Elucidation of metal and support effects during ethanol steam reforming over Ni and Rh based catalysts supported on (CeO₂)-ZrO₂-La₂O₃

Highlights

• ESR studied on Ni and Rh catalysts supported on (CeO₂)-ZrO₂-La₂O₃ mixed oxides.

• Dehydrogenation reactions favoured on Rh catalysts, leading to zero CH₄ production.

• Steam dissociation on ZrO₂ enhances activity and selectivity versus inert supports.

• High oxygen mobility in CeO₂ containing supports promotes water gas shift reaction.

• Rh/CeO₂-ZrO₂-La₂O₃ exhibits the highest performance and lowest carbon deposition.

Abstract

Hydrogen production via steam reforming of biomass derived oxygenates is a promising environmental alternative to the use of fossil fuels. The ethanol steam reforming reaction is investigated over Ni and Rh based catalysts supported on ZrO₂-La₂O₃ and CeO₂-ZrO₂-La₂O₃ mixed oxides, aiming at the elucidation of the role of the metal and the support in the reaction mechanism. Rh versus Ni is shown to be highly active and more selective with no methane production under all conditions studied. CeO₂-ZrO₂-La₂O₃ versus ZrO₂-La₂O₃ is shown to promote efficiently the water gas shift reaction, enhancing hydrogen production substantially. Time on stream studies show that the catalysts on ceria containing supports are highly stable, whereas a gradual deactivation was more evident on the ZrO₂-La₂O₃ supported catalysts. TPO analysis of spent catalysts revealed extremely low amounts of graphitic coke deposited on the Rh catalysts. On Ni, and particularly the ZrO₂-La₂O₃ supported catalyst, larger peaks corresponding to both amorphous and graphitic coke were evident, amounting to higher coke production. The combined effects of metal and support make the catalysts on CeO₂-ZrO₂-La₂O₃ most suitable for the reaction, with Rh/CeO₂-ZrO₂-La₂O₃ showing particularly high activity, selectivity and stability, with minimal CH₄ and CO production resulting at the highest H₂ yield.

Introduction

The increasing energy demand along with the finite supply of fossil fuels and pollution problems, stemming from the extensive use of the latter, have intensified research on alternative renewable energy sources [1]. Hydrogen is a clean energy carrier with high energy density that can be used for the production of electricity in fuel cells or heat. Most of the hydrogen originates currently from fossil fuels, resulting in high CO₂ emissions with well-known negative impacts on the environment [2]. Biomass conversion to hydrogen has the potential to accelerate the latter’s realization as a major, carbon neutral, energy carrier [3]. Out of various liquid sources for hydrogen, bio-ethanol is a sustainable candidate with low toxicity and easy handling [4].

Ethanol steam reforming (ESR) is an endothermic reaction producing CO₂ and H₂, with its reaction pathway shown to strongly depend on the catalyst [[5], [6], [7], [8], [9]] and operating conditions [10,11]. Several parallel reactions take place on the surface of the catalyst along with reforming, resulting in the generation of different by-products, such as acetaldehyde, ethylene, methane, carbon monoxide, acetone and carbon deposits [12]. Dehydrogenation, dehydration, decomposition and polymerization reactions, as presented in various kinetic studies [13], contribute to the formation of these by-products. Due to the complexity of the reaction pathway H₂ production is affected by the operating conditions, with maximum ethanol conversion and H₂ yield being achieved at high temperatures, S/C ratios and contact times [14].

Both transition [[15], [16], [17]] and noble [[18], [19], [20]] metals have been extensively examined as catalysts for ESR reactions, indicating that ethanol activation pathways depend on the metal nature. Rh based catalysts are considered as the most active due to their excellent CC and CH bond scission affinity, high water gas shift (WGS) activity and high resistance towards carbon deposition [21,22]. The reaction pathway over Rh based catalysts is primarily steered by the metal’s oxophilicity leading to the dehydrogenation of ethanol to acetaldehyde, followed by decomposition reactions to yield CH_x and CO species [23]. The CH_x fragments further dehydrogenate on the metal active sites to C species which are most likely to be oxidised to CO_x, thus low CH₄ and high CO_x selectivities are observed [24]. However, the high cost of noble metals has shifted the attention to transition metals such as Ni, with various studies suggesting the latter metal as active for ESR given its effectiveness in catalysing CC and CO bond scissions. Ethanol adsorbed on Ni active sites can dehydrogenate towards acetaldehyde, followed by decomposition reactions for the formation of CH₃ and CO species [[25], [26], [27]]. These methyl groups at lower temperatures desorb as CH₄, with nickel’s methanation activity also contributing to higher methane selectivities. Over all metals, acid sites on the support can promote in parallel ethanol’s dehydration towards ethylene [28], while carbon deposition remains a major issue for the long term stability of Ni catalysts [29,30].

The study of supports based on CeO₂-(ZrO₂) has received particular interest on account of ceria’s redox properties that facilitate the formation of surface and bulk oxygen vacancies, the latter effectively replenished by water from the feed [31]. The well-known oxygen storage capacity (OSC) of CeO₂ and the associated supply of O species to the metal can enhance the oxidation of carbonaceous fragments on the surface of the catalyst and promote the WGS and reforming reactions [32]. Conversion and the H₂ yield are enhanced, while by-products such as CH₄ and coke precursors are largely eliminated [33]. Zirconia supports are of interest independently, due to the oxide’s ability to dissociate water molecules and provide hydroxyl species to the metal [34]. Lastly, among used modifiers, La₂O₃ is known to enhance the stability of Ni catalysts through strong metal–support interactions (SMSI), that lead to an enhancement of the dissociation of water [35] and the mobility of O species in CeO₂ supports [31]. The enhanced catalytic stability during ESR over Ni catalysts with the addition of La₂O₃ has also been reported, attributed to the formation of thin overlayers of La₂O_x on top of Ni particles [36]. Upon reaction with CO₂ the formed lanthanum oxycarbonate reacts with surface carbon cleaning the Ni surface from carbonaceous deposits.

Recently [37], Ni and Rh catalysts supported on ZrO₂-La₂O₃ or CeO₂-ZrO₂-La₂O₃ showed high activity at short contact times in methane and biogas steam reforming at the 400−550 °C range, with the CeO₂-ZrO₂-La₂O₃ supported ones further exhibiting notably stable behaviour at extended 90 h stability tests. The present study reports on the steam reforming of ethanol over these catalysts at a wide range of experimental conditions in a fixed bed reactor, focusing on the investigation of the effect of both the metal and the support on the catalytic activity and selectivity, but also stability and coke formation. The CeO₂-ZrO₂-La₂O₃ supported catalysts are revealed to be particularly stable and active for the reaction, achieving very high H₂ yields at 400 °C. Turnover frequencies over the Ni sample outperform most literature reported values on Ni catalysts, approaching those of the much more expensive noble metal Rh, highlighting the promise of this catalyst for industrial application.