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Ind. Eng. Chem. Res. 2003, 42, 2996-3000
Promotion of the Catalytic Activity of a Ag/Al2O3 Catalyst for N2O Decomposition by the Addition of Rh. A Comparative Activity and Kinetic Study T. N. Angelidis† and V. Tzitzios* Department of Chemistry, Box 116, Aristotle University, Thessaloniki GR-54124, Greece
Preliminary activity tests showed a synergistic effect on the yield of N2O decomposition by the addition of small quantities of rhodium in a Ag/Al2O3 catalyst. An analytical comparative kinetic study over Rh/Al2O3, Ag/Al2O3, and Rh-Ag/Al2O3 was performed in order to explain this effect. In the absence of oxygen, the reaction kinetics over the silver catalyst seemed to follow a L-H mechanism with N2O inhibition; in the case of rhodium, the power law equation was applied. While in the case of the mixed catalyst, a L-H mechanism was followed and the inhibition effect was not observed for the studied condition. From the results of the kinetic study, it seemed that the synergistic effect was connected with an increase of the catalytic activity over both silver and rhodium catalysts, since more active sites were offered and N2O inhibition became less intensive. The presence of oxygen was found to inhibit the reaction over the silver and the mixed catalysts, but the inhibition effect was less intensive in the case of the mixed catalyst. Intoduction Because of its simplicity, N2O decomposition has been used for decades as a probe to evaluate the catalytic activity and other surface properties of various solids.1 In the previous decade, the study of this reaction gained further significance because N2O has been recognized as an environmental pollutant. N2O is one of the important greenhouse gases with a long lifetime in the atmosphere (about 150 years). The greenhouse potential of N2O is 270 times higher than the potential of CO2 (on a weight basis as calculated over a 100-year time period). It is also an important source of stratospheric nitrogen oxides, which contribute to stratospheric ozone destruction.2 Control of N2O emissions has thus become a significant target especially from sources such as stationary and mobile combustion processes or chemical processes, which can be controlled effectively by cleanup methods. Among the various N2O emission control methods catalysis plays a major role especially in the case of a low concentration of N2O and high temperatures. A variety of catalysts (supported or unsupported noble metals, pure oxides, mixed oxides, spinels, perovskites, hydrotalcites, and zeolites) were tested for N2O decomposition.2 The main problem observed for most catalysts was the inhibition of N2O decomposition by the strong adsorption of N2O and particularly oxygen, normally present in many exhaust gases.2 In the literature, silver was extensively studied for the removal of NOx from exhaust gases by hydrocarbons, especially propene3-6 or propene-propane7 mixtures in the presence of excess oxygen. An interesting feature of these studies was that the de-NOx activity is followed by high selectivity (low N2O production) compared manly with noble metal catalysts. Rhodium is known as one of the most active catalysts for N2O decomposi* To whom correspondence should be addressed. Tel.: +302310-997-696. Fax: +30-2310-997-759. E-mail: tzitzios@ chem.auth.gr. † Deceased.
tion.2 Rhodium-exchanged ZSM-5 catalysts have higher activity than the Al2O3 supported ones.9 Recent studies were also devoted to the application of supported (especially on ZrO2) rhodium catalysts for N2O decomposition.10-13 Silver-rhodium catalysts have been tested for CO hydrogenation, CO insertion, ethane and methylclopentane hydrogenolysis, and last year butadiene hydrogenation.14 The aim of the present research work was to study the activity of silver-based catalysts for N2O decomposition and the influence of the addition of rhodium in these catalysts. Since, a promotional effect on the activity of both catalysts (silver and rhodium) was observed, a comparative kinetic study was followed in an effort to explain this synergistic effect. Experimental Section The Ag-based catalysts were prepared by impregnating the support γ-Al2O3 (150 m2 g-1) with a solution of lactic silver produced by dissolution of Ag2O in lactic acid. The method of silver catalyst preparation has been described in detail in another publication.7 The rhodium and mixed catalysts were prepared following exactly the same procedure, with the addition of Rh(NO3)3 in the solution as rhodium precursor. The catalytic systems were characterized by SEM-EDX studies in a previous work.8 The reaction experiments were performed in a quartz reactor containing ∼0.1 g of catalyst. N2O and O2 certified calibration gas mixtures balanced by He were used as reacting gases, and pure He was applied as diluent (all from Air Liquid). Gas flow rates were controlled using mass flow controllers (Brooks Series 5850E). The total pressure in the reactor was near atmospheric, and the partial pressures of the reactants were varied by changing the individual flow rates, while simultaneously adjusting the flow rate of pure He. Reactant and product analyses were performed by gas chromatography (Shimadzu GC 14B equipped with Poropac Q and Molecular Sieve 5A columns). The
10.1021/ie020533b CCC: $25.00 © 2003 American Chemical Society Published on Web 05/31/2003
Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003 2997
Figure 1. Effect of addition of Rh on the catalytic activity of the 5 wt % Ag/Al2O3 catalyst (experimental conditions: pN2O ) 0.01 atm, flow rate 150 mL min-1, and 0.1 g of catalyst).
catalysts were pretreated with H2 at 550 °C for 3 h (flow rate 100 mL min-1) and with a 20% O2 in helium gas mixture for 1 h at 550 °C (flow rate 150 mL min-1). Reduction by H2 was applied in an effort to remove from the catalyst surface the products of silver tarnishing (silver oxides and possibly sulfides) formed during the preparation procedure. The catalyst was treated for 12 h with the reaction mixture before its experimental run and the product analysis was performed after the required time period to reach steady-state. The activity tests were performed keeping constant the partial pressure of the reactant (0.01 atm N2O) and increasing stepwise the reaction temperature with a gas mixture flow rate of 150 mL min-1. From the results of these experiments, the light-off temperatures were determined as the temperature at which 50% N2O decomposition was obtained. For the kinetic measurements, the partial pressures of N2O or O2, were held constant at 0.01 atm, while the pressure of the other gas was varied at constant temperature. These measurements were repeated at five different constant temperatures. The reactor was operated in a differential mode with the conversion not exceeding 6%. The reaction conversion was controlled by varying the catalyst loading (diluting the catalyst with the proper quantity of R-Al2O3) and varying the reactant mixture flow rate between 100 and 300 mL min-1. Separate experimental tests showed that bulk mass transfer and intraparticle mass-transfer resistance could be eliminated by using a gas flow rate greater than 75 mL min-1 and catalyst particles less than 212 µm in size. The N2O decomposition was used to calculate the reaction rate as
rN2O ) (XN2O,in - XN2O,out)Nt/m
(1)
where rN2O is the rate of N2O decomposition (in mol s-1 g-1), XN2O,in is the molar fraction of N2O in the feed, XN2O,out is the molar fraction of N2O in the product, Nt is the total molar gas flow rate (in mol s-1), and m is the catalyst weight (in g). Results and Discussion The influence of rhodium addition in the catalyst was studied by use of a series of mixed catalysts containing 5 wt % Ag and various quantities of rhodium. The results of the activity tests are shown in Figure 1. The activity increases as the rhodium concentration increases up to 0.05 wt % and then remains almost constant. So, the catalyst containing 0.05 wt % Rh and 5 wt % Ag was selected as the one with the better activity. In Figure 2, the catalytic activity of the 5 wt % Ag, 0.05 wt % Rh, and 0.05 wt % Rh-5 wt % Ag
Figure 2. Comparison of the catalytic activity of 5 wt % Ag/Al2O3, 0.05 wt % Rh/Al2O3, mixed 5 wt % Ag-0.05 wt % Rh/Al2O3 catalysts (experimental conditions: pN2O ) 0.01 atm, flow rate 150 mL min-1, and 0.1 g of catalyst).
Figure 3. Influence of the partial pressure of N2O on the rate of N2O decomposition over 5 wt % Ag/Al2O3 (a), 0.05 wt % Rh/Al2O3 (b), and mixed 5 wt % Ag-0.05 wt % Rh/Al2O3 (c) catalysts at various temperatures.
catalysts is compared. As seen clearly, the mixed catalyst presents a better activity than the catalysts containing the separate components at the same concentrations. In an effort to explain the above behavior, an analytical comparative kinetic study follows. Kinetic Studies. (1) Results and Preliminary Kinetic Approach. Steady-state kinetic measurements were performed under differential reactor conditions at various temperatures between 475 and 575 °C for the 5 wt % Ag/Al2O3 catalyst, 400-480 °C for the 0.05 wt % Rh/Al2O3 catalyst, and 350-450 °C for the 5 wt % Ag-0.05 wt % Rh/Al2O3 catalyst. One set of experiments was performed for each catalyst by varying the partial pressure of N2O. The results are illustrated in Figure 3a-c, respectively. A second set of experiments was performed in the presence of oxygen by varying the oxygen partial pressure, keeping constant the partial pressure of N2O at 0.01 atm. These measurements were also performed under differential conditions at various temperatures between 540 and 600 °C for the 5 wt %
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Figure 4. Influence of the partial pressure of O2 on the rate of N2O decomposition over 5 wt % Ag/Al2O3 and 5 wt % Ag-0.05 wt % Rh/Al2O3 catalysts at various temperatures. In the inset, R2 values of the simulation procedure for each temperature are also shown. Table 1. Exponents Calculated by the Application of the Power Law Rate Equation rN2O ) kpN2Om on the Experimental Data for N2O Decomposition in the Absence of Oxygen over the Three Catalysts at Various Temperaturesa 5% Ag temp (°C)
0.05% Rh
5% Ag-0.05% Rh
5% Ag
m
temp (°C)
m
temp (°C)
m
475 500 525 550 575
0.31 0.44 0.48 0.52 0.59
400 420 440 460 480
0.33 0.37 0.38 0.39 0.40
350 375 400 425 450
0.96 0.76 1.29 0.87 1.38
mean SD
0.47 0.10
0.38 0.03
temp (°C)
1.05 0.27
a
The mean value and the respective standard deviation are also shown.
Ag/Al2O3 catalyst and 400-475 °C for the 5 wt % Ag0.05 wt % Rh/Al2O3. The results are illustrated in Figure 4a and b, respectively. The rate of N2O decomposition seems to increase at low partial pressures of N2O and then tends to stabilize at higher partial pressures for the Ag and Rh catalysts. The decline of the rate increase is sharper for the Rh than for the silver catalyst. On the contrary, the rate over the mixed catalyst seems to be a linear function of the partial pressure of N2O. A first approach to the experimental results was made by the power law rate equation:
rN2O ) kpN2OmpO2n
Table 2. Exponents Calculated by Application of the Power Law Rate Equation rN2O ) kpN2OmpO2n on the Experimental Data for N2O Decomposition, in the Presence of Oxygen with Constant N2O Partial Pressure (0.01 atm) over the Silver and the Mixed Catalysts at Various Temperaturesa
(2)
The values of the exponents for the three catalysts are summarized in Table 1 (in the absence of oxygen) and in Table 2 (in the presence of oxygen), as calculated from eq 2 for various temperatures. A comparison of the mean values of the exponent m for the three types of catalysts in Table 1 show that inhibition by N2O is stronger for the rhodium catalyst than for the silver and the mixed catalysts. The inhibition by N2O is less in the case of the mixed catalyst. The same behavior was observed in
5% Ag-0.05% Rh n
temp (°C)
n
540 560 580 600
-0.46 -0.37 -0.35 -0.26
400 425 450 475
-0.36 -0.30 -0.26 -0.20
mean SD
-0.36 0.08
-0.28 0.07
a The mean value and the respective standard deviation are also shown.
the presence of oxygen. The strong inhibition as seen by the negative values of n is larger in the case of the silver catalyst than in the case of the mixed one. (2) Analytical Kinetic Approach. In its simplest form, N2O decomposition can be described as an adsorption of N2O at an active center, followed by decomposition giving formation of N2 and a surface oxygen.2 This surface oxygen can be desorbed by combination with another oxygen atom as molecular oxygen. The above mechanism could be described by the following sequence of reactions:
N2O + * T N2O*
(3)
N2O* f N2 + O*
(3a)
2O* T O2 + 2*
(3b)
Based on the above reaction sequence with the N2O adsorption and O2 desorption steps being at equilibrium and the decomposition step being the rate-determining step with the assumption of steady-state condition and constant number of active sites, the following expression may be easily derived:
Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003 2999
rN2O )
kKN2OpN2O 1 + KN2OpN2O + xKO2pO2
(4)
where rN2O is the N2O decomposition rate (mol N2O s-1 g-1), k ) k′NT is the kinetic constant including the total number of active sites (NT) (mol N2O s-1 g-1), KN2O is the adsorption equilibrium constant of N2O (atm-1), pN2O is the partial pressure of N2O (atm), KO2 is the adsorption equilibrium constant of O2 (atm-1), and pO2 is the partial pressure of O2 (atm). The denominator in the equation represents the distribution between the catalyst active sites that are empty and those that are occupied by N2O and O, respectively. This expression was originally proposed for oxides15 but is applied rather in general. Adsorption of N2O and of O2 results in reaction inhibition. In the absence of oxygen and under differential experimental conditions (low N2O conversion) the resulting equation is
rN2O ) kKN2OpN2O/(1 + KN2OpN2O)
(5)
At very low N2O partial pressures, KN2OpN2O , 1 and the rate becomes a linear function of pN2O:
rN2O ) kKN2OpN2O
(6)
All the above reaction routes were examined for the three catalysts and the selection was made by taking in account the fitting to the experimental data, as well as the calculation of valid values for the kinetic and physical constants. (3) Kinetic Behavior in the Absence of Oxygen. (a) Silver Catalyst. Equation 5 was linearized and fitted on the experimental data, and the values of the adsorption equilibrium constants and kinetic constant were calculated. The fitting of the above model on the experimental data is shown in Figure 3a and the respective R2 values in Table 3. The calculated values of the constants are summarized in Table 3. The experiments at five different constant temperatures permit the calculation of the activation energy and the enthalpies of adsorption of the reactants by the application of Arrhenius (Figure 5a) and van’t Hoff (Figure 5b) equations, respectively. The calculated values are also shown in Table 3. (b) Rhodium Catalyst. The above procedure was applied to the experimental data obtained for N2O decomposition over the rhodium catalyst. Although the fitting of eq 5 was good, the calculated values for KN2O were independent of temperature, an indication that may mean that the calculated values did not represent adsorption equilibrium constants following the van’t Hoff equation. Therefore, the power law equation was applied in the case of the Rh catalyst. The values of kinetic constants resulting from fitting of the power law equation to the experimental data are shown in Table 4. The value of the apparent activation energy calculated by the Arrhenius equation is 71,1 kJ mol-1. The calculated value of the activation energy is lower than the one found in the literature (139 kJ mol-1),16 A possible explanation for this difference is the different Rh loading of the catalyst 0,05 wt % in this study versus 1,85 wt % in ref 16 and the respective different surface coverages.
Figure 5. (a) Estimation of the activation energy of N2O decomposition on 5 wt % Ag/Al2O3 catalyst. (b) Temperature dependency of the adsorption equilibrium constant of N2O and the respective enthalpy of adsorption on 5 wt % Ag/Al2O3 catalyst. Table 3. 5 wt % Ag/Al2O3 Catalyst. R2-, Adsorption Equilibrium Constant, Kinetic Constants, Enthalpy of Adsorption, and Activation Energy As Calculated by the Application of the Selected Kinetic Model on the Experimental Results temp (°C) 475 500 525 550 575
RSQ
k × 10-6 (mol s-1 g-1)
K N 2O (atm-1)
0.992 219 ( 23 0.998 158 ( 6 0.991 109 ( 4 0.972 83 ( 3 0.982 65 ( 1
0.39 ( 0.02 0.47 ( 0.01 0.64 ( 0.03 0.96 ( 0.07 1.55 ( 0.09
∆Hads.,N2O (kJ mol-1)
Eact (kJ mol-1)
-65.3 ( 1.8 73.1 ( 8.4
Table 4. 0.05 wt % Rh/Al2O3 Catalyst. Kinetic Constants and Apparent Activation Energy As Calculated by the Application of the Power Law Equation on the Experimental Results temp (°C)
k1 × 10-5 (mol s-1 g-1)
E1act (kJ mol-1)
400 420 440 460 480
2.3 ( 0.6 3.0 ( 0.6 3.5 ( 0.9 4.1 ( 1.6 4.6 ( 1.7
71.1 ( 6
(c) Mixed Catalyst. In the case of the mixed catalyst, the reaction rate seems to be a linear function of the partial pressure of N2O. This is the case for eq 6, where KN2OpN2O , 1. The application of eq 6 to the experimental data permits the calculation of the slope that is equal to the product KN2Ok. By the application of the Arrhenius equation, Figure 6, the apparent activation energy, which represents the sum of Eact + ∆Hads, was calculated. The fitting of the proposed equation to the experimental data is shown in Figure 3c, and the calculated constants and the apparent activation energy are in Table 5. In conclusion, the initial activity tests show that the N2O decomposition activity of the studied catalysts follows the order
5 wt % Ag-0.05% Rh > 0.05 wt % Rh > 5 wt % Ag The above activity order was confirmed by the kinetic study. A comparison between the rates of N2O decomposition over the silver and the rhodium catalysts (Figure 3a and b) shows that the rate over the rhodium
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Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003
significant increase of the catalyst activity for N2O decomposition. The mixed catalyst was found to be more active than the catalysts with separate active components. A synergistic effect was revealed from the comparative kinetic studies based on the high decomposition activity of the rhodium catalysts that suffered from strong N2O inhibition and the large number of active sites offered by silver, which removes the inhibition effect. Literature Cited Figure 6. Estimation of the apparent activation energy of N2O decomposition on 5 wt % Ag-0.05 wt % Rh/Al2O3 catalyst. Table 5. 5 wt % Ag-0.05 wt % Rh/Al2O3 Catalyst. Standard Deviation, Constant and Apparent Activation Energy As Calculated by the Application of the Selected Kinetic Model to the Experimental Results temp (°C)
kKN2O × 10-5 (mol s-1 g-1)
SD × 10-5
350 375 400 425 450
5.4 8.8 22.6 33.4 46.1
0.4 3.5 5.7 7.2 8.4
Eact.ap (kJ mol-1)
84.4 ( 8
catalyst is ∼10 times higher than over the silver catalyst at the same temperature range. The rhodium catalyst is characterized by strong N2O inhibition. Such an inhibition is less intensive in the case of the silver catalyst. The inhibition effect is completely removed in the case of the mixed catalyst for the N2O partial pressures studied. The absence of inhibition over the mixed catalyst is followed by a slight increase of the decomposition rate compared with the rhodium catalyst (Figure 3b and c). It is possible that over the mixed catalyst a synergistic effect between the two active components (silver and rhodium) is present. The synergistic effect can be explained by the increase of active sites offered for N2O decomposition over the mixed catalyst. Rhodium offers high decomposition activity, while silver offers more active sites for the reaction to proceed. Such an explanation is confirmed by the kinetic study over the silver and the mixed catalyst in the presence of oxygen (Figure 4a and b, respectively). Oxygen inhibition is stronger over the silver catalyst than over the mixed one. This fact is also connected with the increased number of active sites offered by the mixed catalyst. Such a synergistic effect means that each component retains its individual properties and that there are no strong interactions (e.g., alloying) between silver and rhodium. This is in accordance with the findings of early studies on mixed alumina-based silver-rhodium catalysts by H2 chemisorption; TPR and TPO techniques showed that there was no strong interaction between silver and rhodium.14 In conclusion, the addition of small quantities of rhodium to a silver alumina-based catalyst caused a
(1) Kung, H. H. Transition Metal Oxides Surface Chemistry and Catalysis; Elsevier: New York, 1989. (2) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Heterogeneous Catalytic Decomposition of Nitrous Oxide. Appl. Catal., B 1996, 9, 25. (3) Miyadera, T. Alumina-supported silver catalysts for the selective reduction of nitric oxide with propene and oxygencontaining organic compounds. Appl. Catal., B 1993, 2, 199. (4) Martinez-Arias, A.; Fernandez-Garcia, M.; Iglesias-Juez, A.; Anderson, J. A.; Conesa, J. C.; Soria, J. Study of the lean NOx reduction with C3H6 in the presence of water over silver/alumina catalysts prepared from inverse microemulsions. Appl. Catal., B 2000, 28, 29. (5) Bethke, K. A.; Kung, H. H. Supported Ag Catalysts for the Lean Reduction of NO with C3H6. J. Catal. 1997, 172, 93. (6) Meunier, F. C.; Breen, J. P.; Zuzaniuk, V.; Olsson, M.; Ross, J. R.H Mechanistic Aspects of the Selective Reduction of NO by Propene over Alumina and Silver-Alumina Catalysts. J. Catal. 1999, 187, 493. (7) Angelidis, T. N.; Kruse, N. Promotional effect of SO2 on the selective catalytic reduction of NOx with propane/propene over Ag/ Al2O3. Appl. Catal., B 2001, 34, 201. (8) Angelidis, T. N.; Tzitzios, V. Promotion of the catalytic activity of a Ag/Al2O3 catalyst for the N2O+CO reaction by the addition of Rh, a comparative activity tests and kinetic study. Appl. Catal., B 1272 2002, 1-14. (9) Li, Y.; Armor, J. N. Catalytic Decomposition of Nitrous Oxide on Metal Exchanged Zeolites. Appl. Catal., B 1992, 1, L21. (10) Yuzaki, K.; Yarimizu, T.; Aoyagi, K.; Ito, S.; Kunimori, K. Catalytic decomposition of N2O over supported Rh catalysts: effects of supports and Rh dispersion. Catal. Today 1998, 45, 129. (11) Centi, G.; Dall’ Olio, L.; Perathoner, S. In situ activation phenomena of Rh supported on zirkonia samples for the catalytic decomposition of N2O. Appl. Catal., B 2000, 194-195, 79. (12) Centi, G.; Dall’ Olio, L.; Perathoner, S. Oscillating Behavior in N2O Decomposition over Rh Supported on Zirconia Based Catalysts: The Role of Reaction Conditions. J. Catal. 2000, 192, 224. (13) Dann T. W.; Schulz, K. H.; Mann, M.; Collings, M. Supported rhodium catalysts for nitrous oxide decomposition in the presence of NO, CO2, SO2 and CO. Appl. Catal., B 1995, 6, 1. (14) Yuvaraj, S.; Chow, S. C.; Yeh, C. T. Characterization of Silver-Rhodium Bimetallic Nanocrystallites Dispersed on γ-Alumina. J. Catal. 2001, 198, 187. (15) Yamashita, T.; Vannice, A. N2O Decomposition over Manganese Oxides. J. Catal. 1996, 161, 254. (16) Holles, J. H.; Switzer, M. A.; Davis, R. J. Influence of Ceria and Lanthana Promoters on the Kinetics of NO and N2O Reduction by CO over Alumina-Supported Palladium and Rhodium. J. Catal. 2000, 190, 247.
Received for review July 17, 2002 Revised manuscript received February 26, 2003 Accepted April 16, 2003 IE020533B
