Interaction of NO and CO on RhâSiO2 and CeâRhâSiO2 Catalysts

Chapter 12

Interaction of NO and CO on Rh-SiO Ce-Rh-SiO Catalysts

and

2

2

A Transient In Situ IR Spectroscopic Study

Downloaded by UNIV OF QUEENSLAND on October 7, 2015 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch012

Girish Srinivas, Steven S. C. Chuang, and Santanu Debnath Department of ChemicalEngineering,The University of Akron, Akron, OH 44325-3906

In situ infrared spectroscopic studies of NO-CO reaction on Rh/SiO and Ce-Rh/SiO reveal that Si-NCO and Rh-NCO are the dominant species during the reaction at 723 K. The growth rate of Rh-NCO is greater than that of Si-NCO on the Ce-Rh/SiO catalyst at the beginning of the reaction. In contrast, the concentration of Rh-NCO and Si-NCO increases at about the same rate on the prereduced and preoxidized Rh/SiO . Steady-state isotopic transient study at 573 Κ shows that Si-NCO is a spectator species under reaction conditions; the addition of Ce increases the reactivity and decreases the average residence time of intermediates leading to CO over Rh/SiO . 2

2

2

2

2

2

Rh has been successfully used in three-way catalysts to control the emission of NO from automobile exhaust (1-4). Due to the shortage of Rh and the increasingly stringent standards for NO emission, continuing improvement of the catalyst performance and development of substituents remain to be major challenges in the area of environmental catalysis. Investigation of adsorbed species on the Rh catalyst during NO-CO reaction could lead to a better understanding of how Rh catalyzes the reaction and help to improve catalyst performance. Infrared (IR) studies have shown that CO may chemisorb on Rh in the forms of linear CO and bridged CO on the reduced Rh site as well as linear CO and gemdicarbonyl on the Rh site (5-10). The mode of adsorption and stability of the adsorbed CO depend on the surface state and the chemical environment of the catalyst. Adsorption of NO on metals displays three modes of adsorbed NO: a cationic Rh-NO species, a neutral Rh-NO species, and an anionic Rh-NO" species (10-13). In situ infrared studies reveal that adsorbed NO is the dominant species on the Rh surface with a small coverage of NCO and adsorbed CO during NO-CO reaction over Rh/Si02 at 463-523 Κ and NO conversion below 50% (14,15). Increasing NO conversion increases the coverage of adsorbed CO and NCO. Cerium has been an important promoter in three-way catalysts (16-20). The major role of Ce has been identified to be (a) storage of oxygen, (b) stabilization of AI2O3 and metals, (c) promotion of the water-gas shift reaction, (d) suppression of N2O formation and (e) modification of the kinetics (rate law) of the reaction. It remains x

x

+

+

0097-6156/94/0552-0157$08.00/0 © 1994 American Chemical Society In Environmental Catalysis; Armor, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

158

ENVIRONMENTAL CATALYSIS


unclear how Ce modifies the intrinsic kinetics, i.e., the intrinsic rate constant k, and the surface coverage of intermediates Θ, resulting in an increase in the overall reaction rate of the NO-CO reaction, k (the reactivity of intermediates) is the reciprocal of τ ( the residence time of intermediates) if the reaction of intermediates leading to gaseous product is a first-order irreversible reaction (27). Determination of k and Θ requires the use of steady-state isotopic transient technique. Steady-state isotopic pulse technique incorporated with in situ IR has been developed to investigate the effect of Ce on the dynamic behavior of adsorbed species and the residence time of intermediates leading to CO2 during the CO-NO reaction. This study provides an insight into the catalysis of NO-CO reaction under reaction conditions. Experimental The Rh/Si02 catalyst (5 wt% Rh) was prepared by incipient wetness impregnation of RhCl3-3H20 (Alfa) onto a silica support (Strem); the Rh-Ce/Si02 catalyst (atomic ratio of Rh:Ce = 1:1 containing 5 wt% Rh) was prepared by co-impregnating a solution of RhCl3-3H20 (Alfa) and Ce(N03)3*6H20 (Alfa) onto the silica support (Strem). The catalysts were dried in air overnight followed by reduction in flowing H2 at 673 Κ for 8 hours. During pulse CO chemisorption at 303 K, the Rh/Si02 catalyst chemisorbed 17.4 μ mol CO/gm catalyst; and the Ce-Rh/Si02 catalyst chemisorbed 28.5 μ mol CO/gm catalyst. An in situ infrared reactor cell capable of operating upto 873 Κ and 6 MPa was used for the NO-CO reaction. The catalysts were pressed into disks and placed in the reactor cell. The catalysts were further prereduced or preoxidized at 573 Κ before the reaction study. The reaction mixture consisted of CO/Ar (Commercial grade), NO (UHP) and He (UHP) controlled by mass flow meters. An inlet system was designed to introduce an abrupt change in the concentration of CO in the form of a C O pulse or step changefrom CO to C O (27). Since the C O has the same chemical properties as CO, a change in the concentration from C O to C O would not affect the environment of the catalyst surface and the total concentration of all CO species. The C O contained 2% Ar, an inert gas, which was used to determine the effect of flow pattern through the reactor and transportation lines on the transient response. The change in concentration of adsorbed species was monitored by an FT-IR spectrometer with a resolution of 4 cm . The effluent from the reactor was monitored continuously using a Balzers QMG 112 quadrupole mass spectrometer. Steady-state IR spectra were recorded using 32 co-added scans and transient spectra, using 3 co-added scans. 13

l2

13

13

12

12

13

12

-1

Results and Discussion Steady-State NO-CO Reaction on Prereduced and Preoxidized Rh/Si02Figure 1 shows the IR spectra of the prereduced and preoxidized Rh/Si02 catalysts during NO-CO reaction at 723 Κ with a CO:NO = 5:1 ratio (CO: 12.5 cm /min, NO: 2.5 cm /min and He: 28 cm /min). The prereduced catalyst was reduced in flowing hydrogen at 573 Κ for 1 hour and the preoxidized catalyst was oxidized in flowing air at 573 Κ for 1 hour. Both the prereduced and preoxidized catalysts achieved nearly total conversion of NO at 723 K. No NO2 was detected and the amount of CO2, N2 and N 2 O could not be quantified due to overlapping of signals in the mass spectrometer. The spectra for the prereduced Rh show a strong band at 2288 cm" , attributed to an isocyanate species (NCO) adsorbed on the S1O2 support (11-15). A shoulder band at 2173 cm is assigned to an isocyanate species (NCO) on the Rh metal (14,15,22) and a shoulder band at 2358 cnr is due to CO2. A weak band appearing at 2003 cm is due 3

3

3

1

-1

1

In Environmental Catalysis; Armor, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

-1


Figure 1. IR spectra of steady-state NO-CO reaction on prereduced Rh/Si02 and preoxidised Rh/Si02 catalysts.



160


to linear CO on the reduced Rh surface. The intensity of linear CO decreased with reaction time. In contrast, the intensity of NCO species on Rh and S1O2 grew with reaction time, regardless of the reaction temperature. The NCO species remained on the catalyst surface following a long period offlowingHe. The behavior of NCO observed here at 723 Κ is consistent with previous studies at 463-523 Κ (14,15) showing that Si-NCO is very stable in the presence and absence of reactants. The preoxidized catalyst exhibited similar IR spectral features with a low intensity of the isocyanate species. No chemisorbed NO species were observed in the case of either prereduced or preoxidized Rh. Linear CO is the only adsorbed CO present on the surface of both prereduced and preoxidised catalysts at 723 K. The presence of linear CO could be due to the use of a high ratio of CO to NO. The excess CO chemisorbs as linear CO while most of the NO was rapidly converted to products. Hecker and Bell reported that preoxidised Rh is more active for the NO-CO reaction than prereduced Rh catalyst and both catalysts exhibit similar IR spectra after a long break-in period (14). The slow growth of NCO on the preoxidised Rh as compared to that on the prereduced Rh could be due to the high activity of the preoxidized Rh which is more effective than prereduced Rh in removing adsorbed NO and CO before they could be converted to NCO. Steady-State NO-CO Reaction on Ce-Rh/Si02. Figure 2(A) shows the IR spectra of the steady-state reaction on Ce-Rh/Si02 catalyst carried out under the same condition as that of prereduced and preoxidized Rh/Si02 catalysts. Nearly complete conversion of NO was observed. The spectra show prominent bands due to isocyanate species at 2288 and 2178 cm" , attributed to those chemisorbed on S1O2 and on the Rh metal, respectively. The development of these bands is slower and the Rh-NCO band observed at 27 min is more prominent than that observed on prereduced or preoxidized Rh/Si02. The intensity of Rh-NCO, which grows at a higher rate than that of NCO on the support, reached steady-state after 15 min of reaction. The intensity of Si-NCO continued to increase during the reaction. In order to investigate the effect of CO lean and NO lean atmospheres on the CeRh/Si02 catalyst performance, the catalyst was subjected to an abrupt stoppage of CO or NO flow. Figure 2(B) shows the IR spectral features observed following abruptly stopping either CO or NO flow to the reactor at 723 K. Following 11 min of steadystate reaction where Rh-NCO and linear CO slowly approach steady-state, the NO flow was terminated abruptly. Figures 3 (A) and (B) show the mass spectroscopic (MS) analysis of the effluents during the transient experiments. Shutting NO flow to the reactor resulted in the decrease in the CO2 IR band while the remaining bands showed no variation except the slight increase in the linear CO band. The effluents from the reactor indicated a decrease in the CO2 concentration following the stoppage. The IR spectra, following stopping CO flow to the reactor, showed the following: (i) an almost total disappearance of the Rh-NCO band at 2178 cnr (ii) a decrease in the Si-NCO band at 2288 cm , and (in) the disappearance of the linear CO band at 2009 cm . The disappearance of the chemisorbed CO band is accompanied by an enhancement of the Rh(NO) band at 1900 cm" while the CO2 band at 2358 cm" remained present. Stopping CO flow also resulted in a momentary and abrupt enhancement followed by a decrease of the gaseous CO2 concentration in the effluent. This suggests that gaseous CO is an inhibitor for the formation of CO2. Similar observations on the negative order dependence of the reaction rate with respect to the partial pressure of CO on Ce-Rh/Al203 have been reported (18). 1

1

-1

+

-1

1

1

Steady-State Isotopic Transient Study. Transient isotopic studies were performed on Rh/Si02 and Ce-Rh/Si02 catalysts at 573 Κ under near complete conversion of NO. The studies were undertaken by pulsing 10 cm of C O into a 3

13



SRINIVAS ET AL.

NO and CO on Rh-Si0 and Ce-Rh-Si0 2

I

1

2400

2190

1

1

2

1

1—

1980 1770 1560 WAVENUMBER (cm )

1350

-1

2037 1

2400

2190

1

1 min 1

1

1

1980 1770 1560 WAVENUMBER (cm" )

1350

1

Figure 2. (A) IR spectra of steady-state NO-CO reaction on Ce-Rh/Si02 (B) IR spectra following transient of stopping NO flow and CO flow on Ce-Rh/Si02.



162

A

m/e = 28 m/e = 44


CO/NO/He

CO/He

^X^5

Β CO/NO/He

m/e = 28 - - - m/e = 30 m/e = 44

NO/He * . X 1/2

1 I

0

1

1

1

1 «

1 1

r

1

1

2 3 Time (min)

4

5

Figure 3. M S . analysis of the transient on Ce-Rh/Si02 (A) during stopping of NO flow (B) during stopping of CO flow


12. SRINIVAS ET AL.

NO and CO on Rh-Si0 and Ce-Rh-Si0 2

163

2

steady-state flow of CO/Ar-NO-He. The ratio of CO:NO was maintained at 1:1 during the transient study. Figure 4 shows the normalized MS analysis of the transient pulse on Rh/SiC>2. Figures 5 (A) and (B) show the IR spectra and difference spectra during the transient isotopic study on Rh/Si02. Steady-state IR spectra before the pulse shows the following bands: (i) a prominent Si-NCO band at 2295 cm* , and a weak Rh-NCO band at 2160 cm" and, (ii) a prominent CO2 band at 2358 cm" and a weak Rh-NO" band at 1683 cm . Pulsing C O into a CO/Ar flow did not shift the intensity and wavenumber of the Si-NCO band indicating that this species is a spectator species during the formation of CO2. A downward shift of CO2 and Rh-NCO bands immediately, followed by an upward shift back to the original wavenumber can be clearly observed in the difference spectra obtained by subtracting the spectrum taken before the pulse from each transient spectra. The turnover frequency for CO2 formation (TOF) can be expressed in terms of intrinsic rate constant (k) and coverage of intermediate species (0); 1

1

1


-1

13

12

TOF = k*0 = (l/x)*0, where τ is the residence time of the intermediate (2123). Since there was almost total conversion of NO, the TOF determined does not represent the intrinsic activity of the catalyst and the coverage, 0, was not determined. Preliminary study shows that the coverage can be determined at lower temperatures where the conversion is lower; however, the lower temperature may result in a different surface chemistry of the catalysts. The residence times of the carbon-containing species leading to CO2 formation for Rh/Si02 has been calculated from the normalized curves shown in Figure 4, taking into account the residence time of the inert Ar representing the inherent delay due to the reactor and transportation lines (21). Figure 6 shows the MS analysis during the isotopic transient experiment on Ce-Rh/Si02. Symmetry in C O and C O responses indicated that there was no perturbation to the total concentration of gas phase CO (including both C O and CO). The response of the C O and C 0 2 curves indicate that the formation of C02 is faster than the desorption of C 0 . The residence times for C 0 desorption and intermediates leading to C 0 2 formation are smaller than those obtained for Rh/Si02. Figures 7 (A) and (B) show the IR spectra and difference spectra during the isotopic pulse on Ce-Rh/Si02. Steady-state spectra before the pulse show the CO2 band at 2358 cm" , Si-NCO band at 2309 cm" , Rh-NO band at 1913 cm" Rh-NO" band at 1691 cm" and N2O bands at 2241 and 2205 cm" . Introducing a C O pulse into CO/Ar flow resulted in a downward shift of CO2 bands, immediately followed by an upward shift to their original wavenumbers. There was, however, no change in the intensity and wavenumber of the Si-NCO band, indicating that it was a spectator species. Rh-NCO did not exhibit a distinct IR band at 573 K. PreUrninary study at 523 Κ shows that the rate of displacement of Rh-N CO is slower than that of formation of C02. This result suggests that Rh-NCO is also a spectator species, consistent with Hecker and Bell's suggestion (75). Oh has reported (18) that on Ce-Rh/Al203, catalysts containing 0.5 wt% Ce exhibit essentially the same activity as RI1/AI2O3, but catalysts containing 2 wt% or more of Ce show a significandy higher activity than RIÎ/AI2O3. The catalyst used in this study has a Ce loading of 6.8 wt% and exhibits a significant increase in the mtrinsic rate constant as compared to Rh/Si02, agreeing well with reported high activity of CeRI1/AI2O3fromthe study undertaken by Oh (18). 12

13

1 2

13

13

13

13

13

1 3

13

1

1

1

+

1

1

13

12

13

13




164

Figure 4. MS analysis of the transient during isotopic pulse over prereduced Rh/Si02.


12.

NO and CO on Rh-Si0 and Ce-Rh-Si0

SRINIVAS ET AL.

2

165

2

Λ2295 0.31

0.24 min 0.28***"ty

PQ « Ο


%

3.03 **vyijfl^**vsMnM^^ f

After Pulse 2700

2375 2050 1725 WAVENUMBER {cm }

1400

2700

2375 2050 1725 WAVENUMBER (cm"}

1

1400

1

Figure 5. (A) IR spectra during isotopic pulse over prereduced Rh/SiC)2(B) Difference spectra of Figure 5 (A). ι u

0.8

-

— co ---• co

0.6

-

τ,, = 0.09 min

u

\

0.4

2

τ „ =0.05 min

C 0

0.2

\2

1

0 -0.2

1

1

1

0.2 0 -0.2 -0.4

f: /: /"'

m/e = 28 m/e = 44 Ar

-0.6 -0.8 -1 Time (min)

Figure 6. MS analysis of the transient during isotopic pulse over Ce-Rh/Si02.



166

ENVIRONMENTAL

3.94

CATALYSIS

— ~ Λ ~

After Pulse ι

2700

ι

r

2375 2050 1725 WAVENUMBER {cm } 1

1

1400

2700

ι

ι

2375 2050 1725 WAVENUMBER {cm }

1400

1

Figure 7. (A) IR spectra during isotopic pulse over prereduced Rh/Si02(B) Difference spectra of Figure 7 (A).

Conclusions Steady-state isotopic pulse technique has been developed to investigate the kinetics of the NO-CO reaction under reaction conditions. The major chemisorbed species observed during the NO-CO reaction at 723 Κ on Rh/Si02 and Ce-Rh/Si02 is an NCO species on S1O2. The Ce-Rh/Si02 catalyst also showed a higher intrinsic rate constant for C O 2 formation and a lower residence time of intermediates leading to C O 2 than those for Rh/Si02. Combined in situ IR and transient isotopic studies on Rh/Si02 and Ce-Rh/Si02 have shown that the NCO is a spectator species during the NO-CO reaction.

Literature Cited 1. Taylor, K. C. In Catalysis Science and Technology; Boudart, M.; Anderson, J. R., Eds.; Springer Verlag: New York, NY, 1984, Vol. 5;p119. 2. Cooper, B. J.; Evans, W. D. J.; Harrison, B. In Catalysis and Automotive PollutionControl;Crucq, Α.; Frennet, Α., Eds.; Elsevier: New York, NY, 1987, Vol. 30;p117. 3. Funabiki, M.; Yamada, T.; Kayano, K. Catalysis Today. 1981, 10, 33. 4. Farrauto, R. J.; Heck, R. M.; Speronello, Β. K. Chem. and Eng. News. 1992, Sept. 7, 34. 5. Yates, Y. T.; Duncan, T. M.; Vaughn, R. W. J. Chem. Phys. 1979, 71, 3908. 6. Worley, S. D.; Rice, C. Α.; Mattson, G. Α.; Curtis, C.W.; Guinn, J. Α.; Tarrer, A. A. J. Phys. Chem. 1982, 86, 2714. 7. Solymosi, F.; Pasztor, M. J. Phys. Chem. 1985, 89, 4789. 8. Dictor, R. J. Catal. 1988, 109, 89. 9. Chuang, S. S. C.; Pien, S.I. J. Catal. 1992, 755, 618.


12. SRINIVAS ET AL. NO and CO on Rh-Si0 and Ce-Rh-Si0 2

10. 11. 12. 13. 14. 15. 16


17. 18. 19. 20. 21. 22. 23.

2

167

Chuang, S. S. C.; Pien, S.L J. Catal. 1992, 138, 536. Arai, H.; Tominaga, H. J. Catal. 1976, 43, 131. Solymosi, F.; Bansagi, T.; Novak, E. J. Catal. 1983, 112, 83. Solymosi, R; Novak, E. J. Catal. 1990, 125, 112. Hecker, W. C.; Bell, A. T. J. Catal. 1983, 84, 200. Hecker, W. C.; Bell, A. T. J. Catal. 1984, 85, 389. Gandhi, H. S.; Piken, A. G.; Shelef, M.; Delosh, R. G. SAE . 1976 Paper No. 760201. Hindin, S. G. U.S. Patent 3, 870, 455 1973. Oh, S. J. Catal. 1990, 124, 477. Nakamura, R.; Nakai, S.; Sugiyama, K.; Echigoya, E. Bull. Chem. Soc. Japan. 1981, 54, 1950. Fisher, G. B.; Theis, J. R.; Casarella, M . V.; Mahan, S. T. SAE. 1993 Paper No. 931034. Srinivas, G.; Chuang, S. S. C.; Balakos, M. W. AIChE Journal. 1993, 39, 530. Rasko, J.; Solymosi, F. J. Catal. 1981, 71, 219. Biloen, P. J. Mol. Catal. 1983, 21, 17.

RECEIVED

September 29, 1993


Interaction of NO and CO on RhâSiO2 and CeâRhâSiO2 Catalysts

Recommend Documents

Interaction of NO and CO on RhâSiO2 and CeâRhâSiO2 Catalysts