Vanadia-Based Catalysts for the Sulfur Dioxide Oxidation Studied In Situ by Transmission Electron Microscopy and Raman Spectroscopy

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1 Vanadia-Based Catalysts for the Sulfur Dioxide Oxidation Studied In Situ by Transmission Electron Microscopy and Raman Spectroscopy F. Cavalca 1, P. Beato 1 *, J. Hyldtoft 1, K. Christensen 1, and S. Helveg 1 * 1 Haldor Topsoe A/S, Haldor Topsøes Allé 1, DK-2800 Kgs. Lyngby, Denmark *pabb@topsoe.dk *sth@topsoe.dk 1. Electron illumination conditions The electron beam can change the catalyst sample and processes under investigation. To characterize the changes induced by the electron beam and to address the electron illumination conditions under which such changes are negligible, an in situ study of the vanadia-based catalyst is conducted with a systematic variation of the electron illumination conditions. Specifically, the Cs-rich catalyst was exposed to 10 mbar of SO 2 :O 2 = 1:1 at 300 C. This temperature is below the melting point for the vanadia sulfate and thus helped to address morphological changes caused by the electron beam and thermal processes. The primary electron energy was 300 kev and dose-rates of 0.5, 1, and 2 e - /Å 2 /s were employed. For each dose-rate, a previously unexposed area of the sample was examined using an illumination scheme mimicking the actual experiment. That is, the area was repeatedly exposed to the electron beam for ca. 1 min, including ca. 10 sec for locating the specimen and focusing the image and ca. 50 sec for image acquisition, followed by 1 min with the beam blanked off from the gas cell for ca. 1 min. This experiment was performed once. S1

2 Figure S1. Time-resolved TEM images of the Cs-rich sample during gas exposure as a function of the electron dose-rate (indicated in the first frame of each series). Each frame is cropped from the original TEM image. For each row, the as-prepared frames were recorded in vacuum (~ mbar) at room temperature and the subsequent images were extracted from an image series acquired during exposure to 10 mbar SO 2 :O 2 = 1:1 at 300 C. Each frame reports the total accumulated exposure of the illuminated area relative to time = 0, corresponding to the first exposure in gas atmosphere for each area. Figure S1 shows that, at the dose-rate of 0.5 e - /Å 2 /s, no visible changes were apparent in the specimen morphology after 30 min. At a dose-rate of 1 e - /Å 2 /s, changes such as shrinking of material (dashed circles) and deformation of supported structures (full circles) appeared after 2 minutes. Finally, at a dose-rate of 2 e - /Å 2 /s, changes in morphology (full circles) appeared after 1 min. In view of this evaluation, the actual observations reported in the manuscript were obtained using an electron dose-rate of 0.5 e - /Å 2 /s as that allows for a maximum of 30 images to be obtained of a specific specimen area without apparent changes in the sample. In the isothermal and variable-temperature experiments a S2

3 single area is illuminated at most 24 and 26 times, respectively, using the present illumination scheme with repeated exposures of the sample to the electron beam for ca. 1 min. Under these conditions the electron beam should thus have a negligible effect on the morphology of the vanadia-based catalyst. However, it cannot be excluded that the electron beam changes the molecular species in the samples, and thus, that the catalysis or dynamics of the liquid to some extent could have been affected by the electron beam. The post-mortem appearance of the examined areas and of other non-illuminated areas is similar (see Figure S2), and therefore further indicate that the electron beam had negligible effect. Figure S2. TEM images of the Cs-rich sample before (left) and after (right) the isothermal experiment at 450 C. A) reproduces the first and last frames of Fig. 2 in the main text. B) and C) represent areas that were not imaged during the experiment. All scale bars are 50 nm. S3

4 2. Isothermal experiments As described in the Experimental section, four regions were observed by TEM in a dose-fractionated way at nominal times given in Table S1. In the actual experiment, time is spent in moving the sample stage from one region to another as well as to stabilize the stage with respect to drift. Therefore, the actual time at which the region was exposed to the electron beam for imaging differed slightly from the nominal values. The actual times, relative to time t=0 at which the isothermal reaction conditions were established, is therefore denoted on each image. Moreover, as the dose-rate was 0.5 e - /Å 2 /s, the accumulated electron dose on a region is estimated by multiplying the dose-rate by the electron illumination time (1 min) and by the number of images obtained for that region. Therefore, each image is also associated with its number in the series image acquisitions from the specific region. Total accumulated dose in each area from A through D decreases by a factor of min 30 min 45 min 60 min 75 min 90 min 105 min 120 min 135 min A x x x x x x x x x B x x x x x C x x x D x x Table S1. Nominal image acquisition scheme. Different areas of the sample (labelled A through D) receive a twofold-different total electron exposure with respect to each other. A) is imaged roughly every 15 min, B) every ca. 30 min, C) every ca. 60 min and D) every ca. 120 min. S4

5 Figure S3. Time-resolved TEM images of three regions of the Cs-free sample under isothermal conditions. The series contains an image of the as-prepared catalyst in the microscope vacuum (~ mbar) at room temperature (As-prepared) and images of the catalyst during exposure to SO 2 :O 2 = 1:1 at 10 mbar total pressure at 450 C. The times are relative to t = 0 min, corresponding to the time for the establishment of the isothermal conditions. The number of the image in the series is denoted #. The electron dose-rate is 0.5 e - /Å 2 /s. Frame size 280 nm x 280 nm. The solid circles outline convex regions, and the dashed circles outline concave regions. S5

6 Figure S4. Time-resolved TEM images of three regions of the Cs-rich sample under isothermal conditions. Each series contains an image of the as-prepared catalyst in the microscope vacuum at room temperature (As-prepared) and images of the catalyst during exposure to SO 2 :O 2 = 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. The number of the image in the series is denoted #. The electron dose-rate is 0.5 e - /Å 2 /s. Frame size 280 nm x 280 nm. The solid circles outline convex regions, and the dashed circles outline concave regions. S6

7 Figure S5. Time-resolved TEM images of three regions of the Cs-free sample under isothermal conditions. The series contains images of the as-prepared catalyst in the microscope vacuum (~ mbar) at room temperature (As-prepared) and images of the catalyst during exposure to SO 2 :O 2 = 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. The number of the image in the series is denoted #. The electron dose-rate is 0.5 e - /Å 2 /s. Frame size 280 nm x 280 nm. The solid circles outline convex regions, and the dashed circles outline concave regions. In D), support spheres translate relative to the other spheres, which is probably due to sample drift or vanadia restructuring during heating. S7

8 Figure S6. Time-resolved TEM images of three regions of the Cs-rich sample under isothermal conditions. The series contains an image of the as-prepared catalyst in the microscope vacuum (~ mbar) at room temperature (As-prepared) and images of the catalyst during exposure to SO 2 :O 2 = 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. The number of the image in the series is denoted #. The electron dose-rate is 0.5 e - /Å 2 /s. Frame size 280 nm x 280 nm. The solid circles outline convex regions, and the dashed circles outline concave regions. S8

9 3. Variable-temperature experiments. Figure S7. TEM frames of area A of the Cs-free sample during heating (upper row) and cooling (lower row) in SO 2 :O 2 = 1:1 at 10 mbar total pressure. The temperature was varied in the range C in steps of 20 C and TEM images were acquired at each step. The displayed frames were cropped from images in the series spaced 80 C. The image number in the series is denoted by #. The electron doserate is 0.5 e - /Å 2 /s. Solid and dashed circles outline convex and concave regions, respectively. S9

10 Figure S8. TEM frames of area C of the Cs-free sample during heating (upper row) and cooling (lower row) in SO 2 :O 2 = 1:1 at 10 mbar total pressure. The temperature was varied in the range C in steps of 20 C and TEM images were acquired every 80 C. The displayed frames were cropped from the images in the series. The image number in the series is denoted by #. The electron dose-rate is 0.5 e - /Å 2 /s. Solid circles outline convex regions. S10

11 Figure S9. TEM frames of area D of the Cs-free sample during heating (upper row) and cooling (lower row) in SO 2 :O 2 = 1:1 at 10 mbar total pressure. The temperature was varied in the range C in steps of 20 C and TEM images were acquired every 160 C. The image number in the series is denoted by #. The electron dose-rate is 0.5 e - /Å 2 /s. Solid circles outline convex regions. S11

12 Figure S10. TEM frames of area A of the Cs-rich sample during heating (upper row) and cooling (lower row) in SO 2 :O 2 = 1:1 at 10 mbar total pressure. The temperature was varied in the range 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 80 C intervals. The image number in the series is denoted by #. The electron dose-rate is 0.5 e - /Å 2 /s. Solid circles outline convex regions. S12

13 Figure S11. TEM frames of area C of the Cs-rich sample during heating (upper row) and cooling (lower row) in SO 2 :O 2 = 1:1 at 10 mbar total pressure. The temperature was varied in the range C in steps of 20 C and TEM images were acquired every 80 C. The displayed frames were cropped from the images in the series. The image number in the series is denoted by #. The electron dose-rate is 0.5 e - /Å 2 /s. Solid and dashed circles outline convex and concave regions, respectively. S13

14 Figure S12. TEM frames of area D of the Cs-rich sample during heating (upper row) and cooling (lower row) in SO 2 :O 2 = 1:1 at 10 mbar total pressure. The temperature was varied in the range C in steps of 20 C and TEM images were acquired every 160 C. The image number in the series is denoted by #. The electron dose-rate is 0.5 e - /Å 2 /s. Solid and dashed circles outline convex and concave regions, respectively. 4. Control experiments The effect of heating the bare silica in different gas atmospheres in the electron microscope was studied to address the stability of the silica spheres. In different experiments, fresh silica spheres were exposed to either of the following gasses: 0.5mbar O 2 0.5mbar SO 2 S14

15 0.5mbar SO mbar O 2 Vacuum During gas exposure, each sample was heated to 600 C and temperature was kept constant for 1 hour. Images were acquired in situ every 15 minutes using an electron dose-rate of 0.5 e - /Å 2 /s. The images show no modifications in any of these samples. A second set of control experiments was run on the Cs-rich model system. First, a fresh sample was heated in vacuum (~ mbar) first from RT to 350 C in ~10 sec., then from 350 C to 600 C in steps of 10 C. TEM images were acquired at every temperature step, after waiting 15 minutes at each temperature, using a dose-rate of 0.5 e - /Å 2 /s. The TEM images showed no changes. A fresh Cs-rich sample was also exposed to pure O 2 at 0.5 mbar total pressure. The sample was heated first from RT to 350 C in ~10 sec., then from 350 C to 600 C in steps of 10 C. Images were acquired at every temperature step, after waiting 15 minutes at each temperature, using a dose-rate of 0.5 e - /Å 2 /s. Figure S13 shows TEM images from this heating series. At 500 C, the silica spheres start moving relative to each other. At 600 C, the silica spheres transform from having a round shape to a more irregular shape with less distinct boundaries between the spheres. This transformation resembles that observed in Figure S5D. This observation indicates that restructuring of the sample occurs in presence of oxygen and in absence of SO 2 at elevated temperatures. Such morphological change can in part be due to a movement of either the supported phase or the support material or both simultaneously. Figure S13. TEM images of Cs-rich catalyst during to exposure at varying temperature in 100% O 2 at 5 mbar total pressure. Electron dose-rate: 0.5 e - /Å 2 /s. S15

16 Finally, a fresh Cs-rich sample was exposed to pure SO 2 at 0.5 mbar total pressure. The sample was heated first from RT to 350 C in ~10 sec., then from 350 C to 600 C in steps of 10 C. Images were acquired at every temperature step, after waiting 15 minutes at each temperature, using a dose-rate of 0.5 e - /Å 2 /s. TEM images did not show any changes of the catalyst. 5. Effect of the laser beam in Raman spectroscopy The effect of the 514 nm focused laser beam at 10 mw (as measured on the sample with a power meter) on the Cs-rich model system was studied. The sample was loaded in the Raman cell as described in the Experimental section and was heated to 350 C in reaction gas with fluidization. A Raman spectrum of the sample was recorded in this condition, then fluidization was stopped and a second spectrum was recorded on the static sample. The temperature was then raised stepwise up to 500 C, 50 C at a time, and spectra were recorded at each temperature with and without fluidization, as just described. Comparisons of the spectra at each temperature are shown in Figure S14. S16

17 Figure S14. Raman spectra of Cs-rich sample in reaction gas at different temperatures. At 350 C, intensities of the peaks at 1000 and 975 cm -1 are different in the fluidized and static acquisitions. The fluidization causes the grains in the catalyst sample to be in constant motion with respect to the laser beam, thus yielding an averaged spectrum over a number of different particles. This is advantageous compared to static Raman spectroscopy as it reflects the average state of the catalyst instead of the local state of a particular catalyst grain. The spectra at 400 C have similar spectral S17

18 features, but the relative intensities of the peaks at 1085, 1000 and 730 cm -1 is lower in the static acquisition. The spectra acquired at 450 C show the most noticeable differences. Spectral features present in the fluidized acquisition, such as the peaks at 1085 and 1000 and 730 cm -1, are not visible in the static acquisition. Finally, at 500 C both fluidized and static acquisitions have comparable spectral features and all distinct bands observed at lower temperatures are absent. Notably, the static acquisition at 450 C resembles the spectra at 500 C. This suggests that the laser beam has a non-negligible effect on the spectra, causing visible changes in spectrum appearance that might lead to misinterpretation of data. Activity measurements with and without beam did not reveal any measurable change in the catalyst activity. Radiation-induced heating seems to be caused by prolonged exposure to the focused laser beam if the sample is not fluidized, resulting in a local temperature increase that can be estimated between C in the present experimental conditions. This estimate is based on the observation that the static acquisition at 450 C resembles that at 500 C, as noted above. Such beam effect is influenced by sample movement, gas flow, power density distribution and other local factors such as sample absorbance. Although addressing this critical aspect is of utmost importance to perform representative experiments in operando conditions, this aspect has only been touched upon vaguely in previous publications on SO 2 oxidation catalysts. We show that beam effect is reduced by fluidization, but we cannot exclude that effects are completely eliminated. S18