Operando and Postreaction Diffraction Imaging of the La–Sr/CaO Catalyst in the Oxidative Coupling of Methane Reaction

Dorota Matras,Cristina Estruch Bosch,Benjamin Rollins,Antonios Vamvakeros,Nicolas Grosjean,Marco Di Michiel,Simon D M Jacques,Robert J Cernik,Jonathan Wright,Stephen Poulston,Andrew M Beale

doi:10.1021/acs.jpcc.8b09018

Abstract

A La–Sr/CaO catalyst was studied operando during the oxidative coupling of methane (OCM) reaction using the X-ray diffraction computed tomography technique. Full-pattern Rietveld analysis was performed in order to track the evolving solid-state chemistry during the temperature ramp, OCM reaction, as well as after cooling to room temperature. We observed a uniform distribution of the catalyst main components: La2O3, CaO–SrO mixed oxide, and the high-temperature rhombohedral polymorph of SrCO3. These were stable initially in the reaction; however, doubling the gas hourly space velocity resulted in the decomposition of SrCO3 to SrO, which subsequently led to the formation of a second CaO–SrO mixed oxide. These two mixed CaO–SrO oxides differed in terms of the extent of Sr incorporation into their unit cell. By applying Vegard’s law during the Rietveld refinement, it was possible to create maps showing the spatial variation of Sr occupancy in the mixed CaO–SrO oxides. The formation of the Sr-doped CaO species is expected to have an important role in this system through the enhancement of the lattice oxygen diffusion as well as increased catalyst basicity.

Highlights

The continuous depletion of crude oil, coupled with the large reserves of conventional and unconventional natural gas, especially its main component methane, renders it an increasingly important source of hydrocarbons for the chemical industry.[1]
These results obtained through Rietveld refinement were in good agreement with elemental analysis performed by inductively coupled plasma−mass spectrometry (MS) (ICP−MS) (Table 1) for Ca and La loadings, whereas for Sr, the ICP−MS loading was almost double the quantity found through Rietveld refinement
We presented the results from an operando OCM experiment with a La−Sr/CaO catalyst using the X-ray diffraction computed tomography (XRD-CT) technique in order to track the evolving solid-state chemistry under various operating conditions

Summary

INTRODUCTION

The continuous depletion of crude oil, coupled with the large reserves of conventional and unconventional natural gas, especially its main component methane, renders it an increasingly important source of hydrocarbons for the chemical industry.[1]. Apart from the radial gradient, when doubling the total flow of reactive gases [the gas hourly space velocity (GHSV)], we could observe a temporal gradient when the rhombohedral polymorph reappeared due to an increase of the catalyst bed temperature. This phenomenon was attributed to the higher ratio of nonselective reactions when applying a higher GHSV. The solid-state changes introduced during the OCM were maintained after cooling to room temperature, and further analysis of the spent sample showed that radial and axial solid-state changes occurred during the OCM experiment

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

SUMMARY AND CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES