If "operate" does NOT mean "hold" (i.e., if you just need pulses of 24
volts)
then you can wire a capacitor to a switch such that the capacitor charges
to 12 volts and then ADDS to that same 12 volts to operate the relay.

Well, i need a stable 24 v dc voltage, not just a voltage peak to 24 and
back to 12.

Thank you,
73 de Riccardo IK5WQO