CNT‐Supported Mo_xC Catalysts: Effect of Loading and Carburization Parameters

Abstract

Mo x C/CNT catalysts were prepared by carburization of an oxidic Mo precursor impregnated on multiwalled carbon nanotubes (CNTs). The effects of different carburization atmospheres, heating rates, and Mo loadings were tested. The catalysts were characterized by CO-TPD, XRD, N 2 physisorption, SEM, and TEM. The catalytic performance in the steam reforming of methanol (SRM) was used as a sensitive probe to indicate changes in the catalyst surface during catalytic action. Contrarily to the bulk Mo x C catalysts the heating rate during carburization has practically no effect on the catalysts. Instead, Mo loading and carburization atmosphere are the key factors for catalyst structure and performance. The Mo-based activity decreases at loadings >10 wt% at constant product selectivity. The CO 2 /CH 4 product ratio sensitively indicates changes in the catalyst properties at the loadings <20 wt%, where the activity is practically constant. Carburization in 20% CH 4 /H 2 yields 2 nm sized crystallites of cubic α-MoC. Carburization in pure H 2 and in He yields hexagonal β-Mo 2 C with a larger particle size. This phase change is documented in a different catalytic performance in terms of activity and CO 2 /CH 4 selectivity. Thus a multi-parameter toolbox for fine-tuning of catalyst properties is presented.[a]

MoxC/CNT Composites as Active Electrocatalysts for the Hydrogen Evolution Reaction under Alkaline Conditions

“…Subsequently, 1 g functionalized CNTs was impregnated in 50 mL (NH 4 ) 6 Mo 7 O 24 ⋅ 4H 2 O (Sigma‐Aldrich, 99.98 %) aqueous solution with a nominal Mo loading of 20 wt%. The resulting catalyst precursor was dried at 100 °C for 1 h and then thermally treated at 700 °C in N 2 , H 2 or 20 vol% CH 4 /H 2 at a flow rate of 100 sccm following a literature procedure .…”

“…Interestingly, a higher Mo 2 C to C intensity ratio can be observed in Mo x C/CNT‐H 2 compared with Mo x C/CNT‐N 2 suggesting the presence of a larger amount of Mo 2 C nanoparticles. For the sample Mo x C/CNT‐CH 4 /H 2 , the reflections at 36.3°, 42.2°, 61.2° and 73.3° can be assigned to cubic α‐MoC . The weak reflections of α‐MoC are consistent with observations in literature that α‐MoC is less stable than other Mo x C phases and contains a number of carbon defects in the lattice .…”

“…In contrast, Mo x C/CNT‐H 2 has a significantly higher Mo content of 28.3 wt% due to the gasification of both amorphous carbon and CNTs at defect sites in H 2 at 700 °C. Correspondingly, due to the removal of surface carbon, Mo x C/CNT‐H 2 has a higher Mo surface concentration derived from the XPS measurements, which is in good agreement with the literature results .…”

Nanostructured Carbon Material Effect on the Synthesis of Carbon-Supported Molybdenum Carbide Catalysts for Guaiacol Hydrodeoxygenation

“…In addition, when Mo 2 C is prepared by CHR using carbon materials as a support, the formation of coke on the surface of the catalysts, typically associated with the use of hydrocarbons in the carburization step, is avoided [28]. Normally, Mo 2 C supported in metal oxides (TiO 2 , ZrO 2 , CeO 2 , SiO 2 or Al 2 O 3 ) prepared by temperature programmed reduction with hydrocarbons shows deactivation problems due to coke deposition, resulting in a low specific surface area and low stability, and thus low catalytic activity [14].…”

“…Nanofilamentous supports such as carbon nanofibers (CNF) and nanotubes (CNT) were also studied in CHR. For CNT, Frank et al evaluated the CHR conditions (gas flow, heating rate and Mo loading), and they noticed that the defects of the CNT structure were crucial in the Mo x C crystal formation, which led to a highly active catalyst in the steam reforming of methanol (SRM) [28]. Subsequently, aiming to enhance the Mo 2 C/CNT properties in this reaction, they modified the CNT surface with heteroatoms (O, N and S).…”