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Effects of strain induced by Au dispersion in Ba and Ni doped Y2O3 on direct decomposition of NO
Molecular Catalysis 475 (2019) 110488
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Effects of strain induced by Au dispersion in Ba and Ni doped Y2O3 on direct decomposition of NO
T
Tatsumi Ishiharaa,b, , Siman Fangc, Tomoaki Idea ⁎
a
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan c Graduate School of Automotive Science, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan b
ARTICLE INFO
ABSTRACT
Keywords: NO decomposition Y2O3 Ni dopant Strain effects Au dispersion
NO direct decomposition on Y2O3 based oxide was studied and it was found that doping Ba and Ni simultaneously in Y2O3 is effective for achieving the high NO decomposition activity. The optimized amount for Ni and Ba doping is 5 and 0.5–1 mol%, respectively and on this Y2O3 based catalyst showed almost 100% N2 yield at 800 °C and high N2 yield was sustained under co-existence of oxygen. Effects of tensile strain caused by Au dispersion was further studied for increasing low temperature NO decomposition activity and it was found that NO decomposition activity around 500 °C was much increased by dispersion of 1 mol% Au which induced a tensile strain.
1. Introduction In current NOx removal technology, catalytic reduction processes using NH3 or traces of residual hydrocarbons are used in order to selectively reduce N2O, NO, and NO2 gas concentrations in the combustion effluent streams [1,2]. However, such processes become more challenging because lean-burn engines are now more popularly used for increasing fuel efficiency. In contrast to current methods, direct decomposition of NO seems to be a promising way because of simple operation system. The catalytic decomposition of NO is favored by chemical equilibrium and for the catalytic reaction, no reductant is required [3,4]. Therefore, many studies have been focused on the development on the NO direct decomposition reaction activated catalysts such as Cu ion exchanged zeolites [5], perovskites [6,7] and C-type rare earth oxides [8–12]. According to the Teraoka et al, the NO direct decomposition reaction route on perovskite type catalyst is considered as the following equations [13].
⁎
Here [] means oxygen vacancy site. According to this reaction mechanism, at first, one NO molecule was adsorbed on the surface of the catalyst with oxygen ended in the oxygen vacancies (1). After that, adsorption of the second NO molecule at the pair site happened (2).
Corresponding author at: I2CNER, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan. E-mail address: [email protected] (T. Ishihara).
https://doi.org/10.1016/j.mcat.2019.110488 Received 16 April 2019; Received in revised form 19 June 2019; Accepted 21 June 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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This step is also considered as the rate determine step during the NO direct decomposition reaction of perovskite-type oxides catalyst. Then the two adsorbed NO would form the N to N bond followed by the association of formed nitrogen product (3). At the last step, the remained O would form O2 and release from the oxygen vacancies (4). However, in our previous study on Ba doped Y2O3, it seems like that the adsorption of NO at the paired position is relevant faster compared with O2 formation after dissociation. Therefore, removing the surface oxygen seems to be rate determining step. In order to achieve, oxygen desorption, coupling of atomic oxygen on the surface is essentially required and so high mobility of coupling oxygen on the surface is important for high activity to NO decomposition. As a result, the rate determine step of Y2O3 based catalyst is considered as the formation and releasing of O2 from active sites [14]. The limited reaction rate on this step would result in the formation of nitrate and will further cover the active sites of catalyst. Therefore, how to efficiently remove the surface nitrate or the surface remained O is highly required for increasing the NO direct decomposition activity on C-type rare earth oxide catalyst [14]. In the conventional process, we applied reductant gas such as propene cofeeding with NO and remove the surface oxygen [14,15]. It is because the propene could facilitate removing the nitrite and nitrate species formed on the surface of the catalyst through reduction reaction [15]. On the other hand, recently, there have been several reports on the improved oxygen diffusivity by positive effects of tensile strain [16–18]. The strain can be introduced by dispersed metal into metal oxide catalyst because of the difference in thermal expansion coefficient between metal and metal oxide. When metal and metal oxide was connected at high temperature followed by cooling down, it is expected that the residual strain will be introduced at the interface between metal and metal oxide because of difference in thermal expansion coefficient. In this study, we studied Au as a dispersed metal in Y2O3 because of its large thermal expansion coefficient. The thermal expansion coefficient of Au and Y2O3 are αAu = 14.2 × 10−6 K−1 and αY2O3 = 7.2 × 10−6 K−1 respectively [19,20], and so, tensile expansion will be introduced during cooling down of Au dispersed into Y2O3 matrix. In this study, effects of NiO doping in Y2O3 doped with 1 mol% Ba on NO decomposition reaction was studied and it was found that doping NiO is effective for high activity to NO decomposition. In addition, effects of tensile strain by Au dispersion on NO decomposition on the sintering sample of Au particles dispersed into Y2O3.
using a 5 A molecular sieve column for N2 and O2, and a Porapak Q column for N2O. In-situ FT-IR analysis were performed using a diffused reflectance measurement cell with a KBr single-crystal window. Approximately 100 mg of the catalyst powder was placed in the cell prior to acquiring spectra. Spectra were recorded with a JASCO 610 spectrometer (JASCO, Japan) coupled with a Hg-Cd-Te semiconductor (MCT) detector. Background spectra were acquired under He flowing prior to introducing the reactant gas and these were subtracted from subsequent sample spectra recorded at a 4 cm−1 resolution for each spectrum. Pulsed reaction analyses were conducted using a BEL-CAT-4 (Microtrac BEL JAPAN) coupled with an online gas analyzer (BEL MassSP, Microtrac BEL). The pulsed reactor was loaded with 100 mg samples and these samples were pre-treated under a He flow for 10 min prior to each pulse reaction. Before NO pulse reaction, 1000 ppm C3H6 in He was pulsed for removing surface oxygen and then NO (1% in He) was subsequently introduced into the reactor to assess NO adsorption and decomposition. An online mass spectrometric detector was used to monitor the signals at M/Z values of 28 (N2), 30 (NO), 32 (O2) and 44 (N2O). 3. Results and discussion 3.1. NO decomposition and Rate limiting step on Y2O3 doped with Ba2+ and Ni2+ Although high temperature is required, it was found that Ba doped Y2O3 shows reasonable activity to NO decomposition, and reasonable N2 yield is achieved at 1 mol% doped for Y site in Y2O3. Since oxygen vacancy seems to be active site, effects of dopant in (Y0.99Ba0.01)2O3 was studied. Table 1 shows NO decomposition activity and yield of N2, O2, and N2O on 30 mol% doped sample. Obviously, NO conversion as well as N2 and O2 yield was strongly affected by second dopant. Among the examined cation, Ni2+ or Mg2+ showed increased N2 and O2 yield, however, Cu+, Co3+, or Fe3+ doped sample decreased N2 yield significantly. Therefore, it seems that doping divalent cation is effective for increasing NO decomposition activity and this might be assigned to the formation of oxygen vacancy in Y2O3. In particular, N2 yield was achieved 92% at 850 °C on Ni doped catalyst. Therefore, partial substitution of Ni2+ at Y3+ site in Y2O3 seems to be effective for increasing NO decomposition activity. Fig. 1 shows NO decomposition activity at 850 °C as a function of Ni amount added. N2 as well as O2 yields were increased with increasing amount of NiO and became almost independent of NiO higher than 5 mol%. Among the composition studied, the highest N2 yield was achieved at 5 mol% NiO and so optimum amount of NiO addition seems to be 5 mol % of Ni and the optimized composition was
2. Experimental Appropriate amounts of Y(NO3)3･6H2O (99.9%, Kishida Chemical Co, Ltd.) and Ba(CH3COO)2 (99.0%, Kishida Chemical Co., Ltd.), Ni (CH3COO)2･4H2O (Wako, 98 wt%) and 1 mol% HAuCl4 (99.0%, Kishida Chemical Co., Ltd.) were dissolved in deionized water as starting materials and evaporation to dryness with heating and stirring. The gel thus obtained was then subjected to pre-calcination in an exhaust oven at 400 °C for 2 h followed by a second calcination at 800 °C for 6 h. The obtained powder were pressed into disks shape with 20 mm diameter. The prepared disks were seperately sintered at 1100, 1200, and 1480 °C. The control sample (no Au dispersion) was prepared exactly in the same method excepting for the addition of Au. The solution was evaporated to dryness with heating and stirring and finally sintering at 900 °C for 6 h. The obtained catalysts were characterized by XRD (Rigaku Rint-2500, Cu-Kα radiation). NO direct decomposition reaction was performed using a conventional fixed-bed gas flow reactor consisting of a quartz glass tube. An amount of 1 g of catalyst (ca. 0.8 ml) was loaded on a glass-wool support in the reactor tube. The total flow rate of feed gas was 40 cc min−1, corresponding to a gas hourly space velocity of 2983 h−1. The feed gas composition was 5000 ppm NO with He as the balance gas. The reactor was placed in an electric furnace and the reaction temperature was varied from 200 to 800 °C. The gas stream was analyzed by an on-line gas chromatograph equipped with thermal conductivity detector and
Table 1 NO direct decomposition on doped (Y0.69(M)0.3Ba0.01)2O3. Dopant for Y site
Ni Sc Tb Yb Mg – Ce Fe Co Cu Al Bi Nb
2
Conv. (%)
Yield (%)
NO
N2
O2
N2O
NO2
95.3 92.3 91.4 90.1 89.2 87.5 84.3 79.0 72.2 34.7 28.1 1.1 (0.3)
91.8 62.8 77.1 56.6 85.8 78.3 46.9 50.8 41.7 2.6 9.7 1.3 0.5
83.5 56.5 64.6 51.0 75.4 66.9 36.9 33.6 23.4 6.0 1.8 0.3 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
5.9 17.9 13.4 19.5 6.9 10.3 23.7 22.7 24.6 14.4 13.2 0.4 0.2
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Fig. 3. Temperature (Y0.945Ni0.05Ba0.005)2O3.
Fig. 1. NO conversion, N2 and O2 yield on (Y0.99-xNixBa0.01)2O3 at 850 °C as a function of x value.
dependence
of
NO
decomposition
on
is required for high NO decomposition activity. Fig. 3 shows the temperature dependence of NO decomposition on (Y0.945Ni0.05Ba0.005)2O3. Although N2 yield on Y2O3 was as low as 30% at 850 °C, it was 95% by doping just 5 mol % Ni2+ and 0.5 mol% Ba2+ which may introduce oxygen vacancy into Y2O3. Therefore, it can be said that the introduction of oxygen vacancy is effective for achieving the high activity to NO decomposition at high temperature, however, as shown in Fig. 3, at temperature lower than 600 °C, NO decomposition was hardly occurred. Therefore, increase in NO decomposition activity at lower temperature is required. In the conventional catalyst, NO decomposition activity is significantly decreased with co-existence of O2. Therefore, effects of O2 coexistence on NO decomposition activity of (Y0.945Ni0.05Ba0.005)2O3 were studied at 850 °C. Fig. 4 shows N2 yield as a function of O2 partial pressure. Although N2 yield was decreased by co-existence of O2 up to 1%, it was hardly changed by increasing amount of O2 higher than
Fig. 2. NO conversion, N2 and O2 yield on (Y0.95-xNi0.05Bax)2O3 at 850 °C as a function of x value.
(Y0.94Ni0.05Ba0.01)2O3. On this catalyst, NO conversion of 100% was achieved at 800 °C and O2 formation was also observed at temperature higher than 600 °C. As a result, the optimized composition for Ni dope seems to exist around (Y0.94Ni0.05Ba0.01)2O3. It is also noted that BET surface area of this catalyst was 28 m2/g and one reason for the high activity to NO decomposition at temperature higher than 800 °C may be assigned to the reasonably large surface area. Effects of Ba amount in (Y0.95Ni0.05)2O3 on NO decomposition was further studied. Fig. 2 shows NO conversion, N2 and O2 yield as a function of Ba amount. In a similar manner with Ni2+, N2 yield was increased with Ba amount up to 0.5 mol% and then almost independent of Ba amount. Therefore, it seems that doping of Ba2+ higher than 0.5%
Fig. 4. N2 yield in NO decomposition on (Y0.945Ni0.05Ba0.005)2O3 at 850 °C as a function of O2 partial pressure. 3
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Fig. 5. NO pulse reaction on Y2O3 at (a) 400, (b) 500 and (c) 600 °C, and that on (Y0.94Ni0.05 Ba0.01)2O3 at (d) 400, (e) 500 and (f) 600 °C, and NO conversions as functions of pulse numbers on (g) Y2O3 and (h) (Y0.94Ni0.05Ba0.01)2O3.
1 mol%. Therefore, it was found that (Y0.945Ni0.05Ba0.005)2O3 was highly stable and active to NO decomposition under co-existence of O2, although high temperature is required for high N2 yield. In order to further increase in NO decomposition activity at low temperature, detail mechanism on NO decomposition on Ni2+ and Ba2+ doped Y2O3 was studied by using a pulse reaction system. It is also noted that negative effects of oxygen coexistence were decreased by Ni2+ co-doping with Ba2+ Ni and so doped Ni2+ might redox during NO decomposition and has role like oxygen storage material. Since the NO decomposition activity is almost the same, (Y0.94Ni0.05Ba0.01)2O3 was used in the following part. Fig. 5 (a)-(f) show the results of applying 3.6 μl 10 pulses of 1% NO (1.5 nmol NO) to Y2O3 and (Y0.94Ni0.05Ba0.01)2O3 at 400, 500 and 600 °C. Only N2, NO and N2O were detected during these pulse reaction. During the first NO pulse at 400 °C, as shown in Fig. 2 (a) and (d), approximately 70 and 65% NO conversions were achieved over Y2O3 and (Y0.94Ni0.05Ba0.01)2O3, respectively, corresponding to approximately 1 and 0.9 nmol NO consumption, respectively. With increasing pulse numbers, NO conversions decreased, as did those of N2 and N2O.
After 10 pulses, the total NO consumption quantities were determined to be 5.2 nmol for the Y2O3 and 3.4 nmol for (Y0.94Ni0.05Ba0.01)2O3. Fig. 5 (b) and (c) present NO conversion data acquired over Y2O3 at 500 and 600 °C upon injecting 10 pulses of NO at each temperature. At 500 °C, both NO consumption and the generation of N2 and N2O were observed, although much lower levels of N2 formation than that at the initial pulses of NO at 400 °C because of no treatment for removal of surface strongly adsorbed species (may be NO3 species) formed in pulse reaction at 400 °C. The NO conversion decreased with increasing pulses, producing the same trend as observed at 400 °C. The total amount of NO converted at 500 C was 2.3 nmol, which is lower than the amount at 400 °C. At 600 °C, no significant decrease in the NO signal was evident, indicating that NO was not converted over Y2O3 at this temperature. Fig. 5 (e) and (f) present the results of NO pulsed reaction over (Y0.94Ni0.05Ba0.01)2O3 at 500 and 600 °C using 10 pulses having the same amount of NO as the above trials. At 500 °C, the NO conversion decreased during the first 5 pulses and then plateaued. A similar trend was observed at 600 °C over the same catalyst, and the extent of N2 formation became stable from the third pulse. The total amount of NO 4
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Fig. 6. Comparison of XRD patterns between (Y0.94Ni0.05Ba0.01)2O3 and 1 mol% Au-dispersed (Y0.94Ni0.05Ba0.01)2O3 after sintering at 1100, 1200 and 1480 °C. a) wide range and b) narrow range diffraction. Sintering temperature (a) 900 (b) 1100 (c) 1200, and (d) 1480 °C.
consumption over (Y0.94Ni0.05Ba0.01)2O3 at 500 °C was 2.4 nmol, which is almost equal to the NO consumption over Y2O3. However, at 600 °C, NO consumption (2.4 nmol in total for 10 pulses) continued over (Y0.94Ni0.05Ba0.01)2O3, as well as at 700 and 800 °C (data not shown). In our previous study, Y2O3 having a C-type cubic structure typical of rare earth oxides was found to promote the NO decomposition reaction [8,9]. BaO does not promote NO decomposition and increase NO adsorption from basic property, although doping Ba2+ into Y2O3 may increase the oxygen vacancies which are believed to act as the active sites for NO adsorption. With increasing temperature during the pulsed NO experiments, larger amount of NO conversion was observed on Ba and Ni-doped Y2O3. Therefore, it appears that the doping of Y2O3 with Ba2+ and Ni2+ created more active sites for NO adsorption and decomposition. Neither catalyst exhibited oxygen production during the pulse reaction, suggesting that oxygen atom from NO (Oads) remained on the surfaces of both catalysts between 400 and 600 °C, resulting in the deactivation of NO decomposition. Increase in oxygen removal from surface is required for the regeneration of the active site resulting in the increase activity to NO decomposition at low temperature.
Fig. 7. Temperature dependence on NO direct decomposition on (Y0.94Ni0.05Ba0.01)2O3 and 1 mol% Au-dispersed (Y0.94Ni0.05Ba0.01)2O3 sintered at 1100 °C followed by making powder.
3.2. Tensile strain effects for increasing NO decomposition activity at low temperature
particle size of Au by excessively high sintering temperature (higher than meting temperature) and annealed the tensile strain formed by difference in mismatch in thermal expansion coefficient when particle size became larger. Fig. 7 shows the comparison of the NO conversion of Au-dispersed (Y0.94Ni0.05Ba0.01)2O3 (surface area: 1.7 m2 g−1) and no Au dispersed one (surface area: 0.4 m2 g−1). Therefore, the surface area of the catalyst became smaller by two order of magnitude by sintering treatment. Both catalysts shows NO direct decomposition activity from 600 °C, and the oxygen formation was also observed from 600 °C. Comparing with the NO decomposition activity shown in Fig. 3, the smaller activity at higher temperature may be assigned to the decreased surface area by sintering treatment. The catalyst with Au dispersed sample started to show higher N2 and O2 yield than that on no Au dispersed catalyst from 600 and at 850 °C, i.e., N2 and O2 yield of Au catalyst was almost twice higher than that of the control catalyst. Thus it is considered that the tensile strain introduced works positively to oxygen desorption resulting in the increased NO decomposition activity of Y2O3 added with BaO. In case of Pr1.90Ni0.71Cu0.24Ga0.05O4+δ, dispersion of Au induces a tensile strain resulting in the increased oxygen diffusivity [18]. Therefore, in a similar manner with effects of Ni2+ doping, Au dispersed in Y2O3 could also increase oxygen diffusivity in bulk of Y2O3 and the improved NO decomposition activity could be assigned to this increased oxygen vacancy diffusivity. As a result, it is seen that dispersion of Au is effective for increasing NO decomposition activity. Therefore, NO
Fig. 6 (a) shows the XRD patterns of Au-dispersed samples after (a) 1100, (b) 1200, and (c) 1480 °C sintering. In three samples, the main strong peaks are assigned to Y2O3 with rare-earth type C structure (cubic lattice), which is the major component of three samples. Since 1 mol % Au was introduced into catalyst as a dispersed composite, two peaks attributed to Au metal phase were observed at three samples. Because diffraction peaks from Au were observed, the dispersed Au seems to exist as a metallic phase, suggesting that the added Au did not react with Y2O3 phase. Therefore, metallic particles of Au were successfully dispersed into the Y2O3 grains. No peaks belong to Ba can be observed at three samples. It is indicated that Ba was highly dispersed and is likely to have been incorporated in the lattice positions of Y2O3. The comparison between these three samples and no Au dispersed (Y0.94Ni0.05Ba0.01)2O3 sample suggests that the XRD peaks were shifted to lower angle direction suggesting the expansion of lattice parameter as shown in Fig. 6 (b). It is obviously that main diffraction peak from Y2O3 was shifted to a lower angle by dispersion of Au after 1100 °C sintering. This indicates that Y2O3 lattice was tensile after sintering with 1 mol % Au. The large difference in the thermal expansion coefficient can induce tensile strain in Y2O3, resulting in changes in Y2O3 unit lattice length as expected. On the other hand, lower angle shift in diffraction peaks was return to the original angle when sintering was performed at higher temperature. This may be explained by increasing 5
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dispersed (Y0.94Ni0.05Ba0.01)2O3 The nitrosyl species at 1455 cm−1 also increased in intensity from 100 to 500 °C indicating the increasing adsorption amount as temperature increases. However, it is started to decrease in intensity from 600 °C, and this peak is almost disappeared at 700 °C. Considering the temperature dependence of NO decomposition, N2 and O2 might be formed through removal of surface nitrosyl species formed on the catalyst. Similar temperature dependence of bridging nitrate adsorbed on Au-dispersed sample as nitrosyl species was also observed. Its absorbance slowly increased from 100 to 300 °C followed by the decreasing at higher temperature especially at 600 to 700 °C at which NO decomposition reaction was actively proceeded. Most of the adsorbed NO on the oxygen vacancies refer to form hyponitrite instead of nitrate species. Therefore, at 600 °C, a new peak at 1135 cm−1 attributed to hyponitrite species is appeared, and it started to increase intensity from 600 to 700 °C [21]. The operando FTIR spectra of NO adsorption on non-Au dispersed (Y0.94Ni0.05 Ba0.01)2O3 at different temperature was shown in Fig. 9 (b). There are four similar absorbance species at 1135, 1179, 1296 and 1455 cm−1 observed, i.e., the hyponitrite, N–O, bridging nitrate and nitrosyl species, respectively [21–25]. However, different from Audispersed sample, there is a new IR peak observed at 1235 cm−1 which may be attributed to bidentate nitrate became visible from 200 to 700 °C. On the non-Au dispersed catalyst, the desorption of O2 from the oxygen vacancies on the surface is the rate determining step. Therefore, introduction of tensile strain forms oxygen vacancy and high diffusivity resulting in the increased removal of surface nitrosyl and increased NO decomposition activity. In this section, it was confirmed that increase in diffusivity in oxygen vacancy is effective for increasing O2 desorption at decreased temperature. O2-TPD was measured for analysis of desorption property of oxygen, however, up to now, no reasonable desorption curves were obtained and this may suggest that amount of O2 adsorption is small. One reason for this may be small surface area of the catalyst. The change in oxygen adsorption state by applying tensile strain by Au dispersion will be reported in future. Effects of Au dispersion on NO adsorption property was further studied by TPD measurement. Fig. 10 shows TPD profile of NO from (Y0.94Ni0.05Ba0.01)2O3 and Au dispersed catalyst. Obviously, part of NO was desorbed as N2, O2, N2O and NO2 on both catalysts and this suggests that dissociative adsorption of NO was occurred on (Y0.94Ni0.05Ba0.01)2O3. On both catalyst, NO desorption was occurred around 400 °C and at the same temperature, O2, N2, and N2O desorption was observed suggesting that dissociative adsorbed NO was decomposed. Comparing with non Au dispersed sample, desorption
Fig. 8. Effects of Au amount on temperature dependence of NO decomposition of Au dispersed (Y0.94Ni0.05Ba0.01)2O3.
decomposition activity at further higher amount of Au was studied. Fig. 8 shows effects of Au amount on temperature dependence of NO decomposition of Au dispersed (Y0.94Ni0.05Ba0.01)2O3. With increasing amount of Au from 1 to 2 mol%, NO decomposition activity was decreased, in particular, decrease in activity at low temperature was more significantly observed. Therefore, excess amount of Au dispersed seems not to be effective for increasing NO decomposition activity. However comparing with no Au dispersed (Y0.94Ni0.05Ba0.01)2O3, 2 mol% Au dispersed sample shows still higher NO decomposition activity as shown in Fig. 8. Considering from the XRD results showing in Fig. 6, this is because Au particle size was increased with increasing Au amount and so tensile strain was annealed when Au amount became larger. Therefore, it seems that the optimum amount for Au dispersion in (Y0.94Ni0.05Ba0.01)2O3 seems to be exist around 1 mol%. Fig. 9 shows the operando FT-IR spectra of NO adsorption on Audispersed (Y0.94Ni0.05Ba0.01)2O3 from 100 to 700 °C. There are four main adsorption peaks observed at 1135, 1179, 1297 and 1455 cm−1. At 100 °C, there are three absorbed peaks at 1179, 1297 and 1455 cm−1 which are attributed to NeO adsorption on the oxygen vacancies, nitrate (NO3) and nitrosyl (NO2) species [21–25]. NeO species at 1179 cm−1 became more intense with increasing temperature. This suggests that the adsorption amount of NO increased on the Au-
Fig. 9. Operando FTIR spectra of NO adsorbed species on the surface of (a) Au-dispersed (Y0.94Ni0.05Ba0.01)2O3 and (b) no Au dispersed sample from 100 to 700 °C temperature. Labels in figure are reaction temperature. 6
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Fig. 10. Temperature programmed desorption (TPD) profile of NO from (Y0.94Ni0.05Ba0.01)2O3 and Au dispersed catalyst.
temperature of NO as shifted to lower temperature and this is more obviously observed for O2 desorption. Therefore, as suggested by in-situ FT-IR measurement, removal of surface nitrate is accelerated by dispersion of Au which means introduction of tensile strain. At high temperature, large amount of N2 as well as N2O desorption was observed on both catalysts, however, no O2 desorption was occurred. Therefore, oxygen may be remained on the surface. By Au dispersion, amount of N2O desorbed at higher temperature region was decreased and larger amount of O2 and NO2 which may be formed during desorption of O2 and NO was occurred around 400 °C. This may be explained by removal of nitrate species at low temperature on Au dispersed (Y0.94Ni0.05Ba0.01)2O3. As a result, tensile strain induced by Au dispersion is effective for removing nitrate species at lower temperature resulting in the increased NO decomposition activity around 500 °C.
in future work. In any case, this study reveals that (Y0.94Ni0.05Ba0.01)2O3 with tensile strain is highly active to NO decomposition at low temperature. Acknowledgements This study was financially supported by a Grant-in-Aid for Specially Promoted Research (No. 16H06293) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan through the Japan Society for the Promotion of Science. References [1] R. Burch, J.P. Breen, F.C. Meunier, Appl. Catal. B Environ. 39 (2002) 283–303. [2] M.F. Fu, C. Li, P. Lu, L. Qu, M.Y. Zhang, Y. Zhou, M. Yu, Y. Fang, Catal. Sci. Technol. 4 (2014) 14–25. [3] M. Haneda, H. Hamada, Comptes Rendus Chim. 19 (2016) 1254–1265. [4] N. Imanaka, T. Masui, Appl. Catal. A: Gen. 431-432 (2012) 1–8. [5] M. Iwamoto, Catal. Today 29 (1996) 29–35. [6] J. Zhu, D. Xiao, J. Li, X. Yang, Y. Wu, J. Mol. Catal. A 234 (2005) 99–105. [7] B. Zhao, R. Wang, X. Yang, Catal. Commun. 10 (2009) 1029–1033. [8] T. Ishihara, Y. Shinmyo, K. Goto, N. Nishiyama, H. Iwakuni, T. Matsumoto, Chem. Lett. 37 (2008) 318–319. [9] T. Ishihara, K. Goto, Catal. Today 164 (2011) 484–488. [10] T. Masui, S. Uejima, S. Tsujimoto, R. Nagai, N. Imanaka, Catal. Today 242 (2015) 338–342. [11] N. Imanaka, T. Masui, H. Masaki, Adv. Mater. 19 (2007) 3660–3663. [12] Y. Doi, M. Haneda, M. Ozawa, J. Mol. Catal. A: Chem. 383-384 (2014) 70–76. [13] Y. Teraoka, T. Harada, S. Kagawa, J. Chem. Soc. Faraday Trans. 94 (1998) 1887–1891. [14] L. Liu, T. Ishihara, Appl. Catal. A: Gen. 550 (2018) 90–97. [15] L. Liu, K. Murakami, S. Ida, T. Ishihara, Catal. Commun. 100 (2017) 5–9. [16] M. Kubicek, Z. Cai, W. Ma, B. Yildiz, H. Hutter, J. Fleiq, J. ACS Nano 7 (2013) 3276–3286. [17] D. Pergolesi, E. Fabbri, S.N.C. Cook, V. Roddatis, E. Traversa, J.A. Kilner, ACS Nano 6 (2013) 10524–10534. [18] J. Hyodo, K. Tominaga, J.E. Hong, S. Ida, T. Ishihara, J. Phys. Chem. C 119 (2015) 5–13. [19] D. Majumdar, D.J. Chatterjee, Appl. Phys. 70 (1991) 988–992. [20] A. Gauzzi, H.J. Mathieu, J.H. James, B. Kellett, Vacuum 41 (1990) 870–874. [21] S.J. Huang, A.B. Walters, M.A. Vannice, J. Catal. 192 (2000) 29–47. [22] B. Klingenberg, M.A. Vannice, Chem. Mater. 8 (1996) 2755–2768. [23] A. Martinez-Arias, J. Soria, J.C. Conesa, X.L. Seoane, A. Arcoya, R.J. Cataluna, Chem. Soc., Faraday Trans. 91 (1995) 1679–1687. [24] A. Snis, I. Panas, Surf. Sci. 412-413 (1998) 477–488. [25] G. Ghiotti, A. Chiorino, Spectrochim. Acta 49A (1993) 1345–1359.
4. Conclusion NO decomposition on Y2O3 based oxide doped with small amount of Ba2+ was studied and it was found that doping Ni2+ is effective for increasing NO decomposition among the dopants studied and the optimized amount of NiO addition seems to be 5 mol %. On this Ni2+ and Ba2+ doped Y2O3, almost 100% NO conversion was achieved at 800 °C and N2 yield higher than 95% was achieved. Therefore, Ni2+ and Ba2+ doped Y2O3 shows high activity and selectivity to NO decomposition. In particular, (Y0.945Ni0.05Ba0.005)2O3 catalyst shows high N2 yield (> 80%) at 850 °C under coexistence of O2 up to 5%. However, NO decomposition was hardly occurred around 500 °C, however, from pulse reaction, (Y0.94Ni0.05Ba0.01)2O3 shows NO decomposition activity around 400 °C, although no O2 formation was observed. Therefore, it seems that rate determining step of NO decomposition on this catalyst is removal of oxygen which is formed by NO decomposition. Introduction of tensile strain by dispersion of Au is effective for removing the surface nitrate species at lower temperature and so it was found that Au dispersed (Y0.94Ni0.05Ba0.01)2O3 which is obtained by sintering at 1100 °C followed by powdering, showed increase NO decomposition activity around 500 °C. Similar strain effects are also expected for other metals, however, comparing with the case of Au, separation of catalytic activity of metals and strain effects is rather difficult and so in this study, Au was used, however, synergy effect will also be expected in case of other metal like Pd, and this is now under study. The results will be reported
7
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Effects of strain induced by Au dispersion in Ba and Ni doped Y2O3 on direct decomposition of NO
T
Tatsumi Ishiharaa,b, , Siman Fangc, Tomoaki Idea ⁎
a
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan c Graduate School of Automotive Science, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan b
ARTICLE INFO
ABSTRACT
Keywords: NO decomposition Y2O3 Ni dopant Strain effects Au dispersion
NO direct decomposition on Y2O3 based oxide was studied and it was found that doping Ba and Ni simultaneously in Y2O3 is effective for achieving the high NO decomposition activity. The optimized amount for Ni and Ba doping is 5 and 0.5–1 mol%, respectively and on this Y2O3 based catalyst showed almost 100% N2 yield at 800 °C and high N2 yield was sustained under co-existence of oxygen. Effects of tensile strain caused by Au dispersion was further studied for increasing low temperature NO decomposition activity and it was found that NO decomposition activity around 500 °C was much increased by dispersion of 1 mol% Au which induced a tensile strain.
1. Introduction In current NOx removal technology, catalytic reduction processes using NH3 or traces of residual hydrocarbons are used in order to selectively reduce N2O, NO, and NO2 gas concentrations in the combustion effluent streams [1,2]. However, such processes become more challenging because lean-burn engines are now more popularly used for increasing fuel efficiency. In contrast to current methods, direct decomposition of NO seems to be a promising way because of simple operation system. The catalytic decomposition of NO is favored by chemical equilibrium and for the catalytic reaction, no reductant is required [3,4]. Therefore, many studies have been focused on the development on the NO direct decomposition reaction activated catalysts such as Cu ion exchanged zeolites [5], perovskites [6,7] and C-type rare earth oxides [8–12]. According to the Teraoka et al, the NO direct decomposition reaction route on perovskite type catalyst is considered as the following equations [13].
⁎
Here [] means oxygen vacancy site. According to this reaction mechanism, at first, one NO molecule was adsorbed on the surface of the catalyst with oxygen ended in the oxygen vacancies (1). After that, adsorption of the second NO molecule at the pair site happened (2).
Corresponding author at: I2CNER, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan. E-mail address: [email protected] (T. Ishihara).
https://doi.org/10.1016/j.mcat.2019.110488 Received 16 April 2019; Received in revised form 19 June 2019; Accepted 21 June 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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This step is also considered as the rate determine step during the NO direct decomposition reaction of perovskite-type oxides catalyst. Then the two adsorbed NO would form the N to N bond followed by the association of formed nitrogen product (3). At the last step, the remained O would form O2 and release from the oxygen vacancies (4). However, in our previous study on Ba doped Y2O3, it seems like that the adsorption of NO at the paired position is relevant faster compared with O2 formation after dissociation. Therefore, removing the surface oxygen seems to be rate determining step. In order to achieve, oxygen desorption, coupling of atomic oxygen on the surface is essentially required and so high mobility of coupling oxygen on the surface is important for high activity to NO decomposition. As a result, the rate determine step of Y2O3 based catalyst is considered as the formation and releasing of O2 from active sites [14]. The limited reaction rate on this step would result in the formation of nitrate and will further cover the active sites of catalyst. Therefore, how to efficiently remove the surface nitrate or the surface remained O is highly required for increasing the NO direct decomposition activity on C-type rare earth oxide catalyst [14]. In the conventional process, we applied reductant gas such as propene cofeeding with NO and remove the surface oxygen [14,15]. It is because the propene could facilitate removing the nitrite and nitrate species formed on the surface of the catalyst through reduction reaction [15]. On the other hand, recently, there have been several reports on the improved oxygen diffusivity by positive effects of tensile strain [16–18]. The strain can be introduced by dispersed metal into metal oxide catalyst because of the difference in thermal expansion coefficient between metal and metal oxide. When metal and metal oxide was connected at high temperature followed by cooling down, it is expected that the residual strain will be introduced at the interface between metal and metal oxide because of difference in thermal expansion coefficient. In this study, we studied Au as a dispersed metal in Y2O3 because of its large thermal expansion coefficient. The thermal expansion coefficient of Au and Y2O3 are αAu = 14.2 × 10−6 K−1 and αY2O3 = 7.2 × 10−6 K−1 respectively [19,20], and so, tensile expansion will be introduced during cooling down of Au dispersed into Y2O3 matrix. In this study, effects of NiO doping in Y2O3 doped with 1 mol% Ba on NO decomposition reaction was studied and it was found that doping NiO is effective for high activity to NO decomposition. In addition, effects of tensile strain by Au dispersion on NO decomposition on the sintering sample of Au particles dispersed into Y2O3.
using a 5 A molecular sieve column for N2 and O2, and a Porapak Q column for N2O. In-situ FT-IR analysis were performed using a diffused reflectance measurement cell with a KBr single-crystal window. Approximately 100 mg of the catalyst powder was placed in the cell prior to acquiring spectra. Spectra were recorded with a JASCO 610 spectrometer (JASCO, Japan) coupled with a Hg-Cd-Te semiconductor (MCT) detector. Background spectra were acquired under He flowing prior to introducing the reactant gas and these were subtracted from subsequent sample spectra recorded at a 4 cm−1 resolution for each spectrum. Pulsed reaction analyses were conducted using a BEL-CAT-4 (Microtrac BEL JAPAN) coupled with an online gas analyzer (BEL MassSP, Microtrac BEL). The pulsed reactor was loaded with 100 mg samples and these samples were pre-treated under a He flow for 10 min prior to each pulse reaction. Before NO pulse reaction, 1000 ppm C3H6 in He was pulsed for removing surface oxygen and then NO (1% in He) was subsequently introduced into the reactor to assess NO adsorption and decomposition. An online mass spectrometric detector was used to monitor the signals at M/Z values of 28 (N2), 30 (NO), 32 (O2) and 44 (N2O). 3. Results and discussion 3.1. NO decomposition and Rate limiting step on Y2O3 doped with Ba2+ and Ni2+ Although high temperature is required, it was found that Ba doped Y2O3 shows reasonable activity to NO decomposition, and reasonable N2 yield is achieved at 1 mol% doped for Y site in Y2O3. Since oxygen vacancy seems to be active site, effects of dopant in (Y0.99Ba0.01)2O3 was studied. Table 1 shows NO decomposition activity and yield of N2, O2, and N2O on 30 mol% doped sample. Obviously, NO conversion as well as N2 and O2 yield was strongly affected by second dopant. Among the examined cation, Ni2+ or Mg2+ showed increased N2 and O2 yield, however, Cu+, Co3+, or Fe3+ doped sample decreased N2 yield significantly. Therefore, it seems that doping divalent cation is effective for increasing NO decomposition activity and this might be assigned to the formation of oxygen vacancy in Y2O3. In particular, N2 yield was achieved 92% at 850 °C on Ni doped catalyst. Therefore, partial substitution of Ni2+ at Y3+ site in Y2O3 seems to be effective for increasing NO decomposition activity. Fig. 1 shows NO decomposition activity at 850 °C as a function of Ni amount added. N2 as well as O2 yields were increased with increasing amount of NiO and became almost independent of NiO higher than 5 mol%. Among the composition studied, the highest N2 yield was achieved at 5 mol% NiO and so optimum amount of NiO addition seems to be 5 mol % of Ni and the optimized composition was
2. Experimental Appropriate amounts of Y(NO3)3･6H2O (99.9%, Kishida Chemical Co, Ltd.) and Ba(CH3COO)2 (99.0%, Kishida Chemical Co., Ltd.), Ni (CH3COO)2･4H2O (Wako, 98 wt%) and 1 mol% HAuCl4 (99.0%, Kishida Chemical Co., Ltd.) were dissolved in deionized water as starting materials and evaporation to dryness with heating and stirring. The gel thus obtained was then subjected to pre-calcination in an exhaust oven at 400 °C for 2 h followed by a second calcination at 800 °C for 6 h. The obtained powder were pressed into disks shape with 20 mm diameter. The prepared disks were seperately sintered at 1100, 1200, and 1480 °C. The control sample (no Au dispersion) was prepared exactly in the same method excepting for the addition of Au. The solution was evaporated to dryness with heating and stirring and finally sintering at 900 °C for 6 h. The obtained catalysts were characterized by XRD (Rigaku Rint-2500, Cu-Kα radiation). NO direct decomposition reaction was performed using a conventional fixed-bed gas flow reactor consisting of a quartz glass tube. An amount of 1 g of catalyst (ca. 0.8 ml) was loaded on a glass-wool support in the reactor tube. The total flow rate of feed gas was 40 cc min−1, corresponding to a gas hourly space velocity of 2983 h−1. The feed gas composition was 5000 ppm NO with He as the balance gas. The reactor was placed in an electric furnace and the reaction temperature was varied from 200 to 800 °C. The gas stream was analyzed by an on-line gas chromatograph equipped with thermal conductivity detector and
Table 1 NO direct decomposition on doped (Y0.69(M)0.3Ba0.01)2O3. Dopant for Y site
Ni Sc Tb Yb Mg – Ce Fe Co Cu Al Bi Nb
2
Conv. (%)
Yield (%)
NO
N2
O2
N2O
NO2
95.3 92.3 91.4 90.1 89.2 87.5 84.3 79.0 72.2 34.7 28.1 1.1 (0.3)
91.8 62.8 77.1 56.6 85.8 78.3 46.9 50.8 41.7 2.6 9.7 1.3 0.5
83.5 56.5 64.6 51.0 75.4 66.9 36.9 33.6 23.4 6.0 1.8 0.3 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
5.9 17.9 13.4 19.5 6.9 10.3 23.7 22.7 24.6 14.4 13.2 0.4 0.2
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Fig. 3. Temperature (Y0.945Ni0.05Ba0.005)2O3.
Fig. 1. NO conversion, N2 and O2 yield on (Y0.99-xNixBa0.01)2O3 at 850 °C as a function of x value.
dependence
of
NO
decomposition
on
is required for high NO decomposition activity. Fig. 3 shows the temperature dependence of NO decomposition on (Y0.945Ni0.05Ba0.005)2O3. Although N2 yield on Y2O3 was as low as 30% at 850 °C, it was 95% by doping just 5 mol % Ni2+ and 0.5 mol% Ba2+ which may introduce oxygen vacancy into Y2O3. Therefore, it can be said that the introduction of oxygen vacancy is effective for achieving the high activity to NO decomposition at high temperature, however, as shown in Fig. 3, at temperature lower than 600 °C, NO decomposition was hardly occurred. Therefore, increase in NO decomposition activity at lower temperature is required. In the conventional catalyst, NO decomposition activity is significantly decreased with co-existence of O2. Therefore, effects of O2 coexistence on NO decomposition activity of (Y0.945Ni0.05Ba0.005)2O3 were studied at 850 °C. Fig. 4 shows N2 yield as a function of O2 partial pressure. Although N2 yield was decreased by co-existence of O2 up to 1%, it was hardly changed by increasing amount of O2 higher than
Fig. 2. NO conversion, N2 and O2 yield on (Y0.95-xNi0.05Bax)2O3 at 850 °C as a function of x value.
(Y0.94Ni0.05Ba0.01)2O3. On this catalyst, NO conversion of 100% was achieved at 800 °C and O2 formation was also observed at temperature higher than 600 °C. As a result, the optimized composition for Ni dope seems to exist around (Y0.94Ni0.05Ba0.01)2O3. It is also noted that BET surface area of this catalyst was 28 m2/g and one reason for the high activity to NO decomposition at temperature higher than 800 °C may be assigned to the reasonably large surface area. Effects of Ba amount in (Y0.95Ni0.05)2O3 on NO decomposition was further studied. Fig. 2 shows NO conversion, N2 and O2 yield as a function of Ba amount. In a similar manner with Ni2+, N2 yield was increased with Ba amount up to 0.5 mol% and then almost independent of Ba amount. Therefore, it seems that doping of Ba2+ higher than 0.5%
Fig. 4. N2 yield in NO decomposition on (Y0.945Ni0.05Ba0.005)2O3 at 850 °C as a function of O2 partial pressure. 3
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Fig. 5. NO pulse reaction on Y2O3 at (a) 400, (b) 500 and (c) 600 °C, and that on (Y0.94Ni0.05 Ba0.01)2O3 at (d) 400, (e) 500 and (f) 600 °C, and NO conversions as functions of pulse numbers on (g) Y2O3 and (h) (Y0.94Ni0.05Ba0.01)2O3.
1 mol%. Therefore, it was found that (Y0.945Ni0.05Ba0.005)2O3 was highly stable and active to NO decomposition under co-existence of O2, although high temperature is required for high N2 yield. In order to further increase in NO decomposition activity at low temperature, detail mechanism on NO decomposition on Ni2+ and Ba2+ doped Y2O3 was studied by using a pulse reaction system. It is also noted that negative effects of oxygen coexistence were decreased by Ni2+ co-doping with Ba2+ Ni and so doped Ni2+ might redox during NO decomposition and has role like oxygen storage material. Since the NO decomposition activity is almost the same, (Y0.94Ni0.05Ba0.01)2O3 was used in the following part. Fig. 5 (a)-(f) show the results of applying 3.6 μl 10 pulses of 1% NO (1.5 nmol NO) to Y2O3 and (Y0.94Ni0.05Ba0.01)2O3 at 400, 500 and 600 °C. Only N2, NO and N2O were detected during these pulse reaction. During the first NO pulse at 400 °C, as shown in Fig. 2 (a) and (d), approximately 70 and 65% NO conversions were achieved over Y2O3 and (Y0.94Ni0.05Ba0.01)2O3, respectively, corresponding to approximately 1 and 0.9 nmol NO consumption, respectively. With increasing pulse numbers, NO conversions decreased, as did those of N2 and N2O.
After 10 pulses, the total NO consumption quantities were determined to be 5.2 nmol for the Y2O3 and 3.4 nmol for (Y0.94Ni0.05Ba0.01)2O3. Fig. 5 (b) and (c) present NO conversion data acquired over Y2O3 at 500 and 600 °C upon injecting 10 pulses of NO at each temperature. At 500 °C, both NO consumption and the generation of N2 and N2O were observed, although much lower levels of N2 formation than that at the initial pulses of NO at 400 °C because of no treatment for removal of surface strongly adsorbed species (may be NO3 species) formed in pulse reaction at 400 °C. The NO conversion decreased with increasing pulses, producing the same trend as observed at 400 °C. The total amount of NO converted at 500 C was 2.3 nmol, which is lower than the amount at 400 °C. At 600 °C, no significant decrease in the NO signal was evident, indicating that NO was not converted over Y2O3 at this temperature. Fig. 5 (e) and (f) present the results of NO pulsed reaction over (Y0.94Ni0.05Ba0.01)2O3 at 500 and 600 °C using 10 pulses having the same amount of NO as the above trials. At 500 °C, the NO conversion decreased during the first 5 pulses and then plateaued. A similar trend was observed at 600 °C over the same catalyst, and the extent of N2 formation became stable from the third pulse. The total amount of NO 4
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Fig. 6. Comparison of XRD patterns between (Y0.94Ni0.05Ba0.01)2O3 and 1 mol% Au-dispersed (Y0.94Ni0.05Ba0.01)2O3 after sintering at 1100, 1200 and 1480 °C. a) wide range and b) narrow range diffraction. Sintering temperature (a) 900 (b) 1100 (c) 1200, and (d) 1480 °C.
consumption over (Y0.94Ni0.05Ba0.01)2O3 at 500 °C was 2.4 nmol, which is almost equal to the NO consumption over Y2O3. However, at 600 °C, NO consumption (2.4 nmol in total for 10 pulses) continued over (Y0.94Ni0.05Ba0.01)2O3, as well as at 700 and 800 °C (data not shown). In our previous study, Y2O3 having a C-type cubic structure typical of rare earth oxides was found to promote the NO decomposition reaction [8,9]. BaO does not promote NO decomposition and increase NO adsorption from basic property, although doping Ba2+ into Y2O3 may increase the oxygen vacancies which are believed to act as the active sites for NO adsorption. With increasing temperature during the pulsed NO experiments, larger amount of NO conversion was observed on Ba and Ni-doped Y2O3. Therefore, it appears that the doping of Y2O3 with Ba2+ and Ni2+ created more active sites for NO adsorption and decomposition. Neither catalyst exhibited oxygen production during the pulse reaction, suggesting that oxygen atom from NO (Oads) remained on the surfaces of both catalysts between 400 and 600 °C, resulting in the deactivation of NO decomposition. Increase in oxygen removal from surface is required for the regeneration of the active site resulting in the increase activity to NO decomposition at low temperature.
Fig. 7. Temperature dependence on NO direct decomposition on (Y0.94Ni0.05Ba0.01)2O3 and 1 mol% Au-dispersed (Y0.94Ni0.05Ba0.01)2O3 sintered at 1100 °C followed by making powder.
3.2. Tensile strain effects for increasing NO decomposition activity at low temperature
particle size of Au by excessively high sintering temperature (higher than meting temperature) and annealed the tensile strain formed by difference in mismatch in thermal expansion coefficient when particle size became larger. Fig. 7 shows the comparison of the NO conversion of Au-dispersed (Y0.94Ni0.05Ba0.01)2O3 (surface area: 1.7 m2 g−1) and no Au dispersed one (surface area: 0.4 m2 g−1). Therefore, the surface area of the catalyst became smaller by two order of magnitude by sintering treatment. Both catalysts shows NO direct decomposition activity from 600 °C, and the oxygen formation was also observed from 600 °C. Comparing with the NO decomposition activity shown in Fig. 3, the smaller activity at higher temperature may be assigned to the decreased surface area by sintering treatment. The catalyst with Au dispersed sample started to show higher N2 and O2 yield than that on no Au dispersed catalyst from 600 and at 850 °C, i.e., N2 and O2 yield of Au catalyst was almost twice higher than that of the control catalyst. Thus it is considered that the tensile strain introduced works positively to oxygen desorption resulting in the increased NO decomposition activity of Y2O3 added with BaO. In case of Pr1.90Ni0.71Cu0.24Ga0.05O4+δ, dispersion of Au induces a tensile strain resulting in the increased oxygen diffusivity [18]. Therefore, in a similar manner with effects of Ni2+ doping, Au dispersed in Y2O3 could also increase oxygen diffusivity in bulk of Y2O3 and the improved NO decomposition activity could be assigned to this increased oxygen vacancy diffusivity. As a result, it is seen that dispersion of Au is effective for increasing NO decomposition activity. Therefore, NO
Fig. 6 (a) shows the XRD patterns of Au-dispersed samples after (a) 1100, (b) 1200, and (c) 1480 °C sintering. In three samples, the main strong peaks are assigned to Y2O3 with rare-earth type C structure (cubic lattice), which is the major component of three samples. Since 1 mol % Au was introduced into catalyst as a dispersed composite, two peaks attributed to Au metal phase were observed at three samples. Because diffraction peaks from Au were observed, the dispersed Au seems to exist as a metallic phase, suggesting that the added Au did not react with Y2O3 phase. Therefore, metallic particles of Au were successfully dispersed into the Y2O3 grains. No peaks belong to Ba can be observed at three samples. It is indicated that Ba was highly dispersed and is likely to have been incorporated in the lattice positions of Y2O3. The comparison between these three samples and no Au dispersed (Y0.94Ni0.05Ba0.01)2O3 sample suggests that the XRD peaks were shifted to lower angle direction suggesting the expansion of lattice parameter as shown in Fig. 6 (b). It is obviously that main diffraction peak from Y2O3 was shifted to a lower angle by dispersion of Au after 1100 °C sintering. This indicates that Y2O3 lattice was tensile after sintering with 1 mol % Au. The large difference in the thermal expansion coefficient can induce tensile strain in Y2O3, resulting in changes in Y2O3 unit lattice length as expected. On the other hand, lower angle shift in diffraction peaks was return to the original angle when sintering was performed at higher temperature. This may be explained by increasing 5
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dispersed (Y0.94Ni0.05Ba0.01)2O3 The nitrosyl species at 1455 cm−1 also increased in intensity from 100 to 500 °C indicating the increasing adsorption amount as temperature increases. However, it is started to decrease in intensity from 600 °C, and this peak is almost disappeared at 700 °C. Considering the temperature dependence of NO decomposition, N2 and O2 might be formed through removal of surface nitrosyl species formed on the catalyst. Similar temperature dependence of bridging nitrate adsorbed on Au-dispersed sample as nitrosyl species was also observed. Its absorbance slowly increased from 100 to 300 °C followed by the decreasing at higher temperature especially at 600 to 700 °C at which NO decomposition reaction was actively proceeded. Most of the adsorbed NO on the oxygen vacancies refer to form hyponitrite instead of nitrate species. Therefore, at 600 °C, a new peak at 1135 cm−1 attributed to hyponitrite species is appeared, and it started to increase intensity from 600 to 700 °C [21]. The operando FTIR spectra of NO adsorption on non-Au dispersed (Y0.94Ni0.05 Ba0.01)2O3 at different temperature was shown in Fig. 9 (b). There are four similar absorbance species at 1135, 1179, 1296 and 1455 cm−1 observed, i.e., the hyponitrite, N–O, bridging nitrate and nitrosyl species, respectively [21–25]. However, different from Audispersed sample, there is a new IR peak observed at 1235 cm−1 which may be attributed to bidentate nitrate became visible from 200 to 700 °C. On the non-Au dispersed catalyst, the desorption of O2 from the oxygen vacancies on the surface is the rate determining step. Therefore, introduction of tensile strain forms oxygen vacancy and high diffusivity resulting in the increased removal of surface nitrosyl and increased NO decomposition activity. In this section, it was confirmed that increase in diffusivity in oxygen vacancy is effective for increasing O2 desorption at decreased temperature. O2-TPD was measured for analysis of desorption property of oxygen, however, up to now, no reasonable desorption curves were obtained and this may suggest that amount of O2 adsorption is small. One reason for this may be small surface area of the catalyst. The change in oxygen adsorption state by applying tensile strain by Au dispersion will be reported in future. Effects of Au dispersion on NO adsorption property was further studied by TPD measurement. Fig. 10 shows TPD profile of NO from (Y0.94Ni0.05Ba0.01)2O3 and Au dispersed catalyst. Obviously, part of NO was desorbed as N2, O2, N2O and NO2 on both catalysts and this suggests that dissociative adsorption of NO was occurred on (Y0.94Ni0.05Ba0.01)2O3. On both catalyst, NO desorption was occurred around 400 °C and at the same temperature, O2, N2, and N2O desorption was observed suggesting that dissociative adsorbed NO was decomposed. Comparing with non Au dispersed sample, desorption
Fig. 8. Effects of Au amount on temperature dependence of NO decomposition of Au dispersed (Y0.94Ni0.05Ba0.01)2O3.
decomposition activity at further higher amount of Au was studied. Fig. 8 shows effects of Au amount on temperature dependence of NO decomposition of Au dispersed (Y0.94Ni0.05Ba0.01)2O3. With increasing amount of Au from 1 to 2 mol%, NO decomposition activity was decreased, in particular, decrease in activity at low temperature was more significantly observed. Therefore, excess amount of Au dispersed seems not to be effective for increasing NO decomposition activity. However comparing with no Au dispersed (Y0.94Ni0.05Ba0.01)2O3, 2 mol% Au dispersed sample shows still higher NO decomposition activity as shown in Fig. 8. Considering from the XRD results showing in Fig. 6, this is because Au particle size was increased with increasing Au amount and so tensile strain was annealed when Au amount became larger. Therefore, it seems that the optimum amount for Au dispersion in (Y0.94Ni0.05Ba0.01)2O3 seems to be exist around 1 mol%. Fig. 9 shows the operando FT-IR spectra of NO adsorption on Audispersed (Y0.94Ni0.05Ba0.01)2O3 from 100 to 700 °C. There are four main adsorption peaks observed at 1135, 1179, 1297 and 1455 cm−1. At 100 °C, there are three absorbed peaks at 1179, 1297 and 1455 cm−1 which are attributed to NeO adsorption on the oxygen vacancies, nitrate (NO3) and nitrosyl (NO2) species [21–25]. NeO species at 1179 cm−1 became more intense with increasing temperature. This suggests that the adsorption amount of NO increased on the Au-
Fig. 9. Operando FTIR spectra of NO adsorbed species on the surface of (a) Au-dispersed (Y0.94Ni0.05Ba0.01)2O3 and (b) no Au dispersed sample from 100 to 700 °C temperature. Labels in figure are reaction temperature. 6
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Fig. 10. Temperature programmed desorption (TPD) profile of NO from (Y0.94Ni0.05Ba0.01)2O3 and Au dispersed catalyst.
temperature of NO as shifted to lower temperature and this is more obviously observed for O2 desorption. Therefore, as suggested by in-situ FT-IR measurement, removal of surface nitrate is accelerated by dispersion of Au which means introduction of tensile strain. At high temperature, large amount of N2 as well as N2O desorption was observed on both catalysts, however, no O2 desorption was occurred. Therefore, oxygen may be remained on the surface. By Au dispersion, amount of N2O desorbed at higher temperature region was decreased and larger amount of O2 and NO2 which may be formed during desorption of O2 and NO was occurred around 400 °C. This may be explained by removal of nitrate species at low temperature on Au dispersed (Y0.94Ni0.05Ba0.01)2O3. As a result, tensile strain induced by Au dispersion is effective for removing nitrate species at lower temperature resulting in the increased NO decomposition activity around 500 °C.
in future work. In any case, this study reveals that (Y0.94Ni0.05Ba0.01)2O3 with tensile strain is highly active to NO decomposition at low temperature. Acknowledgements This study was financially supported by a Grant-in-Aid for Specially Promoted Research (No. 16H06293) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan through the Japan Society for the Promotion of Science. References [1] R. Burch, J.P. Breen, F.C. Meunier, Appl. Catal. B Environ. 39 (2002) 283–303. [2] M.F. Fu, C. Li, P. Lu, L. Qu, M.Y. Zhang, Y. Zhou, M. Yu, Y. Fang, Catal. Sci. Technol. 4 (2014) 14–25. [3] M. Haneda, H. Hamada, Comptes Rendus Chim. 19 (2016) 1254–1265. [4] N. Imanaka, T. Masui, Appl. Catal. A: Gen. 431-432 (2012) 1–8. [5] M. Iwamoto, Catal. Today 29 (1996) 29–35. [6] J. Zhu, D. Xiao, J. Li, X. Yang, Y. Wu, J. Mol. Catal. A 234 (2005) 99–105. [7] B. Zhao, R. Wang, X. Yang, Catal. Commun. 10 (2009) 1029–1033. [8] T. Ishihara, Y. Shinmyo, K. Goto, N. Nishiyama, H. Iwakuni, T. Matsumoto, Chem. Lett. 37 (2008) 318–319. [9] T. Ishihara, K. Goto, Catal. Today 164 (2011) 484–488. [10] T. Masui, S. Uejima, S. Tsujimoto, R. Nagai, N. Imanaka, Catal. Today 242 (2015) 338–342. [11] N. Imanaka, T. Masui, H. Masaki, Adv. Mater. 19 (2007) 3660–3663. [12] Y. Doi, M. Haneda, M. Ozawa, J. Mol. Catal. A: Chem. 383-384 (2014) 70–76. [13] Y. Teraoka, T. Harada, S. Kagawa, J. Chem. Soc. Faraday Trans. 94 (1998) 1887–1891. [14] L. Liu, T. Ishihara, Appl. Catal. A: Gen. 550 (2018) 90–97. [15] L. Liu, K. Murakami, S. Ida, T. Ishihara, Catal. Commun. 100 (2017) 5–9. [16] M. Kubicek, Z. Cai, W. Ma, B. Yildiz, H. Hutter, J. Fleiq, J. ACS Nano 7 (2013) 3276–3286. [17] D. Pergolesi, E. Fabbri, S.N.C. Cook, V. Roddatis, E. Traversa, J.A. Kilner, ACS Nano 6 (2013) 10524–10534. [18] J. Hyodo, K. Tominaga, J.E. Hong, S. Ida, T. Ishihara, J. Phys. Chem. C 119 (2015) 5–13. [19] D. Majumdar, D.J. Chatterjee, Appl. Phys. 70 (1991) 988–992. [20] A. Gauzzi, H.J. Mathieu, J.H. James, B. Kellett, Vacuum 41 (1990) 870–874. [21] S.J. Huang, A.B. Walters, M.A. Vannice, J. Catal. 192 (2000) 29–47. [22] B. Klingenberg, M.A. Vannice, Chem. Mater. 8 (1996) 2755–2768. [23] A. Martinez-Arias, J. Soria, J.C. Conesa, X.L. Seoane, A. Arcoya, R.J. Cataluna, Chem. Soc., Faraday Trans. 91 (1995) 1679–1687. [24] A. Snis, I. Panas, Surf. Sci. 412-413 (1998) 477–488. [25] G. Ghiotti, A. Chiorino, Spectrochim. Acta 49A (1993) 1345–1359.
4. Conclusion NO decomposition on Y2O3 based oxide doped with small amount of Ba2+ was studied and it was found that doping Ni2+ is effective for increasing NO decomposition among the dopants studied and the optimized amount of NiO addition seems to be 5 mol %. On this Ni2+ and Ba2+ doped Y2O3, almost 100% NO conversion was achieved at 800 °C and N2 yield higher than 95% was achieved. Therefore, Ni2+ and Ba2+ doped Y2O3 shows high activity and selectivity to NO decomposition. In particular, (Y0.945Ni0.05Ba0.005)2O3 catalyst shows high N2 yield (> 80%) at 850 °C under coexistence of O2 up to 5%. However, NO decomposition was hardly occurred around 500 °C, however, from pulse reaction, (Y0.94Ni0.05Ba0.01)2O3 shows NO decomposition activity around 400 °C, although no O2 formation was observed. Therefore, it seems that rate determining step of NO decomposition on this catalyst is removal of oxygen which is formed by NO decomposition. Introduction of tensile strain by dispersion of Au is effective for removing the surface nitrate species at lower temperature and so it was found that Au dispersed (Y0.94Ni0.05Ba0.01)2O3 which is obtained by sintering at 1100 °C followed by powdering, showed increase NO decomposition activity around 500 °C. Similar strain effects are also expected for other metals, however, comparing with the case of Au, separation of catalytic activity of metals and strain effects is rather difficult and so in this study, Au was used, however, synergy effect will also be expected in case of other metal like Pd, and this is now under study. The results will be reported
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