Kinetics of Titania Reduction by Methane-Hydrogen Gas

摘要

Titania is effectively reduced to titanium oxycarbide by the methane-hydrogen-argon gas mixture in the temperature range of 1523-1773K. Reduction of titania to titanium oxycarbide occurs in the following sequence: TiO_2→Ti_5O_9→Ti_4O_7 →Ti_3O_5→Ti_2O_3→(TiO-TiC)_(ss). At 1673-1773K, extent of reduction of titania to titanium oxycarbide, defined as percentage of oxygen removed, achieves more than 85% (according to LECO analysis) in about 90 min. This means that the TiO-TiC solid solution contains about 70 wt% of TiC. The reduction process was modeled using two-interface shrinking core model by approximating the process with two reactions: reduction of titania to Ti_2O_3 and reduction of Ti_2O_3 to the TiC-TiO solid solution. The calculated results are close to the experimental data at temperature range 1473-1773K and methane partial pressure up to 8 kPa. Within the range of methane partial pressure up to 8 kPa the reaction of titania reduction to Ti_2O_3 is of first order with respect to methane and hydrogen partial pressures. The apparent activation energy of the reaction is 124 kJ/mol. The reduction rate of Ti_2O_3 to TiC-TiO solid solution is of first order with respect to methane partial pressure and is independent of hydrogen partial pressure. However, hydrogen suppresses methane cracking and carbon deposition. The apparent activation energy of the reaction is 161 kJ/mol. Beyond 8 kPa in methane partial pressure causes excessive methane cracking with solid carbon deposition, which hinders access of the reducing gas and decreases carbon activity in the system. Similar effect has an increase in temperature above 1723K. To depress the deposition of solid carbon hydrogen partial pressure in the reducing gas should be above 35 kPa (at 1573K and 5 kPa CH_4). Optimum conditions for titanium carbide synthesis from titania and methane-hydrogen-argon gas mixture include temperature in the range 1573-1723K, methane content of 8 vol% and hydrogen content above 35 vol%.