Elsevier

Applied Catalysis B: Environmental

Volume 197, 15 November 2016, Pages 198-205
Applied Catalysis B: Environmental

Reactions of ethanol over CeO2 and Ru/CeO2 catalysts

https://doi.org/10.1016/j.apcatb.2016.03.065Get rights and content

Highlights

  • Ethanol reactions are studied over on Ru/CeO2 by TPD and steam reforming.

  • Main reactions during TPD are dehydrogenation to acetaldehyde and dehydration to ethylene.

  • The presence of Ru switched the desorption products to CO, CO2, CH4 and H2.

  • Shift in reaction selectivity between CO2 and CO during TPD, linked to ESR, is due to WGSR.

Abstract

The reaction of ethanol has been investigated on Ru/CeO2 in steady state conditions as well as with temperature programmed desorption (TPD). High resolution transmission electron microscopy (HRTEM) images indicated that the used catalyst contained Ru particles with a mean size of ca. 1.5 nm well dispersed on CeO2 (of about 12–15 nm in size). Surface uptake of ethanol was measured by changing exposure to ethanol followed by TPD. Saturation coverage is found to be between 0.25 and 0.33 of a monolayer for CeO2 that has been prior heated with O2 at 773 K. The main reactions of ethanol on CeO2 during TPD are: re-combinative desorption of ethanol; dehydrogenation to acetaldehyde; and dehydration to ethylene. The dehydration to ethylene occurs mainly in a small temperature window at about 700 K and it is attributed to ethoxides adsorbed on surface-oxygen defects. The presence of Ru considerably modified the reaction of ceria towards ethanol. It has switched the desorption products to CO, CO2, CH4 and H2. These latter products are typical reforming products. Ethanol steam reforming (ESR) conducted on Ru/CeO2 indicated that optimal reaction activity is at about 673 K above which CO2 production declines (together with that of H2) due to reverse water gas shift. This trend was well captured during ethanol TPD where CO2 desorbed about 50 K below than CO on both oxidized and reduced Ru/CeO2 catalysts.

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 Csingle bondC 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 Csingle bondC 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 Csingle bondC bond dissociation. It is to be noted that Rh has a unique role within the 4d transition metals for Csingle bondH bond dissociation reaction [21], a requirement prior to the Csingle bondC 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 Csingle bondC and Csingle bondH 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 Csingle bondC bond making reactions [29], [30]. The addition of Ru (initially in the form of RuO2) is poised to change these reactions and accelerate the Csingle bondC 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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