Al2O3 NSR Catalysts at Different Ba Loading: Characterization

Jul 31, 2008 - Morphological, textural, and surface properties of several Pt−Ba/Al2O3 NOx storage reduction (NSR) catalysts at different Ba loading ...
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J. Phys. Chem. C 2008, 112, 12869–12878

12869

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

L. Castoldi and L. Lietti Dipartimento di Energia, Centro NEMAS - Nano Engineered Materials and Surfaces; Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy ReceiVed: February 19, 2008; ReVised Manuscript ReceiVed: April 10, 2008

Morphological, textural, and surface properties of several Pt-Ba/Al2O3 NOx storage reduction (NSR) catalysts at different Ba loading in the range 0-30 wt % were characterized by means of X-ray diffraction, highresolution transmission electron microscopy, and Fourier-tranform infrared (FT-IR) spectroscopy using CO, CO2, and CH3CN as probe molecules. Upon increasing the Ba loading, the Pt exposure progressively decreased, accounting for sintering and masking of the Pt particles by Ba, whereas the interaction between Pt and the barium oxide phase increased. After a few cycles of heating in NO2 and subsequent evacuation (conditioning treatment), BaCO3 initially present evolved to Ba(NO3)2 and then decomposed into a well-dispersed nanosized BaO phase. Upon conditioning, a slight sintering of Pt is observed. However, the presence of Ba avoided a more stressed sintering, as it occurred for the Pt/Al2O3 sample. Investigation by CO2 and CH3CN adsorption followed by FT-IR spectroscopy revealed a high heterogeneity at the BaO surface. In particular, upon CO2 adsorption a variety of surface carbonate-like species were formed (mainly bridging and chelating carbonates on Ba sites). The most relevant features upon CH3CN adsorption were the formation of anionic species on strongly basic oxygen ions of Ba2+ O2- pairs and the presence of acetonitrile molecules polarized by highly uncoordinated Ba2+ ions, stable under evacuation at room temperature. The FT-IR characterization with the three test molecules suggests that the best spread of the Ba phase is obtained for a loading between 16 and 23 wt %. 1. Introduction Air pollution is nowadays one of the most important problems that the developed countries have to tackle and try to solve; indeed, the upcoming legislations are more and more strict regarding the emissions of pollutants such as unburned hydrocarbons (HCs), CO2, and NOx, so car manufacturers have to find new strategies to lower the emission of pollutants. In this way, a crucial point seems to be the air/fuel (A/F) ratio: at the moment, this ratio is equal to 14.7 for petrol engines, representing this value the condition in which both the reduction of NOx and the oxidation of CO and HCs can be provided in a satisfactory way using the three-ways catalysts (TWC). Increasing the A/F ratio to 20-25 implies an increased availability of oxygen to provide a more efficient combustion of fuel; unfortunately, the TWC are very responsive to the A/F ratio, and, particularly, an increase of this ratio is critical for NOx conversion. To solve the problem, alternatives to TWC are looked for, and, between the proposals, NOx storage reduction (NSR) catalysts, also called lean NOx traps, seem to be promising. Usually constituted of a high-surface area support (such as γ-Al2O3), a storage phase (an alkaline or earth alkaline metal oxide), and a noble metal (Pt), these catalysts switch between a lean phase (i.e., in excess oxygen) during which NOx * Author to whom correspondence should be addressed. Phone: +39 0116707539; fax: +39 0116707855; e-mail: [email protected].

are stored on the oxidic phase, and a rich phase poor in oxygen, during which NOx are reduced by CO and HC to N2, CO2, and H2O.1–4 During the last decade several studies have been published investigating both the storage and the reduction processes in depth,5–12 however some aspects still remain controversial. Investigations have been carried out in our labs on a Pt-Ba/ Al2O3 catalyst having a composition near a Toyota catalyst, that is, containing 1 wt % of Pt and 16 wt % of Ba. The findings allowed us to present a proposal for the NOx storage pathways, in which the storage upon admission of NO/O2 mixture proceeds on the catalyst through two parallel ways: (i) the “nitrite route”, which implies NO storage on Ba neighboring Pt sites first in the form of nitrite ad-species that are then progressively oxidized to nitrates; and (ii) the “nitrate route”, implying NO oxidation to NO2 on Pt sites, followed by NO2 disproportion on Ba sites with formation of nitrates and NO release in the gas phase.13–17 The studies were thereafter extended to Pt-Ba(x)/Al2O3 catalysts containing different Ba loading in the range 0-30 wt %, to understand how the barium influences the storage capacity and which is the best formulation.18,19 Obtained data indicated that the increase in the Ba loading results in a strong increase of the NOx adsorption and in the percentages of Ba involved in the storage. The increase of the catalyst NOx adsorption capability upon increasing the Ba loading has been associated with the involvement in the NOx storage process of Pt-Ba neighboring species. Specifically, the addition of Ba increases the number of Pt-Ba neighboring species, which could favour

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TABLE 1: Chemical Composition, Morphological and Textural Features of Calcined Catalysts sample

γ-Al2O3 content (% wt/wt)

Pt content (% wt/wt)

Ba content (% wt/wt)

SS (m2/g)

pore volume (cm3/g)

Pt dispersion (%)

dPt (nm)a

Pt/Al2O3 Pt-Ba(5)/Al2O3 Pt-Ba(10)/Al2O3 Pt-Ba(16)/Al2O3 Pt-Ba(23)/Al2O3 Pt-Ba(30)/Al2O3 Ba(16)/Al2O3

99.1 94.1 89.1 82.6 76.4 69.5 83.3

0.99 0.95 0.90 0.83 0.77 0.70 –

– 4.95 10.00 16.67 22.78 29.79 16.67

186 183 176 137 129 108 133

1.02 0.92 0.87 0.81 0.72 0.59 0.82

82 80 75 71 46 39 –

1.3 1.4 1.5 1.5 2.4 2.8 –

a Particle diameter calculated from Pt dispersion evaluated by H2 chemisorption measurements at 273 K15 using the empirical relationship dPt (nm) ) 1.1 / (H/Pt).20

the NOx storage process, resulting in a better utilization of the Ba component. Moreover, the analysis of the collected data also showed that the NOx storage capacity seems to be independent of the oxidative capability of the catalysts.18,19 In spite of the wide number of studies devoted to the understanding of NOx storage reduction pathways, the morphological, structural, and textural properties of Pt-Ba/Al2O3 catalysts have not been characterized so deeply; thus, in this paper the attention has been focused on these aspects by means of several techniques such as X-ray diffraction (XRD), highresolution transmission electron microscopy (HR-TEM) and Fourier-transform infrared spectroscopy (FT-IR). The aim was to carry out a circumstantial physical and chemical characterization of these catalytic systems that, in our opinion, could be very useful to solve some aspects not yet completely clarified concerning the NOx storage reduction mechanisms. 2. Experimental Section 2.1. Sample Preparation. Pt-Ba(x)/Al2O3 catalysts at different Ba loading (x ) wt %) were obtained by impregnation in two sequential steps of a commercial γ-Al2O3 carrier (Versal 250 from La Roche Chemicals, surface area of 200 m2 g-1 and pore volume of 1.4 cm3/g) as follows: the powder γ-Al2O3 carrier was at 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 calcination at 823 K for 5 h, batches of this sample were impregnated with solutions containing various concentrations of Ba(CH3COO)2 (Strem Chemical, 98.5%), appropriately chosen to yield samples with Ba loadings in the range 0-30 wt %. The impregnated samples were initially dried for 12 h at 353 K and then calcined at 823 K for 5 h. The powder materials prepared in this work are listed in Table 1 in term of weight percent for each component. For comparative purposes, the reference binary systems, Pt/Al2O3 and Ba(16)/Al2O3 were also prepared using the same precursors and procedures. 2.2. Characterization Techniques and Procedures. Surface area and pore size distribution were determined by N2 adsorption-desorption at 77 K with the BET method using a Micromeritics TriStar 3000 Instrument. The Pt dispersion was estimated from hydrogen chemisorption at 273 K after reduction in H2 at 573 K using a TPD/R/O 1100 ThermoElectron Corporation instrument. XRD spectra were collected on powder 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. HRTEM analyses were performed using a top entry Jeol JEM 2010 (200 kV) microscope; for analyses, the powdered samples were deposited on a copper grid, coated with porous carbon film.

Absorption/transmission IR spectra were run at room temperature (RT) on a Perkin-Elmer FT-IR 2000 spectrophotometer equipped with a Hg-Cd-Te cryodetector, working in the range 7200-580 cm-1 at a resolution of 2 cm-1 (No. of scans: ∼40). For IR analysis, powder samples were pelletized in selfsupporting disks (10-15 mg cm-2) and placed in a quartz IR cell allowing thermal treatments in vacuo or in a controlled atmosphere. Pellets were initially activated by heating in vacuo at 823 K and subsequently submitted to the conditioning treatment, consisting of few cycles of heating in NO2 at 623 K and outgassing at 823 K as previously reported.17 Pellets were thereafter submitted to redox treatments consisting of (i) heating in dry oxygen at 823 K and evacuation at RT and (ii) heating in H2 at 623 K and evacuation at the same temperature. IR spectra were recorded at room temperature (RT) before and after interaction with CO or CO2 (Matheson, C.P.) or CH3CN (Carlo Erba, RPE). 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. Morphological, Structural, and Textural Characterization. The surface area, pore volume and Pt dispersion for each sample (listed in Table 1) were already discussed in previous works.18,19 Briefly, it is shown that the Pt/Al2O3 sample is characterized by the highest surface area (186 m2/g) and pore volume (1.02 cm3/g). The impregnation with barium solution at increasing concentration results into a progressive decrease in the surface area and pore volume, whereas the pore radius is always in the range 90-110 Å. Also, the Pt exposure, measured by H2 chemisorption at 273 K, progressively decreases upon addition of Ba, moving from 82% in the case of Pt/Al2O3 to 39% in Pt-Ba(30)/Al2O3. The decrease in the Pt exposure has been ascribed to the fast and exothermic decomposition of barium acetate precursor, shown by DTA-TG thermal analysis reported elsewhere,18,19 eventually leading to the sintering of the Pt crystallites and/or to the masking of the Pt crystallites upon addition of the Ba component (vide infra). In Table 1 the mean Pt particle sizes (dPt) calculated from the empirical relationship often used for monometallic catalysts was dPt(nm) ) 1.1/(H/Pt), where H/Pt is the Pt exposure measured from H2 chemisorption,20 are also reported. Progressive increase of the Pt particle sizes (from 1.3 to 2.8 nm) can be observed upon increasing the Ba content. XRD patterns of calcined samples show the presence of micro-crystalline γ-Al2O3 (JCPDS 10-425); in the case of the samples with low Ba content (up to 10 wt %), no other crystalline phases are detected (Figure 1, curve a), thus suggesting that the Ba phase is well-dispersed over the support. In samples with higher Ba loadings, the presence of crystalline BaCO3 Whiterite phase (JCPDS 5-378) is evident (Figure 1, curves b and c).

Pt-Ba/Al2O3 NSR Catalysts at Different Ba Loading

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Figure 1. X-rays diffractograms of calcined Pt-Ba(5)/Al2O3, Pt-Ba(16)/ Al2O3, and Pt-Ba(30)/Al2O3 (curves a, b and c respectively) and of conditioned Pt-Ba(16)/Al2O3 (curve d). • γ-Al2O3 (JCPDS 10-425), * BaCO3 Whiterite (JCPDS 5-378).

Figure 3. HRTEM images of Pt/Al2O3 calcined at 823 K (panel A) and fully conditioned (panel B); original magnification: ×500 K (panel A), ×250 K (panel B).

Figure 2. Panel A: FT-IR spectra of Pt/Al2O3, Pt-Ba(5)/Al2O3, Pt-Ba(10)/Al2O3, Pt-Ba(16)/Al2O3, and Pt-Ba(23)/Al2O3 catalysts calcined at 823 K (curves a-e, respectively). Spectra have been translated along the y-axis for sake of clarity. Panel B: FT-IR spectra of Pt-Ba(10)/Al2O3 before (curve a) and after (curve b) the conditioning treatment with NO2.

The presence of bulky BaCO3 on calcined samples is also clearly revealed by FT-IR spectra, where it is easy to note the presence of intense bands at 1440 cm-1 with shoulders at 1550, 1400, and 1370 cm-1 and of weak bands at 1752 and 1059 cm-1 (Figure 2A), whom intensities increase in parallel with the barium content. Actually, these bands are characteristic of BaCO3 in bulk, as reported in the literature21 and as obtained in a bulky BaCO3 spectrum (not reported) run by us for comparison.

It was shown in previous papers14,17 that the Ba phase undergoes a deep morphological-structural evolution during the conditioning treatment described in the Experimental Section. Indeed, it can be observed both in XRD patterns (Figure 1, curve d) and in IR spectra (Figure 2B) that the features due to crystalline BaCO3 disappeared, whereas no new barium phases were detected. This accounts for an almost complete evolution of BaCO3 to Ba(NO3)2, which was then decomposed into a well-dispersed nanosized BaO phase.9,13,22 Notably, this evolution occurs at temperatures markedly lower than those required for the thermal decomposition of bulky BaCO3.23 Because the presence of such intense and broad bands in the 1600-1300 cm-1 region makes it very hard or impossible to distinguish the absorption of NOx surface species (whose vibrational modes are strongly overlapped with those of carbonates), samples were subjected to conditioning treatment before performing FT-IR experiments. Finally, the morphological and structural features of the calcined samples and their evolution upon conditioning treatment were investigated by HRTEM. For comparative purposes, a Ba-free Pt/Al2O3 sample was first examined. On the calcined sample (Figure 3A) very small

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TABLE 2: Mean Pt Particle Size Measured by HRTEM on Some Calcined and Conditioned Catalysts dPt (nm) sample

calcined sample

conditioned sample

Pt/Al2O3 Pt-Ba(16)/Al2O3 Pt-Ba(30)/Al2O3

16 wt % the Ba phase tends to give unsupported aggregates. The surface acid-base properties were finally investigated by CO2 and CH3CN adsorption at RT followed by FT-IR spectroscopy. The CO2 adsorption provides evidence, in the case of Bafree Pt/Al2O3 sample, of the presence of linear CO2 coordinated VI IV on Alcus and on Alcus , strongly bent CO2 molecules, and two types of hydrogen carbonates. On Ba-containing samples the following are observed: (i) the presence of linear CO2 coordinated to Ba2+ ions, (maximum amount for 16 wt % of Ba); (ii) the marked decrease of the hydrogen carbonates; (iii) the presence of a large variety of surface carbonate-like species in agreement with the high dispersion of the BaO phase: bridged carbonates on Ba and Al sites (maximum amount between 5 and 10 wt % of Ba), bridged, chelating, and very small amounts of monodentate carbonates on Ba sites, all showing lower thermal stability in comparison with the bulky Whiterite phase; and (iv) a progressive increase of the overall amount of these species with the Ba loading, with a sudden rise between 10 and 16 wt % of Ba, followed by a slower rise. The CH3CN adsorption provides evidence, in the case of Bafree Pt/Al2O3 sample, of mainly CH3CN N-bonded to Al3+ sites, along with physisorbed (liquid-like and H-bonded) species. On Ba-containing samples: (i) CH3CN bonded to Al3+ sites decreases rapidly upon increasing the Ba loading and is no longer detected on samples with Ba content g 16 wt %; (ii) a relevant feature is the presence of acetonitrile molecules interacting with highly uncoordinated Ba ions (e.g., isolated or highly dispersed on the alumina support), exhibiting vibrational modes with the same frequencies as physisorbed species but with higher stability; (iii) another relevant feature is the formation on strongly basic O2- sites of anionic species, adsorbed thereafter onto the adjacent cationic sites; (iv) specifically, mainly CH2CN- monomers coordinated on cationic sites were initially observed, rapidly evolving to polymeric anionic species upon increasing the CH3CN coverage. At the catalyst saturation, the presence of at least three types of polymeric anions (one tentatively assigned to dimeric anions coordinated to Al3+ sites, the other ones to different Ba2+ sites) reveals the high surface heterogeneity, already evidenced by CO2 adsorption data; (v) the overall amount of these species progressively increases with the Ba loading up to 23 wt % and then slightly decreases. It is noteworthy that, as for the spread of the Ba phase over the support, the results obtained in the FT-IR characterization with the three test molecules are in agreement each other and suggest that the best spread of the Ba-phase is obtained for a loading between 16 and 23 wt %. Moreover, this conclusion perfectly matches with data concerning the NSR properties presented in previous work,18 which indicated that the NOx

12878 J. Phys. Chem. C, Vol. 112, No. 33, 2008 storage capability increases in parallel with the barium loading up to 23 wt%. Acknowledgment. The authors would like to thank Regione Piemonte for funding this research. References and Notes (1) Matsumoto, S. Catal. Today 1996, 29, 43. (2) Miyoshi, N.; Matsumoto, S.; Katoh, K.; Tanaka, T.; Harada, J.; Takahashi, N.; Yokota, K.; Sugiura, M.; Kasahara, K. SAE Technical Paper 1995, 950809. (3) Shinjoh, H.; Takahashi, N.; Yokota, K.; Sugiura, M. Appl. Catal. B: EnVironmental 1998, 15, 189. (4) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.-i.; Tanizawa, T.; Tanaka, T.; Tateishi, S.-s.; Kasahara, K. Catal. Today 1996, 27, 63. (5) Broqvist, P.; Panas, I.; Fridell, E.; Persson, H. J. Phys. Chem. B 2002, 106, 137. (6) Cant, N. W.; Patterson, M. J. Catal. Today 2002, 73, 271. (7) Fanson, P. T.; Horton, M. R.; Delgass, W. N.; Lauterbach, J. Appl. Catal. B 2003, 46, 393. (8) Laurent, F.; Pope, C. J.; Mahzoul, H.; Delfosse, L.; Gilot, P. Chem. Eng. Sc. 2003, 58, 1793. (9) Piacentini, M.; Maciejewski, M.; Baiker, A. Appl. Catal. B 2005, 59, 187. (10) Szailer, T.; Kwak, J. H.; Kim, D. H.; Hanson, J. C.; Peden, C. H. F.; Szanyi, J. J. Catal. 2006, 239, 51. (11) Szanyi, J.; Kwak, J. H.; Hanson, J.; Wang, C.; Szailer, T.; Peden, C. H. F. J. Phys. Chem. B 2005, 109, 7339. (12) Westerberg, B.; Fridell, E. J. Molec. Catal. A 2001, 165, 249. (13) Frola, F.; Prinetto, F.; Ghiotti, G.; Castoldi, L.; Nova, I.; Lietti, L.; Forzatti, P. Catal. Today 2007, 126, 81. (14) Nova, I.; Castoldi, L.; Lietti, L.; Tronconi, E.; Forzatti, P.; Prinetto, F.; Ghiotti, G. J. Catal. 2004, 222, 377. (15) Nova, I.; Castoldi, L.; Prinetto, F.; Santo, V. D.; Lietti, L.; Tronconi, E.; Forzatti, P.; Ghiotti, G.; Psaro R.; Recchia, S. Top. Catal., 2004, 30, 181. (16) Prinetto, F.; Ghiotti, G.; Nova, I.; Castoldi, L.; Lietti, L.; Tronconi, E.; Forzatti, P. Phys. Chem. Chem. Phys. 2003, 5, 4428.

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