Al2O3 NSR Catalysts: Characterization of Morphological

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Pt-K/Al2O3 NSR Catalysts: Characterization of Morphological, Structural and Surface Properties F. Prinetto, M. Manzoli, S. Morandi, F. Frola, and G. Ghiotti* Dipartimento di Chimica Inorganica, Fisica e dei Materiali, NIS s Nanostructured Interfaces and Surfaces, Centre of Excellence, UniVersita` di Torino, Via Pietro Giuria 7, 10125 Torino, Italy

L. Castoldi, L. Lietti, and P. Forzatti Dipartimento di Energia, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy ReceiVed: September 18, 2009; ReVised Manuscript ReceiVed: NoVember 18, 2009

Morphological, textural, and surface properties of a NSR (NOx storage reduction) Pt-K/Al2O3 model catalyst (Pt 1 wt %; K 5.4 wt %) were characterized by means of XRD, HRTEM, and FT-IR spectroscopy. Thin crystalline K-containing layers, in the form of cubic K2O and monoclinic K2CO3 and very small roundish Pt particles with a mean diameter of 1.5 nm, have been observed. Monoclinic K2CO3 disappears, and a certain degree of Pt sintering occurs (dPt ≈ 3.4 nm) after use. However, the presence of potassium limits the Pt sintering which occurs on the Pt/Al2O3 reference sample (Pt 1 wt %). FT-IR spectra of CO adsorbed at RT, compared with those recorded for Pt/Al2O3, revealed a marked interaction between the Pt and K phases that is much higher than the interaction between the Pt and Ba phases observed for the classic Pt-Ba/Al2O3 catalyst. CO2 adsorption at RT indicated a high heterogeneity of the K phase, evidenced by the formation of a variety of surface-carbonate-like species (mainly bridging carbonates on K sites). Minor amounts of nitrites and nitrates were formed at RT under NO admission, while the uptake was sensibly higher under NO/O2 or NO2 admission; nitrites (mono- and bidentate) and nitrates (ionic and bidentates) were formed in different amounts, both relative and absolute, and the nitrate to nitrite ratio increased in parallel with the NO/O2 ratio. Also, at each contact time, the amount of the stored NOx species increased upon increasing the NO/O2 ratio. 1. Introduction For the protection of the global environment, car manufactures are struggling to lower the carbon dioxide emission and to clean up automobile exhaust gases. The upcoming lean-burn engine technology forces engine combustion to occur at a very high air-fuel (A/F) ratio,1 up to A/F ) 20-25:1; in this way, a better fuel consumption and, consequently, a lower CO2 production should be achieved. Unfortunately, the current three-way catalysts (TWC) are very sensitive to the A/F ratio, and in particular, an increase of this ratio is critical for NOx conversion. To solve the problem, alternatives to TWC are looked for; in this respect, NSR (NOx storage reduction) catalysts, also called lean NOx traps, seem to be promising. Usually constituted by a high-surface-area support (like γ-Al2O3), a storage phase (an alkaline or earth alkaline metal oxide), and a noble metal (Pt), these catalysts operate in conditions that switch between a lean phase (i.e., in excess oxygen), during which NOx are stored on the basic catalyst component, and a short rich phase, during which NOx are reduced by H2, CO, and unburned hydrocarbons (UHC) to N2, CO2, and H2O.2–5 The most common formulation of NSR catalysts exploits barium as a material for the NOx storage, and plenty of methods have been developed both to characterize these catalysts6–10 and to investigate the NOx storage reduction process.11–21 Recently, potassium has gained attention as a storage phase, instead of or together with barium. Recently, Takeuchi et al.1 found higher NOx storage capability for Pt-K/Al2O3 with respect to Pt-Ba/ * To whom correspondence should be addressed. Tel. +39 0116707539. Fax +39 0116707855. E-mail: [email protected].

Al2O3 at temperatures up to 673 K; however, the Ba-containing catalysts seemed to be more able to operate the hydrocarbon conversion. Park et al.22 have shown that (i) the co-loading of potassium oxide with barium oxide enhances the thermal stability of the stored NOx and (ii) after hydrothermal treatment (10% water vapor at 1123 K), K/Al2O3 and K-Ba/Al2O3 preserve their storage capacity better than Ba/Al2O3. A beneficial effect of the potassium with respect to barium about the NOx storage properties was reported also by Toops et al.,23,24 again especially at high temperature (623-673 K). However, although the performances of the K catalyst were promising even in the presence of steam, the same authors underlined the removal of the K phase from the Pt-K/Al2O3 catalyst in the presence of liquid water. This aspect is not negligible concerning the applicability of the potassium in the NSR catalysts as water is known to condense in the catalyst brick at low temperatures. In this work, the results of a characterization study of a Pt-K/ Al2O3 catalyst by means of several techniques (like XRD, HRTEM, and FT-IR) are reported. The aim is to carry out a systematic physical-chemical analysis of this catalytic system that, in our opinion, could be very useful for the comprehension of the NOx storage reduction mechanism occurring on Pt-K/ Al2O3 and/or in the mixed Pt-Ba-K/Al2O3. In particular, in this paper, on one hand, a deep analysis of the platinum phase was achieved because Pt plays a relevant role in both NOx storage and reduction processes; on the other hand, a systematic study of NO/O2 and NO2 adsorption at RT was performed to probe the basic component. Moreover, this study represents the first stage in the investigation of NOx storage reduction at

10.1021/jp909026p  2010 American Chemical Society Published on Web 12/18/2009

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TABLE 1: Chemical Composition and Morphological and Textural Features of a.p. Catalysts sample

Pt content (wt %)

Pt/Al2O3 Pt-K/Al2O3 K/Al2O3 Pt-Ba/Al2O3b

1.0 1.0 1.0

K (or Ba) content (wt %)

SS (m2/g)

pore volume (cm3/g)

Pt dispersion (%)

dPt (nm)a

5.4 5.4 16

197 176 178 137

1.00 0.90 0.84 0.81

71.7 64.8

1.5 1.7

71

1.5

a Particle diameters are calculated from the Pt dispersion evaluated by H2 chemisorption measurements at 273 K14,15 using the empirical relationship dPt (nm) ) 1.1/(H/Pt). b Reference 29.

operating temperatures (623 K; results reported in a second paper). Indeed, the study of the NOx interaction at RT allows easier assignment of the vibrational modes of the NOx species that can be formed on the catalyst surface and an easier investigation of the different paths of their formation. 2. Experimental Section 2.1. Sample Preparation. Pt-K/Al2O3 catalyst (Pt: ∼1 wt %; K: 5.4 wt %) was obtained by impregnation in two sequential steps of a commercial γ-Al2O3 carrier (Versal 250 from UOP, surface area of 200 m2/g and pore volume of 1 cm3/g) as follows: The γ-Al2O3 powder was first impregnated with a solution of Pt(NH3)2(NO2)2 (Strem Chemicals, 5% Pt in ammonium hydroxide) with an appropriate concentration so as to yield 1 wt % Pt metal loading. After drying in air for 12 h at 353 K and calcining in atmospheric air at 773 K for 5 h, the sample was impregnated with a K(CH3COO) (Sigma Aldrich, 99%) solution so as to yield a K content equal to 5.4 wt %. After that, the sample was dried for 12 h at 353 K and then calcined at 773 K for 5 h (sample denoted as prepared, a.p., in the following). For comparative purposes, two reference binary systems, Pt/ Al2O3 (Pt: 1 wt %) and K/Al2O3 (K: 5.4 wt %), were also prepared using the same precursors and procedures. A K loading equal to 5.4 wt % was chosen to obtain a catalyst with a molar loading of the K-phase equivalent to the loading of Ba of the classic Pt-Ba/Al2O3 catalyst (Pt 1 wt % and Ba 16 wt %), previously fully characterized7,15,16 in our laboratories. Actually, the results obtained on this catalyst will be reported in the discussion for comparative purpose. 2.2. Characterization Techniques and Procedures. The surface area and pore size distribution were determined using a Micromeritics TriStar 3000 instrument for volumetric measurements with N2 adsorption-desorption at 77 K on the a.p. samples outgassed overnight at 393 K. XRD spectra were collected on powdered a.p. samples with a Philips powder/thin film diffractometer PW1140 equipped with a PW3020 goniometer using a Cu KR1 (λ ) 1.54) radiation between 10 and 80° 2θ. The Pt dispersion was estimated on samples activated by heating in vacuo and subsequently in dry oxygen at 823 K and finally reduced in H2 at 623 K by (i) hydrogen chemisorption at 273 K using a TPD/R/O 1100 ThermoElectron Corporation instrument and (ii) TEM and HRTEM measurements, performed using a side-entry Jeol JEM 3010 (300 kV) microscope; for analyses, the powdered samples were deposited on a copper grid coated with a porous carbon film. All digital micrographs were acquired by an Ultrascan 1000 camera, and the selected images were processed by Gatan digital micrograph. Absorption/transmission IR spectra were run at room temperature (RT) on a Perkin-Elmer FT-IR 1760-X spectrophotometer equipped with a Hg-Cd-Te cryodetector, working in the range of wavenumbers 7200-580 cm-1 at a resolution of 2 cm-1 (number of scans: ∼40). For IR analysis, powdered samples were compressed in self-supporting disks (10-15 mg

cm-2) and placed in a quartz IR cell allowing thermal treatments in vacuo or in controlled atmosphere. Pellets were initially activated by heating in vacuo at 823 K and subsequently oxidized or reduced, depending on the target of FT-IR analyses. Oxidation treatment consisted of heating in dry oxygen at 823 K and evacuation at RT, reduction treatment of heating in H2 at 623 K, and evacuation at the same temperature. IR spectra were recorded at RT before and after interaction with CO or CO2 (Matheson, C.P.), NO (Praxair, freshly distilled before use), NO/O2 (1:8, 1:6, and 1:4 freshly prepared mixtures), and NO2 (Praxair). In order to evaluate the amounts of the surface species formed on the catalysts, spectra were normalized to the same pellet weight. 3. Results and Discussion 3.1. Textural, Structural, and Morphological Characterization. The surface area, pore volume, and Pt dispersion for each sample are listed in Table 1. Volumetric measurements showed that the impregnation with Pt leaves the γ-Al2O3 surface area almost unchanged (200 m2/g for Al2O3; 197 m2/g for Pt/ Al2O3); the a.p. Pt-K/Al2O3 catalyst has a slightly lower surface area (176 m2/g), similar to that found for the K/Al2O3 catalyst. The impregnation with the potassium solution results into a decrease in the Pt dispersion, measured by H2 chemisorption at 273 K, moving from 71.7% in the case of Pt/Al2O3 to 64.8% for Pt-K/Al2O3. The decrease in the Pt dispersion has been ascribed primarily to the masking of the Pt crystallites upon addition of the K component (vide infra). In Table 1, the mean Pt particle sizes (dPt) calculated from the empirical relationship often used for monometallic catalysts, dPt(nm) ) 1.1/(H/Pt), where H/Pt is the Pt dispersion measured from H2 chemisorption, are also reported. XRD analysis was performed on Pt/Al2O3, K/Al2O3, and Pt-K/Al2O3 a.p. samples. No difference between K/Al2O3 and Pt-K/Al2O3 was observed; hence, for the sake of brevity, we report in Figure 1A only the XRD patterns of the Pt/Al2O3 (curve a) and Pt-K/Al2O3 (curve b) a.p. samples. Broad peaks, indicating a low crystallinity, due to the γ-Al2O3 phase (/, JCPDS 10-425), have been detected on all materials. On the K-containing samples, monoclinic K2CO3 (JCPDS 16-820) and cubic K2O (JCPDS 23-493) phases have been hardly recognized (O and 0, respectively). The FT-IR spectrum of the a.p. Pt-K/Al2O3 sample (Figure 1B, curve a) reveals two intense bands centred around 1570 and 1350 cm-1, almost completely removed by outgassing at 823 K (Figure 1B, curve b); these bands are characteristic of surface bidentate carbonates.25 Conversely, the FT-IR spectrum of bulky K2CO3 (not reported), run by us for comparison, showed only one band at 1420 cm-1 in this spectral region and a thermal stability markedly higher. This feature indicates that either the spectrum of the crystalline K2CO3 phase is hidden in the modes of the surface carbonates and its stability is lowered by the dispersion or that the carbonates detected are related to a crystalline K2CO3 phase dispersed in the form of very thin

Pt-K/Al2O3 NSR Catalysts

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Figure 1. (A) XRD patterns of Pt/Al2O3 (a) and Pt-K/Al2O3 (b) a.p. samples; / γ-Al2O3 (JCPDS 10-425); O monoclinic K2CO3 (JCPDS 16-820); 0 cubic K2O (JCPDS 23-493). (B) FT-IR spectra of Pt-K/ Al2O3, a.p. (a) and after outgassing at 823 K (b).

slabs so that not only is its stability lowered but also its FT-IR spectrum is strongly affected (see below). We emphasize the difference between the a.p. Pt-K/Al2O3 and Pt-Ba/Al2O3 catalysts obtained by the same protocol;7,11,16 indeed, in the latter case, a relevant amount of large agglomerates of bulky BaCO3 were evidenced, and a catalyst activation, consisting of repeating cycles of heating under NO2 at 623 K followed by evacuation at 823 K, was required in order to displace the BaCO3 and to obtain a well-dispersed barium oxide phase. Actually, on the K-containing samples left in the laboratory atmosphere for few months, the amount and thermal stability of the carbonate species slightly increased, even if they showed spectral features similar to those of fresh prepared samples. However, only one cycle of heating under NO2 at 623 K followed by evacuation at 823 K was sufficient for their complete elimination. Morphological and structural characterization of a.p. Pt-Al2O3 and Pt-K/Al2O3 catalysts, activated previously at 823 K and reduced in H2 at 623 K, has been performed by TEM and HRTEM analyses; the characterization was repeated on the same samples submitted to several NOx storage reduction cycles at 623 K (“used” samples). The Pt particle size distributions obtained for freshly reduced Pt/Al2O3 and Pt-K/Al2O3 and for “used” Pt-K/Al2O3 are reported in Figure 2A, B, and C, respectively. In Figure 3, the HRTEM images of freshly reduced Pt-K/Al2O3 (A, A1, and A2) and of “used” Pt-K/Al2O3 (B) are compared. Actually, the

Figure 2. Pt particle size distributions obtained from TEM images of freshly reduced Pt/Al2O3 and Pt-K/Al2O3 (A and B, respectively) and of “used” Pt-K/Al2O3 (C).

freshly reduced Pt/Al2O3 sample shows very small Pt particles having a mean size of 1.1 nm, in particular, the size distribution is centred around 1 nm and shows a tail toward higher sizes (Figure 2A; TEM images not reported for sake of brevity). Conversely, the Pt particle size distribution related to freshly reduced Pt-K/Al2O3 appears more symmetric and ranges between 0.7 and 2.5 nm, the mean size being dPt ) 1.5 nm (Figures 2B and 3A). According with the H2 volumetric measurements, the impregnation with potassium solution results into a slight decrease in the Pt dispersion. The analysis of the “used” Pt-K/Al2O3 catalyst clearly indicates a sintering of the Pt phase; indeed, Pt particles with a size of ∼5 nm can be seen (Figures 2C and 3B), reaching in some very rare cases 10-20 nm. However, a relevant amount of Pt particles with diameter < 5 nm is still present; therefore, the mean Pt particles size is equal to 3.4 nm, that is, slightly more than doubled with respect to the freshly reduced sample. It is worth noting that the presence of the K phase avoids the marked Pt sintering observed on the K-free Pt/Al2O3 submitted to repeated NSR processes, showing Pt particles with a mean size 8 times bigger than those on fresh Pt/Al2O3 (Table 2). A similar behaviour was observed for the Pt-Ba/Al2O3 catalysts;7,11,16 indeed, also in that case, the mean Pt particle size in the Bacontaining catalysts doubled passing from the freshly reduced to the used catalysts.

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Figure 3. HRTEM images of freshly reduced Pt-K/Al2O3 (A, A1, and A2), of “used” Pt-K/Al2O3 (B), and the distance between the diffraction fringes of the images reported in A1 (fA1) and A2 (fA2).

TABLE 2: Mean Pt Particle Size Measured by HRTEM on a.p. and Used Catalysts dPt (nm)

a

sample

a.p. sample

used sample

Pt/Al2O3 Pt-K/Al2O3 Pt-Ba/Al2O3a

1.1 1.5 1.5

8.1 3.4 3.6

Reference 29.

On freshly reduced Pt-K/Al2O3, crystalline phases different from alumina have been identified; the diffraction patterns (obtained by doing the Fourier transform of the HRTEM digital images) and/or the distance between the diffraction fringes (insets in A1 and A2 of Figure 3) revealed the presence of a highly dispersed crystalline thin layer of monoclinic K2CO3 (A1) and cubic K2O (A2) phases. Moreover, a careful analysis (performed on a statistically significant number of images) revealed that the abundance of either crystalline K2CO3 or K2O regions was almost the same. As at the end of the activation and reduction treatments with H2 the FT-IR spectra did not reveal the presence of appreciable amounts of carbonates, these results indicate that a fraction of the K2O layer, when recontacted

with the laboratory atmosphere, is rapidly converted into K2CO3. The two phases are crystalline, but as evidenced by the HRTEM, they are in the form of very thin slabs. An equivalent inspection of different regions of the “used” catalyst revealed mainly the presence of a crystalline cubic K2O phase (not shown). In conclusion, HRTEM images of the freshly reduced samples revealed a K phase well dispersed on the alumina support. This is also confirmed by the comparison of FT-IR spectra of Pt/ Al2O3 and Pt-K/Al2O3 calcined samples outgassed at 823 K in the hydroxyl stretching region (not reported). In the spectrum of the K-free system, four bands are present at 3776, 3755, 3730, and 3682 cm-1 (related to different types of free hydroxyls characteristic of the alumina26), while only one band at 3716 cm-1 is present on the ternary system, with an overall integrated intensity markedly decreased in comparison with that of Pt-Al2O3 free hydroxyls. 3.2. FT-IR Characterization. CO Adsorption. To further characterize the Pt phase and to gain information about its interaction with the potassium phase, increasing pressures of CO (up to 2 kPa) were admitted at RT on reduced Pt-K/Al2O3. The two reference samples (K/Al2O3 and Pt/Al2O3) were also investigated; however, no significant CO uptake was found on



Figure 4. CO admission at RT on freshly reduced Pt/Al2O3 (a), Pt-Ba/ Al2O3 (b), and Pt-K/Al2O3 (c). Solid lines: spectra upon admission of 2 kPa of CO; dotted lines: spectra after subsequent evacuation at RT. For the sake of clarity, spectra have been reported after subtraction of the spectrum before gas admission and translated along the y-axis.

the Pt-free K/Al2O3 sample, whereas no differences with respect to data reported in previous works7,16 were pointed out by the CO admission on reduced Pt/Al2O3. Briefly, on reduced Pt/Al2O3 (Figure 4, curve a),the main observed features were (i) an intense peak slightly blue shifting with coverage (3 cm-1) up to 2090 cm-1 ascribed to νCO of linear carbonyls formed on Pt0 particles, stable under vacuum at RT; (ii) two minor bands at 1845 and 1630 cm-1, the latter weaker and broader, that could be, respectively, assigned to ν(CO) of bridged and multibridged CO; the intensity ratio between bridged and linear carbonyls is that expected for this metal, which shows a preference for bonding of CO in linear configuration; and (iii) a band at 2215 cm-1, with a shoulder at 2230 cm-1, almost completely reversible upon evacuation at RT due to CO adsorbed onto Al3+ Lewis acid sites.26 Moving to the reduced Pt-K/Al2O3 catalyst (Figure 4, curve c, and Figure 5A), no bands due to CO adsorbed onto Al3+ Lewis acid sites are observed, in accord with the high coverage of the alumina support by the K phases. As for CO on the metal phase, three main peaks, blue shifting upon increasing the CO coverage, are observed in the ν(CO) region of linear and bridged carbonyls on Pt0, one shifting from 2015 to 2045 cm-1, a second one shifting from 1945 to 1955 cm-1, and a third one shifting from 1690 to 1745 cm-1. The CO outgassing at increasing temperature, subsequent to the adsorption (Figure 5B), causes the decrease of the three peaks. The peaks at 2045 and 1955 cm-1 are sensibly consumed already at 473 K (the former more markedly) and shifted to lower frequency, thus giving rise to an unresolved asymmetrical band, still present at 573 K at around 1880 cm-1 and then completely removed at 623 K. Also, the peak at 1745 cm-1 decreases in intensity upon increasing the temperature and shifts to lower frequencies. However, it is difficult to state the temperature of its complete removal. Actually, in parallel, partially superimposed to it, a new band increases at 1570 cm-1, accompanied by another one at 1340 cm-1 and a third weaker band at 1080 cm-1 (not reported), easily assignable to ν(CdO), νasym(OCO), and νsym(OCO) modes, respectively, of bidentate carbonates. This indicates a certain amount of CO dismutation on the metal favored by the temperature increase; the CO2 thus formed diffuses on the oxide phase, giving rise to carbonates stable up to 823 K.

Figure 5. (A) CO admission at RT on freshly reduced Pt-K/Al2O3 up to 2 kPa (a-f) and subsequent evacuation at RT (g, dash). (B) Evacuation at RT (g) and at increasing temperatures (373, 423, 473, 523, and 623 K (h-l, respectively)).

The band at 2015-2045 cm-1 is easily assigned to linear carbonyls on Pt0 particles. A comparison with the results reported for the reference Pt/Al2O3 catalyst (Figure 4, curve a) reveals a ν(CO) of linear carbonyls markedly red shifted upon passing from the K-free to the K-containing catalyst, indicating a certain interaction of Pt with the basic K phase. Similar red shifts have been observed in the literature27–29 for ν(CO) of linear carbonyls adsorbed on metal particles passing from alumina to supports with more basic character like Mg(Al)O mixed oxides. The red shift is justified by an increased back-donation to 2π* antibonding orbitals of CO as a result of a simultaneous electron density transfer from the oxide ions to the metal. This effect can be also interpreted by invoking a decrease in the ionization potential of the Pt valence orbitals, directly induced by the Coulomb potential of the oxygen ions.30 The assignment of the band at 1945-1955 cm-1 is not straightforward. A similar feature has been observed for Pt0 supported on Mg(Al)O mixed oxide;27,28 also in this case, a component was present as a shoulder on the low-frequency side of the peak assigned to linear carbonyls on the metal particle facets. A series of hypotheses have been made to assign the component at low frequency; among them are (i) CO linearly bonded to metal sites at the border of the particles therefore showing electronic properties more affected by the interaction with the support, (ii) CO bonded to these peripherical Pt sites but interacting through a π bond with the cations of the support, and (iii) CO linearly bonded to metal sites belonging to very small metal clusters.

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Figure 6. FT-IR spectra upon admission of 2 kPa of CO and subsequent outgassing at RT on reduced Pt-K/Al2O3; comparison between a.p. (a) and the “used” sample (b).

Also in our case, we could assign this band to CO linearly bonded to Pt surface sites at the border with the K phase (whatever the particle size) or belonging to very small particles and/or clusters, for which the electronic interaction with the K phase is particularly strong. It is worth of noting that the two components assigned to linear carbonyls (2015-2045 and 1945-1955 cm-1) have about the same intensity, indicating that the ratio between the related Pt sites is near unity. Finally, the band at 1690-1745 cm-1 is assigned to bridged carbonyls, markedly red-shifted with respect to those observed on Pt/Al2O3 (1845 cm-1) because of the strong interaction of Pt with the basic phase. However, if the assignment is right, again, what is worth noting is that this band shows intensity comparable to that of bands related to linear carbonyls. As known, surface Pt0 sites, both on extended crystal faces and on facets of Pt particles supported on SiO2, Al2O3, and Mg(Al)O, show a preference for bonding CO in a linear configuration.31 Once more, this behavior can be attributed to the influence of the K phase on the metal particles, which enhances the electron density of the Pt atoms at the surface, favoring the formation of bridged carbonyls over linear ones. Actually, an IR study of CO adsorbed on Pt electrodes at different electrical potential32 has shown that for a negative potential (referred to as a hydrogen electrode), bridged carbonyls are preferentially formed. The CO admission was repeated, following the same protocol, on the “used” Pt-K/Al2O3 catalyst (Figure 6, curve b). The main feature is the dramatic lowering of the integrated intensity of the three peaks related to Pt carbonyls, revealing a decreased accessibility of Pt to CO. This observation matches well with HRTEM images showing that a sintering process of Pt particles is acting during the catalyst “use”, moving the mean Pt particle size from 1.5 to 3.4 nm. Actually, by a rough calculation in which the Pt particles are considered as spheres with diameters of 1.5 and 3.4 nm, it is possible to calculate that the sintering involves a ratio between the Pt surface area before and after “use” equal to 2.6. Assuming now that the integrated intensity of the bands related to Pt carbonyls is proportional to the number of bonded Pt and considering that two Pt atoms correspond to one bridged carbonyl, we have roughly estimated a ratio between the Pt exposed before and after “use” equal to 2.9, in good agreement with the previous calculation. As the Pt phase plays a relevant role in both NOx storage and reduction processes, it is interesting to compare the influence

Prinetto et al. of the K phase with that of the Ba phase added to the Pt/Al2O3 catalyst by comparison of the spectra of adsorbed CO (see Figure 4). Actually, linear carbonyls with ν(CO) at 2045 cm-1 (at maximum CO coverage) were also observed for the Pt-Ba/ Al2O3 catalyst (Figure 4, curve b) and were markedly red shifted with respect to linear carbonyls formed on Pt/Al2O3, indicating an interaction of Pt with the basic Ba phase. In the case of the Ba-containing catalyst, the presence of a shoulder at 1945-1955 cm-1 has been observed and ascribed to linear carbonyls formed on Pt atoms strongly interacting with the basic oxygen anions of the Ba phase. However, on the Pt-K sample, this feature shows an intensity comparable to that of the 2045 cm-1 band. This means that the fraction of Pt sites strongly interacting with the basic component is much higher on Pt-K than that on Pt-Ba catalyst. Also in the case of the Pt-Ba/Al2O3 catalyst, the presence of a band at 1750 cm-1 has been observed and ascribed to bridged carbonyls. Again, an increment in the relative amount of bridged carbonyls is observed upon passing from the Ba-free to Pt-Ba system, but for Pt-K, this increment is markedly higher. This still indicates that the relative amount of metallic atom in very strong interaction with the basic phase is much higher than that for Pt-Ba. As on Pt-Ba and Pt-K catalysts, both fresh and used, the mean size of the Pt particles is comparable (see Table 2); these results confirm the assignment of the band at 1945-1955 cm-1 to Pt clusters, not detectable by HRTEM, present in major amount in the case of the K-containing sample. CO2 Adsorption. CO2 is often used as a probe molecule to characterize metal oxides, giving hydrogen-carbonate species when CO2 interacts with hydroxyl groups or carbonate-like species when the interaction occurs with the basic oxygen anions. Moreover, CO2 can be adsorbed on metal oxides, yielding end-on surface complexes; this interaction occurs at Lewis acid sites by a σ-charge donation from one of the oxygen lone pair orbital. In this experiment, CO2 is admitted at RT at a pressure increasing up to the catalyst saturation (pCO2 ∼ 0.2 kPa) on both reduced and oxidized Pt-K/Al2O3 and, for comparison, on the reference Pt/Al2O3 and K/Al2O3 samples. No differences between reduced and oxidized samples were observed, and very similar results were obtained on both the K-containing samples. The spectrum of the K-free Pt-Al2O3 reference sample (Figure 7, curve a) shows (i) two bands at 2345 and 2358 cm-1, completely reversible under outgassing at RT, due to linear CO2 adsorbed on coordinatively unsaturated surface (cus) Al3+ ions in octahedrical and tetrahedrical configuration, respectively;26 (ii) a group of bands in the range of 1780-1860 cm-1 correlated with bands at 1200-1180 cm-1, reversible upon outgassing at RT, ascribed to strongly bent CO2 molecules interacting with highly cus cations;25,26 and (iii) bands stable under vacuum at RT at 1646, 1480, 1440, and 1230 cm-1, easily assigned to the ν(CdO), ν(C-O), and δ(COH) modes, respectively, of two types of hydrogen carbonates formed on the γ-alumina support and showing only separated ν(C-O) modes, as reported by Morterra et al.26 Passing to K/Al2O3 and Pt-K/Al2O3, the spectra (Figure 7, curves b and c, respectively) show (i) a band at 2345 cm-1 completely reversible under outgassing at RT, assigned to CO2 weakly interacting with the K+ ions; actually, CO adsorption revealed a complete coverage of the Al2O3 support; and (ii) an envelope of at least five strongly overlapped components (at about 1710, 1685, 1660, 1645, and 1610 cm-1), a second envelope centred at 1320 cm-1, and a third one where three components are distinguishable (at about 1100, 1070, and 1010


Figure 7. FT-IR spectra upon admission at RT of CO2 (0.2 kPa, solid curves) and subsequent evacuation at RT (dotted curves) on Pt/Al2O3, K/Al2O3, and Pt-K/Al2O3 catalysts (a-c respectively) oxidized at 823 K. For the sake of clarity, spectra have been reported after subtraction of the spectrum before gas admission and translated along the y-axis.

cm-1). The three envelopes of bands observed can be related to a variety of surface carbonates on the potassium phase; the first envelope (at higher wavenumbers) can be assigned to the ν(CdO) modes, the second one to the νasym (OCO) modes, and the third one to the νsym (OCO) modes of bidentate carbonates. Such a variety of ν(CdO) modes indicates a high surface heterogeneity, but the heavy superposition of the components in the two envelopes due to νasym(OCO) and νsym(OCO) modes made a correlation between the components very hard. However, on the basis of the average separation of the ν(CdO) and νasym(OCO) modes (ν ) 300 cm-1), they can be assigned mainly to bridged carbonates. The comparison with Pt/Al2O3 (Figure 7, curve a), indicates that all of the species formed on the K-containing samples are formed on the K phase. Analogous experiments performed on the Ba-containing samples11 (not reported for brevity) showed very similar results, that is, the formation of a high variety of carbonates, mainly bidentates, on the Ba phase but with a more important fraction of chelating species than that on the K phase, and a small amount of monodentate carbonates were also detected. All of these features indicate a slightly higher basicity of the Mn+O2- pairs in the Ba phase. The thermal stability of the surface carbonates was finally tested by evacuation at increasing temperature. Figure 8 reports the FT-IR spectrum of Pt-K/Al2O3 upon admission of CO2 (pCO2 ) 0.2 kPa), evacuation at RT (curve a), and outgassing at increasing temperature at a step of 50 K in the 373-673 K range (curves b-h). Bridged carbonates (bands at 1710, 1685, 1660, 1320, and 1100 cm-1) were already removed at 423 K, while chelating carbonates (bands at 1650-1530, 1400-1300, and 1070-1010 cm-1) showed resistance up to 673 K. NOx Adsorption. For the sake of clarity, the NOx surface species and the related IR bands formed upon adsorption of NO, NO/O2, and NO2 at RT on the Pt/Al2O3, K/Al2O3, and Pt-K/Al2O3 are reported in Table 3. NO Adsorption. FT-IR spectra of NO (peq ) 0.5 kPa) adsorbed at RT on Pt/Al2O3, K/Al2O3, and Pt-K/Al2O3 activated at 823K are displayed in Figure 9. All spectra were run on the samples evacuated and subsequently oxidized at 823 K. For the reference Pt/Al2O3 sample (Figure 9, curve a), the predominant feature is the band centred at 1800 cm-1 assignable


Figure 8. FT-IR spectra of Pt-K/Al2O3 catalyst oxidized at 823 K upon admission of CO2 at RT (pCO2 ) 0.2 kPa), the subsequent evacuation at RT (a), and at increasing temperature at a step of 50 K in the 373-673 K range (b-h). For the sake of clarity, spectra have been reported after subtraction of the spectrum before gas admission.

to Ptδ+ mononitrosyls, formed upon partial reduction of Pt2+ ions by NO at RT.16 On K/Al2O3 and Pt-K/Al2O3 (Figure 9, curves b and c, respectively), bands assignable to the presence of nitrites and nitrates are formed. In particular, two peaks with correlated intensity at 1470 (shoulder at 1455 cm-1) and at 1295 cm-1 are present on both K/Al2O3 (with negligible intensity) and Pt-K/Al2O3 and assigned to νasym(NO2) and νsym(NO2), respectively, of monodentate nitrates.15,16,33 A band at 1260 cm-1, present only on Pt-K/Al2O3, is assigned to νasym(NO2) of bidentate nitrites. The amount of surface NOx is higher on Pt-K/ Al2O3 than that on K/Al2O3, thus indicating a role of Pt in the storage. Moreover, the absence of nitrites and nitrates on Pt/ Al2O3 shows the importance of the contemporary presence of the noble metal and the basic surface for their formation. In any case, since the growth of these species is extremely limited, we are not interested in the discussion of the mechanism formation. NO/O2 Interaction. The nature, time evolution, and thermal stability of species formed after admission at RT of different NO/O2 mixtures (NO/O2 1:8, 1:6, and 1:4; pNO ) 0.5 kPa) on oxidized Pt-K/Al2O3 and K/Al2O3 samples were then analyzed, and the results are displayed in Figure 10. On the Pt/Al2O3 reference sample, only the 1:4 NO/O2 mixture has been analyzed. With respect to the admission of NO alone, bands with an intensity of one or two orders of magnitude higher were observed, indicating that the presence of oxygen strongly favors the formation of surface NOx species (vide infra). On both K-containing samples, the admission of NO/O2 mixtures with different ratios clearly revealed that the type of species formed and their absolute and relative amounts strongly depend on the NO/O2 ratio. First, we examine the results obtained for the Pt-K/Al2O3 oxidized system (Figure 10A, B, and C). When the 1:8 NO/O2 mixture was admitted (Figure 10A), only nitrites were formed on Pt-K/Al2O3, both bidentate (bands at 1242-1246 and 1325 cm-1, related to the νasym(NO2) and νsym(NO2) modes, respectively) and monodentate (a band at 1495 cm-1 with a shoulder at ∼1540 cm-1 and a broad band in the 1100-1000 cm-1 region, related to ν(NdO) and ν(N-O) modes, respectively). Both linear and bidentate nitrites slowly increased upon increasing

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TABLE 3: NOx Species and Related IR Bands Formed upon Adsorption of NO, NO/O2, or NO2 at RT on Pt/Al2O3, K/Al2O3, and Pt-K/Al2O3 Samples band positions (cm-1) sample Pt/Al2O3

K/Al2O3 and Pt-K/Al2O3

NOx species

NO

bidentate nitrites

traces

bridging bidentate nitrates

traces

chelating bidentate nitrates

traces

monodentate nitrates

n.f.

NO2δ+ Pt mononitrosyls ionic bidentate nitrites

n.f. 1800 n.d. 1260 n.f.

monodentate nitrites monodentate nitrates

1470-1450 1290

bridging bidentate nitrates

a

NO/O2

chelating bidentate nitrates

n.f.

ionic nitrates

n.f.

NO2 n.f.

a

1320 1230 1645, 1615, 1590 1254-1256 1070-1060 1564 1295 1040-1030 1470-1460 1400-1380 1960 1860 1330-1320 1260-1230 1565-1505 1100-1010 n.f.

n.f.

1615 1270 n.d.b 1565-1530 1317 1040-1000 1395, 1365-1370 1040

1615 1270 n.d. 1565-1530 1317 1040-1000 1395, 1365-1370 1040

1630, 1610, 1580 1250-1240 1060-1000 1570 1292 1060-1000 1500-1420 1400-1300 ∼2000 n.f. n.f. n.f.

assignments νsym (NO2) νasym (NO2) ν(NdO) νasym (NO2) νsym (NO2) ν(NdO) νasym (NO2) νsym (NO2) νasym (NO2) νsym (NO2) νasym (NO2) ν(NO) νsym (NO2) νasym (NO2) ν(NdO) ν(N-O) νasym (NO2) νsym (NO2) ν(NdO) νasym (NO2) νsym (NO2) ν(NdO) νasym (NO2) νsym (NO2) νasym (NO3) νsym (NO3)

n.f. ) not formed. b n.d. ) band not detectable because it is superimposed on other bands.

Figure 9. FT-IR spectra upon admission of 0.5 kPa of NO on Pt/ Al2O3, K/Al2O3, and Pt-K/Al2O3 catalysts at RT (a-c, respectively). Spectra have been reported after subtraction of the spectrum before gas admission and translated along the y-axis.

the contact time, and no traces of nitrates could be recognized also after long contact times (2 h). When the 1:6 NO/O2 mixture was used (Figure 10B), nitrites and nitrates simultaneously appeared, namely, (i) bidentate nitrites, initially the main species present, showing a complex band shifting from 1260 to 1235 cm-1 related to νasym(NO2) modes and a weak band at around 1322 cm-1 related to νsym(NO2) modes, (ii) linear monodentate nitrites, showing two complex absorptions, one in the 1550-1500 cm-1 region due to ν(NdO) modes and another one in the 1100-1010 cm-1 region due to ν(N-O) modes, and (iii) ionic nitrates, showing two bands at 1394 and 1371 cm-1 related to the νasym(NO3) vibration split into two components for the loss of symmetry

caused by interaction with the surface (νsym(NO3) modes expected at around 1030-1040 cm-1). Upon increasing the contact time, all of the species increase together; the bidentate nitrites reach their maximum intensity after 1 h of contact with the gas phase (Figure 10B, curve e) and then slightly decrease, whereas linear nitrites and ionic nitrates further rise. No further increment in the band intensities is recorded after 3 h of contact (curve g), and no significant changes in the spectrum are noted under outgassing at RT. When the 1:4 NO/O2 mixture is used (see Figure 10C), (i) bidentate nitrites are first formed (band shifting from 1260 to 1239 cm-1, νasym(NO2)), increasing in intensity until 5 min of contact with the gas mixture, (ii) after 5 min, also, linear nitrites and ionic nitrates appear, and (iii) by prolonging the contact time, monodentate nitrites do not increase further, whereas bidentate nitrites are consumed, ionic nitrates further increase, and bidentate nitrates (bands at 1567-1572 and 1613 cm-1 [ν(NdO)] and 1317 and 1270 cm-1 [νasym(NO2) and νsym(NO2), respectively]) appear and increase until reaching the catalyst saturation after 2 h of contact with the gas phase. For these surface nitrates νsym(NO2) vibrations are expected at 1040-1000 cm-1, partially superimposed to those of monodentate nitrites still present. However, it is worth noting that in this case, nitrates (ionic and bidentate) are the predominant species at saturation. We next examine the results after admission of NO/O2 mixtures on K/Al2O3; (i) for the 1:8 mixtures, again, only nitrites (spectra not reported) were formed with a rate not significantly different with respect to that of the Pt-K/Al2O3 catalyst; (ii) conversely, significant changes were noted under admission of the other NO/O2 mixtures (Figure 10D and E). The main difference is a lower rate of nitrate formation with respect to Pt-K/Al2O3, as valued by comparing their intensities at the same contact times or by comparing the nitrate to nitrite intensity ratios at each contact time.



Figure 10. FT-IR spectra upon admission at RT of NO/O2 mixtures at different NO/O2 ratios (pNO) 0.5 kPa) at increasing contact times, 10 s (a) and 1 (b), 5 (c), 15 (d), 30 (e), 60 (f), 120 (g), and 180 min (h). (A, B, and C) Pt-K/Al2O3 catalyst upon admission of 1:8, 1:6, and 1:4 mixtures of NO/O2, respectively. (D and E) K/Al2O3 catalyst upon admission of 1:6 and 1:4 mixtures of NO/O2, respectively. (F) Pt/Al2O3 catalyst upon admission of a 1:4 NO/O2 mixture. For the sake of clarity, spectra have been reported after subtraction of the spectrum before gas admission, and some spectra were omitted.

However, on both K-containing catalysts, it can be seen that the nitrate to nitrite ratio increases in parallel with the NO/O2 ratio. Also, at each contact time, the NOx uptake increases upon increasing the NO/O2 ratio. Finally, we examine the results obtained by admission of the 1:4 NO/O2 mixture on the Pt/Al2O3 catalyst (Figure 10F). As

previously reported by us,16 under this condition, surface bidentate nitrites (1320, 1230 cm-1) are the main species present after 5 min of contact even if nitrates of various types are formed (several bands in the range of 1700-1000 cm-1; see Table 3 for the peak positions and assignments). However, by prolonging the contact time, an evolution of nitrites to nitrates takes place,

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so that the main surface species observed after 30 min of contact with the mixture are bridging and chelating bidentate nitrates. Monodentate nitrates, NO2δ+ species coordinated to Lewis acidic sites and Pt2+ mononitrosyls (at 1860 cm-1), were detected in smaller amounts. By comparison of the band positions of species formed on Pt-K/Al2O3 and on Pt/Al2O3, it is clear that the K phase only is interested in the adsorption of monodentate nitrites and ionic nitrates. Conversely chelating nitrites, bridging, and chelating nitrates observed on the Pt-K/Al2O3 system could be adsorbed both on alumina and the K phase. However, looking at the intensities of the related bands on the two systems (note the different absorbance scale in Figure 10C and F) and taking into account that a large fraction of the alumina is covered by the K phase, we can reasonably conclude that on Pt-K/Al2O3 systems, only a very minor fraction of these NOx species is bound to alumina. NOx Adsorption Reaction Routes. Different routes can be suggested for the NOx formation by NO/O2 mixtures. A reaction route leading to the formation of nitrites as stable intermediates followed by their subsequent oxidation to nitrates is

2NO + 0.5O2 + Os2- f 2NO2-

(1)

2NO2- + O2 f 2NO3-

(2)

2-

where Os is a surface oxygen anion. A second reaction route takes into account the NO2 formation in the gas phase following the reaction

NO + 0.5O2 f NO2

(3)

Notably, this reaction is the sum of the NO dimerization followed by oxidation to NO2. Nitrites and nitrates can be thus formed following the reaction

2NO2 + Os2- f NO3- + NO2-

(4)

leading to equal amounts of nitrates and nitrites, the latter being oxidized to nitrates following reaction 2, the overall reaction being

2NO2 + Os2- + 0.5O2 f 2NO3-

(5)

Alternatively, nitrites can be oxidized to nitrates following the reaction

NO2- + NO2 f NO3- + NO

(6)

which, combined with reaction 4, leads to the dismutation process

3NO2 + Os2- f 2NO3- + NO

Figure 11. FT-IR spectra after admission at RT of NO2 (0.5 kPa) on Pt-K/Al2O3 and on K/Al2O3 catalysts (A and B, respectively) recorded at increasing contact times, 10 s (a) and 2 (b), 5 (c), 7 (d), and 10 min (e). Spectra have been reported after subtraction of the spectrum before gas admission.

(7)

Looking at reaction 3, it is well known that, while the thermodynamic equilibrium is completely displaced to the right at RT, in the gas phase, the reaction rate at RT is not very high. The mechanism proposed for NO oxidation previews the NO dimerization followed by oxidation. At high oxygen concentration, the dimerization is the rate-determining step; actually, the dimerization depends on the probability that two NO molecules can interact. Conversely, at low oxygen concentration, the ratedetermining step is the dimer oxidation. We always work in excess of oxygen; therefore, the rate-determining step can be considered the NO dimerization. Upon increasing the NO partial pressure, the dimerization rate and thus the NO2 formation rate are increased. On this basis, the different results obtained using the three different mixtures can be first related to the increased rate of NO2 formation. In particular, the formation of nitrites only in the case of the 1:8 mixture indicates that at this NO/O2

ratio, reaction 3 in the gaseous phase can be neglected; the nitrite formation then follows reaction 1. Furthermore, on one hand, the observation that only nitrites are formed suggests that reaction 2 at RT is actually very slow despite the oxygen excess both on Pt- containing and Pt-free catalysts. On the other hand, at RT, Pt does not seem to affect the rate of reaction 1. Looking at the other two mixtures (1:6 and 1:4), the formation of increasing amounts of nitrates can be related to the formation of increasing amounts of NO2 in the gas phase (reaction 3), followed by reaction 4 and eventually by reaction 6 rather than by reaction 2. However, in these cases, the higher rate of nitrate formation indicates a catalytic effect of the Pt on the NO2 formation already at RT. This is confirmed by the examination of results obtained by direct adsorption of NO2. NO2 Adsorption. Hereafter, the results obtained by admitting 0.5 kPa of NO2 at RT at increasing contact time are presented. The spectra after admission of NO2 (0.5 kPa) on Pt-K/Al2O3 and on K/Al2O3 catalysts (Figure 11A and B, respectively) show that immediately high amounts of nitrates, both of ionic (1395 and 1365 cm-1 due to the split of the νasym(NO3) mode and 1040 cm-1 due to νsym(NO3) mode) and bidentate type, but low amounts of nitrites, both of monodentate and bidentate type, are formed. By increasing the contact time, nitrates increase rapidly, whereas nitrites remain constant or decrease (actually, it is not easy to understand exactly their fate, owing to the


J. Phys. Chem. C, Vol. 114, No. 2, 2010 1137 thermal stability. When the temperature reaches 423 K, nitrites go on desorbing, whereas bands of ionic nitrates are slightly enhanced. At temperatures above 473 K, bidentate nitrates appear, exhibiting a ν(NdO) mode at 1535 cm-1, a νasym(NO2) mode at 1317 cm-1, and a νsym(NO2) mode at 1005 cm-1. The formation of bidentate nitrates has been the most important finding of the temperature-programmed desorption experiments, and it has been very useful to correctly assign the bands formed upon admission of the 1:4 NO/O2 mixture and of NO2. Proceeding with the experiment, at 573 K, also nitrates begin to be removed, and when the temperature reaches 723 K, no more traces of residual species can be seen at the catalyst surface. Concerning the thermal stability of nitrate species formed by admission both of the 1:4 NO/O2 mixture (not shown in the figure) and of NO2 (Figure 12B), the outgassing at increasing temperature in the RT-723 K range indicates that no relevant removal of adsorbed species is recorded until the temperature reaches 523 K. Then, bidentate and ionic nitrates are progressively removed together; particularly, the major fraction of nitrates is desorbed between 623 and 673 K; at 673 K, bidentate nitrates are completely removed, whereas a temperature of 723 K is needed to remove the residual ionic nitrates, which show higher thermal stability with respect to the bidentate ones. 4. Conclusions

Figure 12. FT-IR spectra upon desorption at increasing temperatures of the Pt-K/Al2O3 sample previously saturated at RT with the 1:6 NO/ O2 (A) or saturated with NO2 (B); desorption at RT (a), 373 (b), 423 (c), 473 (d), 523 (e), 573 (f), 623 (g), 673 (h), and 723 K (i). For the sake of clarity, spectra have been reported after subtraction of the spectrum before gas admission, and some spectra were omitted.

superposition of their weak bands with those very intense bands of the nitrates). The admission of NO2 was performed also on the reference Pt/Al2O3 and Al2O3 samples; the obtained results (not reported), already examined by us in previous work,16 show that nitrites were not observed, while chelating and bridging nitrates were formed immediately and in higher amounts than after prolonged contact with the NO/O2 mixture. It is interesting to emphasize that by using NO2 on all of the systems, the saturation is reached in times markedly shorter than those by using the NO/O2 mixture (10 min versus 2 h). It is also interesting to note that minor differences in the amount of NOx species adsorbed and in the adsorption rate are observed for the Pt-free catalysts. These results clearly indicate that, starting from NO2, the nitrates formation proceeds rapidly, probably through reactions 4 and 6 as suggested in the previous paragraph, that is, in this case, nitrites are formed by NO2 dismutation together with nitrates and are rapidly oxidized to nitrates by NO2 in excess. Thermal Stability of the NOx Adsorbed Species. Interesting information has also been pointed out by the temperatureprogrammed desorption experiments performed after outgassing at RT of the Pt-K/Al2O3 sample saturated with the 1:6 NO/O2 mixture (Figure 12A). Indeed, by increasing the temperature to 373 K, both of the nitrite species start to be removed, whereas bands of ionic nitrates remain unaffected, thus showing a higher

The combined use of XRD, HRTEM, and FT-IR techniques allowed us to deeply investigate the textural, structural, morphological, and surface properties of the Pt-K/Al2O3 catalyst. The characterization of the Pt phase by HRTEM and CO adsorption at RT followed by FT-IR spectroscopy provides evidence that (i) the platinum phase is well dispersed on the alumina support in the form of nanosized roundish particles having a mean dimension equal to 1.5 nm, (ii) the catalyst used in the NSR process causes a sintering of the Pt particles, but less dramatic than that observed on the K-free Pt/Al2O3 sample, (iii) the potassium phase completely covers the alumina support, and (iv) Pt is strongly interacting with the basic potassium phase. The surface acid-base properties were investigated by CO2 adsorption at RT followed by FT-IR spectroscopy. A lower basicity of the Mn+O2- surface pairs of Pt-K/Al2O3 with respect to Pt-Ba/Al2O3 has been put in evidence. The NOx adsorption at RT showed that (i) in the absence of oxygen, NO is adsorbed in very limited amounts; (ii) the presence of oxygen strongly favors the NO adsorption, giving rise to surface nitrites and/or nitrates; (iii) the absolute and relative amounts of nitrites and nitrates strongly depend on the NO/O2 ratio; in particular, at the lowest ratio used (1:8), nitrites are the only species formed, and at the highest one (1:4), nitrates are the predominant species at the saturation; (iv) a comparison with the results of analogous measurements on the K/Al2O3 system shows a catalytic effect of Pt already at RT in nitrates formation; and (v) the NO2 adsorption clearly indicates that nitrate formation proceeds rapidly both in the absence and in the presence of platinum. References and Notes (1) Takeuchi, M.; Matsumoto, S. Top. Catal. 2004, 28, 151. (2) Matsumoto, S. Catal. Today 1996, 29, 43. (3) Miyoshi, N.; Matsumoto, S.; Katoh, K.; Tanaka, T.; Harada, J.; Takahashi, N.; Yokota, K.; Sugiura, M.; Kasahara, K. SAE Tech. Pap. 1995, 950809. (4) Shinjoh, H.; Takahashi, N.; Yokota, K.; Sugiura, M. Appl. Catal., B 1998, 15, 189.

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