Tunable Carbon Dioxide Activation Pathway over Iron Oxide Catalysts: Effects of Potassium

Abstract

The activation of CO 2 to form CO, known as the reverse water−gas shift (RWGS) reaction, is an initial and essential step of numerous CO 2 hydrogenation reactions. However, the reaction is challenging, and the underlying CO 2 activation mechanism is usually overlooked when designing new heterogeneous catalysts. In this work, we investigate the effect of potassium, a common alkali metal promoter, on the iron oxide catalysts using a series of (quasi) in situ characterization techniques, kinetic analysis, and first-principles calculations. It is revealed that introducing potassium increases the surface basicity and steers the preferred CO 2 activation reaction route from a "redox" pathway to an "associative" pathway. Understanding the controlling factors and competitiveness of the two CO 2 activation mechanisms can potentially benefit the rational design of CO 2 hydrogenation catalysts.

Structure–Activity Relationships of Copper- and Potassium-Modified Iron Oxide Catalysts during Reverse Water–Gas Shift Reaction

“…37 Iron carbides are excluded from the reacted samples, which is also consistent with our previous Mossbauer spectrum analysis on the iron-based catalysts. 28 It is noted that the XRD signals for Al-, Cu-, and K-related phases are absent for the reaction-treated catalysts. Rietveld refinement of the crystal structures was performed using the program GSAS-II, with the results summarized in Table S4.…”

“…It is worthwhile to mention that AlFe has a 2-fold higher activity than the pure Fe 2 O 3 catalyst, which highlights the structural promotional effect of Al. 28 It is also noted, according to the literature, that the combination of Fe 2 O 3 and Cu yields much improved WGS/RWGS activities, attributed to their synergistic effect. 40 1KnCuAlFe exhibits much improved RWGS activity as compared with the nCuAlFe ones, and the addition of Cu still promotes the reaction.…”

“…The first is alkali metals such as sodium (Na) and potassium (K), which mainly facilitate the CO 2 adsorption step in their cationic form. 26,27 A recent study has also shown that the RWGS reaction proceeds via a redox mechanism over iron oxide catalysts but changes to an associative mechanism with the addition of K. 28 The second one includes some transition metals, such as copper (Cu) and nickel (Ni). 29 These components, in their metallic form during the reaction, can promote the H 2 dissociation step.…”

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

“…Several approaches can be applied to modify the structure of metal oxides for the facile adsorption and dissociation of CO 2 molecules, mainly, through tailoring their physicochemical (e.g., improving surface basicity) [182,183] and electronic properties (e.g., doping and creating defect sites) [182].…”

“…The basic sites of metal oxide catalysts can be generated by pre-treatment with basic promoters such as La 2 O 3 , K 2 O, Na 2 O, and MgO [183]. Tian et al [182] reported that introducing potassium into iron oxide catalyst increased the surface basicity. Thus, alkali metals can be incorporated into the metal oxides to tune the concentration of surface basic sites.…”