Direct Spectroscopic Evidence of CO Spillover and Subsequent

Aug 13, 2012 - Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. Environ. Sci. Technol...
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Direct Spectroscopic Evidence of CO Spillover and Subsequent Reaction with Preadsorbed NOx on Pd and K Cosupported Mg−Al Mixed Oxides Yexin Zhang,† Xiao Wang,† Zhongpeng Wang,† Qian Li,† Zhaoliang Zhang,*,† and Limin Zhou‡ †

School of Chemistry and Chemical Engineering, University of Jinan, 106 Jiwei Road, Jinan 250022, China Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

‡

S Supporting Information *

ABSTRACT: The CO adsorption and subsequent reaction with preadsorbed NOx on Pd and K cosupported Mg−Al mixed oxides (Pd−K/MgAlO, 1/8/100 w/w) were investigated using in situ FTIR spectroscopy. During CO adsorption, a peculiar and well-defined IR band at 2160 cm−1 was observed. Several elaborately designed experiments such as the competitive adsorption of CO and CO2 demonstrated that the 2160 cm−1 band was exclusively assigned to a carbonyl species on K sites due to the CO spillover from Pd to K, which results from a strong Pd−K interaction based on temperatureprogrammed reduction with H2 experiments. Importantly, the spillover of CO is found to be involved in the reduction of preadsorbed NOx from temperature-programmed surface reactions with CO. Thus, all adsorbed NOx can be reduced by CO before desorption. Like the process of “pumping” CO by Pd from the atmosphere to “irrigate the field” of the nitrates/nitrites, the adsorbed NOx at not only K sites adjacent to Pd but also at the remote K sites can be reduced into N2 and N2O effectively.

1. INTRODUCTION Nitrogen oxides (NOx) from the emission of boilers and vehicles contribute much to the depletion of tropospheric ozone, and production of photochemical smog and acid rain.1 Recently, it is urgent in China to reduce the total amount of NOx because of the stringent demand of the National Twelfth Five-Year Plan. Among the control technologies, catalytic reduction with various reducing agents prevails, in which CO (one of the main pollutants and reductants) is generally employed in the reduction of NOx from vehicle exhaust.2 Commonly, the reduction of NOx occurs on the surface of a catalyst, where NOx is preliminarily adsorbed and then reduced by reducing agent. Therein, spillover is often regarded as a mechanistic step, which involves the transportation of reducing agent to the sites of adsorbed NOx.3 In the case of the reduction of NOx with H2,4−6 hydrogen spillover has been extensively studied. However, the similar spillover for the reduction of NOx with CO is scarcely reported.7−10 In a limited literature, CO spillover was studied using temperatureprogrammed desorption (TPD)11 and/or isotope labeling.12 The FTIR spectroscopy was usually used to detect the surface adsorbed species of CO, such as methoxides, formates, and carbonates.13,14 He et al.15 observed the spillover of CO from a Rh (100) surface to a subsequently deposited Cu overlayer using FTIR. However, no direct evidence of CO spillover is observed by FTIR on noble metal-supported mixed oxides. © 2012 American Chemical Society

For a long time, Mg−Al mixed oxides (MgAlO) have received significant attention for NOx adsorption.16−20 Especially, K-supported MgAlO (K/MgAlO) showed improved NOx adsorption at higher temperature.16 On the other hand, noble metal Pd has been reported to be more effective than Pt for NOx reduction at lower temperature.17 In this work, on the basis of the characterization results of X-ray powder diffraction (XRD), elemental analysis, and temperature-programmed reduction with H2 (H2−TPR), the CO adsorption and subsequent reaction with preadsorbed NOx on Pd and K cosupported MgAlO (Pd−K/MgAlO) were studied using in situ FTIR spectroscopy−mass spectrometer (MS). The CO spillover from Pd to K sites was first discovered and confirmed to play an important role in the reduction of preadsorbed NOx into N2 and N2O.

2. EXPERIMENTAL SECTION MgAlO with molar ratio of 3:1 was prepared as described elsewhere.21,22 More details are given in the Supporting Information (SI). The Pd/MgAlO (1/100 w/w) and K/ MgAlO (8/100 w/w) catalysts were prepared by incipient Received: Revised: Accepted: Published: 9614

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wetness impregnation method with an aqueous solution of Pd(NO3)2 and K2CO3, respectively, followed by drying overnight and calcination at 850 °C for 2 h. The Pd−K/ MgAlO (1/8/100 w/w) catalyst was prepared by incipient wetness impregnation of Pd/MgAlO with a aqueous solution of K2CO3, followed by drying overnight and calcinations at 850 °C for 2 h. To investigate the combination property among Pd, K, and the support, Pd−K/MgAlO was also washed with hot water at 80 °C. After filtration, drying, and calcination at 850 °C for 2 h, the obtained sample was denoted as Pd−K/ MgAlO_w. Powder XRD patterns were recorded on a Rigaku D/max-rc diffractometer. The elemental compositions were measured by inductively coupled plasma−optical emission spectrometry (ICP−OES, IRIS Advantage, Thermo, USA) and X-ray fluorescence spectrometry (XRF, ZSX Primus II, Rigaku, Japan). Surface area and pore size distribution were determined by N2 adsorption−desorption at 77 K with the BET method using a Micromeritics ASAP 2020 instrument after outgassing at 300 °C for 5 h prior to analysis. The H2−TPR experiments were performed in a quartz reactor with a thermal conductivity detector (TCD) to monitor the H2 consumed. A 50-mg sample was pretreated in O2 at 500 °C for 1 h and cooled to room temperature. H2−TPR was conducted at 10 °C/min in a flow of 5 vol % H2 + N2 (30 mL/ min). To quantify the total amount of H2 consumed during these experiments, CuO was used as a calibration reference. The Pd dispersion was measured by H2−O2 titration in a quartz reactor with a TCD to monitor H2.23 Prior to H2−O2 titration, a 50-mg sample was reduced in 5 vol % H2 + N2 at 500 °C for 2 h. N2 was passed over the sample for 1 h. After cooling to 120 °C, the O2 titration was performed, followed by flushing with N2, and then the introduction of pure H2 pulses (10 μL). The stoichiometry of Pd/H2 = 2/3 was assumed. The in situ FTIR spectra were recorded on a Bruker Tensor 27 spectrometer over 400−4000 cm−1 after 16 scans at a resolution of 4 cm−1. Background spectra were obtained without samples before each experiment in a He flow at room temperature. The sample was pressed into a thin selfsupporting wafer with a thickness of 7.5 mg/cm2. The wafer was loaded into an in situ infrared transmission cell which is capable of operating up to 500 °C and equipped with gas flow system. The sample was pretreated at 500 °C for 1 h in He flow (100 mL/min). After that, the in situ FTIR experiments of CO adsorption or the reduction of preadsorbed NOx with CO were performed. In the case of CO adsorption, the pretreated sample was cooled to 100 °C in He, and then 2000 ppm CO + He (100 mL/min) was introduced. When the spectrum was not changed, the sample was heated to 500 °C in the CO flow at the rate of 10 °C/min. As to the reduction of the preadsorbed NOx with CO, the pretreated sample at 500 °C was cooled to 300 °C in He, and then 1000 ppm NO + 5 vol % O2 + He (100 mL/min) was admitted. After saturation of NOx adsorption at 300 °C, the sample was flushed by a He flow and then cooled to 100 °C. At 100 °C, the feed gas was switched to 2000 ppm CO + He (100 mL/min), and the sample was heated to 500 °C at the rate of 10 °C/min.24 Meanwhile, the product of N2 was analyzed by a quadruple MS (OmniStar 200, Balzers) with a m/ z of 14.

Figure 1. XRD patterns of MgAlO, K/MgAlO, Pd/MgAlO, Pd−K/ MgAlO, and Pd−K/MgAlO_w.

MgAlO, and Pd−K/MgAlO_w. For all samples, periclase-type Mg(Al)Ox (JCPDS 45-0946)25 and spinel-type MgAl2O4 (JCPDS 21-1152) phases were observed. The PdO (JCPDS 41-1107) phase was merely detected for Pd-containing catalysts. Furthermore, the absence of any K phase for Pd− K/MgAlO and K/MgAlO suggests that most K2CO3 precursor is decomposed and K is well dispersed on the surface, which is promoted by the interaction with MgAlO support.21,26 The amount of 8 wt % has been determined to be the highest limit for high dispersion of K on MgAlO.21 On the other hand, XRF shows that all the K species on Pd−K/MgAlO_w can be removed after washing procedure (Table S1). The Pd dispersions for Pd−K/MgAlO and Pd/MgAlO were determined as 7.4% and 16.7% by H2−O2 titration, respectively. These relatively low Pd dispersions may be due to the hightemperature treatment at 850 °C. Additionally, the loss of Pd dispersion with K loading can be explained by two facts: (1) the specific surface areas of samples are decreased by K loading (Table S2); and (2) the PdO particles are partly covered by K, which also implies the intimate contact between Pd and K.27 However, the latter factor is determinate, which can be confirmed by the restoration of Pd dispersion (14.0%) for Pd−K/MgAlO_w after K removal with the washing procedure. The redox properties of catalysts were characterized by H2− TPR (Figure 2). The consumed hydrogen amounts (Table S3) are close to the stoichiometry for a complete Pd2+ → Pd0 reduction. K/MgAlO does not show any H2−TPR peaks.21 Pd/ MgAlO shows a single reduction peak at 63 °C, whereas the reduction peak of Pd−K/MgAlO was shifted to 118 °C. The similar phenomenon was observed on acidic SiO2−Al2O3 by Pellegrini et al.27 This suggests a strong chemical interaction between Pd and K oxides, which results in the decrease in the reducibility of PdO. In the case of Pd−K/MgAlO_w, the H2− TPR profile (peak temperature) is similar to that of Pd/ MgAlO, which confirms that the K has been completely removed (Table S1). Combined with XRD results, it is reasonable that no chemical bond (complex of Pd and K) was formed between Pd and K in spite of the presence of the strong Pd−K interaction.27 3.2. In Situ FTIR of CO Adsorption. Figure 3 shows the in situ FTIR spectra of CO adsorption during temperatureprogrammed heating on Pd/MgAlO and Pd−K/MgAlO.

3. RESULTS AND DISCUSSION 3.1. Characterization of Samples. Figure 1 shows the XRD patterns of MgAlO, K/MgAlO, Pd/MgAlO, Pd−K/ 9615

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adsorbed CO on K sites was also proposed in some experimental and calculation works.31−34 Therefore, several elaborately designed experiments were performed and are discussed in the following. 3.3. Assignment of the Band at 2160 cm−1. As indicated above, it seems plausible to assign the band at 2160 cm−1 to the adsorbed CO on Pd2+ considering two facts: (i) the presence of Pd2+ may be stabilized by the Pd−K interaction, which lowers the reducibility of PdO (Figure 2); and (ii) the band is not found on Pd−K/MgAlO after H2 reduction at 500 °C (Figure S3). Nevertheless, Mckinney35 found that the adsorbed CO on oxidized Pd sites was completely desorbed at 218 °C, which is much lower than that observed in this work (300 °C as shown in Figure S1). To rule out the possibility of the adsorbed CO at Pd2+ sites, the CO adsorption on Pd/MgAlO after oxidation with O2 was characterized by in situ FTIR at various temperatures. As shown in Figure 4, the band at 2160 cm−1

Figure 2. H2−TPR curves over Pd/MgAlO, Pd−K/MgAlO, and Pd− K/MgAlO_w.

Figure 4. In situ FTIR spectra of CO (2000 ppm in He) adsorption on Pd/MgAlO after oxidation with 2000 ppm O2 + He for 20 min at various temperatures.

Figure 3. In situ FTIR spectra of CO (2000 ppm in He) adsorption on Pd/MgAlO (A) and Pd−K/MgAlO (B).

is not observed besides those at 2100−1900 cm−1 which can be assigned to the adsorbed CO on Pd+ and/or Pd0.30 These lowvalence Pd species originate from the reduction of Pd2+ by CO during CO adsorption. The impossibility of the adsorbed CO at Pd2+ sites is further demonstrated by the result of Figure 5, which shows that the adsorbed CO is not removed until 450 °C in H2 atmosphere while Pd2+ has been reduced. The H2−TPR result of Pd−K/MgAlO_w confirmed that no complex of Pd and K was formed on MgAlO surface, even though a strong Pd−K interaction exists. Furthermore, Figure S3 shows that if the Pd−K interaction was destroyed by H2 reduction (XRD shows the presence of Pd metal, not given here), the band at 2160 cm−1 would not be observed; however, as shown in Figure 5, it is still present in the flow of H2. This confirmed that the adsorbed CO is not bridge-bound on the interacting Pd and K sites (Pd−CO−K+). Iordan et al.36 reported that the carbonyl species on K sites at 2168 cm−1 can be held to 500 °C in the CO atmosphere, which

During CO adsorption, the doublet bands of gaseous CO (2171 and 2119 cm−1) are observed.28 For Pd/MgAlO (Figure 3A), the bands at 1966 cm−1 (below 300 °C) and 1920 cm−1 (above 300 °C) are assigned to the 2-fold bridged carbonyl species on Pd (100)27,29 and Pd (111),27 respectively. However, these adsorbed species on Pd0 are not obvious on Pd−K/MgAlO in Figure 3B. Significantly, a well-defined band at 2160 cm−1 appears evidently above 300 °C, and the intensity reaches maximum at 400 °C. After purging with He, the band does not vanish until 300 °C (Figure S1), suggesting that the band should be related to a chemically adsorbed CO, which is peculiar because it is not observed on either Pd/MgAlO (Figure 3A) or K/MgAlO (Figure S2). Prior to further discussion, it is essential to reveal at which sites the adsorbed CO at 2160 cm−1 is located. Tessier et al.30 thought it was at Pd2+ sites. Alternatively, the assignment to the 9616

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greatly, suggesting the preference for CO2 adsorption on K sites. Therefore, no band at 2160 cm−1 is observed. Lastly, the assignment of the adsorbed CO on K sites can be confirmed by the absence of adsorbed CO on Pd−K/MgAlO_w, on which all K species have been washed away (Figure S5). 3.4. Formation Mechanism of the Adsorbed CO on K Sites. It is surprising that the adsorbed CO on K sites is observed on Pd−K/MgAlO but not on K/MgAlO, which definitely suggests that a Pd-catalyzed chemical route is responsible for the formation of the adsorbed CO species rather than a direct adsorption route. As pointed out above, the Pd−K interaction is indispensable for the formation of the adsorbed CO on K sites. Based on the result of Figure 2, the Pd−K interaction can weaken the adsorption of CO on Pd, which results in the adsorbed CO on Pd to be unstable and transferable. This is also the reason for the difficulty to trap the IR signal of the adsorbed CO on Pd for Pd−K/MgAlO (Figure 3B). As soon as the adsorbed CO is transferred to the K sites, the carbonyl species is formed. Therefore, it can be deduced that the formation of adsorbed CO on K sites is due to the spillover of adsorbed CO from Pd. This can be confirmed by the fact of Figure S3, that the carbonyl species cannot be formed on the reduced Pd−K/ MgAlO due to the loss of the Pd−K interaction after H2−TPR. The continued CO spillover can result in the accumulation of carbonyl species on K sites, as indicated by the increase in intensity of the band at 2160 cm−1 from 300 to 400 °C in Figure 3B. At higher temperatures, the carbonyl species is subject to desorption. Therefore, the gradual decrease of the band in intensity is observed. 3.5. In Situ FTIR of the Reduction of Preadsorbed NOx with CO. Figure 7 shows the in situ FTIR spectra of the

Figure 5. Evolution of the FTIR band of adsorbed CO at 2160 cm−1 on Pd−K/MgAlO during heating in 2000 ppm H2 + He. The adsorbed CO is in situ obtained by heating Pd−K/MgAlO to 400 °C in 2000 ppm CO + He and then cooling to 200 °C.

is in agreement with our results (Figure 3B). Consequently, the band at 2160 cm−1 is most possible to be assigned to the adsorbed CO on K sites. To confirm the hypothesis, the CO adsorption on Pd−K/MgAlO after CO2 adsorption was characterized using in situ FTIR. As expected, the band at 2160 cm−1 is not detected since the K sites are occupied in advance by CO2 (Figure S4). The similar situation is observed in the competitive adsorption of CO and CO2 on Pd−K/ MgAlO. As shown in Figure 6, two kinds of carbonates, chelating bidentate carbonate (1545 and 1346 cm−1) and ionic carbonate (1408 cm−1), still remained after thermal treatment in He at 500 °C,22 which may originate from the CO2 contamination in air. After both CO and CO2 are introduced, the intensity of the chelating bidentate carbonates increased

Figure 7. In situ FTIR spectra of the reduction of the preadsorbed NOx with CO (2000 ppm in He) on Pd−K/MgAlO.

reduction of preadsorbed NOx with CO on Pd−K/MgAlO. After NOx adsorption, both the ionic nitrites (1243 cm−1) and the ionic nitrates (1370 cm−1) were observed.22 When the sample is heated to 280 °C in the flow of CO, the band of carbonyl species on K sites at 2160 cm−1 appears, and then increases to maximum in intensity at 400 °C, which indicates the occurrence of CO spillover. It is during this period that the bands of ionic nitrites/nitrates vanished while the band of ionic carbonate (1408 cm−1) became dominant. This confirms the

Figure 6. In situ FTIR spectra of the competitive adsorption of 1000 ppm CO and 1000 ppm CO2 in He on Pd−K/MgAlO. 9617

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contemporaneous presence of the reduction of preadsorbed NOx by CO and the adsorption of the produced CO2 on Pd− K/MgAlO. As the amount of CO2 formed is limited, this is not enough to deactivate the K sites (The chelating bidentate carbonate is not changed in Figure 7 in comparison with the great increase in Figure 6 after the atmosphere is switched from He to 1000 ppm CO + 1000 ppm CO2 + He). In the flow of He, however, the nitrites/nitrates always stay there in the similar heating process (Figure S6), suggesting that the reduction of the preadsorbed NOx is not initiated by the thermal decomposition. Furthermore, the doublet bands at 2240 and 2211 cm−1 are observed at 280−300 °C in Figure 7, which can be assigned to gaseous N2O, since the doublet is in coincidence with the characteristic bands of gaseous N2O (Figure S7A) and vanished quickly with the purging of He (Figure S7B). In addition to N2O, the main product of N2 is detected by MS below 350 °C (Figure S8). This is consistent with the observation in Figure 7 that all the bands of nitrites/ nitrates are substituted by those of carbonates above 350 °C, indicating the completion of the reduction. The reduction of adsorbed NOx released the active K sites, on which the carbonyl species would be accumulated from the continued CO spillover. Correspondingly, the band at 2160 cm−1 became significant at higher temperatures (above 350 °C in Figure 7). The above results unambiguously indicate that the reduction of adsorbed NOx involves the CO spillover. Under the Pd−K interaction, the adsorbed CO on Pd becomes transferrable and easily spills over onto K sites to form the carbonyl species, which further reduces the preadsorbed NOx into N2 and N2O. This reaction route is an entirety in which Pd activates gaseous CO. Because the CO spillover is not limited by the sites of the intimate contact between Pd and K, CO can be transferred to the K sites with a certain distance from the Pd sites one by one. Therefore, all adsorbed NOx can be reduced by CO before desorption, as observed in Figure 7. Like the process of “pumping” CO from the atmosphere to “irrigate the field” of the nitrates/nitrites, the adsorbed NOx not only at K sites adjacent to Pd but also at the remote K sites can be reduced into N2 and N2O effectively. In the future, further work may be needed to confirm the proposed mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S3 and Figures S1−S8 as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: + 86 531 89736032; fax: + 86 531 89736032. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21077043, 21007019, and 21107030), the Development Program of the Science and Technology of Shandong Province (2011GSF11702), and State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (KF2009-13). 9618

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