Reactivity and reaction intermediates for acetic acid adsorbed on CeO2(1 1 1)

doi:10.1016/j.cattod.2015.03.033

Catalysis Today

Volume 253, 15 September 2015, Pages 65-76

https://doi.org/10.1016/j.cattod.2015.03.033 Get rights and content

Highlights

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Adsorption and transformations of acetic acid were studied upon a model CeO₂(1 1 1) surface.
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Multiple spectroscopic techniques probe adsorbate species resulting from acetic acid adsorption.
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Speciation of desorption products depends upon surface reduction prior to adsorption.
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Acetate at a vacancy precursor to ketene but reaction with μ-acetate leads to acetone.

Abstract

Adsorption and reaction of acetic acid on a CeO₂(1 1 1) surface was studied by a combination of ultra-high vacuum based methods including temperature desorption spectroscopy (TPD), soft X-ray photoelectron spectroscopy (sXPS), near edge X-ray absorption spectroscopy (NEXAFS) and reflection absorption IR spectroscopy (RAIRS), together with density functional theory (DFT) calculations. TPD shows that the desorption products are strongly dependent upon the initial oxidation state of the CeO₂ surface, including selectivity between acetone and acetaldehyde products. The combination of sXPS and NEXAFS demonstrate that acetate forms upon adsorption at low temperature and is stable to above 500 K, above which point ketene, acetone and acetic acid desorb. DFT and RAIRS show that below 500 K, bridge bonded acetate coexists with a moiety formed by adsorption of an acetate at an oxygen vacancy, formed by water desorption.

Graphical abstract

Introduction

Cerium oxide has been widely recognized as a catalytic material and employed in catalytic systems primarily because of its reducibility. This property makes it able to store and release oxygen, and so is used during changes in catalytic cycles in an automotive three-way catalysts. Working with a precious metal catalyst, CeO₂ and especially mixed Ce-based oxides, can buffer oxygen availability during oxidation of CO, reduction of NO, and water gas shift reactions [1]. However, ceria is also interesting for its acid–base properties that make it able to catalyze reactions such as dehydrogenation, ketonization, and dehydration reactions even without the necessity for a catalytic metal [2]. In these reactions that typically involve organic oxygenates, the redox capability of the CeO₂ may enter into the reaction pathways, and this can be expected to lead to a selectivity that varies depending upon the reduction of the ceria surface. It is difficult to discern these changes for polycrystalline CeO₂ under reactor conditions but studies of model surfaces can help relate the reaction selectivity to surface structure, redox processes occurring on the surface, and identity of adsorbed surface species present on the surface during reactions.

With this goal in mind, we and others have explored reaction pathways for C1 and C2 oxygenates reacting at single crystal surfaces [3] and CeO₂ nanoparticles with well-defined crystallographic terminations [4], [5]. In studies of ethanol dehydrogenation by these CeO₂ nanoparticles, dehydrogenation to acetaldehyde was favored over dehydration to ethylene but the selectivity was found to depend upon surface structure [4]. Sensitivity to structure and to the extent of reduction was found in TPD studies from CeO₂(1 0 0) and CeO₂(1 1 1) of ethanol and other alcohols [6]. Reactions of acetaldehyde demonstrated weak interactions with a fully oxidized CeO₂(1 1 1) but introduction of vacancies by reduction leads to formation of the enolate form [7]. This enolate is identified as an intermediate for coupling reaction to crotonaldehyde, the primary reaction product observed over CeO₂ nanoparticles. During in situ DRIFTS studies of CeO₂ nanoparticles during reaction, targeted to identify reaction intermediates and pathways, it was found that the oxidized product of these reactions, acetate, was frequently observed. This is of practical importance, since it has been shown that carboxylates are the primary intermediate from reactions of aldehydes that lead to long chain ketones via ketonization [8]. Cerium oxide has been shown to ketonize acetic acid to acetone, but depending upon conditions the ceria can undergo transformation to form bulk cerium acetate [9]. Mechanisms for ketonization of carboxylic acids and the acid enolization pathway leading to ketone have been of intense interest and are still a matter of debate [10], [11], [12]. Clarification of acetate and its role as spectator, intermediate or deactivator can be aided by model studies aimed at identifying the temperature evolution of adsorbed acetic acid. Identification of surface intermediates by IR methods during C2 reactions requires better identification of possible reaction intermediates by their spectroscopic signature. First principles density functional theory (DFT) calculations now make it possible to examine bonding configurations of surface intermediates in detail when combined with surface science techniques. Comparisons of vibrational spectra simulated using DFT calculations with experimental FTIR spectra obtained for formate, carbonates and acetaldehyde have clarified the interpretation of the experimental spectra, which enables more reliable detection of the surface species [7], [13], [14].

To obtain a better understanding of the interactions between acetic acid and cerium oxide catalysts, we have now combined UHV based measurements on a model CeO₂(1 1 1) surface with DFT computations. Surface science measurements, performed as a function of temperature on a surface with known structure, provide information about the evolution of the acetic acid on the surface and provide signatures of bonding configurations. DFT is used to predict models for the surface bonding configurations and to simulate vibrational spectra that can be compared directly with experimental RAIRS spectra. From this we have identified the stages of acetic acid deprotonation and decomposition. Measurements are performed upon both oxidized and reduced surfaces to identify the important role of oxygen vacancies upon the reaction products. From these studies, we find that bridge bonded acetate coexists with another type of acetate that is bonded by its interaction with oxygen vacancies.

Section snippets

Experimental and computational methods

Experiments were performed in three separate ultra-high vacuum chambers described previously, one used for reflection absorption infra-red spectroscopy (RAIRS) [7], one based at the National Synchrotron Light Source (NSLS) for soft X-ray photoelectron spectroscopy (sXPS) and near edge X-ray absorption fine structure (NEXAFS), and a third for performing temperature programmed desorption (TPD). In each of the UHV systems the model CeO₂ catalyst were prepared as thin films deposited on Ru(0 0 0 1)

Desorption products

TPD experiments were performed on ceria samples with different levels of reduction. Fig. 1 shows the TPD obtained from the fully oxidized surface following adsorption at 175 K. Acetic acid (mass 45) desorbs in two states near 210 K and 520 K. Acetic acid fragmentation pattern presents two intense fragments at 43 and 45 amu and a less intense parent peak at 60 amu. All three masses were followed to discriminate against acetone which would contribute at mass 43. The lower temperature state corresponds

Discussion

The combination of desorption measurements and surface spectroscopies allows us to put together a detailed description of the transformations that acetic acid undergoes on a CeO₂ surface as shown in Scheme 1. This scheme summarizes the desorption products that are observed and shows in an approximate way the adsorbate species and their evolution with temperature. It is clear that lattice oxygen participates in the temperature-dependent transformations and that oxygen vacancies play an important

Conclusions

Adsorption and transformations of acetic acid were studied upon a model CeO₂(1 1 1) surface. Near 175 K acetic acid adsorbs to form a mix of molecular acetic acid and deprotonated acetate. Acetic acid desorbs at low temperature along with water that leads to reduction of the CeO₂ surface. The resulting oxygen vacancies form strong adsorption sites that trap acetate. Adsorbate states are best described by a mixture of bridge bonded μ-acetate and acetate bonded at an oxygen vacancy (acetate/V_o), and

Acknowledgements

This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. F.C. Calaza and T.-L. Chen were sponsored by an appointment to the Oak Ridge National Laboratory Postdoctoral Research Associates Program administered jointly by Oak Ridge Institute for Science and Education and Oak Ridge National

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¹: Current address: Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany.

²: Current address: Applied Materials, Inc., Gloucester, MA 01930, USA.

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Reactivity and reaction intermediates for acetic acid adsorbed on CeO2(1 1 1)

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental and computational methods

Desorption products

Discussion

Conclusions

Acknowledgements

Surf. Sci. Rep.

J. Catal.

Appl. Catal. A: Gen.

Surf. Sci.

Surf. Sci.

J. Catal.

Comput. Mater. Sci.

Surf. Sci.

J. Electron Spectrosc. Relat. Phenom.

Surf. Sci.

Chem. Phys. Lett.

Surf. Sci. Rep.

Catal. Today

Coord. Chem. Rev.

Appl. Surf. Sci.

Spectrosc. Acta A: Mol. Biomol. Spectrosc.

Appl. Catal. A: Gen.

J. Mol. Catal. A: Chem.

J. Catal.

Chemsuschem

ACS Catal.

Top. Catal.

J. Am. Chem. Soc.

ACS Catal.

ACS Catal.

ACS Catal.

ACS Catal.

J. Phys. Chem. C

Phys. Chem. Chem. Phys.

Reactivity and reaction intermediates for acetic acid adsorbed on CeO₂(1 1 1)