I am working on CO2 and nitrite (NO2-) co-electrochemical reduction to produce urea. I've tried several catalysts (some similar to those from papers), but none of them is capable of producing urea.

I use 1H NMR to detect urea, and the proton peak of urea is at ~5.6 ppm. But in some catalysts, one peak appears at ~7.4 ppm, which can be originated from the proton in -CONH2 group. Besides, there are no other peaks. So I don't know what this product is.

Does anybody have any idea what it might be? Or maybe someone working on this topic may have the same problem?

Thank you.