Evaluation of the performance of Ni/La2O3 catalyst prepared from LaNiO3 perovskite-type oxides for the production of hydrogen through steam reforming and oxidative steam reforming of ethanol

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Abstract

This paper studies the performance of LaNiO3 perovskite-type oxide precursor as a catalyst for both steam reforming and oxidative steam reforming of ethanol. According to results of temperature-programmed desorption of adsorbed ethanol and by carrying out diffuse reflectance infrared Fourier transform spectroscopy analyses of ethanol steam reforming, ethanol decomposes to dehydrogenated species like acetaldehyde and acetyl, which at moderate temperatures, convert to acetate by the addition of hydroxyl groups. Demethanation of acetate occurs at higher temperatures, leading to a steady state coverage of carbonate. Catalyst deactivation occurs from the deposition of carbon on the surface of the catalyst. Both thermogravimetric and scanning electron microscopy analyses of postreaction samples indicate that lower reaction temperatures and lower H2O/EtOH ratios favor the deposition of filamentous carbon. However, less carbon formation occurs when the H2O/EtOH ratio is increased. Increasing reaction temperature or including O2 in the feed suppresses filamentous carbon formation.

Graphical abstract

DRIFTS spectra recorded as a function of time on stream under the reaction mixture ethanol + water at 773 K (Figure below) showed that the coverage of carbonate species on the catalyst surface remained constant and could not be responsible for the deactivation observed. Catalyst deactivation was determined to be due to the deposition of carbon.

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Introduction

The development of new processes for the production of chemicals and fuels from biomass requires new catalysts tailored to convert these feedstocks with high conversion rates, selectivities, and stabilities. Hydrogen emerges as an energy carrier with high potential and if produced from renewable sources, may contribute to the sustainable production of energy, as it can be directly converted electrochemically in PEM fuel cells to produce electricity for use in transportation applications and portable power devices. Hydrogen can be produced through the steam reforming of biomass-derived liquids such as bioethanol, a water and ethanol mixture that may be obtained by biomass fermentation [1], [2], [3]. However, there are currently no viable commercial catalysts for bio-ethanol steam reforming.

Different catalysts, including metal oxides [4], [5], [6], [7], mixed metal oxides [8], [9], [10], supported base metals (Ni, Co, Cu) [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] and supported noble metals (Pd, Pt, Rh, Ru, Ir) [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], have been extensively studied for the steam reforming (SR), partial oxidation (POX) and oxidative steam reforming (OSR) of ethanol.

In spite of their lower activity relative to supported metal catalysts, metal oxides are capable of producing hydrogen free of CO as well as carbon deposits, depending on the reaction conditions used [19], [26]. However, a wide range of undesirable by-products (e.g., ethene, acetaldehyde and acetone) is formed during steam reforming of ethanol over metal oxides in comparison with supported metal catalysts, depending on the metal oxide properties.

Supported metal catalysts are more active and selective to hydrogen than metal oxides but they undergo significant losses in activity with time on stream (TOS) [6], [29]. Catalyst deactivation during ethanol conversion reactions may be associated with:

  • metal particle sintering;

  • metal oxidation (mainly for Co- and Ni-based catalysts);

  • carbon deposition, including both filamentous carbon and amorphous carbon covering the metallic particle and the support.

The type of carbon formed and the mechanism of catalyst deactivation depends on the nature of the metal employed.

Both types of carbon are formed on Co- and Ni-based catalysts [8], [10], [13], [31]. Galeti et al. [31] studied the effect of reaction temperature on the performance of CuCoZnAl catalyst during SR. The nature of the carbon deposits formed was evaluated by Raman spectroscopy and temperature-programmed oxidation (TPO) experiments. CuCoZnAl strongly deactivated during SR at 673 and 773 K, whereas the activity remained unchanged at 873 K. Regardless of the reaction temperature used, Raman spectra revealed the presence of two types of carbon: ordered (graphitic) and disordered (amorphous or filamentous) carbon. Scanning electron microscopy (SEM) analysis confirmed the formation of carbon filaments after reaction at 773 K, while amorphous carbon was present at 873 K. They proposed that coke was removed from the surface of the catalyst by the reverse of Boudouard reaction, improving catalyst stability at this high temperature.

For supported noble metal catalysts, filamentous carbon is not formed. In this case, the deactivation mechanism is still a matter of debate. According to Erdõhelyi et al. [21], the catalytic deactivation during steam reforming of ethanol was due to an inhibiting effect caused by surface acetate species. Platon et al. [28] studied the deactivation of Rh/Ce0.8Zr0.2O2 catalysts during low temperature ethanol steam reforming. The results did not reveal any significant metal particle sintering or carbon deposits. However, an important build-up of carbonaceous intermediates was observed, which may be a contributing factor in the deactivation of the catalyst. These carbonaceous intermediates were less stable at higher reaction temperatures. Among different reaction intermediates, acetone and ethene were the main ones suggested to be responsible for catalyst deactivation.

Recently, Lima et al. [25], [26] have studied the performance of Pt/CeZrO2 catalyst during steam reforming of ethanol at 773 K, which significantly deactivated during the reaction. An accumulation of acetate species on the Pt/CeZrO2 catalyst surface was also observed but this seemed to be symptomatic and not the root cause for the deactivation of the Pt/CeZrO2 catalyst. They proposed that the presence of the metal promotes the decomposition of acetate species to CO and CHx, which may be further dehydrogenated to H and C. The CHx species formed may lead to the blockage of the Pt-support interface, deleteriously impacting the acetate turnover rate and thereby leading to a steadily increasing inventory of its steady state coverage, as well as catalyst deactivation. Therefore, a proper balance between the rate of acetate decomposition and the rate of desorption of CHx species is necessary in order to avoid catalyst deactivation.

Suppressing carbon formation during ethanol conversion reactions is therefore a main issue in catalyst development. As deactivation due to carbon deposition has often been generally observed, efforts have been undertaken to develop new catalysts that are more resistant to coke formation. An interesting class of material is the perovskite-type oxide (ABO3); these mixed oxides are able to produce very small metal particles upon reduction [32], [33]. Taking into account the catalyst deactivation mechanism proposed [25], [26], carbon formation could be decreased or inhibited on these highly dispersed metal particles [34], [35]. In addition, perovskites may exhibit structural defects due to deficiencies of cations at the A or B sites or of oxygen anions [36]. The oxygen vacancies are more common and affect directly the catalytic properties of the material. The literature reports that the higher reducibility and oxygen storage/release capacity of perovskite-type oxides catalysts may promote the mechanism of continuous removal of carbonaceous deposits from the active sites in reactions like partial oxidation and the CO2 reforming (i.e., dry reforming) of methane [32], [33], [36]. LaNiO3, a more easily reducible perovskite-type oxide, displayed high activity and good stability during CO2 reforming of methane [37]. However, there are only a few studies about perovskites employed as catalysts [38], [39] for the steam reforming of ethanol.

Urasaki et al. [38] studied the performance of Co and Ni catalyst supported on perovskite-type oxides for the steam reforming of ethanol. The perovskite-type oxides tested (LaAlO3, SrTiO3 and BaTiO3) exhibited low activity and tended to produce C2H4 as the main product. SR was investigated on a series of La0.6Sr0.4Co1−xFexO3 perovskite-type oxides [39]. The activation treatment of the sample significantly affected the activity and selectivity obtained. For the unreduced samples, acetone, ethene and acetaldehyde were the main products observed until 800 K, whereas ethanol was completely converted to syngas only above 900 K. The reduction at 873 K favored ethanol conversion to syngas and acetone production at low temperature (600–900 K). Long-term stability tests were not conducted in this study.

The aim of this work is to study the performance of LaNiO3 perovskite-type oxide precursor, following activation, as a catalyst for both SR and OSR of ethanol reactions. The LaNiO3 perovskite-type oxide catalyst precursor has never been used in the SR or OSR of ethanol for hydrogen production. Therefore, a study of the reaction mechanism was carried out in order to shed light on the mechanistic pathways of ethanol reactions using a combination of reaction testing, temperature programmed desorption (TPD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements taken under suitable reaction conditions. The second goal of this work was to gain insight into the deactivation mechanism by analyzing the nature of carbon formed by scanning electron microscopy (SEM) and the amount of carbon formed by thermogravimetric analysis. The holdup of adsorbed species during reaction was assessed by DRIFTS experiments.

Section snippets

Catalyst preparation

The precursor oxide was prepared by a precipitation method [40]. Aqueous solutions of lanthanum and nickel nitrates (Aldrich) in appropriate quantities to give perovskites with the desired formula were added rapidly to an aqueous solution of sodium carbonate, with vigorous stirring. The resultant mixed precipitate was washed and filtered under vacuum, until free of contaminating ions. The washed sample was dried in air at 333 K for 20 h, crushed and then calcined in two stages, first at 823 K for 3

Catalyst characterization

The BET surface area of the perovskite-type oxide was very low (3.8 m2/g), which is typical of these materials prepared after calcination at high temperature. The XRD pattern of the calcined sample is displayed in Fig. 1a. The diffraction lines at 2θ = 23.2, 32.8, 40.5, 41.2, 47.0, 53.0, 53.6, 58.0, 68.7, 69.7° are characteristic of the LaNiO3 rhombohedral phase (JCPDF 330711) [41], indicating that a perovskite structure was the main phase obtained after calcination.

The TPR profile of LaNiO3 is

Conclusions

Activation of LaNiO3 perovskite-type oxide led to the formation of La2O3-supported Ni0 particles. In TPD of adsorbed ethanol and by carrying out DRIFTS of ethanol steam reforming as a function of temperature, ethanol was suggested to decompose to dehydrogenated species like acetaldehyde and acetyl in the low temperature range, and can further decompose to gas phase byproducts. At moderate temperatures, acetate was suggested to form by the addition of–OH to the dehydrogenated species. At higher

Conflict of interest statement

There is no conflict of interest.

Acknowledgements

This work received financial support of CTENERG/FINEP-01.04.0525.00. CAER acknowledges the Commonwealth of Kentucky for financial support. The authors thank Victor Teixeira da Silva for providing the multi-walled carbon nanotubes. We would like to thank the Material Laboratory from Instituto Militar de Engenharia for the SEM images.

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