Supported Molybdenum Carbide Nanoparticles as an Excellent Catalyst for CO2 Hydrogenation

Marc Figueras,Ramón A Gutiérrez,Francesc Illas,Pedro J Ramírez,Francesc Viñes,José A Rodriguez

doi:10.1021/acscatal.1c01738

Marc Figueras, Ramón A Gutiérrez + Show 4 more

Open Access

https://doi.org/10.1021/acscatal.1c01738

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Abstract

Experiments under controlled conditions show that MoCx nanoclusters supported on an inert Au(111) support are efficient catalysts for CO2 conversion, although with a prominent role of stoichiometry. In particular, C-deficient nanoparticles directly dissociate CO2 and rapidly become deactivated. On the contrary, nearly stoichiometric nanoparticles reversibly adsorb/desorb CO2 and, after exposure to hydrogen, CO2 converts predominantly to CO with a significant amount of methanol and no methane or other alkanes as reaction products. The apparent activation energy for this process (14 kcal/mol) is smaller than that corresponding to bulk δ-MoC (17 kcal/mol) or a Cu(111) benchmark system (25 kcal/mol). This trend reflects the superior ability of MoC1.1/Au(111) to bind and dissociate CO2. Model calculations carried out in the framework of density functional theory provide insights into the underlying mechanism suggesting that CO2 hydrogenation on the hydrogen-covered stoichiometric MoCx nanoparticles supported on Au(111) proceeds mostly under an Eley–Rideal mechanism. The influence of the Au(111) is also analyzed and proven to have a role on the final reaction energy but almost no effect on the activation energy and transition state structure of the analyzed reaction pathways.

Highlights

Since the 1970s, transition-metal carbides have been suggested as an alternative to catalysts based on expensive late transition metals for a wide variety of hydrogenation reactions.[1−4] In many new catalysts, metal carbides nanoparticles (NPs) are dispersed on oxides and zeolites[5,6] but little is known about the specific phenomena which determine the hydrogenation capabilities of these systems
Experimental studies for H2 adsorption on NPs of MoC0.6 and MoC1.1 supported on Au(111), complemented by computational modeling, have shown that these systems are extremely efficient for the cleavage of the H−H bond at room temperature and the storage of H adatoms.[9]
For H2/ MoC1.1/Au(111), the results of temperature-programmed desorption (TPD) showed H2 evolution between 350 and 450 K and a H/C ratio close to two, a value much larger than the one observed on bulk rock-salt δ-MoC surfaces, pointing out that MoC1.1 NPs can act as H sponges, where H2 can be adsorbed or desorbed, posing them as hot hydrogen reservoirs for catalytic applications.[9]

Summary

Introduction

Since the 1970s, transition-metal carbides have been suggested as an alternative to catalysts based on expensive late transition metals for a wide variety of hydrogenation reactions.[1−4] In many new catalysts, metal carbides nanoparticles (NPs) are dispersed on oxides and zeolites[5,6] but little is known about the specific phenomena which determine the hydrogenation capabilities of these systems. We examine the performance of MoCx/ Au(111) surfaces in the catalytic hydrogenation of carbon dioxide. This chemical transformation was chosen as a test due to its relevance for the control of air pollution and the production of high value chemicals in C1 catalysis.[10,11] It is nowadays well-accepted that many activities involving the burning of fossil fuels in our industrial society have led to an excessive concentration of CO2 in the atmosphere and major environmental problems.[10] In order to mitigate these harmful effects, CO2 capture, storage, and, specially, its conversion to valuable chemicals or commodity goods have become an urgent need.[11,12]

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