Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Reexamination Certificate
2000-09-25
2002-04-02
Wong, Edna (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
C204S157300, C204S158200, C210S748080, C423S021100, C423S263000, C502S005000, C502S302000, C502S303000, C502S350000
Reexamination Certificate
active
06365007
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of photocatalysts for the degradation of organic pollutants. The present invention provides a novel semiconductor photocatalyst for use in such degradation, as well as a process for preparing such photocatalyst.
BACKGROUND OF THE INVENTION
Semiconductor photocatalysis for water and air purification has evoked considerable interest during the last decade. The application of such catalysts for the degradation of contaminants has been used successfully for the degradation of various organic compounds such as alkanes, aliphatic and aromatic carboxylic acids, aliphatic and aromatic halogenated compounds, alkenes, phenols, amines, surfactants, pesticides, heterocyclic compounds, as well as for the reductive deposition of heavy metals.
Titanium dioxide (TiO
2
) is a chemically inert compound, stable under illumination conditions, relatively inexpensive and has proven to be an especially useful semiconductor for environmental applications (Schwitzbegel, J. et al.,
J. Phys. Chem.,
99 (1995), 5633). Excitation of TiO
2
by light energy leads to the formation of an electron-hole pair and in the presence of a suitable scavenger or surface states, the electron or hole are trapped, recombination is prevented and subsequent redox reactions occur. The photocatalytic degradation of organic materials by TiO
2
particles in aqueous media and in the presence of oxygen, is initiated by the formation of free hydroxide radicals (OH) which are capable to degrade organic compounds into water and carbon dioxide.
Effective degradation of organic pollutants is accomplished when the pollutant is preconcentrated at the semiconductor surface. There are known in the art several methods for enhancing interfacial electron-transfer reactions of TiO
2
, wherein the properties of the TiO
2
particles have been modified by selective surface treatments such as surface chelation (J. Moser et al., Langmuir, 7 (1991), 3012), surface derivatization (Hong, A. P., et al.,
J. Phys. Chem.,
91 (1987), 6245) and selective doping of the crystalline matrix (Lee, W. et al.,
Mater. Res. Bull
., 28 (1993), 1127).
TiO
2
can be obtained by a sol-gel reaction, upon heating a gel containing it to a temperature of above 450° C. By dispersing or dissolving substances in such a matrix, a solid TiO
2
with homogeneously mixed substances may eventually be obtained. Furthermore, a TiO
2
solid prepared by the sol-gel method is characterized by a very large specific surface area and high pore volume, rendering this solid an excellent absorbent of various, molecules or ions which bind to its surface. The large specific surface area makes the TiO
2
attractive for catalytic applications.
Commercial products coated with TiO
2
having photoelectrochemically active surfaces have now entered the market. For example, the Japanese company Toto is marketing tiles for operating rooms and other hospital applications. Such tiles are also being used in rest rooms where they breakdown organic compounds deposited on the tiles and thus reduce the amount of microorganisms which grow on said organic compounds and thereby eliminate malodors. Another use of TiO
2
coated products is for street lighting appliances, from which organic pollutants emitted from vehicles have to be removed.
SUMMARY OF THE INVENTION
The adsorption of organic compounds onto the semiconductor surface is a critical step in the photodegradation process. Therefore, the preconcentration of the organic pollutant at the semiconductor surface is an especially important feature. It has been found, in accordance with the present invention, that such preconcentration can be enhanced by incorporating lanthanide ions such as europium or praseodymium ions, in a titanium dioxide matrix, e.g. by means of the sol-gel method mentioned above. The lanthanide ions incorporated in such a titanium dioxide matrix are capable of forming Lewis acid-base complexes with organic moieties such as double bonds, acids, amines, aldehydes, esters, ethers, thiols, alcohols, etc., and, hence, impart to the titanium dioxide photocatalyst enhanced activity for the degradation of organic compounds bearing such moieties.
It is an object of this invention to provide novel TiO
2
semiconductor photocatalysts (a photocatalyst being a catalyst induced to perform a catalytic activity by light) doped with oxides of lanthanide metals, which exhibit enhanced activities for the degradation of organic compounds and particularly organic pollutants.
It is a further object of the present invention to provide a process for the preparation of such photocatalysts.
The present invention provides a photocatalyst comprising TiO
2
doped with at least one lanthanide oxide. Preferably the molar ratio Ti:M being from about 100:0.5 to about. 100:5. At times, the lanthanide oxides may be incorporated as hydrates.
All elements with atomic numbers from 58 to 71 are known as lanthanide series, including Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. These elements are inner-transition elements. i.e. their electron configuration feature the filling of 4f orbitals and are capable to form complexes with electron donors.
Preferred lanthanide oxides for use in accordance with the invention are europium oxide, praseodymium oxide and ytterbium oxide. At times other metals may be incorporated into the photocatalyst, such as sodium, magnesium, iron, cobalt or palladium. Such other metals may be incorporated as ions, oxides and other metal containing compounds as complexes. Incorporation of such other metals should be at a level which will not seduce the catalytic activity of the photocatalyst.
As shown hereinbelow, TiO
2
photocatalyst doped with a lanthanide oxide, according to the present invention, exhibit enhanced activity, as compared to undoped TiO
2
photocatalyst, in degradation of functionalized organic compounds. The term “functionalized organic compound” denotes any compound having a functional group which may be a donor of electrons such as a double bond, carboxylic group, carbonyl group, aldehyde group, amine group, thiol, hydroxyl, ether, heterocyclic organic substrates, phosphate, phosphonate, etc. Organic compounds with functionalized groups may include a variety of noxious substances such as herbicides, intermediates used for the preparation of herbicides, various organic compounds used in industrial synthesis, pollutants, e.g. such emitted by vehicles or industrial combustors or power generators, airborne compounds with a malodor, chemical warfare gases such as nerve gas, mustard. gas, etc.
A specific example of functionalized organic compounds which may be degraded by the photocatalyst of the invention are a variety of aromatic compounds, e.g. such present in herbicides which include, inter alia, p-chlorophenoxyacetic acid, salicylic acid, trans-cinnamic acid, aniline and p-nitrobenzoic acid. Other specific examples include a variety of oily pollutants, e.g. in water.
By another aspect, the present invention provides a process for preparation of the above TiO
2
-based photocatalyst. The process of the invention comprises: mixing a titanium (Ti) containing compound with a lanthanide metal (M) containing compound, to form a gel, the molar ratio Ti:M being from about 100:0.5 to about 100:5; drying the gel thus obtained; and subjecting it to calcination.
The titanium containing compound is typically isopropyl titanoate or titanium chloride. The lanthanide compound may either be lanthanide oxide or may be a precursor compound from which the lanthanide oxide may be obtained. A lanthanide precursor compound may for example be acetyloxy lanthanide or lanthanide nitrate.
The sol is typically prepared in an alcohol solution, the solvent being a lower alkanol. The gel may be formed by adding a catalytic amount of a strong acid to the sol.
The heat treatment may include a first drying step to cause evaporation of the alcohol and then a second calcination step which may be at a temperature of at least about 600° K at times above about 800° K.
The photocatalyst of the invention
Ranjit Koodali
Willner Itamar
Fish & Richardson P.C.
Wong Edna
Yissum Research Development Company of the Hebrew University of
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