The effect of hydrocarbon structure on nickel catalyst deactivation in steam reforming of hexane and benzene

Open Access
Author:
Kim, Kyungsoo
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 11, 2013
Committee Members:
  • Yongsheng Chen, Dissertation Advisor
  • Yongsheng Chen, Committee Chair
  • Caroline Elaine Clifford, Committee Member
  • Semih Eser, Committee Member
  • Chunshan Song, Committee Member
  • Turgay Ertekin, Committee Member
  • Adrianus C Van Duin, Special Member
Keywords:
  • steam reforming
  • catalyst
  • deactivation
  • hydrocarbon
  • hexane
  • benzene
  • nickel
  • XANES
Abstract:
Steam reforming of hexane and benzene for hydrogen production has been carried out on Ni and Rh catalysts at 800 oC with and without sulfur to understand the deactivation mechanisms in steam reforming reactions and how hydrocarbon structure affects the processes. Three catalysts were synthesized using incipient wetness impregnation method: 2% and 10%Ni catalysts supported on 20%CeO2-modified Al2O3 (denoted as “2Ni/CeAl” and “10Ni/CeAl”, respectively), and 2%Rh catalyst supported on the same support (denoted as “2Rh/CeAl”). The 10Ni/CeAl catalyst is the focus of the study, and the 2Ni/CeAl and 2Rh/CeAl catalysts were used as control samples. It is found that catalyst deactivates much faster in hexane reforming than in benzene reforming on 10Ni/CeAl with the presence of sulfur poison. Temperature programmed oxidation (TPO) and X-ray absorption near edge structure (XANES) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) are used together to understand the catalyst deactivation mechanisms and the effects of hydrocarbon structure on catalyst deactivation. Sulfur K-edge XANES measured and estimated the sulfur species formed on the catalyst after steam reforming reaction. The sulfur species formed on the catalyst are the same for benzene and hexane reforming, i.e., Ni sulfide and sulfate, which do not correlate with catalyst deactivation once carbon starts to deposit on the catalyst. Formation of metal sulfide is faster in hexane reforming than in benzene reforming. It is likely that that benzene has higher reactivity with Ni, which leads to delayed sulfur poisoning in benzene reforming. TPO measured the amounts of carbon deposits on metal and support due to their different reactivity toward oxidation. Carbon deposition on the catalyst increases with time in both benzene and hexane reforming. More carbon is formed in hexane reforming than in benzene reforming. Carbon deposition locations are quite different as well. In benzene reforming, more carbon is deposited on the metal while in hexane reforming it is on the contrary. Carbon K-edge XANES measured the structure of carbon deposits on the used catalysts and it is found that the carbon deposits have no significant structural difference in benzene and hexane reforming. However, TEM analysis combined with EDX elemental mapping revealed that there is significant structural difference between carbon formed on the Ni and carbon on the support. Results from both reactions support a carbon deposition mechanism, i.e., carbon is formed on the metal first, and then partially dehydrogenated hydrocarbons formed on metal migrate over to the support to form coke on the support. In summary, the difference in catalyst deactivation of steam reforming over Ni catalyst with the presence of sulfur poison for benzene and hexane reforming is well explained by the metal reactivity with different structured hydrocarbons. The hydrocarbons with higher reactivity (such as benzene) protects the catalyst by delaying the formation of Ni sulfide through more competitive absorption on metal, leading to more available Ni sites for steam reforming and gasification. Higher gasification activity allows less carbon deposits on the metal and also less migration of partially dehydrogenated hydrocarbons to the support, which helps maintain steam activation on the support and stable catalyst activity for steam reforming reaction.