Structure and performance of zeolite supported Pd for complete methane oxidation

Ida Friberg,Adam H Clark,Phuoc Hoang Ho,Nadezda Sadokhina,Glen J Smales,Jungwon Woo,Xavier Auvray,Davide Ferri,Maarten Nachtegaal,Oliver Kröcher,Louise Olsson

doi:10.1016/j.cattod.2020.11.026

Abstract

The influence of zeolite support materials and their impact on CH4 oxidation activity was studied utilizing Pd supported on H-beta and H-SSZ-13. A correlation between CH4 oxidation activity, Si/Al ratio (SAR), the type of zeolite framework, reduction-oxidation behaviour, and Pd species present was found by combining catalytic activity measurements with a variety of characterization methods (operando XAS, NH3-TPD, SAXS, STEM and NaCl titration). Operando XAS analysis indicated that catalysts with high CH4 oxidation activity experienced rapid transitions between metallic- and oxidized-Pd states when switching between rich and lean conditions. This behaviour was exhibited by catalysts with dispersed Pd particles. By contrast, the formation of ion-exchanged Pd2+ and large Pd particles appeared to have a detrimental effect on the oxidation-reduction behaviour and the conversion of CH4. The formation of ion-exchanged Pd2+ and large Pd particles was limited by using a highly siliceous beta zeolite support with a low capacity for cation exchange. The same effect was also found using a small-pore SSZ-13 zeolite due to the lower mobility of Pd species. It was found that the zeolite support material should be carefully selected so that the well-dispersed Pd particles remain, and the formation of ion-exchanged Pd2+ is minimized.

Highlights

Methane is a highly valued fuel and the main component in natural and biogas, it is a greenhouse gas with a global warming potential 28 times higher than that of CO2 over a 100-year period [1]
Transmission QuickEXAFS measurements were performed at the SuperXAS beamline [39] at the Swiss Light Source (SLS) using 15 cm long ion chambers filled with 1 bar N2 and 1 bar Ar
Despite similar Si/Al ratio (SAR), the PdS43 sample exhibited a higher number of strong Brønsted acid sites than the PdB40 sample, which was seen in the deconvolution of the NH3-TPD. 77 % of the desorbed NH3 of the PdS43 was released in the high temperature peak associated with Brønsted acid sites

Summary

Introduction

Methane is a highly valued fuel and the main component in natural and biogas, it is a greenhouse gas with a global warming potential 28 times higher than that of CO2 over a 100-year period [1]. The formation of ion-exchanged Pd2+ can be facilitated under lean conditions [29,30,31,32,35], at high temperatures, and for certain zeolite types, in the presence of water vapour [36,37,38] Another complexity of zeolite-supported Pd catalysts is that Pd can rearrange to form large Pd◦ particles on the external zeolite surface under rich conditions and re-disperse into small PdO species and/or monodispersed Pd ions under lean conditions [29,30,31,32,35]. By combining catalytic activity measurements with a variety of characterization techniques (XANES, EXAFS, NH3-TPD, SAXS, STEM and NaCl-titration), we can correlate the level of CH4 oxidation to the Si/Al ratio (SAR), the type of zeolite framework, and the reduction-oxidation behaviour and nature of Pd species

Catalyst preparation

Characterization and catalytic activity measurements

Ex-situ characterization

Steady state CH4 oxidation

CH4 oxidation under alternating rich and lean pulses

Conclusions