Elsevier

Catalysis Today

Volume 283, 1 April 2017, Pages 104-109
Catalysis Today

In situ XANES study of Cobalt in Co-Ce-Al catalyst applied to Steam Reforming of Ethanol reaction

https://doi.org/10.1016/j.cattod.2016.02.029Get rights and content

Highlights

  • Co5Ce10Al catalyst is highly active in SRE reaction.

  • XANES showed structural changes from CoAl2O4 to CoO and Co.

  • Ce decrease the thermal stability of CoAl2O4.

Abstract

The effect of ceria in cobalt-ceria-alumina catalyst was studied using in situ X-ray near edge spectroscopy (XANES) at Co K-edge and Ce LIII-edge. The introduction of ceria in this catalyst resulted in a significant removal of Co from CoAl2O4 spinel phase to Co and CoO phases. The Co K-edge revealed the symmetry changes in Co according to the temperature and atmosphere in reduction process. We also showed the stability of Co sites as well the oxidation state of ceria in operando steam reforming of ethanol (SRE) reaction. After reduction of Ce4+ to Ce3+, no significant changes were observed by XANES. Gas Chromatography (GC) analysis showed a high ethanol conversion at 500 °C, high hydrogen yield and low formation of undesired products as methane and ethylene. The results showed a Co/Ce/Al2O3 catalyst as promising material to be applied in hydrogen production in SRE reaction.

Introduction

The growing need of a more rational use of natural resources has drawn high interest in hydrogen (H2) production, especially from renewable sources [1]. The use of H2 on fuel cells in automobiles involves the production of this gas at the time of use, to avoid problems related to its storage and transportation.

The hydrogen production from methanol and hydrocarbons is widely studied in reforming reactions; however, the reforming of these compounds releases CO2, which is harmful to the environment [2], [3]. But hydrogen can also be obtained from bio-ethanol, for example, in a sustainable and clean way [4], through the Steam Reforming of Ethanol (SRE), a very simple catalytic process. This reaction also releases CO2, but the crops that produce bio-ethanol (for SRE), such as sugarcane and corn, require CO2 to grow and develop, balancing the emission of this gas [2], [3], [5].

Steam Reforming of Ethanol consists in reacting ethanol and water, at moderate temperatures, to produce CO2 and H2. Literature reports different mechanisms for SRE, under the influence of different catalysts supported in different materials [2], [3], [4], [5], [6], [7], [8], [9], [10]. The general reaction is represented in Eq. (1) [5].C2H5OH + 3H2O  2CO2 + 6H2

The reaction pathway follows through dehydrogenation of ethanol, acetaldehyde steam reforming reaction and water-gas shift reaction (WGSR). Parallel reactions may occur in Steam Reforming of Ethanol reaction. Decomposition of ethanol produces methane, carbon monoxide and hydrogen; and the dehydration of ethanol produces ethylene, which can lead to coke formation on catalyst surface, causing its deactivation [11].

SRE has been widely studied and a large variety of materials can be used under soft reaction conditions. The great interest on the development of new active catalysts has contributed to understand the mechanisms, to enhance the hydrogen selectivity while the coke formation over the material can be minimized [4].

Choosing the catalysts is essential to SRE, because they may improve the selectivity to desired product and minimize the parallel reactions. Ideally, catalyst must maximize ethanol conversion and hydrogen selectivity, convert the CO produced to CO2 and avoid coke formation over catalyst surface. Many transition metals have been studied as catalyst active phase as Ni, Co, Cu, Fe [12]. For this purpose, the non-noble metals Ni and Co have showed efficient catalytic activity for SRE reaction [5]. The interest in cobalt catalyst has been reinforced thanks to their activity to break Csingle bondC bonds [13], and their low cost, high activity and selectivity to hydrogen [12]. It is then noteworthy that many authors [12], [13], [14] have demonstrated that cobalt is an alternative to nickel due to the effectiveness of this catalyst in the SRE.

Many different supports can be used in SRE, which can play an active hole in the reaction depending on composition, morphology and particle sizes. In particular, ceria has been widely studied as active support in SRE with reports showing that different particles sizes have significant activity, especially at high temperatures. However, the Csingle bondC bond cleavage activity was seen to be low [15], [16], [17]. Also, Song and Ozkan [18] reported on the positive effect of ceria to avoid deactivation by coke, due to the high oxygen mobility in ceria oxide, which improves the catalytic stability. All these interesting properties has highlighted the outstanding catalytic properties of Co/ceria system in SRE [19].

Several techniques have been used to characterize inorganic solids in order to obtain morphological, structural and/or textural information of catalysts [20]. Among other techniques, XAS (X-Ray Absorption Spectroscopy) in the near edge region XANES (X-Ray Absorption Near Edge Structure) provides information of the oxidation state of absorber atom, the spatial arrangement of neighborhood and the density of unoccupied states [21], [22]. Moreover, the high penetration of hard X-rays allows to investigate catalysts under operando conditions [23].

With all that in mind, the goal of this work was to study the structure of Co-supported on ceria-alumina catalysts by XANES during activation under H2 and in SRE reaction. We also aimed at studing the catalytic activity for Steam Reforming of Ethanol reaction, seeking for the optimal conditions to achieve high ethanol conversion, hydrogen selectivity and catalyst stability. We show that the presence of ceria affects the catalyst structure allowing hydrogen production in our catalytic tests under SRE conditions.

Section snippets

Catalyst preparation

The catalyst was prepared by the wet impregnation method. An aqueous solution of CoCl2·6H2O and Ce(NO3)3·6H2O was added to Al2O3 (all Sigma-Aldrich reactants). The mixture was stirred over 1 h and then dried overnight at 100 °C. The solid obtained was calcined at 800 °C over 8 h in air to remove residual components. We defined the initial concentrations to obtain a final solid with molar composition of 5% of CoO, 10% of Ce2O3 and 85% of Al2O3. The sample was named Co5Ce10Al.

Catalytic reaction

Preliminary catalytic

XANES study at Co K-edge

From XANES data at Co K-edge the information about geometry and oxidation state can be attributed by qualitative analysis. Fig. 1 shows the XANES of Co K-edge of some cobalt compounds. The Co from Co5Ce10Al sample is in CoAl2O4 structure, which has spinel geometry where the cobalt ions (Co2+) occupy only tetrahedral (Td) sites while the aluminum ions are in the octahedral (Oh) sites. The other common compound with spinel structure is Co3O4 with two thirds of the cobalt ions occupying Oh sites

Conclusions

We studied the influence of Ce on Co/CeO2/Al2O3 catalyst. The Ce presence contributed to activate the catalyst and even with the presence of CoAl2O4 we obtained a sample with a mixture of compounds which presented high hydrogen selectivity by SRE reaction. The interaction between Co and Ce showed a significant improvement in the reducibility of Co and an important hole to avoid the formation of CoAl2O4 again.

The catalyst evaluation showed high ethanol conversion followed by high hydrogen yield

Acknowledgements

We thank Dr. Daniela Zanchet and Felipe Moreira Pinto for the GC analysis. We also thank the financial support from FAPESP (proc. 2013/18062-2 and 2014/2666-1) and the Brazilian Synchrotron Light Laboratory (LNLS) at CNPEM for the use of XAFS1 beamline installation (internal proc.). Narcizo Souza-Neto is acknowledged for the manuscript revision.

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