الفهرس 
Hydrogen EnergyOnly 14 pages are availabe for public view
 |  | from | 216 |  |  |
| | | from | 216 | | |
Abstract
Highly pure amorphous nanosilica was successively extracted from rice husk and tested as catalyst support for NiO particles during methane decomposition reaction. The catalytic performance of the Ni/SiO2 catalyst was significantly changed by adding Al, Mg, La and Ce oxides to SiO2 support. The TPR results demonstrated that the addition of secondary oxide to SiO2 promoted the NiO dispersion with different extents. The catalytic activity and stability of the catalysts were primarily dependent on the nature of the support. Except Ni/SiO2-CeO2, all Ni-based catalysts exhibited higher activity at the initial period of reaction, thereafter, the catalytic activity and stability decreased with increasing the reaction time. This behavior could be attributed to the decrease in the number of accessible active Ni0 sites due to the deposition of carbon on their surfaces, which could isolate the active sites and/or caused pore plugging, preventing the access of fresh CH4 to the catalyst surface. The experimental results illustrated that the lowest H2 yield and poor stability were obtained over the Ni/SiO2-MgO catalyst. The strong metal-support interaction via formation of MgxNi(1–x)O solid solution was the main reason for the fast depletion of activity and stability of this catalyst.
On the other hand, the Ni/SiO2-CeO2 catalyst exhibited a moderate activity and pronounced stability for catalytic decomposition of methane. The facile reduction of NiO particles together with the higher dispersion were the main reasons for its high catalytic performance. It could thus suggest that the Ni/SiO2-CeO2 catalyst has a great potential for application on the hydrogen production by methane decomposition.
Several types of carbon nano-structured materials were formed on the surface of spent catalysts. Most of the carbon deposited on the Ni/SiO2 catalyst was in the form of graphene sheets, reflecting the aggregation of Ni particles. While, MWCNTs were obtained over all binary oxides- supported Ni catalysts; implying the tight size distribution of Ni particles.
Based on the promising results of Ni/SiO2-CeO2 catalyst, a series of nickel (50 wt %) supported on SiO2-CeO2 of different silica/ceria loadings were prepared by a wet impregnation method. The solid fresh and spent catalysts were analyzed structurally and chemically through XRD, N2- adsorption desorption, surface area, H2-TPR, TGA-DTG, TEM techniques as well as Raman spectroscopy. The XRD results suggested that the dispersion of NiO on the catalyst surface is improved by increasing the ceria content. The TPR results demonstrated that the increase of ceria content in the catalyst structure most probably facilitates the reducibility of NiO species supported on different SiO2-CeO2 supports. The obtained catalytic activity and stability results indicated that all catalysts containing ceria exhibit good catalytic stabilities over all the reaction time and their catalytic activity is enhanced by increasing the time-on-stream. Moreover, the Ni/100%CeO2 catalyst exhibited the highest catalytic activity and stability for hydrogen and MWCNTs production over the whole time-on-stream studied, being thus considered as the best catalyst with the optimal amount of ceria among the other catalyst members of this (SiO2-CeO2) series. The superior activity for this catalyst is mainly related to the formation of a large number of small Ni particles highly dispersed on the surface of ceria presented by pre-reduction of the catalyst for 1 h. On the other hand, the increase of activity of the other ceria containing catalysts by increasing the time-on-stream could confirm that a significant portion of the evolved hydrogen is consumed to complete the reduction of nickel oxide species during the catalytic run. This resulted in the formation of a further number of active Ni0 sites on the surface of the catalysts, leading to better accessibility of CH4 to the Ni metals.
MWCNTs were formed on the surfaces of the over all spent catalysts, while as the growth of carbon nanotubes was not uniform throughout in the diameter. Some nanoparticles were found to locate at the tips or in the bodies of the tubes. The variation in diameter of these CNTs was attributed to the sintering and attrition of the Ni particles under the action of carbon dissolved. In addition, some agglomerates of the Ni-containing particles were observed on the Ni/25%SiO2-75%CeO2, and Ni/75%SiO2-25%CeO2 catalysts, indicating lower dispersion profile of the metal particles used for CNTs deposition.
Based on previous literature data, 40%Ni supported on 100%CeO2 and 50%SiO2-50%CeO2 catalysts promoted by (10 wt %) loading of Cu were prepared by co-impregnation method. The TPR results implied that the reducibility of NiO species in these two supported catalysts has been increased than the unpromoted catalysts.
It was found that the combination of Ni and Cu promoter in 40%Ni-10%Cu/100%CeO2 catalyst is most suitable for the methane decomposition reaction under study. Such combination resulted in maximum catalytic stability, hydrogen and carbon nanotubes yield. However a decreased was observed in the net catalytic activity of 40%Ni-10%Cu/50%SiO2-50%CeO2 catalyst, maintaining the catalytic stability. These findings could be attributed to that, when NiO species deeply engulfed in the CuO lattice are reduced in a big number, the Ni dispersion level is enhanced, leading thus to higher catalytic activity.
The addition of Cu promoter to Ni/100%CeO2 catalyst caused also the formation of Ni-Cu alloy, which separated the active Ni sites from each other. The adsorbed carbon-containing species had thus more difficulty to react with each other on the metal surface, leading the activity of the catalyst to be improved. Lower amounts of encapsulating carbon species that causes blocking the access of methane to the active sites were formed and the catalyst stability was improved. The same reason was valid for the maintaining the catalyst stability by addition of Cu for Ni/50% SiO2-50%CeO2 catalyst without the formation of Ni-Cu alloy, as evidenced by TEM results. The large amount of copper particles blocked the active sites on the catalyst surface leading to non-improved catalytic activity for this catalyst.
Comparing the yield of deposited carbon after the addition of Cu with the corresponding unpromoted catalysts revealed that the nature and properties of the supports and their interaction with metals play a prime important role in the catalytic activity and stability.
Several types of carbon nano-structured materials were formed on the surface of spent catalysts. Carbon nano fibers (CNFs); “Bifurcate carbon filaments” were formed over the 40%Ni-10%Cu/100%CeO2 catalyst, where the Ni-Cu alloy particles responsible for the activity toward methane decomposition were not at the tip of the carbon filaments. CNFs were also obtained over 40%Ni-10%Cu/50%SiO2-50%CeO2 catalyst.
In conclusion of this study, the 40%Ni-10%Cu/100%CeO2 catalyst followed by 50%Ni/100%CeO2 and 50%Ni/50%SiO2-50%CeO2 catalysts are having a great potential for application in hydrogen production, from methane decomposition reaction.