S T R U C T U R E O F M A T E R I A L S

S C I E N T I F I C H I G H L I G H T S

1 4 0 H I G H L I G H T S 2 0 2 2 I

Deciphering the role of metal oxides in deNOx reactions

Developing low-temperature catalysts for the abatement of nitric oxides (NOx) in vehicle exhaust is critical to reduce harmful emissions. Here, an investigation with advanced spectroscopic tools revealed the role of the different components responsible for the low-temperature activity of Mn- based catalysts.

Every day when car engines are started, harmful nitric oxides (NOx) are emitted. These emissions can be controlled by the installation of a catalytic converter that transform NOx into harmless N2. Currently, technologies such as selective catalytic reduction of NOx with ammonia (NH3-SCR) are able to effectively reduce emissions when using a catalyst based on Cu-exchanged zeolites [1]. Unfortunately, those catalysts are not efficient during cold start and when the exhaust is at low temperatures (less than 200°C). Therefore, there is a strong need to develop catalysts that are able to operate at low temperatures. In this respect, manganese-based mixed oxides show promising activity at low temperatures.

Typically, Mn-based catalysts are prepared with other metal oxides, such as Ce and Ti oxides, that act as support, dopants or promoters. Over the past decades, the role of the different oxides on catalytic activity and selectivity has been under debate. To shed light on this debate, in this work 30 different catalysts were systematically prepared with different compositions of Mn, Ce and Ti oxides. The catalyst structures

were resolved by a multi-technique approach and their catalytic performances were investigated under conditions relevant for real application in a mobile exhaust.

In solving the catalyst structure, sketched in Figure 132a and Figure 132b, the X-ray absorption spectroscopy measurements at beamline BM30B were critical. These data, together with additional characterisation results, show that when Mn, Ce and Ti are in the formulations, the catalyst structure is highly porous and amorphous, with a homogeneous composition of all metal atoms at the nanoscale, resulting in a spacing of the Mn oxides by Ce and Ti oxide species. For the binary Mn and Ti, there is a clear enrichment of Mn on the catalyst surface, which causes a proximity of Mn oxides at the nanoscale. This is clearly observed by the larger peak area corresponding to the Mn-Mn scattering for the Mn and Ti catalyst, in comparison with the stronger disorder of the Mn, Ce and Ti system, as observed in the extended X-ray absorption fine structure (EXAFS) spectra in Figure 132d and Figure 132e.

To assess the implication of catalyst structure on catalytic performance, the activity of the samples was measured in the NH3-SCR reaction at low temperature (150°C). Specific activities (NO converted per surface area and per time) in Figure 133a show a linear relationship for the binary MnTi and also a pseudolinear relationship for the ternary samples but with a lower slope. It is inferred that Mn oxide species in the binary MnTi catalysts have similar activity, and those are more active than in the MnCeTi ternary system. Therefore, it is

Fig. 132: a) MnTi binary catalysts, where amorphous layers of Mn oxide

are on the surface of crystalline TiO2 and (b) MnCeTi ternary

catalysts, where the metal oxides are amorphous and well mixed. Mn

K-edge (c) X-ray absorption near- edge structure (XANES), (d) EXAFS

and (e) FT-EXAFS spectra of a binary Mn0.35Ce0.00Ti0.65 and a ternary

Mn0.37Ce0.04Ti0.51 .