Combining Theory and Experiment for Multitechnique Characterization of Activated CO₂ on Transition Metal Carbide (001) Surfaces

Abstract

Early transition metal carbides (TMC; TM = Ti, Zr, Hf, V, Nb, Ta, Mo) with face-centered cubic crystallographic structure have emerged as promising materials for CO 2 capture and activation. Density functional theory (DFT) calculations using the Perdew−Burke−Ernzerhof exchange−correlation functional evidence charge transfer from the TMC surface to CO 2 on the two possible adsorption sites, namely, MMC and TopC, and the electronic structure and binding strength differences are discussed. Further, the suitability of multiple experimental techniques with respect to (1) adsorbed CO 2 recognition and (2) MMC/TopC adsorption distinction is assessed from extensive DFT simulations. Results show that ultraviolet photoemission spectroscopies (UPS), work function changes, core level X-ray photoemission spectroscopy (XPS), and changes in linear optical properties could well allow for adsorbed CO 2 detection. Only infrared (IR) spectra and scanning tunnelling microscopy (STM) seem to additionally allow for MMC/TopC adsorption site distinction. These findings are confirmed with experimental XPS measurements, demonstrating CO 2 binding on single crystal (001) surfaces of TiC, ZrC, and VC. The experiments also help resolving ambiguities for VC, where CO 2 activation was unexpected due to low adsorption energy, but could be related to kinetic trapping involving a desorption barrier. With a wealth of data reported and direct experimental evidence provided, this study aims to motivate further basic surface science experiments on an interesting case of CO 2 activating materials, allowing also for a benchmark of employed theoretical models.

Diversity of Adsorbed Hydrogen on the TiC(001) Surface at High Coverages

“…This result has been analyzed by estimating core-level shifts values obtained using the initial state model as well as the Janak-Slater model, as described in previous work. 40,41 The present estimates reveal that a C-H configuration has a C 1s binding energy only 0.27 eV higher than the bare surface C in the carbide. In agreement with the trend seen in the XPS measurements for low coverages of H on TiC(001).…”

Critical effect of carbon vacancies on the reverse water gas shift reaction over vanadium carbide catalysts

“…These computational simulations forecasted that some of these TMCs, such as ZrC and HfC, would be able to capture CO 2 even at 473 K from sources with a low CO 2 content, such as atmospheric air, with a current CO 2 partial pressure, p CO 2 , of 40.5 Pa [17]. The theoretical predictions became afterwards experimentally confirmed on TiC, ZrC, and VC samples [18]. Further, several TMCs were found to be able to activate CO 2 , resulting in a partially negatively charged bent molecular species.…”

Supported Molybdenum Carbide and Nitride Catalysts for Carbon Dioxide Hydrogenation

“…Recently, transition metal carbides have attracted attention as promising catalysts for the conversion of CO 2 into CO, methanol, methane, and other hydrocarbons (Nagai et al, 1998b;Solymosi et al, 2002;Porosoff et al, 2014;Posada-Pérez et al, 2014, 2016aXu et al, 2014;Chen et al, 2015Chen et al, , 2016Gao et al, 2016;Liu et al, 2016;Han et al, 2019;Reddy et al, 2019). It has been reported through computational work that transition metal (e.g., Ti, Mo, and V) carbide surfaces are able to uptake and activate CO 2 (Posada-Pérez et al, 2014Kunkel et al, 2016Kunkel et al, , 2018Kunkel et al, , 2019Liu et al, 2017). In liquid phase, metals (Pd, Cu, Co, Fe) supported on Mo 2 C have been tested for the hydrogenation of CO 2 in dioxane under 40 bars, at 135-200 • C (Chen et al, 2015(Chen et al, , 2016.…”