Surface Oxidation of Supported Ni Particles and Its Impact on the Catalytic Performance during Dynamically Operated Methanation of CO2

Benjamin Mutz,Wolfgang Kleist,Maarten Nachtegaal,Oliver Müller,Andreas Gänzler,Jan-Dierk Grunwaldt,Ronald Frahm

doi:10.3390/catal7090279

Abstract

The methanation of CO2 within the power-to-gas concept was investigated under fluctuating reaction conditions to gather detailed insight into the structural dynamics of the catalyst. A 10 wt % Ni/Al2O3 catalyst with uniform 3.7 nm metal particles and a dispersion of 21% suitable to investigate structural changes also in a surface-sensitive way was prepared and characterized in detail. Operando quick-scanning X-ray absorption spectroscopy (XAS/QEXAFS) studies were performed to analyze the influence of 30 s and 300 s H2 interruptions during the methanation of CO2 in the presence of O2 impurities (technical CO2). These conditions represent the fluctuating supply of H2 from renewable energies for the decentralized methanation. Short-term H2 interruptions led to oxidation of the most reactive low-coordinated metallic Ni sites, which could not be re-reduced fully during the subsequent methanation cycle and accordingly caused deactivation. Detailed evaluation of the extended X-ray absorption fine structure (EXAFS) spectra showed surface oxidation/reduction processes, whereas the core of the Ni particles remained reduced. The 300-s H2 interruptions resulted in bulk oxidation already after the first cycle and a more pronounced deactivation. These results clearly show the importance and opportunities of investigating the structural dynamics of catalysts to identify their mechanism, especially in power-to-chemicals processes using renewable H2.

Highlights

The power-to-chemicals concept is an important strategy for future renewable energy systems based on chemical energy storage
CO2 simulated as H2 dropouts were performed and resulted in detailed insights into the catalyst deactivation mechanism
Thirty-second short-term H2 dropouts showed surface oxidation and reduction occurring after the seventh modulation cycle monitored in the center of the catalyst bed

Summary

Introduction

The power-to-chemicals concept is an important strategy for future renewable energy systems based on chemical energy storage. The splitting of water to produce H2 and the catalytic conversion of CO2 to methane, alkanes, methanol or higher alcohols are the main steps to generate a chemical energy carrier [1,2,3,4]. Both steps need to withstand fluctuations in supplied electricity from wind and solar plants, which occur temporarily and fluctuate on a time scale of seconds to days. The most relevant catalyst support material in industrial methanation reactions is reported to be γ-Al2 O3 with a high surface area [19,21,22]

Methods

Results

Conclusion