Iron-Copper Bimetallic Catalysts for Direct Carbon Dioxide Hydrogenation to Higher Hydrocarbons
Open Access
- Author:
- Wang, Wenjia
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 14, 2019
- Committee Members:
- Chunshan Song, Dissertation Advisor/Co-Advisor
Chunshan Song, Committee Chair/Co-Chair
Randy Lee Vander Wal, Committee Member
Michael John Janik, Outside Member
Xiaoxing Wang, Committee Member
Mort D Webster, Program Head/Chair - Keywords:
- CO2 hydrogenation
bimetallic catalyst
hydrocarbon - Abstract:
- The catalytic CO2 hydrogenation to C2+ higher hydrocarbons has attracted increasing attention due to both environmental and sustainability concerns. Past efforts have focused on modifying Fe-based catalysts and promoters with or without support. Cu-based catalyst is well known for its activity in water-gas shift (WGS) as well as reverse water-gas shift (RWGS) reaction and does not lead to CO2 methanation compared with other metals such as Co, Ni and Ru. Potassium (K) has been known as an effective promoter for CO and CO2 hydrogenation to olefin-rich higher hydrocarbons over Fe-based catalysts. Thus, this work aims at clarifying the effect of combining Fe and Cu, and influence of addition of K promoter on the activity and product selectivity towards C2+ hydrocarbons from CO2 hydrogenation, and developing a fundamental understanding on the structure and properties that affect the catalytic performance. The Fe-Cu bimetallic catalysts with various compositions, with or without K promoter, were prepared and examined in CO2 hydrogenation at relatively mild reaction conditions (573 K and 1.1 MPa). The combination of Fe and Cu led to a synergistic promotion of CO2 conversion and C2+ hydrocarbon formation rate when the Cu/(Fe + Cu) atomic ratio was 0.17 for γ-Al2O3 supported Fe-Cu catalysts. XRD results and high resolution TEM images demonstrate the presence of the metallic and alloy phases in the reduced Fe-Cu(0.17) catalysts. The strong interaction between Fe and Cu was observed from H2-TPR profiles. The complementary roles of each metal component in the bimetallic catalysts led to the well-dispersed metals in the alloy particles. The combination of Fe and Cu promoted the adsorption towards moderately and strongly adsorbed H2 (Type III + IV). The increased amount of moderately and strongly adsorbed H2 appears to correlate to the observed synergetic effect on C2+ hydrocarbon promotion. FT-IR results showed that adsorbed CO was enhanced in Fe-Cu bimetallic catalysts. Existence of formate and formic acid species in Fe-Cu bimetallic catalysts also proved the CO2 direct hydrogenation to C2+ hydrocarbons. The in situ XAS results showed that Fe peaks in both Fe and Fe-Cu catalysts were similar to that of Fe3O4, but Fe-Cu catalyst was more reducible to Fe0. The addition of K into Fe-Cu bimetallic catalysts led to a strong promotion of CO2 conversion and C2+ hydrocarbon formation rate. K is proved to be an effective promoter for Fe-Cu for enhancing C2+ hydrocarbon formation, which leads to promotion of C-C chain growth and light olefin yield, as well as suppression of CH4. K promoted Fe-Cu catalyst enhanced the adsorption towards all types of adsorbed CO2 (Type I, II, III and IV). The increased amount of moderately and strongly adsorbed CO2 appears to correlate to the observed synergetic effect on C5+ hydrocarbon promotion. Kinetic study demonstrated that CO were formed as a primary product from CO2 hydrogenation (reverse water-gas shift reaction; RWGS) and the produced CO was then hydrogenated to hydrocarbons (FTS). Direct hydrogenation on Fe catalyst was suggested but the contribution of such route would be relatively small. Light olefins and paraffins (C2 - C4) would form simultaneously from hydrogenation, but some of these olefins could be further hydrogenated to paraffins.