J. Phys. Chem. 1995,99, 16727-16735
16727
Pulse Reaction Studies of Transient Nature of Adsorbates during NO-CO Reaction over Rh/SiOz Raja Krishnamurthy and Steven S. C. Chuang* Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325-3906 Received: June 13, 1995; In Final Form: August 28, 1995@
The transient nature of adsorbates for the reaction of NO with CO over a 4 wt % Rh/Si02 catalyst has been studied by in situ infrared spectroscopy combined with pulse transient techniques. Dynamic behavior of infrared-observable Rh-NO-, Rh-NO+, Rh+(CO)2, and linear and bridged C O reveals that dissociation of h'; Rh' chemisorbs CO and NO as Rh+(C0)2 and Rh-NO', respectively; and Rh-NO- oxidizes Rho to R Rh+(CO)* can either undergo reductive agglomeration or react with adsorbed oxygen to produce C02 at 473-523 K. At 573 K, Rh is in the reduced state where linear C O and bridged CO react with the oxygen produced from dissociation of Rh-NO- to produce C 0 2 . The catalyst in the reduced surface state is more active for the formation of N20 and C02 products, and the response of N20 formation leads that of COZ formation at temperatures below the light-off temperature. The rapid C02 response during the pulse reaction studies indicates that the reaction can quickly reach steady state at temperatures above 473 K when the reaction conditions are altered.
Introduction The reaction of NO with CO (NO-CO reaction) over Rh catalysts has been extensively studied for more than two decades due to its importance in the control of automotive exhaust emissions.'-'* Many studies have suggested that C02 is formed by the reaction of adsorbed CO with adsorbed oxygen formed by the dissociation of adsorbed N0.6-'6 N2 is formed by the combination of adsorbed nitrogen. However, little work has been done to identify the type of adsorbed NO and CO involved in the formation of C02 and N2 over supported Rh catalysts. Investigation of the transient nature and reactivities of various modes of adsorbed CO and NO may provide insight into the nature of active sites and the surface state of the catalyst during the NO-CO reaction since the modes of adsorbed CO and NO are closely related to the nature of adsorption ~ i t e s . ~ - ' ' - ~ ~ The transient nature and reactivity of adsorbates may be studied by steady-state isotopic or pulse transient coupled with in situ infrared (IR) methods which allow the simultaneous measurement of variation in concentration of IR-observable adsorbates and gaseous reactants and p r o d u ~ t s . ~ 'Our , ~ ~earlier steady-state step I3CO transient studies have revealed that the residence time of intermediates for C02 formation decreases with increase in temperature, and a rapid exchange between the gaseous CO and adsorbed CO occurs during NO-CO reaction on the supported Rh catalyst.29 Due to the rapid exchange between the gaseous CO and various forms of adsorbed CO, steady-state isotopic transient studies cannot be used to determine the transient nature of adsorbed CO toward formation of c02. The pulse transient technique, which consists of the sudden introduction of one reactant into another reactant, may be used to determine the nature of various adsorbates under transient conditions. Information on the transient nature of adsorbates could reveal not only the relative reactivity of various adsorbates but also the change in behavior of the adsorbates and catalyst surface due to variation in reactant concentration. The latter provides the essential information for a better understanding of the reaction kinetics under transient conditions which often occur in catalytic converters.
* To whom all correspondence @
should be addressed. Abstract published in Aduunce ACS Abstrucrs, October 15, 1995.
In the catalytic converter, Rh metal or Rh oxide is in a complex environment in which ceria, NiO, Si02, and various additives are present in the )'-A1203 w a s h ~ o a t . " . ' * . ~ The ~-~~ function of Si02 is to reduce the subsurface diffusion of Rh metal into the y-AlzO3, which can result in the loss of active Rh surface sites.34-36 Due to the inertness of Si02 support and lack of metal-support interaction, Rh/Si02 has been studied extensively for NO-CO reaction.6,' 1.29,34.37.38 However, the transient behavior of adsorbed CO and NO remains far from being understood. This paper reports the results of the study of the transient nature of IR-observable adsorbates on the Rh/ Si02 catalyst during pulse condition. The results of this study provide further understanding of the transient kinetics and mechanism of the NO-CO reaction on Rh catalyst.
Experimental Section Catalyst Preparation and Characterization. The RWSiO:! catalyst containing 4 wt % Rh was prepared by incipient wetness impregnation of large pore silica support (Strem, 300 m2/g)using RhC13.3H20 (Alfa Chemicals) solution. The ratio of the solution to the weight of support material used is 1 cm3 to 1 g. After impregnation, the catalyst was dried overnight in air at 303 K and then reduced in flowing hydrogen at 673 K for 8 h. Pulse CO chemisorption studies at 303 K showed that the reduced Rh/SiOz catalyst chemisorbed 55.3 ymol CO/g of catalyst, corresponding to a crystallite size of 63 A.38 An in situ infrared (IR) reactor cell capable of operating upto 873 K and 6 MPa, equipped with CaF2 windows, was used for the pulse reaction studies.28 The catalyst was pressed into a self-supporting disk (10 mg) and placed in the reactor cell and further reduced in situ at 673 K for 2 h before the reaction study. The reactant gases used were CO (Linde, Commercial Grade), NO, and He (Linde, UHP); the gas flows were controlled by mass flow meters. The effluent of the IR reactor cell was monitored continuously using a Balzers QMG 112 quadrupole mass spectrometer (MS). NO Pulse in CO/He Flow. The catalyst disk in the IR cell was reduced in flowing hydrogen at 673 K for 1 h and cooled to the desired reaction temperature. The catalyst surface was exposed to a CO/He flow that resulted from a combination of a 10 cm3/minCO flow and a 30 cm3/min helium flow. Helium,
0022-3654/95/2099-16727$09.00/0 0 1995 American Chemical Society
Krishnamurthy and Chuang
16728 J. Phys. Chem., Vol. 99, No. 45, I995 an inert gas, was used to keep a low concentration of the ionized species in the MS vacuum chamber for high sensitivity to reaction products. Helium (He) also served as a carrier for introducing a 10 cm3 pulse of NO to the IR cell. The change in concentration of the adsorbates on the catalyst surface was monitored by a Fourier transform infrared (FTIR) spectrometer interfaced with an IBM PC. Four scans, collected at the rate of 1 scads, were coadded to obtain the change in concentration of the adsorbates under transient conditions. The total time between each coadded spectrum, however, was approximately 10 s due to the additional time needed to store the data on the IBM PC. The gaseous responses of the effluents during the pulse were monitored by the MS. The species monitored by the MS were mle = 22 for C02, mle = 30 for NO, mle = 28 for CO, mle = 44 for C02 and N20, and mle = 46 for N02. The secondary ionization of COZ at mle = 22 was used to separate the C02 and N20 components of the mle = 44 peak obtained during the NO-CO reaction. The ratio of the mle = 22 peak to the mle = 44 peak for C02 was obtained from separate calibration studies by injection of known amount of CO2 into helium carrier. The IR and MS responses were collected simultaneously in real time. The gaseous species in the IR reactor required 10 s to travel through the transportation lines from the reactor to the MS ionization chamber, resulting in a delay of 10 s in the MS response in comparison to the IR spectra. The MS response curves represent the change in concentration of each species monitored as a function of time. The mle intensity, I, for each species can be converted to the rate of elution by a responding factor. The responding factor, Cal,, for each species is determined by calibration runs using the following equation:28
a
Wavenumber(cm-')
b
I " '
I
I
3 c
.C
mie=22 X20 I1
where Cal, is the responding factor for species i, NO is the number of moles of the species injected in the calibration run, and I(t) is the intensity of mle species. J bI(t) dt is obtained from the integration of the area under the response curve resulting from the pulse injection during the calibration run. The responding factor is a function of ionizing current, vacuum in the ionization chamber, and the pumping speed and opening of the gas inlet system. The ratio of responding factors for the two different species is independent of the operating conditions, which hence requires only a calibration pulse for one of the ratioed species for an experimental run. CO Pulse in NOMe Flow. The catalyst surface was exposed to a 40 cm3/min NO/He flow by the combination of a 10 cm3/ min NO flow and a 30 cm3/min helium flow after the NO pulse in the CO/He experiment. The CO pulse in the NOMe flow study was carried out by pulsing 10 cm3 of CO into the 30 cm3/min flow of He to the IR cell while maintaining a steadystate flow of NO at 10 cm3/min.
Results NO Pulse in COEIe Flow. Figure l a shows the IR spectra obtained during the first NO pulse into a steady-state flow of CO/He over Rh/Si02 catalyst at 473 K. The initial spectrum obtained on flowing CO/He over the catalyst surface showed a linear CO band at 2056 cm-] and weak gaseous C02 bands at 2361 and 2330 cm-I. On pulsing 10 cm3 of NO into the COl He stream, gaseous NO was found to enter the reactor about 33 s after the pulse and completely leave the reactor at approximately 76 s as indicated by the emergence and diminishing of gaseous NO bands at 1916 and 1845 cm-l. Thirty-three seconds is the time required for the 10 cm3 pulse of NO to
I
Time (sec)
Figure 1. (a) Transient IR spectra of first NO pulse in CO/He flow at 473 K. (b) MS analysis of the effluents from the reactor for (a). x20 indicates that the intensity of the response curve shown is 20 times the actual MS intensity.
travel through the transportation lines from the pulse valve to the IR reactor. It should be noted that 10 cm3 pulse of NO was brought by 30 cm3/min He carrier and mixed with 10 cm3/ min CO flow into the reactor. The use of high He flow results in a high ratio of NO to CO between 43 and 64 s. Pulsing NO caused the following changes in the IR spectra: (i) the adsorption of NO as NO- at 1642 cm-I, (ii) the decrease in the intensity of linear CO at 2056 cm-', (iii) the gradual formation of gem-dicarbonyl bands at 2090 and 2032 cm-I, (iv) the formation of Rh-NCO band at 2186 cm-I, (v) production of N20 shown by IR bands at 2240 and 2201 cm-I, and (vi) the formation of C02 shown by increase in the intensity of the bands at 2361 and 2330 cm-I. The high-wavenumber NO- species in the range 1700-1750 cm-I and the NO+ species in the range 1900-1950 cm-I that were previously observed in a steadystate flow of NO and CO were not found in this s t ~ d y . ~ ~ . ~ ~ The observed variation in the intensity of various CO and NO adsorbates during the NO pulse suggests the following
J. Phys. Chem., Vol. 99, No. 45, 1995 16729
NO-CO Reaction over Rh/Si02 reaction scheme:
Rho-NO-
+ 2Rh0 - (02-)2Rhf + Rho-N Rh' + 2CO - Rh+(CO)2
(3) (4)
Rh-NO- is the anionic NO species formed by the transfer of a partial negative charge from the Rh metal to NO; the RhNO- species as a whole should be considered a neutral species. Note that some of the above steps may not be considered elementary steps. The reduced Rh crystallite surface adsorbs linear CO which is replaced by the NO- species upon exposure to gaseous NO. The replacement process is evidenced by the disappearance of linear CO band and an mle = 28 peak in the MS response, shown in Figure lb. Although the mle = 28 response corresponds to both CO and N2, a major portion of the mle = 28 peak can be attributed to desorbed linear CO since only a small amount of N2 is formed during the reaction of CO and NO at 473 K. Also, the MS responding factor is approximately 12 times greater for CO than N2, resulting in a larger response for CO than N2. NO- has been shown to bind on the reduced Rho sites.24-26.38 Figure l a shows that the NO- band exhibited an initial increase and then a decrease in intensity during NO pulse. The decrease in the intensity of the NO- band accompanied by the downward shift in its wavenumber suggests that NO- chemisorbs on the surface of Rh crystallites, resulting in the dipole-dipole interaction between neighboring NO-. The dipole-dipole interaction causes the downward shift in wavenumber of adsorbates with decrease in coverage of adsorbate^.^^ Similar variation of intensity with wavenumber has also been observed for linear and bridged CO.?' Both linear CO and NO- species have been shown to adsorb on the same type of reduced Rh sites.38 gem-Dicarbonyl is known to be bonded on the Rh+ site. The gradual formation of gem-dicarbonyl and the disappearance of linearly adsorbed CO reflect the slow transformation of Rho site to Rh+ site which is associated with oxygen from dissociated NO- as suggested in reaction 3. NO- is the most likely species to dissociate to adsorbed N and 0 since NO- is formed by the donation of an electron from the metal to the antibonding orbital of NO, resulting in a weakening of the N-0 b ~ n d . ~ . ~ O Dissociation of NO- is further supported by the slow decrease in the NO- intensity and absence of gaseous product formation after 86 s of the NO pulse as shown in Figure 1 a. The absence of gaseous reactants and products suggests that the decrease in NO- intensity is due to the dissociation of NO- to adsorbed nitrogen and adsorbed oxygen. The latter further contributes to the gradual growth of gem-dicarbonyl during the same period. gem-Dicarbonyl can be produced from CO adsorption on Rh' sites on oxidized or reduced Rh catalyst^.'^-^^.^'-^^ Formation of gem-dicarbonyl over reduced Rh surface requires the involvement of OH on the support surface to produce oxidized Rh+ site.4' The absence of variation in the infrared intensity of OH group during gem-dicarbonyl formation indicates that gemdicarbonyl is produced from the direct adsorption of CO on the Rh+ site as suggested in reaction 4. Since the adsorption of CO on Rh+ sites as gem-dicarbonyl is a rapid process,42the gradual formation of gem-dicarbonyl, reaction 4,may be limited by reaction 3, the NO dissociation step, which oxidizes Rho to Rh+ sites. The Rh-NCO band at 2186 cm-' increased in its intensity immediately after the introduction of NO to the reactor. The weak Rh-NCO band prior to the NO pulse was formed during the steady-state NO-CO reaction which was used to check the
catalyst activity. The isocyanate peak at 2186 cm-' also overlapped with the gaseous N20 bands at 2201 and 2240 cm-' between 43 and 160 s. The isocyanate band broadened after the removal of N20. The isocyanate spilled over from the Rh metal surface to the Si02 support surface as shown by the increase in the intensity of the band at 2296 cm-' and a corresponding decrease in intensity of the Rh-NCO band in the range 2185-2200 cm-I. The gaseous responses to first and second NO pulse for the mle ratios corresponding to C02 (mle = 22), CO and N2 (mle = 28), NO (mle = 30), C02 and N20 (mle = 44), and NO2 (mle = 46) were monitored by the MS as shown in Figure lb. The N20 (mle = 44) and C02 (mle = 44) response can be separated by using the response ratio of the primary ionization (mle = 44) and the secondary ionization (mle = 22) of C02. Repeated calibration by injection of known amount of C02 and monitoring the mle = 44 and 22 responses found the ratio of mle = 44 to mle = 22 areas to be 50.8. The dashed curve that corresponds to the contribution of C02 to the mle = 44 response is obtained by multiplying the mle = 22 response by 50.8. Figure l b also shows the rate of N20 and C02 product formation during the NO pulse which is obtained by multiplying the MS intensity with the responding factor for the species obtained from the calibration runs. A comparison of the shape of N20, CO2 (dashed mle = 44 curve), and NO response curves reveals that the front of the N2O response led those of NO and C02. The results clearly show that the reaction of adsorbed NO with adsorbed N is faster than the reaction of adsorbed CO with adsorbed 0 in which adsorbed N and adsorbed 0 are produced from NO dissociation. IR spectra between 43 and 86 s show that N20 was formed before the appearance of gem-dicarbonyl. The results suggest that the reaction Rho-NO-
+ Rho-N
-
2Rh0
+ N20
(5)
takes place on the reduced Rh site prior to the formation of gem-dicarbonyl. The reaction continued after gaseous NO left the reactor as evidenced by the infrared band and tailing response of NzO. As gaseous NO leaves the reactor, a part of the Rh+ sites is reduced back to the Rho sites as shown by the reappearance of linear CO at 2060 cm-' and a tail in the CO and C02 responses (dashed mle = 44 curve) in Figure lb. The close match between the mle = 44 response and the dashed C02 response in the tailing region indicates the gradual formation of COz upon completion of the pulse. The formation of C02 and the reduction process for Rh, leading to the reappearance of linear CO, may be described by the relation: 2Rh+(C0)2
+ 02-- CO, + 2Rho-CO (linear CO) + CO (6)
It should be noted that the conversion of gem-dicarbonyl to linear CO did not lead to the decrease in gem-dicarbonyl intensity after NO pulse into the steady-state CO flow. The change in gem-dicarbonyl intensity appears to be determined by the relative rates of reactions 3, 4, and 6. Following the first NO pulse, the catalyst surface under flowing CO atmosphere contained gem-dicarbonyl at 2032 and 2090 cm-I, linear CO at 2060 cm-I, Rh-NCO species at 2195 cm-I, and Si-NCO species at 2296 cm-I. The catalyst was further exposed to the second NO pulse, of which IR spectra are shown in Figure 2. Results of the second NO pulse are similar to that of the first NO pulse except that significantly less N20 was formed. Lower N20 formation and the presence of more Rh+(C0)2 during the second NO pulse suggest that
Krishnamurthy and Chuang
16730 J. Phys. Chem., Vol. 99, No. 45, 1995
a
I
Time-
1
1
Sec 540
0.8 Q)
150 139 129 118 107 97 86 75 64 54 43 32 0
3
f 0.6
0.4 0.2
0'
I
2400
I
I
I
2000
I
I
0.8
-
48
-
108
97
0.6
86
5:
s
4 0.4
0.2
-
Coin1
0
2400 1
2000
1600
Wavenumber(cm")
1600
Wavenumber(cm") Figure 2. Transient IR spectra of second NO pulse in Come flow at 473 K. TABLE 1: Conversions (%) Obtained during NO Pulse in COMe and CO Pulse in NO/He on Rh/SiOz Catalyst NO pulse in CO/He CO pulse in NOme NO conversion pulse CO conversion pulse temu (Ki 1st 2nd 3rd 1st 2nd 3rd 1.1 3.7 1.7 '1 2.6 413 11.8 6.3 3.2 3.7 5.9 5.9 523 8.4 12.3 14.8 a 15 42.0 573 39.5
b
I
I
'
E
Y
'' Experimental run was not conducted.
N 2 0 formation (reaction 5) is favored on the reduced Rh surface. Table 1 lists the conversion of NO and CO which were determined as
x = (NO, - NJJNO,
d0522
1.5
(7)
I
I
x20
,
where N is the moles of A fed to the reactor and N A is the moles of A leaving the reactor. The amount of product formed from either NO or CO pulses is listed in Table 2. The overall reaction for C02, N20, and N2 formation can be written as follows: 0
(9) 2N'(,,
+ 2C0,g)
-
N2(,,
+ 2C',(g)
50
100 150 200 Time (sed
250
300
Figure 3. (a) Transient IR spectra of NO pulse in CO/He flow at 523 K. (b) MS analysis of the effluents from the reactor for (a).
(10)
Under steady-state reaction conditions, formation of a N20 molecule should be accompanied by the formation C02 molecule. Significantly more N20 formation than C02 indicates that majority of the 0 produced by reaction 8 during the pulse remains on the catalyst surface. Following NO pulses to steady-state CO flow and CO pulse to NO flow studies at 473 K, the reactor temperature is raised to 523 K while the catalyst is exposed to helium flow. An increase in the reactor temperature from 473 to 523 K in He flow resulted in the removal of most of the adsorbates from the surface prior to the exposure of catalyst to CO flow at 523 K. Exposure to CO flow results in the formation of linear CO at 2060 cm-' and weak gem-dicarbonyl bands at 2030 and 2090 cm-I. The formation of linear CO is due to the partial reduction of the Rh+ sites to Rho crystallites by adsorbed CO. Reductive agglomeration of the Rh+ sites to Rh crystallites in the presence of adsorbed CO has been shown to occur at temperatures above 448 K.43
Figure 3a and b, shows the transient IR spectra of adsorbed species and the MS response of gaseous products, respectively, during the NO pulse studies at 523 K. The catalyst surface prior to the NO pulse contains gem-dicarbonyl, linear CO, and Si-NCO which were produced from the earlier runs. The variations in the IR intensity of adsorbates and gaseous concentration shown in Figure 3a,b for the study at 523 K are parallel to, in some aspects, those observed at 473 K. Careful comparison of the transient IR spectra and MS responses at 473 and 523 K reveals the following subtle differences: (i) the formation of smaller fraction of Rh+ sites and larger fraction of Rho sites resulting in a lower gem-dicarbonyl intensity and a higher intensity of NO- adsorbed on reduced Rh crystallite surface at 523 K; (ii) smaller amount of N20 produced at 523 K; (iii) faster rate of spillover of the isocyanate species from Rh to Si at 523 K; and (iv) the smaller lead time of N20 to CO2 response at 523 K. A linear CO band at 2060 cm-' begins to reappear on prolonged exposure of the catalyst surface to
J. Phys. Chem., Vol. 99, No. 45, 1995 16731
NO-CO Reaction over RWSi02
TABLE 2: Amounts @mol) of N20 and COz Formed during NO Pulse in CO/He and CO Pulse in NO/He on FWSiO2 Catalyst NO pulse in CO/He CO pulse in NO/He COz formation
N20 formation
COz formation
NzO formation
temp (K)
1st
2nd
3rd
1st
2nd
3rd
1st
2nd
3rd
1st
2nd
3rd
473 523 573
214.7 58.2 333.0
68.4 22.1 146.9
7.6 18.0 a
9.4 11.2 153.6
9.4 11.7 156.1
12.2 10.5 a
0.3 3.3
1.9 3.9 22.8
0.0 0.0 0.0
2.1 3.0 171.8
1.7 1.5 168.1
0.8 0.5 163.1
0.0
'' Experimental run was not conducted.
a 1 0.8
3B 0.6 5:
2 0.4 0.2
n 2000
2400
1600
Wavenumber(cm'')
.-A Y
P
,
E
U
0
50
100
150
200
Time (sed Figure 4. (a) Transient IR spectra of NO pulse in COlHe flow at 573 K. (b) MS analysis of the effluents from the reactor for (a).
CO/He flow due to the reductive agglomeration of the Rh+sites in the presence of CO at 523 K.43 Figure 4a shows the IR spectra of the adsorbed species during the NO pulse in CO/He flow at 573 K, which is higher than the light-off temperature of 543 K for the reaction determined from previous study.44 The catalyst surface contains only linear CO before and after NO pulse. Bands for gem-dicarbonyl and NO+ species which are associated with Rh+ sites were not observed during the run at 573 K. During the NO pulse, NO
adsorbed as NO- at 1693 cm-I; linear CO was depleted, and N20 and C02 were produced. After gaseous NO left the reactor, linear CO reappeared and Rh-NCO decreased its intensity. Figure 4b shows that the tail in the C02 peak is much less than that observed at 473 and 523 K due to the higher rate of COz formation at this temperature and the absence of adsorbed gemdicarbonyl. The C02 response leads the N20 response, which suggests that the reaction of CO with adsorbed oxygen is faster than the reaction of NO with adsorbed nitrogen. CO Pulse in NO/He Flow. The CO pulse experiment was carried out after the NO pulse in COMe studies at 473 K without further reduction of the catalyst. The catalyst was not reduced between experiments in order to study the catalyst under practical conditions similar to those existing in an automotive catalytic converter where the catalyst is not periodically reduced. Figure 5a shows the IR spectra obtained during the first CO pulse into a flow of NO/He over Rh/SiO2 catalyst surface at 473 K. Steady-state NO flow through the reactor resulted in a band at 1534 cm-' corresponding to a bidentate nitrato specie^,^' a low-intensity NO- band at 1642 cm-I, an NO+ species at 1911 cm-' which overlaps with the P-branch of the gaseous NO band resulting in a higher intensity of the 1911 cm-' band as compared to the 1845 cm-' band, and an Si-NCO band at 2301 cm-I formed during the previous studies of NO pulse in CO/He at 473 K. The spikes between 1800 and 1400 cm-' were observed only at 473 K when the catalyst surface was exposed to gaseous NO. During the CO pulse, gaseous CO enters the reactor at about 33 s and completely leaves the reactor at 76 s as indicated by the emergence and disappearance of gaseous CO bands at 2116 and 2176 cm-'. CO adsorbs gradually on the surface as gem-dicarbonyl at 2090 and 2030 cm-I, indicating the presence of Rh+ sites which are associated with oxygen from dissociated NO. The rate of growth in gemdicarbonyl in this study is higher than that of gem-dicarbonyl produced from pulsing NO into a steady-state CO flow shown in Figure la. The results reveal that steady-state NO flow oxidized the Rh surface to facilitate gem-dicarbonyl formation. The change in the IR spectra of adsorbed species can be further discerned from the difference spectra shown in Figure 5b obtained by subtracting the first spectrum at 0 s from all the spectra in Figure 5a. The difference spectra unravel that the decrease in intensity of adsorbed NO+ at 1911 cm-' is accompanied by increase in the intensity of gem-dicarbonyl and C02 during the CO pulse. The decrease in the intensity of adsorbed NO+ could be due to either the replacement of the NO+ species by gem-dicarbonyl or the dissociation of adsorbed NO+ to adsorbed N and 0. The former step is more feasible than the latter since both Rh+(CO)l and Rh-NO+ adsorb on similar oxidized Rh sites on the surface,38 and the bonding structure of Rh-NO+ does not favor d i s s o ~ i a t i o n .A ~ ~careful examination of the difference spectra also shows that COl formation occurs earlier than a significant decrease in the intensity of the NO+ band, indicating that the 0 in C02 is not obtained from dissociation of NO+ species. The low probability for dissociation of NOf is also supported by the donation of antibonding electrons from NO to the metal, resulting in an increase in N - 0 bond strength for adsorbed NO+ species."
Krishnamurthy and Chuang
16732 J. Phys. Chem., Vol. 99, No. 45, 1995
a
I
1
1
0.8
0.8
I
I
I
I
I
1
I
B
e
k
r
I
Qb
a,
2 0.6 b 2
a 0.6
4
B
'
4 0.4
0.4
0.2
0.2
U
0
2400
Wavenumber(cm")
2000
e
Pulsg
1600
Wavenumber(cm")
b
,
b
Time
,
Sec
l t
t
0.8 a,
2
4
0.6
0
B
'
-
kTz
119 108 87
65
0.2
44
33
2 c 2 0 b O
'
'
l6bO
Wavenumber(cm"1
.C
0
'
-
55
1911
Y h
-
76
0.4
d e = 30
-
97
C
F
100 150 200 Time (sec) Figure 6. (a) Transient IR spectra of CO pulse in NO/He flow at 523 K. (b) MS analysis of the effluents from the reactor for (a). 0
-
0.08
I
0
I
50
100
150
200
Time (sed Figure 5. (a) Transient IR spectra of CO pulse in NO/He flow at 473 K. (b) Difference spectra of the CO pulse in NO/He flow at 473 K for (a). (c) MS analysis of the effluents from the reactor for (a).
Figure 5c shows the MS response of the effluents from the reactor corresponding to Figure 5a. Most of the mle = 4 4 peak corresponds to COz as indicated by the close match between
50
solid and dashed mle = 44 curve. A slightly higher mle = 44 base line at 473 K than at 523 and 573 K and gaseous N20 bands in Figure 5a indicate that a small amount N20 was present in the background prior to the pulse. Figure 5c shows N20 formation lags the COz formation, revealing that the reduction of surface to Rho is needed for N20 formation. The C02 response also closely follows the CO response. IR spectra in Figure 5a also show that the rise and fall of COr and gemdicarbonyl occur at about the same time. These observations suggest that gem-dicarbonyl may react rapidly with the readily available oxygen from dissociated NO during the CO pulse into steady-state NO flow. Our recent isotopic labeled study has also provided the evidence to support that gem-dicarbonyl is an active precursor for CO;! formation.45 IR spectra and MS responses obtained during the CO pulse studies at 523 K are shown in Figure 6a,b. MS responses for CO, COz, and Nz0 appeared at about the same time during the CO pulse. Although responses of the reactants and products at 523 K are essentially the same as those at 473 K except that
NO-CO Reaction over Rh/Si02
J. Phys. Chem., Vol. 99, No. 45, I995 16733 a
higher NO conversion is achieved at higher temperature, several distinct differences were observed in the IR spectra. The catalyst surface for the study at 523 K started with a weak bidentate nitrato band at 1534 cm-I, a low-intensity NO- band at 1660 cm-I, an NO+ band at 1916 cm-' which overlaps with the gaseous NO band, and the Si-NCO species at 2301 cm-' formed during the earlier CO pulse studies. Exposure of the catalyst surface to a CO pulse resulted in the formation of RhNCO species at 2189 cm-I, gem-dicarbonyl at 2090 and 2030 cm-I, linear CO on Rh+ at 2102 cm-I, a decrease in the intensity of the adsorbed NOf species at 1916 cm-I, and the emergence of NO- band at 1693 cm-I. The formation and disappearance of adsorbates follow the sequence gem-dicarbonyl, isocyanate on Rh surface, and NO-, in which the former appeared and disappeared earlier than the latter. In contrast, no obvious change in the isocyanate and NO- species has been observed at 473 K. The most interesting observation from the results in Figure 6a is that the increase and decrease in the intensity of NOband are accompanied by a corresponding decrease and increase in the intensity of NO+ during the CO pulse into the steadystate NO flow. NO- and NO+ are known to adsorb on the surface of reduced Rh crystallites and oxidized Rh sites, respectively. The contrasting changes in the IR intensity of NO+ and NO- during CO pulse suggest that the CO pulse reduces the Rh+ site to Rho sites by removing readily available oxygen on the Rh surface produced from exposure to a steady-state NO flow. In the absence of CO and presence of steady-state NO flow, the Rho site is gradually oxidized to Rh+ site by dissociation of NO- (reaction 3), which appears to be the ratelimiting step for the slow oxidation process. The formation of Rh-NO+ may be depicted as follows: Rh+
1 0.8
4E
0.6
0
B
4 0.4
0.2
n 2400
2000
1600
Wavenumber(cm")
d e = 30
d e = 28
+ NO - Rh - NO'
(11) Heating the catalyst surface in He flow from 523 to 573 K resulted in a decrease in the intensity of the Si-NCO band. Figure 7a shows the IR spectrum of N20, NO-, and Si-NCO prior to CO pulse at 573 K. Pulsing CO resulted in the disappearance of NO- at 1660 cm-I, the adsorption of linear and bridged CO at 2048 and 1946 cm-I, respectively, and the formation of N20 and C02 between 32 and 75 s. The formation of linear and bridged CO and the absence of NO+ and Rh+(C0)2 indicate that the catalyst surface was in a reduced state, and oxygen from dissociated NO was not able to oxidize the Rh surface at 573 K. The MS response of the effluents from the reactor in Figure 7b shows that C02 response leads the CO response. Our previous isotopic I3CO transient studies have also shown that the residence time of intermediates for I3CO2 is less than the I3CO residence time at 573 K.29
Discussion The infrared-active nature of adsorbed CO and NO allows the use of in situ infrared spectroscopy to study their dynamics on the catalyst surface during reaction. Various modes of adsorbed CO, NO, and isocyanate on supported metal catalysts have been observed by in situ IR stud^.^.^'-*^.^ It has been well established that the modes of adsorbed CO and NO are very sensitive to the surface state of their adsorption sites which are influenced by their chemical environment, Le., temperature and partial pressure of adsorbing gases. The reduced Rh crystallite surface chemisorbs CO as linear (Rho-CO) and bridged CO; the Rh+ site adsorbs CO as gem-dicarbonyl and linear CO (Rhf-C0).17-22 In NO adsorption, NO adsorbs on the reduced Rh crystallite surface as NO- and as NO+ on the Rh' ~ i t e . ? ~ -The * ~ adsorption of Rh+(N0)2 on Rh/A1203 has also been reported in the l i t e r a t ~ r e . ~ ~ . ~ ~
Time (sed Figure 7. (a) Transient IR spectra of CO pulse in NO/He flow at 573 K. (b) MS analysis of the effluents from the reactor for (a).
The modes of adsorbates reflect the surface states of their adsorption sites. The presence of Rh+(C0)2 and Rh-NO+ at 473 and 523 K indicates that part of the Rh surface is in the Rh+ state during NO-CO reaction below light-off temperature. In contrast, these species are absent above the light-off temperature, indicating that most of the Rh surface is in the reduced state. The difference in surface states and adsorbates above and below light-off temperature suggests that two different mechanisms are operating for the reaction of NO with CO on Rh/SiOZ. The following reaction steps were proposed to explain the product formation and observed changes in adsorbates below the light-off temperature.
(A)
adsorption and NO dissociation
+ Rho-CO,,, - Rho-NO- + CO(,, Rho-NO- + 2Rho - (02-)2Rhf + Rho-N
step 1. NO,,, step 2.
Krishnamurthy and Chuang
16734 J. Phys. Chem., Vol. 99, No. 45, 1995
+ NO -.Rh-NO' step 4. Rh-NO' + 2CO - Rh+(C0)2 + NO(,, step 5. Rh+ + 2 c o - R h + ( c 0 ) 2
tion of Rho to Rh+ site, and vice versa, below the light-off temperature is not a rapid process which can be clearly observed by the gradual change in the IR intensity of adsorbates on these sites. In the Rho Rh+ process, the linear CO adsorbed on Rho desorbs while Rho is converted to Rh+ by dissociated NO. Desorption of CO is evidenced not only by the m/e = 28 peak in Figure l b but also by the absence of clear IR bands for surface reaction and product formation (B) adsorbed CO at 43 s in Figure la. Oxidation of Rho leading to desorption of linear CO has also been observed at room step 6 . 2Rh+(CO), 02temperature on Rh/AlzO3 catalyst.24 However, oxidation/ reduction of the Rh site appears to lead to a direct exchange of CO, ~ R ~ O - C O (linear CO) co adsorbed NO+ and NO- as evidenced by the gradual decrease of one form of adsorbed NO and an increase of the other form step 7. Rh+(CO)2 20,, 2 c 0 , Rh+ shown in Figure 6a. Above the light-off temperature, Rh+(C0)2 and Rh-NO+ are Rho-N 2Rh0 N 2 0 step 8. Rho-NOabsent; the CO2 response leads CO and N20 responses during the NO pulse into CO flow and the CO pulse into NO flow. Rho-N N2 2Rh0 step 9. Rho-N The results suggest (i) oxygen from dissociated NO is not strongly bound to Rh, precluding conversion of the Rho to Rh+ Steps 1-6 and 8 have been discussed in the previous section and are also suggested by a number of earlier s t ~ d i e s . ~ ~ . ~site, ~ , and ~ ~ (ii) , ~ oxygen on the Rh surface, which may be in the reduced state, rapidly reacts with linear and bridged CO to form Replacement of linear CO by NO has been observed on Rh/ COz. The reaction mechanism on Rh/Si02 above the light-off Si02 at 303 and 373 K and on Rh/A1203 at 303 K.24 Adsorption temperature may resemble that described for single crystal of NO on the reduced Rh has been found to result' in the surfaces. On Rh( 111) surface bridged NO dissociates to form formation Rh+ site that chemisorbs either NOf or gemN and 0 above 350 K.47,48 A lying down or highly adsorbed dicarbonyl. The presence of both NO and CO may result in inclined adsorbed NO species was suggested as a precursor for the formation Rh+(NO)(CO), which appears to be stable at room NO dissociation over Rh(100) single crystal surface near 170 temperature.26 The Rh+(NO)(CO) species was not detected K.49 The mode of adsorbed NO on single crystal surfaces at during this study at 473 K. temperatures above the light-off temperature remains unclear. Step 6 shows the reductive agglomeration of Rh+ sites to Rho crystallites in the presence of CO known to occur at Comparison of C02 and N20 responses with other catalytic temperatures above 448 K.43 The reductive agglomeration of reactions shows that the time constant of response for NORh+ sites results in the conversion of adsorbed gem-dicarbonyl CO reaction is significantly smaller than methanation reacspecies to C02 product and the adsorption of linear CO on Rh t i ~ n , ~Fischer-Tropsch ~.~' ~ y n t h e s i s , ~oxygenate ~-~~ synthecrystallite. This step for the conversion of gem-dicarbonyl is sis:7.56.57 and hydroformylation reaction27in the same temperature slower than the formation of gem-dicarbonyl at 473 K. Step 7 range. Steady-state isotopic transient study of NO-C0/I3CO is supported by our recent study which shows that Rh+(13CO)2 showed that the residence time (Le., time constant or t) of is rapidly converted to I3CO2 in the presence of CO and NO intermediates for C02 formation decreased from 12 s at 473 K mixture at 573 K.45However, the nature of adsorbed oxygen to less than 0.5 s at 573 K.29 As reaction temperature increased, involved in the step 7 remains to be investigated. Step 8, the the time constant for C02 response decreased in both steadyformation of N20 in the presence of reduced Rh crystallites, is state isotopic transient and pulsing NO into a CO/He flow supported by formation of N20 prior to gem-dicarbonyl in the studies. In contrast, an instantaneousresponse for C02 response NO pulse into CO and a low N20 formation during the CO was observed for pulsing CO into a NO/He flow. The NO pulse pulse into NO where the initial surface was in an oxidized state. into a CO/He flow mimics the steady-state NO-CO reaction The formation of nitrogen in step 9 that was proposed in the more closely than the CO pulse into a NO/He flow does. The literature could not be supported by the data obtained in this small time constant of C02 responses in these studies indicates study.6.10-16 The most likely rate-determining step for the that the reaction can rapidly reach a new steady state when the formation of products is the NO dissociation step due to the reaction conditions (temperature, partial pressure of reactants) slow rate of the dissociation of NO to form adsorbed N and 0. are altered, especially at temperatures above the light-off Many studies over supported Rh catalysts have suggested that temperature. Therefore, the rate law determined for the steadythe adsorbed NO- species is the most likely precursor for NO state NO-CO reaction above the light-off temperature is dissociation over supported Rh catalysts.6*8 The contrasting applicable for describing the kinetics of reaction under transient changes in NO- and NO+ at 523 K and the gradual disappearconditions. ance of the NO- species at 573 K during the CO pulse in steadyAll the adsorbates observed under the condition of pulsing state NO/He flow indicate that the NO- dissociation is not a NO into a CO/He flow and pulsing CO into a NO/He flow are rapid step. Comparison of the reactant and product responses identical to those found in the steady-state NO-CO-He flow reveals that COz response lags the CO response on the average c o n d i t i ~ n . ~The ~ , ~coverage of these adsorbates depends on of 2-3 s during pulsing NO into CO flow below light-off the relative rates of their formation and consumption which are temperature; the C02 response essentially matched the CO greatly influenced by reaction condition (Le., partial pressure response during pulsing CO into NO flow. In the former case, of reactants and temperature). At given temperature, the adsorbed 0 is not readily available while in the latter case coverage of adsorbate is varying during the pulse condition while adsorbed 0 is readily available for the formation of C02. These the coverage of adsorbate remains constant during steady-state results indicate that NO dissociation is a slow step (i.e., the reaction and steady-state isotopic transient studies. In the rate-determining step) for COz formation. present study, the ratio of gaseous CO to NO varies from 0 to The change in infrared intensity of adsorbates during pulsing 3 and then back to 0 for pulsing NO into a C o m e flow and a studies reflects not only the variation in adsorbate concentration but also alteration in state of catalyst surface. The transformareversed ratio for pulsing CO into a NO/He flow. As a result, step 3. Rh'
+
+ +
+
+ -
+
+
+
-
-
+
NO-CO Reaction over Rh/SiO:! the conclusion drawn from this study can only be applicable to the above feed stoichiometry of NO to CO.
Conclusions In situ infrared study of the pulse NO-CO reaction reveals that below the light-off temperature both Rho and Rh+ sites are present; above the light-off temperature Rho sites are predominant on the catalyst surface during pulsing NO into CO flow and pulsing CO into NO flow. Below the light-off temperature, dissociation of Rh-NO- oxidizes Rho to Rh+;Rh+ chemisorbs CO as Rht(C0)2 and NO as Rh-NO+. Rh-NO+ can be further replaced by CO to form Rh+(C0)2, which can undergo reductive agglomeration to form Rho-CO (linear CO) and COz. Rh+(CO)z can also rapidly react with oxygen from dissociated NO to produce C02. Above the light-off temperature, the Rh surface remained in primarily the reduced state during NOCO reaction. The species observed during the reaction are linear CO, bridged CO, and Rh-NO-. The product formation responded rapidly to changes in reactant concentration. Except for pulse NO into CO below the light-off temperature, all the C02 responses exhibit a small time constant, revealing the short lifetime of intermediates for C02 formation. Acknowledgment. R.K. is grateful for the financial support from the Department of Chemical Engineering, The University of Akron. References and Notes Eng. 1975, 11, 1. (2) Campbell, C. T.: White, J. M. Appl. Surf: Sci. 1978, I , 347. (3) Dubois, L. H.; Hansma, P. K.; Somorjai, G. A. J. Catal. 1980, 65, 318. (4) Taylor, K. C.; Schlatter, J. C. J. Caral. 1980, 63, 53. (5) Williamson, W. B.; Stepien, H. K.; Gandhi, H. S. Environ. Sci. Technol. 1980, 14, 319. (6) Hecker, W. C.; Bell, A. T. J. Caral. 1983, 84, 200. (7) Root, T. W.; Fisher, G. B.: Schmidt, L. D. J. Chem Phys. 1986, 85, 4679. (8) Oh, S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J. Caral. 1986, 100, 360. (9) Peden, C. H. F.; Goodman, D. W.; Blair, D. S.: Berlowitz, P. J.; Fisher, G. B.; Oh, S. H. J. Phys. Chem. 1988, 92, 1563. (10) Cho, B. K.; Shanks, B. H.; Bailey, J. E. J. C a r d 1989, 115, 486. (11) Oh, S. H. J. Catal. 1990, 124, 477. (12) Taylor, K. C. Caral. Rev.-Sci. Eng. 1993, 35, 457. (13) Cho, B. K. J. Caral. 1994, 148, 697. (14) Ng, K. Y. S.: Belton, D. N.; Schmeig, S. J.: Fisher, G. B. J. Catal. 1994, 146, 394. (15) Belton, D. N.; Schmeig, S. J. 1.Caral. 1993, 144, 9. (16) Schwartz, S. B.; Fisher, G. B.; Schmidt, L. D. J. Phys. Chem. 1988, 92, 389. (17) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504. (18) Worley, S. D.; Rice, C. A,; Mattson, G. A.; Curtis, C. W.; Guin, J. A.; Tamer, A. R. J. Phys. Chem. 1982, 86, 2714. (19) Li, Y. E.; Gonzalez, R. D. J. Phys. Chem. 1988, 92, 1589. (20) Yates, J. T., Jr.; Duncan, T. M.; Worley, S. D.; Vaughan, R. W. J. Chem. Phys. 1979, 70, 1219. (21) Chuang, S. S. C.; Pien, S. I. J. C a r d 1992, 135, 618. (1) Shelef, M. Caral. Rev.-Sci.
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