Elsevier

Applied Catalysis B: Environmental

Volume 195, 15 October 2016, Pages 16-28
Applied Catalysis B: Environmental

Steam reforming of acetone over Ni- and Co-based catalysts: Effect of the composition of reactants and catalysts on reaction pathways

https://doi.org/10.1016/j.apcatb.2016.04.047Get rights and content

Highlights

  • Co, Ni-based catalysts were oxidized in steam reforming of acetone at low temperatures.

  • Ni catalyst was reduced and active for reforming of acetone in H2 at low temperatures.

  • The reaction pathway was strongly influenced by reduction of the Co-Ni surface.

  • Steam reforming of acetone was not possible at low temperatures over the Ni catalyst.

  • The fragments of acetone activation were hydrogenated to CH4 at low temperatures.

Abstract

The properties of Ni/Co/Co-Ni/MgAl2O4 catalysts in the steam reforming of acetone (SRA) were investigated regarding the metallic composition and nature of catalytic site. The catalysts were characterized by nitrogen physisorption, X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy, and temperature programmed reduction and desorption of acetone. Experimental data revealed that the acetone conversion pathway on the Co, Co-Ni, or Ni catalysts was strongly dependent on the nature of the metal, reaction temperature, and the oxidation state of the metal atoms in nanoparticles surface atoms. Reaction data indicated that the acetone decomposition on reduced metal catalysts at high temperatures (>350 °C) occurred mainly via the Hsingle bondC and Csingle bondCO bonds cleavage, leading to the formation of CO, H2, and C on the metal surface. At low temperatures (200 °C) and in the presence of H2 in the reactor feed, the Ni catalyst catalyzed the hydrogenation of the CO and CHx species formed from acetone activation on the metallic sites, producing CH4. For Co-containing catalysts, at low temperatures (200–350 °C) the metal nanoparticles surface was in a higher oxidation degree and promoted the oxidation of acetone. At high temperatures (>350 °C), the hydrogenation of CHx and CO species to CH4 was determined by the nanoparticle oxidation degree, which decreased in the order Ni > Co-Ni > Co. With increased temperature, the CHx species decomposed to C and H2, instead of being hydrogenated to CH4. The oxidation of C by H2O was favored on Co-containing catalysts. The reaction pathways are discussed based on theoretical data obtained from the literature.

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 Csingle bondH, Csingle bondC, and Osingle bondH 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 Csingle bondC bonds. However, the high cost of noble metals has shifted attention to Ni and Co catalysts, which are also effective in breaking Csingle bondC 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 Csingle bondH and Csingle bondC 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 single bondCHx and single bondCO, leading to the formation of equimolar mixtures of CH4 and CO [39]. At high temperatures, the steam reforming of ethanol occurs via oxidation of single bondCO and decomposition of single bondCHx 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 Csingle bondC 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], Csingle bondH scission is favored relative to Csingle bondC; therefore, the decomposition to C and H2 might restrict the formation of CH4 via hydrogenation of single bondCHx 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 NiOsingle bondNi 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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