"Of these compounds, oxides and sulfates of a metal selected from silicon,titanium, zirconium and vanadium, such as silicon oxide, titanium oxide, vanadium oxide, zirconium oxide, titanium sulfate, titanyl sulfate, zirconium sulfate and vanadyl sulfate are particularly preferred in the present invention.The above polyvalent metal compound is used in such an amount that the total amount of the polyv" . . . . .