1 research outputs found
Structure of Mo<sub>2</sub>C<sub><i>x</i></sub> and Mo<sub>4</sub>C<sub><i>x</i></sub> Molybdenum Carbide Nanoparticles and Their Anchoring Sites on ZSM‑5 Zeolites
Mo carbide nanoparticles supported
on ZSM-5 zeolites are promising catalysts for methane dehydroaromatization.
For this and other applications, it is important to identify the structure
and anchoring sites of Mo carbide nanoparticles. In this work, structures
of Mo<sub>2</sub>C<sub><i>x</i></sub> (<i>x</i> = 1, 2, 3, 4, and 6) and Mo<sub>4</sub>C<sub><i>x</i></sub> (<i>x</i> = 2, 4, 6, and 8) nanoparticles are identified
using a genetic algorithm with density functional theory (DFT) calculations.
The ZSM-5 anchoring sites are determined by evaluating infrared vibrational
spectra for surface OH groups before and after Mo deposition. The
spectroscopic results demonstrate that initial Mo oxide species preferentially
anchors on framework Al sites and partially on Si sites on the external
surface of the zeolite. In addition, Mo oxide deposition causes some
dealumination, and a small fraction of Mo oxide species anchor on
extraframework Al sites. Anchoring modes of Mo carbide nanoparticles
are evaluated with DFT cluster calculations and with hybrid quantum
mechanical and molecular mechanical (QM/MM) periodic structure calculations.
Calculation results suggest that binding through two Mo atoms is energetically
preferable for all Mo carbide nanoparticles on double Al-atom framework
sites and external Si sites. On single Al-atom framework sites, the
preferential binding mode depends on the particle composition. The
calculations also suggest that Mo carbide nanoparticles with a C/Mo
ratio greater than 1.5 are more stable on external Si sites and, thus,
likely to migrate from zeolite pores onto the external surface of
the zeolite. Therefore, in order to minimize such migration, the C/Mo
ratio for zeolite-supported Mo carbide nanoparticles under hydrocarbon
reaction conditions should be maintained below 1.5