ARTICLE pubs.acs.org/IECR
Influence of the MgCo2O4 Preparation Method on N2O Catalytic Decomposition Miguel A. Zamudio, Samir Bensaid, Debora Fino, and Nunzio Russo* Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy ABSTRACT: MgCo2O4 spinel catalysts were prepared via the solution combustion synthesis (SCS) and coprecipitation (CP) methods. The produced powder catalysts were characterized by XRD, BET, FESEM, TPD, and XPS. The performance of these catalysts toward the decomposition of N2O to N2 and O2 was evaluated in a temperature programmed reaction (TPRe) apparatus in the absence and in the presence of oxygen (W/F = 0.03 g 3 s/cm3). The catalyst prepared by the CP method has been found to provide the best activity; the half conversion temperature (T50) of nitrous oxide was 380 and 415 °C in the absence and in the presence of oxygen, respectively. The MgCo2O4 spinel catalysts were directly deposited, by in situ SCS and CP, over ceramic honeycomb monoliths and tested in a lab-scale test rig. Again in this case, a prevalent activity of the catalytic monolith prepared by CP has been observed. The higher activity of the catalyst prepared by CP could be correlated with its higher surface area and its higher capacity of enriching the surface with the more reactive suprafacial, weakly chemisorbed, oxygen species.
1. INTRODUCTION Nitrous oxide (N2O) is an environmental pollutant which, during the past decade, has been recognized as a potential contributor to ozone destruction in the stratosphere (greenhouse potential 310 times higher than CO2) and has been acknowledged as a relatively strong greenhouse gas.1,2 Its continuously increasing concentration in the atmosphere (which mainly appears to be caused by human activities) is due to natural and anthropogenic sources (0.2-0.3% per year), as well as to a long atmospheric residence time (150 years). The emissions that can be reduced in the short term are the ones from chemical production processes, including, above all, adipic acid production and nitric acid plants, while the application of these catalysts in the automotive field is presently not envisaged, because N2O emissions are not individually regulated yet by the NEDC legislation. Ongoing agreements and forthcoming regulations call for the development of efficient and economical systems for N2O mitigation, all of which encourage us to develop catalysts that would decompose it into nitrogen and oxygen, to protect our global environment. Catalytic removal of N2O from anthropogenic sources would be one of the possible solutions. The catalytic decomposition of N2O, by means of several catalytic systems, has been the subject of increasing fundamental and applied research.1,3-14 Additionally, the kinetic modeling of catalytic decomposition based on some experiments using mixed metal oxide catalysts prepared from calcined hydrotalcite-type precursor has been conducted.15,16 In recent years, spinel-type oxides have been the subject of increasing research because of their catalytic properties.3-8 The synthesis procedure of spineltype oxides, which determines the final oxide features, plays a crucial role in the performance of these catalysts.4,5 The commercial use of zeolites has been limited by their poor hydrothermal stability;14 conversely, the zeolite’s susceptibility to sulfur poisoning, widely reported in the literature pertaining to r 2010 American Chemical Society
their application in the automotive field as DeNOx catalysts,25 is less relevant for the decomposition of N2O present in the offgases of adipic acid and nitric acid production plants, since they are sulfur-free.1 Only a few studies on the decomposition of N2O on monolithic catalysts have been reported so far.17-19 The present paper concerns the characterization, catalytic activity tests and reaction mechanism assessment of a MgCo2O4 spinel catalyst synthesized by two different methods: solution combustion synthesis and coprecipitation.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. The MgCo2O4 spinel catalyst was prepared by two different synthesis methods: Solution combustion synthesis (SCS) and coprecipitation (CP).3,4 SCS, as starting point, involves the preparation of an aqueous solution containing magnesium nitrate (Aldrich, 99% purity) and cobalt nitrate (Aldrich, 98% purity) as precursors of the desired spinel and glycine as sacrificial fuel (Aldrich, 98.5%). According to this method, the reagents are dissolved in distilled water, and this solution is placed in a crucible in an oven at 600 °C for a few minutes; the solution is then quickly brought to boil and it then froths and swells until the chemical reaction starts. The CP synthesis method involved the preparation of an aqueous solution containing the same metal nitrate precursors. In our tests, a water solution of 15 wt % K2CO3 was then added, drop by drop, at room temperature until the pH of the solution reached 9. The key step was to stop the stirring when the color of Special Issue: IMCCRE 2010 Received: March 17, 2010 Accepted: June 1, 2010 Revised: May 27, 2010 Published: June 10, 2010 2622
dx.doi.org/10.1021/ie100658w | Ind. Eng. Chem. Res. 2011, 50, 2622–2627
Industrial & Engineering Chemistry Research the slurry changed. The obtained precipitate was washed until the filtrate was neutral, then dried at 100 °C for 7 h, and this was followed by calcinations at 400 °C in air for 2 h.5 The catalysts were analyzed by X-ray diffraction (PW1710 Phillips diffractometer) in order to assess their purity and crystalline structure. A field emission scanning electron microscope (FESEM, Leo 50/50 VP Gemini column) was employed to analyze the microstructure of the crystal aggregates of the catalysts. The specific surface area of the prepared catalysts was evaluated from the linear parts of the BET plot of the N2 isotherms, using a Micromeritics ASAP 2010 analyzer. The XPS spectra were recorded using a PHI 5000 Versa Probe with a scanning ESCA microscope equipped with an Al monochromatic X-ray source (1486.6 eV, 25.6 W), a beam diameter of 100 μm, a neutralizer at 1.4 eV 20 mA, and at FAT analyzer mode. All the binding energies were referenced to the C1s peak at 284 eV of the surface carbon. The individual components were obtained by curve fitting. 2.2. Catalytic Monolith Preparation and Characterization. As far as the catalytic monolith preparation is concerned, cordierite honeycombs produced by Chauger (diameter, 34 mm; length, 25 mm; cell density, 200 cpsi) were catalyzed according to the two different procedures. The deposition of MgCo2O4, by means of SCS, was carried out according to the procedure detailed in one of our previous papers.20 As far as the catalytic monolith prepared by means of the coprecipitation method is concerned, the deposition procedure was done as follows: the catalyst powder, prepared by CP, was dispersed in a water solution, and this resulted in a slurry into which the bare monolith was dipped. The excess of slurry remaining in the channels was forced out by blowing a controlled air flux through the monolith.21 In both cases, a final calcination step was performed at 400 °C for 2 h in air. The amount of deposited spinel catalyst was 10 wt %, referred to the monolith weight. 2.3. Catalytic Activity Assessment. The activity of the prepared catalysts in powder was analyzed by a temperature programmed reaction (TPRe), according to a standard operating procedure: A gas mixture (5000 ppmv N2O; 0 or 5 vol % O2, He = balance) was fed, at a constant rate of 100 mL 3 min-1, via a set of mass flow controllers, to the catalytic fixed-bed microreactor enclosed in a quartz tube, placed in an electric oven. The tubular quartz reactor was loaded with 50 mg of pelletized catalyst (W/F = 0.03 g 3 s/cm3). The activity of the monoliths was tested using a ceramic tubular reactor, placed in a temperature programmed furnace. The total gas flow was 12 NL 3 min-1 (30000 h-1), with the same two compositions adopted for the test on powder. The reaction temperature for the powder and monolith catalyst tests was controlled by a PID-regulated oven, and varied from 200 to 1050 at 5 °C min-1 rate. The outlet gas composition was analyzed through NOx/N2O NDIR (ABB) and chemiluminescence NO/NOx (Ecophysics) analyzers as a function of the bed temperature. The temperature corresponding to half N2O conversion (T50) was taken as an index of the activity of each tested catalyst, both in powder form or supported on monolith: the lower the T50, the more active the catalyst. The runs were repeated three times (with a 10 °C maximum difference in T50) in order to obtain an average T50 value for each catalyst. Blank nitrous oxide decomposition runs, in the absence of any catalyst and in the presence of only SiO2 or the bare monolith,
ARTICLE
Figure 1. XRD patterns of fresh powder catalysts: (A) MgCo2O4 synthesized by SCS, (B) MgCo2O4 synthesized by CP.
were also carried out to examine the catalytic effect of the spinel catalysts more clearly. Some further temperature programmed desorption analyses were performed on both the prepared spinel catalysts in a Thermoquest TPD/R/O 1100 analyzer, equipped with a thermal conductivity (TCD) detector.22 X-ray diffraction was once again employed on the catalysts, which underwent TPD analysis, to check whether the spinel structure had been retained or not and to check for the possible appearance of new phases. 2.4. Catalytic Monolith Aging Procedure. Finally, the structural stability of the supported catalysts was investigated through the following aging protocol: thermal aging in air atmosphere, at 700 °C for 24 h, with 3% vol of water vapor. These conditions are representative of the moisture concentration in the off-gases from the adipic acid and nitric acid production plants,1 while the temperature is well above the typical ones reached by the offgases of the adipic acid and nitric acid processes, and could be considered as conservative of an operating system willing to approach the T50, as later demonstrated by the XRD results on the aged monoliths. After the aging protocol, the possible loss of catalytic activity due to prolonged catalyst operation was checked, both in the absence and presence of oxygen. The tests on the monoliths, after the aging treatment, were carried out under the same operating conditions as the experiments on fresh monoliths using the same apparatus and analysis systems reported above.
3. RESULTS AND DISCUSSION The XRD diffraction patterns (reported in Figure 1) of the powder catalysts correspond to the desired spinel structure (JPCDS card PDF 81-0671) without diffraction peaks related to other phases. The BET specific surface area values are reported in Table 1. The high specific surface area obtained with the spinel catalyst prepared by the coprecipitation method (143 m2/g), that is three times higher than the homologue powder synthesized by SCS (52 m2/g), seems to be one of the governing parameters for the catalytic activity, in agreement with the reaction mechanism reported below. The difference between the catalyst surface areas, induced by the two synthesis procedures, is also evident considering both the pore volume and pore size values, which are summarized in Table 1. The coprecipitation method allows a catalyst to have a higher porosity than that prepared by SCS, and with smaller pores (10.9 nm versus 27.3 nm, respectively). 2623
dx.doi.org/10.1021/ie100658w |Ind. Eng. Chem. Res. 2011, 50, 2622–2627
Industrial & Engineering Chemistry Research
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Table 1. Results of the Catalyst Characterization Tests Concerning the BET Specific Surface Area, Porosimetry, Catalytic Activity, Temperature Programmed Desorption, and Reaction (on Fresh Powder Catalysts, Fresh Supported Catalysts, and Aged Supported Catalysts) TPD O2 BET area
Pore volume
Pore diameter
desorption
T50 no O2
T50 5% O2
T50 no O2
T50 5% O2
T50 no O2
T50 5% O2
(m2/g)
(cm3/g)
(Å)
(mmol/g)
(°C)a
(°C)a
(°C)b
(°C)b
(°C)c
(°C)c
905
990
MgCo2O4 SCS
52.3
0.04
273
0.06
440
470
535
740
630
782
MgCo2O4 CP
143.8
0.18
109
1.33
380
415
480
675
572
701
catalyst no catalyst
a
Tests on fresh powder catalysts. b Testss on fresh supported catalysts. c Tests on aged supported catalysts.
Figure 3. N2O conversion to N2 and O2 over the two unsupported spinel catalysts for different feed compositions: (top) 5000 ppm N2O and helium as the balance; (bottom) 5000 ppm N2O, 5 vol % O2, and helium as the balance. (9,0) MgCo2O4 by CP; (b,O) MgCo2O4 by SCS; (1,3) no catalyst.
Figure 2. FESEM views of the catalyst microstructure: (A) MgCo2O4 synthesized by SCS, (B) MgCo2O4 synthesized by CP.
Figure 2, in which the SEM images are reported, seems to confirm the data previously expressed. In fact, the morphology the MgCo2O4 prepared by CP (Figure 2B) presents a spongy and regular structure, with the presence of micrometric and submicrometric pore diameters. On the contrary, the MgCo2O4 prepared by SCS (Figure 2A) appears to have an irregular surface with several, not uniformly distributed, pores and larger crystal grains. The activity tests on the powder catalysts show that the MgCo2O4 spinel obtained by means of the coprecipitation method exhibits the best catalytic performance. Figure 3 compares the catalytic decomposition of N2O in the absence and in the presence of oxygen, for the two spinel oxides. Decomposition
runs in the absence of any catalyst and in the presence of only the inert (SiO2) are also reported. The catalytic decomposition of N2O to N2 and O2 on MgCo2O4 prepared by the CP method becomes detectable from 250 °C onward, with complete conversion occurring before 500 °C. For the other spinel catalyst prepared by SCS, the catalytic decomposition starts in the 320360 °C range and reaches the total conversion in the 600650 °C range. When oxygen is present in the feed stream, the T50 shift is rather small (
