Interaction of SO2 with ZnO Nanoshapes: Impact of Surface Polarity

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Interaction of SO With ZnO Nanoshapes: Impact of Surface Polarity Si Luo, Jue Liu, and Zili Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02155 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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The Journal of Physical Chemistry

Interaction of SO2 with ZnO Nanoshapes: Impact of Surface Polarity Si Luo,a Jue Liu,b Zili Wua* a Chemical

Science Division and Center for Nanophase Materials Sciences, Oak Ridge National

Laboratory, Oak Ridge, 37830, TN, United States b Neutron

Scattering Division, Oak Ridge National Laboratory, Oak Ridge, 37830, TN, United

States

Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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ABSTRACT As petroleum related products dominates the global fuel market, acid gas resistant materials are in demand. ZnO is one of the widely used materials as SO2 sorbent and catalyst/catalyst support. To understand the surface structure sensitivity in interaction with SO2, morphology controlled ZnO wire and plate with nonpolar/polar facets have been investigated as model materials in this work. Morphology and crystal structure of nano- to micro- sized ZnO wire and plate were confirmed by SEM, XRD and Raman characterizations. SO2 interaction with the ZnO plate (dominated by polar facet {0001}) and ZnO wire (dominated by nonpolar facet {1010}) were investigated by in situ IR coupled with temperature programmed desorption (TPD) as well as XPS. SO2 interaction with ZnO resulted in more sulfate species on the wire than on the plate while some sulfates and sulfites still remained on both surfaces after heating to 350 °C. TPD showed higher SO2 desorption capacity on the ZnO plate surface at lower temperature than on the nonpolar wire surface, indicating a stronger interaction between SO2 and the nonpolar surface of ZnO. Acid-base property probed by adsorption microcalorimetry and reducibility of ZnO probed by H2-TPR were analyzed to understand the difference in SO2 behavior on different ZnO surfaces. The similar reducibility of the two ZnO surfaces from H2-TPR indicates the SO2 – ZnO interaction is more related to the surface structure and acid-base property of ZnO, supported by the higher density of both acid and base sites probed by NH3 and CO2 on the ZnO wire than on the plate. The results from this work suggest that polar surface of ZnO can be the preferred facet when engineering ZnO-based materials with enhanced SO2-resistance.


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1. INTRODUCTION Sulfur dioxide (SO2) is a well-known toxic gas that is harmful to environment and human health. Acid rain resulted from accessed SO2 generated in human activity has attracted great attention to the concern of environmental pollution.1-2 SO2 is mainly generated by the burning of natural fuel in auto mobile device and the industrial process related to petroleum. As a corrosive acid gas, SO2 causes great loss in the industry instrumentation. Moreover, the very reactive nature of SO2 makes it a poison of many functional catalysts, among which the three-way catalyst in auto exhaust system is a classic example. SO2 could deactivate the metal/oxide catalysts and thus great efforts are needed to remove it or develop SO2-resistant catalysts, which remains a big challenge. 3-5 ZnO is a semiconducting metal oxide that has been widely used as sorbent, gas sensor, corrosion protector, catalyst/catalyst support, et. al.6-9 It is safe, stable and cheap, making it popular in various practical applications. It is a common sorbent in industry to remove SO2 and the component of catalysts used in many petroleum related process.9-13 Due to this fact, a lot of studies focusing on the SO2 interaction with ZnO were conducted in the last decades. DFT study indicated that SO2 poisoning could be converted on oxygen-deficient (1010) facet and that Znterminated (0001) polar facet is less active towards SO2 than O-terminated polar facet.14-16 Surface science study on the ZnO single crystal in UHV show that the oxygen site is active to form SO32-, SO42- upon interaction with SO2, while the SO2 bond to Zn site is very weak (only below 100 K); Zn terminated (0001) facet of ZnO is less reactive towards SO2 than polycrystalline ZnO.17-20 The investigation on powder ZnO materials found that ZnO presents high chemisorption capacity with multilayer adsorption; on the other hand, there were more binding sites for weakly adsorbed SO2 on large particles while the strongly bound species were 3 ACS Paragon Plus Environment


not size dependent; polycrystalline ZnO promotes the formation of sulfates due to low coordination sites and high mobility.8, 18-19, 21-22, However, the knowledge obtained from the studies on specific single crystal facets in UHV and from DFT calculation is not enough to understand the facts observed in the case of more complicated practical powder ZnO materials due to the so-called material and pressure gap.23 In recent years, ZnO materials with well controlled shapes have been successfully synthesized.24-27 These shape-controlled powder materials provide great opportunity to study the interaction of SO2 with ZnO with different morphology and surface structure. Information gained through these studies will be beneficial to design optimized catalyst for SO2-related processes. In this study, we chose the wire-like and plate-like ZnO as they expose mainly the nonpolar ({1010}) and polar ({0001}) facets, respectively and investigate their interactions with SO2 via a suite of approaches including temperature programmed desorption (TPD), reduction (TPR), in situ IR spectroscopy and XPS. In principle, the polar facet can be either oxygen terminated or Zn terminated, while the nonpolar facet exposes stoichiometric Zn and O atoms (see scheme in Figure 1).25-27 Both polar and nonpolar facets are stable in the powder ZnO materials and the difference in surface structure is instrumental in providing insights into the interaction mechanism of ZnO with SO2.

2. EXPERIMENTAL 2.1. Materials synthesis


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ZnO wire was synthesized by a surfactant assisted alcohol-thermal procedure according to reference.25 In a typical synthesis, 3.0 g of polyvinylpyrrolidone (PVP) (Sigma–Aldrich; M = 55,000) was dissolved in 180 ml of ethanol and then 0.75 g of zinc acetate dihydrate was slowly added into the solution. The resulting reaction mixture was stirred for several minutes, followed by the addition of 4.5 g of NaOH. A turbid solution was stirred and loaded into autoclave. It was then sealed and maintained at 80 °C for 24 h. The obtained material was washed with ethanol and distilled water for several times, respectively. ZnO wire was obtained followed by a calcination at 350°C in air for 3 h. ZnO plate was synthesized by surfactant directed crystallization according to the reported method.25 2.0 g of Polyvinylpyrrolidone (PVP) (Sigma–Aldrich; M = 40,000) was dissolved in 200 ml of pentanol, and then 8 ml of a 0.15 M aqueous NaOH solution and 6 ml of a 0.1 M ethanol solution of Zn(NO3)2·6H2O were sequentially added. The reaction mixture was then kept at 90 °C for 1 h. The obtained material was washed with ethanol and distilled water for several times, respectively. The ZnO plate was obtained followed by a calcination at 350 °C in air for 3 h. 2.2. XRD, SEM and Raman characterization X-ray diffraction (XRD) patterns were collected with a PANalytical X’Pert Pro system using Cu Kα radiation. Diffractograms were obtained at incident angles for 2θ = 5–80°. Scanning electron microscope (SEM) images were collected using a Zeiss Merlin system operating at 1.00 kV ~5.0 kV. Raman spectra were collected using an in-house-built multiple-wavelength Raman system that includes the laser excitation at λ = 532 nm. Raman scattering was collected using fiber optics connected directly to the spectrograph stage of the triple Raman spectrometer 5 ACS Paragon Plus Environment


(Princeton Instruments Acton Trivista 555). Brunauer-Emmett-Teller (BET) surface areas of the ZnO samples were measured via nitrogen adsorption at -196 oC using a Micrometrics Gemini 275 system. 2.3. In situ IR/TPD In situ IR experiments coupled with TPD typically includes firstly, pretreatment of catalysts to remove the impurities and prepare stoichiometric surfaces by O2 treatment at 350 °C for 1h ; secondly, adsorption of gas (1%SO2/Ar/He) at room temperature for 30 min and probe the surface species; thirdly, switch to inert gas and conduct TPD by heating with specific rate (10°C/min) and probe the desorption process by MS. IR spectra were taken at different temperatures to detect the remained surface species and reveal the binding strength of various surface species. IR spectra were collected using Thermo Nicolet Nexus 670 spectrometer in Diffuse Reflectance mode (DRIFTS) continuously throughout SO2 adsorption/desorption cycle. The outlet gases from the DRIFTS reactor (Pike Technologies HC-900) were analyzed using quadrupole mass spectrometer (Omnistar GSD-301 O2, Pfeiffer Vacuum). 2.4. Post-reaction X-ray photoelectron spectroscopy (XPS) analysis The surface SOx species was analyzed by XPS on ZnO samples after interaction with SO2 (1%SO2/2%Ar/He) for 1h at different temperatures. The samples were transferred quickly from the treatment cell to the vials which were purged with Ar for 5min and sealed for XPS analysis. X-ray photoelectron spectroscopy (XPS) measurements were performed under operating pressure of 3.0 × 10-7 mbar using an Al Kα micro fused monochromatized source (1486.6 eV) with a step size of 0.1 eV over 50 scans. XPS spectra were deconvoluted by using CASA XPS software. 6 ACS Paragon Plus Environment

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2.5. Adsorption microcalorimetry and TPR The heat of adsorption of NH3 and CO2 were measured using an adsorption microcalorimeter (Setaram Sensys Evo DSC) coupled with a Micromeritics 3Flex Characterization Analyzer.28 Typically, a sample of about 50 mg ~100 mg was loaded into one side of a custom-made quartz bi-tube and the tube was loaded onto the microcalorimeter. Each sample was evacuated and heated at 350 °C for 1 h, dosed with 500 mmHg of O2, evacuated for 30 min each for two rounds, and cooled down to 30 °C (CO2 adsorption) or 150 °C (NH3 adsorption). The temperature was held for 1 h to reach thermal equilibrium. Subsequently, the sample was exposed to small doses of NH3 or CO2 and then evacuated for 1 h at the same temperature. The adsorption heat was measured by the DSC microcalorimeter at the same time. After evaluation, small doses of with NH3 or CO2 were introduced again until it reached the saturation pressure to derive the reversible adsorption amount. The density of acid and base sites was determined by the amount of irreversibly adsorbed NH3 and CO2, respectively.28

3. RESULTS AND DISSCUSSION 3.1. Oriented growth and crystal structure The as-synthesized ZnO wire and plate were characterized by SEM with images shown in Figure 1. The wire showed an average length of around 200 nm and a diameter of ~20 nm. While the ZnO plate presented a hexagonal plate with diameter of 130~150 nm and thickness of 30~50 nm. To further confirm the crystal structure and facets exposed on the ZnO wire and plate,



Figure 1. SEM images of ZnO wire (a, b) and plate (c, d) and (e) the corresponding scheme of the ZnO crystal shape and surface structure (Red represents oxygen atoms and blue represents Zn atoms). XRD patterns were collected and plotted in Figure 2. The main diffraction peaks at 31.8°, 34.5°, and 36.3° were attributed to the (100), (002) and (101) planes of wurtzite crystal structure for both ZnO wire and plate. The (100), (101) planes were associated with the nonpolar facet {101 0} and the peak (002) was associated with the polar facet {0001}. The noticeably different sharpness and intensities of XRD patterns of the nonpolar facets {1010} and the polar facets {0001} indicated the different crystal shape and growth orientation. The ratio of polar (002) vs. nonpolar (100) facets was 0.49 and 0.90 for ZnO wire and plate, respectively, indicating more polar facets exposed on the ZnO plate. The (002) peak for ZnO wire was sharp and intense, indicating the growth direction of wire was along [002]. For the plate, the polar (002) peak was weaker in intensity and broader, while the nonpolar (100) and (101) peaks were sharp and intense, proving the scale along [100] and [101] was enlarged and the growth along [002] was


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suppressed.21, 27 The crystal shape and corresponding facets exposed on ZnO wire and plate were shown in the scheme in Figure 1. Typically, the ZnO hexagonal crystal has an end surface of Zn terminated (0001) (Zn(0001)) polar surface and/or the oxygen terminated (0001) (O-(0001)) polar surface, while the other six side surfaces are equivalent nonpolar {1010} facets. The percentage of polar and nonpolar facets on the ZnO plate and wire can thus be roughly estimated with the dimensions from the SEM measurements. Apparently the ZnO wire is dominated by nonpolar facet (> 95%) while the plate has much larger proportion of polar facet (~60%). As for the coordination situation on the surfaces, only three coordinated oxygen atoms are exposed on the polar O(0001) facet, giving the negative polarity; While on the Zn-(0001) facet, only three coordinated Zn atom are exposed, giving the positive polarity. For the nonpolar {1010} facets, threecoordinated Zn and O atoms are exposed in stoichiometric manner.29-30 Theoretical oxygen density on the polar O-(0001) is 10.9 atoms/nm2 and for the nonpolar (1010) facets is 5.9 atoms/nm2. According to the size dimension measured in SEM, the nonpolar/polar surface area ratio is ~0.7 and 24 on the ZnO plate and wire, respectively. Hence the average oxygen density could be calculated. For the ZnO wire, O-terminated plate and Zn-terminated plate, the average theoretical surface oxygen density is 6.1, 8.9 and 2.4 atoms/nm2, respectively.



Figure 2. XRD pattern (a) and Raman spectra (b) of ZnO wire and plate Raman spectra of ZnO wire and plate excited by a laser line at 532 nm were shown in Figure 2. Both samples exhibit Raman peaks at 332, 440 and 582 cm-1 that can be assigned to the second order 2E2(M), first order E2(high) and E(LO) modes, respectively. The broad peak around 1100 cm-1 is due to the luminescence peak.31 E1(LO) mode is a forbidden mode from both the a and c faces. The ratio of E1(LO) (574~580) mode to the E2 modes for ZnO wire is much larger than that for ZnO plate, contributable to a higher density of oxygen vacancies on ZnO wire.32 The higher background of ZnO plate than the wire is likely due to fluorescence from trace impurity introduced during the synthesis process. A baseline-correction and normalization were done on the spectra to confirm that the correct interpretation of the Raman spectra can still be obtained regardless of the different baseline, as shown in Figure S1. 3.2.

Surface structure dependence of the interaction of SO2 with ZnO revealed by in situ IR/TPD and XPS

Figure 3. SO2 TPD for 2 cycles on ZnO wire (a) and ZnO plate (b) after adsorption at Room temperature for 30 min.


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The adsorption and desorption behaviors of adsorbates are descriptive for the surface properties of sorbent and catalyst materials. Herein, we conducted the temperature programmed desorption (TPD) experiments to study the behavior of SO2 on different ZnO facets. Figure 3 presents the TPD profiles of SO2 from ZnO wire and plate after 2 cycles of adsorption desorption. On ZnO wire, the temperature dependent desorption of SO2 showed a broad peak with two main features at ~100 and > 300 °C. These desorption peaks are indicative of the interaction strength of SO2 with ZnO wire: the stronger the binding strength, the higher the desorption temperature. In contrast, the SO2 TPD profile on ZnO plate only presents a major peak at 100 °C with much weaker tailing at higher temperatures, indicating that the SO2 species on ZnO plate was mainly weakly adsorbed. There are two possible factors that could result in this difference between the two ZnO surfaces. First, considering the significantly larger ratio of nonpolar to polar facets exposed on ZnO wire than on the plate, it could be that the SO2 interaction with polar facets mainly generates weakly bound species while strongly bound species come from SO2 adsorption on ZnO nonpolar facets. On the other hand, the species with higher binding energy may be formed at the defected surface sites and the ZnO wire with smaller size presents more defects than the plate as indicated by the Raman result (Figure 2). For the 2nd cycle of the TPD profile, the amount of SO2 desorption from both ZnO wire and plate increases compared to the first TPD, more significant for the wire than the plate. This indicates more of the adsorbed SO2 species are retained on the ZnO wire surface then the plate after heating to 350 °C in the first cycle. The amount of desorbed SO2 could be calculated from the integration of the area of the TPD spectra. Desorption capacity of SO2 on ZnO wire and plate normalized to surface area is 297 (448 for 2nd cycle) and 469 (552 for 2nd cycle) µmol/m2, respectively. The higher SO2 desorption capacity on ZnO plate indicated there were more and weaker binding sites



present on the polar facets of ZnO than on the nonpolar facets. In the second run, desorption capacity increased by 50% and 18% on the ZnO wire and plate, respectively. Although some of the strong basic sites on ZnO were occupied by SOx after the 1st cycle, these sites may still be able adsorb SO2 but with weaker interaction strength, leading to the increased SO2 desorption at lower temperature in the 2nd cycle.

The interaction of SO2 with ZnO could generate various species on the surface, IR spectroscopy is very helpful to identify the nature of surface species during the adsorption and desorption processes. Figure 4 shows the in situ IR spectra for SO2 adsorption at room temperature and desorption from ZnO wire at elevated temperatures from 25 to 350 °C. The spectra were obtained after SO2 adsorption with the IR spectra from the pretreated ZnO sample at specific temperatures as the background. Hence positive peaks shown here were attributed to the adsorbed surface species. During the adsorption process, characteristic bands centered at 1376 and 1163 cm-1 were due to gas phase SO2 and the peak at 1338 cm-1 was assigned to physisorbed SO2.21, 33 The intense peaks at 1293, 1318 cm-1 and the shoulder at 1244 cm-1 were associated with chemisorbed SO2 on the ZnO surfaces.34-35 On the other hand, the building up of sulfites was observed with the characteristic bands at 833, 961 and 1002 cm-1 for S O vibration.22, 33 The remaining bands were associated with sulfate species: the broad peak at 1000~1200 cm-1 were assigned to S O while the peaks at 1356, 1163 and 1393 cm-1 were assigned to S=O in SO42-.22, 33, 36 During the TPD, there was a distinct shift of the shoulder from 1322 to 1252 cm-1 with increasing temperature, due to the stronger chemisorption replaced the weakly adsorbed SO2.34 On the other hand, as temperature increased, some of the surface sulfite/sulfate species decomposed and desorbed from the surface as evidenced by the decrease of the characteristic IR bands of S O and S=O in SO32-/SO42- as assigned and summarized in


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Table 1. Meanwhile some species remained on the surface after desorption at 350 °C. The peak at 1178 cm-1 became more intense as temperature was increased. This peak could be due to bulk sulfates that are generated from the reaction of ZnO with SO2 species at elevated temperatures.37

Figure 4. In situ IR spectra of SO2 (a) adsorption from 0.5 min to 30 min and (b) desorption from ZnO wire at elevated temperatures from 25 to 350 °C.



Table 1. Summary of IR Band Assignments of Adsorbed SOx on ZnO wire IR wavenumbers (cm-1)

Assignment

Frequencies (ref)

1376, 1163

SO2 gas

1350,115033

1338

physisorbed SO2

133621

1322/1318, 1244/1252,

chemisorbed SO2

1320, 1250, 34 1304, 1135 35

1289/1293, 1130 1130, 1244/1252,

S=O, surface sulfates

1230, 1120 1130,22, 33

(SO42-) 1358/1356, 1408/1393

S O, surface sulfates

1250,37 1360, 140036

1047/1066, 1178

(SO42-)

1060, 118322, 33

1178

S O, surface sulfates

116037

961, 1000/1002, 1047

S O, surface sulfite

951,1003,105622, 33

(SO32-)

The IR spectra of SO2 adsorption on ZnO plate do not give any evident features and thus are only shown in the supporting information (Figure S2). The lack of IR signal, also observed in other work,25 is likely due to the electronic effects of the polar facets on ZnO plate. To study the surface species formed from SO2 interaction with ZnO plate, we turned to XPS investigation and compared with ZnO wire. Figure 5 present the XPS spectra of S 2p and O 1s core levels of ZnO wire and plate after exposing to SO2 at elevated temperatures ranging from 25 to 350 °C and then cooled down to room temperature in He before transfer to the XPS chamber with a brief exposure to air. Control experiments have been conducted to compare the desorption profiles of


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SO2 from ZnO with and without the brief exposure to air. The very similar TPD results proved that the exposure didn’t impact the surface speciation resulted from SO2 treatment. In the XPS S 2p spectra, there were two S 2p3/2 peaks observed with binding energy at 166.9 and 169.0 eV. The higher binding energy peak was attributed to metal sulfate (SO42-) while the lower binding energy peak was assigned to sulfite species (SO32-).17, 21 Deconvolution of S 2p spectra has been done to differentiate the different sulfur species observed on the ZnO samples. Detailed parameters used for the deconvolution of S 2p was shown in Table S1. The quantified fraction of sulfites and sulfates was summarized in Table 2. It could be seen that more sulfites were generated on ZnO plate at different temperatures while sulfates were the main species on the nonpolar wire at lower temperatures. Thus, it could be concluded that the polar facet of ZnO promotes the formation of sulfite species while nonpolar facet provides more sites for sulfate formation. When the temperature was increased to 350 °C, similar amount of sulfite and sulfate were remained on both ZnO surfaces. There were two peaks present in the O1s core level spectra. The main peak with a binding energy of 530.6 eV was assigned to the lattice oxygen of ZnO, while the peak at higher binding energy (~532.0 eV) was attributed to oxygen in sulfate/sulfite species.21 The distinct difference in the relative intensity of these two peaks indicates the difference in coverage of surface sulfate/sulfite species. It’s apparent that more sulfate/sulfite species were present on the ZnO wire than on the plate. Generally, the formation of surface sulfite requires the interaction of SO2 with lattice oxygen or hydroxyl group on the surface. (𝑆𝑂2(𝑔𝑎𝑠) + 𝑂𝐻 ― (𝑠𝑖𝑡𝑒) →𝑆𝑂3𝐻 ― (ads); 𝑆𝑂2(𝑔𝑎𝑠) + 𝑂2 ― (𝑙𝑎𝑡𝑡𝑖𝑐𝑒) ↔𝑆𝑂23 ― (ads))21 While the formation of surface sulfate results from the coordination of SO2 with two lattice oxygen sites. (𝑆𝑂2(𝑔𝑎𝑠) + 2𝑂 ― (𝑠𝑖𝑡𝑒) →𝑆𝑂24 ― (ads)) 21



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Therefore, significant amount of sulfite could be formed on both stoichiometric nonpolar facet and Zn- or O-terminated polar facet of ZnO. However, the formation of sulfate would not occur on the Zn-terminated polar facet as there is no two adjacent oxygen sites present. The fact that much less sulfate was detected on ZnO plate than on the wire indicates that the polar facet of ZnO plate is partially Zn-terminated with a mixture of O-termination.

Figure 5. Ex situ XPS spectra of ZnO wire (a: S 2p, b: O 1s) and plate (c: S 2p, d: O 1s) after treated in 1% SO2 at 25, 100, 200 and 350 °C. Table 2. Summary of quantified sulfites and sulfates calculated from S 2p Spectra deconvolution. sample

ZnO wire

ZnO plate

Sulfate (SO42-):

Sulfite (SO42-):

Sulfate (SO42-):

Sulfite (SO42-):

total S

total S

total S

total S

25 °C

0.70

0.30

0.46

0.54

100 °C

0.71

0.29

0.28

0.72


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200 °C

0.45

0.55

0.44

0.56

350 °C

0.45

0.55

0.37

0.63

3.3.

Acid-base property and reducibility characterization

Figure 6. Heat of adsorption of CO2 (a) and NH3 (b) on ZnO wire and plate SO2 is known as an acid gas with strong interaction with basic sites on solid materials. Therefore, the acid and base properties of ZnO wire and plate are very beneficial to understand the different behaviors in the interaction with SO2. Strength and density of acidic and basic sites of ZnO wire and plate were probed by NH3 and CO2 adsorption microcalorimetry and the heat of adsorption as a function of coverage (µmol/m2) was plotted in Figure 6. The initial heat of adsorption of CO2 was similar on ZnO wire and plate (134 and 129 kJ/mol, respectively). At low CO2 coverage (up to 0.4 µmol/m2), the enthalpy of CO2 adsorption on ZnO plate was essentially indistinguishable from that on the wire, indicating the similar strength of the basic site present on the plate and wire. On the other hand, the heat of adsorption of CO2 on ZnO wire was constant (75 ~ 90 kJ/mol) from coverages from 0.3 to 1.0 µmol/m2. In contrast, the heat of adsorption decreased almost linearly on ZnO plate at coverages from 0 to 1.1 µmol/m2. 17 ACS Paragon Plus Environment

This result was


reflected in the quantified irreversible differential adsorption of CO2: 2.1 and 0.8 µmol/m2 for wire and plate, respectively, indicating larger amount of the strong basic site present on ZnO wire. This conclusion is consistent with the SO2 TPD results (Figure 3.) where the ZnO wire had more strongly bound species desorbing at high temperature (300 °C) than on the plate. For the adsorption of ammonia, the heat of adsorption on ZnO plate was constantly higher than on the wire at most adsorption coverages, indicating stronger acidic sites present on the plate than on the wire. The density of acid sites characterized by the irreversible differential adsorption was higher for ZnO wire: 4.5 and 3.4 µmol/m2 for wire and plate, respectively. The quantified adsorption capacities of CO2 and NH3 based on surface area of ZnO samples were listed in Table 3, which showed that the density of both acidic and basic sites is higher on the ZnO wire. Considering the significant higher ratio of nonpolar/polar facets on the ZnO wire than on the plate, it is reasonable to conclude that the nonpolar facet of ZnO presents higher density of acid and base sites than the polar facet while the acid sites on the polar facet shows higher binding strength towards NH3. Interestingly, the density of basic sites probed by SO2 TPD was orders of magnitude higher than that probed by CO2 adsorption microcalorimetry. This could be due to the huge difference in the pressure of adsorbate used in the two experiments: a few torr for CO2 vs atmospheric pressure for SO2. Furthermore, the density of basic sites on ZnO plate was higher than on the wire as probed by SO2-TPD, different from CO2 probing. This difference is likely due to 1) some sulfate/sulfite species were still retained (more on the ZnO wire) on the surface of ZnO after SO2-TPD, thus the TPD result is not revealing the total density of surface sites; 2) the acidity difference between the two molecules: SO2 and CO2. Since SO2 is significantly more acidic than CO2, it could be that the density of weak basic sites is


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higher on the polar ZnO facet and thus can be readily probed by SO2, while the density of stronger basic sites is higher on the ZnO wire than the plate and thus is probed by CO2.

Table 3. Summary of BET surface area, CO2, NH3 and SO2 adsorption/desorption capacities on ZnO wire and plate

BET surface

CO2 adsorption

NH3 adsorption

SO2 desorption

area(m2/g)

(µmol/m2)

(µmol/m2)

(µmol/m2)

ZnO wire

34

2.1

4.5

297 (448 2nd)

ZnO plate

17

0.8

3.4

469 (552 2nd)

Figure 7. Temperature programed reduction of ZnO wire and plate by H2 probed by thermal conductivity detector from 50 to 700 °C (normalized to mass (a) and surface area (b) of ZnO sample).



The formation of sulfates involves the oxidation of adsorbed SO2 by ZnO, thus the reducibility is one of the key factors affecting the formation of sulfates during the interaction of SO2 with ZnO materials. Temperature programmed reduction (TPR) by H2 measurement was conducted on the ZnO wire and plate to characterize the reducibility of the materials. Figure 7 showed the TPR profiles that were normalized to either the mass or the surface area of the two samples. ZnO is a very stable metal oxide that needs quite high temperature (>600°C) to get fully reduced.9 Therefore, the TPR at lower temperature range was aimed to measure the reducibility of the surface/subsurface oxygen. Only one broad peak between 350 to 550 °C was observed in the H2-TPR profile for ZnO plate while two small features at 356 and 458 °C and one sharp reduction peak at 566 °C were seen for ZnO wire. The small broad peaks are most likely due to loosely bound surface oxygen removal while the sharp peak could be caused by partial reduction of ZnO lattice oxygen, according to the literature.38-40 The more pronounced reduction peaks on ZnO wire proved the better reducibility of ZnO wire than the plate at 350 ~ 650 °C temperature range. One the other hand, it could be seen that up to 350 °C, ZnO wire consumed more H2 than plate when normalized to mass. Yet they showed almost the same result when normalized to surface area from room temperature to 350 °C. Evidently, the polar and nonpolar facets of ZnO present a very similar reducibility in terms of surface/subsurface oxygen. As mentioned previously, there were significantly more sulfates formed on the ZnO wire than on the plate. Therefore, this result could be due to the structural difference on the polar/nonpolar facets of ZnO, other than the reducibility of the different facets. On the ZnO wire with nonpolar facet, stoichiometric Zn and O atoms were exposed on the surface along with some defects (indicated by Raman measurement), adjacent surface oxygen sites were available for the formation of SO42-. While on the polar facets, the mixed Zn- and O-terminated surfaces provide much less


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sites for the SO42- formation. Considering that ZnO wire presents a larger portion of nonpolar facets than plate (95% and 40%), it’s reasonable to observe more sulfates on ZnO wire as a result. Putting all the results together, it is apparent that SO2 interacts more strongly with ZnO wire than plate, which can be due to 1) the presence of higher density and strong basic sites on the nonpolar surface on the wire than the polar surface dominated on the plate; 2) more surface defects present on ZnO wire that could interact strongly with SO2. To regenerate the nonpolar surface, Ling et. al15 conducted DFT study on the interaction of SO2 with ZnO nonpolar (1010) and found that ZnO poisoned by SO2 could be regenerated by reducing the (1010) facet. Our TPR results showed that with a considerable reduction of the ZnO particles requires a pretty high temperature (~700 °C), thus in general applications it is not practical to use such a pretreatment. Instead, our results suggest that ZnO particles dominated by polar surface such as the plate can be a good candidate for application in SO2 environment as the interaction is relatively weak. This is consistent with previous surface science study by Rodriguez et. al18. They reported the investigation on SO2 adsorption on ZnO single crystal in UHV system and showed that SO2 adsorption on Zn-terminated (0001) plane only happened below 200K. However, under ambient condition as for the ZnO plate here, even the Zn-terminated ZnO surface is prone to be covered with hydroxyl groups from water dissociation, which provides base sites for SO2 adsorption to form sulfite/sulfate species. To conclude, SO2 poisoning could be minimized on reduced nonpolar facets and ideally Zn-terminated polar facets, while the regular nonpolar facets of ZnO is more reactive towards SO2 to form strongly bond sulfates. As for the size/morphology dependent properties of ZnO towards the interaction with SO2, the root could be the different ratios of polar/nonpolar facets exposed when the size/morphology of ZnO particles changes. 21 ACS Paragon Plus Environment


CONCLUSIONS Shape-controlled ZnO particles with polar ({0001}) and nonpolar ({1010}) facets have been synthesized and the crystal structure, acid-base and redox properties were characterized. The surface structure dependence of SO2 interaction with ZnO was systematically investigated by in situ IR/TPD and XPS. The nature, density and binding strength of adsorbed species from SO2 interaction were found dependent on the shape of ZnO particles and can be explained by the differences in acid-base property and surface structure of the polar and nonpolar facets. The stronger basic sites present on nonpolar facet results in more strongly bound sulfate species on ZnO wire, whereas the polar surface is less basic and thus interacts less strongly with SO2 to produce more sulfite species. Thus the polar surface of ZnO can be easily regenerated after SO2 exposure. The presence of both sulfate and sulfite species on ZnO plate may suggest that the polar ({0001}) surface is a mixture of Zn- and O-termination. The insights on the nature and strength of the SOx species over different ZnO surfaces from this work provide important inputs for future theoretical calculations and for the rational design of SO2-resistant catalysts/sorbents. Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Raman spectra with baseline correction and normalization, in situ IR spectra on ZnO plate during SO2 TPD, detailed parameters of deconvolution of S 2p XPS spectra in Figure 5 AUTHOR INFORMATION Corresponding Author


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*Email: [email protected]. Telephone: 865-576-1080 Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by U.S. Department of Energy, Office of Science, Basic Energy Sciences. Part of the work including the IR and Raman was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

REFERENCE (1) Fundamentals of Air Pollution (Fifth Edition), Vallero, D., Ed. Academic Press: Boston, 2014. (2) Greenberg, N.; Carel, R. S.; Derazne, E.; Bibi, H.; Shpriz, M.; Tzur, D.; Portnov, B. A. Different effects of long-term exposures to SO2 and NO2 air pollutants on asthma severity in young adults. J Toxicol Env Heal A 2016, 79, 342-351. (3) Cai, H.; Liu, Y.-f.; Gong, J.-k.; E, J.; Geng, Y.-h.; Yu, L.-p. Sulfur poisoning mechanism of three way catalytic converter and its grey relational analysis 2014, 21, 4091-4096.



(4) Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E. Overview of the fundamental reactions and degradation mechanisms of NOx storage/reduction catalysts AU. Catalysis Reviews 2004, 46, 163-245. (5) Wise, H., Mechanisms of catalyst poisoning by sulfur species. In Studies in Surface Science and Catalysis 1991, 68, 497-504. (6) Baei, M. T.; Tabar, M. B.; Hashemian, S. Zn12O12 Fullerene-like Cage as a Potential Sensor for SO2 Detection. Adsorpt Sci Technol 2013, 31, 469-476. (7) Jing, L. Q.; Xu, Z. L.; Sun, X. J.; Shang, J.; Cai, W. M.; Du, Y. G.; Fu, H. G. Photocatalytic activity of ZnO and TiO2 particles and their deactivation and regeneration. Chinese J Catal 2003, 24, 175-180. (8) Carnes, C. L.; Klabunde, K. J. Synthesis, isolation, and chemical reactivity studies of nanocrystalline zinc oxide. Langmuir 2000, 16, 3764-3772. (9) Sasaoka, E.; Hirano, S.; Kasaoka, S.; Sakata, Y. Stability of zinc oxide high-temperature desulfurization sorbents for reduction. Energ Fuel 1994, 8, 763-769. (10) Li, Z.; Qin, W.; Zhao, W. J.; Wu, X. H. Synthesis of flower-like Al doped ZnO microstructures by hydrothermal process and analysis of their gas sensing properties. Funct Mater Lett 2014, 7, 04. (11) Kowsari, E.; Bazri, B. Synthesis of rose-like ZnO hierarchical nanostructures in the presence of ionic liquid/Mg2+ for air purification and their shape-dependent photodegradation of SO2, NOx, and CO. Appl Catal a-Gen 2014, 475, 325-334. (12) Jing, L. Q.; Xin, B. F.; Yuan, F. L.; Wang, B. Q.; Shi, K. Y.; Cai, W. M.; Fu, H. G. Deactivation and regeneration of ZnO and TiO2 nanoparticles in the gas phase photocatalytic oxidation of n-C7H16 or SO2. Appl Catal a-Gen 2004, 275, 49-54. (13) Siriwardane, R. V. C.; Daniel C.; Stiegel, G. J.; Gupta, R. P.; Turk, B. S. Durable zinc oxide-based regenerable sorbents for desulfurization of syngas in a fixed-bed Reactor. University of North Texas Libraries, Digital Library, crediting UNT Libraries Government Documents Department 2001. (14) Singh, S.; Singh, J.; Singh, G.; Kaura, A.; Tripathi, S. K. Interaction of NO2 and SO2 with ZnO [1010] surface using density functional theory. Aip Conf Proc 2016, 1731, 050056. (15) Ling, L. X.; Han, P. D.; Wang, B. J.; Zhang, R. G. DFT study on the regeneration mechanism of ZnO surface during the desulfurization of H2S. Fuel Process Technol 2013, 109, 49-56. (16) Prades, J. D.; Cirera, A.; Morante, J. R. Ab initio calculations of NO2 and SO2 chemisorption onto non-polar ZnO surfaces. Sensor Actuat B-Chem 2009, 142, 179-184. (17) Onsten, A.; Stoltz, D.; Palmgren, P.; Yu, S.; Claesson, T.; Gothelid, M.; Karlsson, U. O. SO2 interaction with Zn(0001) and ZnO(0001) and the influence of water. Surf Sci 2013, 608, 31-43. (18) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.; Dvorak, J. Chemistry of SO2 and NO2 on ZnO(0001)-Zn and ZnO powders: changes in reactivity with surface structure and composition. J Mol Catal a-Chem 2001, 167, 47-57. (19) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.; Kuhn, M. Reaction of SO2 with ZnO(000(1))-O and ZnO powders: photoemission and XANES studies on the formation of SO3 and SO4. Surf Sci 1999, 442, 400-412. (20) Chaturvedi, S.; Rodriguez, J. A.; Jirsak, T.; Hrbek, J. Surface chemistry of SO2 on Zn and ZnO: Photoemission and molecular orbital studies. J Phys Chem B 1998, 102, 7033-7043.


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Wu, C. M.; Baltrusaitis, J.; Gillan, E. G.; Grassian, V. H. Sulfur dioxide adsorption on ZnO nanoparticles and nanorods. J Phys Chem C 2011, 115, 10164-10172. (22) Li, J.; Shang, J.; Zhu, T. Heterogeneous reactions of SO2 on ZnO particle surfaces. Sci China Chem 2011, 54, 161-166. (23) Freund, H. J.; Kuhlenbeck, H.; Libuda, J.; Rupprechter, G.; Baumer, M.; Hamann, H. Bridging the pressure and materials gaps between catalysis and surface science: clean and modified oxide surfaces. Top Catal 2001, 15, 201-209. (24) Sun, Y. H.; Chen, L. M.; Bao, Y. F.; Zhang, Y. J.; Wang, J.; Fu, M. L.; Wu, J. L.; Ye, D. Q. The applications of morphology controlled ZnO in Catalysis. Catalysts 2016, 6, 12. (25) Zhang, H.; Sun, J. M.; Liu, C. J.; Wang, Y. Distinct water activation on polar/non-polar facets of ZnO nanoparticles. J Catal 2015, 331, 57-62. (26) Gobo, F.; Goto, T.; Long, T. F.; Yin, S.; Sato, T. Mild solution synthesis of plate-like and rod-like ZnO crystals. Res Chem Intermediat 2011, 37, 211-217. (27) Mclaren, A.; Valdes-Solis, T.; Li, G. Q.; Tsang, S. C. Shape and size effects of ZnO nanocrystals on photocatalyticactivity. J Am Chem Soc 2009, 131, 12540. (28) Ushakov, S. V.; Navrotsky, A. Direct measurements of water adsorption enthalpy on hafnia and zirconia. Appl Phys Lett 2005, 87, 164103. (29) Zhang, L. P.; Xu, S. H.; Yu, X. J.; Tan, L. X.; Wang, Q. N.; Yang, L. N. Shape-controlled synthesis of ZnO microstructures: The effects of inorganic shape directing and pH altering agents. J Nanosci Nanotechno 2014, 14, 5415-5420. (30) Golic, D. L.; Brankovic, Z.; Daneu, N.; Bernik, S.; Brankovic, G. Solvothermal syntheses of nano- and micro-sized ZnO powders with a controllable morphology. J Sol-Gel Sci Techn 2012, 63, 116-125. (31) Yoshikawa, M.; Inoue, K.; Nakagawa, T.; Ishida, H.; Hasuike, N.; Harima, H. Characterization of ZnO nanoparticles by resonant Raman scattering and cathodoluminescence spectroscopies. Appl Phys Lett 2008, 92, 113115. (32) Khan, A. Raman Spectroscopic Study of the ZnO Nanostructures. J Pak Mater Soc 2010, 4, 1. (33) Thompson, M. M.; Palmer, R. A. In situ Fourier transform infrared diffuse reflectance and photoacoustic spectroscopy characterization of sulfur-oxygen species resulting from the reaction of SO2 with CaCO3. Applied Spectroscopy 1988, 42, 945-951. (34) Siriwardane, R. V.; Woodruff, S. In situ Fourier transform infrared characterization of sulfur species resulting from the reaction of water vapor and oxygen with zinc sulfide. Ind Eng Chem Res 1997, 36, 5277-5281. (35) Hussla, I.; Heidberg, J. Adsorption of SO2 on Cscl(Film) under ultrahigh-vacuum - a far and mid infrared spectroscopic study. J Electron Spectrosc 1986, 39, 213-222. (36) Waqif, M.; Bazin, P.; Saur, O.; Lavalley, J. C.; Blanchard, G.; Touret, O. Study of ceria sulfation. Appl Catal B-Environ 1997, 11, 193-205. (37) Luo, T.; Gorte, R. J. Characterization of SO2-poisoned ceria-zirconia mixed oxides. Appl Catal B-Environ 2004, 53, 77-85. (38) Derrouiche, S.; La Fontaine, C.; Thrimurtulu, G.; Casale, S.; Delannoy, L.; Lauron-Pernot, H.; Louis, C. Unusual behaviour of Au/ZnO catalysts in selective hydrogenation of butadiene due to the formation of a AuZn nanoalloy. Catal Sci Technol 2016, 6, 6794-6805. (39) Wang, W. X.; Li, X. K.; Zhang, Y.; Zhang, R.; Ge, H.; Bi, J. C.; Tang, M. X. Strong metalsupport interactions between Ni and ZnO particles and their effect on the methanation performance of Ni/ZnO. Catal Sci Technol 2017, 7, 4413-4421. 25 ACS Paragon Plus Environment


(40) Lv, Y. H.; Yao, W. Q.; Ma, X. G.; Pan, C. S.; Zong, R. L.; Zhu, Y. F. The surface oxygen vacancy induced visible activity and enhanced UV activity of a ZnO1-x photocatalyst. Catal Sci Technol 2013, 3, 3136-3146.

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Interaction of SO2 with ZnO Nanoshapes: Impact of Surface Polarity

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