Abstract
Cu-ZnO-based catalysts are of importance for CO2 utilization to synthesize methanol. However, the mechanisms of CO2 activation, the split of the C=O double bond, and the formation of C-H and O-H bonds are still debatable. To understand this mechanism and to improve the selectivity of methanol formation, the combination of strong electronic adsorption (SEA) and atomic layer deposition (ALD) was used to form catalysts with Cu nanoparticles surrounded by a non-uniform ZnO layer, uniform atomic layer of ZnO, or multiple layers of ZnO on porous SiO2. N2 adsorption, H2 temperature-programmed reduction (H2-TPR) X-ray diffraction (XRD), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDX), CO-chemisorption, CO2 temperature-programmed desorption (CO2-TPD), X-ray adsorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) were used to characterize the catalysts. The catalyst activity was correlated to the number of metallic sites. The catalyst of 5 wt% Cu over-coated with a single atomic layer of ZnO exhibited higher methanol selectivity. This catalyst has comparatively more metallic sites (smaller Cu particles with good distribution) and basic site (uniform ZnO layer) formation, and a stronger interaction between them, which provided necessary synergy for the CO2 activation and hydrogenation to form methanol.
Highlights
Carbon dioxide (CO2 ) is considered to be the most severe greenhouse gas by the amount of anthropogenic emission (36 billion tons per year in 2017 [1]), and the prominent free carbon source in the future, on the other hand
ZnO prepared by atomic layer deposition (ALD) was to the attributable the formation of acase, Cu–Zn our case, the increase the lattice constant upon not of large enough to be In detected by X-ray diffraction (XRD)
The non-condensable gases in the stream were monitored by a bubble flow column installed at the exit of the reactor, and analyzed by an off-line Agilent Technologies 7890A gas chromatograph instrument equipped with two thermal conductivity detectors (TCDs) and a flame ionization detector (FID)
Summary
Carbon dioxide (CO2 ) is considered to be the most severe greenhouse gas by the amount of anthropogenic emission (36 billion tons per year in 2017 [1]), and the prominent free carbon source in the future, on the other hand. Cu/ZnO/ZrO2 catalyst [6] prepared by the precipitation–reduction method showed a CO2 conversion of 23% and selectivity to MeOH of 57%. Kanai and coworkers [18] found that parts of ZnO migrated onto the surface to form Cu–Zn alloy, and that Cu+ species formed in the vicinity of ZnOx were regarded as active sites for MeOH synthesis. The right structures must be able to perform two functions at the same time, pulling one O away to form H2 O and inserting H atoms in to form methanol Based on this thinking, we planned to make two extremely different catalysts with the help of the atomic layer deposition (ALD) method. Structure (EXAFS) and their performance for CO2 hydrogenation to form methanol was tested using a fixedand bedDiscussion reactor
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