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Abstract
AbstractCatalyst deactivation involves a complex interplay of processes taking place at different length and time scales. Understanding this phenomenon is one of the grand challenges in solid catalyst characterization. A process contributing to deactivation is carbon deposition (i. e., coking), which reduces catalyst activity by limiting diffusion and blocking active sites. However, characterizing coke formation and its effects remains challenging as it involves both the organic and inorganic phase of the catalytic process and length scales from the atomic scale to the scale of the catalyst body. Here we present a combination of hard X‐ray imaging techniques able to visualize in 3‐D the distribution, effect and nature of carbon deposits in the macro‐pore space of an entire industrially used catalyst particle. Our findings provide direct evidence for coke promoting effects of metal poisons, pore clogging by coke, and a correlation between carbon nature and its location. These results provide a better understanding of the coking process, its relation to catalyst deactivation and new insights into the efficiency of the industrial scale process of fluid catalytic cracking.
Highlights
Carbon deposits on catalysts are an unwanted side product of any chemical reaction where hydrocarbons react over heterogeneous catalysts
The good agreement with earlier work[38,50,55] confirms that a typical, i. e., representative, aged fluid catalytic cracking (FCC) catalyst particle was investigated and, in agreement with bulk X-ray diffraction (XRD) data and SR-based in-situ SAXS/WAXS/DSC measurements, the comparison of values before and after calcination confirms that no morphological changes other than coke removal took place
We have estimated the total amount of coke in the catalyst particle to 2.37 vol.% (1.68 wt.% assuming the density of graphite), a value that was very close to typically reported amounts (0.7–1.5 wt.%)[5] for commercially used equilibrium catalyst (ECAT), and in excellent agreement with the 1.75 wt.% previously reported for ECAT with high carbon content,[12] confirming validity of our segmentation approach
Summary
With the aim to successfully tackle the challenge of visualization of weak absorbing organic phases by hard X-rays, we present here a unique characterization methodology to reveal, identify, and assess the effects of carbon deposits within single catalyst bodies at the macro-pore scale by using nondestructive, hard X-ray holotomography in differential contrast mode We further combined this method with X-ray fluorescence (XRF) tomography data recorded for the same catalyst particle to reveal spatial correlations between coke deposits and structurally and/or chemically different regions in a commercially used FCC catalyst particle that was used as an archetypical example for a hierarchically complex porous catalyst body. The 3D distribution of metal deposits has been studied at tens of nanometers precision.[50]
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