FTIR Spectroscopy Study of CO Adsorption on PtNaMordenite

Langmuir 2005, 21, 11821-11828

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FTIR Spectroscopy Study of CO Adsorption on Pt-Na-Mordenite Mihail Mihaylov,† Kristina Chakarova,† Konstantin Hadjiivanov,*,† Olivier Marie,‡ and Marco Daturi*,‡ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, and Laboratoire Catalyse et Spectrochimie, CNRS-ISMRA, Universite´ de Caen, 6, bd. Mare´ chal Juin, 14050, Caen Cedex, France Received July 11, 2005. In Final Form: August 22, 2005 Different carbonyls are formed after CO adsorption at ambient temperature on a Pt-Na-mordenite (Pt-Na-MOR) sample. Pt3+(CO)2 dicarbonyls (νs at 2205 cm-1 and νas at 2167 cm-1) are decomposed without formation of monocarbonyls. The respective mixed-ligand species, Pt3+(12CO)(13CO), formed after 12CO-13CO coadsorption, display bands at 2192 and 2131 cm-1, in excellent agreement with the theoretically calculated values. Pt2+-CO species absorb at 2145 cm-1 and are not able to accept a second CO molecule. Pt+-CO carbonyls are characterized by a band at 2111 cm-1. Under CO equilibrium pressure, these species are converted into dicarbonyls (νs at 2135 cm-1 and νas at 2101 cm-1). The respective mixed-ligand species, Pt+(12CO)(13CO), manifest bands at 2123 and 2069 cm-1, in good agreement again with the theory. Different carbonyls of metallic platinum are observed below 2100 cm-1. In addition, weakly adsorbed CO was registered as Na+-CO complexes (2177 and 2165 cm-1) and Na+-OC-Na+ species (2138 cm-1). It was found that during desorption of CO platinum was reduced, ultimately to metal. However, heating in a NO + O2 mixture leads to reoxidation of the metal particles and restoration of the initial state of the sample.

1. Introduction Platinum is one of the most used and most studied metals in the field of catalysis. Reports on the application of platinum in new catalytic processes appear regularly. That is why, there are thousands of studies devoted to characterization of different platinum-containing catalysts. However, the efforts have been, as a rule, concentrated on metal platinum particles.1-10 Recently, some authors11,12 indicated that in several cases, mainly oxidation reactions, the catalytically active sites could be platinum cations or Ptn+/Pt0 redox couples could operate in the processes. For that reason, the characterization of different surface cationic sites, underestimated so far, is of great importance for understanding the mechanism of redox catalytic reactions involving platinum catalysts. IR spectroscopy of probe molecules is one of the most powerful techniques for characterization of catalyst surfaces.13-15 The most frequently used IR probe is CO.13 * To whom correspondence should be addressed. Fax: 003593 8705024. E-mail: [email protected]. † Bulgarian Academy of Sciences. ‡ Universite ´ de Caen. (1) Hollins, P. Surf. Sci. Rep. 1992, 16, 51. (2) Bourane, A.; Dulaurent, O.; Chandes, K.; Bianchi, D. Appl. Catal. A 2001, 214, 193. (3) Welch, P. C.; Mills, P. S. W.; Mason, C.; Hollins, P. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 151. (4) Anderson, J. A.; Chong, F. K.; Rochester, C. H. J. Mol. Catal. A 1999, 140, 65. (5) Gracia, F.; Li, W.; Wolf, E. E. Catal. Lett. 2003, 91, 235. (6) Venezia, A. M.; Liotta, L.; Deganello, G.; Terreros, P.; Pen˜a, M.; Fierro, J. L. G. Langmuir 1999, 15, 1176. (7) Crocoll, M.; Kureti, S.; Weisweiler, W. J. Catal. 2005, 229, 480. (8) Tang, C.; Zou, S.; Weaver, M. J. Surf. Sci. 1998, 412/413, 344. (9) Vu, T. N.; van Gestel, J.; Gilson, J. P.; Collet, C.; Dath, J. P.; Duchet, J. C. J. Catal. 2005, 231, 453. (10) Raddi de Araujo, L. R.; Schmal, M. Appl. Catal. A 2002, 235, 139. (11) Schiesser, W.; Vinek, H.; Jentys, A. Catal. Lett. 2001, 73, 67. (12) Burch, R.; Sullivan, J. A. J. Catal. 1999, 182, 489. (13) Hadjiivanov, K.; Vayssilov, G. Adv. Catal. 2002, 47, 307.

It can provide information on the oxidation and coordination state of the cations and their Lewis acidities. Some complications of the use of CO as a probe can arise from the reductive properties of this molecule. Indeed, it has been reported that cationic platinum is slowly reduced in CO atmosphere.16,17 The number of studies devoted to CO adsorption on nonreduced platinum catalysts is restricted.11,12,16-30 It is generally accepted that surface carbonyls of cationic platinum are observed above 2100 cm-1, but there is no consensus on the oxidation state of the adsorption center. Often the carbonyl bands have been simply assigned as due to Ptδ+-CO species. (14) Kno¨zinger, H. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1997; Vol. 2, p 707. (15) Davydov, A. Molecular Spectroscopy of Oxide Catalyst Surfaces; Wiley: Chichester, U.K., 2003. (16) Hadjiivanov, K.; Saint-Just, J.; Che, M.; Tatibouet, J. M.; Lamotte, J.; Lavalley, J.-C. J. Chem. Soc., Faraday Trans. 1994, 90, 2277. (17) Hadjiivanov, K. J. Chem. Soc., Faraday Trans. 1998, 94, 1901. (18) Chakarova, K.; Mihaylov, M.; Hadjiivanov, K. Micropor. Mesopor. Mater. 2005, 81, 305. (19) Chakarova, K.; Mihaylov, M.; Hadjiivanov, K. Catal. Commun. 2005, 6, 466. (20) Solomennikov, A.; Davydov, A. Kinet. Katal. 1984, 25, 403. (21) Shpiro, E.; Tkachenko, O.; Jaeger, N.; Ekloff, G.; Grunert, W. J. Phys. Chem. B 1998, 102, 3798. (22) Stakheev, A.; Shpiro, E.; Tkachenko, O.; Jaeger, N.; SchulzEkloff, J. J. Catal. 1997, 169, 382. (23) Marchese, L.; Boccuci, M. R.; Coluccia, S.; Lavagnino, S.; Zecchina, A.; Bonneviot, L.; Che, M. Stud. Surf. Sci. Catal. 1989, 48, 653. (24) Primet, M.; Basset, J. M.; Mathieu, M. V.; Prettre, M. J. Catal. 1973, 29, 213. (25) Alexeev, O.; Graham, G. W.; Kim, D. W.; Shelef, M.; Gates, B. C. Phys. Chem. Chem. Phys. 1999, 1, 5725. (26) Holmgren, A.; Andersson, B.; Duprez, D. Appl. Catal. B 1999, 22, 215. (27) Anderson, J. A.; Rochester, C. H. Catal. Today 1991, 10, 275. (28) Kustov, L.; Sachtler, W. M. H. J. Mol. Catal. 1992, 71, 233. (29) Novakova, J. Phys. Chem. Chem. Phys. 2001, 3, 2704. (30) Yamasaki, Y.; Matsuoka, M.; Anpo, M. Catal. Lett. 2003, 91, 111.

10.1021/la051877k CCC: $30.25 © 2005 American Chemical Society Published on Web 11/09/2005

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Figure 1. FTIR spectra of CO adsorbed on Na-MOR. Equilibrium pressure of 2400 (a), 1470 (b), 530 (c), 270 (d), and 133 Pa CO (e). The spectra are background corrected and the gas-phase CO subtracted.

The aim of this work is to investigate the species produced after CO adsorption on cationic platinum sites in Pt-Na-MOR and to draw conclusions on the applicability of this probe molecule for characterization of cationic platinum sites. 2. Experimental Section The Pt-Na-mordenite (Pt-Na-MOR) sample was prepared by a conventional ion-exchange procedure. Briefly, 1.00 g of NaMOR was suspended in 10 mL of a 0.0055 M solution of Pt(NH3)4Cl2 and stirred for 3 days at ambient temperature. Then the precipitate was thoroughly washed with distilled water and dried at 353 K for 10 h. The IR spectra were recorded on Nicolet Magna IR 550 and Nicolet Avatar 360 spectrometers at a spectral resolution of 2 cm-1 and accumulating 64 scans. Self-supporting pellets (ca. 15 mg cm-2) were prepared from the sample powder and treated directly in the purpose-made IR cells. The latter were connected to a vacuum-adsorption apparatus with a residual pressure below 10-3 Pa. Prior to the adsorption measurements, the samples were activated in two different ways. Activation procedure 1 consisted in 1 h thermal treatment under vacuum at a given temperature. Activation procedure 2 was performed by initial heating of the sample in dry oxygen (13.3 kPa, 1 h, 673 K), followed by 1 h evacuation at the same temperature. In all cases, the temperature was reached by heating at a rate of 2 K min-1. Carbon monoxide (>99.997) and nitrogen monoxide (>99.9) were supplied by Air Liquide, France. Labeled carbon monoxide (13C isotopic purity of 92.9 at. %) was delivered by CEA-ORES, France. Before use, carbon monoxide and oxygen were passed through a liquid nitrogen trap. Except specially specified, the adsorption of CO was performed at ambient temperature.

3. Results and Discussion 3.1. Adsorption of CO on Activated Na-MOR. To be able to distinguish between the carbonyl species formed with the support and the platinum carbonyls, initially we studied CO adsorption on the Na-MOR sample. Introduction of CO (2.4 kPa equilibrium pressure) to the sample (activation procedure 2) led to the appearance of two principal bands, at 2165 and 2138 cm-1 (Figure 1, spectrum a). A high frequency shoulder at 2177 cm-1 and a weak feature at 2112 cm-1 were also distinguished. Decrease in the equilibrium CO pressure led to a decrease in intensity of all bands (Figure 1, spectra b-e) and their disappearance after short evacuation. The bands at 2177 and 2165 cm-1 are assigned to two kinds of Na+-CO species.31-34 Usually these species are (31) Marie, O.; Thibault-Starzyk, F.; Massiani, P. J. Catal. 2005, 230, 28.

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detected at low temperatures, but the electrophilicity of sodium cations in zeolites is high enough to form carbonyl complexes even at ambient temperature under CO equilibrium pressure.34 The band at 2138 cm-1 has been initially assigned to physically adsorbed CO.31 However, the stability of the species is too high for physically adsorbed CO. The appearance of the band below 2143 cm-1 (the frequency at which gaseous CO absorbs) could be due to some π-back-bonding. We can rule out this hypothesis, since there is no π-donor in the system. Another possibility for explaining the low wavenumber is O-bonded CO.13 In this case, however, the frequency is expected even at lower wavenunbers, down to 2111 cm-1.13 Busca et at.32,33 have recently proposed this band to be due to Na+-OC-Na+ species. We agree with this interpretation. 3.2. Initial Characterization of the Pt-Na-MOR Sample. The IR spectrum of the Pt-Na-MOR sample recorded after evacuation at 473 K (activation procedure 1) contained bands at 1962, 1860, and 1633 cm-1 originating from overtones and combinations of the zeolite skeletal vibrations.35 A sharp band at 3744 cm-1 (silanol groups) and a weaker one at 3686 cm-1 were registered in the OH region. Since the band at 3686 cm-1 was not registered with a hydrogen-reduced sample, it most probably characterized Ptn+-OH species. A series of intense bands at 3326, 3275, 3210, and 3115 cm-1 were detected at lower frequencies and attributed to N-H modes of residual ammonium ions.36 Evacuation at 573 K led to almost full disappearance of the N-H bands. In addition, a weak hydroxyl band at 3628 cm-1 appeared due to liberation of these species from adsorbed ammonia. Usually the acidic zeolite hydroxyls are observed around 3610 cm-1,31 but in our case, the wavenumber was higher probably due to zeolite modification by guest platinum species. The spectrum of the sample initially treated in oxygen (13.3 kPa, 1 h, 673 K) and then evacuated for 1 h at 673 K (activation procedure 2) is similar. In this case, however, neither traces of N-H modes nor the band at 3686 cm-1 were registered. 3.3. Adsorption of CO on Activated Pt-Na-MOR. Adsorption of CO on the Pt-Na-MOR sample (activated by the “oxidative” procedure 2) was initially performed on doses successively added to the cell. The first dose provoked the appearance of three bands at 2205, 2168, and 2098 cm-1 and a broad feature with two maxima at 2140 and 2120 cm-1 (Figure 2, spectrum a). With the increase in amount of CO adsorbed on the sample, the bands at 2205 and 2168 cm-1 increased in concert, the band at 2098 cm-1 was gradually shifted to 2102 cm-1 and the band at 2140 cm-1 developed and was settled at 2136 cm-1 (Figure 2, spectra b-g). In addition, two weak bands, at 2218 and 2186 cm-1, were also registered. A careful analysis of the spectra showed that at higher coverages the band at 2168 cm-1 started to develop faster than the 2205 cm-1 band. This can be explained by the superimposing of the band at 2168 cm-1 with the band at 2165 cm-1 observed with the support. Most probably, under these conditions, the Na+-OC-Na+ band at 2138 cm-1 contributed to the (32) Salla, I.; Montanari, T.; Salagre, P.; Cesteros, Y.; Busca, G. J. Phys. Chem. B 2005, 109, 915. (33) Salla, I.; Montanari, T.; Salagre, P.; Cesteros, Y.; Busca, G. Phys. Chem. Chem. Phys. 2005, 7, 2526. (34) Hadjiivanov, K.; Massiani, P.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 1999, 1, 3831. (35) Hadjiivanov, K.; Saussey, J.; Freysz, J.-L.; Lavalley, J.-C. Catal. Lett. 1988, 52, 103. (36) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds; Mir: Moscow, 1966.

Study of CO Adsorption on Pt-Na-Mordenite

Figure 2. FTIR spectra of CO adsorbed on activated Pt-NaMOR. Progressively increasing amount of CO introduced into the cell (a-g) and after evacuation (h). The spectra are background corrected. The second derivative of spectrum h is shown in the inset.

absorbance around 2136 cm-1. Short evacuation hardly affected the spectrum. The band at 2102 cm-1 was back shifted to 2097 cm-1 (Figure 2, spectrum h). It is known that second derivatives of the spectra are very useful for exact determination of the maxima of overlapping bands. The second derivative of spectrum “h” is presented in the inset of Figure 2. Note that a band at 2148 cm-1 can be distinguished in this case. The bands stable toward evacuation at 2218, 2205, 2186, 2168, 2148, and 2138 and ca. 2100 cm-1, were not detected with Na-MOR and are assigned to different platinum carbonyls. According to data from the literature1-10 we assign the band around 2100 cm-1 to Pt0-CO species. Indeed, a blue shift of the band maximum with the coverage increase is typical of CO adsorbed on metal particles. The bands at higher frequencies are generally assigned to cationic platinum carbonyls. In recent studies of CO adsorption on Pt-H-ZSM-5, we detected a set of bands at 2211 and 2175 cm-1 and assigned them to Pt3+(CO)2 dicarbonyls.18,19 This suggests that the intense bands at 2205 and 2168 cm-1 observed in this study also characterize Pt3+(CO)2 species. The set of two weak bands at 2218 and 2186 cm-1 are assigned to another family of Pt3+(CO)2 dicarbonyls. Additional arguments in support of these interpretations will be provided below. The other bands registered in this study strongly differ from those reported with Pt-H-ZSM-5 and will be assigned later. However, for the sake of convenience, the main observed bands and their assignments are summarized in Table 1. Some experiments were performed on a sample activated in vacuo (activation procedure 1). The concentration of residual ammonia species was negligible when the activation was performed at 573 K. That is why we studied CO adsorption on a 573 K activated sample. Adsorption of a small dose of CO provoked the appearance of the same bands as those already described (2204, 2168, 2147, and 2100 cm-1), but with very different relative intensities (Figure 3, spectrum a). In this case, a broad Pt0-CO band around 2100 cm-1 dominated in the spectrum, whereas the other bands appeared with reduced intensities. The results imply that the activation treatment 1 results in the reduction of a significant fraction of deposited platinum to metal. The next two portions of CO led mainly to development of the bands at 2204, 2168, and 2100 cm-1 and appearance of a band at 2139 cm-1 (Figure 3, spectra b,c). Increase of the CO equilibrium pressure up to 1.3 kPa provoked additional rise in intensity of the bands at 2168 (settled at 2166 cm-1) and 2139 cm-1 (Figure 3, spectrum d). These are the bands associated with the

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support and disappeared after evacuation at ambient temperature (Figure 3, spectrum e). The second derivatives of some spectra are presented in the inset of Figure 3. Interestingly, there was no band corresponding to the strong Pt0-CO band at 2100 cm-1. This suggests that the band consists of many overlapping bands due to heterogeneity of the metal platinum sites. 3.4. Adsorption of CO on Pt-Na-MOR Oxidized by NO + O2. It is well-known that a NO + O2 mixture is a stronger oxidizing agent than is oxygen itself.37 To obtain additional information on the oxidation state of the cationic platinum sites and the possibility of reoxidation of metallic platinum, we tried to reoxidize a sample evacuated at 573 K (procedure 1) and subjected once to a CO adsorptiondesorption cycle. The pellet was placed in a NO (765 Pa initial partial pressure) + O2 (1530 Pa initial partial pressure) mixture and then heated for 1 h at 623 K under this atmosphere. After that the gas phase was evacuated at the same temperature. Adsorption of CO on the sample thus treated was performed on small doses again (see Figure 4, spectra a-c). The picture is similar to that registered with the sample activated by procedure 2. However, the overall intensity of all bands was reduced. Note also that no bands in the 2150-2100 cm-1 region appeared which indicated that the respective Ptn+ sites had been oxidized. The Pt0CO band was registered at lower frequencies. We should like to note that no broad band around 2100 cm-1 was detected in this case. Thus, the results imply that, on average, the oxidation state of platinum in the reoxidized sample was higher than in the evacuated one. Only a small fraction of metallic platinum was left as evidenced by the carbonyl band at 2089 cm-1. The position of this band suggests that it was formed with the participation of relatively big particles,7 which is consistent with their expected resistance toward oxidation. After maintaining CO equilibrium pressure in the IR cell (Figure 4, spectrum d), bands due to CO adsorbed on the support (2165 and 2138 cm-1) and at 2226, 2216, and 2185 cm-1 developed. The set of bands at 2216 and 2185 cm-1 were already assigned to Pt3+(CO)2 species. The carbonyls associated with the support as well as the band at 2226 cm-1 were removed by evacuation (see Figure 4, spectrum e and difference spectrum “d-e”). In this case bands at 2141 and 2133 cm-1 were also detected. The band at 2226 cm-1 characterizes unstable species and is attributed to CO adsorbed on extraframework Al3+ sites.13 The latter have been formed during the sample treatment. The bands at 2141 and 2133 cm-1 are assigned to carbonyls of reduced platinum sites. Evidently, reduction has occurred under CO equilibrium pressure. 3.5. Thermal Desorption Experiments. In a new set of experiments, where the pellet was reoxidized, we studied the stability of the carbonyl complexes. Adsorption of CO on the sample led to the appearance of the already described bands at 2205, 2167, 2138, and 2096 cm-1 (Figure 5, spectrum a). Decrease in the equilibrium pressure/evacuation resulted in a decrease in intensity (Figure 5, spectrum b), and disappearance (Figure 5, spectrum c) of the carbonyl bands at 2167 and 2138 cm-1 associated with the support itself. The disappearance of the band at 2138 cm-1 revealed two weak bands at 2141 and 2131 cm-1 (the exact positions of these bands were determined by the second derivative of the spectrum). The 2096 cm-1 band was shifted to 2092 cm-1. Note that the band at 2205 cm-1 remained unaffected. (37) Hadjiivanov, K. Catal. Rev.-Sci. Eng. 2000, 42, 71.

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Table 1. Assignment of the IR Carbonyl Bands Observed in This Study bands carbonyl bands on the support 2177 cm-1, 2165 cm-1 2138 cm-1 cationic platinum carbonyls 2205 and 2167 cm-1 2146 cm-1 2111 cm-1 2135 and 2101 cm-1 carbonyls of metallic platinum 2102-2000 cm-1 1870 cm-1 a

The dicarbonyl structure proven by

assignment

comments

two kinds of Na+-CO species Na+-OC-Na+ species

disappear during evacuation disappear during evacuation

νs and νas of Pt3+(CO)2 speciesa νCO of Pt2+-CO νCO of Pt+-CO νs and νas of Pt+(CO)2 species*

complex-specified geminal species stable to 473 K stable to 623 K form dicarbonyls site-specified geminal species

νCO of Pt0-CO νCO of Pt0-CO- Pt0

different sites on reduced samples

12CO-13CO

coadsorption.

Figure 3. FTIR spectra of CO adsorbed on Pt-Na-MOR evacuated at 573 K. Progressively increasing amount of CO introduced into the cell (a-c), under equilibrium pressure of 3870 Pa (d) and after evacuation (e). The spectra are background corrected. The second derivatives of spectra a and c are shown in the inset.

Figure 4. FTIR spectra of CO adsorbed on Pt-Na-MOR oxidized in a NO + O2 mixture. Progressively increasing amount of CO introduced into the cell (a-c), under equilibrium pressure of 320 Pa (d) and after evacuation (e). The spectra are background corrected and the gas-phase CO subtracted.

Evacuation at 373 K leads to an almost 2-fold decrease in intensity of the bands at 2205 and 2167 cm-1 (Figure 5, spectrum d). The bands at 2141 and 2131 cm-1 are not more observable and a band at 2145 cm-1 becomes dominating in the region. New bands at 2111 and 1985 cm-1 emerge. The band assigned to carbonyls of metallic platinum (2092 cm-1) is shifted to 2082 cm-1 and slightly increases in intensity. Evacuation at 473 K (Figure 5, spectrum e) leads to the full disappearance of the bands at 2205 and 2167 cm-1. Two weak bands in the region, namely at 2216 and 2184 cm-1, become observable. The band at 2145 cm-1 decreases in intensity. The band at 2111 cm-1 additionally develops,

Figure 5. FTIR spectra of CO adsorbed on activated Pt-NaMOR. Equilibrium CO pressure of 2670 (a) and 267 Pa (b) and after evacuation at 293 (c), 373 (d), 473 (e), 573 (f), and 623 K (g). The spectra are background corrected and the gas-phase CO subtracted.

and the Pt0-CO band at 2082 cm-1 decreases in intensity and is further shifted to 2071 cm-1. A new band at 2050 cm-1 emerges, whereas the 1985 cm-1 band disappears. Evacuation at 573 K (Figure 5, spectrum f) provokes total disappearance of all features above 2150 cm-1. The band at 2145 cm-1 has strongly reduced intensity. The band at 2111 cm-1 also slightly decreases in intensity. The band at 2071 cm-1 totally disappears, and two bands, at 2087 and 2050 cm-1, remain in the spectrum below 2100 cm-1. The bands at 2111, 2085, and 2050 cm-1 are still well observable after evacuation at 623 K (Figure 5, spectrum g). A shoulder of the 2111 cm-1 band, namely at 2121 cm-1, is clearly seen. After this treatment the band at 2145 cm-1 disappears. The above results allowed us to make the following conclusions and suggestions: (1) The bands at 2205 and 2167 cm-1 change synchronously, which confirms the supposition that they characterize one species. Only at high equilibrium CO pressures the band at 2167 cm-1 is overlaps with the band at 2165 cm-1 arising from Na+-CO species. (2) The band at 2145 cm-1 demonstrates an independent behavior, which implies that it characterizes linear platinum monocarbonyls. (3) The band at 2111 cm-1 also seems to be independent and characterizes another monocarbonyl species. (4) The bands at 2145 and 2111 cm-1 (as well as a row of bands at lower frequencies) are produced during CO desorption experiments. This shows that these species are formed either as a result of surface reduction or by decomposition of polycarbonylic species. The next set of experiments was designed to check whether the bands at 2145 and 2111 cm-1 were produced


Figure 6. FTIR spectra of CO adsorbed on activated Pt-NaMOR. Adsorption of CO followed by evacuation at 473 K (a) and subsequent introduction of progressively increasing amount of CO into the cell (b-h). The spectra are background corrected. The second derivatives of spectra a, f, and h are shown in the inset.

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Figure 8. FTIR spectra of 12CO-13CO isotopic mixture (molar ratio of ca. 1:1) adsorbed on activated Pt-Na-MOR. Progressively increasing amount of isotopic mixture introduced into the cell (a-f) and under equilibrium pressure of 53 Pa (g). The spectra are background corrected and the gas-phase CO subtracted. Table 2. Observed and Calculated Frequencies of the IR Carbonyl Bands of Ptn+(CO)2 Species after Substitution of 12CO by 13CO species Pt3+(12CO)

2

Pt3+(12CO)(13CO) Pt3+(13CO)2 Pt+(12CO)2 Pt+(12CO)(13CO) Pt+(13CO)2

Figure 7. FTIR spectra of CO adsorbed on activated Pt-NaMOR (continuation from Figure 6). Spectrum (a) corresponds to spectrum h from Figure 6 and spectrum (b) is registered after subsequent evacuation. The spectra are background corrected. The second derivatives of the spectra are presented in the inset.

by decomposition of polycarbonylic species. CO was adsorbed on a sample activated by procedure 2 and then evacuated at 473 K. As a result, bands at 2146, 2111, 2084, and 2050 cm-1 were registered (Figure 6, spectrum a). Then CO was added to the system on small doses (Figure 6, spectra b-g) up to an equilibrium pressure of 27 Pa (Figure 6, spectrum h). This resulted in one feature with a maximum at 2101 cm-1, which evidently consisted of many bands. Analysis of the second derivatives of the spectra indicates that the band at 2111 cm-1 progressively loses intensity and two other bands, at 2135 and 2101 cm-1, develop at its expense (see the inset, Figure 6). On the contrary, the band at 2146 cm-1 seems to remain unaffected. These results suggest that the monocarbonyls detected at 2146 cm-1 are not able to accommodate any additional CO molecule, whereas the monocarbonyls displaying a band at 2111 cm-1 are converted, in the presence of more CO, into dicarbonyls. The latter species seem to be characterized by symmetric modes at 2135 cm-1 and respective antisymmetric vibrations at 2101 cm-1. It was also found that subsequent evacuation even at ambient temperature led to development of the band at 2111 cm-1 and decrease in intensity of the bands at 2135 and 2101 cm-1 (Figure 7). Here, a good coincidence between the results obtained by a computer deconvolution of the bands (the peaks presented by dotted lines in Figure 7) and those based on the second derivatives of the spectra (see the inset of Figure 7) is well seen.

observed bands cm-1

2205 and 2167 2192 and 2130 cm-1 2152 and 2120 cm-1 2135 and 2101 cm-1 2123 and 2069 cm-1 2087 and 2050 cm-1

calculated frequencies 2193 and 2129 cm-1 2155 and 2116 cm-1 2123 and 2065 cm-1 2087 and 2054 cm-1

Another observation that deserves noting is that the band at 2204 cm-1 appeared with negligible intensity as compared to the experiments before thermal desorption, which indicates that the main part of the respective sites had been reduced. 3.6. Coadsorption of 12CO and 13CO on Pt-NaMOR. To prove/reject the polycarbonylic structure of the species formed, we studied the coadsorption of 12CO and 13 CO (molar ratio of ca. 1:1) on a sample reoxidized with a NO + O2 mixture. Initially, we aimed at establishing whether the bands at 2205 and 2167 cm-1 corresponded to two types of sites or to polycarbonylic species. If the bands characterize two kinds of Ptn+-CO species, the adsorption of the isotopic mixture had to result in the appearance of four bands with similar intensities: two for Ptn+-12CO at 2205 and 2167 cm-1 and two for corresponding Ptn+-13CO complexes at ca. 2155 and 2116 cm-1. If the bands at 2205 and 2167 cm-1 characterize dicarbonyls, six bands had to be registered after adsorption of the isotopic mixture: two for Ptn+(12CO)2, two for Ptn+(13CO)2, and two for Ptn+(12CO)(13CO) species.13 Using an approximate force field model,38 we calculated the expected frequencies as follow: 2205 and 2167 cm-1 for Ptn+(12CO)2; 2155 and 2116 cm-1 for Ptn+(13CO)2; and 2193 and 2129 cm-1 for Ptn+(12CO)(13CO) (Table 2). In addition, for the bands due to mixed-ligand species, the composition of the isotopic mixture should ensure a twice as high intensity as that for the other bands. If the species under consideration were tricarbonyls, the resulting spectrum should be even more complicated. The first small dose of a 12CO-13CO isotopic mixture introduced into the IR cell leads to the appearance of a row of low-intensity bands at 2205, 2192, 2167, 2152, 2130, 2120, and 2094 cm-1 (Figure 8, spectrum a). All bands increase in concert with the amount of the CO mixture (38) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London, 1975.

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Figure 9. FTIR spectra of 12CO-13CO isotopic mixture (molar ratio of ca. 1:1) adsorbed on activated Pt-Na-MOR. Introduction of isotopic mixture, followed by evacuation at 473 K (a), subsequent introduction of isotopic mixture (40 Pa equilibrium pressure) (b) and evacuation at 573 K (c). The spectra are background corrected.

Figure 10. Second derivatives of the spectra presented in Figure 9.

added into the system (Figure 8, spectra b-g). The appearance of the six bands at 2205, 2192, 2167, 2152, 2130, and 2120 cm-1, as well as the relative intensities of the bands, confirm the dicarbonyl structure of the species under consideration. The subsequent purpose of the 12CO-13CO coadsorption experiments was to prove that the band at 2111 cm-1 characterized monocarbonyls converted into dicarbonyls with bands at 2135 and 2100 cm-1. After adsorption of 12 CO-13CO isotopic mixture, the sample was evacuated at 473 K. As a result, bands at 2146, 2110, 2096, 2072, 2060, 2035, and 2001 were registered (Figure 9, spectrum a). Based on the isotopic shift factor we can identify the following corresponding pairs of bands: 2146 cm-1 f 2096 cm-1; 2110 cm-1 f 2061 cm-1; and 2035 cm-1 f 2001 cm-1. The isotopic shift for the 2035 cm-1 band is lower than theoretically expected. This could be due to the fact that the respective vibrations are not pure CO stretches but are coupled with other vibrations. Similarly, it has been well established that the isotopic shift of the socalled tilted CO is much smaller than the theoretically expected values.13 Then the 12CO-13CO isotopic mixture (40 Pa equilibrium pressure) was added to the sample. As a result, one broad feature was registered in the spectrum (Figure 9, spectrum b). However, according to the second derivative (Figure 10, spectrum b), the band at 2110 cm-1 (as well as the corresponding band at 2060 cm-1) disappeared from the spectrum and several new bands appeared or strongly increased in intensity, at 2138, 2123, 2087, 2069, and 2050 cm-1.

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Figure 11. FTIR spectra of CO adsorbed on reduced Pt-NaMOR. Equilibrium CO pressure of 1303 (a), 507 (b), and 227 Pa (c), after evacuation at 293 (d) and 573 K (e). The spectra are background corrected and the gas-phase CO subtracted.

Using the approximate force field model,38 assuming that dicarbonyls are detected at 2135 and 2101 cm-1, one can calculate the bands for mixed ligand species at 2123 and 2065 cm-1 and those for species having two 13CO ligands, at 2087 and 2054 cm-1 (Table 2). Comparison with the second derivatives presented on Figure 10 shows that we have observed bands at similar frequencies which proves the proposed dicarbonyl structure. 3.7. Adsorption of CO on reduced Pt-Na-MOR. The sample was reduced by hydrogen (13.3 kPa, 1 h, 673 K) and then evacuated at the same temperature for 30 min. Subsequent CO adsorption led to the appearance of one strong band at 2096 cm-1 with a shoulder at 1990 cm-1 assigned to Pt0-CO species (Figure 11, spectrum a). A weak band at 1870 cm-1 was also visible and assigned to bridged platinum carbonyls. Four bands were detected at higher frequencies, namely at 2226 (w), 2173 (sh), 2163, and 2136 cm-1. The appearance of these bands on the reduced sample proved the above propositions about them as not being related to platinum. The band at 2226 cm-1 was assigned to Al3+-CO species, the bands at 2173 and 2163 cm-1, to two kinds of Na+-CO species and the band at 2136 cm-1, to Na+-OC-Na+ species. After evacuation the bands above 2100 cm-1 disappeared and the 2096 cm-1 band was shifted to 2083 cm-1 (Figure 11, spectrum e). This kind of shift is typical of CO adsorbed on metals. The results obtained show that the platinum was reduced after the treatment applied. 3.8. Coadsorption of CO and H2O on Pt-Na-MOR. The aim of the next set of experiments was to establish whether water replaced CO from the platinum cations. CO was adsorbed on a reoxidized sample and then evacuated. As a result, two intense bands at 2205 and 2168 cm-1 and two weaker bands at 2218 and 2186 cm-1 as well as a broad feature centered at about 2100 cm-1 remained in the spectrum (Figure 12, spectrum a). Then small amounts of water were added successively into the cell (Figure 12, spectra b-d). As a result, bands evidencing formation of adsorbed water appeared: deformation modes (around 1637 cm-1) and OH stretching vibrations (broad tailed band with a maximum around 3640 cm-1). The spectrum in the carbonyl region was also affected. The weak bands at 2218 and 2186 cm-1 quickly disappeared, and then the bands at 2205 and 2168 cm-1 decreased in intensity in parallel, to disappear completely. The broad feature as a whole increased in intensity but was eroded at 2137 cm-1 which indicated some changes in the Pt+(CO)2 species. It is well-known that water, as being able to transmit electrons, causes a red shift of the CO stretching frequency


Figure 12. FTIR spectra of CO and H2O coadsorbed on activated Pt-Na-MOR. Adsorption of CO, followed by evacuation (a) and after introduction into the cell of progressively increasing amount of water (b-d). The spectra are background corrected. The spectra in the region of H-O-H deformation modes are shown in the inset.

Figure 13. FTIR spectra of CO adsorbed at low temperature on activated Pt-Na-MOR. Equilibrium pressure of 133 Pa CO adsorbed at 293 K (a), after cooling the sample to 100 K, followed by evacuation at 100 K (b, c) and at increasing temperatures (d, e). The spectra are background corrected.

when adsorbed on the same site or in vicinity.13 The principal new carbonyl band around 2115 cm-1 on the water-precovered sample is too low to correspond to the set of bands at 2205 and 2167 cm-1. That is why we infer that water generally replaces CO in the dicarbonylic species. The band at 2115 cm-1 most probably corresponds to mixed water-carbonyl complexes formed with the participation of Pt+ cations. The results obtained can be important when assigning carbonyl bands formed under “wet” atmosphere, as is the case of most operando studies. 3.9. Low-Temperature CO Adsorption on Pt-NaMOR. Low-temperature adsorption was performed in order to check the possibility of insertion of an additional CO molecule, especially in the dicarbonyl complexes characterized by bands at 2205 and 2167 cm-1. Initially, CO (133 Pa equilibrium pressure) was adsorbed at ambient temperature and the species already discussed appeared (Figure 13, spectrum a). Cooling the sample in this CO atmosphere down to 100 K led to the appearance of two strong bands at 2167 and 2134 cm-1 associated with CO adsorption on the support (Figure 13, spectrum b). In this case, the 2134 cm-1 band appeared at lower frequencies, probably due to temperature/coverage effects. The band at 2205 cm-1 was shifted to 2207 cm-1. Evacuation at 100 K and elevated temperatures strongly eroded the bands at 2167 and 2134 cm-1 (Figure 13, spectra c-e). The band at 2207 cm-1 was gradually back shifted to its original position without noticeable changes in intensity. There-

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fore, the small shift of 2 cm-1 of the band at 2205 cm-1 was a temperature effect. Thus, the results show that no third molecule can be inserted into the dicarbonyls characterized by bands at 2205 and 2167 cm-1. Unfortunately, we are not able to draw definite conclusion about the other species, because of the fact that the possible polycarbonyl bands are masked by the strong bands in the region due to CO adsorbed on the support. 3.10. Carbonyls of Platinum Cations in Pt-NaMOR: Comparison with Pt-H-ZSM-5. Platinum is among the elements that form the so-called nonclassical carbonyls; that is, carbonyls with the participation of cations.39 The most typical oxidation states of platinum are 2+ and 4+, but compounds of Pt3+ and Pt+ are also known.40 The two bands at 2205 and 2168 cm-1 were proven by 12 CO-13CO coadsorption to characterize dicarbonyl species. Water was found to replace CO as a ligand from these Ptn+(CO)2 species. In principle, H2O can displace CO from carbonyls in which the M-CO bond has mainly a σ-character and the π-back-donation is very weak.13 When the π-back-donation is significant, as is expected to be the case of the carbonyls of Pt2+ and Pt+, it is CO that replaces H2O.17 Indeed, these species are observed on oxidized samples, which suggests a high oxidation state of platinum in them. Moreover, in our experiments, we observed at least three lower oxidation states of platinum: (i) metallic platinum, forming carbonyls detected around 2100 cm-1, (ii) linear carbonyls of cationic platinum manifesting a band at 2111 cm-1, and (iii) linear carbonyls of cationic platinum with a band at 2146 cm-1. All of this strongly suggests that platinum cations in the Ptn+(CO)2 species are in a high oxidation state: 3+ or 4+. Similar dicarbonyls (2221 and 2200 cm-1) were observed recently with Pd-H-ZSM-5 samples and assigned to Pd3+(CO)2 species.41,42 Taking into account the analogous electron configuration of palladium and platinum, as well as their similar carbonyl chemistry, we inferred that the species under consideration were Pt3+(CO)2 dicarbonyls. However, we cannot totally rule out the possibility of Pt4+(CO)2 species. The Pt3+(CO)2 dicarbonyls belong to the so-called complex-specified dicarbonyls,43 i.e., they are decomposed without producing a measurable fraction of linear Pt3+CO species. It is believed, that the complex-specified species are produced because of reaching a stable electron configuration. There are many examples of such species, the most typical one being the well-known rhodium gemdicarbonyls.44 Similar Pt3+(CO)2 dicarbonyls were observed recently with Pt-H-ZSM-5, the respective IR bands being at 2211 and 2175 cm-1.18,19 In our case, the IR bands are at frequencies lower by 6-7 cm-1. This could be due to two reasons: (i) enhanced π-back-donation or (ii) weaker σ-bonding. The first hypothesis can easily be ruled out, since it presupposes an enhancent stability of the complexes. However, the stability of the Pt3+(CO)2 dicarbonyls in Pt-Na-MOR is lower that the stability of these species in Pt-H-ZSM-5: in the latter case, they are still observed after evacuation at 473 K,18 whereas in Pt-Na-MOR, (39) Aubke, F.; Wang, C. Coord. Chem. Rev. 1994, 137, 483. (40) Ripan, R.; Ceteanu, I. Inorganic Chemistry; Mir: Moscow, 1972. (41) Aylor, W.; Lobree, L. J.; Reimer, J. A.; Bell, A. T. J. Catal. 1997, 172, 453. (42) Chakarova, K.; Ivanova, E.; Hadjiivanov, K.; Klissurski, D.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 2004, 6, 3702. (43) Hadjiivanov, K.; Ivanova, E.; Klissurski, D. Catal. Today 2001, 70, 75. (44) Miessner, H.; Burkhardt, I.; Gutschick, D.; Zecchina, A.; Morterra, C.; Spoto, G. J. Chem. Soc. Faraday Trans. 1990, 86, 2321.

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they are decomposed at this temperature (see Figure 5). Therefore, Pt3+ cations are more electrophilic in a ZSM-5 matrix as compared to MOR. Indeed, analysis of many literature data45-47 suggests that cations exchanged in ZSM-5 are characterized by a very high electrophilicity. The angle between the two CO ligands can be calculated according to the formula

Iasym/Isym ) tan2(θ/2) 38 where Isym and Iasym are integrated intensities of the bands due to symmetric and antisymmetric vibrations, respectively; 2θ is the angle between the two CO groups. Using this formula, we calculated the angle between the CO molecules to be 102°. This angle is somewhat larger that the angle between the CO molecules in the complexes produced in Pt-H-ZSM-5 (97°). These observations can be helpful in the determination of the location of platinum cations in different zeolites. It should be noted that Solomennikov and Davydov20 observed bands at 2205 and 2175 cm-1 after CO adsorption on oxidized PtY. Although the authors assigned these bands to two kinds of Ptn+-CO species, analysis of their results showed that the bands changed in concert. This suggests that Pt3+(CO)2 species are also produced in PtY as well. However, there are no reports indicating formation of similar dicarbonyls with oxide-supported platinum. At present, we can only speculate that the low coordination of cations in zeolites favors formation even of complexspecified dicarbonyls. Let us now discuss the carbonyls characterized by a band at 2146 cm-1. The relatively low frequency of this band allows us to assign it to Pt2+-CO species. It was found that these carbonyls were not able to accept an additional CO molecule. In contrast, Pt2+(CO)2 species were reported with Pt-H-ZSM-5. The latter are sitespecified, i.e., are formed because of the low coordination of the Pt2+ cations. They are also decomposed via monocarbonyls. It was reported that in order to form sitespecified dicarbonyls, the cation has to possess a sufficiently big cationic radius. The “critical” cationic radius depends on the position and is different for different (45) Hadjiivanov, K.; Kantcheva, M.; Klissurski, D. J. Chem. Soc., Faraday Trans 1996, 92, 4595. (46) Hadjiivanov, K.; Kno¨zinger, H. J. Phys. Chem. B 1998, 102, 10936. (47) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751.

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zeolites. Thus, Mn2+ cations (radius of 80 Å48) in MnZSM-5 form dicarbonyls, whereas in MnY monocarbonyls only are produced even after low-temperature CO adsorption.49 It is believed that small cations penetrate the O-ring of the oxygen atoms to which are coordinated and thus, for steric reasons, are not able to coordinate two guest molecules. Evidently, the Pt2+ cations (radius of 80 Å48) are too small to form dicarbonyls in Pt-Na-MOR. One more family of carbonyl species were found with the reduced sample. They are Pt+-CO monocarbonyls (2111 cm-1) that are converted into Pt+(CO)2 dicarbonyls (2135 and 2101 cm-1) under CO equilibrium pressure. Here again, the picture is different from that observed with Pt-H-ZSM-5. In the latter case, conversion between complex-specified Pt+(CO)2 species (2120 and 2091 cm-1) and tricarbonyls (2150 and 2110 cm-1) was reported. However, these results are in general agreement with the results observed with the Pt2+ cations. It is known that the decrease in oxidation state results in an increase of the cationic radius. That is why, the Pt+ cations in PtNa-MOR are able to absorb up to two CO molecules each. However, the cationic radius of Pt+ seems to be not big enough for simultaneous adsorption of three CO molecules when the cation is exchanged in MOR. Evidently, the location of platinum cations is very important for the type of carbonyls they from. 4. Conclusion Adsorption of CO on unreduced Pt-Na-MOR leads to formation of various carbonyls involving platinum cations in different oxidation states. Pt3+ cations form complexspecified dicarbonyls. These cations are reduced during evacuation of preadsorbed CO but are again formed after reoxidation of the sample by a NO + O2 mixture. Pt2+ cations form monocarbonyls only. Under CO, the Pt+ cations coordinate two CO molecules but lose one of these ligands even after evacuation at ambient temperature. The use of a 12CO-13CO isotopic mixture combined with analysis of the second derivatives of the spectra is very useful for proving the polycarbonyl structures. Acknowledgment. This work was supported by the Bulgarian National Research Foundation (project X-1205) and the Rila project 02/04 (French-Bulgarian joint research program). LA051877K (48) Hadjiivanov, K.; Ivanova, E.; Kantcheva, M.; Cifitikli, E.; Klissurski, D.; Dimitrov, L.; Kno¨zinger, H. Catal. Commun. 2002, 3, 313.

FTIR Spectroscopy Study of CO Adsorption on PtNaMordenite

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