CATALYST AND USE THEREOF IN THE PRODUCTION OF VLNILO ACETATE Field of the Invention The present invention relates to a process for the production of vinyl acetate and to a new catalyst for use in such a process. Background of the Invention Vinyl acetate is generally prepared commercially by contacting acetic acid and ethylene with molecular oxygen in the presence of an active catalyst for the production of vinyl acetate. The catalyst suitable for use in the production of the vinyl acetate can comprise a Group VIII metal, for example palladium, an alkali metal acetate as a promoter, for example sodium or potassium acetate, and an optional co-promoter, for example sodium acetate. cadmium or gold In particular, US 5185308 describes a process for the preparation of vinyl acetate by the catalytic oxidation of ethylene in the presence of acetic acid. The catalyst used is a palladium catalyst supported and promoted with gold and potassium acetate. Acetic acid, useful as a feedstock in the production of vinyl acetate, can be prepared by various methods commonly practiced in the industry, for example, by the liquid phase carbonylation of methanol and / or a reactive derivative of the same, in the presence of a Group VIII noble metal catalyst, an alkyl promoter iodide and a finite concentration of water. The acetic acid produced is then used as a reactant in the production of vinyl acetate. In this way, the process tends to be a two-stage process. US 3373189 relates to a process for the production of vinyl acetate from ethylene and oxygen using a palladium catalyst. US 4188490 describes a process for the production of acetic acid and vinyl acetate using a palladium catalyst on a zinc oxide support. US 3637818 describes a process for the production of acetaldehyde, acetic acid and vinyl acetate by oxidation of ethylene in the presence of a noble metal and manganese or cobalt oxides. These processes are carried out in the liquid phase. Summary of the Invention It has now been found that vinyl acetate can be produced directly from a reactant mixture comprising ethylene, optionally water and an oxygen-containing gas, without the need to initially produce acetic acid as a separate step from the overall process. Through the use of a modified palladium catalyst, vinyl acetate can be produced directly in a single-step process. Accordingly, the present invention provides a process for the production of vinyl acetate which comprises reacting ethylene with a gas containing oxygen, and optionally water, in the presence of a catalyst comprising (1) a catalyst support, (2) palladium , (3) an acid, (4) at least one acetic acid catalyst promoter, and (5) at least one vinyl acetate catalyst promoter and / or co-promoter. According to another embodiment of the present invention, there is provided a catalyst for use in the production of vinyl acetate comprising (1) a catalyst support, (2) palladium, (3) an acid, (4) at least one promoter of acetic acid catalyst and (5) at least one promoter 'and / or vinyl acetate catalyst co-promoter. Detailed Description of the Invention The present invention provides a new and inexpensive route for the production of vinyl acetate. The process is not only highly selective for the production of vinyl acid, but does not require the independent and separate production of acetic acid which is then used as a co-reactant in the process. On the contrary, the use of the modified palladium catalyst results in the in situ oxidation of the reactants to produce acetic acid which is then oxidized with ethylene to vinyl acid. The bifunctional nature of the catalyst is translated into a direct process. The present invention provides a process for the production of vinyl acid from ethylene, a gas containing oxygen and optionally water. The ethylene may be substantially pure or may be mixed with one or more of nitrogen, methane, ethane, carbon dioxide, hydrogen and low levels of alkenes or C3 / C4 alkanes. The gas containing oxygen can be air or a gas richer or poorer in molecular oxygen than air. Suitably, the gas can be oxygen diluted with a suitable diluent, such as, for example, nitrogen or carbon dioxide. Preferably, the gas containing oxygen is oxygen. Optionally, water can be co-fed into the reaction chamber. When water is present in the reaction chamber, it may be present in an amount of up to 50% by volume, preferably 10 to 30% by volume. A small amount of acetic acid can also be introduced into the reaction chamber. Suitably, the acetic acid can be introduced through a recycle stream. When it is desired to introduce acetic acid, it may be present in an amount of up to 50% by volume, preferably 50 to 20% by volume. The catalyst of the present invention comprises palladium. The concentration of palladium may be greater than 0.5% by weight, preferably greater than 1% by weight based on the total weight of the catalyst. The concentration of palladium can be as high as 10% by weight for fixed bed or fluid bed applications. The catalyst of the present invention is a supported catalyst. Suitable catalyst supports may comprise porous silica, alumina, silica / alumina, titania, zirconia or carbon. Preferably the support is silica. Suitably the support can have a pore volume of 0.2 to 3.5 ml per g of support, a surface area of 5 to 800 m2 per g of support and an apparent density of 0.3 to 1.5 g / ml . For catalysts used in fixed bed processes, the support usually has dimensions of 3 to 9 mm and can be presented in spherical form, as tablets, as extrudates, as pills or in any other suitable form. For catalysts to be used in fluid bed processes, the support can normally have a particle size distribution such that at least 60% of the catalytic particles have a particle diameter of less than 200 microns, preferably such that at least 50% are less than 105 microns and no more than 40% of the catalytic particles have a diameter less than 40 microns. The catalyst composition comprises at least one catalyst promoter for acetic acid. Suitable promoters include compounds containing selenium, titanium, tellurium and / or vanadium. Preferably the catalyst promoter for acetic acid is an oxide, acetate or acetylacetonate of at least one of the aforementioned metals. Preferably, the content of catalyst promoter for acetic acid in the final catalyst is up to 10% by weight. In addition to the palladium compound and the acetic acid promoter, the catalyst comprises at least one promoter and / or catalyst co-promoter for vinyl acid, preferably both a promoter and copromotor. Suitable promoters include gold, copper and / or nickel and cadmium acetate. A preferred promoter is gold. Suitable sources of gold include gold chloride, tetrachloroauric acid HAuCl4, NaAuCl4, KAuCl4, dimethylol acetate, barium acetoaurate or gold acetate. The preferred gold compound is HAuCl 4. The metal may be present in an amount of 0.1 to 10% by weight in the finished catalyst. Suitable co-promoters include alkali metal or alkaline earth metal salts, preferably an acetate salt, such as potassium or sodium acetate. Preferably, the co-promoter content in the final catalyst is from 0.1 to 9.5% by weight as acetate. A preferred catalyst component (5) is gold and either sodium acetate or potassium acetate. The catalyst composition comprises an acid. Preferably the acid is a strong acid. Suitable acids include heteropolyacids which may include silicotungstic acid, Phosphotungstic acid, phosphomolybdic acid, silico acid olíbdico, tungstomolybdofosfórico acid, tungstomolibdosilícico acid, tungstovanadophosphorico acid, tungstovanadosilícico acid, molibdovanadofosfórico acid, molibdovanadosilícico acid, borotúngstico acid, boromolybdic acid, tungstomolibdobórico acid, molibdoalumínico acid, tungstoalumínico acid, molibdotungstoalumínico acid, molibdogermánico acid, tungstogermanic acid, molybdenotungstogermanic acid, molybdicitic acid, tungstotitanic acid, molybdenotungstenic acid, cericmolybdic acid, cerictumic acid, cericmolybdenotunic acid, molybdocobalt acid, tungstocobalt acid, molibdotungstocobalt acid, phosphonobioic acid, siliconiobic acid, and silicotanthic acid. Among these, silicotungstic acid, phosphotungstic acid, phosphomolybdic acid, silicomolybdic acid, tungstomolybiphosphoric acid, tungstomolibosilicic acid, tungstovanadofosphoric acid, tungstovanadosilicic acid, molybdovanadosilicic acid, borotungstic acid, boromolybdic acid and boromolibototungstic acid are especially preferred. Preferably, the acid content in the final catalyst is up to 50% by weight. The final catalyst composition can be suitably optimized to maximize the rate of vinyl acid production while maximizing selectivity. The catalyst of the present invention can be suitably prepared by the method described in detail in GB-A-1559540. In the first stage of the preparation process, the support is impregnated with a solution containing the required palladium and metal promoter, for example gold, in the form of soluble salts. Examples of such salts include the soluble halide derivatives. The impregnating solution is preferably an aqueous solution and the volume of solution used is such that it corresponds to 50-100% of the pore volume of the support, preferably 95-99% of the pore volume for fixed-bed catalysts or 50- 99% of the pore volume for fluid bed catalysts. After impregnation, the wet support is treated, optionally, with an aqueous solution of an alkali metal salt selected from alkali metal silicates, carbonates or hydroxides, to develop a -
metal shell structure, familiar to experts in the field. The amount of alkali metal salt used is such that, once the solution has been contacted with the impregnated support for a time comprised between 12 and 24 hours, the pH of the solution is suitably from 6.5 to 9, 5, preferably from 7.5 to 8, measured at 25 ° C. The preferred metal salts are sodium silicate, sodium carbonate and sodium hydroxide. During the above described treatment, it is believed that they precipitate or are incorporated on the support: palladium and promoter, for example gold, and hydroxides. Alternatively, the impregnated support can be dried at ambient pressure or reduced pressure and at temperatures between ambient temperature and 150 ° C, preferably between 60 and 120 ° C, before the reduction of the metals. To convert said materials to the metallic state, the support impregnated with a reducing agent such as ethylene, hydrazine, formaldehyde or hydrogen is treated. If hydrogen is used, it will normally be necessary to heat the catalyst at temperatures between 100 and 300 ° C in order to effect complete reduction. Once the steps described above are carried out, the reduced catalyst is washed with water and then dried. The dry support is then impregnated with the required amount of catalyst co-promoter for vinyl acetate, for example aqueous alkali metal acetate, and catalyst promoter for acetic acid, for example aqueous selenium containing compound, and then dried. The dry support is further treated with the appropriate amount of heteropolyacid dissolved in water and the final product is dried. The method of catalyst preparation can be varied to optimize catalyst performance based on maximizing yield and selectivity to vinyl acid. The vinyl acid preparation using the catalyst of the present invention is usually carried out by contacting ethylene, water and a gas containing oxygen, such as oxygen or air, with the catalyst at a temperature of 100 to 400 ° C, preferably from 140 to 210 ° C, and at a pressure of 1 to 20 bar manometry, preferably from 6 to 15 bar manometry. The process can be carried out in a fixed bed or fluidized bed reactor and is preferably carried out in the gas phase. The present invention will now be illustrated with reference to the following examples. Example 1 is an example according to the present invention. Comparative examples A, B and C are outside the scope of the invention wherein the process used a catalyst also outside the present invention: Example A because it does not contain a promoter or co-promoter of a catalyst for vinyl acid; Example B because it does not contain an acid or a catalyst promoter for acetic acid, - and Example C because the catalytic component of acetic acid and the catalytic component of vinyl acid are in separate beds. Example 1 (a) Preparation of the catalyst. In an aqueous solution containing 1.7 g of sodium tetrachloropaladalate (II) and 1.5 g of hydrated sodium tetrachloroaurate (III), dissolved in 34 g of water, 68.4 g of a porous silica support (KA) are placed 160 of Sud Chemie) which has a particle size of 5 to 7 mm, to absorb the entire solution. The resulting support is added to 76.5 g of an aqueous solution containing 6.5 g of sodium metasilicate and completely covered by the solution. The mixture is allowed to stand for 18 hours and then 20 g of 99% hydrazine hydrate are added to the solution to reduce Pd and Au. The resulting mixture is allowed to stand for 4 hours or until the reduction is completed. The support is separated from the solution and washed with deionized water until chloride ion is no longer present in the effluent using an AgN03 solution. The resulting support is dried at 60 ° C for 24 hours. 10 g of the dried support is added to an aqueous solution containing 0, 0071 g of selenate (VI) of potassium and 0.51 g of potassium acetate dissolved in 5 g of water. The support absorbs the entire solution and then dries at 60 ° C for 24 hours. Then, an aqueous solution containing 3.3 g of hydrotuned silicotungstic acid dissolved in 5 g of water is added to the support and the solution is completely absorbed. The resulting support containing Pd, Au, selenium salt and tungstenic acid is dried at 60 ° C for 24 hours. Production of vinyl acid. 5 g of the resulting catalyst are uniformly distributed in 60 ml of glass beads (size 1 mm) in a reaction tube. In the unit, a mixture of ethylene, oxygen, water vapor and inert gas is introduced in a volume ratio of 40: 6: 31: 23, at a temperature of 160 ° C and at a pressure of 8 bar gauge and at a speed of flow of 15.8 LN / hr to effect the reaction. The effluent is analyzed online by gas phase chromatography. The space-time yield of vinyl acetate was 84.5 g / hr.L, and a selectivity of 76% was obtained based on the carbon balance. The selectivity at C02 was 22%. Comparative Example A Preparation of the catalyst. Sodium tetrachloropaladalate (II) (9.5 g) is dissolved in water (90 g). A porous silica support (Sudchemie KA160, particle size 5 to 7 mm) (180 g) is impregnated with this aqueous solution until all the solution is absorbed. The resulting support is added to an aqueous solution (170 g) containing sodium metasilicate (17.7 g). The mixture is allowed to stand for 18 hours and then 20 g of 99% hydrazine hydrate are added to the solution to reduce the palladium. The resulting mixture is allowed to stand for 4 hours or until the reduction is complete. The support is separated from the solution and washed with deionized water until chloride ion is no longer present in the effluent using an AgN03 solution. The resulting support is dried at 60 ° C for 48 hours. Potassium selenate (VI) (0.164 g) is dissolved in water (10 g) and the solution is completely absorbed by 20 g of the dry support. The support is then dried again at 60 ° C for 24 hours. An aqueous solution containing silicotungstic acid hydrate (6.3 g) dissolved in water (10 g) is then added to the support and the solution is completely absorbed. The resulting support containing palladium, selenium salt and silicotungstic acid is dried at 60 ° C for 24 hours. 5 g of the resulting catalyst are uniformly distributed in 60 ml of glass beads (size 1 mm) in a reaction tube and a mixture of ethylene, oxygen, water vapor and an inert gas in a volume ratio is introduced into the unit. of 40: 6: 31: 23 at a temperature of 150 ° C and a pressure of 8 bar gauge and a flow rate of 15.5 LN / hr to effect the reaction. The effluent is analyzed online by gas phase chromatography. As a result, the following data were obtained: Space-time yield of acetic acid: 235 g / hr.L; space-time yield of vinyl acetate: 2.1 g / hr.L; space-time yield of acetaldehyde: 3.5 g / hr.L; and space-time yield of carbon dioxide: 21.2 g / hr.L. The overall selectivity to acetic acid was 87% and the overall selectivity to vinyl acetate was 0.54%. Comparative Example B (a) Preparation of the catalyst. A catalyst for vinyl acetate was prepared according to US 5185308 with nominal charges of 0.9 Pd, 0.4 Au and 7% by weight KOAc on KA160. 2.5 g of the resulting catalyst are uniformly distributed in 60 ml of glass beads (size 1 mm) in a reaction tube and a mixture of ethylene, oxygen, steam and an inert gas is introduced into the unit in a volume ratio of 47: 7: 19: 27, at a temperature of 150 ° C and a pressure of 8 bar gauge and at a flow rate of 21, lLN / hr, to effect the reaction. The effluent is analyzed online by gas chromatography. As a result, the following data were obtained: space-time yield of carbon dioxide: 119.6 g / hr.L; no acetic acid, vinyl acetate or other by-products were detected. Comparative example C (a) Preparation of the catalyst The catalysts were prepared according to comparative examples A and B. (b) Production of vinyl acetate. A reaction tube is filled with catalyst 1 (5 g) and catalyst 2 (2.5 g) and glass beads (60 ml, size 1 mm). Catalyst 1, uniformly distributed in glass beads (32 ml), is placed on top of the tube and catalyst 2 on glass beads (16 ml) is placed on the bottom. Between the catalyst section 1 and the catalyst section 2 there were 2 ml of glass beads and 5 ml in each of the upper and lower parts of the tube. In the unit, a mixture of ethylene, oxygen, water vapor and an inert gas is introduced in a volume ratio of 50: 8: 21: 21 at a temperature of 150 ° C and at a pressure of 8 bar gauge and at a speed of flow of 15.8 LN / hr, to effect the reaction. The effluent is analyzed line by chromatography in gas phase. The following results were obtained: space-time yield of vinyl acetate: 115 g / hr.L; 77% selectivity based on the carbon balance; C02 selectivity; of 11%; and 5% acetic acid selectivity. As minor by-products, ethyl acetate and ethanol were obtained. It can be appreciated that only reaction selectivities are achieved which are comparable to those achieved from the claimed process when a two-step process is carried out.