Title : Carbon deposits in a single catalyst particle as studied by correlated 3D X-ray microscopy and pore network modeling
Abstract:
Catalyst deactivation is one of the major problems in heterogeneous catalysis. Solid catalysts can be deactivated both reversibly and irreversibly by a reduction of accessibility or by destruction/deactivation of the catalyst’s active sites. An example for (typically) reversible deactivation is carbon deposition in the pore structure of a catalyst. Here various carbon species (also called ‘coke’) are deposited in the pore structure of the catalyst and can cover the active sites of the catalyst or block pores causing diffusion limitations for both products and reaction species. Mapping and modeling of the catalyst pore system can help to understand the coking process and its effects by studying the changes in a catalyst’s pore structure and interconnectivity related to carbon deposition. In this context pore network (PN) modeling was shown to be a powerful tool to obtain quantitative morphological and topological information about individual catalyst particles.
In this study, a spent industrial fluid catalytic cracking (FCC) catalyst particle was used as an example for a coked hierarchically complex porous catalyst body. X-ray holotomography on the same catalyst particle before and after coke removal via extensive calcination was used to map changes in the macro-porosity of the catalyst. Next, two pore networks were generated from these two correlated data sets (before and after calcination) to quantify changes of the macropore structure such as pore clogging and a decrease of connectivity caused by carbon deposits. We observed a clear increase of porosity, pore size, total volume, and the number of nodes (branching points of the pore network) being accessible from the particle surface when comparing these pore networks, evidencing the presence of coke in the macro-pore structure of the spent FCC catalyst.
An example of pore narrowing and blockage is displayed in Figure 1 highlighting the effects of carbon deposition in a sub-volume of the catalyst particle. Figure 1d displays how after calcination, i.e. coke removal, the path connecting the yellow and green nodes (blue) is shorter than before calcination (red path), providing direct, visual evidence for the pore clogging effect of carbon deposits.
Key Points:
- By combining three separate state-of-the-art X-ray microscopy measurements we obtained correlated 3-D maps of carbon deposits, macro-pore space, zeolite domains, and poisoning metals present in the catalyst. This correlated information allowed deepening our understanding of the coking process and its deactivating effects during fluid catalytic cracking.
- By correlating the 3-D maps of carbon deposits and metals we were also able to pinpoint the effect of coke promotion by poisoning metals such as Ni and identify regions that are most active in coke formation.
- This developed approach also allowed us to distinguish two types of carbon deposits based on their density and location, i.e. i) surface carbon deposited during the FCC process, which is denser and mainly aromatic in nature, ii) non-surface carbon formed by cracking and dehydrogenation reactions during FCC, which was found to have a lower density.
