The Mechanism of Potassium Promoter: Enhancing the Stability of Active Surfaces

Abstract

Owing to their outstanding effect on improving catalytic reactivity, alkali-metal promoters have been widely used in industry [1,2] and extensively studied in academia. [3][4][5][6][7][8][9][10][11][12] Potassium-promoted iron catalysts for Fischer-Tropsch synthesis (FTS) and ammonia synthesis are the most representative examples of this effect. Owing to the complexity of real catalytic systems, the microscopic understanding of the alkalimetal promotion effect is still an elusive and challenging subject.In the past decades, most experimental and theoretical studies have focused on the co-adsorption systems involving alkali-metal atoms, for example, K + CO/metal. Five types of alkali-metal-adsorbate interactions were proposed to explain the alkali-metal promotion effect: 1) substrate-mediated charge transfers, [3] 2) direct bonding between alkali metal and adsorbate, [4] 3) electrostatic interactions, [5] 4) alkalimetal-induced molecular polarization, [6] and 5) nonlocal alkali-metal-induced enhancement of the surface electronic polarizability. [7] Although these studies represent excellent surface science, they are not immediately relevant to catalysis, because alkali-metal salts (or oxides) rather than metallic alkalis are used in heterogeneous catalysis. More importantly, previous studies only highlighted the alkali-metal-induced effect on the electronic structure of metallic substrate and coadsorbed molecules, while the more intriguing aspect, the alkali-metal effect on the surface structure of catalysts, has never been taken into account. On the basis of these proposals, the drastic changes in catalytic activity and selectivity caused by very low loadings of alkali-metal promoters cannot be reasonably explained. [8] As metallic iron is the active catalyst in ammonia synthesis, [9] iron-based FTS catalysts show an rich phase chemistry of metal, oxides, and carbides under reaction conditions.

ε-Iron carbide as a low-temperature Fischer–Tropsch synthesis catalyst

“…7b). We propose that the strong promoter-iron interaction in this multiply promoted catalyst stabilizes the metallic iron NPs by lowering the surface energy 48 , which alters the tendency of carbidation despite of its small size.…”

Metal organic framework-mediated synthesis of highly active and stable Fischer-Tropsch catalysts

“…The Fe@C catalysts were tested in the FTS reaction at 613 K, 20 bar, H 2 /CO ¼ 1 and gas hourly space velocity (GHSV) of 30,000 h À 1 (space velocity based on catalyst bed volume). Owing to the outstanding effect of potassium promotion on iron-based catalyst for FTS, namely improving activity and olefins to paraffins ratio [33][34][35] , a series of alkali-promoted samples were are also synthesized.…”

Selective formation of light olefins from CO2 hydrogenation over Fe–Zn–K catalysts

“…Preparation methods and promoter addition show impacts on the catalytic activity. It is well recognized that potassium addition in iron catalysts enhances the production of olefinic hydrocarbons in CO hydrogenation [6,10,11]. This alkali promotion is also indispensable to raise light olefin selectivity in CO 2 hydrogenation and has been discussed in detail [3,7,12].…”