New approaches to improving catalyst stability over Pt/ceria during ethanol steam reforming: Sn addition and CO2 co-feeding

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Abstract

To promote long-term stability of Pt/CeO2 catalyst for ethanol steam reforming, two approaches were examined. Sn was added to Pt to suppress carbon formation. Although the catalyst with high Sn content exhibited improved stability, acetaldehyde selectivity was prohibitive. DRIFTS experiments revealed that Sn inhibited the ability of Pt to facilitate steam-assisted forward acetate decomposition reaction to carbonate, the precursor to CO2 formation. However, CO2 co-feeding was more effective, not only in promoting long-term catalyst stability, but also in maintaining high H2 selectivity. DRIFTS experiments indicate that the kinetic influence of CO2 can be explained as a competition with ethanol for adsorption sites, leading to a suppression in the rate of formation of coke precursors.

Introduction

Hydrogen production through steam reforming of ethanol (SR) is an attractive technology since ethanol is a renewable raw material obtained from fermentation of various agricultural products (e.g., sugar cane) [1], [2], [3]. Although SR has been extensively studied in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9], the development of suitable catalysts is still a significant challenge from the standpoint of catalyst deactivation.

Ethanol reactions on transition metal surfaces comprise a complex system, including several reaction intermediates [10], [11]. Several reaction pathways may occur, depending on reaction conditions and choice of the catalyst, which in turn lead to the formation of different by-products like CO, acetaldehyde, acetone, ethene, and methane [2], [3]. Some of these reactions may lead to coke formation, which can induce catalyst deactivation. The main reactions that may contribute to coke formation are: (i) ethanol dehydration to ethylene, followed by polymerization to coke (reactions (1), (2)); (ii) the Boudouard reaction (reaction (3)); (iii) the reverse of carbon gasification (reaction (4)) and (iv) CH4 decomposition (reaction (5)), as detailed below:C2H5OH  C2H4 + H2OC2H4  coke2CO  CO2 + CCO + H2  H2O + CCH4  C + 2H2

The extent of each reaction depends on the reaction conditions and the nature of the metal and the support. While low reaction temperature favors the formation of carbon through reactions (3), (4) carbon formation via reaction (5) is the main route at high temperatures.

The formation of carbon is likely the main issue in applying ethanol conversion reactions to produce H2 for PEM fuel cell applications. Regardless of the metal and the support used, significant deactivation during SR was reported in the open literature over Pt [8], [12], [13], Pd [8], [14], Rh [8], [9], [15], [16], Ru [8], [17], Ir [8], [18], Co [19], [20], [21], [22], [23], and Ni [7], [24], [25], [26] based catalysts. The types of carbonaceous deposits formed depend on the nature of the metal selected. On Ni- or Co-based catalysts, carbon formed during reaction diffuses through the Ni or Co crystallite, nucleating carbon filaments which grow behind the metal particle, lifting it from the support and altering the catalyst structure [22], [26], [27]. However, this, carbon formation does not necessarily induce catalyst deactivation.

On noble metal-based catalysts, carbon diffusion does not take place and thus carbon may encapsulate the metal particle or cover the support. Roh et al. [15] studied the deactivation and regeneration of Rh/CeZrO2 catalyst during SR and proposed that catalyst deactivation was due to carbonaceous deposition. The catalyst could be completely regenerated and recover its initial activity after treatment under O2/He mixture above 473 K. Platon et al. [16] suggested that a significant buildup of reaction intermediates over Rh/CeZrO2 during SR, leads to catalyst deactivation. Erdohelyi et al. [8] proposed that the deactivation of supported noble metal catalysts was caused by the accumulation of acetate-like species over the support, which was suggested to inhibit the migration of ethoxy species from the support to the metal particles and thus, its decomposition. Recently, we have studied the deactivation mechanism of Pt/CeZrO2 during SR [12]. The catalyst was found to deactivate at all of the reaction conditions studied. According to the reaction mechanism proposed, ethanol adsorbs dissociatively as ethoxy species, followed by dehydrogenation to acetaldehyde and acetyl species. The dehydrogenated species may oxidize to acetate species, via the addition of support-bound hydroxyl groups. The decomposition reactions of dehydrogenated and acetate species are promoted by the metal–support interface. The unbalance between the rate of the decomposition reaction and the rate of desorption of CHx species as CH4 leads to the accumulation of carbon deposits and obstruction of the Pt–support interface, resulting in catalyst deactivation. The loss of the Pt–support interface results in an increasing steady-state coverage of acetate species with respect to time on stream (TOS).

The nature of the support may also strongly affect the product distribution and catalyst stability during SR since it independently exhibits activity for this reaction [1], [2]. Acidic supports such as alumina promote the dehydration of ethanol to ethylene, a precursor to coke [7], while redox supports like ceria and ceria-containing mixed oxides improve catalyst stability due to their high oxygen storage capacities [12], [15], [18], [28], [29], [30]. Cai et al. [18] proposed that ceria inhibited coke deposition through oxygen transfer from ceria to Ir particles, thereby contributing to carbon removal from Ir particles. Along similar lines, Song and Ozkan [30] suggested that the high oxygen mobility of ceria suppresses carbon deposition and keeps the particle surface clean.

Different strategies have been adopted to minimize carbon deposition. One approach is to increase the steam/ethanol ratio of the feed, which should favor the gasification of carbon with water (Eq. (4)). However, the effectiveness of this strategy depends on the rate of the gasification reaction, which is generally slow [31]. For example, Rh/CeZrO2 catalyst significantly deactivated during SR even at a high steam-to-ethanol molar ratio (H2O/ethanol = 8.0) [15]. We also studied the effect of the H2O/ethanol ratio on the performance of a Pt/CeZrO2 catalyst during SR. Increasing H2O/ethanol ratio from 2.0 to 10.0 decreased the catalyst deactivation rate but not enough to achieve long-term catalyst stability [12]. Another strategy is to add promoters such as alkali or alkali-containing supports, which accelerate the gasification reaction [31]. However, the addition of 0.5 wt.% of potassium to Rh/CeZrO2 catalyst only slightly improved ethanol conversion but the catalyst still underwent significant deactivation. Adding more K (5 wt.%) strongly decreased catalyst activity [15]. Alkali compounds like K may also neutralize the acid sites on alumina, thereby decreasing the formation of ethylene and, in turn, coke formation. Domok et al. [32] suggest that adding K to Pt/Al2O3 lowers the stability of the acetate species, thus reducing its poisoning effect. Despite these attempts, there is yet to be developed an effective procedure that prevents or at least significantly inhibits or mitigates carbon formation during ethanol conversion reactions to produce hydrogen.

The aim of this work is to evaluate two different approaches to minimize or prevent carbon formation during SR of ceria-supported Pt catalyst. The first envisions the control of the size of the site ensembles via the addition of a second metal. PtSn and PtRe catalysts have been extensively used in hydrocarbon reforming reactions [33], since coke formation requires a minimum ensemble of metal atoms [31]. Taking into account the geometric effect, the addition of Sn may dilute the metal atoms by alloying and thereby inhibit coke formation.

The second approach is based on the Boudouard reaction (reaction (3)). The addition of CO2 to the feed may shift the equilibrium in favor of reactants and help to diminish carbon formation. This CO2 may be recirculated from the outlet stream of the PEM fuel cell.

Section snippets

Catalyst preparation

Cerium (IV) ammonium nitrate was calcined in a muffle furnace at 1073 K for 1 h. The ramp rate used was 5 K/min. Pt (1 wt.%) was added to CeO2 by incipient wetness impregnation using an aqueous solution of H2PtCl6, whereas tin (0.6 and 2.6 wt.%) was added using an aqueous solution of SnCl2 (Merck). After impregnation, the samples were dried at 393 K and calcined under flowing air (50 mL/min) at 673 K for 2 h. The following catalysts were obtained: Pt/CeO2; PtSn/CeO2 (atomic ratio = 1.0); and PtSn4/CeO2

Reactions

The support plays an important role in the SR of ethanol. Recently, we demonstrated that CeO2 is a highly active material for the SR of ethanol, producing hydrogen free of CO as well as carbon deposits [34]. Therefore, we decided to investigate the performance of CeO2 on SR.

Conclusions

Differences in activity and selectivities were observed between high and low surface area unpromoted ceria materials during ethanol steam reforming, confirming that the oxide carrier is active. However, the selectivities to ethylene and acetaldehyde were high over the low and high surface area materials, respectively. Improved initial yields of hydrogen were obtained by adding Pt to ceria, and in agreement with this, initial ethylene and acetaldehyde selectivities were correspondingly much

Acknowledgements

This work received financial support of CTENERG/FINEP-01.04.0525.00. CAER acknowledges the Commonwealth of Kentucky for financial support.

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  • Cited by (0)

    1

    Present address: Universidade Estadual do Oeste do Paraná - Unioeste, Campus de Toledo, Rua da Faculdade, 645, Jd. La Salle, CEP 85903-000, Toledo, Brazil.

    2

    Present address: Universidade Federal Fluminense, Rua Passo da Pátria, 156, Niterói, RJ CEP 24210-240, Brazil.

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