Abstract

The utilization of operando spectroscopy has allowed us to watch the dynamic nature of supported metal nanoparticles. However, the realization that subtle changes to environmental conditions affect the form of the catalyst necessitates that we assess the structure of the catalyst across the reactant/product gradient that exists across a fixed bed reactor. In this study, we have performed spatial profiling of a Pd/Al2O3 catalyst during NH3 oxidation, simultaneously collecting mass spectrometry and X-ray absorption spectroscopy data at discrete axial positions along the length of the catalyst bed. The spatial analysis has provided unique insights into the structure–activity relationships that govern selective NH3 oxidation—(i) our data is consistent with the presence of PdNx after the spectroscopic signatures for bulk PdNx disappear and that there is a direct correlation to the presence of this structure and the selectivity toward N2; (ii) at high temperatures, ≥400 °C, we propose that there are two simultaneous reaction pathways—the oxidation of NH3 to NOx by PdO and the subsequent catalytic reduction of NOx by NH3 to produce N2. The results in this study confirm the structural and catalytic diversity that exists during catalysis and the need for such an understanding if improvements to important emission control technologies, such as the selective catalytic oxidation of NH3, are to be made.

Highlights

  • Advances in the design and optimization of heterogeneous catalysts for sustainable transformations and environmental protection require a precise understanding of structure−activity relationships

  • The Pd/Al2O3 catalyst used in this study has been thoroughly characterized with the structural information reported elsewhere.[4]

  • Prior to performing NH3 oxidation, the catalyst was loaded into the Spaci-FB reactor and treated in H2 at 400 °C before cooling down under He

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Summary

■ INTRODUCTION

Advances in the design and optimization of heterogeneous catalysts for sustainable transformations and environmental protection require a precise understanding of structure−. We have utilized a method that merges these approaches, i.e., Spaci-FB-XAFS,[3] a minimally invasive technique that allows monitoring of the gas-phase concentrations as well as the temperatures along a reactor bed (SpaciFB),[26,27] with X-ray absorption fine structure (XAFS) spectroscopy,[3] which provides information on the local Pd speciation Using this methodology, it was possible to profile the catalyst properties along a fixed catalytic bed and obtain both chemical and structural information on the Pd speciation. The analysis was performed using MCR-ALS developed by Tauler et al.[39]

■ RESULTS
■ ACKNOWLEDGMENTS
■ REFERENCES
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