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ZnO catalysts
"surface science
Surface Science 269/270 (1992) 514-519 North-Holland
IR study of NO reduction by CO on Pt/ZnO catalysts F. Boccuzzi, G. Ghiotti, A. C h i o r i n o a n d E. G u g l i e l m i n o t t i Dtpart~mento (It Chumca lnorgamca, Ctumlca FIswa e Ctumtca det Matenah, eta P G~u~Ja Z 10125 Tonno, Italy Received 29 August 1991; accepted for pubhcatmn 22 October 1991
An FTIR study of the N O - C O interaction, from RT up to 533 K, was done on two differently pretreated P t / Z n O samples, the first, mddly reduced, formed of small pure Pt particles dispersed on ZnO, and the second, strongly reduced, presenting PtZn alloyed metal parhcles. On the mddly reduced one by heating m NO-CO atmosphere at 373 K a qmte strong band assigned to an lsocyanate speoes is formed, produced by reactmn of CO molecules with mtrogen atoms, coming from the NO decomposition At the same time the intensity of the CO band grows up to that of the wrgln sample, showing thai the Isocyanate speoes spd~ over from the metal to the support. On strongly reduced samples the isocyanate speoes band is quite weak and a CO 2 band is produced. The different behavlour wdl be discussed taking into account the different surface composmon and electromc properhes of the two samples.
1. Introduction Noble metals supported on reducible oMdes have been extensively studied m the last years, in view of their activity in the synthesis of hydrocarbons from CO and H , [1,2] However, in spite of the interest that these systems can pre~cnt also for the catalytic rcductton of NO. only few papers [3] can be found on the latter topic. The reduction of NO by CO has been the subject of extensive research in recent years; the catalysts used are usually supported noble metal catalysts (Rh, Pt, Pd, Ru) [4]. Recently it was shown that the Pt catalyst activity for this reaction strongly depends on the support and that for catalysts supported on n-~pe sem:cend,:ctors the activity depends on the reduction pretreatment temperature [51. However, the mechanism of the reaction on supported catalysts has not been tinambiguously determined; for example, dtsagreement persists about the intermediates of th~s reaction and on the dependence of thetr produchon on the surface chemical composition of the cataly.,t. In this paper we will present an FTIR study of the N O - C O mterachon on two differently
pretreated P t / Z n O samples, showing a different surface composition [6].
2. Experimental P t / Z n O samples wcrc prepared, as previously dewrlbed [7], by photodcposit~on on commercial Z n O Kadox, the Pt content was -~ 1.01 at% P t / P t + ZnO. The IR spectra were run at R T with a Perkin Elmer F T I R 1760 at a resolution of 4 c m - t ; the number of scans (100-200) was changed according to the transmission of the sample with the aim of obtainmg similar levels of noise. High-purity 02, H2, CO from Matheson were used without further purJfwafion, whetea~ NO from Matheson was freshly dtstilled before use. h n n expe:iments were carr;ed out The z. .c.t.~. n r~n..... on samples already submitted to the follov, mg thermal and chemical treatments: (2) after oxldatton and evacuation at 623 K the sample was mildly reduced wtth a H z / N 2 0.5% mLxture at 493 K and then outgassed at the same temperature (sample labeled PR~493 ); 0i) same treat-
00"~9-6028/92/$05 00 ~, 1992 - Elsc~lLr Science Publishers B V All rights reserved
F Boccuzzt et al / N O reduction by C O on P t / Z n O c atah'~ts
ments above but lastly the sample was reduced m pure H 2 at 573 K and than outgassed at the same temperature (sample labeled PR573).
3. Results
CO-NO coadsorption on Pt / ZnO samples The effect of the NO adsorption and sttbsequent CO inlet on the IR transmission spectra of a PR~493 sample is shown in fig. la. The inlet of 10 Torr of CO on the preadsorbed NO, showing only one band at 1780 cm -t, causes the depletion of this band and the growth of an absorption at 2097 cm -t with a weak component at 2132 c m without changes in the overall transmission; the frequencies of the CO absorptions are the same
B
';15
ot that observed on a virgin sample treated m the same way, as it car, be seen more clearly in fig lc, c u ~ c 2, however the mtegral~d intensity of the CO stretching band is nearly, hallcned ( / = ~ ) cm -~) m comparison w~th that observed on the virgin sample (I = 20 c m - ~, fig. 4a of ref. [6]). Fig. lb shows the effect of heating in N O - C O atmosphere on the transmission spectra of a PRI493 sample, at increasing temperatures from RT to 513 K; only minor changes in the transmission are observed. As for the vibrational band changes, the heating at 423 K (fig. lc, curve 3) produces an asymmetric band at ~ 2220 cm-~, I = 2.4 cm-~; at the same time a strong intensification of the band at 2097 cm-~ and a decrease of the band at 2132 cm - t are also observed. After this treatment the integrated intensity of the 2150-2000 cm-= region is almost coincident
b 3
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1
q
2097
,4000 8
3'500
3000
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--
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i
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o 8000
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!
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~
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3500
3000
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2000
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2000
~700
-t
Pig 1 IR spectra ot the N O - C O interachon on a PR1493 sample Section (a~ - traqsnnsslon spectra after R F mteracuons curve l, background, curve 2, after contact with 10 Torr of NO, curve 3, after inlet ot !0 Torr of CO. Section (b) - transmission spectra, run at R T after m~eractlons at temperatures up to 533 K curve 1, al 423 K, " w e ?, at 493 K, curve 3, at 533 K Section (c) absorbance spectra, obtained after subtraction of the background curve 1, 10 Aurr of NO, curve 2, Inlet of 10 Torr of CO, curve 3 after h e a a n g at 423 K, curve 4, after t~eatlng at 493 K
F. Boccuzzt et al / NO reductton by CO on Pt / ZnO catalysts
516
with that measured on a similar sample after heating at the same temperature in pure CO atmosphere, I = 22 cm -t, in comparison with I = 23 c m - l for the clean sample [b]. By further heating up to 493 K (fig. lc, curve 4) a strong growth of the band at 2220 cm-~ is observed (1 = 16.6 c m - l ) , and simultaneously the band at 2132 c m - t completely disappears. The integrated intensity of the 2150-2000 cm-~ region is now 29 c m - ~, slightly higher than on clean samples treated at the same temperature in pure CO, I = 26 c m - t (curve 3 of ref. [6]). A further heating up to 533 K causes the complete depletion of the bands at 2220 and 2097 c m - t (fig. lb, curve 3).
The effect of the NO adsorption and of the subsequent CO inlet on the IR transmission spectra of the sample PR573 is shown in fig. 2a, curves 1 and 2 respectively. NO adsorption produces a significant increase of IR transmission and, at the same time, a band of medium intensity at 1812 c m - ~, with other weaker components, at 1900 and 1675 cm - t , assigned to the NO stretching of molecules adsorbed on different surface sites [8]. The CO inlet on this saml;le at R T causes a small decrease in the transmission and the depletion of the NO stretching band; it yields, also, a CO stretching band at 2078 c m - t , a very weak c o m p o n e n t at 2132 cm-~ and a very weak absorption at 2220 c m - t , observed on the more
t.50 I b gT
c
o.2AI
0 75
0
2078
O0 4000
1.50
3500
3000
2500 cm -1
2000
-
1500
iO00
1812
~T
2
2340
I
0.75
0.00 4000
,
3500
3000
2500 CB - i
2000
t500
~000
2400
L:~00
2000 cm - t
,
t700
Fig 2 IR ',pectra of the N O - C O mtcrachon on a PR573 ',ample Sechon (a) - transmission spectra after RT interaction,, curve 1. background, curve 2, after cop'.act vvlth 10 Torr of NO, cu~,e 3, after m i t t of 10 Torr of CO Section (b) - transm,sslon spectra, run at RT, after mterachon,, at temperature~ up to 493 K curve 1, at 373 K; curve 2, at 423 K, curve 3, at 493 K Section (c) ab~orbance ',pectra, obtained after ',uhtract~on of the background o! curve 1, N O - C O mterachon at RT, cu~,e 2, after heating at 373 K c u ~ e 3 alter heating at 473 K
F Boccuzzt et al. / NO reductton by CO on P t / Z n O catalysts
mildly reduced sample only after thermal treatment at 423 K and a band at 2340 cm-~, due to CO 2 formation (fig. 2a, curve 3). The frequency and the intensity of the CO stretching band (v = 2078 cm-~, I = 18.4 cm-~) are higher than that observed for a virgin sample reduced at the same temperature (v = 2067 cm -l, I = 4.2 c m - ' , ref. [61). By heating at 373 K (fig. 2b, curve 1) the transmission decreases while no changes are observed in the vibrational features of the adsorbed species (fig. 2c, curve 2); at 423 K a further reduction of transmission is detected (fig. 2b, curve 2); after heating at 473 K the decrease in the IR transmission is also accompanied by a significant weakening of the CO band (fig. 2c, curve 3, 1 = 6.5 c m - I ) . By heating at 493 K the band is completely depleted and also the transmission is further reduced (not shown).
4. Discussion
The effects of NO adsorption on differently pretreated P t / Z n O ~amples have been prevlouslv illustrated [8]: on the basis of the frequency of the NO band, looking at our previous characterization of the same samples by H R T E M microscopy and CO chem~sorption [6] and to literature data of NO adsorption on monocrystalline and supported Pt samples, reported and discussed in ref. [8], we assigned the 1780 cm -1 band to NO molecules chemisorbed on top on stepped Pt sites, and the 1812 cm -I band to NO adsorbed on Pt sites on ~.~,e PtZn alloy surface, perturbed by oK gcn atoms produced in the NO dissociation ana chemisorbed nearby. The CO interaction at RT on NO precovered PRt493 produces the depletion of the NO stretching mode and the growth of CO stretching modes; the intensities of the CO bands are, however, as exposed in the results section, sigmficantly weaker than on clean, virgin samples. The experiments of heating in N O - C O atmosphere lead to formation of isocyanate species. These are produced by reaction of CO molecules with N ~.toms coming from the decomposition of
517
NO. In fact a band at 2220 cm-~ appears m the sample PR~493 by heating at 423 K, and intensifies sigmficantly at 493 K: on the basis of Its frequency and thermal stability and by comparison with literature data [9], the band can be assigned to the asymmetric stretching of an isocyanate species. We observed that after interaction at RT, when the isocyanate species is still absent, the intensity of the CO band --- ~1 of that observed on a virgin sample while, after heating at 423 or 493 K and during the parallel growth of the isocyanate band at 2220 era-l, the integrated intensity of the CO band becomes similar to that observed on a virgin sample. These findings indicate that some NG is dissociated at RT on this sample on some Pt surface sites or that the CO coadsorption induces the decomposition of the NO leaving oxygeh atoms on the surface of the Pt pameles, being the nitrogen chemisorbed amount usually considered negligible [10]. At T > 423 K a cleaning of the Pt particles from the adsorbed ox3gen atoms, with production of C O , in the gas phase, occurs, as shown by the growth of the CO stretching band; at the same time the products of the reaction N + CO ~ NCO migrate from the metal to the support. The occurrence of the spillover to the support Is indicated by the fact that the CO band shows almost the same frequency and the same l n t e n s ~ of the virgin sample: this fact suggests that CO and NCO are not coadsorbed on the metallic particles. The NCO band grows up during the heating up to 493 K, while by heating at 533 K, both the CO and NCO bands are depleted. As for the sample PR573 the C O - N O interaction produces a weak band at 2220 c m - l almost unaffected ai,-o by heating up to 473 K. The formation and stabihty of isocyanate species in the NO + CO reaction on different Pt supported catalysts (Pt on SIO 2, MgO, A[203, etc ) have bcen previously studied [9]; tt was shown that the stabihty of this species strongly depends on the support: it decomposes rapidly on P t / T i O 2 at 573 K, whde it is extremely stable on P t / S i O 2 up to T > 723 K. In our case the d e c o m p o s m o n of the ~socyanate species occurs at a very low temperature, at 533 K m few minutes the band is
518
F Boccuzzl et al / NO reductton by CO on Pt / ZnO catalysts
completely depleted. It seems therefore confirmed, as previously suggested [9], that the isocyanate species are adsorbed on the support and that their stability is primarily determined by the chemical and electronic properties of the support. Therefore the low amount of the isocyanate species observed on our strongly reduced sample can indicate that the anionic species is far less stabilized on the electro, licher samples. There are other differences in the interaction N O - C O on the sample PR573 so that it can be useful to discuss briefly. The inteI action with NO alone causes, in this case, a significai, t gain in the IR transparency, this phenomenon being interpreted [8] as due to an electron trapping process on oxygen atoms produced by NO dissociation, occurring on the defective ZnO phase and on the PtZn alloy particles. Moreover the CO inlet produces the depletion of the 1812 c m - I NO band and the growth of a CO stretching band at higher frequency and with a higher intensity than on a virgin sample. At the same time, and without significant changes by heating up to 423 K, a CO z band of medium intensity appears. The remark that the CO band observed after NO interaction has a higher frequency and a higher intensity as that shown on a virgin sample pretreated m the same way could be an indication that the reaction of dissociation of NO occurring on this sample causes a change, as a consequence of the heat produced, in the surface composition of the alloy, with an enrichment in Pt. Moreover, it can be hypothesized that NO dissociation on the alloy produces oxygen atoms adsorbed preferentially on the more electropositive element of the alloy, on Zn atoms, while nitrogen atoms, as on pure Pt, recombine and desorb immediately; therefore Pt sates remain free. We do not have direct evidences, in our case, confirming a different chemisorption of the nitrogen and oxygen atoms on the two components of the alloy; however there are some evidences m the hterature ,that th~s p h e n o m e n o n sometimes occurs: on PtRh [i1] there are Auger and LEED evidences that after NO dissociation oxygen atoms are preferentially adsorbed on Rh. In our case, CO adsorbed on Pt sites can easdy react w~th oxygen atoms adsorbed on neighboring
Zn atoms, producing CO 2, without significant production of other intermediates or byproducts. In conclusion the differences observed for the N O - C O interaction on the two differently pretreated samples can be explained by two factors: (i) the easier dissociation of the NO molecules on the a~loy surface, mainly determined by an electron transfer from the electron rich Zn surface atoms of the metallic phase to the antibonding orbitals of the NO and (ii) the adsorption of the oxygen atoms produced in the dissociation on the same atoms, with absence of competition for CO chemisorption on Pt sites, different than on pure Pt. O n monocrystalline Pt samples [10] it has been quite firmly d e t e r m i n e d that the NO dissociation is the rate determining step and between supported Pt catalysts the most active one is P t / T i O z reduced at high temperature while the less active one is P t / S i O z, not active at all [5]. This observation can support the idea that the alloying of Pt with elements characterized by a work function lower than the pure element can make easier the NO dissociation and the CO oxidation; in fact PtTi phases have been identified on high-temperature reduced P t / T i O . samples [i2].
Acknowledgement We gratefully acknowledge financial support flom C N R "Progetto Fina!izzato Chimica Fine e Secondaria II".
References [1] [2] [~] {4]
G L Hailer and D E Resasco, Adv Catal 36 t1989) 173. M A. Vanmce and C C Twu, J. Catal 82 (1983) 213 N K Pande and A I Bell, J Catal 97(1986) 137 W.F Egelhoff, Jr, m The Chemical Physics of Sohd Surfaces and Heterogeneous Catalysis, Vol 4, Eds D A King and D.P Woodruff (Elsevier, Amsterdam, 1982) ch 9, p. 397 [5] A Kudo, M Steinberg, A J Bard, A Campiov M A Fox, T E Mallouk, S E Webber and J M. White. J Catal 125 (1990) 565 [ill F Boccuzzi, G Ghlottl, A Chiormo and L Marchcse, Surf Scl 233 (199{))141
F Boccuzz: et al / NO reductton by CO on P t / Z n O catalysts
[7] F. Boccuzzl, A. Chlormo and G Ghlottl, Surf Scl. 209 (1989) 77 [8] F Boccuzzz and E Gughelmmo|~l, Surf Scl, m press [9] F. Solymosi, L. Volgyesl and J Sarkany, J. Catal. 54 (1978) 336 [10] Th. Fink, J.P. Dath, M.R. Bassett, R. lmbihl and G Ertl, Surf. Scl. 245 (1991) 96, and references tlaerem.
519
[11] H Htrano, T Yamada, K Tanaka, J Stera and B E Nzeuwenhuys, Surf Scl. 222 (1989) L804 [12] L Wang, G W Qlas, H O Ye, K H K u o a n d Y k Chcn m: Proc. 9th Int Congr on Catalysis, Calgary, Eds. M J. Phdhps and M Ternan, Vol. Ill (1988) p 1253.
Surface Science 269/270 (1992) 514-519 North-Holland
IR study of NO reduction by CO on Pt/ZnO catalysts F. Boccuzzi, G. Ghiotti, A. C h i o r i n o a n d E. G u g l i e l m i n o t t i Dtpart~mento (It Chumca lnorgamca, Ctumlca FIswa e Ctumtca det Matenah, eta P G~u~Ja Z 10125 Tonno, Italy Received 29 August 1991; accepted for pubhcatmn 22 October 1991
An FTIR study of the N O - C O interaction, from RT up to 533 K, was done on two differently pretreated P t / Z n O samples, the first, mddly reduced, formed of small pure Pt particles dispersed on ZnO, and the second, strongly reduced, presenting PtZn alloyed metal parhcles. On the mddly reduced one by heating m NO-CO atmosphere at 373 K a qmte strong band assigned to an lsocyanate speoes is formed, produced by reactmn of CO molecules with mtrogen atoms, coming from the NO decomposition At the same time the intensity of the CO band grows up to that of the wrgln sample, showing thai the Isocyanate speoes spd~ over from the metal to the support. On strongly reduced samples the isocyanate speoes band is quite weak and a CO 2 band is produced. The different behavlour wdl be discussed taking into account the different surface composmon and electromc properhes of the two samples.
1. Introduction Noble metals supported on reducible oMdes have been extensively studied m the last years, in view of their activity in the synthesis of hydrocarbons from CO and H , [1,2] However, in spite of the interest that these systems can pre~cnt also for the catalytic rcductton of NO. only few papers [3] can be found on the latter topic. The reduction of NO by CO has been the subject of extensive research in recent years; the catalysts used are usually supported noble metal catalysts (Rh, Pt, Pd, Ru) [4]. Recently it was shown that the Pt catalyst activity for this reaction strongly depends on the support and that for catalysts supported on n-~pe sem:cend,:ctors the activity depends on the reduction pretreatment temperature [51. However, the mechanism of the reaction on supported catalysts has not been tinambiguously determined; for example, dtsagreement persists about the intermediates of th~s reaction and on the dependence of thetr produchon on the surface chemical composition of the cataly.,t. In this paper we will present an FTIR study of the N O - C O mterachon on two differently
pretreated P t / Z n O samples, showing a different surface composition [6].
2. Experimental P t / Z n O samples wcrc prepared, as previously dewrlbed [7], by photodcposit~on on commercial Z n O Kadox, the Pt content was -~ 1.01 at% P t / P t + ZnO. The IR spectra were run at R T with a Perkin Elmer F T I R 1760 at a resolution of 4 c m - t ; the number of scans (100-200) was changed according to the transmission of the sample with the aim of obtainmg similar levels of noise. High-purity 02, H2, CO from Matheson were used without further purJfwafion, whetea~ NO from Matheson was freshly dtstilled before use. h n n expe:iments were carr;ed out The z. .c.t.~. n r~n..... on samples already submitted to the follov, mg thermal and chemical treatments: (2) after oxldatton and evacuation at 623 K the sample was mildly reduced wtth a H z / N 2 0.5% mLxture at 493 K and then outgassed at the same temperature (sample labeled PR~493 ); 0i) same treat-
00"~9-6028/92/$05 00 ~, 1992 - Elsc~lLr Science Publishers B V All rights reserved
F Boccuzzt et al / N O reduction by C O on P t / Z n O c atah'~ts
ments above but lastly the sample was reduced m pure H 2 at 573 K and than outgassed at the same temperature (sample labeled PR573).
3. Results
CO-NO coadsorption on Pt / ZnO samples The effect of the NO adsorption and sttbsequent CO inlet on the IR transmission spectra of a PR~493 sample is shown in fig. la. The inlet of 10 Torr of CO on the preadsorbed NO, showing only one band at 1780 cm -t, causes the depletion of this band and the growth of an absorption at 2097 cm -t with a weak component at 2132 c m without changes in the overall transmission; the frequencies of the CO absorptions are the same
B
';15
ot that observed on a virgin sample treated m the same way, as it car, be seen more clearly in fig lc, c u ~ c 2, however the mtegral~d intensity of the CO stretching band is nearly, hallcned ( / = ~ ) cm -~) m comparison w~th that observed on the virgin sample (I = 20 c m - ~, fig. 4a of ref. [6]). Fig. lb shows the effect of heating in N O - C O atmosphere on the transmission spectra of a PRI493 sample, at increasing temperatures from RT to 513 K; only minor changes in the transmission are observed. As for the vibrational band changes, the heating at 423 K (fig. lc, curve 3) produces an asymmetric band at ~ 2220 cm-~, I = 2.4 cm-~; at the same time a strong intensification of the band at 2097 cm-~ and a decrease of the band at 2132 cm - t are also observed. After this treatment the integrated intensity of the 2150-2000 cm-= region is almost coincident
b 3
I"T
1
q
2097
,4000 8
3'500
3000
~00 lob
--
4
i
2220
o 8000
~_500
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O.sA
I
--
!
<2 1780
~
u3
J 0 4000
3500
3000
~oo ca
2000
15o0
lO00
2,400
2200
2000
~700
-t
Pig 1 IR spectra ot the N O - C O interachon on a PR1493 sample Section (a~ - traqsnnsslon spectra after R F mteracuons curve l, background, curve 2, after contact with 10 Torr of NO, curve 3, after inlet ot !0 Torr of CO. Section (b) - transmission spectra, run at R T after m~eractlons at temperatures up to 533 K curve 1, al 423 K, " w e ?, at 493 K, curve 3, at 533 K Section (c) absorbance spectra, obtained after subtraction of the background curve 1, 10 Aurr of NO, curve 2, Inlet of 10 Torr of CO, curve 3 after h e a a n g at 423 K, curve 4, after t~eatlng at 493 K
F. Boccuzzt et al / NO reductton by CO on Pt / ZnO catalysts
516
with that measured on a similar sample after heating at the same temperature in pure CO atmosphere, I = 22 cm -t, in comparison with I = 23 c m - l for the clean sample [b]. By further heating up to 493 K (fig. lc, curve 4) a strong growth of the band at 2220 cm-~ is observed (1 = 16.6 c m - l ) , and simultaneously the band at 2132 c m - t completely disappears. The integrated intensity of the 2150-2000 cm-~ region is now 29 c m - ~, slightly higher than on clean samples treated at the same temperature in pure CO, I = 26 c m - t (curve 3 of ref. [6]). A further heating up to 533 K causes the complete depletion of the bands at 2220 and 2097 c m - t (fig. lb, curve 3).
The effect of the NO adsorption and of the subsequent CO inlet on the IR transmission spectra of the sample PR573 is shown in fig. 2a, curves 1 and 2 respectively. NO adsorption produces a significant increase of IR transmission and, at the same time, a band of medium intensity at 1812 c m - ~, with other weaker components, at 1900 and 1675 cm - t , assigned to the NO stretching of molecules adsorbed on different surface sites [8]. The CO inlet on this saml;le at R T causes a small decrease in the transmission and the depletion of the NO stretching band; it yields, also, a CO stretching band at 2078 c m - t , a very weak c o m p o n e n t at 2132 cm-~ and a very weak absorption at 2220 c m - t , observed on the more
t.50 I b gT
c
o.2AI
0 75
0
2078
O0 4000
1.50
3500
3000
2500 cm -1
2000
-
1500
iO00
1812
~T
2
2340
I
0.75
0.00 4000
,
3500
3000
2500 CB - i
2000
t500
~000
2400
L:~00
2000 cm - t
,
t700
Fig 2 IR ',pectra of the N O - C O mtcrachon on a PR573 ',ample Sechon (a) - transmission spectra after RT interaction,, curve 1. background, curve 2, after cop'.act vvlth 10 Torr of NO, cu~,e 3, after m i t t of 10 Torr of CO Section (b) - transm,sslon spectra, run at RT, after mterachon,, at temperature~ up to 493 K curve 1, at 373 K; curve 2, at 423 K, curve 3, at 493 K Section (c) ab~orbance ',pectra, obtained after ',uhtract~on of the background o! curve 1, N O - C O mterachon at RT, cu~,e 2, after heating at 373 K c u ~ e 3 alter heating at 473 K
F Boccuzzt et al. / NO reductton by CO on P t / Z n O catalysts
mildly reduced sample only after thermal treatment at 423 K and a band at 2340 cm-~, due to CO 2 formation (fig. 2a, curve 3). The frequency and the intensity of the CO stretching band (v = 2078 cm-~, I = 18.4 cm-~) are higher than that observed for a virgin sample reduced at the same temperature (v = 2067 cm -l, I = 4.2 c m - ' , ref. [61). By heating at 373 K (fig. 2b, curve 1) the transmission decreases while no changes are observed in the vibrational features of the adsorbed species (fig. 2c, curve 2); at 423 K a further reduction of transmission is detected (fig. 2b, curve 2); after heating at 473 K the decrease in the IR transmission is also accompanied by a significant weakening of the CO band (fig. 2c, curve 3, 1 = 6.5 c m - I ) . By heating at 493 K the band is completely depleted and also the transmission is further reduced (not shown).
4. Discussion
The effects of NO adsorption on differently pretreated P t / Z n O ~amples have been prevlouslv illustrated [8]: on the basis of the frequency of the NO band, looking at our previous characterization of the same samples by H R T E M microscopy and CO chem~sorption [6] and to literature data of NO adsorption on monocrystalline and supported Pt samples, reported and discussed in ref. [8], we assigned the 1780 cm -1 band to NO molecules chemisorbed on top on stepped Pt sites, and the 1812 cm -I band to NO adsorbed on Pt sites on ~.~,e PtZn alloy surface, perturbed by oK gcn atoms produced in the NO dissociation ana chemisorbed nearby. The CO interaction at RT on NO precovered PRt493 produces the depletion of the NO stretching mode and the growth of CO stretching modes; the intensities of the CO bands are, however, as exposed in the results section, sigmficantly weaker than on clean, virgin samples. The experiments of heating in N O - C O atmosphere lead to formation of isocyanate species. These are produced by reaction of CO molecules with N ~.toms coming from the decomposition of
517
NO. In fact a band at 2220 cm-~ appears m the sample PR~493 by heating at 423 K, and intensifies sigmficantly at 493 K: on the basis of Its frequency and thermal stability and by comparison with literature data [9], the band can be assigned to the asymmetric stretching of an isocyanate species. We observed that after interaction at RT, when the isocyanate species is still absent, the intensity of the CO band --- ~1 of that observed on a virgin sample while, after heating at 423 or 493 K and during the parallel growth of the isocyanate band at 2220 era-l, the integrated intensity of the CO band becomes similar to that observed on a virgin sample. These findings indicate that some NG is dissociated at RT on this sample on some Pt surface sites or that the CO coadsorption induces the decomposition of the NO leaving oxygeh atoms on the surface of the Pt pameles, being the nitrogen chemisorbed amount usually considered negligible [10]. At T > 423 K a cleaning of the Pt particles from the adsorbed ox3gen atoms, with production of C O , in the gas phase, occurs, as shown by the growth of the CO stretching band; at the same time the products of the reaction N + CO ~ NCO migrate from the metal to the support. The occurrence of the spillover to the support Is indicated by the fact that the CO band shows almost the same frequency and the same l n t e n s ~ of the virgin sample: this fact suggests that CO and NCO are not coadsorbed on the metallic particles. The NCO band grows up during the heating up to 493 K, while by heating at 533 K, both the CO and NCO bands are depleted. As for the sample PR573 the C O - N O interaction produces a weak band at 2220 c m - l almost unaffected ai,-o by heating up to 473 K. The formation and stabihty of isocyanate species in the NO + CO reaction on different Pt supported catalysts (Pt on SIO 2, MgO, A[203, etc ) have bcen previously studied [9]; tt was shown that the stabihty of this species strongly depends on the support: it decomposes rapidly on P t / T i O 2 at 573 K, whde it is extremely stable on P t / S i O 2 up to T > 723 K. In our case the d e c o m p o s m o n of the ~socyanate species occurs at a very low temperature, at 533 K m few minutes the band is
518
F Boccuzzl et al / NO reductton by CO on Pt / ZnO catalysts
completely depleted. It seems therefore confirmed, as previously suggested [9], that the isocyanate species are adsorbed on the support and that their stability is primarily determined by the chemical and electronic properties of the support. Therefore the low amount of the isocyanate species observed on our strongly reduced sample can indicate that the anionic species is far less stabilized on the electro, licher samples. There are other differences in the interaction N O - C O on the sample PR573 so that it can be useful to discuss briefly. The inteI action with NO alone causes, in this case, a significai, t gain in the IR transparency, this phenomenon being interpreted [8] as due to an electron trapping process on oxygen atoms produced by NO dissociation, occurring on the defective ZnO phase and on the PtZn alloy particles. Moreover the CO inlet produces the depletion of the 1812 c m - I NO band and the growth of a CO stretching band at higher frequency and with a higher intensity than on a virgin sample. At the same time, and without significant changes by heating up to 423 K, a CO z band of medium intensity appears. The remark that the CO band observed after NO interaction has a higher frequency and a higher intensity as that shown on a virgin sample pretreated m the same way could be an indication that the reaction of dissociation of NO occurring on this sample causes a change, as a consequence of the heat produced, in the surface composition of the alloy, with an enrichment in Pt. Moreover, it can be hypothesized that NO dissociation on the alloy produces oxygen atoms adsorbed preferentially on the more electropositive element of the alloy, on Zn atoms, while nitrogen atoms, as on pure Pt, recombine and desorb immediately; therefore Pt sates remain free. We do not have direct evidences, in our case, confirming a different chemisorption of the nitrogen and oxygen atoms on the two components of the alloy; however there are some evidences m the hterature ,that th~s p h e n o m e n o n sometimes occurs: on PtRh [i1] there are Auger and LEED evidences that after NO dissociation oxygen atoms are preferentially adsorbed on Rh. In our case, CO adsorbed on Pt sites can easdy react w~th oxygen atoms adsorbed on neighboring
Zn atoms, producing CO 2, without significant production of other intermediates or byproducts. In conclusion the differences observed for the N O - C O interaction on the two differently pretreated samples can be explained by two factors: (i) the easier dissociation of the NO molecules on the a~loy surface, mainly determined by an electron transfer from the electron rich Zn surface atoms of the metallic phase to the antibonding orbitals of the NO and (ii) the adsorption of the oxygen atoms produced in the dissociation on the same atoms, with absence of competition for CO chemisorption on Pt sites, different than on pure Pt. O n monocrystalline Pt samples [10] it has been quite firmly d e t e r m i n e d that the NO dissociation is the rate determining step and between supported Pt catalysts the most active one is P t / T i O z reduced at high temperature while the less active one is P t / S i O z, not active at all [5]. This observation can support the idea that the alloying of Pt with elements characterized by a work function lower than the pure element can make easier the NO dissociation and the CO oxidation; in fact PtTi phases have been identified on high-temperature reduced P t / T i O . samples [i2].
Acknowledgement We gratefully acknowledge financial support flom C N R "Progetto Fina!izzato Chimica Fine e Secondaria II".
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