(699b) Effects of Morphology and Site Proximity on Sorption-Enhanced Steam Methane Reforming Using Hybrid Ni-CaO Based Nanofibers

Hydrogen production is an essential component of the refining and petrochemical industries. Hydrogen also represents a critical energy carrier as energy portfolios shift from carbon-based to renewable technology. Currently 95% of the hydrogen in the US is produced via steam-methane reforming (SMR), with the byproducts being CO and CO₂. Sorption enhanced steam-methane reforming (SE-SMR) is an attractive adaptation of SMR technology. SE-SMR can produce a much higher hydrogen purity stream (>85%) at lower operating temperatures (500-600°C) and pressures (1-10 atm) compared to conventional steam reforming. The primary drawbacks of SE-SMR materials are the high temperatures required for sorbent regeneration and loss of sorption capacity and catalytic activity over repeated sorption-regeneration cycles. This work focuses on developing hybrid catalyst-sorbent materials that improve reaction and sorption rates by reducing the size and increasing the dispersion of metal and metal oxide domains.

Site proximity and morphology changes were achieved with nanofibrous metal oxides produced via electrospinning. Calcium oxide, nickel oxide, and magnesium oxide nanofibers were synthesized via electrospinning using calcium acetate, nickel acetate, and magnesium acetate precursors. Electrospinning solutions consisted of the salt precursor and polyvinylpyrrolidone (PVP) dissolved in an ethanol and water mixture. Electrospinning processing was conducted at 12.5-30 kV, 0.25-2 mL/hr extrusion rate, and 3-20 inch separation between the collector plate and the tip of the extrusion needle. Salt-PVP nanofibers were finally calcined at 600°C (2°C/min ramp rate) for 2 hours to decompose PVP and produce oxide nanofibers. MgO-supported Ni catalysts were also synthesized via incipient wetness impregnation of MgO powder with nickel nitrate solutions and calcined at 650°C (5°C/min ramp rate) for 5 hours.

Materials were characterized via scanning electron microscopy (SEM), x-ray diffraction (XRD), and N₂ physisorption. XRD spectra confirmed the formation of the desired metal oxide phases. SEM images indicated nanofiber diameters of approximately 80-200 nm, 65-95 nm, and 100-300 nm for calcined calcium oxide, nickel oxide, and magnesium oxide, respectively. The nanofibrous materials exhibited higher porosity, smaller crystallites, and smaller metal oxide domain sizes than their supported particle analogs.

The reactivity and catalytic activity of the nanofibrous materials were tested using CO₂ thermogravimetry, temperature-programmed reduction, and steam-methane reforming experiments. Calcium oxide nanofibers exhibited higher rates and higher saturation CO₂ capacities (78% vs 31%) compared to bulk calcium oxide powder. The increased rate and conversion of the calcium oxide nanofibers result form their smaller more reactive domains. Temperature-programmed reduction experiments were used to characterize Ni-based materials. Commercial Ni-based reforming catalyst exhibited reduction peaks at 500°C and 720°C, however, nickel oxide nanofibers exhibited a single reduction peak centered at 350°C. The much lower reduction temperature results from smaller NiO domains that are more evenly dispersed throughout the nanofiber compared to the particle analogs. The performance of absorber materials for SE-SMR was tested in a plug flow reactor at 500-650°C, 12-80 kPa of H₂O, 12-80 kPa of CH₄, and 1-5 atm total pressure. Catalyst-sorbent beds comprised of nanofibrous materials operated for longer periods of time and could be regenerated at milder conditions compared to beds consisting of supported Ni-catalysts and bulk CaO powder.

The results of this work indicate a promising opportunity for the application of nanofibrous materials in SE-SMR. Smaller, more reactive metal oxide domains allow for these materials to achieve stoichiometric CO₂ capacities without mass transfer restricted sorption rates. These hybrid catalyst-sorbent materials also retain their maximum capacity and catalytic activity over multiple cycles because they are less susceptible to particle agglomeration and morphology changes during thermal regeneration. These improvements in SE-SMR catalyst-sorbent materials has the potential to reduce costs associated with reforming and potentially lead to economically competitive distributed H₂ generation by (i) replacing expensive alloys required for high temperatures and pressures with less expensive materials of construction, (ii) eliminating water-gas shift reactors and downstream hydrogen purification, (iii) reducing carbon deposition in the reactor, and (iv) reducing the size of heat exchange equipment.