Vanadia-Based Catalysts for the Sulfur Dioxide Oxidation Studied In

Article pubs.acs.org/JPCC

Vanadia-Based Catalysts for the Sulfur Dioxide Oxidation Studied In Situ by Transmission Electron Microscopy and Raman Spectroscopy F. Cavalca, P. Beato,* J. Hyldtoft, K. Christensen, and S. Helveg* Haldor Topsoe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark S Supporting Information *

ABSTRACT: The industrial production of sulfuric acid involves the oxidation of sulfur dioxide, which is catalyzed by a silica-supported phase consisting of V2O5 species dissolved in a pyrosulfate melt with Na, K, and Cs added as promoters. As the molten phase is only present during the catalytic reaction and solidifies at room temperature, in situ studies are necessary to address the working state of the SO2 oxidation catalyst. Here we combine transmission electron microscopy (TEM) and Raman spectroscopy to study in situ a vanadia-based SO2 oxidation catalyst upon activation and reaction in an SO2/ O2 gas mixture. The observations reveal that the vanadia phase dynamically redistributes on the support upon heating in an SO2/O2 mixture. Surprisingly, the vanadia phase can disperse into partially crystalline islands on convex surfaces of the silica support and into a molten state on concave areas of the support. The presence of Cs was found to lower the temperature for the pyrosulfate formation and stabilize vanadium in the active VV state by forming linked structures at low temperature. Combining these in situ studies with activity measurements leads to the proposal that the linked structures stabilize the catalyst in the active state. (VO)2O(SO4)44−, VO2(SO4)23−, β-VOSO4, NaVO(SO4)2, K4(VO)3(SO4)5, and KV(SO4)2. Moreover, dynamic transformations of vanadia among some of these species can occur upon changes in the operation conditions.28 However, the dimeric vanadium complex (VO)2O(SO4)44− is commonly regarded as the important species for catalyzing the SO2 oxidation reaction. The functionality of this complex has been proposed to involve the attachment of O2 and SO2 to the active vanadium complex followed by the oxidation of SO2 into SO3.16−23 In order to make the catalytically active complexes more accessible to the gas phase and thereby optimize the activity, it is important to enlarge the gas−melt interface and, hence, the dispersion of the molten film on the porous support. Moreover, the catalytic activity can be enhanced by the addition of different alkali metals, e.g., Na, K, Rb, and Cs.28,29 Among these alkali metals, Cs gives the most significant activity enhancement at low reaction temperatures.29 The promotion by Cs is attributed to a lowering of the temperature at which the alkali metal sulfate transforms into the pyrosulfate melt.30 Whether the Cs promoter atoms only affect the sulfate melting point or play additional roles on the availability of different vanadia complexes as well as on their catalytic turnover frequencies is still a subject of debate.28,29 Here, we address the role of Cs promoters by examining the dynamic transformation of the vanadia−sulfate precursor into

1. INTRODUCTION It is important to reduce emissions of sulfur dioxide from, e.g., power plants and mining industries to protect ecosystems and human health.1 The removal of SO2 is favorably done by oxidation into SO3, followed by condensation into sulfuric acid, which is one of the most important bulk chemicals. In the industrial plants, the SO2 oxidation reaction proceeds over a catalyst that typically consists of vanadia and alkali metal sulfates, dispersed on a porous silica carrier.2,3 It has long been realized that the as-prepared catalyst contains a solid, crystalline phase, which transforms into a molten film under operating conditions due to an uptake of gaseous sulfur oxides.3−5 Therefore, the SO2 oxidation catalysts differ from most heterogeneous catalysts by having the catalytically active phase in the liquid state and are often referred to as supported liquid-phase (SLP) catalysts.3,6 The realization that the active state of vanadium is the molten state emphasizes the need for studying the vanadiabased catalyst in situ to obtain relevant information about the structure, composition, and distribution of the catalytically active species. In situ studies of SO2 oxidation catalysts are still rare partly due to the harsh reaction conditions under which the experimental equipment is more prone to corrosion. So far, in situ studies have mainly been based on spectroscopy using nuclear magnetic resonance (NMR),7−11 electron spin resonance (EPR),12−15 and Raman scattering16−23 as well as on thermal methods.24−27 These studies revealed that the molten pyrosulfate salts can contain different vanadia compounds including © XXXX American Chemical Society

Received: October 24, 2016 Revised: December 19, 2016 Published: December 19, 2016 A

DOI: 10.1021/acs.jpcc.6b10711 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

and a background subtraction was performed by extrapolating a power law fitted to the spectra in the energy range preceding the V L2,3 ionization edge. The catalysts were studied in situ using a Philips CM300 FEG-ST transmission electron microscope. The microscope is equipped with a differentially pumped vacuum system and permits in situ observations of samples during exposure to reactive gases at up to ca. 15 mbar and temperatures of up to ca. 900 °C.35 Specifically, this microscope is uniquely dedicated to sulfur-containing gases that may corrode interior parts of the microscope and cross-contaminate sulfur-free experiments.36 Microelectromechanical devices (E-chips from Protochips) were used as sample support. In the experiments, the catalysts were examined either at base vacuum pressure (∼1 × 10−6 mbar) or during exposure to a constant gas mixture of SO2 and O2 at elevated temperatures. Therefore, prior to catalyst loading, the E-chips were temperature-calibrated as described by Hansen et al.36 The E-chips were subsequently loaded by dispersing the catalyst powder in dry state onto the heater, and the E-chips were inserted into the microscope by means of a Protochips Aduro holder. The microscope was operated at a primary electron energy of 300 keV. Individual images and time-lapsed series of TEM images were recorded using a 1024 pixel × 1024 pixel TIETZ Fastscan F-114 CCD camera. The electron beam dose-rate was measured in vacuum (∼1 × 10−6 mbar) by means of the CCD operated at an effective pixel size corresponding to 0.56 nm/pixel and illuminated by the electron beam for 1 s. It is important to address the role of the electron beam on the in situ observations in order to distinguish dynamic changes in the sample induced by the electron beam and by the reaction environment, respectively. Electron beams can have a strong reducing effect on vanadium oxides in vacuum37 and can, in general, induce changes on the material in the presence of gas.34,38,39 Therefore, an empricial approach was undertaken to address the role of the electron beam on the present observations. Specifically, the actual experiments employed a dose-fractioned imaging procedure to evaluate the effect of the accumulated electron dose. With this procedure, the individual images were acquired using an electron dose-rate that was selected by observing the Cs-rich catalyst during exposure to the SO2/O2 gas at 300 °C. As this temperature is just below the melting point of alkali pyrosulfates,24,40 temperature-induced modifications in the sample in the absence of the beam should be negligible at this temperature. Therefore, any sample modifications observed by TEM should reflect an effect of the electron beam rather than the reaction environment. Under such conditions, the role of the electron dose-rate was examined by illuminating the sample at 0.5, 1, and 2 e−/Å2/s (Supporting Information, Figure S1). For each dose-rate, a previously unexposed area of the sample was examined using an illumination scheme mimicking the dose-fractioned imaging procedure in the actual experiment. That is, the area was repeatedly exposed to the electron beam for ca. 1 min, including ca. 10 s for locating the specimen and focusing the image and ca. 50 s for image acquisition, followed by 1 min with the beam blanked off. Visual inspection of the TEM images showed no apparent changes of the sample for an electron dose-rate of 0.5 e−/Å2/s for up to 30 image acquisitions, whereas slight modifications of the sample occurred at higher electron dose-rates and for fewer image acquisitions (Figure S1). Thus, with an electron dose-rate of 0.5 e−/Å2/s, the electron beam should have a negligible effect on the

vanadia−pyrosulfate upon activation of an SO2 oxidation catalyst. The transformation is addressed through an investigation of the transient response of a Cs-free and a Csrich catalyst upon heating in an SO2/O2 mixture. The observations were made in situ by means of transmission electron microscopy (TEM) and Raman spectroscopy, which provide spatial and molecular insight into the dynamic transformation of the vanadia-based catalysts upon activation.

2. EXPERIMENTAL SECTION 2.1. Materials. Industrial catalysts for the SO2 oxidation reaction are typically composed of vanadium oxides and alkali metal pyrosulfates (M = Na, K, Cs) dispersed on diatomaceous earth.31 Diatomaceous earth has a marked inhomogeneous morphology that makes it difficult to pinpoint representative structures at the high spatial resolutions accessible by TEM. To mitigate this situation, a more well-defined silica material consisting of ca. 100 nm wide nonporous silica spheres (Fiber Optic Center, Inc.), also called Stöber spheres,32 was used as support. The catalyst was prepared by first calcining the silica substrate in ambient air at 600 °C for 6 h. The silica was then treated with ammonia solution to activate the surface.33 Specifically, 2 g of Stöber spheres were added to 100 mL of 0.1 M NH3 and then stirred at 300 rpm for 15 min. The residue was filtered, washed with water, and dried at 80 °C for 60 min. Subsequently, the activated silica was split into two portions that were impregnated by aqueous solutions, consisting of vanadyl sulfate and alkali pyrosulfate, to produce two different catalyst samples, including one enriched with Cs (labeled “Csrich” in the following) and one without any Cs (labeled “Csfree”). Specifically, the impregnation liquor was made targeting a nominal composition of the Cs-free sample with 4.2 wt % V as well as K at a molar ratio of K/V = 3.5 and of the Cs-rich catalyst with 4.2 wt % V as well as K and Cs at molar ratios of K/V = 3.5 and Cs/V = 0.4, respectively. The actual composition of the impregnated catalysts was within 10% from the nominal composition as determined by inductively coupled plasma mass spectrometry (ICP-MS). The activity of the Cs-rich catalyst was measured in a plugflow reactor at different SO2 and O2 partial pressures with N2 as balance at a total pressure of 1 bar and at the temperatures of 500 and 600 °C. The gas exiting the reactor was monitored using an infrared SO2-gas analyzer, showing a constant composition after roughly 6 h on stream. After 48 h on stream, the samples were cooled to room temperature in the reaction gas in less than a minute. Afterward the catalysts were examined by ICP-MS for the molar content of V, S, Cs, and K. 2.2. Transmission Electron Microscopy (TEM). The catalyst samples were investigated ex situ using an FEI Titan 80300 transmission electron microscope.34 The microscope was operated at a primary electron energy of 300 keV and with a base vacuum of ∼1 × 10−7mbar. In TEM mode, images were acquired using a charge-coupled device (CCD) camera (Gatan US1000). In scanning TEM (STEM) mode, images were acquired at a semiconvergence angle of β ∼ 9.4 mrad. Moreover, in STEM mode, concurrent electron energy-loss spectroscopy (EELS) was performed using a postcolumn Gatan Imaging Filter Tridiem 863. Specifically, EELS analysis of the samples was performed using an energy dispersion of 0.3 eV/ channel, corresponding to the energy range of 100−700 eV and an exposure time of 5 s per spectrum at a dose-rate of ca. 3 e−/ Å2/s. Spectra were processed using DigitalMicrograph. The energy scale was calibrated using the O K-edge as a reference, B


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a heating−cooling cycle, the sample was cooled from 360 °C to room temperature, and the microscope was evacuated to ∼1 × 10−6 mbar. The sample was then inspected by TEM at an electron dose-rate of ∼3 e−/Å2/s. In the isothermal and variable-temperature experiments, a single area was illuminated at ∼0.5 e−/Å2/s for ca. 1 min at most 24 or 26 times. Consistent with the examination of beaminduced sample alterations, the effect of the electron beam on the observations should be negligble. However, it cannot be excluded that the electron beam changes the molecular species in the samples and, thus, that the catalysis and dynamics of the liquid to some extent could be affected by the electron beam. An examination of previously unexposed areas showed no significant difference in morphology with respect to the imaged areas (Figure S2). Moreover, the dose-fractioned experiments revealed variations in the evolution of different areas. This heterogeneity might be due to local variations in the concentration of the active phase or in the chemical composition of the active phase or to the different electron illumination conditions. Although the different effects are difficult to disentangle, the following Results sections will refer to observations made in at least two regions to exclude the impact of the accumulated electron dose. Finally, in order to study the effect of the individual experimental variables on the support and catalysts, control experiments were conducted by heating either the bare silica spheres or the Cs-rich sample in either vacuum, pure O2, pure SO2, or a mixture of SO2 and O2 (see Supporting Information and Figure S13). 2.3. Raman Spectroscopy. Raman spectroscopy was used to analyze the chemical state of the model catalyst under exposure to SO2 oxidation reaction conditions. The Cs-free and Cs-rich samples were crushed in a mortar and filtered in a multistage copper net sieve to obtain a 150−200 μm sieve fraction. A ∼100 mg portion of a sieved sample was loaded in a Linkam CCR1000 stage, which allows for operando observations by combining Raman spectroscopy and conversion measurements.23 The SO2 conversion was measured by a Maihak S700 infrared SO2-gas analyzer attached to the gas outlet. The reactor was first flushed with inert gas (He) before experiment. Given the hygroscopic nature of the sample and the corresponding propensity for sulfuric acid formation upon SO2 conversion, the sample was preheated in air at 600 °C for 6 h. Moreover, all gases are flown through a Drierite desiccant filter (anhydrous calcium sulfate) to reduce the residual moisture content as much as possible. Subsequently, isothermal and variable-temperature experiments were pursued on both Cs-free and Cs-rich catalysts. Gas composition was checked using the IR analyzer prior to the experiment. In the isothermal experiments, the Cs-free and Csrich catalysts were first exposed to air at 600 °C for 6 h. The temperature was subsequently lowered to 450 °C, and the gas feed was switched to 30 mL/min of 10% SO2 and 10% O2 in N2 at atmospheric pressure. The temperature was maintained for 3 h, during which Raman spectra were recorded at fixed time intervals of 15 min. For the variable-temperature experiments, the same pretreatment at 600 °C was undertaken. The temperature was subsequently lowered to 350 °C at a rate of ∼20 °C/min at 350 °C, and the gas feed was switched to 30 mL/min of 10% SO2 and 10% O2 in N2 at atmospheric pressure. In this mixture, the samples were then heated in steps of 10 °C from 350 to 500 °C at a rate of ∼20 °C/min and thereafter cooled to 350 °C in steps of 10 °C. Spectra were recorded after 5 min stabilization at each temperature step.

morphology of the vanadia-based catalyst for up to 30 image acquisitions. Experiments were conducted at either isothermal or variabletemperature conditions as outlined below. Each experiment was repeated three times, each time employing a new E-chip loaded with fresh as-prepared catalyst. TEM observations reported in this article from one of these experiments are representative of the others. After insertion of an E-chip with a fresh load of catalyst, an in situ experiment was conducted in the following way: First, four separate areas of the sample (labeled A, B, C, and D) were identified in the microscope at the base vacuum pressure of ∼1 × 10−6 mbar at room temperature. The interspacing between areas was greater than the largest electron beam illumination. This initial ex situ inspection was done with an incident electron dose-rate of ∼3 e−/Å2/s and accumulated exposure time of less than 1 min per area. Within this electron exposure, no visual changes of the sample morphology were noticed in TEM images recorded at a magnification corresponding to an image pixel size of 0.56 nm/pixel. Following the initial inspection, the SO2/O2 gas mixture was introduced by first dosing 5 mbar SO2 and subsequently adding 5 mbar O2. Thereafter, the sample was heated in the gas. The isothermal experiments aimed at visualizing the dynamical behavior of the two different catalysts upon a fast temperature increase. Specifically, a sample was heated in the SO2/O2 gas to an operation temperature of 450 or 600 °C at roughly ∼100 °C/s. The temperature was subsequently kept constant for ca. 6 h. After reaching the operation temperature (time t = 0), the thermal drift of the sample prevailed and decayed within ca. 15 min to below 1 nm/min. This drift rate is sufficiently low to avoid smearing of nanoscale features in the electron micrographs, and acquisition of TEM images commenced thereafter. Specifically, TEM images of the four areas of interest were acquired in such a way that all areas were imaged after ca. 15 min, and subsequently area A was imaged at intervals of ca. 15 min, B of ca. 30 min, C of ca. 60 min, and D of ca. 120 min. For each image, the area of interest was exposed to the electron beam at a dose-rate of ∼0.5 e−/Å2/s for ca. 1 min, including roughly 10 s for locating and focusing and 50 s for exposure of the CCD for image acquisition. Between the consecutive image acquisitions the area was left unexposed to the electron beam. After about 6 h, the sample was abruptly cooled to room temperature, and the microscope was evacuated to the base pressure of ca. ∼1 × 10−6 mbar. Under these conditions, TEM images of the samples were acquired at ∼3 e−/Å2/s. The variable-temperature experiments aim at addressing the dynamic response of the two different catalysts to minor temperature changes upon heating and cooling. Four areas of interest were chosen before the experiment and monitored in a dose-fractioned way: First, a sample was heated in the SO2/O2 gas from room temperature to an operation temperature of 360 °C at roughly ∼100 °C/s, and the temperature was subsequently kept constant for 15 min for drift stabilization. The temperature was then increased in steps of 20 °C from 360 to 600 °C and thereafter lowered in 20 °C steps to 360 °C. At each step, the temperature was kept stable for about 5 min for drift stabilization and imaging. Area A was imaged at every temperature step, area B was imaged every second step, area C every fourth step, and area D every 16th step. Each image was acquired at a dose-rate of ∼0.5 e−/Å2/s for an illumination time of about 1 min, which accounts for the time necessary to locate and focus the specimen area and to acquire a TEM image. After C


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The Journal of Physical Chemistry C The Raman spectra were recorded with a Jobin Yvon Horiba LabRamanHR, equipped with an Ar-ion laser. Samples were excited using the 514 nm laser line at 10 mW (output power). A 10× objective lens, a 200 μm wide slit, and no aperture or filter were used. Spectra were recorded in the spectral range 1200−550 cm−1 in autoscanning mode with a total acquisition time of ∼14 min, averaging five spectra into one. The Raman spectra were acquired with the Linkam CCR1000 stage operated in fluidized bed mode to reduce laser heating effects (see Supporting Information and Figure S14).23 Spectra were analyzed by fitting Gaussian functions to a spectral window containing the bands of interest, while the background was fitted by a constant. First, the fitting parameters, including the Gaussian center position, peak width, and peak height, were determined by fitting the first spectrum of each series. In the case of vanadium compounds, three bands were identified as representative of three vanadium compounds in the sample. The remaining spectra in the whole spectrum series were then fitted with the Gaussian center position and width constrained, while the peak height was allowed to vary. Finally, the area of the fitted Gaussian curves for each spectrum was extracted and compared. The area of each band was compared with the sum of the areas of the three bands for each spectrum in order to calculate their relative contribution to the total, as follows Ir, i =

Ai ∑j Aj

where (melt) represents the gaseous (g) species dissolved in the liquid (l) pyrosulfate. Reactions 2−7 show that the composition of the molten phase is closely related to the composition of the gas environment. In industrial converters, the partial pressure of SO2 and O2 is about 100 mbar at the inlet. The partial pressures of SO2 and O2 decrease, and the partial pressure of SO3 increases along the converter. Therefore, the molten phase will attain a varying composition through the industrial converter and across the individual catalyst pellets. In contrast, the gas cell employed in the electron microscope resembles a differential reactor with a gas composition close to that entering the microscope,41,42 and the Raman cell is more closely described as a continuously stirred tank reactor. In the electron microscope, the total pressure is at the level of a few mbar, and in the Raman setup, the total pressure is 1 bar. On the contrary, for both gas cells, the partial pressures and temperature match the conditions found in the upper beds of an industrial converter. In view of these considerations, caution must be exercised to ensure that the composition of the molten phase under investigation is relevant and is properly compared to conditions in the industrial converter. To compare the observations made by the different characterization techniques, the composition of the molten phase of the Cs-rich catalyst is therefore characterized by its sulfur content after the exposure to SO2 and O2 partial pressures ranging from those used in typical technical applications to those available in the electron microscope. Such gas exposures were carried out in separate experiments, and the spent Cs-rich catalyst material was analyzed by ICP MS with respect to the sulfur content (Table 1). The sulfur

(1)

where Ir,i is the relative contribution and Ai is the area of the ith band, respectively. In this work, only the relative intensity of spectral features is discussed, and the change in Raman cross section that may arise upon a phase transition from solid to liquid and vice versa is not addressed. Previous in situ Raman spectroscopy studies of SO2 oxidation catalysts and model catalysts 16−22 were performed in atmospheres of O2/N2 and SO2/N2 and of SO2/O2/N2 in the ratio 0.4:4:95.6. The latter composition contains all the reactant gas species for SO2 oxidation, but the partial pressure of SO2 is 10 times lower than that of O2 compared to the feed gas to the first bed of a typical industrial SO2 oxidation reactor operating with SO2/O2/N2 in the ratio 10:10:80. In this study, we use a ratio of 10:10:80 to be able to directly compare results on model systems and industrial catalysts in the upper beds of a simulated catalytic converter.

Table 1. α Parameter Measured as a Function of Partial Pressure of SO2 and O2 at 500 and 600 °C

(2)

O2 (g) ⇌ O2 (melt)

(3)

2SO2 (melt) + O2 (melt) ⇌ 2SO3(melt)

(4)

SO3(melt) ⇌ SO3(g)

(5)

SO24 −(s) + SO3(melt) ⇌ S2 O27 −(l)

(6)

2− S2 O27 −(l) + SO3(melt) ⇌ S3O10 (l)

(7)

T (°C)

SO2 (mbar)

O2 (mbar)

α

a b c d e f

500 500 500 600 600 600

0.5 5 100 0.5 5 100

0.5 5 100 0.5 5 100

1.69 1.80 2.05 1.16 1.48 1.79

content can be expressed by the parameter α, defined as twice the number of moles of sulfur (S) relative to the moles of alkali metal (alk), i.e.

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Vanadium-Containing Phase. The as-prepared catalyst takes up sulfur oxide upon exposure to the reacting gases, SO2 and O2. Hereby, the sulfate transforms into pyrosulfate as described by the following reactions SO2 (g) ⇌ SO2 (melt)

label

α := 2S/alk

(8)

A distribution of sulfates, pyrosulfates, and higher pyrosulfates is established at a constant temperature as described by eqs 6 and 7. For this case, α = 1 corresponds to a distribution centered on stoichiometric sulfate (M2SO4); α = 2 corresponds to a distribution centered on stoichiometric pyrosulfate (M2S2O7); and 1 < α < 2 reflects a shift of equilibria in eqs 6 and 7, corresponding to a mixture of sulfates, pyrosulfates, and higher pyrosulfates. This latter situation with a reduced sulfur content represents operation at higher temperatures and at lower SO2 and O2 partial pressures due to a more pronounced decomposition of the pyrosulfates. Table 1 shows that α is approximately 2 at SO2 and O2 partial pressures of 100 mbar at 500 and 600 °C. That is, the molten phase is dominated by pyrosulfates at the upper beds of the industrial converters and in the Raman reactor. Table 1 also shows that α is close to or below 2 at SO2 pressures of 5 mbar D


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Figure 1. Electron microscopy of the silica support and as-prepared Cs-rich catalyst. (a) Cropped TEM image of bare silica support particles. (b) Cropped TEM image of the as-prepared catalyst with the vanadia phase located at convex and concave regions, as outlined by a solid and dashed circle, respectively, superimposed on the image. (c) Cropped STEM image of the as-prepared Cs-rich model catalyst. (d) EELS spectra at the V L3 (515 eV), V L2 (523 eV), and O K (534 eV) ionization edges acquired at the positions indicated in (c). The spectra show that vanadium is present in the concave interstitial region between spheres (spectrum 1) as well as on the convex area as protrusions on the silica spheres (spectra 2−5). Spectrum 6 is acquired outside the catalyst as a reference.

Figure 2. Time-resolved TEM image series of the Cs-free (upper row) and Cs-rich (lower row) catalyst. The series consist of an image of the asprepared catalyst in the microscope vacuum at room temperature and images of the catalyst during exposure to SO2:O2 = 1:1 at 10 mbar total pressure and to an elevated temperature of 450 °C. The times are relative to t = 0 min, corresponding to the time for the establishment of the isothermal conditions, and the number (#) of the image in the series is denoted as well. Electron dose-rate 0.5 e−/Å2/s. Moreover, solid circles outline convex regions, and dashed circles outline concave regions.

3.2. Electron Microscopy. 3.2.1. Electron Microscopy of the As-Prepared Catalyst. The silica support and the asprepared Cs-rich catalyst were characterized by electron microscopy. Figure 1a shows that the bare support particles appear round in the projected TEM images with a diameter of ca. 100 nm. Agglomerates of spherical silica particles therefore established a support surface with distinct convex and concave regions. After impregnation and calcination, Figure 1b shows that the vanadia phase appears as smaller particles and corrugations in the convex regions (solid circle) and as an

or lower. Thus, a mixture of sulfates, pyrosulfates and higher sulfates is expected for electron microscopy experiments at 500 and 600 °C (a,b,d,e). Noticeably, at 5 mbar (b,e), α attains values just 10−15% lower than at 100 mbar (c,f). In fact, at 500 °C and 5 mbar of SO2 and O2, α attains a value that matches α at 600 °C and 100 mbar of SO2 and O2. The α value of the molten phase in TEM experiments at SO2 and O2 partial pressures of 5 mbar, respectively, should therefore mimic a liquid composition present under industrial conditions. E


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Figure 3. Time-resolved TEM images of Cs-free (upper row) and Cs-rich (lower row). The series contains an image of the as-prepared catalyst in the microscope vacuum at room temperature and images of the catalyst during exposure to SO2:O2 = 1:1 at 10 mbar total pressure and to an elevated temperature of 600 °C. The times are relative to t = 0 min, corresponding to the time for the establishment of the isothermal conditions, and the number (#) of image acquisitions is denoted as well. Electron dose-rate 0.5 e−/Å2/s. Moreover, solid circles outline convex regions, and dashed circles outline concave regions. For the Cs-free catalyst, a support sphere translated relative to the other spheres within the first 25 min, probably due to sample tilt or vanadia restructuring during heating.

additional phase that fills the concave region between the silica spheres (dashed circle). The presence of vanadium in these structures is confirmed by a combined STEM-EELS analysis at the V L2,3 and O K ionization edges (Figure 1c,d). In these spectra, the O K-edge is relatively more intense than the V L2,3edge due to the oxygen contribution from both the vanadium phase and the SiO2 support. Therefore, the intensity of the V L2,3-edge increases, and the O K-edge decreases in the spectra acquired from the core of the silica particle toward its surface (spectra 2 to 5 in Figure 1d). Thus, the vanadia phase covered only part of the silica surface with an inhomogeneous distribution. This heterogeneity in loading of the silica carrier by the vanadia phase hampers the interpretation of dynamic changes observed at the edges of the silica particles during exposure to a SO2/O2 gas at an elevated temperature. Therefore, each experiment monitors four different regions using the dose-fractioned electron illumination scheme described in the Experimental section. 3.2.2. TEM of the Catalysts at Isothermal Reaction Conditions. The transient response of the Cs-free and Csrich catalyst is addressed by rapidly heating a catalyst to 450 and 600 °C, respectively, in the SO2/O2 gas and by subsequent monitoring of the structural changes of the catalyst by means of time-resolved TEM. Figures 2 and 3 illustrate the main observations made in one region of the sample, and TEM images of the additional regions are depicted in Supporting Information Figures S3−S6. For the Cs-free sample, the corrugated vanadia phase in convex regions of the silica first transformed into smaller particles with a darker contrast and width of ca. 10 nm (Figures 2 and S3B). Subsquently, these particles transformed (Figures 2 and S3B) into more extended and faceted structures indicating that some areas of the convex surface developed crystalline character (Figures 2 and S3B). Concurrent structural dynamics were also observed in concave regions at the interstitial space between neighboring silica particles (Figures 2, S3A, and S3D). Accumulation of material with contrast comparable to that of

the silica spheres is observed in such areas in the course of time. As α ∼ 1.80 at 500 °C in the electron microscope (Table 1), a higher content of pyrosulfates, and therefore the presence of a molten phase, is expected. The material accumulated in the interstitial regions is therefore attributed to a molten phase. Noticeably, as the interstitial regions provide concave surfaces, representing a simple pore structure, the preferred migration of a molten vanadia phase to such regions can be explained by a lowering of the melt’s surface energy. For the Cs-rich catalyst sample, the convex regions tend to develop extended facets that also exhibit varying contrast levels (Figures 2, S4A, and S4B). The faceting indicates that the vanadia-related phase on the convex part of the silica spheres is of crystalline nature. In contrast to the Cs-free sample, the transient particle formation in the initial stage was not observed in any of the monitored areas. Moreover, the extended facets tend to restructure with time to shorter and more compact features (Figures 2 and S4A). Also the concave regions change in the course of time by an apparent accumulation of material in the interstitial space (Figures 2, S4A, and S4B). Unlike the Cs-free catalyst, the material is associated with a uniformly darker contrast compared to the silica spheres (Figures 2, S4A, and S4B). Following the reasoning above, the material accumulated in the concave regions is tentatively attributed to a molten phase, and the darker appearance is accounted for by mass−thickness contrast owing to the higher atomic number of Cs. In summary, the Cs-free and Cs-rich catalysts at 450 °C both tend to develop faceted and crystalline phases at convex surface regions and accumulate materials in a molten state at their concave regions. The apparent coexistence of molten and solid phases is likely sensitive to the reaction conditions. To address such a dynamic behavior of the vanadia phase, the isothermal experiments were also conducted at the higher temperature of 600 °C. At this temperature α is ca. 1.48 in the electron microscope (Table 1), which suggests a lower content of pyrosulfates and thus a lower amount of the molten phase and a F


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Figure 4. TEM frames of the Cs-free sample during heating (upper row) and cooling (lower row) in SO2:O2 = 1:1 at 10 mbar total pressure. The temperature was varied in the range 360−600 °C in steps of 20 °C, and TEM images were acquired at each step. The displayed frames were cropped from the images in the series at every 40 °C. Frame size 195 nm × 514 nm. The image number in the series is denoted by #. Electron dose-rate 0.5 e−/Å2/s. Solid and dashed circles outline convex and concave regions, respectively. The silica particle in the lower part of the image moves during heating probably due to thermal sample drift or vanadia restructuring.

Figure 5. TEM frames of the Cs-rich sample during heating (upper row) and cooling (lower row) in SO2:O2 = 1:1 at 10 mbar total pressure. The temperature was varied in the range 360−600 °C in steps of 20 °C, and TEM images were acquired at each step. The displayed frames were cropped from the images in the series at every 40 °C. Frame size 195 nm × 514 nm. The image number in the series is denoted by #. Electron dose-rate 0.5 e−/Å2/s. Solid and dashed circles outline convex and concave regions, respectively. The silica particle in the lower part of the image moves during heating probably due to thermal sample drift or vanadia restructuring.

higher content of sulfates as compared to the isothermal experiments at 450 °C.

For the Cs-free catalyst, a markedly different transient behavior is observed at 600 °C compared to 450 °C. G


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Figure 6. Time-resolved Raman spectra of Cs-free (left) and Cs-rich (right) samples exposed to technical air and to 10% SO2 and 10% O2 in N2 (process gas) at 450 °C. Both samples were pretreated in air for 6 h at 600 °C prior to experiment. Spectra at 0 min are acquired as soon as the gas feed was switched from air to process gas. Peaks marked with an asterisk (*) are artifacts from the CCD.

Irrespective of the as-prepared state, the in situ observations at 600 °C reveal that all the convex silica surfaces are round and exhibit uniform contrast (Figures 3, S5A, S5B, and S5D). This observation suggests that the vanadia phase in the as-prepared catalyst has either migrated away from these regions or desorbed from the surface. Moreover, also the concave regions behave differently at 600 °C. The TEM image series show that the interstitial regions extended in size, and the accumulated material apparently shrank as the temperature was increased to 600 °C (Figures 3, S5A, S5B, and S5D). The Cs-rich catalyst in the SO2/O2 gas at 600 °C also undergoes transformations that resemble those observed for the Cs-free catalyst. That is, convex surfaces obtain rounded shapes with uniform contrast (Figures 3 and S6A−S6D). Moreover, the darker material accumulated in the concave regions shrinks over the observation period at a comparable rate for the two catalysts. In summary, the compact vanadia phases in SO2 and O2 at 600 °C has been depleted from convex and concave regions in both the Cs-free and Cs-rich catalysts. The behavior of the vanadia phase in the two catalysts at 450 and 600 °C can be rationalized by, on the one hand, faster mass transport at higher temperature, whereby material accumulation in the concave regions is promoted over the convex regions and, on the other hand, considering the lower value of α at higher temperature reflecting a loss of sulfur oxide from the molten phase that therefore shrinks in size. From the images, it cannot be excluded that some vanadium, potassium, or cesium also

desorbs from the catalyst under the present conditions. However, as will be discussed below, material may reappear on convex surfaces upon cooling to 350 °C, showing that desorption of material from the support was certainly not complete under the present conditions. 3.2.3. TEM of the Catalyst at Variable-Temperature Reaction Conditions. To further address the thermal onset for dynamic transformations of the vanadia phase, the catalyst was examined by TEM during heating from 360 to 600 °C and during subsequent cooling in the SO2/O2 gas. Figures 4 and 5 show a TEM image series of a region of the Cs-free and Cs-rich catalyst, and image series of additional regions are shown in Supporting Information Figures S7−S12. For the Cs-free catalyst, the convex regions are decorated by the vanadia phase in the form of particles and surface corrugations (Figures 4, S7, and S9). No noticeable changes in this as-prepared vanadia phase are observed upon heating to 360 °C. The observation is consistent with the inactivity of the catalyst at this low temperature.16−22 Upon further heating up to 480 °C, a shrinkage of the larger particles in the convex areas is observed (Figures 4, S7, and S9), and material accumulates in the concave regions of the silica surface (Figure 4). Although this latter observation was only observed in one image series (Figure 4), the dynamic restructuring of the vanadia phase resembles qualitatively the observations in the isothermal experiment at 450 °C (Figure 2). Furthermore, from 560 °C and onward, the material is again depleted from the silica support in the concave regions (Figure 4). This observation H


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The Journal of Physical Chemistry C Table 2. Band Assignments for Raman Spectra According to References 16−22, and 43−49 band wavenumber (cm−1) ∼1080 ∼1060 ∼1050 1022 1000 990 970 940 840 780 730 668 616 ∼602

bond type ν1(S2O72−) ν(VO) terminal stretch ν(VO) terminal stretch ν(VO) ν(VO) ν1(SO42−) ν(SO42−) S−O terminal stretch ν(S−O) bridging in S−O−V V−O−V ν(S2O72−) ν4(SO42−) ν4(SO42−) D2 defects

assignment

reference

pyrosulfate ion (VVO)2O(SO4)44− (D and lD) VVO2(SO4)23− (M and lM) molten VIV crystalline K4(VO)3(SO4)5 crystalline VIV (VOSO4, VO(SO4)34−, VO(SO4)22−) M2SO4 (M = K, Cs) VVO2(SO4)23− (M, lM) (VVO)2O(SO4)44−, VVO2(SO4)23−, and their linked forms (M, D, lM and lD) (VVO)2O(SO4)44− (D, lD) pyrosulfate ion VVO2(SO4)23− M2SO4 (M = K, Cs) trisiloxane rings

17,18,43 17,20,49 16,17,20 19 45 18,19,44 16,20,22,46 20 20,21 17,21 17,18,43 19,20,22 16,20,46 47,48

resembles those of the isothermal experiment at 600 °C (Figure 3). Interestingly, during cooling from 600 to 360 °C in the SO2/O2 gas, restructuring of the Cs-free catalyst occurs, which becomes evident as vanadia structures appear on convex regions of several silica spheres in one region below 420 °C (Figure 4). These observations show that structures comparable to those of the vanadia phase in the as-prepared catalyst may reappear after heating and could reflect a shift of reactions 6 and 7 corresponding to an uptake of sulfur oxide from the gas phase at lower temperatures. Thus, this finding indicates that at 600 °C the vanadia phase was at least partly dispersed across the silica and indistinguishable by TEM. Further support for this redispersion phenomenon is obtained from the Cs-rich catalyst. For the Cs-rich catalyst, heating from RT to 360 °C does not result in marked changes (Figures 5, S11, and S12). Upon heating above 360 °C, particles and corrugations on the convex surface regions became rounder, shrank in size, and eventually vanished at a temperature in the range 360−520 °C (Figures 5, S11, and S12). In the concave regions, accumulated material tends to deplete the interstitial space as the temperature exceeds 440 °C (Figures 5, S11, and S12), as for the Cs-free catalyst. Upon cooling the Cs-rich sample from 600 °C to below 480 °C, a single region showed the formation of small particles on or near convex surface regions (Figure 5). Moreover, partial filling of the interstitial spaces in concave regions occurred as well (Figures 5 and S11). In some of the concave regions, the accumulated materials exhibited strong contrast (Figures 5 and S11) which is consistent with either heavy element content or a crystalline phase. In summary, the Cs-free and Cs-rich catalysts behave qualitatively similar in the variable-temperature experiments. On the convex regions, the vanadia phase is present as particles or surface corrugations by heating in SO2/O2 to 360 °C, and it disappears at higher temperatures. For the concave regions, material in the interstitial space tended to disappear upon heating to temperatures higher than 450 °C. Upon cooling, some particles reappear on convex regions. For each catalyst, the four different areas (Figures 4, 5, and S7−S12) showed different onsets for the restructuring and differences in the structural changes. This variation is likely due to the heterogeneous loading of the silica carrier with the vanadia phase and to the short time at each temperature step that may prevent the establishment of a steady state of the sample. However, the observations confirm the findings in the

isothermal experiments of a temperature-dependent dispersion and state of the vanadia phase. 3.3. Raman Spectroscopy. 3.3.1. Raman Spectroscopy at Isothermal Conditions. Figure 6 shows time-resolved spectra for the two model catalysts under isothermal conditions. Generally, the spectral features of the two systems are quite similar. Therefore, the band assignment is done collectively for both samples according to the literature data listed in Table 2.16−22,43−49 In air at 450 °C, bands at ca. 1050, 940, 840, and 668 cm−1 are visible in both samples. These bands are associated with the molten monomeric VV complex VVO2(SO4)23−.19,20 The presence of a band centered around 780 cm−1 is typical for bridging V−O−V bonds and is assigned to the dimeric vanadium complex (VO)2O(SO4)44−.17,21 For the sake of clarity, in the following the monomeric complex VVO2(SO4)23− and the dimeric complex (VO)2O(SO4)44− will be labeled M and D, respectively. However, since both complexes can be present either isolated or in polymeric sulfate-linked form, the linked forms of M and D will be labeled lM and lD, respectively. For example, the band centered at 840 cm−1 is representative of bridging S−O−V bonds present in M and lM, as well as in D and lD.20,21 Said band is more intense in the presence of lM and lD since they contain a high number of bridging S−O−V bonds. When the band at 840 cm−1 is predominant, all compounds are present. When both bands at 840 and 780 cm−1 are visible, the species D, lM, and lD are present. Finally, when the band at 780 cm−1 is predominant, D is mostly present. After 15 min of exposure to process gas, the spectrum of the Cs-free sample has a sharp peak at 990 cm−1, assigned to the vanadyl bond (VO) in crystalline VIV compounds such as VOSO4, VO(SO4)34−, and VO(SO4)22−18,19,44 and a peak at 1020 cm−1 assigned to the vanadyl bond in molten VIV compounds.19 Furthermore, the 1050 cm−1 band assigned to the vanadyl bond in M disappears, while the 1060 cm−1 band corresponding to the vanadyl bond in molten D appears.17,20,49 The latter compound is regarded as the active species for catalyzing the SO2 oxidation reaction.18,44 This shows that molten VV compound and VIV compounds (both liquid and solid) can coexist simultaneously under SO2 oxidation reaction at 450 °C. The Cs-rich sample shows identical spectral features, but their appearance and disappearance occur at different times. The pyrosulfate (S2O72−) band at 1080 cm−117,18 is also visible in all spectra. The pyrosulfate is more pronounced in the Cs-rich catalyst compared to the Cs-free. Both catalysts contain I


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Figure 7. Raman spectra of Cs-free (left) and Cs-rich (right) model systems at 350 °C in air and subsequently during heating in process gas (10% SO2 and 10% O2 in N2) from 350 °C (red curve) to 470 °C (blue curve). The lower graphs represent the relative area of the bands at 780, 840, and 990 cm−1, obtained by fitting Gaussian curves to the peaks. A representative Gaussian fit of the peaks at 780 and 840 cm−1 is shown in the Cs-rich plot at 370 °C. Spectra are unscaled and unprocessed. Laser excitation wavelength λ = 514.5 nm. Peaks marked with an asterisk (*) are artifacts from the CCD.

same features as at 450 °C in air in the isothermal experiments. The bands at 840 and 780 cm−1 indicate the presence of vanadium sulfate and pyrosulfate species in the as-prepared sample. After switching to process gas, both samples exhibit sharp peaks at ∼1085 cm−1 (pyrosulfate, S2O72−), ∼1065 cm−1 (vanadyl bond in D and lD), 1022 cm−1 (vanadyl bond in molten VIV), and 1000 cm−1 (crystalline K4(VIVO)3(SO4)5). The latter peak presents a shoulder at 990 cm−1 assigned to crystalline VIV species.44 The peak at 1050 cm−1 disappears, while the peak at 1060 cm−1 appears in both samples, which indicates the formation of D in both samples. The relatively lower intensity of the 840 cm−1 band in the Cs-free sample indicates that D is mainly present in isolated form in the Cs-free and in more linked form (lD) in the Cs-rich sample. Finally, peaks at 970 and 615 cm−1 are associated with free sulfate in M2SO4 (M = K, Cs).16,20,46 During heating in process gas, sulfate compounds such as M2SO4 (M = K, Cs) disappear, and pyrosulfate forms, as reflected by the disappearance of the bands at 615 and 970 cm−1 (ν1, ν4 of SO42−) around 360 °C and the correspondent increase in intensity of the bands at 1080 and 730 cm−1 (pyrosulfate).43 The full melting of crystalline VIV species can be identified by the disappearance of the sharp peak at 990 cm−1 around a critical temperature Tc, occurring at 460 °C for the Cs-free and 440 °C for the Cs-rich sample, respectively. At the same temperature, the peak at 1022 cm−1 (associated with molten VIV species) disappears, indicating that both crystalline and liquid VIV species are not present in the melt and that full oxidation of vanadium

2 wt % V and K to a 3.5 K/V molar ratio, while the Cs-rich catalyst contains, in addition, Cs to a 0.4 Cs/V molar ratio. The higher alkali metal loading in the Cs-rich sample might explain the higher alkali pyrosulfate signal for this sample. The most striking difference in the evolution of the spectra for the two samples is the time delay in the appearance of the 990 cm−1 peak (crystalline VIV compounds), which is already observed after 15 min in the Cs-free, while it appears first after 30 min in the Cs-rich. Likewise, the Raman spectra support the observation of crystalline structures made in the TEM. Previous investigations also showed that the precipitation of crystalline compounds occurs in more reducing atmospheres (with higher SO2 partial pressure) and low temperatures50 and that precipitation is more pronounced if the catalyst is poorly or not dispersed at all.28 3.3.2. Raman Spectroscopy at Variable-Temperature Conditions. Raman spectra were recorded at variable-temperature conditions to monitor the evolution of the catalyst composition. First, a spectrum was recorded at 350 °C in technical air, then the gas was exchanged to 10% SO2 and 10% O2 in N2, and subsequently spectra were recorded during stepwise heating (Figure 7) and cooling (Figure 8). Figure 7 shows that heating of either of the two catalysts in air to 350 °C leads to visible bands at ∼1080 cm−1 (pyrosulfate, S2O72−), 1050 cm−1 (vanadyl bond in M and lM), 940 cm−1 (terminal sulfate groups in M and lM), 840 cm−1 (S−O−V bonds in M, IM, D, and lD), and 780 cm−1 (V−O−V bonds in D and lD). The spectrum at 350 °C in technical air has the J


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Figure 8. Raman spectra acquired during cooling of Cs-free (left) and Cs-rich (right) model systems from 470 °C (red curve) to 350 °C (blue curve) in process gas (10% SO2 and 10% O2 in N2). The lower graphs represent the relative intensity of the bands at 780, 840, and 990 cm−1. Spectra are unscaled and unprocessed. Laser excitation wavelength λ = 514.5 nm. Peaks marked with an asterisk (*) are artifacts from the CCD.

the crystalline species (990 cm−1), while D remains at the same level, indicating that linked chains are mainly formed of M units (lM). In both samples the contribution of the band associated with crystalline vanadium drops monotonically throughout the entire temperature ramp. In the Cs-free sample, at 400 °C the three bands are equally contributing to the vanadium compound distribution, and at temperatures between 400 and 450 °C the band associated with linked complexes (840 cm−1) becomes dominant. Increase of the 1060 cm−1 peak and decrease of the 1050 cm−1 peak begin at 440 °C and at 400 °C in the Cs-free and Cs-rich samples, respectively. This indicates that the dimerization of M into D in the two samples onsets at different temperatures. Both 1050 and 1060 cm−1 bands are present over the whole temperature range, but deconvolution was not attempted due to the strong overlap, broadening, and red shifts with increasing temperature. The species associated with the two bands are in equilibrium, and this equilibrium shifts with temperature. In the same temperature range the signal associated with D grows faster than lD and lM in the Csrich sample, indicating that D forms but does not link. Between 450 and 470 °C, the Cs-free sample exhibits high D content, with a lower degree of linking than the Cs-rich sample in the same temperature range. During the cooling phase, Figure 8 shows that the processes observed in the heating phase are only partly reversible. As the temperature is lowered below Tc = 460 °C and Tc = 440 °C for the Cs-free and Cs-rich samples, respectively, features related to crystalline VIV species (990 cm−1) and pyrosulfate (1080 and 730 cm−1) grow in intensity. This indicates that VIV species accumulate and precipitate out of the melt and that

compounds occurs above Tc. The spectrum at 450 °C for the Cs-rich catalyst does not show the peak at 990 cm−1 observed in the isothermal experiments after 60 min of exposure to process gas. This suggests that the sample may not have reached a steady state within the acquisition time in the variable-temperature experiment. Finally, a growth of the characteristic (V−O−V) broad band centered at 780 cm−1 can be noted at high temperature, suggesting an increase in concentration of the active dimeric complex D.17 Assuming that all vanadium is contained either in D, lD, M, lM, or crystalline VIV species, spectra were further analyzed by fitting Gaussian curves to the most prominent bands associated with such compounds (780, 840, and 990 cm−1, respectively) and quantifying the area underlying each Gaussian. Specifically, the analysis focuses on the bands at 780 cm−1, corresponding to D and lD, at 840 cm−1, corresponding to M, D, lM, and lD, and at 990 cm−1, corresponding to crystalline VIV (Table 2). It has to be noted that the band at 840 cm−1 is associated with the S− O−V bond present in both M and D complexes and that the amount of these bonds increases when they link via bridging sulfate groups. Therefore, the intensity of the band at 840 cm−1, relative to 780 cm−1, is related to the degree of linking of vanadium (V) compounds. Results are shown in the lower part of Figure 7. The quantification results (Figure 7) show that the band associated with D and lD (780 cm−1) dominates with respect to that associated with the linked complexes lM and lD (840 cm−1) in the Cs-free sample at temperatures below 400 °C. Instead, in the Cs-rich samples lM and lD dominate the vanadium compound distribution and grow at the expense of K


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The Journal of Physical Chemistry C accumulation of free molten pyrosulfate occurs. Precipitated VIV compounds deplete the melt of vanadium. Dissolved vanadium is therefore available in lower concentration in the melt for bonding with pyrosulfates to form D. As a result, the intensities of the bands at 780 and 840 cm−1 decrease as temperature is lowered. At the end of the cooling cycle, the spectrum of the sample at 350 °C differs from that of the sample at 350 °C before the first heating in SO2/O2 as seen in Figure 7. The differences include, for both samples, the absence of the shoulder at 970 cm−1 and the band at 615 cm−1 (both relative to crystalline M2SO4) after cooling. Furthermore, the peak at 1000 cm−1 (assigned to crystalline K4(VO)3(SO4)5) is also absent. This demonstrates that the sample has undergone irreversible modifications during the heating cycle and that the crystalline vanadium compounds formed upon cooling in process gas are different from those found after treatment in air. Furthermore, the peak assigned to the vanadyl bond shifts from ∼1050 to ∼1060 cm−1 indicating a transition from M to D upon cooling. The formation of D occurs at a lower temperature (∼410 °C) in the Cs-rich rather than in the Csfree sample (440 °C). The minor contribution of the band at 940 cm−1 for the spectra at low temperature suggests that D is dominating the vanadium distribution in both catalysts; however, in the Cs-rich catalyst the relatively higher intensity of the 840 cm−1 band evidences that D is involved in linked chains (lD), whereas in the Cs-free sample D is mainly isolated. Figure 8 includes a quantification of the relative intensity of the bands at 780, 840, and 990 cm−1 by the Gaussian fitting procedure for both samples during the cooling phase. This analysis shows similar trends compared to the heating phase in the same temperature range (Figure 7). The relative contribution of crystalline VIV species in the Cs-rich sample at low temperature is lower than in the Cs-free, owing to the higher contribution of linked complexes to the vanadium distribution. 3.4. Activity Measurements. Correlated with the Raman spectroscopy data, the composition of the gas exiting the reactor was monitored by an IR analyzer, and the SO2 conversion was obtained as a function of temperature. A turnover frequency (TOF) was obtained by normalizing the conversion by the weight and the vanadium content of the sample loaded in the Raman reactor. As these parameters were not accurately measured, the TOFs can only be compared qualitatively. Therefore, Figure 9 shows TOFs for the Cs-rich and Cs-free sample loaded in the Raman reactor on an arbitrary scale. In Figure 9, curves exhibit an abrupt slope change at a well-defined break temperature (Tb), which is attributed to the onset of precipitation of inactive VIV species.50 Tb occurs at ∼460 and ∼430 °C for the Cs-free and Cs-rich catalysts, respectively. Noticeably, Tb corresponds well to the temperature at which the 990 cm−1 peak assigned to VIV has a detectable signal, i.e., at below ca. 460 °C and ca. 440 °C for the Cs-free and Cs-rich samples, respectively, as seen in Figure 8. This indicates that the precipitation of VIV compounds can be the cause of the activity loss below Tb.

Figure 9. Temperature dependence of the turnover frequency (TOF) for the SO2 oxidation reaction for the Cs-rich and Cs-free samples. The TOF is obtained from conversion measurements during cooling from 500 to 350 °C of the Cs-free and Cs-rich samples in process gas.

catalyst and provide additional new insight about the dynamic transformations of the vanadia phase. We combine in situ TEM with operando Raman spectroscopy in order to correlate the catalytic performance of the catalyst samples with their molecular structure and the spatial dynamics of the melt. To validate this approach it is important to consider carefully the applied experimental conditions for each of the techniques. The dynamic behavior revealed by TEM and Raman matches qualitatively. The temperature for the solid-melt transition is around 480 °C for the Cs-free and Cs-rich samples in the TEM experiment, while in the Raman experiments the temperature at which the VIV peak disappears is ca. 460 °C and ca. 440 °C for the Cs-free and Cs-rich samples, respectively. A more precise comparison of the two techniques is difficult because the heterogeneous loading causes large differences in the local observations provided by TEM and because the applied heating rates differed markedly (∼100 °C/s in the electron microscope versus ∼20 °C/min in the Raman cell). However, it is important to also consider the shift in the extent of the SO2 oxidation reaction upon heating. At higher temperatures (600 °C), α approaches ∼1.48 and ∼1.79 for the TEM and Raman experiments, respectively. This reflects that the TEM experiments are conducted with a low amount of catalyst compared to the surrounding gas volume and are therefore corresponding to conditions of negligible conversion and SO3 partial pressure. In contrast, the Raman experiments represent conversions up to ∼80% and a correspondingly higher SO3 partial pressure at high temperatures. At higher temperatures, more marked deviations between the two methods are indeed expected because the reaction equations 6−7 will be shifted depending on the SO3 pressure. That is, in the TEM experiments, the low conversion and low SO3 partial pressure effectively imply a more reducing SO2-rich atmosphere yielding more solid VIV

4. DISCUSSION It is well-known that the catalytic active state of vanadia-based catalysts for the SO2 oxidation reaction is dynamically responding to changes in the reaction environment. The vanadia phase changes from a solid sulfate to a molten pyrosulfate state that disperses across the support.3,51 The present “live” observations corroborate this picture of the active L


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The Journal of Physical Chemistry C compounds16 and a higher degree of pyrosulfate decomposition, as also indicated in the isothermal TEM experiment at 600 °C. The conditions in the two types of gas cells approach each other at lower temperatures, where the conversion drops to ∼10% at about 410 °C in the Raman setup. The resulting convergency in α at lower temperatures therefore allows us to correlate the TEM and Raman observations. The correlation of TEM and Raman data reveals that molten phases contain the dimeric (VVO)2O(SO4)44− complex (D), which is commonly believed to be the catalytic important species. The molten phase dynamically disperses on the support with a strong dependency on the support morphology in such a way that it preferentially migrates into interstitial spaces of concave regions in both the Cs-free and Cs-rich catalyst. This preference is attributed to capillary forces that tend to minimize the surface free energy of the melt. Moreover, at lower temperatures (450 °C), the melt coexists with crystalline phases containing VIV species. The crystalline materials separate out during the activation onto convex support surfaces, and a portion of crystalline material may also be immersed in the molten phase as well. Thus, the dynamic behavior of the vanadia phase shows that the melt surface and, in turn, the gas accessibility to the catalytic active VV species can be optimized by employing a highly corrugated support, i.e., a silica carrier characterized by a high porosity made up by narrow and short pores. The variable-temperature experiments also reveal that a molten phase emerges in the Cs-rich catalyst at lower temperatures than in the Cs-free. Thus, Cs seems to promote the activation of the catalyst by reducing the temperature onset for the transition from sulfate to pyrosulfate, in agreement with ref 49. The observations also suggest an additional role of the Cs promoter, as the Cs-rich catalyst contains a higher abundance of linked VV complexes (lM and lD) compared to the Cs-free catalyst. When the VIV concentration in the melt exceeds the solubility limit, precipitation of crystalline VIV compounds occurs.52 We speculate that the presence of vanadium in linked VV chains hinders the formation of VIV compounds and therefore the depletion of the melt from active VV species. One additional role of Cs might therefore be that of stabilizing vanadium in the active VV state by forming linked structures at low temperature. The presence of these linked structures could also explain the more viscous behavior often observed for the Cs-rich catalysts.30 We also observe that the temperature of transformation between M and D is lowered in the presence of Cs. This finding is consistent with the fact that Cs is known to favor the formation of pyrosulfates and higher sulfates more than other smaller alkali cations,40 which might shift the equilibrium reaction eq 921,50 to the left

low temperatures due to an increase of the melt viscosity and crystallization. The apparent activation energies calculated from the Arrhenius plot in Figure 9 are ∼48 kJ/mol and ∼145 kJ/ mol for T > Tc and T < Tc, respectively, and match well with those obtained for supported model catalysts and industrial catalysts.28,29 The role of Cs is therefore that of stabilizing vanadium in the VV state and preventing VIV formation at low temperature. Finally, we cannot exclude that Cs has the additional effect of lowering the activation energy of one or more steps in the catalytic cycle, as a consequence of its less polarizing character compared to other alkali metal precursors.

5. CONCLUSIONS A realistic model system for the SO2 oxidation catalyst was prepared to investigate the effect of Cs on the chemical and structural dynamics of the active phase. The model system was analyzed by means of in situ TEM and operando Raman spectroscopy in a reactive atmosphere composed of SO2/O2. By combining structural, spectroscopic, and catalytic data, dynamic insight was provided about the relationship between the catalyst structure and functionality. Specifically the observations demonstrate that the surface roughness of the support plays an important role in the dispersion of the active phase in industrial SO2 oxidation catalysts. Moreover, the precipitation of solid material from the melt and the formation of VIV species occur concurrently with an abrupt loss of activity. The precipitation and activity drop occur at lower temperature for the Cs-rich than for the Cs-free catalyst. Importantly, the Cs has more linked vanadia-species than the Cs-free below this transition, indicating that the degree of linking in the melt has an influence on the formation of deactivation products.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10711. Electron illumination conditions, isothermal experiments, variable-temperature experiments, control experiments, and effect of the laser beam in Raman spectroscopy (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] ORCID

S. Helveg: 0000-0002-0328-8295 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Haldor Topsoe A/S for supporting this work. The authors thank S. Ullmann for support with the in situ electron microscopy experiments.

2VO2 (SO4 )32 − + S2O27 − ⇌ (VO)2 O(SO4 )44 − + 2SO24 − (9)

The reactivity data shown in Figure 9 reveal that the activity of the Cs-rich catalyst is higher than that of the Cs-free catalyst at low temperatures (below ∼430 °C). In particular, Tb ∼ 460 °C for the Cs-free catalyst, whereas Tb ∼ 430 °C for the Cs-rich catalyst. It has been shown that the breaking point in the Arrhenius plot can be removed by accounting for only the VV in the sample.50 The activity below Tb decreases because of the formation of VIV compounds and consequent depletion of the melt from active VV. Moreover, mass transport is also limited at

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REFERENCES

(1) Pope, C. A.; Rodermund, D. L.; Gee, M. M. Mortality Effects of a Copper Smelter Strike and Reduced Ambient Sulfate Particulate Matter Air Pollution. Environ. Health Perspect. 2007, 115 (5), 679− 683. (2) Frazer, J. H.; Kirkpatrick, W. J. A New Mechanism for the Action of the Vanadium Pentoxide-Silica-Alkali Pyrosulfate Catalyst for the

M


Article

The Journal of Physical Chemistry C Oxidation of Sulfur Dioxide. J. Am. Chem. Soc. 1940, 62 (7), 1659− 1660. (3) Villadsen, J.; Livbjerg, H. Supported Liquid-Phase Catalysts. Catal. Rev.: Sci. Eng. 1978, 17, 203−272. (4) Topsøe, H. F. A.; Nielsen, A. The Action of Vanadium Catalysts in the Sulfur Trioxide Synthesis. Catal. Today 2006, 111, 12−18. (5) Topsøe, H. F. A.; Nielsen, A. A New Procedure for Sulphuric Acid Manufacture by the Contact Process. Catal. Today 2006, 111 (1− 2), 19−21. (6) Rony, P. R. Supported Liquid-Phase Catalysts. Chem. Eng. Sci. 1968, 23 (9), 1021−1034. (7) Balzhinimaev, B. S.; Ivanov, A. A.; Lapina, O. B.; Mastikhin, V. M.; Zamaraev, K. I. Mechanism of Sulphur Dioxide Oxidation over Supported Vanadium Catalysts. Faraday Discuss. Chem. Soc. 1989, 87, 133−147. (8) Lapina, O. B.; Mastikhin, V. M.; Shubin, A. A.; Eriksen, K. M.; Fehrmann, R. Multinuclear NMR Studies of V2O5-Cs2S2O7 Melts. J. Mol. Catal. A: Chem. 1995, 99 (2), 123−130. (9) Kozyrev, V.; Balzhinimaev, B. S.; Boreskov, G. K.; Ivanov, A. A.; Mastikhin, V. M. ESR Studies of Slow Relaxations Relaxations. React. Kinet. Catal. Lett. 1982, 20 (1−2), 53−57. (10) Folkmann, G. E.; Eriksen, K. M.; Fehrmann, R.; Gaune-Escard, M.; Hatem, G.; Lapina, O. B.; Terskikh, V. Conductivity, NMR Measurements, and Phase Diagram of the K2SO7-V2O5 System. J. Phys. Chem. B 1998, 102 (1), 24−28. (11) Lapina, O. B.; Terskikh, V. V.; Shubin, A. A.; Eriksen, K. M.; Fehrmann, R. High-Temperature Multinuclear Magnetic Resonance Studies of Vanadia Catalysts for SO2 Oxidation. Colloids Surf. A 1999, 158 (1−2), 255−271. (12) Masters, S. G.; Chrissanthopoulos, A.; Eriksen, K. M.; Boghosian, S.; Fehrmann, R. Catalytic Activity and Deactivation of SO2 Oxidation Catalysts in Simulated Power Plant Flue Gases. J. Catal. 1997, 166, 16−24. (13) Eriksen, K. M.; Fehrmann, R.; Bjerrum, N. J. ESR Investigations of Sulfuric Acid Catalyst Deactivation. J. Catal. 1991, 132 (1), 263− 265. (14) Oehlers, C.; Fehrmann, R.; Masters, S. G.; Eriksen, K. M.; Sheinin, D. E.; Bal’zhinimaev, B. S.; Elokhin, V. I. In-Situ Spectroscopic Studies and Modelling of Crystallization Processes of Sulphuric Acid Catalysts. Appl. Catal. A 1996, 147, 127−144. (15) Rasmussen, S. B.; Eriksen, K. M.; Fehrmann, R. EPR and UV/ VIS Spectroscopic Investigations of VO2+ Complexes and Compounds Formed in Alkali Pyrosulfates. J. Chem. Soc. Dalt. Trans 2002, 87−91. (16) Christodoulakis, A.; Boghosian, S. Molecular Structure of Supported Molten Salt Catalysts for SO2 Oxidation. J. Catal. 2003, 215 (1), 139−150. (17) Boghosian, S.; Chrissanthopoulos, A.; Fehrmann, R. Structure of Vanadium Oxosulfato Complexes in V2O5-M2S2O7-M2SO4 (M = K, Cs) Melts. A High Temperature Spectroscopic Study. J. Phys. Chem. B 2002, 106, 49−56. (18) Giakoumelou, I.; Parvulescu, V.; Boghosian, S. Oxidation of Sulfur Dioxide over Supported Solid V2O5/SiO2 and Supported Molten Salt V2O5-Cs2SO4/SiO2 Catalysts: Molecular Structure and Reactivity. J. Catal. 2004, 225 (2), 337−349. (19) Giakoumelou, I.; Caraba, R. M.; Parvulescu, V. I.; Boghosian, S. First in Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts. Catal. Lett. 2002, 78, 209−214. (20) Boghosian, S. Vibrational Modes and Structure of vanadium(V) Complexes in M2SO4−V2O5 (M = K or Cs) Molten Salt Mixtures. J. Chem. Soc., Faraday Trans. 1998, 94, 3463−3469. (21) Boghosian, S.; Borup, F.; Chrissanthopoulos, A. Vanadium (V) Complexes in Molten Salts of Interest for the Catalytic Oxidation of Sulphur Dioxide. Catal. Lett. 1997, 48, 145−150. (22) Pârvulescu, V. I.; Paun, C.; Pârvulescu, V.; Alifanti, M.; Giakoumelou, I.; Boghosian, S.; Rasmussen, S. B.; Eriksen, K. M.; Fehrmann, R. Vanadia-Silica and Vanadia-Cesium-Silica Catalysts for Oxidation of SO2. J. Catal. 2004, 225 (1), 24−36.

(23) Beato, P.; Schachtl, E.; Barbera, K.; Bonino, F.; Bordiga, S. Operando Raman Spectroscopy Applying Novel Fluidized Bed MicroReactor Technology. Catal. Today 2013, 205, 128−133. (24) Hatem, G.; Eriksen, K. M.; Gaune-Escard, M.; Fehrmann, R. SO2 Oxidation Catalyst Model Systems Characterized by Thermal Methods. Top. Catal. 2002, 19, 323−331. (25) Hansen, N. H.; Fehrmann, R.; Bjerrum, N. J. Complex Formation in Pyrosulfate Melts. 1. Potentiometric, Cryoscopic, and Spectrophotometric Investigations of the Systems Potassium DisulfatePotassium Sulfate and Potassium Disulfate-Potassium Sulfate-Vanadium Pentoxide in the Temperature Range 410−450 °C. Inorg. Chem. 1982, 21 (2), 744−752. (26) Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. Complex Formation in Pyrosulfate Melts. 2. Calorimetric Investigations of the Systems V2O5-K2S2O7, V2O5·K2S2O7-K2SO4, V2O5·2K2S2O7-K2SO4 and V2O5·3K2S2O7-K2SO4 at 430 °C. Inorg. Chem. 1986, 25 (8), 1132−1137. (27) Hatem, G.; Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. Complex Formation in Pyrosulfate Melts. 3. Density and Conductometric Measurements of the System Vanadium Pentoxide-Potassium Pyrosulfate in the Temperature Range 350−490°C. J. Phys. Chem. 1987, 91 (1), 195−203. (28) Eriksen, K. M.; Karydis, D. A.; Boghosian, S.; Fehrmann, R. Deactivation and Compound Formation in Sulfuric Acid Catalysts and Model Systems. J. Catal. 1995, 155, 32−42. (29) Boghosian, S.; Fehrmann, R.; Bjerrum, N. J.; Papatheodorou, G. N. Formation of Crystalline Compounds and Catalyst Deactivation during SO2 Oxidation in V2O5-M2S2O7 (M = Na, K, Cs) Melts. J. Catal. 1989, 119, 121−134. (30) Doering, F. J.; Berkel, D. A. Comparison of Kinetic Data for K/ V and Cs/V Sulfuric Acid Catalysts. J. Catal. 1987, 103, 126−139. (31) Christensen, K. A. Chemical Product Design: Toward a Perspective Through Case Studies; Ng, K. M., Gani, R., Dam-Johansen, K., Eds.; Elsevier, 2007; Vol. 23. (32) Stö ber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62−69. (33) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pérez-Ramírez, J. On the Introduction of Intracrystalline Mesoporosity in Zeolites upon Desilication in Alkaline Medium. Microporous Mesoporous Mater. 2004, 69 (1−2), 29−34. (34) Jinschek, J. R.; Helveg, S. Image Resolution and Sensitivity in an Environmental Transmission Electron Microscope. Micron 2012, 43 (11), 1156−1168. (35) Hansen, P. L.; Helveg, S.; Datye, A. K. Atomic-Scale Imaging of Supported Metal Nanocluster Catalysts in the Working State. Adv. Catal. 2006, 50, 77−95. (36) Hansen, L. P.; Johnson, E.; Brorson, M.; Helveg, S. Growth Mechanism for Single- and Multi-Layer MoS2 Nanocrystals. J. Phys. Chem. C 2014, 118 (39), 22768−22773. (37) Su, D.; Wieske, M.; Beckmann, E.; Blume, A. Electron Beam Induced Reduction of V 2O5 Studied by Analytical Electron Microscopy. Catal. Lett. 2001, 75 (1), 81−86. (38) Kisielowski, C.; Wang, L. W.; Specht, P.; Calderon, H. A.; Barton, B.; Jiang, B.; Kang, J. H.; Cieslinski, R. Real-Time SubÅngstrom Imaging of Reversible and Irreversible Conformations in Rhodium Catalysts and Graphene. Phys. Rev. B 2013, 88 (2), 24305. (39) Wagner, J. B.; Cavalca, F.; Damsgaard, C. D.; Duchstein, L. D. L.; Hansen, T. W. Exploring the Environmental Transmission Electron Microscope. Micron 2012, 43 (11), 1169−1175. (40) Tandy, G. H. The Role of Alkali Sulphates in Vanadium Catalysts for Sulphur Dioxide Oxidation. J. Appl. Chem. 1956, 6 (2), 68−74. (41) Vesborg, P. C. K.; Chorkendorff, I.; Knudsen, I.; Balmes, O.; Nerlov, J.; Molenbroek, A. M.; Clausen, B. S.; Helveg, S. Transient Behavior of Cu/ZnO-Based Methanol Synthesis Catalysts. J. Catal. 2009, 262 (1), 65−72. (42) Mølgaard Mortensen, P.; Hansen, T. W.; Wagner, J. B.; Degn Jensen, A. Modeling of Temperature Profiles in an Environmental N


Article

The Journal of Physical Chemistry C Transmission Electron Microscope Using Computational Fluid Dynamics. Ultramicroscopy 2015, 152, 1−9. (43) Kalampounias, A. G.; Tsilomelekis, G.; Boghosian, S. Liquid Phase Dynamics of Molten M2S2O7 (M = K, Cs): A Temperature Dependent Raman Spectroscopic Study. Vib. Spectrosc. 2013, 65, 66− 73. (44) Selbin, J. The Chemistry of Oxovanadium(IV). Chem. Rev. 1965, 65 (2), 153−175. (45) Fehrmann, R.; Boghosian, S.; Papatheodorou, G. N.; Nielsen, K.; Berg, R. W.; Bjerrum, N. J. Crystal Structure and Infrared and Raman Spectra of K4(VO)3(SO4)5. Inorg. Chem. 1989, 28 (10), 1847− 1853. (46) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds; Wiley, 1963. (47) Yuan, P.; He, H. P.; Wu, D. Q.; Wang, D. Q.; Chen, L. J. Characterization of Diatomaceous Silica by Raman Spectroscopy. Spectrochim. Acta, Part A 2004, 60 (12), 2941−2945. (48) Humbert, B.; Burneau, A.; Gallas, J. P.; Lavalley, J. C. Origin of the Raman Bands, D1 and D2, in High Surface Area and Vitreous Silicas. J. Non-Cryst. Solids 1992, 143, 75−83. (49) Christodoulakis, A.; Machli, M.; Lemonidou, A.; Boghosian, S. Molecular Structure and Reactivity of Vanadia-Based Catalysts for Propane Oxidative Dehydrogenation Studied by in Situ Raman Spectroscopy and Catalytic Activity Measurements. J. Catal. 2004, 222 (2), 293−306. (50) Lapina, O. B.; Bal’zhinimaev, B. S.; Boghosian, S.; Eriksen, K. M.; Fehrmann, R. Progress on the Mechanistic Understanding of SO2 Oxidation Catalysts. Catal. Today 1999, 51, 469−479. (51) Livbjerg, H.; Christensen, T.; Hansen, T.; Villadsen, J. Theoretical Foundation of Cluster Formation in Supported LiquidPhase Catalysts. Sadhana 1987, 10, 185−216. (52) Bunimovich, G. A.; Vernikovskaya, N. V.; Strots, V. O.; Balzhinimaev, B. S.; Matros, Y. S. SO2 Oxidation in a Reverse-Flow Reactor: Influence of a Vanadium Catalyst Dynamic Properties. Chem. Eng. Sci. 1995, 50 (4), 565−580.

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