There is now ample evidence from an assortment of experiments, especially those involving the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) technique, that a variety of neutral–neutral reactions possess no activation energy barrier and are quite rapid at very low temperatures. These reactions include both radical–radical systems and, more surprisingly, systems involving an atom or a radical and one ‘stable’ species. Generalizing from the small but growing number of systems studied in the laboratory, we estimate reaction rate coefficients for a larger number of such reactions and include these estimates in a new network of gas‐phase reactions for use in low‐temperature interstellar chemistry. Designated osu.2003, the new network is available on the World Wide Web and will be continually updated. A table of new results for molecular abundances in the dark cloud TMC‐1 (CP) is provided and compared with results from an older (new standard model; nsm) network.

Abstract. The formation of molecular hydrogen via the recombination of hydrogen atoms on interstellar grain surfaces has been investigated anew. A detailed Monte Carlo procedure known as the continuous-time random-walk method has been used. This Monte Carlo approach has two advantages over the stochastic master equation method: it treats random walk on a surface correctly, and it can easily be used for inhomogeneous surfaces. The recombination efficiency for H 2 formation as a function of surface temperature and grain size has been calculated for a variety of grain surfaces with a flux of hydrogen atoms representative of diffuse interstellar clouds. The surfaces studied include homogeneous olivine and amorphous carbon, characterized by single energies for the diffusion barrier and binding energy of H atoms, inhomogeneous versions of these two surfaces with distributions of H-atom diffusion barriers and binding energies, and a variety of mixed surfaces of olivine and carbon. For the homogeneous surfaces, we confirm that the temperature range for efficient formation of H 2 is very small. At temperatures near peak efficiency, there is little dependence on grain size. At temperatures higher than those of peak efficiency, the Monte Carlo procedure exhibits smaller efficiencies for molecular hydrogen formation than the master equation method in the limit of large grain sizes. For various types of inhomogeneous and mixed surfaces, the major effect we find is an increase in the temperature range over which the efficiency of molecular hydrogen formation is high. Efficient formation of H 2 in diffuse interstellar clouds now seems possible with inhomogeneous grains.

The influence of different iron carbides on the activity and selectivity of iron-based Fischer−Tropsch catalysts has been studied. Different iron carbide phases are obtained by the pretreatment of a binary Fe/SiO 2 model catalyst (prepared by coprecipitation method) to different gas atmospheres (syngas, CO, or H 2 ). The phase structures, compositions, and particle sizes of the catalysts are characterized systematically by XRD, XAFS, MES, and TEM. It is found that in the syngas-treated catalyst only χ-Fe 5 C 2 carbide is formed. In the CO-treated catalyst, Fe 7 C 3 and χ-Fe 5 C 2 with a bimodal particle size distribution are formed, while the H 2 -treated catalyst exhibits the bimodal size distributed ε-Fe 2 C and χ-Fe 5 C 2 after a Fischer−Tropsch synthesis (FTS) reaction. The intrinsic FTS activity is calculated and assigned to each corresponding iron carbide based on the phase composition and the particle size. It is identified that Fe 7 C 3 has the highest intrinsic activity (TOF = 4.59 × 10 −2 s −1 ) among the three candidate carbides (ε-Fe 2 C, Fe 7 C 3 , and χ-Fe 5 C 2 ) in typical medium-temperature Fischer−Tropsch (MTFT) conditions (260−300 °C, 2−3 MPa, and H 2 /CO = 2). Moreover, FTS over ε-Fe 2 C leads to the lowest methane selectivity.