Hydrogen production from catalytic steam reforming of bio-oil over nano NixMgyO solid solution

Abstract

Hydrogen production from bio-resource is a promising option. In order to economically and practically derive hydrogen from biomass on a sustainable scale, novel catalysts are needed to be developed with properties of effective and inexpensive. In this study, initial works include the preparation of Ni/MgO catalysts via different methods including co-precipitation, hydrothermal treatment and wet-impregnation. These catalysts formed solid solutions after calcination at 600 ℃. It was found that hydrothermal treatment increased the specific surface area of the catalyst from 49.7 m2/g to 79.8 m2/g. In addition, the total pore volume and t-plot micropore volume of the hydrothermally treated Ni/MgO (NixMgyO-hydro) increased by a great extent. In the 20 h methanol steam reforming tests, NixMgyO solid solutions prepared via different methods were examined for their catalytic performance, stability and resistance to carbon deposition. Amongst all the catalysts tested, the NixMgyO-hydro catalyst exhibited the highest conversion rate of 97.4mol% with no carbon deposition. This was particularly true when the steam-to-carbon ratio (S/C) was 3. When S/C was 1, similarly, the NixMgyO-hydro showed the highest performance and the lowest amount of carbon deposition. Characterizations of the spent NixMgyO-hydro revealed that it had very low portion of highly ordered carbon on its surface. It is attributed to the rapid removal of atomic carbon, which led to the prevention of carbon accumulation and subsequent transformation into highly ordered structure. The carbon removal mechanism was confirmed by CO2-TPD analysis. The strong basic sites on the NixMgyO-hydro surface enhanced the reaction between deposited carbon and adsorbed CO2. In addition, the catalytic activity of NixMgyO-hydro catalyst was compared with the Ni/γ-Al2O3 catalyst and several other commercial catalysts. Its outstanding performance in steam reforming of methanol was further verified. Although the NixMgyO-hydro catalyst showed good performance in the steam reforming of methanol, it was not the case for ethanol reforming. The NixMgyO-hydro catalyst showed low hydrogen yield and serious carbon deposition during ethanol steam reforming. The low hydrogen yield was caused by the suppression of water-gas shift reaction (WGSR) at high temperatures, whilst the carbon fouling was due to the existence of C-C bonds in ethanol and high selective conversion towards ethylene. Therefore, the modification of the NixMgyO-hydro catalyst was carried out to overcome these drawbacks. Various elements, i.e., Ce, La and Co, were as catalytic promoters and individually added to the NixMgyO-hydro catalyst. Most of the modified catalysts exhibited much higher hydrogen yield at 700 ℃ due to the enhancement of WGSR. Some catalysts, such as Ce- and Co-modified catalysts, showed significant increase in hydrogen yields, which were higher than 80mol% after 30 h of reaction. It is worth mentioning that the La-modified catalysts promoted the hydrogen yield to 53mol% even at low temperature condition (500 ℃), whilst it was only 12.5mol% with the unmodified catalyst at the same temperature. The reason for this was due to the lack of suitable acid sites on La surface, which led to the accelerated formation of acetaldehyde. The advantage of acetaldehyde is it could be decomposed at very low temperature. The formation of carbon on Ce- and La-modified catalysts was also suppressed. The Ce element showed outstanding oxygen storage and release capability to improve the gasification of carbon deposition. Similarly, La2O3 would form La2O2CO3 species which could achieve carbon removal by offering CO2. Subsequently, the modified catalysts were tested with acetic acid (HAc) and phenol as feedstock, both of which are the most common-seen compounds in bio-oil. The results of these tests, such as catalytic performance and anti-carbon abilities, were consistent with the findings in ethanol steam reforming. Most of the modified catalysts showed very high hydrogen yields above 80mol%, which were only 61.9mol% and 73.7mol% for the unmodified NiMgO catalyst in the steam reforming of HAc and phenol, respectively. The better resistant abilities of the modified catalysts over carbon deposition were also confirmed in the steam reforming of HAc and phenol. In order to determine the performance of the catalysts in steam reforming of actual bio-oil, all of the modified catalysts were evaluated based on their performance in the reforming of major model compounds of bio-oil. Three hydrothermally treated catalysts, i.e., 1%Ce/NiMgO, 2%La/NiMgO and 2%Co/NiMgO, were selected and tested. All three catalysts showed carbon conversions above 90mol% and hydrogen yield in excess of 70mol% after 100 h test. The amounts of carbon deposition on these catalysts were also within an acceptable range. It can therefore be concluded that the NixMgyO solid solution with proper modification, i.e. addition of suitable promoter, could be developed as a promising catalyst for hydrogen production via the steam reforming of bio-oil.

Date of Award	12 Nov 2016
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Tao Wu (Supervisor), Cheng Heng Pang (Supervisor) & Edward Lester (Supervisor)