Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Exploring the Interaction of Ammonia with Supported Vanadia Catalysts by Continuous Wave and Pulsed Electron Paramagnetic Resonance Spectroscopy Valeria Lagostina, Maria Cristina Paganini, Mario Chiesa,* and Elio Giamello Department of Chemistry and NIS Centre, University of Torino, via Giuria 7, I-10125 Torino, Italy
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ABSTRACT: Continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopy are used to investigate the nature of vanadium species on V2O5/TiO2 catalysts prepared via wet impregnation and gas-phase grafting routes. CW EPR experiments show in both cases the formation of VO2+ vanadyl ions upon reaction with ammonia; moreover, phase memory time filtered echo-detected EPR experiments at X- and Q-band frequencies provide direct experimental evidence for the reduction of the TiO2 substrate and the formation of Ti3+ ions. X-band hyperfine sublevel correlation (HYSCORE) experiments are used to investigate the local environment of VO2+ ions. The direct nitrogen coordination of residual ammonia fragments in the equatorial plane of the vanadyl ion is demonstrated.
1. INTRODUCTION The electronic and molecular structure of supported vanadium species on titanium dioxide (TiO2) has been extensively investigated, with the aim of understanding its remarkable catalytic activity toward many important reactions, ranging from the partial oxidation of light alcohols and the oxidative dehydrogenation (ODH) of light alkanes1−3 to the selective catalytic reduction (SCR) of NOx in the presence of ammonia.4−7 The SCR of NO with NH3 is widely employed for reduction of NOx emissions from stationary sources with efficiencies in NOx abatement up to 80−100%.6 The most well-known and commercially applied catalyst is vanadium oxide, well-dispersed on a titanium dioxide support, often promoted by other components such as WO3−.5,8,9 Despite the fact that this catalyst has been industrially used for 50 years, many aspects concerning the nature of the catalytic active sites, reaction intermediates, and mechanisms are still debated, and a recent perspective can be found in ref 6. Moreover, the catalyst preparation method is a critical factor in determining the interaction between the active components (vanadium) and the support (TiO2). Different preparation methods for supported V2O5/TiO2 catalysts have been reported, using a variety of V precursors (V-oxalate, ammonium metavanadate, grafting of VOCl3, grafting of V-alkoxide, and even thermal spreading of crystalline V2O5 onto TiO2),10,11 although apparently the coordination of the hydrated surface vanadium oxide species cannot be controlled by the specific V-precursor or preparation method.12 Common to typical heterogeneous catalysts, the heterogeneity of the support, the plurality, and low concentration of © XXXX American Chemical Society
the active surface sites, combined with difficulties in investigating the catalysts under reaction conditions, are just a few examples of the challenges that need to be faced in order to achieve a proper understanding of the catalytic sites and, consequently, the rational design of the catalyst. In the specific case of vanadia-supported catalysts, open issues are concerned with the role of isolated vs oligomeric surface vanadia sites, the nature of adsorbed ammonia species, as well as the nature of surface reaction intermediate complexes. The characterization challenge requires thus the use of specific spectroscopic methods capable of providing new molecular level insights. Because of its specific capability to describe the local topology of paramagnetic centers such as V4+ which are formed during a catalytic turnover via reduction of the active V5+ species, electron paramagnetic resonance (EPR) spectroscopy can be used as an additional, selective tool. EPR spectroscopy has been largely applied in the past for the characterization of these catalytic systems,13−16 and recently, operando EPR studies7 provided evidence for the existence of a +5/+4 redox cycle under operational conditions. The application of the technique to vanadia/titania is limited in practice to the detection of the spectra due to V4+ ions characterized by the presence of a single unpaired electron in the 3d orbitals (3d1, S = 1/2). Conventional CW EPR spectra are characterized by a distinctive hyperfine pattern arising from the interaction of Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Received: July 22, 2018 Revised: September 27, 2018 Published: October 14, 2018 A
DOI: 10.1021/acs.jpcc.8b07027 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Experimental (black) and simulated (red) X- and Q-band CW-EPR spectra of the V2O5/TiO2 impregnated sample. (a) X-band EPR spectrum of the oxidized sample. (b) X-band EPR spectrum recorded after reaction with ammonia at 200 °C. (c) Corresponding computer simulation. (d) Q-band EPR spectrum recorded after reaction with ammonia. (e) Corresponding computer simulation. All spectra were recorded at 77 K. The asterisk indicates a spurious cavity signal, while the open circle indicates a radical impurity.
the electron magnetic moment with the nuclear spin of 51V (I = 7/2). Analysis of the spin-Hamiltonian parameters can reveal important details regarding the nature of the V4+ species, allowing to discriminate between V4+ and VO2+ species, their local symmetry, and the degree of covalency of V−O bonds.17 A key aspect to be elucidated in V/TiO2 systems is, however, the local coordination environment of the formed V4+ species, including the presence of V−O−V linkages or coordinated ammonia residues as well as the possible presence of reduced species in the TiO2 support. Such information can be conveniently obtained by the use of pulse EPR methodologies and in particular by so-called hyperfine techniques, which allow detecting additional small hyperfine couplings arising from neighboring nuclei through partial spin delocalization.18 Electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopy are ideally suited to detect such small hyperfine coupling with sub-MHz resolution, which might be decisive for discriminating between different structures. Hyperfine techniques have been successfully applied to a number of different vanadium-containing heterogeneous catalysts providing evidence for the topology of VO2+ sites,19,20 specific metal−support interactions,21,22 the presence of V−O−V linkages,23,24 and the interaction with nitrogencontaining ligands.25,26 In this work, we apply pulse EPR methodologies to characterize the nature of surface V species on a TiO2 support generated by two different V precursors (ammonium metavanadate and grafting of VOCl3) upon reaction with ammonia, aimed at establishing the effect of NH3 and the nature of surface reaction intermediate complexes.
2.2. Catalyst Preparation. Two different methods were used, consisting of a wet impregnation procedure and the grafting via vapor-phase reaction with VOCl3 at room temperature. In a typical impregnation procedure 2.5 mg of ammonium metavanadate was dissolved in 0.5 mL of a 0.2 M oxalic acid solution. This solution was added to a water suspension of TiO2 (100 mg) and stirred for 2 h at room temperature. Excess water was evaporated using a water bath and the resulting solid dried overnight and subsequently calcined in air at 400 °C. The vanadium content of this catalyst was ≈2 wt % as V2O5. The vapor-phase grafting reaction was performed at room temperature exposing the TiO2 powder, previously activated at 200 °C under vacuum and oxidized at 300 °C with 100 mbar of O2, to the VOCl3 vapors in a quartz cell equipped with an EPR tube. The cell was evacuated after the reaction to remove excess VOCl3 and the reaction products (HCl). The vanadium content of this catalyst was ≈1.7 wt % as V2O5. Both samples were reacted with ammonia (50 mbar) in situ at 200 °C. 2.3. EPR Experiments. CW- and Pulse-EPR spectra were recorded at X- and Q-band frequencies on a Bruker ELEXYS 580 EPR spectrometer, equipped with helium gas-flow cryostat from Oxford Inc. The magnetic field was measured with a Bruker ER035 M NMR gaussmeter. X-band (9.8 GHz) CWEPR spectra were recorded at 23 dB power attenuation, 100 kHz modulation frequency, and 0.3 mT modulation amplitude. Q-band experiments (33.8 GHz) were performed with 25 dB power attenuation, 50 kHz modulation frequency, and 0.1 mT modulation amplitude. All spectra were recorded at 77 K. Electron Spin−Echo (ESE) Detected EPR. The experiments were carried out with the pulse sequence π/2−τ−π−τ−echo, with mw pulse lengths tπ/2 = 16 ns and tπ = 32 ns at both Xand Q-band. Different τ values were used, which are specified in the figure captions. X-Band Hyperfine Sublevel Correlation (HYSCORE).27 HYSCORE experiments were carried out with the pulse sequence π/2−τ−π/2−τ1−π−τ2−π/2−τ−echo. The length of the π/2 pulses was tπ/2 = 16 ns, while the length of the mixing π pulse was tπ = 16 ns. The larger excitation bandwidth of the π pulse allows suppressing diagonal peaks, which complicates the line-shape analysis. The time intervals t1 and t2 were varied in steps of 16 ns starting from 96 to 3288 ns. Different τ values were chosen, which are specified in the figure captions. An
2. EXPERIMENTAL SECTION 2.1. Support Preparation. Anatase TiO2 powders were prepared by sol−gel synthesis. In a typical synthesis, titanium(IV) isopropoxide was mixed with a solution of isopropyl alcohol in water and stirred at ambient temperature to complete the hydrolysis with formation of a gel. The gel was left aging for 20 h at room temperature, dried at 340 K, and eventually calcined in air at 770 K for 2 h. The resulting sample is a white powder, with anatase structure and surface area of about 50 m2 g−1. B
DOI: 10.1021/acs.jpcc.8b07027 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Experimental (black) and simulated (red) X- and Q-band CW-EPR spectra of the VOCl3/TiO2 grafted sample. (a) X-band EPR spectrum of the oxidized sample. (b) X-band EPR spectrum recorded after reaction with ammonia at 200 °C. (c) Corresponding computer simulation. (d) Q-band EPR spectrum recorded after reaction with ammonia. (e) Corresponding computer simulation. All spectra were recorded at 77 K. The asterisk indicates a spurious cavity signal, while the open circle indicates a radical impurity.
Table 1. Spin-Hamiltonian Parameters Derived from the Simulation of the EPR Spectra Presented in Figures 1−5a samples wi-TiO2
VOCl3−TiO2
gx
species VO2+ aggregate Ti3+ VO2+ (I) VO2+ (II) aggregate
1.9745 1.96 1.967 1.9510 1.962 1.96
± ± ± ± ± ±
0.0006 0.01 0.001 0.0006 0.004 0.01
gy
gz
|Ax|
|Ay|
|Az|
% weight
1.9745 ± 0.0006
1.938 ± 0.002
1.967 ± 0.001 1.9731 ± 0.0006 1.995 ± 0.004
1.89 ± 0.01 1.948 ± 0.002 1.945 ± 0.002
173 ± 5 140 ± 5 158 ± 5 -
173 ± 5 150 ± 5 135 ± 5 -
490 ± 10 472 ± 5 500 ± 10 -
13.2 84.8
