Abstract
Abstract. This study reports on a sensor concept to measure in situ sulfur poisoning (sulfidation) of refinery catalysts, in this case, of commercial silica pellets loaded with highly dispersed nickel. Catalyst pellets were poisoned in diluted H2S between 100 and 400 °C and the sulfidation of the catalyst was observed. During this process, nickel sulfides are formed on the catalyst according to X-ray diffraction spectra and energy dispersive X-ray spectroscopy data. The sulfidation kinetics was quantitatively described by a shrinking core model. Representative catalyst pellets were electrically contacted, and their impedance was recorded in situ during sulfidation. At the beginning, the particles are highly insulating and behave capacitively. Their conductivity increases by decades during sulfidation. At high temperatures, an almost constant slope in the double-logarithmic representation vs. time can be found. At low temperatures, the conductivity remains constantly low for a long time but changes then rapidly by decades, which is also indicated by the phase that drops from capacitive to ohmic behavior. Since nickel sulfides exhibit a lower conductivity than nickel, the conductivity increase by decades during sulfidation can only be explained by electrically conducting percolation paths that form during sulfidation. They originate from the increased volume of sulfides compared to the pure nickel metal.
Highlights
In industry, heterogeneous catalysts are used to accelerate chemical reactions and to enhance product selectivity
The front moves very slowly into the particle; even after 60 h, only an 0.8 mm thick NiS layer from the particle edge has been formed. These observations can be explained if one assumes that formation and growth of the reaction front during sulfidation at high temperatures depends on the diffusion through the already formed nickel sulfide layer
There, coking and resistance change occur in the same timely dimensions but instead of sulfidation coke is deposited on the surface of the catalyst. This can be measured in almost the same manner with the sulfur sensor at low temperatures (< 150 ◦C)
Summary
Heterogeneous catalysts are used to accelerate chemical reactions and to enhance product selectivity. Sulfur poisoning is typically observed during reforming of heavy gasoline, if the feedstock (low-octane gasoline) has not been deeply desulfurized previously (Oudar, 1980; Bartholomew et al, 1979). Another example is the nickel-catalyzed-steam reforming of hydrocarbons to syngas (Jess and Depner, 1999). Today’s standards in industry are ex situ methods to monitor the sulfur content of a fixed bed or even models that estimate it. They do not allow for an immediate response.
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