Passivation agents and conditions for Mo2C and W2C: Effect on catalytic activity for toluene hydrogenation

•CO2 and H2O not suitable agents for passivation of Mo2C.•Toluene hydrogenation activity of Mo2C best after 0.1% O2 passivation and reduction.•Activity of Mo2C fully recoverable, and activity of W2C only partially recoverable.•Enhanced activity after intervening operation at 21 bar H2/toluene and 300–400 °C.•Surface carbide formation may be possible at 300–400 °C and 21 bar H2/toluene.

The passivation and reactivation of Mo2C and W2C were investigated by thermogravimetry with effluent gas analysis, and by comparison of the toluene hydrogenation activity of fresh carbides with those of passivated and reactivated carbides (21 bar, H2:toluene = 33, 150 or 200 °C, WHSV = 10 or 20 g gcat−1 h−1). Contrary to the literature, CO2 and H2O were unsuitable passivation agents for Mo2C. Mo2C and W2C reacted readily with O2 at 40 °C. The mass gained during oxidation could be quantitatively removed through reduction in 1 bar H2 at 300 °C (Mo2C) or 400 °C (W2C). The rate of toluene conversion was fully recovered only for Mo2C that had been passivated with an initial O2 concentration of 0.1 vol.% and not for other passivation conditions or for W2C. Intervening operation of toluene hydrogenation at 300 or 400 °C enhanced the activity of samples that were previously oxidized and reduced or harshly reduced. This enhancement suggests that carbide formation is possible at milder temperatures than typically employed.

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