http://support.spring8.or.jp/Report_JSR/PDF_JSR_28B/2016B1626.pdf

2016B1626

BL37XU

Diffusion Engineering of an Ammonia Slip Catalyst Through X-rayTomography
Johannes Beila b, Henning O. Sørensena, Kim Dalbya, Anna P. Molinab, Ton Janssensb,
Robert Feidenhans’laaUniversity of Copenhagen, bHaldor Topsoe A/S
Abstract
A high throughput nano-tomography study of over 30 Ammonis Slip Catalyst (ASC) samples at aresolution of ~100 nm was conducted in order to investigate the structural changes due to different
preparation methods and in a further step, calculate the effective diffusion through the layer.
Keywords：

Catalysis, Material Science, Nanotomography, Gas Transport in Porous Media,Diffusion Modeling

Background and Objective
The current technology for removal of harmful NOx gases from diesel exhaust is based on the selective
catalytic reduction by ammonia (NH3-SCR), in which NOx is selectively reduced to N2, according to 4 NO
+ 4 NH3 + O2 → 4 N2 + 6 H2O. In practical applications, this reaction is done with a 5-10% of excessammonia, which is removed afterwards in the ammonia slip catalyst by oxidation of ammonia with oxygento N2. Typical operation conditions for these catalysts are 200-500 °C, with NO and NH3 inlet concentrations
on the order of 100-1000 ppm, and 5-10% O2 and water. Standard catalysts for SCR are V2O5/TiO2 andCu- or Fe-zeolites [1], which are typically applied on the walls of a monolithic support, with channels on theorder of 1-2 mm in size.
Ammonia slip catalysts (ASC) are also monolithic catalysts, containing an ammonia oxidation catalyst,
which usually is Pt, combined with an SCR catalyst. The function of the SCR catalyst is to enhance theselectivity to N2, since the majority of ammonia is oxidized to NOx over the Pt catalyst. The latest designsof the ASC contain a layered structure, in which the oxidation catalyst is covered with a thin SCR layer. It
has been shown that such a layered design results in a significantly better selectivity to N2 [2].
As a consequence of the layered catalyst design, the diffusion of NH3 and NO through the SCR layer has
a direct impact on the performance of the final product. If the diffusion through the SCR layer is difficult,the ammonia will not reach the oxidation catalyst, resulting in a low conversion of ammonia. Making athinner SCR layer will improve the ammonia conversion, but then the selectivity to N2 decreases, which isnot desirable. A faster diffusion through the SCR layer will allow for a sufficient ammonia conversion
without compromising the selectivity towards N2.
One cause for a slow diffusion of NH3 and NO is the pore structure of the SCR layer. The effective
diffusion through the SCR layer is determined by the contributions of the bulk diffusion of these components
and the Knudsen diffusion with decreases with the pore size in the SCR layer, for pore sizes diameter well
below 1μm. The best diffusion properties are thus obtained by creating an SCR layer with sufficient pores ofabout 1 μm in diameter.
To produce an SCR layer with the desired pore size, organic pore modifiers can be used. These modifiers
are deposited together with the catalyst particles in the SCR layer. Then these are burnt off, leaving free space
between the particles, and in this way extra pores are created in the SCR layer. The aim of the project is torelate the differences in performance to the pore structure in the SCR layer. By using different amounts andtypes of organic pore modifiers, SCR layers with different pore structures can be made. Without advanced
characterization methods, it is difficult to relate the effect of the pore modification to the performance, since
many of the relevant parameters of the pore space, such as connectivity and tortuosity are inherently 3D.
Experiments
Using the experimental setup for nano-tomography at the beamline BL37XU enabled us to acquire
tomograms from 6 different preparation types (Figure1). For each type we managed to image up to 5 samples.
The setup worked mostly without any problems. Over all, we recorded and reconstructed 30 high quality
tomograms with a voxel size of 40.5 nm within 24 hours. At the moment of the writing of this report, we areworking on advanced algorithms for pore structure analysis.

－133－

Figure 1: Slices from six different samples which have been prepared with different pore modifications. Thechanges in pore structure are clearly visible. Note for example the larger particles in B, or the large
agglomerates in F.
Results and Discussion
The high image quality at BL37XU gave important qualitative insight into the effect of different
preparation methods of the ASC monolith (Figure 1). Since we are only interested in the diffusion trough one
thin layer which is part of a multilayer catalyst, established laboratory techniques such as mercury intrusion
porosimetry are not applicable here, since they rely on a powder made form the entire monolith and are thus
insensitive to specific areas of the sample. In contrast, as demonstrated, using high resolution X-ray
tomography can give local information, since we were able to extract a single layer by hand from the monolithand image it at BL37XU. However, the image quality is not good enough to use the established workflow
for X-ray based analysis of the pore size distribution, which is to segment the image and then use a shrinking
ball algorithm to measure the distance of each pore-voxel to the next wall. It will therefore be necessary todevelop segmentation free methods to estimate the pore size distribution.
Next Steps
We are currently involved in a research effort together with Computer Scientists from the Danish Technical
University (DTU) to develop an appropriate segmentation free method to calculate the pore size distribution
from the tomograms. Once successfully, we can use this to calculate the diffusivity through the layer [4][5]and compare those results to experimental performance measurements.
References:[1] Nova, I.; Tronconi, E. Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts; Nova, I.;
Tronconi, E., Eds.; Springer Science Business Media: New York, 2014.
[2] Scheuer, A.; Hauptmann, W.; Drochner, A.; Gieshoff, J.; Vogel, H.; Votsmeier, M. Dual Layer
Automotive Ammonia Oxidation Catalysts: Experiments and Computer Simulation. Appl. Catal. BEnviron. 2012, 111-112, 445–455.
[3] J.-D. Grunwaldt, J.B. Wagner, R.E. Dunin-Borkowski, „Imaging catalysts at work: A hierarchicalapproach from the macro- to the meso- and nano-scale”, ChemCatChem 5, 62–80 (2013).
[4] A. Takeuchi, K. Uesugi, Y. Suzuki, J. Phys.: Conf. Series 186 (2009) 012020[5] Johnson, Claes. Numerical solution of partial differential equations by the finite element method. CourierCorporation, 2012.
[4] J.W. Beeckman and L.L. Hegedus, Ind. Eng. Chem. Res., 30:969–978, 1991.
[5] E. Tronconi, Catalysis Today, 34:421–427, 1997.

－134－