Activating Molybdenum Carbide Nanoparticle Catalysts under Mild Conditions Using Thermally Labile Ligands

Abstract

Transition-metal carbides are promising low-cost materials for various catalytic transformations due to their multifunctionality and noble-metal-like behavior. Nanostructuring transition-metal carbides offers advantages resulting from the large surface-area-to-volume ratios inherent in colloidal nanoparticle catalysts; however, a barrier for their utilization is removal of the long-chain aliphatic ligands on their surface to access active sites. Annealing procedures to remove these ligands require temperatures greater than the catalyst synthesis and catalytic reaction temperatures and may further result in coking or particle sintering that can reduce catalytic performance. One way to circumvent this problem is by replacing the long-chain aliphatic ligands with smaller ligands that can be easily removed through low-temperature thermolytic decomposition. Here, we present the exchange of native oleylamine ligands on colloidal α-MoC1–x nanoparticles for thermally labile tert-butylamine ligands. Analyses of the ligand exchange reaction by solution 1H NMR spectroscopy, FT-IR spectroscopy, and thermogravimetric analysis–mass spectrometry (TGA-MS) confirm the displacement of 60% of the native oleylamine ligands for the thermally labile tert-butylamine, which can be removed with a mild activation step at 250 °C. Catalytic site densities were determined by carbon monoxide (CO) chemisorption, demonstrating that the mild thermal treatment at 250 °C activates ca. 25% of the total binding sites, while the native oleylamine-terminated MoC1–x nanoparticles showed no available surface binding sites after this low-temperature treatment. The mild pretreatment at 250 °C also shows distinctly different initial activities and postinduction period selectivities in the CO2 hydrogenation reaction for the ligand exchanged MoC1–x nanoparticle catalysts and the as-prepared material.

The Surface Chemistry of Colloidal Nanocrystals Capped by Organic Ligands

“…The objective can be to further improve the binding affinity, to improve the colloidal solubility, or to design cleaner nanocrystal syntheses. On the other hand, one can also design thermally labile ligands, , photosensitive ligands, or ligands that allow for the coupling of cargo, etc. Here we focus on the binding affinity aspect.…”

Supported transition metal (Mo, W) carbide and nitride catalysts for lignin hydrodeoxygenation: interplay of supports, structure, and catalysis

“…48,49 Several advanced methods have been developed to make carbide synthesis more moderate and controllable. Herein, liquid or solid organic carbon materials were used as both reducing agent and C source to obtain Mo or W carbide nanoparticles (NPs), such as urea, 50,51 amines [52][53][54][55] and resorcinol/ Catalysis Science & Technology Mini review formaldehyde (RF) polymeric layer. 56 Leonard and his coworkers synthesized some Mo carbide NPs with different crystal structures and morphologies by changing the kind or amount of amine and the calcining temperature.…”

“…52 Baddour and his co-workers reported a continuous-flow synthesis of α-MoC 1−x NPs with multimodal morphology by using a millifluidic (mF) reactor under the exceptionally mild conditions of 320 °C and 0.27 MPa N 2 . 54,55 When the RF polymeric layer was used for the synthesis of W carbide NPs, 45,57 the phase transition went through the pathway WO…”

“…52 Baddour and his co-workers reported a continuous-flow synthesis of α-MoC 1− x NPs with multimodal morphology by using a millifluidic (mF) reactor under the exceptionally mild conditions of 320 °C and 0.27 MPa N 2 . 54,55 When the RF polymeric layer was used for the synthesis of W carbide NPs, 45,57 the phase transition went through the pathway WO x ( x ≤ 3) → W → W 2 C → W x C (1 < x < 2) → WC, but when melamine was used, the precursor passed through the WC x N y intermediate and was subsequently converted to W 2 C, 58 probably because melamine contains both C and N atoms. In addition, some other methods used non-thermal energy for carburization, such as plasma carburization (PC), 59 mechanochemical method, 17 etc.…”

Crystal-Phase Engineering in Heterogeneous Catalysis

“…Unlike pure metals, which are limited to the fcc, bcc, and hcp crystal phases, metal carbides and nitrides exhibit greater structural complexity and a wider range of crystal phases due to the diverse coordination of metal and nonmetal atoms as well as nonuniform molar ratios. , While controlling temperature, pressure, or size is typically sufficient for achieving specific crystal-phase transitions or growth in pure metals, controlling the crystal phases of metal carbides and nitrides often requires adjustments in the proportions of different elements and the use of unconventional synthesis methods such as solvothermal processes. , Additionally, because of this, further research efforts are needed to fully uncover the impact of crystal phases on a chemical reaction, particularly when considering variations in the composition ratio. Currently, there is relatively limited research on the crystal phases of metal carbonitrides, with most of them centered around molybdenum atoms.…”