Catalyst for Fischer-Tropsch Synthesis and Method of Producing Hydrocarbon Field of the Invention The present invention relates to a catalyst for producing hydrocarbons from a synthesis gas containing hydrogen and carbon monoxide as main components and a method for producing hydrocarbons using such a catalyst. Background of the Invention Reactions for synthesizing hydrocarbons from a synthesis gas containing hydrogen and carbon monoxide are referred to as "Fischer-Tropsch synthesis (FT synthesis)" and are well-known in the prior art. Fischer-Tropsch synthesis is carried out using a catalyst obtained by loading an active metal such as iron and cobalt on a support such as silica and alumina (for example, as disclosed in Japanese Patent Laid-Open Publication No. 4-227847). The FT synthesis reaction is specified by carbon monoxide conversion (CO conversion), methane selectivity, and chain growth probability a. The term "lower methane selectivity" means that methane-generating reaction, i.e., a side reaction of Fischer-Tropsch reaction, is suppressed to below. The term "chain growth probability a " is used as a measure -1of the molecular weights of the resulting hydrocarbons, and higher chain growth probability a, i.e., close to 1.0 means that higher molecular weight hydrocarbons can be produced. 5 FT synthesis products are usually hydrocracked in the subsequent stage and then used as clean liquid fuels. In recent years, among such liquid fuels, middle distillates such as kerosene and light gas oil are in higher demand. In order to enhance the yield of these middle distillates, 10 lower methane selectivity and higher chain growth probability a are required. Therefore, the development of FT synthesis reactions of higher CO conversion, lower methane selectivity, higher a is held up a target in the industry and promoted by improving FT synthesis catalysts. 15 However, not only there is a tendency that methane selectivity increases as CO conversion increases, but also CO conversion and chain growth probability a are in a trade-off relation. More specifically, a catalyst with low methane selectivity in high CO conversion regions and 20 high chain growth probability a has not been developed yet. This is the biggest hindrance in utilizing FT synthesis and a method of producing clean liquid fuels using the synthesis practically. Any discussion of documents, acts, materials, 25 devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common 30 general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. Disclosure of the Invention 35 Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be 2 understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 5 The present invention preferably provides a novel FT synthesis catalyst with low methane selectivity in high CO conversion regions and a high chain growth probability a so as to eliminate the hindrance to practical application 10 of the FT synthesis. As a result of extensive research made by the present inventors, they completed the present invention upon finding that the above problems were able to be overcome with a catalyst which comprises two or more types of 15 precursor compounds containing a specific active metal loaded on a silica-based support containing 0.03 percent by mass or more and 0.30 percent by mass or less of an alkali metal and/or an alkaline earth metal. That is, according to a first aspect of the present 20 invention, there is provided a catalyst for Fischer Tropsch synthesis which comprises two or more types of precursor compounds containing an active metal selected from the group consisting of cobalt, nickel, and ruthenium loaded on a silica-based support containing 0.03 to 0.30 25 percent by mass of an alkali metal and/or an alkaline earth metal wherein said two or more types of precursor compounds are the combination of two precursor compounds which are nitrate and formate, nitrate and acetate, and nitrate and acetylacetonate. 3 According to another aspect of the present invention, there is provided the catalyst wherein the catalyst is produced by impregnating two or more types of precursor compounds containing a metal selected from cobalt, nickel, 5 and ruthenium onto a silica-based support containing 0.03 to 0.30 percent by mass of an alkali metal and/or an alkaline earth metal, followed by drying and calcination. According to yet another aspect of the present invention, there is provided the catalyst wherein the 10 alkali metal and/or the alkaline earth metal are one or more types of alkali metals selected from lithium, sodium, and potassium and/or one or two types of alkaline earth metals selected from magnesium and calcium. According to yet another aspect of the present 15 invention, there is provided the catalyst wherein the precursor compounds containing a metal selected from cobalt, nickel, and ruthenium are compounds selected from nitrate, hydrochloride, sulfate, formate, acetate, propionate, oxalate, and acetylacetonates, of the metal. 20 According to yet another aspect of the present invention, there is provided the catalyst wherein the metal selected from cobalt, nickel, and ruthenium is loaded in an amount of 3 to 50 percent by mass, in terms of metal based on the silica-based support. 25 According to still another aspect of the present invention, there is provided the catalyst wherein the silica-based support has an average particle size of 10 pm to 10 mm and a specific surface area of 100 to 500 m 2 /g. 4 The present invention also relates to a method of producing hydrocarbon wherein hydrocarbon is synthesized by reacting hydrogen with carbon monoxide using the foregoing catalyst. The present invention will be described in more details below. The term "silica-based support" used herein denotes silica or a support containing silica as a main component modified with an alkali metal and/or an alkaline earth metal. Alkali metals used for modifying silica are preferably lithium, sodium, and potassium. Alkaline earth metals used for modifying silica are preferably magnesium and calcium. No particular limitation is imposed on the method for modifying silica with an alkali metal and/or an alkaline earth metal. Therefore, any conventional modifying method such as impregnation methods or metal alkoxide methods may be selected. Particularly preferred methods are impregnation methods. Among impregnation methods, the most preferred is an incipient wetness method. After silica is impregnated with an alkali metal and/or an alkaline earth metal, it is dried and calcined so as to be modified therewith. No particular limitation is imposed on the method -5for drying the impregnated silica which, therefore, may be natural dry in the air and deaeration dry under vacuum. Drying may be generally carried out at a temperature of from 100 to 200 "C, preferably 110 to 150 "C for 0.5 to 48 hours, preferably 5 to 24 hours under the air atmosphere. Calcination may be generally carried out at a temperature of from 300 to 600 "C, preferably 400 to 450 "C for 0.5 to 10 hours, preferably 1 to 5 hours under the air atmosphere. The amount of an alkali metal and/or an alkaline earth metal used for modifying silica is 0.03 percent by mass or more and 0.30 percent by mass or less, preferably 0.04 percent by mass or more and 0.20 percent by mass or less, and more preferably 0.05 percent by mass or more and 0.13 percent by mass or less. An alkali metal and/or an alkaline earth metal in an amount of less than 0.03 percent by mass would be ineffective in providing decreased methane selectivity and enhanced chain growth probability a , while those in an amount of more than 0.30 percent by mass would provide decreased CO conversion. Silicas used in the present invention may be those with an average pore diameter of from 8 to 20 nm, preferably 10 to 18 nm, and more preferably 11 to 16 nm. The term "average pore size" used herein denotes a value measured by a nitrogen absorbing method. No particular limitation is imposed on the shape -6of the silica and silica-based support used in the present invention. Therefore, the silica and silica-based support may be in any shape suitable for the intended processes, selected from various shaped products such as spherical or 5 pulverized products and column-like molded products. No particular limitation is imposed on the average particle size of the silica and silica-based support. Any of those having an average particle size of generally from 10 pm to 10 mm, preferably 50 pm to 5 mm may be selected depending 10 on the intended processes. No particular limitation is imposed on the specific surface area of the silica and silica-based support. Any of those having a specific surface area of generally from 100 to 500 m 2 /g, preferably 200 to 400 m 2 /g may be used. 15 The term "precursor compound containing a metal selected from cobalt, nickel, and ruthenium, impregnated onto a silica-based support" denotes all compounds containing in molecules such a metal in the form of salt 20 or complex. Although no particular limitation is imposed on the type of the compound, preferred are nitrate, hydrochloride, sulfate, formate, acetate, propionate, oxalate, and acetylacetonates. The present invention is characterized by using two or more types of precursor 25 compounds containing any of the above-exemplified active metals. Although two types of precursor compounds are 7 generally combined for easy production, three or more precursor compounds may be combined if necessary. No particular limitation is imposed on the combination of precursor compounds. Preferred combinations of two 5 precursor compounds are nitrate and formate; nitrate and acetate; and nitrate and acetylacetonates. Particularly preferred combinations are nitrate and formate; and nitrate and acetate. Most preferred combination is nitrate and acetate. 10 The advantageous effects of the present invention can be achieved by impregnating two or more types of precursor compounds containing a specific active metal onto a silica-based support containing a specific amount of an alkali metal and/or an alkaline earth metal. 15 Active metals to be loaded on a silica-based support are those selected from cobalt, nickel, and ruthenium. Among these, more preferred are cobalt and ruthenium, and most preferred is cobalt. In general, the active metal is loaded in the form of a metal oxide on a silica-based 20 support by dipping the silica-based support into a solution of two or more precursor compounds containing the metal such that the compounds are impregnated onto and loaded on the support, followed by drying and calcination. 8 No particular limitation is imposed on the drying method which, therefore, may be natural dry in the air and deaeration dry under vacuum. Drying may be generally carried out at a temperature of from 100 to 200 0 C, preferably 110 to 150 0 C for 0.5 to 48 hours, preferably 5 to 24 hours under the air atmosphere. Calcination may be generally carried out at a temperature of from 300 to 600 0 C, preferably 400 to 450 0 C for 0.5 to 10 hours, preferably 1 to 5 hours under the air atmosphere. No particular limitation is imposed on the amount of the active metal loaded on a silica-based support. However, the active metal is loaded in an amount of generally 3 to 50 percent by mass, preferably 5 to 40 percent by mass, and particularly preferably 10 to 30 percent by mass in terms of metal based on the silica-based support. Since the active metal loaded in an amount of less than 3 percent by mass would result in a catalyst with poor activity and the active metal loaded in an amount of more than 50 percent by mass would be extremely aggregated, the advantageous effects of the present invention may not be obtained. If necessary, a promoter such as zirconia and lanthania may be loaded on a silica-based support. The amount of such a promoter is generally from 1 to 20 percent by mass in terms of metal ,based on the silica-based support. -9- Hydrocarbons can be synthesized from hydrogen and carbon monoxide at a high CO conversion, low methane selectivity, and high a using the catalyst of the present invention. When the catalyst of the present invention is used for an FT synthesis reaction, it is preferably reduced with hydrogen beforehand. No particular limitation is imposed on feed stocks for an FT synthesis reaction conducted using the catalyst of the present invention as long as they are synthesis gases containing hydrogen and carbon monoxide as main components. However, the molar ratio of hydrogen to carbon monoxide is from 1.5 to 2.5, preferably from 1.8 to 2.2. The catalyst of the present invention is applicable to any process known as reaction processes for FT synthesis, such as fixed bed-, supercritical fixed bed-, slurry bed-, and fluidized bed-type reaction processes. Although not restricted, preferred processes are fixed bed-, supercritical fixed bed-, and slurry bed-type reaction processes, particularly preferred are fixed bed- and supercritical fixed bed-type reaction processes, and most preferred is fixed bed-type reaction processes. No particular limitation is imposed on the reaction conditions of an FT synthesis reaction which, -10therefore, may be carried out under conditions which have conventionally been employed. The reaction may be carried out at a temperature of generally from 200 to 280 0 C and gas hourly space velocity of 1, 000 to 3, 000 h 1 . Applicability in the Industry As described above, an FT-synthesis reaction whose methane selectivity in high CO conversion regions is low and chain growth probability a is high can be accomplished using the FT synthesis catalyst of the present invention obtained by impregnating two or more types of precursor compounds containing a metal selected from cobalt, nickel, and ruthenium onto a silica-based support containing 0.03 percent by mass or more and 0.30 percent by mass or less of an alkali metal and/or an alkaline earth metal, followed by drying and calcination. Best Modes for Carrying out the Invention The present invention will be described in more details with reference to the following Examples and Comparative Examples but are not limited thereto. (Example 1) 5.0 g of silica with an average pore diameter of 15.2 nm and a specific surface area of 320 m 2 /g was -11impregnated with an aqueous solution containing sodium acetate in an amount corresponding to 0.04 percent by mass in terms of metal sodium of the silica and magnesium nitrate in an amount corresponding to 0.04 percent by mass in terms of metal magnesium of the silica by an incipient wetness method. Thereafter, the water was dried out at a temperature of 120 0 C over the night. The silica was modified by 2 hour calcination at a temperature of 450 0 C thereby obtaining a silica-based support loading sodiumandmagnesium. The alkali metal and alkaline earth metal content of the silica-based support was analyzed using a metal analyzing apparatus. As a result, it was confirmed that the total content of the metals was 0.08 percent by mass. The silica-based support was impregnated with a solution containing cobalt nitrate in an amount corresponding to 10.0 percent by mass in terms of metal cobalt of the support and a cobalt acetate in an amount corresponding to 10.0 percent by mass of in terms of metal cobalt of the support by an incipient wetness method. Thereafter, the water was dried out at a temperature of 120 0 C over the night. The silica-based support was calcined at a temperature of 450 0 C for 2 hours thereby obtaining a catalyst loading cobalt. The resulting catalyst was charged into a fixed-bed circulation-type reactor and reduced under a hydrogen gas stream at a temperature of 400 0 C for 2 hours before a reaction was initiated. -12- Thereafter, a feed stock mixed gas containing hydrogen and carbon monoxide at a molar ratio of 2/1 was supplied at a gas hourly space velocity of 2, 000 h 1 , and the reaction was initiated at a temperature of 250 0 C and a pressure of 1 MPa. The gas composition at the outlet of the reactor was analyzed using a gas chromatography with time. The CO conversion, methane selectivity, and chain growth probability a were calculated using the resulting analyzed data in accordance with a conventional method. The results are shown in Table 1. (Example 2) Example 1 was repeated except that silica of an average pore diameter of 12.8 nm and a specific area surface of 347 m 2 /g was used thereby determining the CO conversion, methane selectivity, and chain growth probability a. The results are shown in Table 1. (Example 3) Example 1 was repeated except for the use of a silica-based support containing 0.04 percent by mass of an alkali metal and an alkaline earth metal obtained using an aqueous solution containing sodium acetate in an amount corresponding to 0. 02 percent by mass in terms of metal sodium of the silica and magnesium nitrate in an amount corresponding to 0. 02 percent by mass in terms -13of metal magnesium of the silica, thereby determining the CO conversion, methane selectivity, and chain growth probability a . The results are shown in Table 1. (Example 4) Example 1 was repeated except for the use of a silica-based support containing 0.16 percent by mass of an alkali metal and an alkaline earth metal obtained using an aqueous solution containing sodium acetate in an amount corresponding to 0. 08 percent by mass in terms of metal sodium of the silica and magnesium nitrate in an amount corresponding to 0.08 percent by mass in terms of metal magnesium of the silica, thereby determining the CO conversion, methane selectivity, and chain growth probability a. The results are shown in Table 1. (Example 5) Example 1 was repeated except for the use of a silica-based support containing 0.10 percent by mass of an alkali metal and an alkaline earth metal obtained using an aqueous solution containing sodium acetate in an amount corresponding to 0.05 percent by mass in terms of metal sodium of the silica and magnesium nitrate in an amount corresponding to 0.05 percent by mass in terms of metal magnesium of the silica, thereby determining -14the CO conversion, methane selectivity, and chain growth probability a. The results are shown in Table 1. (Example 6) Example 1 was repeated except for the use of a silica-based support containing 0.24 percent by mass of an alkali metal and an alkaline earth metal obtained using an aqueous solution containing sodium acetate in an amount corresponding to 0. 12 percent by mass in terms of metal sodium of the silica and magnesium nitrate in an amount corresponding to 0. 12 percent by mass in terms of metal magnesium of the silica, thereby determining the CO conversion, methane selectivity, and chain growth probability a. The results are shown in Table 1. (Comparative Example 1) A silica-based support having an average pore diameter of 15.2 nm and a specific surface area of 320 m 2 /g and containing an alkali metal and an alkaline earth metal in a total amount of 0.02 percent by mass was impregnated with an aqueous solution containing cobalt nitrate in an amount corresponding to 10.0 percent by mass in terms of metal cobalt of the silica-based support and cobalt acetate in an amount corresponding to 10.0 percent by mass in terms of metal -15cobalt of the silica-based support by an incipient wetness method. Thereafter, the water is dried out at a temperature of 120 0 C over the night. The support was calcined at a temperature of 450 0 C for 2 hours thereby obtaining a catalyst loading cobalt. The resulting catalyst was charged into a fixed-bed circulation-type reactor and reduced under a hydrogen gas stream at a temperature of 400 0 C for 2 hours before a reaction was initiated. Thereafter, a feed stock mixed gas containing hydrogen and carbon monoxide at a molar ratio of 2/1 was supplied at a gas hourly space velocity of 2,000 h~1, and the reaction was initiated at a temperature of 250 C and a pressure of 1 MPa. The gas composition at the outlet of the reactor was analyzed using a gas chromatography with time. The CO conversion, methane selectivity, and chain growth probability a were calculated using the resulting analyzed data in accordance with a conventional method. The results are shown in Table 1. (Comparative Example 2) Comparative Example 1 was repeated except that a silica-based support having an average pore diameter of 12.8 nm and a specific surface area of 347 m 2 /g and containing an alkali metal and an alkaline earth metal in a total amount of 0.02 percent by mass was used thereby determining the CO conversion, methane -16selectivity, and chain growth probability a. The results are shown in Table 1. (Comparative Example 3) Example 1 was repeated except for the use of a silica-based support containing 0.32 percent by mass of an alkali metal and an alkaline earth metal obtained using an aqueous solution containing sodium acetate in an amount corresponding to 0.16 percent by mass in terms of metal sodium of the silica and magnesium nitrate in an amount corresponding to 0.16 percent by mass in terms of metal magnesium of the silica, thereby determining the CO conversion, methane selectivity, and chain growth probability a. The results are shown in Table 1. (Comparative Example 4) Example 1 was repeated except that before modification, only cobalt nitrate in an amount corresponding to 20.0 percent by mass in terms of metal cobalt of the silica was loaded on a silica-based support thereby determining the CO conversion, methane selectivity, and chain growth probability a. The results are shown in Table 1. It is confirmed from the results shown in Table 1 that high CO conversion, low methane selectivity, and -17high chain growth probability a are satisfied at the same time using a catalyst obtained by loading two or more precursor compounds containing a metal selected from cobalt, nickel, and ruthenium on a silica-based support containing 0.03 percent by mass or more and 0. 30 percent by mass or lower of an alkali metal and/or an alkaline earth metal. Table 1 CO conversion % Methane Chain growth Catalyst selectivity % probability a Example 1 89.3 13.6 0.89 Example 2 90.0 14.0 0.87 Example 3 84.0 13.7 0.88 Example 4 85.0 13.6 0.89 Example 5 90.0 9.0 0.91 Example 6 83.3 14.0 0.89 Comparative 85.2 17.2 0.86 Example 1 Comparative 90.0 20.0 0.81 Example 2 Comparative 78.0 11.0 0.89 Example 3 Comparative 64.0 13.5 0.88 Example 4 -18-