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Role of the Acid Site for Selective Catalytic Oxidation of NH over Au/NbO Mingyue Lin, Baoxiang An, Nao Niimi, Yohei Jikihara, Tsuruo Nakayama, Tetsuo Honma, Takashi Takei, Tetsuya Shishido, Tamao Ishida, Masatake Haruta, and Toru Murayama ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04272 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019
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Role of the Acid Site for Selective Catalytic Oxidation of NH3 over Au/Nb2O5 Mingyue Lin,*,† Baoxiang An,† Nao Niimi,‡ Yohei Jikihara,‡ Tsuruo Nakayama,‡ Tetsuo Honma,# Ta† † † † † kashi Takei, Tetsuya Shishido, Tamao Ishida, Masatake Haruta, and Toru Murayama*, †
Research Center for Gold Chemistry, Graduate School of Urban Environmental Science, Tokyo Metropolitan University, 11 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan ‡
NBC Meshtec Inc., 2-50-3 Toyoda, Hino, Tokyo 191-0053, Japan
#
Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan KEYWORDS: gold, niobium oxide, ammonia, acid sites, selective catalytic oxidation Supporting Information such as CuO/Al2O3,9-10 Ce/V/TiO2,11 MoO3/SiO2,12 Fe2O3-TiO2,13 Fe2O3-ZrO2,13 Fe2O3-Al2O3,13 MnO2,14 and MnOx-TiO215 were found to have good N2 selectivities, but the operating temperatures were very high (T>300oC). Zeolite-based catalysts containing transition metals such as Fe-exchanged zeolites (ZSM-5,16-19 mordenite,16 ferrierite,16 Beta16), Cu-exchanged zeolites (ZSM5,19 Beta20), Co-exchanged ZSM-5,19 and Mn-exchanged ZSM-519 were reported to be active at high temperatures (T>400oC) with high N2 selectivities. There is a technical vacancy for air purification from 25oC to 200oC to solve low-concentration NH3 emissions in local but closed spaces such as hospitals, toilets and feedstock farms (Figure S1). The US Occupational Safety and Health Administration has set a limit of 50 ppm over an 8-h work day and 40-h work week for ammonia vapor in ambient air.21 To develop air purification systems in a living environment, efficient catalysts with high N2 selectivities at low temperatures are needed. Gold catalysts are well known for their high catalytic activity at low temperatures, especially for oxidation of CO even at -70oC.22 However, SCO of NH3 over gold catalysts has rarely been investigated. In this study, the important effect of MOx supports was taken into consideration. Various MOx supported gold catalysts were screened for SCO of NH3 (Figure 1 and Table S2). From the comparison at 150oC, Au/ZrO2, Au/Fe2O3, and Au/CeO2 showed high NH3 conversions, but their selectivities to N2 were low due to the production of N2O. In contrast, Au/SiO2 showed extremely high selectivity to N2. Since SiO2 is an acidic metal oxide (isoelectric point ≈ 1.8),23 we assumed that a support possessing acid sites may be good for increasing N2 selectivity. Hydrated niobium oxide (Nb2O5·nH2O) has high acid strength (H0 ≤ -5.6) and possesses Brønsted and Lewis acid sites on its surface, making it attractive as a support to improve N2 selectivity.24 Its surface acidity and surface area are controllable by heat treatment temperatures and crystalline structures.25 Three crystalline niobium oxides (Figure S2), including amorphous, orthorhombic (called T-phase) and deformed orthorhombic structures (denoted as Nb2O5-A, Nb2O5-T, and Nb2O5-DO, respectively) were synthesized to investigate the effects of crystalline structure and acidity of Nb2O5 on SCO of NH3. Sol immobilization method
ABSTRACT: Selective catalytic oxidation (SCO) of NH3 to harmless N2 and H2O is an ideal technology for its removal. To develop air purification systems in living environment, catalysts that can work at room temperature with high selectivities to N2 are required. However, it is a technical challenge since the reported catalysts either needed high operating temperatures or showed low selectivities to N2. In this study, we firstly demonstrated that acidic metal oxides supported gold catalysts showed good N2 selectivities compared with that of other metal oxides supported gold catalysts. A gold catalyst with niobium oxide synthesized by the hydrothermal method as a support showed high catalytic activity and high selectivity to N2 at low temperatures (18% NH3 conversion with 100% N2 selectivity at 25oC) and at high temperatures (100% NH3 conversion with 95% N2 selectivity at 245oC). Important roles of Brønsted acid sites and formation of active oxygen sites in improving N2 selectivity were revealed in this study. To the best of our knowledge, this is the first report of efficient catalysts that presented high NH3 conversion with high N2 selectivity at 25oC which will offer great scopes for commercial applications related to control of odors. In addition, this breakthrough in finding acid sites would greatly affect N2 selectivity and catalytic activity will provide a new trend in designing efficient catalysts not only for SCO of NH3, but also for the other selective catalytic oxidation.
KEYWORDS: gold, niobium oxide, ammonia, acid sites, selective catalytic oxidation Ammonia (NH3) has a well-known pungent odor. Its emission is hazardous to human lives and may result in degradation of the environment.1 Selective catalytic oxidation (SCO) of NH3 in the presence of O2 to produce N2 and H2O has been considered as one of the most promising methods for solving NH3 pollution. Various kinds of catalysts for SCO of NH3 have been studied, which can be divided into three main groups: supported noble metals, metal oxides (MOx) and zeolites (Figure S1 and Table S1). Supported noble metals such as Pt,2-4 Pd,5 Rh,4-5 Ir,4 Au6 and Ag7-8 have been reported to be active at temperatures around 250oC but less selective to N2 with byproducts of NOx. MOx-based catalysts
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using a thiolate-protected gold colloid (d = 1.8 ± 0.3 nm) was applied to prepare Au/Nb2O5 catalysts.26 Thermogravimetricdifferential thermal analysis (TG-DTA) of thiolate-protected gold colloid was shown in Figure S3. It suggested that alkanethiolates of gold colloid could be removed by the calcination temperatures above ca. 270oC. According to the results of XAFS shown in Figure S4(c), the peak of Au-S interaction (R = 1.90 Å) on Au/Nb2O5-DO didn’t appear which suggested that almost all alkanethiolates of gold colloid were removed by the calcination of 300°C for 2 h.
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Figure 2. Effect of reaction temperatures on (a) NH3 conversion and (b) N2 selectivity over Au/Nb2O5 with different crystalline structures. Symbols in (a) are the same as (b). The gold loading amount was 1 wt%. Reaction conditions were same as described in Figure 1. The effect of Brønsted and Lewis acid sites was investigated by introducing NaOH to block Brønsted acid sites on Au/Nb2O5-DO and Au/Nb2O5-T.24, 27 Acid amounts were calculated by NH3-TPD measurements (Figure S7 and Table S5). Au/Nb2O5-DO, having a high BET surface area, gave a much higher acid amount (0.37 mmol g-1) than that of Au/Nb2O5-T (0.053 mmol g-1). After NaOH treatment, acid amounts of Au/Nb2O5-DO and Au/Nb2O5-T drastically decreased to 0.18 and 0.024 mmol g-1, respectively. The adsorption-desorption equilibrium time of NH3 on the support also decreased in proportion to the decrease in acid amounts after NaOH treatment (Figure S8 and Table S5). N2 selectivity remarkably decreased from 95% to 32% on Au/Nb2O5-DO and decreased from 79% to 36% on Au/Nb2O5-T at 250oC after NaOH treatment (Table S5). To clearly distinguish the Brønsted and Lewis acid sites on Au/Nb2O5, pyridine adsorption was conducted (Figure 3). The ratio of Brønsted acid sites (1540 cm-1)27 and Lewis acid sites (1445 cm-1)27 based on the band area (B/L) suggested that the amount of Brønsted acid sites on Au/Nb2O5-DO was much larger than that of Au/Nb2O5-DO_NaOH (Figure 3(c)). It also suggested that the Brønsted acid sites, which remained to adsorb pyridine at high temperatures, on Au/Nb2O5-DO were blocked after NaOH treatment. From a comparison of Au/Nb2O5-DO and Au/Nb2O5DO_NaOH shown in Figure 3(d), the roles of different acid sites can be proposed: Lewis acid sites would be responsible for catalytic activity above 150oC and would not play an important role in N2 selectivity at all temperatures, whereas Brønsted acid sites would be active and favorable for N2 selectivity at all temperatures. Catalytic activity and N2 selectivity of Au/Nb2O5-T showed the same decreasing trends after NaOH treatment (Figure S9). The results of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) (Figure S10 and Figure S11) showed that NaOH treatment for both Au/Nb2O5-DO and Au/Nb2O5-T did not change the crystalline structures and oxidation states of Au and Nb, respectively. Thus, Brønsted acid sites play a critically important role in improving catalytic activity and N2 selectivity. To elucidate the reaction pathway, in situ FTIR experiments of NH3 adsorption and interaction between pre-adsorbed NH3 and O2 on different catalysts at different temperatures were conducted. In situ FTIR spectra of Au/Nb2O5-DO at 60oC is shown in Figure S12. For comparison, in situ FTIR experiments of Au/CeO2 at different temperatures were conducted (Figure S13) because Au/CeO2 mostly possesses Lewis acid sites (Figure S14) and showed high catalytic activity with low N2 selectivity (Figure 1), while Au/Nb2O5-DO possesses both Brønsted and Lewis acid sites (Figure 3(a)) and showed high catalytic activity and high N2 selectivity for SCO of NH3 (Figure 1 and Figure 2). After introducing O2, the observation of intermediates of HNOad and N2H4ad
Figure 1. NH3 conversions and N2 selectivities over Au/MOx at 150oC. The gold loading amount was 1 wt%. Reaction conditions: catalyst, 0.15 g; 50 ppm NH3 and 20% O2 balanced with Ar; space velocity, 40000 mL h-1 gcat-1. The results of SCO of NH3 over Au/Nb2O5-DO, Au/Nb2O5-T, and Au/Nb2O5-A are shown in Figure 1, Figure 2 and Table S3. Figure S5 shows the time course of NH3 conversions over Au/Nb2O5-DO with different gold loading amounts at different reaction temperatures. The values of conversion shown in Figure 2 were plotted after it reached the plateau at each temperature. Nb2O5-DO showed no activity even at 250oC (Figure 2(a)). In contrast, the catalytic activity was significantly enhanced by the deposition of gold nanoparticles. Au/Nb2O5-DO showed better catalytic activity than that of Au/Nb2O5-T and Au/Nb2O5-A with a similar gold loading amount (ca. 1 wt%) and size of gold nanoparticles (ca. 2.8 nm, see Figure S4, Figure S6, Table S3 and Table S4). The turnover frequency (TOF) of Au/Nb2O5-DO (7.3E-05 μmol(NH3) μmol(Au)-1 s-1) was much higher than that of Au/Nb2O5T (2.5E-05 μmol(NH3) μmol(Au)-1 s-1) at 25oC based on all gold used (Table S3). It is noteworthy that Au/Nb2O5-DO worked at 25oC with 18% NH3 conversion and 100% N2 selectivity. Besides, its catalytic activity increased with the gold loading amount increased from 1.1 wt% to 2.8 wt% over Nb2O5-DO, suggesting that gold surface or its perimeter interface was involved in the reaction. In addition to the good activity, Au/Nb2O5-DO showed 100% N2 selectivity at temperatures from 25oC to 200oC and showed 95% N2 selectivity at 245oC, while Au/Nb2O5-T and Au/Nb2O5-A showed 100% N2 selectivity at temperatures less than 150oC and their N2 selectivities decreased to 79% at 250oC and to 73% at 315oC. In addition, the order of catalytic activity for NH3-SCO at less than 150oC was Au/Nb2O5-DO > Au/Nb2O5-T > Au/Nb2O5-A. We have reported the CO oxidation by those catalysts,26 and the order of the catalytic activity for CO oxidation was the same as that for NH3-SCO. These results suggested that the crystalline structure of the support is important for both NH3 and CO oxidation, requiring the activation of oxygen.
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on Au/Nb2O5-DO and intermediate of HNOad on Au/CeO2 suggested the reaction mechanism is different on different acid sites.
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Figure 3. In situ FTIR spectra of pyridine adsorption on (a) Au/Nb2O5-DO, (b) Au/Nb2O5-DO_NaOH at different temperatures. (c) Ratios of Brønsted and Lewis acid sites based on the band area over Au/Nb2O5-DO and Au/Nb2O5-DO_NaOH. (d) Comparison of NH3 conversions and N2 selectivities on Au/Nb2O5-DO and Au/Nb2O5-DO_NaOH. From the in situ FTIR measurements, hydrazine mechanism has been proposed for Brønsted acid sites.28-29 NH2ad is formed in the reaction between the adsorbed NH3 and active oxygen species (eq. 1). N2H4ad intermediate is formed from the recombination of surface NH2ad (eq. 2), which will be subsequently oxidized by O2 to form N2 and H2O (eq. 3). (1) NH4+ + O → NH2ad + H2O NH2ad + NH2ad → N2H4ad (2) (3) N2H4ad + O2 → N2 + 2H2O Imide mechanism has been proposed for Lewis acid sites.30-32 Adsorbed NH3 would firstly react with the active oxygen species to form NHad (eq. 4), which are subsequently oxidized by active oxygen species to form HNOad intermediate (eq. 5). N2 and H2O are formed in the reaction between HNOad and NHad (eq. 6). (4) NH3ad + O → NHad + H2O NHad + O → HNOad (5) (6) HNOad + NHad → N2 + H2O 2HNOad → N2O + H2O (7) Compared with the hydrazine mechanism proposed for Brønsted acid sites, the role of active oxygen species is more important for N2 selectivity in the imide mechanism proposed for Lewis acid sites. Typical MOx supports for gold catalysts such as ZrO2, Fe2O3, and CeO2 showed high catalytic activity not only for non-selective NH3 oxidation (Figure 1) but also for CO oxidation,33-36 which are capable of the formation of sufficient active oxygen species. Thus, these catalysts are favorable for the formation of HNOad and it produces N2O (eq. 7) as a result. On the other hand, N2H4ad intermediate can be directly oxidized by O2 to form N2 and H2O (eq. 3). Therefore, Brønsted acid sites are good for N2 selectivity. In summary, we demonstrated that Au/Nb2O5-DO which possesses both Brønsted and Lewis acid sites showed high NH3 conversion and high selectivity to N2 at the temperatures ranging
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from 25oC to 200oC. We also revealed that the reaction sites of the gold nanoparticulates and Brønsted acid sites play important roles in selective catalytic oxidation of NH3. Most of the adsorbed NH3 on the support would be a spectator but Brønsted acid sites around the perimeter interface between gold and Nb2O5 might be the active sites for NH3 oxidation where the O2 adsorbed and activated. In addition, the catalytic activity of Au/Nb2O5 decreased drastically if a high concentration of NH3 was introduced without O2. This phenomenon suggested that the adsorbed NH3 around perimeter of gold would hinder the access of O2. As far as we know, this is the first report on the unique features of Au/Nb2O5 catalysts for SCO of NH3 with high catalytic activity as well as high N2 selectivity at 25oC. This is a breakthrough technology for applications in air purification systems and it will be of great significance in expanding design approaches of novel catalysts for selective catalytic oxidation.
AUTHOR INFORMATION Corresponding Author *
[email protected], *
[email protected] ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: *****. Experimental procedures, summary of the reported catalysts, physical properties of Au/MOx and their catalytic activity for SCO of NH3 conversion, characterization of Au/Nb2O5 (XRD, HAADF-STEM, XANES, XAFS, XPS), effect of the acidity of the support of Au/Nb2O5 catalysts on NH3-SCO (NH3-TPD), in situ FTIR measurements and reaction mechanism.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This study was supported by the platform for technology and industry project of Tokyo metropolitan government.
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