- Email: [email protected]
Catalytic combustion of methane over ruthenium supported on zinc aluminate spinel
Applied Catalysis A: General 453 (2013) 349–357
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Catalytic combustion of methane over ruthenium supported on zinc aluminate spinel Janina Okal ∗ , Mirosław Zawadzki Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 19 September 2012 Received in revised form 29 November 2012 Accepted 16 December 2012 Available online 5 January 2013 Keywords: Methane combustion Ruthenium Zinc aluminate RuO2 agglomeration Deactivation
a b s t r a c t Activity of the Ru/ZnAl2 O4 catalysts, H2 -reduced or air-aged at 700 ◦ C, for the combustion of methane under O2 -rich conditions was studied in this work. High surface area ZnAl2 O4 spinel was synthesized by the unconventional co-precipitation method. Catalysts were prepared using Ru(NO)(NO3 )3 as metal precursor and characterized by H2 chemisorption, O2 uptake, BET, XRD and TEM. The size of ruthenium particles in fresh catalysts varied from 1.1 to 1.5 nm with the metal loading from 0.5% to 4.5 wt.%. Airaging treatment caused severe agglomeration of the Ru phase and formation of the well-crystallized RuO2 oxide. Under reaction conditions, highly dispersed Ru species were easily oxidized and RuO2 oxide was the active phase for methane combustion. The fresh catalysts were more effective than aged samples in terms of light-off temperature and temperature needed for the complete methane conversion. The mean crystallite size of the RuO2 , formed during combustion reaction over fresh catalysts, depended on metal loading and was lower (21–27 nm) as compared to that formed during aging process (21–40 nm), which leads to higher activity. However, stable catalytic activity was observed only for aged catalysts. The specific reaction rate (mol gRu −1 s−1 ) for the fresh catalysts was found to decrease about 50% when the Ru loading increases from 0.5% to 4.5%. The apparent activation energies Eapp on Ru/ZnAl2 O4 catalysts were in range of 120–129 kJ/mol and did not depend on the metal loading and catalyst pre-treatment. Catalytic activity could be partly explained by the changes in the morphology and the crystallite size of the RuO2 phase and may suggest a structure sensitivity of CH4 combustion over ruthenium. Catalyst deactivation observed for the fresh catalysts, originate mainly from severe modifications of the Ru phase but not from changes in the ZnAl2 O4 support structure. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Catalytic combustion of hydrocarbons has received considerable attention due to its practical applications in both power generation and pollutant abatement [1]. Among all the hydrocarbon fuels methane is the most difficult to be oxidized [1,2]. In general, supported noble metals such as Pt, Pd and Rh are well established as efficient catalysts for methane combustion [1–4]. Among these catalysts, palladium has been widely recognized as the most reactive in methane combustion under oxygen excess. The nature of the support has also been investigated and alumina supported metal catalysts are the most studied systems [1,2,5,6]. At high temperature required for the complete oxidation of methane, ␥-alumina frequently undergoes transition into ␣ phase causing a drastic decrease in the surface area, and the sintering noble metal contribute to the thermal deactivation of the catalysts [1]. The search
∗ Corresponding author. Tel.: +48 71 34 350 20; fax: +48 71 34 410 29. E-mail address: [email protected] (J. Okal). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.12.040
of new catalytic materials that can allow efficient oxidation of methane at lower temperatures still remains a major challenge. Supported ruthenium catalysts have received much interest over the past years, because of their high activity in many oxidation reactions. For example, ruthenium showed good reactivity in the low-temperature oxidation of volatile organic compounds (VOCs), such as ethyl acetate, acetaldehyde, toluene, propylene and propane [7–10]. Catalytic oxidation of these VOCs is usually complete at temperatures below 400 ◦ C, but methane combustion is not possible at these temperatures [1–4]. Thus, most studies on ruthenium have been carried out for more reactive VOCs, at conditions in which the thermal stability of the support is not so important as in the case of methane. In the literature, it is rarely found that ruthenium or bimetallic ruthenium catalysts were tested for the application of methane combustion [11]. More frequently ruthenium catalysts were applied in the partial oxidation of methane to synthesis gas [12,13]. A study by Ryu et al. [11] on bimetallic Pd–Ru system revealed that addition of ruthenium to palladium catalyst was favorable for the methane combustion in the presence of H2 S. This was ascribed to enhance dispersion of the Pd in the presence of ruthenium and also greater poisoning resistance
350
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
of ruthenium. Generally, it was stated in the literature that use of the Ru-based catalysts for the methane combustion is limited by their high-temperature stability. Labhsetwar et al. [14] reported that ruthenium shows high thermal stability when is incorporated in perovskite type structure and then can be used for high temperature applications like methane combustion. However, the major limitation of perovskites application is their low surface area. In recent years, nanocrystalline spinel materials like ZnAl2 O4 have gained considerable interest in the field of catalysis since they may be used as supports for noble metals to substitute traditional materials such as ␥-alumina. The zinc aluminate is a very interesting material from the catalytic point of view because of its desirable properties like the high thermal and chemical stability, hydrophobic behavior, high mechanical resistance, low acidity and good metal dispersion capacity [15]. In addition, ZnAl2 O4 spinel is capable to interact strongly with the metal component which causes an enhancement of the metal particles stability [16]. So far, there are only scarce literature data concerning the application of the zinc aluminate spinel as support material in VOCs oxidation [15,17]. Moreover, this material was never tested as the support for noble metals used in combustion of methane. Recently, we found that ruthenium supported over the ZnAl2 O4 spinel showed good reactivity in propane [18] and butane combustion [19]. In this study, the catalytic performance of ruthenium supported on ZnAl2 O4 spinel was evaluated in the total oxidation of methane. Catalysts containing various amounts of ruthenium were prepared, and the influence of the pre-treatment procedure on their physicochemical characteristics and catalytic activity in methane combustion at low concentration has been investigated. Detail structural studies, including used catalyst samples, are used to correlate the intrinsic properties of the catalyst materials to their catalytic performances. To our knowledge, no catalytic results for methane combustion over Ru/ZnAl2 O4 systems are available in the literature. Also, little is known about the stability of these catalysts at the relatively high temperatures needed for CH4 oxidation. 2. Experimental 2.1. Preparation of the Ru/ZnAl2 O4 catalysts The detailed procedure for the support and catalysts preparation was described previously [18]. The ZnAl2 O4 spinel was prepared by the unconventional co-precipitation method using aqueous solution of Zn(NO3 )2 and Al(NO3 )3 (molar ratio Al:Zn = 2:1). The precipitate was filtered off, washed with water, dried in air and finally calcined at 550 ◦ C for 3 h. Next, the support was crushed and sieved into the pieces of 0.3–0.5 mm. The Ru catalysts were obtained by the incipient-wetness impregnation method using an aqueous solution of Ru(NO)(NO3 )3 as metal precursor. Impregnated samples were dried at 110 ◦ C and finally reduced in H2 at 500 ◦ C for 5 h. The reduced catalyst is hereafter referred to as fresh catalyst. After reduction, half of the catalyst sample was air-aged at 700 ◦ C for 3 h. 2.2. Catalysts characterization The ruthenium content was determined with the ICP-AES method. In catalysts heated in air even at 700 ◦ C no ruthenium loss was noticed. Some details on the equipment and procedures used for catalyst characterization can be found elsewhere [18]. Nitrogen adsorption–desorption isotherms at temperature of liquid nitrogen were used to determine the textural properties (porosity and specific surface area) of the obtained samples. Measurements were carried out using an Autosorb-1 Quantachrome Instruments system. X-ray diffraction (XRD) patterns were
collected using DRON-3 diffractometer with a Ni-filtered Cu K␣ radiation. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were recorded on a Philips CM20 SuperTwin microscope. Ruthenium dispersion was determined by hydrogen chemisorption at 100 ◦ C using conventional static volumetric equipment. The extent of the Ru oxidation at temperatures up to 700 ◦ C was estimated by the O2 uptake measurements employing the same apparatus. 2.3. The catalytic activity measurements Combustion of methane over fresh and aged Ru catalysts was examined at atmospheric pressure using an apparatus, which consists of a flow measuring and control system, reactor and an on-line analysis system. The flow system is equipped with a mass-flow controllers (MKS) and a set of valves, which allows introduction of the gas mixture to the reactor. The reactor consists of a 350 mm long quartz tube with inner diameter of 10 mm. The reactor with the catalyst sample, placed in the middle of the reactor on a quartz wool bed, was located in a programmable electric furnace. The catalyst temperature was controlled by a Pt–Rh thermocouple mounted internally. A mixture of methane and synthetic air was fed at a flow rate of 14 l/h and catalysts were packed to a constant volume to give a gas hourly space velocity GHSV of 32,000 h−1 for all studies. The volumetric gas composition was 2000 ppm CH4 in air. The reaction products were analyzed on-line using a gas chromatograph (Chromatron GCHF 18.3) equipped with a flame ionization detector (FID) for the determination of CH4 analysis and TCD detector for CO and CO2 analysis. Prior to each catalytic experiment, sample of the catalyst (400 mg) was conditioned at 200 ◦ C for 2 h with the reaction mixture. Measurements were taken as the sample was heated stepwise at the temperature range of 200–700 ◦ C. Analyses were made at each temperature until steady-state activity was attained, and at least two consistent analyses were taken and data were averaged. The conversion (XCH4 ) was calculated from the inlet and outlet concentration of CH4 , respectively, assuming 100% mass balance. Reaction rates were obtained from measured values of XCH4 according to the equation; rate = XCH4 F/W , where F is the molar flow rate of methane (mol s−1 ) and W is the catalyst weight (g). The conversion data were reproducible within 5% accuracy. 3. Results and discussion 3.1. Thermal stability of the ZnAl2 O4 support The thermal stability of the ZnAl2 O4 support was evaluated after treatment in air for 3 h at temperatures up to 1050 ◦ C. XRD patterns of the support heated at 550, 700, 900 and 1050 ◦ C, are shown in Fig. 1. In all cases, thermal treatment yielded the nanocrystalline material with zinc aluminate spinel structure (JCPD 05-0669) without any impurities like ZnO or Al2 O3 . Very broad diffraction peaks for 550 ◦ C heated ZnAl2 O4 support become more intense and sharper after treatment at higher temperatures indicating increase in the degree of crystallinity. The results of the mean crystallite size calculations show an exponential-like growth relationship to the calcination temperature. As shown in Fig. 2, the crystallite size increases with temperature from about 3 to 13 nm. Taking into account the nitrogen adsorption–desorption isotherms, shown in Fig. 3, and calculated values of both the specific surface area SBET and average pore diameter Dp , listed in Table 1, one can see that all heated support samples are typical mesoporous materials. The isotherms are of type IV and show the hysteresis loop of H1/H2 (after heating up to 900 ◦ C) or H1/H3 (after heating at 1050 ◦ C) type which is characteristic for
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
351
Table 2 BET surface areas, Ru dispersions and metal particle size of fresh Ru/ZnAl2 O4 catalysts and the mean crystallite size of RuO2 in aged catalyst samples. Catalyst
SBET (m2 /g)
0.5% Ru/ZnAl2 O4 Fresh 210 Aged 148 1% Ru/ZnAl2 O4 Fresh 212 Aged 146 4.5% Ru/ZnAl2 O4 Fresh 214 Aged 141
Metal dispersiona (H/Ru)
dav (chem.) (nm)
d (TEM) (nm)
d (XRD) (nm)
0.71
1.1
n.d.b
n.d. RuO2 ; 21
0.69
1.2
n.d.
n.d. RuO2 ; 28
0.56
1.5
1.2–2.5
n.d. RuO2 ; 40
a Defined as the ratio of exposed Ru surface atoms to the total number of Ru atoms, assuming H/Rus = 1. Characterization data of fresh catalysts from previous study, Ref. [18]. b No Ru◦ was detected.
Fig. 1. X-ray diffraction patterns of ZnAl2 O4 support after heat treatment in air at 550 ◦ C (a), 700 ◦ C (b), 900 ◦ C (c) and 1050 ◦ C (d).
Fig. 2. Dependence of average crystallite size () and the specific surface area () of ZnAl2 O4 support on the heating temperature.
mesoporous materials, according to IUPAC classification. Only in the case of 550 ◦ C heated support, adsorption takes place also at very low relative pressures which revealed some contribution from microporosity. It should be noted that the nitrogen isotherms show a sharp increase of adsorbed nitrogen volume at high partial pressures (P/Po ∼ 0.6) which corresponds to a large mesopore volume with a relatively narrow distribution of pore diameters in the range 1–30 nm with a maximum at about 6–12 nm. This type of adsorption is attributed to interparticle mesoporosity and it could be assumed that nanoporous structure of zinc aluminate support is formed by the agglomeration of smaller monodispersed particles inherent in the 550 ◦ C heated sample or larger particles created after subsequent heat treatment. In the first case it forms more uniform pores in the micro/mesoporous range, while the second produces random-size pores in the mesoporous range. Heat treatment leads to some decrease of specific surface area and total pore volume and increase of pore diameter but even after heating at 1050 ◦ C, ZnAl2 O4 support maintained significant value of both SBET and Vtotal , i.e. 67 m2 /g and 0.28 cm3 /g, respectively, and Dp was changed a little. In line with XRD data, these results indicate also on good thermal stability of zinc aluminate support with spinel structure. 3.2. Characterization of the fresh and aged Ru/ZnAl2 O4 catalysts
Fig. 3. N2 adsorption–desorption isotherms of ZnAl2 O4 support after heat treatment at 550 ◦ C (a) and 1050 ◦ C (b) with corresponding pore size distributions (as inset).
Table 1 Morphological characteristics of the ZnAl2 O4 support. Heating temperature (◦ C)
dav (XRD) (nm)
550 700 900 1050
3 5 9 13
Specific surface area SBET (m2 /g) 216 149 81 67
Pore volume (VBJH ) (cm3 /g)
Pore diameter (Dp ) (nm)
0.40 0.39 0.32 0.28
5.6 9.5 11.2 12.4
The main characteristics of the fresh and aged Ru/ZnAl2 O4 catalysts are summarized in Table 2. All fresh Ru catalysts have a similar BET surface area as the 550 ◦ C heated support (216 m2 /g), while aged samples as the 700 ◦ C air-heated support (149 m2 /g). The fresh catalysts have a very high dispersion even at the high Ru loading. The ruthenium particle size changes from about 1.1 to 1.5 nm by increasing the metal loading from 0.5 to 4.5 wt.%. Additionally, only in the 4.5% Ru catalyst small metal particles were detected by HRTEM studies [18]. XRD patterns of fresh catalysts contained no peaks from the ruthenium phase. In XRD patterns of aged samples (not shown) characteristic peaks of RuO2 were detected at 2 of 28.1◦ , 35.1◦ and 54.3◦ (JCPDS 88-0322), indicating that agglomeration of the Ru phase occurred during aging process. The mean crystallite size of RuO2 oxide for the aged 0.5%, 1% and 4.5% Ru catalysts was 21, 28 and 40 nm, respectively (Table 2, last column). The Ru catalysts were tested for their activity toward methane combustion in O2 -rich atmosphere, hence studies of the interactions between the ruthenium and oxygen are essential. The results of the O2 uptake measurements at temperatures up to 700 ◦ C by the fresh Ru catalysts are presented in Table 3. Already at RT, the O2 uptake by the 0.5% Ru and 1% Ru catalysts is high and the O/Ru ratio of 1.73 and 1.50, respectively indicates that 85 or 75% of the total amount of ruthenium was oxidized to RuO2 . For the
352
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
Table 3 Volumetric O2 uptake at temperatures 20–700 ◦ C by the fresh Ru/ZnAl2 O4 catalysts and mean crystallite sizes of RuO2 calculated from XRD. O/Rua
Mean crystallite size of RuO2 (nm)
42.7 48.0 50.0 51.5 50.5 52.0 51.0 51.0
1.73 2.00 2.02 2.08 2.04 2.10 2.06 2.06
– – – – 6 10 20 21
20 150 200 250 350 500 600 700
74.2 86.6 96.0 99.0 101.5 100.5 100.5 99.5
1.50 1.75 1.96 2.00 2.05 2.03 2.03 2.01
– – – 4 10 16 20 28
20 150 200 250 350 500 600 700
147.9 230.0 268.0 282.0 420.0 481.1 480.0 452.2
0.66 1.04 1.21 1.27 1.89 2.16 2.14 2.03
– – 6 8 10 18 32 40
Catalyst
Temperature (◦ C)
0.5% Ru/ZnAl2 O4
20 150 200 250 350 500 600 700
1% Ru/ZnAl2 O4
4.5% Ru/ZnAl2 O4
Uptake of oxygen (mol O2 /g cat.)
a Ratio of the number of O atoms adsorbed to the total numbers of Ru atoms in the catalyst.
4.5% Ru catalyst, the O2 uptake at RT was much lower and may be attributed mainly to dissociative chemisorption [18]. Observed rise of the oxygen uptake with temperature points to a bulk oxidation of ruthenium. The complete oxidation of small ruthenium particles in the 0.5% Ru catalyst was achieved at 150 ◦ C and in the 1% and 4.5% Ru catalysts at about 250 ◦ C and 350 ◦ C, respectively. In samples oxidized at 250–700 ◦ C, the O/Ru ratio was slightly higher than the theoretical value, calculated by assuming the total oxidation of Ru to RuO2 . Recent published literature data [20,21] indicate that
additional adsorption of oxygen may occur also on the surface of the crystalline RuO2 oxide. XRD patterns of the 0.5% and 4.5% Ru/ZnAl2 O4 catalysts, recorded after oxidation at 200–700 ◦ C, are shown in Fig. 4. For the 0.5% Ru catalyst, at temperatures below or at 250 ◦ C (Fig. 4a), no diffraction patterns due to RuO2 or metallic Ru species were detected which indicate on well dispersion of the oxidized Ru species. When the temperature was raised to 350–500 ◦ C, diffraction peak of the crystalline RuO2 oxide (2 at 28.1◦ ) was observed with intensity increasing with temperature. For the 4.5% Ru catalyst already at 200 ◦ C, diffraction peaks of the RuO2 oxide were visible (Fig. 4b). The XRD and O2 uptake data indicate that highly dispersed metallic ruthenium species in the fresh samples were extensively oxidized by oxygen in this temperature range. The aggregation of the RuO2 particles to larger RuO2 crystallites occurs when the Ru catalysts were treated in oxygen especially above 350 ◦ C. This is evidenced by the increased intensity of the XRD peaks of the ruthenium oxide with temperature. The mean crystallite sizes of the RuO2 oxide calculated from XRD data of all catalysts are presented in Table 3, last column. The crystallite size of RuO2 ranged between 6–21 nm, 4–28 nm and 6–40 nm for the 0.5%, 1% and 4.5% Ru catalyst, respectively. The important finding is that RuO2 crystallite size depends strongly not only on temperature but also on the metal loading and above 500 ◦ C, the RuO2 crystallites are much larger in the high-loaded catalyst than in the low-loaded samples. 3.3. Methane combustion activity over the fresh and aged catalysts Methane conversion as a function of reaction temperature, over the fresh 4.5% Ru/ZnAl2 O4 catalyst as well as over the 550 ◦ C heated support and in an empty reactor is shown in Fig. 5. In the absence of the catalyst or support, the conversion curve corresponds to a homogeneous combustion curve published in the literature. Methane conversion over the ZnAl2 O4 spinel starts at about 450 ◦ C, i.e. at temperature up to 200 ◦ C higher than that over the Ru catalyst confirming that Ru phase is an active component for the methane combustion. Under O2 -rich conditions all Ru catalysts showed the complete oxidation of CH4 to CO2 , i.e. no CO was detected.
Fig. 4. X-ray diffraction patterns of the 0.5% (a) and 4.5% Ru/ZnAl2 O4 catalyst (b) obtained at the indicated oxygen treatment temperature.
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
353
Table 4 Catalytic properties of ruthenium during methane combustion over the fresh and aged Ru/ZnAl2 O4 catalysts.a Catalyst
BET of used catalyst (m2 /g)
Catalytic activity in terms of conversion temperature (◦ C) T10
Fresh ZnAl2 O4 0.5% Ru/ZnAl2 O4 1% Ru/ZnAl2 O4 4.5% Ru/ZnAl2 O4
128 198 200 197
T50
T95
Aged
Fresh
Aged
Fresh
Aged
Fresh
Aged
145 140 138
540 390 375 350
– 435 460 480
650 430 412 385
– 480 500 530
770 500 490 450
– 550 575 600
a The reaction conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
Fig. 5. Methane conversion as a function of temperature over the fresh 4.5% Ru/ZnAl2 O4 catalyst (), ZnAl2 O4 support (䊉), and in empty reactor (). Experimental conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
The effect of metal loading on methane conversion over the fresh and aged catalysts is compared in Fig. 6. As shown, all fresh catalysts exhibit the better performance in the methane combustion than the aged samples in terms of light-off temperature
Fig. 6. Methane conversion over the fresh (a) and aged (b) Ru/ZnAl2 O4 catalysts with Ru loading of 0.5% (,), 1% (,), and 4.5% (,䊉) as a function of reaction temperature. Experimental conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
and temperature needed for the complete methane conversion. Depending on the Ru loading in the fresh catalysts methane conversion started at between 250 and 350 ◦ C, while over the aged samples at between 330 and 400 ◦ C. Over the fresh 0.5%, 1% and 4.5% Ru/ZnAl2 O4 catalyst the complete CH4 oxidation occurs at 500, 490 and 450 ◦ C, respectively, whereas over the aged catalysts at 550, 575 and 600 ◦ C, respectively. Thus, for the fresh catalysts activity increases with the Ru loading, but opposite for the aged catalysts activity shows the reverse order with the Ru loading. To compare the oxidation activity of the catalyst samples, we measured from the light-off curves the values of temperature for conversion of 10%, 50% and 95%. The obtained T10 , T50 and T95 values for the fresh and aged Ru samples are given in Table 4. For all Ru catalysts conversion of methane rise rapidly as the reaction temperature increased (Fig. 6) which is also reflected in the Arrhenius plots. Fig. 7 shows, for example, the Arrhenius plots obtained for the fresh and aged 4.5% Ru/ZnAl2 O4 catalyst. The specific reaction rate is expressed as moles of CH4 converted per gram of the catalyst and per second. The turnover frequencies values (TOF, molecules of methane reacted per surface Ru per second) of the catalysts cannot be taken into account since the particle size of the Ru phase can change during course of the reaction (see Table 3). From the Arrhenius plots, apparent activation energies Eapp of 129 and 121 kJ/mol for the fresh and aged 4.5% Ru/ZnAl2 O4 catalyst, respectively were obtained. Similar Eapp values were found for the 0.5%–1% Ru/ZnAl2 O4 catalysts (125–120 kJ/mol). The activation energies obtained in this study are slightly higher than that found in the literature for the others noble metals. Reported values are between 60–100 kJ/mol for methane combustion over platinum [3,22,23], and 70–100 kJ/mol over palladium catalysts [23–25]. However, comparative data for Ru catalysts are not available in the literature. Additionally, it is clear from Fig. 7 that at identical Ru loading, the air-aged catalyst is less active for methane combustion than the fresh catalyst. The large differences in the
Fig. 7. Arrhenius plots for the combustion of methane over the fresh () and aged (䊉) 4.5% Ru/ZnAl2 O4 catalyst.
354
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
Table 5 The specific reaction rate at 375 ◦ C and apparent activation energy (Eapp ) for methane combustion over the fresh Ru/ZnAl2 O4 catalysts.a Catalyst
0.5% Ru/ZnAl2 O4 1% Ru/ZnAl2 O4 4.5% Ru/ZnAl2 O4
Eapp (kJ/mol−1 )
Specific reaction rate mol CH4 gcat −1 s−1
mol CH4 gRu −1 s−1
0.04 0.11 0.32
13.0 11.2 7.03
124 121 129
a The reaction conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
specific activity may indicate that the number of the active sites is different in these samples. In fact, the characterization data of the 4.5% Ru/ZnAl2 O4 catalyst before activity studies, show that airaging at 700 ◦ C leads to the large aggregation of the Ru phase. The mean crystallite size of the RuO2 oxide in the aged sample amount to 40 nm and it was much higher than that of after oxidation of the fresh catalyst at 350–500 ◦ C (10–18 nm) (Table 3). Here, we assume that under O2 -rich reaction conditions highly dispersed Ru clusters present in the fresh catalyst transforms to RuO2 crystallites with sizes similar to that obtained only in the oxygen atmosphere. Our activity results are consistent with studies of Mitsui et al. [7], who found that activity of the Ru/␥-Al2 O3 and Ru/ZrO2 catalysts for combustion of ethyl acetate, acetaldehyde and toluene was higher on the reduced catalysts than on the asoxidized at 400 ◦ C. We also found previously strong decline activity in oxidation of propane over the Ru/␥-Al2 O3 catalysts oxidized at 600 ◦ C [26]. Some additional information about activity of the Ru catalysts, as a function of the metal loading, could be obtained by considering the specific reaction rate at given temperature, expressed as moles of CH4 converted per gram of Ru or per gram of the catalyst and per second. Table 5 shows that at the reaction temperature of 375 ◦ C (in the kinetic region), increase of the Ru loading in the fresh catalysts causes the rise of the reaction rate, expressed per gram of the catalyst, which is probably associated simply with an increase in the number of active sites. However, when the reaction rate is expressed per g of Ru, activity is about 50% lower i.e. changes from 13.0 to 7.03 mol gRu −1 s−1 when the Ru loading increase from 0.5 to 4.5 wt.%, respectively. As shown in Table 3, the crystallite size of the Ru phase slightly varied with the metal loading at heating range of 350–500 ◦ C, i.e. from 6–10 nm to 10–18 nm for the 0.5% and 4.5% Ru catalyst, respectively. Thus, our catalytic data may suggest structure sensitivity of methane combustion over the ruthenium catalysts. Rather large structure sensitivity of methane combustion has been reported in literature not only over Pd [1,27,28] catalysts but also over Pt [27,29] catalysts, which does not form a bulk oxide [6]. Interestingly, the specific activity of the Ru catalysts in the methane combustion reaction is comparable with the Pt catalysts but it is much lower than that reported for the most active Pd catalysts. For example, very recently Kinnunen et al. [6] report a methane conversion rate (at 300 ◦ C) of 7 mol gPt −1 s−1 for the fresh 2.3–4.1% Pt/Al2 O3 catalysts and 4–2 mol gPt −1 s−1 for the air-aged at 900 ◦ C samples. At the same temperature, methane conversion rates of the fresh 2.3% Pd/Al2 O3 catalyst, with different preparation parameters, varied between 566 and 426 mol gPd −1 s−1 and after aging at 900 ◦ C varied between 39 and 32 mol gPd −1 s−1 [6]. Significantly lower methane conversion rates were reported by Zhu et al. [30] for the fresh 5% Pd/SiO2 catalyst (at 280 ◦ C, 72 mol gPd −1 s−1 ) and 10% Pd/ZrO2 catalyst (71 mol gPd −1 s−1 ). Previous literature comparative studies of the methane combustion over Pt, Pd and Rh/Al2 O3 catalysts showed that in an oxidizing feed stream, catalytic activity ranking is given by Pd > Rh > Pt [23].
Fig. 8. Methane conversion over the fresh (open symbols) and aged (close symbols) Ru/ZnAl2 O4 catalysts with Ru loading of 0.5% (a), and 4.5% (b) as a function of reaction temperature; first and second run. Experimental conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
3.4. Catalyst stability tests After the first run of the methane combustion, the fresh and aged catalysts with the Ru loading of 0.5% and 4.5%, were cooled down to 200 ◦ C and the second run was carried out at the same experimental conditions. As shown in Fig. 8, in the second run both fresh catalysts are slightly less active, which is evident from the shift of the corresponding conversion curves toward higher temperatures. In the second run, the T50 value for the fresh 0.5% Ru catalyst (Fig. 8a) is only 10 ◦ C higher (T50 = 435 ◦ C) than in the first run (T50 = 425 ◦ C), but for the fresh 4.5% Ru catalyst (Fig. 8b) differences in T50 values are larger and amount to 35 ◦ C. Results indicate that some deactivation of the fresh catalysts take place under reaction conditions. Deactivation process is probably caused by the progressive agglomeration of the Ru phase in the fresh Ru catalysts, especially at the higher Ru loading. For both aged catalysts, the T50 values in the first and second run were nearly the same indicating high stability of the aged samples. After the second run, to demonstrate stability of the Ru phase, we measured CH4 combustion as a function of time on stream at constant temperature (third run). Fig. 9 shows methane conversion over the fresh and aged catalysts as a function of time on stream at 500 ◦ C (0.5% Ru catalyst, Fig. 9(a), and at 450 ◦ C and 600 ◦ C (4.5% Ru catalyst, Fig. 9(b). At these temperatures conversion of methane was high but less than 100%, thus providing a more sensitive indication of changes of the catalyst performance with time on stream. For both aged catalysts, it is observed that methane conversion does not change after 7 h time on stream, thereby indicating very high stability of these samples. For the fresh 0.5% and 4.5% Ru samples, methane conversion only slightly decreased showing that both at low and high Ru content, catalyst materials were already thermally stabilized during the first and second runs. 3.5. Characterization of used Ru catalysts The literature data show that drop of the BET surface area during high temperature methane combustion may deactivate metal catalysts to some extent [1]. In our study, no major differences were observed in the BET surface areas of the catalyst samples before and
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
355
Fig. 11. X-ray diffraction patterns of the fresh catalysts after methane combustion tests; 0.5% Ru/ZnAl2 O4 (a), 1% Ru/ZnAl2 O4 (b), and 4.5% Ru/ZnAl2 O4 catalyst (c).
Fig. 9. Methane conversion over the fresh (open symbols) and aged (close symbols) 0.5% (a) and 4.5% Ru/ZnAl2 O4 catalysts (b) as a function of time on stream, obtained at indicated reaction temperature. Experimental conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
after catalytic tests (cf. Tables 2 and 4). As shown in Table 4, the BET surface areas varied between 197 and 200 m2 /g and 138–145 m2 /g for the used fresh and aged Ru/ZnAl2 O4 catalysts, respectively. Therefore, we suppose that observed some catalysts deactivation is not primary caused by the changes in the support structure but rather by modifications of the active ruthenium phase. Fig. 10 shows XRD patterns of the 0.5% Ru catalyst recorded for the freshly reduced sample (pattern a) and after stability tests at 500 ◦ C over the fresh and aged sample. In the used fresh sample (pattern b) diffraction peak of RuO2 phase (at 2 of 28.1◦ ) is clearly observed, indicating that ruthenium species become oxidized under reaction conditions. The mean crystallite size of the RuO2 oxide was estimated to be 16 nm and it was higher than that of after oxidation of the fresh catalyst at 500 ◦ C (10 nm, Table 3). The XRD pattern of the aged 0.5% Ru catalyst after stability test (pattern c) is very similar to that after the aging treatment at 700 ◦ C (Fig. 4), supporting formation of the stable RuO2 oxide in aging process (21 nm) and thus, during the methane combustion reaction no further agglomeration of the Ru phase was observed.
Fig. 10. X-ray diffraction patterns of the 0.5% Ru/ZnAl2 O4 catalyst reduced in hydrogen at 500 ◦ C (a), and after stability test at 500 ◦ C over the fresh catalyst (b), and over the aged catalyst (c).
Fig. 11 presents XRD patterns of all fresh Ru/ZnAl2 O4 catalysts recorded after second catalytic runs. Diffraction peaks of the crystalline RuO2 phase, with intensities increasing with the metal loading, are clearly visible. It appears that during the prolonged time of the methane oxidation reaction, Ru clusters present in the fresh catalysts transforms to rather large RuO2 crystallites. The mean size of RuO2 crystallites increases from about 16 to 22 and to 27 nm for the 0.5%, 1% and 4.5% Ru catalyst, respectively. HRTEM images and SAED patterns (insets) of the fresh 1% Ru and 4.5% Ru catalysts before and after methane combustion reaction (shown in Figs. 12 and 13, respectively) indicate also on drastic changes in the morphology of the Ru phase. In the HRTEM image of the fresh 1% Ru/ZnAl2 O4 catalyst only nanocrystalline ZnAl2 O4 patches can be observed (Fig. 12a) evidencing high Ru dispersion (see Table 2). The SAED pattern (inset to Fig. 12a) contains only rings from zinc aluminate spinel structure. In the HRTEM image of the used sample occasionally thin RuO2 particles with lattice fringes of distance 0.317 nm were observed. The SAED pattern (inset in Fig. 12b) additionally confirms the presence of crystalline RuO2 particles. In the case of the fresh 4.5% Ru catalyst small metal particles (1.2–2.5 nm) could be identify by the HRTEM image (Fig. 13a) and by the SAED pattern (inset in Fig. 13a). After the catalytic test, the SAED pattern (inset in Fig. 13b) contains strong reflections which could be assigned to the RuO2 oxide structure. The HRTEM image in Fig. 13b shows that the morphology of the RuO2 crystallites is rodlike (as characteristic for the bulk RuO2 oxide) with large particle dimensions of about 15 nm × 60 nm. In line with XRD data, comparison of Figs. 12 and 13b (at different magnification) reveals that RuO2 crystallites are smaller and more irregular for the used 1% Ru catalyst (particle dimensions of about 14 nm × 28 nm) than those observed in the used 4.5% Ru catalyst. The different morphologies of the ruthenium oxide particles in the deactivated catalysts with low and high Ru loading, clearly suggest that a strong structure sensitive effect may occur for combustion of methane on Ru oxide. It can be noted that support material, before and after methane combustion, does not show any sintering or agglomeration of the ZnAl2 O4 particles. TEM images of the aged catalysts after catalytic tests were similar to that before testing (not shown). This TEM analysis confirms the properties described by BET surface area measurements and X-ray diffraction. In summary, our characterization and activity data evidence that the crystalline RuO2 oxide is the active phase for the methane combustion reaction. The O2 uptake and XRD results show that highly dispersed ruthenium present in the reduced Ru/ZnAl2 O4 catalysts is easy to oxidize and it remains in fully oxidized state at elevated temperatures and under the CH4 /air reaction mixture or the O2 atmosphere. In agreement with our previous data [18] amorphous
356
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
Fig. 12. Representative HRTEM images and SAED patterns (insets) of the 1% Ru/ZnAl2 O4 catalyst reduced at 500 ◦ C (a) and after catalytic tests in methane combustion (b).
Fig. 13. Representative HRTEM images and SAED patterns (insets) of the 4.5% Ru/ZnAl2 O4 catalyst reduced at 500 ◦ C (a) and after catalytic tests in methane combustion (b).
RuO2 is formed at first and then it is transformed to the crystalline RuO2 oxide. The rate of this transformation is accelerated at higher temperatures and at higher Ru loading. Catalytic results show that when the Ru oxide was amorphous or weakly crystalline, the rate of methane combustion was very low (at T < 300–350 ◦ C, Fig. 6a). Also, it is evidenced that during the CH4 combustion over the fresh Ru catalysts progressive agglomeration of the ruthenium occurs and this process rise with the Ru loading (Fig. 11). Observed some catalyst deactivation of the fresh catalysts (Fig. 8) may originate therefore, mainly from severe modifications of the Ru phase and not from changes in the ZnAl2 O4 support structure. Previously, we found that during propane oxidation at T ≤ 260 ◦ C, only insignificant aggregation of the oxidized ruthenium species occurred and low-loaded Ru/ZnAl2 O4 catalysts formed a very stable catalyst for the low-temperature propane oxidation [18]. Generally, catalyst deactivation of the supported Ru catalysts by sintering is a serious problem with their using in oxidation atmosphere [10,31–33]. Recently, Liu et al. [34] suggested that sintering of the Ru/Al2 O3 catalysts heated under the CH4 /O2 mixture at high reaction temperature may have occurred due to the volatilization from well-dispersed ruthenium oxide species, or migration of tiny ruthenium oxide particles over the surface of the support. Additionally, some Ru loss in unsupported RuO2 oxide and in supported Ru catalysts, by the volatilization of RuO4 above 800 ◦ C, was observed in air-atmosphere [31]. In the present study we perform catalytic tests at lower temperature and we do not observed Ru loss in the fresh and aged Ru/ZnAl2 O4 catalysts. Additionally, XRD and TEM data could partly explain activity results presented in Table 5. Namely, some decrease in the intrinsic reactivity of the fresh Ru/ZnAl2 O4 catalysts with the Ru loading may be caused not only by the increase of the RuO2 crystallite size, which possesses slightly lower specific activity than smaller RuO2 crystallites, but also by the different morphology of the Ru oxide particles (Figs. 12 and 13). Moreover, more reactive Ru species on the catalysts with low Ru loading may also suggest their higher reducibility,
which favors the reduction process required for the relevant C H bond activation step in redox cycles using lattice oxygen involved in CH4 catalysis. Balint et al. [13] proposed that CH4 is selectively oxidized to CO2 and H2 O over bulk RuO2 probably by a Mars van Krevelen redox-type mechanism. In the first stage some of the bulk RuO2 is reduced by CH4 forming substoichiometric oxide (RuO2−x ). This process is interpreted as a creation of surface oxygen vacancies (surface defects) [35]. Next, in parallel with partial RuO2 reduction, CH4 is quickly oxidized to CO2 and H2 O. Under oxygen rich conditions, the oxide layer is not completely reduced to metal because the reoxidation (refilling of oxygen vacancies) with gas-phase O2 is a fast process [13]. Hu and Ruckenstein [36] suggested that on the oxidized metal surface, the reaction between methane and oxygen might occur via the Eley–Rideal mechanism: that is, methane in the gas phase (or weakly adsorbed on the surface of the catalyst) reacts with strongly adsorbed or lattice oxygen. We suppose that activity of Ru catalysts in the oxidation of methane should be related to the ruthenium-oxygen (Ru O) bond strength of the ruthenium oxide. The bond strength of Ru O (481 ± 63 kJ/mol) is much higher than that of Pd O (380 ± 84 kJ/mol) [34], therefore activity of Ru catalysts in methane combustion found in this study is also lower than that reported for the most active Pd catalysts [1,6,30]. 4. Conclusion We have investigated for the first time combustion of methane over the Ru/ZnAl2 O4 catalysts under O2 -rich conditions. The H2 -reduced Ru/ZnAl2 O4 catalysts exhibited higher catalytic performance in the methane combustion as compared to the air-aged samples. Good activity was attributed to the presence of large amount of active sites on smaller RuO2 crystallites (16–27 nm), formed during methane combustion reaction, as compared to that formed in aging process at 700 ◦ C (21–40 nm). The Ru/ZnAl2 O4 catalysts aged at 700 ◦ C were very stable during the long-term activity tests. Thermally induced deactivation of the H2 -reduced
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
Ru catalysts under the CH4 /air reaction mixture, results mainly from the loss of the catalytic surface area due to crystallite growth of the RuO2 phase. The ZnAl2 O4 spinel with high surface area (∼200 m2 /g) applied as the support material in the metal catalysts, proved to be suitable for high temperature operation due to lack of sintering at elevated temperatures (up to 700–900 ◦ C). Finally, further studies are necessary to improve stability of the Ru phase in the supported Ru catalysts for their possible applications in the complete combustion of dilute methane (lean methane-air mixtures) for its emission control. Moreover, the behavior of the Ru/ZnAl2 O4 catalysts for the methane combustion in the presence of sulfur compounds and water should be also investigated. Acknowledgments The authors thank very much Mrs. L. Krajczyk for TEM investigations and A. Cielecka for chemisorption measurements. References [1] P. Gélin, M. Primet, Appl. Catal. B: Environ. 39 (2002) 1–37. [2] O. Demoulin, B. Le Clef, M. Navez, P. Ruiz, Appl. Catal. A: Gen. 344 (2008) 1–9. [3] A. Janbey, W. Clark, E. Noordally, S. Grimes, S. Tahir, Chemosphere 52 (2003) 1041–1046. [4] T.V. Choudhary, S. Banerjee, V.R. Choudhary, Appl. Catal. A: Gen. 234 (2002) 1–23. [5] P. Hurtado, S. Ordónez, H. Sastre, F.V. Díez, Appl. Catal. B: Environ. 47 (2004) 85–93. [6] N.M. Kinnunen, J.T. Hirvi, M. Suvanto, T.A. Pakkanen, J. Mol. Catal. A: Chem. 356 (2012) 20–28. [7] T. Mitsui, K. Tsutsui, T. Matsui, R. Kikuchi, K. Eguchi, Appl. Catal. B: Environ. 81 (2008) 56–63. [8] N. Kamiuchi, T. Mitsui, H. Muroyama, T. Matsui, R. Kikuchi, K. Eguchi, Appl. Catal. B: Environ. 97 (2010) 120–126.
357
[9] D. Homsi, S. Aouad, J.E. Nakat, B.E. Khoury, P. Obeid, E. Abi-Aad, A. Aboukaïs, Catal. Commun. 12 (2011) 776–780. [10] J. Okal, M. Zawadzki, W. Tylus, Appl. Catal. B: Environ. 101 (2011) 548–559. [11] Ch.K. Ryu, M.W. Ryoo, I.S. Ryu, S.K. Kang, Catal. Today 47 (1999) 141–147. [12] C. Elmasides, X.E. Verykois, J. Catal. 203 (2001) 477–486. [13] I. Balint, A. Miyazaki, K. Aika, J. Catal. 220 (2003) 74–83. [14] N.K. Labhsetwar, A. Watanabe, T. Mitsuhashi, H. Haneda, J. Mol. Catal. A: Chem. 223 (2004) 217–223. [15] W. Walerczyk, M. Zawadzki, J. Okal, Appl. Surf. Sci. 257 (2011) 2394–2400. ˛ nski, ´ [16] M. Zawadzki, W. Mi´sta, L. Kepi Vacuum 63 (2001) 291–296. [17] W. Walerczyk, M. Zawadzki, Catal. Today 176 (2011) 159–162. [18] J. Okal, M. Zawadzki, Appl. Catal. B: Environ. 105 (2011) 182–190. [19] J. Okal, M. Zawadzki, L. Krajczyk, Catal. Today 176 (2011) 173–176. [20] H. Wang, W.F. Schneider, D. Schmidt, J. Phys. Chem. C 113 (2009) 15266–15273. [21] Y.D. Kim, A.P. Seitsonen, S. Wendt, J. Wang, C. Fan, K. Jacobi, H. Over, G. Ertl, J. Phys. Chem. B 105 (2001) 3752–3758. [22] T.F. Garetto, C.P. Apesteguia, Catal. Today 62 (2000) 189–199. [23] P. Hurtado, S. Ordónez, A. Vega, F.V. Díez, Chemosphere 55 (2004) 681–689. [24] T.R. Baldwin, R. Burch, Appl. Catal. 66 (1990) 337–358. [25] F.H. Ribeiro, M. Chow, R.A. Dalla Betta, J. Catal. 146 (1994) 537–544. [26] Okal, M. Zawadzki, Catal. Lett. 132 (2009) 225–234. [27] R.F. Hicks, H. Qi, M.L. Young, R.G. Lee, J. Catal. 122 (1990) 280–294. [28] K. Fujumoto, F.H. Ribeiro, M. Avalos-Borja, E. Jglesia, J. Catal. 179 (1998) 431–442. [29] I.E. Beck, V.I. Bukhtiyarov, O.Y. Pakharukov, V.I. Zaikovsky, V.V. Kriventsov, V.N. Parmon, J. Catal. 268 (2009) 60–67. [30] G. Zhu, K-i. Fujimoto, D.Y. Zemlyanov, A.K. Datye, F.H. Ribeiro, J. Catal. 225 (2004) 170–178. [31] R. Lanza, S.G. Järas, P. Canu, Appl. Catal. A: Gen. 325 (2007) 57–67. [32] J. Assmann, V. Narkhede, N.A. Breuer, M. Muhler, A.P. Seitsonen, M. Knapp, D. Crihan, A. Farkas, G. Mellau, H. Over, J. Phys.: Condens. Matter 20 (2008) 184017–184040. [33] J.T. Kiss, R.D. Gonzalez, J. Phys. Chem. 88 (1984) 892–897. [34] Y. Liu, F.Y. Huang, J.M. Li, W.Z. Weng, Ch.R. Luo, M.L. Wang, W.S. Xia, Ch.J. Huang, H.L. Wan, J. Catal. 256 (2008) 192–203. [35] H. Madhavaram, H. Idriss, S. Wendt, Y.D. Kim, M. Knapp, H. Over, J. Assmann, J. Catal. 202 (2001) 296–307. [36] Y.H. Hu, E. Ruckenstein, J. Phys. Chem. B 102 (1998) 230–233.
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Catalytic combustion of methane over ruthenium supported on zinc aluminate spinel Janina Okal ∗ , Mirosław Zawadzki Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 19 September 2012 Received in revised form 29 November 2012 Accepted 16 December 2012 Available online 5 January 2013 Keywords: Methane combustion Ruthenium Zinc aluminate RuO2 agglomeration Deactivation
a b s t r a c t Activity of the Ru/ZnAl2 O4 catalysts, H2 -reduced or air-aged at 700 ◦ C, for the combustion of methane under O2 -rich conditions was studied in this work. High surface area ZnAl2 O4 spinel was synthesized by the unconventional co-precipitation method. Catalysts were prepared using Ru(NO)(NO3 )3 as metal precursor and characterized by H2 chemisorption, O2 uptake, BET, XRD and TEM. The size of ruthenium particles in fresh catalysts varied from 1.1 to 1.5 nm with the metal loading from 0.5% to 4.5 wt.%. Airaging treatment caused severe agglomeration of the Ru phase and formation of the well-crystallized RuO2 oxide. Under reaction conditions, highly dispersed Ru species were easily oxidized and RuO2 oxide was the active phase for methane combustion. The fresh catalysts were more effective than aged samples in terms of light-off temperature and temperature needed for the complete methane conversion. The mean crystallite size of the RuO2 , formed during combustion reaction over fresh catalysts, depended on metal loading and was lower (21–27 nm) as compared to that formed during aging process (21–40 nm), which leads to higher activity. However, stable catalytic activity was observed only for aged catalysts. The specific reaction rate (mol gRu −1 s−1 ) for the fresh catalysts was found to decrease about 50% when the Ru loading increases from 0.5% to 4.5%. The apparent activation energies Eapp on Ru/ZnAl2 O4 catalysts were in range of 120–129 kJ/mol and did not depend on the metal loading and catalyst pre-treatment. Catalytic activity could be partly explained by the changes in the morphology and the crystallite size of the RuO2 phase and may suggest a structure sensitivity of CH4 combustion over ruthenium. Catalyst deactivation observed for the fresh catalysts, originate mainly from severe modifications of the Ru phase but not from changes in the ZnAl2 O4 support structure. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Catalytic combustion of hydrocarbons has received considerable attention due to its practical applications in both power generation and pollutant abatement [1]. Among all the hydrocarbon fuels methane is the most difficult to be oxidized [1,2]. In general, supported noble metals such as Pt, Pd and Rh are well established as efficient catalysts for methane combustion [1–4]. Among these catalysts, palladium has been widely recognized as the most reactive in methane combustion under oxygen excess. The nature of the support has also been investigated and alumina supported metal catalysts are the most studied systems [1,2,5,6]. At high temperature required for the complete oxidation of methane, ␥-alumina frequently undergoes transition into ␣ phase causing a drastic decrease in the surface area, and the sintering noble metal contribute to the thermal deactivation of the catalysts [1]. The search
∗ Corresponding author. Tel.: +48 71 34 350 20; fax: +48 71 34 410 29. E-mail address: [email protected] (J. Okal). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.12.040
of new catalytic materials that can allow efficient oxidation of methane at lower temperatures still remains a major challenge. Supported ruthenium catalysts have received much interest over the past years, because of their high activity in many oxidation reactions. For example, ruthenium showed good reactivity in the low-temperature oxidation of volatile organic compounds (VOCs), such as ethyl acetate, acetaldehyde, toluene, propylene and propane [7–10]. Catalytic oxidation of these VOCs is usually complete at temperatures below 400 ◦ C, but methane combustion is not possible at these temperatures [1–4]. Thus, most studies on ruthenium have been carried out for more reactive VOCs, at conditions in which the thermal stability of the support is not so important as in the case of methane. In the literature, it is rarely found that ruthenium or bimetallic ruthenium catalysts were tested for the application of methane combustion [11]. More frequently ruthenium catalysts were applied in the partial oxidation of methane to synthesis gas [12,13]. A study by Ryu et al. [11] on bimetallic Pd–Ru system revealed that addition of ruthenium to palladium catalyst was favorable for the methane combustion in the presence of H2 S. This was ascribed to enhance dispersion of the Pd in the presence of ruthenium and also greater poisoning resistance
350
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
of ruthenium. Generally, it was stated in the literature that use of the Ru-based catalysts for the methane combustion is limited by their high-temperature stability. Labhsetwar et al. [14] reported that ruthenium shows high thermal stability when is incorporated in perovskite type structure and then can be used for high temperature applications like methane combustion. However, the major limitation of perovskites application is their low surface area. In recent years, nanocrystalline spinel materials like ZnAl2 O4 have gained considerable interest in the field of catalysis since they may be used as supports for noble metals to substitute traditional materials such as ␥-alumina. The zinc aluminate is a very interesting material from the catalytic point of view because of its desirable properties like the high thermal and chemical stability, hydrophobic behavior, high mechanical resistance, low acidity and good metal dispersion capacity [15]. In addition, ZnAl2 O4 spinel is capable to interact strongly with the metal component which causes an enhancement of the metal particles stability [16]. So far, there are only scarce literature data concerning the application of the zinc aluminate spinel as support material in VOCs oxidation [15,17]. Moreover, this material was never tested as the support for noble metals used in combustion of methane. Recently, we found that ruthenium supported over the ZnAl2 O4 spinel showed good reactivity in propane [18] and butane combustion [19]. In this study, the catalytic performance of ruthenium supported on ZnAl2 O4 spinel was evaluated in the total oxidation of methane. Catalysts containing various amounts of ruthenium were prepared, and the influence of the pre-treatment procedure on their physicochemical characteristics and catalytic activity in methane combustion at low concentration has been investigated. Detail structural studies, including used catalyst samples, are used to correlate the intrinsic properties of the catalyst materials to their catalytic performances. To our knowledge, no catalytic results for methane combustion over Ru/ZnAl2 O4 systems are available in the literature. Also, little is known about the stability of these catalysts at the relatively high temperatures needed for CH4 oxidation. 2. Experimental 2.1. Preparation of the Ru/ZnAl2 O4 catalysts The detailed procedure for the support and catalysts preparation was described previously [18]. The ZnAl2 O4 spinel was prepared by the unconventional co-precipitation method using aqueous solution of Zn(NO3 )2 and Al(NO3 )3 (molar ratio Al:Zn = 2:1). The precipitate was filtered off, washed with water, dried in air and finally calcined at 550 ◦ C for 3 h. Next, the support was crushed and sieved into the pieces of 0.3–0.5 mm. The Ru catalysts were obtained by the incipient-wetness impregnation method using an aqueous solution of Ru(NO)(NO3 )3 as metal precursor. Impregnated samples were dried at 110 ◦ C and finally reduced in H2 at 500 ◦ C for 5 h. The reduced catalyst is hereafter referred to as fresh catalyst. After reduction, half of the catalyst sample was air-aged at 700 ◦ C for 3 h. 2.2. Catalysts characterization The ruthenium content was determined with the ICP-AES method. In catalysts heated in air even at 700 ◦ C no ruthenium loss was noticed. Some details on the equipment and procedures used for catalyst characterization can be found elsewhere [18]. Nitrogen adsorption–desorption isotherms at temperature of liquid nitrogen were used to determine the textural properties (porosity and specific surface area) of the obtained samples. Measurements were carried out using an Autosorb-1 Quantachrome Instruments system. X-ray diffraction (XRD) patterns were
collected using DRON-3 diffractometer with a Ni-filtered Cu K␣ radiation. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were recorded on a Philips CM20 SuperTwin microscope. Ruthenium dispersion was determined by hydrogen chemisorption at 100 ◦ C using conventional static volumetric equipment. The extent of the Ru oxidation at temperatures up to 700 ◦ C was estimated by the O2 uptake measurements employing the same apparatus. 2.3. The catalytic activity measurements Combustion of methane over fresh and aged Ru catalysts was examined at atmospheric pressure using an apparatus, which consists of a flow measuring and control system, reactor and an on-line analysis system. The flow system is equipped with a mass-flow controllers (MKS) and a set of valves, which allows introduction of the gas mixture to the reactor. The reactor consists of a 350 mm long quartz tube with inner diameter of 10 mm. The reactor with the catalyst sample, placed in the middle of the reactor on a quartz wool bed, was located in a programmable electric furnace. The catalyst temperature was controlled by a Pt–Rh thermocouple mounted internally. A mixture of methane and synthetic air was fed at a flow rate of 14 l/h and catalysts were packed to a constant volume to give a gas hourly space velocity GHSV of 32,000 h−1 for all studies. The volumetric gas composition was 2000 ppm CH4 in air. The reaction products were analyzed on-line using a gas chromatograph (Chromatron GCHF 18.3) equipped with a flame ionization detector (FID) for the determination of CH4 analysis and TCD detector for CO and CO2 analysis. Prior to each catalytic experiment, sample of the catalyst (400 mg) was conditioned at 200 ◦ C for 2 h with the reaction mixture. Measurements were taken as the sample was heated stepwise at the temperature range of 200–700 ◦ C. Analyses were made at each temperature until steady-state activity was attained, and at least two consistent analyses were taken and data were averaged. The conversion (XCH4 ) was calculated from the inlet and outlet concentration of CH4 , respectively, assuming 100% mass balance. Reaction rates were obtained from measured values of XCH4 according to the equation; rate = XCH4 F/W , where F is the molar flow rate of methane (mol s−1 ) and W is the catalyst weight (g). The conversion data were reproducible within 5% accuracy. 3. Results and discussion 3.1. Thermal stability of the ZnAl2 O4 support The thermal stability of the ZnAl2 O4 support was evaluated after treatment in air for 3 h at temperatures up to 1050 ◦ C. XRD patterns of the support heated at 550, 700, 900 and 1050 ◦ C, are shown in Fig. 1. In all cases, thermal treatment yielded the nanocrystalline material with zinc aluminate spinel structure (JCPD 05-0669) without any impurities like ZnO or Al2 O3 . Very broad diffraction peaks for 550 ◦ C heated ZnAl2 O4 support become more intense and sharper after treatment at higher temperatures indicating increase in the degree of crystallinity. The results of the mean crystallite size calculations show an exponential-like growth relationship to the calcination temperature. As shown in Fig. 2, the crystallite size increases with temperature from about 3 to 13 nm. Taking into account the nitrogen adsorption–desorption isotherms, shown in Fig. 3, and calculated values of both the specific surface area SBET and average pore diameter Dp , listed in Table 1, one can see that all heated support samples are typical mesoporous materials. The isotherms are of type IV and show the hysteresis loop of H1/H2 (after heating up to 900 ◦ C) or H1/H3 (after heating at 1050 ◦ C) type which is characteristic for
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
351
Table 2 BET surface areas, Ru dispersions and metal particle size of fresh Ru/ZnAl2 O4 catalysts and the mean crystallite size of RuO2 in aged catalyst samples. Catalyst
SBET (m2 /g)
0.5% Ru/ZnAl2 O4 Fresh 210 Aged 148 1% Ru/ZnAl2 O4 Fresh 212 Aged 146 4.5% Ru/ZnAl2 O4 Fresh 214 Aged 141
Metal dispersiona (H/Ru)
dav (chem.) (nm)
d (TEM) (nm)
d (XRD) (nm)
0.71
1.1
n.d.b
n.d. RuO2 ; 21
0.69
1.2
n.d.
n.d. RuO2 ; 28
0.56
1.5
1.2–2.5
n.d. RuO2 ; 40
a Defined as the ratio of exposed Ru surface atoms to the total number of Ru atoms, assuming H/Rus = 1. Characterization data of fresh catalysts from previous study, Ref. [18]. b No Ru◦ was detected.
Fig. 1. X-ray diffraction patterns of ZnAl2 O4 support after heat treatment in air at 550 ◦ C (a), 700 ◦ C (b), 900 ◦ C (c) and 1050 ◦ C (d).
Fig. 2. Dependence of average crystallite size () and the specific surface area () of ZnAl2 O4 support on the heating temperature.
mesoporous materials, according to IUPAC classification. Only in the case of 550 ◦ C heated support, adsorption takes place also at very low relative pressures which revealed some contribution from microporosity. It should be noted that the nitrogen isotherms show a sharp increase of adsorbed nitrogen volume at high partial pressures (P/Po ∼ 0.6) which corresponds to a large mesopore volume with a relatively narrow distribution of pore diameters in the range 1–30 nm with a maximum at about 6–12 nm. This type of adsorption is attributed to interparticle mesoporosity and it could be assumed that nanoporous structure of zinc aluminate support is formed by the agglomeration of smaller monodispersed particles inherent in the 550 ◦ C heated sample or larger particles created after subsequent heat treatment. In the first case it forms more uniform pores in the micro/mesoporous range, while the second produces random-size pores in the mesoporous range. Heat treatment leads to some decrease of specific surface area and total pore volume and increase of pore diameter but even after heating at 1050 ◦ C, ZnAl2 O4 support maintained significant value of both SBET and Vtotal , i.e. 67 m2 /g and 0.28 cm3 /g, respectively, and Dp was changed a little. In line with XRD data, these results indicate also on good thermal stability of zinc aluminate support with spinel structure. 3.2. Characterization of the fresh and aged Ru/ZnAl2 O4 catalysts
Fig. 3. N2 adsorption–desorption isotherms of ZnAl2 O4 support after heat treatment at 550 ◦ C (a) and 1050 ◦ C (b) with corresponding pore size distributions (as inset).
Table 1 Morphological characteristics of the ZnAl2 O4 support. Heating temperature (◦ C)
dav (XRD) (nm)
550 700 900 1050
3 5 9 13
Specific surface area SBET (m2 /g) 216 149 81 67
Pore volume (VBJH ) (cm3 /g)
Pore diameter (Dp ) (nm)
0.40 0.39 0.32 0.28
5.6 9.5 11.2 12.4
The main characteristics of the fresh and aged Ru/ZnAl2 O4 catalysts are summarized in Table 2. All fresh Ru catalysts have a similar BET surface area as the 550 ◦ C heated support (216 m2 /g), while aged samples as the 700 ◦ C air-heated support (149 m2 /g). The fresh catalysts have a very high dispersion even at the high Ru loading. The ruthenium particle size changes from about 1.1 to 1.5 nm by increasing the metal loading from 0.5 to 4.5 wt.%. Additionally, only in the 4.5% Ru catalyst small metal particles were detected by HRTEM studies [18]. XRD patterns of fresh catalysts contained no peaks from the ruthenium phase. In XRD patterns of aged samples (not shown) characteristic peaks of RuO2 were detected at 2 of 28.1◦ , 35.1◦ and 54.3◦ (JCPDS 88-0322), indicating that agglomeration of the Ru phase occurred during aging process. The mean crystallite size of RuO2 oxide for the aged 0.5%, 1% and 4.5% Ru catalysts was 21, 28 and 40 nm, respectively (Table 2, last column). The Ru catalysts were tested for their activity toward methane combustion in O2 -rich atmosphere, hence studies of the interactions between the ruthenium and oxygen are essential. The results of the O2 uptake measurements at temperatures up to 700 ◦ C by the fresh Ru catalysts are presented in Table 3. Already at RT, the O2 uptake by the 0.5% Ru and 1% Ru catalysts is high and the O/Ru ratio of 1.73 and 1.50, respectively indicates that 85 or 75% of the total amount of ruthenium was oxidized to RuO2 . For the
352
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
Table 3 Volumetric O2 uptake at temperatures 20–700 ◦ C by the fresh Ru/ZnAl2 O4 catalysts and mean crystallite sizes of RuO2 calculated from XRD. O/Rua
Mean crystallite size of RuO2 (nm)
42.7 48.0 50.0 51.5 50.5 52.0 51.0 51.0
1.73 2.00 2.02 2.08 2.04 2.10 2.06 2.06
– – – – 6 10 20 21
20 150 200 250 350 500 600 700
74.2 86.6 96.0 99.0 101.5 100.5 100.5 99.5
1.50 1.75 1.96 2.00 2.05 2.03 2.03 2.01
– – – 4 10 16 20 28
20 150 200 250 350 500 600 700
147.9 230.0 268.0 282.0 420.0 481.1 480.0 452.2
0.66 1.04 1.21 1.27 1.89 2.16 2.14 2.03
– – 6 8 10 18 32 40
Catalyst
Temperature (◦ C)
0.5% Ru/ZnAl2 O4
20 150 200 250 350 500 600 700
1% Ru/ZnAl2 O4
4.5% Ru/ZnAl2 O4
Uptake of oxygen (mol O2 /g cat.)
a Ratio of the number of O atoms adsorbed to the total numbers of Ru atoms in the catalyst.
4.5% Ru catalyst, the O2 uptake at RT was much lower and may be attributed mainly to dissociative chemisorption [18]. Observed rise of the oxygen uptake with temperature points to a bulk oxidation of ruthenium. The complete oxidation of small ruthenium particles in the 0.5% Ru catalyst was achieved at 150 ◦ C and in the 1% and 4.5% Ru catalysts at about 250 ◦ C and 350 ◦ C, respectively. In samples oxidized at 250–700 ◦ C, the O/Ru ratio was slightly higher than the theoretical value, calculated by assuming the total oxidation of Ru to RuO2 . Recent published literature data [20,21] indicate that
additional adsorption of oxygen may occur also on the surface of the crystalline RuO2 oxide. XRD patterns of the 0.5% and 4.5% Ru/ZnAl2 O4 catalysts, recorded after oxidation at 200–700 ◦ C, are shown in Fig. 4. For the 0.5% Ru catalyst, at temperatures below or at 250 ◦ C (Fig. 4a), no diffraction patterns due to RuO2 or metallic Ru species were detected which indicate on well dispersion of the oxidized Ru species. When the temperature was raised to 350–500 ◦ C, diffraction peak of the crystalline RuO2 oxide (2 at 28.1◦ ) was observed with intensity increasing with temperature. For the 4.5% Ru catalyst already at 200 ◦ C, diffraction peaks of the RuO2 oxide were visible (Fig. 4b). The XRD and O2 uptake data indicate that highly dispersed metallic ruthenium species in the fresh samples were extensively oxidized by oxygen in this temperature range. The aggregation of the RuO2 particles to larger RuO2 crystallites occurs when the Ru catalysts were treated in oxygen especially above 350 ◦ C. This is evidenced by the increased intensity of the XRD peaks of the ruthenium oxide with temperature. The mean crystallite sizes of the RuO2 oxide calculated from XRD data of all catalysts are presented in Table 3, last column. The crystallite size of RuO2 ranged between 6–21 nm, 4–28 nm and 6–40 nm for the 0.5%, 1% and 4.5% Ru catalyst, respectively. The important finding is that RuO2 crystallite size depends strongly not only on temperature but also on the metal loading and above 500 ◦ C, the RuO2 crystallites are much larger in the high-loaded catalyst than in the low-loaded samples. 3.3. Methane combustion activity over the fresh and aged catalysts Methane conversion as a function of reaction temperature, over the fresh 4.5% Ru/ZnAl2 O4 catalyst as well as over the 550 ◦ C heated support and in an empty reactor is shown in Fig. 5. In the absence of the catalyst or support, the conversion curve corresponds to a homogeneous combustion curve published in the literature. Methane conversion over the ZnAl2 O4 spinel starts at about 450 ◦ C, i.e. at temperature up to 200 ◦ C higher than that over the Ru catalyst confirming that Ru phase is an active component for the methane combustion. Under O2 -rich conditions all Ru catalysts showed the complete oxidation of CH4 to CO2 , i.e. no CO was detected.
Fig. 4. X-ray diffraction patterns of the 0.5% (a) and 4.5% Ru/ZnAl2 O4 catalyst (b) obtained at the indicated oxygen treatment temperature.
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
353
Table 4 Catalytic properties of ruthenium during methane combustion over the fresh and aged Ru/ZnAl2 O4 catalysts.a Catalyst
BET of used catalyst (m2 /g)
Catalytic activity in terms of conversion temperature (◦ C) T10
Fresh ZnAl2 O4 0.5% Ru/ZnAl2 O4 1% Ru/ZnAl2 O4 4.5% Ru/ZnAl2 O4
128 198 200 197
T50
T95
Aged
Fresh
Aged
Fresh
Aged
Fresh
Aged
145 140 138
540 390 375 350
– 435 460 480
650 430 412 385
– 480 500 530
770 500 490 450
– 550 575 600
a The reaction conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
Fig. 5. Methane conversion as a function of temperature over the fresh 4.5% Ru/ZnAl2 O4 catalyst (), ZnAl2 O4 support (䊉), and in empty reactor (). Experimental conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
The effect of metal loading on methane conversion over the fresh and aged catalysts is compared in Fig. 6. As shown, all fresh catalysts exhibit the better performance in the methane combustion than the aged samples in terms of light-off temperature
Fig. 6. Methane conversion over the fresh (a) and aged (b) Ru/ZnAl2 O4 catalysts with Ru loading of 0.5% (,), 1% (,), and 4.5% (,䊉) as a function of reaction temperature. Experimental conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
and temperature needed for the complete methane conversion. Depending on the Ru loading in the fresh catalysts methane conversion started at between 250 and 350 ◦ C, while over the aged samples at between 330 and 400 ◦ C. Over the fresh 0.5%, 1% and 4.5% Ru/ZnAl2 O4 catalyst the complete CH4 oxidation occurs at 500, 490 and 450 ◦ C, respectively, whereas over the aged catalysts at 550, 575 and 600 ◦ C, respectively. Thus, for the fresh catalysts activity increases with the Ru loading, but opposite for the aged catalysts activity shows the reverse order with the Ru loading. To compare the oxidation activity of the catalyst samples, we measured from the light-off curves the values of temperature for conversion of 10%, 50% and 95%. The obtained T10 , T50 and T95 values for the fresh and aged Ru samples are given in Table 4. For all Ru catalysts conversion of methane rise rapidly as the reaction temperature increased (Fig. 6) which is also reflected in the Arrhenius plots. Fig. 7 shows, for example, the Arrhenius plots obtained for the fresh and aged 4.5% Ru/ZnAl2 O4 catalyst. The specific reaction rate is expressed as moles of CH4 converted per gram of the catalyst and per second. The turnover frequencies values (TOF, molecules of methane reacted per surface Ru per second) of the catalysts cannot be taken into account since the particle size of the Ru phase can change during course of the reaction (see Table 3). From the Arrhenius plots, apparent activation energies Eapp of 129 and 121 kJ/mol for the fresh and aged 4.5% Ru/ZnAl2 O4 catalyst, respectively were obtained. Similar Eapp values were found for the 0.5%–1% Ru/ZnAl2 O4 catalysts (125–120 kJ/mol). The activation energies obtained in this study are slightly higher than that found in the literature for the others noble metals. Reported values are between 60–100 kJ/mol for methane combustion over platinum [3,22,23], and 70–100 kJ/mol over palladium catalysts [23–25]. However, comparative data for Ru catalysts are not available in the literature. Additionally, it is clear from Fig. 7 that at identical Ru loading, the air-aged catalyst is less active for methane combustion than the fresh catalyst. The large differences in the
Fig. 7. Arrhenius plots for the combustion of methane over the fresh () and aged (䊉) 4.5% Ru/ZnAl2 O4 catalyst.
354
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
Table 5 The specific reaction rate at 375 ◦ C and apparent activation energy (Eapp ) for methane combustion over the fresh Ru/ZnAl2 O4 catalysts.a Catalyst
0.5% Ru/ZnAl2 O4 1% Ru/ZnAl2 O4 4.5% Ru/ZnAl2 O4
Eapp (kJ/mol−1 )
Specific reaction rate mol CH4 gcat −1 s−1
mol CH4 gRu −1 s−1
0.04 0.11 0.32
13.0 11.2 7.03
124 121 129
a The reaction conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
specific activity may indicate that the number of the active sites is different in these samples. In fact, the characterization data of the 4.5% Ru/ZnAl2 O4 catalyst before activity studies, show that airaging at 700 ◦ C leads to the large aggregation of the Ru phase. The mean crystallite size of the RuO2 oxide in the aged sample amount to 40 nm and it was much higher than that of after oxidation of the fresh catalyst at 350–500 ◦ C (10–18 nm) (Table 3). Here, we assume that under O2 -rich reaction conditions highly dispersed Ru clusters present in the fresh catalyst transforms to RuO2 crystallites with sizes similar to that obtained only in the oxygen atmosphere. Our activity results are consistent with studies of Mitsui et al. [7], who found that activity of the Ru/␥-Al2 O3 and Ru/ZrO2 catalysts for combustion of ethyl acetate, acetaldehyde and toluene was higher on the reduced catalysts than on the asoxidized at 400 ◦ C. We also found previously strong decline activity in oxidation of propane over the Ru/␥-Al2 O3 catalysts oxidized at 600 ◦ C [26]. Some additional information about activity of the Ru catalysts, as a function of the metal loading, could be obtained by considering the specific reaction rate at given temperature, expressed as moles of CH4 converted per gram of Ru or per gram of the catalyst and per second. Table 5 shows that at the reaction temperature of 375 ◦ C (in the kinetic region), increase of the Ru loading in the fresh catalysts causes the rise of the reaction rate, expressed per gram of the catalyst, which is probably associated simply with an increase in the number of active sites. However, when the reaction rate is expressed per g of Ru, activity is about 50% lower i.e. changes from 13.0 to 7.03 mol gRu −1 s−1 when the Ru loading increase from 0.5 to 4.5 wt.%, respectively. As shown in Table 3, the crystallite size of the Ru phase slightly varied with the metal loading at heating range of 350–500 ◦ C, i.e. from 6–10 nm to 10–18 nm for the 0.5% and 4.5% Ru catalyst, respectively. Thus, our catalytic data may suggest structure sensitivity of methane combustion over the ruthenium catalysts. Rather large structure sensitivity of methane combustion has been reported in literature not only over Pd [1,27,28] catalysts but also over Pt [27,29] catalysts, which does not form a bulk oxide [6]. Interestingly, the specific activity of the Ru catalysts in the methane combustion reaction is comparable with the Pt catalysts but it is much lower than that reported for the most active Pd catalysts. For example, very recently Kinnunen et al. [6] report a methane conversion rate (at 300 ◦ C) of 7 mol gPt −1 s−1 for the fresh 2.3–4.1% Pt/Al2 O3 catalysts and 4–2 mol gPt −1 s−1 for the air-aged at 900 ◦ C samples. At the same temperature, methane conversion rates of the fresh 2.3% Pd/Al2 O3 catalyst, with different preparation parameters, varied between 566 and 426 mol gPd −1 s−1 and after aging at 900 ◦ C varied between 39 and 32 mol gPd −1 s−1 [6]. Significantly lower methane conversion rates were reported by Zhu et al. [30] for the fresh 5% Pd/SiO2 catalyst (at 280 ◦ C, 72 mol gPd −1 s−1 ) and 10% Pd/ZrO2 catalyst (71 mol gPd −1 s−1 ). Previous literature comparative studies of the methane combustion over Pt, Pd and Rh/Al2 O3 catalysts showed that in an oxidizing feed stream, catalytic activity ranking is given by Pd > Rh > Pt [23].
Fig. 8. Methane conversion over the fresh (open symbols) and aged (close symbols) Ru/ZnAl2 O4 catalysts with Ru loading of 0.5% (a), and 4.5% (b) as a function of reaction temperature; first and second run. Experimental conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
3.4. Catalyst stability tests After the first run of the methane combustion, the fresh and aged catalysts with the Ru loading of 0.5% and 4.5%, were cooled down to 200 ◦ C and the second run was carried out at the same experimental conditions. As shown in Fig. 8, in the second run both fresh catalysts are slightly less active, which is evident from the shift of the corresponding conversion curves toward higher temperatures. In the second run, the T50 value for the fresh 0.5% Ru catalyst (Fig. 8a) is only 10 ◦ C higher (T50 = 435 ◦ C) than in the first run (T50 = 425 ◦ C), but for the fresh 4.5% Ru catalyst (Fig. 8b) differences in T50 values are larger and amount to 35 ◦ C. Results indicate that some deactivation of the fresh catalysts take place under reaction conditions. Deactivation process is probably caused by the progressive agglomeration of the Ru phase in the fresh Ru catalysts, especially at the higher Ru loading. For both aged catalysts, the T50 values in the first and second run were nearly the same indicating high stability of the aged samples. After the second run, to demonstrate stability of the Ru phase, we measured CH4 combustion as a function of time on stream at constant temperature (third run). Fig. 9 shows methane conversion over the fresh and aged catalysts as a function of time on stream at 500 ◦ C (0.5% Ru catalyst, Fig. 9(a), and at 450 ◦ C and 600 ◦ C (4.5% Ru catalyst, Fig. 9(b). At these temperatures conversion of methane was high but less than 100%, thus providing a more sensitive indication of changes of the catalyst performance with time on stream. For both aged catalysts, it is observed that methane conversion does not change after 7 h time on stream, thereby indicating very high stability of these samples. For the fresh 0.5% and 4.5% Ru samples, methane conversion only slightly decreased showing that both at low and high Ru content, catalyst materials were already thermally stabilized during the first and second runs. 3.5. Characterization of used Ru catalysts The literature data show that drop of the BET surface area during high temperature methane combustion may deactivate metal catalysts to some extent [1]. In our study, no major differences were observed in the BET surface areas of the catalyst samples before and
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
355
Fig. 11. X-ray diffraction patterns of the fresh catalysts after methane combustion tests; 0.5% Ru/ZnAl2 O4 (a), 1% Ru/ZnAl2 O4 (b), and 4.5% Ru/ZnAl2 O4 catalyst (c).
Fig. 9. Methane conversion over the fresh (open symbols) and aged (close symbols) 0.5% (a) and 4.5% Ru/ZnAl2 O4 catalysts (b) as a function of time on stream, obtained at indicated reaction temperature. Experimental conditions; mass of catalyst or support, 400 mg; particle size, 0.3–0.5 mm; methane concentration: 2000 ppm in air; GHSV: 32,000 h−1 and pressure: atmospheric.
after catalytic tests (cf. Tables 2 and 4). As shown in Table 4, the BET surface areas varied between 197 and 200 m2 /g and 138–145 m2 /g for the used fresh and aged Ru/ZnAl2 O4 catalysts, respectively. Therefore, we suppose that observed some catalysts deactivation is not primary caused by the changes in the support structure but rather by modifications of the active ruthenium phase. Fig. 10 shows XRD patterns of the 0.5% Ru catalyst recorded for the freshly reduced sample (pattern a) and after stability tests at 500 ◦ C over the fresh and aged sample. In the used fresh sample (pattern b) diffraction peak of RuO2 phase (at 2 of 28.1◦ ) is clearly observed, indicating that ruthenium species become oxidized under reaction conditions. The mean crystallite size of the RuO2 oxide was estimated to be 16 nm and it was higher than that of after oxidation of the fresh catalyst at 500 ◦ C (10 nm, Table 3). The XRD pattern of the aged 0.5% Ru catalyst after stability test (pattern c) is very similar to that after the aging treatment at 700 ◦ C (Fig. 4), supporting formation of the stable RuO2 oxide in aging process (21 nm) and thus, during the methane combustion reaction no further agglomeration of the Ru phase was observed.
Fig. 10. X-ray diffraction patterns of the 0.5% Ru/ZnAl2 O4 catalyst reduced in hydrogen at 500 ◦ C (a), and after stability test at 500 ◦ C over the fresh catalyst (b), and over the aged catalyst (c).
Fig. 11 presents XRD patterns of all fresh Ru/ZnAl2 O4 catalysts recorded after second catalytic runs. Diffraction peaks of the crystalline RuO2 phase, with intensities increasing with the metal loading, are clearly visible. It appears that during the prolonged time of the methane oxidation reaction, Ru clusters present in the fresh catalysts transforms to rather large RuO2 crystallites. The mean size of RuO2 crystallites increases from about 16 to 22 and to 27 nm for the 0.5%, 1% and 4.5% Ru catalyst, respectively. HRTEM images and SAED patterns (insets) of the fresh 1% Ru and 4.5% Ru catalysts before and after methane combustion reaction (shown in Figs. 12 and 13, respectively) indicate also on drastic changes in the morphology of the Ru phase. In the HRTEM image of the fresh 1% Ru/ZnAl2 O4 catalyst only nanocrystalline ZnAl2 O4 patches can be observed (Fig. 12a) evidencing high Ru dispersion (see Table 2). The SAED pattern (inset to Fig. 12a) contains only rings from zinc aluminate spinel structure. In the HRTEM image of the used sample occasionally thin RuO2 particles with lattice fringes of distance 0.317 nm were observed. The SAED pattern (inset in Fig. 12b) additionally confirms the presence of crystalline RuO2 particles. In the case of the fresh 4.5% Ru catalyst small metal particles (1.2–2.5 nm) could be identify by the HRTEM image (Fig. 13a) and by the SAED pattern (inset in Fig. 13a). After the catalytic test, the SAED pattern (inset in Fig. 13b) contains strong reflections which could be assigned to the RuO2 oxide structure. The HRTEM image in Fig. 13b shows that the morphology of the RuO2 crystallites is rodlike (as characteristic for the bulk RuO2 oxide) with large particle dimensions of about 15 nm × 60 nm. In line with XRD data, comparison of Figs. 12 and 13b (at different magnification) reveals that RuO2 crystallites are smaller and more irregular for the used 1% Ru catalyst (particle dimensions of about 14 nm × 28 nm) than those observed in the used 4.5% Ru catalyst. The different morphologies of the ruthenium oxide particles in the deactivated catalysts with low and high Ru loading, clearly suggest that a strong structure sensitive effect may occur for combustion of methane on Ru oxide. It can be noted that support material, before and after methane combustion, does not show any sintering or agglomeration of the ZnAl2 O4 particles. TEM images of the aged catalysts after catalytic tests were similar to that before testing (not shown). This TEM analysis confirms the properties described by BET surface area measurements and X-ray diffraction. In summary, our characterization and activity data evidence that the crystalline RuO2 oxide is the active phase for the methane combustion reaction. The O2 uptake and XRD results show that highly dispersed ruthenium present in the reduced Ru/ZnAl2 O4 catalysts is easy to oxidize and it remains in fully oxidized state at elevated temperatures and under the CH4 /air reaction mixture or the O2 atmosphere. In agreement with our previous data [18] amorphous
356
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
Fig. 12. Representative HRTEM images and SAED patterns (insets) of the 1% Ru/ZnAl2 O4 catalyst reduced at 500 ◦ C (a) and after catalytic tests in methane combustion (b).
Fig. 13. Representative HRTEM images and SAED patterns (insets) of the 4.5% Ru/ZnAl2 O4 catalyst reduced at 500 ◦ C (a) and after catalytic tests in methane combustion (b).
RuO2 is formed at first and then it is transformed to the crystalline RuO2 oxide. The rate of this transformation is accelerated at higher temperatures and at higher Ru loading. Catalytic results show that when the Ru oxide was amorphous or weakly crystalline, the rate of methane combustion was very low (at T < 300–350 ◦ C, Fig. 6a). Also, it is evidenced that during the CH4 combustion over the fresh Ru catalysts progressive agglomeration of the ruthenium occurs and this process rise with the Ru loading (Fig. 11). Observed some catalyst deactivation of the fresh catalysts (Fig. 8) may originate therefore, mainly from severe modifications of the Ru phase and not from changes in the ZnAl2 O4 support structure. Previously, we found that during propane oxidation at T ≤ 260 ◦ C, only insignificant aggregation of the oxidized ruthenium species occurred and low-loaded Ru/ZnAl2 O4 catalysts formed a very stable catalyst for the low-temperature propane oxidation [18]. Generally, catalyst deactivation of the supported Ru catalysts by sintering is a serious problem with their using in oxidation atmosphere [10,31–33]. Recently, Liu et al. [34] suggested that sintering of the Ru/Al2 O3 catalysts heated under the CH4 /O2 mixture at high reaction temperature may have occurred due to the volatilization from well-dispersed ruthenium oxide species, or migration of tiny ruthenium oxide particles over the surface of the support. Additionally, some Ru loss in unsupported RuO2 oxide and in supported Ru catalysts, by the volatilization of RuO4 above 800 ◦ C, was observed in air-atmosphere [31]. In the present study we perform catalytic tests at lower temperature and we do not observed Ru loss in the fresh and aged Ru/ZnAl2 O4 catalysts. Additionally, XRD and TEM data could partly explain activity results presented in Table 5. Namely, some decrease in the intrinsic reactivity of the fresh Ru/ZnAl2 O4 catalysts with the Ru loading may be caused not only by the increase of the RuO2 crystallite size, which possesses slightly lower specific activity than smaller RuO2 crystallites, but also by the different morphology of the Ru oxide particles (Figs. 12 and 13). Moreover, more reactive Ru species on the catalysts with low Ru loading may also suggest their higher reducibility,
which favors the reduction process required for the relevant C H bond activation step in redox cycles using lattice oxygen involved in CH4 catalysis. Balint et al. [13] proposed that CH4 is selectively oxidized to CO2 and H2 O over bulk RuO2 probably by a Mars van Krevelen redox-type mechanism. In the first stage some of the bulk RuO2 is reduced by CH4 forming substoichiometric oxide (RuO2−x ). This process is interpreted as a creation of surface oxygen vacancies (surface defects) [35]. Next, in parallel with partial RuO2 reduction, CH4 is quickly oxidized to CO2 and H2 O. Under oxygen rich conditions, the oxide layer is not completely reduced to metal because the reoxidation (refilling of oxygen vacancies) with gas-phase O2 is a fast process [13]. Hu and Ruckenstein [36] suggested that on the oxidized metal surface, the reaction between methane and oxygen might occur via the Eley–Rideal mechanism: that is, methane in the gas phase (or weakly adsorbed on the surface of the catalyst) reacts with strongly adsorbed or lattice oxygen. We suppose that activity of Ru catalysts in the oxidation of methane should be related to the ruthenium-oxygen (Ru O) bond strength of the ruthenium oxide. The bond strength of Ru O (481 ± 63 kJ/mol) is much higher than that of Pd O (380 ± 84 kJ/mol) [34], therefore activity of Ru catalysts in methane combustion found in this study is also lower than that reported for the most active Pd catalysts [1,6,30]. 4. Conclusion We have investigated for the first time combustion of methane over the Ru/ZnAl2 O4 catalysts under O2 -rich conditions. The H2 -reduced Ru/ZnAl2 O4 catalysts exhibited higher catalytic performance in the methane combustion as compared to the air-aged samples. Good activity was attributed to the presence of large amount of active sites on smaller RuO2 crystallites (16–27 nm), formed during methane combustion reaction, as compared to that formed in aging process at 700 ◦ C (21–40 nm). The Ru/ZnAl2 O4 catalysts aged at 700 ◦ C were very stable during the long-term activity tests. Thermally induced deactivation of the H2 -reduced
J. Okal, M. Zawadzki / Applied Catalysis A: General 453 (2013) 349–357
Ru catalysts under the CH4 /air reaction mixture, results mainly from the loss of the catalytic surface area due to crystallite growth of the RuO2 phase. The ZnAl2 O4 spinel with high surface area (∼200 m2 /g) applied as the support material in the metal catalysts, proved to be suitable for high temperature operation due to lack of sintering at elevated temperatures (up to 700–900 ◦ C). Finally, further studies are necessary to improve stability of the Ru phase in the supported Ru catalysts for their possible applications in the complete combustion of dilute methane (lean methane-air mixtures) for its emission control. Moreover, the behavior of the Ru/ZnAl2 O4 catalysts for the methane combustion in the presence of sulfur compounds and water should be also investigated. Acknowledgments The authors thank very much Mrs. L. Krajczyk for TEM investigations and A. Cielecka for chemisorption measurements. References [1] P. Gélin, M. Primet, Appl. Catal. B: Environ. 39 (2002) 1–37. [2] O. Demoulin, B. Le Clef, M. Navez, P. Ruiz, Appl. Catal. A: Gen. 344 (2008) 1–9. [3] A. Janbey, W. Clark, E. Noordally, S. Grimes, S. Tahir, Chemosphere 52 (2003) 1041–1046. [4] T.V. Choudhary, S. Banerjee, V.R. Choudhary, Appl. Catal. A: Gen. 234 (2002) 1–23. [5] P. Hurtado, S. Ordónez, H. Sastre, F.V. Díez, Appl. Catal. B: Environ. 47 (2004) 85–93. [6] N.M. Kinnunen, J.T. Hirvi, M. Suvanto, T.A. Pakkanen, J. Mol. Catal. A: Chem. 356 (2012) 20–28. [7] T. Mitsui, K. Tsutsui, T. Matsui, R. Kikuchi, K. Eguchi, Appl. Catal. B: Environ. 81 (2008) 56–63. [8] N. Kamiuchi, T. Mitsui, H. Muroyama, T. Matsui, R. Kikuchi, K. Eguchi, Appl. Catal. B: Environ. 97 (2010) 120–126.
357
[9] D. Homsi, S. Aouad, J.E. Nakat, B.E. Khoury, P. Obeid, E. Abi-Aad, A. Aboukaïs, Catal. Commun. 12 (2011) 776–780. [10] J. Okal, M. Zawadzki, W. Tylus, Appl. Catal. B: Environ. 101 (2011) 548–559. [11] Ch.K. Ryu, M.W. Ryoo, I.S. Ryu, S.K. Kang, Catal. Today 47 (1999) 141–147. [12] C. Elmasides, X.E. Verykois, J. Catal. 203 (2001) 477–486. [13] I. Balint, A. Miyazaki, K. Aika, J. Catal. 220 (2003) 74–83. [14] N.K. Labhsetwar, A. Watanabe, T. Mitsuhashi, H. Haneda, J. Mol. Catal. A: Chem. 223 (2004) 217–223. [15] W. Walerczyk, M. Zawadzki, J. Okal, Appl. Surf. Sci. 257 (2011) 2394–2400. ˛ nski, ´ [16] M. Zawadzki, W. Mi´sta, L. Kepi Vacuum 63 (2001) 291–296. [17] W. Walerczyk, M. Zawadzki, Catal. Today 176 (2011) 159–162. [18] J. Okal, M. Zawadzki, Appl. Catal. B: Environ. 105 (2011) 182–190. [19] J. Okal, M. Zawadzki, L. Krajczyk, Catal. Today 176 (2011) 173–176. [20] H. Wang, W.F. Schneider, D. Schmidt, J. Phys. Chem. C 113 (2009) 15266–15273. [21] Y.D. Kim, A.P. Seitsonen, S. Wendt, J. Wang, C. Fan, K. Jacobi, H. Over, G. Ertl, J. Phys. Chem. B 105 (2001) 3752–3758. [22] T.F. Garetto, C.P. Apesteguia, Catal. Today 62 (2000) 189–199. [23] P. Hurtado, S. Ordónez, A. Vega, F.V. Díez, Chemosphere 55 (2004) 681–689. [24] T.R. Baldwin, R. Burch, Appl. Catal. 66 (1990) 337–358. [25] F.H. Ribeiro, M. Chow, R.A. Dalla Betta, J. Catal. 146 (1994) 537–544. [26] Okal, M. Zawadzki, Catal. Lett. 132 (2009) 225–234. [27] R.F. Hicks, H. Qi, M.L. Young, R.G. Lee, J. Catal. 122 (1990) 280–294. [28] K. Fujumoto, F.H. Ribeiro, M. Avalos-Borja, E. Jglesia, J. Catal. 179 (1998) 431–442. [29] I.E. Beck, V.I. Bukhtiyarov, O.Y. Pakharukov, V.I. Zaikovsky, V.V. Kriventsov, V.N. Parmon, J. Catal. 268 (2009) 60–67. [30] G. Zhu, K-i. Fujimoto, D.Y. Zemlyanov, A.K. Datye, F.H. Ribeiro, J. Catal. 225 (2004) 170–178. [31] R. Lanza, S.G. Järas, P. Canu, Appl. Catal. A: Gen. 325 (2007) 57–67. [32] J. Assmann, V. Narkhede, N.A. Breuer, M. Muhler, A.P. Seitsonen, M. Knapp, D. Crihan, A. Farkas, G. Mellau, H. Over, J. Phys.: Condens. Matter 20 (2008) 184017–184040. [33] J.T. Kiss, R.D. Gonzalez, J. Phys. Chem. 88 (1984) 892–897. [34] Y. Liu, F.Y. Huang, J.M. Li, W.Z. Weng, Ch.R. Luo, M.L. Wang, W.S. Xia, Ch.J. Huang, H.L. Wan, J. Catal. 256 (2008) 192–203. [35] H. Madhavaram, H. Idriss, S. Wendt, Y.D. Kim, M. Knapp, H. Over, J. Assmann, J. Catal. 202 (2001) 296–307. [36] Y.H. Hu, E. Ruckenstein, J. Phys. Chem. B 102 (1998) 230–233.