Industrial Ni-Based Catalyst Development for Carbon Dioxide Reforming of Methane
Date
2018-07-12
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Type
Thesis
Degree Level
Doctoral
Abstract
This research work was conducted in three main phases. In the first phase, Ni and Co monometallic and Ni-Co bimetallic catalysts were prepared by using both co-precipitation and impregnation methods. Cylindrical and spherical shaped catalysts were made using the Ni-Co bimetallic catalyst with and without the addition of Boehmite as the binding material. The shaped catalysts were comparably as strong as the commercial spherical alumina ones. The catalysts were also stable and active for carbon dioxide reforming of methane (CRM) reaction in the range of 800 to 900 °C. However, the mass transfer could limit the performance of the spherical catalyst for CRM reaction. Study showed that the external mass transfer limitation could be neglected at high superficial velocity. However, the internal mass transfer was still restricting the performance of the spherical catalyst. Temperature gradients within the spherical catalyst radius were negligible. In the second phase, performance of the Ni-Co bimetallic, Ni monometallic, Co monometallic, and Co-Op commercial catalysts in powder form for steam carbon dioxide reforming of methane (SCRM) reaction at 850 °C were evaluated with various biogas feed compositions. Steam content in the feed gas was the key factor affecting the catalytic performance. The Ni-Co bimetallic, Ni monometallic, and Co monometallic catalysts could not handle a biogas feed composition with more than 12, 15, and 6 mol% of steam content, respectively. The Ni-Co bimetallic catalyst was the most stable than the other evaluated catalysts in a certain range of the steam content. The Ni-Co catalyst and biogas feed 5, which contains about 33% of CH4, 21.5 mol% of CO2, 12 mol% of H2O, 3.5 mol% of H2 and 30 mol% of N2, represent the best combination, not only to produce syngas with the desired H2/CO ratio (1.8 to 2) but also to convert more than 70% of CO2 of the biogas feed. In the third phase, to understand the sulfur poisoning mechanism, CRM reaction over Ni and Co catalysts in the presence of SO2 was studied. CH4 and CO2 conversions increased and H2 and CO along with the by-products of H2S, elemental sulfur and water were produced as soon as SO2 was added to the CRM feed. Mg-Al-Ox support plays a key role in the SO2 poisoning period by providing additional active sites for methane dissociation. Ni, Co, and S K-edges XAS (X-ray Absorption Spectroscopy) of the catalysts showed that species such as sulfide (S2-), sulfite (S4+) and sulfate (S6+) could be formed during the SO2 poisoning. Produced H2S during CRM in the presence of SO2, is likely from the reaction between S2- and H+ intermediates on the catalysts’ surface. When CH4 also dissociated on the metallic sites or sulfur-support intermediates, hydrogen-intermediates reacted with sulfide to produce H2S. Also, when CO2 dissociated on the support, the produced O-intermediate could have reacted with SO2 to form sulfite or sulfate. Monometallic and bimetallic sites and catalyst preparation methods have different impacts on the poisoning mechanism. Co-containing catalysts facilitated sulfate formation while Ni-monometallic catalysts facilitated sulfide formation.
Description
Keywords
Ni-based catalysts, Shaped catalyst, CO2 conversion, Dry reforming of methane, Steam reforming of methane, Biogas feed, Effect of SO2 on catalytic performance, In-situ XAS study
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemical and Biological Engineering
Program
Chemical Engineering