METHOD OF FORMING A COATING

Abstract

A method is provided of forming a coating within an internal pathway. The method comprises: providing a body having an inlet and an outlet and an internal surface which defines an internal pathway extending within the body between the inlet and the outlet; streaming a mixture of a gas and a fluid along at least a part of a length of the internal pathway, the fluid comprising one or more substances for forming a solid coating on the internal surface, the fluid being a liquid solution of said one or more substances in a solvent or being a dispersion with at least one of said one or more substances being solid particles dispersed in a liquid continuous phase; during said streaming of the mixture, applying localised heat progressively along said at least a part of the length of the internal pathway. The progressive application of localised heat causes, within said at least a part of the length of the internal pathway, formation from the one or more substances of a solid coating on the internal surface.

Claims

1. A method of forming a coating within an internal pathway, comprising: providing a body having an inlet and an outlet and an internal surface which defines an internal pathway extending within the body between the inlet and the outlet; streaming a mixture of a gas and a fluid along at least a part of a length of the internal pathway, the fluid comprising one or more substances for forming a solid coating on the internal surface, the fluid being a liquid solution of said one or more substances in a solvent or being a dispersion with at least one of said one or more substances being solid particles dispersed in a liquid continuous phase; during said streaming of the mixture, applying localised heat progressively along said at least a part of the length of the internal pathway; wherein said progressive application of localised heat causes, within said at least a part of the length of the internal pathway, formation from the one or more substances of a solid coating on the internal surface; and wherein the body is formed from a material selected from the group consisting of: silica, steel, titanium, aluminium, and plastics.

2-4. (canceled)

5. A method according to claim 1, wherein the body is porous and the internal pathway is formed by a plurality of interconnecting internal spaces within the body and defined by the internal surface.

6. A method according to claim 5, wherein the body includes an impermeable portion which defines a perimeter of the internal pathway between the inlet and the outlet.

7. A method according to claim 1, wherein the body is elongate having a length and an external surface extending along the body length, wherein said at least a part of the length of the internal pathway is co-extensive with at least a part of the length of the body, wherein said progressive application of localised heat along the internal pathway comprises application of localised heat to the external surface of the body progressively along said at least a part of the length of the body.

8. A method according to claim 7, wherein the localised application of heat to the external surface of the body is applied by a heater which surrounds a portion of the length of the body so that heat is applied to the external surface of the body all around the portion, and wherein there is progressive relative movement between the body and the heater so that successive portions of the length of the body are surrounded by and heated by the heater.

9-10. (canceled)

11. A method according to claim 1, wherein the one or more substances comprise a solute in the solvent or in the liquid continuous phase, the solute undergoing thermal decomposition to form a decomposition product during said localised heating, the coating comprising the decomposition product.

12. A method according to claim 11, wherein the solute comprises a metallic cation, and wherein the decomposition product is a metal oxide.

13. A method according to claim 12, wherein the metallic cation is selected from the group consisting of: titanium, zinc, aluminium, magnesium, calcium, scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium, germanium, strontium, yttrium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, antimony, tellurium, barium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, thallium, lead, bismuth, zirconium, and lanthanum or actinium group metals.

14. A method according to claim 13, wherein the metal oxide is a porous solid and the metallic cation is selected from the group consisting of: titanium, zinc, aluminium, magnesium, calcium, scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium, germanium, strontium, yttrium, niobium, molybdenum, cadmium, indium, tin, antimony, tellurium, barium, tantalum, tungsten, thallium, lead, bismuth, zirconium, and lanthanum or actinium group metals.

15. A method according to claim 12, wherein the solute comprises an anion or a ligand, and wherein the anion or the ligand is selected from the group consisting of: nitrate, acetate, acetyl acetonate, nitrite, chloride, citrate, ammonia, carbonyl, cyclopentadienyl and its derivatives, and anions of organic acids including amino acids.

16. A method according to claim 1, wherein said fluid is said dispersion, the coating comprising the solid particles.

17. A method according to claim 16, wherein the solid particles are selected from, or independently selected from when there are solid particles of more than one said substances, the group consisting of: silicon dioxide, aluminium oxide, titanium dioxide, cerium oxide, zirconium oxide, iron oxide, carbon, mixed oxides, or combinations thereof.

18. (canceled)

19. A method according to claim 1, wherein after said formation of the coating, the body is heated to stabilise the coating.

20. A method according to claim 1, wherein the coating is porous and wherein after the formation of the coating, the coating is impregnated with metal particles.

21. A method according to claim 20, wherein the metal particles are selected from the group consisting of particles of: platinum, palladium, rhodium, osmium, iridium, ruthenium, copper, silver, cobalt, iron, nickel and gold.

22. A method according to claim 20, wherein said impregnation comprises filing said at least part of the length of the internal pathway with a suspension of the metal particles in a liquid and evaporating the liquid.

23. A method according to claim 20, wherein the impregnation comprises filing said at least part of the length of the internal pathway with a solution comprising a solute comprising metal cations, and heating the solution to cause decomposition of the solute so that the metal cations form the metal particles.

24. (canceled)

25. A method according to claim 20, wherein after said formation of said coating and before said impregnation of the coating with the metal particles, the body is heated to stabilise the coating.

26. A method according to claim 1, wherein the coating comprises a porous component and metal particles entrapped in the porous component.

27-30. (canceled)

31. A method according to claim 1, wherein the coating is porous and wherein the fluid comprises a molecule in solution which acts to increase the mean pore diameter in the coating.

32-38. (canceled)

Description

[0052] The following is a more detailed description of embodiments of the invention, by way of example, with reference to the appended drawings, in which:

[0053] FIG. 1 is a schematic representation of an apparatus for performing the method;

[0054] FIG. 2 is graph showing the effect of different temperatures for the stabilising heating step;

[0055] FIGS. 3a and 3B are cross-sectional scanning electron microscope images showing a porous coating on an internal surface of a capillary tube; and

[0056] FIGS. 4a and 4b are cross-sectional schematic representations of bodies with more than one channel.

EXAMPLE 1

[0057] Referring to FIG. 1, the apparatus used to perform the method of Example 1 comprised a syringe pump 10, a gas mass-flow controller 12, a T-junction 14, a stepper motor 16, a vertical furnace 18 and an extraction hood 20. The syringe pump 10 was connected to the T-junction 14 by a first conduit 22 and the gas mass-flow controller 12 was connected to the T-junction by a second conduit 24.

[0058] FIG. 1 also shows a tube 26 (the body in this Example) that was provided with a coating. The tube 26 was formed from 316 L stainless steel and had a length of 5 m. The tube 26 had an external diameter of 1.55 mm and an internal diameter of 1.27 mm. The current method was used to provide a coating on the internal surface of the tube 26 adjacent the internal channel of the tube 26.

[0059] As a first step, a porous coating of zinc oxide was formed on the internal surface of the tube 26. Firstly, the syringe pump 10, the gas mass-flow controller 12, the first and second conduits 22, 24, the T-junction 14 and the tube 26 were placed in a desiccator to remove any traces of water. (This prevents precipitation of the zinc solution as zinc hydroxide.) The syringe pump 10 was then filled with a solution of zinc (II) nitrate hexahydrate (28.6 g) in 50 ml of ammonia solution (28 wt % in water). The gas mass-flow controller 12 was connected to a source of dry compressed air. An inlet 28 of the tube 26 was connected to the T-junction 14. The syringe pump 10 was then operated to pump the zinc solution to the T-junction 14 at a flow rate of 500 l min.sup.1. The gas mass-flow controller 12 was operated to feed air to the T-junction 14 at a flow rate of 4 ml min.sup.1 (STP). At the T-junction 14, the zinc solution and the air mixed to form a mixture. The mixture was streamed continuously into and through the tube 26 to a tube outlet 28. The mixture is believed to have progressed through the tube 26 in the form of slugs of the solution filling the cross-section of the tube 26, the slugs being separated from one another by air.

[0060] While the mixture of the air and the zinc solution was streamed through the tube 26, the tube 26 was fed into the vertical furnace 18 by the stepper motor 16 at a rate of 5 mm s1, starting with the tube outlet 30 and progressing towards the tube inlet 28. The vertical furnace 18 was maintained at a temperature of 350 C. Hence, successive portions of the length of the tube 26 passed into, passed through for a predetermined residence period, and passed out of the vertical furnace 18.

[0061] The feeding of the tube 26 through the vertical furnace 18 was continued until no further feeding was possible (leaving only a minimum length of the tube 26 located adjacent the tube inlet 28 that was not heated in the vertical furnace 18). Streaming of the mixture of the air and the zinc solution dispersed through the tube 26 was continued for the whole of this process.

[0062] The heating in the vertical furnace 18 caused the zinc solution to undergo thermal decomposition to form zinc oxide. Specifically, zinc ammonium hydroxide dissolved in the solution decomposes to form zinc oxide. The zinc oxide formed a coating on the internal surface of the tube 26.

[0063] Gas and solution passing out of the tube outlet 30 entered the extraction hood 20.

[0064] The zinc oxide coating produced in this way was found not to be stable and up to 70% could be removed by washing.

[0065] As a second step, to stabilise the zinc oxide coating, the tube 26 was heated in an oven for 4 hours. To optimize this step, four different experiments were performed at respective different temperatures with four different tubes 26 each provided with a zinc oxide coating as described above. Temperatures of from 550 C. to 950 were tested. In each case, after heating for 4 hours at the test temperature, the tube 26 was flushed with 20 ml of isopropanol or acetone at a flow rate of 100 ml min.sup.1 to remove loosely bound zinc oxide. The results are shown in FIG. 2 which shows the mass of the remaining zinc oxide coating after washing on the y-axis, and the temperature used for the heating step on the x-axis. It was found that heating at a greater temperature has a greater stabilising effect on the zinc oxide coating. A temperature of 800 C. was chosen for routine experiments as this temperature was effective at stabilising the coating while it could be readily achieved using inexpensive equipment and without undue energy expenditure.

[0066] After the formation and stabilization of the zinc oxide coating in the first and second steps as described above, the zinc oxide coating, which was porous, was then impregnated with catalytic palladium nanoparticles in a third step.

[0067] To impregnate the porous zinc oxide coating with palladium nanoparticles, the tube 26 was first filled with a solution of palladium (II) acetate dissolved in acetone to form a column of the palladium solution in the internal pathway. The concentration of the palladium (II) acetate was calculated to give a final palladium metal loading of 5 wt % with respect to the total weight of the zinc oxide coating. The tube outlet 30 was closed and the tube inlet 28 was left open. The tube 26, together with the column of palladium (II) acetate solution, was then advanced into an oven heated at 300 C. at a rate of 5 mm s.sup.1 starting with the open tube inlet 28. During this process, the acetone solvent evaporated and the palladium (II) acetate decomposed to form palladium metal nanoparticles entrapped in the porous zinc oxide coating. The rate of advancement of the tube 26 into the oven was sufficiently slow so that evaporation of the acetone solvent took place only at the end of the column of palladium solution closest to the open tube inlet 28. In this way, evaporation in the body of the column, which is undesirable because it could cause ejection of the palladium solution, was avoided.

[0068] The coating is shown in FIGS. 3a and 3b. The coating is shown at 32 and the wall of the tube is shown at 34. The coating 32 has an even thickness and is about 18 m thick.

[0069] The tube may be used to perform conversion of nitrobenzene into aniline, 2-methyl-3-butyn-2-ol into 2-methyl-3-buten-2-ol with the palladium nanoparticles catalysing the reaction or cinnamaldehyde into cinnamyl alcohol with platinum nanoparticles catalysing the reaction.

EXAMPLE 2

[0070] In Example 2, a commercial SiO.sub.2 sol was used (Ludox 30 wt %) supplied by Sigma-Aldrich as a coating precursor. The sol was diluted 30-fold to obtain a 1 wt % SiO.sub.2 sol which was used for the coating method. The method was performed using the apparatus described in Example 1 and shown in FIG. 1.

[0071] The tube 26 that was coated in Example 2 was identical to the tube 26 coated in Example 1. The tube 26 was washed with petroleum spirit and acetone and dried prior to the coating.

[0072] The 1 wt % SiO.sub.2 sol was pumped from the syringe pump 10 to the T-junction 14 at a rate of 150 l min.sup.1. Air was passed to the T-junction 14 by the gas mass-controller 12 at a flow rate of 4 ml min.sup.1 (STP). At the T-junction 14, the sol and the gas mixed to form a mixture of air and sol. This mixture was streamed into and through the tube 26. It is believed that the mixture progressed through the tube 26 in the form of slugs of sol, filling the cross-section of the tube, separated from one another by pockets of air.

[0073] During the streaming of the mixture of the air and the sol through the tube 26, successive portions of the length of the tube were moved into, through and out of the vertical furnace 18 at a constant displacement velocity of 3 mm s.sup.1. The furnace temperature was 180 C. As the tube passed though the vertical furnace 18, the liquid continuous phase of the sol evaporated and the silicon dioxide particles of the sol formed a coating on the internal surface of the tube 26.

[0074] After the silicon dioxide coating had been formed on the internal surface of the tube 26, the tube was subjected to an annealing step at 350 C. for 4 hours in an oven to stabilise the coating. The mass of the coating obtained was around 200 mg.

[0075] After the silicon dioxide coating had been stabilised, the coating was impregnated with platinum nanoparticles. The platinum nanoparticles were first obtained by dissolving hexachloroplatinic acid (1 g) in 50 ml ethylene glycol and heating the solution to 160 C. under reflux and with stirring for 4 h to obtain a stable dispersion of platinum nanoparticles. This dispersion of platinum nanoparticles was diluted with sufficient ethylene glycol calculated to give a 12 wt % platinum loading (with respect to the weight of the silicon dioxide coating) assuming full incorporation of the platinum nanoparticles into the silicon dioxide coating.

[0076] The diluted dispersion of platinum nanoparticles was introduced into the tube 26 until the tube 26 was filled with the platinum dispersion which formed a column within the tube 26. The tube outlet 30 was closed and the tube inlet 28 was left open. The tube was then introduced into an oven starting with the open tube inlet 28 at a rate of 5 mm s.sup.1. The oven temperature was 350 C. The rate of introduction was sufficiently slow so that evaporation of the ethylene glycol dispersant occurred only at the end of the column of the platinum dispersion (and not within the body of the column). In this way, the dispersant evaporated in a controlled manner, without formation of air bubbles in the body of the column and ejection of the dispersion, and the platinum nanoparticles were left incorporated into the porous silicon dioxide coating.

[0077] Example 2 was repeated several times to study the effect of varying various parameters. It was found that an increase in gas flow rate reduces the coating yield. Increasing the heating temperature used to stabilise the coating had no effect on the coating. Instead of using air as the gas used to form the mixture of gas and fluid, helium or nitrogen may be used but there is no effect on the coating.

[0078] If a tube 26 with a different internal diameter is used, a coating of similar properties may be obtained by increasing or decreasing either the flow rate of the silicon dioxide sol, or the concentration of the silicon dioxide sol, in either case so as to keep the flow rate or the concentration proportional to the internal volume of the tube, while keeping the gas flow rate the same.

EXAMPLE 3

[0079] Using the same apparatus described in Example 1 with reference to FIG. 1, and a similar methodology, an internal surface of a tube 26 was provided with a coating of magnesium oxide. Instead of the zinc solution used in Example 1, an aqueous solution of 1 wt % Mg(NO.sub.3).sub.2 was used as a coating precursor. The solution was displaced into the stainless tube 26 at a flow rate of 100 l min.sup.1 and the air flow rate was 4 ml min.sup.1. The tube 26 was displaced into the vertical furnace 18 at a velocity of 2 mm s.sup.1 and the furnace temperature was 350 C. The heating caused the magnesium nitrate to decompose to magnesium oxide. The magnesium oxide coating obtained had a mass of 140 mg. After the coating was formed, it was heated at 500 C. for 4 hours to stabilise the coating.

EXAMPLES 4a and 4b

[0080] Examples 4a and 4b are examples of the use of the method to form a coating within a body 36, 38 which has a plurality of channels extending therethrough.

[0081] In Example 4a, the body 36 consisted of a quartz tube 40 and two stainless steel tubes 42, 44 inserted in the quartz tube 40. The quartz tube 40 had a length of 50 cm, an outside diameter (OD) of 6 mm and an inside diameter (ID) of 4 mm. The two stainless steel tubes 42, 44 had outer diameters of 3 mm and 1.5 mm respectively, and each had a wall thickness of 0.2 mm. The two stainless steel tubes 42, 44 were inserted coaxially into the quartz tube 40, as shown in FIG. 4a.

[0082] The quartz tube 40 was connected to gas and fluid flows via a T-junction as in Example 1.

[0083] 1 wt % SiO.sub.2 sol was pumped from the syringe pump 10 to the T-junction 14 at a rate of 150 l min.sup.1. Air was passed to the T-junction 14 by the gas mass-controller 12 at a flow rate of 4 ml min.sup.1 (STP). At the T-junction 14, the sol and the gas mixed to form a mixture of the air and the sol. This mixture was streamed into and through the quartz tube 40 and the stainless tubes 42, 44 inserted into it.

[0084] During the streaming of the mixture of the air and the sol, successive portions of the length of the quartz tube 40 (with the stainless steel tubes 42, 44 therein) were moved into, through and out of the vertical furnace 18 at a constant displacement velocity of 3 mm The furnace temperature was 180 C. As the body passed though the vertical furnace 18, the liquid continuous phase of the sol evaporated and the silicon dioxide particles of the sol formed a coating on the internal surface of the quartz tube 40 and on the inner and outer surfaces of the steel tubes 42, 44. About 80 mg of the coating was obtained inside the body 36.

[0085] In Example 4b, the body 38 was made of a 6 mm OD quartz tube 46 with 3 stainless steel tubes 48, each having an OD of 1.6 mm and an ID of 1.2 mm, inserted into the quartz tube 46, in a generally triangular configuration as shown in FIG. 4b. The method was conducted as described above for Example 4a. About 70 mg of the coating was obtained including about 30 mg inside the stainless tubes 48.