Hydrogen production by ethanol steam reforming: Effect of the synthesis parameters on the activity of Ni/TiO2 catalysts

https://doi.org/10.1016/j.ijhydene.2013.12.178Get rights and content

Highlights

  • The activity of Ni/TiO2 catalysts in ethanol steam reforming for H2 production was studied.

  • The availability of metallic Ni is an essential condition to reach high performance.

  • Ni particles stabilization through strong metal-support interactions is also important.

  • In order to satisfy these requirements, the synthesis conditions must be properly tuned.

Abstract

Ethanol steam reforming is an attracting process to produce hydrogen in a sustainable way. In this work the performance of Ni/TiO2 catalysts in ethanol steam reforming was studied. In particular, the effect of the synthesis procedure on the properties of the catalysts and on their activity was deeply investigated. On the basis of the experimental results, it was evidenced that TiO2-supported Ni systems are very sensitive to the synthesis parameters. We found that a proper thermal activation by calcination at 800 °C allows to obtain stable catalysts by means of strong interactions between the active phase and the support, preserving the catalyst from sintering phenomena. Nevertheless, the synthesis conditions must be properly tuned in order to prevent Ni incorporation in scarcely reducible structures which would depress catalytic activity.

Introduction

One of the main challenges for scientists today is to reduce the dependence from fossil fuels. But how can we meet the ever growing world energy demand in a clean and sustainable way? Renewables can be the answer. Hydrogen is the ideal candidate to solve environmental problems. It could be used for generation of electricity in fuel cells and as fuel for transportation, virtually without CO2 emissions, being water the only oxidation product [1]. Hydrogen has great advantages as compared with fossil fuels, because it is a good transportation fuel, has the highest utilization efficiency and can be converted to useful energy forms through several processes [2]. Nowadays 47% of global hydrogen is produced from natural gas, 30% from oil, 19% from coal and the remaining fraction via water electrolysis [3]. Therefore about 96% of hydrogen derives from the conversion of fossil resources, which means the co-production of CO2. New sources and new processes are necessary to produce hydrogen in a sustainable way.

The steam reforming of biofuels is an attracting topic for researchers. Ethanol emerged as a good candidate for hydrogen production because it is a renewable source and the CO2 produced in the steam reforming process is consumed by biomass during its growth [4]. Moreover ethanol is easy to store, handle and transport because of its low volatility and atoxicity [5].

Ethanol steam reforming (ESR) is an endothermic reaction (ΔH2980=+347.4KJmol1) that leads to the formation of H2 and CO2 according to the stoichiometric reaction, Eq. (1):CH3CH2OH+3H2O6H2+2CO2including the water-gas shift of the intermediate CO (ΔH2980=41KJ/mol), which further increases the hydrogen yield (Eq. (2)):CO+H2OCO2+H2

However the overall process is quite complex and several side reactions can take place, such as ethanol dehydrogenation (CH3CH2OH → CH3CHO + H2), dehydration (CH3CH2OH → C2H4 + H2O) and decomposition (CH3CH2OH → CH4 + CO + H2), leading to the formation of acetaldehyde, ethylene and methane respectively. These by-products compete for hydrogen atoms, thus lowering the overall H2 yield [6], [7]. It is well known that both the active phase and the support play a key role in determining the productivity and selectivity of the reaction, because different catalysts induce different pathways for H2 production [5], [8], [9]. Several metal-based catalysts [4], [10], [11], [12] have been proposed for the steam reforming of alcohols. Nickel is particularly attractive because of its high activity and selectivity in breaking C–C bonds and its lower cost if compared with noble metals; it also catalyses the water-gas shift reaction in order to remove adsorbed CO from the surface and the gas phase [13], [14], [15], improving H2 yield and its purification. However, some challenges related to metal sintering and coke deposition are still open; therefore the choice of the support is crucial. The synergism between the support and the metal is fundamental in order to stabilize the active phase and decrease the rate of coke formation [16], [17].

TiO2 is used for its well-known ability to interact with metals [18]. The so-called strong metal-support interaction (SMSI) effect, firstly introduced by Tauster et al. [19] and observed for group VIII metals over reducible support materials, deeply affects the properties of a catalyst [20], [21], [22], [23]. In our case, strong metal-support interaction proved of outstanding importance in the case of zirconia- and silica-supported Ni samples for steam reforming reactions. Indeed, higher Ni dispersion and stronger interaction of the metal with the support allowed to improve carbon balance during testing at low temperature, i.e. decreased catalyst coking [24], [25]. By contrast, as for a TiO2-based sample, we found that the catalyst showed negligible activity because of Ni incorporation in the anatase lattice [26]. We concluded that this could be due to the preparation method, in which the active phase was added to an uncalcined support.

The aim of the present work is to study the effect of the synthesis parameters (i.e. Ni addition on Ti(OH)4 or TiO2; ii. calcination temperature) on the properties of the catalyst, in particular on the Ni-support interaction, and on their activity. The samples were used as catalysts for the ESR reaction for hydrogen production. The reaction was carried out at relatively low temperature, i.e. 500 °C, in order to limit the energy input to the process.

Section snippets

Catalysts preparation

Ti(OH)4 was prepared by a conventional precipitation method [26]. Part of the titanium hydroxide was calcined at 500 or 800 °C. The active phase was added to the support through the incipient wetness impregnation technique using Ni(NO3)2∗6H2O (Sigma–Aldrich, purity ≥98.5%), in the proper concentration in order to obtain a 8 wt% Ni loading.

Four different samples were synthesized. Two samples were prepared by adding Ni on the Ti(OH)4 support (TH) and calcining at 500 °C (THNi500) or 800 °C (TH

Results and discussion

We recently reported that a Ni/TiO2 catalyst prepared by impregnating the non-calcined support was almost completely inactive in glycerol steam reforming [26]. We supposed that all nickel was incorporated in the anatase lattice. This hypothesis is in agreement with previous reports [31]. Two different substitution mechanisms have been suggested to explain the incorporation of Ni2+ in anatase: occupation of nickel ions in octahedral interstitial positions charge-compensated by Ti4+ vacancies or

Conclusions

In this work the activity of Ni/TiO2 catalysts in ethanol steam reforming was studied. Our results highlighted that two parameters in particular affect the catalytic performance of TiO2-supported Ni samples. First of all, nickel reducibility is an essential condition to obtain high performing catalysts, since it is related to the availability of metallic Ni, which is the active phase in ESR. For our samples, Ni incorporation (both in the anatase lattice of the support or in a scarcely reducible

Acknowledgements

The authors are indebted to Dr. Cesare Biffi for collecting activity data. The financial support of Regione Lombardia (project “M4H2 - Materiali innovativi per la produzione di H2 da fonti rinnovabili”), Regione Lombardia – INSTM (RU of Venice and RU of Milan) is gratefully acknowledged. G.C. acknowledges founding under project “Nanomateriali per l’energia” from MiSE-ICE-CRUI.

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