Oxide catalyst, process for producing oxide catalyst, process for producing unsaturated acid, and process for producing unsaturated nitrile

A particulate oxide catalyst for use in the gas-phase catalytic oxidation reaction or the gas-phase catalytic ammoxidation reaction of propane or isobutane,

• the oxide catalyst containing a Mo compound, a V compound, a Nb compound, a compound of at least one element selected from the group consisting of Sb and Te, a W compound, and an optional compound of at least one element selected from the group consisting of Mn, B, Ti, Al, Ta, an alkali metal, an alkaline earth metal, La, Ce, Pr, Yb, Co, Y, and Sc, at atomic ratios represented by the following formula (0), wherein
• an average intensity of W present in a region within 5 μm from a surface toward a center of the particle of the oxide catalyst is equal to or greater than 1.08 times that of W present in the whole oxide catalyst:
• CMo:CV:CW:CNb:CX:CZ=1:a:w:c:x:z (0)
• wherein CMo represents the atomic ratio of Mo;
• CV represents the atomic ratio of V;
• CW represents the atomic ratio of W;
• CNb represents the atomic ratio of Nb;
• CX represents the atomic ratio of at least one element selected from the group consisting of Sb and Te;
• CZ represents the atomic ratio of at least one element selected from the group consisting of Mn, B, Ti, Al, Ta, an alkali metal, an alkaline earth metal, La, Ce, Pr, Yb, Co, Y, and Sc;
• and a, w, c, x, and z fall within the ranges of 0.01≦a≦1, 0<w≦2, 0.01≦c≦1, 0.01≦x≦1, and 0≦z≦1, respectively, and wherein the particulate oxide catalyst has the ability to catalyze the gas-phase catalytic oxidation reaction or the gas-phase catalytic ammoxidation reaction of propane or isobutane.