Patent Application
20080096042
This invention relates to a process for the preparation of surface-oxidised metallic wools and relevant fibres deriving from them, the products obtained, and their use as catalysts, supports for catalysts, sensors and friction materials. The coating of these products with ceramic oxides can give rise to metallic core composite materials which are useful in various applications, especially in the catalytic field. Wools and fibres deriving from them can be coated later in a separate plant, or immediately after their formation in the same manufacturing plant.
Metallic wools and fibres have high potential for the production of composite materials for use in the catalytic field and for other major applications. In those composite materials, metallic constitutes the core which is coated with materials of different nature, such as ceramic oxides with high surface and porosity.
For the formulation of these composite materials, it is essential to form an interphase that creates continuity between the two different components, because the final material must feature high chemical and mechanical stability besides the other properties. Problems of adherence to the metallic surface of layers of materials of different nature are very important, and can preclude the production of a good finished product. Pre-treatments of the metallic (acid treatment, heat treatment, electrochemical treatment, etc.), designed to attack the surface firstly from a morphological standpoint, by increasing its roughness, and secondly from a chemical standpoint, by creating layers of oxide, are therefore required to make it suitable to receive the growth of the second component.
The development of composite materials for catalytic applications obtained by coating metallic surfaces with ceramic oxide compounds is still very limited. Relatively few examples could be found in the patent and scientific literature, and they are limited to rigid metallic structures or “formed” structures with geometrical shape constraints (e.g. honeycombs, fans, propellers, etc.) which require further processing before they can be used. These structures are not always easily adaptable to the reactors or housings of various shapes and sizes into which they must be introduced to realize the required chemical transformations. They are also expensive, and the finished catalytic system has a high cost due to the nature and processing of the raw material (pre-treatment of the metallic surface and coating process).
A new process has now been discovered which allows controlled oxidation of metallic wools and fibres during their production, and which produces metallic materials with morphologically and chemically modified surfaces with no need for complex, expensive pre-treatments in view of their coating.
The metallic wools and fibres obtainable by the process according to the invention are suitably corrugated and oxidised at the surface, and can be used as new materials in various fields of application.
The main field of application is the catalysis. Composite materials obtainable according to the invention, suitably coated with porous material, such as ceramic material, can be used directly as catalysts or as supports for active dispersed phases (Pt, Pd, Rh, Ag, Cu, etc.).
The use of catalytic materials obtainable according to the invention is particularly advantageous because these materials improve the thermal control of reactors as a result of more effective heat transfer and easy elimination of the heat produced by highly exothermic reactions. In fact, the thermal conductivity of catalysts with metallic core is much greater than that of conventional ceramic catalysts. Moreover these characteristics make the catalytic systems obtainable with the invention more stable homogeneous under the most severe conditions of use (e.g. catalytic converters), and allow better control of the chemical in terms of selectivity to the desired products. The absence of rigid shape constraints of the metallic fibres and wools also makes the finished materials highly adaptable to the reactors or housings in which they will be positioned.
This invention also relates to the preparation of composite catalytic materials with metallic core obtained by coating the metallic surfaces and the fibres of various nature deriving from them (e.g. stainless steel, steel, aluminium, bronze, brass, copper, zinc, etc.) with a second ceramic oxide material (e.g. silica, silica-alumina, alumina, etc.). To obtain a suitable coating, the metallic wools and fibres must be prepared by oxidising a micrometric thickness layer (50<thickness/nm<150) of their surface of creating an oxide whose nature is associated with the nature of the metallic wool and/or fibre (e.g. ZnO on zinc wool/fibre, Fe2O3 on steel wool/fibre, CuO on copper wool/fibre, etc.). The metallic fibre/wool can be oxidised at the step of manufacture of the wools/fibres with an optimised process depending on temperature, time and humidity. The layer of metallic oxide formed produces good adherence and assures of the ceramic oxide component with which the fibre/wool is subsequently coated, allowing continual shift from metallic to ceramic material.
Depending on the nature and properties of the starting wools/fibres, composite materials are prepared by different methods, in particular with sol-gel, sol, deep-coating, or impregnation methods. Depending on some parameters which can be controlled during the preparatory step, the layer of coating ceramic is more or less thick and homogeneous. The coating of the metallic surface with the ceramic oxide can be homogeneous or non homogeneous. By controlling the heterogeneity of the coating, new “rosary catalysts” can be obtained which have catalytic zones coated with ceramic oxide, and bare metallic zones able to transfer heat to the surrounding reaction fluid, on the same fibre. The composite materials obtained have surfaces with chemical, morphological and structural properties typical of ceramic oxide materials; in other words, they are microcrystalline and have a large surface area and porosity. These properties make the materials suitable for use mainly as catalysts or supports for active catalytic phases.
The invention is particularly advantageous in a variety of fields, especially:
in the environmental and energy field, where composite catalytic materials with a metallic core can be useful to eliminate pollutant compounds in exhaust gases (e.g. catalytic converters for motor vehicles), and to produce energy for industrial or domestic use in processes of total combustion of fossil fuels (catalysts for flameless boilers operating on a catalytic combustion system);
applications in the field of industrial chemistry relate to exothermic chemical processes, in which better temperature control means better kinetic control of the reaction and greater stability of the catalyst;
in the field of sensors for the detection of toxic and/or dangerous gases which use metallic materials coated with thin layers of active material, creating active oxide films (SnO2, Ga2O3, V2O5, etc.);
in the field of non-asbestos friction materials; the oxidised metallic fibres can be coated with suitable resins, to produce friction materials to be used in the manufacture of vehicle brake pads and clutches and other similar applications.
For catalytic uses, the high thermal conductivity of the metallic that constitutes the core of the catalytic materials formed allows elimination of the heat developed by the reactions, reducing the stresses on the catalytic material, which can work for longer time (increased durability) with more controlled selectivity. In fact, the high local temperature that can be reached during the exothermic reaction may lead to transformation of the active phase present on the surface of the catalyst (sintering), with loss of catalytic activity. Temperature also plays an important role in the kinetics of the single reactions which can take place on the catalyst bed, or more generally in the reactor, and consequently in the selectivity of the process. Rapid elimination of evolved heat not only improves catalytic performance but also preserves the structure of the catalyst over time.
The process according to the invention for the production of metallic wools with a controlled degree of surface oxidation consists of dry scraping, with a set of grooved tools or knives, a taut metallic wire made to slide between two rollers, characterised in that said process includes regulation of the wire-scraping temperature to obtain controlled oxidation of the wool.
The metallic wire scraping temperature is maintained at between approx. 200 and 700° C.
In this way, oxidation percentages of between 0.1% and 0.3% w/w can be generated, corresponding to oxide thicknesses ranging between approx. 10 and 120 nm.
The metallic wire, before cutting, is preferably made to slide on wire guides, with temperature regulation of both the wire guides and the air in the cutting zone.
The composition of the air in the cutting zone is modified by varying, for example, the water content from 0 to 7% and the oxygen content to 5 and 30% v/v.
The metallic wire is preferably made to advance at a constant speed, typically between 30 and 90 metres/minute.
If, due to the nature of the material, controlled oxidation cannot be completely performed in the cutting zone, oxidation can be performed or completed in the same plant, downstream of scraping, by treatment at a controlled temperature with blowing of air having a defined humidity level.
The metallic wire normally has a diameter of between approx. 3.25 and approx. 3.40 mm, said grooved knives are preferably made of K 100 steel with between 30 and 250 grooves per inch, and the wire is scraped to a diameter of 0.6 mm.
The material used to make the metallic wire can be chosen, according to the fields of use, from among the group constituted by aluminium, steel, stainless steel, copper, brass, zinc and other metallics or alloys.
According to a preferred form of embodiment of the process according to the invention, the wool obtained by scraping said metallic wire can be conveyed to a metallic-bladed mill which chops it into fibres 500-5.000 microns long.
The wool and the fibres deriving from it can be coated with the second component subsequently in another plant, or immediately after their formation in the same manufacturing plant. After scraping, the surface-oxidised metallic wire can be made to slide in baths containing ceramic oxide suspensions of a defined nature and concentration, or sprayed continuously with similar suspensions, to coat it with oxide material. Said second oxide component fixes to the newly-formed metallic oxide, which acts as interface between the metallic and the second oxide. On completion of the cycle, the composite material created is dried and possibly calcined.
The metallic wire coatings can have an homogeneous thickness or an non homogeneous coating with bare metallic areas and areas covered with ceramic, giving rise to “rosary” materials.
The invention is illustrated in greater detail by the following example.
The results obtained with metallic fibres deriving from various kinds of metallic wool (aluminium, steel, stainless steel, copper, brass and zinc), on the basis of optimised modification of their surfaces by creating thin layers of oxides, are shown in Tables 1-2 and
Table 1 shows the nature of the fibres and the code numbers given to them.
a-3d are scanning electron microscope microphotographs of a steel wool (SO1AE, Table 1). Two enlargements have been chosen: panoramic, to give an idea of the shape and morphology of the fibres obtained from the metallic wools, and greatly enlarged, to show the surface roughness of the fibres caused by the mechanical and thermal stresses undergone during formation.
At the same time microanalyses (SEM-EDS=scanning electron microscopy with energy-dispersive X-ray spectroscopy) were performed before (
The surface oxidation process is obviously activated by temperature, and begins at different times, according to the nature of the fibre. Surface oxidation begins at much lower temperatures for copper and steel than for zinc, brass, stainless steel and aluminium. The different shape of the thermogravimetric curves clearly shows that the kinetics of oxidation, proceeds with very different activation energy depending on the nature of the fibre.
Table 2 shows the temperatures required to obtain surface oxidation of the fibre amounting to 0.1% in weight (T0.1%), together with the quantity of oxygen adsorbed at those temperatures.
aThe main metallic constituent of each fibre was considered (see Table 1).
bFor fibre SOA1, the heating ramp was extended to 600° C. (step 2), with isotherm at 600° C. (step 3).
cM = Cu, Zn.
dFor the SOINOX fibre, the heating ramp was extended to 900° C. (step 2), with isotherm at 900° C. (step 3).
The quantity of oxygen adsorbed was experimentally determined (meqoxygen/gfibre); this value can be elaborated to determine the quantity of oxide formed as a percentage of the starting metallic (% w/w), on the basis of the assumed stoichiometry for the oxide in formation. As will be seen, for copper and steel fibres the quantity of the oxide formed is more than one order of magnitude greater than for other metallic fibres which are more difficult to oxidise.
On the basis of the TGA results (quantity of oxygen adsorbed according to time and temperature of treatment) and values of surface areas of the fibres, the thickness of the oxide layer created can be calculated. The example given in
As will be seen from
It is therefore possible to create a layer of oxide of the required height for each fibre by optimising the time, temperature, and humidity of the oxidising treatment performed. Complete coating of the surface of the metallic fibre with a nanometric layer of oxide is useful to create the interphase between the metallic and a second component (e.g., a ceramic oxide) which will coat the metallic fibre, giving rise to the future final composite material.
The fibres with the surface oxidised in a controlled way were coated with porous ceramic material (e.g. alumina, silica, zirconia, silica-alumina, etc.), and different methods of preparation and deposit of the ceramic material (e.g. the “sol”, “sol-gel” or “impregnation” method) were compared.
The formation of ceramic phases deposited on the materials has been proved by thermogravimetric analyses conducted with programmed temperatures (10° C./min. in the temperature range 40° to 600° C.) in an air atmosphere.
As a result of the ceramic phase coating of the metallic fibres, the composite materials created developed a large surface area. Table 3 shows the values of the specific surface area (measured with nitrogen at temperature of −196° C.) for some stainless steel fibres coated with silica-alumina by the sol-gel method. The surface area values are higher for the material created from oxidised fibre than from fibre which is merely washed, as expected. For the materials shown in Table 3, the percentage in weight of ceramic compared with metallic fibre, and the corresponding thickness of ceramic developed, assuming an homogeneous deposit on the metallic surface, are also shown.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2004A001964 | Oct 2004 | IT | national |
| 05015937.5 | Jul 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/11025 | 10/13/2005 | WO | 7/11/2007 |
