MXene-supported Single-Atom and Nano Catalysts for 1,3-Butadiene Hydrogenation
Résumé
In the last decades, 2D materials have gained great attention in catalysis, owing to their large specific surface area, mechanical robustness and flexibility, and unique physical and chemical properties [1]. MXenes, a bourgeoning class of 2D transition metal carbides or nitrides, have a general formula of Mn+1XnTx (n=1−4), where M is an early transition metal, X is C or N, and Tx indicates surface termination group(s) such as −O, −OH, −F or −Cl, which depend on the etchants used in the synthesis process [2]. Since their discovery in 2011 with the Ti3C2-based system [3], MXenes have shown great potential in various fields including electrocatalysis for evolution or reduction reactions [4]. However, the literature on the application of MXenes in thermal catalysis remains scarce. In this work, we have used butadiene hydrogenation under gas flow conditions as a simple model selective hydrogenation reaction, though of applied interest for polymerization.
We have used two types of Ti3C2 MXenes, noted CMX and PMX, prepared with HF or LiF/HCl etchants, respectively, as supports for metal (Pt, Pd) single atoms or nanoparticles deposited by aqueous impregnation of metal salts. From SEM and STEM images, the MXenes present a multilayer structure where each layer itself contains a few stacked nanosheets. Aberration-corrected STEM analyses of the as-prepared low-loading (0.1 wt%) Pt/MXene and Pd/MXene catalysts exclusively show single atoms on both MXenes. An increase in metal loading (1 wt%) leads to a fraction of nanoparticles.
Before the reaction, the catalyst was delaminated, dispersed on quartz wool, and reduced in hydrogen. The bare MXenes showed no hydrogenation capability. PMX-supported low-loading single-atom catalysts are less active than 1 wt% metal-loaded catalysts, but they exhibit a higher selectivity towards butenes, even at the same conversion. Depending on the metal, 1-butene (Pd) or 2-butenes (Pt) are the main products. Moreover, compared to conventional alumina-supported catalysts, the MXene-supported ones are much more selective to butenes. This could be due to the high thermal conductivity of MXenes [5], which prevents the appearance of hot spots in the catalyst bed during exothermic reactions.
The structural properties and surface compositions of metal/MXenes powders were studied in detail by XRD, XPS, and DRIFTS to correlate them with the catalytic performances and assess the influence of the MXene synthesis etchant, as will be shown in this contribution.
References:
[1]C. Anichini, et al. Chem. Soc. Rev. 47 (2018) 4860.
[2]K. Kannan, et al. Catalysts 10 (2020) 495.
[3]M. Naguib, et al. Adv. Mater. 23 (2011) 4248.
[4]H. Wang, et al. J. Mater. Chem. A 8 (2020) 10604.
[5]L. Chen, et al. Materials 11 (2018) 1701.