20006
J. Phys. Chem. 1996, 100, 20006-20014
Role of Lattice Oxygen in the Combustion of Methane over PdO/ZrO2: Combined Pulse TG/DTA and MS Study with 18O-Labeled Catalyst Christian A. Mu1 ller, Marek Maciejewski, Rene´ A. Koeppel, Reto Tschan, and Alfons Baiker* Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zu¨ rich, Switzerland ReceiVed: June 27, 1996; In Final Form: September 17, 1996X
The contribution of a redox mechanism involving lattice oxygen in the catalytic combustion of methane over PdO/ZrO2 catalysts, prepared from amorphous Pd-Zr alloys, has been studied by means of gas pulse methods, including a novel technique “pulse thermal analysis”, and using labeled catalysts containing Pd18O. Special emphasis was devoted to the influence of the isotope exchange (scrambling) of reactants and products, especially O2 and CO2, with the catalyst on the quantity of 18O-containing reaction products. Substantial amounts of H218O and C18O16O were detected during pulses of a reactant mixture consisting of methane and 16O2 in a ratio 1:4 at 300 and 500 °C. The effect of the oxygen exchange of molecular oxygen with the solid phase proved to be negligible due to its low extent. At 300 °C, at least 20% of the CO2 formed originated from the redox mechanism involving lattice oxygen. At 500 °C, oxygen exchange of CO2 with the catalyst became predominant and precluded determining reliably the proportion of CO2 formed by the redox process. The results indicate that a substantial part of methane is oxidized via a redox process. Consequently, this reaction has to be taken into account when interpreting the catalytic behavior of palladium-based catalysts and explaining the structure-activity relations previously observed.
1. Introduction Since the publication of the original work by Mars and van Krevelen,1 various oxidation reactions have been shown to occur via direct interaction of reactants with lattice oxygen.2-14 A redox mechanism, where the reactant is oxidized by lattice oxygen and the reduced catalyst is subsequently reoxidized by gas phase oxygen, is according to refs 15-19 believed to be possible for all oxidation reactions carried out over oxidic catalysts. Known Mars and van Krevelen-type catalysts are bismuth molybdate,2,3,5,6,9,12 molybdena in general,10-13 and vanadia catalysts.1,8,15,17 Although redox-type mechanisms are widely accepted, the experimental verification is difficult and mostly based on the comparison of the respective rates of reduction and oxidation of the catalyst by the gas phase constituents1,8,12,13 or the use of labeled molecules.2-6 The latter, mostly performed by labeling of the catalyst, provides more reliable information, since a direct interaction of gaseous constituents with the solid phase can be shown by the detection of labeled molecules in the gas phase. The interpretation of the results is, however, complicated by the occurrence of oxygen exchange between oxygen containing gas phase components and the solid catalyst (so-called scrambling) without participation of a redox mechanism. The lack of experiments where this phenomenon has been taken into account in a quantitative way prompted us to investigate the Pd-catalyzed combustion of methane, a reaction possibly submitted to a redox mechanism, by means of 18O labeling. Special emphasis was devoted to the effects caused by the isotopes exchange of reactants and products. Most literature on scrambling deals with the exchange of lattice oxygen with gas phase oxygen.20-26 To our knowledge, no reports exist in the literature concerning the exchange reactions between lattice oxygen of palladium oxide and other oxygen-containing components, such as carbon dioxide, especially in combination with a catalytic reaction. * Corresponding author. Tel: (+41-1) 6323153. Fax: (+41-1) 6321163. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, December 1, 1996.
S0022-3654(96)01903-X CCC: $12.00
Catalytic combustion offers an efficient way for complete combustion of fuels over wide air/fuel ratios without significant NOx or soot formation and permits system designs with improved combustion efficiency and energy recovery.27-33 Increasing interest in catalytic combustion led to high research activity in this field. Palladium-based materials are reported to be among the most active catalysts for catalytic combustion.27,29,32-34 The unique activity of palladium based catalysts, only comparable with Pt, has attracted much interest.35-64 Despite concentrated efforts to determine the phenomena occurring during the complete oxidation of methane, there are still unsolved problems to examine. A very interesting property of palladium-based catalysts is the enhanced activity which is observed after reduction of previously oxidized palladium.37,40,55,59,60 We recently reported that the rise in catalytic performance can be correlated with changes in the Pd particle size.59 Several chemical properties of the catalyst such as the ease of decomposition, the rate of reduction by methane and hydrogen, and the reoxidation behavior subsequent to reduction in function of the particle size were related to the catalytic activity. As a sound explanation for these findings, a redox mechanism, including the reduction of palladium oxide by methane followed by reoxidation of metallic palladium by oxygen, was proposed. Such a redox or so-called Mars and van Krevelen mechanism was previously suggested by other groups for the combustion of methane over palladium catalysts34,46,49,55 without providing experimental evidence. Together with the conventional thermal analysis applied for the characterization of the reactivity of the catalysts, a novel pulse thermal analysis (PTA) method65,66 was used for elucidating the catalytic reactions and the isotope exchange. PTA is based on the injection of a small quantity of the gaseous reactant into the carrier gas stream and simultaneous monitoring of the changes of mass, enthalpy, and gas composition (mass spectrometry) resulting from the very small progresses of reaction. The course of the reaction, depending on the applied pulse © 1996 American Chemical Society
Combustion of Methane over PdO/ZrO2 volume, can be investigated at any temperature and degree of conversion. Due to the very high sensitivity of PTA, changes of mass or enthalpy in the range of 0.01 mg or 0.1 J g-1, respectively, can be monitored, providing the possibility of investigating even the differences in adsorption behavior under reaction conditions of very small amounts of injected gases. The aim of the present work was to determine whether the combustion of methane over palladium supported on zirconia is subjected to a redox mechanism, involving lattice oxygen. The studies were pursued by means of conventional pulse experiments and the novel PTA method using 18O-labeled catalysts. The influence of temperature and feed composition on the reaction mechanism was investigated. Special attention was given to the scrambling of gaseous components, i.e., the exchange of gaseous, oxygen-containing molecules with lattice oxygen, because of its masking effect on the amount of labeled products originating from the redox process. 2. Experimental Section 2.1. Catalyst Preparation. The amorphous alloys used as catalyst precursors were prepared from the premixed melt of the pure constituents by rapid quenching using the technique of melt spinning. The resulting ribbons were ground into flakes of 100-400 µm size. The as-prepared amorphous alloys with the nominal composition Pd25Zr75 and Pd33Zr67 were activated by oxidation in air in a furnace at 350 °C, until full oxidation was accomplished. The oxidation resulted in a drastic increase in the BET surface area from ca. 0.1 to 24.4 m2 g-1 (for a Pd25Zr75 alloy). XRD and TEM measurements revealed that the catalysts consisted of small intergrown, poorly crystalline domains of zirconia (monoclinic and tetragonal) and palladium oxide. The specific Pd surface area measured by CO chemisorption over Pd25(ZrO2)75 amounted to ca. 3.7 m2 g-1 (7.9 × 10-5 mol g-1 Pd exposed)60 after reduction at 300 °C, indicating a Pd dispersion of 5.0%. Note that the specific Pd surface area does not reflect the amount of Pd exposed under reaction conditions, where PdO is the predominant palladium phase. However, it provides an estimate of the maximum possible Pd surface area. For the preparation of the 18O-labeled catalysts, the fully oxidized Pd-Zr amorphous alloys were slowly reduced in a closed quartz reactor under pure hydrogen at room temperature for 20 min. Subsequently, the temperature was increased to 400 °C with a rate of 10 °C min-1, followed by cooling with a rate of ca. 20 °C min-1 to 50 °C. At this temperature the atmosphere in the system was changed to argon and the sample was heated with 10 °C min-1 to 400 °C in order to decompose possibly formed palladium hydride. After cooling in argon to 50 °C, the obtained Pd/ZrO2 was heated with 10 °C min-1 to 500 °C in an atmosphere containing 30 vol % of 18O2 (balance of Ar). After 30 min of reaction at 500 °C, the concentration of 18O2 was increased to 50 vol % and the sample was kept under these conditions until the pressure did not further decrease, indicating completion of the Pd oxidation (ca. 30 min). Decomposition experiments confirmed that the palladium oxide phase of the Pd33Zr67-based catalyst contained ca. 19% Pd18O (average value of three experiments). 2.2. Pulse Thermal Analysis (PTA). Thermoanalytical investigations (DSC/TG) were carried out at atmospheric pressure using a Netzsch STA 409 thermoanalyzer equipped with PtRh10/Pt thermocouples and Pt crucibles and a Mettler TA 2000C. The Netzsch thermoanalyzer was equipped with a pulse device suitable for PTA. By this means, the changes of mass and enthalpy of the samples during a pulse could be
J. Phys. Chem., Vol. 100, No. 51, 1996 20007 followed. The STA 409 thermoanalyzer was coupled via a heated capillary with a Balzers QMG 420 quadrupole mass spectrometer. PTA was carried out isothermally (at 300 and 500 °C) or in nonisothermal mode with a heating rate of 4 °C min-1 in the range 30-930 °C. Measurements were performed using 100 mg of Pd33Zr67 after activation in air and labeling with 18O. The volume of injected gases (mixture of methane and oxygen in the ratio 1:4 for catalytic reaction, pure and diluted carbon dioxide for the isotopic exchange) was 1.0 or 0.25 mL, respectively. Argon with a flow rate of 50 mL min-1 was used as a carrier gas. 2.3. Pulse Measurements in Fixed-Bed Microreactor. Pulse measurements were carried out in a continuous fixedbed microreactor (quartz, 4 mm i.d.) operated at atmospheric pressure using a Balzers GAM 400 quadrupole mass spectrometer for gas analysis. As not stated otherwise, the tests were performed using 100 mg of Pd33Zr67-based catalyst. A volume of 2 mL reaction gas mixture of various compositions, consisting of methane (99.995%) or carbon monoxide (99.995%), respectively, and oxygen (99.999%), was injected into an inert gas flow of 285 mL min-1 helium, which was previously purified by an oxysorb and a hydrosorb column. The time span between each pulse lasted 10 min. The mass spectrometer was calibrated using carbon dioxide (mass/charge ) 44), oxygen (mass/charge ) 32) and methane (mass/charge ) 15). The calibration factors of CO2 and O2 species were assumed to be the same, independent of the isotope composition. The amount of a specific component was determined by integrating its time-dependent MS signal. The concentration of water could not be measured satisfactorily due to its adsorption on tube surfaces and resulting delays in the signal, causing a low signal-to-noise ratio. The methane conversion was calculated on the basis of a carbon balance including carbon dioxide and methane. 3. Results 3.1. Investigation of Reduction and Oxidation of Palladium Oxide/Zirconia Catalysts by Methane and Oxygen. A redox mechanism for the combustion of methane over palladium oxide/zirconia catalysts demands both the reduction and oxidation of the catalyst by the constituents of the reaction atmosphere to be of similar rate under steady state conditions. However, these rates cannot be measured discretely under reaction conditions. The rate of reduction with methane and subsequent reoxidation with oxygen of a PdO/ZrO2 catalyst prepared by oxidation of an amorphous Pd33Zr67 alloy was therefore studied by PTA at temperatures 300 and 500 °C, using alternating pulses of 0.25 mL of CH4 and 1.0 mL of O2 (Figure 1 and Table 1). Comparing the MS signals of CO2 and H2O (lower parts of Figure 1), we notice a larger width of the water peak. This phenomenon is explained by a strong adsorption of water by the catalyst and by a stronger retention of H2O in the apparatus due to adsorption effects. In the upper part of Figure 1, the TG and the DSC curve are depicted. The steps in the TG curve resulting from the reduction of PdO and the oxidation of Pd are well discernible (compare Table 1). The continuous weight loss during the reduction-reoxidation pulses originates from a permanent release of water evolving from the zirconia support. A major difference between the tests at 300 and 500 °C lies in the varying oxidation behavior. At 500 °C, the weight changes due to reduction and oxidation during pulses are almost equal from the beginning of the experiment. Similarly, the degree of the reduction at 300 °C during methane pulses is independent of the number of redox cycles (compare the weight loss in Table 1 and the constancy of the exothermal effects on
20008 J. Phys. Chem., Vol. 100, No. 51, 1996
Mu¨ller et al.
TABLE 1: Weight Gain and Weight Loss during PTA Measurements at 300 and 500 °C with a Catalyst Derived from Pd33Zr67a pulse 1 CH4 weight gain/% weight loss/% differenceb/% weight gain/% weight loss/% difference/%
2 O2
3 CH4
-0.104
0.154
-0.038
-0.089
0.188
-0.015
6 O2
7 CH4
0.079 0.151
500 °C 0.173
0.162 0.200
5 CH4
300 °C 0.065
0.044 0.148
4 O2
-0.072
0.084 0.149
0.196 0.200
-0.004
8 O2
-0.065 0.187
0.198
-0.011
Pulse volume: CH4, 0.25 mL; O2, 1.0 mL. Catalyst weight, 100 mg; Ar carrier gas flow 50 mL min-1. b Calculated as the difference between the weight gain and the previous weight loss. a
Figure 1. Comparison of the catalytic behavior of palladium oxide/ zirconia during alternating pulses of CH4 and O2 with pulse volume ratio 1 (methane):4 (oxygen) at temperatures 300 and 500 °C investigated by PTA. Upper part, TG and DSC curve; lower part, MS signals of H2O (m/z ) 18, bar values: 300 °C, 5 × 10-12 A; 500 °C, 10-11 A) and CO2 (m/z ) 44, bar values: 300 and 500 °C, 2 × 10-12 A); catalyst weight, 100 mg; Ar flow rate, 50 mL min-1.
the DSC curve in Figure 1). However, the extent of the reoxidation during oxygen pulses is substantially lower for the first pulse and steadily increases for subsequent pulses. The weight loss during reduction and the weight gain during oxidation converge toward each other with increasing number of pulses (Table 1). At 300 °C, the rate of oxidation is too slow for the slightly reduced catalyst and increases only after ongoing reduction, giving rise to the amount of metallic palladium to be reoxidized. Throughout this work, the investigation of the catalytic reaction is based on pulse methods. It has been previously established by investigating the catalytic activity in microreactor experiments that palladium oxide is not reduced under reaction conditions.60 However, these experiments have been long-term measurements to investigate the steady-state conversion. In order to check the changes of the oxidation state of palladium in the PdO/ZrO2 catalyst by PTA, 1 mL pulses of a mixture CH4:O2 ) 1:4 were passed over a Pd33Zr67-based catalyst containing 18O (Figure 2). 18O-labeled gaseous molecules, such as H2O (mass 20), CO2 (mass 46), and CO2 (mass 48), were detected while passing the pulses over the catalyst bed (see lower part of Figure 2). The upper parts of Figure 2 confirm the stability of the composition of the active phase during pulses of the reaction mixture. Except a constant weight loss originating from water released from zirconia, no decrease of the
Figure 2. Catalytic behavior of palladium oxide/zirconia containing Pd18O during pulses of reaction gas mixture (CH4:16O2 ) 1:4) at 300 and 500 °C (for comparison: total weight loss upon reduction, 4.4%). Upper part, TG and DSC curve; lower part, MS signals of CO2 (m/z ) 44 [bar values: 300 and 500 °C, 10-12 A] and m/z ) 46 [bar: 300 and 500 °C, 5 × 10-12 A]); catalyst weight, 100 mg; pulse volume, 1 mL; Ar flow rate, 50 mL min-1.
mass due to reduction of PdO was observed during pulses at 300 and 500 °C. Note that the total weight loss upon reduction of PdO would amount to 4.4%. The investigation of the catalytic combustion of methane over palladium oxide/zirconia by pulse methods therefore provides valid information about the condition of PdO during reaction conditions. The PTA technique proved to be very sensitive, showing even a small weight gain during pulses, which is attributed to the adsorption of gaseous components on the catalyst surface. This behavior is further supported by the DSC curve. A prominent peak mostly caused by the heat of reaction is followed by an endothermic event hardly visible in Figure 2. We conclude that this effect has to be assigned to the endothermic process of desorption taking place after the pulse. 3.2. Determination of 18O Content in the Reaction Products under Different Conditions. During PTA experiments, part of the gaseous reaction mixture bypasses the catalyst bed. In order to draw quantitative conclusions about the amount of CO2 produced via a mechanism involving lattice oxygen, further experiments were carried out in a fixed-bed microreactor. The results of pulse experiments performed with a Pd33Zr67based catalyst containing 18O and CH4:O2 ) 1:4 at 300 °C are depicted in Figure 3 and listed in Table 2. Species containing 18O are detected for both CO and O (water not being 2 2 measured). The amount of 18O-containing species in CO2 is
Combustion of Methane over PdO/ZrO2
J. Phys. Chem., Vol. 100, No. 51, 1996 20009
TABLE 2: Results of Pulse Experiments with CH4:16O2 ) 1:4 Carried Out at 300 °C with a Catalyst Derived from Pd33Zr67 Containing Pd18Oa pulse CH4/mol CO2(mass44)/mol CO2(mass46)/mol CO2(mass48)/mol O2(mass32)/mol O2(mass34)/mol conversion (%) Sib total amount of 18Oc/mol relative amount of 18O in catalyst (%)d
1
2
3
4
5
6
7
8
1.0 × 10-5 1.1 × 10-5 6.6 × 10-6 7.8 × 10-7 6.0 × 10-5 2.2 × 10-7 64 0.58 8.4 × 10-6 97
1.2 × 10-5 1.1 × 10-5 4.9 × 10-6 4.4 × 10-7 6.5 × 10-5 1.4 × 10-7 58 0.43 5.9 × 10-6 95
1.2 × 10-5 1.2 × 10-5 4.6 × 10-6 3.3 × 10-7 6.6 × 10-5 2.4 × 10-7 58 0.40 5.5 × 10-6 93
1.2 × 10-5 1.2 × 10-5 4.0 × 10-6 2.5 × 10-7 6.7 × 10-5 2.1 × 10-7 57 0.33 4.7 × 10-6 91
1.3 × 10-5 1.3 × 10-5 3.6 × 10-6 1.8 × 10-7 6.9 × 10-5 2.1 × 10-7 57 0.28 4.2 × 10-6 89
1.3 × 10-5 1.3 × 10-5 3.3 × 10-6 1.4 × 10-7 6.8 × 10-5 1.7 × 10-7 56 0.25 3.8 × 10-6 88
1.2 × 10-5 1.3 × 10-5 2.9 × 10-6 1.2 × 10-7 6.8 × 10-5 1.8 × 10-7 57 0.22 3.3 × 10-6 87
1.3 × 10-5 1.3 × 10-5 3.0 × 10-6 1.3 × 10-7 6.9 × 10-5 2.6 × 10-7 57 0.23 3.5 × 10-6 85
a Catalyst weight, 100 mg; He carrier gas flow 285 mL min-1. b Si is defined as the ratio between the amount of CO2 (m/z ) 46) and CO2 (m/z ) 44). c Calculated as the sum of 18O contained in CO2 (mass 46 and 48) and O2 (mass 34). No measurements could be made for water. d Calculated on the basis of the total amount of 18O initially present in the catalyst (2.7 × 10-4 mol) and the cumulative amount of 18O detected in CO2 and O2 (no water included).
Figure 3. Results of pulses of reaction gas mixture (CH4:16O2 ) 1:4) obtained from microreactor experiments performed at 300 °C with palladium oxide/zirconia containing Pd18O: (a) formation of CO2 (mass 44, 46, 48); (b) O2 (mass 32) detected during pulses. He flow rate, 285 mL min-1; pulse volume, 2 mL; catalyst weight, 100 mg.
decreasing with increasing number of pulses (Figure 3a). The catalyst is deprived of its 18O, which is replaced by 16O supplied to the system as a part of the reaction atmosphere, thus augmenting the amount of CO2 with mass 44. This fact is also evidenced by the increasing amount of unreacted molecular oxygen with mass 32, indicating a substitution of 18O by 16O (Figure 3b). A peculiar property of the catalyst is the considerable mobility of oxygen within the palladium oxide phase. Assuming that only the first three monolayers of palladium oxide are accessible to oxygen exchange, contrary to the experimental findings, no 18O should be detected in the reaction products after approximately four pulses without contribution of diffusion of 18O in the palladium oxide phase (amount of palladium exposed ca. 10-4 mol g-1 60). A lowering of the amount of 18O in the reaction products to zero has actually not been achieved even with a large number of pulses (ca. 25), probably because of the 18O contained in zirconia, as discussed later, and a certain amount of 18O found in normal oxygen (ca. 0.2%). The conversion of methane, being almost constant with every pulse except the first one, reaches 60% already at 300 °C (Table 2). As a measure of the degree of exchanged 18O, the ratio between the amount of CO2 containing one 18O atom (mass 46) and CO2 containing two 16O atoms (mass 44), Si, is listed in Table 2. A remarkable value of 0.58 is achieved for the first pulse at 300 °C, showing over one-third of the totally detected CO2 to contain oxygen stemming from the catalyst. The influence of the reactant mixture has been studied at 300 °C using Pd33Zr67-based catalysts (Figure 4). Feeds with CH4:
Figure 4. Influence of the feed composition (CH4:O2 ) 1:4 and 1:2) at 300 °C on (a) formation of CO2 (mass 46), outlier for pulse number 3 (CH4:O2 ) 1:2) in brackets; (b) Si (defined as the ratio between CO2 (mass 46) to CO2 (mass 44)). He flow rate, 285 mL min-1; pulse volume, 2 mL; catalyst weight, 100 mg.
O2 of 1:4 and 1:2 have been used with the same pulse volume (2 mL). For both conditions, the same behavior regarding the amount of 18O-containing CO2 (mass 46) produced as a function of pulse number is observed. The larger amount of methane pulsed to the system for a feed ratio 1:2 led to a higher amount of CO2 produced (Figure 4a), while the overall conversion decreased only marginally (average conversion ratio 1:4, 58%; ratio 1:2, 52%). The varying ratio of methane to oxygen pulsed to the system did not significantly influence the ratio between CO2 (mass 46) and CO2 (mass 44), Si (Figure 4b, definition see Appendix). The fact that Si was not considerably affected either by the amount of methane or the amount of oxygen, pulsed simultaneously, is astonishing and will be discussed in detail later. The influence of the temperature on the course of the methane combustion investigated by pulses performed with a ratio CH4: O2 ) 1:4 is illustrated in Figure 5 with catalysts based on Pd33Zr67 for 300 °C and Pd25Zr75 for 500 °C. The conversion was 58% (mean value) for 300 °C and reached 88% (mean value) at 500 °C, which is also evidenced by a rise in CO2 formation (see Figure 5a for mass 46). Note the steeper slope for points measured at 300 °C compared to the results obtained at 500 °C. These findings can be explained by considering the oxygen diffusion in palladium oxide to be faster at 500 °C, resulting in a better relaxation before each pulse and thus extending the amount of 18O present within the first few monolayers. The comparison of the ratio CO2 (mass 46) to CO2 (mass 44), Si,
20010 J. Phys. Chem., Vol. 100, No. 51, 1996
Figure 5. Influence of the temperature with CH4:O2 ) 1:4: (a) formation of CO2 (mass 46); (b) ratio Si. He flow rate, 285 mL min-1; pulse volume, 2 mL; catalyst weight, 100 mg; catalysts: 300 °C, Pd33Zr67 precursor; 500 °C, Pd25Zr75 precursor.
Figure 6. Isotope exchange (scrambling) of molecular oxygen over palladium oxide/zirconia containing Pd18O investigated by PTA (pulses of pure 16O2): (a) TG curve and O2 (mass 34) evolution as a function of the temperature; (b) evolution of O2 (mass 32 and 34) during decomposition of PdO without superimposed signals due to pulses. Ar flow rate, 50 mL min-1; heating rate, 4 K min-1; sample weight, 100 mg.
for temperatures 300 and 500 °C shows this value to be unaffected by the temperature under the conditions used (Figure 5b). 3.3. Influence of O2 and CO2 Scrambling. In the preceding section, we have reported that during a reactant pulse several product species containing 18O are produced. Generally, three major steps responsible for the presence of 18O in the reaction products can be distinguished: the isotope exchange (scrambling) of molecular oxygen leading to molecular oxygen containing 18O and thus producing labeled CO2 during a subsequent reaction (Langmuir-Hinshelwood model); the reaction of methane or carbon containing intermediates with the catalyst (redox reaction); finally, the scrambling of CO2, leading to the formation of CO2 containing 18O and thus masking the previous reaction steps. The scrambling of molecular oxygen with catalyst oxygen has been studied by PTA using a catalyst based on Pd33Zr67 and is illustrated by the curve corresponding to m/z ) 34 in Figure 6a. PdO/ZrO2 containing 18O was exposed to pulses containing pure oxygen (mass 32) while subjected to a constant heating rate. The contact of gas phase oxygen with the catalyst surface led to the formation of oxygen 16O18O (mass 34, Figure 6a) and 18O2 (mass 36). At temperatures below 300 °C, the scrambling of oxygen was negligible. The small peak visible at ca. 260 °C was caused by the natural content of the 18O isotope in molecular oxygen (0.2%). With increasing temperature, scrambling became more prominent, leading to a maximum in peak height at ca. 560 °C. The decreasing peak maxima
Mu¨ller et al.
Figure 7. Scrambling of CO2 investigated by PTA (pulses of pure C16O2) over a Pd33Zr67-based catalyst containing Pd18O: (a) TG curve and CO2 (mass 46) evolution as a function of the temperature (peaks are labeled by numbers representing the Si value (ratio of the area of the signal m/z ) 46 to the area of m/z ) 44, i.e., the ratio of CO2 [mass 46] to CO2 [mass 44]); (b) magnification for the pulse at 210 °C (TG and DSC curve [upper part]; MS signals [lower part], bar values: m/z ) 44, 10-11 A; m/z ) 46, 5 × 10-11 A). Ar flow rate, 50 mL min-1; heating rate, 4 K min-1; sample weight, 100 mg.
were probably caused by depletion of 18O in the catalyst and/ or an inhibited adsorption of oxygen. At temperatures around 600 °C, the decomposition of palladium oxide started. This was evidenced by a continuous evolution of oxygen (mass 32 and mass 34, Figure 6b), and a simultaneous loss of mass indicated by the TG curve in Figure 6a. The evolution of oxygen during decomposition of PdO was superimposed by the scrambling occurring during pulses. Even after a major part of palladium oxide had decomposed, pulses of oxygen led to the appearance of 18O in the product gas. The occurrence of oxygen scrambling shown in Figure 6 is also evidenced by the detection of labeled oxygen molecules in the experiment referred to in Figure 3b and Table 2. The extent of the influence of labeled oxygen during reactant pulses can be estimated using the relation derived in the Appendix and thus be compared to the actual amount of labeled CO2 measured. Assuming no chemical preferences to exist in methane oxidation by 16O or 18O, the differential formation of the respective CO2 species can be formulated in terms of probabilities depending on the fraction of 16O and 18O present in the gas phase at the moment i. During the passing of the reactant pulse over the catalyst bed, the percentage of 18O in the gas phase increases, therefore ri g ri-1, where i - 1 denotes the moment before i and ri the double ratio of 18O to 16O in the gas phase (for definitions see Appendix). The amounts of CO2 produced containing none or a single 18O atom are summarized to give Si (the amount of C18O2 produced is not important for these considerations). It was shown, that ri g Si ∀ i. Consequently, if oxygen scrambling would be the only source for 18O in CO2 produced, the measured ratio Si (Table 2) could not be larger than the measured ratio ri. For both experiments performed at 300 and 500 °C with CH4:O2 ) 1:4, the ratio ri is much smaller than Si (values for the first pulse: 300 °C, ri ) 3.8 × 10-3, Si ) 0.58; 500 °C, ri ) 0.04, Si ) 0.48). The influence of oxygen scrambling is therefore found to be negligible and will not be considered in following calculations. The presence of 18O in the products can also be caused by isotopic oxygen exchange of CO2 and has therefore been investigated by PTA with a catalyst based on Pd33Zr67 using pulses of CO2 (Figure 7). The numbers added to the peaks are the values of the ratio Si (CO2 (mass 46):CO2 (mass 44)). The oxygen scrambling of CO2 is significant already at temperatures near room temperature (Figure 7a). The ascending temperature gives rise to an almost linear increase of the intensity of the
Combustion of Methane over PdO/ZrO2
Figure 8. Scrambling of CO2 investigated by pulses of CO2 (mass 44) over palladium/zirconia containing Pd18O: (a) values of Si and r*i (definitions, see text) for 500 °C with alternating pulses of CO2/O2 and CO2; (b) ratio Si for various mixtures at different temperatures (reaction gas, CH4:O2 ) 1:4, 50% mixture, CH4:CO2:O2 ) 1:1:6; alternating, CO2/O2 and CO2 [see a]). He flow rate, 285 mL min-1; pulse volume, 2 mL; catalyst weight, 100 mg; catalysts: 300 °C, Pd33Zr75 precursor; 500 °C, Pd25Zr75 precursor.
m/z ) 46 signal, resulting in a maximal exchange around 500 °C. The ratio Si increases according to a maximum around the same temperature. A marked difference exists between the value of Si for pulses at 300 and 500 °C. This difference has to be assumed even larger if one considers the ongoing removal of 18O during proceeding pulses in Figure 7. At temperatures when PdO decomposition occurs (600-800 °C, see TG curve), the scrambling is enforced again. At temperatures above 800 °C, the exchange of 18O is continuously observed. For the illustration of the sensitivity of the applied pulse thermal analysis, the TA and MS curves obtained during pulses at 210 °C are magnified in Figure 7b. In the lower part, the MS signals of all CO2 species are depicted (note that for better presentation, different scales are used for the respective signals of m/z ) 44 and 46). The weight gain evidenced by the TG curve is caused by the adsorption of CO2. A concomitant exothermic effect on the DSC curve is followed by a endothermic event which is presumably caused by the desorption of CO2. The exchange of oxygen between carbon dioxide and palladium oxide is of much more importance than the scrambling of oxygen in the system gas phase catalyst. Several investigations have been undertaken to clarify the influence of CO2 scrambling on the distribution of labeled products of the pulses. To test the competition in the exchange between O2 and CO2, alternating pulses of pure CO2 and a mixture CO2:O2 ) 1:2 (equals the ratio of these molecules after full conversion with a feed ratio CH4:O2 ) 1:4) have been injected to the microreactor (Figure 8a). Obviously, the presence of oxygen does not disturb the ratio Si, although the absolute amount of labeled CO2 decreases as a consequence of the smaller amount of CO2 pulsed. In Figure 8b, a comparison between pulses of several gaseous mixtures at different temperatures is made (experiments performed at 300 °C with a Pd33Zr67-derived catalyst and at 500 °C with a Pd25Zr75-based catalyst; pulse volume, 2 mL). A significant gap exists between the scrambling at 300 and 500 °C. This is in contrast to the findings emerging from the comparison of pulses of reaction mixture at 300 and 500 °C, where no major changes in Si were observed as a function of the temperature (Figure 5b). A “50% mixture” consisting of CH4:CO2:O2 ) 1:1:6 (equals the ratio of these molecules after 50% conversion with a feed ratio CH4:O2 ) 1:4) yields the same ratio Si than a pulse of reaction mixture with the nominal composition. The difference of the curve obtained by pulsing alternatingly CO2 and a CO2/O2 mixture to the other two curves
J. Phys. Chem., Vol. 100, No. 51, 1996 20011
Figure 9. Influence of the pulse volume and the pulse composition on the scrambling of CO2 at 300 °C (the curve fit was made with the empty circles representing pulses of 0.25 mL of CO2).
measured at 500 °C is attributed to a larger amount of exchanged for the CO2/O2 pulses. One is to suspect that the volume pulsed to the system has an impact on the amount of exchanged 18O and thus on the ratios between labeled and unlabeled species. Figure 9 depicts an experiment to determine the influence of both the volume of the pulse and its composition at 300 °C. As a reference, 0.25 mL pulses of pure CO2 have been injected (odd pulse numbers), alternating with 1 mL pulses of varying gas mixtures. A direct comparison between pulses of 0.25 and 1 mL of pure CO2 (first five pulses) reveals only marginal differences in the ratio Si, although the pulse volume has been changed by a factor of 4. Additions of N2 and O2, respectively, gave similar results for both molecules. With regard to Si, oxygen seems to have the same impact as nitrogen. The influence of CO2 scrambling has been found to be a major importance in determining the amount of CO2 produced by a possible redox mechanism. Pulses of pure CO2 resulted in an amount of labeled carbon dioxide comparable to that found with pulses of the reaction gas mixture. These results were independent of the volume of the pulses for the conditions applied. At 500 °C, the scrambling of CO2 occurred to such an extent that a redox mechanism could not be evidenced unambiguously (compare Figure 5b and Figure 8b). However, at 300 °C the scrambling of CO2 diminished to such an extent that distinction between Si for pulses of reaction mixture and pure CO2 became possible. The effect of the CO2 exchange was estimated using the expression derived in the Appendix. The parameters in eq 16 were assumed to be x ) 0.3 (Figure 8b, 300 °C) and y ) 0.5 (Figure 5b, 300 °C), which indicates a part of R ) 19% of CO2 produced by a redox mechanism during pulses of CH4:O2 ) 1:4 at 300 °C. 18O
4. Discussion The combustion of methane over PdO/ZrO2 catalysts prepared by total oxidation of a Pd-Zr amorphous alloy proceeds partly via a redox or so-called Mars and van Krevelen mechanism. This fact is evidenced by the detection of labeled reaction products such as C18O16O, C18O2, and H218O after pulses of reaction mixture (CH4 and 16O2) to a catalyst system containing 18O. The problems encountered in elucidating the participation of the redox mechanism, caused by the restriction to labeled catalysts and scrambling effects, could be solved on the basis of the combined use of conventional pulse techniques and PTA, and a balancing of the isotope labeled species. The need for pulses is justified by the use of labeled catalysts, which would be deprived of their 18O too fast if steady-state experiments
20012 J. Phys. Chem., Vol. 100, No. 51, 1996 would be performed. The short duration of a pulse and the small amount of reaction mixture guarantee a sufficient number of pulses before the level of labeled reaction products drops below significant values. However, for comparative studies of the behavior of a catalyst under different conditions, only the first pulse can be used, because depending on the parameters such as pulse volume and composition, the same state of the catalyst before contacting with the reaction mixture is only assured for the initial pulse. Furthermore, one has to pay attention to the fact that the values obtained by integrating the time-dependent mass spectrometric signal are averaged values and do not represent the momentaneous state of the oxygen exchange. The as-obtained results are therefore regarded as lower bounds. The experiments based on conventional pulses were additionally supported by applying a novel PTA method. The proposed redox mechanism and the oxygen exchange of O2 and CO2 established by our measurements are competing sources for the incorporation of 18O into reaction product molecules. Scrambling is an effect masking the formation of 18O-containing molecules via a Mars and van Krevelen mechanism by altering the amount of labeled molecules produced. The redox mechanism can occur only in combination with PdO, since pure zirconia is virtually inactive (0.9% conversion at 530 °C47). Oxygen scrambling of gas phase molecular oxygen, however, is known to take place on both palladium oxide and zirconia in the temperature range where our investigations were made.21,23 The detection of 18O-containing molecules after the decomposition of PdO in Figures 6 and 7 additionally leads to the conclusion that scrambling occurs on zirconia for both O2 and CO2. Furthermore, the transfer of lattice oxygen via the interface of zirconia and palladium oxide has to be considered. The reason for the low amount of Pd18O (ca. 20%) in the catalysts after oxidation of Pd/Zr16O2 with 97 vol % 18O2 lies probably in the transport of oxygen through the interface between palladium oxide and zirconia and the scrambling of 18O over zirconia. The influence of the support on the catalytic 2 activity of palladium-based methane combustion catalysts is widely discussed37,40,47 and has gained a new aspect by the observed high mobility of oxygen in zirconia. However, in contrast to the extent of scrambling, the source of oxygen exchange is of less importance with respect to the discrimination of a redox mechanism. The calculation of mass balances shown in the Appendix indicates that the influence of molecular oxygen scrambling can be neglected. In contrast, the scrambling of CO2 over both zirconia and palladium oxide has to be taken into consideration. The oxygen exchange of CO2 at 500 °C proves to be so massive that the confirmation of a redox mechanism is not possible for this temperature. No distinction can be made between a redox mechanism and the scrambling of CO2 as the source for 18O in CO2. However, the relative amount of labeled CO2 for pulses of the feed mixture remains almost constant for reaction temperatures of 300 and 500 °C, whereas the extent of CO2 scrambling decreases substantially at 300 °C. This leads to the conclusion that the additionally produced labeled CO2 at 300 °C is formed via a redox mechanism. A rough calculation on the basis of the experimental results revealed that at 300 °C ca. 20% of the totally produced CO2 are formed via a redox mechanism. Reasoning about this value, one has to keep in mind that only 20% of palladium oxide present before the first pulse are Pd18O. Even the exclusive occurrence of a Mars and van Krevelen mechanism, in which all CO2 is produced via lattice oxygen, would lead to a considerable amount of CO2 with mass 44. Additionally, levelling occurs throughout the pulse. Assuming a redox mechanism to be valid, the exchange of catalyst oxygen
Mu¨ller et al. with gas phase oxygen takes place over the entire duration of the pulse and leads to a lower amount of Pd18O and consequently to a lower total amount of CO2 containing 18O. Therefore, the contribution of a redox mechanism is generally underestimated based on pulse methods and the assessed amount of 20% CO2 produced by a redox mechanism at 300 °C represents a conservative estimate of the real quantity. The redox mechanism found represents a link between the catalytic reaction and the state of the palladium containing phase. As shown before,60 several bulk chemical and morphological properties have direct impact on the catalyst behavior. An enhanced mobility of lattice oxygen through the solid-gas interface, indicated for example by faster decomposition of PdO or better reducibility, is expected to induce an increased catalytic performance during oxidation reactions. It has, however, to be emphasized that the influence of the redox mechanism is restricted by other parameters which also have to be optimized, namely the specific surface area and the mass transport. Moreover, the overall conversion is determined by the interplay between the surface reaction of adsorbed reactants and the Mars and van Krevelen mechanism. 5. Conclusions The combustion of methane over PdO/ZrO2 catalysts prepared by total oxidation of Pd-Zr amorphous alloys has been shown to occur partly via a redox or so-called Mars and van Krevelen mechanism. The experiments were based on pulse methods using a catalyst labeled with 18O to give Pd18O. As a new method for investigations of gas-solid reactions, pulse thermal analysis (PTA) has been introduced and proved to be a potent tool for this kind of experiments. Due to the high sensitivity of the PTA method, even weight changes and exo- and endothermal effects occurring during adsorption and desorption can be followed. The analysis of the results, showing high amounts of combustion products containing 18O, was complicated by the isotope exchange of molecular oxygen and carbon dioxide with Pd18O. The influence of oxygen scrambling of gas phase O2 could be neglected because of its low extent, whereas the scrambling of CO2 allowed the unambiguous confirmation of a redox mechanism only for experiments performed at 300 °C due to the prominent oxygen exchange of CO2 at 500 °C. A minimum of 20% of the total amount of CO2 was estimated to be produced by a redox mechanism. Appendix Influence of Oxygen Scrambling. Assuming the oxidation of methane to carbon dioxide to proceed by surface reaction with adsorbed oxygen via an intermediate formation of carbon monoxide, the amount of CO2 species with masses 44, 46, and 48, respectively, can be described by the following scheme: CO(mass 28) 1/(1 + ri*)
1/(1 + ri*)
CO2(mass 44)
ri*/(1 + ri*)
CH4
CO2(mass 46)
ri*/(1 + ri*)
1/(1 + ri*)
CO(mass 30)
ri*/(1 + ri*)
CO2(mass 48)
The expressions assigned to the arrows describe the probabilities of the respective pathway to occur. r*i is defined as the ratio between the amounts of 18O (ni18) and 16O (ni16) in the gas phase oxygen at the moment i, and thus can be measured:
r*i ) ni18/ni16
(1)
Since scrambling of oxygen occurs throughout the entire
Combustion of Methane over PdO/ZrO2
J. Phys. Chem., Vol. 100, No. 51, 1996 20013
rip(44)i g Si∑p(44)i - ((p(44)1 + ... + p(44)i-1)ri-1) (9)
SCHEME 1
i
Remembering that r*i g r*i-1, and thus
ri g ri-1 we get
rip(44)i g Si∑p(44)i - ((p(44)1 + ... + p(44)i-1)ri) (10) i
Further rearrangements result in
ri∑p(44)i g Si∑p(44)i
(11)
ri g Si or 2r*i g Si.
(12)
i
i
and thus
duration of the pulse, r*i is always larger than r*i-1. The value ri, which represents the ratio of the amount of CO2 (mass 46) to the amount of CO2 (mass 44) built at the moment/point i, is calculated as the ratio of the probabilities to obtain the respective products:
2r*i (1 + r*i)2 ri ) ) ) 2r*i 1 p(44)i (1 + r*i)2 p(46)i
(2)
It is to be shown that
ri g Si ∀ i
(3)
The above statement is true for every i when at least once ri g Si, which is so for r1 ) S1. We have shown that if CO2 (mass 46) is produced only from oxygen 18O formed due to the scrambling of molecular oxygen, the ratio of CO2 (mass 46) to CO2 (mass 44) cannot be larger than the double ratio of 18O to 16O in the gas phase leaving the catalyst bed. A value for Si exceeding the value of ri can thus not be explained by the oxygen isotope exchange between lattice oxygen and gas phase oxygen. Influence of Carbon Dioxide Scrambling. The isotopic exchange between 16O in CO2 (mass 44) with 18O in the solid catalyst leading to the formation of CO2 (mass 46) and the reverse process [scrambling of CO2 (mass 46) with 16O in the catalyst] can be schematically illustrated in Scheme 2. The SCHEME 2
with
S1 )
p(46)1 p(46)1 + ... + p(46)i ; ... ; Si ) p(44)1 p(44)1 + ... + p(44)i
(4)
Si represents the ratio of the amounts of CO2 with mass 46 and CO2 with mass 44 totally built until the moment i. It can therefore be measured mass spectrometrically. We want to show that, if only a surface reaction occurs using adsorbed oxygen, the ratio between both species of CO2 (mass 46 to mass 44) cannot be larger than the double ratio of both isotopes of oxygen in the gas phase. The situation is graphically represented in Scheme 1. We see that
d ) (ri∑p(44)i) - (Si∑p(44)i) i
values ci denote the respective amount of carbon dioxide as defined above. The scrambling of CO2 as measured by pulses of pure CO2 to the labeled catalyst is expressed by x, which is defined as
x ) c2/c1
(5)
i
(13)
which we assumed to be also
x ) c4/c3
but also
d ) (p(44)1 + ... + p(44)i-1)(ri - Si-1)
(6)
Recombination thus yields
The amounts of CO2 (mass 46) and CO2 (mass 44) measured during the experiments of pulses of reaction mixture yield the value for y, which stands for
rip(44)i ) Si∑p(44)i - ((p(44)1 + ... + p(44)i-1)Si-1) (7)
y)
i
Assuming that
c2 + c3 c1 + c4
(14)
One can show that
ri-1 g Si-1 is valid at least for one i, we obtain
(8)
c3 + c4 c4 y-x ) ) )z c1 + c2 c2 1 - xy
(15)
20014 J. Phys. Chem., Vol. 100, No. 51, 1996 The percentage of CO2 (mass 46) built by a redox mechanism, taking into account the scrambling of carbon dioxide, is therefore expressed as
R)
(c3 + c4) (c3 + c4) + (c1 + c2)
)
y-x z ) (16) 1 + z 1 + y - xy - x
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