2 Emulsion a Emulsion _ Emulsion c ~cm3/g) A 100 - - 0.028 B 94.7 5.3 - 0.100 C 90 10.0 - O. 119 D 81.818.2 - 0.231 E 75 25.0 - 0.319 F 82 13.2 5.0 0.220 G 82 8.2 10.0 0.210 H 94.7 - 5.3 0.083 I 90.0 - 10.0 0.190 J 75.0 - 18.2 0.432 K 90.0 5.0 5.0 0.127 The relation between the pore size and the total pore volume was determined on the coatings a, ~, y,~ and formed from the baths A, C, E, G
and J. The results are shown in Figure 1 which reveals that the coating formed from the emulsion-free bath is somewhat lower in porosity but that the coatings formed from the emulsion-containing baths have a high porosity.
Curve y ~ndicates that the use of emulstion a composed of high molecular weight substance of low glass transition temperature gives coating pores which are predominan~ly 30 to 70 ~` in pore size and which scarcely include pores at least 1000 A in pore size and suited for the passage of the reaction gas through the coating. As represented by Curve ~, the emulsion b composed of high molecular weight substance of high glass transition temperature affored a coating formed with a relatively large number of pores not smaller than 100 A
and having good gas permeability.

9~7 Reference Example 2 a. Preparation of carriers Steel panels of SUS 304 (JIS), 2mm x 33 mm x 50 mm, were immersed in a molten aluminum bath at 680C for 20 minutes to obtain steel panels coated with aluminum. Each of the panels was then heat-treated in a nitrogen gas atmosphere at 800C for one hour to cause aluminum to diffuse through the panel to form an aluminum alloy surface layer on the panel. The panel was then immersed in 200 mQ of aqueous solution containing 10 wt. % of NaOH at 80C for 3 hours to dissolve out aluminum from the alloy to render the surface layer porous. Subsequently the steel panel was washed with water, dried in air and exposed to nitrogen gas containing 3 vol. % of oxygen at 300C for 3 hours to oxidize the porous surface layer. In this way, catalyst bases of porous steel were formed.
Some of the catalyst bases were immersed in the coating bath A
prepared in Reference Example 1 at room temperature for 10 minutes, then with-drawn from the bath and dried at 90C for one hour. The immersing and drying procedures were repeated 3 times to form a 7 - to 10 - micron-thick p~rous silica coating on the bases. The resulting bases were then baked in air at 600C for one hour to remove the high molecular weight organic component, whereby a carrier (a) was obtained. In the same manner as above, carriers (c), (d), (g) and (;) were produced with use of coating baths, C, D, G and J
prepared in Reference Example 1.
b. Measurement of the strength of carriers The carrier (a) obtained was placed into a stirring container filled with silica gel crushed to 60- to 80-mesh sizes, and the silica gel was stir-red to wear away the surfaces of pieces of carrier.(a). Variations in the weight of the carrier (a) were measured at specified time interval to deter-mine the average thi kness of the carrier ~a) worn aNay. The same proceu~re as above was repeated with use of the carriers Cc), (d), (g) and (j) to determine the relation between the average wear thickness and the stirring time. The results are shown in Figure 2. Generally wear resistance, namely mechanical strength, decreases with increasing porosity. Figure 2 shows that the carriers ~c), (d), ~g) and (j) prepared with use of the emulsion-containing baths have higher porosity than the carrier (a) obtained with use of the emulsion-free bath and are nevertheless comparable thereto in strength.
Example l a. Preparation of catalysts Steel Raschig rings, 21 mm in diameter and 20 mm in height, were used as a material for the catalyst base. Six pieces of catalyst base were pre-pared by forming a porous layer on the rings in the same manner as in Reference Example 2. The mixture of the colloidal silica and the emulsion ~ used in Reference Example l, ammonium salt of Ti-(OH)2 ~OCH~CH3) COOH]2, and a mixture of dibutyltin laurate and emulsion a were mixed together in varying proportions to prepare four kinds of coating baths L, M, N and O as listed in Table 3.
Of the six pieces of catalyst base previously prepared, one was placed into each of the baths L and M, and two into each of the baths N and 0. The pieces were thereafter subjected to repeated immersion-drying treatment and to baking step under the same conditions as in Reference Example 2 to form a porous coating on the pieces to obtain six carriers. The five carriers except for one obtained with use of the bath O were caused to support TiO2 and/or V205. For the support of TiO2, the carrier was immersed in liquid tetraisopropyl titanate at room temperature for 10 minutes, withdrawn there-from, then allowed to stand in saturated water vapor at room temperature for 12 hours to hydrolyze the titanate and thereafter dried at 100C. For the support of V205, the carrier was immersed in a solution of 1 mole of NH403 in 1 liter of 15 vol. % aqueous solution of monoethanolamine at room temperature for 10 minutes and thereafter baked in air at 300 C for one hour.
For the support of both TiO2 and V205, the carrier was made to support the former first. In this way, five catalysts 1, m, n-l, n-2 and o-l were obtained as listed in Table 3. The carrier having neither of these compounds supported thereon is also listed as a catalyst 0-2.
Table 3 Bath Composition of bath Active component Catalyst (wt. parts) supported (wt. pts) SiO2 TiO2 SnO2* Emul-**TiO2 V205 sion a L 100 - - - - 2.3 5.5 M 82 - - 18 2.3 5.5 m N 57 25 - 18 - 5.5 n-l N 57 25 - 18 2.3 5.5 n-2 0 57 11 14 18 - 5.5 o-l 0 57 11 14 18 - - o-2 (comparison) * Calculated as the ploportion of the metal oxide from the proportion of the corresponding organic metal compound used for the preparation of the bath.
**Proportion of the high molecular weight substance concerned.
b.Activity test The catalysts were tested for activity with use of a quartz re-actor tube of the flow type. The catalyst 1 was placed into the reactor tube fixed in position, and a test exhaust gas of the composition shown in Table 4 was passed through the reactor tube at a rate of 15 m/hr per unit geometric surface area of the catalyst.

_g_ ~1311'~7 Table 4 Component of gas Proportion ~vol. %) NO 0.02 S2 0.02 NH3 0.02

2 5.0 H20 10 . O
C2 10. 0 N2 Balance 10Denitration efficency was calculated from the difference between the NO concentration at the inlet of the reactor tube and that at the outlet thereof. The same procedure was repeated at varying reaction temperatures to determine denitration efficiencies at the temperatures.
Similarly denitration efficiencies were determined with use of the catalysts ; m, n-l, n-2, o-I and o-2. Figure 3 showing the results reveals that the catalysts incorporating V205 all have high activity. Particularly high activity is available with the catalysts composed of a carrier prepared with use of the emulsion-containing coating bath and having V2O5 supported on the carrier.
20The catalysts obtained in this example were exposed to air containing 4000 ppm of sulfuric acid vapor at 400 C for 2 hours and were thereafter tested for denitration efficiency in the same manner as above. The results are shown in Figure 4. ComparisDD between Figure 3 and Figure 4 indicates that the catalysts 1, m, n-l, n-2 and o-2 exhibit somewhat reduced activity at temperatures of not higher than 350 C, whereas the catalyst o-l incor-porating SnO2 in the coating retains high activty and has high resistance to sulfuric acid.

li3119'~

Example 2 a. Preparation of catalysts A number of pieces of the same catalyst base as used in Example 1 were immersed in the same coating bath O as used in Example 1 and sub-sequently dried, each a different number of times. The pieces were there-after baked and caused to support V2O5 in the same manner as in Example 1 to prepare many catalysts having varying coating thicknesses.
b. Relation between coating thickness and denitration efficiency.
The catalysts were tested for denitration efficiency at 300 C
under the same conditions as in Example 1 to determine th~ relation between the coating thickness and the denitration efficiency. Curve t in Figure 5 represents the results.
In the same manner as above the relation between the coating thick-ness and denitration efficiency was determined with use of cordierite Raschig rings and alumina Raschig rings of the same shape as used in Example 1. Curves u and v in Figure 5 represent the results. Figure 5 indicates that the denitration efficiency increases with increasing coating thickness but that little or no improvement is achieved with coating thicknesses of 20 or larger. Comparison between Curve t and Curves u, v reveals that the catalyst base of porous steel per se has noticeable denitrating activity.
Example 3 a. Preparation ~f catalysts Ten pieces of catalyst base were prepared in the same manner as in Reference Example 2 except that a steel material SS 41 (JIS) was used. The pieces were coated under the same conditions as in Reference Example 2 with use of the coating bath L prepared in Example 1 to obtain ten carriers.
Of these carriers, one was immersed in 200 mQ of 2N oxalic acid solution of NH4VO3 (1.0 mole/liter) at room temperature for 30 minutes, ~131197 then withdrawn from the solution and thereafter dried at 100 C for one hour to obtain a V-incorporating catalyst.
Another carrier was immersed in a n-butyl alcohol solution of tetra-n-butyl titanate (1.5 moles/liter) under the same conditions as above and dried under the same conditions to prepare a catalyst having Ti incorporated therein~
The remaining eight carriers were treated in the same manner as above with use of the titanate solution at varying concentrations as shown in Table 5 to cause the carriers to support Ti. Subsequently the resulting carriers were similarly treated with use of the same metavanadate solution as above at the varying concentrations listed in Table 5 to cause the carriers to support V. Thus eight catalysts were prepared which incorporated both Ti and V.
b. Activity test The catalysts were tested for denitration efficiency at 350 C in the same manner as in Example 1. The catalysts were treated with sulfuric acid vapor in the same matter as in Example 1 and thereafter tested for denitration efficiency. Table 5 also shows the results.

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ll;~l~L97 Table 5 shows that the catalysts incorporating both the Ti compound and V compound have higher activity than those containing only one of them and are more resistant to sulfuric acid.
Example 4 a. Preparation of catalysts Seven pieces of the same catalyst base as used in Reference Example 2 with a porous surface layer and the coating bath O prepared in Example 1 were used to obtain seven carriers in the same manner as in Reference Example 2. The carriers were respectively immersed in the metal salt-containing solutions ~A) to ~G) shown in Table 6 at room temperature for 10 minutes, then dried at 100C for one hour and further baked at 300C for one hour to obtain a catalyst incorporating Fe, catalysts incorporating Cu, catalyst incorporating Sb, catalyst incorporating Sb and Fe, catalyst incorporating W and Fe and catalyst incor-porating Cr.
b. Activity test The catalysts were tested for denitrating efficiency at 300C and 350C in the same manner as in Example 1 except that the gas was passed at a rate of 24 m/hr per unit geometric surface area of the catalyst. Table 6 shows the results.

113~19'7 Table 6 Soln. Metal salt : Concn. (mole/ Denitration efficiency ~%) liter) 300 C 350 C
(A) FeS04 0 5 42 65 (B) CuS04 0.5 45 65 (C) CuCQ2 0.5 59 71 (D) SbBr4 0.25 65 83 FeBr2 0.25 ~E) SbBr4 0.5 40 60 (F) NH4W3 0.25 54 79 FeS04 0.25 (G) (NH4)2CR207 0.25 68 87 Fe2S4 0.25 As apparent from Table 6, the catalysts have increased activity at higher temperatures.
Example 5 a. Preparation of catalysts Four pieces of the same catalyst base as formed in Reference Example 2 with a porous surface layer and the four kinds of coating baths 1 to 4 listed in Table 7 and prepared in the same manner as in Reference Example 2 were used to obtain four carriers in the same manner as in Refer,-ence Example 2. Table 7 also shows the compositions of the coatings formed on the pieces.

1~31i97 Table 7 Bath Composition of bathComposition of coating ~wt. %) -- (wt. %) SiO2 TiO2* SnO2* Emul- SiO2 TiO2 SnO2 sion a 5711 14 18 69.5 13.4 17.1 2 5715 10 18 69.5 18.3 12.2

3 57 7 18 18 69.5 8.5 22.0

4 5720 - 18 74.0 26.9 * The same as in Table 3.
** The same as in Table 3.
Aqueous solutions of NH4VO3 having varying concentrations of S0 to 1000 mg/liter calculated as V were prepared, and the carriers were made to support V with use of the solutions in the same manner as in Example 4 to determine the relation between the V compound concentration of the solution and the amount of V supported on the carrier per unit amount of adsorbed substances in the coating. The amount of V supported is expressed by:
Weight of V supported on carrier Weight of coating x ~TiO2 content + 1 SnO2 content). The results 20 are shown in Figure 6, which shows that the V compound is strongly adsorbed on the coating, with an increasing amo~t supported on the carrier with the increase in V concentration.
b. Activity test The catalysts were tested for denitration efficiency at 300 C
in the same manner as in Example 1 to determine the relation between the denitrating efficiency and the amount of V supported on the carrier per unit amount of the adsorbed substances in the coating. The results are given in Figure 7, which reveals that the denitration efficiency increases with the increase in the amount of V supported per unit amount of adsorbed substances and reaches the highest level when the latter is in the range of 0.01 to 0.02 which corresponds to 0.15 to 1.5% in terms of the weight of V.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for producing a denitrating catalyst comprising forming a porous catalyst base by treating a metal material having an aluminium alloy surface layer with an aluminum-dissolving solu-tion to dissolve out aluminium; coating the porous catalyst base in a silica containing coating bath; drying the coated base; immersing the thus obtained carrier in a solution containing the active com-ponent, and drying the resulting carrier having an active component thereon.

2. A process as defined in claim 1 wherein the coating bath is an acid colloidal silica.

3. A process as defined in claim 1 wherein the coating bath is a mixture of colloidal silica and containing an emulsion of high molecular weight substance, and the base is baked after having been immersed in the bath and dried.

4. A process as defined in claim 3 wherein the high molecular weight substance is contained in an amount of 10 to 50 parts by weight per 100 parts by weight of SiO2.

5. A process as defined in claim 4 wherein the coating bath is a colloidal silica containing a titanium compound.

6. A process as defined in claim 5 wherein the content of the titanium compound calculated as TiO2 is 40 to 100 parts by weight per 100 parts by weight of SiO2.

7. A process as defined in claim 4 wherein the coating bath is a colloidal silica containing a tin compound.

8. A process as defined in claim 7 wherein the content of the tin compound calculated as SnO2 is 30 to 70 parts by weight per 100 parts by weight of SiO2.

9. A process as defined in claim 1 wherein the active com-ponent is a vanadium compound.

10. A process as defined in claim 9 wherein the vanadium com-pound is supported on the carrier in an amount of 0.15 to 1.5% by weight calculated as V.

11. A process as defined in claim 1 wherein the active com-ponent is a mixture of a vanadium compound and a titanium compound.

12. A process as defined in claim 1 wherein the active com-ponent is a compound selected from the group consisting of sulfates and halides of iron, copper and antimony, tungstates and chromates.

13. A process as defined in claim 1 wherein the coating has a thickness of 7 to 20 µ.

14. A denitrating catalyst having a porous coating, produced by the process of claim 1, the porous coating having a thickness of 7 to 20 µ, an improved porosity obtained by adding high molecu-lar weight substance and baking, and being able to support the active component by immersing.

15. A denitrating catalyst having a porous coating, produced by the process of claim 9, the porous coating supporting vanadium compound in an amount of 0.15 to 1.5% by weight calculated as vanadium.

16. A denitrating catalyst having a porous coating, produced by the process of claim 11, the porous coating supporting a mixture of a vanadium compound and a titanium compound.

17. A denitrating catalyst having a porous coating, produced by the process of claim 12, the porous coating supporting a compound selected from the group consisting of sulfates and halides of iron, copper and antimony, tungstates and chromates.