Tetragonal Rutile SnO2 Solid Solutions for NOx-SCR by NH3

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Tetragonal Rutile SnO2 Solid Solutions for NOx‑SCR by NH3: Tailoring the Surface Mobile Oxygen and Acidic Sites by Lattice Doping Jingyan Zhang,† Yaqian Liu,† Yue Sun,† Honggen Peng,† Xianglan Xu,† Xiuzhong Fang,† Wenming Liu,† Jianjun Liu,‡ and Xiang Wang*,† †

College of Chemistry, Institute of Applied Chemistry, Nanchang University, Nanchang, Jiangxi 330031, China Jiangxi Baoan New Material Technology Corporation, Ltd., Pingxiang, Jiangxi 337000, China

‡

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S Supporting Information *

ABSTRACT: To understand the active sites requirements for NOx-SCR by NH3 and for designing better catalysts, four SnO2-based solid solution samples with the SnO2 lattice doped by metal cations possessing redox ability (Ce4+ and Cu2+) or acidity (In3+ and W6+) have been purposely designed. It is revealed that the metal cations have entered into the SnO2 matrix to form a tetragonal rutile solid solution phase. Compared with individual SnO2, all the modified catalysts possess higher surface areas, lower crystallinity, more abundant surface defects, facile oxygen, and acidic sites. Therefore, they display improved reaction performance. Both surface facile oxygen and acidic sites play vital roles, and the balance between them controls the reaction. For the catalysts modified by Ce4+ and Cu2+, a better balance can be generated. Therefore, they display the best performance among all the samples. NOx-SCR by NH3 on the SnO2-based solid solutions follows a Langmuir−Hinshelwood mechanism.

1. INTRODUCTION Nitrogen oxides (NOx) coming from various combustion processes are harmful to the air atmosphere and human health.1,2 To date, several technologies3−6 have been adopted for the elimination of NOx from different sources. Among these, NOx selective catalytic reduction (SCR) by NH3 is the most feasible technology to abate NOx from both still sources and diesel engine emissions but still faces challenges. For instance, how to maintain acceptable NOx conversion at both the low and high temperature regions and how to restrict the byproducts (such as N2O, etc.) formation are still problems that need to be solved. Up to now, many catalysts have been reported in the literature, which can be divided majorly into three categories, such as supported precious metals,7,8 zeolitebased materials,2,9,10 and metal oxide catalysts.11,12 Precious metal catalysts have been testified to depict high efficiency for NOx-SCR, but the limited source and high costs have restricted their wide applications. Zeolite-based catalysts, such as Fe,9,13 Co,14 and Cu15 exchanged SSZ-13, ZSM-5, SAPO-34, and MOR, have also attracted much attention and been proven to be promising for NOx removal. However, the procedures to prepare this type of catalysts are generally complicated and consume a lot of time. Moreover, sometimes the structure-directing templates are expensive for the synthesis of some special zeolites. Furthermore, zeolite materials with more potent resistance to sulfur poisoning and better thermal stability are still desirable. Therefore, it is of great necessity to develop more feasible NOx-SCR catalysts with lower costs. © XXXX American Chemical Society

As an n-type semiconductor, SnO2 possesses rich deficient surface oxygen and Lewis acidic sites. Its lattice oxygen can also be reduced easily and involved in redox reactions. Furthermore, SnO2 has both excellent thermal and physical chemical stability because of it high melting point at 1630 °C.16−19 Our previous studies have demonstrated that a bunch of metal cations17−22 can enter into the crystal structure of rutile SnO2 to generate noncontinuous solid solutions, which can stabilize the specific surface areas and the metastable surface deficient oxygen sites at higher temperature, hence achieving catalysts with evidently enhanced reaction performance. To understand deeper the relationship between reactivity and structure of solid solution catalysts, a simple and feasible XRD extrapolation method has been developed by our group for the first time to quantify the lattice capacity of different cations in SnO2 matrix recently.18,21 It has been commonly accepted that a practical catalyst for effective NOx-SCR by NH3 requires the coexistence of both redox and acidic sites, and the balance between these two kinds of sites is regarded to decide the reaction performance.23−25 When using Al2O3, TiO2 and hexagonal WO3 (HWO) to support CeO2 to prepare catalysts for NOx-SCR by NH3, Tang and coauthors23 revealed that because HWO support contains both acidic sites and reducible active oxygen species, which can Received: Revised: Accepted: Published: A

May 23, 2018 June 29, 2018 July 4, 2018 July 4, 2018 DOI: 10.1021/acs.iecr.8b02288 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

reached around 8.0, which was then stirred for another 10 h and followed by centrifugation. The precipitate was washed completely by using DDI water until the filtrate was Cl− free, which was indicated by a TDS less than 20 ppm. The achieved solids were dried at 110 °C for ∼12 h, and followed by calcination at 550 °C in air atmosphere for 4 h to get the ultimate catalysts, with a heating ramp of 2 °C min−1. The catalysts are named SnIn9-1, SnW9-1, SnCe9-1, and SnCu9-1 according to the chemical compositions. The element compositions of the catalysts have been confirmed by ICP to be same to the original ratios adopted for sample preparations, as listed in Table 1. An unmodified SnO2 was prepared either by following the same procedure for comparison study.

improve the CeO2 dispersion and strengthen its interaction with the support, CeO2/HWO displays thus the best activity among all the catalysts. When studying ceria-based solid solutions for the reduction of NO with NH3, Dong and coauthors24 found that by doping Ti4+ or Sn4+ cations into the cubic fluorite CeO2 lattice, both the surface acidity and oxygen storage capacity can be improved, hence obtaining catalysts with much higher activity and broadened windows in comparison with the unmodified MnOx/CeO2. Recently, it was reported by Li and coauthors that Ce−Sn composite oxide catalysts26 prepared with a hydrothermal method showed high performance as well as good H2O and SO2 resistance for NOxSCR by NH3. The reason is ascribed to the synergism between Ce and Sn, which improves both the surface oxygen activity and the Lewis acidity of the catalysts, hence promoting their ability to adsorb and activate the NH3 species. When using Nb to modify SnO2−CeO2 for the same reaction, Zhang et al. discovered that the addition of Nb is able to improve the abundance of active surface oxygen species and the NH3 adsorption ability,27 which generates catalysts with significantly enhanced performance. On the basis of the above discussion, it is apparent that SnO2-based solid solutions can match most of the active site requirements for NOx-SCR by NH3 and deserve to be investigated systematically in more detail. Therefore, four SnO2-based solid solution catalysts with the tetragonal rutile SnO2 lattice matrix modified by metal cations possessing varied chemical properties have been prepared in this work by a traditional coprecipitation method. In detail, with the incorporation of In3+ and W6+ cations, the method is expected to generate more acidic sites; and with the incorporation of Ce4+ and Cu2+ cations, it is expected to produce more redox sites for the resulting catalysts. Indeed, the results in this study have testified that by the introduction of these secondary metal cations, all the modified binary catalysts display better activity than the individual SnO2, among which the catalysts promoted by Ce4+ and Cu2+cations show the best reaction performance. By using different characterization techniques, such as N2 adsorption−desorption, XRD, XPS, TEM, STEM-mapping, Raman, H2-TPR, O2-TPD, and NH3-TPD. The structural and textural properties, surface acidity, and redox behaviors of the catalysts have been investigated. On the basis of these results, the effects of the metal cation dopants on the catalytic performance have been assessed.

Table 1. XRD Quantification Results of the Catalysts lattice parameters

catalysts SnO2 SnIn9-1 SnW9-1 SnCe9-1 SnCu9-1

Sn/M molar ratio by ICP

a (Å)

c (Å)

8.9/1.1 8.9/1.1 9.1/0.9 9.0/1.0

4.720 4.721 4.709 4.740 4.730

3.218 3.224 3.216 3.225 3.217

α/β/ average γ crystallite (deg) sizea (nm) 90 90 90 90 90

8.4 5.6 5.0 5.9 5.2

average particles sizeb (nm) 10.1 7.6 5.6 8.1 6.5

a Obtained with Scherrer’s equation based on XRD (110) peak of SnO2. bEstimated from the TEM results.

2.2. Reaction Performance Evaluation and Catalyst Characterization. The reaction performance of the catalysts has been evaluated for NOx-SCR by NH3. The physical chemical nature of the catalysts has also been characterized by means of different techniques to see the doping effects of metal cations with different properties. The detailed information about the used equipment types and models, the operation condition, the experimental procedures, and parameters is included in the Supporting Information file.

3. RESULTS AND DISCUSSION 3.1. Reaction Performance Test. The catalysts have been tested by NOx-SCR with NH3 as the reductant and the profiles are shown in Figure 1. Figure 1a displays the NOx conversion on the catalysts at different temperatures. Pure SnO2 possesses certain activity for NOx reduction, and the highest 38% NOx conversion is achieved at 400 °C due to the coexistence of both surface active oxygen and acidic sites.28 After the incorporation of the secondary cations, the NOx conversion on all the modified catalysts has been improved more or less. Notably, with the modification by Ce4+ and Cu2+ cations, the reaction performance can be significantly improved. Over SnCe9-1, the maximum NOx conversion around 80% can be achieved at 300 °C; and over SnCu9-1, the maximum NOx conversion around 70% can even be achieved at a low temperature of 200 °C, indicating its superior activity at low temperature. In contrast, on SnIn9-1 and SnW9-1, the promotional effect is much weaker. On the basis of these results, it is reasonable to deduce that the incorporation of Ce4+ and Cu2+ into SnO2 to produce additional redox sites is more effective to enhance the reaction performance than the incorporation of In3+ and W6+ cations to create additional acidic sites. In addition, as shown in Figure 1b, N2 is the major product and the amounts of N2O

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. A traditional coprecipitation method has been used to synthesize the SnM9-1 (M = In, W, Ce, and Cu) solid solution samples having an atomic ratio of Sn/M = 9/1. All the chemicals are obtained from reliable commercialized sources and used directly. Ce(NO3)3·6H2O (AR), InN3O9·4H2O (AR) and SnCl4·5H2O (AR) were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Aqueous ammonia solution (25−28 wt %) was supplied by Xilong Chemical Company (Guangdong, China). Na2WO4·2H2O (AR) and Cu(NO3)2·3H2O (AR) were provided by Tianjing Chemical Company (Tianjing, China). To prepare a sample, the calculated amount of 0.5 mol L−1 SnCl4 and the secondary metal salts (0.5 mol L−1) solutions were mixed together and then stirred ∼2 h at ambient temperature to get a homogeneous solution mixture. Under constant stirring, the aqueous ammonia solution was then added dropwise into the mixed solution until the pH B

DOI: 10.1021/acs.iecr.8b02288 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. NOx-SCR by NH3 over the catalysts: (a) NOx conversion, (b) N2 and N2O concentration.

samples change slightly in comparison with pure SnO2 (Table S1), which provides additional proof to testify the possible formation of SnO2 based solid solutions.18,21 Theoretically, to effectually form a solid solution structure between two metal oxides, the two metal cations must meet the following two basic requirements:18,30

byproduct in the entire tested temperature region are negligible over all the catalysts, testifying that these SnO2based catalysts, either modified or unmodified, are highly selective to N2. In summary, metal cations with different chemical properties can pose varied modification effects on the reaction performance of SnO2, among which Ce4+ and Cu2+ cations have significant positive influence. 3.2. XRD Measurements. To analyze the phase structures of the catalysts, they have been tested by XRD. The patterns in Figure 2 demonstrate that for all the catalysts, three strongest

(1) (R1 − R2)/R1 < 30% (R1 and R2 are the radii of the two metal cations, R1 > R2); (2) X1 − X2 ≤ 0.4 (X1 and X2 are the electronegativities of the two metal cations). In this study, the Sn4+ cations in the rutile SnO2 lattice have a coordination number (CN) of 6, whose radius is 0.69 Å. XPS results displayed in Figure S1 have proven that the secondary metal cations are present predominantly as In3+, W6+, Ce4+, and Cu2+ cations, which will be discussed in more details in the XPS part. For these cations possessing also a CN of 6, their corresponding radii are 0.80, 0.62, 0.87, and 0.73 Å in sequence, which can meet the first requirement. Furthermore, all the secondary metal cations have close valence states to Sn4+, which enables them to meet the second requirement. Therefore, there is high possibility for them to be doped into the SnO2 lattice, hence forming solid solutions and evading detection by XRD. The unmodified SnO2 displays very sharp diffraction peaks, proving that it is well-crystallized. In contrast, the diffraction peaks of SnIn9-1, SnW9-1, SnCe9-1, and SnCu9-1 are broadened, indicating that the incorporation of the secondary metal oxide hinders the crystallization in the high temperature calcination. To see the trend more clearly, the crystallite sizes of the catalysts based on the SnO2 (110) plane diffraction are quantified in Table 1. Obviously, all the solid solution samples have smaller sizes than that of the individual SnO2, testifying strongly that the addition of the secondary metal cations can prevent them from crystallizing. 3.3. N2 Sorption Measurement. N 2 adsorption− desorption has been used to measure the textural properties of the samples. Figure S2a shows that similar to pure SnO2, all of the modified catalysts depict also type IV isotherms and H2type hysteresis loops, which provides indirect evidence to prove that the secondary metal cations might have entered into the rutile SnO2 matrix to form tetragonal solid solutions, thus posing little change to its texture properties. Figure S2b

Figure 2. XRD study on the catalysts.

peaks at 26.6°, 33.8°, and 51.7°,29 which are typical diffractions attributed to the tetragonal rutile SnO2 (PDF-ICDD 41-1445) phase, are observed but with altered intensities. Diffraction peaks related to the secondary metal oxides are not observed for all the catalysts, although their molar percentage is as high as 10%, implying that the metal cations might enter into the SnO2 lattice to generate a solid solution phase. The lattice parameters of all the modified samples listed in Table 1 are very similar to that of the individual SnO2, which testifies that they still have the tetragonal rutile phase structure of SnO2 and confirms the possible formation of noncontinuous solid solutions. Moreover, the 2θ and d values corresponding to the three strongest peaks of the SnO2 phase in all the modified C


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neously with Sn and O elements. Taking into account of the preparation method and the theoretical match of the radii and electronegativities of the secondary metal cations with Sn4+, STEM-mapping results provide strong evidence to confirm that SnO2-based solid solutions have been formed for all the modified catalysts. 3.5. Raman Study on the Catalysts. Figure 5 shows the Raman spectra of all the catalysts. It is observed that all the modified catalysts display similar Raman spectra to pure SnO2, and no Raman vibrations related to the secondary metal oxides are observed, testifying that they have similar phase composition to pure SnO2, which has proven again the formation of tetragonal rutile SnO2-based solutions in all the modified catalysts. SnO2 has a tetragonal rutile structure and belongs to the D144h space group. According to the group theory, eight vibration modes corresponding to Γ = A1g + A2g + 2A2u + B1g + B2g + 2B1u + Eg + 4Eu are present in the cell structure, but some modes such as A2g, B1g, and Eu are Raman silent.33 Therefore, in Figure 5, the strongest and typical A1g peak together with five weaker Eg, 2A2u, B1u, and B2g peaks are observed for pure SnO2. For all the modified catalysts, the strongest A1g peak is always observed in the spectra except that it is broadened, as testified by the larger half width listed in Table 3. Also, the A1g peak has some slight position shift. In addition, one or two of the weak bands disappear for some of the modified catalysts. According to former publications, the shift and broadening of the A1g peak reflect the size decrease of the nanoparticles,33 which is consistent with our XRD and TEM results. Moreover, it was reported that the B1u peak at ∼550 cm−1 (labeled as α) and A2u peak at ∼680 cm−1 (labeled as β) are related to the lattice disorder and surface defects,34 such as SnO2 surface and lattice oxygen vacancies. In addition, it was pointed out that the sum of the integrated areas of these two peaks divided by the integrated peak area of the A1g peak (labeled as γ) can reflect the concentration of surface defects.32 Therefore, the integrated peak ratios, (α + β)/γ have been quantified for the catalysts in Table 3. It is apparent that all the solid solution catalysts have much higher ratios than the individual SnO2, especially for SnCu9-1, SnCe9-1, and SnW9-1, demonstrating that more lattice and surface defects have been created by forming solid solutions, which is believed to be favorable to the reaction performance. In summary, the Raman results are consistent with the XRD, TEM, and N2 sorption results, testifying that the incorporation of In3+, W6+, Ce4+, and Cu2+ into the SnO2 lattice can impede the growth of the grain sizes, increase the surface areas, and create more defects such as oxygen vacancies in the prepared solid solution catalysts. 3.6. H2-TPR and O2-TPD Studies on the Catalysts. The H2-TPR technique has been used to study the redox behavior alteration of the samples by forming a solid solution structure. Figure 6 depicts that individual SnO2 displays a big reduction peak at 667 °C ascribed to the reduction of Sn4+ to Sn0.18,20 After forming a solid solution structure with the other metal cations, this peak of all the samples shifts evidently to lower temperatures, proving that the lattice oxygen becomes more reducible. It is mentioned here that SnCu9-1 shows two low temperature peaks positioned at 142 and 219 °C, which correspond to the stepwise reduction of Cu2+ cations in the lattice into metallic Cu;17 SnW9-1 shows also an extra peak at

displays that a mesoporous structure is existing in the bulk of all the catalysts. However, as exhibited in Table 2, by the Table 2. Nitrogen Sorption Results of the Catalysts catalysts

surface area (m2 g−1)

mean pore size (nm)

mean pore volume (cm3)

SnO2 SnIn9-1 SnW9-1 SnCe9-1 SnCu9-1

23 47 79 57 59

9.7 5.8 3.5 6.1 4.2

0.08 0.09 0.07 0.11 0.09

incorporation of the secondary metal cations, the average pore sizes of all the modified catalysts become smaller than that of the individual SnO2. While the average pore volumes of all the modified catalysts have no evident change, their specific surface areas are improved evidently compared with that of the unmodified SnO2, which is in accordance to the crystallite size results, demonstrating that the incorporation of the secondary metal cations can restrict the crystallization of the catalysts significantly. It is noted here that SnW9-1 possesses the highest surface area (79 m2 g−1) in all the samples, but its activity for NOxSCR by NH3 is not the best. In addition, it is obvious that the activity of all the catalysts has no proportional relation to their surface areas. Therefore, although we believe that the higher surface areas of the modified catalysts can contribute to the reaction activity, it is not the major factor to decide the activity. The change of the surface acidic and mobile oxygen sites and the balance between the two types of sites induced by the addition of the secondary metal cations are believed to play vital roles for the reaction performance, which will be discussed in details with extra experimental evidence.23,24 3.4. HR-TEM and STEM-Mapping Study on the Catalysts. To understand deeper the catalyst structures, HRTEM has been employed to identify the morphologies, grain sizes, and crystallite growth of the catalysts. Figure 3 depicts that all the catalysts consist of irregular round grains with different average sizes, which are listed in Table 1. Again, the modified samples possess smaller average grain sizes than that of pure SnO2, indicating that the incorporation of the metal cations into its lattice can also impede the agglomeration of the initial crystallites. In addition, the grain sizes of all the catalysts have a merely slight increase compared with the corresponding crystallite sizes, testifying that the secondary aggregation of the initial crystallites at high temperature is very mild. It is worth mentioning here that in the HRTEM images (Figure 3 b, e, h, k, and n) of all the catalysts, both (110) and (101) facets with d-spacings of 0.33 and 0.26 nm, which are characteristics of tetragonal rutile SnO2 (Table S1), are exposed.31 Any lattice fringe corresponding to the secondary metal oxides is not detected. Considering the high ratio of these metal oxides, it is reasonable to believe that the cations have entered into the SnO2 lattice to generate solid solutions,32 which indeed confirms the Raman and XRD results. To confirm the solid solution structure present in the modified samples, SnCu9-1 and SnCe9-1, the two catalysts with the best reaction performance, are further investigated by STEM-mapping, with the images exhibited in Figure 4. It can be seen that Cu and Ce, the major secondary metal cations of SnCu9-1 and SnCe9-1, respectively, distribute very homogeD


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Figure 3. HR-TEM pictures and particle size distribution profiles of (a, b, c) SnO2, (d, e, f,) SnIn9-1, (g, h, i) SnW9-1, (j, k, l) SnCe9-1, and (m, n, o) SnCu9-1.

860 °C, which belongs to the reduction of W6+ cations in the solid solution lattice into metallic W.35 For all the catalysts, except for the major reduction peak, a shoulder peak below 400 °C is detected also, which pertains to the reduction of the surface facile oxygen species over the

catalysts.18 For clarification, this part of the reduction has been enlarged in Figure 6b. As a typical n-type semiconductor, pure SnO2 contains generally a large amount of deficient surface oxygen species, but which will be mostly suppressed if the calcining temperature is higher than 400 °C because of good E


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Figure 4. STEM-mapping images of SnCu9-1 and SnCe9-1 catalysts.

affecting the NOx-SCR performance, which may facilitate the oxidizing of NO into NO2, which is a crucial step.2,40 O2-TPD has been adopted to test further the mobile oxygen nature of the catalysts with the profiles depicted in Figure S3. All the catalysts depict multiple oxygen desorption peaks. For easy comparison, the peaks are classified into two categories. In detail, the peak below 250 °C is classified as α type, and the peak above 250 °C is classified as β type. Pure SnO2 shows a small α peak around 85 °C corresponding to the desorption of facile surface oxygen; and a broadened β peak centered around 400 °C ascribed to the desorption of fragile surface lattice oxygen.41 As shown in Table 5, after the formation of solid solutions, the integrated areas of the α peaks of SnCe9-1 and SnCu9-1 are significantly improved, testifying the generation of much larger amount of loosely bonded surface oxygen species. However, over SnW9-1, this loosely bonded surface oxygen species is absent, which could affect its ability to oxidize NO into NO2, an important step for the reaction.40,42,43 In addition, the β peak integrated areas for all the modified catalysts increase, especially for SnW9-1, SnCe9-1, and SnCu91, testifying the presence of a larger quantity of facile surface lattice oxygen species. The total amount of the desorbed oxygen for the catalysts follows the sequence of SnCu9-1 > SnCe9-1 > SnW9-1 > SnIn9-1 > SnO2, which is well consistent with the reaction performance displayed in Figure 1. In brief, O2-TPD provides extra evidence to H2-TPR to demonstrate that the entering of the metal cations into SnO2 matrix can induce the generation of far more active oxygen sites favorable to the NOx-SCR reaction. The abundance of surface facile oxygen sites is thus believed to be one of the important elements to determine the catalytic performance. 3.7. Surface Characteristics of the Catalysts Investigated by XPS. XPS experiments have been performed to discern the oxidation states and surface feature of the samples doped by varied metal cations. The XPS spectra are illustrated in Figure 7 and Figure S1, and the quantified results are also listed in Table 6. Notably, a C 1s internal standard has been used to calibrate all the binding energies. As shown in Figure S1a, a set of doublet peaks at ∼495 and ∼486 eV corresponding to Sn 3d3/2 and Sn 3d5/2 are observed for all the samples, which are typical for Sn4+. It is noticed that for different catalysts, the two peaks have slight binding energy shifts with the changing of doped metal cations. Therefore, the binding-energy gap (ΔE) between them have been calculated for different samples. As exhibited in Table 6, in contrast to the

Figure 5. Raman spectra of the catalysts.

Table 3. Raman Results of the Catalysts integrated peak areas

catalysts

Raman shift of A1g peak (cm−1)

fwhm of Raman line (cm−1)

α peak (a.u.)

β peak (a.u.)

γ peak (a.u.)

(α + β)/γ


633 634 612 630 618

19.5 30.0 53.8 32.6 22.8

0 10.8 17.3 6.3 6.4

3.1 0 0 1.9 18.5

95.2 99.1 100.0 45.2 83.1

0.03 0.11 0.17 0.18 0.30

crystallization.18,36−39 It is apparent that by forming a solid solution structure, this surface deficient oxygen on all the modified catalysts can be stabilized and the amount is increased, as also testified by the quantified results in Table 4. This is in agreement with the fact that all the modified catalysts exhibit better reaction performance than the individual SnO2. Furthermore, if being normalized by the surface areas, SnCu9-1, the catalyst possessing the best reaction performance, owns obviously the largest amount of H2 uptake amount per m2, indicating it has the highest density of deficient surface oxygen sites. On the basis of H2-TPR results, we believe that the mobile surface oxygen of the catalysts is important and is one of the major elements F


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Figure 6. H2-TPR study on the catalysts: (a) complete profile; (b) partly enlarged profile.

Table 4. H2-TPR Quantification Results consumed H2 amount below 400 °C catalysts

mmol gcat


−1

−2

(×10 )

−2

mmol m

2.1 5.4 12.2 8.6 13.5b

consumed H2 amount above 400 °C −4

(×10 )

9.1 11.5 15.4 15.1 22.9

mmol gcat−1

mmol gSnO2−1

O/Sn molar ratioa

13.2 11.8 10.7 11.5 11.4

13.2 13.2 13.2 13.2 13.2

2.0 2.0 2.0 2.0 2.0

Calculated from the H2 reduction peak above 400 °C. bH2 uptake for Cu2+ reduction is excluded.

a

eV for In 3d5/2 and In 3d3/2, which fit well to the binding energies for In2O3,44 proving the presence of In3+ in the matrix of SnIn9-1. Figure S1c presents doublet peaks at ∼36 and 38 eV for W 4f7/2 and W 4f5/2, which are nearly the same as those for WO3,45 indicating the presence of W6+ in the matrix of SnW9-1. Figure S1d presents doublet peaks at ∼898 and 917 eV for Ce 3d5/2 and Ce 3d3/2, which are in line with the those reported for CeO2,46 indicating the presence of Ce4+ in the lattice of SnCe9-1. Figure S1e presents doublet peaks at ∼934 and 954 eV for Cu 2p3/2 and Cu 2p1/2, which indicates the presence of Cu2+ in the matrix of SnCu9-1.47,48 The unsymmetrical O 1s spectroscopy of each catalyst indicates the existence of multiple kinds of surface oxygens having varied chemical environments. The major O 1s peak at 530 eV is attributed to the surface lattice oxygen (Olatt), and the ∼531 eV O 1s peak is characteristic for the facile surface oxygen.2 Therefore, the O 1s signal of each catalyst has been deconvoluted and integrated. As shown in Table 6, all the modified catalysts have a higher Oads/(Olatt + Oads) ratio than the unmodified SnO2, especially for SnCu9-1 and SnCe9-1, the two catalysts with the best NOx-SCR performance. In summary, the XPS measurements are consistent with the Raman, H2-TPR, and O2-TPD results, confirming that the formation of a solid solution structure creates larger amounts of surface facile oxygen species, which contributes to the NOxSCR performance. 3.8. NH3-TPD Analysis of the Catalysts. It was reported formerly that surface acidic sites play a vital role for NOx-SCR by NH3,49,50 which can adsorb and activate NH3 molecules. Therefore, NH3-TPD has been employed to identify the acidic sites on the catalyst surfaces. The TPD profiles are shown in Figure S4.

Table 5. O2-TPD Quantification Results oxygen desorption amount (a.u.) catalysts

α peak

β peak

total amount (a.u.)


4 3

29 31 65 63 57

33 34 65 75 100

12 43

Figure 7. XPS O 1s spectra of the catalysts.

individual SnO2, all the modified catalysts have slightly decreased ΔE, proving that the chemical environment of the lattice Sn4+ has been altered after the secondary metal cation doping.18 Figure S1b presents doublet peaks at ∼445 and 452 G


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Industrial & Engineering Chemistry Research Table 6. XPS Quantification Results of the Catalysts integrated area of O 1s peak (a.u.)

Oads/(Oads + Olatt)

Sn 3d B.E. (eV)

catalysts

Oads

Olatt

molar ratio (%)

Sn 3d3/2

Sn 3d5/2

ΔE (eV)a


28 29 30 44 55

100 96 95 93 90

22 23 24 32 38

495.59 494.95 495.62 495.07 494.69

487.15 486.54 487.21 486.64 486.29

8.44 8.41 8.41 8.43 8.40

a

The B.E. gap between Sn 3d3/2 and Sn 3d5/2 signals.

It is well-known that SnO2 possesses Lewis acidity.51 Therefore, a small amount of NH3 desorption is observed in its profile between 300 and 550 °C. In contrast, with the incorporation of the secondary metal cations to form solid solutions, the NH3 desorption amount on all the modified catalysts is significantly improved, demonstrating the generation of more abundant surface acidic sites. For easy comparison, the desorption peaks are categorized into three groups. The α group represents NH3 adsorbed on weak acidic sites, the β group represents NH3 adsorbed on acidic sites with intermediate strength and the γ group represents NH3 adsorbed on strong acidic sites. On the base of the quantification results in Table 7, obviously, by the doping of

synergism between the two categories of sites control the catalytic performance.23−25 Therefore, the same thing could occur to the SnO2-based solid solution catalysts investigated in this study. 3.9. Evaluating Sulfur and Water Vapor Resistance of the Catalysts. In most of the exhaust from diesel engines, a certain amount of sulfur oxide and water vapor are present. The sulfur and water vapor tolerance ability are critical aspects to decide the application possibility of an NOx-SCR catalyst.53,54 Hence, the stability of SnCu9-1 has been evaluated at 200 °C in the presence of the two poisoning gases. As exhibited in Figure 8, after the NOx conversion was

Table 7. NH3-TPD Quantification Results of the Catalysts NH3 desorption amount (a.u.) catalysts

α peak

β peak

γ peak

total amount (a.u.)


2 5 8 8 11

5 63 48 61 44

12 14 25 29 45

19 82 81 98 100

the cations into the SnO2 matrix to generate solid solutions, the amount of acidic sites with intermediate strength has been significantly increased. Taking into account the fact that the reaction tests in this study are performed in the temperature region of 100−550 °C, it is believed that all the acidic sites make a contribution to the reaction. The total amount of the desorbed NH3 follows the order of SnCu9-1 > SnCe9-1 > SnW9-1 > SnIn9-1 > SnO2, which is in accordance to the reaction performance, testifying that the surface acidity is also very important and could be another factor to influence the reaction performance. It is noted here that although it is initially expected to generate more acidic sites with the incorporation of In3+ and W6+ cations and produce more redox sites with the incorporation of Ce4+ and Cu2+ cations for the resulted catalysts, SnCu9-1 and SnCe9-1 possess obviously more acidic sites than SnW9-1 > SnIn9-1. Some former studies have demonstrated that the amount of surface acidic sites is related to the quantity of surface defects.25,52 The quantified Raman results in Table 3 proved that the surface defective site amount of the catalysts follows the order of SnCu9-1 > SnCe9-1 > SnW9-1 > SnIn9-1 > SnO2, in good accordance to the NH3TPD results. Therefore, it is believed that the amount of surface acidic sites on the catalysts is mainly decided by the surface defects, but not the nature of the doping cations. Indeed, it was pointed out previously that for an effective catalyst to selectively reduce NOx by NH3, the balance between surface acidic and oxidative sites amount and the

Figure 8. Stability test of SnCu9-1 in the presence of H2O or/and SO2 for NOx-SCR by NH3 at 200 °C.

stabilized at ∼65% for 5 h, the introduction of 50 ppm of SO2 in the feed gas had no negative effect on the NOx conversion during a 7 h test. However, by increasing the SO 2 concentration to 100 ppm, NOx conversion decreased to ∼50%, but which will be completely recovered after removing SO2. Following this, the introduction of 5% water vapor decreased the NOx conversion to ∼41%, but which will be completely regenerated again after removing the water vapor. When 50 ppm of SO2 and 5% water vapor were added together into the feed, the NOx conversion dropped now to 21%. However, the conversion recovered again to the initial ∼65%, which remained also stable in the following 30 h test. What is observed in this series of experiments has testified that both SO2 and water vapor have no permanent impact on the structure and properties of the SnCu9-1 catalysts. The observed deactivation is in fact temporary, due to the occupation of some surface active sites by the SO2 and H2O molecules. Some former studies showed that CuO can be deactivated by sulfur due to the easy formation of copper H


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Figure 9. In situ DRIFT study on SnCu9-1 catalyst at 200 °C: (a) in a 50 mL min−1 500 ppm of NO + 5% O2/N2 flow; (b) in a 50 mL min−1 500 ppm of NH3/N2 after saturation in the 500 ppm of NO + 5% O2/N2 flow.

Figure 10. In situ DRIFT study on the SnCu9-1 catalyst at 200 °C (a) in a 50 mL min−1 500 ppm of NH3/N2 and (b) in a 50 mL min−1 500 ppm of NO + 5% O2/N2 after saturation in the 500 ppm of NH3/N2 flow.

sulfate or sulfide.55 However, in this study, when Cu2+ cations entered into the rutile SnO2 lattice to generate a solid solution structure, they were thus protected by the rigid crystalline lattice and avoided the direct contact with the SO2 molecules, thus becoming resistant to sulfur deactivation. 3.10. In Situ DRIFTS Study on the Reaction Intermediates. To identify the possible reaction mechanisms, in situ DRIFTS experiments have been performed with SnCu91 catalyst in two ways.56,57 First, adsorbed NOx intermediates will be produced over the catalyst surface, and then gas phase NH3 will be introduced. Second, adsorbed NH3 intermediates will be generated over the catalyst surface, and then gas phase NOx will be introduced. The surface reaction in each step was monitored by in situ DRIFTS, which is described in detail here. 3.10.1. Reaction between Gas Phase NH3 and Adsorbed NOx Surface Species. For this series of experiments, the catalyst was treated first at 300 °C in a 30 mL min−1 N2 feed for 1 h to remove any possible surface impurity, and then cooled to 200 °C. The 0 min spectroscopy in Figure 9a shows that the catalyst surface is clean without any IR peak. Afterward, 500 ppm of NO + 5% O2/N2 gas mixture was flowed into the in situ cell. It is evidently observed that two peaks at 1002 and 1180 cm−1 ascribed to monodentate nitrate

and bridged nitrite in sequence, and two peaks at 1278 and 1520 cm−1 assigned to bidentate nitrate started to form.43,58,59 Interestingly, the 1180 cm−1 band corresponding to bridged nitrite increased in the first 20 min, and then diminished and disappeared completely after 25 min. However, the bidentate nitrate bands at 1278 and 1529 cm−1 increased continually with reaction time until it was stabilized after 40 min. These results have testified that NOx species could adsorb first on SnCu9-1 surface as bridged nitrite, which was then oxidized into bidentate nitrate. Afterward, the gas mixture was replaced by a pure N2 flow and flushed 1 h at the same temperature, then a 500 ppm of NH3 /N2 flow was introduced. Figure 9b shows that the bidentate nitrate peaks at 1281 and 1529 cm−1 decreased and were stabilized after 20 min. By increasing the exposure time in NH3 gas, the 1529 cm−1 peak also shifted to the lower frequency gradually (NH2 species).60 In addition, the monodentate nitrate band at 1005 cm−1 became smaller and disappeared. Simultaneously, new bands at 3377, 3265, 1619, 1216, and 850 cm−1 appeared. The peaks at 3377 and 3265 cm−1 can be attributed to the N−H stretching vibration modes, and the peaks at 1619 and 1216 cm−1 can be assigned to the NH3 coordinated on Lewis acidic sites.60,61 All these I


Article

Industrial & Engineering Chemistry Research results have indicated that both bidentate nitrate and monodentate nitrate are the active surface intermediates, which react easily with NH3. On the basis of the peak intensity, it seems that bidentate nitrate could be the major adsorbed NOx intermediate. 3.10.2. Reaction between Gas Phase NOx and Adsorbed Surface NH3 Species. Similar to the above experimental process, the catalyst surface was cleaned under the same condition for this series of experiments. However, 500 ppm of NH3/N2 gas flow was introduced first to create an adsorbed NH3 intermediate on the catalyst surface. Figure 10a depicts that a peak at 850 cm−1 corresponding to weakly bounded or NH3 in the gas phase, two bands at 1618 and 1225 cm−1 belonging to NH3 adsorbed on Lewis acidic sites, and a peak at 1464 cm−1 assigned to the vibration of NH4+ species attached on Brønsted acidic sites57 are detected. Furthermore, the peaks at 3360, 3245, and 3130 cm−1 are also observed, which are ascribed to the stretching vibration of N−H for coordinated NH3. After 40 min exposure of the catalyst surface, the intensity of all the adsorbed NH3 species was stabilized. Afterward, the gas mixture was replaced by a pure N2 flow and flushed for 1 h at the same temperature, then a 500 ppm of NO + 5% O2/N2 mixture feed was flowed into it. Figure 10b shows that the bands at 3360, 3245, 3130 (N−H stretching vibration), 1618, 1225 (NH3 adsorbed on Lewis acidic sites), and 1464 cm−1 (NH4+ species adsorbed on Brønsted acid sites) decreased by increasing the exposure time in 500 ppm of NO + 5%O2/N2, indicating that coordinated NH3 and ionic NH4+ have both joined the NOx-SCR reaction. In addition, the bidentate nitrate peaks at 1538 and 1278 cm−1, and the monodentate nitrate peak at 1005 cm−1 appeared,26 the intensity of which increased with the exposure time. After the catalyst surface was exposed to 500 ppm of NO + 5%O2 /N2 gas mixture for 40 min, the DRIFTS spectrum is similar to that of SnCu9-1 exposed directly in 500 ppm of NO + 5%O2 /N2 in Figure 9a, which proves the complete consumption of the adsorbed surface NH3 species, and the adsorption of NOx species started. In summary, gas phase NOx can adsorb on the SnCu9-1 surface to form a large quantity of bidentate nitrate and a small amount of monodentate nitrate intermediates. Both are reactive to NH3 species. However, we believe that the adsorbed bidentate nitrate could play the major role for the reaction since its amount is much larger than that of the monodentate nitrate. Although bridged nitrite was also detected, which was oxidized quickly into bidentate nitrate and does not attend the NOx-SCR process, as reported previously.43 On the other hand, gas phase NH3 can also adsorb on SnCu9-1 surface to form ionic NH4+ and coordinated NH3 intermediates, which are reactive to NOx species. Therefore, it is concluded that the NOx-SCR by NH3 over the SnCu9-1 catalyst follows a Langmuir−Hinshelwood pathway26,43 involving the adsorption of both NOx and NH3 reactants on the catalyst surface.

(1) It has been revealed that all the metal cations can enter into the rutile SnO2 lattice to produce a pure solid solution phase, as testified by XRD, Raman, HR-TEM, and STEM-mapping results. Consequently, all the modified samples own higher specific surface areas and lower crystallinity in comparison with pure SnO2. (2) Raman results have testified that more surface defects are formed on the modified catalysts. In addition, larger amounts of both surface active oxygen and acidic sites have also been generated, as evidenced by H2-TPR, O2TPD, XPS, and NH3-TPD results. (3) A catalyst with higher NOx-SCR performance usually possesses richer surface active oxygen and acidic sites. Therefore, the balance between these two categories of surface sites is believed to control the reaction performance. For the two catalysts modified by Ce4+ and Cu2+ cations, a much better balance can be reached. Therefore, they display the best catalytic performance of all the catalysts. (4) In Situ DRIFTS study over SnCu9-1 catalyst has substantiated that the preadsorption of NO + O2 can mainly form bidentate nitrate intermediate on its surface, which is reactive to NH3; and the adsorption of NH3 on its surface can form both adsorbed NH3 and NH4+ intermediates, which are reactive to NO + O 2. Therefore, it is concluded that NOx-SCR by NH3 reaction on the SnO2-based solid solution catalysts in this study follows a Langmuir−Hinshelwood mechanism involving the adsorption of both NOx and NH3 reactants on the catalyst surface.

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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.iecr.8b02288. Experimental details of SnO2, SnIn9-1, SnW9-1, SnCe91, and SnCu9-1 characterization, performance evaluation. XPS spectra of Sn 3d, In 3d, W 4f, Ce 3d, and Cu 2p, N2 adsorption−desorption profiles, O2-TPD profiles, NH3-TPD profiles, and a table listing 2θ and d values of three strong peaks (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Honggen Peng: 0000-0001-9133-5727 Xiang Wang: 0000-0002-4096-6147 Notes

The authors declare no competing financial interest.

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CONCLUSIONS In this study, with the purpose to gain deeper fundamental understanding on the active sites requirements for NOx-SCR by NH3 and to design better catalysts, a series of SnO2-based solid solutions with the tetragonal rutile SnO2 matrix doped by metal cations possessing redox ability (Ce4+ and Cu2+) or acidity (In3+ and W6+) have been intentionally designed.

ACKNOWLEDGMENTS The authors appreciate the financial support by the National Science Foundation of China (21567016, 21666020), the National Key Research and Development Program of China (2016YFC0209302), the Education Department of Jiangxi Province (GJJ150016, KJLD14005), and the Natural Science Foundation of Jiangxi Province (20171BAB213013). J


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Tetragonal Rutile SnO2 Solid Solutions for NOx-SCR by NH3

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