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Context: The HD and H2 molecules play important roles in the cooling ofprimordial and very metal-poor gas at high redshift. Aims: Grain surface andgas phase formation of HD and H2 is investigated to assess the importance oftrace amounts of dust, 10^{-5}-10^{-3} Zo, in the production of HD and H2.Methods: We consider carbonaceous and silicate grains and include bothphysisorption and chemisorption, tunneling, and realistic grain surfacebarriers. We find, for a collapsing gas cloud environment with coupled chemicaland thermal balance, that dust abundances as small as 10^{-5} solar lead to astrong boost in the H2 formation rate due to surface reactions. As a result ofthis enhancement in H2, HD is formed more efficiently in the gas phase throughthe D+ + H2 reaction. Direct formation of HD on dust grains cannot compete wellwith this gas phase process for dust temperatures below 150 K. We also deriveup-to-date analytic fitting formulae for the grain surface formation of H2 andHD, including the different binding energies of H and D. Results: Grain surfacereactions are crucial to the availability of H2 and HD in very metal-poorenvironments. Above metallicities of 10^{-5} solar, the grain surface routedominates the formation of H2, which in turn, drives the formation of HD in thegas phase. At dust temperatures above 150 K, laboratory experiments andtheoretical modelling suggest that H2 formation on grains is suppressed whileHD formation on grains is not.

HD and H2 formation in low-metallicity dusty gas clouds at high redshift