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Manganese-promoted cobalt oxide as efficient and stable non-noble metal catalyst for preferential oxidation of CO in H2 stream
Applied Catalysis B: Environmental 102 (2011) 207–214
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Manganese-promoted cobalt oxide as efficient and stable non-noble metal catalyst for preferential oxidation of CO in H2 stream Qinghong Zhang, Xianhong Liu, Wenqing Fan, Ye Wang ∗ State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history: Received 29 July 2010 Received in revised form 27 November 2010 Accepted 30 November 2010 Available online 5 December 2010 Keywords: Carbon monoxide Preferential oxidation High-purity hydrogen Cobalt oxide Manganese oxide
a b s t r a c t The modification by MnOx significantly enhanced the activity and stability of Co3 O4 for the preferential oxidation of CO in H2 -rich stream. The MnOx -promoted Co3 O4 catalyst with a Mn/Co molar ratio of 1/8 exhibited the best performance; CO conversion was 56% at 40 ◦ C and increased to >90% as the temperature was raised to >75 ◦ C. This catalyst could provide 100% CO conversion at 125–175 ◦ C. CO conversions of 97% and 100% remained unchanged for 100 h at 100 and 125 ◦ C, respectively, whereas the deactivation was observed over Co3 O4 alone. Our characterizations using XRD, XPS and TEM indicated that the modification of Co3 O4 with MnOx caused the formation of smaller Mnx Co3−x O4 solid-solution particles. The O2 -TPD, CO-TPD, and pulse reaction studies suggest that the incorporation of MnOx into Co3 O4 increased the amounts of reactive oxygen species and adsorbed CO species over catalyst surfaces and enhanced the regeneration ability of the reduced catalyst by O2 . All these enhancements are proposed to be responsible for the improved catalytic performance of the MnOx -promoted Co3 O4 . © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is well known that CO (0.5–2 wt%) in H2 produced mainly by reforming of hydrocarbons, followed by the water–gas shift (WGS) reaction, can poison the Pt-based anode in the polymer electrolyte membrane (PEM) fuel cell. The preferential oxidation (PROX) of CO in H2 -rich stream is regarded as one of the simplest and cost-effective routes for the removal of CO in H2 . Many catalysts, typically noble metal-based and transition metal oxide-based catalysts, have been reported for the PROX of CO [1–5]. Among the noble metal-based catalysts, supported Pt and Au catalysts show promising performances [6–9]. Conventional Pt catalysts typically work at higher temperatures (>150 ◦ C) [6], and recent work has attempted to promote the low-temperature activity of the supported Pt catalysts by adding FeOx [7,9], CoOx [10] and other promoters or using peculiar supports such as FSM-16 [11]. On the other hand, supported Au catalysts such as Au/MnOx , Au/Fe2 O3 Au/TiO2 , and Au/CeO2 could provide good CO conversions at lower temperatures but H2 oxidation occurred seriously at temperatures >80 ◦ C, resulting in lower O2 selectivity for CO oxidation [12–20]. Because of the high cost and limited availability of noble metals, the catalysts based on transition metal oxides or composite oxides are promising alternatives for the PROX of CO [1,2,5]. Cu-
∗ Corresponding author. Tel.: +86 592 2186156; fax: +86 592 2183047. E-mail address: [email protected] (Y. Wang). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.11.043
based supported or composite oxides, especially CuO–CeO2 , have been studied intensively for the PROX of CO [1,2,5,13,21–29]. Cobalt oxide (Co3 O4 ) was known to exhibit good catalytic performance for the oxidation of CO by O2 at lower temperatures [30–34]. There only exist a few studies on the PROX of CO in H2 -rich stream over Co3 O4 -based catalysts such as Co3 O4 /ZrO2 and Co3 O4 –CeO2 [35–42]. These Co3 O4 -based catalysts typically work effectively at 150–200 ◦ C, providing similar or even better performances than the CuO–CeO2 catalyst. The Co3+ site on catalyst surfaces appears to play a key role in the oxidation of CO both in the absence and the presence of H2 , and the lattice oxygen may participate in the transformation of CO [31,34,36,37]. However, the Co3 O4 -based catalysts usually deactivate with time on stream during the reaction [31,33,34,38,39]. For the PROX of CO in H2 stream, several reasons, e.g., the partial reduction of Co3+ and the formation of carbonates, have been proposed to be responsible for the deactivation of Co3 O4 -based catalysts [37–39]. To date, none has succeeded in demonstrating a catalyst with satisfactory combination of activity, selectivity and stability for the PROX of CO in H2 stream. Recently, we found that the modification of Co3 O4 with MnOx could significantly increase its activity at lower temperatures and widen the operation temperature region. The stability of Co3 O4 was also remarkably enhanced by the modification with MnOx . Herein, we report the superior catalytic performances of the MnOx promoted Co3 O4 catalysts in the PROX of CO in H2 . The effects of MnOx modification will be discussed in detail on the basis of catalyst characterizations and pulse reaction studies.
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2. Experimental 2.1. Catalyst preparation The MnOx -promoted Co3 O4 samples (denoted as Mn–Co–O hereafter) with different Mn/Co atomic ratios were prepared by a co-precipitation method. Typically, an aqueous solution of Na2 CO3 were added dropwise into the mixed aqueous solution of Co(NO3 )2 and Mn(NO3 )2 with a fixed Mn/Co ratio under vigorous agitation, and then a suspension was obtained. The pH of the suspension was kept at 8.5–9.0. After further aging for 4 h, the solid was recovered by filtration, followed by washing with hot deionized water and drying at 80 ◦ C for 24 h. The catalyst was finally calcined in air at 350 ◦ C for 5 h. The Co3 O4 and MnO2 samples were prepared with the same procedure.
2.2. Catalyst characterization N2 physisorption was performed at −196 ◦ C on a Micromeritics Tristar 3000 surface and porosimetry analyzer to obtain the surface areas of samples. Powder X-ray diffraction (XRD) patterns were collected on a Panalytical X’ pert Pro diffractometer using Cu K␣ radiation (40 kV, 30 mA). X-ray photoelectron spectroscopic (XPS) measurements were performed on a Quantum 2000 Scanning ESCA Microprobe (Physical Electronics) using Al K␣ radiation (1846.6 eV) as X-ray source. The binding energy was calibrated using C 1s photoelectron peak at 284.6 eV as a reference. Transmission electron microscopy (TEM) was measured with a Tecnai F30 electron microscope (Phillips Analytical) operated at an acceleration voltage of 300 kV. Oxygen temperature-programmed desorption (O2 -TPD) measurements were performed on a Micromeritics AutoChem 2920 II instrument. Typically, after pretreated by a 20 vol% O2 /He gas flow at 350 ◦ C for 1.5 h, the sample was purged by high-purity He and was then cooled down to 50 ◦ C in He flow. The adsorption of O2 was performed in a 5 vol% O2 /He gas flow for 1 h at 50 ◦ C. After purge by high-purity He, the sample was heated from 50 ◦ C to 900 ◦ C at a rate of 10 ◦ C min−1 , and the desorbed O2 was monitored using a mass spectrometer (Pfeiffer Vacuum ThermoStar GSD 301 T2). Carbon monoxide temperature-programmed desorption (COTPD) was measured with a similar procedure on the Micromeritics AutoChem 2920 II instrument. After pretreatment at 350 ◦ C in a 20% O2 /He flow, followed by purge with He and cooling down to 50 ◦ C, the sample was exposed to 5% CO/He at 50 ◦ C for 1 h, and then the gas-phase CO was removed by He purge. The temperature was subsequently raised to 700 ◦ C at a rate of 10 ◦ C min−1 . The desorbed CO and CO2 were monitored by the mass spectrometer.
2.3. Catalytic reactions The PROX of CO was performed on a fixed-bed reactor operated at atmospheric pressure. The catalyst was pretreated in an O2 -containing He gas flow (O2 , 5 vol %) at 200 ◦ C for 40 min before reaction. After the catalyst was cooled down to a certain reaction temperature, the reactant gas containing 1% CO, 1% O2 , and 50% H2 in He was introduced into the reactor. Typically, 100 mg of catalyst was used, and the total gas flow rate was 50 mL min−1 , corresponding to a weight hourly space velocity (WHSV) of 30,000 mL g−1 h−1 . The inlet and outlet gas compositions were analyzed by an on-line gas chromatograph equipped with TCD and FID. H2 , O2 , CO, and CO2 were separated using a carbon molecular sieve (TDX-01) column. CO and CO2 were further converted to methane by a methanation reactor and analyzed using a FID detector. The carbon balance in each run was 100 ± 3%. The conversion of CO and the O2 selectivity
Fig. 1. Effect of the atomic ratio of Mn/(Co + Mn) on catalytic behaviors for the PROX of CO in H2 . (A) CO conversion; (B) O2 selectivity for CO oxidation. Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , 48% He; total flow rate, 50 mL min−1 .
for CO oxidation were calculated by the following equation: CO conversion (%) =
Selectivity (%) = =
1 − [CO]out × 100% [CO]in
1/2[CO2 ] × 100% [O2 ]in − [O2 ]out 1/2([CO]in − [CO]out ) × 100% [O2 ]in − [O2 ]out
3. Results and discussion 3.1. Effect of the modification of Co3 O4 by MnOx on catalytic behaviors Several research groups have demonstrated that Co3 O4 is a promising catalyst for the PROX in H2 , but relatively high temperatures (150–200 ◦ C) are required to attain a high activity [35–46]. Our results (Fig. 1) show that the single Co3 O4 exhibits very low CO conversions (<10%) at temperatures ≤75 ◦ C. The modification of Co3 O4 by MnOx significantly enhanced CO conversions at these temperatures (Fig. 1A). The Mn–Co–O catalyst with an atomic Mn/(Co + Mn) ratio of 0.11, i.e., Mn/Co ratio of 1/8, exhibited the highest CO conversions; CO conversions over this catalyst reached 56% and 92% at temperatures of 40 ◦ C and 75 ◦ C, respectively. These values are significantly higher than those obtained over the single Co3 O4 and MnO2 catalysts. The O2 selectivities at ≤75 ◦ C over these catalysts were 100% (Fig. 1B). An increase in temperature to 100 ◦ C increased the CO conversion to 97% over the Mn–Co–O (Mn/Co = 1/8) catalyst, but the O2 selectivity was slightly decreased to 93%. Further comparisons in catalytic behaviors among the Mn–Co–O catalysts (with Mn/Co ratios of 1/8 and 2/8) and the single Co3 O4 and MnO2 are shown in Fig. 2. Over the Mn–Co–O catalyst with
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Fig. 3. Changes of catalytic performances with time on stream for Co3 O4 and Mn–Co–O (Mn/Co = 1/8) in the PROX of CO in H2 . Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , 48% He; total flow rate, 50 mL min−1 ; reaction temperature, 100 ◦ C.
Fig. 2. Dependence of catalytic performances on reaction temperature for the PROX of CO in H2 over Co3 O4 , MnO2 , and Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8. (A) CO conversion; (B) O2 selectivity for CO oxidation. Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , 48% He; total flow rate, 50 mL min−1 .
a Mn/Co ratio of 1/8, the CO conversion remained ≥97% at temperatures of 100–200 ◦ C, and 100% CO conversion was achieved at 125–175 ◦ C. A further increase in temperature to 250 ◦ C gradually decreased the CO conversion likely due to the competitive oxidation of H2 at higher temperatures. On the other hand, the highest CO conversions over the single Co3 O4 and MnO2 catalysts were 91% and 95%, respectively at 175 ◦ C. The CO conversion decreased rapidly as the temperature was raised to >200 ◦ C over these single oxides. It is of interest to note that, when compared to that over Co3 O4 or MnO2 alone, the decreases in CO conversion at higher temperatures (>200 ◦ C) are less significant over the Mn–Co–O catalysts. Moreover, as shown in Fig. 2B, the decrease in O2 selectivity for CO oxidation over the Mn–Co–O catalyst was also less remarkable than that over Co3 O4 or MnO2 alone at higher temperatures (>200 ◦ C). Fig. 2 also demonstrates that the temperature range, in which CO conversions are higher than 90%, becomes significantly wider (75–225 ◦ C) after the modification of Co3 O4 by MnOx with a Mn/Co ratio of 1/8 or 2/8. Moreover, our Mn–Co–O catalyst could provide 100% CO conversion at 125–175 ◦ C although the O2 selectivity for CO oxidation was 50–75% at these temperatures. The complete conversion of CO is an important requirement for PROX catalysts because the PEM fuel cell is also sensitive to low-level CO impurity (<20 ppm) in H2 stream [47]. The temperature range (125–175 ◦ C) of the present Mn–Co–O catalyst, over which 100% CO conversion could be achieved, may also be suitable for the CO-PROX process, which should ideally lie between a WGS reactor (typically working at 200–250 ◦ C) and a PEM fuel cell (typically operating at 80 ◦ C).
at 100 ◦ C, although CO conversion was not high. In contrast, the Mn–Co–O (Mn/Co = 1/8) catalyst was very stable under the same reaction conditions. No significant changes in both CO conversion and O2 selectivity was observed for a 100 h of reaction over the Mn–Co–O (Mn/Co = 1/8) catalyst. Because H2 produced from steam reforming generally contains H2 O and CO2 , we have investigated the influences of H2 O and CO2 on the catalytic performances of our catalyst for the PROX of CO in H2 . Fig. 4 compares the catalytic performances of the Mn–Co–O (Mn/Co = 1/8) catalyst in the absence and the presence of CO2 (10 vol.%) and/or H2 O (10 vol.%) in reactant stream at different reac-
3.2. Stability of the MnOx -promoted Co3 O4 catalyst As mentioned earlier, the deactivation [31,33,34,38,39] with time on stream is a serious problem for the Co3 O4 -based CO-PROX catalysts. Fig. 3 shows that Co3 O4 alone deactivates quite rapidly
Fig. 4. Effect of the presence of H2 O and/or CO2 in reactant stream on catalytic performances of Mn–Co–O (Mn/Co = 1/8) catalyst. Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , He as balance; total flow rate, 50 mL min−1 .
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tion temperatures. The addition of CO2 and/or H2 O decreased the catalytic activity at temperatures < 150 ◦ C, while the selectivity was not significantly decreased. The activity decreased in the sequence of adding CO2 > adding H2 O > adding (CO2 + H2 O). The inhibiting effect by the addition of CO2 was also reported over other catalysts such as CuO–CeO2 , and the decrease in activity was attributed to the adsorption of CO2 on the active site forming carbonates [48]. Over our Mn–Co–O catalyst, the inhibiting effect of H2 O is stronger than that of CO2 . We speculate that this might be related to the serious blockage of the active site by H2 O at ≤100 ◦ C, suppressing the activation of CO and O2 on the catalyst. The inhibiting effects of both CO2 and H2 O became less significant at higher temperatures. At temperatures of 150–200 ◦ C, the conversion of CO kept at ∼100% even for the reactant stream containing both CO2 and H2 O. We further investigated the stability of the Mn–Co–O (Mn/Co = 1/8) catalyst at two different contact times (WHSVs = 30,000 and 120,000 mL h−1 g−1 ) at 125 ◦ C. As shown in Fig. 5, at a WHSV of 30,000 mL h−1 g−1 , CO conversion was always 100% during the reaction for 100 h. No CO could be detected at the outlet of the reactor. The increase in WHSV to 120,000 mL h−1 g−1 decreased CO conversion from 100% to ∼90%, but both CO conversion and O2 selectivity were sustained during the long-term reaction. These observations clearly suggest that our catalyst is quite stable under these conditions. However, when H2 O (10 vol.%) and CO2 (10 vol.%) were added into the reactant steam, the CO conversion began to decline gradually after 40 h of reaction (Fig. 5). Further enhancement in catalyst stability in the presence of H2 O and CO2 is needed in future studies. 3.3. Characterization of MnOx -promoted Co3 O4 catalysts To understand the nature of the significant promoting effect of MnOx on catalytic performances of Co3 O4 , we investigated the structures of the Mn–Co–O catalysts by several techniques. Fig. 6 shows the XRD patterns of the Mn–Co–O samples with different Mn/Co ratios. The diffraction peaks of single Co3 O4 and MnOx could be ascribed to cubic spinel phase Co3 O4 and rutile-like -MnO2 [44–46]. In the spinel structure of Co3 O4 , Co2+ and Co3+ ions are located in tetrahedral and octahedral coordination environments, respectively [49]. For the Mn–Co–O samples with different Mn/Co ratios, only diffraction peaks attributable to the cubic spinel phase of Co3 O4 could be observed. The diffraction peaks of Co3 O4 became
Fig. 5. Changes of catalytic performances with time on stream for Mn–Co–O (Mn/Co = 1/8) catalyst under different reaction conditions. Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , 48% He; reaction temperature, 125 ◦ C.
Fig. 6. XRD patterns for Mn–Co–O samples together with single Co3 O4 and MnOx .
weaker and broader in the presence of MnOx . The broadening of the diffraction peaks for the Mn–Co–O samples suggests the decreases in the crystalline sizes. We have estimated the crystalline sizes of Co3 O4 and the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8 by the Scherrer equation. The result shows that the crystalline size for Co3 O4 was ∼19 nm, which was decreased to 12 nm and 8.1 nm for the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8, respectively. As shown in Fig. 6, the shift of diffraction peaks belonging to Co3 O4 to smaller 2 angles was observed in the presence of MnOx with a Mn/Co ratio of 1/8, indicating an enlargement of the Co3 O4 unit cell. Similar observations were reported by Morales et al. [50,51], and the formation of a Mnx Co3−x O4 solid solution phase was proposed [44,50,51]. It can be expected that, in the Mnx Co3−x O4 solid solution, there may exist electron transfer between Co2+ and Mn4+ . Thus, a part of Co2+ and Mn4+ may be changed into Co3+ and Mn3+ , respectively [44,50,51]. The lattice expansion (2 decreasing) ˚ in the spinel strucoccurred due to the replacement of Co3+ (0.61 A) ˚ with a slightly larger radius. The formation ture by Mn3+ (0.65 A) of the Mnx Co3−x O4 solid solution was further confirmed by X-ray absorption fine structure (XAFS) spectroscopy through analyzing the difference between the Mn–Co–O and Co3 O4 samples in the coordination structures around Co and Mn atoms and the valence state of Mn species in the Mn–Co–O sample [50,51]. Many other studies also demonstrated the formation of Mnx Co3−x O4 solid solution in the Mn–Co–O binary oxides prepared by co-precipitation followed by calcination [44,50,52,53]. Moreover, the Hausmannite Mn3 O4 , which can be viewed as MnII MnIII 2 O4 and possesses a spinel-type crystalline structure containing close-packed oxide ions with tetrahedral sites occupied by Mn2+ and octahedral sites by Mn3+ , can also be easily prepared [54]. The similar crystalline structures of Co3 O4 and Mn3 O4 also allow us to consider that the formation of Mnx Co3−x O4 solid solution in our Mn–Co–O catalyst is reasonable. Fig. 7 shows the TEM micrographs of Co3 O4 and the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8. It is clear that both Co3 O4 and the Mn–Co–O samples are composed of nanoparticles with uniform sizes. The sizes of Co3 O4 and Mn–Co–O samples with Mn/Co of 1/8 and 2/8 were estimated to be 20, 9.6, and 7.8 nm, respectively. These sizes were close to those calculated by the Scherrer equation from XRD results. Thus, it can be concluded that the introduction
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Fig. 8. XPS spectra for Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8 together with single Co3 O4 and MnOx . (A) Co 2p; (B) Mn 2p.
Fig. 7. TEM micrographs for Co3 O4 (A) and Mn–Co–O samples with Mn/Co ratios of 1/8 (B) and 2/8 (C).
of MnOx to Co3 O4 leads to the formation of smaller Mnx Co3−x O4 solid solution particles. The specific surface area obtained from N2 sorption at −196 ◦ C increased from 52 m2 g−1 for Co3 O4 to 102 and 104 m2 g−1 for the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8, respectively. The specific surface area of the Mn–Co–O sample with a Mn/Co ratio of 2/8 (104 m2 g−1 ) is lower than that estimated from the particle size by assuming a spherical particle model (127 m2 g−1 ), and we think that such a deviation is acceptable by considering the experimental errors and that the Mn–Co–O (Mn/Co = 2/8) particles are not all perfectly spherical (Fig. 7). The Co 2p and Mn 2p XPS spectra for the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8 are shown in Fig. 8 together with single Co3 O4 and MnO2 . The binding energy of Co 2p3/2 for Co3 O4 was 779.6 eV, consistent with that reported in another work [55]. The binding energy of Co 2p3/2 increased after the addition of MnOx to Co3 O4 . This observation suggests the increase in fraction of Co3+ /Co2+ in the Mn–Co–O catalysts. On the other hand, as compared to that in single MnO2 , the binding energy of Mn 2p3/2 in the Mn–Co–O decreased slightly, and this may be due to the partial reduction of Mn4+ and the smaller difference in the binding energy between Mn4+ and Mn3+ [56]. These results suggest an interaction between CoOx and MnOx species in the Mn–Co–O samples, which leads to the electron transfer from CoOx to MnOx This is in agreement with the formation of Mnx Co3−x O4 solid solution in the Mn–Co–O samples, which could also be deduced from the XRD results. The surface Mn/Co ratios in the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8 were estimated from XPS, and the values were 1/8.4 and 1/4.1, respectively, indicating that manganese was homogeneously distributed in the solid solutions in these samples. The O2 -TPD has been performed to gain insight into the nature of oxygen species over the Mn–Co–O samples. Fig. 9 shows that the desorption of O2 occurs in two temperature regions over our Mn–Co–O samples. It is reported that the desorption peaks at temperatures lower than 300–350 ◦ C are generally attributable to the surface adsorbed oxygen species (␣-oxygen species) such as O2 −
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Fig. 9. O2 -TPD profiles for Co3 O4 and Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8.
and O− , while the desorption peaks at higher temperatures can be ascribed to lattice oxygen species (-oxygen species) [57,58]. Co3 O4 exhibited a weak desorption peak at 217 ◦ C, which was mainly ascribed to the O2 − species [58]. The peak belonging to ␣oxygen shifted to higher temperatures after the addition of MnOx to Co3 O4 , implying the change of O2 − to O− [57,58]. Moreover, the intensity of the desorption at >350 ◦ C increased significantly in the presence of MnOx . It is reasonable to attribute the desorption peaks at 350–650 ◦ C to surface lattice oxygen. Thus, the presence of MnOx enhanced the mobility of surface lattice oxygen of the catalyst. The activation of O2 on catalyst surfaces is generally a key step for CO oxidation. A kinetic study of CO oxidation over Co3 O4 revealed a higher reaction order of O2 (0.28) than that of CO (0.12), and this might imply a more important role of O2 activation in determining the reaction rate [34]. The correlation of the O2 -TPD results with the catalytic performances suggests that the abundant oxygen species over the MnOx -promoted Co3 O4 catalyst may be one point contributing to its higher catalytic activity. Fig. 10 shows the CO-TPD profiles for the Mn–Co–O samples together with Co3 O4 . In addition to CO, the desorption of CO2 , which should be produced during the surface reaction between the adsorbed CO and the oxygen species, was also observed over Co3 O4 and the Mn–Co–O samples. The desorption of CO started from ∼160 ◦ C over Co3 O4 , and this starting temperature shifted to lower temperatures after the addition of MnOx . As compared to Co3 O4 , the Mn–Co–O samples showed a significantly larger amount of CO2 desorption (Fig. 10B). The intensity of CO desorbed was also increased in the presence of MnOx . The total amount of CO and CO2 desorbed from the Mn–Co–O samples (Mn/Co = 1/8 and 2/8) per surface area was 1.5–1.8 times that from Co3 O4 alone. These observations imply that the modification of Co3 O4 with MnOx increases the CO adsorption ability. As shown in Fig. 10B, CO2 desorption peaks appeared in both lower-temperature region (140–190 ◦ C) and higher-temperature region (300–350 ◦ C). The comparison of the CO2 desorption peaks (Fig. 10B) with the oxygen desorption results (Fig. 9) suggests that the CO2 desorbed in the lower-temperature region over Co3 O4 might be formed by the reaction of ␣-oxygen species with the adsorbed CO, since ␣-oxygen is the main oxygen species over Co3 O4 (Fig. 9). On the other hand, a larger amount of CO2 was desorbed in the higher-temperature region over the Mn–Co–O samples. These
Fig. 10. CO-TPD profiles for Co3 O4 and Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8. (A) CO desorbed, (B) CO2 desorbed.
CO2 desorbed may be contributed by the reaction of CO adsorbed with the lattice oxygen species (-oxygen species), since -oxygen is the dominant oxygen species on the surfaces of the Mn–Co–O samples at ≥300 ◦ C. Therefore, we speculate that both types of oxygen species on the Mn–Co–O catalyst, i.e., the surface adsorbed oxygen species (␣-oxygen species) and the lattice oxygen species (-oxygen species), may contribute to the oxidation of CO. As shown in Fig. 10, Some CO2 desorbed at high temperatures, and this might cause catalyst deactivation. The reactivity of CO with the oxygen species over the Mn–Co–O catalyst was investigated by the pulse reaction technique. Fig. 11A compares the conversion of CO pulses to CO2 over the Mn–Co–O (Mn/Co = 1/8) with that over Co3 O4 at 40 ◦ C. The amount of CO2 formed decreased with the number of successive CO pulses over both catalysts, and the reaction almost stopped after ∼10 and 20 pulses of CO over Co3 O4 and the Mn–Co–O, respectively. The integral amount of CO2 formed during the reaction of CO pulses over the Mn–Co–O was ∼4 times higher than that over Co3 O4 (Table 1). After the first run of reactions, the catalysts were regenerated in O2 flow at the same temperature for 30 min, and the second run of successive pulse reactions was started after purge with He. Fig. 11A shows that Co3 O4 cannot be regenerated by O2 at 40 ◦ C and no CO2 was formed during the second run of reactions. On the other hand, the CO pulse could be oxidized to CO2 during the reaction with the Mn–Co–O in the second and the third runs of Table 1 The amount of CO2 formed during the successive reactions of CO pulses with Co3 O4 and the Mn–Co–O (Mn/Co = 1/8). Catalyst
Co3 O4 Mn–Co–O Co3 O4 Mn–Co–O
Temperature (◦ C)
40 40 100 100
Amount of CO2 (mol) 1st run
2nd run
3rd run
0.628 3.32 2.16 5.86
0 1.97 1.58 5.81
0 1.63 1.40 5.80
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tivity for CO oxidation remained unchanged for a 100 h reaction at 100 ◦ C or 125 ◦ C over the Mn–Co–O catalyst, whereas the deactivation was observed over Co3 O4 . The presence of MnOx caused the formation of smaller Mnx Co3−x O4 solid solution particles. This led to the increased amounts of both active oxygen species and adsorbed CO. Furthermore, the reduced MnOx -promoted catalyst exhibited significantly higher O2 activation abilities particularly at lower temperatures. These are proposed to contribute to the enhanced activity and stability of the MnOx -promoted Co3 O4 catalysts. Acknowledgements This work was supported by the NSF of China (Nos. 20625310, 20773099, 20873110 and 20923004), the National Basic Research Program of China (No. 2010CB732303), the Key Scientific Project of Fujian Province (No. 2009HZ0002-1), and the Research Fund for the Doctoral Program of Higher Education (No. 20090121110007). References
Fig. 11. Successive reactions of CO pulses with Co3 O4 () and the Mn–Co–O (Mn/Co = 1/8) (䊉) at 40 ◦ C (A) and 100 ◦ C (B). Reaction conditions: catalyst, 0.10 g; He carrier gas, 80 mL min−1 ; volume of pulse, 0.63 mL; CO partial pressure in the pulse, 5.07 kPa (He balance).
pulse reactions although the integral amounts of CO2 produced decreased gradually in the next two runs. With increasing the reaction and regeneration temperature to 100 ◦ C, the integral amount of CO2 formed in the first run of pulse reactions increased over both catalysts. The Mn–Co–O still exhibited remarkably higher CO2 formation activity than Co3 O4 alone. Co3 O4 could also be regenerated to some extent at 100 ◦ C, but the integral amount of CO2 formed decreased in the second and the third runs. On the other hand, the integral amounts of CO2 formed in the second and the third runs over the Mn–Co–O (Mn/Co = 1/8) were almost the same with that in the first run. Thus, the reduced Mn–Co–O could be regenerated almost completely at 100 ◦ C. The significant larger amount of CO2 formed in above pulse reactions over the Mn–Co–O catalyst also indicates that the MnOx promotion increased the concentration of reactive oxygen species for the oxidation of CO to CO2 . This is consistent with O2 -TPD results. As described above, the comparison of O2 -TPD (Fig. 9) with CO-TPD (Fig. 10B) suggests that both the surface adsorbed oxygen species (such as O2 − and O− ) and the lattice oxygen species have the ability to react with the CO adsorbed on catalyst surfaces. Moreover, the pulse reaction results further suggest that the reduced Mn–Co–O catalyst can activate O2 more easily than Co3 O4 alone especially at lower temperature to regenerate the reactive oxygen species for the oxidation of CO. Even at 100 ◦ C, the reduced CoOx could not be completely regenerated by O2 . We think that this is one main reason for the deactivation of Co3 O4 at lower temperatures. 4. Conclusions
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Manganese-promoted cobalt oxide as efficient and stable non-noble metal catalyst for preferential oxidation of CO in H2 stream Qinghong Zhang, Xianhong Liu, Wenqing Fan, Ye Wang ∗ State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history: Received 29 July 2010 Received in revised form 27 November 2010 Accepted 30 November 2010 Available online 5 December 2010 Keywords: Carbon monoxide Preferential oxidation High-purity hydrogen Cobalt oxide Manganese oxide
a b s t r a c t The modification by MnOx significantly enhanced the activity and stability of Co3 O4 for the preferential oxidation of CO in H2 -rich stream. The MnOx -promoted Co3 O4 catalyst with a Mn/Co molar ratio of 1/8 exhibited the best performance; CO conversion was 56% at 40 ◦ C and increased to >90% as the temperature was raised to >75 ◦ C. This catalyst could provide 100% CO conversion at 125–175 ◦ C. CO conversions of 97% and 100% remained unchanged for 100 h at 100 and 125 ◦ C, respectively, whereas the deactivation was observed over Co3 O4 alone. Our characterizations using XRD, XPS and TEM indicated that the modification of Co3 O4 with MnOx caused the formation of smaller Mnx Co3−x O4 solid-solution particles. The O2 -TPD, CO-TPD, and pulse reaction studies suggest that the incorporation of MnOx into Co3 O4 increased the amounts of reactive oxygen species and adsorbed CO species over catalyst surfaces and enhanced the regeneration ability of the reduced catalyst by O2 . All these enhancements are proposed to be responsible for the improved catalytic performance of the MnOx -promoted Co3 O4 . © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is well known that CO (0.5–2 wt%) in H2 produced mainly by reforming of hydrocarbons, followed by the water–gas shift (WGS) reaction, can poison the Pt-based anode in the polymer electrolyte membrane (PEM) fuel cell. The preferential oxidation (PROX) of CO in H2 -rich stream is regarded as one of the simplest and cost-effective routes for the removal of CO in H2 . Many catalysts, typically noble metal-based and transition metal oxide-based catalysts, have been reported for the PROX of CO [1–5]. Among the noble metal-based catalysts, supported Pt and Au catalysts show promising performances [6–9]. Conventional Pt catalysts typically work at higher temperatures (>150 ◦ C) [6], and recent work has attempted to promote the low-temperature activity of the supported Pt catalysts by adding FeOx [7,9], CoOx [10] and other promoters or using peculiar supports such as FSM-16 [11]. On the other hand, supported Au catalysts such as Au/MnOx , Au/Fe2 O3 Au/TiO2 , and Au/CeO2 could provide good CO conversions at lower temperatures but H2 oxidation occurred seriously at temperatures >80 ◦ C, resulting in lower O2 selectivity for CO oxidation [12–20]. Because of the high cost and limited availability of noble metals, the catalysts based on transition metal oxides or composite oxides are promising alternatives for the PROX of CO [1,2,5]. Cu-
∗ Corresponding author. Tel.: +86 592 2186156; fax: +86 592 2183047. E-mail address: [email protected] (Y. Wang). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.11.043
based supported or composite oxides, especially CuO–CeO2 , have been studied intensively for the PROX of CO [1,2,5,13,21–29]. Cobalt oxide (Co3 O4 ) was known to exhibit good catalytic performance for the oxidation of CO by O2 at lower temperatures [30–34]. There only exist a few studies on the PROX of CO in H2 -rich stream over Co3 O4 -based catalysts such as Co3 O4 /ZrO2 and Co3 O4 –CeO2 [35–42]. These Co3 O4 -based catalysts typically work effectively at 150–200 ◦ C, providing similar or even better performances than the CuO–CeO2 catalyst. The Co3+ site on catalyst surfaces appears to play a key role in the oxidation of CO both in the absence and the presence of H2 , and the lattice oxygen may participate in the transformation of CO [31,34,36,37]. However, the Co3 O4 -based catalysts usually deactivate with time on stream during the reaction [31,33,34,38,39]. For the PROX of CO in H2 stream, several reasons, e.g., the partial reduction of Co3+ and the formation of carbonates, have been proposed to be responsible for the deactivation of Co3 O4 -based catalysts [37–39]. To date, none has succeeded in demonstrating a catalyst with satisfactory combination of activity, selectivity and stability for the PROX of CO in H2 stream. Recently, we found that the modification of Co3 O4 with MnOx could significantly increase its activity at lower temperatures and widen the operation temperature region. The stability of Co3 O4 was also remarkably enhanced by the modification with MnOx . Herein, we report the superior catalytic performances of the MnOx promoted Co3 O4 catalysts in the PROX of CO in H2 . The effects of MnOx modification will be discussed in detail on the basis of catalyst characterizations and pulse reaction studies.
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2. Experimental 2.1. Catalyst preparation The MnOx -promoted Co3 O4 samples (denoted as Mn–Co–O hereafter) with different Mn/Co atomic ratios were prepared by a co-precipitation method. Typically, an aqueous solution of Na2 CO3 were added dropwise into the mixed aqueous solution of Co(NO3 )2 and Mn(NO3 )2 with a fixed Mn/Co ratio under vigorous agitation, and then a suspension was obtained. The pH of the suspension was kept at 8.5–9.0. After further aging for 4 h, the solid was recovered by filtration, followed by washing with hot deionized water and drying at 80 ◦ C for 24 h. The catalyst was finally calcined in air at 350 ◦ C for 5 h. The Co3 O4 and MnO2 samples were prepared with the same procedure.
2.2. Catalyst characterization N2 physisorption was performed at −196 ◦ C on a Micromeritics Tristar 3000 surface and porosimetry analyzer to obtain the surface areas of samples. Powder X-ray diffraction (XRD) patterns were collected on a Panalytical X’ pert Pro diffractometer using Cu K␣ radiation (40 kV, 30 mA). X-ray photoelectron spectroscopic (XPS) measurements were performed on a Quantum 2000 Scanning ESCA Microprobe (Physical Electronics) using Al K␣ radiation (1846.6 eV) as X-ray source. The binding energy was calibrated using C 1s photoelectron peak at 284.6 eV as a reference. Transmission electron microscopy (TEM) was measured with a Tecnai F30 electron microscope (Phillips Analytical) operated at an acceleration voltage of 300 kV. Oxygen temperature-programmed desorption (O2 -TPD) measurements were performed on a Micromeritics AutoChem 2920 II instrument. Typically, after pretreated by a 20 vol% O2 /He gas flow at 350 ◦ C for 1.5 h, the sample was purged by high-purity He and was then cooled down to 50 ◦ C in He flow. The adsorption of O2 was performed in a 5 vol% O2 /He gas flow for 1 h at 50 ◦ C. After purge by high-purity He, the sample was heated from 50 ◦ C to 900 ◦ C at a rate of 10 ◦ C min−1 , and the desorbed O2 was monitored using a mass spectrometer (Pfeiffer Vacuum ThermoStar GSD 301 T2). Carbon monoxide temperature-programmed desorption (COTPD) was measured with a similar procedure on the Micromeritics AutoChem 2920 II instrument. After pretreatment at 350 ◦ C in a 20% O2 /He flow, followed by purge with He and cooling down to 50 ◦ C, the sample was exposed to 5% CO/He at 50 ◦ C for 1 h, and then the gas-phase CO was removed by He purge. The temperature was subsequently raised to 700 ◦ C at a rate of 10 ◦ C min−1 . The desorbed CO and CO2 were monitored by the mass spectrometer.
2.3. Catalytic reactions The PROX of CO was performed on a fixed-bed reactor operated at atmospheric pressure. The catalyst was pretreated in an O2 -containing He gas flow (O2 , 5 vol %) at 200 ◦ C for 40 min before reaction. After the catalyst was cooled down to a certain reaction temperature, the reactant gas containing 1% CO, 1% O2 , and 50% H2 in He was introduced into the reactor. Typically, 100 mg of catalyst was used, and the total gas flow rate was 50 mL min−1 , corresponding to a weight hourly space velocity (WHSV) of 30,000 mL g−1 h−1 . The inlet and outlet gas compositions were analyzed by an on-line gas chromatograph equipped with TCD and FID. H2 , O2 , CO, and CO2 were separated using a carbon molecular sieve (TDX-01) column. CO and CO2 were further converted to methane by a methanation reactor and analyzed using a FID detector. The carbon balance in each run was 100 ± 3%. The conversion of CO and the O2 selectivity
Fig. 1. Effect of the atomic ratio of Mn/(Co + Mn) on catalytic behaviors for the PROX of CO in H2 . (A) CO conversion; (B) O2 selectivity for CO oxidation. Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , 48% He; total flow rate, 50 mL min−1 .
for CO oxidation were calculated by the following equation: CO conversion (%) =
Selectivity (%) = =
1 − [CO]out × 100% [CO]in
1/2[CO2 ] × 100% [O2 ]in − [O2 ]out 1/2([CO]in − [CO]out ) × 100% [O2 ]in − [O2 ]out
3. Results and discussion 3.1. Effect of the modification of Co3 O4 by MnOx on catalytic behaviors Several research groups have demonstrated that Co3 O4 is a promising catalyst for the PROX in H2 , but relatively high temperatures (150–200 ◦ C) are required to attain a high activity [35–46]. Our results (Fig. 1) show that the single Co3 O4 exhibits very low CO conversions (<10%) at temperatures ≤75 ◦ C. The modification of Co3 O4 by MnOx significantly enhanced CO conversions at these temperatures (Fig. 1A). The Mn–Co–O catalyst with an atomic Mn/(Co + Mn) ratio of 0.11, i.e., Mn/Co ratio of 1/8, exhibited the highest CO conversions; CO conversions over this catalyst reached 56% and 92% at temperatures of 40 ◦ C and 75 ◦ C, respectively. These values are significantly higher than those obtained over the single Co3 O4 and MnO2 catalysts. The O2 selectivities at ≤75 ◦ C over these catalysts were 100% (Fig. 1B). An increase in temperature to 100 ◦ C increased the CO conversion to 97% over the Mn–Co–O (Mn/Co = 1/8) catalyst, but the O2 selectivity was slightly decreased to 93%. Further comparisons in catalytic behaviors among the Mn–Co–O catalysts (with Mn/Co ratios of 1/8 and 2/8) and the single Co3 O4 and MnO2 are shown in Fig. 2. Over the Mn–Co–O catalyst with
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Fig. 3. Changes of catalytic performances with time on stream for Co3 O4 and Mn–Co–O (Mn/Co = 1/8) in the PROX of CO in H2 . Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , 48% He; total flow rate, 50 mL min−1 ; reaction temperature, 100 ◦ C.
Fig. 2. Dependence of catalytic performances on reaction temperature for the PROX of CO in H2 over Co3 O4 , MnO2 , and Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8. (A) CO conversion; (B) O2 selectivity for CO oxidation. Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , 48% He; total flow rate, 50 mL min−1 .
a Mn/Co ratio of 1/8, the CO conversion remained ≥97% at temperatures of 100–200 ◦ C, and 100% CO conversion was achieved at 125–175 ◦ C. A further increase in temperature to 250 ◦ C gradually decreased the CO conversion likely due to the competitive oxidation of H2 at higher temperatures. On the other hand, the highest CO conversions over the single Co3 O4 and MnO2 catalysts were 91% and 95%, respectively at 175 ◦ C. The CO conversion decreased rapidly as the temperature was raised to >200 ◦ C over these single oxides. It is of interest to note that, when compared to that over Co3 O4 or MnO2 alone, the decreases in CO conversion at higher temperatures (>200 ◦ C) are less significant over the Mn–Co–O catalysts. Moreover, as shown in Fig. 2B, the decrease in O2 selectivity for CO oxidation over the Mn–Co–O catalyst was also less remarkable than that over Co3 O4 or MnO2 alone at higher temperatures (>200 ◦ C). Fig. 2 also demonstrates that the temperature range, in which CO conversions are higher than 90%, becomes significantly wider (75–225 ◦ C) after the modification of Co3 O4 by MnOx with a Mn/Co ratio of 1/8 or 2/8. Moreover, our Mn–Co–O catalyst could provide 100% CO conversion at 125–175 ◦ C although the O2 selectivity for CO oxidation was 50–75% at these temperatures. The complete conversion of CO is an important requirement for PROX catalysts because the PEM fuel cell is also sensitive to low-level CO impurity (<20 ppm) in H2 stream [47]. The temperature range (125–175 ◦ C) of the present Mn–Co–O catalyst, over which 100% CO conversion could be achieved, may also be suitable for the CO-PROX process, which should ideally lie between a WGS reactor (typically working at 200–250 ◦ C) and a PEM fuel cell (typically operating at 80 ◦ C).
at 100 ◦ C, although CO conversion was not high. In contrast, the Mn–Co–O (Mn/Co = 1/8) catalyst was very stable under the same reaction conditions. No significant changes in both CO conversion and O2 selectivity was observed for a 100 h of reaction over the Mn–Co–O (Mn/Co = 1/8) catalyst. Because H2 produced from steam reforming generally contains H2 O and CO2 , we have investigated the influences of H2 O and CO2 on the catalytic performances of our catalyst for the PROX of CO in H2 . Fig. 4 compares the catalytic performances of the Mn–Co–O (Mn/Co = 1/8) catalyst in the absence and the presence of CO2 (10 vol.%) and/or H2 O (10 vol.%) in reactant stream at different reac-
3.2. Stability of the MnOx -promoted Co3 O4 catalyst As mentioned earlier, the deactivation [31,33,34,38,39] with time on stream is a serious problem for the Co3 O4 -based CO-PROX catalysts. Fig. 3 shows that Co3 O4 alone deactivates quite rapidly
Fig. 4. Effect of the presence of H2 O and/or CO2 in reactant stream on catalytic performances of Mn–Co–O (Mn/Co = 1/8) catalyst. Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , He as balance; total flow rate, 50 mL min−1 .
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tion temperatures. The addition of CO2 and/or H2 O decreased the catalytic activity at temperatures < 150 ◦ C, while the selectivity was not significantly decreased. The activity decreased in the sequence of adding CO2 > adding H2 O > adding (CO2 + H2 O). The inhibiting effect by the addition of CO2 was also reported over other catalysts such as CuO–CeO2 , and the decrease in activity was attributed to the adsorption of CO2 on the active site forming carbonates [48]. Over our Mn–Co–O catalyst, the inhibiting effect of H2 O is stronger than that of CO2 . We speculate that this might be related to the serious blockage of the active site by H2 O at ≤100 ◦ C, suppressing the activation of CO and O2 on the catalyst. The inhibiting effects of both CO2 and H2 O became less significant at higher temperatures. At temperatures of 150–200 ◦ C, the conversion of CO kept at ∼100% even for the reactant stream containing both CO2 and H2 O. We further investigated the stability of the Mn–Co–O (Mn/Co = 1/8) catalyst at two different contact times (WHSVs = 30,000 and 120,000 mL h−1 g−1 ) at 125 ◦ C. As shown in Fig. 5, at a WHSV of 30,000 mL h−1 g−1 , CO conversion was always 100% during the reaction for 100 h. No CO could be detected at the outlet of the reactor. The increase in WHSV to 120,000 mL h−1 g−1 decreased CO conversion from 100% to ∼90%, but both CO conversion and O2 selectivity were sustained during the long-term reaction. These observations clearly suggest that our catalyst is quite stable under these conditions. However, when H2 O (10 vol.%) and CO2 (10 vol.%) were added into the reactant steam, the CO conversion began to decline gradually after 40 h of reaction (Fig. 5). Further enhancement in catalyst stability in the presence of H2 O and CO2 is needed in future studies. 3.3. Characterization of MnOx -promoted Co3 O4 catalysts To understand the nature of the significant promoting effect of MnOx on catalytic performances of Co3 O4 , we investigated the structures of the Mn–Co–O catalysts by several techniques. Fig. 6 shows the XRD patterns of the Mn–Co–O samples with different Mn/Co ratios. The diffraction peaks of single Co3 O4 and MnOx could be ascribed to cubic spinel phase Co3 O4 and rutile-like -MnO2 [44–46]. In the spinel structure of Co3 O4 , Co2+ and Co3+ ions are located in tetrahedral and octahedral coordination environments, respectively [49]. For the Mn–Co–O samples with different Mn/Co ratios, only diffraction peaks attributable to the cubic spinel phase of Co3 O4 could be observed. The diffraction peaks of Co3 O4 became
Fig. 5. Changes of catalytic performances with time on stream for Mn–Co–O (Mn/Co = 1/8) catalyst under different reaction conditions. Reaction conditions: catalyst, 0.10 g; reactant composition: 1% CO, 1% O2 , 50% H2 , 48% He; reaction temperature, 125 ◦ C.
Fig. 6. XRD patterns for Mn–Co–O samples together with single Co3 O4 and MnOx .
weaker and broader in the presence of MnOx . The broadening of the diffraction peaks for the Mn–Co–O samples suggests the decreases in the crystalline sizes. We have estimated the crystalline sizes of Co3 O4 and the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8 by the Scherrer equation. The result shows that the crystalline size for Co3 O4 was ∼19 nm, which was decreased to 12 nm and 8.1 nm for the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8, respectively. As shown in Fig. 6, the shift of diffraction peaks belonging to Co3 O4 to smaller 2 angles was observed in the presence of MnOx with a Mn/Co ratio of 1/8, indicating an enlargement of the Co3 O4 unit cell. Similar observations were reported by Morales et al. [50,51], and the formation of a Mnx Co3−x O4 solid solution phase was proposed [44,50,51]. It can be expected that, in the Mnx Co3−x O4 solid solution, there may exist electron transfer between Co2+ and Mn4+ . Thus, a part of Co2+ and Mn4+ may be changed into Co3+ and Mn3+ , respectively [44,50,51]. The lattice expansion (2 decreasing) ˚ in the spinel strucoccurred due to the replacement of Co3+ (0.61 A) ˚ with a slightly larger radius. The formation ture by Mn3+ (0.65 A) of the Mnx Co3−x O4 solid solution was further confirmed by X-ray absorption fine structure (XAFS) spectroscopy through analyzing the difference between the Mn–Co–O and Co3 O4 samples in the coordination structures around Co and Mn atoms and the valence state of Mn species in the Mn–Co–O sample [50,51]. Many other studies also demonstrated the formation of Mnx Co3−x O4 solid solution in the Mn–Co–O binary oxides prepared by co-precipitation followed by calcination [44,50,52,53]. Moreover, the Hausmannite Mn3 O4 , which can be viewed as MnII MnIII 2 O4 and possesses a spinel-type crystalline structure containing close-packed oxide ions with tetrahedral sites occupied by Mn2+ and octahedral sites by Mn3+ , can also be easily prepared [54]. The similar crystalline structures of Co3 O4 and Mn3 O4 also allow us to consider that the formation of Mnx Co3−x O4 solid solution in our Mn–Co–O catalyst is reasonable. Fig. 7 shows the TEM micrographs of Co3 O4 and the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8. It is clear that both Co3 O4 and the Mn–Co–O samples are composed of nanoparticles with uniform sizes. The sizes of Co3 O4 and Mn–Co–O samples with Mn/Co of 1/8 and 2/8 were estimated to be 20, 9.6, and 7.8 nm, respectively. These sizes were close to those calculated by the Scherrer equation from XRD results. Thus, it can be concluded that the introduction
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Fig. 8. XPS spectra for Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8 together with single Co3 O4 and MnOx . (A) Co 2p; (B) Mn 2p.
Fig. 7. TEM micrographs for Co3 O4 (A) and Mn–Co–O samples with Mn/Co ratios of 1/8 (B) and 2/8 (C).
of MnOx to Co3 O4 leads to the formation of smaller Mnx Co3−x O4 solid solution particles. The specific surface area obtained from N2 sorption at −196 ◦ C increased from 52 m2 g−1 for Co3 O4 to 102 and 104 m2 g−1 for the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8, respectively. The specific surface area of the Mn–Co–O sample with a Mn/Co ratio of 2/8 (104 m2 g−1 ) is lower than that estimated from the particle size by assuming a spherical particle model (127 m2 g−1 ), and we think that such a deviation is acceptable by considering the experimental errors and that the Mn–Co–O (Mn/Co = 2/8) particles are not all perfectly spherical (Fig. 7). The Co 2p and Mn 2p XPS spectra for the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8 are shown in Fig. 8 together with single Co3 O4 and MnO2 . The binding energy of Co 2p3/2 for Co3 O4 was 779.6 eV, consistent with that reported in another work [55]. The binding energy of Co 2p3/2 increased after the addition of MnOx to Co3 O4 . This observation suggests the increase in fraction of Co3+ /Co2+ in the Mn–Co–O catalysts. On the other hand, as compared to that in single MnO2 , the binding energy of Mn 2p3/2 in the Mn–Co–O decreased slightly, and this may be due to the partial reduction of Mn4+ and the smaller difference in the binding energy between Mn4+ and Mn3+ [56]. These results suggest an interaction between CoOx and MnOx species in the Mn–Co–O samples, which leads to the electron transfer from CoOx to MnOx This is in agreement with the formation of Mnx Co3−x O4 solid solution in the Mn–Co–O samples, which could also be deduced from the XRD results. The surface Mn/Co ratios in the Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8 were estimated from XPS, and the values were 1/8.4 and 1/4.1, respectively, indicating that manganese was homogeneously distributed in the solid solutions in these samples. The O2 -TPD has been performed to gain insight into the nature of oxygen species over the Mn–Co–O samples. Fig. 9 shows that the desorption of O2 occurs in two temperature regions over our Mn–Co–O samples. It is reported that the desorption peaks at temperatures lower than 300–350 ◦ C are generally attributable to the surface adsorbed oxygen species (␣-oxygen species) such as O2 −
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Fig. 9. O2 -TPD profiles for Co3 O4 and Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8.
and O− , while the desorption peaks at higher temperatures can be ascribed to lattice oxygen species (-oxygen species) [57,58]. Co3 O4 exhibited a weak desorption peak at 217 ◦ C, which was mainly ascribed to the O2 − species [58]. The peak belonging to ␣oxygen shifted to higher temperatures after the addition of MnOx to Co3 O4 , implying the change of O2 − to O− [57,58]. Moreover, the intensity of the desorption at >350 ◦ C increased significantly in the presence of MnOx . It is reasonable to attribute the desorption peaks at 350–650 ◦ C to surface lattice oxygen. Thus, the presence of MnOx enhanced the mobility of surface lattice oxygen of the catalyst. The activation of O2 on catalyst surfaces is generally a key step for CO oxidation. A kinetic study of CO oxidation over Co3 O4 revealed a higher reaction order of O2 (0.28) than that of CO (0.12), and this might imply a more important role of O2 activation in determining the reaction rate [34]. The correlation of the O2 -TPD results with the catalytic performances suggests that the abundant oxygen species over the MnOx -promoted Co3 O4 catalyst may be one point contributing to its higher catalytic activity. Fig. 10 shows the CO-TPD profiles for the Mn–Co–O samples together with Co3 O4 . In addition to CO, the desorption of CO2 , which should be produced during the surface reaction between the adsorbed CO and the oxygen species, was also observed over Co3 O4 and the Mn–Co–O samples. The desorption of CO started from ∼160 ◦ C over Co3 O4 , and this starting temperature shifted to lower temperatures after the addition of MnOx . As compared to Co3 O4 , the Mn–Co–O samples showed a significantly larger amount of CO2 desorption (Fig. 10B). The intensity of CO desorbed was also increased in the presence of MnOx . The total amount of CO and CO2 desorbed from the Mn–Co–O samples (Mn/Co = 1/8 and 2/8) per surface area was 1.5–1.8 times that from Co3 O4 alone. These observations imply that the modification of Co3 O4 with MnOx increases the CO adsorption ability. As shown in Fig. 10B, CO2 desorption peaks appeared in both lower-temperature region (140–190 ◦ C) and higher-temperature region (300–350 ◦ C). The comparison of the CO2 desorption peaks (Fig. 10B) with the oxygen desorption results (Fig. 9) suggests that the CO2 desorbed in the lower-temperature region over Co3 O4 might be formed by the reaction of ␣-oxygen species with the adsorbed CO, since ␣-oxygen is the main oxygen species over Co3 O4 (Fig. 9). On the other hand, a larger amount of CO2 was desorbed in the higher-temperature region over the Mn–Co–O samples. These
Fig. 10. CO-TPD profiles for Co3 O4 and Mn–Co–O samples with Mn/Co ratios of 1/8 and 2/8. (A) CO desorbed, (B) CO2 desorbed.
CO2 desorbed may be contributed by the reaction of CO adsorbed with the lattice oxygen species (-oxygen species), since -oxygen is the dominant oxygen species on the surfaces of the Mn–Co–O samples at ≥300 ◦ C. Therefore, we speculate that both types of oxygen species on the Mn–Co–O catalyst, i.e., the surface adsorbed oxygen species (␣-oxygen species) and the lattice oxygen species (-oxygen species), may contribute to the oxidation of CO. As shown in Fig. 10, Some CO2 desorbed at high temperatures, and this might cause catalyst deactivation. The reactivity of CO with the oxygen species over the Mn–Co–O catalyst was investigated by the pulse reaction technique. Fig. 11A compares the conversion of CO pulses to CO2 over the Mn–Co–O (Mn/Co = 1/8) with that over Co3 O4 at 40 ◦ C. The amount of CO2 formed decreased with the number of successive CO pulses over both catalysts, and the reaction almost stopped after ∼10 and 20 pulses of CO over Co3 O4 and the Mn–Co–O, respectively. The integral amount of CO2 formed during the reaction of CO pulses over the Mn–Co–O was ∼4 times higher than that over Co3 O4 (Table 1). After the first run of reactions, the catalysts were regenerated in O2 flow at the same temperature for 30 min, and the second run of successive pulse reactions was started after purge with He. Fig. 11A shows that Co3 O4 cannot be regenerated by O2 at 40 ◦ C and no CO2 was formed during the second run of reactions. On the other hand, the CO pulse could be oxidized to CO2 during the reaction with the Mn–Co–O in the second and the third runs of Table 1 The amount of CO2 formed during the successive reactions of CO pulses with Co3 O4 and the Mn–Co–O (Mn/Co = 1/8). Catalyst
Co3 O4 Mn–Co–O Co3 O4 Mn–Co–O
Temperature (◦ C)
40 40 100 100
Amount of CO2 (mol) 1st run
2nd run
3rd run
0.628 3.32 2.16 5.86
0 1.97 1.58 5.81
0 1.63 1.40 5.80
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tivity for CO oxidation remained unchanged for a 100 h reaction at 100 ◦ C or 125 ◦ C over the Mn–Co–O catalyst, whereas the deactivation was observed over Co3 O4 . The presence of MnOx caused the formation of smaller Mnx Co3−x O4 solid solution particles. This led to the increased amounts of both active oxygen species and adsorbed CO. Furthermore, the reduced MnOx -promoted catalyst exhibited significantly higher O2 activation abilities particularly at lower temperatures. These are proposed to contribute to the enhanced activity and stability of the MnOx -promoted Co3 O4 catalysts. Acknowledgements This work was supported by the NSF of China (Nos. 20625310, 20773099, 20873110 and 20923004), the National Basic Research Program of China (No. 2010CB732303), the Key Scientific Project of Fujian Province (No. 2009HZ0002-1), and the Research Fund for the Doctoral Program of Higher Education (No. 20090121110007). References
Fig. 11. Successive reactions of CO pulses with Co3 O4 () and the Mn–Co–O (Mn/Co = 1/8) (䊉) at 40 ◦ C (A) and 100 ◦ C (B). Reaction conditions: catalyst, 0.10 g; He carrier gas, 80 mL min−1 ; volume of pulse, 0.63 mL; CO partial pressure in the pulse, 5.07 kPa (He balance).
pulse reactions although the integral amounts of CO2 produced decreased gradually in the next two runs. With increasing the reaction and regeneration temperature to 100 ◦ C, the integral amount of CO2 formed in the first run of pulse reactions increased over both catalysts. The Mn–Co–O still exhibited remarkably higher CO2 formation activity than Co3 O4 alone. Co3 O4 could also be regenerated to some extent at 100 ◦ C, but the integral amount of CO2 formed decreased in the second and the third runs. On the other hand, the integral amounts of CO2 formed in the second and the third runs over the Mn–Co–O (Mn/Co = 1/8) were almost the same with that in the first run. Thus, the reduced Mn–Co–O could be regenerated almost completely at 100 ◦ C. The significant larger amount of CO2 formed in above pulse reactions over the Mn–Co–O catalyst also indicates that the MnOx promotion increased the concentration of reactive oxygen species for the oxidation of CO to CO2 . This is consistent with O2 -TPD results. As described above, the comparison of O2 -TPD (Fig. 9) with CO-TPD (Fig. 10B) suggests that both the surface adsorbed oxygen species (such as O2 − and O− ) and the lattice oxygen species have the ability to react with the CO adsorbed on catalyst surfaces. Moreover, the pulse reaction results further suggest that the reduced Mn–Co–O catalyst can activate O2 more easily than Co3 O4 alone especially at lower temperature to regenerate the reactive oxygen species for the oxidation of CO. Even at 100 ◦ C, the reduced CoOx could not be completely regenerated by O2 . We think that this is one main reason for the deactivation of Co3 O4 at lower temperatures. 4. Conclusions
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The modification of Co3 O4 by MnOx significantly promoted the preferential oxidation of CO in H2 -rich stream. The catalyst with a Mn/Co molar ratio of 1/8 afforded the best catalytic performance. The CO conversion was enhanced significantly especially at lower temperatures, and the temperature region, in which CO conversion was >90%, was significantly widened due to the presence of MnOx in Co3 O4 . The MnOx -promoted Co3 O4 could also catalyze the complete oxidation of CO, providing 100% CO conversion at 125–175 ◦ C. Moreover, the catalyst stability was markedly enhanced by MnOx modification; the conversion of CO and O2 selec-
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