Two-dimensional (2D) carbides, nitrides, and carbonitrides known as MXenes are emerging materials with a wealth of useful applications. However, the range of metals capable of forming stable MXenes is limited mostly to early transition metals of groups 3−6, making the exploration of properties inherent to mid or late transition metal MXenes very challenging. To circumvent the inaccessibility of MXene phases derived from mid-to-late transition metals, we have developed a synthetic strategy that allows the incorporation of such transition metal sites into a host MXene matrix. Here, we report the structural characterization of a Mo 2 CT x :Co phase (where T x are O, OH, and F surface terminations) that is obtained from a cobalt-substituted bulk molybdenum carbide (β-Mo 2 C:Co) through a two-step synthesis: first an intercalation of gallium yielding Mo 2 Ga 2 C:Co followed by removal of Ga via HF treatment. Extended X-ray absorption fine structure (EXAFS) analysis confirms that Co atoms occupy Mo positions in the Mo 2 CT x lattice, providing isolated Co centers without any detectable formation of other cobalt-containing phases. The beneficial effect of cobalt substitution on the redox properties of Mo 2 CT x :Co is manifested in a substantially improved hydrogen evolution reaction (HER) activity, as compared to the unsubstituted Mo 2 CT x catalyst. Density functional theory (DFT) calculations attribute the enhanced HER kinetics of Mo 2 CT x :Co to the favorable binding of hydrogen on the oxygen terminated MXene surface that is strongly influenced by the substitution of Mo by Co in the Mo 2 CT x lattice. In addition to the remarkable HER activity, Mo 2 CT x :Co features excellent operational and structural stability, on par with the best performing non-noble metal-based HER catalysts. Overall, our work expands the compositional space of the MXene family by introducing a material with site-isolated cobalt centers embedded in the stable matrix of Mo 2 CT x . The synthetic approach presented here illustrates that tailoring the properties of MXenes for a specific application can be achieved via substitution of the host metal sites by mid or late transition metals.

Development of efficient catalysts for the direct hydrogenation of CO2 to methanol is essential for the valorization of this abundant feedstock. Here we show that a silica-supported Cu/Mo2CTx (MXene) catalyst achieves a higher intrinsic methanol formation rate per mass Cu than the reference Cu/SiO2 catalyst with a similar Cu loading. The Cu/Mo2CTx interface can be engineered owing to the higher affinity of metallic Cu for the partially reduced MXene surface (in preference to the SiO2 surface) and the mobility of Cu under H2 at 500 C.Increasing the reduction time, the Cu/Mo2CTx interface becomes more Lewis acidic due to the higher amount of Cu + sites dispersed onto the reduced Mo2CTx and this correlates with an 2 increased rate of CO2 hydrogenation to methanol. The critical role of the interface between Cu and Mo2CTx is further highlighted by DFT calculations that identify formate and methoxy species as stable reaction intermediates.

Early transitional metal carbides are promising catalysts for hydrogenation of CO2. Here, a two-dimensional (2D) multilayered 2D-Mo2C material is prepared from Mo2CTx of the MXene family. Surface termination groups Tx (O, OH, and F) are reductively de-functionalized in Mo2CTx (500 °C, pure H2) avoiding the formation of a 3D carbide structure. CO2 hydrogenation studies show that the activity and product selectivity (CO, CH4, C2–C5 alkanes, methanol, and dimethyl ether) of Mo2CTx and 2D-Mo2C are controlled by the surface coverage of Tx groups that are tunable by the H2 pretreatment conditions. 2D-Mo2C contains no Tx groups and outperforms Mo2CTx, β-Mo2C, or the industrial Cu-ZnO-Al2O3 catalyst in CO2 hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO2 hydrogenation.

This work critically assesses the electrocatalytic activity, stability, and nature of the active phase of a two-dimensional molybdenum carbide (MXene) with single-atomic iron sites, Mo 2 CT x :Fe (T x are surface terminating groups O, OH, and F), in the catalysis of the oxygen reduction reaction (ORR). X-ray absorption spectroscopy unequivocally confirmed that the iron single sites were incorporated into the Mo 2 CT x structure by substituting Mo atoms in the molybdenum carbide lattice with no other detectable Fe-containing phases. Mo 2 CT x :Fe, the first two-dimensional carbide with isolated iron sites, demonstrates a high catalytic activity and selectivity in the oxygen reduction to hydrogen peroxide. However, an analysis of the electrode material after the catalytic tests revealed that Mo 2 CT x :Fe transformed in situ into a graphitic carbon framework with dispersed iron oxyhydroxide (ferrihydrite, Fh) species (Fh/C), which are the actual active species. This experimental observation and the results obtained for the titanium and vanadium 2D carbides challenge previous studies that discuss the activity of the native MXene phases in oxygen electrocatalysis. Our work showcases the role of 2D metal carbides as precursors for active carbon-based (electro)catalysts and, more fundamentally, highlights the intrinsic evolution pathways of MXenes in electrocatalysis.

Alumina and aluminosilicates, prepared under various synthesis conditions, play a central role in heterogeneous catalysis with a broad range of industrial applications. We report herein the atomic-scale structure of alumina layers obtained by atomic layer deposition (ALD) of trimethylaluminum onto partially dehydroxylated silica. Such a detailed insight into the atomic structure of the species formed with increasing Al content was gained using a variety of one-and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments involving 27 Al, 1 H and 29 Si nuclei. Multi-component fittings of the 1D and 2D experimental datasets allowed us to show that at 3.4 wt% of deposited Al, a sub-monolayer containing [4] Al(3Si), [4] Al(4Si) and [5] Al(2Si) species forms on the silica surface, with most of these sites carrying OH groups. The films obtained after additional ALD cycles (depositing 9.2 or 15.4 wt% Al) feature characteristics of an amorphous alumina phase with a high concentration of [5] Al species and abundant OH groups. The most probable species at the interface between silica and alumina are [4] Al(2Si), [4] Al(3Si) and [5] Al(2Si). 15 N dynamic nuclear polarization surface-enhanced NMR spectroscopy ( 15 N DNP SENS) and infrared spectroscopy using 15 N-labeled pyridine as a probe molecule reveal that aluminum oxide layers on amorphous silica contain both strong Brønsted and strong Lewis acid sites, whereby the relative abundance and nature of these sites, and therefore the acidity of the surface, evolve with increasing thickness of the alumina films (controlled by the number of ALD cycles). This study provides the first in-depth atomic-scale description of (sub) nanometerscale aluminum oxide films prepared by ALD as a function of their growth on a partially dehydroxylated silica support, opening the way to molecular-level understanding of the catalytic activity of such heterogeneous catalysts with tailored acidity.

The development of stable Ni-based dry reforming of methane (DRM) catalysts is a key challenge owing to the high operating temperatures of the process and the propensity of Ni for...

The ethene-to-propene reaction on Ni catalysts correlates with the formation of alkylated aromatic species. The deactivation of surface Ni aluminate sites can be reversed by calcination, while the deactivation of Ni silicate sites is irreversible.

Two-dimensional carbides and nitrides (MXenes) represent an excellent platform for the fundamental catalysis studies due to their well-defined and tunable structure and theoretically predicted activity in numerous catalytic reactions. However, the scope of metals capable of forming stable MXenes is limited mostly to early transition metals, making the investigation of properties inherent to mid or late transition metal carbides/nitrides challenging. Here, we present the synthesis and structural characterization of the Mo2CTx:M phases (M = Co, Fe; Tx = O, OH, F surface terminations), where M substituents occupy molybdenum positions in the Mo2CTx lattice, providing isolated Co/Fe sites. Our experimental and DFT studies demonstrate that metal substitution renders Mo2CTx:M active catalysts for the hydrogen evolution reaction. Overall, our work expands the compositional space of the MXene family by introducing materials with isolated metal sites incorporated into the stable matrix of Mo2CTx. The synthetic approach discussed here illustrates that tailoring the properties of MXenes for a specific application can be achieved via targeted substitution of the host metal.