Search

In hydrocarbon reforming processes, coke formation on the catalyst usually reduces reaction rates. We show that when subjected to thermal treatment, a commercial nickel catalyst, overcoated with alumina, ALD exhibited both higher activity per g Ni and higher carbon formation rates than an uncoated catalyst. During the temperature‐programmed reaction in a CH4+CO2 atmosphere, the uncoated catalyst deactivated rapidly from carbon buildup, but the overcoated catalyst displayed an increase in catalytic activity per g Ni, despite generating two times the surface carbon. The unexpected phenomenon was investigated via TEM/EDS, TGA/DSC, SEM, XRD and Raman spectroscopy. We hypothesize that this may be due to (a) formation of a thicker than expected 'quasi‐ALD' overcoat of amorphous alumina, (b) crystallization of ALD overcoat into nanofibers that act as secondary supports for migrating Ni, and (c) the ability of ALD overcoat to isolate carbon as carbon nano‐onions (CNOs). [ABSTRACT FROM AUTHOR]

• Ni/Al 2 O 3 stability for CH 4 dry reforming (DRM) higher with alumina ALD overcoats. • Rates of Ni sintering and C deposition lower on overcoated than uncoated catalyst. • Nickel aluminate phase unstable under DRM conditions used. • Slow reduction of NiAl 2 O 4 cause of gradual catalyst activity increase. • Proposed to stabilise alumina overcoated Ni based catalysts by avoiding NiAl 2 O 4. A catalyst consisting of 2 wt% Ni supported on a commercially available transition alumina is modified using TMA-H 2 O ALD cycles to deposit thin alumina overcoats on the catalyst, and it is then investigated for the catalytic dry reforming of methane (DRM) at 700 °C. Highly dispersed Ni rapidly sinters to form bulk Ni particles on the uncoated catalyst. In contrast, deposition of an alumina overcoat by ALD significantly lowers the rate of Ni sintering, and also lowers the propensity towards carbon deposition during DRM. Additionally, it is experimentally demonstrated that the Ni aluminate (NiAl 2 O 4) spinel phase is unstable under the present DRM conditions and slowly undergoes reduction to metallic Ni and Al 2 O 3. Slow reduction of the Ni2+ from NiAl 2 O 4 is proposed as the origin of the large increase in DRM activity observed for the alumina-overcoated Ni catalysts. [ABSTRACT FROM AUTHOR]

Atomic layer deposition, deactivation, carbon nano onions, alumina nanofibers, DRM Keywords: Atomic layer deposition; deactivation; carbon nano onions; alumina nanofibers; DRM EN Atomic layer deposition deactivation carbon nano onions alumina nanofibers DRM 2959 2959 1 07/21/21 20210707 NES 210707 B The Front Cover b shows an alumina-overcoated Ni/Al SB 2 sb O SB 3 sb catalyst undergoing disintegration during dry methane reforming due to carbon buildup under the overcoat. [Extracted from the article]

This study investigates changes in dispersion with time-on-stream of a Ni catalyst coated with alumina by Atomic Layer Deposition (ALD) in Dry Reforming of Methane (DRM) conditions. A 20 wt% commercial Ni catalyst is coated with 5, 10, and 20 ALD cycles and tested for DRM at 650 °C, 1 atm for 40 h. Using an in-situ H 2 –CO pulse chemisorption technique, it is found that the rate of decline in catalyst dispersion is more rapid in the uncoated catalyst (~0.11%h−1) than 5-ALD catalyst (~0.025%h−1). TEM images before and after reaction show that the average particle size for the uncoated catalyst increases from 8.5 nm to 24.5 nm, indicating sintering, whereas the 5-ALD catalyst retained the initial particle size. The reduced particle size also explains the 50% reduction in carbon formation-rate in the 5-ALD catalyst. The developed sequential H 2 –CO chemisorption technique reliably measures in-situ dispersion in uncoated and ALD coated catalysts with exposed active sites. Image 1 • A dual H 2 –CO chemisorption method for in-situ estimation of catalyst metal dispersion. • Rate of dispersion loss much lower in ALD-coated Ni catalysts than uncoated catalyst. • Chemisorption results contrasted with TEM. • Unusual CO adsorption and H 2 desorption patterns for ALD catalysts. [ABSTRACT FROM AUTHOR]