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Direct nitrous oxide decomposition with CoOx-CeO2 catalysts
Applied Catalysis B: Environmental 106 (2011) 416–422
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Direct nitrous oxide decomposition with CoOx -CeO2 catalysts Ewa Iwanek a,∗,1 , Krzysztof Krawczyk a , Jan Petryk a , Janusz W. Sobczak b , Zbigniew Kaszkur b a b
Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 28 March 2011 Received in revised form 2 May 2011 Accepted 28 May 2011 Available online 6 June 2011 Keywords: Nitrous oxide decomposition Co3 O4 -CeO2 catalysts Catalyst state Reduction Phase interface
a b s t r a c t The focus of the performed studies were CoOx -CeO2 oxide catalysts for nitrous oxide decomposition. All CoOx -CeO2 systems exhibit similar or higher activity than the undoped cobalt catalyst. It has been found that in temperatures up to 800 ◦ C in a N2 O–Ar stream cobalt in these catalysts is in the form of Co3 O4 . At higher temperatures it is reduced to CoO. In a N2 O–O2 –Ar stream Co3 O4 is the main cobalt-containing phase in the entire studied temperature range. The obtained results revealed that the activity of CoOx CeO2 systems with a high cobalt loading increases with temperature only up to 800 ◦ C in a N2 O–Ar stream. Upon further temperature increase the activity of these catalysts decreases, as in the case of the undoped cobalt catalyst. This is due to the reduction of Co3 O4 to CoO. Hence, when oxygen is present in the feed and cobalt is in the form of Co3 O4 , the activity is higher. In contrast, the activity of catalysts with the cobalt molar ratio no greater than 0.64 is the same in both N2 O–Ar and N2 O–O2 –Ar streams and increases with temperature in the entire studied range (700–850 ◦ C). It has been demonstrated that at 850 ◦ C in a N2 O–Ar stream CoOx -CeO2 systems contain two types of CoO, which require different conditions to be oxidized. This is a result of a different strength of interaction with CeO2 . It can be concluded that the activity of CoOx -CeO2 systems results from the activity of Co3 O4 and of the cobalt oxide–ceria interface. The share of each component is determined by the cobalt content. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitrous oxide is said to be the most harmful ozone depletor which is not a volatile organic compound [1,2]. Moreover, the global warming potential, GWP, of nitrous oxide is approximately 300 times higher than that of carbon dioxide [3]. Due to its high GWP, as well as substantial emissions, this gas significantly contributes to global warming. The main industrial source of N2 O are nitric acid plants. Nitrous oxide forms in them as a by-product of ammonia oxidation on Pt-Rh gauze. It has been experimentally established that in ammonia oxidation the selectivity to N2 O on oxide catalysts which contain cobalt oxide, is much lower than on the Pt-Rh catalyst [4,5]. Therefore, catalytic systems containing Co3 O4 are the subject of studies as potential nitrous oxide decomposition catalyst [6–8]. Currently, the most frequently implemented method of nitrous oxide abatement is high temperature N2 O decomposition [9]. This process is carried out directly downstream of the Pt-Rh gauze, where a selective N2 O-decomposition catalyst is placed. The temperature of this stream is approximately 800 ◦ C and higher.
∗ Corresponding author. E-mail address: [email protected] (E. Iwanek). 1 Nee Wilczkowska. 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.05.049
Undoped cobalt oxide catalysts have several flaws. The main problem associated with their application in high temperature nitrous oxide decomposition is the decrease of their activity at temperatures exceeding 800 ◦ C [10]. This results from the reduction of the active phase to CoO [10,11]. Furthermore, the grains of the catalyst tend to sinter and form clusters [10]. What is more, the presence of oxygen in the feed suppresses the activity of the undoped cobalt catalyst under certain conditions [10]. Supports, such as Al2 O3 [12] or CeO2 [13–17], are used in order to increase the activity, as well as the mechanical and thermal stability, of the cobalt spinel. Ceria is added to Co3 O4 also due to the fact that Co-Ce catalysts exhibit a greater specific area and better redox properties than the undoped cobalt spinel [14,15,18]. According to literature data concerning cobalt catalysts, the addition of CeO2 [7] or alkali metal ions, such as Cs+ [18] and K+ [19,20], has a positive effect on the activity of Co3 O4 in N2 O decomposition. It has been evidenced that the oxygen vacancies on the surface of CeO2 exhibit a high mobility [19]. Therefore, the oxygen adsorbed by CeO2 can be swiftly transported to the active center [21]. The ease of adsorbing oxygen from the gas phase by cerium oxide and transporting it to other catalyst components, is the reason why it is added to Co-based oxidation catalysts. However, there is little information regarding the oxidation of this type of catalysts. In this work the oxidation of CoOx -CeO2 catalytic systems, in which Co3 O4 is thermally reduced to CoO, was investigated.
E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422
The aim of this work was to establish how the Ce/Co ratio in CoOx -CeO2 catalysts affects their activity in high temperature N2 O decomposition, as well as their thermal stability and other properties. The influence of oxygen in the feed on the activity of CoOx -CeO2 catalysts was established. Nitrogen physisorption, XPS, XRD and TG-DTA-MS studies were performed in order to determine the effect of reaction temperature and oxygen content on the state of the catalyst under high temperature nitrous oxide decomposition conditions.
2. Experimental 2.1. Catalyst preparation The CoOx -CeO2 catalysts were prepared from ceria (Riedel-de Haen, anhydrous, pure) and cobalt nitrate, Co(NO3 )2 ·6H2 O (POCH Gliwice, pure for analysis). The mixture was heated to 400 ◦ C in air until the nitrate decomposed. To 20.0 g of the obtained oxides 1.0 g of (NH4 )2 CO3 and 0.3 g of glycerin were added. The mixture was ground and sieved (0.4 mm). Tablets (1.70 cm × 0.3 cm) were pressed in two consecutive steps applying the force of 5 and 10 tons, respectively. Thus formed tablets were calcined at 850 ◦ C for 48 h, cooled, and crushed. The 2.0–2.5 mm grains were used in activity measurements. Catalysts with the following molar cobalt contents (%Co), i.e. 100 × Co/(Co + Ce), were made: 2, 8, 32, 64, 86, 92, 96 and 98. The catalysts have been given symbols, in which the number denotes the molar Co content (%Co). Catalysts from undoped CeO2 (Riedel-de Haen, anhydrous, pure) and Co3 O4 , obtained from the thermal decomposition of cobalt nitrate (POCH Gliwice, pure for analysis), were used as reference materials. 2.2. Activity measurements In order to perform activity measurements, the catalysts were placed in a cylindrical flow reactor made from quartz glass. The measurements were carried out under atmospheric pressure at a constant temperature (700, 750, 800 or 850 ◦ C) and gas hourly space velocity, i.e. 8 × 104 h−1 . The catalyst bed volume was 0.5 ml. The reaction mixture was passed through the catalyst bed from the top. Two inlet stream compositions were used, namely 5% N2 O, 95% Ar and 5% N2 O, 5% O2 , 90% Ar. The N2 O content in the inlet and outlet mixtures was established using gas chromatography (KONIK HRGC 4000B). The presented values are the average of 6 measurements whose results differed by less than 3%. The characterization measurements were performed on grains from the lowest layer of the catalyst bed. After the tests, the catalyst bed was cooled in flowing argon (900 ml min−1 ) at the rate of 80 ◦ C min−1 . A TG-MS analysis showed that this procedure does not lead to a change in mass nor any m/z signal. The presence of oxygen at 850 ◦ C has a beneficial effect on the activity of CoOx -CeO2 catalysts with molar cobalt contents of 86% or higher, as in the case of the undoped cobalt catalyst. In contrast, the activity of catalysts containing no more than 64% cobalt is not influenced by the addition of oxygen to the inlet stream. In order to establish why there is no influence of oxygen on the activity of these catalysts, the characterization experiments were performed mainly on Ce32Co and Ce64Co. 2.3. Catalyst characterization 2.3.1. Specific surface area and porosity measurements The specific surface area and pore distribution measurements of the fresh and used catalysts were performed using ASAP 2020 (Micromeritics). The nitrogen physisorption experiments were car-
417
ried out at T = 77 K and the p/p0 range of 0.01–1.00. The SBET values were calculated using the Brunauer–Emmett–Teller equation. 2.3.2. TG-DTA-MS measurements Thermogravimetric measurements were carried out on NETZSCH STA 449C apparatus equipped with a NETZSCH QMS 403C quadruple mass spectrometer. The reference material was electrocorundum of the same fraction as the catalyst. The following two gases were passed through the thermobalance: argon (Multax, N5.0) and oxygen (Multax, N5.0). The total gas flow through the furnace with the sample was 100 ml min−1 . Throughout the measurements, a channeltron detector recorded the following m/z signals: 2, 18, 28, 30, 32, 44, and 46. Data were analyzed using NETZSCH Proteus Thermal Analysis software. The sample mass was 150 mg. In measurements performed in accordance with program 1, a sample (fresh catalyst or after catalytic tests) was heated up to 1100 ◦ C, and cooled to 200 ◦ C at 5 ◦ C min−1 . The feed consisted of 18% O2 in argon. The oxygen content was sufficient to allow a full oxidation of a reduced sample and a complete reduction during the heating phase, as well as another full oxidation during the cooling phase. The Co3 O4 content in the samples was determined based on the mass change resulting from oxygen uptake by the sample and the mass of oxygen evolved during the reduction of Co3 O4 to CoO. In measurements performed in accordance with program 2, a sample was heated up to 1000 ◦ C at 10 ◦ C min−1 in argon. Next, the sample was cooled to 650 ◦ C at 0.2 ◦ C min−1 in a stream composed of 2.5% O2 , 97.5% argon. In the case of Ce32Co an additional measurement was performed. The three parts of program 3 were as follows: 1) heating a fresh sample to 1000 ◦ C with the heating rate of 20 ◦ C min−1 , in a stream containing 5% oxygen, 2) cooling it to 850 ◦ C with the cooling rate of 5 ◦ C min−1 , 5% O2 , 95% Ar, 3) increasing the oxygen content by 2% every 30 min, from 5 to 90%, T = 850 ◦ C. 2.3.3. XPS measurements The XPS measurements were carried out with a scanning photoelectron spectrometer PHI 5000 VersaProbe (Physical Electronics USA/ULVAC). Monochromatized Al K␣ radiation was used (1486.6 eV). The spectra of the Ce 3d, Co 2p, O 1s, C 1s regions were measured with a pass energy of 23.5 eV in 0.1 eV increments. The C 1s (BE = 284.7 eV) signal was used to calibrate the binding energy (BE) scale. The O 1s signal components were fitted using AVANTAGE v. 4.51 (Thermo Scientific) after a Shirley background subtraction. The XPS measurements of all studied CoOx -CeO2 catalysts were performed after catalytic tests at 850 ◦ C in a N2 O–Ar stream. Furthermore, this technique was applied to investigate the surface of a Ce64Co sample operating at the same temperature in a N2 O–O2 –Ar stream. 2.3.4. XRD measurements XRD measurements were performed using a hybrid RigakuDenki Geigerflex diffractometer. The source of radiation was a Cu lamp (40 kV, 40 mA). The beam divergence was approximately 0.1◦ . The experiments were performed in the scattering angle range of 20–125◦ with a 0.2◦ min−1 step. An internal standard (quartz) was used. In order to determine if cobalt is incorporated into the ceria lattice in catalysts with molar cobalt contents no greater than 0.32, measurements with fresh Ce2Co, Ce8Co and Ce32Co catalysts, as well as with CeO2 and Co3 O4 , were performed. The CeO2 lattice constant values in the following fresh catalysts: CeO2 , Ce2Co, Ce8Co and Ce32Co were calculated based on a linear extrapolation of values obtained from all reflections to cos() = 0. The mean square deviation of atoms from the nodes of CeO2 lattice was calculated on
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a N2O conversion[%]
N2O conversion[%]
100
75
50
100 90 80 70 60
25 50 0
0
25
50
75
100
Cobalt content [%mol] 0
25
50
75
100
b
Cobalt content [%mol]
N 2O conversion[%]
Fig. 1. Dependence of N2 O conversion on the cobalt content in the catalyst; feed: 5% N2 O, 95% Ar; T = 700 ◦ C (pluses), 750 ◦ C (squares), 800 ◦ C (triangles), or 850 ◦ C (circles). Data for Co3 O4 taken from [10].
the basis of the Debye–Waller factor (LAZY-PULVERIX). The Co3 O4 lattice constant was established using the (4 4 0) reflection. In samples of catalysts after activity tests at 850 ◦ C in a N2 O–Ar stream, the lattice constant of CoO and the crystallite size of this phase were determined. The CoO lattice constant was calculated on the basis of (1 1 1), (2 0 0) and (2 2 0) reflections.
100 90 80 70 60 50
0
25
a
The calcination of the studied catalysts was carried out at 850 ◦ C to minimize their sintering during catalytic tests. One of the consequences of this was a low surface area of the catalysts. The
2.0 1.5 1.0 0.5 0 0
20
40
60
80
100
Cobalt content [mol%]
b
0.25 0.20 0.15 0.10 0.05 0.00
3.2. Physisorption measurements
100
dependence of SBET on the cobalt content is shown in Fig. 3a. The largest specific surface area is exhibited by CeO2 (1.9 m2 g−1 ), whereas that of Co3 O4 is the smallest (0.6 m2 g−1 ). The surface of all the studied catalysts diminished by approximately 0.2 m2 g−1 dur-
Specific surface area [m2/g]
The decomposition of nitrous oxide on all studied catalysts leads to the formation of N2 and O2 , regardless of the reaction temperature and oxygen content in the feed. The curves in Fig. 1 depict the influence of the cobalt content in the catalyst on N2 O conversions in a N2 O–Ar stream at 700 ◦ C, 750 ◦ C, 800 ◦ C and 850 ◦ C on the studied catalysts. The activity of all CoOx -CeO2 systems is much higher than that of the undoped CeO2 catalyst. It can be seen that even 2% of cobalt in the catalyst results in higher N2 O conversions (Fig. 1). The catalysts containing 8% of cobalt or more exhibit higher activities than the undoped cobalt catalyst at 800 ◦ C and 850 ◦ C. It is noteworthy that regardless of the reaction temperature the highest activity was noted for Ce86Co (Fig. 1). The influence of oxygen in the feed on nitrous oxide conversion at 800 and 850 ◦ C is shown in Fig. 2. At 800 ◦ C, as in the case of lower temperatures (results not shown), the N2 O conversion values are not influenced by the inlet stream composition, or are affected only slightly. At 850 ◦ C the values obtained on Ce2Co, Ce8Co, Ce32Co and Ce64Co do not exhibit a dependence on the feed composition. In contrast, the N2 O conversions noted on catalysts containing more than 64% cobalt are higher in the presence of oxygen (Fig. 2). The influence of 5% oxygen in the feed on the nitrous oxide conversion is the bigger, the greater the cobalt content. The differences obtained at 850 ◦ C for Ce86Co, Ce96Co and the undoped cobalt catalyst in N2 O–O2 –Ar and N2 O–Ar streams are 3%, 11% and 21% respectively. It can be assumed that the positive influence of oxygen in the feed on the activity of catalysts with a high cobalt content probably results from the fact that cobalt is kept in the form of Co3 O4 , as in the undoped cobalt catalyst [10].
75
Fig. 2. Influence of inlet stream composition on N2 O conversion at (a) 800 ◦ C and (b) 850 ◦ C; feed: argon and 5% N2 O (circles), 5% N2 O, 5% O2 (squares). Data for Co3 O4 taken from [10].
Pore area [m 2/g]
3.1. Activity measurements
50
Cobalt content [%mol]
3. Results
10
13
17
24
38
63
110
232
699
Average pore radius[Å] Fig. 3. Nitrogen physisorption results: (a) dependence of specific surface area on the cobalt content, (b) pore distribution of CeO2 (white), Ce2Co (gray) and Co3 O4 (black).
E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422 Table 1 Results obtained during TG-DTA-MS measurements; program 1. Catalyst symbol
T [◦ C]
Feed
Ce2Co Ce8Co Ce32Co Ce64Co Ce64Co Ce64Co Ce64Co Ce64Co Ce86Co Ce96Co
850 850 850 850 800 750 700 850 850 850
5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 5% O2 , 90% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar
419
TG [%]
a 106 Co3 O4 content [%]
Ce86Co
105
0 0 0 0 100 100 100 91 0 0
104 103
Ce64Co
102 Ce32Co
101
II
I ing catalytic tests. The grains of the undoped cobalt catalyst [10] and the CoOx -CeO2 catalysts which contained at least 86% of cobalt were partially agglomerated. In contrast, grains of catalysts containing 64% cobalt or less did not form clusters under operating conditions. The results of porosimetric analysis show that the pore distributions of the undoped CeO2 catalyst and of the undoped cobalt catalyst differ (Fig. 3b). The former has mostly large pores, (Fig. 3b white columns), whereas Co3 O4 has mostly small ones (Fig. 3b black columns). Small pores are also dominant in all CoOx -CeO2 systems. This is true even in the case of the catalyst which contains 2% cobalt (Fig. 3b gray columns).
100 99 900
850
b
750
700
TG [%] 100.5 100.0
II
99.5 99.0
I
3.3. TG-DTA-MS measurements Thermogravimetric measurements have shown that CeO2 heated up to 1100 ◦ C does not undergo any changes in an argon stream or an argon-oxygen mixture. In contrast, upon heating of the samples, the TG curves of all studied CoOx -CeO2 catalysts show a mass loss accompanied by an m/z = 32 signal increase and an endothermic signal on the DTA curve. This transformation has been identified as the thermal reduction of Co3 O4 to CoO. These studies have shown that in all fresh CoOx -CeO2 catalysts cobalt is in the form of Co3 O4 . The results of experiments carried out in accordance with program 1 have revealed that during nitrous oxide decomposition at 800 ◦ C, as well as at lower temperatures, cobalt is in the form of Co3 O4 in all CoOx -CeO2 catalysts. As an example, the appropriate values for Ce64Co have been given in Table 1. However, in samples of CoOx -CeO2 catalysts operating at 850 ◦ C, the Co3 O4 content strongly depends on the reaction mixture composition. After operating at this temperature in a N2 O–Ar stream, the sample contains only CeO2 and CoO regardless of the cobalt content (Table 1). Hence, the activity of CoOx -CeO2 catalysts has been ascribed mainly to the interaction of CeO2 with cobalt oxide (CoO at 850 ◦ C or Co3 O4 at lower temperatures). Samples of Ce8CO, Ce32Co, Ce64Co, Ce86Co and the undoped cobalt catalyst after catalytic experiments in a N2 O–Ar stream were tested according to program 2. The mass changes of Ce32Co, Ce64Co and Ce86Co during the cooling stage are depicted in Fig. 4a. It can be seen that in the case of the first two CoOx -CeO2 catalysts there are two phases of rapid mass increase (Fig. 4a I and II). Therefore, there are two steps of CoO oxidation. Their contribution depends on the cobalt content in the catalyst. For Ce8Co, which contains little cobalt, the signal connected with the first step is minute. Moreover, it was not possible to detect the second oxidation step on Ce86Co. The TG curve obtained for Ce86Co is practically the same as that noted for the undoped cobalt catalyst. Therefore, the first oxidation step was assigned to the oxidation of CoO (to Co3 O4 ) weakly interacting with CeO2 . The second step, which takes place at lower temperatures, is the oxidation of CoO (to Co3 O4 ) strongly interacting with ceria. Thermally reduced fresh catalysts containing 32%
800
Temperature [°C]
98.5
0
Ce32Co 30
60
90
Oxygen content [%] Fig. 4. TG curves obtained during (a) cooling of Ce32Co, Ce64Co and Ce86Co; program 2, (b) increase of oxygen content in the gas surrounding Ce32Co; T = 850 ◦ C; program 3.
and 64% cobalt are also oxidized in two steps. Hence, it can be concluded that the formation of two types of CoO is characteristic for the catalysts themselves, and it is not a property acquired during N2 O decomposition. The TG curve of Ce32Co during a measurement performed according to program 3 is shown in Fig. 4b. In this experiment, the sample was kept at a constant temperature, i.e. 850 ◦ C, and the oxygen content in the stream introduced into the furnace with the sample was increased from 5% to 90%. The mass of the sample, similarly as in the case of program 2, increases in two steps. When the oxygen content equals 8%, the CoO weakly interacting with CeO2 begins to oxidize (Fig. 4b I). Next, despite the gradually increasing oxygen content, the oxidation rate diminishes and then increases again (Fig. 4b II). 3.4. XPS measurements The Co 2p region of the XPS spectra of Ce64Co operating at 850 ◦ C with the two studied reaction mixture compositions are shown in Fig. 5. Intensive satellites can be seen in the spectrum of Ce64Co working in a N2 O–Ar stream. They are located approximately 6 eV away from the main signals (Fig. 5). Hence, cobalt on the surface of this sample is in the form of CoO. The analysis of XPS results revealed that at 850 ◦ C in a N2 O–Ar stream, the only cobaltcontaining species found on the surface of CoOx -CeO2 catalysts is CoO, irrespective of the cobalt content. In contrast, a spectrum typical for the higher cobalt oxide was acquired for the Ce64Co sample operating in a N2 O–O2 –Ar stream (Fig. 5). In this spectrum, the
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E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422 Table 3 The CoO lattice constant and crystallite size in samples after catalytic tests at 850 ◦ C in a 5% N2 O, 95% Ar stream, calculated from XRD results.
Co2p3/2 Co2p1/2 shake-up
Intensity [a.u.]
shake-up
N2O-Ar
N2O-O2-Ar 810
800
790
780
770
Binding energy[eV] Fig. 5. The Co 2p region of XPS spectra of Ce64Co after tests with different feed compositions.
satellites are less intensive and are shifted approximately 10 eV in the direction of higher binding energy values than the main signals. No differences were noted in the Ce 3d region of CoOx -CeO2 catalysts, regardless of the cobalt content (results not shown). The obtained spectra are typical for CeO2 . Moreover, in spectra of Ce64Co samples after operating in the two studied reaction mixtures, no components corresponding to Ce3+ can be seen (results not shown). 3.5. XRD measurements The results of the XRD measurements show that there is no phase which contains both Ce and Co in the studied catalysts (Fig. 6). In all fresh CoOx -CeO2 catalysts including Ce2Co (Fig. 6b) the only cobalt-containing phase is Co3 O4 . The intensity of Co3 O4 signals increases with the cobalt content. Lattice constants of both CeO2 and Co3 O4 were established based on measurements with an internal standard. The resulting values are given in Table 2. The CeO2 ˚ The same lattice constant in the undoped ceria catalyst is 5.416 A. value was noted for: Ce2Co, Ce8Co and Ce32Co (Table 2). The mean square deviation of ions from the nodes is approximately 0.2 for all four abovementioned catalysts. Hence, it can be stated that the incorporation of cobalt into the ceria lattice does not occur in the studied catalysts. The lattice constant of Co3 O4 in Ce8Co, Ce32Co, as well as the undoped cobalt catalyst, was determined. It equals 8.09 A˚ (Table 2). In the case of Ce2Co, the Co3 O4 lattice constant was not established due to the fact that the spinel content was too small. The diffraction pattern of Ce64Co after catalytic tests in N2 O–Ar and N2 O–O2 –Ar streams is shown in Fig. 7. The XRD results indicate that in all CoOx -CeO2 catalysts operating at 850 ◦ C in N2 O–Ar and N2 O–O2 –Ar streams, cobalt is present mainly in the form of CoO Table 2 Lattice parameters of CeO2 and Co3 O4 , as well as the mean square deviation of ceria atoms from the nodes in the ceria lattice, calculated from XRD results. Catalyst symbol
˚ aCeO2 [A]
MSDCeO2
˚ aCo3 O4 [A]
CeO2 Ce2Co Ce8Co Ce32Co Co3 O4
5.416 5.416 5.416 5.416 –
0.2 0.2 0.2 0.2 –
– – 8.09 8.09 8.09
Catalyst symbol
˚ aCoO [A]
dCoO [nm]
Ce32Co Ce64Co Ce86Co Ce96Co Co3 O4
4.26 4.26 4.26 4.26 4.26
36 38 42 46 49
and Co3 O4 , respectively. To exemplify this, the diffraction pattern of Ce64Co is shown in Fig. 7. The values of the CoO lattice constant, as well as the crystallite size of CoO particles, in CoOx -CeO2 catalysts operating at 850 ◦ C in a N2 O–Ar stream is given in Table 3. No dependence of the CoO lattice constant on the catalyst composition has been found in the XRD studies. However, the results clearly indicate that the presence of ceria influences the crystallite size of CoO formed during the thermal reduction of Co3 O4 . It has been found that the higher the cobalt content, the larger the CoO crystallites. 4. Discussion The cobalt spinel (Co3 O4 ) is the active phase in the undoped cobalt catalyst [10]. Hence, under the following conditions: T = 850 ◦ C, feed: N2 O–Ar, under which Co3 O4 is to a large extent reduced to CoO, the activity of the catalyst is strongly suppressed [10]. The activity of CoOx -CeO2 catalysts with a high cobalt loading is also lower at 850 ◦ C than at 800 ◦ C. In contrast, the N2 O conversions obtained on CoOx -CeO2 catalysts containing no more than 86% cobalt increase in the whole studied temperature range (Fig. 1). According to literature data, CeO2 can influence the amount of oxygen available for transition metal oxides [22]. However, based on the XRD (Fig. 6a), XPS (Fig. 7) and TG-DTA-MS (Table 3) results obtained within this work, it can be stated that at 850 ◦ C the cobalt spinel in the lowest layer of the catalyst bed is reduced to CoO, regardless of the cobalt content. The reduction occurs both on the surface and in the bulk of the grains. When oxygen is present in the feed, cobalt is in the form of Co3 O4 . Nevertheless, the nitrous oxide conversions noted on CoOx -CeO2 catalysts containing no more than 64% cobalt, are the same in N2 O–Ar and N2 O–O2 –Ar streams in the entire studied temperature range. Based on these results, the activity of these catalysts has been ascribed to the interaction of cobalt oxides (CoO at 850 ◦ C and Co3 O4 at lower temperatures) with CeO2 on the phase interface. The nitrous oxide conversions noted at 850 ◦ C on CoOx -CeO2 catalysts with a high cobalt content (Ce92Co, Ce96Co, Ce98Co), as well as the undoped cobalt catalyst, are smaller than those obtained at 800 ◦ C. The difference is the bigger, the higher the cobalt content. An addition of oxygen in the feed leads to higher activities on these catalysts, as well as on Ce86Co. This implies that the activity of Co3 O4 contributes to their activity, when enough oxygen is present in the feed to maintain the cobalt in the form of the higher cobalt oxide. The XRD and TG-DTA-MS results show that the only two phases in the fresh catalysts are Co3 O4 and CeO2 . No phase containing both Ce and Co is formed in catalyst containing ceria and cobalt oxides upon preparation nor during the reaction. It has been found that the higher the cobalt content in the catalysts, the bigger the crystallite size of CoO formed when Co3 O4 is reduced to CoO. The increase of the thermal stability of CoO due to interactions with CeO2 has been observed by Luo et al. [23]. The results of our thermogravimetric measurements carried out according to programs 2 and 3 have shown that when Co3 O4 in Ce32Co is thermally reduced to CoO, two distinct types of CoO can be identified. In program 2, the temperature was lowered at a constant oxygen con-
E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422
a
421
b *
CeO2 Co3 O4
*
* *
*
*
*
*
*
Intensity [a.u.]
*
30
30
Ce32Co
35
35 Scattering angle [°]
40
40
Ce8Co Ce2Co 25 25
35 35
45 45
55 55
65 65
75 75
Scattering angle [°] Fig. 6. Diffraction patterns of fresh catalysts, (a) scattering angle range: 25–75◦ , (b) enlargement of the 30–40◦ region.
centration, i.e. 2.5%. Two steps of oxidation can be observed on the TG curve. The thermogravimetric experiment performed in accordance with program 3 revealed that the two steps of CoO oxidation to Co3 O4 in Ce32Co are also observed at a constant temperature, i.e. 850 ◦ C, and a regular increase of the oxygen content. As in the TG curves obtained during program 2 measurements, the two steps have been assigned to two types of CoO: weakly and strongly interacting with CeO2 . The two-step oxidation has also been observed for Ce8Co and Ce64Co. The ratio of oxygen uptake in these two steps depends on the cobalt content. In N2 O decomposition, the oxygen atom which is a part of a nitrous oxide molecule has to combine with another oxygen atom and desorb from the surface. According to literature data the Co O bond on the ceria–cobalt oxide interface is elongated because of
the interaction with ceria [23]. Hence it can be broken more easily. Based on the results obtained within this work a probable scheme of molecular oxygen formation on CoOx -CeO2 catalysts has been proposed (Fig. 8). This mechanism takes into account the fact that catalysts with a molar cobalt content no higher than 64% contain CoO which strongly interacts with CeO2 . One way of O2 formation is via the Eley–Rideal mechanism (Fig. 8a), that is as a result of direct interaction of the N2 O molecule with active oxygen. As indicated by the scheme, a nitrous oxide molecule can adsorbs on a cobalt cation adjacent to active oxygen on the phase interface (Langmuir–Hinshelwood mechanism; Fig. 8b). An oxygen molecule can form from the two oxygen atoms and desorb. Regardless of the O2 formation mechanism, the active oxygen is restored from the gas phase (Fig. 8).
CeO2 CoO *
**
*
*
*
*
*
Co3O 4
*
Intensity[a.u.]
*
N2O-O2-Ar
N2O-Ar
15
30
45
60
75
90
105
Scattering angle[º] Fig. 7. Diffraction patterns of Ce64Co after tests with different feed compositions.
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E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422
a
O O O Co Co Ce Ce
O Ce Ce Ce
O Co Co
Ce
anion migration
N2 O
O O O Co Co Ce Ce Ce
b
O O Co Co
O-
O
Ce Ce Ce
Co
Ce Ce
N2O
Ce
O Co
O Ce Ce
Acknowledgements
O2 O
O
O2
O
O
N2
N N O
As the cobalt content increases, the beneficial influence of the cobalt oxide–ceria interface on the activity of the catalyst decreases. Simultaneously, the share of the activity of Co3 O4 in the overall activity increases. Both influences are noticeable in the activity of the catalyst containing 86% cobalt. This may be the reason why it exhibits the highest activity among the studied systems.
N2 + O2
N N O
O Co
O
O Ce
Ce
Ce
Ce
Ce
Ce
O
anion migration
Ce
Co
This work has been carried out within the Research Project N N205 012434, sponsored by the Ministry of Science and Higher Education.
O
O Co
Ce
Ce
References Ce
OFig. 8. Scheme of nitrous oxide decomposition on a Co-Ce catalyst via (a) the Eley–Rideal mechanism, (b) the Langmuir–Hinshelwood mechanism.
5. Conclusions The studies have shown that CoOx -CeO2 catalysts are in general more active in high temperature nitrous oxide decomposition than the undoped cobalt catalyst. It has been found that catalysts with a Co/(Co + Ce) ratio no higher than 0.64 sinter less than those with a higher cobalt content. No phase containing Co and Ce is formed in CoOx -CeO2 catalysts. Undoped CeO2 exhibits much lower activity than all of the other systems. The results of TG-DTA-MS experiments performed in an oxygen-containing stream reveal that at 850 ◦ C in a N2 O–Ar stream two types of CoO coexist in CoOx -CeO2 catalysts, i.e. weakly and strongly interacting with CeO2 . Hence, in CoOx -CeO2 catalysts operating at 850 ◦ C, cobalt is present in the form of CoO and a CoO-CeO2 system formed on the cobalt oxide–ceria interface. Therefore, the activity of CoOx -CeO2 catalysts originates from the interaction of cobalt oxide with CeO2 . In the case of both the undoped cobalt catalyst and CoOx -CeO2 catalysts, Co3 O4 is reduced to CoO at 850 ◦ C in a N2 O–Ar stream. At lower temperatures and in oxygen-containing streams cobalt is present as Co3 O4 . However, the presence of oxygen in the feed does not affect the activity of catalysts containing up to 64% Co in the entire temperature range. For the other catalysts, it has been observed that the higher the cobalt content, the higher the increase of activity at 850 ◦ C upon the addition of oxygen to the feed. The increase has been attributed to the change of the oxidation state of cobalt.
[1] A.R. Ravishankara, J.S. Daniel, R.W. Portmann, Science 326 (2009) 123–125. [2] D.J. Wuebbles, Science 326 (2009) 56–57. [3] M. Elrod, “Greenhouse Warming Potential Model” Based on J Chem. Ed., vol. 76, 1999, pp. 1702–1705. [4] J. Petryk, E. Kołakowska, Appl. Catal. B 24 (2000) 121–128. [5] J. Petryk, Z. Kowalczyk, E. Kołakowska, S. Jodzis, Pol. J. Chem. 79 (2005) 1699–1703. [6] C. Ohnishi, K. Asano, S. Iwamoto, K. Chikama, M. Inoue, Catal. Today 120 (2007) 145–150. [7] L. Xue, H. He, C. Liu, C. Zhang, B. Zhang, Environ. Sci. Technol. 43 (2009) 890–895. [8] P. Stelmachowski, F. Zasada, G. Maniak, P. Granger, M. Inger, M. Wilk, A. Kotarba, Z. Sojka, Catal. Lett. 130 (2009) 637–641. [9] J. Pérez-Ramírez, F. Kapteijn, K. Schöffel, J.A. Moulijn, Appl. Catal. B 44 (2003) 117–151. [10] E. Wilczkowska, K. Krawczyk, J. Petryk, J.W. Sobczak, Z. Kaszkur, Appl. Catal. A 389 (2010) 165–172. [11] M. Pottier, ADEME/InfoMil, NOXCONF 2001: collection of papers “International conference industrial atmospheric pollution”, 21st and 22nd March 2001, Paris (France), ADEME (France)/InfoMil (The Netherlands), 2001. [12] J. Jansson, J. Catal. 194 (2000) 55–60. [13] L. Xue, C. Zhang, H. He, Y. Teraoka, Appl. Catal. B 75 (2007) 167–174. [14] M. Kang, M.W. Song, Ch.H. Lee, Appl. Catal. A 251 (2003) 143–156. [15] H. Wang, J.L. Ye, Y. Liu, Y.D. Li, Y.N. Qin, Catal. Today 129 (2007) 305–312. [16] L.F. Liotta, G. Di Carlo, G. Pantaleo, G. Deganello, Catal. Commun. 6 (2005) 329–336. [17] L.F. Liotta, M. Ousmane, G. Di Carlo, G. Pantaleo, G. Deganello, G. Marcì, L. Retailleau, A. Giroir-Fendler, Appl. Catal. A 347 (2008) 81–88. [18] S. Imamura, M. Shono, N. Okamoto, R. Hamada, S. Ishida, Appl. Catal. A 142 (1996) 279–288. [19] N. Pashaa, N. Lingaiaha, N. Seshu Babua, P. Siva Sankar Reddya, P.S. Sai, Catal. Commun. 10 (2008) 132–136. [20] K. Asano, C. Ohnishi, S. Iwamoto, Y. Shioya, M. Inoue, Appl. Catal. B 78 (2008) 242–249. [21] A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, London, GBR, 2002, p. 15. [22] A.C. Gluhoia, N. Bogdanchikovab, B.E. Nieuwenhuys, J. Catal. 229 (2005) 154–162. [23] J.Y. Luo, M. Meng, X. Li, X.G. Li, Y.Q. Zha, T.D. Hu, Y.N. Xie, J. Zhang, J. Catal. 254 (2008) 310–324.
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Direct nitrous oxide decomposition with CoOx -CeO2 catalysts Ewa Iwanek a,∗,1 , Krzysztof Krawczyk a , Jan Petryk a , Janusz W. Sobczak b , Zbigniew Kaszkur b a b
Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 28 March 2011 Received in revised form 2 May 2011 Accepted 28 May 2011 Available online 6 June 2011 Keywords: Nitrous oxide decomposition Co3 O4 -CeO2 catalysts Catalyst state Reduction Phase interface
a b s t r a c t The focus of the performed studies were CoOx -CeO2 oxide catalysts for nitrous oxide decomposition. All CoOx -CeO2 systems exhibit similar or higher activity than the undoped cobalt catalyst. It has been found that in temperatures up to 800 ◦ C in a N2 O–Ar stream cobalt in these catalysts is in the form of Co3 O4 . At higher temperatures it is reduced to CoO. In a N2 O–O2 –Ar stream Co3 O4 is the main cobalt-containing phase in the entire studied temperature range. The obtained results revealed that the activity of CoOx CeO2 systems with a high cobalt loading increases with temperature only up to 800 ◦ C in a N2 O–Ar stream. Upon further temperature increase the activity of these catalysts decreases, as in the case of the undoped cobalt catalyst. This is due to the reduction of Co3 O4 to CoO. Hence, when oxygen is present in the feed and cobalt is in the form of Co3 O4 , the activity is higher. In contrast, the activity of catalysts with the cobalt molar ratio no greater than 0.64 is the same in both N2 O–Ar and N2 O–O2 –Ar streams and increases with temperature in the entire studied range (700–850 ◦ C). It has been demonstrated that at 850 ◦ C in a N2 O–Ar stream CoOx -CeO2 systems contain two types of CoO, which require different conditions to be oxidized. This is a result of a different strength of interaction with CeO2 . It can be concluded that the activity of CoOx -CeO2 systems results from the activity of Co3 O4 and of the cobalt oxide–ceria interface. The share of each component is determined by the cobalt content. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitrous oxide is said to be the most harmful ozone depletor which is not a volatile organic compound [1,2]. Moreover, the global warming potential, GWP, of nitrous oxide is approximately 300 times higher than that of carbon dioxide [3]. Due to its high GWP, as well as substantial emissions, this gas significantly contributes to global warming. The main industrial source of N2 O are nitric acid plants. Nitrous oxide forms in them as a by-product of ammonia oxidation on Pt-Rh gauze. It has been experimentally established that in ammonia oxidation the selectivity to N2 O on oxide catalysts which contain cobalt oxide, is much lower than on the Pt-Rh catalyst [4,5]. Therefore, catalytic systems containing Co3 O4 are the subject of studies as potential nitrous oxide decomposition catalyst [6–8]. Currently, the most frequently implemented method of nitrous oxide abatement is high temperature N2 O decomposition [9]. This process is carried out directly downstream of the Pt-Rh gauze, where a selective N2 O-decomposition catalyst is placed. The temperature of this stream is approximately 800 ◦ C and higher.
∗ Corresponding author. E-mail address: [email protected] (E. Iwanek). 1 Nee Wilczkowska. 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.05.049
Undoped cobalt oxide catalysts have several flaws. The main problem associated with their application in high temperature nitrous oxide decomposition is the decrease of their activity at temperatures exceeding 800 ◦ C [10]. This results from the reduction of the active phase to CoO [10,11]. Furthermore, the grains of the catalyst tend to sinter and form clusters [10]. What is more, the presence of oxygen in the feed suppresses the activity of the undoped cobalt catalyst under certain conditions [10]. Supports, such as Al2 O3 [12] or CeO2 [13–17], are used in order to increase the activity, as well as the mechanical and thermal stability, of the cobalt spinel. Ceria is added to Co3 O4 also due to the fact that Co-Ce catalysts exhibit a greater specific area and better redox properties than the undoped cobalt spinel [14,15,18]. According to literature data concerning cobalt catalysts, the addition of CeO2 [7] or alkali metal ions, such as Cs+ [18] and K+ [19,20], has a positive effect on the activity of Co3 O4 in N2 O decomposition. It has been evidenced that the oxygen vacancies on the surface of CeO2 exhibit a high mobility [19]. Therefore, the oxygen adsorbed by CeO2 can be swiftly transported to the active center [21]. The ease of adsorbing oxygen from the gas phase by cerium oxide and transporting it to other catalyst components, is the reason why it is added to Co-based oxidation catalysts. However, there is little information regarding the oxidation of this type of catalysts. In this work the oxidation of CoOx -CeO2 catalytic systems, in which Co3 O4 is thermally reduced to CoO, was investigated.
E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422
The aim of this work was to establish how the Ce/Co ratio in CoOx -CeO2 catalysts affects their activity in high temperature N2 O decomposition, as well as their thermal stability and other properties. The influence of oxygen in the feed on the activity of CoOx -CeO2 catalysts was established. Nitrogen physisorption, XPS, XRD and TG-DTA-MS studies were performed in order to determine the effect of reaction temperature and oxygen content on the state of the catalyst under high temperature nitrous oxide decomposition conditions.
2. Experimental 2.1. Catalyst preparation The CoOx -CeO2 catalysts were prepared from ceria (Riedel-de Haen, anhydrous, pure) and cobalt nitrate, Co(NO3 )2 ·6H2 O (POCH Gliwice, pure for analysis). The mixture was heated to 400 ◦ C in air until the nitrate decomposed. To 20.0 g of the obtained oxides 1.0 g of (NH4 )2 CO3 and 0.3 g of glycerin were added. The mixture was ground and sieved (0.4 mm). Tablets (1.70 cm × 0.3 cm) were pressed in two consecutive steps applying the force of 5 and 10 tons, respectively. Thus formed tablets were calcined at 850 ◦ C for 48 h, cooled, and crushed. The 2.0–2.5 mm grains were used in activity measurements. Catalysts with the following molar cobalt contents (%Co), i.e. 100 × Co/(Co + Ce), were made: 2, 8, 32, 64, 86, 92, 96 and 98. The catalysts have been given symbols, in which the number denotes the molar Co content (%Co). Catalysts from undoped CeO2 (Riedel-de Haen, anhydrous, pure) and Co3 O4 , obtained from the thermal decomposition of cobalt nitrate (POCH Gliwice, pure for analysis), were used as reference materials. 2.2. Activity measurements In order to perform activity measurements, the catalysts were placed in a cylindrical flow reactor made from quartz glass. The measurements were carried out under atmospheric pressure at a constant temperature (700, 750, 800 or 850 ◦ C) and gas hourly space velocity, i.e. 8 × 104 h−1 . The catalyst bed volume was 0.5 ml. The reaction mixture was passed through the catalyst bed from the top. Two inlet stream compositions were used, namely 5% N2 O, 95% Ar and 5% N2 O, 5% O2 , 90% Ar. The N2 O content in the inlet and outlet mixtures was established using gas chromatography (KONIK HRGC 4000B). The presented values are the average of 6 measurements whose results differed by less than 3%. The characterization measurements were performed on grains from the lowest layer of the catalyst bed. After the tests, the catalyst bed was cooled in flowing argon (900 ml min−1 ) at the rate of 80 ◦ C min−1 . A TG-MS analysis showed that this procedure does not lead to a change in mass nor any m/z signal. The presence of oxygen at 850 ◦ C has a beneficial effect on the activity of CoOx -CeO2 catalysts with molar cobalt contents of 86% or higher, as in the case of the undoped cobalt catalyst. In contrast, the activity of catalysts containing no more than 64% cobalt is not influenced by the addition of oxygen to the inlet stream. In order to establish why there is no influence of oxygen on the activity of these catalysts, the characterization experiments were performed mainly on Ce32Co and Ce64Co. 2.3. Catalyst characterization 2.3.1. Specific surface area and porosity measurements The specific surface area and pore distribution measurements of the fresh and used catalysts were performed using ASAP 2020 (Micromeritics). The nitrogen physisorption experiments were car-
417
ried out at T = 77 K and the p/p0 range of 0.01–1.00. The SBET values were calculated using the Brunauer–Emmett–Teller equation. 2.3.2. TG-DTA-MS measurements Thermogravimetric measurements were carried out on NETZSCH STA 449C apparatus equipped with a NETZSCH QMS 403C quadruple mass spectrometer. The reference material was electrocorundum of the same fraction as the catalyst. The following two gases were passed through the thermobalance: argon (Multax, N5.0) and oxygen (Multax, N5.0). The total gas flow through the furnace with the sample was 100 ml min−1 . Throughout the measurements, a channeltron detector recorded the following m/z signals: 2, 18, 28, 30, 32, 44, and 46. Data were analyzed using NETZSCH Proteus Thermal Analysis software. The sample mass was 150 mg. In measurements performed in accordance with program 1, a sample (fresh catalyst or after catalytic tests) was heated up to 1100 ◦ C, and cooled to 200 ◦ C at 5 ◦ C min−1 . The feed consisted of 18% O2 in argon. The oxygen content was sufficient to allow a full oxidation of a reduced sample and a complete reduction during the heating phase, as well as another full oxidation during the cooling phase. The Co3 O4 content in the samples was determined based on the mass change resulting from oxygen uptake by the sample and the mass of oxygen evolved during the reduction of Co3 O4 to CoO. In measurements performed in accordance with program 2, a sample was heated up to 1000 ◦ C at 10 ◦ C min−1 in argon. Next, the sample was cooled to 650 ◦ C at 0.2 ◦ C min−1 in a stream composed of 2.5% O2 , 97.5% argon. In the case of Ce32Co an additional measurement was performed. The three parts of program 3 were as follows: 1) heating a fresh sample to 1000 ◦ C with the heating rate of 20 ◦ C min−1 , in a stream containing 5% oxygen, 2) cooling it to 850 ◦ C with the cooling rate of 5 ◦ C min−1 , 5% O2 , 95% Ar, 3) increasing the oxygen content by 2% every 30 min, from 5 to 90%, T = 850 ◦ C. 2.3.3. XPS measurements The XPS measurements were carried out with a scanning photoelectron spectrometer PHI 5000 VersaProbe (Physical Electronics USA/ULVAC). Monochromatized Al K␣ radiation was used (1486.6 eV). The spectra of the Ce 3d, Co 2p, O 1s, C 1s regions were measured with a pass energy of 23.5 eV in 0.1 eV increments. The C 1s (BE = 284.7 eV) signal was used to calibrate the binding energy (BE) scale. The O 1s signal components were fitted using AVANTAGE v. 4.51 (Thermo Scientific) after a Shirley background subtraction. The XPS measurements of all studied CoOx -CeO2 catalysts were performed after catalytic tests at 850 ◦ C in a N2 O–Ar stream. Furthermore, this technique was applied to investigate the surface of a Ce64Co sample operating at the same temperature in a N2 O–O2 –Ar stream. 2.3.4. XRD measurements XRD measurements were performed using a hybrid RigakuDenki Geigerflex diffractometer. The source of radiation was a Cu lamp (40 kV, 40 mA). The beam divergence was approximately 0.1◦ . The experiments were performed in the scattering angle range of 20–125◦ with a 0.2◦ min−1 step. An internal standard (quartz) was used. In order to determine if cobalt is incorporated into the ceria lattice in catalysts with molar cobalt contents no greater than 0.32, measurements with fresh Ce2Co, Ce8Co and Ce32Co catalysts, as well as with CeO2 and Co3 O4 , were performed. The CeO2 lattice constant values in the following fresh catalysts: CeO2 , Ce2Co, Ce8Co and Ce32Co were calculated based on a linear extrapolation of values obtained from all reflections to cos() = 0. The mean square deviation of atoms from the nodes of CeO2 lattice was calculated on
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a N2O conversion[%]
N2O conversion[%]
100
75
50
100 90 80 70 60
25 50 0
0
25
50
75
100
Cobalt content [%mol] 0
25
50
75
100
b
Cobalt content [%mol]
N 2O conversion[%]
Fig. 1. Dependence of N2 O conversion on the cobalt content in the catalyst; feed: 5% N2 O, 95% Ar; T = 700 ◦ C (pluses), 750 ◦ C (squares), 800 ◦ C (triangles), or 850 ◦ C (circles). Data for Co3 O4 taken from [10].
the basis of the Debye–Waller factor (LAZY-PULVERIX). The Co3 O4 lattice constant was established using the (4 4 0) reflection. In samples of catalysts after activity tests at 850 ◦ C in a N2 O–Ar stream, the lattice constant of CoO and the crystallite size of this phase were determined. The CoO lattice constant was calculated on the basis of (1 1 1), (2 0 0) and (2 2 0) reflections.
100 90 80 70 60 50
0
25
a
The calcination of the studied catalysts was carried out at 850 ◦ C to minimize their sintering during catalytic tests. One of the consequences of this was a low surface area of the catalysts. The
2.0 1.5 1.0 0.5 0 0
20
40
60
80
100
Cobalt content [mol%]
b
0.25 0.20 0.15 0.10 0.05 0.00
3.2. Physisorption measurements
100
dependence of SBET on the cobalt content is shown in Fig. 3a. The largest specific surface area is exhibited by CeO2 (1.9 m2 g−1 ), whereas that of Co3 O4 is the smallest (0.6 m2 g−1 ). The surface of all the studied catalysts diminished by approximately 0.2 m2 g−1 dur-
Specific surface area [m2/g]
The decomposition of nitrous oxide on all studied catalysts leads to the formation of N2 and O2 , regardless of the reaction temperature and oxygen content in the feed. The curves in Fig. 1 depict the influence of the cobalt content in the catalyst on N2 O conversions in a N2 O–Ar stream at 700 ◦ C, 750 ◦ C, 800 ◦ C and 850 ◦ C on the studied catalysts. The activity of all CoOx -CeO2 systems is much higher than that of the undoped CeO2 catalyst. It can be seen that even 2% of cobalt in the catalyst results in higher N2 O conversions (Fig. 1). The catalysts containing 8% of cobalt or more exhibit higher activities than the undoped cobalt catalyst at 800 ◦ C and 850 ◦ C. It is noteworthy that regardless of the reaction temperature the highest activity was noted for Ce86Co (Fig. 1). The influence of oxygen in the feed on nitrous oxide conversion at 800 and 850 ◦ C is shown in Fig. 2. At 800 ◦ C, as in the case of lower temperatures (results not shown), the N2 O conversion values are not influenced by the inlet stream composition, or are affected only slightly. At 850 ◦ C the values obtained on Ce2Co, Ce8Co, Ce32Co and Ce64Co do not exhibit a dependence on the feed composition. In contrast, the N2 O conversions noted on catalysts containing more than 64% cobalt are higher in the presence of oxygen (Fig. 2). The influence of 5% oxygen in the feed on the nitrous oxide conversion is the bigger, the greater the cobalt content. The differences obtained at 850 ◦ C for Ce86Co, Ce96Co and the undoped cobalt catalyst in N2 O–O2 –Ar and N2 O–Ar streams are 3%, 11% and 21% respectively. It can be assumed that the positive influence of oxygen in the feed on the activity of catalysts with a high cobalt content probably results from the fact that cobalt is kept in the form of Co3 O4 , as in the undoped cobalt catalyst [10].
75
Fig. 2. Influence of inlet stream composition on N2 O conversion at (a) 800 ◦ C and (b) 850 ◦ C; feed: argon and 5% N2 O (circles), 5% N2 O, 5% O2 (squares). Data for Co3 O4 taken from [10].
Pore area [m 2/g]
3.1. Activity measurements
50
Cobalt content [%mol]
3. Results
10
13
17
24
38
63
110
232
699
Average pore radius[Å] Fig. 3. Nitrogen physisorption results: (a) dependence of specific surface area on the cobalt content, (b) pore distribution of CeO2 (white), Ce2Co (gray) and Co3 O4 (black).
E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422 Table 1 Results obtained during TG-DTA-MS measurements; program 1. Catalyst symbol
T [◦ C]
Feed
Ce2Co Ce8Co Ce32Co Ce64Co Ce64Co Ce64Co Ce64Co Ce64Co Ce86Co Ce96Co
850 850 850 850 800 750 700 850 850 850
5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar 5% N2 O, 5% O2 , 90% Ar 5% N2 O, 95% Ar 5% N2 O, 95% Ar
419
TG [%]
a 106 Co3 O4 content [%]
Ce86Co
105
0 0 0 0 100 100 100 91 0 0
104 103
Ce64Co
102 Ce32Co
101
II
I ing catalytic tests. The grains of the undoped cobalt catalyst [10] and the CoOx -CeO2 catalysts which contained at least 86% of cobalt were partially agglomerated. In contrast, grains of catalysts containing 64% cobalt or less did not form clusters under operating conditions. The results of porosimetric analysis show that the pore distributions of the undoped CeO2 catalyst and of the undoped cobalt catalyst differ (Fig. 3b). The former has mostly large pores, (Fig. 3b white columns), whereas Co3 O4 has mostly small ones (Fig. 3b black columns). Small pores are also dominant in all CoOx -CeO2 systems. This is true even in the case of the catalyst which contains 2% cobalt (Fig. 3b gray columns).
100 99 900
850
b
750
700
TG [%] 100.5 100.0
II
99.5 99.0
I
3.3. TG-DTA-MS measurements Thermogravimetric measurements have shown that CeO2 heated up to 1100 ◦ C does not undergo any changes in an argon stream or an argon-oxygen mixture. In contrast, upon heating of the samples, the TG curves of all studied CoOx -CeO2 catalysts show a mass loss accompanied by an m/z = 32 signal increase and an endothermic signal on the DTA curve. This transformation has been identified as the thermal reduction of Co3 O4 to CoO. These studies have shown that in all fresh CoOx -CeO2 catalysts cobalt is in the form of Co3 O4 . The results of experiments carried out in accordance with program 1 have revealed that during nitrous oxide decomposition at 800 ◦ C, as well as at lower temperatures, cobalt is in the form of Co3 O4 in all CoOx -CeO2 catalysts. As an example, the appropriate values for Ce64Co have been given in Table 1. However, in samples of CoOx -CeO2 catalysts operating at 850 ◦ C, the Co3 O4 content strongly depends on the reaction mixture composition. After operating at this temperature in a N2 O–Ar stream, the sample contains only CeO2 and CoO regardless of the cobalt content (Table 1). Hence, the activity of CoOx -CeO2 catalysts has been ascribed mainly to the interaction of CeO2 with cobalt oxide (CoO at 850 ◦ C or Co3 O4 at lower temperatures). Samples of Ce8CO, Ce32Co, Ce64Co, Ce86Co and the undoped cobalt catalyst after catalytic experiments in a N2 O–Ar stream were tested according to program 2. The mass changes of Ce32Co, Ce64Co and Ce86Co during the cooling stage are depicted in Fig. 4a. It can be seen that in the case of the first two CoOx -CeO2 catalysts there are two phases of rapid mass increase (Fig. 4a I and II). Therefore, there are two steps of CoO oxidation. Their contribution depends on the cobalt content in the catalyst. For Ce8Co, which contains little cobalt, the signal connected with the first step is minute. Moreover, it was not possible to detect the second oxidation step on Ce86Co. The TG curve obtained for Ce86Co is practically the same as that noted for the undoped cobalt catalyst. Therefore, the first oxidation step was assigned to the oxidation of CoO (to Co3 O4 ) weakly interacting with CeO2 . The second step, which takes place at lower temperatures, is the oxidation of CoO (to Co3 O4 ) strongly interacting with ceria. Thermally reduced fresh catalysts containing 32%
800
Temperature [°C]
98.5
0
Ce32Co 30
60
90
Oxygen content [%] Fig. 4. TG curves obtained during (a) cooling of Ce32Co, Ce64Co and Ce86Co; program 2, (b) increase of oxygen content in the gas surrounding Ce32Co; T = 850 ◦ C; program 3.
and 64% cobalt are also oxidized in two steps. Hence, it can be concluded that the formation of two types of CoO is characteristic for the catalysts themselves, and it is not a property acquired during N2 O decomposition. The TG curve of Ce32Co during a measurement performed according to program 3 is shown in Fig. 4b. In this experiment, the sample was kept at a constant temperature, i.e. 850 ◦ C, and the oxygen content in the stream introduced into the furnace with the sample was increased from 5% to 90%. The mass of the sample, similarly as in the case of program 2, increases in two steps. When the oxygen content equals 8%, the CoO weakly interacting with CeO2 begins to oxidize (Fig. 4b I). Next, despite the gradually increasing oxygen content, the oxidation rate diminishes and then increases again (Fig. 4b II). 3.4. XPS measurements The Co 2p region of the XPS spectra of Ce64Co operating at 850 ◦ C with the two studied reaction mixture compositions are shown in Fig. 5. Intensive satellites can be seen in the spectrum of Ce64Co working in a N2 O–Ar stream. They are located approximately 6 eV away from the main signals (Fig. 5). Hence, cobalt on the surface of this sample is in the form of CoO. The analysis of XPS results revealed that at 850 ◦ C in a N2 O–Ar stream, the only cobaltcontaining species found on the surface of CoOx -CeO2 catalysts is CoO, irrespective of the cobalt content. In contrast, a spectrum typical for the higher cobalt oxide was acquired for the Ce64Co sample operating in a N2 O–O2 –Ar stream (Fig. 5). In this spectrum, the
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E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422 Table 3 The CoO lattice constant and crystallite size in samples after catalytic tests at 850 ◦ C in a 5% N2 O, 95% Ar stream, calculated from XRD results.
Co2p3/2 Co2p1/2 shake-up
Intensity [a.u.]
shake-up
N2O-Ar
N2O-O2-Ar 810
800
790
780
770
Binding energy[eV] Fig. 5. The Co 2p region of XPS spectra of Ce64Co after tests with different feed compositions.
satellites are less intensive and are shifted approximately 10 eV in the direction of higher binding energy values than the main signals. No differences were noted in the Ce 3d region of CoOx -CeO2 catalysts, regardless of the cobalt content (results not shown). The obtained spectra are typical for CeO2 . Moreover, in spectra of Ce64Co samples after operating in the two studied reaction mixtures, no components corresponding to Ce3+ can be seen (results not shown). 3.5. XRD measurements The results of the XRD measurements show that there is no phase which contains both Ce and Co in the studied catalysts (Fig. 6). In all fresh CoOx -CeO2 catalysts including Ce2Co (Fig. 6b) the only cobalt-containing phase is Co3 O4 . The intensity of Co3 O4 signals increases with the cobalt content. Lattice constants of both CeO2 and Co3 O4 were established based on measurements with an internal standard. The resulting values are given in Table 2. The CeO2 ˚ The same lattice constant in the undoped ceria catalyst is 5.416 A. value was noted for: Ce2Co, Ce8Co and Ce32Co (Table 2). The mean square deviation of ions from the nodes is approximately 0.2 for all four abovementioned catalysts. Hence, it can be stated that the incorporation of cobalt into the ceria lattice does not occur in the studied catalysts. The lattice constant of Co3 O4 in Ce8Co, Ce32Co, as well as the undoped cobalt catalyst, was determined. It equals 8.09 A˚ (Table 2). In the case of Ce2Co, the Co3 O4 lattice constant was not established due to the fact that the spinel content was too small. The diffraction pattern of Ce64Co after catalytic tests in N2 O–Ar and N2 O–O2 –Ar streams is shown in Fig. 7. The XRD results indicate that in all CoOx -CeO2 catalysts operating at 850 ◦ C in N2 O–Ar and N2 O–O2 –Ar streams, cobalt is present mainly in the form of CoO Table 2 Lattice parameters of CeO2 and Co3 O4 , as well as the mean square deviation of ceria atoms from the nodes in the ceria lattice, calculated from XRD results. Catalyst symbol
˚ aCeO2 [A]
MSDCeO2
˚ aCo3 O4 [A]
CeO2 Ce2Co Ce8Co Ce32Co Co3 O4
5.416 5.416 5.416 5.416 –
0.2 0.2 0.2 0.2 –
– – 8.09 8.09 8.09
Catalyst symbol
˚ aCoO [A]
dCoO [nm]
Ce32Co Ce64Co Ce86Co Ce96Co Co3 O4
4.26 4.26 4.26 4.26 4.26
36 38 42 46 49
and Co3 O4 , respectively. To exemplify this, the diffraction pattern of Ce64Co is shown in Fig. 7. The values of the CoO lattice constant, as well as the crystallite size of CoO particles, in CoOx -CeO2 catalysts operating at 850 ◦ C in a N2 O–Ar stream is given in Table 3. No dependence of the CoO lattice constant on the catalyst composition has been found in the XRD studies. However, the results clearly indicate that the presence of ceria influences the crystallite size of CoO formed during the thermal reduction of Co3 O4 . It has been found that the higher the cobalt content, the larger the CoO crystallites. 4. Discussion The cobalt spinel (Co3 O4 ) is the active phase in the undoped cobalt catalyst [10]. Hence, under the following conditions: T = 850 ◦ C, feed: N2 O–Ar, under which Co3 O4 is to a large extent reduced to CoO, the activity of the catalyst is strongly suppressed [10]. The activity of CoOx -CeO2 catalysts with a high cobalt loading is also lower at 850 ◦ C than at 800 ◦ C. In contrast, the N2 O conversions obtained on CoOx -CeO2 catalysts containing no more than 86% cobalt increase in the whole studied temperature range (Fig. 1). According to literature data, CeO2 can influence the amount of oxygen available for transition metal oxides [22]. However, based on the XRD (Fig. 6a), XPS (Fig. 7) and TG-DTA-MS (Table 3) results obtained within this work, it can be stated that at 850 ◦ C the cobalt spinel in the lowest layer of the catalyst bed is reduced to CoO, regardless of the cobalt content. The reduction occurs both on the surface and in the bulk of the grains. When oxygen is present in the feed, cobalt is in the form of Co3 O4 . Nevertheless, the nitrous oxide conversions noted on CoOx -CeO2 catalysts containing no more than 64% cobalt, are the same in N2 O–Ar and N2 O–O2 –Ar streams in the entire studied temperature range. Based on these results, the activity of these catalysts has been ascribed to the interaction of cobalt oxides (CoO at 850 ◦ C and Co3 O4 at lower temperatures) with CeO2 on the phase interface. The nitrous oxide conversions noted at 850 ◦ C on CoOx -CeO2 catalysts with a high cobalt content (Ce92Co, Ce96Co, Ce98Co), as well as the undoped cobalt catalyst, are smaller than those obtained at 800 ◦ C. The difference is the bigger, the higher the cobalt content. An addition of oxygen in the feed leads to higher activities on these catalysts, as well as on Ce86Co. This implies that the activity of Co3 O4 contributes to their activity, when enough oxygen is present in the feed to maintain the cobalt in the form of the higher cobalt oxide. The XRD and TG-DTA-MS results show that the only two phases in the fresh catalysts are Co3 O4 and CeO2 . No phase containing both Ce and Co is formed in catalyst containing ceria and cobalt oxides upon preparation nor during the reaction. It has been found that the higher the cobalt content in the catalysts, the bigger the crystallite size of CoO formed when Co3 O4 is reduced to CoO. The increase of the thermal stability of CoO due to interactions with CeO2 has been observed by Luo et al. [23]. The results of our thermogravimetric measurements carried out according to programs 2 and 3 have shown that when Co3 O4 in Ce32Co is thermally reduced to CoO, two distinct types of CoO can be identified. In program 2, the temperature was lowered at a constant oxygen con-
E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422
a
421
b *
CeO2 Co3 O4
*
* *
*
*
*
*
*
Intensity [a.u.]
*
30
30
Ce32Co
35
35 Scattering angle [°]
40
40
Ce8Co Ce2Co 25 25
35 35
45 45
55 55
65 65
75 75
Scattering angle [°] Fig. 6. Diffraction patterns of fresh catalysts, (a) scattering angle range: 25–75◦ , (b) enlargement of the 30–40◦ region.
centration, i.e. 2.5%. Two steps of oxidation can be observed on the TG curve. The thermogravimetric experiment performed in accordance with program 3 revealed that the two steps of CoO oxidation to Co3 O4 in Ce32Co are also observed at a constant temperature, i.e. 850 ◦ C, and a regular increase of the oxygen content. As in the TG curves obtained during program 2 measurements, the two steps have been assigned to two types of CoO: weakly and strongly interacting with CeO2 . The two-step oxidation has also been observed for Ce8Co and Ce64Co. The ratio of oxygen uptake in these two steps depends on the cobalt content. In N2 O decomposition, the oxygen atom which is a part of a nitrous oxide molecule has to combine with another oxygen atom and desorb from the surface. According to literature data the Co O bond on the ceria–cobalt oxide interface is elongated because of
the interaction with ceria [23]. Hence it can be broken more easily. Based on the results obtained within this work a probable scheme of molecular oxygen formation on CoOx -CeO2 catalysts has been proposed (Fig. 8). This mechanism takes into account the fact that catalysts with a molar cobalt content no higher than 64% contain CoO which strongly interacts with CeO2 . One way of O2 formation is via the Eley–Rideal mechanism (Fig. 8a), that is as a result of direct interaction of the N2 O molecule with active oxygen. As indicated by the scheme, a nitrous oxide molecule can adsorbs on a cobalt cation adjacent to active oxygen on the phase interface (Langmuir–Hinshelwood mechanism; Fig. 8b). An oxygen molecule can form from the two oxygen atoms and desorb. Regardless of the O2 formation mechanism, the active oxygen is restored from the gas phase (Fig. 8).
CeO2 CoO *
**
*
*
*
*
*
Co3O 4
*
Intensity[a.u.]
*
N2O-O2-Ar
N2O-Ar
15
30
45
60
75
90
105
Scattering angle[º] Fig. 7. Diffraction patterns of Ce64Co after tests with different feed compositions.
120
422
E. Iwanek et al. / Applied Catalysis B: Environmental 106 (2011) 416–422
a
O O O Co Co Ce Ce
O Ce Ce Ce
O Co Co
Ce
anion migration
N2 O
O O O Co Co Ce Ce Ce
b
O O Co Co
O-
O
Ce Ce Ce
Co
Ce Ce
N2O
Ce
O Co
O Ce Ce
Acknowledgements
O2 O
O
O2
O
O
N2
N N O
As the cobalt content increases, the beneficial influence of the cobalt oxide–ceria interface on the activity of the catalyst decreases. Simultaneously, the share of the activity of Co3 O4 in the overall activity increases. Both influences are noticeable in the activity of the catalyst containing 86% cobalt. This may be the reason why it exhibits the highest activity among the studied systems.
N2 + O2
N N O
O Co
O
O Ce
Ce
Ce
Ce
Ce
Ce
O
anion migration
Ce
Co
This work has been carried out within the Research Project N N205 012434, sponsored by the Ministry of Science and Higher Education.
O
O Co
Ce
Ce
References Ce
OFig. 8. Scheme of nitrous oxide decomposition on a Co-Ce catalyst via (a) the Eley–Rideal mechanism, (b) the Langmuir–Hinshelwood mechanism.
5. Conclusions The studies have shown that CoOx -CeO2 catalysts are in general more active in high temperature nitrous oxide decomposition than the undoped cobalt catalyst. It has been found that catalysts with a Co/(Co + Ce) ratio no higher than 0.64 sinter less than those with a higher cobalt content. No phase containing Co and Ce is formed in CoOx -CeO2 catalysts. Undoped CeO2 exhibits much lower activity than all of the other systems. The results of TG-DTA-MS experiments performed in an oxygen-containing stream reveal that at 850 ◦ C in a N2 O–Ar stream two types of CoO coexist in CoOx -CeO2 catalysts, i.e. weakly and strongly interacting with CeO2 . Hence, in CoOx -CeO2 catalysts operating at 850 ◦ C, cobalt is present in the form of CoO and a CoO-CeO2 system formed on the cobalt oxide–ceria interface. Therefore, the activity of CoOx -CeO2 catalysts originates from the interaction of cobalt oxide with CeO2 . In the case of both the undoped cobalt catalyst and CoOx -CeO2 catalysts, Co3 O4 is reduced to CoO at 850 ◦ C in a N2 O–Ar stream. At lower temperatures and in oxygen-containing streams cobalt is present as Co3 O4 . However, the presence of oxygen in the feed does not affect the activity of catalysts containing up to 64% Co in the entire temperature range. For the other catalysts, it has been observed that the higher the cobalt content, the higher the increase of activity at 850 ◦ C upon the addition of oxygen to the feed. The increase has been attributed to the change of the oxidation state of cobalt.
[1] A.R. Ravishankara, J.S. Daniel, R.W. Portmann, Science 326 (2009) 123–125. [2] D.J. Wuebbles, Science 326 (2009) 56–57. [3] M. Elrod, “Greenhouse Warming Potential Model” Based on J Chem. Ed., vol. 76, 1999, pp. 1702–1705. [4] J. Petryk, E. Kołakowska, Appl. Catal. B 24 (2000) 121–128. [5] J. Petryk, Z. Kowalczyk, E. Kołakowska, S. Jodzis, Pol. J. Chem. 79 (2005) 1699–1703. [6] C. Ohnishi, K. Asano, S. Iwamoto, K. Chikama, M. Inoue, Catal. Today 120 (2007) 145–150. [7] L. Xue, H. He, C. Liu, C. Zhang, B. Zhang, Environ. Sci. Technol. 43 (2009) 890–895. [8] P. Stelmachowski, F. Zasada, G. Maniak, P. Granger, M. Inger, M. Wilk, A. Kotarba, Z. Sojka, Catal. Lett. 130 (2009) 637–641. [9] J. Pérez-Ramírez, F. Kapteijn, K. Schöffel, J.A. Moulijn, Appl. Catal. B 44 (2003) 117–151. [10] E. Wilczkowska, K. Krawczyk, J. Petryk, J.W. Sobczak, Z. Kaszkur, Appl. Catal. A 389 (2010) 165–172. [11] M. Pottier, ADEME/InfoMil, NOXCONF 2001: collection of papers “International conference industrial atmospheric pollution”, 21st and 22nd March 2001, Paris (France), ADEME (France)/InfoMil (The Netherlands), 2001. [12] J. Jansson, J. Catal. 194 (2000) 55–60. [13] L. Xue, C. Zhang, H. He, Y. Teraoka, Appl. Catal. B 75 (2007) 167–174. [14] M. Kang, M.W. Song, Ch.H. Lee, Appl. Catal. A 251 (2003) 143–156. [15] H. Wang, J.L. Ye, Y. Liu, Y.D. Li, Y.N. Qin, Catal. Today 129 (2007) 305–312. [16] L.F. Liotta, G. Di Carlo, G. Pantaleo, G. Deganello, Catal. Commun. 6 (2005) 329–336. [17] L.F. Liotta, M. Ousmane, G. Di Carlo, G. Pantaleo, G. Deganello, G. Marcì, L. Retailleau, A. Giroir-Fendler, Appl. Catal. A 347 (2008) 81–88. [18] S. Imamura, M. Shono, N. Okamoto, R. Hamada, S. Ishida, Appl. Catal. A 142 (1996) 279–288. [19] N. Pashaa, N. Lingaiaha, N. Seshu Babua, P. Siva Sankar Reddya, P.S. Sai, Catal. Commun. 10 (2008) 132–136. [20] K. Asano, C. Ohnishi, S. Iwamoto, Y. Shioya, M. Inoue, Appl. Catal. B 78 (2008) 242–249. [21] A. Trovarelli (Ed.), Catalysis by Ceria and Related Materials, Imperial College Press, London, GBR, 2002, p. 15. [22] A.C. Gluhoia, N. Bogdanchikovab, B.E. Nieuwenhuys, J. Catal. 229 (2005) 154–162. [23] J.Y. Luo, M. Meng, X. Li, X.G. Li, Y.Q. Zha, T.D. Hu, Y.N. Xie, J. Zhang, J. Catal. 254 (2008) 310–324.