24V ceramic heater to work on 12V?
Yes, but it'll only use a quarter of the power (10 watts) and put out a quarter of the heat.

Per Ohm's law, V=IR, with half the voltage, I'll draw half the current. Per P=IV, with half voltage and half current, it'll consume one quarter power.

Note that this is approximate since, as @Brian Drummond pointed out, the resistance of the heater may be lower when cooler. This will increase the power at lower voltage, but only slightly.