Carbon dioxide dominates the Venusian atmosphere. While I think it is possible to use the thermal energy in the atmosphere to split carbon dioxide to carbon and oxygen, the process may be too slow to cool down the planet. Also, the thermal energy needs to go somewhere.
Oxygen has three stable isotopes and other radioactive and unstable ones. After the oxygen is recovered from the splitting reaction above, oxygen-16 is going to be the dominant isotope as it is on Earth. My question is that is it possible to carry out a nuclear disproportionation reaction? Two Oxygen-16 atoms suck up huge amount of thermal energy to become one oxygen-18 and one oxygen-14. Oxygen-14 decays with a half life of over one minute to stable nitrogen-14. Creating nitrogen is the main goal.
If this transfer of neutron is possible, then transfer one more from oxygen-18 to oxygen-19. Here hydrogen can take part to become deuterium. Oxygen-19 decays with a half life of just under one minute to stable fluorine-19. Fluorine itself is then reacted with the carbon from the first reaction to create fluorocarbons. They are stored and shipped from Venus to Mars for the Martian global warming effort.
This whole question describes a hypothetical situation so I think only worldbuilding stackexchange site would accept this.
All the while, the Venusian atmospheric temperature cools. In the big picture, the huge amount of thermal energy in the atmosphere may be exploited but how? A way -- maybe foolish -- is to exploit the energy to make atoms in need, nitrogen and fluorine.
So after reading answers and comments, at this AD 2012 moment, it is possible to only use the thermal energy in the atmosphere to split carbon dioxide to carbon and oxygen. Then allotropes of carbon can be made. Oxygen is not a greenhouse gas and fixing the carbon removes carbon dioxide. So the bit by bit, the thermal energy is lost to space. As noted above this CO2-> C + O2 is too slow to ever cool down Venus significantly. But at least synthetic diamond and carbon nanotubes can be possible and help set up bases on other celestial bodies including the Moon and Mars.