Do you have evidence for this? Aside from the oath of office which requires them to protect and defend the constitution, I am unaware of any additional oath required for impeachments.
Senate Resolution 479 - An original resolution amending the Rules of Procedure and Practice in the Senate when sitting on impeachment trials. 99th Congress (1985-1986).
Section IX:
"I ______________ do solemnly swear that the return made by me upon the process issued on the _____ day of _____, by the Senate of the United States against, _______________, is truly made, and that I have performed such service as prescribed within: So Help Me God."