Ni Single Atom Catalysts for CO2 Activation.

Marie-Mathilde Millet,Aliaksei Mazheika,Sabine Wrabetz,Sergey V Levchenko,Andrey Tarasov,Gerardo Algara-Siller,Robert Schlögl,Detre Teschner,Friedrich Seitz,Elias Frei,Frank Girgsdies

doi:10.1021/jacs.8b11729

Abstract

We report on the activation of CO2 on Ni single-atom catalysts. These catalysts were synthesized using a solid solution approach by controlled substitution of 1–10 atom % of Mg2+ by Ni2+ inside the MgO structure. The Ni atoms are preferentially located on the surface of the MgO and, as predicted by hybrid-functional calculations, favor low-coordinated sites. The isolated Ni atoms are active for CO2 conversion through the reverse water–gas shift (rWGS) but are unable to conduct its further hydrogenation to CH4 (or MeOH), for which Ni clusters are needed. The CO formation rates correlate linearly with the concentration of Ni on the surface evidenced by XPS and microcalorimetry. The calculations show that the substitution of Mg atoms by Ni atoms on the surface of the oxide structure reduces the strength of the CO2 binding at low-coordinated sites and also promotes H2 dissociation. Astonishingly, the single-atom catalysts stayed stable over 100 h on stream, after which no clusters or particle formation could be detected. Upon catalysis, a surface carbonate adsorbate-layer was formed, of which the decompositions appear to be directly linked to the aggregation of Ni. This study on atomically dispersed Ni species brings new fundamental understanding of Ni active sites for reactions involving CO2 and clearly evidence the limits of single-atom catalysis for complex reactions.

Highlights

Noble metals stabilized on metal oxides are one of the most widely used heterogeneous catalysts applied in the industry.[1]
Through the synthesis of phase pure precursors and catalysts, we prepared for the first time Ni single atom catalysts using wet chemistry
A Ni surface enrichment of the solid solutions is observed, by both physical and chemical analysis methods, in agreement with the prediction made from first principle calculations.[24]

Summary

Introduction

Noble metals stabilized on metal oxides are one of the most widely used heterogeneous catalysts applied in the industry.[1]. Reducing the complexity of such systems can be achieved by the largest conceivable reduction of both the size and the size distribution of the metal particles by the preparation of “single atom catalysts” (SACs). While the denomination of SACs is relatively recent,2e the concept of “isolated active site within a solid catalyst” was introduced much earlier according to Thomas[3] and became prominent thanks to the early work of Grasselli et al.[4] In recent years, many studies were conducted on single atom catalysts, dealing with supported noble metals like Pt,2e,5 Au,2d,5a,6 Ir,[7] and Pd8 but rarely using cheaper and more earth-abundant transition metals. Vogt et al provided some new Received: October 31, 2018

Results

Discussion

Conclusion