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

Abstract

The passivation and reactivation of Mo 2 C and W 2 C was investigated by thermogravimetry with effluent gas analysis, and by comparison of the toluene hydrogenation activity of fresh carbides with that of passivated and reactivated carbides (20 bar, H 2 : toluene = 33, 150 or 200 °C, WHSV = 10 or 20 g g cat-1 min-1). Contrary to the literature, CO 2 and H 2 O were unsuitable passivation agents for Mo 2 C. Mo 2 C and W 2 C reacted readily with O 2 at 40 °C. The mass gained during oxidation could be quantitatively removed through reduction in 1 bar H 2 at 300 °C (Mo 2 C) or 400 °C (W 2 C). The rate of toluene conversion was fully recovered only for Mo 2 C that had been passivated with an initial O 2 concentration of 0.1 vol.% and not for other passivation conditions or for W 2 C. 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.

Synergistic Effect of Molybdenum and Tungsten in Highly Mixed Carbide Nanoparticles as Effective Catalysts in the Hydrogen Evolution Reaction under Alkaline and Acidic Conditions

“…According to Vegard's Law, a gradual contraction can be expected with increasing tungsten content, shifting the reflections to higher angles. [32] As shown in Figure 2b, the (101) reflections indeed shift from 2θ = 39.5°(Mo 2 C) to 2θ = 40.2°( W 2 C). The observed gradual shift indicates well-controlled unit cell volume contraction resulting from the increasing tungsten content in the carbides.…”

“…Mo 2 C and W 2 C share a very similar structure, with Mo 2 C having a larger unit cell volume of 37.21 Å 3 than W 2 C (36.78 Å 3 ). According to Vegard's Law, a gradual contraction can be expected with increasing tungsten content, shifting the reflections to higher angles . As shown in Figure b, the (101) reflections indeed shift from 2θ=39.5° (Mo 2 C) to 2θ=40.2° (W 2 C).…”

“…All the samples exhibited the characteristic diffraction pattern of the orthorhombic structure, and the difference between β-Mo 2 C and β-W 2 C lies only in the random 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 or ordered distribution of carbon in the octahedral gaps of the close packing, which makes their diffraction patterns difficult to distinguish. [32] Notably, with increasing tungsten content, the reflection at around 2θ = 40°shifted slightly to higher angles. Mo 2 C and W 2 C share a very similar structure, with Mo 2 C having a larger unit cell volume of 37.21 Å 3 than W 2 C (36.78 Å 3 ).…”

“…Figure a shows the X‐ray diffraction (XRD) patterns of the as‐prepared samples. All the samples exhibited the characteristic diffraction pattern of the orthorhombic structure, and the difference between β‐Mo 2 C and β‐W 2 C lies only in the random or ordered distribution of carbon in the octahedral gaps of the close packing, which makes their diffraction patterns difficult to distinguish . Notably, with increasing tungsten content, the reflection at around 2θ=40° shifted slightly to higher angles.…”

Non‐oxidative Coupling of Methane to Ethylene Using Mo2C/[B]ZSM‐5

“…)C H 4 /H 2 .T his step takes about 16 hours with the temperature rampingf rom 298 Kt o6 23 Ka t2Kmin À1 ,t hen to 973 Ka t1Kmin À1 ,a nd held at 973 Kf or 8hours.A fter this step, the system was cooled to the reaction temperature in He. [35] After the temperature was stabilized, ar eaction gas consistingo f5 %( vol) CH 4 /He was fed into the system and the products are analyzed with an online GC equipped with TCD and FID detectors (Figure9). Due to thermodynamic limitations, the conversiono fm ethane around 1% with a9 0%+ selectivity to ethylene instead of benzene.…”