NANO-Tailoring Nickel Catalyst Surfaces for CO2 reforming of Methane

Natural gas reforming, while of critical importance to the gas processing industry has several drawbacks not least of which is the high energy requirement, contributing significantly to a plant’s CO₂ footprint. The case of CO₂ or Dry Reforming of Methane (DRM) is therefore of particular interest because of its potential as a viable route to combined CH₄ and CO₂ (Green House Gases) utilization. Commercialization efforts, however, have met with little success, partly owing to the propensity of conventional nickel-based dry reforming catalysts to deactivate rapidly via carbon deposition or ‘coking’, and sintering. Mitigating these issues has been the objective of several investigations into synthesizing novel yet inexpensive catalysts over the years. One of the more recent approaches is the use of Atomic Layer Deposition (ALD) to deposit nanometer-thick overlayers of either Alumina, Titania or Silica on Nickel oxide with the objective of anchoring down the Nickel whilst retarding the deposition of carbon. Previous studies [1,2] have demonstrated this effect on several catalytic systems, including unsupported nickel oxide nanoparticles.

This work investigates the potency of increasing ALD layers on high-loading, commercial Nickel catalyst activity, and resistance to deactivation. The 20% wt. (nominal) technical catalyst is ALD coated with 1, 5, 10 and 20 cycles of Tri-Methyl Aluminum-H₂O pulses in a static-bed coater unit. Catalyst deactivation is induced by treating the uncoated and coated catalysts in a 1:1:8 CH₄:CO₂:He environment at 650°C, a temperature at which both coking and sintering contribute comparably. Dispersion as a function of available Nickel sites is monitored over the course of the experiment via a combined CO-H₂ chemisorption technique. Uncoated catalyst showed a steep decline in dispersion over 40h of DRM from 4% to less than 1%. After 40h TOS, no nickel active sites were detected by our chemisorption technique. However, of the coated catalysts, the 5 cycle ALD catalyst could maintain a stable, albeit low dispersion (~1%) even after 40 hours on stream. Although the rates of carbon deposition per gram catalyst from Temperature Programmed Oxidation is comparable with uncoated commercial catalyst, the sustained activity of the ALD catalysts hint at the nature and location of carbon deposits themselves not being similar. High-Resolution Transmission Electron Microscopy (HR-TEM) of the spent catalysts highlight these differences, especially the larger particles sizes for commercial catalyst. The effect of regeneration on a 20 cycle ALD coated catalyst during a 500 hour long time on stream run reveals the possibility of fine-tuning the porous ALD overcoat to allow controlled access of reactants to the underlying Nickel. These findings lends credence to the use of ALD as a relatively inexpensive, scalable method of protecting conventional catalysts to withstand the harsh, deactivating conditions of Natural gas reforming.

Keywords: Atomic layer Deposition; CO₂; Methane reforming

References

[1] Baktash, E., Littlewood, P., Schomäcker, R., Thomas, A., & Stair, P. C., Applied Catalysis B: Environmental (2015) 122

[2] O’Neill, B. J., Jackson, D. H. K., Lee, J., Canlas, C., Stair, P. C., Marshall, C. L., Elam, J. W., Kuech, T. F., Dumesic, J. A., Huber, G. W., ACS Catalysis (2015) 1804