Hydrogen reaction with diamond

Hamers and coworkers showed that undoped diamond (001) surfaces undergo [2 + 2] cycloaddition reactions with a prototypical alkene, cyclopentene [95]. The (001) surfaces of diamond, silicon, and germanium share a similar bonding motif in which pairs of atoms can bond to each other via a strong a bond and a weaker tt bond forming a dimer. Normally, the formation of dimers on the diamond surface is prevented by hydrogen chemisorption however, the diamond (001) surface was made reactive for the addition reaction by heat treatment in the UHV at 1375 K. At these temperatures, the surface is cleaned of impurities and surface hydrogen desorbs. This leads to the formation of carbon atoms paired into C=C dimers with a partial 7t bond [95]. This surface is essentially an extended array of alkene-like C=C... 

In a typical use of this method, a mixture of hydrogen and methane is fed into a reaction chamber at a pressure of about 1.33 kPa (10 torr). The substrate upon which diamond forms is at about 950°C and Hes about 1 cm away from a tungsten wine at 2200°C. Small diamond crystals, 1 mm or so in si2e, nucleate and grow profusely on the substrate at a rate around 0.01 mm /h to form a dark, rough polycrystalline layer with exposed octahedral or cubic faces, depending on the substrate temperature. 

Fig. 7 Dependence of IR band intensities on H2 partial pressure during ethene hydrogenation catalyzed by Ir4/y-Al203 at 288 K and 760 Torr (40 Torr C2H4, 50-300 Torr H2, and the balance He). The bands at 2990 (diamonds) and 2981 cnr (squares) were chosen to represent di-cr-bonded ethene and that at 1635 cnr (circles) to represent water on the y-AbOs support. These IR bands were chosen as the best ones to minimize error caused by overlap with other bands. The triangles represent the reaction rate expressed as a turnover frequency (TOF), the rate of reaction in units of molecules of ethene converted per Ir atom per second. The data indicate a correlation of the band intensities with the TOF, consistent with the suggestion that the ligands represented by the bands are reaction intermediates (but the data are not sufficient to identify the reaction intermediates) [39]...

---