КИНЕТИКА И КАТАЛИЗ, 2011, том 52, № 5, с. 727-731
УДК 541.128.3:542.943.7:547.211
THE EFFECT OF THE NATURE OF THE SUPPORT ON CATALYTIC PROPERTIES OF RUTHENIUM SUPPORTED CATALYSTS IN PARTIAL OXIDATION OF METHANE TO SYN-GAS
© 2011 r. T. P. Maniecki*, K. Bawolak, P. Mierczynski, P. Kaczorowski, W. K. JoZwiak
Institute of General and Ecological Chemistry, Technical University of Lodz, Poland
*E-mail:tmanieck@p.lodz.pl Received 06.11.2009
The effect of the carrier on catalytic properties of ruthenium supported catalysts in partial oxidation of methane (POM) was investigated. A variety of supports differed in texture and reducibility(Al2O3, SiO2, TiO2, Cr2O3, CeO2 and Fe2O3) were used. The catalyst activity is governed by ruthenium phase formation (RuO2 ^ Ru0), and it depends on redox properties of the support as well as support—ruthenium phase interaction. The activity of Ru supported catalysts decreases in the order Al2O3 « SiO2 > Cr2O3 > TiO2 > CeO2 > Fe2O3. No significant effects of the specific surface area and porosity of catalysts on the methane conversion and selectivity of CO formation were found. The selectivity of CO2 formation (total oxidation of CH4) under conditions of POM (a ratio of CH4/O2 = 2) is associated with the contribution of reducible support oxides into the catalytic performance.
Partial oxidation of methane has been extensively investigated for many years due to various advantages among which the low cost plays a significant role.
Irreducible (SiO2, Al2O3, La2O3, Y2O3, MgO [1-3]) as well as reducible (ZrO2, TiO2, CeO2 [4-6]) oxides can be used in partial oxidation of methane (POM) as supports for metal catalysts. Ruckenstein and Wang found that with irreducible supports higher activity and selectivity can be achieved than with reducible oxides. A possible reason is that a fraction of the active phase (ruthenium or nickel) is covered by oxygen from the support material [7, 8]. The strong metal-support interaction (SMSI) effects were observed also for TiO2 based catalysts [9]. Experiments [9] with VIII group metals reported by Nakagawa et al. showed that for high activity in POM reaction the reduction of active phase is needed. A low activity of catalysts supported on reducible support is related to the presence of ruthenium oxide in the form of a surface oxide on the support which is difficult to reduce. The existence of the SMSI effects was proved by Ruckenstein and Wang [6, 7] who found SMSI in the case of catalysts on irreducible supports. They observed that the nature of the supports as well as the calcination temperature influence the formation of perowskite (LaRhO3) or spinel (MgRh2O4) like structures which can be involved in
Статья написана по материалам конференции "Механизмы каталитических реакций" (29 июня—2 июля 2009 г., Новосибирск). Статья публикуется на английском языке в авторском варианте.
redox cycle of POM reaction [9]. The authors compared the effects of support precursors on reaction yield and found that the kind of precursor has no significant influence on the catalytic activity of rhodium supported catalysts [9], while modifying their stability and resistance to carbonaceous deposits.
The influence of the carrier on catalytic properties of platinum and palladium supported catalysts in POM reaction was studied by Choundary et al. [10]. They found that activity of platinum catalysts was higher than that observed with palladium systems. The order of decreasing activity for supported platinum catalysts was: Gd2O3 > Dy2O3 > Er2O3 > Sm2O3 > >Pr6On > Nd2O3.
Literature data suggest that the yield of syn-gas depends upon the conditions of the process, the method of catalyst preparation, the nature and the amount of an active phase. Correspondingly, the comparison of experimental data from different works is difficult or nearly impossible. For this reason, it seems important to compare the behavior of ruthenium catalysts dispersed on different oxides (Al2O3, SiO2, TiO2, Cr2O3, CeO2, Fe2O3) in POM.
EXPERIMENTAL
Catalyst preparation
Ruthenium supported catalysts were prepared according to the usual wet-impregnation procedure. Commercial oxides, such as silica, alumina, titania,
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dV/dr 6х10-2
4x10
2x10
10
100
Fig. 1. Pore-size distribution of different supports. CH4
Al2O3
1000 Radius, Â
0 100 200 300 400 500 600 700 800 900 Temperature, °C
Fig. 2. TPSR reactants concentration profiles for 5%Ru/Al2O3 catalyst heated at 10°C/min in the POM reaction.
ceria, chromia and Fe2O3were used as supports. Ruthenium was introduced from an aqueous solution of RuCl3. Two sets of catalysts containing 0.5 and 5% of ruthenium were prepared. The catalysts were dried and then calcined at 400° C for 4 h in air.
Methods of characterization
Temperature programmed reduction (TPR-H2) of supports and ruthenium supported catalysts was conducted using the automatic unit (TPO-TPR system AMI-1, Altamira Instruments) in the H2 flow at
THE EFFECT OF THE NATURE OF THE SUPPORT ON CATALYTIC
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10°C/min to 900°C. The samples (ca. 0.1 g) were reduced in a 60 cm3/min stream of hydrogen (5% H2 + + 95% Ar) or methane (5% CH4 + 95% He). Thermal conductivity detector (TCD) or mass spectrometer was used.
The BET surface area and porosity (Dollimore-Heal method) associated with supports calcined at 500 and 900°C were determined using the commercial Sorptomatic 1900 unit. Prior to the low temperature nitrogen adsorption—desorption cycle, the samples were preheated at 250°C for 12 h.
The activity tests in POM reaction were carried out in a flow quartz reactor loaded with 0.1g catalyst samples. Both reactants — methane (5% CH4 + He) and oxygen (5% O2 + He) — were fed in a gaseous stream of CH4 and O2 (molar ratio = 2) with a flow rate of 100 cm3 min-1. The reaction was studied in a temperature range of 25 to 900°C.
The gas chromatography analysis of reactants and products mixture (CH4, O2, CO2, CO) was carried out using GC Varian 3300 (Varian Inc) instrument equipped with CTR-1 column (helium as a carrier gas, 35°C) and TCD detector (130 mA, 120°C). The analysis of hydrogen was carried out employing the CHROM-4 gas chromatograph unit (Laboratorni Pristroje Praha) packed with 4 A molecular sievear-gon, 110°C). The data were monitored by a thermal conductivity detector (100 mA, 120°C).
RESULTS AND DISCUSSION
The effect of calcination temperature on the specific surface area of various supports is shown in the Table. There are considerable differences among oxides as evidenced by a wide range of values of surface area recorded for the supports calcined at 500°C (20650 m2/g) and for the supports preheated at 900°C (1450 m2/g). The increasing calcination temperature results in a decrease of specific surface area that is especially pronounced for oxides Cr2O3 or Fe2O3 which seem to be very sensitive to sintering. When specific surface areas of oxides and their catalytic activity are compared, no evident relation between activity in POM reaction and support surface area can be traced. The pore-size distribution in samples of various supports is presented in Fig. 1. The differences in the porosity of the oxides with pore sizes in the range of 20 to 600 A are fairly discernable.
The preliminary catalytic activity tests in the POM reaction indicated that pure supports show no catalytic activity. Only for Cr2O3 a negligibly small activity could be observed [11].
The profiles of the temperature programmed surface reaction (TPSR) for the 5%Ru/Al2O3 catalyst are
2
d
13 a •Si
S
CH4 \ CJ
H2O i \
H2
CO
CO2 1 1 1 1
0 100 200 300 400 500 600 700 800 900 Temperature, °C
Fig. 3. TPR-CH4 profiles for 5%Ru/Al2O3 catalysts.
given in Fig. 2. In the 300-450°C temperature range full oxidation of methane occurs:
CH4 + 2O2 ^ CO2 + 2H2O.
At a temperature near to 450°C the concentrations of reactants change considerably and the evolution of carbon monoxide and hydrogen in POM reaction is observed:
CH4 +1/2O2 ^ CO + 2H2.
This rather rapid ignition effect is associated with the reduction of ruthenium oxide by methane according to equitation:
2RuO2 + CH4 ^ 2Ru + CO2 + 2H2O, 3RuO2 + 2CH4 ^ 3Ru + 2CO + 4H2O.
In the temperature range 450-750°C the selectivity towards CO2 decreases due to enhancement of methane conversion in the POM reaction indicating that the POM reaction occurs on the surface of metallic ruthenium.
This suggestion was confirmed by the TPR-CH4 experiment. An example of a typical TPR-CH4 run on 5% Ru/Al2O3 catalyst is presented in Fig 3. Based on
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o
CD
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^ f\ A
D 0-4 C
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100 200 300 400 500 600 700 800 900 0 (b) 1.0
100 200 300 400 500 600 700 800 900
(d)
Al2O3, ' //Cr2O3
SiO2—jj
/\TiO2 CeO21
0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600 700 800 900
Temperature, °C
Fig. 4. Temperature dependences of methane conversion and selectivity to carbon monoxide for 0.5% Ru (a, b) and 5% Ru (c, d) supported catalysts.
the TPR profile, the rapid methane consumption occurs at 480°C. At the same temperature the peaks due to CO and CO2 evolution are observed. Above 500°C only the thermal decomposition of methane can be
The surface area of different supports preheated in air at 500 and 900°C for 5 h
Support Specific surface area, m2/g
500°C 900°C
SiO2 650 450
AI2O3 130 75
TiO2 80 20
CeO2 25 8
Fe2O3 12 <1
Cr2O3 20 3
traced as evidenced by evaluation of hydrogen and simultaneous consumption of methane (see CH4 and H2 profiles). This process took place according to equation:
CH4 ^ C + 2H2.
The effects of rapid consumption of CH4 and evaluation of H2, CO, CO2, H2O at 470°C can be identified during POM reaction and at the same temperature in reactions that accompany TPR-CH4 processes. Occurrence of both reactions in the course of ruthenium oxide reduction indirectly confirm the analysis of TPR-CH4 profiles,. During those processes the simultaneous release of CO, CO2, water and CH4 consump
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