Different Reaction Mechanisms of Ammonia Oxidation Reaction on Pt

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23102−23111

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Different Reaction Mechanisms of Ammonia Oxidation Reaction on Pt/Al2O3 and Pt/CeZrO2 with Various Pt States Mengmeng Sun,*,†,‡ Jingying Liu,§ Chang Song,∥ Yoshito Ogata,⊥ Hanbing Rao,† Xiaoqing Zhao,† Haidi Xu,*,‡ and YaoQiang Chen‡ †

College of Science, Sichuan Agricultural University, No. 46 Xinkang Road, Yaan 625014, China Institute of New Energy and Low-Carbon Technology and §College of Chemistry, Sichuan University, Chengdu 610065, China ∥ Graduate School of Design and ⊥Faculty of Design, Kyushu University, Fukuoka 815-8540, Japan Downloaded via BUFFALO STATE on July 18, 2019 at 07:44:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: NH3 emissions were limited strictly because of the threat for human health and sustainable development. Pt/Al2O3 and Pt/CeZrO2 were prepared by the impregnation method. Differences in surface chemical states, reduction ability, acid properties, morphological properties, reaction mechanisms, and ammonia oxidation activity were studied. It indicated that Pt species states were affected by different metal−support interactions. The homogeneously dispersed Pt species over Pt/Al2O3 exposed Pt(111) because of weak metal−support interactions; there even existed an obvious interface between Pt and Al2O3. While obscure even an overlapped interface was observed over Pt/ CeZrO2, resulting in the formation of PtO because of the oxygen migration from CeZrO2 to Pt species (confirmed by CO-FTIR, the cycled H2-TPR and transmission electron microscopy results). It was noteworthy that different reaction mechanisms were induced by different states of Pt species; NH was the key intermediate species for ammonia oxidation reaction over Pt/Al2O3, but two kinds of intermediates, N2H4 and HNO, were observed for Pt/CeZrO2. It consequently resulted in the obvious distinction of the NH3-SCO catalytic performance; the light-off temperatures of NH3 over Pt/Al2O3 and Pt/CeZrO2 were 231 and 275 °C, respectively, while the maximum N2 selectivity (65%) was obtained over Pt/CeZrO2, it was obviously better than that over Pt/Al2O3. KEYWORDS: platinum, ammonia oxidation reaction, reaction mechanism, support material, interface performances.12−14 However, it was not clear whether the different Pt states had an effect on the NH3-SCO catalytic performance. The reaction mechanism should be investigated to understand the effect of different Pt states on the catalytic performance clearly. In addition, some scientific work was done to explore the reaction mechanism of ammonia oxidation. For Pt catalysts, the NH mechanism was proposed mostly. Mieher and Ho15 studied the coadsorption of oxygen and ammonia over Pt(111). The intermediate species NH2, NH, and OH were identified. They thought that oxygen was dissociated first, and then, ammonia was stripped by the oxygen atoms. Following that, the nitrogen atoms were reacted with oxygen to form NO. Moreover, the reaction mechanisms on Pt(100) and Pt(111) surfaces were also studied through density functional theory simulations.16,17 The result also proved the NH mechanism, in which NH3 was dehydrogenated progressively to form N atoms, and resulting N atoms were converted to N2, NO, and N2O. However, the ammonia oxidation reaction mechanism over the supported Pt catalysts

1. INTRODUCTION Nitrogen oxide from the vehicle exhaust was seriously harmful to human health. Extremely low emission limit of NOx was required to meet stricter and stricter regulations.1,2 The slipping NOx was removed usually by the catalytic reduction reaction with ammonia as a reducing agent.3 Large amounts of the reducing agent were used to enhance the conversion of NOx, which consequently resulted in leaking NH3 into the air.4 NH3 had become an important reason of threatening human health and sustainable development; thus, it was necessary to prevent the unreacted-NH3 from slipping.5 NH3 could react with O2 to effectively cope with the NH3 release.6,7 The desired product was N2, but the byproducts N2O and NO were also feasible.8 Thus, the ideal selective catalytic oxidation of NH3 (NH3-SCO) catalysts should possess both high activity and selectivity toward N2 in low temperatures. The noble metals Pt, Pd, and Rh presented good ammonia oxidation activity below 300 °C, especially Pt.9−11 However, extremely low N2 selectivity was obtained by Pt/Al2O3 because of the high level by-produced N2O and NO. However, the influence factors of high activity and selectivity have not been investigated clearly. It was well known that Pt states could be modified efficiently by the support material because of the metal−support interaction, which led to different catalytic © 2019 American Chemical Society

Received: February 1, 2019 Accepted: June 10, 2019 Published: June 10, 2019 23102

DOI: 10.1021/acsami.9b02128 ACS Appl. Mater. Interfaces 2019, 11, 23102−23111

Research Article

ACS Applied Materials & Interfaces

Figure 1. Conversion of NH3 (a); selectivity of N2 (b); N2O formation (c); and NOx formation (d). gases of 200 ppm NH3, 8% O2, 5% H2O, and N2 at 400 °C. The catalytic performance was tested in 100 000 h−1 space velocity as the temperature decreased. The tailpipe was connected to a Fourier transform infrared (FTIR, Antaris IGS-6700) instrument to get the gas compositions. 2.4. Characterization of Catalysts. Before O2-TPD measurement, the 0.10 g weigh sample was activated for 1 h under He flow. After that, the temperature was cooled from 400 to 100 °C. Then, O2 was adsorbed until saturation, and the unnecessary O2 was blowed by He. The O2 desorption peak was tested by a thermal conductivity detector (TCD) with the 10 °C·min−1 rate to 800 °C under He. Before the H2-TPR experiment, the 0.10 g weigh sample was activated for 1 h under N2 flow. Afterward, the temperature was cooled from 400 °C to room temperature, and the sample was deoxidized under reduction gases 5% H2/N2 with the 8 °C·min−1 rate of elevated temperature to 700 °C using a TCD. The cycled H2-TPR was measured with the same rate of elevated temperature to 400 °C, and then, the temperature was cooled to test again. CO-FTIR spectra were collected with an FTIR spectrometer (Thermo Nicolet 6700). The catalyst pretreatment for 30 min at 400 °C was made under N2 in the cell, and then, the background spectrum was recorded at room temperature. After that, CO/N2 (1 vol %) was adsorbed at 30 °C until saturation, and the adsorption result was tested after blowing excess CO using N2 (99.999%). CO-FTIR spectra were obtained by scanning 100 times. The CO-FTIR of both samples was also obtained after pretreating by NH3 reduction and retreating by N2 after NH3 reduction, respectively. To obtain the Pt species state in the reaction process, CO-FTIR was also tested as the above steps after the sample was treated for 30 min by the simulated gases of 200 ppm NH3, 8% O2, and N2 at 300 °C. Before transmission electron microscopy (TEM) (Tecnai G2 F20 S-TWIN) measurement, the sample was activated for 30 min by the simulated gases of 200 ppm NH3, 10% O2, 5% H2O, and N2 at 400 °C. An appropriate amount of the sample was added into the ethanol aqueous and dispersed in a centrifugal machine. The sample was measured with a scanning mode, and the accelerating voltage was 200 kV. CO chemisorption measurement was tested by a thermal conductivity detector (TCD). For Pt/Al2O3, 0.15 g of the sample was reduced for 30 min by H2 at 300 °C. Then, the temperature decreased to the room temperature, Pt dispersion was tested by COpulse chemisorption. For Pt/CeZrO2, the test method as the refs.19,20

had been seldom investigated because of the complexity of the supported catalysts. In addition, Pt states could be modified through the different metal−support interaction, which might lead to different reaction mechanisms.18 In this work, different support materials Al2O3 and CeZrO2 were used to modify the Pt states and displayed different catalytic performances of ammonia oxidation reaction, and the reaction mechanisms were further investigated over different Pt states, which have not been studied on the supported catalysts. It would provide deeper understanding for the relationship of Pt species states and the reaction mechanism, which is useful to obtain an excellent catalyst for ammonia oxidation reaction.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ce0.9Zr0.1O2 Material. First, Ce(NO3)3·6H2O and ZrO(NO3)2 as precursors were dissolved to prepare the Ce0.9Zr0.1O2 material. After that, ammonia water solution was added into the salt solution during the stirring process, and pH was maintained at approximately 10. Then, the clean precipitate was obtained by washing and filtering. The final Ce0.9Zr0.1O2 material with the mass ratio of CeO2/ZrO2 9:1 was gained after drying in the oven at 120 °C and calcining in the muffle oven at 600 °C for 3 h. The final Ce0.9Zr0.1O2 material was referred to as CeZrO2. 2.2. Catalysts Preparation. At first, an aqueous solution of Pt(NO3)2 was impregnated on the CeZrO2 (114 m2/g) support material (Pt with 1.0 wt %). The sample powder was obtained by drying in the oven at 120 °C and calcining for 3 h in the muffle oven at 550 °C. After that, the right amount of water was added into the powders to gain slurry. The 160 g/L monolithic catalyst was obtained by coating the slurry on 2.2 mL honeycomb cordierite and calcined for 3 h in the muffle oven at 550 °C, which was called as Pt/CeZrO2. The monolithic catalyst Pt/Al2O3 was prepared with the same method. The Al2O3 (129 m2/g) material was supplied by Sinocat Environmental Technology Co., Ltd. 2.3. Catalytic Performance Measurements. The quartz-glass reactor was vertically installed in a cylindrical furnace and used a thermocouple to control the temperature of the furnace, which was put in a middle place between the furnace and reactor. The monolithic catalyst was placed in the constant temperature zone of the reactor. On the top of the catalyst, another thermocouple was used to display the temperature of the catalyst. Before the activity performance, the sample was activated for 1 h under the simulated 23103


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the temperature went up, that is, first increased and then decreased. It was noteworthy that Pt/CeZrO2 had a stronger ability to suppress the formation of N2O than Pt/Al2O3. The highest N2O approached 60 ppm over Pt/Al2O3 at 300 °C, whereas the maximum N2O emission was only 26 ppm over Pt/CeZrO2 at 325 °C. The larger amounts of N2O emission over Pt/Al2O3 at low temperature led to its lower N2 selectivity. The significant difference of N2O emission was not associated with the ability of N2O decomposition (Figure S1), which might be due to the different Pt states and reaction mechanism. From Figure 1d, the value of NOx was relatively low below 275 °C. Therefore, controlling the production of N2O was an effective method to improve N2 selectivity in low temperatures. Unlike Pt/Al2O3, Pt/CeZrO2 had a stronger ability to suppress the generation of byproduct NOx above 275 °C. In conclusion, Pt/Al2O3 and Pt/CeZrO2 catalysts displayed obviously different catalytic performances. A better activity but lower N2 selectivity was obtained by Pt/Al2O3, whereas Pt/ CeZrO2 possessed higher N2 selectivity but lower activity. It indicated that the different metal−support interactions must exist in both catalysts, which led to different catalytic performances and reaction mechanisms. The reason that Pt/ Al2O3 showed better activity but Pt/CeZrO2 was beneficial to improve N2 selectivity should be explained clearly, which was very meaningful for designing catalysts for ammonia oxidation reaction. 3.2. O2-TPD. O2-TPD measurements were investigated to understand and identify the different oxygen species over both catalysts. As shown in Figure 3, three kinds of oxygen species:

Before CO-pulse chemisorption, CO2 was adsorbed to avoid the CO adsorption on the CeZrO2 material. The in situ adsorption−desorption of NH3 was investigated using an FTIR (Thermo Nicolet 6700) spectrometer. At first, the sample was activated for 40 min with N2, and then, the temperature was dropped from 400 °C down to 80 °C. In addition, NH3 (200 ppm) was adsorbed until adsorption capacity saturation. Afterward, the desorption spectra were collected in 80−500 °C. The saturated NH3 and O2 reaction process was also monitored. The catalyst was activated for 40 min in N2, and then, the temperature dropped from 400 to 275 °C. After that, NH3 was adsorbed until the maximum adsorption capacity, and then, the reaction process was measured when O2 was the inlet. The spectra were also obtained by subtracting the backgrounds. Furthermore, the in situ NH3 and O2 reaction was also investigated with the same instrument. First, the catalyst was activated by 200 ppm NH3, 8% O2, and N2. Secondly, the temperature was dropped from 400 to 80 °C under N2. Then, the in situ reaction spectra were measured under the simulated gases in 80−400 °C.

3. RESULTS AND DISCUSSIONS 3.1. Catalytic Activity. The NH 3 conversion, N 2 selectivity, and N2O and NOx formation results are shown in Figure 1. For Pt/CeZrO2, T50 and T90 of NH3 were 275 and 307 °C, respectively. However, Pt/Al2O3 presented much better activity than Pt/CeZrO2 in terms of T50 and T90 of NH3 (231 and 247 °C), which was a significant improvement for the ammonia oxidation reaction. Furthermore, the kinetic analysis results with the determination coefficient (R2) above 0.98 are showed in Figure 2 and Table 1. The TOF value was 0.12 s−1

Figure 2. Arrhenius plots of ammonia conversion turnover frequency.

Table 1. Kinetic Analysis Resultsa reaction NH3 +

O2b

samples Pt/Al2O3 Pt/CeZrO2

rates (mol/g·s) −6

6.3 × 10 4.0 × 10−7

TOF (s−1)

Ea (kJ/mol)

0.12 0.0077

211.7 391.6

Figure 3. O2-TPD curves for both samples.

The Pt content of Pt/Al2O3 and Pt/CeZrO2 was 5.13 × 10−5 mol/g. Catalyst bed temperature is 220 °C.

α, β, and γ were observed on the surface of Pt/CeZrO2. The oxygen species α was attributed to the surface chemical adsorbed oxygen under 250 °C, and oxygen species γ with a higher desorption temperature above 700 °C was ascribed to lattice oxygen, but oxygen species β was because of the oxygen vacancy.21,22 As for Pt/Al2O3, only α and β oxygen species were found, which indicated that lattice oxygen γ desorption was extremely hard because of the strong Al−O bond strength.23 When comparing the peak areas of the two catalysts, larger amount of vacancy oxygen and lattice oxygen species were obtained over Pt/CeZrO2. Moreover, the vacancy oxygen and lattice oxygen species of Pt/CeZrO2 still existed after treated at 400 °C, but none of oxygen species were present over Pt/Al2O3 above 400 °C.

a

b

for Pt/Al2O3, which was significantly larger than that for Pt/ CeZrO2 (0.0077 s−1), and the rate values of Pt/Al2O3 and Pt/ CeZrO2 were 6.3 × 10−6 and 4.0 × 10−7 mol/g·s, respectively. In addition, the Ea value of Pt/Al2O3 was 211.7 kJ/mol, which was only half of that over Pt/CeZrO2 (391.6 kJ/mol). It was that the Pt species with a stronger intrinsic activity on Pt/ Al2O3. In Figure 1b, N2 selectivity of Pt/CeZrO2 was higher than that of Pt/Al2O3. In detail, the highest N2 selectivity (about 65%) was obtained by Pt/CeZrO2, while the highest value for Pt/Al2O3 was only 50%. Figure 1c shows the generated value of N2O for both catalysts. It displayed the same tendency as 23104


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Figure 4. TPR profiles of Pt species on both catalysts (a); re-reduction profile of Pt/CeZrO2 (b).

Figure 5. CO-FTIR spectra prepared by N2 (a); CO-FTIR results reduced by NH3 (b); CO-FTIR results after prepared under N2 after reduced by NH3 (c).

3.3. H2-TPR. The redox properties of Pt/Al2O3 and Pt/ CeZrO2 are depicted in Figure 4. From Figure 4a, the peak of Pt/Al2O3 was assigned to Pt species reduction. Different from Pt/Al2O3, the peak intensity of Pt/CeZrO2 was extremely high, which was mainly due to the hydrogen spillover phenomenon leading to Pt oxide and surface CeO2 species reduction.24 The reduction peak of Pt/CeZrO2 was located at 212 °C, which greatly shifted to 152 °C for Pt/Al2O3. As claimed in the literature, Pt/CeZrO2 possessed a stronger metal−support interaction because of Pt−O−Ce bond, which enhanced the platinum−oxygen bond strength, leading to weaker reduction ability of Pt species.25,26 The better reduction ability was obtained by Pt/Al2O3 because of the weaker platinum−oxygen bond strength. In this work, both catalysts with different support materials contributed to the different reduction ability of Pt species, leading to different catalytic performances. The cycled H2-TPR was measured to prove the transfer phenomenon of oxygen species from CeZrO2 to Pt species, which was presented in Figure 4b. Three peaks were observed in 50−900 °C. The H2 consumption peak located at 212 °C was 355.5 μmol/g, which was much larger than the theoretical amounts of PtO reacted H2 (51.2 μmol/g). It was mainly

because of the hydrogen spillover leading to some surface CeO2 reduction. The H2 consumption at 430 and 900 °C belonged to the remaining CeO2 reduction.27−29 Only the peak (212 °C) was found when the catalyst powder was reduced below 400 °C. Afterward, re-reduction was measured after the temperature decreased to room temperature. A new peak appeared at the lower temperature (102 °C) that belonged to some regenerative Pt oxide species reduction. Its H2 consumption was 39.6 μmol/g, which indicated that about 80% Pt species were oxidized again because of the effective transfer of oxygen species from CeZrO2 to Pt species. The Pt oxide states of Pt/CeZrO2 were kept strongly because of the effect of the support material, which was a main reason for the weakened NH3-SCO performance. 3.4. CO-FTIR Measurement. The active platinum state could be one of the key factors to affect the catalytic performance.30 After the samples were pretreated in N2, the CO-FTIR result for Pt species was provided in Figure 5a. The bands of 2062 cm−1 were CO bonded on metallic Pt states, and the bands of 2097 cm−1 belonged to mild Pt oxidation states.31 Both metallic Pt and mild Pt oxidation states were observed over Pt/CeZrO2 and Pt/Al2O3 catalysts. But the Pt 23105


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oxidation states because of the existed excess O2, while Pt/ Al2O3 had a strong ability to keep the metallic Pt state because of the little bit of reduction gas NH3. 3.5. TEM Results. To provide more direct evidence for Pt species in the reaction atmosphere, TEM and high-resolution TEM (HRTEM) were measured and displayed in Figure 7.

species of Pt/CeZrO2 contained a larger amount of mild Pt oxidation states, whereas more Pt metallic states existed in Pt/ Al2O3. Furthermore, a lower coordination metallic Pt was observed at 2012 cm−1 on Pt/CeZrO2.32 It marked that the Pt species of Pt/CeZrO2 had a smaller Pt particle, whereas Pt/ Al2O3 did not have the smaller Pt particle after being treated by N2. The results suggested that the Pt species exposed states could be influenced by the support material.12−14 CO adsorption behaviors were measured after reduction by NH3 (shown in Figure 5b). The band at 2053 cm−1 was because the metallic Pt was found on Pt/Al2O3, but the mild Pt oxidation states almost all disappeared. However, Pt oxide species (2097 cm−1) was still observed over Pt/CeZrO2. It indicated that the reduction of Pt oxide species was much harder because of the effect of support CeZrO2. Moreover, the different electronic states of Pt species over both samples also existed clearly. For the Pt species of Pt/Al2O3 with more electrons, the band of CO absorption appeared at 2053 cm−1, whereas the CO absorption band of Pt/CeZrO2 was located at a higher wavenumber (2058 cm−1).33,34 Figure 5c shows CO-FTIR results of pretreatment in N2 after reduction. In addition, the flowing N2 was prepared by the deoxidizing agent. Compared with the results in Figure 5b, the results of 2059 and 2062 cm−1 was because metallic Pt obviously shifted to higher wave numbers than that of Pt/ Al2O3 and Pt/CeZrO2 pretreated by NH3. It was worthwhile to point out that not only the metallic Pt (2062 cm−1) but also large amounts of Pt oxidation states (2093 cm−1) over Pt/ CeZrO2 were observed clearly. It could be deduced that the generation of Pt oxidation states belonged to oxygen species transfer function.35 According to the results of the catalytic performance, the presence of mild Pt oxidation states over Pt/ CeZrO2 weakened the oxidation ability for NH3, thus leading to higher N2 selectivity. However, Pt0-active states almost remained intact over Pt/Al 2 O 3 , which was the main contributor to better NH3-SCO activity. To obtain the Pt species states in the NH3-SCO reaction, the CO-FTIR was tested after the catalyst treatment by the diesel vehicle simulation gas. Two CO absorption peaks of 2091 and 2165 cm−1 were found on Pt/CeZrO2, which were mild Pt oxidation states and Pt oxidation states (Figure 6). However, Pt species of Pt/Al2O3 mainly existed in metallic Pt state (2059 cm−1), with just a little bit of mild Pt oxidation (2091 cm−1). It could clearly show the Pt species states in the reaction process. All of the Pt species of Pt/CeZrO2 were

Figure 7. TEM results for Pt/CeZrO2 (a); TEM results for Pt/Al2O3 (b); HRTEM results for Pt/CeZrO2 (c) and Pt/Al2O3 (d).

Before measurement, the sample was pretreated by simulated exhaust gas to make the catalyst keep the same states as the reaction process. Figures 7a,b present the TEM results. For Pt/ CeZrO2, Pt species was highly dispersed on the support and was found in 0.8−2.0 nm (1.21 nm average diameter), and the particle size of Pt species with a mean size of 2.45 nm was located at 1.0−4.0 nm from Pt/Al2O3. However, the higher dispersion over Pt/CeZrO2 did not have an ability to promote the NH3-SCO activity. It was because that the smaller Pt particle size over Pt/CeZrO2 was obtained as a result of the strong Pt−O−Ce bond, which made Pt species present as inactive states.35 The HRTEM images were observed further over Pt/CeZrO2 (Figure 7c) and Pt/Al2O3 (Figure 7d). As for Pt/CeZrO2, the obscure even overlapped interface between Pt species and CeO2 particle was observed clearly, indicating that the interface oxygen species was very active. Moreover, the dspacing of Pt species over Pt/CeZrO2 was 0.29 nm, which proved PtO formation because of the oxygen species transfer function.30 Combining with CO-FTIR results, a good consistency was showed, the lattice oxygen species of CeZrO2 could transfer to Pt species, making Pt species become oxidation states. In addition the Pt species of Pt/ CeZrO2 with the weaker reduction ability (confirmed by H2TPR) promoted the high N2 selectivity. However, the interface between Pt species and Al2O3 remained clearly after being treated by the reaction gas, and Pt0 species of Pt/Al2O3 could be remained, which was the same result with CO-FTIR treated by reaction gas. Furthermore, Pt(111) with 0.23 nm d-spacing was found on Pt/Al2O3.34 According to the references, the

Figure 6. CO-FTIR spectra of the two catalysts prepared by the diesel vehicle simulation gas. 23106


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higher dispersion (74%) was found. In addition, the Pt particle size of Pt/Al2O3 obtained by CO chemisorption was about 2.49 nm, it was decreased to 1.37 nm for Pt/CeZrO2 due to higher Pt dispersion on Pt/CeZrO2. In addition the CO adsorption results could be related with the TEM results. However, the Pt species of Pt/CeZrO2 with higher dispersion and smaller particle size did not display a better activity because of the Pt oxidation states with lower intrinsic activity. Thus, Pt species existed states was the main factor to affect NH3-SCO activity. 3.7. NH3 Adsorption−Desorption. NH3 adsorption− desorption behaviors were displayed in Figure 8. For Pt/Al2O3, the bands at 1490, 1389, and 1692 cm−1 belonged to NH3 bonded on Brønsted acid sites at 80 °C,40−44 whereas the adsorption peaks of 2240 and 3000−3500 cm−1 were NH3

bonded on Lewis sites.45−47 The area of adsorption peaks decreased with the increase of temperature, indicating that ammonia was reacted or desorbed. It was also worth noting that the nitrates species (1603 1560, and 2048 cm−1) were produced with the increase of temperature, which proved that the reaction of NH3 and oxygen species indeed existed.48 It was interesting that NH2 and −NH intermediates were presented at 1583 and 1452 cm−1, which could be activated and reacted to form the nitrates species.49 Combined with the activity, the productions were N 2 , N 2 O, and NO x . Furthermore, the NH mechanism was also proved on Pt(111),15,17 and the presence of Pt(111) was also proved by the TEM results. FTIR spectra of NH3 adsorbed on Pt/CeZrO2 are showed in Figure 8b.50 In detail, the adsorption peaks of 1300, 1200− 1000, and 3500−3000 cm−1 belonged to ammonia bonded on Lewis sites.50,51 Correspondingly, the Brønsted peak was at 1567 and 1401 cm−1.31 The area of NH3 adsorbed decreased with the increase of temperature, which was the same with that of Pt/Al2O3. It was interesting that the new peak of 1328 cm−1 appeared suddenly at 250 °C and then disappeared with the reaction progress, which must be an intermediate species for ammonia oxidation reaction. According to references, the band was belonged to N2H4 species,52 and N2H4 mechanism was found when the oxygen species came from CeZrO2. The N2H4 species might be formed by the combination of two NH2 species produced by the dehydrogenation of NH3, which indicated that ammonia oxidation reaction mechanism over Pt/CeZrO2 was different from that over Pt/Al2O3. 3.8. In Situ Reaction FTIR Results of Adsorbed NH3 with O2. Figure 9a shows the DRIFTS spectra about NH3 and O2 reaction over Pt/Al2O3. The adsorption peaks of 3000−

Figure 8. Ammonia adsorption−desorption results on Pt/Al2O3 (a); Ammonia adsorption−desorption results on Pt/CeZrO2 (b).

Figure 9. DRIFTS results about the reaction of adsorbed NH3 and O2 on Pt/Al2O3 (a) and Pt/CeZrO2 (b) at 275 °C.

oxygen dissociation was the rate-determining step in the NH3SCO reaction.36,37 Moreover, the presence of Pt(111) for Pt/ Al2O3 could increase the dissociation probability of oxygen molecules, and more atomic oxygen could enhance the ability of dehydration of ammonia, which led to a higher NH3-SCO activity.30,38,39 3.6. CO Chemisorption. From Table 2, the Pt dispersion was 41% for Pt/Al2O3, while Pt species of Pt/CeZrO2 with a Table 2. Pt Particle Size and Pt Dispersion for Catalysts CO adsorption

TEM

samples

SABET (m2/g)

Pt particle size (nm)

Pt dispersion (%)

Pt particle size (nm)

Pt/CeZrO2 Pt/Al2O3

114 129

1.37 2.49

74 41

1.21 2.45

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ACS Applied Materials & Interfaces 3500 and 1273 cm−1 were ammonia bonded on the Lewis sites, and the Brønsted acid sites (1692, 1490 and 1394 cm−1) were also found.40−47 With the reaction progress, the area of ammonia adsorption peaks in Lewis sites disappeared quickly, which suggested that ammonia bonded on Lewis sites could be activated more easily. The NH2 species (1583 cm−1) was also presented, which showed a good consistency with the NH3 adsorption−desorption spectra. It suggested that the reaction mechanism remained unchanged because of the strong oxygen dissociation ability of Pt(111). Furthermore, some new species would appear gradually with the reaction progress. Accordingly, NO species (1544 cm−1) was found at 0 min, and the intensity increased significantly with reaction progress.53 However, the production NO could not be found in the results of catalytic performance below 275 °C. However, the asymmetric N−N−O stretching modes of N2O (2240 cm−1) was observed at 1−7 min,54 which indicated that the NO might be a key species for N2O formation. Figure 9b shows the results of Pt/CeZrO2. The bands of 3000−3500 and 1000−1200 cm−1 were ammonia bonded on Lewis sites, whereas ammonia bonded on Brønsted sites was found at 1627 cm−1.31,50 The same with Pt/Al2O3, the ammonia bonded on Lewis sites was a major contributor to NH3-SCO activity when the adsorption peak area was decreased significantly. It was very interesting that an adsorbed nitroxyl (HNO) species (1470 cm−1) appeared at 5 min, without increasing as the reaction progress, which could be an intermediate species for ammonia oxidation.55 At the same time, the product N2O was observed at 5 min and disappeared at 7 min, with much shorter time, which indicated that Pt/ CeZrO2 had a stronger ability to suppress N2O formation than Pt/Al2O3. Afterward, the NO species (1030, 1223, and 1518 cm−1) were observed clearly.56−58 Compared with the results of NH3 adsorption−desorption spectra over Pt/CeZrO2, the N2H4 mechanism was produced when the adsorbed NH3 reacted with the oxygen species from CeZrO2, whereas the ammonia oxidation reaction would follow the HNO mechanism when NH3 was reacted with gaseous oxygen species. 3.9. FTIR Spectra of the in Situ Reaction of NH3 and O2. From Figure 10, only the adsorption peaks of NH3 species on Pt/Al2O3 and Pt/CeZrO2 were displayed because of NH3 and O2 being unreacted at 80 °C. For Pt/Al2O3, the intensities of adsorption NH3 species (1653 and 3000−3500 cm−1) were decreased with the reaction progress and disappeared until 150 °C, and −NH2 (1581 cm−1) and −NH (1455 cm−1) were found. It marked that the NH3 species was activated to −NH2 and −NH followed the NH mechanism. In addition, NO species (1555 and 1431 cm−1) and N2O species (1734 cm−1) showed at 250 °C, and the intensity of production increased as the temperature increases.53,59 According to the results of activity performance, NO was not produced at 250 °C, indicating that NO was the key intermediate for generation of N2O. As shown in Figure 10b, the intensity of NH3 adsorption species (1577, 1414, 1392, 1296, and 1000−1200 cm−1) was reduced with the reaction progress on Pt/CeZrO2 and disappeared at 200 °C.50,51 Compared with that of Pt/Al2O3, NH3 adsorbed on Pt/CeZrO2 had a weaker ability to be dehydrogenated because of the different Pt states. Afterward, the intermediates HNO (1470 cm−1) and N2H4 (1328 cm−1) over Pt/CeZrO2 were observed at 250 °C.52,55 It indicated that the HNO and N2H4 mechanism might coexist during the

Figure 10. FTIR spectra of the in situ reaction of NH3 with O2 over Pt/Al2O3 (a) and Pt/CeZrO2 (b).

ammonia oxidation reaction, which was different from the reaction mechanism over Pt/Al2O3. Subsequently, NO formation (1547, 1518, 1440, 1221, and 1030 cm−1) was presented at higher temperature (until 300 °C), and the intensity was increased with the increasing temperature. 3.10. Reaction Mechanism over Pt/Al2O3 and Pt/ CeZrO2. According to the results of TEM and CO-FTIR prepared by the reaction gas, the different Pt species states are showed in Figure 11. Pt species of Pt/Al2O3 was presented

Figure 11. Reaction mechanism and Pt states over Pt/Al2O3 (a) and Pt/CeZrO2 (b).

mainly as Pt0 active states, and the existence of Pt(111) species was proved by the TEM measurements. The active Pt(111) could improve O2 dissociation to O as reaction (1) and promote ammonia dehydrogenation as reactions (2−4).36,37 It was beneficial to NH and NH2 formation, and facilitated the products N2O, NO, and N2 generation.

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O2 → 2O

(1)

NH3 + O → NH 2 + OH

(2)


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ACS Applied Materials & Interfaces NH 2 + O → NH + OH

(3)

NH + O → N + OH

(4)

4. CONCLUSIONS Pt/Al2O3 and Pt/CeZrO2 catalysts with different metal− support interactions led to different Pt states. Pt/Al2O3 contained Pt0 species mainly due to Pt species and Al2O3 weak interaction, and even the obvious interface existed between Pt and Al2O3. It also should be noted that Pt(111) with extremely an high activity was observed by TEM results. However, the overlapped interface was presented obviously because of the transfer of oxygen species from CeZrO2 to Pt species, which resulted in PtO formation (CO-FTIR and TEM). More importantly, different reaction mechanisms were induced by different Pt states because of the distinctive ability for dissociation of O2. In detail, the ammonia oxidation reaction over Pt/Al2O3 followed the NH mechanism, whereas the N2H4 mechanism and HNO mechanism coexisted over Pt/ CeZrO2. The NH mechanism would easily lead to the formation of byproduct NO, while N2 was promoted by N2H4 and HNO mechanism. Finally, the obvious distinction of the NH3-SCO catalytic performance was obtained over Pt/ Al2O3 and Pt/CeZrO2. Pt/Al2O3 with T50 = 231 °C showed a better activity than Pt/CeZrO2 (T50 = 275 °C); correspondingly, the TOF value over Pt/Al2O3 was 0.12 s−1 that is remarkably larger than that over Pt/CeZrO2 (0.0077 s−1), while the N2 selectivity (65%) was obtained over Pt/CeZrO2, that is obviously higher than that over Pt/Al2O3.

For Pt/Al2O3, the existence of Pt(111) was a positive factor for active atomic oxygen species. Moreover, larger amount of active oxygen species would promote the generation of NO following the reaction (5). At low temperature, NO formation led to production of byproduct N2O through reaction (6), and at higher temperature, NO with high desorption rates was the main byproduct. In conclusion, the larger amounts of active oxygen species was a key factor for the dehydrogenation of ammonia toward NH mechanism, which led to the better NH3-SCO activity. N + O → NO

(5)

N + NO → N2O

(6)

As for Pt/CeZrO2, the main PtO species were presented clearly because of the interface oxygen transfer interaction, which were confirmed by CO-FTIR and TEM results. It was not beneficial to active atomic oxygen formation and suppressed ammonia dehydrogenation and NO formation; thus, the ammonia oxidation reaction mechanism over Pt/ CeZrO2 was different from that over Pt/Al2O3. From the in situ reaction of NH3 and O2, the intermediate species HNO and N2H4 were observed, which indicated that not just one reaction mechanism for ammonia oxidation existed over Pt/ CeZrO2. With the active oxidation species from the lattice oxygen species, the ammonia oxidation reaction was presented as the N2H4 mechanism following the reactions (7−11).60,61 In the first step, NH2 species were produced by abstraction of H from adsorbed NH3 (7). Then, the recombination of NH2 species led to the formation of the N2H4 intermediate (8) and then reacted with O to produce N2 (9) and/or N2O (10). At the same time, the HNO mechanism also appeared when the active oxygen species from oxygen molecules, which was proved by the adsorbed NH3 and O2 in situ reaction.62 At the beginning, the adsorbed NH3 reacted with active oxygen species to generate NH (12), and then, HNO was generated by combining NH with the oxygen atom (13). However, limited HNO intermediates were observed because of the weaker ability of PtO to activate O2, and the lack of active oxidation species would suppress the formation of byproducts N2O (15) and NO (16), leading to the higher N2 selectivity (14). Furthermore, the NH mechanism could not be excluded over the Pt/CeZrO2 catalyst. NH3 + O → NH 2 + OH

(7)

2NH 2 → N2H4

(8)

N2H4 + 2O → N2 + 2H 2O

(9)

2N2H4 + 6O → 2N2O + 4H 2O

(10)

NH 2 + 2O → NO + H 2O

(11)

NH3 + O → NH + H 2O

(12)

NH + O → HNO

(13)

HNO + NH → N2 + H 2O

(14)

2HNO → N2O + H 2O

(15)

HNO + O → NO + OH

(16)

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b02128. N2O conversion over Pt/Al2O3 and Pt/CeZrO2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.S.). *E-mail: [email protected] (H.X.). ORCID

Mengmeng Sun: 0000-0003-0177-1723 Xiaoqing Zhao: 0000-0001-9675-5114 Haidi Xu: 0000-0002-6045-5600 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support for the study described in this article from National Natural Science Foundation Project in China under contract number 21802099.

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Different Reaction Mechanisms of Ammonia Oxidation Reaction on Pt

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