In situ electron paramagnetic resonance (EPR) – a unique tool for analysing structure and reaction behaviour of paramagnetic sites in model and real catalysts

Thomas Risse, Dirk Hollmann and Angelika Brückner

DOI: 10.1039/9781782622697-00001

The majority of catalytic reactions involves reduction and/or reoxidation steps in which electrons are transferred between catalysts and substrates. EPR spectroscopy can sensitively probe the local environment of paramagnetic catalytic sites as well as their behavior in catalytic redox processes since it can be applied under a wide range of conditions. After a short summary of the most important application examples of in situ EPR in redox catalysis, the main features of EPR spectra such g and A matrices and approaches of their evaluation are presented using model systems that contain Au atoms deposited on MgO single crystal surfaces. To illustrate the versatility of in situ EPR for deriving structure – reactivity relationships in catalysis, two application examples are presented in more detail: (1) Analysis of heterogeneous supported VOx/TiO2 catalysts during oxidative dehydrogenation of propane in the gas phase and (2) Study of photocatalytic water splitting over a homogeneous catalytic system comprising an iridium photosensitizer complex and an iron carbonyl catalyst.

1 Introduction

EPR spectroscopy has been used to characterize catalysts since very early on, because of its ability to provide detailed information on paramagnetic species such as their geometric and electronic structure or their chemical environment. In terms of the systems being investigated, EPR covers a wide range of applications from heterogeneous, via homogeneous catalysts all the way to enzymatic systems. Paramagnetic species in catalytic systems range from sites considered crucial for catalytic turnover, such as transition metal ions, to paramagnetic reaction intermediates. Analyzing their behavior in situ, i.e. under conditions as close as possible to those of a catalytic reaction, can be most helpful for deriving structure–reactivity relationships and reaction mechanisms. Nevertheless, applications of in situ EPR spectroscopy for monitoring catalytic reactions are not as numerous compared to other common methods such as vibrational spectroscopy or X-ray techniques, due to the fact that it is restricted to systems containing unpaired electrons. However, for catalytic redox processes in which electrons are transferred between catalyst and reactants, in situ EPR (or operando EPR, as it is called when together with the EPR spectrum catalytic conversion/selectivity data are measured) is a unique tool, since it can visualize this electron transfer directly as long as paramagnetic species are involved. This has been illustrated in the past for a variety of hydrocarbon conversion reactions. Thus, mixed oxide and oxynitride bulk phases such as vanadium–phosphorus oxides, vanadium–molybdenum oxides, mixed VTiSbSi oxides, VAlON and VZrON oxynitrides or heteropolyacids of defined structure have been used for selective oxidation of alkanes and aromatics. Such reactions have also been monitored by in situ EPR over supported vanadia and bismuth molybdate catalysts while supported chromia and nickel catalysts were studied during non-oxidative aromatization of alkanes as well as during interaction with butenes. Another important class of heterogeneous catalytic systems analysed by in situ EPR comprises zeolites containing transition metal ions either incorporated in the framework or located in pore positions. Many of such catalysts have been used to remove nitrogen oxides (NOx) from exhaust gases. Relevant examples comprise the use of Co–BEA and Fe–ZSM-5 for selective catalytic reduction of NOx, but also Cu–ZSM-5 has been widely used for the same purpose by Kucherov et al., whose work has been reviewed in ref. 1. The interaction of benzene with oxygen has been recently studied on the same type of catalysts to understand the gas-phase oxidation of benzene to phenol. For a more comprehensive selection of application examples for in situ EPR in heterogeneous catalysis the reader is referred to a number of reviews. Surprisingly, the use of in situ EPR spectroscopy for elucidating structure–reactivity relationships in homogeneous redox catalysis is even more limited than in heterogeneous catalysis.

In EPR spectroscopy the sample is placed in an external magnetic field to lift the degeneracy of the electron spin states and microwave radiation is used to induce magnetic dipole transitions between these states. Historically, most of the information has been obtained using continuous wave EPR spectroscopy operating at a microwave frequency of [approximately equals to]10 GHz in the so-called X-band. Spectrometers of this kind are still by far the most abundant ones, however, the last decades have seen a significant diversification of experimental capabilities, namely, the commercial availability of spectrometers operating at different microwave frequencies as well as pulse spectrometers, which enables the use of these techniques also outside of laboratories dedicated to instrumental developments in EPR spectroscopy.

The aim of this chapter is to acquaint the reader with the basic principles and application opportunities of in situ EPR in redox catalysis. The introductory part starts with a presentation of instrumental aspects and experimental procedures (Section 2), followed by a discussion of the most important parameters, the g and A matrix components, that can be derived from EPR spectra (Section 3). Here we will focus on the examples of Au atoms and O2•- radicals deposited on a MgO(001) surface, which both represent the simplest case of paramagnetic species with a single unpaired electron and a spin of S = 1/2. We will restrict ourselves to the analysis of cw EPR spectra being still the most common ones and typically the initial step in an EPR spectroscopic investigation. This discussion will be amended by only very few examples of pulse EPR techniques (Section 4), which provide valuable additional information on hyperfine interactions in paramagnetic species that can reflect peculiarities of their environment with much higher resolution than cw EPR.

In the application oriented part of this chapter (Section 5), we present two case studies, which illustrate the potential of in situ EPR for deriving structure–reactivity relationships in both heterogeneous and homogeneous catalytic redox processes, whereby special benefits arising from the coupling of in situ EPR with other techniques are explicitly pointed out. The first example is dedicated to a supported VOx/TiO2 catalyst. Such catalysts are of paramount importance for a variety of heterogeneous catalytic redox processes. Thus, V2O5/TiO2 modified by WO3 (also known as EUROCAT oxide) is an industrial catalyst for selective catalytic reduction of NOx in power plant exhaust gases, which has been studied in a round robin test by many European catalysis laboratories. Besides, there is a multitude of papers, among them those containing operando EPR studies in which the use of supported V2O5/TiO2 catalysts is described for a variety of catalytic oxidation reactions. In the first case study, the influence of surface sulfates on the structure of VOx species dispersed on the surface of titania and their catalytic behaviour in the oxidative dehydrogenation of propane to propene is analyzed by operando EPR coupled with UV-vis diffuse reflectance and laser-Raman spectroscopy. The second case study illustrates the special benefits of in situ EPR, supported by vibrational spectroscopy, for elucidating the mechanism of photocatalytic water reduction in a homogeneous catalytic system. A similar approach has been used very recently to unravel different wavelength dependent electron transfer mechanisms in plasmonic Au/TiO2 water splitting catalysts. The selection of these case studies was done with the aim to highlight the versatility of in situ EPR spectroscopy for a wide variety of reaction conditions, as long as paramagnetic species are involved.

2 Basic experimental considerations

Prior to a discussion of the appearance of EPR spectra it is appropriate to spend some time on experimental considerations. Herein, the discussion will be restricted to cw spectroscopy. With respect to pulse spectroscopy the interested reader is referred to the literature (e.g. ref. 22). EPR spectroscopy probes the properties of paramagnetic species by means of magnetic dipole transitions. Because of the small oscillator strength of such magnetic dipole transitions resonators are widely used in EPR spectroscopy to enhance the magnetic field strength at the sample. The properties of the resonator, which can be characterized by quantities such as the quality factor, filling factor or conversion efficiency, crucially determine the performance of the experiment. The use of resonators typically implies that monochromatic radiation has to be used, which forms a standing wave pattern inside the resonator. Additionally, the resonator structures lead to a separation of the magnetic from the electric field component helping to reduce undesired electric dipole excitations. The reader is referred to the literature for a more comprehensive discussion.

The EPR spectrum is obtained by sweeping the external magnetic field. Despite the fact that resonator structures are used, the amount of absorbed radiation is still very small. To improve the signal-to-noise ratio of the spectrum, cw-EPR spectrometers use a lock-in detection scheme. To this end, the external magnetic field is modulated with a known frequency and phase, which allows phase sensitive detection of the corresponding Fourier component of the detector signal. While this sounds like a pure technicality it has the important consequence that cw-EPR spectra are recorded as the first derivative of the absorption spectrum, more precisely: the first derivative of the imaginary part of the complex high frequency susceptibility by the field (dχ"/dB) as a function of the magnetic field. The number of spins may be determined from such a spectrum by double integration. This has to be done with care to avoid artefacts, e.g. due to imperfections of the baseline. Because of the direct relation of the EPR signal with the susceptibility of paramagnetic samples, the intensity of the EPR signal should obey Curie's law, which predicts an inverse proportionality of the magnetic susceptibility and, hence, the EPR signal intensity with temperature. A simple and often used test for Curie behavior is to plot the EPR signal intensity multiplied by the sample temperature (I x T) as a function of the temperature. For a paramagnetic sample with a constant number of spins, this product should be constant and deviations from this may be discussed as changes in the amount of paramagnetic species detected by the spectroscopy at different temperatures. For catalytic systems containing species, which may adopt paramagnetic as well as diamagnetic states, such as transition metal centres, temperature dependent measurements of the EPR signal intensity are a rather simple test to monitor the amount of paramagnetic species for different conditions. It is important to note that it is the signal area of an individual species and not simply the peak-to-peak amplitude of a line or the integral intensity of the entire spectrum, which needs to be considered. The latter two quantities may be used under appropriate conditions, too. The analysis of the temperature dependent intensity as outlined above also assumes that the experimental conditions will not induce additional changes in the measured spectrum. To this end, saturation effects may be one source of error, because relaxation times are a function of temperature. Unfavourable relaxation properties may also be responsible for the difficulty to observe paramagnetic species at certain temperatures. It is well known that not all paramagnetic species can be observed by EPR at all temperatures. Prominent examples are certain transition metal ions such as Co2+ or Ni2+, which typically require very low temperature to be observed. The relaxation behaviour of paramagnetic centres is one of the important issues to be considered in detail when one tries to apply EPR spectroscopy as an operando technique in catalysis. Despite the importance of relaxation phenomena for EPR, a more detailed discussion goes beyond the scope of this introductory part and the interested reader is referred to the literature on this topic.

3 Cw EPR spectroscopy of spin 1/2 systems

3.1 Paramagnetic species on solid surfaces

The key step in the interpretation of EPR spectra is to identify the various underlying physical effects by analysing appropriate EPR spectra. In order to get acquainted with the language typically used in EPR spectroscopy within this introductory chapter we will restrict the discussion to systems with a spin doublet ground state. Placing such a doublet state in a magnetic field leads to a splitting of the two spin states S = [+ or -] 1/2 according to Zeeman interaction, which gives rise to a linear dependence of the energy difference between the spin states from the magnetic field. This shall be exemplified using Au atoms, which are characterized by an unpaired electron in the 6s orbital leading to a 2S ground state. In the particular example presented here Au atoms are deposited on a single crystalline MgO(001) surface at 30 K to prevent diffusion of the atoms, which would lead to the formation of nanoparticles. The experiments were done under ultrahigh vacuum conditions (p < 1 x 10-10 mbar) to ensure that the results are not perturbed by the interaction with molecules from the gas phase. For the magnetic field being oriented parallel to the surface, Au atoms on MgO(001) show an EPR spectrum as presented in Fig. 1a.

In contrast to the simple consideration based on the Zeeman interaction of the doublet ground state, it does not show a single line, but four non-equidistant lines of considerably different amplitude. When observing such a spectrum it is not obviously clear that these four lines correspond to the same paramagnetic species. However, a closer look into the properties of Au atoms helps to rationalize the appearance of the spectrum. Au consists exclusively of the isotope 197Au having a nuclear spin of I = 3/2. The unpaired electron couples to the nuclear spin of the Au atom. This is called hyperfine interaction. Therefore, it is expected that the Zeeman line splits into four peaks due to the interaction of the unpaired electron with the 4 different nuclear spin states of Au. Note that a statistically picked Au atom within the ensemble will have a certain nuclear spin state and give rise to one of the four lines observed in the spectrum. Because of the equipartition of the nuclear spin states even at 30 K, all four nuclear spin states are populated equally and thus the Boltzmann weight of each line is identical for an ensemble of spins as observed here. A double integration of the spectrum reveals that within experimental error the four lines have the same intensity despite the fact that the observed signal amplitudes are different. The latter is due to differences in line width observed for the different lines.

The hyperfine interaction plays a pivotal role for the analysis of EPR spectra of many catalytically important systems, because it often allows extracting valuable information on the electronic structure as well as on the environment of the paramagnetic centres. From a qualitative point of view the preceding discussion seems to be sufficient to understand the appearance of the EPR spectrum of Au atoms. However, additional experiments at different angles between the surface and the magnetic field summarized in the lower diagram of Fig. 1a reveal a significant dependence of the resonance positions on the orientation of the magnetic field. A more detailed description is required to understand this behaviour, but also to extract information on the structural and electronic properties of the species. The framework to achieve this goal is the description of the system by a so-called spin Hamiltonian (eqn (1)), which – for the current discussion – consists of two terms reflecting the electron Zeeman and hyperfine interactions.