(630c) Nickel-Based Dry Reforming Catalysts Modified By Atomic Layer Deposition: Effect of Reaction Conditions on ALD Films & Measuring Deactivating Catalysts at Isoconversion

The challenges facing heterogeneous catalysts for high temperature hydrocarbon transformations have been extensively documented, including sintering and carbon deposition as deactivation pathways for methane dry reforming (DRM) catalysts. Overcoating supported metal catalysts with an ultrathin oxide layer by Atomic Layer Deposition (ALD) has been shown to dramatically lower rates of both sintering and carbon deposition.[1] Although ALD is an increasingly viable and attractive option to modify high surface area materials,[2] it is known that the as-deposited film often changes phase, morphology or chemical composition as it is heated to high temperatures. This can impact the catalyst activity over time. Therefore, the behaviors of ALD thin films under the harsh conditions required to drive certain catalytic reactions, such as DRM, require further insight.

When investigating rates of heterogeneous catalyst deactivation in a fixed bed, it is common to set a fixed Gas Hourly Space Velocity (GHSV) and measure the change in conversion with time on stream. Although it is well known that the catalyst activity does not map to the measured reaction rate at higher conversions, rates of catalyst deactivation are regularly equated to rates of conversion change in deactivation studies with constant GHSV. Furthermore, rates of catalyst deactivation processes are often dependent upon the surrounding environment, and large changes in conversion as a catalyst deactivates can lead to large changes in the reaction mixture composition, with consequent influence on the rate of catalyst deactivation. It can thus be a challenge to accurately measure or compare rates of catalyst deactivation and obtain scientific insight into such processes.

In this work, a range of Ni-based catalysts were prepared and tested for DRM with and without alumina ALD overcoats. The surface areas of catalysts are lower after overcoating, but increase as the materials are heated and pores are generated in the overcoat. This process is investigated extensively using a commercially available Ni/Al₂O₃ catalyst. We show that, when alumina overcoated nickel catalysts are used for the DRM reaction, the activity of the catalyst slowly increases over long timescales, consistent with reduction of Ni²⁺ in NiAl₂O₄. Additionally, synthesis strategies are developed to target favorable catalyst properties for a given set of reaction conditions.[1]

In addition, a new experimental approach to testing heterogeneous catalysts will be presented, whereby the GHSV is automatically changed with time to obtain a constant conversion. This allows precise changes in catalyst activity to be measured regardless of conversion and the dependence of the deactivation rate on the reaction atmosphere to be easily elucidated. To demonstrate this approach, we show for the Ni-catalyzed DRM reaction that the main source of carbon deposition is CO disproportionation, which does not deactivate the catalyst. We also measure the rate of deactivation at high temperatures, attributed to Ni sintering, and calculate an apparent activation energy of 160 kJ mol^-1 for this process.[4]

[1] J. Lu, B. Fu, M.C. Kung, G. Xiao, J. Elam, H.H. Kung, P.C. Stair, Science, 335 (2012), 1205–1208.

[2] B. J. Oneill, D. H. K. Jackson, J. Lee, C. Canlas, P. C. Stair, C. L. Marshall, J. W. Elam, T. F. Kuech, J. A. Dumesic, and G. W. Huber, ACS Catal., 5, 3 (2015), 1804–1825.

[3] P. Littlewood, E. Weitz, T.J. Marks, P.C. Stair, Ind. Eng. Chem. Res. 58 (2019) 2481–2491 doi: 10.1021/acs.iecr.8b04320

[4] P. Littlewood, S. Liu, E. Weitz, T.J. Marks, P.C. Stair, Catal. Today, (accepted 2019) dio: 10.1016/j.cattod.2019.03.040