Comparing gas-phase and grain-catalyzed H2 formation
  Because H2 formation on dust grain surfaces completely dominates gas-phase H2 formation in local molecular clouds, it is often assumed that gas-phase formation is never important. In fact, it is the dominant mechanism in a number of cases. In this paper, I briefly summarize the chemistry of gas-phase H2 formation, and show that it dominates for dust-to-gas ratios less than a critical value D_cr. I also show that D_cr is simple to calculate for any given astrophysical situation, and illustrate this with a number of examples, ranging from H2 formation in warm atomic gas in the Milky Way to the formation of protogalaxies at high redshift. 
Simon C.O. Glover