Catalyst and catalyst group

A catalyst group, comprising:

• 200 or more ring-shaped catalysts, each having a straight body part and a hollow body part, which is suitable for a gas-phase catalytic oxidation reaction of an olefin or a tertiary butanol to produce a corresponding unsaturated aldehyde and/or unsaturated carboxylic acid, wherein the catalyst group satisfies (1) and (2):
• (1) the catalyst group comprises a catalyst (A) in which a length of the straight body part is shorter than a length of the hollow body part, the straight body part is present between a surface comprising a first end part of the hollow body part and a surface comprising a second end part of the hollow body part, and at least at one end part, a region from an end part of the straight body part to an end part of the hollow body part is linear and/or concavely curved;
• and
• (2) an upward ratio by a shaking test of the catalyst group is 70% or less,
• wherein a number ratio of the catalyst (A) to the ring-shaped catalysts is at least 10%,
• wherein a proportion of concave curve in the region of the catalyst (A) is at least 40% of a total number of the first end part and the second end part with a remainder being linear, and
• wherein the upward ratio is measured by a shaking test of the catalyst group, involving randomly selecting 100 ring-shaped catalysts from the catalyst group and placing the 100 ring-shaped catalysts in a stainless steel tray having a width of 296 mm, a depth of 231 mm, and a height of 49 mm, mounting the stainless steel tray in a digital shaker, and, after shaking for 1 minute at a reciprocatory shaking width of 10 mm and a shaking speed of 350 reciprocations/min, defining a number of the ring-shaped catalysts having an upward hollow body part per 100 ring-shaped catalysts as the upward ratio.