Catalyst design for electrochemical CO2 reduction through DFT calculations
Date of Issue2018-02-06
School of Materials Science and Engineering
Electrochemically converting CO2 into low-coordinated carbides provides a sustainable way to utilize CO2 but, unfortunately, requires effective catalysis. The catalytic activity of currently employed metallic catalysts is largely hindered by the low energy efficiency (high overpotential) and poor product selectivity (low Faradaic efficiency). In this thesis, by taking advantage of the density functional theory (DFT) calculations, three strategies – creation of low-coordinated atomic sites, surface alloying, and phase transformation – are proposed to design effective catalysts towards CO2 electrochemical reduction (CO2ER). Large numbers of low-coordinated atomic sites are contained in experimentally synthesized nanoparticles (NPs), whose catalytic roles towards CO2ER have never been theoretically investigated. Hypothesizing these low-coordinated sites are more active in catalyzing CO2 reduction, we modelled four types of low-coordinated Cu sites – the plain (100) site, the edgy (211) site, the cornered (532) site, and the defective vac-(111) site – and compared their catalytic activity with the full-coordinated (111) site. For all the mono-carbide products, significantly lowered energetics of reaction intermediates (as high as 74.5% decrease) were found on the low-coordinated sites, estimating decreased overpotential for CO2ER. Moreover, on the low-coordinated sites, formation of methane was found to be more thermally advantageous than the formation of methanol, implying the generation of hydrocarbons is likely proceeded on these sites. Finally, the unwanted hydrogen evolution reaction (HER) was also found to estimate lowered overpotentials (as high as 66%) on the low-coordinated sites, suggesting that high operation potentials are required to suppress this competing reaction. Design of surface alloys (SAs) is also proposed to be effective in achieving lowered reaction overpotential for catalyzing CO2ER. In order to screen out a decent Cu SA catalyst, we have systematically studied the stability and reaction performance of all transition-metal doped Cu SAs. By computing the segregation energy and mixing energy, only eight transition metals (Au, Ag, Zn, Cd, Sc, Y, Pd, and Pt) were found to stably alloy Cu at the topmost layer. For the catalytic activity, SA of ZnCu was found to remain almost the same reaction overpotential for CO2ER as Cu, but considerably enlarged (200%) the one for the competing HER. Dopants of Au, Ag, Pd, and Pt resulted in enlarged overpotential for CH4 generation; while dopants of Sc and Y showed no activity towards CO2ER. When doping low concentration Cd into Cu surface, the generated Cu-Cd-Cu bonding cannot change the selectivity for CH4; however, when doping high concentration Cd, the formed Cd-Cd-Cu site can selectively and effectively catalyze CO2 into HCOOH, with the prevention of yielding hydrocarbons as well as the unwanted HER. The effect of phase transformation on the catalytic activity of metals towards CO2ER is also explored. Using Cu (selectivity of hydrocarbon), Au (selectivity of CO), and Pb (selectivity of HCOOH) as representatives, the catalytic performances of these metals in crystal symmetry of fcc, bcc, 2H, 4H, and sc have been investigated. It is found that only passive metals (Au) were stable in sc phase under CO2ER conditions. For other orientations, the binding affinity towards CO2ER intermediates followed the trend of fcc < hcp-type (2H and 4H) < bcc, suggesting a general guidance can be utilized in tuning the catalytic activity of metals. For Au and Pb, since their original symmetry is fcc, converting the fcc phase into bcc type can result in a 25.2% and 21.3%, respectively, decreased onset potential for catalyzing CO2ER.