Investigating the Trimethylaluminium/Water ALD Process on Mesoporous Silica by In Situ Gravimetric Monitoring

Abstract

A low amount of AlOx was successfully deposited on an unordered, mesoporous SiO2 powder using 1–3 ALD (Atomic Layer Deposition) cycles of trimethylaluminium and water. The process was realized in a self-built ALD setup featuring a microbalanceand a fixed particle bed. The reactor temperature was varied between 75, 120, and 200 °C. The self-limiting nature of the deposition was verified by in situ gravimetric monitoring for all temperatures. The coated material was further analyzed by nitrogen sorption, inductively coupled plasma-optical emission spectroscopy, powder X-ray diffraction, high-resolution transmission electron microscopy, attenuated total reflection Fourier transformed infrared spectroscopy, and elemental analysis. The obtained mass gains correspond to average growth between 0.81–1.10 Å/cycle depending on substrate temperature. In addition, the different mass gains during the half-cycles in combination with the analyzed aluminum content after one, two, and three cycles indicate a change in the preferred surface reaction of the trimethylaluminium molecule from a predominately two-ligand exchange with hydroxyl groups to more single-ligand exchange with increasing cycle number. Nitrogen sorption isotherms demonstrate (1) homogeneously coated mesopores, (2) a decrease in surface area, and (3) a reduction of the pore size. The experiment is successfully repeated in a scale-up using a ten times higher substrate batch size.

Design of an active and stable catalyst for dry reforming of methane via molecular layer deposition

“…The pore sizes measured by NLDFT changed from 6.6 nm to 3.6 nm after 6 ALD cycles. The change in surface area and pore size is in agreement with the detailed ALD study on the growth of alumina layers on mesoporous silica studies by Strempel et al [39] The Al content increased from 0 to 13.7% after 3 ALD cycles and to 19.2% after 6 ALD cycles. This is expected, as the uptake of aluminium will decrease with the number of ALD cycles due to the reduced surface area of the substrate after each cycle.…”

“…The detailed process description and growth characteristics via in-situ mass gain analysis of alumina ALD on high surface area silica gel is explained elsewhere. [39] During Alucone MLD, the first half cycle is similar to alumina ALD.…”

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“…In addition, N 2 sorption measurements show no change the in surface area during ALD. In contrast to ALD on porous material on which the surface area drastically decreases due to the blocking of micropores 29 or even mesopores, 30 no change is expected for V 2 O 5 as it contains no micro-or meso-pores and already has a relatively low surface area. Possible reason for the decrease in gpc is incomplete removal of ligands on phosphorus, which prevent further reaction or the previously reactive surface sites were not regenerated in the second half cycle.…”

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“…A detailed description of the set-up is given elsewhere. 38,39 High surface area SiO 2 powder was filled into a tubular fixed bed reactor made of a quartz tube. The ALD process was carried out under a continuous total gas flow of 100 mL/min at atmospheric pressure and a deposition temperature of 80 °C.…”