A synthesis gas mixture of 1:1 carbon monoxide and hydrogen below ~250C is thermodynamically unfavorable to stay as such. It is more thermodynamically favorable for the recombination of these elements in to hydrocarbons or alcohols (or water and carbon dioxide). However, left to its own devices, syngas will not recombine in a timely way without the coaxing of selected transition metal oxides. This is why the syngas reactions into the various fuels we are discussing here, are highly exothermic in operation. Some existing plants are controlling the excessive heat production of the reactors by creating steam to co-generate power for the system/grid. While some amount of added thermal energy is needed for the catalysts to become active; once they are active, cooling is needed or the reaction will provide enough thermal energy to deactivate and/or decompose catalyst.  This is why pressure is the main driver to promote these reactions so as to 'economically force' the still thermodynamically favorable reaction to occur at a rate that is efficient enough for the designed reactor without over heating.

Transition metal oxides have typically been used to recombine syngas in various ways. There has been some extensive research on other more exotic combinations of transition metals on solid supports or slurry phases, nanotech applications, or patented complexes. While 'nanotech' sounds fancy enough to get funded, I have seen some projects using the same chemical mechanisms of its prior art with an increase in the surface area of the catalyst or the same catalyst in a molecularly restricted space/sieve* which increases the selectivity of the product fuel.

The mechanism of each transition metal oxide is worth some mimetic organization as we are interpreting, referring, or manipulating the technologies as we sort through patents and post.

• Polymerization: chain growth by the linking of -CH2- (iron oxide) (ex: Fischer-Tropsch reaction.
• Methanization: chain termination by the addition of -CH3 (nickel oxide, ruthenium, typicall of Group VIII elements) (ex: Sabatier Reaction)
• Hydrogenation: addition of -H (different chemical mechanism from methanization, but in syngas conversion in environments (nickel oxide, ruthenium, alumina) they may be seemingly close in their operation when building linear chains)
• Hydroxylation: chain termination -OH (zinc oxide, copper oxide- oxidized pennies and brass? ponder.) the addition of a hydroxyl group to an alcohol.
• Dehydration: the subtraction of a water molecule from a compound -H2O (Al2O3) (ex: dehydration of methanol to dimethyl ether (DME)). Also alumina was typically used as a support for solid state catalysts, then some slurry phase processes seemed to be able to replace some of the need for alumina as a support. However, it was found that alumina was needed in just smaller amounts because it promoted a type of p-bonding occurrence in the surrounding mechanism of the recombination activity of the transition metal oxides- in the case that it might keep water from forming in the reaction. 
• Mixed Mechanism: Some research and processes have mixed the transition metal oxide mechanics for example a combination of Fischer Tropsch and hydroxylation tends to yield a mixture of higher alcohols.

The recombination of carbon/carbon bonds will exclude the oxygen in the reaction with carbon monoxide, creating an oxygen free radical to grab the hydrogen in the syngas to produce water which will inherently be in your product fuel stream.

Nitrogen:

Air: ~70-80% nitrogen, ~20% oxygen.

-Dilution: The chances that a carbon monoxide, a hydrogen, and your transition metal will all be concentrated within a couple angstroms are greatly reduced, thus reducing the catalytic activity and yield of the system. 

-Deactivation: NOx and NH3 can be created in the reactor, the later creates a basic environment which has the potential to deacivate/decompose some catalysts.

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*Hengye USA produces a wide variety of molecular sieves http://www.hengyeusa.com/ (they didn't pay me to promo, but you can start there if you are interested).