The aim of this project, that is an extension of the last year project, is the development of active, selective and stable catalysts for syngas (H2+CO) production by tri-reforming of methane or ethanol (CH4-TR or C2H5OH-TR).
The CH4-TR process consists of a combination of the endothermic steam reforming (CH4-SR) and the dry reforming (CH4-DR) reactions with the exothermic partial oxidation of methane (CH4-CPO). It results energetically efficient, gives less coke deposition and uses greenhouse CO2. On the other hand, the employment of biomass as energy source is gaining great importance, because it contains high amount of oxygenates, in particular C2H5OH, that should be used for the production of H2. However, the reforming processes lead to catalyst deactivation by coke deposition. The major goal is to prevent the formation of coke on the catalyst surface.
The new perspective in the field of syngas production deals with the adoption of new Ni-based catalysts supported on ZrO2 effective for the TR process, obtained with the addition of small amount of noble metal Rh promoter. Ni/ZrO2 catalysts will be prepared with different methods at various Ni (0.5-10 wt%) and Rh loading (Rh, 0.05-0.5 wt%). These catalysts will be studied in terms of structure, morphology and catalytic performances. The expected role of Rh on the catalyst activity is to prevent carbon deposition by delaying carbon mobility and nucleation process. The effect of Rh on Ni particle sizes and properties will be investigated. Materials characterization will be performed with Atomic Absorption, XRD, nitrogen adsorption/desorption, FESEM, UV-vis, XPS, TPR, Raman, and operando and in situ FTIR. The catalytic activity will be studied in a flow apparatus changing feed ratio, reaction temperature, contact time and catalyst activation treatment. Results will be analysed in order to elucidate relationships between composition, structure and catalytic performances of materials for the TR reaction.
H2 and syngas are historically produced from fossil fuels and the exploitation of biomass-derived streams causes novel challenges. The increasing global demand for more environmentally friendly processes and materials is a motivation to look for innovative conversion technologies. Among the various technologies, one of the most investigated is the catalytic conversion of methane to syngas (CO+H2) as building block to produce chemicals and liquid fuels. Methane tri-reforming (CH4-TR) is a new important route for syngas production that combines the utilization of greenhouse gases as CH4 and CO2, in the presence of water steam and oxygen, therefore allowing the direct use of tail gases emitted from fossil fuel-fired power plants [22]. As CO2 is considered the main contributor to greenhouse effects, one challenge of the CH4-TR process is to decrease gaseous CO2 emission from industrial plants.
Additionally, there is growing interest in the syngas production from biomass as a potential source of renewable energy. Among biomolecules derived from biomass, ethanol is of particular interest because of its low toxicity, easily production from biomass waste, high hydrogen content, and ease of handling [12]. Therefore, it can be used as alternative to methane in the syngas production. The new perspective brought to the topic deals with the adoption for industrial applications of new catalytic systems for this promising process by using as feedstock natural gas (CH4) or C2H5OH from biomass (bio-ethanol).
Ni-based catalysts are established systems for all methane reforming reactions, economically preferable to noble metal based catalysts due to the high cost and low abundance of the noble metals. Up to now Ni catalysts, however, are susceptible to coking or sintering of the active metal particles [16-20] with ensuing loss of activity and selectivity. Efficient Ni catalysts for the CH4 reforming reaction require control over the electronic and structural properties of the nickel nanoparticles, which could be achieved by the careful selection of the support and synthesis procedure. In this context, the use of the support ZrO2, whose properties were extensively investigated in various project by our group, gives the opportunity to easily direct the preparation of Ni or Ni,Rh-supported catalysts.
Therefore, this research project is focused on the engineering and testing of nanostructured and long lasting Ni-based catalysts supported on ZrO2 modified by the addition of noble-metal promoter (Rh). We expected that, since carbon formation and decomposition are structure-sensitive, the proximity between Rh and Ni particles favours the gasification of C adsorbed species and, thanks to the H-spillover effect due to Rh atoms, maintains Ni in reduced state. This fact should result in the improvement of activity and selectivity of the tri-reforming process.
The achievement of the goal is supported by the knowledge that our group developed on Ni/ZrO2 and Rh/ZrO2 systems. During the last two projects on Ni/ZrO2 system, we tuned the preparation method to obtain catalysts with high metal dispersion due to strong Ni-support interaction and good catalytic performances [32]. We have obtained similar results on Rh/ZrO2 applied for CH4-CPO [15], where high metal dispersion were obtained. As it is well known that metal dispersion is one of the key aspect directing catalytic behavior, we will try to apply our background on the preparation methodologies to obtain well dispersed bimetallic Ni,Rh/ZrO2 catalysts. Our goal is to clarify which properties of Ni metal particles were affected by Rh addition and exerted a key role for the catalytic behavior.
In turning feasible the tri-reforming technology, specifically that of oxygenate hydrocarbons, another challenge persists in understanding the reaction mechanisms. It is reported that the decomposition of oxygenates might produce CH4. In particular, depending on active metal in the catalyst, at low temperature ethanol undergoes oxidative dehydrogenation to acetaldehyde or decomposition to CH4 and CO, the selectivity being determined by the conversion of the intermediate acetaldehyde [34]. At higher temperatures the produced CH4 should become the reactant for subsequent reforming, yielding syngas. The final outlet gas composition will depend on the predominance of side reactions like reverse water gas shift or Boudouard equilibrium. Therefore, we are searching for an innovative catalysts that combine a good activity for ethanol decomposition to CH4 and a high activity for CH4 reforming reactions (steam-, dry-, oxidative- or tri-): this could be a promising way for the valorization of biomass.
[34] B. Zhang, X.Tang, Y. Li, W. Cai, Y. Xu, W. Shen, Catal. Comm. 4 (2006) 367-372