Evolution of Water Diffusion in a Sorption-Enhanced Methanation Catalyst

Renaud Delmelle,Andreas Borgschulte,Arndt Remhof,Joris Proost,Andre Heel,Jasmin Terreni

doi:10.3390/catal8090341

Abstract

Sorption-enhanced methanation has consequent advantages compared to conventional methanation approaches; namely, the production of pure methane and enhanced kinetics thanks to the application of Le Châtelier’s principle. In this paper, we address the question of the long-term stability of a sorption-enhanced methanation catalyst-support couple: Ni nanoparticles on zeolite 5A. Compared to most conventional methanation processes the operational conditions of sorption-enhanced methanation are relatively mild, which allow for stable catalyst activity on the long term. Indeed, we show here that neither coking nor thermal degradation come into play under such conditions. However, a degradation mechanism specific to the sorption catalysis was observed under cyclic methanation/drying periods. This severely affects water diffusion kinetics in the zeolite support, as shown here by a decrease of the water-diffusion coefficient during multiple cycling. Water diffusion is a central mechanism in the sorption-enhanced methanation process, since it is rate-limiting for both methanation and drying.

Highlights

The key issues of intermittency and dispersion of primary renewable electricity sources find an answer in power-to-gas (P2G) strategies, where the excess of renewable electricity is converted into synthetic gas fuels, using hydrogen produced by water electrolysis as a primary reactant [1]
The reaction temperature is above 250 ◦ C, resulting in a thermodynamically limited conversion yield of less than 96% [10], which is further reduced by finite kinetics
We show in this publication that the degradation of the catalytic activity relevant during the reaction phase of a sorption catalyst is negligible at optimized conditions

Summary

Introduction

The key issues of intermittency and dispersion of primary renewable electricity sources find an answer in power-to-gas (P2G) strategies, where the excess of renewable electricity is converted into synthetic gas fuels, using hydrogen produced by water electrolysis as a primary reactant [1]. We show in this publication that the degradation of the catalytic activity relevant during the reaction phase of a sorption catalyst is negligible at optimized conditions We attribute this to the encapsulation of the Ni-particle in the inner of the zeolite structure, preventing irreversible carbon growth, but allowing exchange of reactants and products to and from the active surface, respectively. This approach allows for both equilibrium and kinetic analyses through real-time monitoring of the specimen mass change This reflects the evolution of the Sabatier reaction, because water is one of its products and is entirely adsorbed on the zeolite support as long as the reaction is sorption-enhanced. We used this experimental approach to measure the equilibrium CO2 and H2 O uptake capacity at conditions relevant for methanation. The catalyst surface chemistry, crystal structure and water desorption kinetics

Results and Discussion

Sketch of diffusion: diffusion: water local

Experimental

Conclusions