UNDERSTANDING OF CATALYST DEACTIVATION CAUSED BY SULFUR POISONING AND CARBON DEPOSITION IN STEAM REFORMING OF LIQUID HYDROCARBON FUELS
Open Access
- Author:
- Xie, Chao
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 14, 2011
- Committee Members:
- Chunshan Song, Dissertation Advisor/Co-Advisor
Chunshan Song, Committee Chair/Co-Chair
Yongsheng Chen, Committee Member
Andre Louis Boehman, Committee Member
Michael John Janik, Committee Member - Keywords:
- Hydrogen
reforming
Nickel
Rhodium
Deactivation
Sulfur poisoning
Carbon deposition - Abstract:
- The present work was conducted to develop a better understanding on the catalyst deactivation in steam reforming of sulfur-containing liquid hydrocarbon fuels for hydrogen production. Steam reforming of Norpar13 (a liquid hydrocarbon fuel from Exxon Mobile) without and with sulfur was performed on various metal catalysts (Rh, Ru, Pt, Pd, and Ni) supported on different materials (Al2O3, CeO2, SiO2, MgO, and CeO2-Al2O3). A number of characterization techniques were applied to study the physicochemical properties of these catalysts before and after the reactions. Especially, X-ray absorption near edge structure (XANES) spectroscopy was intensively used to investigate the nature of sulfur and carbon species in the used catalysts to reveal the catalyst deactivation mechanism. Among the tested noble metal catalysts (Rh, Ru, Pt, and Pd), Rh catalyst is the most sulfur tolerant. Al2O3 and CeO2 are much better than SiO2 and MgO as the supports for the Rh catalyst to reform sulfur-containing hydrocarbons. The good sulfur tolerance of Rh/Al2O3 can be attributed to the acidic nature of the Al2O3 support and its small Rh crystallites (1-3 nm) as these characteristics facilitate the formation of electron-deficient Rh particles with high sulfur tolerance. The good catalytic performance of Rh/CeO2 in the presence of sulfur can be ascribed to the promotion effect of CeO2 on carbon gasification, which significantly reduced the carbon deposition on the Rh/CeO2catalyst. Steam reforming of Norpar13 in the absence and presence of sulfur was further carried out over CeO2-Al2O3 supported monometallic Ni and Rh and bimetallic Rh-Ni catalysts at 550 and 800 °C. Both monometallic catalysts rapidly deactivated at 550 °C, and showed poor sulfur tolerance. Although ineffective for the Ni catalyst, increasing the temperature to 800 °C dramatically improved the sulfur tolerance of the Rh catalyst. Sulfur K-edge XANES revealed that metal sulfide and organic sulfide are the dominant sulfur species on the used Ni catalyst, while sulfonate and sulfate predominate on the used Rh catalyst. The superior sulfur tolerance of the Rh/CeO2-Al2O3 catalyst at 800 °C may be associated with its capability in sulfur oxidation. It is very likely that the oxygen-shielded sulfur structure of sulfonate and sulfate can suppress the poisoning impact of sulfur on Rh through inhibiting direct rhodium-sulfur interaction. Although the Rh-Ni catalyst exhibited better sulfur tolerance than the monometallic ones at 550 °C, its catalytic performance was inferior compared with the Rh catalyst in the sulfur-containing reaction at 800 °C probably due to the severe carbon deposition on the bimetallic catalyst. The last part of this work focuses on the influence of sulfur on the carbon deposition in steam reforming of liquid hydrocarbon fuels over CeO2-Al2O3 supported monometallic Ni and Rh catalysts at 800 ºC. Though abundant carbon deposits can accumulate on the pure CeO2-Al2O3 support due to fuel thermal cracking, the metal addition substantially mitigated the carbon deposition in the sulfur-free reaction. The presence of sulfur increased the carbon deposition on both catalysts, which has a much more significant impact for the Ni catalyst. Our results indicate that (I) the presence of sulfur can suppress carbon gasification and promote the formation of graphitic carbon on reforming catalysts, and (II) the Rh catalyst possesses stronger capability to maintain carbon gasification activity than the Ni catalyst in the presence of sulfur.