(17d) Analyzing CO2 Hydrogenation Activity of Ga-Cu Alloys By Experimental and Computational Methods

Climate change is an environmental curse caused by excessive CO₂ releases in the atmosphere caused by the anthropogenic combustion of fossil fuels. Researchers currently focus on making value-added chemical products from CO₂ such as methanol, DME, formic acid, etc., on reducing greenhouse gases (GHG) and ameliorate global warming. The catalytic hydrogenation of CO₂ represents one viable option for utilizing CO₂. In recognition of the growing need for methanol (produced mainly from syngas) and the global effort to reduce greenhouse gas emissions, researchers have a terrific opportunity to convert CO₂ captured from a wide range of sources using heterogeneous catalysts. Catalysts commonly used in CO₂ hydrogenation include Cu, Pd, In, Ni, and bimetallic catalysts (Cu-Zn, Pd-In, and Ga-Cu). Several metals supported on oxides have shown promise as catalysts for CH₃OH synthesis from CO₂. Despite the wide use of oxide-supported metals as catalysts for CO₂ hydrogenation reactions, very little comprehensive knowledge of the mechanisms and key steps that contribute to activity and selectivity. Creating a better catalyst by design depends on the understanding of atoms at the atomic level. Towards that goal, we carried out the first principle density functional theory calculations for the major reactions that can form methanol and DME from CO₂.

Experiments were conducted in 16-channel Flowrence XD from Avantium at temperature of 225-300^oC, 50 bar pressure and CO₂ to H₂ ratio of 1:4. Vienna Ab Initio Simulation Package (VASP) was used to calculate the reaction kinetics using the first principle density functional theory (DFT). The climbing image nudged elastic band method (CI-NEB) was used to identify the transition state structures of the elementary reactions. The following equation was used to calculate the binding energy of all the species present in CO₂ hydrogenation: E_bind = E_{adsorbate+surface -Eadsorbate -Esurface}

where E_{adsorbate+surface} is the total energy of the adsorbate on metal surface, E_adsorbate, and E_surface are the total energy of adsorbate in the gas phase and the bare metal surface

Research has been devoted to the development of catalysts that can promote the methanol synthesis process from CO₂ hydrogenation. It has recently been found experimentally that a Ga-Cu intermetallic catalyst exhibits high activity, selectivity, and stabilities for this reaction. We present here a combined density functional theory and kinetic modeling study in order to provide insight into the reactions related to methanol and dimethyl ether (DME) formation over Ga-Cu (111). The activation barriers and adsorption energies of the major primaries in CO₂ hydrogenation reaction mechanism have been calculated via DFT calculations on Ga-Cu (111) surfaces. Many reaction intermediates are involved in the CH₃OH production from formate and carboxyl reaction pathways, and it can occur in various ways. An understanding of the active intermediates in CO₂ hydrogenation and their role in CH₃OH formation can only be achieved with experimental observations and theoretical calculations combined. We demonstrate that methanol can be synthesized using HCOO* as well as COOH* reaction pathway rather than just HCOO* over various catalyst surfaces. We found that the HCOO* formation is possible reaction pathway for the formation of methanol, because energy barrier is lower than the carboxyl pathway. Further observed that the formation of methanol via hydrogenation of CH₃O is kinetically favorable compared to the hydrogenation of CH₂OH. This analysis provides insight into the role surface structure play in defining the nature of Ga-Cu catalysts used in DME and methanol syntheses. Experimental results showed that the selectivity of methanol was higher in equal percentages of Ga and Cu, but the selectivity of CO was higher in Ga-rich surfaces. There is an interesting fact that the DME can only be observed with addition of Ga to the Cu. According to the DFT calculations, the Cu rich surface had the lowest activation energy for methanol formation, and the Ga rich surface had the lowest activation energy for DME formation. Experimental observations and DFT results are in good agreement. By applying microkinetic modelling techniques to the calculated activation barriers presented in the current study, future studies will be able to better understand the mechanism of methanol and DME synthesis and the relative importance of the identified intermediates.