Dynamic Effects of Electrified Fast Pulse Heating for Shale Gas Chemistries
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Light olefins and synthesis gas are essential building blocks for the modern chemical industry. They have been produced at an increasing rate and are expected to continue increasing in the near future. The shale gas revolution ensures abundant feedstock to produce these chemicals, but due to their energy intensity and environmental impact, it is urgent to optimize and electrify them for sustainability. This challenging task calls for the development of advanced catalysts, electrified reactors, and novel processes. This dissertation focuses on developing high-performance catalysts and novel dynamic processes for natural gas related chemistries.
The performance of a catalyst is determined by the catalyst structure. Therefore, structural dynamics play a central role in the catalyst activity and stability. I first investigate the structures and active sites of two catalysts for ethane and propane dehydrogenation reactions. A novel earth-abundant-metal (EAM) based Co/SiO2 catalyst was synthesized via strong electrostatic adsorption using the widely available Co(NO3)2 as the precursor. We demonstrate that a simple high-temperature pretreatment (900 °C) in an inert atmosphere enhances the initial activity of the Co/SiO2 catalyst. X-ray absorption near-edge spectroscopy (XANES), temperature-programmed reduction (TPR), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) suggest that highly dispersed Co(II) clusters are more active than Co0 or CoOx nanoparticles. Investigations using Fourier transform infrared (FTIR), isopropanol (IPA) temperature-programmed desorption, and density functional theory (DFT) calculations suggest that high-temperature treatment significantly increases the density of active Lewis acid sites, possibly via surface dehydroxylation of the catalyst. Based on this structural dynamics, we develop a strategy to regenerate the catalyst for multiple cycles.
Secondly, we investigated PtSn/SiO2. We employed in situ/operando techniques, including UV-vis, CO diffuse reflective infrared Fourier transform spectroscopy (CO-DRIFTS), near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), and operando XAS, to elucidate the structural dynamics of PtSn/SiO2 catalysts under reduction and working conditions. Our investigation reveals that the synthesis procedures and the initial catalyst structure strongly influence the interactions between Pt, Sn, and SiO2 support. Exposure to H2 causes a reversible Sn-OH formation observed by modulation excitation spectroscopy (MES). Two catalysts prepared using two methods exhibit comparable surface properties and PDH performance, attributed to the dynamic migration of Sn species and formation of a Pt-rich metal surface under reductive atmospheres.
The second part of the dissertation focuses on developing dynamic processes to intensify the reactions. A programmable Rapid Pulse Joule Heating (RPH) flow reactor is developed, using carbon fiber paper (CFP) as the heating element, where the catalyst is coated. The reactor can reach a temperature ramp rate of ~14000 °C/s, allowing rapid temperature cycling in the reaction zone. Dry reforming of methane (DRM) is first tested in the reactor over a PtNi/SiO2 catalyst. Dynamic electrification can increase syngas productivity and reaction rate. Extensive characterizations suggest pulse heating creates an in situ catalyst regeneration strategy that eliminates coke formation and suppresses sintering and phase segregation, resulting in improved catalyst stability under many conditions.
The RPH reactor is then applied to a more complex reaction network: CO2 hydrogenation, where two competing pathways, reverse water gas shift and methanation reaction, coexist. RPH operation at 1 bar over a Ni/Al2O3 catalyst increases the reaction rate and shifts the product selectivity toward CO over CH4 at low reaction temperatures (<500 °C). Through transient kinetics and operando pulse heating DRIFTS, we propose that the selectivity change is due to the transient coverages of *CO and *H over the Ni surface during temperature pulsing, which facilitates CO desorption and suppresses the deep deoxygenation and hydrogenation to CH4.
The activity of bimetallic catalysts depends on the surface composition, which is prone to change under the steady-state reaction environment. Lastly, we apply the RPH to two bimetallic catalysts, PtNi and CuNi, to understand their structural evolution under dynamic conditions. Characterizations using NAP-XPS, XRD, and TEM suggest slower kinetics of phase segregation under pulse heating compared with steady-state heating in different gas environments, presenting a potential way to retain the bimetallic catalyst structure during reaction.