Difference between 7. Qc2 vs 7. Qa4 in the Catalan This is a position from the main tabia of Catalan Opening:

 [FEN "rnbq1rk1/ppp1bppp/4pn2/8/2pP4/5NP1/PP2PPBP/RNBQ1RK1 w - - 0 7"]
 [StartPly "12"]
 1. d4 Nf6 2. c4 e6 3. g3 d5 4. Nf3 Be7 5. Bg2 O-O 6. O-O dxc4


The main move for white is 7. Qc2 with the intention of grabbing a pawn back on c4; however, the same result can be achieved with 7. Qa4.

Why is 7. Qc2 so much more popular? (~10:1 ratio in BigDatabase from Chessbase, with 10065 vs 1186 as of Dec 21, 2019)

Does Qc2 prevent some lines that Qa4 does not? Isn't the idea behind both of them the same?

Reason for asking: I am playing Catalan with both black and white, and study the theory on the materials from Victor Bologan. In the materials, I have, the 7. Qa4 idea is not mentioned; however, I often face it, and wonder if there is any particular idea (or even concrete line) that I should be aware of.
As you mentioned, 7.Qc2 a6 8.Qxc4 and 7.Qa4 a6 8.Qxc4 lead to the same position.

However, after 7.Qc2 a6, white has the option to play 8.a4, which is recommended by GM Avrukh in Grandmaster Repertoire 1A: The Catalan. According to the Game Database of ChessTempo, 8.a4 is a bit more popular than 8.Qxc4.
Having the choice of 8.a4 and 8.Qxc4 is probably the main reason why 7.Qc2 is played so much more often than 7.Qa4.

On the other hand, 7.Qc2 allows 7....b5, a move that became popular on the highest level only recently. For example, see Ding Liren-Carlsen.
So, if white prefers to play 8.Qxc4 after 7....a6, it makes sense to choose 7.Qa4, which occurred in Caruana-Duda.

After 7.Qa4, there are some sidelines you might consider as black: 7....Qd5 (played in Adly-Aronian (blitz)) and 7....b6.



      [StartPly "12"]

      [FEN ""]
      1.d4 Nf6 2.c4 e6 3.g3 d5 4.Nf3 Be7 5.Bg2 O-O 6.O-O dxc4 7.Qc2 (7.Qa4 a6 (7...b6)(7...Qd5) 8.Qxc4) a6 (7...b5) 8.a4 (8.Qxc4)