Understanding the Product Selectivity of Syngas Conversion on ZnO Surfaces with Complex Reaction Network and Structural Evolution

Abstract

Recently, a bifunctional oxide–zeolite (OX-ZEO) catalyst was widely studied experimentally, which can selectively convert syngas to light olefins. The performance of OX-ZEO is exceptional, while the mechanism is controversial. In this work, we have first developed an algorithm based on graph theory to establish a complete reaction network for syngas conversion to methanol, ketene, and methane. Combined with density functional theory (DFT) calculations, the activity and selectivity of syngas conversion over zinc oxide (ZnO) are systematically studied by a reaction phase diagram. The key intermediate, ketene, is observed in experiments, which has been first confirmed theoretically in this work. The evolution of ZnO surfaces is found to be a key factor of diverse product selectivity. It is found that methanol production is more favored over the ZnO surfaces with a low oxygen vacancy concentration. As the oxygen vacancy increases, the main product evolves gradually from methanol to ketene and finally to methane. Accordingly, the overall reaction activity increases too. Our prediction from the reaction phase diagram is finally verified by microkinetic modeling.

Light olefin synthesis from a diversity of renewable and fossil feedstocks: state-of the-art and outlook

“…Zinc oxide reduction to metallic Zn should be avoided; it leads to the CH 2 * hydrogenation to methane. 411 Higher number of oxygen vacancies promotes the ketene formation and further LO production.…”

H2 Activation on Pristine and Substitutional ZnO(101̅0) and Cr2O3(001) Surfaces by Density Functional Theory Calculations

“…Different from transition-metal catalysts, the bifunctional OX-ZEO catalysts separate the activation of syngas on the oxide part from the subsequent carbon chain growth and hydrogenation on the zeolite part. For the oxide part, the metal–oxide mixtures and/or complex metal oxides were found to be more active than the separated metal oxides. − Identification of the active sites and their role in syngas conversion remain challenging. − Mechanistic investigation and gaining an understanding of the intrinsic activity of prototype oxides such as ZnO and Cr 2 O 3 for syngas activation and the influence of the defects such as heterometal atoms are valuable for understanding the mechanism behind this phenomenon but not well explored yet.…”

Toward Understanding and Simplifying the Reaction Network of Ketene Production on ZnCr2O4 Spinel Catalysts

“…The first step is to construct all possible reaction paths. We used a novel algorithm developed by us recently inspired from graph theory . In the course of pathway generating, the elementary steps and total reactions are given as input .…”

“…According to the Sabatier volcano, it suggests that the catalyst with high activity should have appropriate reactivity with key intermediates . According to our previous work about syngas conversion on ZnO, catalysts with G CO* ∼ −1 eV together with a G O* ∼ −1 eV is promising for ketene production. As shown in Table S1, the 111-Cr–O v 0.50 surface exhibits promising candidates for ketene formation in terms of intermediate adsorption energies of CO* and O*.…”

“…We used a novel algorithm developed by us recently inspired from graph theory. 25 In the course of pathway generating, the elementary steps and total reactions are given as input. 31 The involving substances are regarded as nodes and elementary steps are edges that connect each node.…”