Effect of different RuO2 contents on selective catalytic oxidation of ammonia over RuO2-Fe2O3 catalysts

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 47, Issue 2, February 2019 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2019, 47(2), 215223

RESEARCH PAPER

Effect of different RuO2 contents on selective catalytic oxidation of ammonia over RuO2-Fe2O3 catalysts WANG Hui-min, NING Ping, ZHANG Qiu-lin*, LIU Xin, ZHANG Teng-xiang, HU Jia, WANG Lan-ying Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China

Abstract:

A series of RuO2-Fe2O3 catalysts varied in RuO2 loading were prepared by sol-gel method and used for selective catalytic

oxidation of ammonia to nitrogen. The results indicated that all the RuO2-Fe2O3 catalysts showed an excellent low-temperature catalytic activity and the RuO2 loading played a key role in catalytic activity of ammonia oxidation. Moreover, the characterizations of BET, XRD, H2-TPR and DRIFTS were employed to investigate the relation between the physicochemical property of catalysts and catalytic activity. The research results elucidated that the introduction of RuO2 increased the surface area. The synergistic effect between RuO2 and Fe2O3 enhanced the redox property and the catalytic activity of ammonia oxidation. Meanwhile, the RuO 2 loading gave the significant effect on surface acidity of catalysts. Lewis acid sites were predominant on the catalyst surface. Key words:

sol-gel method; selective catalytic oxidation of ammonia; synergistic effect; acid sites

Ammonia (NH3), as one of the main industrial pollution, has resulted in a series of serious environmental problems. In addition, ammonia slip from diesel vehicle exhaust retreatment system is another major source of ammonia[1,2]. Excessive inhalation of NH3 will lead to lung diseases and even death in human, so the effective removal of NH 3 is an urgent problem to be solved. Compared with traditional adsorption, absorption and decomposition methods, selective catalytic oxidation of ammonia (NH3-SCO) as a more effective and environmental-friendly NH3 treatment method has attracted extensive attention in recent years[3–5]. The main reaction is as follows. 4NH3 + 3O2 → 2N2 + 6H2O (1) In this process, the selection of catalytic materials is particularly important. In the past studies, catalysts used in ammonia catalytic oxidation mainly include noble metal based catalysts, transition metal oxide catalysts and zeolite catalysts. Among them, noble metals such as Pt, Rh, Pd, Ir and Ag-based catalysts have good NH3 catalytic activity at low temperature, but all of them generally have low N2 selectivity[6–9]. Transition metal oxides like CuO, CoOx and MnOx have higher catalytic oxidation activity of ammonia, but their N2 selectivity is as low as that of precious metals[10–12]. In addition, the zeolite catalysts (Pd-Y, Rh-ZSM-5 and Cu-ZSM-5) modified with precious metals and transition

metals have high N2 selectivity within a wide temperature window. Unfortunately, the reaction conditions are rather harsh, and NH3 can only be fully transformed at a high reaction temperature in general[13–15]. In recent years, iron oxide (Fe2O3), as a potential transition metal oxide, has been widely used in catalytic fields such as NO reduction[16], toluene oxidation[17], and CO oxidation[18], which has also been recognized in the field of NH3-SCO[19,20]. Furthermore, the noble metal ruthenium has been applied for ammonia oxidation due to its excellent oxidation performance and relatively low cost[21,22]. Cui et al[21,22] found that RuO2-CuO/Al-ZrO2 and CuO/RuO2 catalyst showed superior catalytic performance at low temperature when the RuO2 loaded up to 5%30% and 70%95%, respectively. It could achieve the 100% conversion of NH3 at 200°C. Considering the economy of catalytic material preparation, in this study, we prepared a series of RuO2-Fe2O3 composite oxide catalysts with low RuO2 content for NH3-SCO reaction. This experiment mainly studied the influence of different RuO2 content on the catalytic activity of the catalysts. In addition, this study explored the influence of RuO 2 content on the structural characteristics of the catalyst through a series of characterization methods. In situ diffuse reflectance infrared spectroscopy was used to study the acid sites on the surface of catalysts and deduce the surface reaction process.

Received: 08-Oct-2018; Revised: 07-Dec-2018. Foundation items: Supported by the National Natural Science Foundation of China (21307047). *Corresponding author. Tel: 13668788376, E-mail: [email protected]. Copyright  2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

WANG Hui-min et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 215223

Fig. 1

Influence of RuO2 loading of RuO2-Fe2O3 catalysts on SCO catalytic performance (a): NH3 conversion; (b): N2 selectivity; (c): NO yield; (d): NO2 yield; (e): N2O yield

1 1.1

Experimental Catalysts preparation

RuO2-Fe2O3 composite oxide catalysts were synthesized following a sol-gel method. First, iron nitrate (Fe(NO3)3·9H2O) and ruthenium chloride (RuCl3·3H2O) were dissolved in 60 mL deionized water and stirred at room temperature for a certain time until completely dissolved. Then, citric acid (C6H8O7·H2O) was added to the above solution with two times the total number of metal ions (n(C6H8O7·H2O)/n(Fe3++Ru3+)=2), and continuously stirred for a period of time to form a uniform mixture solution. Next, the mixed solution was heated at 80°C in water bath and then dried in the oven for 72 h. Finally, the dry gel was placed in the tube furnace calcined at 550°C for 3 h

under nitrogen atmosphere, and then calcined at 500°C for 4 h in air to get the required RuO2-Fe2O3 composite oxides. The prepared catalysts was labeled as xRuO2-Fe2O3 with different content of RuO2 (x is the mass percentage of RuO2, x=0.5%, 1%, 1.5% and 2%). In addition, for comparison, pure Fe2O3 sample were prepared by the same method. 1.2

Characterization of the catalysts

The Bruker D8 X-ray diffractometer was used for the characterization test of X-ray diffraction (XRD). Cu K was the diffraction source (λ =0.15406 nm), and the required working voltage and current were 40 kV and 40 mA, respectively. The scanning angle was from 20°to 85°with a scan rate of 6(°)/min and a resolution of 0.02°per step.

WANG Hui-min et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 215223

Fig. 2

XRD patterns of pure Fe2O3 and RuO2-Fe2O3 catalysts

heated to 600°C at the rate of 8°C/min under N2 atmosphere and the amount of desorbed NH3 was measured by a quadrupole mass spectrometer (Hiden) at the same time. In situ diffuse reflection infrared spectroscopy (In situ DRIFTS) experiment was measured in the Fourier transform infrared spectrometer (Nicolet 6700). The sample was placed in IR cell pretreating with N2 for 20 min to remove surface impurities. After the sample was cooled to room temperature, the condition is set according to the required test conditions. The spectra were collected by OMNIC software, and the background spectrum was subtracted from each sample spectrum acquired at the same temperature. Then, a gas mixture comprising of 0.06% NH3 in N2 (100 mL/min) was passed through the samples for 30 min. After turning off the NH3, all spectra were collected at 250°C in N2 as a function of time.

The specific surface area (BET), pore size, and pore volume were measured on TriStar II type 3020 equipment. Before the test, the samples were degassed for 4 h at 300°C under the condition of vacuum. Then N2 adsorption-desorption were 1.3 Catalytic activity tests carried out with high purity nitrogen as adsorption gas, and the specific surface area of the catalyst was calculated by The catalytic activity evaluation of NH 3-SCO was adopted Brunauer-Emmett-Teller (BET) equation. following the temperature-programmed reaction method, and H2 temperature-programmed reduction (H2-TPR) was the temperature range of the test was 150350°C. The 0.4 measured in a self-made quartz-tube (i. d. = 4 mm) reaction mL catalyst with a particle size of 4060 mesh was packed system. Firstly, 30 mg sample was packed in the quartz tube in a quartz tube (i.d.=6 mm) fixed-bed micro-reactor for and pretreated at 300°C with high purity nitrogen purging 40 temperature-programmed measurement. The mixture min to remove the surface impurities. Later, after cooling to components (volume fraction) were 0.08% NH3, 5% O2, and the required temperature, the sample was exposed to 5% Ar as balance gas, with a total flow of 400 mL/min. The H2/Ar mixture gas heating from 50°C to 800°C with a heating concentration of NH3 and NOx (NO, NO2) was detected on rate of 8°C/min. The H2 consumption was detected by thermal GXH-1050E NH3 analyzer and ECOMJ2KN flue gas conductivity detector (TCD). analyzer, respectively. The concentration of N 2O was NH3 temperature-programmed desorption (NH3-TPD) detected by the electron capture detector (ECD). NH3 experiment was carried out in a fixed bed reactor equipped conversion and N2 selectivity were calculated by the with a quartz U-tube. In front of the adsorption of NH3, each following formula: sample was pretreated at 400°C with N2 for 1 h. After cooling c(NH3 ) − c(NH3 ) inlet outlet xNH3 (%)= ×100(%) (2) to room temperature, NH3 was introduced for 40 min, and c(NH3 ) inlet then N2 was introduced for 20 min. Finally, the catalyst was c(NH3 ) − c(NH3 ) − c(NO)outlet − c(NO2 ) −2c(N2 O) outlet outlet inlet outlet sN2 (%)= ×100(%) (3) c(NH3 ) − c(NH3 ) inlet

Fig. 3

outlet

(a) N2 adsorption-desorption isotherms and (b) pore-size distribution of different catalysts

WANG Hui-min et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 215223

wNO (%)=

c(NO)outlet

×100(%) c(NH3 ) inlet c(NO2 )outlet wNO2 (%)= ×100(%) c(NH3 ) inlet c(N2 O)outlet ×100(%) wN2O (%)= c(NH3 )

(5) (6)

inlet

2 2.1

Pore structural parameters of different samples

Sample

ABET /(m2·g–1)

Pore volume v/(cm3·g–1)

Fe2O3

14.3

0.15

0.5%RuO2-Fe2O3

16.0

0.16

1%RuO2-Fe2O3

14.2

0.13

1.5%RuO2-Fe2O3

15.6

0.14

2%RuO2-Fe2O3

19.1

0.16

Results and discussion NH3-SCO performance of catalysts

Figure 1 displayed the catalytic performance evaluation results for NH3-SCO over pure Fe2O3 and RuO2-Fe2O3 catalysts. Fe2O3 catalyst exhibited a mild activity, which achieved 100% conversion for NH3 and 79% selectivity for N2 at 300°C. It was worth noting that catalytic performance of catalysts was greatly improved with the addition of RuO2. It showed preferable low-temperature activity, and the catalytic performance was obviously correlated to the increase of RuO2 contents. When the RuO2 loading reached to 0.5% or 1%, the complete NH3 conversion occurred at 250°C and the N2 selectivity was greater than 80% at 350°C (Figure 1(a) and Figure 1(b)). RuO2-Fe2O3 catalysts with 1.5% RuO2 showed the superior catalytic performance, which accomplished 100% NH3 conversion and 89% N2 selectivity at around 225°C. When RuO2 content further increased to 2%, the catalytic activity could not be improved, but exhibited a declined tendency. However, the negative effect had little influence on the N2 selectivity. In general, the catalytic performance increased with the increase of RuO2 contents but decreased with the excessive RuO2, of which the 1.5% was the best loading. Interestingly, the change tendency of N2 selectivity was contrary to NH3 conversion but all RuO2-Fe2O3 catalysts displayed excellent low-temperature activity and N2 selectivity. Meanwhile, the detection results indicated that by-products NO (Figure 1(c)), NO2 (Figure 1(d)) and N2O (Figure 1(e)) were also found. For Fe2O3 catalyst, the main by-products NO and N2O were detected when the reaction temperature beyond 250°C, and only few NO2 formed. By compared, only few N2O produced for the RuO2-Fe2O3 catalysts when the reaction temperature was less than 250°C, which was correlated to the high N2 selectivity. When the reaction temperature was greater than 250°C, the yield of N2O decreased, and the main by-products were NO and NO2. Namely, the addition of RuO2 altered the formation tendency of by-products. 2.2

Table 1

(4)

XRD analysis

XRD analysis illustrated the structural characteristics of Fe2O3 and RuO2-Fe2O3 catalysts and was displayed in Figure 2.

For pure Fe2O3 sample, the sharp diffraction peaks located at 24.3°, 33.3°, 35.7°, 41.0°, 49.6°, 54.2°, 62.6° and 64.1° were attributed to the typical -Fe2O3 structure (JCPDS 33-0644), which were respectively corresponding to (012), (104), (110), (113), (024), (116), (214) and (300) planes[23], revealing a well crystallinity of Fe2O3. Additionally, the typical peaks presented on RuO2-Fe2O3 catalysts were similar to pure Fe2O3 catalyst and no diffraction peaks of Ru species were observed. The results revealed that Ru species were highly dispersed on the surface of catalyst or existed with an amorphous structure, so that could not be detected by XRD [24]. To further explore the influence on the structure of Fe 2O3 by incorporating RuO2, the diffraction peak of (104) plane was amplified and showed in Figure 2(b). It could be observed that the diffraction peak located at 33.3° presented a slight positive shift for 0.3°when the RuO2 contents reached to 2%. This phenomenon could be ascribed to the partial Ru species with smaller metal ions radius (Ru4+: 0.062 nm) successfully incorporated into Fe2O3 lattice structure with larger metal ions radius (Fe3+: 0.065 nm) resulting in the change of Fe2O3 lattice parameters[25]. However, this phenomenon was disappeared on the RuO2-based catalyst with low contents of RuO2 (<2%), which might be due to the distribution of ruthenium species on the surface or in pores of Fe2O3 in the form of ruthenium oxide. 2.3

BET analysis

The N2 adsorption-desorption curves and the corresponding pore size distribution of Fe2O3 and RuO2-Fe2O3 catalysts were presented in Figure 3. As shown in Figure 3(a), all samples exhibited the type IV adsorption-desorption isotherms with H3 type hysteresis loop, which were typical characteristics of mesoporous materials according to the IUPAC [26] classification . The central distribution of pore size for pure Fe2O3 sample located at 48 nm and the addition of a small amount of RuO2 (<2%) displayed a weak effect on it. Combining the results of the pore volume and specific surface area of catalysts from Table 1, it was found that the influence of small contents RuO2 on pore structure was not obvious. This phenomenon could be related to the high dispersion of RuO2[18].

WANG Hui-min et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 215223

of cooperativity between RuO2 and Fe2O3, which could greatly enhance the reduction property. This result was correlated to the improvement catalytic performance. Interestingly, a new low-temperature reduction peak appeared with the addition of RuO2 and its intensity increased along with the increase of RuO2. This peak could be attributed to the reduction of RuO2 species. 2.5

Fig. 4

H2-TPR profiles of different catalysts

However, the pore size declined to 38 nm and the pore volume slightly increased over 2% RuO2-Fe2O3 catalyst, which was correlated to the increase of the specific surface area. Furthermore, the reasons of pore size decline were not only the partial incorporation of Ru species into Fe 2O3 lattice structure, but also the formation of micropore due to the accumulation of Ru species. On the other hand, more pores might be formed during the calcination process, which was slightly smaller than pure Fe2O3. Thus, the contents of RuO2 indeed affected the pore size distribution. Table 1 showed the pore structure parameters of Fe2O3 and RuO2-Fe2O3 samples. As shown in Table 1, the specific surface area of pure Fe2O3 was 14.3 m2/g. In addition, all RuO2-Fe2O3 catalysts apart from 1%RuO2-Fe2O3 exhibited an enhanced specific surface area compared to pure Fe 2O3. The specific surface area reached the highest value when RuO 2 loading was 2%. It could be speculated that the large specific surface area was beneficial to the dispersion of active sites, thus facilitating the improvement of catalytic activity. 2.4

H2-TPR

H2-TPR profiles of Fe2O3 and RuO2-Fe2O3 catalysts were portrayed in Figure 4 to investigate the redox performance according to the different reduction peaks location. There were two reduction peaks in pure Fe2O3 catalyst. The low-temperature reduction peak appeared at approximately 380°C was assigned to the reduction of Fe2O3 to Fe3O4, and the high-temperature reduction peak started from 600°C was assigned to the further reduction of Fe3O4 to FeO and Fe0[27]. Moreover, the reduction peak position belonging to Fe2O3 in RuO2-Fe2O3 catalysts obviously shifted to lower temperature compared with pure Fe2O3 catalyst. This result demonstrated that the addition of RuO2 accelerated the Fe species reduction, simultaneously revealed the occurrence of hydrogen spillover from Ru atoms to Fe2O3[28]. Namely, the positive impact of Ru species on the reduction of Fe species illustrated the existence

NH3-TPD

Figure 5 gave the results of NH3-TPD experiments over Fe2O3 and RuO2-Fe2O3 catalysts. According to the previous literatures, the NH3 desorption peak below 200°C belong to the weakly adsorbed NH3, especially when the desorption temperature was lower than 100°C, which might be due to the physical/weakly adsorbed NH3 species[29,30]. The desorption peak happened above 250°C was generally ascribed to the desorption of NH3 bound to the medium or strong acid sites[30]. There were two desorption peaks on pure Fe2O3 sample corresponding to the physical/weakly adsorbed NH3 species at weak acid sites and chemical adsorbed NH3 at medium acid sites, respectively. Interestingly, the acidic environment on the catalysts surface changed apparently with the addition of RuO2. When the content of RuO2 was greater than 1%, the desorption amount of ammonia species on the medium acid sites increased significantly, which was reflected in the obvious increase of the intensity of the desorption peak. In addition, the weak acid sites of all RuO2-Fe2O3 catalysts also increased slightly compared with that of pure Fe2O3. It was well recognized that the increase of surface acid sites facilitated the adsorption of NH3, so that more NH3 could be adsorbed and activated by reactive metal. Combined with the results of the activity tests (Figure 1), RuO2, as the main active sites, not only promoted the adsorption of NH3, but also improved the ability of NH3 to oxidize with surface active oxygen. This conclusion was further proved in the following study on in situ DRIFTS.

Fig. 5

NH3-TPD profiles of different catalysts

WANG Hui-min et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 215223

Fig. 6

In situ DRIFTS of different catalysts exposed to NH3 for various times at 250°C

(a): Fe2O3; (b): 0.5%RuO2-Fe2O3; (c): 1%RuO2-Fe2O3; (d): 1.5%RuO2-Fe2O3; (e): 2%RuO2-Fe2O3

2.6

In situ DRIFTS study of NH 3 adsorption

In situ DRIFTS experiments further explored the types of acid sites on the catalyst surface and the changes of adsorbed species as a function of time. Figure 6 depicted in situ DRIFTS of NH3 adsorption over Fe2O3 and RuO2-Fe2O3 catalysts at 250°C. As could be seen from Figure 6(a), some strong bands appeared on pure Fe2O3 sample after feeding the NH3 into the reaction cell. The bands located at 3362, 3235 and 3133 cm–1 were assigned to the N–H stretching vibration modes of coordinated NH3. The band at 1191 and 1601 cm–1 could be attributed to the symmetric and asymmetric bending vibrations of coordinated NH3 linked to Lewis acid sites[31,32]. With the increase of adsorption time, the intensity of all bands increased and remained stable after 10 min. In addition, a new weak band appeared at 1438 cm–1, which was attributed to the

NH+4 ion bound to Brønsted acid sites[33]. Compared with Figure 6(a), the experimental results in Figure 6(b)–6(e) mainly studied the influence of different RuO2 contents of RuO2-Fe2O3 catalysts on the surface acid sites and the ability of NH3 adsorption. A new band at 1822 cm–1 appeared on all of the RuO2-Fe2O3 samples, which could be attributed to the nitrosyl (–HNO) species[7,34]. Moreover, the bands ascribed to Lewis acid sites on 0.5%RuO2-Fe2O3 and 2%RuO2-Fe2O3 shifted from 1601 cm–1 to 1585 and 1588 cm–1[35], respectively. Compared with Fe2O3, another difference of RuO2-Fe2O3 catalysts was mainly reflected in the change of acid strength. With increasing the content of RuO 2, the peak intensity of Lewis acid sites on RuO2-Fe2O3 catalysts gradually decreased, and the peak intensity of nitrosyl (–HNO) species increased. Among them, the changes were most obvious when the content of RuO2 increased to 1.5% or 2%.

WANG Hui-min et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 215223

Interestingly, further increasing the RuO2 content resulted in the disappearance of Brønsted acid sites on Fe2O3 surface. The above results exhibited that the addition of RuO2 enhanced the activation of lattice oxygen on the catalyst surface. On the one hand, it could be seen from the results of NH3-TPD that the addition of RuO2 increased the acid sites on the surface of the catalysts, while the Lewis acid sites and Brønsted acid sites for ammonia adsorption species showed a decreasing trend in DRIFTS. On the other hand, the emergence of –HNO species also indicated that the adsorbed NH3 species were activated and oxidized by active oxygen species. In general, NH3 adsorbed on the acid sites of RuO2-Fe2O3 catalysts was rapidly dehydrogenated to generate amide (–NH2) species, and –NH2 was further dehydrogenated to form imide (–NH) species, resulting in a decrease in the number of adsorbed ammonia species detected by infrared spectrometer[36]. After that, –NH was rapidly oxidized with the active atomic oxygen activated by RuO2 on the catalyst to generate –HNO species. The main dehydrogenation and oxidation reactions were as follows. NH3 → NH2 + H (7) NH2 → NH + H (8) NH + O → HNO (9) The whole process happened so quickly that some steps were not well monitored by infrared spectroscopy experiments. At the same time, it was confirmed that more active sites existed on the surface of RuO2-Fe2O3 catalysts combined with the results of activity test. These active sites led to the oxidation reaction between the adsorbed NH3 species and the active oxygen species on the surface of the catalysts, which contributed to the excellent activity of RuO2-Fe2O3 catalyst. In addition, according to the previous reports[7,36], –HNO species were the intermediates of in-situ selective catalytic reduction of NO (i-SCR). That is to say, –HNO was further oxidized to NO species, and NO reacted with amide (–NH2) to form N2. The reaction mechanism of i-SCR was generally considered to be a relatively common reaction pathway in NH3-SCO reaction.

3

Conclusions

study further showed that the Lewis acid sites was the main acid species on the catalyst surface, and the dehydrogenation and oxidation reaction occurred on the surface of active RuO2-Fe2O3: NH3 species adsorbed on Lewis acid sites were firstly dehydrogenated to form –NH species, and then –NH was oxidized rapidly by the active atomic oxygen activated by ruthenium species on the catalyst surface.

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