Understanding the Origin of Structure Sensitivity in Nano Crystalline Mixed Cu/Mg−Al Oxides Catalyst for Low‐Pressure Methanol Synthesis

Abstract

Cu nanoparticles of size 5-10 nm supported on MgÀ Al mixed oxide were prepared by the sol-gel method. Cu loading was varied from 2.5 to 10 wt % on the support to investigate the effect on particle size and activity/selectivity of the catalyst. The Cu/MgÀ Al catalysts containing small copper nanoparticles favor high selectivity of methanol, while the rate of CO formation was higher for larger copper particles. The high methanol selectivity (~99 %) and methanol formation rate (0.016 mol g Cu À 1 h À 1 ) over the 4.8Cu/MgÀ Al catalyst was due to the combined effect of the presence of high Cu dispersion, Cu surface area, and strong interaction between small Cu particles with MgÀ Al support. The high stability of the catalyst was attributed to the strong binding of the Cu cluster (À 179.7 kJ/mol) to the MgO/γ-Al 2 O 3 support, as shown by the DFT study. Additionally, the adsorption energy calculated using DFT showed preferential adsorption of CO 2 and H 2 at the Cu/MgO(100) active site (À 120.9 kJ/mol, À 130.4 kJ/mol) compared to the Cu/γ-Al 2 O 3 (100) (À 64.2 kJ/mol, À 85.7 kJ/mol)active site. The high selectivity of the catalyst towards methanol can be attributed to the higher stability of the formate (HCOO) intermediate (À 257.2 kJ/mol) compared to the carboxylate (COOH) intermediate (À 131.0 kJ/mol).

Steering the Aspects of MgO-Induced Structure Sensitivity in Cu-Based Catalysts for CO2-Rich Syngas Conversion to Dimethyl Ether: Cu/Zn Ratio and Lattice Parameters

“…Also, the optimized amount of MgO (20 mol %) (as evidenced from previous results 4 ) is responsible for properly dispersing the Cu species, resulting in a higher surface area. Crystallite size data (Table S3 of the Supporting Information) also demonstrate that the smallest crystallite size was possessed by the CZ [2]−M(20) catalyst, which is in agreement with the surface area value of the catalyst. The lower the crystallite size, the higher the dispersion and, hence, surface area.…”

“…All bifunctional catalysts show a pore size distribution with mesopore sizes of around 10 nm, possibly attributed to the intergrain structures of MSC. As seen from Table S3 of the Supporting Information, maximum surface areas of 32.2 and 99.7 m 2 /g for MSC and hybrid catalyst, respectively, were observed for the CZ [2]−M(20) catalyst. In each group of Cu/ Zn ratio, the catalyst with 20 mol % Mg possesses the largest surface area.…”

“…Also, the amount of ZnO and MgO might be insufficient to properly disperse the amount of Cu in the catalyst matrix. However, catalyst CZ [2]−M(20) possesses a Cu/Zn ratio of 2, nearing the Cu/Zn ratio of 2.33, which is believed an ideal ratio for methanol synthesis. Also, the optimized amount of MgO (20 mol %) (as evidenced from previous results 4 ) is responsible for properly dispersing the Cu species, resulting in a higher surface area.…”

“…25 The trends obtained in surface area values of the catalyst can also be elucidated upon the presence of the amount of different diluents (ZnO and MgO) in the catalyst and their average ionic radii. For this, three catalysts CZ [2]−M (10), CZ [3]− M(20), and CZ [6]−M (30) with identical Cu content but different ratios of Zn/Mg are considered. The ionic radius values for Cu 2+ , Zn 2+ , and Mg 2+ are 0.73, 0.74, and 0.72 Å, respectively.…”

“…Implementing more energy-competent technologies or vehicles with high mileage (low gasoline/diesel consumption) can aid to mitigate this urgent issue of concern. However, to curb the rise in atmospheric CO 2 , implementing carbon capture utilization and storage (CCUS) may represent a potential option to recycle emitted CO 2 . , Although CCUS emerges to be a decisive technology to abate CO 2 emissions, the issues raised by the stability and suitability of massive topographical reservoirs call for definite alternatives. The realization of a carbon circular economy via CO 2 conversion into chemicals and fuels with renewable hydrogen is likely to portray a decisive role on the horizon of a sustainable future.…”