The oxidation of carbon monoxide on platinum-supported binary oxide catalysts

Applied Catalysis A: General 192 (2000) 51–55

The oxidation of carbon monoxide on platinum-supported binary oxide catalysts Gülten Gürdaˇg a,∗ , Thomas Hahn b a

Department of Chemical Engineering, Faculty of Engineering, University of Istanbul, 34850 Avcilar, Istanbul, Turkey. b Institut für Chemische Verfahrenstechnik, Universität Karlsruhe, Kaiserstraße 12, D-76128 Karlsruhe, Germany. Received 14 December 1998; received in revised form 26 July 1999; accepted 27 July 1999

Abstract The kinetics of the carbon monoxide oxidation on Pt-supported MeSb2 O6 (Me = Ni, Co and Cu) have been investigated in a recycle reactor at 80◦ C. The catalysts contain 1% Pt (weight)-supported on NiSb2 O6 , CoSb2 O6 and CuSb2 O6 . The reaction rates were measured at different carbon monoxide contents in air from 0 to 1% (vol.). The results were compared with that of Pt/SnO2 /Al2 O3 catalyst which showed a synergistic effect on carbon monoxide oxidation under the same conditions. Metal antimonates, MeSb2 O6 (Me = Ni, Co and Cu) had no catalytic activity for CO oxidation at high and low CO contents under our reaction conditions. The catalytic activity observed on Pt/NiSb2 O6 , Pt/CoSb2 O6 and Pt/CuSb2 O6 catalysts in the zero order reaction region at high CO contents is very low or not measurable compared with that on the Pt/SnO2 /Al2 O3 catalyst. Therefore in the catalytic oxidation of carbon monoxide over Pt supported on different metal antimonates, no or only a low degree synergism is present compared to Pt-supported on SnO2 . ©2000 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide oxidation; Platinum catalysts; Metal antimonates

1. Introduction The catalytic oxidation of carbon monoxide on transition metal surfaces has gained increasing importance in recent years in industrial chemistry and automobile emission control. CO oxidation on platinum group metals also serves as a model catalytic reaction. It has now been shown that the CO oxidation mechanism is of the Langmuir–Hinshelwood-type [1–3]. The reaction takes place between dissociatively adsorbed oxygen and molecularly adsorbed carbon monoxide. CO2 desorbs immediately after formation. In partial oxidation of hydrocarbons and oxidation of CO mixed oxides and supported transition metals ∗

Corresponding author.

as catalyst are often used [4–6]. The results obtained with single phases of oxides containing two or several elements contributed to the development of multiphase oxidation catalysts. In most instances, it was observed that the single metal oxide shows no or lower activity than that of two or more components. The synergism between oxide phases was explained by means of the spillover model by Delmon et al. [7–9]. According to the spillover mechanism, the oxygen species, dissociatively adsorbed, may spillover onto the metal and react there with the chemisorbed carbon monoxide [10–11]. Fattore et al. [12] reported that they have observed higher selectivity in production of acrolein from propene on FeSbO4 and Sb2 O4 than on each phase alone and FeSbO4 and Sb2 O4 play the roles of oxygen donor and oxygen acceptor, respec-

0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 3 3 2 - 4

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G. Gürdaˇg, T. Hahn / Applied Catalysis A: General 192 (2000) 51–55

tively. Kulshreshtha et al. [13] found that a synergistic effect for CO oxidation reaction over Mn2 O3 + SnO2 system. A similar type of synergistic effect was reported by Straguzzi et al. [14] for the Fe2 O3 + Sb2 O5 system where antimony cations play the role of oxygen carrier. They have investigated the selective oxidation of 1-butene and CO over a series of MSbO4 , where M = Fe, Al, Cr, Co and Rh at 400◦ C and found that CoSb2 O6 was unselective for 1-butene oxidation, but had higher activity than the rest of the catalysts investigated for carbon monoxide oxidation. It has now been known that synergism can be observed not only in mixed oxide systems, but also on Pt/SnO2 and Pt/Sb2 O4 catalysts in various degrees [10,11,15,16]. Oxidation reaction takes place on these catalysts under conditions where neither Pt nor SnO2 and Sb2 O4 alone are catalytically active [17,18]. After Lintz et al. [10,11,16] observed the synergistic effect over the Pt/SnO2 catalyst on CO oxidation, they [15] used Pt/Sb2 O4 as a catalyst in the same reaction. But they did not obtain a high enough rate enhancement on this catalyst or synergism between Pt and Sb2 O4 phases, although Sb2 O4 clearly produces oxygen spillover in partial oxidation reactions at higher temperatures. It is known that CoSb2 O6 is an active catalyst at 400◦ C for CO oxidation [14]. This information motivated us to investigate the synergism between Pt and MeSb2 O6 (Me = Ni, Co and Cu) phases by using CO oxidation as a model catalytic reaction over Pt/MeSb2 O6 catalysts and by comparing the results with the values obtained on Pt/SnO2 /Al2 O3 [16] under identical conditions.

2. Experimental 2.1. Catalyst preparation The MeSb2 O6 used for catalyst preparation was provided from BASF in powder form. During the catalyst preparation, NiSb2 O6 and CoSb2 O6 powders were pressed as a cylindrical tablets, without any additives. CuSb2 O6 powder was pressed by addition of 2.5% stearic acid to increase the durability of the tablet. The cylindrical MeSb2 O6 tablets are 6–7 mm in length and 4 mm in diameter. The NiSb2 O6 and CoSb2 O6 tablets were calcined for 1 h in air at 600◦ C

before impregnation with platinum. CuSb2 O6 tablets were calcined for 10 h to decompose the stearic acid under the same conditions. Platinum tetramine hydroxide solution was supplied from Degussa, Hanau was diluted with NH4 OH and impregnation solution contained 23.2 g/l platinum at pH = 10. Platinum content of impregnation solution was determined by a spectrophotometrical method [19]. The tablets calcined at 600◦ C were then subjected to impregnation with a predetermined amount of that solution, and dried for 30 min at 80◦ C. These impregnation-drying stages were repeated until the tablet contained 1% Pt (weight). Then MeSb2 O6 tablets, platinum impregnated and dried at 80◦ C were calcined for 1 h in air at 350◦ C. The catalysts used in this work contained 1% Pt (weight) and were used without further treatment. The Pt-supported metal antimonates were investigated by the X-ray diffraction method. It was determined from a previous X-ray diffraction investigation that under identical conditions of catalyst preparation (drying of platinum impregnation solution at 80◦ C and calcining at 350◦ C ), decomposition to metallic platinum occurred [20]. But, there was no difference between X-ray diffraction patterns of MeSb2 O6 powder and Pt/MeSb2 O6 catalysts prepared in this work, probably due to their low platinum content.

2.2. Apparatus and kinetic measurements Reaction rate measurements were carried out in a tubular glass reactor (30 cm in length and 2.4 cm in diameter) which is heated electrically. A membrane pump was used to obtain gradientless reactor with recycle ratio of 25. The recycle reactor contained a frit layer and during the kinetic measurements, 1 g of the catalyst was placed upon this frit. Temperature was measured by a thermoelement (Chromel-Alumel) placed just above the catalyst tablets on frit layer of the reactor (Fig. 1). The exit of the system was open to the atmosphere. The impurities such as ferrous pentacarbonyl compounds in CO were decomposed by heating them at 300◦ C in a carbonyl reactor. Constant gas flow was maintained by mass flow controller (Brooks-Rosemount) for each gas component (O2 , CO and diluent gas, N2 ). During the reaction rate measurements, the gas mixture was fed to the gra-

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method [10,15] with diluted reactants (1 vol% CO or 20 vol% O2 in N2 , respectively) at 80◦ C.

3. Results and discussion

Fig. 1. Flow diagram of the carbon monoxide oxidation system.

dientless reactor or to analytical equipments. The CO and CO2 concentrations were measured by using two non-dispersive IR spectrometers (Ultramat 1, Siemens and Uras 2T, Hartmann and Braun, respectively). For the purpose of oxygen concentration measurements, a mechano-magnetic device (Magnos 6G, Hartmann and Braun) was used. The total gas flow rate (180 ml/min) was measured at the system outlet by a soap bubble gas flow meter. For the determination of catalytic activity in the oxidation of carbon monoxide, the following mass specific reaction rate statement (Eq. (1)) was used. rm =

1 dξ mcat dt

(1)

where mcat is the total catalyst mass and ␰ is the extent of reaction. The reaction rate measurements were made at 80◦ C. While the mole fraction of O2 in each feed gas mixture was kept constant at a 20 vol%, the CO mole fraction was varied from 0 to 1 vol%, with N2 for the balance of the feed. The sorption capacities of catalysts related to carbon monoxide and oxygen were determined by a titration

It is known that [11,15,16] two different kinetic regime regions can be observed during the oxidation of carbon monoxide. At low CO contents (in the first region), the oxidation reaction is first order with respect to carbon monoxide and practically temperature-independent. At high CO contents (second region), the reaction rate is zero order with respect to carbon monoxide and temperature-dependent. Synergism can be observed for the second region (zero-order reaction region) depending on the oxide component of the catalyst. Oscillations between two kinetic regime regions have also been previously observed. The oxygen and carbon monoxide sorption capacities of the catalysts prepared and used in this work and of the Pt/SnO2 Al2 O3 catalyst used again with excess oxygen at the same temperature in a previous work [16] are given in Table 1. In this work [16], Grass and Lintz published that neither SnO2 nor Pt alone were catalytically active at high CO concentrations (zero-order reaction region with respect to CO; viz. second region) and the CO oxidation reaction over Pt/Al2 O3 catalyst is first order with respect to CO at low CO concentrations (first region). As was mentioned above, they found synergism between Pt and SnO2 on CO oxidation between Pt and SnO2 over the Pt/Al2 O3 catalyst. As can be seen from Table 1, the ratio of the sorption capacity, nad,O2 /nad,CO , on Pt-supported metal antimonates is approximately 1, while this ratio is one

Table 1 The sorption capacities of the catalysts Catalyst

Pt/NiSb2 O6 Pt/CoSb2 O6 Pt/CuSb2 O6 Pt/SnO2 //Al2 O3 a

0.98% Pt, 25.0% SnO2 .

Platinum content (weight %)

1 1 1 0.98a

Sorption capacity (mol/g)a 10−5 nad,CO

nad,O2

0.61 0.43 0.20 2.62

0.67 0.40 0.16 13.51

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G. Gürdaˇg, T. Hahn / Applied Catalysis A: General 192 (2000) 51–55

Fig. 2. Reaction rate of CO oxidation on Pt/NiSb2 O6 and Pt/SnO2 /Al2 O3 catalysts as a function of CO concentration.

Fig. 4. Reaction rate of CO oxidation on Pt/CuSb2 O6 and Pt/SnO2 /Al2 O3 catalysts as a function of CO concentration.

magnitude higher on Pt/SnO2 /Al2 O3 . This means that after all the Pt surface of the Pt/SnO2 /Al2 O3 catalyst is covered with CO at high CO contents, there are sorption sites on the SnO2 surface for the dissociative adsorption of oxygen, but unfortunately not so much on Pt/MeSb2 O6 catalysts. The reaction rates as a function of carbon monoxide content in gas phase in the recycle reactor on Pt-supported metal antimonates in comparison to those [16] on Pt/SnO2 /Al2 O3 are shown in Figs. 2–4. Metal antimonates, MeSb2 O6 (Me = Ni, Co and Cu), had no catalytic activity for CO oxidation at low and high CO contents under our reaction conditions. The results show that, for high CO concentrations, the reaction rates are significantly lower on Pt/NiSb2 O6 and Pt/CoSb2 O6 catalysts than on the Pt/SnO2 /Al2 O3 catalyst, indicating very low synergism. Lintz et al. [11,15,16] reported that at high CO contents (zero-order reaction region with respect to

CO), oxidation of CO occurs at the phase boundary of metal and oxide components of the catalyst. Oxygen adsorbed on oxide phase, migrating to phase boundary reacts with CO adsorbed on Pt. Since the rate of the reaction also increases with the magnitude of the boundary, it may be assumed that for the catalysts prepared in this work the phase boundary is small or oxygen migration in oxide phase of NiSb2 O6 and CoSb2 O6 is low. Fig. 2 also shows oscillations between two kinetic regimes of zero and first order as CO concentration increases. The maximum and minimum reaction rates for the same feed CO concentration are a function of CO concentration in the recycle reactor. As is seen on Fig. 4, Pt/CuSb2 O6 showed no activity in the region of zero order with respect to carbon monoxide indicating that no synergism exists between Pt and CuSb2 O6 for this catalyst under our working conditions.

4. Conclusion

Fig. 3. Reaction rate of CO oxidation on Pt/CoSb2 O6 and Pt/SnO2 /Al2 O3 catalysts as a function of CO concentration.

It can be deduced from Table 1 and Figs. 2–4 that the activities of platinum-supported metal antimonates is proportional to sorption capacities of carbon monoxide and oxygen of these catalysts. The low oxygen and carbon monoxide sorption capacity values of Pt/NiSb2 O6 , Pt/CoSb2 O6 and Pt/CuSb2 O6 combined with small or no activity values of these catalysts indicate that there is no synergism between Pt and MeSb2 O6 phases. But a rate enhancement with these catalysts, especially with Pt/NiSb2 O6 by changing the conditions of the

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