(617e) From Mechanistic to Kinetic Analyses of Ethanol Steam Reforming Over Ir/CeO2 Catalyst

From mechanistic to kinetic analyses of ethanol steam reforming over Ir/CeO₂ catalyst

Fagen Wang^a,b, Weijie Cai^a,b, Hélène Provendier^b, Claude Descorme^b, Yves Schuurman^b, Wenjie Shen^a, Claude Mirodatos^b*

^aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

^b Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON, UMR 5256, CNRS; Université Claude Bernard Lyon 1), 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France.

Despite a large amount of papers dedicated to the ethanol steam reforming (ESR) targeting the production of hydrogen for downstream uses like PEMFC, relatively few ones report a kinetic analysis of the reaction, based on a mechanistic and ageing study for monitoring the kinetic modeling process. To that end, this work presents a combined approach between i) the identification of active sites and their potential ageing, ii) the critical mechanistic steps and iii) a kinetic analysis, accounting for the main mechanistic features.

From thorough investigations carried out over a series of Ir/CeO₂ catalysts [1,2], we demonstrated that the ethanol reforming proceeds essentially via a bi-functional way, involving at least two types of active sites (metal and support) covered with reacting intermediates considered as precursors of the reaction products. Thus, ethoxy species formed from the reaction of ethanol with the ceria surface, further oxidized into acetate adspecies accumulate on the ceria grains while CH_x and carbonyl pools build up on the Ir particles. The transfer of the C₂adspecies from ceria towards the metal particles, which corresponds to the C-C bond breaking, is strongly monitored by the state of the ceria surface and of the interface between metal and ceria. Again these two parameters are found to be related to the structure (e.g., the density of surface defects on the ceria and the size of the metal particles) and the texture of the catalyst (as deduced from measurements of microporosity and BET surface of the ceria).

Another feature which makes difficult any kinetic analysis of ESR is the occurrence of catalyst ageing, which may render questionable kinetic data collected without accounting for these dynamic processes. Thus, various causes of deactivation were identified, depending on reaction temperature and time on stream. The initial, fast but rather limited deactivation process was ascribed essentially to a loss of ceria surface (smoothing by loss of micro-porosity and/or roughness in the presence of steam), coinciding with an active phase build-up formed by a monolayer of carbonaceous reacting intermediates. In addition, a progressive and long-term deactivation was found to superimpose, originating from structural changes at the ceria/Ir interface, linked to the Ir particles sintering and the ceria restructuring. The continuous build-up of an encapsulating layer of carbon at moderate temperature, coming from C₂ intermediate polymerization (essentially related to the side reaction of ethanol dehydration into ethylene), was found not to contribute significantly to the catalyst deactivation, at least under the operating conditions investigated in this study. This rather stable graphite like layer formed progressively on stream could be suppressed by simple catalyst reoxidation from time to time to avoid potential diffusion limitations.

All these key statements offered us strong guidelines for designing an advanced kinetic modeling of this complex process.

A first conventional kinetic study of ethanol steam reforming over Ir/CeO₂ catalyst was carried out by analyzing the changes in ethanol conversion and selectivity under steam reforming conditions, by varying the main operating parameters (temperature, water/ethanol molar ratio and partial pressure of products). The apparent activation energy of the reaction was measured to be ca 58 kJ/mol, in line with values reported in the literature [3]. The apparent reaction orders of ethanol and water were estimated to be 0.6 and 0.5, respectively, which might indicate in a first approximation that the corresponding adsorption steps are not determining, but participate to the overall rate of conversion via the active ethoxy species and hydroxyl groups. The ethanol conversion was found inhibited by the addition of the main gaseous products in the reaction feed (negative apparent orders, equal to – 0.9, -0.4, -0.4 and -0.3 for H₂, CO₂, CO, and CH₄, respectively), which were consistent with some features of the reaction mechanism. The CO₂ addition would inhibit the ethoxy and acetate migration from ceria to Ir particles upon increasing the carbonate concentration on the ceria. The CO addition would increase the carbonyl concentration at the Ir-ceria interface, thus inhibiting the decomposition of acetate and methyl fragments into CO. The CH₄addition would favor its steam reforming at the expenses of the decomposition of acetate species. As for the detrimental effect of hydrogen addition, a partial reduction of the ceria surface might inhibit key reaction steps like the oxidation of ethoxy to acetate species.

A more advanced kinetic analysis, based on a microkinetic approach, was then carried out by assuming two distinct types of active sites, while the conventional approach based on the Langmuir-Hinshelwood-Hougen-Watson theory generally considers only one type of active site (see for instance [3] or [4]). This analysis will be presented in details during the conference (calculations are still under process), showing the discrimination of various models based on the above mechanistic assumptions. The selected model is considered to be robust enough to explore advanced engineering solutions for the hydrogen production from ethanol reforming, such as using micro-reactors as a tool for process intensification, as described in [5].  

References:

1- Ageing analysis of a model Ir/CeO2 catalyst in ethanol steam reforming. F. Wang, W. Cai, Tana, H. Provendier , Y. Schuurman, C. Descorme, C. Mirodatos^., W. Shen, Appl. Catal. B, 2012,125,546.

2- Oxidative steam reforming of ethanol over Ir/CeO2 catalysts: A structure sensitivity analysis. W. Cai, F. Wang, C. Daniel, A. C. van Veen, Y. Schuurman, C Descorme, H. Provendier, W. Shen, C. Mirodatos, J. Catal. 2012, 286,137.

3- Ethanol Steam Reforming over Rh(1%)MgAl₂O₄/Al₂O₃: A Kinetic Study, C. Graschinsky,. M. Laborde, N. Amadeo, A. Le Valant, N. Bion, F. Epron, D. Duprez, Ind. Eng. Chem. Res. 2010, 49, 12383

4- Experimental, kinetic and 2-D reactor modeling for simulation of the production of hydrogen by the catalytic reforming of concentrated crude ethanol (CRCCE) over a Ni-based commercial catalyst in a packed-bed tubular reactor. E. Akpan, A. Akande, A. Aboudhier, H. Ibrahim, R. Idem, Chem. Eng. Sci. 2007, 62, 3112.

5- Hydrogen production from ethanol steam reforming in a micro-channel reactor. W. Cai, F. Wang, A. van Veen, C. Descorme, Y. Schuurman, W. Shen, C. Mirodatos, Int. J. Hydrogen Energy, 2010, 35, 1153