ARTICLE pubs.acs.org/IECR
Thermochemical Stability of Alumina-Supported Rh-LaCoO3 Catalysts for Tar Conversion Paola Ammendola,† Elena Cammisa,‡ Luciana Lisi,†,* and Giovanna Ruoppolo† † ‡
Istituto di Ricerche sulla Combustione CNR, P.le Tecchio, 80-80125 Naples, Italy Dipartimento di Ingegneria Chimica, Universita Federico II, P.le Tecchio, 80-80125 Naples, Italy ABSTRACT: The thermal and chemical stability of alumina-supported catalysts (Rh/Al2O3, LaCoO3/Al2O3, Rh-LaCoO3/Al2O3) have been investigated by performing cycles of tar conversion at 700 °C followed by regeneration of the catalyst by oxidation of coke deposited on the surface up to 800 °C. Structural and chemical modifications underwent by the catalysts upon conversion/ regeneration cycles have been studied by BET, TPR/TPO, and DRIFT analyses. The catalysts containing rhodium are highly stable maintaining the original performance and chemical properties of the fresh sample also after several cycles, dispersion of rhodium on Al2O3 surface being sufficiently preserved also at 800 °C. On the contrary, LaCoO3/Al2O3 catalyst undergoes a partial deactivation after the first cycles related to the irreversible migration of cobalt into the alumina lattice.
1. INTRODUCTION In the past decade the depletion of fossil fuels shifted the attention of the scientific community toward the study of renewable sources. Biomass represents one of the most available renewable fuels which can be converted into bio-oil or syngas.1,2 A clean syngas can be produced by biomass gasification for energy purposes or for production of fine chemicals provided that suitable conditioning of the biomass-derived gas is performed. Indeed, the biomass gasification at 700 900 °C produces, in addition to the gas phase containing H2, CO, CO2 and light hydrocarbons, a solid residue (char) and a mixture of condensable hydrocarbons, mostly aromatics, called tar.1 4 Condensation of tar in downstream pipes and filters represents a severe drawback and must be necessarily avoided.1 4 Tars can be removed through primary processes, directly within the gasifier, or secondary processes operated in a separate reactor downstream from the gasifier. The more common methods for removing tars include physical means, such as filtration, or chemical processes, such as thermal cracking or catalytic conversion. Filtration generally requires multiple expensive downstream unit operations, while thermal cracking must be carried out at very high temperatures. Catalytic conversion of tars can be operated at significantly lower temperatures (650 850 °C) and, at the present, is the best technology to remove tar and, at the same time, to increase the total gas yield.5 16 A large variety of catalysts have been proposed for tar conversion ranging from natural materials as dolomite or olivine7,11,16,17 to synthetic catalysts.9,10,13 15,18,19 The addition of transition metals as Fe or Ni to olivine has also been investigated.5,6,8 More recently, Rh-based catalysts have been proposed as highly performing catalysts for tar conversion.12,14,15,18,20 These catalysts also inhibit coke formation and show a very good resistance to poisoning by sulfur compounds, often present in the biomass.12,14,15,19,21 The incorporation of rhodium in small amounts into a matrix of a perovskite-like oxide (LaCoO3) further improved the performance of the Rh/Al2O3 catalyst.14,15,20 Nevertheless, the activity is not the only parameter to be considered, the catalyst lifetime being important as well. Indeed, r 2011 American Chemical Society
during its use the catalyst undergoes several redox and thermal cycles and, consequently, permanent or reversible deactivation can occur. To investigate the thermal and chemical stability of the alumina-supported rhodium-based catalysts cycles of tar conversion followed by regeneration of the temporarily deactivated catalyst due to coke deposition have been carried out. The possible modifications of the structural and redox properties of both LaCoO3 and Rh upon repeated cycles have been also studied by BET, TPR/TPO, and DRIFT techniques and related to the performance after each cycle.
2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. The alumina support was a commercial La stabilized (3 wt % La2O3) γ-Al2O3 (Puralox SCF140-L3, SASOL). LaCoO3 at 20 wt % was dispersed on the support by wet impregnation. Stoichiometric amounts of La(NO3)3 3 xH2O (Aldrich, >99.9%) and Co(NO3)2 3 6H2O (Fluka, g99%) were dissolved in a water solution containing alumina, followed by the addition of a few drops of nitric acid. The solution was completely evaporated, and then the powder sample was dried for 2 h at 120 °C and calcined in flowing air (150 Ncm3 3 min 1) for 3 h at 800 °C (heating rate = 10 °C 3 min 1). A fraction of calcined LaCoO3/Al2O3 powder was impregnated with a Rh(NO3)3 3 xH2O (Riedel-de-Ha€en) solution using an amount of rhodium corresponding to 1 wt %. The mixture was dried and then calcination was repeated. A 1 wt % Rh/Al2 O3 was prepared impregnating the alumina support using the same rhodium precursor. In Table 1 a list of the Special Issue: Russo Issue Received: July 28, 2011 Accepted: October 5, 2011 Revised: September 28, 2011 Published: October 05, 2011 7475
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Table 1. Metal Content and BET Surface Area of the Catalysts sample
Rh content
Co content
surface area
(% wt)
(% wt)
(BET) (m2 3 g‑1)
4.7
143 101
Al2O3 LaCoO3/Al2O3 Rh/Al2O3
0.84
Rh-LaCoO3/Al2O3
0.80
134 4.7
101
catalysts and their physical and chemical characteristics are reported. 2.2. Catalyst Characterization. Rhodium and cobalt contents were quantitatively determined using an ICP MS Agilent 7500 instrument. Catalyst surface area measurements were carried out according to the BET method by N2 adsorption at 77K with a Quantachrome Autosorb 1-C analyzer. H2 TPR and TPO experiments were carried out with a Micromeritics Autochem II TPD/TPR analyzer. Approximately 100 mg of catalyst was reduced with a 2% H2/Ar mixture (50 Ncm3 3 min 1) at 10 °C 3 min 1 up to 800 °C after a 1 h in situ pretreatment in air (50 Ncm3 3 min 1) at 800 °C. After the first TPR experiment the sample was reoxidized under 0.5% O2/He mixture heating at 10 °C 3 min 1 up to 800 °C. The TPR/TPO cycle was repeated according to this procedure until TPR profiles are superimposable. The nature of Rh species was investigated by DRIFT spectroscopy using CO as a probe molecule. DRIFT experiments were performed on a Perkin-Elmer Spectrum GX spectrometer with a spectral resolution of 4 cm 1 averaged over 50 scans. For each experiment finely ground powder sample was placed into the ceramic cup of a commercial high-temperature Pike DRIFT cell equipped with a ZnSe window and connected to mass flow controlled gas lines. Prior to the experiment the sample was reduced with a 2% H2/N2 mixture for 1 h at 800 °C and then cooled down to room temperature under the same mixture. After 15 min Ar purging a background spectrum was recorded; then CO was adsorbed at room temperature (2%CO/N2 mixture, 100 Ncm3 3 min 1) for 30 min and any excess CO was removed by flushing with Ar (100 Ncm3 3 min 1) prior to recording the DRIFT spectrum. 2.3. Catalytic Tests. An experimental setup consisting of two fixed bed quartz reactors (i.d. = 1 cm, length = 60 cm) which can be heated independently in two different electric furnaces was used to test the catalysts. The biomass sample (0.5 g of maple wood chips with the following proximate composition: 8.9 wt % moisture, 74.6 wt % volatiles, 15.9 wt % fixed carbon, 0.6 wt % ash, and the following chemical composition estimated on dry basis: 46.28 wt % C, 5.56 wt % H, 47.52 wt % O, 0 wt % N) was heated up in the first furnace under pure N2 (99.998%) constant flow rate (12 NL 3 h 1) regulated with a Brooks mass flow controller (SLA 5850). The gaseous and condensable products formed during biomass pyrolysis in the first reactor passed through the second reactor containing the catalyst (0.50 g) kept at 700 °C. Then the stream was sent to a two stage-condenser, in the first stage (a pear-shaped flask of 75 mL at room temperature) heavier tars were collected, whereas the second one (three Erlenmeyer of 50 mL) was used to collect lighter ones at 20 °C, following the CEN/TS 15439 (2006) procedure for sampling and analysis of tars. Finally, the permanent gases were sent to a microgas-chromatograph (Agilent 3000A), equipped with four different independent channels (with the following
Figure 1. TPR profiles of LaCoO3/Al2O3 catalyst (each TPR is followed by a TPO).
four columns: OV-1, alumina, PLOT-U, and MS5A), and a TCD. The lines between the first and second reactor as well as between the second reactor and the first stage of the tars condensation train were heated between 300 and 400 °C to avoid the condensation of tarry compounds. The duration of the whole catalytic test, related to the biomass pyrolysis process (heating at 5 °C min 1 to 800 °C, 30 min at 800 °C) was about 3 h. The sampling train was weighed before and after each experiment for the quantitative evaluation of the condensable tars yield. The analysis of the condensed tars, after diluting with isopropyl alcohol, was performed off line in a gas chromatograph (HP 9600 series), equipped with a HP 35 phenylethylmethyl siloxane column and a FID detector. The char yield was estimated by weighing the biomass residue in the first reactor after pyrolysis. At the end of each experiment the amount of coke deposited on the catalyst was determined by oxidation of the material after cooling down under inert atmosphere. The oxidation was performed following a heating ramp of 5 °C 3 min 1 from ambient temperature up to 800 °C under O2/He mixture (0.5% vol) at constant flow rate (17 NL 3 h 1). The concentration of released carbon dioxide was measured by the micro-GC. The numerical integration of the signal provided the stoichiometric amount of coke formed during the previous catalytic tar conversion experiment. This operation also allows the regeneration of the catalyst surface. The duration of the regeneration cycle was about 3.5 h. Catalytic activity was thus estimated as the difference between the yields and type of gas, liquid, and solid products obtained with the different catalysts. For all experiments the mass balance was closed with a maximum error of (10% wt.
3. RESULTS AND DISCUSSION In Table 1 the metal content of the catalyst and their surface area are reported. The actual metal content is close to the nominal one. The deposition of rhodium results in a slight 7476
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Figure 2. TPR profiles of Rh/Al2O3 catalyst (each TPR is followed by a TPO).
Figure 3. TPR profiles of Rh-LaCoO3/Al2O3 catalyst (each TPR is followed by a TPO).
reduction of the surface area of the alumina support, whereas the deposition of LaCoO3 causes a larger decrease. No significant effect on BET surface areas of either conversion/regeneration or TPR/TPO cycles were observed for all samples thus showing a good structural stability. TPR/TPO cycles were performed in order to reproduce the redox cycles the catalyst undergoes under reaction conditions. Indeed, the catalyst is exposed to a reducing atmosphere under the tar conversion reaction (the oxygen is just that contained in the biomass pyrolysis products in addition to that of the supported metal oxide) and to an oxidizing atmosphere under regeneration conditions when oxygen oxidizes coke on the surface and in case reoxidizes the alumina-dispersed metal. In Figures 1 3 TPR profiles of LaCoO3/Al2O3, Rh/Al2O3, and Rh-LaCoO3/Al2O3 are reported. In each figure the TPR analysis carried out on the fresh sample (1st TPR) is compared with the following ones (second and third TPR) carried out after a TPO to reoxidize the sample. The first TPR profile of LaCoO3/ Al2O3 (Figure 1) has the typical features of a perovskite-like oxide dispersed on alumina, as previously reported,14 showing two peaks with a roughly 1:2 ratio of the corresponding areas associable to the reduction of cobalt from +3 to +2 and +2 to 0, respectively. The H2 uptake (0.3 mmol H2 3 g 1) is well below the theoretical value estimated for reduction of all Co3+ to metallic cobalt which suggests that a significant fraction of the transition metal is not reducible and thus not available for the catalytic reaction, as previously reported.14 The second TPR carried out after a TPO up to 800 °C shows a reduction of these two peaks balanced by the appearance of a new peak at higher temperature typical of cobalt which diffused in the alumina matrix forming an aluminate with spinel structure (CoAl2O4).14 The subsequent TPO/TPR cycle does not significantly modify the profile of the second TPR curve suggesting that the cobalt migration basically takes place upon the first cycle. The H2 consumption slightly decreases in the following cycles due to the thermal and redox treatments that likely promote a further interaction between cobalt and alumina leading to a lower reducibility of the metal. Nevertheless, the possibility that cobalt is not efficiently reoxidized by O2 in the intermediate TPO cannot be excluded.
The first TPR cycle of Rh/Al2O3 catalyst (Figure 2) shows two signals related to surface RhOx and to RhAlOx14 at low and high temperature, respectively. The following TPR/TPO cycles result in a shift toward lower temperature of the signal attributed to RhOx surface species which also increases in intensity suggesting that the redox treatment promotes some rhodium mobility. The total H2 uptake corresponds to the complete reduction of rhodium from +3 to 0. The amount of consumed H2 does not significantly change in the subsequent TPR cycles. The TPR profiles of the catalyst containing both LaCoO3 and the noble metal (Figure 3) show a medium temperature peak corresponding to the reduction of cobalt in the perovskite-like oxide and a high temperature peak related to the reduction of cobalt in the aluminate.14 A signal at low temperature is associated to highly dispersed rhodium species.14 Their fraction is much higher compared to that observed for the Rh/Al2O3 sample due to a barrier effect of the LaCoO3 layer which inhibits rhodium migration.14 The comparison of the first TPR with the following ones does not evidence significant differences of the medium and high temperature peaks, but a slight modification of the low temperature signal can be noticed. Although the amount of surface species related to this signal is not affected by the TPR/ TPO cycles, this peak is clearly composed of two contributions which were attributed to the aggregate rhodium species (lower temperature) and the isolated rhodium species (higher temperature), respectively, as previously reported.20 The comparison of the TPR profiles suggests that the TPR/TPO cycles promote the aggregation of the surface species as shown by the increase of the intensity ratio between the low temperature and the high temperature peak. On the basis of this finding, we can also suppose that the shift of the low temperature peak toward lower temperatures observed in the second and the third cycle of Rh/Al2O3 is associated to a greater aggregation of RhOx particles. In Figure 4 the DRIFT spectra of Rh/Al2O3 as fresh, after three TPR/TPO cycles, and after three conversion/regeneration cycles are reported. The bands observed in the spectra have been attributed to CO adsorbed on the noble metal. In particular, the doublet bands at 2090 and 2016 cm 1 correspond to gemdicarbonyl species on isolated Rh+ sites,20 whereas the band at 7477
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Figure 4. DRIFT spectra of Rh/Al2O3 as fresh, after three TPR/TPO cycles, and after three tar conversion/regeneration cycles.
Figure 5. DRIFT spectra of Rh-LaCoO3/Al2O3 as fresh and after three tar conversion/regeneration cycles.
around 2060 cm 1 was assigned to the linearly adsorbed CO on Rh crystallites or clusters in the zero oxidation state.22 24 In the spectrum of the fresh catalyst the bands ascribed to gemdicarbonyls on isolated rhodium sites prevail, whereas an enhanced segregation of rhodium was observed after the cycles the sample underwent, as shown by the well-detectable band at 2060 cm 1 present both in the spectrum of the sample after three TPR/TPO cycles and in that after three conversion/regeneration cycles. This result also implies that the TPR/TPO cycles well simulate the thermal and redox cycles the catalyst experienced during the tar conversion and the following regeneration. In addition, it is also in good agreement with the TPR/TPO findings. The aggregation of rhodium surface species compared
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to that of the fresh catalyst was confirmed by the DRIFT spectra recorded on the Rh-LaCoO3/Al2O3 as fresh and after three tar conversion/regeneration cycles (Figure 5). The increase of the intensity of the band at 2060 cm 1, already present in the fresh sample, due to the linearly adsorbed CO on Rh larger particles, points out that the redox and thermal cycles the material underwent promotes some metal particles aggregation. The pyrolysis of biomass produces a wide range of organic compounds (light hydrocarbons included), as previously reported.14 In Table 2 the results of all catalytic tests are reported as solid (char and coke), liquid, and gas yields. All catalysts are able to effectively convert tar mainly into gaseous species leaving a low fraction of carbonaceous compounds on the catalyst surface expressed as coke. The LaCoO3/Al2O3 catalyst, which provides performance comparable to that of the Rh-containing catalysts in the first cycle, clearly shows a considerable activity loss in the second cycle which, however, is not significantly further reduced in the third cycle. Indeed, in the second and third cycle a rather large decrease of gas yield was observed balanced by the appearance of condensable products, mainly consisting in water (liquid yield) and by an increased amount of coke deposited on the surface. Also the nature of light hydrocarbons (Table 3), produced by the cracking reactions of the tarry compounds promoted by the catalyst, changes, ethane and ethylene being substituted by heavier species as n-C4 and n-C5 and also by cyclic compounds. On the other hand, when rhodium is present in the catalyst formulation the catalyst deactivation after the first conversion cycle is definitely less important. For both Rhcontaining samples just a limited decrease of gas yield was observed after the first conversion cycle balanced by a slight increase of coke yield. No liquids were collected and no light hydrocarbons were detected in the gas phase for both Rhcontaining catalysts in all cycles experienced. Moreover, the Rh-LaCoO3/Al2O3 catalyst was tested up to six cycles, and a stable performance, similar to that observed after the first cycle, was obtained showing that the little modifications the material underwent after the first cycle do not change further. The same sample was also tested using half the amount of material in the reactor, that is, 0.25 g instead of 0.50 g, since it was shown that this catalyst was able to provide the same tar conversion also with half the contact time compared to the contact time used in this work.20 In fact, the stable performance observed in the catalytic tests carried out using 0.5 g sample could have been ascribed to the availability of a fraction of catalyst not involved in the previous cycles and thus not deactivated. The results reported in Table 2 show that, even using half the contact time, the same solid, liquid, and gas yields were obtained, thus suggesting that the catalyst was not permanently deactivated and that it is perfectly regenerated after each coke oxidation cycle even when the whole catalytic bed is involved in the reactions. The permanent gases distribution obtained for the three catalysts in all cycles is reported in Figures 6 8. Both H2 and CO yields are reduced after the first cycle for the LaCoO3/Al2O3 catalyst (Figure 6), although H2 decreases more than CO. CO2 is quite constant in all cycles while the amount of light hydrocarbons, methane included, increases. This is an indication that sites active for the reforming reaction are partly deactivated upon the first conversion/regeneration treatment, whereas cracking reactions still occur. The stable performance of Rh/Al2O3 catalyst are also shown by the rather constant H2 and CO yields evaluated after the first cycle. 7478
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Table 2. Solid, Liquid, and Gas Yields Obtained in the Different Cycles of Tar Conversion for All Catalysts (Coke Was Oxidized between One Cycle and the Following) solid yield (wt %) liquid yield gas yield catalyst LaCoO3/Al2O3
Rh/Al2O3
char
coke
(wt %)
(wt %)
1st
18.3
3.6
2nd
18.0
5.6
11.9
56.8
3rd
18.2
5.1
12.4
55.9
1st 2nd
16.5 18.6
2.0 2.7
75.9
76.5 71.7
3rd
18.6
2.9
72.3
1st
17.0
1.3
75.0
2nd
17.0
1.7
72.4
3rd
17.2
2.1
71.0
Rh-LaCoO3/Al2O3a 1st
Rh-LaCoO3/Al2O3
a
cycle
17.5
1.1
71.1
2nd
16.5
1.6
71.6
3rd
17.7
1.7
70.3
Figure 7. Permanent gases yields obtained in the three tar conversion/ regeneration cycles for the Rh/Al2O3 catalyst.
A half amount (0.25 g) of catalyst was used in this test.
Table 3. Light Hydrocarbons (μmol) Detected in the Gas Phase during the Different Cycles of Tar Conversion for the LaCoO3/Al2O3 Catalyst cycle C2H2 C2H4 C2H6 C3H6/C3H8 nC4 nC5 1-pentene 1-hexene 1st 2nd
0 0
2.1 0.2
2.8 0
0 0
0 0 30.8 2.9
0 0
0 0
3rd
0
0.2
0
0
32.2 2.8
6.0
0
Figure 8. Permanent gases yields obtained in the three tar conversion/ regeneration cycles for the Rh-LaCoO3/Al2O3 catalyst.
Figure 6. Permanent gases yields obtained in the three tar conversion/ regeneration cycles for the LaCoO3/Al2O3 catalyst.
The slight decrease of syngas yield is balanced by an increase of CO2 yield, being CH4 yield rather constant (Figure 7). The Rh-LaCoO3/Al2O3 catalyst shows a rather stable performance in syngas production following the first cycle (Figure 8). The CO2 formation does not change in the following cycle and is lower than that provided by Rh/Al2O3 with carrying on the conversion/regeneration treatments. The same yields (not shown) were obtained also using half the contact time with Rh-LaCoO3/Al2O3.
Figure 9. Coke oxidation profiles of LaCoO3/Al2O3 catalyst after each conversion/regeneration cycle.
In conclusion, the Rh-based catalysts are more stable, which is also confirmed by the TPR and DRIFT results. On the other 7479
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coming from biomass pyrolysis into oxidized products. On the other hand, for all the used catalysts the metal availability to form carbide species decreases with conversion/regeneration cycles, limiting the increase of low temperature peak.
Figure 10. Coke oxidation profiles of Rh/Al2O3 catalyst after each conversion/regeneration cycle.
4. CONCLUSIONS The thermochemical stability of alumina-supported catalysts (Rh/Al2O3, LaCoO3/Al2O3, Rh-LaCoO3/Al2O3) has been investigated in the conversion of tar produced by biomass decomposition. Repeated cycles of tar conversion at 700 °C followed by regeneration of the catalyst by oxidation of coke deposited on the surface up to 800 °C were carried out in order to test the catalysts lifetime. All catalysts show a good structural stability; however, the catalysts containing rhodium also show a satisfactory physical and chemical stability, dispersion of rhodium on Al2O3 surface being preserved even at 800 °C. They maintain the original performance and chemical properties of the fresh sample also after several cycles. On the contrary, the significant modification of the redox properties of cobalt in the LaCoO3/Al2O3 catalyst after the first conversion/regeneration cycle is related to a partial deactivation due to the irreversible migration of cobalt into the alumina lattice. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: (+39) 081-7682279. Fax: (+39) 081-5936936.
’ ACKNOWLEDGMENT This work was financially supported by MiSE-CNR “Carbone pulito-CO2 Capture” Project, Italy. ’ REFERENCES Figure 11. Coke oxidation profiles of Rh-LaCoO3/Al2O3 catalyst after each conversion/regeneration cycle.
hand, the significant and irreversible modification of the redox properties of cobalt after the first TPR/TPO experiment could explain the decreased performance of this catalyst after the first conversion/regeneration cycle. For all catalysts the amount of coke produced during each cycle was evaluated by the estimation of the CO2 produced during the coke oxidation performed on the used catalysts. The total amount of coke is reported in Table 2. It increases with proceeding cycles for all catalysts due to a greater occurrence of cracking reactions which leave carbonaceous residues on the catalyst surface, even if this phenomenon is definitely more important for the sample not containing rhodium. The coke oxidation profiles (Figures 9 11) show two peaks, which in a previous paper20 were attributed to the formation of metal carbide (low temperature peak) and graphitic carbon (high temperature peak), respectively. It is worth noting that in all cases after the first cycle, the high temperature peak mainly increases, while the peak associated to metal carbide is more constant or it even decreases, as observed for Rh-LaCoO3/ Al2O3. The observed increase of graphitic carbon production can be related to the worse ability to convert the organic species
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