Reactions of ethanol over CeO2 and Ru/CeO2 catalysts
Graphical abstract
Introduction
Hydrogen generation from renewables and its use as a clean fuel is attractive since upon oxidation only water is formed while the released energy is the highest known per unit weight of a chemical compound (120.7 kJ/g). Because low molecular weight oxygenates can be stored and distributed readily they are used as hydrogen carriers for on-board generation. Methanol and ethanol, in particular, are promising liquid feeds for onboard hydrogen production due to their high hydrogen to carbon (H/C) atomic ratio. Steam reforming of oxygenated hydrocarbons for the production of hydrogen is thermodynamically more favored at relatively low temperatures compared with that of hydrocarbons [1]. A major advantage of ethanol production from biomass and its use for the energy generation is the absence of carbon emissions because carbohydrates are initially formed by photosynthesis. Producing hydrogen from the catalytic decomposition of ethanol has been investigated for almost two decades now. The two main reactions are partial oxidation (Reaction A), and steam reforming (Reaction B).CH3CH2OH + 3/2O2 → 3H2 + 2CO2 ΔH = −553 kJmol−1 (A)CH3CH2OH + 3H2O → 6H2 + 2CO2 ΔH = +174 kJmol−1 (B)
Reaction B, despite being energetically more demanding than Reaction A, yields more hydrogen and can be achieved with high selectivity. The reaction requires a catalyst with three main properties. First, it should be capable of breaking the CC bond of the adsorbed ethoxide (formed upon dissociative adsorption on the catalysts surface). Second, it needs to be active for water gas shift (WGS) and reforming reactions. Third, it needs to have fast hydrogen–hydrogen bond generation kinetics [2]. Therefore, a combination of a reducible metal oxide and metals capable of both CC bond dissociation and water gas shift reaction is needed.
CeO2 is one of the most active redox binary metal oxides [3] known. Part of the reasons is the low activation energy for O anions diffusion [4] and the relatively low oxygen vacancy formation energy [5]. In addition to its use in automobile catalytic converters [6], and as a WGSR support [7] it was also, for the same reasons, found to be an excellent support for reforming of ethanol. Our groups have previously studied the reactions of ethanol oxidation and steam reforming by infrared (IR) spectroscopy, and temperature programmed desorption (TPD), over CeO2 [8], Pd/CeO2 [8], Pt/CeO2 [9], Rh/CeO2 [10], Pt-Rh/CeO2 [11], Pd-Rh/CeO2 [12], Au/CeO2 [13], Au-Rh/CeO2 [14] and Rh/CeO2-ZrO2 [15]. The most active of these catalysts was the Rh-Pd/CeO2 [16]. Many other researchers have also worked on this reaction and a large number of catalysts based on other metals and oxides have been synthesized and tested [17], [18], [19], [20]. While the use of non-noble metals is possible, noble metals, in particular Pt and Pd are unique, as they do not form coke and therefore are not easily poised to deactivation. In brief, a metal such as Rh is needed for efficient CC bond dissociation. It is to be noted that Rh has a unique role within the 4d transition metals for CH bond dissociation reaction [21], a requirement prior to the CC bond dissociation reaction, and Pt (or Pd) is needed for fast hydrogen–hydrogen recombination reactions, while CeO2 provides the active support for the redox process. Ru is akin to Rh in many properties (such as for CC and CH bond braking reactions). Its introduction onto CeO2 and (CeZr)O2 has shown benefits for hydrogen production from glycerol [22]. Ru based catalysts have been studied previously for steam reforming of ethanol [23], oxidative steam reforming [24], CO2 activation [25], and ammonia synthesis [26]. Previously we have synthesized and conducted preliminary studies on Ru-Pt/CeO2 on the water gas shift reaction [27] and found it be of high activity.
In this work, we limit our attention on CeO2 and Ru/CeO2 in order to determine the reaction mechanisms of ethanol where investigation of elementary steps can be extracted. Among the many reactions that ethanol can have on the surface of CeO2 are dehydrogenation to acetaldehyde [28], dehydration to ethylene, oxidation to acetates, condensation to higher aldehydes, and other CC bond making reactions [29], [30]. The addition of Ru (initially in the form of RuO2) is poised to change these reactions and accelerate the CC bond dissociation reaction resulting in WGS and reforming reactions. We also attempt to link TPD results to catalytic performances in order to probe into the reforming reaction.
Section snippets
Catalyst preparation
Cerium oxide (CeO2) was prepared via precipitation from a solution of white crystalline cerous nitrate, (Ce(NO3)3·6H2O) (100 g), in deionised water (0.40 L) with mild stirring. The temperature was kept constant at 373 K (pH 8-8.5) and ammonia (0.91 mol L−1) was added drop wise (ca. 30 mL). The resulting white precipitate was collected by filtration, washed with deionised water and left to dry in an oven for 12 h (373 K) before being calcined (773 K) for 4 h under air-flow. Ruthenium (III) chloride (0.5008
TPD results of oxidised and reduced CeO2
Fig. 1 presents TPD results following surface saturation with ethanol of oxidized ceria (CeO2) at 300 K. Two main desorption regions are present, one at low temperature ∼475 K and the other at high temperature ∼ 640–650 K. In addition, minor desorption features appear at ∼560 K as shown in Fig. 1B. Fig. 1A shows the main desorption products; ethanol (CH3CH2OH, m/z = 31, 29, 27, 43), acetaldehyde (CH3CHO, m/z = 29, 43), and ethylene (m/z = 27, 28). The major desorption species is unreacted ethanol
Conclusions
Temperature programmed desorption of ethanol was compared over CeO2 and Ru/CeO2 and complemented by ethanol steam reforming catalytic experiments. Ru metal particles are well dispersed on CeO2 with a mean particle size of 1.5 nm. Three main conclusions can be extracted from this study. First, the reduced CeO2 surface is more active for both dehydrogenation and dehydration reactions of ethanol than the non-reduced CeO2. Second, the addition of Ru considerably shifted the reaction from these
Acknowledgments
J.L. is Serra Húnter Fellow and is grateful to ICREA Academia program and MINECO grant ENE2015-63969.
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