Chemistry reaction question

why does it initially increase but then decrease? i thought of it as the reaction occurs the pressure will increase

I suppose this question should state: “C2H4 and H2 gases react together”.

When nickel catalyst is added, it takes up space therefore there is some volume inside the container taken up by the catalyst. This means that particles of gases have less space to move around, so the pressure inside cylinder initially increases.

During the course of the reaction, C2H4 and H2 (2 gas molecules) react together to form C2H6 (1 gas molecule). This means the pressure decreases.