Steam reforming of acetone over Ni- and Co-based catalysts: Effect of the composition of reactants and catalysts on reaction pathways
Graphical abstract
Introduction
The need to reduce greenhouse gases emissions, combined with an increasing demand for energy, has generated substantial interest in developing of alternative routes for energy production, especially from renewable feedstocks [1]. Hydrogen is important for the production of clean fuels by hydroprocessing. It is an effective alternative to fossil fuels and can be used in highly efficient systems such as fuel cells to produce energy in an environmentally friendly fashion [2]. Hydrogen can be produced from the biomass-derivatives gasification process by steam reforming.
The pyrolysis of biomass produces a liquid “bio-oil” composed of oxygenated compounds and H2O. The main light molecules are aldehydes, alcohols, carboxylic acid, cresols, and ketones, here collectively denoted CxHyOz, as well as carbohydrates and lignin [3]. The light fraction, can be directly steam reformed to produce H2, which has received special attention due to the associated environmental benefits [4], [5]. In the steam reforming process, the catalyst assists the cleavage of CH, CC, and OH bonds, with the fragments recombining to produce CO, CO2, and H2. Noble metals such as Rh and Ir are promising catalysts due to their greater ability in breaking CC bonds. However, the high cost of noble metals has shifted attention to Ni and Co catalysts, which are also effective in breaking CC bonds and highly active in steam reforming [6], [7], [8], [9], [10]. Ni-based catalysts have been used in steam reforming of model bio-oil molecules such as acetone and acetic acid.
A series of studies have investigated the influence of independent variables including the reaction temperature, space velocity, and steam-to-carbon molar ratio (S/C) in the H2 yields and carbon deposition [5], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. The performance of a catalyst is intrinsically dependent on the nature of the active metal, the metal particle size, the support, and the promoters [7], [8], [25], [31], [32], [33], [34], [35], [36], [37].
Due the complexity of bio-oil, model molecules have been studied in steam reforming reactions [18], [38]. Ethanol, as an example of a biomass derived light molecule, has been extensively studied in steam reforming [6]. Reforming of bio-oil light fraction compounds involves the cleavage of CH and CC bonds, which, compared to methane, are easier to activate and require lower temperatures. The pathway for reforming these compounds, as proposed for ethanol, is different from the one extensively described for CH4 steam reforming [39]. Previous studies of reforming of ethanol have revealed that the pathway depends on the nature of the metal [6], [40]. On Ni/Al2O3 catalysts at low temperatures (<650 K), the decomposition of ethanol occurs via the acetaldehyde intermediate, which decomposes to species such as CHx and CO, leading to the formation of equimolar mixtures of CH4 and CO [39]. At high temperatures, the steam reforming of ethanol occurs via oxidation of CO and decomposition of CHx to C and H2. The pathway for this reaction on Co/SiO2 is apparently different to that on Ni-based catalysts, where the adsorbed CHx species formed by the cleavage of CC bonds at low temperature are strongly adsorbed, hindering the hydrogenation to CH4 [40].
The mechanism for steam reforming of acetone, another bio-oil component, was proposed using DFT calculations for Co [41] catalysts. Different from the mechanism proposed for ethanol [42], CH scission is favored relative to CC; therefore, the decomposition to C and H2 might restrict the formation of CH4 via hydrogenation of CHx species. Although Co-based catalysts are able to activate H2O for reforming of CxHyOz compounds, producing H2 [43], [44], Co adsorbs O and OH more strongly than Ni. Adsorbed O and OH species on Co have negative free energies [45], indicating greater coverage of these species on Co than on Ni. Thermodynamic analysis of the oxidation and re-reduction of cobalt nanoparticles shows that they are likely to be oxidized in the presence of an H2O-H2 mixture [46]. This indicates a high susceptibility to oxidation for Co, opposed to Ni, which is favored with decreasing particle size [46].
In this work, the influence of the nature of the metal on catalytic properties and the reaction pathway for steam reforming of acetone was evaluated using temperature-resolved characterization of the reaction and the oxidation state of the metal particles in Ni-, Co-Ni-, and Co-based catalysts. Using experimental measurements and theoretical data from the literature, evaluation of the dependence of the pathway for steam reforming of acetone on the oxi-reduction properties of the metal and their influence on selectivity and carbon accumulation with time on stream is made.
Section snippets
Catalyst preparation
The catalysts were prepared by incipient wetness impregnation of Ni(NO3)2·6H2O and Co(NO3)2·6H2O onto a MgAl2O4 support synthesized by the sol-gel method, as previously described by Avila-Neto et al. [47]. The appropriate amounts of the Ni and Co salts were dissolved in ethanol in order to give total metal loadings of 8 wt.%. In the case of the bimetallic sample, the Co and Ni salts were diluted in ethanol to obtain a sample with 4 wt.% of Co and 4 wt.% of Ni. The solution was then added to the
Textural properties and XRD analysis of the MgAl2O4 support and catalysts
The XRD patterns obtained for the previously calcined MgAl2O4 support and the supported Co, Co-Ni, and Ni samples are shown in Fig. 1. As previously reported [44], [47], the diffraction peaks of the support at 31.3, 36.9, 44.9, 59.5, and 65.4° correspond to the spinel MgAl2O4 structure (ICSD collection code 40030). The Co and Co-Ni samples showed diffraction peaks similar to those of the support, suggesting the formation of spinel Co3O4 and/or NiCo2O4 structures with diffraction peaks
Discussion
The parameters that control the activity and selectivity of metal nanoparticles in reforming reactions that involve reactants with oxi-reducing properties have been identified as the nature of the metal and reactants, temperature, and metal particle size [6]. The changes in free energy during the oxidation reaction (metal + O ↔ metal − O) depend on the curvature of the metal particle, reactant composition, and the nature of the metal [55]. Oxidation of the metal is favored by smaller particle size
Conclusions
Co- and Ni-based catalysts were partially oxidized in contact with SRA reaction mixtures (acetone/H2O) at room temperature. The thermodynamic equilibrium for reduction of the oxidized surface strongly depended on temperature, the nature of the metal, and the redox potentials of the reactants. The reduction of NiO indicated the presence of oxygen vacancies on NiONi core shell-like nanoparticles, which were able to cleave H2, with subsequent reduction of NiO. The Ni nanoparticles became reduced
Acknowledgements
The authors are grateful for the financial support provided by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, process 2011-50727-9 and 2013/10858-2), Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq, Project FNI E02/16), the Brazilian Synchrotron Light Laboratory (LNLS) for use of DXAS beamline (proposal XAFS1-16215) and Brazilian Nanotechnology National Laboratory for the use of TEM facilities (proposal TEM-18527). The authors also gratefully acknowledge the
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