The catalyzed etching of diamond by CO$\sb2$ in the presence of first-row transition metal oxides was studied and it was observed to occur in the presence of NiO, $\rm Fe\sb2O\sb3,\ Co\sb3O\sb4$ with a strong CO$\sb2$ dependence. The apparent activation energies were found to be similar for NiO, $\rm Fe\sb2O\sb3,\ Co\sb3O\sb4$ in the presence of CO$\sb2$ while they are different under an argon atmosphere. A three-step qualitative model was proposed involving the adsorption of CO$\sb2$ onto the metal oxide surface, the transfer of oxygen to the diamond surface, and the then desorption of CO from the diamond surface. The analysis of the model and the experimental facts indicates that the transfer of oxygen is most likely the rate determining step for the catalytic etching of diamond in the presence of NiO, $\rm Fe\sb2O\sb3,\ Co\sb3O\sb4.$ Several of the metal oxides were also observed to be able to etch diamond directly for which a similar model was proposed.
The etching of diamond by CO$\sb2$ and H$\sb2$O in the presence of $\rm Cr\sb2O\sb3$ was studied. HCl, H$\sb2$O, H$\sb2,$ and $\rm CH\sb3Cl$ were found to be able to enhance the etching rate of diamond by CO$\sb2$ to different extents with only small amounts added. With $\rm Cr\sb2O\sb3$ present, water was observed to be able to etch diamond at a much lower temperature than water alone. A large difference was found between the apparent activation energies when HCl and H$\sb2$O were introduced into the reaction system respectively, the latter being similar to the activation energy for water etching alone. The formation of surface hydroxyl is believed to be essential for the reaction to happen.