In the recent years, as the awareness of environmental protection grew, many countries started regulating industrial and automobile emissions through legislation, prodding the industry and academia to come up with an environmentally friendly catalyst. As the number of automobiles multiplied, automobile emission has become a major source of air pollution. In the U.S. automobile emission accounts for nearly 60% of air pollution, whereas industrial exhaust accounts for 17%. Automobile exhaust fumes contain carbon monoxide (CO), unburned hydrogen carbonate (HC), and nitrogen oxides (NO.sub.x) which is mainly NO and NO.sub.2. Nitrogen oxide is well known for damaging ozone (O.sub.3) in our atmosphere. The O.sub.3 layer filters out much of the ultraviolet radiation; however, damage to the O.sub.3 layer results, which thereby exposes the earth surface to an excess amount of ultraviolet which is responsible for causing skin cancer.
Most of the catalysts available today are designed to reduce the amount of carbon monoxide (CO) and hydrogen carbonate (HC) in automotive vehicle exhaust gas. However, there is much demand for a catalyst that can simultaneously treat CO, HC and NO.sub.x. The conversion of CO and HC is an oxidizing reaction, while the conversion of NO.sub.x to inert N.sub.2 is a reduction reaction. Therefore, a catalyst that can treat all three ingredients must be an oxidizing agent as well as a reductive agent. Moreover, such a catalyst is efficient under a small window of operating conditions of the automobile engine, e.g. air/fuel ratio should be kept at a strict stoichiometric relation. Leaded gasoline which is now used in some countries could poison automobile exhaust catalysts. The sulfide in exhaust gases can also poison catalyst. Western countries (e.g. the U.S.) has enacted the automobile emission standard which stipulated lead-free gasoline as standard motor fuel.
A description of the catalyst for purification of automobile exhaust gases in which noble metals e.g. Pd, Pt, Ru and rare earth metals are used as the active ingredients has been reported in the literature. However, these catalysts are undoubtedly more costly. Other catalysts using non-noble metals as the active ingredients may also be found in the literature. But the common problem for these catalysts is low conversion rate, especially at low temperature, and poor durability. Some catalysts should be equipped with oxygen sensor and heating unit, the active ingredients and the proportion thereof differ greatly different from those of present invention.
Chinese patent application No. 85109694.8 discloses a nonmetal honeycomb-shaped combustion catalyst which applied double-carrier and oxide of Cu, V, Mn, Co, Ni, or oxide/mixture of several elements above mentioned, and adopted rare-earth elements, e.g. La, Ce, Pr, Sm, Nd, Yt as active ingredients.
Chinese patent application No. 98105063.9 discloses a catalyst similar to Chinese Patent No. 85109894.8. that consists of composite carrier made from honeycomb-shaped ceramics materials coated with aluminum oxide, and active ingredients which are one or multi oxides (or mixture made of these oxides) of nonnoble metals e.g. Zr, Co, Ni, Mn, Cu, Cr and the oxide of a rare-earth element. But the active oxides are reduced by hydrogen, carbon monoxide, or hydrocarbon oxides during the preparation process in which a portion of the active oxides of the metal elements are converted to metal elements. The catalyst calcine technology is not acceptable at 700-1100.degree. C. As the temperature is increased to 865.degree. C., .gamma.-Al.sub.2 O.sub.3, becomes crystalline and shows low activity and stability. Since .gamma.-Al.sub.2 O.sub.3 is amorphous (indefinite) at 200-600.degree. C. with high activity and dispersion. When the temperature is up to 1100.degree. C., .gamma.-Al.sub.2 O.sub.3 becomes non-active .alpha.-Al.sub.2 O.sub.3 and is easily separated from the carrier.
EP 9393517A2 discloses a catalytic oxidization/reduction converter employed to treat the exhaust waste from an internal combustion engine. It could burn completely the unburned hydrocarbon and reduce the NO.sub.x to the elements nitrogen and oxygen. As a catalyst, it adopted the oxides of many metals e.g. Cr, Mn, Fe,Co, Ni,Cu, Zn, Sn, Ba, La, and Ce. The publication reported only a catalyst with chemical component Cu 26%, Cr 21%, BaO 11% (by weight). The converter must be accompanied by a heater operating at 200-400.degree. C. to clean up the exhaust gases at the exit of the muffler.
U.S. Pat. No. 4,519,992 of Alkhazov et. al discloses a method for purifying hydrogen sulfide in which the following oxides are used in the catalyst: TiO.sub.2 10-30%, Fe.sub.2 O.sub.3 20-30%, ZnO 20-25% (by weight ), and air is served to oxidize the hydrogen sulfide. The catalyst is prepared by reacting chemically aqueous solutions of these metal's salts (such as hydrochloride) with ammonia water to generate hydroxide, then well-mixing, drying, and calcining into the relevant oxides.
GB 2059934A provides a catalyst to purify N.sub.2 O that contains one or multi-oxides e.g. Fe.sub.2 O.sub.3, CoO, CuO, Cr.sub.2 O.sub.3 MnO.sub.2 and NiO as active ingredients. The document describes some catalysts which employed at the most three metal elements indicated above as active ingredients. The catalyst is made by soaking the carrier with the aqueous solutions of these metal's salts (such as nitrate), then drying, calcining to make the salts decompose into the relevant oxides.
There is a need, therefore, to develop a catalyst applied to purify industrial waste gaseous (such as sulfide) and automotive vehicle exhaust gases that should be highly effective, long lasting and can start at low temperatures.