Soot Combustion over Niobium-Doped Cryptomelane (K-OMS-2) Nanorods—Redox State of Manganese and the Lattice Strain Control the Catalysts Performance

Piotr Legutko,Janusz Janas,Zbigniew Sojka,Joanna Gryboś,Jakub Szlachetko,Xuehua Yu,Monika Fedyna,Andrzej Kotarba,Andrzej Adamski,Anna Wach,Zhen Zhao

doi:10.3390/catal10121390

Abstract

A series of Nb-doped (0–23 wt%) cryptomelane catalyst (Nb-K-OMS-2) was synthesized and thoroughly characterized by XRD, TEM/EDX, XRF, XPS, XAS, UV-Vis, and Raman techniques corroborated by the work function measurements. The obtained catalysts were tested for soot oxidation (Printex U) in model tight and loose contact modes. It was shown that the catalytic properties of the Nb-K-OMS-2 are controlled by the amount of Nb dopant in a strongly non-monotonous way. The introduction of niobium gives rise to the strain in the cryptomelane lattice, accompanied by significant Mn+3/Mn+4 ratio variations and concomitant work function changes. The isotopic exchange experiments revealed that the catalytic activity of the Nb-OMS-2 catalysts in soot combustion proceeds via the pathways, where both the activated suprafacial 18O and the surface 16O2− species participate together in the reaction. The niobium doping level controls the non-monotonous changes of the catalyst work function and the lattice strain, and variations of these parameters correlate well with the observed deSoot activity. To our best knowledge, the role of the lattice strain of the cryptomelane catalysts was documented for the first time in this study.

Highlights

During combustion of fossil fuels in diesel engines soot particles are produced, being one of main components of the particulate matter (PM) [1]
A tight dependence of the soot oxidation on the catalyst work function was observed previously for other oxide systems [55,56,57], and this study provides yet another argument confirming the general merit of the work function as one of the prime descriptors of the deSoot activity
Detailed investigations of niobium-doped cryptomelane catalysts (K-OMS-2) for soot oxidation showed that their properties are controlled by the amount of Nb dopant in a strongly non-monotonous way

Summary

Introduction

During combustion of fossil fuels in diesel engines soot particles are produced, being one of main components of the particulate matter (PM) [1]. The soot particles are removed using ceramic DPFs filters, which can lower soot emission up to 99%. Their use involves a mechanical filtration and combustion of the accumulated soot particles followed by subsequent regeneration [6,7]. The combustion temperature of soot particles (550–650 ◦ C) is much higher than the temperatures of diesel exhaust gases (200−450 ◦ C), intensive fundamental and applied studies are carried out on the development of soot oxidation catalysts that may work at the relatively low temperature window. One of the strategies for diesel soot oxidation consists of the application of catalytic particle filters (CDPF) based on noble metals [8,9]. The shortage of the platinum group metals imposes a high price of such CDPF catalysts, their cheaper alternatives are still receiving significant research attention [10]

Methods

Results

Conclusion