Degree

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Document Type

Dissertation

Abstract

Dry reforming of methane (DRM) is a potential industrial solution to greenhouse emissions whereby CH4 and CO2 are reacted on a transition metal catalyst to produce syngas, a feed stock for higher value chemical products. Ni can be used as the catalyst for DRM replacing the high cost and low abundance noble metals (Pt, Pd, Rh). However, the harsh process conditions of DRM (T > 700°C, P of 1-10 bar) readily deactivate Ni through coking and sintering. Ni supported on reducible oxides, specifically CeO2 and CeO2-ZrO2 (CZO), have been widely studied to combat the rapid deactivation during operation. However, CeO2 and CZO enhance the reverse water gas shift reaction by converting the valuable H2 product to water and decreasing the selectivity. In this study, we synthesized Ni-CeO2-ZrO2/Al2O3 (Ni/CZA) catalysts that were coated in an ultra-thin Al2O3 overlayer by atomic layer deposition, with the purpose to reduce deactivation and improve selectivity. A low conversion screening method revealed that the addition of the ALD overlayer improved the DRM rate and lowered the coking rate. In high conversion experiments the ALD coated catalyst had a H2/CO ratio ~1 and improved catalyst lifetime compared to the uncoated Ni/CZA catalyst. Post reactor experiment characterization of the ALD coated catalyst revealed the active site was a Ni with >0 oxidation state and a Ni-Ce-Al mixed oxide was the most likely surface species. A 0.5nm thick overcoat was found to be the optimal thickness.

The effect of doping Ni with Ga and altering the DRM feed conditions on catalyst stability and activity were also tested. Ni-Ga/Al2O3 catalysts were synthesized with Ni:Ga molar ratios from 3:1 to 1:3. The low conversion screening method revealed that high Ga content reduced both DRM and coking rates. The 3:1 Ni:Ga/Al2O3 catalyst was tested in a high conversion reactor revealing an active, selective, and seemingly stable catalyst. However, the post-reaction characterization revealed large amounts of coke formation had occurred and the most likely active state is Ni embedded on carbon filaments. Ni/CZO, Ni:Ga/Al2O3, Ni/Al2O3, and Ni/MgO-Al2O3 were tested in experiments where the CO2:CH4 feed ratio was altered from stoichiometric 1:1 to CO2 rich 1.2:1. All catalysts experienced a change in the DRM and coking rates with an increase in CO2 pressure. Specifically, Ni/CZO and Ni/Al2O3 experienced an increase in DRM rate and a decrease in coking rate. Separately, CO or H2O were co-fed with a stoichiometric DRM feed over the Ni/CZO catalyst and no product inhibition by CO or H2O was observed. The CO2 rate order was shown to be ~1 for all catalysts.

Date

3-24-2025

Committee Chair

Dooley, Kerry

DOI

10.31390/gradschool_dissertations.6780

Download

Share

COinS