Ind. Eng. Chem. Res. 2004, 43, 3133-3136
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Behavior of Fresh and Deactivated Combustion Promoter Additives Marta C. N. A. de Carvalho,† Edisson Morgado, Jr.,‡ Henrique S. Cerqueira,‡ Neuman S. de Resende,† and Martin Schmal*,† NUCAT Programa de Engenharia Quı´mica, COPPE/UFRJ, Centro de Tecnologia, Bl.G/115, Ilha do Funda˜ o, 21945-970 Rio de Janeiro, Brazil, and Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (Cenpes), Pesquisa e Desenvolvimento do Abastecimento, Tecnologia em FCC, Ilha do Funda˜ o, Av. Jequitiba´ 950, 21949-900 Rio de Janeiro, Brazil
This work reports the behavior of fresh and deactivated monometallic (Pt, Pd) and bimetallic (Pd-Ce) alumina-based combustion promoters prepared in the laboratory. The different additives were tested in carbon monoxide oxidation reaction before and after hydrothermal deactivation. Among the tested samples the best activity in CO oxidation was obtained for the additive containing 660 ppm of palladium. The addition of 15% of cerium oxide to the palladium-based additive did not increase its performance. Introduction Fluid catalytic cracking (FCC) is one of the most profitable petroleum refining processes because it converts low-value residual oil into valuable products, such as liquefied petroleum gas (C3-C4 alkenes) and gasoline, adjusting market offer to demand.1-2 Together with the desired cracking reactions, there is also the formation of coke (heavy hydrocarbons retained in the pore structure of the catalyst after reaction). This coke temporarily deactivates the catalyst, which continuously circulates between the riser (FCC reactor) and the regenerator. In the regenerator the coke is converted into CO, CO2, H2O, SOx, and NOx compounds. The coke can be oxidized to CO and/or CO2, in accordance with the following reactions:
C + O2 ) CO2
(1)
1 O ) CO 2 2
∆H ) - 26.4 Kcal‚mol-1 (2)
1 O ) CO2 2 2
∆H ) - 67.6 Kcal‚mol-1 (3)
C+ CO +
∆H ) - 94 Kcal‚mol-1
The heat produced during CO oxidation is 2.6-fold higher than the heat produced for its formation. Hence, the complete burn of coke into CO2 produces the maximum energy. In the FCC regenerator, the coked catalyst is distributed over a partly fluidized bed generated by a counter flow of well-distributed air from the bottom. Such catalyst bed is divided into two main regions of different gas-solid densities. At total combustion operation, it is important that the burning of the coke mostly occurs in the regenerator dense phase; otherwise, unconverted CO escapes from this region and remains reacting with the excess oxygen in the upper region, where the combustion gases are separated from the catalyst (diluted phase), thus causing undesirable
hot spots that can affect the metallurgical limits of the equipment. Similarly, in the partial combustion operation, nonreacted oxygen that escapes from the dense phase to react with CO in the diluted phase must be prevented. Such undesirable phenomenon is usually quantified by the temperature difference between the dense and diluted regenerator phases, known as “afterburning”. For controlling “afterburning”, CO combustion promoters are used worldwide in FCC operations. Most commercial combustion promoter additives contain between 300 and 800 ppm of platinum, supported on alumina or mixed oxides.2-7 A metal content above 1000 ppm in the additive increases sintering of the metallic phase. In addition, gaseous sulfur and nitrogen oxide emissions [SOx; NOx] must be controlled as well in the FCC unit.8 As long as CO promotes the reduction of NOx to N2 in the regenerator conditions, the platinumbased additive inhibits this reaction because it consumes the available CO. Besides, because of its high oxidation capacity,9 it also favors the intermediate conversion of HCN and NH3 into NOx. New combustion promoters have been developed to minimize this effect;10 alternative noble metals, such as iridium, osmium, palladium, rhodium, ruthenium, and gold, usually have been studied,5,10,11 but also copper, cobalt, magnesium, or iron oxides supported on titania.12 Moreover, the influence of cerium oxide associated to noble metals has been shown to increase the activity in the CO oxidation reaction under reducing conditions by lowering the total energy of activation and consequently decreasing the CO inhibition effect and the dependence on the oxygen partial pressure.13 The main goal of the present work is to establish a protocol for testing combustion-promoter additives and to evaluate the relative performance of different labmade additives using distinct noble metals Pt, Pd, and Pd-Ce supported on alumina. Experimental Section
* To whom correspondence should be addressed. E-mail:
[email protected]. † NUCAT Programa de Engenharia Quı´mica, COPPE/UFRJ, Centro de Tecnologia. ‡ Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (Cenpes), Pesquisa e Desenvolvimento do Abastecimento.
Platinum Additives Preparation. A γ-Al2O3 support with specific surface of 220 m2/g was prepared at Petrobras R&D Center for this study. The support was impregnated with a solution of hexachloroplatinic acid (H2PtCl6 supplied by Aldrich) in distilled water using an incipient wetness method, in which the volume of
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the added Pt precursor solution was equal to the pore volume in the solid, measured by nitrogen adsorption at -196 °C in the ASAP2002 equipment. The solution containing the amount of required Pt was dripped onto the support under constant kneading. Two nominal Pt contents were established: 300 and 700 ppm. The impregnated materials were dried at 120 °C for 18 h and calcined under air flow (120 mL/min) at 500 °C (heating rate of 5 °C/min) for 2 h. Palladium Additive Preparation. The palladium additives were also prepared by the incipient wetness impregnation method, as described above. Because the cost of palladium is approximately half that of the platinum, the palladium additive was prepared with a nominal content of 600 ppm Pd, which is comparable in cost to the platinum additive of 300 ppm Pt. The impregnation solution was prepared with palladium chloride (PdCl2 supplied by Aldrich) dissolved in distilled and deionized water. These samples were dried and calcined as described above for the Pt additive. Cerium-Palladium Bimetallic Additive Preparation. Cerium was impregnated over the alumina support in three successive steps by incipient wetness method, by using a solution of cerium nitrate (Ce(NO3)3 supplied by Aldrich). The concentration was adjusted to achieve a final content of 15 wt % Ce dry-basis. This sample was dried at 70 °C for 20 h and calcined under air flow (120 mL/min) at 550 °C for 4 h. The pore volume of the Ce/Al2O3 system, determined by titration with water, was 0.20 mL/g. With this value, the required amount of PdCl2 solution was prepared for impregnation (incipient wetness) on the Ce/Al2O3 support to obtain 600 ppm of Pd. The resulting bimetallic system was then dried at 70 °C and calcined at 550 °C (heating rate of 5 °C/min) for 4 h. CO Chemisorption. The metallic dispersion of the different additives was calculated by means of CO chemisorption analyses carried out in an ASAP 2000Q. The samples were previously reduced with pure H2 at 350 °C for 1 h, evacuated for 1 h, cooled to room temperature, and then submitted to pure carbon monoxide atmosphere at RT to obtain the CO isotherms. The metallic dispersion was calculated by the irreversible amount of adsorbed CO, considering linear CO adsorption. The bulk metal content of the additives was determined by X-ray fluorescence (XRF). CO Combustion. The activity of the different CO combustion promoters was determined in an experimental setup equipped with a furnace and a quartz microreactor online with a gas chromatograph (Chrompack CP9001), equipped with a Haysep column and a thermal conductivity detector using He as carrier gas. The column temperature programming was as follows: 30 °C for 9 min, and 200 °C for 20 min, with heating rate of 10 °C/min. Gaseous mixtures of 20% CO in He and 20% O2 in He were used as reactants with flow rates of 80 and 150 mL/min, respectively. Under these conditions, the CO/O2 molar ratio was 1.9 (slightly oxidant condition). The gaseous flow rate was controlled through mass flow meters (MKS Instruments, 247C) and the products were analyzed by GC. The catalyst bed in the reactor was composed of 5 mg of the additive (as such or steam deactivated) diluted and homogenized in 1200 mg of an equilibrium FCC catalyst (e-cat) from the REDUC refinery (Rio de Janeiro). In the cases in which the additives were predeactivated, they were submitted to a lab fixed-bed
Table 1. Metal Content, Surface Area, and Dispersion of Combustion-Promoter Additives sample Pt300 Pt700 Pd600 Pd600Ce15
Pt (wt %)
Pd (wt %)
Ce (wt %)
surface area (m2/g)
dispersion (%)
15.3
218 220 174 176
36.5 32.8 56.0 54.6
0.032 0.075 0.066 0.061
steamer operating at 760 °C for 6 h under 100% steam prior to testing. Initially, the sample was heated under nitrogen flow (60 mL/min) at a heating rate of 10 °C/min, in three sequential steps of temperature: 150, 450, and 620 °C, remaining at each temperature for 30 min. Then the catalyst was cooled to the starting reaction temperature, 200 °C, and the reactants started flowing into the reactor under the desired flow rates. The reaction temperature ranged from 200 to 700 °C, with increments of 25, 50, and 100 °C. The reaction time for each reaction temperature was 15 min. and the products were analyzed by GC. The conversion data were calculated on a carbon basis for each temperature step to establish the light-off (ignition) curve. Results and Discussion Table 1 presents the denomination of the additives, as well as the BET values and the active metal contents. CO chemisorption data show that the additives containing platinum present about 35% of metallic dispersion. For additives containing palladium the relative amount of active sites was higher, close to 55%. These differences might be attributed to differences in the Pt- and Pd-support interactions. The amount of H2 and CO irreversibly adsorbed on Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts, and the CO/H2 ratio, together with the dispersion values, have been previously reported.14,15 The metal oxide addition caused suppression of H2 chemisorption capacity. The CO adsorption stoichiometry changed depending on the nature of the oxide. The CO/ H2 ratio showed a CO stoichiometry between bridged (CO/H2 ) 1) and linear (CO/H2 ) 2) for Pd/Al2O3, whereas for Pd/CeO2/Al2O3 it was 1.06,15 which indeed exhibits a linear bridge adsorption. Infrared measurements on Pd/CeO2/Al2O3 showed adsorption bands at 1971 (bridged) and 2077 cm-1 (linear) in good agreement with the chemisorption value. The results of CO conversion are compared in Figures 1 and 2. It shows a common pattern of the curves with a steep increase in conversion representing the ignition phenomenon. The performance of an additive can be measured by the rate at which it promotes the conversion of CO into CO2. For a given conversion, the additive is more active when the reaction temperature is lower.16 The results of fresh additives (Figure 1) show that with increasing platinum content from 320 to 750 ppm the catalytic activity increases at temperatures below 425 °C, however above this temperature both additives exhibit equivalent conversion level. The additive Pd600, containing 660 ppm of Pd, exhibited the highest activity at temperatures below 450 °C but was equivalent to the platinum-based counterparts at higher temperatures. This remarkable performance, at least comparable to the platinum-based additive with slightly higher metal loading (Pt700), is related to the number of available metallic sites at the surface, as estimated by considering the metal content and the dispersion of the metallic
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Figure 1. CO conversion vs reaction temperature: fresh additives.
Figure 2. CO conversion vs reaction temperature: hydrothermally deactivated additives. Table 2. Reaction Temperatures for Different Isoconversion Values in the CO Oxidation reaction temperature (°C)
conversion (%)
Pt300D
Pt700D
Pd600D
Pd600Ce15D
5 10 15 20 30
405 426 437 442 449
408 418 423 427 434
391 407 414 419 426
406 428 442 449 458
particles (Table 1). Indeed, the mass of additive was approximately constant in the testing reactor, and the Pd600 additive exhibited the highest amount of available active sites for the CO oxidation reaction. On the other hand, the bimetallic additive containing cerium (Pd600Ce15) shows a clear drop in activity compared to that of the monometallic additives, even at high temperatures. The hydrothermally deactivated additives (assigned with a “D” extension in Figure 2) were correspondingly less active by shifting the ignition temperatures to higher values. The same relative performances were observed, as described above, with somewhat better discrimination of the samples. The additive Pd600D showed the best performance, whereas the bimetallic additive Pd600Ce15D showed the worst activity among the deactivated samples. The relative performances can also be seen in Table 2, presenting the reaction temperatures at constant conversion for comparison. The turnover frequency (TOF) was calculated for different reaction temperatures, based on CO conversion and CO chemisorption data. Figure 3 shows that the
Figure 3. Turnover frequency vs reaction temperature: hydrothermally deactivated samples.
Pt300 additive is clearly distinguishable from the others, reaching the highest turnover frequency for all temperatures, whereas the bimetallic Pd600Ce15D had the lowest activity. Pt700 and Pd600 additives showed intermediate behavior with equivalent TOF values. These results evidence that palladium was negatively influenced by cerium in the bimetallic system, although metallic dispersion was not significantly affected. Similar negative effect has been previously observed17 with a 0.5%Pd-2%Ce/alumina catalyst in CO oxidation at temperatures lower than 250 °C. This suggests that the presence of cerium species modifies the nature of the palladium sites, provoking modifications in the Pd-CO bonding, and affecting its performance.18,19 The behavior of Pd/CeO2/Al2O3 catalysts in the oxidation of hydrocarbons has been studied,14 but in principle stabilizes and activates the catalyst for these coupled oxidation systems. Lee et al.20 observed an inhibiting effect in the oxidation of light hydrocarbons, increasing the CO formation of Pd/Al2O3 catalyst in the presence of promoters such as CeO2 and K2O, which induced the reforming reaction (WGS). This was attributed to the enhancing basicity. Bensalem et al.21 have shown that the crystalline structure of CeO2 affected the oxidation reactions, and that ceria is in the reduced state (Ce3+) in the presence of water. XPS results14 evidenced Pd0 after reduction in situ, but after oxidation, it exhibits palladium oxide. According to Burch et al.,22 PdO/Pd0 interfaces are formed, which are the active sites. However, the catalyst containing CeO2 showed the same behavior, and in addition indicated the formation of palladium species in the highest oxidation state, probably PdO2, after the oxidation.14 Besides, there are strong evidences of sinterization. This is not surprising, because during the activation period the oxi-chloride species present on the calcined samples, such as PdxOyClz, facilitate the coalescence of particles, favoring the formation of larger particles. According to the literature,22 the oxidation is favored by larger PdO/Pd0 interfaces and large particles. The dispersion of Pd after reduction decreased but was still comparatively high in the presence of CeO2. Therefore, besides migration of Pd particles over CeO2, there is a migration phenomenon of CeO2 over Pd particles that may result in Pd encapsulation by ceria and/or sintering during reduction. It turns out that large ceria crystallites were formed, thus harming its redox properties as well as its performance in the promotion of CO oxidation reaction.
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Conclusions The use of hydrothermally deactivated samples in the activity tests was more adequate in discriminating additives in the CO oxidation reaction under lightly oxidant condition (CO to O2 ratio of 1.9). The additive containing 660 ppm of palladium (Pd600) showed activity equal to or better than that of the corresponding platinum-based counterparts (320 and 750 ppm Pt). This could be related to its higher amount of active sites per mass of additive. On the other hand, the additive containing 320 ppm of platinum (Pt300) showed the best activity per active site (TOF). The use of 15 wt % cerium oxide in the Pd-based additive did not play the desired role as oxidation promoter. On the contrary, it was less active, which might be attributed to the specific conditions in this work, leading to the formation of inappropriate cerium species. Literature Cited (1) Biswas, J.; Maxwell, I. R. Recent process-related and catalyst-related developments in fluid catalytic cracking. Appl. Catal. 1990, 63, 197. (2) Guisnet, M.; Mignard, S. Fluid catalytic cracking: process, catalyst and chemistry. Actual. Chim. 2000, 2, 14. (3) Chester, A. W. (Mobil). Conversion of carbon monoxide. U.S. Patent 4,181,600, 1980. (4) Kennedy, J. V.; Dight, L. B. (Engelhard). Catalytic cracking. U.S. Patent 4,214,978, 1980. (5) Meguerian, G. H.; Lorntson, J. M.; Vasalos, I. A. (Standard Oil Company). Catalytic cracking with reduced emission of noxious gas. U.S. Patent 4,350,615, 1982. (6) Petty, R. H.; Bartley, B. H. (Texaco). Fluid catalytic cracking catalyst. U.S. Patent 4,414,138, 1983. (7) Avidan, A. A. (Mobil). Circulating fluid bed combustion with CO combustion promoter. U.S. Patent 4,915,37, 1990. (8) Cheng, W. C.; Kim, G.; Peters, A. W.; Zhao, X.; Rajagopalan, K.; Ziebarth, M. S.; Pereira, C. J. Environmental FCC Technology. Catal. Rev.-Sci. Eng. 1998, 40, 39. (9) Yaluris, G.; Peters, A. W. Studying the chemistry of the FCCU regenerator in the laboratory under realistic conditions. In Designing Transportation fuels for a Cleaner Environment; Reynolds, J. G., Khan, M. R., Eds.; Applied Energy Technology Series; Taylor & Francis: Philadelphia, PA, 1999. (10) Guczi, L.; Horvath, D.; Paszti, Z.; Peto, G. Effect of treatments on gold nanoparticles: Relation between morphology, electron structure and catalytic activity in CO oxidation. Catal. Today 2002, 72, 101.
(11) Hodge, N. A.; Kiely, C. J.; Whyman, R.; Siddiqui, M. R. H.; Hutchings, G. J.; Pankhurst, Q. A.; Wagner, F. E.; Rajaram, R. R.; Golunski, S. E. Microstructural comparison of calcined and uncalcined gold/iron-oxide catalysts for low-temperature CO oxidation.Catal. Today 2002, 72, 133. (12) Larsson, P. O.; Andersson, A.; Wallenberg, L. R.; Svensson, B. Combustion of CO and Toluene: characterization of Copper Oxide Supported on Titania and Activity Comparisons with Supported Cobalt, Iron and Manganese Oxides. J. Catal. 1996, 163, 279. (13) Holmgren, A.; Azarnoush, F.; Fridell, E. Influence of pretreatment on the low-temperature activity of Pt/ceria. Appl. Catal. B: Env. 1999, 22, 49. (14) A. L. Guimara˜es; Dieguez, L. C.; Schmal, M. Surface Sites of Pd/CeO2/Al2O3 Catalysts in the Partial Oxidation of Propane. J. Phys. Chem. B 2003, 107 (18), 4311. (15) Noronha, F. B.; Baldanza, M. A. S.; Monteiro, R. S.; Aranda, D. A G.; Ordine A.; Schmal, M. The nature of metal oxide on adsorptive and catalytic properties of Pd/MeOx/Al2O3 catalysts. Appl. Catal. A: Gen. 2001, 210, 275. (16) Paulis, M.; Gandı´a, L. M.; Gil, A.; Sambeth, J.; Odriozola, J. A.; Montes, M. Influence of the surface adsorption-desorption processes on the ignition curves of volatile organic compounds (VOCs) complete oxidation over supported catalysts. Appl. Catal. B: Env. 2000, 26, 37. (17) Tenchev, K. K.; Petrov, L. A.; Savelieva, G. A.; Sass, A. S. Oscillations during the interaction between carbon monoxide and oxygen on palladium-containing catalysts. Appl. Catal. A: Gen. 1992, 83, 31. (18) Monteiro, R. S.; Dieguez, L. C.; Schmal, M. The role of Pd precursors in the oxidation of carbon monoxide over Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts. Catal. Today 2001, 1, 77. (19) Bunluesin, T.; Gorte, R. J.; Graham, G. W. Studies of the water-gas-shift reaction on ceria-supported Pt, Pd, and Rh: implications for oxygen-storage properties. Appl. Catal. B: Env. 2000, 15, 107. (20) Lee, C. H.; Chen, Y. W. Effect of additives on PdAl2O3 for CO and propylene oxidation at oxygen-deficient conditions. Appl. Catal. B: Environ. 1998, 17, 279. (21) Bensalem, A.; Bozon-Verduraz, F.; Delamar, M. Bugli, G. Preparation and characterization of highly dispersed silica-supported ceria. Appl. Catal. A: Gen. 1995, 121, 81. (22) Burch, R.; Crittle, D. J.; Hayes, M. J. C-H bond activation in hydrocarbon oxidation on heterogeneous catalysts. Catal. Today 1999, 47, 229.
Received for review December 8, 2003 Revised manuscript received February 20, 2004 Accepted April 2, 2004 IE034292Z
