(560hz) Sequential H2-CO Pulse Chemisorption for Estimating in-Situ Ni Dispersion : Application on ALD-Coated Catalyst
Measuring catalytic properties in-situ provides much more valuable information about catalyst deactivation mechanisms than ex-situ measurements. In this work, a binary in-situ H2-CO pulse chemisorption technique was used to study the changes in metallic Ni dispersion. Catalysts coated through Atomic Layer Deposition (ALD) technique are known to provide coking and sintering resistance. A commercial 20% Ni on alumina catalyst was coated with 5,10 and 20 cycles of alumina using ALD process. The uncoated commercial catalyst was then compared with the ALD catalysts for changes in dispersion, which was measured periodically in 40 h of Dry Reforming of Methane (DRM) reaction at 650 °C and 1 atm. Plotting the dispersion for the uncoated and the 5-ALD catalyst shows that the rate of decrease of dispersion is much faster in the uncoated catalyst than for the 5-ALD catalyst. For the 5-ALD catalyst, the dispersion dropped at the rate of ~0.02% per hour. For the uncoated catalyst, it was about 5 times higher at ~0.11% per hour. After 40 h, the uncoated commercial catalyst did not show any chemisorption results whereas the 5-ALD catalyst still showed measurable dispersion, indicating exposed Ni active sites. TEM imaging before and after the reaction provide a more direct measurement of the crystallite size. The average crystallite size for the uncoated commercial catalyst increased from 8.5 nm to 24.5 nm during the 40 h of DRM, indicating sintering phenomenon. Under similar conditions, the 5-ALD catalyst retained the initial crystallite size, and the post-reaction crystallite size was ~8.7 nm. Carbon formation was measured by doing a Temperature-Programmed-Oxidation (TPO) after the 40 h of DRM, and the 5-ALD catalyst showed a lesser rate of coke formation than the uncoated catalyst, which could be due to the lesser crystallite size of the 5-ALD catalyst. The chemisorption technique described herein can be quite helpful for core-shell systems where in-situ chemisorption is to be performed, and this is a much more faster and cheaper alternative to in-situ imaging of catalysts.