Catalysis Volume 24

A Review of Recent Literature

By J. J. Spivey, M. Gupta

The Royal Society of Chemistry

Copyright © 2012 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-375-5

Contents

Preface James J. Spivey and Mayank Gupta, v,
Recent advances in imaging and monitoring of heterogeneous catalysts with Raman spectroscopy Vanesa Calvino-Casilda and Miguel A. Bañares, 1,
Catalytic reforming of logistic fuels at high-temperatures Olaf Deutschmann, 48,
Coverage dependent adsorption properties of atomic adsorbates on late transition metal surfaces Spencer Miller, Carmeline Dsilva and John R. Kitchin, 83,
Green oxidation catalysis with metal complexes: from bulk to nano recyclable hybrid catalysts Cristina Freire, Clara Pereira and Susana Rebelo, 116,
Selective oxidation of o-xylene to phthalic anhydride: from conventional catalysts and technologies toward innovative approaches Fabrizio Cavani, Aurora Caldarelli, Silvia Luciani, Carlotta Cortelli and Federico Cruzzolin, 204,
Asymmetric organocatalyzed Morita-Baylis-Hillman reactions Gabriela Guillena, Diego J. Ramón and Miguel Yus, 223,
Catalytic applications of mesoporous silica-based materials Rafael Luque, Alina Mariana Balu, Juan Manuel Campelo, Maria Dolores Gracia, Elia Losada, Antonio Pineda, Antonio Angel Romero and Juan Carlos Serrano-Ruiz, 253,
Polarization-dependent total reflection fluorescence extended X-ray absorption fine structure and its application to supported catalysis Kiyotaka Asakura, 281,
Modeling oxidation of Pt-based alloy surfaces for fuel cell cathode electrocatalysts Rafael Callejas-Tovar, Wenta Liao, Julibeth M. Martinez de la Hoz and Perla B. Balbuena, 323,

CHAPTER 1

Recent advances in imaging and monitoring of heterogeneous catalysts with Raman spectroscopy

Catalysis is a complex multidisciplinary science that enables efficient performance in energy, automotive, chemical and pharmaceutical industries; most chemical reactions are catalyzed and it is a science that cannot be understood without spectroscopy. Spectroscopy is the enabling tool for knowledge-based design of highly efficient and stable catalysts. This review presents the progress of operando Raman spectroscopy during reaction and temperature-programmed treatments for heterogeneous catalysts (solid-gas and solid-liquid), with particular emphasis on the combination with other techniques, by extending it to space-resolved analyses and as a tool for mechanism investigation and monitoring in the liquid phase. Operando techniques are a key tool to understand catalysis and for monitoring and controlling catalytic processes. We summarize the most relevant research lines where Raman spectroscopy is applied in catalysis, challenges, hurdles and opportunities. This review outlines the versatility of Raman spectroscopy, for real-time analyses, in situ variable-programmed investigations and reaction studies. Spectroscopic information can be enhanced in a quantitative or qualitative manner, i.e., by using high-throughput Raman setups or by combining several spectroscopic techniques in a sample, respectively. This compilation outlines the posibilities of signal enhancement by resonance or SERS, and expanding it to mapping. We also comment developments for Raman imaging of profiles during catalyst synthesis and during reaction. Finally, this review summarizes the progress made in the liquid phase, to study catalyst synthesis, to monitor and investigate reaction mechanism and progress. The simultaneous combination of Raman with other complementary techniques is presented for these three lines of development. The current scenario presents an extraordinary perspective on opportunities for future developments.

1 Introduction

Catalysis cannot be understood without spectroscopy. Spectroscopic techniques for characterization of catalysts in the working state are powerful, because they provide fundamental information about catalyst structures, including surface structures, under the appropriate conditions. Such characterizations have permitted major advances in catalysis, as they can be the basis for the design or discovery of new catalysts. Catalysis has gained importance and popularity in chemical technology since it enables the determination of relationships between catalytic activity and catalyst structure at the atomic scale. The need for characterization of catalysts during reaction has been highlighted and demonstrated by several authors. In particular, Raman spectroscopy is one of the most powerful tools used to characterize working catalysts since it normally works in reflectance mode, typically uses visible radiation and may work under high pressures and temperatures above 1000 °C (with the appropriate excitation wavelength). Time-resolved transient temperature or pressure response experiments can be also carried out by Raman spectroscopy and reaction kinetics data can be measured directly and correlated with the spectroscopic data. In addition, catalytic reactors are easily accessible to spectroscopy using quartz fiber optics. The Raman experiments can be carried out with static controlled atmosphere or under flowing mixtures of gases to mimic the conditions in a catalytic reactor. It is also possible to study reactions in the liquid phase or under supercritical conditions.

Operando spectroscopic techniques are suitable for studying, monitoring and controlling homogeneous and heterogeneous catalysts in real-time under working conditions such as high pressures and temperatures in the gas and liquid phase, so that they are kept at its optimum performance. The operando methodology combines in situ spectroscopy during reaction with simultaneous activity measurement in a cell that meets the requirements of both, in situ cell and catalytic reactor. The advantage of operando methodologies is that catalytic activity/selectivity changes can be directly linked to electronic and structural changes of catalytic active sites and to changes of adsorbed molecules. They are nowadays commonly used to obtain mechanistic insight into the active site and the related reaction mechanism. Thus, operando spectroscopic methodologies have now become efficient tools for the design of advanced catalytic materials. Real-time spectroscopic feedback would be used as a multivariate control parameter, which modulates reaction condition parameters to keep the catalyst operating at its optimum performance rather than submitting it to deactivation/regeneration cycles. This would result in much higher product selectivity and typically longer catalyst operation time since relatively aggressive regeneration cycles is avoided; for instance thermal peaks during coke calcination.

New instrumental developments combining multiple spectroscopic techniques into one operando set-up have emerged during the last years, giving ample opportunities to reach a more detailed understanding of many relevant catalytic systems. In the present paper, an overview of the literature on the most representative examples of using Raman operando as a single-technique as well as those relating to combining Raman operando with other spectroscopic techniques are presented. Its application for imaging and monitoring during catalytic operation or catalyst synthesis is also presented.

A large number of monographs and review articles on Raman spectroscopy in heterogeneous catalysis have been published to date so this work is not aiming at bringing a thorough review, but presents current progress and opportunities for Raman spectroscopy based on the significant progress of in situ and operando studies during the last decade for both liquid and gas phase reactions.

2 Operando spectroscopy for developing catalysts and catalytic processes

In situ spectroscopic methodologies bring an insight on the state of catalytic materials, their structure, surface structure and adsorbed species under controlled environment. As the experimental facilities progress, there is an evident evolution on in situ studies that get closer and closer to the catalytic event. This evolution has become more apparent in the literature after the term “operando” that was first published in 2002. Since this term were coined as described elsewhere. A qualitative change has become apparent in the last decade to further consolidate in situ spectroscopy of the working catalyst. The term “operando” is a common term in literature, but it is appropriate to put its concept in perspective. The term “in situ,” Latin for “on site,” implies that the sample is analyzed at the location (the cell) where it has been treated or is being treated. In situ is quite a versatile term and several levels of such experiments are described in literature (Fig. 1) that imply different levels of approach to catalytic life conditions:

(a) “In situ” spectroscopy: implies that the spectra are recorded of a sample at the same location at which it has been or is being treated – typically a spectroscopic cell-. In many in situ studies, though, the temperature or gas phase may have changed at the moment of acquisition.

(b) Variable-conditions “in situ” spectroscopy: transformations occurring during the variation of a parameter, such as partial pressure of a component, temperature, etc. are monitored spectroscopically. Temperature-programmed processes are a typical case, like TPR-Raman spectroscopy, in which Raman spectra characterize the reduction of a sample, TPO-Raman spectroscopy, or any temperature-programmed reaction with an adsorbate or a probe molecule (TPSR). In the last few years, a powerful variant of variable-conditions “in situ” spectroscopy is becoming increasingly important: modulation excitation; in this case, the signal-to-noise and time resolution can be significantly improved.

(c) Reaction “in situ” spectroscopy, in which the catalysts is exposed to the temperature, pressure and flow of reactants used in the reaction. This is an increasingly important approach to assess the state of the catalyst during reaction. However, in this approach, no online activity measurement is typically made, or if it is, activity values are significantly lower that it should correspond to the system. This is due to the fact that spectroscopic criteria dominate in the design of the in situ cell. On occasions, there are significant temperature gradients, or the catalyst is as a wafer for reactions runs on powder catalysts, and many other possible cases.

(d) “Operando” spectroscopy of the working catalyst is a hyphenated technique, since spectroscopic measurement is simultaneously combined with additional analyses to determine conversion/selectivity data, e.g., on line mass spectrometry or chromatography. Thus, it is possible to demonstrate that the spectra correspond to an operating catalyst. Due to simultaneous quantitative analysis of the reaction progress, structure and activity can be correlated. The term “operando” is Latin for “working”. In the operando methodology, the operando cell must be a cell that delivers reaction kinetics data that match those obtained in the corresponding conventional reactor and be adequate for simultaneous spectroscopic analyses. Since it was first proposed, a key requirement was that the operando cell would be kinetically relevant thus comparisons with conventional reactor activity data or Arrhenius plots were reported using operando cells. Meunier has reported detailed analyses of kinetic aspects of many operando cells.

While “operando” is a rather new term, several groups had already executed experimental approaches using the ideas of this concept. For instance, an operando EPR cells was presented in 1991 by Fehrmann et col. for sulfuric acid catalyst deactivation. Operando DRIFTS-QMS was reported by Bañares et al. in 1994 to monitor the controlled decomposition of clusters as precursors of self-supported high area metal systems based on cobalt and other element, like zinc, titanium, molybdenum and their activity for hydrogenation reactions. Treating the organometallic compound in reaction feed for butadiene hydrogenation or for crotonaldehyde hydrogenation, it is possible to observe how the organometallic compounds based on carbonyl ligands progressive decomposes, releasing CO. DRIFT spectra show the progressive transformation into a metallic surface based on the IR bands of carbonyl ligands, that shift from frequencies characteristic of CO ligands, to those of chemisorbed CO on a metal surface. Such structural transformation runs parallel to the rise of hydrogenation catalytic activity, thus, to the birth of an active metal catalyst out of an inert organometallic compound. That DRIFT cell was modified to be able to obtain quantitative conversion values, like those obtained in a fixed-bed microreactor; alas, such changes are only briefly commented in those papers. Fortunately, a very detailed description and much more thorough study on how to obtain quantitative conversion modifying com- mercial DRIFT cells was recently reported by Meunier. In the case of Raman spectroscopy, the first paper using the operando approach was reported by Hill et al. they described the design and use of a Raman cell that would perform like a catalytic reactor for the high-pressure liquid phase ammoxidation of propylene. Interestingly their first papers using the term “operando” were also for Raman spectroscopy and for ammoxidation reaction, in these cases, it was for the gas-phase ammoxidation of propane into acrylonitrile.

Many authors have demonstrated the need for characterization of catalysts at work; its progress has been summarized in three recent volumes of Advances in Catalysis and in the compilation in Chemical Society Reviews. In particular, Raman is one of the most powerful tools for operando study of working catalysts. Raman experiments can be carried out at virtually any temperature and pressure, without interference from the gas phase, with increasingly higher time-resolutions. Thus, reaction kinetic data can be measured directly and correlated with the spectroscopic data. A number of monographs and review articles on Raman spectroscopy in heterogeneous catalysis have been published and have been reviewed. Very recently several exciting reviews address specific areas of progress for in situ Raman spectroscopy.

Raman spectroscopy can be used to investigate the state of the catalyst (its bulk and surface structure), of the reactants and of the adsorbed molecules. When reactions happen in the liquid phase, Raman can be used as an efficient tool for monitoring reaction progress. Such approach has a dual value, this is a tool for understanding catalysis, but this is also a tool for monitoring/controlling catalysts. Raman spectra during reaction deliver real-time information on the state of the catalyst and/or reaction progress, which may in turn be used as a feedback signal to control the reaction.

3 Variable-programmed in situ and operando Raman

3.1 Variable-programmed in situ Raman

Detailed revisions of variable-programmed in situ and operando Raman studies has been done recently, so we will only present representative studies as well as their interplay with other complementary approaches. Investigation of the state of catalysts under variable-programmed conditions brings insight under several kind of treatments. These are of critical relevance to catalysts or during TPR and TPO cycles, or during TPSR upon adsorption of a probe molecule. The Raman studies connect typical profiles (e.g., reduction or desorption profiles, among others) with structural changes in the catalyst structure and surface species. For instance, the anomalous reduction profiles of dispersed vanadium oxide on silica at vanadia coverage close to its dispersion limit. These are due to the different behavior of dispersed vanadium oxide species due to the presence of neighboring vanadium sites. These, reduce at low coverage, however, reduction at higher coverage triggers structural rearrangement of surface vanadium oxide species. Upon removal of oxide ions, surface vanadium oxide species rearrange and aggregate into nanocrystalline V2O5 (Fig. 2). Such transformation occurs at a temperature lower than at which highly dispersed vanadia reduces. The aggregation of dispersed vanadia into nano-V2O5 would facilitate their reduction, since the removal of a V-O-Si bond upon reduction (which releases H2O) would be compensated by the rearrangement of a neighboring V-O-Si bond into a V-O-V bond. Since, silica does not stabilize polymerized surface vanadium oxide species, this rearrangement leads to segregated V2O5 nanocrystals. V2O5 nanocrystals eventually reduce as temperature increases during reduction; then, dispersed vanadium oxide that did not rearrange, reduces. Such a scenario does not occur at lower coverages, and has been uncovered with TPR-Raman measurement.

TPR-Raman may also bring detailed insight on other phenomena occurring during reduction. For instance, Lewandowska et al. report on the TPR-Raman-QMS reduction of alumina-supported vanadium oxide catalysts prepared from different precursors. In that occasion, the catalysts prepared with vanadyl sulfate exhibited a very sharp reduction peak. This would indicate that surface vanadium oxide species are particularly well dispersed on alumina with this precursor, and that it would render a pretty narrow distribution of states of surface vanadium oxide species. However, total H2 consumption per vanadium leads to an oxidation state below V3+, which is unlikely on alumina, where, V3+ ions tend to be highly stabilized. Raman spectra confirm the reduction of dispersed vanadium oxide species during TPR, which is also evidenced by TPR-UV-vis-QMS measurements. Most interestingly, the on-line mass spectrometer confirms that H2S is concomitantly produced to water (Fig. 3). This is indicative that the reduction of both, surface sulfate species and surface vanadium oxide species occur in the same temperature range. It could not have been told in the absence of simultaneous spectroscopic confirmation on the reduction of vanadium species and online analysis of effluent gases. The evolution of reduced S-species from these catalysts during TPR has also been reported by Auroux’s group.

(Continues…)Excerpted from Catalysis Volume 24 by J. J. Spivey, M. Gupta. Copyright © 2012 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.