Electronic-Structure-Dependent Performance of Single-Site Potassium

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

Electronic-Structure-Dependent Performance of Single-Site Potassium Catalysts for Formaldehyde Emission Control Junxiao Chen,† Jiayi Gao,† Yaxin Chen,† Xiaona Liu,† Chao Li,† Weiye Qu,† Zhen Ma,†,‡ and Xingfu Tang*,†,‡ †

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Shanghai Key Laboratory of Atmospheric Particle Pollution & Prevention (LAP3), Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China ‡ Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China S Supporting Information *

ABSTRACT: Depending on the dispersion degree and electronic states, alkali metals such as potassium can function as either promoters or inhibitors in catalytic reactions, but the underlying reasons for that are still obscure. Herein, we fabricated two kinds of single-site potassium catalysts (K1/HMO and K1/HWO) by using hollandite-type manganese oxide (HMO) and hexagonal tungsten oxide (HWO) as supports. K1/HMO with the partially charged single K ions efficiently enhances the ability to activate both surface lattice oxygen and molecular oxygen, thereby leading to a higher catalytic activity for HCHO oxidation. In contrast, K1/HWO is less active than HWO due to the electron-lean K ions. This work provides a feasible explanation for the functions played by alkali metals in catalytic oxidation, and thus assists the design of efficient catalysts promoted by alkali metals.

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adsorbed on K+ can increase kinetic barriers for HCHO oxidation.18 Therefore, the effects of K+ in catalytic oxidation are still controversial. For low-temperature HCHO oxidation, the Mars−van Krevelen model has been widely accepted to describe the reaction mechanism, in which the activation of O2 and surface lattice oxygen is critical.19,20 Our recent work has shown that K ions with the high density of electronic states facilitated the adsorption and activation of O2, thus enhancing the oxidation ability of surface lattice oxygen.11,12 Therefore, the effects of K+ on low-temperature catalytic oxidation of HCHO may originate from the electronic states of K+, which are closely associated with the activation of oxygen species. Herein, we explored the influence of K+ on HCHO oxidation by atomically dispersing K ions on HMO and hexagonal tungsten oxides (HWO). HMO and HWO have tunnel structures that can provide rooms for small ions to disperse within.11,12,21,22 Synchrotron X-ray diffraction (SXRD) and extended X-ray absorption fine structure (EXAFS) spectra were utilized to determine the crystal structures and the immediate structures of K+. Electronic structures of single K+ were analyzed by X-ray absorption near-edge structure (XANES) spectra and X-ray photoelectron spectra (XPS), and thus the structure− activity correlation was established. This work provides a feasible explanation for the functions played by alkali metals in

INTRODUCTION Formaldehyde (HCHO), as an indoor air pollutant, is released from building and furnishing materials as well as consumer products.1 It has been proven to be severely harmful to human health and the environment.2 Hence, the effective abatement of indoor HCHO is of great importance. Heterogeneous catalysis is one of the most efficient technologies to remove HCHO by complete oxidation.3,4 Transition metal oxide catalysts due to low costs have been intensively investigated for this purpose.5−9 In particular, MnO2 is one of the most efficient catalysts for the abatement of HCHO.6,7 Especially microporous Hollanditetype manganese oxides (HMO) are favorable for adsorbing and activating HCHO molecules due to the suitable sizes of the pores, thereby resulting in good catalytic performance in HCHO abatement.10 Our recent work demonstrated that the loading of alkali metal ions such as K+ greatly improved the catalytic activity of HMO in HCHO oxidation at relatively low temperatures.11,12 In fact, K+ functions as a promoter or an inhibitor in different cases.11,13−16 For instance, Bai et al.14 introduced K+ into Ag/Co3O4 catalyst and found an obvious improvement of the catalytic activity in HCHO oxidation. Wang et al.17 observed that the addition of K+ to Pt/Al2O3 catalysts promoted the catalytic activity in dichloromethane oxidation due to the accelerated decomposition of formate intermediates. On the other hand, Santiago et al.15 found that the addition of too much K+ decreased the activity of a manganese/cerium oxide catalyst toward the wet oxidation of phenol. Wang et al.16 reported a “seesaw effect” of interlayered K+ in manganese oxides for HCHO oxidation: K+ can help with the activation of adsorbed O2, but H2O favorably © XXXX American Chemical Society

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June 23, 2018 August 19, 2018 August 28, 2018 August 28, 2018 DOI: 10.1021/acs.iecr.8b02815 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research catalytic oxidation and thus assists the design of efficient catalysts promoted by alkali metals.

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EXPERIMENTAL SECTION Catalyst Preparation. HMO was synthesized by a fluxing method.11,23 A 400 mL aqueous solution containing manganese sulfate (MnSO 4 , 0.150 mol), ammonium persulfate [(NH 4 ) 2 S 2 O 8 , 0.150 mol], and ammonium sulfate [(NH4)2SO4, 0.750 mol] was refluxed at 100 °C for 12 h, followed by filtrating, washing, and drying at 110 °C for 12 h. Then, the solid was calcined in air at 400 °C for 4 h. HWO was prepared by a hydrothermal method.21,24 Briefly, a solution prepared by mixing (NH4)10W12O41 (0.7 mmol), (NH4)2SO4 (63.0 mmol), oxalic acid (23.3 mmol), and deionized water (80 mL) was sufficiently mixed and was transferred to a 100 mL autoclave, and the autoclave was put into an oven and rotated in the oven at 180 °C for 12 h. The obtained slurry was filtered, washed with deionized H2O, and dried at 105 °C. K1/HMO and K1/HWO were prepared by impregnation.12,22 A corresponding potassium precursor (KCl, 2.9 mmol or K2SO4, 1.4 mmol) was dissolved in deionized water (50 mL), and HMO or HWO (2.00 g) was added to the solution under stirring. The solution was then evaporated at 80 °C and the powders were calcined in air at 350 °C for 12 h. The final powders were washed with deionized water at room temperature and dried at 80 °C. The chemical and physical properties of these catalysts are listed in Table S1 in the Supporting Information. Catalytic Evaluation. The catalytic oxidation of HCHO was performed in a fixed-bed quartz reactor (i.d. = 8 mm) under atmospheric pressure. The mixed feed gas contained 140 ppm HCHO, 10.0 vol % O2, and was balanced by N2. The total flow rate was 1000 mL·min−1. For each run, a specified amount of catalyst (40−60 mesh) was charged. Gas analysis was undertaken by using an online Agilent 7890B gas chromatograph equipped with a flame ionization detector (FID). The HCHO conversion (XHCHO) was calculated from the proportion of corresponding CO2 peak area to the maximum peak area. For kinetics analysis, XHCHO was kept below 15%. Materials Characterization. SXRD patterns were recorded at BL14B of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 0.6887 Å. X-ray absorption spectra (XAS; including XANES and EXAFS) at the K K-edge of these samples were measured at BL4B7A of the Beijing Synchrotron Radiation Facility (BSRF) with an electron beam energy of 2.21 GeV and a ring current of 300−450 mA. Data analyses were conducted by using the IFEFFIT 1.2.11 software package. XPS analysis was undertaken on a Kratos Axis Ultra-DLD system with a charge neutralizer and a 150 W Al (Mono) X-ray gun (1486.6 eV) with a delay-line detector (DLD). The binding energies of the samples were calibrated according to C 1s XPS at a binding energy of 284.6 eV. Hydrogen temperatureprogrammed reduction (H2-TPR) and oxygen temperatureprogrammed desorption (O2-TPD) experiments were performed on a 2920 adsorption instrument (Micromeritics) with a thermal conductivity detector (TCD). H2-TPR was conducted at 10 °C·min−1 in a 50 mL·min−1 flow of 10 vol % H2 in Ar. O2TPD was conducted at 10 °C·min−1 in a flow of 50 mL·min−1 He.

Figure 1. (a) XHCHO as a function of reaction temperature (T) over K1/ HMO, HMO, K1/HWO, and HWO. Reaction conditions: 140 ppm HCHO, 10 vol % O2, and was balanced by N2. Catalyst mass: 0.05 g of HMO or K1/HMO and 0.5 g of HWO or K1/HWO. (b) Arrhenius plots for the turnover frequency (TOF) of HCHO oxidation over K1/ HMO and HMO, K1/HWO and HWO.

(T) over different catalysts. The activities of catalysts increase following the sequence of K1/HMO > HMO > HWO > K1/ HWO. Clearly, K+ in K1/HMO has a promotional effect, whereas K+ in K1/HWO exhibits an inhibiting effect. The reaction kinetics of HCHO oxidation over these catalysts were studied at XHCHO < 15% after eliminating both mass and thermal diffusion limitations, and the corresponding turnover frequencies (TOFs) are shown in Figure 1b. The apparent activation energies (Ea) for K1/HMO, HMO, HWO, and K1/HWO are 97, 98, 97, and 98 kJ·mol−1, respectively (Table S2). These catalysts have nearly the same Ea, implying the same reaction path for these catalysts. However, the pre-exponential factor (Γ) for K1/ HMO is 4.5 × 1012 s−1, larger than the corresponding value (1.2 × 1012 s−1) for HMO, while the pre-exponential factor (2.4 × 106 s−1) for K1/HWO is much smaller than the corresponding value (1.1 × 107 s−1) for HWO. As the pre-exponential factor is assumed to be proportional to the number of active sites, the different XHCHO changes between HMO and HWO after K+ loading mainly result from the changes in the number of active sites. It has been proven that the tunnel openings of HMO serve as the catalytically active sites in HCHO oxidation for its high surface energy.25,26 Similarly, the HWO rod grows along the [001] direction, indicating that the surface energy of the {001} top facets is much higher than that of the {100} side facets.22 Therefore, it is convincing that the active sites of both HWO and HMO are located at the openings of tunnels. Furthermore, according to the Mars−van Krevelen model, the activation of molecular oxygen and active lattice oxygen are vital for lowtemperature HCHO oxidation.11,12 Therefore, the K+ loading may result in some changes of the openings of tunnels, especially the oxygen species of HMO and HWO.

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RESULTS AND DISCUSSION Effects of Single Potassium Ions on Catalytic Activity. Figure 1a shows XHCHO as a function of reaction temperature B

DOI: 10.1021/acs.iecr.8b02815 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Geometric Structures of Single Potassium Ions. The position of K ions was determined with Rietveld refinement analyses of SXRD patterns and the EXAFS spectra at the K Kedge of the samples. Figure 2 shows the SXRD patterns of K1/

EXAFS spectra were used to precisely determine the local structures of K+ in the tunnels, and the related spectra at the Kedge are shown in Figure 3. Some structural parameters

Figure 3. FT EXAFS spectra of K1/HMO (a) and K1/HWO (b) at the K K-edges with k2 weighting. Red, purple, and yellow balls represent O, K, and Cl atoms, respectively.

Figure 2. SXRD patterns of (a) K1/HMO and (b) K1/HWO. (insets) Models showing K+ in HMO (a) and HWO (b) tunnels. Red, green, violet, and purple balls represent O, Mn, W, and K atoms, respectively; the green and violet octahedra represent MnO6 and WO6, respectively.

obtained by fitting the spectra with theoretical models are listed in Tables S5 and S6. The curve-fitting of R-space and inverse Fourier transform (FT) spectra are presented in Figures S2 and S3. The first shells of K1/HMO and K1/HWO are assigned to the K−O bonds, with average bond lengths of ∼2.88 and 2.64 Å, respectively, while the average bond lengths of the first shells of KCl and K2SO4 are ∼3.05 Å (K−Cl bonds) and 2.52 Å (K−O bonds).27 Therefore, the local environments of K atoms in K1/ HMO and K1/HWO are different from that of the precursor (KCl and K2SO4, respectively). Further analysis shows that the coordination number (CN) of the first shells is 8 in K1/HMO and 6 in K1/HWO, due to the structure of KO8 polyhedron and KO6 plane hexagon, as shown in the insets of Figure 3, parts a and b, respectively. These results are in accordance with those from SXRD refinement analyses. There is no K−K bond in K1/ HMO or K1/HWO, suggesting that K ions in both catalysts are arranged in an alternate stacking model along the tunnel direction, forming “single-atom” K-loaded catalysts and exposing single K ions at the openings, as reported in our previous work.12 Figure S4 shows the scanning transmission electron microscopic (STEM) images together with energy-dispersive X-ray spectroscopy (EDX) mappings of K1/HMO and K1/HWO. Obviously, both K1/HMO and K1/HWO grow along the (001) directions to form rod-shaped morphologies, exposing the tunnel opening on the top facets with higher surface energy. These tunnel openings are active sites for catalytic HCHO oxidation. The high dispersions of K+ in K1/HMO and K1/ HWO can be observed in the EDX mapping images. According to our recent reports,8,19 parts of K+ are confirmed to be exposed on the (001) top facets of HMO or HWO. These conclusions

HMO and K1/HWO, and the corresponding lattice parameters and coordination configuration gained from Rietveld refinement analyses are listed in Tables S3 and S4, respectively. As shown in Figure 2a, after the K+ loading, the tetragonal structure with the I4/m space group of HMO (Table S3) remains unchanged. This tetragonal structure is constructed with one-dimensional square tunnels of ∼4.7 × 4.7 Å2 along the [001] direction.12 According to the Rietveld refinement analyses, the loaded K ions are located at the center of the tetragonal prism structures consisting of eight tunnel oxygen ions, as shown in the inset of Figure 2a. Similarly, constructed by corner-sharing WO6 octahedral, K1/ HWO with the space group P6/mmm (Table S3) possesses approximately 5.4 Å size hexagonal tunnels oriented along the caxis. The loaded K ions occupying the (0,0,0) sites are centered at a plane consisting of six tunnel oxygen ions,21 as shown in the inset of Figure 2b. Therefore, K ions in K1/HMO and K1/HWO are both located in the tunnels. K ions at the opening of the tunnels were evidenced with the temperature-programmeddesorption procedure after saturation adsorption of NO at 250 °C, which is an efficient method to confirm the presence of surface K+ ions.8 As shown in Figure S1, desorbed NOx molecules were obviously observed for both K1/HMO and K1/HWO, whereas the desorption amount of NOx on the HMO or HWO can be ignored under the same conditions, indicating that the NOx molecules adsorbed on surface K+.11 Hence, it is convincing that parts of K atoms are exposed at the openings of the tunnels, which function as active sites for HCHO oxidation.12 C

DOI: 10.1021/acs.iecr.8b02815 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research are consistent with the results obtained from SXRD and EXAFS data. Electronic Structures of Single Potassium Ions. K ions are in the tunnels and coordinated with the oxygen around them, which are associated with the interactions between K and O atoms especially through the orbital hybridization. The pre-edge features of XANES spectra were used to explore the electronic structures of K+ ions, and the K K-edge XANES spectra of K1/ HMO and K1/HWO together with their precursors (KCl and K2SO4) are shown in Figure 4. Both K1/HMO and K1/HWO

Figure 5. K 2p XPS results for K1/HMO and KCl (a), and K1/HWO and K2SO4 (b).

indicating an electron transfer from Mn to K. In contrast, the W 4f core-level binding energy shifts down, indicative of an electron transfer from K to W. Hence, the K atoms in K1/HMO have higher electron density, but the K atoms in K1/HWO have lower electron density than K+. Generally, the outmost electron configuration of a potassium atom is 3p64s13d0 at the ground state, which easily releases one electron to become a more stable electron configuration of 3p64s03d0. Such changes of the electron density of K atoms make them more active in K1/ HMO but more stable in K1/HWO.21 Thus, the d−sp orbital hybridizations result in different electronic states of K atoms in K1/HMO and K1/HWO, which possibly change their activities. Effects of Potassium with Hybridized Orbitals on Surface Lattice Oxygen. Considering the strong d−sp orbital hybridization between K ions and O species, the change of electronic state of K ions is accompanied by the change of the oxygen species, including surface lattice oxygen.12 Thus, O 1s XPS was employed to understand the change of oxygen species, and the results are shown in Figure 6, parts a and b, for HMO and K1/HMO, HWO and K1/HWO, together with their differences, respectively. It is clear that, after K+ loading, the binding energy shifts down for K1/HMO (Figure 5a) but it shifts up for K1/HWO compared with that of HMO and HWO, respectively, which means that the O atoms gain electrons in K1/ HMO while they lose electrons in K1/HWO. Therefore, the proportion of different species must change differently. Further fitting results were obtained to show the change of oxygen species. As shown in Figure 5a, after K ions are trapped, the O 1s XPS intensity of K1/HMO decreases in the binding energy range of 530.5−534 eV but increases around the binding energy of 528−530.5 eV, suggesting that the proportion of surface defect oxygen (Osd) decreases, while the proportion of surface lattice oxygen (Osl) increases.29 Similarly, in Figure 5b, after K ion loading, the O 1s XPS of K1/HWO intensity increases in the binding energy range of

Figure 4. XANES spectra of K1/HMO and KCl (a) and K1/HWO and K2SO4 (b) at the K K-edges. (insets) Spectral fitting edges: the pink (a) and violet (b) shadows show the 1s → 3d transition, and models of KO8 (a) and KO6 (b) structural motifs.

exhibit different K K-edge XANES spectra compared with KCl and K2SO4, respectively. As expected, because of the 1s → 3d dipole-forbidden transitions for the coordination geometry (Oh) of KO6 octahedron motif,28 neither KCl nor K2SO4 shows preedge absorption peaks, indicating the absence of d−sp orbital hybridization.27 Note that the pre-edge absorption peaks of the K K-edge XANES spectra of K1/HMO and K1/HWO are obviously observed in the insets of Figure 4. The presence of the 1s → 3d transitions in K1/HMO and K1/HWO comes from the KO8 polyhedron with a D4h symmetry and the planar KO6 configuration with a Dih6 symmetry, respectively, suggesting that the K ions possess d−sp-hybridized orbitals,28 which can lead to the change of the electronic states of K atoms and O atoms.13 K 2p XPS was adopted to detect the oxidation states of K+, and the results are shown in Figure 5 corresponding to K ions of K1/ HMO and K1/HWO. The K 2p core-level binding energies of KCl and K2SO4 are both at ∼292.5 eV, indicating a normal oxidation state of +1 in KCl and K2SO4. However, the binding energy shifts down to 291.9 eV for K1/HMO, as shown in Figure 5a, while it shifts up by 0.3 eV for K1/HWO in Figure 5b. As shown in the XPS of Mn 2p and W 4f in Figure S5, after K+ loading, the Mn 2p core-level binding energy shifts up, D

DOI: 10.1021/acs.iecr.8b02815 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. H2-TPR profiles of K1/HMO and HMO (a) and K1/HWO and HWO (b).

Figure 6. O 1s XPS results for K1/HMO and HMO (a), and K1/HWO and HWO (b) together with their differences (dotted lines).

lattice oxygen and thus the redox ability, while that between K ions and HWO poisons them, in accordance with the results from O2-TPD (Figure S7), thus leading to the different performance of the catalysts in HCHO oxidation. After alkali metal loading, the activities of oxygen species are changed according to the electronic states of K ions. In summary, we introduced K+ into the tunnels of HMO and HWO to evaluate the influence of K ions on catalytic HCHO oxidation. The combination of characterization techniques demonstrated that, in both catalysts, K+ ions were located in the tunnels, forming “single-atom” K-loaded catalysts. In addition, the single K atoms had hybridized frontier d−sp orbitals through strong interactions with lattice oxygen, resulting in change of the electronic states of K ions and O atoms. For K1/HMO, the K ions gain electrons and the proportion of active surface lattice oxygen (Osl) increases, while for K1/HWO, the K ions lose electrons and the proportion of active surface lattice oxygen decreases. The K ions also influence the oxidation properties of both surface and bulk oxygen species. Since Osl with nucleophilic properties can attack the electrophilic carbonyl group of HCHO readily, the K+ loading improves the catalytic activity in K1/ HMO while it is a poison for K1/HWO. This work provides a feasible explanation for the function played by alkali metals in catalytic oxidation and thus assists the design of efficient catalysts promoted by alkali metals.

531.5−535 eV but decreases around the binding energy of 528− 531.5 eV, suggesting the proportion of surface defect oxygen (Osd) increases while the proportion of surface lattice oxygen (Osl) decreases. On the basis of the Mars−van Krevelen model,19,30 the Osl species are of great importance for lowtemperature oxidation of HCHO because Osl with nucleophilic properties can attack the electrophilic carbonyl group of HCHO readily.31 Therefore, the increase of the proportion of Osl species in K1/HMO improves the catalytic performance in HCHO oxidation compared with that of HMO, whereas the decrease of the proportion of Osl species in K1/HMO leads to the lower catalytic activity (compared with that of HWO), as shown Figure 1. Thus, the effects of K ions can be partly from the changes of the proportion of Osl. The oxidation abilities of catalysts are considered as a major feature of their catalytic performance, and H2-TPR was adopted to study the change of oxidation capacities of catalysts after K+ trapping to confirm the effects of the electron density of K ions on the oxygen species. Parts of the results are shown in Figure 7 (see more information in Figure S6). As for HMO, the first and second peaks are located at about 269 and 279 °C, respectively. After K+ trapping, the corresponding peak shifts forward to about 230 and 257 °C in K1/HMO. These peaks are assigned to the reduction of surface lattice oxygen and bulk lattice oxygen, respectively,10 suggesting that K+ loading can make the reduction of both surface and bulk lattice oxygen happen at lower temperatures; i.e., the oxidizing abilities of both surface and bulk lattice oxygen in K1/HMO are higher. However, as shown in Figure 7b, after K+ trapping in the tunnel of HWO, the first and second peaks assigned to surface lattice oxygen and bulk lattice oxygen shift backward from 525 and 629 °C to 536 and 651 °C, respectively. The shift indicates that the loading of K ions makes the oxidation abilities of both surface lattice oxygen and bulk lattice oxygen in K1/HWO lower. Therefore, electron transfer between K ions and HMO enhances the activation of

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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.8b02815. Calculation of TOF; properties, pre-exponential factors, and activation energies of K1/HMO, HMO, K1/HWO, and HWO; crystallographic data of K1/HMO and K1/ HWO; EXAFS and inverse FT EXAFS spectra of K1/ HMO, KCl, and K1/HWO; STEM and EDX mapping E

DOI: 10.1021/acs.iecr.8b02815 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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images of K1/HMO and K1/HWO; XPS results for K1/ HMO, KCl, K1/HWO, and K2SO4; TPD and TPR profiles of HMO, K1/HMO, HWO, and K1/HWO (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-65642997. Fax: +86-21-31248935. E-mail: [email protected]. ORCID

Zhen Ma: 0000-0002-2391-4943 Xingfu Tang: 0000-0002-0746-1294 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC (21477023 and 21777030). The SXRD measurements were conducted at the SSRF.

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REFERENCES

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DOI: 10.1021/acs.iecr.8b02815 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX