Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonelectrolytic coating before forming nonmetal...
Reexamination Certificate
2000-03-13
2003-03-18
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonelectrolytic coating before forming nonmetal...
C205S210000, C205S212000, C205S229000, C205S316000, C205S322000, C204S290010, C204S294000
Reexamination Certificate
active
06533916
ABSTRACT:
The present invention relates to diamond-coated electrodes, termed diamond electrodes below for short, which can be used in reactions of organic compounds, and to a process for producing such diamond electrodes.
Carbon electrodes, such as graphite or recently also glassy carbon, are known in electrochemistry and are used industrially on account of their relatively good electrical conductivity. Diamond, as a further modification of carbon, on account of its rare occurrence and its properties as an insulator, was previously of no great importance in industrial use.
However, in the interim, it has been found that particularly modified diamond electrodes behave similarly to the conventional graphite electrodes in reactivity and selectivity in organic solvents, such as methanol, THF, DMF or acetonitrile. In this respect, the advantages of the diamond-coated electrodes can be made utilizable. This is especially the inert chemical behavior toward aggressive chemicals (e.g. hydrogen fluoride). Moreover, diamond electrodes have the advantage that a broad potential range can be used, in water for example in a range of from −1.2 V to 2.3 V, without the solvent being attacked. In addition to the advantages of diamond compared with graphite, the disadvantages of graphite may also be eliminated by using diamond electrodes. These are, firstly, the mechanical disadvantages of graphite, for example abrasion and swelling of the noncrystalline layers, secondly the chemical disadvantages, for example corrosion, especially with respect to water, intercalation of organic material into the graphite layers, the layer lattice expansion on hydrogenation and the possible oxidation of the graphite. In the reaction of organic compounds, hitherto predominantly graphite electrodes have been used. Thus, for example, in a capillary cell which was developed by Beck and Guthke in 1969, electrochemical oxidations were carried out on graphite electrodes, such as for example the methoxylation of furan to dimethoxydihydrofuran, or the Kolbe electrolysis of adipic monoesters to form 1,10-sebacic esters. However, when graphite is used, due to the rough surface and the graphite abrasion during the electrolysis, graphite particles can lead to short circuits. Graphite blocks covered with metal foils have proved to be insufficiently stable. The metal foils become wavy and flake off (F. Wenisch, H. Nohe, H. Hannebaum, D. Degner, R. K. Horn, M Stroezel,
AlChE Symposium Series
75, (1979) 14; H. Nohe,
AlChE Symposium Series
75, (1979) 69). In addition, numerous oxidations of aromatics at graphite are known (D. Degner,
Topics in Current Chemistry
148, 1988, 3-95). U.S. Pat. No. 5,399,247 discloses the decomposition of organic compounds in waste water using diamond electrodes. According to this patent, the decomposition of aqueous organic residue components, for example hydroquinone, proceeds very readily. Phenol may also be decomposed very readily at diamond electrodes. For the preparative synthesis of organic compounds, diamond electrodes have not been used hitherto, however. This was due to the fact, inter alia, that it has not been possible hitherto to coat sufficiently large areas of electrode surfaces with diamond. In addition, the layers have not been stable enough hitherto in organic solvents.
The principal areas of use of diamond layers have hitherto been in machine tool construction, or in the case of electrically conductive diamond layers, in the sensor sector also. In electrochemistry, they are also being used as electrode material, for example for its reducing nitrate to ammonia.
U.S. Pat. No. 5,399,247 describes, as mentioned briefly above, for example the use of diamond electrodes for decomposing industrial waste waters, as exemplified by photographic developer baths. JP 09013188 relates to the use of oxidation, reduction and halogenation for modifying the surfaces of diamond electrodes.
In addition, there are numerous studies on diamond electrodes with analytical application for cyclic voltammetry (CV). Here, the diamond electrodes require only a very small area of about 10 mm
2
or wires having a length in the mm region and also only a very thin diamond layer obtained by simple coating. In this case use is made of the property of passivating the coated metal in water, this metal generally being titanium. In organic media, even with low water contents, such a passivation does not succeed. In this respect the electrodes are not sufficiently resistant to aggressive materials. This type of diamond electrode was thus not suitable for use in reactions of organic compounds. This case requires extensive virtually pore-free dense diamond layers on the corresponding support electrodes.
With respect to producing diamond layers, in parallel to the high-pressure/high-temperature synthesis (HPHT synthesis), low-pressure processes for producing diamond have also been developed, which were firstly considered uneconomic and not pursued further. Based on this early work, in the 1980s, activated gas-phase deposition processes (chemical vapor deposition, CVD) for synthesizing diamond layers were developed up to market readiness.
It is an object of the present invention, in view of the above prior art, to provide a diamond electrode which is suitable for using in reactions of organic compounds, and to provide a process for its production.
We have found that this object is achieved by a diamond electrode and a process for its production as defined in the independent claims. Expedient developments are specified in the subclaims.
Accordingly, the present invention relates to a process for producing a diamond-coated electrode, in which the process comprises at least the following steps:
a) Cleaning the surface of the substrate;
b) Seeding the surface of the substrate at a high diamond nucleation density;
c) Coating the surface of the substrate with diamond.
This procedure prevents or substantially suppresses the formation of pores in the diamond layer. Pores occurred in previously known coatings with diamond in comparatively high numbers. Pores of this type are formed in the coating process by contaminants on the surface to be coated. The diamond layer cannot grow over these areas, so that uncoated and thus corrosion-susceptible areas are present on the surface of the substrate to be coated.
Only now by the seeding according to the invention at a high diamond nucleation density and preferably repeated coating, i.e. performed at least twice, of the surface of the substrate with diamond (step c)), coupled with an adequate cleaning procedure prior to each coating step has it become possible to create an adequate extensive, pore-free and dense diamond layer on the surface of the corresponding substrate and thus to create an electrode suitable for use in the reaction of organic compounds. B y means of th e process of the invention it is possible to produce area in the range from 3000 to 300,000 mm
2
, further preferably from 10,000 to 300,000 mm
2
For the purposes of the invention, pore-freeness means that no pore is detectable. The porosity is monitored by applying hydrofluoric acid to the diamond layer produced, in which case if pores are present, considerable hydrogen formation would occur instantaneously. By means of the process of the invention, diamond layer thicknesses for industrial electrode materials are produced in a range from 1 &mgr;m to 50 &mgr;m. In the case of silicon electrodes, diamond layers having a thickness of from 0.25 &mgr;m to 10 &mgr;m are possible.
Preferably, in the case of smooth electrode surfaces, the electrode substrate, prior to the first cleaning step, is preferably mechanically roughened. This is preferably performed by sandblasting, preferably at a pressure of from 5 to 7 bar. This roughening of the surface initially achieves a very good anchoring of the diamond layer still to be applied. When graphite is used for the substrate, in contrast, roughening is not necessary, since here even in the initial state, sufficient roughness is present. A certain roughness is necessary to achieve suffici
Fryda Matthias
Hampel Alexander
Klages Claus-Peter
Merk Claudia
Puetter Hermann
BASF - Aktiengesellschaft
Bell Bruce F.
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