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Effects of Cu oxidation states on the catalysis of NO+CO and N2O+CO reactions
Journal of Physics and Chemistry of Solids 125 (2019) 64–73
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Effects of Cu oxidation states on the catalysis of NO+CO and N2O+CO reactions
T
Hirone Iwamotoa,b, Satoshi Kameokaa,∗, Ya Xuc, Chikashi Nishimurac, An Pang Tsaia a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan c National Institute of Materials Science, Tsukuba, 305-0047, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu Cu2O CuO NO + CO reaction N2O + CO reaction
The relationship between the oxidation state of bulk Cu and catalytic activity during the NO + CO and N2O + CO reactions were investigated, using commercially available Cu, Cu2O and CuO powders. Metallic Cu powder was found to catalyze both reactions in a stable manner without changing its oxidation state. At elevated temperatures, the catalytic activities of the Cu2O and CuO increased with time, in conjunction with reduction by CO to metallic Cu in both cases. The data confirm that metallic Cu exhibits much high activity and stability than either Cu2O or CuO for both the NO + CO and N2O + CO reactions. In addition, Cu2O species (denoted as Cu2O*) formed via oxidation of metallic Cu species obtained by reduction of CuO with CO at low temperature (< 300 °C) showed catalytic activity during both reactions as a result of the high surface area and structural disorder of this material.
1. Introduction NOx emissions from automobile engines continue to be an important environmental concern, and the reaction of these oxides with CO over a catalyst is an effective means of eliminating such emissions. At present, precious metals such as Rh, Pt and Pd are widely used for this purpose [1–4], although this is costly and it would be beneficial to identify less expensive alternatives [1,5]. Many studies have focused on the addition of base elements to the catalyst with the aim of reducing the amount of precious metal required without affecting the catalytic performance. However, this approach still requires some amount of one or more precious metals [6,7]. Cu and Cu oxides are promising materials for NOx reduction and hence their catalytic activities during the NO + CO reaction have been studied extensively, typically in the form of supported catalysts [8–21]. In previous reports, there has been significant discussion regarding the active oxidation state during the NO + CO reaction. As an example, London et al. [13] investigated the reaction mechanism associated with CuO/SiO2 using infrared spectroscopy and proposed that a partially reduced surface having both Cu0 and Cu+ sites effectively promoted the NO + CO reaction. In a recent study, Jiang et al. [15,16] confirmed that the activity of CuO/TiO2 was enhanced by pretreatment in a reducing atmosphere, and that Cu2+ species were reduced by gaseous CO throughout the reaction at high temperatures. Similar phenomena have
∗
been observed in the case of supported CuO catalysts on different materials [17,18]. Therefore, these reports commonly conclude that a material with a greater concentration of reduced Cu species will exhibit higher activity and stability. However, these prior investigations focused primarily on the oxidation states at the outermost surfaces of supported particles that are typically nano-sized. Even so, because the catalytic behavior of Cu particles is greatly affected by the support material, the effect of the substrate should also be assessed, so it is difficult to compare experimental data for the NO + CO reaction over supported catalysts, or to assess the contribution of Cu or/and Cu oxides. Recently, our own group found that Cu powder exhibits high catalytic activity for the NO + CO reaction even at low surface areas (< 1 m2/g) [22]. Such experiments using the bulk catalyst is one of the best approaches to studying the essential reactivity of metals or their oxides. As an example, Jernigan et al. [23] investigated the relationship between Cu oxidation states and the CO oxidation reaction on thin film samples and reported that metallic Cu shows higher catalytic activity compared with Cu2O and CuO. From these results, it is evident that metallic Cu is the active species for CO oxidation in this scenario. Zou et al. [24] studied the effect of the crystal plane on the catalytic activity of Cu2O nanocrystals with different structures, while Auxilia et al. [25] investigated the dependence of NO + CO activity at low temperatures (< 300 °C) on CuO facets using nanoplate samples. However, to date,
Corresponding author. E-mail address: [email protected] (S. Kameoka).
https://doi.org/10.1016/j.jpcs.2018.10.013 Received 21 May 2018; Received in revised form 17 September 2018; Accepted 7 October 2018 Available online 08 October 2018 0022-3697/ © 2018 Elsevier Ltd. All rights reserved.
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each trial, a 5 mm-high portion of catalyst was transferred into a quartz tube having an inner diameter of 4 mm. Each sample was pretreated under a 50 ml/min flow of 100% H2 (for Cu) or 100% He (for Cu2O and CuO) at 400 °C for 1 h. The reaction gases were 0.5 vol% NO + 0.5 vol % CO in He or 0.5 vol% N2O + 0.5 vol% CO in He and were introduced to the reactor at a flow rate of 50 ml/min. The gaseous products exiting the reactor were monitored by gas chromatography (GC) with a TCD (Shimadzu, GC-14B). The reaction temperature range was 100–600 °C and each trial was monitored for at least 15 min after achieving the desired reaction temperature. The catalytic activities during the NO + CO reaction were evaluated based on the NO and CO conversions. During NO reduction by CO, the two reaction represented below as Eqs. (1) and (2) should be considered. For instance, the reaction follows a consecutive reaction pathway wherein NO is first reduced to N2O, which is then further reduced to N2.
there have been no studies of the NO + CO reaction over bulk Cu or Cu oxides that have compared activities. This is unfortunate, because elucidating the role of metallic Cu and/or Cu oxides in the NO + CO reaction could lead to the design of Cu alloys to replace precious metals. From a fundamental viewpoint, the present study is devoted to an investigation of the effects of oxidation states of copper and their transformations on the NO + CO and N2O + CO reactions, using commercial Cu, Cu2O or CuO powders. The activity of Raney Cu was also examined for comparison purposes, to gain a better understanding of the role of surface area. 2. Experimental 2.1. Catalyst samples This work focused on the catalytic properties of Cu and Cu oxides, so highly pure Cu (TOYOTA), Cu2O (Wako, 039–04375), and CuO (Wako, 038–04345) powders were directly used as catalysts. A Raney Cu catalyst having a high surface area was also prepared. An Al2Cu alloy ingot was produced as a precursor, by melting a mixture of 99.9 wt% Al and 99.9 wt% Cu in an argon atmosphere using an arc furnace, following by annealing the as-cast ingot for 72 h at 500 °C under argon. The annealed ingot was crushed to obtain particle sizes in the range of 38–63 μm and dispersed in an aqueous 10 wt% NaOH solution for 24 h at room temperature to remove Al. The leached powder was removed by filtration, washed with distilled water until the filtrate was no longer alkaline, and dried at 40 °C for 72 h. The compositions of the resulting materials and of the precursor alloy were determined by inductively coupled plasma (ICP) analysis. After the leaching treatment, the majority of the Al (97.5%) was removed, although a small amount remained. The resulting specimens exhibited high surface areas as a result of their porous structures (Table 1).
Cu powder Cu2O powder CuO powder Raney Cu
0.30 0.13 0.13 0.11
0.0875 3.04 17.7 5.72
0.0716 0.881 4.90 –
(4)
3. Results 3.1. Reduction of Cu oxides by CO Fig. 1 shows the result of CO-TPR using Cu2O and CuO powders, from which it is evident that only a single peak appeared in each reduction process, indicating that the reduction occurred in one step. Both Cu oxides transitioned to metallic Cu as a result of the reduction, presumably by the following reactions.
Specific surface area (m2/g)
0.102 0.836 0.705 9.94
(3)
Thus, the generation of N2 as shown in Eq. (2) more likely proceeds via the process in Eq. (4) as an intermediate reaction. Therefore, it is important to consider the N2O + CO reaction to fully understand the NO + CO reaction. Thus, the catalytic activities in the present work were evaluated based on both N2O and CO conversions. Variations in conversion over time at specific temperatures were also monitored, and so conditions were chosen under which the conversion was not overly rapid so as to allow for changes in activity to be observed. Therefore, the most appropriate temperature was determined for each catalyst. The catalytic performance for the isothermal stability of Cu, Cu2O and CuO for NO + CO and N2O + CO reaction were performed at 300, 500 and 200 °C, respectively. In each trial, the specimen was heated from ambient to the desired reaction temperature under the reaction atmosphere.
Table 1 Catalyst masses employed and specific surface areas before and after reaction trials.
After N2O + CO
[N2] (mol. ) × 100 [N2 O] (mol. ) + [N2] (mol. )
N2O + CO → N2 + CO2
The reaction tests were carried out in a fixed bed flow reactor. In
After NO + CO
(2)
The N2O produced will also react with CO, as in Eq. (4).
2.3. Reaction analysis
As prepared
2 NO + 2 CO → N2 + 2 CO2
[N2 sel (%)] =
X-ray diffraction (XRD, Rigaku Ulitima IV) with Cu-Kα radiation (40 kV–40 mA) was used to assess each sample before and after the catalytic reaction for structural analysis and phase identification. In situ XRD analysis of the CuO powder was also performed (PANalytical Empyrean). In preparation for analysis, the sample was transferred into a silicone container and held at room temperature for 30 min under vacuum, then heated at 200 °C under a 10 vol% CO in He atmosphere at 1 atm pressure. The Brunauer–Emmett–Teller (BET) specific surface areas of the catalysts provided in Table 1 were determined using a surface area analyzer (BEL Japan, Belsorp-max) in conjunction with Kr adsorption. CO temperature programmed reduction (TPR) was performed using 1 vol% CO in He as the reducing gas at a flow rate of 30 ml/min and a heating rate of 5 °C/min. The consumption of CO was monitored with a thermal conductivity detector (TCD).
Mass of catalyst (g)
(1)
Because the N2O produced by Eq. (1) is a greenhouse gas, the N2 selectivity of the catalyst is also important. This selectivity is calculated based on the relative amounts of N2 and N2O produced as shown in Eq. (3).
2.2. Characterization
Catalyst
2 NO + CO → N2O + CO2
Cu2O + CO → 2 Cu + CO2
(5)
CuO + CO → Cu + CO2
(6)
Fig. 2 presents the in situ XRD pattern acquired from CuO following heating at 200 °C under a 10 vol% CO/He flow. The original powder exhibited a single CuO phase but, after 5 min at 200 °C, Cu (111) and (200) peaks appeared and their relative intensities increased over time. Since no Cu2O peaks were observed during the reduction process, the CuO is thought to have been reduced directly to Cu without passing through the intermediate Cu2O under a CO atmosphere at 200 °C, via the reaction in Eq. (7). These results are good agreement with previous 65
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Fig. 1. Data from the temperature programmed reduction of CuO and Cu2O under a flow of 1 vol% CO in He.
Fig. 2. In situ XRD patterns generated by CuO under a 10 vol% CO in He atmosphere at (i) room temperature and after heating at 200 °C for (ii) 5, (iii) 30, (iv) 60, and (v) 90 min.
reports [26,27]. 2 CuO + CO → Cu2O + CO2
(7) Fig. 3. (a) NO and (b) CO conversions and (c) N2 selectivity during the NO + CO reaction over Cu, Cu2O, and CuO as functions of temperature.
3.2. NO + CO reaction metallic Cu diffraction peaks during XRD analysis (data not shown), indicating that the oxides were reduced.
3.2.1. Activity and selectivity Fig. 3 summarizes the catalytic activities as indicated by the NO and CO conversions during the NO + CO reaction over the Cu, Cu2O, and CuO powders. It is clear that a much higher NO conversion was obtained from the Cu and CuO as compared to the Cu2O over the entire temperature range. The NO conversion reached 100% at 400 °C and 500 °C in the case of the Cu and CuO, respectively, while the NO conversion by the Cu2O was essentially zero up to 450 °C. Almost no hysteresis in the activity or selectivity data was apparent between heating and cooling when using the Cu powder, whereas the activity and selectivity on cooling were significantly different from those during heating for the Cu2O and CuO. That is, only the Cu exhibited stable catalytic properties, as indicated by high activity and selectivity. Nevertheless, none of the three materials showed 100% selectivity for N2 and each generated a small amount of N2O under all conditions. Following these trials, each of the three catalysts generated primarily
3.2.2. Isothermal stability of catalytic properties Isothermal conversion and N2 selectivity data were acquired at various temperatures for the three catalysts to obtain additional insights into the origin of their catalytic properties. Fig. 4 plots catalytic activity as a function of time on-stream for the Cu powder at 300 °C. Although the conversion and selectivity both decreased slightly with time on-stream, both remained relatively high (NO conversion: 88%–85%) at the 300 min mark. It should be noted that the NO and CO conversions were nearly equal, indicating that the reaction proceeded in a stoichiometric manner, as represented by Eq. (2). There were also no significant changes in the oxidation state, as demonstrated by the XRD patterns in Fig. 4 (c). This result implies that the catalytic reaction was promoted solely by metallic Cu. 66
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Fig. 4. Results from the NO + CO reaction over Cu at 300 °C for 288 min: (a) NO and CO conversions, (b) N2 selectivity, and (c) the XRD patterns generated by the catalyst (i) before and (ii) after the reaction.
Fig. 5. Results from the NO + CO reaction over Cu2O at 500 °C for 499 min: (a) NO and CO conversions, (b) N2 selectivity, and (c) the XRD patterns generated by the catalyst (i) before and (ii) after the reaction.
67
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Fig. 6. Results from the NO + CO reaction over CuO at 200 °C for 1106 min: (a) NO and CO conversions, (b) N2 selectivity, and (c) the XRD patterns generated by the catalyst (i) before the reaction, (ii) after the reaction for 350 min and (iii) after the reaction for 1106 min.
strong metallic Cu peak.
Data for the same reaction but using Cu2O at 500 °C are shown in Fig. 5. Unlike the Cu results, the conversions were initially very low and increased over time (Fig. 5(a)). In contrast, the N2 selectivity was relatively constant (Fig. 5(b)). Following a 500 min reaction, the NO conversion was 82% higher than the initial value, while the N2 selectivity only increased from 70% to 85%. It is also of interest that the CO conversion values were slightly higher than the NO conversions over the entire temperature range. Following the reaction, the color of the catalyst layer had clearly changed, and the XRD pattern in Fig. 5(c) demonstrates that the original single Cu2O phase was transitioned to metallic Cu during the reaction. In fact, this catalyst took on an obvious brown coloration following the reaction, meaning that much of the original Cu2O was reduced. Fig. 6 plots the results obtained during the NO + CO reaction over CuO at 200 °C. In the initial 200 min, both the NO and CO conversions increased rapidly while the N2 level was initially less than 20% but gradually increased over time. It is important to note that the NO conversions were higher than the CO conversions over the entire time span. After a 350 min reaction, a large amount of the original CuO looked to be reduced to Cu2O, as demonstrated by the decrease of the diffraction peaks of CuO and the appearance of the diffraction peaks of Cu2O as shown in Fig. 6(c-ii). In the early stage of the reaction, the NO and CO conversions plateaued at values close to 100%, while the N2 selectivity only reached 51% and was not steady. As the reaction was prolonged to 1106 min, the CO conversion and N2 selectivity both increased. Fig. 6(c-iii) shows that diffraction peaks due to metallic Cu appeared during this time frame. Fig. 7 shows the results of the same test performed at 250 °C for 197 min. In contrast to the reaction at 200 °C, the CO and NO conversions were much higher in the beginning (up to 100%) while the N2 selectivity was almost constant at 95% toward the end of the reaction. One obvious difference between the XRD patterns (Fig. 7(c)) of the catalysts following trials at 200 and 250 °C is that the latter contains a
3.3. The N2O + CO reaction Fig. 8 presents catalytic activities measured during the N2O + CO reaction over the Cu, Cu2O, and CuO powders. In the case of the Cu, the N2O conversion was above 90% even at 200 °C and reached 100% at 400 °C, although considerable performance degradation was observed during the cooling process. The N2O conversion over the Cu2O was much lower but exhibited an upward hysteresis that was very similar to the results obtained during the NO + CO reaction over Cu2O. The catalytic properties of the CuO were more complex; between 350 and 500 °C, the N2O conversion reached a maximum at approximately 300 °C. It should also be noted that the N2O conversion trends did not match the CO conversion patterns. The results obtained with Cu at 300 °C at various times are summarized in Fig. 9(a). Both the N2O and CO conversions decreased slightly over time but remained above 80% for the duration of the reaction. The XRD patterns contained only metallic Cu peaks even after the reaction, as shown in Fig. 10(a). The results for Cu2O at 500 °C are shown in Fig. 9(b), which demonstrated that both the N2O and CO conversions rapidly increased with time on-stream and that the CO conversion was higher than the N2O conversion throughout. The majority of catalyst was reduced to Cu during the reaction, as shown in Fig. 10(b). Fig. 9(c) shows the result from the N2O + CO reaction over CuO at 200 °C. Both the N2O and CO conversions rapidly increased with time while the CO conversion remained higher than the N2O conversion over the entire time range. A portion of the catalyst was reduced to both Cu2O and Cu following the reaction, as can be seen from Fig. 10(c), which is very different from the outcome following the NO + CO reaction over the same period of time, following which only Cu2O was observed. 68
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Fig. 7. Results from the NO + CO reaction over CuO at 250 °C for 197 min: (a) NO and CO conversions, (b) N2 selectivity, and (c) the XRD patterns generated by the catalyst (i) before and (ii) after the reaction.
4. Discussion
Cu phase. Although N2O present as a by-product was observed during the NO + CO reaction over the Cu powder, the results for the larger quantity of Cu and for the Raney Cu suggest that 100% N2 selectivity can be obtained in the case that a sufficient number of active sites exist.
4.1. Cu powder As shown in Fig. 4, Cu powder held at 300 °C showed almost no change in activity and selectivity with time on-stream during the NO + CO reaction and there was no difference in the oxidation state of the Cu before and after the reaction (Fig. 4(c)). Moreover, there was no visible hysteresis in the conversion during heating and cooling and the N2 selectivity was higher than 85% at all temperatures. Therefore, it is clear that Cu powder is a stable catalyst for the NO + CO reaction via the process in Eq. (2) over a wide range of temperatures. In contrast, data from the N2O + CO reaction at the same temperatures demonstrate that the activity gradually decreased over time (Fig. 9(a)). Since the composition of the product gases deviated from the expected stoichiometric ratio (N2O: CO = 1: 1) with a slight excess of N2O, it might be possible that there are changes in the nature of active sites and/or in the reaction mechanism. The reduced activity evident during cooling in Fig. 8 could result from the same phenomenon. Nevertheless, metallic Cu definitely exhibited outstanding activity, as indicated by a conversion as high as 91% even at 200 °C. With regard to the reaction in Eq. (2) (as a product of reactions (1) and (4)), the high N2O + CO activity observed also demonstrates excellent N2 selectivity during the NO + CO reaction over this catalyst. The selectivity for N2 and N2O was assessed by performing the NO + CO reaction over a Raney Cu catalyst or a larger quantity of Cu powder. Fig. 11 shows the catalytic activities obtained from the Raney Cu and standard Cu, using a 10 mm high catalyst layer (0.6 g). For both samples, the NO conversion reached 100% at a lower temperature than was required for the Cu powder under standard conditions. Moreover, a N2 selectivity of 100% was achieved at a temperature higher than 300 °C for the smaller amount of Cu powder and at 250 °C for Raney Cu. As shown in Fig. 11(c), the XRD patterns of the Raney Cu before and after the catalytic reaction only contain peaks resulting from a metallic
4.2. Cu2O powder Fig. 5 demonstrates that the catalytic activity of the Cu2O gradually increased with time on-stream. As shown in Fig. 5(c-ii), metallic Cu was found after the NO + CO reaction over the Cu2O, so it is reasonable to assert that the activity of this material originated from metallic Cu rather than the Cu2O. The higher CO conversion relative to NO conversion over the entire time on-stream (as in Fig. 3(a) and (b)) can be understood as follows. If we assume only the two reactions given in Eqs. (1) and (2) occur, the excess CO conversion observed in Fig. 8(a) could be consumed by the reduction of Cu2O. Consequently, the amount of Cu2O should decrease with time on-stream such that, towards the end of the reaction time, the CO and NO conversions would be expected to converge. The hysteresis in the NO conversion on cooling in Fig. 3(a) was higher than that on heating, and this can be explained by the increased amount of Cu above 500 °C, as confirmed by the CO-TPR results in Fig. 1. The trends in the activity exhibited by the Cu2O during the N2O + CO reaction over Cu2O in Figs. 8 and 9(b) are similar to those resulting from the NO + CO reaction. Metallic Cu was obtained following the reaction summarized in Fig. 10 (b-ii), which provides evidence that the N2O + CO reaction was also catalyzed by metallic Cu. In conclusion, the Cu2O powder itself did not catalyze the NO + CO and N2O + CO reactions but rather was first reduced to Cu by CO. 4.3. CuO powder The data indicate that the NO + CO reaction mechanism over the CuO powder was much more complicated than that over the Cu or Cu2O because reduction of CuO itself takes place during the NO + CO 69
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atmosphere make it difficult to conclude that Cu2O was formed through the direct reduction of CuO as in Eq. (7). Therefore, a possible path to the formation of Cu2O is the instantaneous oxidation of Cu by NO as in Eq. (8), following the reduction of CuO to Cu as in Eq. (6). 2 Cu + 2 NO → Cu2O + N2O
(8)
4 Cu + 2 NO → 2 Cu2O + N2
(8a) 0
Balkenende et Al. [33] confirmed the rapid oxidation of Cu to Cu+ as a result of NO exposure. Eqs. (6) and (8) (or (8a)) can be expressed in a different form without Cu, as in Eq. (9) (or (9a)). 2 CuO +2 NO + 2 CO → Cu2O + N2O + 2 CO2
(9)
4 CuO + 2 NO + 4 CO → 2 Cu2O + N2 + 4 CO2
(9a)
In order to verify the redox behaviors of CuO, we performed several pretreatments for the CuO powder: (a) the pretreatment under 0.5 vol% CO/He flow (50 ml/min) at 300 °C for 2 h, (b) (a) followed by the oxidation treatment under 0.5 vol% NO/He flow (50 ml/min) at 250 °C for 2 h, (c) (a) followed by the oxidation treatment under 0.5 vol% N2O/He flow (50 ml/min) at 250 °C for 2 h and (d) (b) followed by the NO + CO reaction under the same condition in Fig. 3. S-Fig. 1 and SFig. 2 show the XRD pattern for CuO after each pretreatment and the NO and CO conversions during the NO + CO reaction over the each redox pretreated sample, respectively. As shown in S-Fig. 1, only diffraction peaks from metallic Cu were observed after the CO treatment at 300 °C, indicating that reduction of CuO completely proceeds. Interestingly, the metallic Cu obtained by the reduction of CuO with CO (hereafter denoted by Cu*) exhibited significantly high catalytic performance for NO + CO reaction in comparison with the conventional Cu powder (S-Fig. 2). Subsequently, the Cu* was readily oxidized to Cu2O (hereafter denoted by Cu2O*) even at lower temperatures (e.g., 250 °C) than the conventional Cu powder. It should be noted that the oxidation of conventional Cu powder with NO or N2O proceed above 400 °C (see S-Fig. 3). We performed the same catalytic experiment of the NO + CO reaction for the Cu2O* sample (after pretreatment (b)). As shown in S-Fig. 2, there was a hysteresis between heating and cooling process in the conversion curves. Although the activity of Cu2O* sample on the heating process was lower than that of Cu* sample, the activity of Cu2O* sample on the cooling process was almost the same as Cu* sample. These results suggest that Cu2O* is readily reduced to Cu* during NO + CO reaction and Cu* works as catalytic active species. According to CO-TPR measurements (see S-Fig. 4), Cu2O* is readily reduced to Cu* below 250 °C. As shown in Fig. 3, bulk Cu2O powder hardly exhibited catalytic activity for NO + CO reaction below 400 °C although Cu2O* showed the high catalytic activity even at 200 °C. The XRD peaks of Cu2O* are much broader than those of conventional Cu2O (see Fig. 6 (c-ii)). The
Fig. 8. (a) N2O conversions and (b) CO conversions during the N2O + CO reaction over Cu, Cu2O and CuO as functions of temperature.
reaction [28–30]. Rodriguez et al. have reported the dependence of the reduction pathway for CuO bulk powders on the flow rate of reducing agent (H2 or CO) [26,31,32]. A direct transformation pathway for CuO reduction (CuO → Cu) when there was a large supply of CO, while they showed a sequential step pathway involving one intermediate (CuO → Cu2O → Cu) with a limited supply of CO. At present stage, we cannot decide strictly about pathway for the CuO reduction because the redox process of CuO species drastically changes in the composition of NO/CO and the temperature under the transient reaction conditions. The COTPR and in situ XRD results acquired following exposure to a CO
Fig. 9. N2O and CO conversions over time using (a) Cu at 300 °C for 330 min, (b) Cu2O at 500 °C for 491 min, and (c) CuO at 200 °C for 364 min. 70
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Fig. 10. XRD patterns generated by (a) Cu, (b) Cu2O, and (c) CuO (i) before and (ii) after the N2O + CO reaction at constant temperature.
Fig. 11. (a) NO conversions, (b) CO conversions, and (c) N2 selectivity during the NO + CO reaction over Cu powder (0.30 g or 0.60 g) and Raney Cu as functions of time. (c) The XRD patterns of Raney Cu (i) before and (ii) after the reaction. 71
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peak broadening of XRD indicates that high density of defects (such as vacancies), disorder and lattice strains existed in Cu2O*, which are supposed act active sites. Moreover, the BET surface area after NO + CO reaction of Cu2O* (17.7 m2/g) is much higher than that of conventional Cu2O (3.0 m2/g), indicating that the considerable surface roughness and defects induced by the reduction process on Cu2O*. Consequently, Cu2O* plays an important role for a high catalytic performance even at lower temperatures. The subsequent reduction of CuO can be expected. As the amount of Cu2O* in the system increased, complete oxidation of Cu* to Cu2O* becomes difficult because the majority of the NO will be converted via catalysis over the Cu2O* and sufficient NO cannot be supplied for the oxidation of Cu. In Fig. 6(a) and (b), the CuO was maintained for a far longer time span (1106 min) at 200 °C, while the N2 selectivity continuously increased to a maximum of 88%. Simultaneously, the CO conversion also slowly approached 100%. In the XRD pattern acquired after the reaction (Fig. 6(iii)), broad Cu2O peaks similar to those observed in Fig. 6(ii) were present. However, metallic Cu was not observed at shorter time on-stream values, such as 350 min. Fig. 6(b) demonstrates a further increase in the N2 selectivity past the 350 min mark. These results can be explained by the continuous formation of Cu2O* and Cu* via Eqs. (6) and (8) (or (8′)). During the reaction at 250 °C (Fig. 7), both the NO and CO conversions reached 100% at 141 min and the catalyst consisted largely of metallic Cu, as shown in Fig. 7(c). At 250 °C, the reduction of the CuO via the process in Eq. (6) was faster than the oxidation of Cu via Eq. (8) (or (8′)), as a result of catalysis by the Cu and the unstable, partially oxidized Cu2O*. In the case of the N2O + CO reaction at 200 °C, the CO conversion was higher than the N2O conversion, and this is attributed to the reduction of CuO. The XRD pattern obtained after the reaction (Fig. 10(cii)) showed the presence of both Cu and Cu2O* while only Cu2O* was present following the NO + CO reaction using almost the same holding time and reaction temperature. The reduction of CuO by CO deeply occurs under N2O + CO, whereas it is suppressed under NO + CO due to the NO adsorbs strongly on Cu. Consequently, Cu was formed via Eq. (6) and partially oxidized according to Eq. (10). Based on the above discussion concerning the NO + CO reaction, the N2O + CO reaction might be catalyzed by Cu obtained by reduction of Cu2O* with CO. 2 Cu + N2O → Cu2O + N2
Fig. 12. Scheme summarizing the relationship between the bulk oxidation states and catalytic reactions in the NO + CO and N2O + CO systems. Cu* and Cu2O* denote activated species via reduction of CuO with CO and oxidation of Cu* with NO or N2O, respectively.
N2 selectivity greatly depends on the number of active sites in the system. Similarly, for the N2O + CO reaction, metallic Cu clearly exhibits high catalytic performance while Cu2O powder must be reduced to activate the reaction. In the case of the CuO, it is possible that Cu2O* also acted as a catalyst, because improved N2 selectivity was obtained during the NO + CO reaction as the quantity of Cu2O* was increased. 5. Conclusion We investigated the relationships between oxidation state and catalytic activity of bulk Cu, CuO and Cu2O during the NO + CO and N2O + CO reactions. Under a NO + CO atmosphere, metallic Cu was the most stable catalyst and the reaction proceeded while generating primarily N2. In contrast, the Cu2O and CuO were eventually reduced to Cu using a flow with a NO: CO = 1: 1 composition, and this Cu promoted the conversion of NO to N2. Similarly, in the case of the N2O + CO reaction, only Cu promoted the reduction of N2O and no change was observed in the oxidation state of the metal, while the Cu2O and CuO were not capable of catalysis during these two reactions until they were reduced to Cu. However, Cu2O* formed via the reduction of CuO did catalyze both reactions to some extent, likely due to the high surface area and structural disorder of this compound. Finally, it can be concluded that the most active and stable bulk catalyst for the NO + CO reaction and related processes is metallic Cu rather than Cu oxides.
(10)
4.4. Transformations of copper species in the NO + CO and N2O + CO reactions Fig. 12 presents a scheme summarizing proposed mechanisms for the NO + CO and N2O + CO reactions over Cu, Cu2O and CuO. The NO + CO reaction is activated by Cu and only small amounts of the Cu are oxidized by the NO and N2O. Thus, the Cu powder is a stable catalyst for the NO + CO reaction. According to the reaction mechanism proposed by London et al. [12], partial oxidation of the Cu surface can be assumed because the formation of N2O is observed in the presence of both Cu0 and Cu+. However, the Cu2O powder having a relatively low surface area showed low activity until it was reduced to Cu. This result confirms that metallic Cu was much effective than Cu2O for the NO + CO reaction. Cu2O* formed via oxidation of metallic Cu species obtained by reduction of CuO with CO at low temperature (< 300 °C) shows high catalytic activity during both reactions as a result of the high surface area and structural disorder of this material. No catalytic reaction was observed over the CuO powder, which immediately transformed into Cu2O* or Cu in the reaction atmosphere. In the case of the oxide powders, the N2 selectivity increased as Cu or Cu2O* was formed. The amount and surface area of the sample evidently both significantly affect the N2 selectivity, as shown in Fig. 11. Therefore, it appears that either Eq. (1) or (2) occurs on the catalysts, such that the
Acknowledgements This work was supported in part by Grant-in-Aid for Scientific Research ((A) 15H02299, (B) 18H01783) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2018.10.013. References [1] M. Iwamoto, H. Hamada, Removal of nitrogen monoxide from exhaust gases
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of Cu oxidation states on the catalysis of NO+CO and N2O+CO reactions
T
Hirone Iwamotoa,b, Satoshi Kameokaa,∗, Ya Xuc, Chikashi Nishimurac, An Pang Tsaia a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan c National Institute of Materials Science, Tsukuba, 305-0047, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu Cu2O CuO NO + CO reaction N2O + CO reaction
The relationship between the oxidation state of bulk Cu and catalytic activity during the NO + CO and N2O + CO reactions were investigated, using commercially available Cu, Cu2O and CuO powders. Metallic Cu powder was found to catalyze both reactions in a stable manner without changing its oxidation state. At elevated temperatures, the catalytic activities of the Cu2O and CuO increased with time, in conjunction with reduction by CO to metallic Cu in both cases. The data confirm that metallic Cu exhibits much high activity and stability than either Cu2O or CuO for both the NO + CO and N2O + CO reactions. In addition, Cu2O species (denoted as Cu2O*) formed via oxidation of metallic Cu species obtained by reduction of CuO with CO at low temperature (< 300 °C) showed catalytic activity during both reactions as a result of the high surface area and structural disorder of this material.
1. Introduction NOx emissions from automobile engines continue to be an important environmental concern, and the reaction of these oxides with CO over a catalyst is an effective means of eliminating such emissions. At present, precious metals such as Rh, Pt and Pd are widely used for this purpose [1–4], although this is costly and it would be beneficial to identify less expensive alternatives [1,5]. Many studies have focused on the addition of base elements to the catalyst with the aim of reducing the amount of precious metal required without affecting the catalytic performance. However, this approach still requires some amount of one or more precious metals [6,7]. Cu and Cu oxides are promising materials for NOx reduction and hence their catalytic activities during the NO + CO reaction have been studied extensively, typically in the form of supported catalysts [8–21]. In previous reports, there has been significant discussion regarding the active oxidation state during the NO + CO reaction. As an example, London et al. [13] investigated the reaction mechanism associated with CuO/SiO2 using infrared spectroscopy and proposed that a partially reduced surface having both Cu0 and Cu+ sites effectively promoted the NO + CO reaction. In a recent study, Jiang et al. [15,16] confirmed that the activity of CuO/TiO2 was enhanced by pretreatment in a reducing atmosphere, and that Cu2+ species were reduced by gaseous CO throughout the reaction at high temperatures. Similar phenomena have
∗
been observed in the case of supported CuO catalysts on different materials [17,18]. Therefore, these reports commonly conclude that a material with a greater concentration of reduced Cu species will exhibit higher activity and stability. However, these prior investigations focused primarily on the oxidation states at the outermost surfaces of supported particles that are typically nano-sized. Even so, because the catalytic behavior of Cu particles is greatly affected by the support material, the effect of the substrate should also be assessed, so it is difficult to compare experimental data for the NO + CO reaction over supported catalysts, or to assess the contribution of Cu or/and Cu oxides. Recently, our own group found that Cu powder exhibits high catalytic activity for the NO + CO reaction even at low surface areas (< 1 m2/g) [22]. Such experiments using the bulk catalyst is one of the best approaches to studying the essential reactivity of metals or their oxides. As an example, Jernigan et al. [23] investigated the relationship between Cu oxidation states and the CO oxidation reaction on thin film samples and reported that metallic Cu shows higher catalytic activity compared with Cu2O and CuO. From these results, it is evident that metallic Cu is the active species for CO oxidation in this scenario. Zou et al. [24] studied the effect of the crystal plane on the catalytic activity of Cu2O nanocrystals with different structures, while Auxilia et al. [25] investigated the dependence of NO + CO activity at low temperatures (< 300 °C) on CuO facets using nanoplate samples. However, to date,
Corresponding author. E-mail address: [email protected] (S. Kameoka).
https://doi.org/10.1016/j.jpcs.2018.10.013 Received 21 May 2018; Received in revised form 17 September 2018; Accepted 7 October 2018 Available online 08 October 2018 0022-3697/ © 2018 Elsevier Ltd. All rights reserved.
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each trial, a 5 mm-high portion of catalyst was transferred into a quartz tube having an inner diameter of 4 mm. Each sample was pretreated under a 50 ml/min flow of 100% H2 (for Cu) or 100% He (for Cu2O and CuO) at 400 °C for 1 h. The reaction gases were 0.5 vol% NO + 0.5 vol % CO in He or 0.5 vol% N2O + 0.5 vol% CO in He and were introduced to the reactor at a flow rate of 50 ml/min. The gaseous products exiting the reactor were monitored by gas chromatography (GC) with a TCD (Shimadzu, GC-14B). The reaction temperature range was 100–600 °C and each trial was monitored for at least 15 min after achieving the desired reaction temperature. The catalytic activities during the NO + CO reaction were evaluated based on the NO and CO conversions. During NO reduction by CO, the two reaction represented below as Eqs. (1) and (2) should be considered. For instance, the reaction follows a consecutive reaction pathway wherein NO is first reduced to N2O, which is then further reduced to N2.
there have been no studies of the NO + CO reaction over bulk Cu or Cu oxides that have compared activities. This is unfortunate, because elucidating the role of metallic Cu and/or Cu oxides in the NO + CO reaction could lead to the design of Cu alloys to replace precious metals. From a fundamental viewpoint, the present study is devoted to an investigation of the effects of oxidation states of copper and their transformations on the NO + CO and N2O + CO reactions, using commercial Cu, Cu2O or CuO powders. The activity of Raney Cu was also examined for comparison purposes, to gain a better understanding of the role of surface area. 2. Experimental 2.1. Catalyst samples This work focused on the catalytic properties of Cu and Cu oxides, so highly pure Cu (TOYOTA), Cu2O (Wako, 039–04375), and CuO (Wako, 038–04345) powders were directly used as catalysts. A Raney Cu catalyst having a high surface area was also prepared. An Al2Cu alloy ingot was produced as a precursor, by melting a mixture of 99.9 wt% Al and 99.9 wt% Cu in an argon atmosphere using an arc furnace, following by annealing the as-cast ingot for 72 h at 500 °C under argon. The annealed ingot was crushed to obtain particle sizes in the range of 38–63 μm and dispersed in an aqueous 10 wt% NaOH solution for 24 h at room temperature to remove Al. The leached powder was removed by filtration, washed with distilled water until the filtrate was no longer alkaline, and dried at 40 °C for 72 h. The compositions of the resulting materials and of the precursor alloy were determined by inductively coupled plasma (ICP) analysis. After the leaching treatment, the majority of the Al (97.5%) was removed, although a small amount remained. The resulting specimens exhibited high surface areas as a result of their porous structures (Table 1).
Cu powder Cu2O powder CuO powder Raney Cu
0.30 0.13 0.13 0.11
0.0875 3.04 17.7 5.72
0.0716 0.881 4.90 –
(4)
3. Results 3.1. Reduction of Cu oxides by CO Fig. 1 shows the result of CO-TPR using Cu2O and CuO powders, from which it is evident that only a single peak appeared in each reduction process, indicating that the reduction occurred in one step. Both Cu oxides transitioned to metallic Cu as a result of the reduction, presumably by the following reactions.
Specific surface area (m2/g)
0.102 0.836 0.705 9.94
(3)
Thus, the generation of N2 as shown in Eq. (2) more likely proceeds via the process in Eq. (4) as an intermediate reaction. Therefore, it is important to consider the N2O + CO reaction to fully understand the NO + CO reaction. Thus, the catalytic activities in the present work were evaluated based on both N2O and CO conversions. Variations in conversion over time at specific temperatures were also monitored, and so conditions were chosen under which the conversion was not overly rapid so as to allow for changes in activity to be observed. Therefore, the most appropriate temperature was determined for each catalyst. The catalytic performance for the isothermal stability of Cu, Cu2O and CuO for NO + CO and N2O + CO reaction were performed at 300, 500 and 200 °C, respectively. In each trial, the specimen was heated from ambient to the desired reaction temperature under the reaction atmosphere.
Table 1 Catalyst masses employed and specific surface areas before and after reaction trials.
After N2O + CO
[N2] (mol. ) × 100 [N2 O] (mol. ) + [N2] (mol. )
N2O + CO → N2 + CO2
The reaction tests were carried out in a fixed bed flow reactor. In
After NO + CO
(2)
The N2O produced will also react with CO, as in Eq. (4).
2.3. Reaction analysis
As prepared
2 NO + 2 CO → N2 + 2 CO2
[N2 sel (%)] =
X-ray diffraction (XRD, Rigaku Ulitima IV) with Cu-Kα radiation (40 kV–40 mA) was used to assess each sample before and after the catalytic reaction for structural analysis and phase identification. In situ XRD analysis of the CuO powder was also performed (PANalytical Empyrean). In preparation for analysis, the sample was transferred into a silicone container and held at room temperature for 30 min under vacuum, then heated at 200 °C under a 10 vol% CO in He atmosphere at 1 atm pressure. The Brunauer–Emmett–Teller (BET) specific surface areas of the catalysts provided in Table 1 were determined using a surface area analyzer (BEL Japan, Belsorp-max) in conjunction with Kr adsorption. CO temperature programmed reduction (TPR) was performed using 1 vol% CO in He as the reducing gas at a flow rate of 30 ml/min and a heating rate of 5 °C/min. The consumption of CO was monitored with a thermal conductivity detector (TCD).
Mass of catalyst (g)
(1)
Because the N2O produced by Eq. (1) is a greenhouse gas, the N2 selectivity of the catalyst is also important. This selectivity is calculated based on the relative amounts of N2 and N2O produced as shown in Eq. (3).
2.2. Characterization
Catalyst
2 NO + CO → N2O + CO2
Cu2O + CO → 2 Cu + CO2
(5)
CuO + CO → Cu + CO2
(6)
Fig. 2 presents the in situ XRD pattern acquired from CuO following heating at 200 °C under a 10 vol% CO/He flow. The original powder exhibited a single CuO phase but, after 5 min at 200 °C, Cu (111) and (200) peaks appeared and their relative intensities increased over time. Since no Cu2O peaks were observed during the reduction process, the CuO is thought to have been reduced directly to Cu without passing through the intermediate Cu2O under a CO atmosphere at 200 °C, via the reaction in Eq. (7). These results are good agreement with previous 65
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Fig. 1. Data from the temperature programmed reduction of CuO and Cu2O under a flow of 1 vol% CO in He.
Fig. 2. In situ XRD patterns generated by CuO under a 10 vol% CO in He atmosphere at (i) room temperature and after heating at 200 °C for (ii) 5, (iii) 30, (iv) 60, and (v) 90 min.
reports [26,27]. 2 CuO + CO → Cu2O + CO2
(7) Fig. 3. (a) NO and (b) CO conversions and (c) N2 selectivity during the NO + CO reaction over Cu, Cu2O, and CuO as functions of temperature.
3.2. NO + CO reaction metallic Cu diffraction peaks during XRD analysis (data not shown), indicating that the oxides were reduced.
3.2.1. Activity and selectivity Fig. 3 summarizes the catalytic activities as indicated by the NO and CO conversions during the NO + CO reaction over the Cu, Cu2O, and CuO powders. It is clear that a much higher NO conversion was obtained from the Cu and CuO as compared to the Cu2O over the entire temperature range. The NO conversion reached 100% at 400 °C and 500 °C in the case of the Cu and CuO, respectively, while the NO conversion by the Cu2O was essentially zero up to 450 °C. Almost no hysteresis in the activity or selectivity data was apparent between heating and cooling when using the Cu powder, whereas the activity and selectivity on cooling were significantly different from those during heating for the Cu2O and CuO. That is, only the Cu exhibited stable catalytic properties, as indicated by high activity and selectivity. Nevertheless, none of the three materials showed 100% selectivity for N2 and each generated a small amount of N2O under all conditions. Following these trials, each of the three catalysts generated primarily
3.2.2. Isothermal stability of catalytic properties Isothermal conversion and N2 selectivity data were acquired at various temperatures for the three catalysts to obtain additional insights into the origin of their catalytic properties. Fig. 4 plots catalytic activity as a function of time on-stream for the Cu powder at 300 °C. Although the conversion and selectivity both decreased slightly with time on-stream, both remained relatively high (NO conversion: 88%–85%) at the 300 min mark. It should be noted that the NO and CO conversions were nearly equal, indicating that the reaction proceeded in a stoichiometric manner, as represented by Eq. (2). There were also no significant changes in the oxidation state, as demonstrated by the XRD patterns in Fig. 4 (c). This result implies that the catalytic reaction was promoted solely by metallic Cu. 66
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Fig. 4. Results from the NO + CO reaction over Cu at 300 °C for 288 min: (a) NO and CO conversions, (b) N2 selectivity, and (c) the XRD patterns generated by the catalyst (i) before and (ii) after the reaction.
Fig. 5. Results from the NO + CO reaction over Cu2O at 500 °C for 499 min: (a) NO and CO conversions, (b) N2 selectivity, and (c) the XRD patterns generated by the catalyst (i) before and (ii) after the reaction.
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Fig. 6. Results from the NO + CO reaction over CuO at 200 °C for 1106 min: (a) NO and CO conversions, (b) N2 selectivity, and (c) the XRD patterns generated by the catalyst (i) before the reaction, (ii) after the reaction for 350 min and (iii) after the reaction for 1106 min.
strong metallic Cu peak.
Data for the same reaction but using Cu2O at 500 °C are shown in Fig. 5. Unlike the Cu results, the conversions were initially very low and increased over time (Fig. 5(a)). In contrast, the N2 selectivity was relatively constant (Fig. 5(b)). Following a 500 min reaction, the NO conversion was 82% higher than the initial value, while the N2 selectivity only increased from 70% to 85%. It is also of interest that the CO conversion values were slightly higher than the NO conversions over the entire temperature range. Following the reaction, the color of the catalyst layer had clearly changed, and the XRD pattern in Fig. 5(c) demonstrates that the original single Cu2O phase was transitioned to metallic Cu during the reaction. In fact, this catalyst took on an obvious brown coloration following the reaction, meaning that much of the original Cu2O was reduced. Fig. 6 plots the results obtained during the NO + CO reaction over CuO at 200 °C. In the initial 200 min, both the NO and CO conversions increased rapidly while the N2 level was initially less than 20% but gradually increased over time. It is important to note that the NO conversions were higher than the CO conversions over the entire time span. After a 350 min reaction, a large amount of the original CuO looked to be reduced to Cu2O, as demonstrated by the decrease of the diffraction peaks of CuO and the appearance of the diffraction peaks of Cu2O as shown in Fig. 6(c-ii). In the early stage of the reaction, the NO and CO conversions plateaued at values close to 100%, while the N2 selectivity only reached 51% and was not steady. As the reaction was prolonged to 1106 min, the CO conversion and N2 selectivity both increased. Fig. 6(c-iii) shows that diffraction peaks due to metallic Cu appeared during this time frame. Fig. 7 shows the results of the same test performed at 250 °C for 197 min. In contrast to the reaction at 200 °C, the CO and NO conversions were much higher in the beginning (up to 100%) while the N2 selectivity was almost constant at 95% toward the end of the reaction. One obvious difference between the XRD patterns (Fig. 7(c)) of the catalysts following trials at 200 and 250 °C is that the latter contains a
3.3. The N2O + CO reaction Fig. 8 presents catalytic activities measured during the N2O + CO reaction over the Cu, Cu2O, and CuO powders. In the case of the Cu, the N2O conversion was above 90% even at 200 °C and reached 100% at 400 °C, although considerable performance degradation was observed during the cooling process. The N2O conversion over the Cu2O was much lower but exhibited an upward hysteresis that was very similar to the results obtained during the NO + CO reaction over Cu2O. The catalytic properties of the CuO were more complex; between 350 and 500 °C, the N2O conversion reached a maximum at approximately 300 °C. It should also be noted that the N2O conversion trends did not match the CO conversion patterns. The results obtained with Cu at 300 °C at various times are summarized in Fig. 9(a). Both the N2O and CO conversions decreased slightly over time but remained above 80% for the duration of the reaction. The XRD patterns contained only metallic Cu peaks even after the reaction, as shown in Fig. 10(a). The results for Cu2O at 500 °C are shown in Fig. 9(b), which demonstrated that both the N2O and CO conversions rapidly increased with time on-stream and that the CO conversion was higher than the N2O conversion throughout. The majority of catalyst was reduced to Cu during the reaction, as shown in Fig. 10(b). Fig. 9(c) shows the result from the N2O + CO reaction over CuO at 200 °C. Both the N2O and CO conversions rapidly increased with time while the CO conversion remained higher than the N2O conversion over the entire time range. A portion of the catalyst was reduced to both Cu2O and Cu following the reaction, as can be seen from Fig. 10(c), which is very different from the outcome following the NO + CO reaction over the same period of time, following which only Cu2O was observed. 68
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Fig. 7. Results from the NO + CO reaction over CuO at 250 °C for 197 min: (a) NO and CO conversions, (b) N2 selectivity, and (c) the XRD patterns generated by the catalyst (i) before and (ii) after the reaction.
4. Discussion
Cu phase. Although N2O present as a by-product was observed during the NO + CO reaction over the Cu powder, the results for the larger quantity of Cu and for the Raney Cu suggest that 100% N2 selectivity can be obtained in the case that a sufficient number of active sites exist.
4.1. Cu powder As shown in Fig. 4, Cu powder held at 300 °C showed almost no change in activity and selectivity with time on-stream during the NO + CO reaction and there was no difference in the oxidation state of the Cu before and after the reaction (Fig. 4(c)). Moreover, there was no visible hysteresis in the conversion during heating and cooling and the N2 selectivity was higher than 85% at all temperatures. Therefore, it is clear that Cu powder is a stable catalyst for the NO + CO reaction via the process in Eq. (2) over a wide range of temperatures. In contrast, data from the N2O + CO reaction at the same temperatures demonstrate that the activity gradually decreased over time (Fig. 9(a)). Since the composition of the product gases deviated from the expected stoichiometric ratio (N2O: CO = 1: 1) with a slight excess of N2O, it might be possible that there are changes in the nature of active sites and/or in the reaction mechanism. The reduced activity evident during cooling in Fig. 8 could result from the same phenomenon. Nevertheless, metallic Cu definitely exhibited outstanding activity, as indicated by a conversion as high as 91% even at 200 °C. With regard to the reaction in Eq. (2) (as a product of reactions (1) and (4)), the high N2O + CO activity observed also demonstrates excellent N2 selectivity during the NO + CO reaction over this catalyst. The selectivity for N2 and N2O was assessed by performing the NO + CO reaction over a Raney Cu catalyst or a larger quantity of Cu powder. Fig. 11 shows the catalytic activities obtained from the Raney Cu and standard Cu, using a 10 mm high catalyst layer (0.6 g). For both samples, the NO conversion reached 100% at a lower temperature than was required for the Cu powder under standard conditions. Moreover, a N2 selectivity of 100% was achieved at a temperature higher than 300 °C for the smaller amount of Cu powder and at 250 °C for Raney Cu. As shown in Fig. 11(c), the XRD patterns of the Raney Cu before and after the catalytic reaction only contain peaks resulting from a metallic
4.2. Cu2O powder Fig. 5 demonstrates that the catalytic activity of the Cu2O gradually increased with time on-stream. As shown in Fig. 5(c-ii), metallic Cu was found after the NO + CO reaction over the Cu2O, so it is reasonable to assert that the activity of this material originated from metallic Cu rather than the Cu2O. The higher CO conversion relative to NO conversion over the entire time on-stream (as in Fig. 3(a) and (b)) can be understood as follows. If we assume only the two reactions given in Eqs. (1) and (2) occur, the excess CO conversion observed in Fig. 8(a) could be consumed by the reduction of Cu2O. Consequently, the amount of Cu2O should decrease with time on-stream such that, towards the end of the reaction time, the CO and NO conversions would be expected to converge. The hysteresis in the NO conversion on cooling in Fig. 3(a) was higher than that on heating, and this can be explained by the increased amount of Cu above 500 °C, as confirmed by the CO-TPR results in Fig. 1. The trends in the activity exhibited by the Cu2O during the N2O + CO reaction over Cu2O in Figs. 8 and 9(b) are similar to those resulting from the NO + CO reaction. Metallic Cu was obtained following the reaction summarized in Fig. 10 (b-ii), which provides evidence that the N2O + CO reaction was also catalyzed by metallic Cu. In conclusion, the Cu2O powder itself did not catalyze the NO + CO and N2O + CO reactions but rather was first reduced to Cu by CO. 4.3. CuO powder The data indicate that the NO + CO reaction mechanism over the CuO powder was much more complicated than that over the Cu or Cu2O because reduction of CuO itself takes place during the NO + CO 69
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atmosphere make it difficult to conclude that Cu2O was formed through the direct reduction of CuO as in Eq. (7). Therefore, a possible path to the formation of Cu2O is the instantaneous oxidation of Cu by NO as in Eq. (8), following the reduction of CuO to Cu as in Eq. (6). 2 Cu + 2 NO → Cu2O + N2O
(8)
4 Cu + 2 NO → 2 Cu2O + N2
(8a) 0
Balkenende et Al. [33] confirmed the rapid oxidation of Cu to Cu+ as a result of NO exposure. Eqs. (6) and (8) (or (8a)) can be expressed in a different form without Cu, as in Eq. (9) (or (9a)). 2 CuO +2 NO + 2 CO → Cu2O + N2O + 2 CO2
(9)
4 CuO + 2 NO + 4 CO → 2 Cu2O + N2 + 4 CO2
(9a)
In order to verify the redox behaviors of CuO, we performed several pretreatments for the CuO powder: (a) the pretreatment under 0.5 vol% CO/He flow (50 ml/min) at 300 °C for 2 h, (b) (a) followed by the oxidation treatment under 0.5 vol% NO/He flow (50 ml/min) at 250 °C for 2 h, (c) (a) followed by the oxidation treatment under 0.5 vol% N2O/He flow (50 ml/min) at 250 °C for 2 h and (d) (b) followed by the NO + CO reaction under the same condition in Fig. 3. S-Fig. 1 and SFig. 2 show the XRD pattern for CuO after each pretreatment and the NO and CO conversions during the NO + CO reaction over the each redox pretreated sample, respectively. As shown in S-Fig. 1, only diffraction peaks from metallic Cu were observed after the CO treatment at 300 °C, indicating that reduction of CuO completely proceeds. Interestingly, the metallic Cu obtained by the reduction of CuO with CO (hereafter denoted by Cu*) exhibited significantly high catalytic performance for NO + CO reaction in comparison with the conventional Cu powder (S-Fig. 2). Subsequently, the Cu* was readily oxidized to Cu2O (hereafter denoted by Cu2O*) even at lower temperatures (e.g., 250 °C) than the conventional Cu powder. It should be noted that the oxidation of conventional Cu powder with NO or N2O proceed above 400 °C (see S-Fig. 3). We performed the same catalytic experiment of the NO + CO reaction for the Cu2O* sample (after pretreatment (b)). As shown in S-Fig. 2, there was a hysteresis between heating and cooling process in the conversion curves. Although the activity of Cu2O* sample on the heating process was lower than that of Cu* sample, the activity of Cu2O* sample on the cooling process was almost the same as Cu* sample. These results suggest that Cu2O* is readily reduced to Cu* during NO + CO reaction and Cu* works as catalytic active species. According to CO-TPR measurements (see S-Fig. 4), Cu2O* is readily reduced to Cu* below 250 °C. As shown in Fig. 3, bulk Cu2O powder hardly exhibited catalytic activity for NO + CO reaction below 400 °C although Cu2O* showed the high catalytic activity even at 200 °C. The XRD peaks of Cu2O* are much broader than those of conventional Cu2O (see Fig. 6 (c-ii)). The
Fig. 8. (a) N2O conversions and (b) CO conversions during the N2O + CO reaction over Cu, Cu2O and CuO as functions of temperature.
reaction [28–30]. Rodriguez et al. have reported the dependence of the reduction pathway for CuO bulk powders on the flow rate of reducing agent (H2 or CO) [26,31,32]. A direct transformation pathway for CuO reduction (CuO → Cu) when there was a large supply of CO, while they showed a sequential step pathway involving one intermediate (CuO → Cu2O → Cu) with a limited supply of CO. At present stage, we cannot decide strictly about pathway for the CuO reduction because the redox process of CuO species drastically changes in the composition of NO/CO and the temperature under the transient reaction conditions. The COTPR and in situ XRD results acquired following exposure to a CO
Fig. 9. N2O and CO conversions over time using (a) Cu at 300 °C for 330 min, (b) Cu2O at 500 °C for 491 min, and (c) CuO at 200 °C for 364 min. 70
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Fig. 10. XRD patterns generated by (a) Cu, (b) Cu2O, and (c) CuO (i) before and (ii) after the N2O + CO reaction at constant temperature.
Fig. 11. (a) NO conversions, (b) CO conversions, and (c) N2 selectivity during the NO + CO reaction over Cu powder (0.30 g or 0.60 g) and Raney Cu as functions of time. (c) The XRD patterns of Raney Cu (i) before and (ii) after the reaction. 71
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peak broadening of XRD indicates that high density of defects (such as vacancies), disorder and lattice strains existed in Cu2O*, which are supposed act active sites. Moreover, the BET surface area after NO + CO reaction of Cu2O* (17.7 m2/g) is much higher than that of conventional Cu2O (3.0 m2/g), indicating that the considerable surface roughness and defects induced by the reduction process on Cu2O*. Consequently, Cu2O* plays an important role for a high catalytic performance even at lower temperatures. The subsequent reduction of CuO can be expected. As the amount of Cu2O* in the system increased, complete oxidation of Cu* to Cu2O* becomes difficult because the majority of the NO will be converted via catalysis over the Cu2O* and sufficient NO cannot be supplied for the oxidation of Cu. In Fig. 6(a) and (b), the CuO was maintained for a far longer time span (1106 min) at 200 °C, while the N2 selectivity continuously increased to a maximum of 88%. Simultaneously, the CO conversion also slowly approached 100%. In the XRD pattern acquired after the reaction (Fig. 6(iii)), broad Cu2O peaks similar to those observed in Fig. 6(ii) were present. However, metallic Cu was not observed at shorter time on-stream values, such as 350 min. Fig. 6(b) demonstrates a further increase in the N2 selectivity past the 350 min mark. These results can be explained by the continuous formation of Cu2O* and Cu* via Eqs. (6) and (8) (or (8′)). During the reaction at 250 °C (Fig. 7), both the NO and CO conversions reached 100% at 141 min and the catalyst consisted largely of metallic Cu, as shown in Fig. 7(c). At 250 °C, the reduction of the CuO via the process in Eq. (6) was faster than the oxidation of Cu via Eq. (8) (or (8′)), as a result of catalysis by the Cu and the unstable, partially oxidized Cu2O*. In the case of the N2O + CO reaction at 200 °C, the CO conversion was higher than the N2O conversion, and this is attributed to the reduction of CuO. The XRD pattern obtained after the reaction (Fig. 10(cii)) showed the presence of both Cu and Cu2O* while only Cu2O* was present following the NO + CO reaction using almost the same holding time and reaction temperature. The reduction of CuO by CO deeply occurs under N2O + CO, whereas it is suppressed under NO + CO due to the NO adsorbs strongly on Cu. Consequently, Cu was formed via Eq. (6) and partially oxidized according to Eq. (10). Based on the above discussion concerning the NO + CO reaction, the N2O + CO reaction might be catalyzed by Cu obtained by reduction of Cu2O* with CO. 2 Cu + N2O → Cu2O + N2
Fig. 12. Scheme summarizing the relationship between the bulk oxidation states and catalytic reactions in the NO + CO and N2O + CO systems. Cu* and Cu2O* denote activated species via reduction of CuO with CO and oxidation of Cu* with NO or N2O, respectively.
N2 selectivity greatly depends on the number of active sites in the system. Similarly, for the N2O + CO reaction, metallic Cu clearly exhibits high catalytic performance while Cu2O powder must be reduced to activate the reaction. In the case of the CuO, it is possible that Cu2O* also acted as a catalyst, because improved N2 selectivity was obtained during the NO + CO reaction as the quantity of Cu2O* was increased. 5. Conclusion We investigated the relationships between oxidation state and catalytic activity of bulk Cu, CuO and Cu2O during the NO + CO and N2O + CO reactions. Under a NO + CO atmosphere, metallic Cu was the most stable catalyst and the reaction proceeded while generating primarily N2. In contrast, the Cu2O and CuO were eventually reduced to Cu using a flow with a NO: CO = 1: 1 composition, and this Cu promoted the conversion of NO to N2. Similarly, in the case of the N2O + CO reaction, only Cu promoted the reduction of N2O and no change was observed in the oxidation state of the metal, while the Cu2O and CuO were not capable of catalysis during these two reactions until they were reduced to Cu. However, Cu2O* formed via the reduction of CuO did catalyze both reactions to some extent, likely due to the high surface area and structural disorder of this compound. Finally, it can be concluded that the most active and stable bulk catalyst for the NO + CO reaction and related processes is metallic Cu rather than Cu oxides.
(10)
4.4. Transformations of copper species in the NO + CO and N2O + CO reactions Fig. 12 presents a scheme summarizing proposed mechanisms for the NO + CO and N2O + CO reactions over Cu, Cu2O and CuO. The NO + CO reaction is activated by Cu and only small amounts of the Cu are oxidized by the NO and N2O. Thus, the Cu powder is a stable catalyst for the NO + CO reaction. According to the reaction mechanism proposed by London et al. [12], partial oxidation of the Cu surface can be assumed because the formation of N2O is observed in the presence of both Cu0 and Cu+. However, the Cu2O powder having a relatively low surface area showed low activity until it was reduced to Cu. This result confirms that metallic Cu was much effective than Cu2O for the NO + CO reaction. Cu2O* formed via oxidation of metallic Cu species obtained by reduction of CuO with CO at low temperature (< 300 °C) shows high catalytic activity during both reactions as a result of the high surface area and structural disorder of this material. No catalytic reaction was observed over the CuO powder, which immediately transformed into Cu2O* or Cu in the reaction atmosphere. In the case of the oxide powders, the N2 selectivity increased as Cu or Cu2O* was formed. The amount and surface area of the sample evidently both significantly affect the N2 selectivity, as shown in Fig. 11. Therefore, it appears that either Eq. (1) or (2) occurs on the catalysts, such that the
Acknowledgements This work was supported in part by Grant-in-Aid for Scientific Research ((A) 15H02299, (B) 18H01783) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2018.10.013. References [1] M. Iwamoto, H. Hamada, Removal of nitrogen monoxide from exhaust gases
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