(621eh) Insights into Sulfuric Acid Catalyst Surface Using Microscopic Characterization Techniques

Introduction

Heterogeneous
catalysis is of great significance in many process technologies that are used
for gas conversion and processing in chemical industries. Various catalyst
particle shapes shown in Figure 1 are sold commercially by vendors for a wide
range of process technogies.¹

Figure 1. Commercial catalyst shapes and
sizes.

Sulfuric
acid (H₂SO₄) catalyst manufacturers offer a variety of
catalyst shapes for the air oxidation of SO₂ to SO₃ using
adiabatic multi-stage reactor systems. These shapes include solid and hollow
cylinders as well as various those involving hollow multi-lobe ribs, such as
5-lobe and 6-lobed shapes (Figure 2). The choice of one catalyst shape over
another for a given sulfuric acid manufacturing process are typically based on various
objectives, such as maximizing reaction light-off and catalyst activity, minimizing
pressure drop and catalyst attrition, and achieving high dust capacity. 

Figure 2.
Commercial sulfuric acid catalyst 
shapes.

With
the increasing demand for sustainable processes, a need exists for development
of new catalysts having higher activity and improved atom economy. Past
research on catalytic materials has provided valuable insights on catalyst
composition and catalyst structure. 
However, synthesis of next-generation catalysts for sustainable
processes requires a more fundamental understanding of the material multi-scale
structure, such as the nanoscale active sites and catalytic surfaces,
micro-scale to macro-scale pore size distribution, and the 3-D connectivity of
the catalyst pore network. Commercial H₂SO₄ catalysts behave
as supported liquid phase (SLP) catalysts since they consist of a solid porous
structure in which the active components are dissolved in a high boiling point metal
salt that is dispersed throughout the porous matrix. At typical reaction temperatures,
these inorganic metal salts can constrict the pores, which can increase the
intra-particle transport resistances with reduced catalyst activity^2,3,4.  This
observation suggests it is important to obtain a fundamental understanding
between catalyst activity and catalyst structure over various length-scales
ranging from sub-microscopic to the particle-scale.

Several
microscopy techniques have been employed over recent years to study the internal
structure of the catalysts⁵. Some of these techniques include Electron
Tomography, X-ray Micro Computed Tomography, and FIB/SEM Tomography.
Transmission electron microscopes, unlike SEM's, produce flat 2-D images of the
sample and only thin sections of the sample can be viewed. Electron Tomography
is an extension of the TEM in which an electron beam is transmitted through the
center of the sample at incremental degrees of rotation. This produces data
that can be used for 3-D reconstruction of the sample, but is limited to a low
depth perspective. X-ray Micro Computed Tomography is a non-destructive
technique in which virtually cross-sectioned images of the sample can be
produced and reconstructed in 3-D to view the fine internal structure of the
sample. FIB/SEM Tomography is a destructive technique, which involves the ion
milling of the sample surface to obtain micrographs at different
cross-sections. The images of the slices obtained using FIB/SEM can be stacked
to build a volume using image reconstruction software.

Objectives. The main objective of this presentation
is threefold: (1) to create a 3-D pore network model of a commercial H₂SO₄
catalyst a combination of 2-D SEM images along with image processing software; (2)
to use the resulting 3-D pore network as the basis for prediction of intraparticle transport-kinetic effects for SO₂ oxidation
in a porous catalyst under typical process conditions; and (3) to compare the
model predictions for catalyst performance to those from simpler models that
use either a constant macropore porosity or combined micropore-macropore porosities.

Methodology.
A relatively new
3-D electron microscopy technique is the Serial Block Face Scanning Electron
Microscopy (SBF-SEM), which has gained a lot of popularity in the medical and
materials research field in the recent years⁶. In the SBF-SEM
approach, slicing of the samples is carried out using an in-situ ultramicrotome inside the SEM
chamber and micrographs of each section are obtained using the SEM. This
technique produces high-resolution images, which are indispensable to study the
microscopic and submicroscopic structures on heterogeneous catalyst surfaces.

Figure
3 illustrates SEM micrographs of sulfuric acid catalyst containing V₂O₅.
 These micrographs are stacked to build a
volume and construct a 3-D pore network model. The data from 3-D image
reconstruction will be used to simulate transport effects in 2-D and 3-D
catalyst pore network models as shown in the Figure 4.

 SHAPE  \* MERGEFORMAT

Figure 3. SEM of sulfuric
acid catalyst containing V₂O₅. 

Figure 4. 3D multiphysics control volumes
(solid and voids) construction from 2D SEM

                                                                                                               images

Results and Discussion

Figure
5 illustrates results from a case study of numerical modeling of fluid and
electrical currents though geometries based on synchrotron X-ray tomographic
images of reservoir rocks using Avizo and COMSOL⁷.
Similar approach will be followed to develop 3D pore network model for SO₂
oxidation reaction. These are based upon COMSOL Multiphysics for modeling of
transport-kinetic interactions needed to guide the synthesis of improved
catalyst compositions for next-generation sustainable processes. 

                                                                                                    Figure 5a. Boundary conditions for 2D pore scale geometry;  Figure 5b. In-pore
velocity 

                                                                                                                     Profile⁷.

References

1.     
Rase,
H. F. (2000). Handbook of Commercial Catalysts: Heterogeneous Catalysts,
CRC Press.

2.     
Brüsewitz, R. and Hesse, D.
(1992)," Problems in use of supported liquid-phase catalysts in fluidized
bed reactors." Chem. Eng. Technol., 15: 385?389. doi: 10.1002/ceat.270150604

3.     
Zhao,
F., Fujita, S., Arai, M. (2006). "Developments and applications of supported
liquid phase catalysts", Curr. Org.
Chem., 10:1681-1695.

4.     
Chen,
O., Rinker, R. (1978) ?Modelling of diffusion-limited, homogeneous reactions in
supported liquid-phase catalysts?, Chemical Engineering Science, 33:9, 1201-1209.

5.     
Gai,
P. L. (2001). "Developments of electron microscopy methods in the study of
catalysts." Current Opinion in Solid State and Materials Science 5(5):
371-380.

6.     
Kremer,
A., Lippens, S., Bartunkova,
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broad biological context.? Journal of Microscopy. doi: 10.1111/jmi.12211

7.     
M.B.
Bird, S. B. (2014). Numerical modeling of fluid and electrical currents though
geometries based on synchrotron X-ray tomographic images of reservoir rocks
using Avizo and COMSOL . Computers
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