Nanostructured Praseodymium Oxide: Preparation, Structure, and

Table 1: Overview of the Synthesis Parameters (Molar Ratios of the ... Samples of the catalysts were inserted in the reactor as follows: 0.1 g Pr oxid...
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J. Phys. Chem. C 2008, 112, 3054-3063

Nanostructured Praseodymium Oxide: Preparation, Structure, and Catalytic Properties Yulia Borchert,† Patrick Sonstro1 m,† Michaela Wilhelm,‡ Holger Borchert,† and Marcus Ba1 umer*,† Institute of Applied and Physical Chemistry, UniVersity of Bremen, Leobener Str., 28359 Bremen, Germany, and Ceramic Materials and Components, UniVersity of Bremen, Am Biologischen Garten 2/IW3, 28359 Bremen, Germany ReceiVed: August 27, 2007; In Final Form: NoVember 15, 2007

Nanostructured praseodymium oxides were successfully prepared via four different methods: two traditional methods (calcination of praseodymium nitrate and sol-gel method with propylene oxide) and two more sophisticated, modern techniques (citrate method and modified Pechini method). Powder X-ray diffraction revealed that all synthesis methods led to praseodymium oxide Pr6O11 with cubic fluorite-like structure. The temperature necessary for the formation of the crystalline oxide phase, however, was dependent on the method and synthesis parameters. The size of the nanocrystalline domains was in the range of some 10 nm in all cases. The catalytic properties of the nanostructured oxides were studied choosing CO oxidation as a first test reaction. According to infrared spectroscopy, the surface of all samples was covered with monodentate carbonate species after the synthesis. After exposure to CO, two types of bidentate carbonates were observed on the oxide surface, and under the feed of both CO and O2, carbon dioxide was observed by IR spectroscopy as product in the gas phase at temperatures from 300 °C on. The activity with respect to CO oxidation was further investigated in a catalytic test reactor. The maximum conversion of CO was reached at ∼550 °C, and it was ∼95-96% independent of the synthesis method. At moderate temperatures (∼350-500 °C), the activities of the catalysts prepared in the present work were dependent on the synthesis method and synthesis parameters, only to a small extent, but all of them were more active than commercial Pr6O11. The differences between the various samples prepared in this study can be explained by an influence of the synthesis on the oxygen ion mobility. Mechanistically, the results of our work suggest that CO oxidation occurs through the adsorption of CO as a bidentate carbonate, which is then transformed into a monodentate carbonate finally desorbing as CO2.

1. Introduction Rare-earth (lanthanide) oxides currently attract considerable attention because of their potential as catalysts for technically important processes, such as the partial oxidation of methane and its steam and dry reforming to syngas,1,2 oxidative coupling of methane to C2 products,3,4 oxidation of CO,5 and the reduction of NO by methane in the presence of O2.6,7 Moreover, rareearth oxides are also considered as promoters in three-way catalysts.8 Praseodymium oxide has a special position within the series of the rare-earth oxides, as it forms a homologous series with a large number of stoichiometrically defined oxides: PrnO2n-2, with n ) 4, 7, 9, 10, 11, 12, ∞.9,10 The extreme cases are Pr2O3 and PrO2. Pr2O3 can adopt a hexagonal or cubic structure,11,12 while PrO2 exhibits a fluorite-like (cubic) structure.12 Another phase also stable at ambient temperature and in air is Pr6O11, which can be considered as an oxygen-deficient modification of PrO2. Pr6O11 and other phases with fluorite-like structure in the homologous series of oxides have praseodymium cations with variable valence states (3+ and 4+). These oxides have the highest oxygen ion mobility within the series of lanthanide oxides, because the variety of stable phases enables fast changes * Corresponding author. Tel.: +(49) 421-2182500. Fax: +(49) 4212184918. E-mail: [email protected]. † Institute of Applied and Physical Chemistry. ‡ Ceramic Materials and Components.

in the oxidation state of praseodymium.13 This aspect renders this system very attractive for catalytic and other applications requiring high oxygen ion mobility. Heterogeneous solid catalysts of praseodymium oxide are often produced by the thermal decomposition of inactive solid precursors, praseodymium nitrate being predominantly used commercially. The phases formed in each case depend on the decomposition temperature and the gas atmosphere applied. Praseodymium oxides obtained via the calcination of the nitrate were intensively investigated with respect to their structure by differential thermal analysis, thermal gravimetric analysis (DTA/TGA),14 and X-ray diffraction (XRD).14 In addition to that, the formation of praseodymium oxides from organic precursors, such as Pr acetate,12,15 oxalate,12,15,16 malonate,17 and citric acid complexes,11 was reported in the literature. Catalytic studies with praseodymium oxides published so far deal with the decomposition of 2-propanol,18 the oxidation of carbon monoxide5,19 or of hydrogen,13 and the oxidative coupling of methane,20 but questions concerning the surface species under reaction conditions and reaction mechanisms are still not satisfactory answered. Even for comparably simple reactions, such as CO oxidation, the reaction mechanism is still controversially discussed. Some studies assume a Langmuir-Hinshelwood-like mechanism where adsorbed CO is supposed to react with surface lattice oxygen,5 whereas other studies report indications for a Mars van Krevelen mechanism involving the diffusion of lattice oxygen to the surface.19

10.1021/jp0768524 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/05/2008

Nanostructured Praseodymium Oxide As mentioned above, the preparation method can strongly influence various material properties such as the phase composition, the size of grains, the Me4+/Me3+ ratio, and the specific surface area, i.e., factors possibly influencing the catalytic activity. In this context, the preparation of nanostructured oxides is a promising new strategy to specifically tune catalytic properties. Interesting results were already obtained for the more intensively studied ceria system. Nanostructured cerium dioxide was, for example, found to have enhanced electronic conductivity, which is of interest for applications requiring mixed ionic electronic conductivity.21 Furthermore, by using a modified Pechini method, where a large organic matrix with uniformly distributed cations is formed during the synthesis, it was possible to prepare nanostructured, Pr- or Gd-doped ceria with high oxygen ionic conductivity as a promising material for syngas generation technology.22-24 For pure praseodymium oxides, however, the influence of the microstructure on catalytic properties was not systematically investigated until now. Against this background, the aim of the present work was a systematic investigation of the influence of different synthesis methods (calcination of Pr nitrate, sol-gel method, citrate method, and modified Pechini method) as well as of various synthesis parameters on the structural, surface, and catalytic properties of nanostructured praseodymium oxide, where CO oxidation was selected as a catalytic test reaction. 2. Experimental Section 2.1. Preparation of Samples. Praseodymium oxide samples were prepared via two traditional methods (calcination of the nitrate and a sol-gel method) and two more sophisticated, modern techniques (citric method and polymerized precursor route/modified Pechini method). The praseodymium precursor used as a starting material for the synthesis in all cases was praseodymium nitrate Pr(NO3)3‚6H2O (99.9%, Aldrich). 2.1.1. Calcination of Salt (C Sample). The praseodymium nitrate Pr(NO3)3‚6H2O was calcined under air at 500 °C. This temperature was chosen on the basis of the results from the thermal analysis (see Section 3.1.1.), also being in accordance with the calcination temperatures reported in the literature.14 2.1.2. Modified Sol-Gel Method with Propylene Oxide (SG Samples). In the first step of this synthesis method, clear, green Pr3+ solutions were obtained by dissolving Pr(NO3)3‚6H2O in ethanol. After the addition of propylene oxide, gel formation occurred as expected according to other studies.25,26 The concentration of the Pr3+ solution and the molar ratio of propylene oxide to Pr were systematically varied, as reported in Table 1. The gel was aged and dried at 90 °C for 3 days. The resulting xerogel was ground to powder and calcined under air at temperatures in the range of 300-550 °C. The calcination temperatures were selected on the basis of results from the thermal analysis (see Section 3.1.1.). 2.1.3. Modified Pechini Method (PM Samples). As a more sophisticated method, the polymerized precursor route (modified Pechini method)22,27 was employed for the preparation of nanostructured praseodymium oxide. Ethylene glycol (EG) and citric acid (CA) were taken as polymerization agents, and ethylenediamine (ED) was used as an additional chelating agent. The preparation proceeded as follows. The citric acid was dissolved in ethylene glycol under stirring and heating at 80 °C. Separately, praseodymium nitrate Pr(NO3)3‚6H2O was dissolved in distilled water, yielding a clear, green solution. The ethylene glycol solution was added to the praseodymium nitrate solution first, and the ethylenediamine was subsequently added dropwise under stirring. Then, the solvent was evaporated at

J. Phys. Chem. C, Vol. 112, No. 8, 2008 3055 TABLE 1: Overview of the Synthesis Parameters (Molar Ratios of the Reactants) for All Prepared Samples SG method with propylene oxide

Pr/C3H6O

c(Pr) in ethanol

SG-1 SG-2 SG-3 SG-4 SG-5

1:10 1:15 1:20 1:15 1:15

0.6 M 0.6 M 0.6 M 1M 2M

Pechini method

CA/Pr

CA/EG

CA/ED

PM-1 PM-2 PM-3 PM-4 PM-5

3.75:1 3.75:1 3.75:1 4.5:1 2.5:1

1:3 1:3 1:3 1:3 1:3

1:1 1:2 1:3 1:1 1:1

citrate method CM-1 CM-2 CM-3 CM-4 CM-5

CA/Pr

Pr/HNO3

1.5:1 1:1 2:1 3:1 2.5:1

1:0.072 1:0.072 1:0.072 1:0.072 1:0.072

80-100 °C, until a resin (polymerized precursor) was obtained. The resin was calcined by slowly heating up to 600 °C. The molar ratio of EG/CA/ED was systematically varied, as summarized in Table 1. 2.1.4. Citrate Method (CM Samples). To prepare samples by this method,9,11,28 praseodymium nitrate Pr(NO3)3‚6H2O was added under stirring to different amounts of citric acid dissolved in ethanol (see Table 1 for the molar ratios). The resulting sol was dried at 80-100 °C and calcined under air at temperatures in the range of 350-600 °C. 2.2. Sample Characterization. All powder samples were investigated by differential thermal analysis and thermal gravimetric analysis (DTA/TGA), BET surface area analysis, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIRS). The thermal analysis (DTA/TGA) was carried out with a STA 503 apparatus of Ba¨hr Thermoanalyse GmbH. The analysis was performed in the temperature range up to 1000 °C with a heating rate of 10 °C/min in air. The structure, crystallinity, and phase composition of the samples as well as the crystallite size were investigated by powder XRD with an X’PertPro MPD diffractometer (Philips) using monochromatic Cu KR radiation (λ ) 1.5418 Å). The 2θ scanning region was 5-85°. The particle morphologies and sizes were characterized by TEM using a transmission electron microscope EM420 (Philips). The surface area of the oxides was measured by N2 BET using an automatic surface area analyzer (Model 4200, βeta Scientific). Adsorbed species on the surface before and after exposing the samples to gases were studied by FTIR spectroscopy using a FTS-60A FTIR spectrometer (Biorad). The IR spectrometer was equipped with a controlled gas supply system and a heatable reaction cell (DRIFTS geometry), allowing in situ measurements under continuous gas feed. For the IR spectroscopic measurements, the samples were either studied as prepared or pretreated with hydrogen or oxygen at 400 °C (maximum temperature). A continuous gas feed of 4 vol % CO in Ar was used to study the CO adsorption. 2.3. Catalytic Experiments. Catalytic experiments regarding CO oxidation were carried out in the reaction cell of the IR spectrometer (4 vol % CO + 2 vol % O2 in Ar) as well as in a laboratory reactor. The laboratory reactor (fixed bed type) was equipped with a controlled gas supply and heating system as well as with a photometric detector (Hartmann & Braun URAS

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Figure 1. DTA and TGA curves of Pr(NO3)3‚6H2O (a) and a sample (CM-4) obtained by the citrate method (b).

3G) for analysis of the CO2 content at the exit side. Samples of the catalysts were inserted in the reactor as follows: 0.1 g Pr oxide grains with diameters in the range of 0.45-0.71 mm (obtained by sieving crushed pellets of the praseodymium oxides) were mixed with 0.4-0.8 mm grains of quartz (1.9 g). The samples were placed on quartz wool and exposed to a continuous gas flow (47 mL/min total flow rate) of 3.3 vol % CO and 1.7 vol % O2 in Ar (∼1:1 ratio of CO/O). Apart from the pristine catalysts, samples were treated with H2 or O2 at 600 °C prior to the catalytic experiments in the laboratory reactor. At this high temperature, both treatments result in a loss of oxygen and the formation of oxygen vacancies in the lattice (see Section 3.2.).29 In the case of the experiments in the IR cell, the samples were first treated with H2, presumably leading to a slight reduction (see Section 3.2.4.1), and then shortly with oxygen (1 min 1 vol % O2 in Ar) at 400 °C, which was the maximum temperature there. In this way, an oxygen-rich surface was obtained without reoxidizing the bulk. 3. Results and Discussion 3.1. Structural Properties. 3.1.1. DTA and TGA. In Figure 1, the DTA and TGA curves obtained for praseodymium nitrate hydrate (a) and for a sample which was prepared by the citric method, but not yet calcined (b) are depicted. The DTA curve of praseodymium nitrate hydrate exhibits endothermic peaks at 73, 184, 234, and 350 °C due to the stepwise release of water (Figure 1a). The other endothermic peaks at 394, 409, and 477 °C correspond to the decomposition of the nitrate. Above

Borchert et al. 477 °C, the final decomposition product PrOx is formed. The observed temperatures agree well with the literature.14 Figure 1b shows that the sample prepared by the citrate method decomposes via three endothermic processes at 70, 132, and 150 °C and four exothermic processes at 199, 268, 408, and 525 °C. The endothermic peaks are due to the release of water and solvent from the organic matrix finally resulting in a resinsa dense solution with uniformly distributed praseodymium cations in the organic matrix. The small peaks at 199 and 268 °C indicate the beginning decomposition of the organic matrix. At 408 and 525 °C, two intense exothermic signals are observed. The first peak can be assigned to the combustion of the organic matrix and the formation of carbonates.11,15,30 Above 525 °C, no further peaks are observed and PrOx can be obtained as the final product. Thermal analysis was also performed with samples prepared by the two other synthesis methods (data not shown for brevity). The data allowed the determination of the temperatures of the phase transformation into the final product: In the case of the modified Pechini method, the final product was obtained at ∼450-490 °C, and in the case of the sol-gel method, the final product was obtained at ∼460 °C. 3.1.2. BET. The surface area of all samples ranged between 5 and 28 m2/g (see Table 2) depending on the calcination temperature and synthesis method. Not surprisingly, the surface area decreased with increasing calcination temperature. 3.1.3. TEM and XRD. In Figure 2, a representative TEM image is shown for a sample which was prepared by the solgel method. After calcination at temperatures above ∼440 °C, the material is composed of nanometer-sized domains with typical diameters in the range of some 10 nm. The clearly visible lattice fringes prove that the nanodomains are crystalline. Similar morphologies were observed for all samples. (The sample in Figure 2 exhibited the smallest crystallite sizes.) Thus, all synthesis methods allow the obtainment of praseodymium oxide with crystalline domains in the nanometer size regime. Apart from TEM, all samples were investigated by XRD. Table 2 summarizes the phase composition of the samples and the average sizes of the domains of Pr6O11, which turned out to be the main phase in most cases (see below). The domain sizes were roughly estimated from the width of all reflections of the Pr6O11 phase by means of the Scherrer equation, where other effects leading to broadening of the reflections were neglected. According to these data, the average size depends on the calcination temperature and synthesis method (see Table 2). Within the accuracy of the evaluation, the particle sizes as determined by XRD agree well with the TEM data. In the following, the results obtained for the different preparation techniques will be described in more detail. Calcination of praseodymium nitrate at 500 °C resulted in praseodymium oxide Pr6O11 (ICSD, 42-1121) with cubic fluorite-like structure. The shape of the background indicates that a small amount of an amorphous phase might be present as well (Figure 3a). The surface area of the sample was 7 m2/g. The samples obtained by the sol-gel method with different ratios between the reactants and calcined at 340-350 °C consisted of a crystalline phase of cubic Pr6O11, a small amount of monoclinic Pr2O2CO3 (SG-2, SG-3), and an amorphous phase (see Table 2). The further calcination of these samples at 540 °C resulted in crystalline Pr6O11 with a small amount of PrO2 (ICSD, 24-1006) (Table 2). The other samples (SG-1, SG4, and SG-5) calcined at 420, 440, and 460 °C consisted of crystalline praseodymium oxide and a very small amount of monoclinic Pr2O2CO3 (SG-4) (see Table 2 and Figure 3a). The

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TABLE 2: XRD and BET Data sample

T, °C

BET, m2/g

C-1 PM-1

500 400

7 17.5

PM-3

500 300 490 310

16.4 15.8 19.3

PM-4

490 630 370

PM-5

590 300

PM-2

15.7 16.7

SG-2

490 590 300 420 340

10.6

SG-3

540 350

27.7

SG-4

540 440

23.1

SG-1

SG-5 CM-1

CM-2

CM-3 CM-4 CM-5

12.8

460 250 400

18.1

590 250 390

16.6 9.1

590 300 590 350 540 330 540

8.4

20.4 28.9 15.3

phase composition Pr6O11 + amorphous phase Pr2O2CO3 monoclinica (dominant); Pr6O11 cubic + amorphous phase Pr6O11 cubic + PrO2 cubic (small amount) amorphous phase + Pr6O11 or PrO2 (small amount) Pr6O11 cubic + Pr2O2CO3 hexagonal (small amount) amorphous phase + Pr oxide and Pr2O2CO3 monoclinica (small amount) Pr6O11 cubic + Pr2O2CO3 hexagonal (small amount) Pr6O11 cubic + PrO2 cubic (small amount) Pr2O2CO3 monoclinica (dominant); Pr6O11 cubic + amorphous phase Pr6O11 cubic + PrO2 cubic (small amount) amorphous phase + Pr2O2CO3 monoclinica and Pr oxides (small amount) Pr6O11 cubic Pr6O11 cubic + PrO2 cubic (small amount) amorphous phase Pr6O11 cubic Pr6O11 cubic + Pr2O2CO3 monoclinica (small amount) Pr6O11 cubic + PrO2 cubic (small amount) Pr6O11 cubic + Pr2O2CO3 monoclinica + amorphous phase Pr6O11 cubic + PrO2 cubic (small amount) Pr6O11 cubic + Pr2O2CO3 monoclinica (small amount) Pr6O11 cubic amorphous phase Pr2O2CO3 monoclinica (dominant) + Pr oxides (small amount) + amorphous phase Pr6O11 cubic amorphous phase Pr2O2CO3 monoclinica (dominant) + Pr oxide (small amount) + amorphous phase Pr6O11 cubic amorphous phase Pr6O11 cubic + Pr2O2CO3 hexagonal (small amount) amorphous phase + Pr2O2CO3 monoclinica (small amount) + Pr oxide (small amount) Pr6O11 cubic + PrO2 cubic amorphous phase Pr6O11 cubic + Pr2O2CO3 hexagonal (small amount)

average size (Pr6O11), Å 400 255 335

530 470

440 310 600 370 180 490

440

690 620 385 385

a

It is noted that for Pr2O2CO3 with monoclinic structure, no data were available in the Inorganic Crystal Structure Database (ICSD). Therefore, the positions of the reflections were calculated using the crystallographic parameters of monoclinic Pr2O2CO3 taken from another literature source (a ) b ) 4.019 Å, c ) 13.31 Å, β ) 91.17° (Sawyer, J.O.; Caro, P.; Eyring, L. Monatsh. Chem. 1971, 102, 333)). The obtained positions were very close to those of monoclinic La2O2CO3 (ICSD, 23-0322). Pr2O2CO3 with a monoclinic structure was observed at low temperatures (below 450 °C), whereas at higher temperature (above 450 °C), a Pr2O2CO3 phase with a hexagonal structure was formed. This is in analogy to La2O2CO3 (Sawyer et al. Monatsh. Chem. 1971, 102, 333), where the phase transition from monoclinic to hexagonal structure occurs at 520-525 °C.

temperature of formation of Pr6O11 and the phase composition were found to depend crucially on the praseodymium-topropylene oxide ratio and on the concentration of the Pr solution used for this synthesis method. The best results were obtained with a praseodymium-to-propylene oxide ratio of 15 and a highly concentrated Pr solution (1-2 M) (see Tables 1 and 2). In the case of the modified Pechini method, calcination up to 400 °C was insufficient to obtain crystalline praseodymium oxide as the main phase. Amorphous contributions or monoclinic Pr2O2CO3 dominated here (see Table 2). The transition to crystalline praseodymium oxide Pr6O11 with cubic structure took place at ∼500 °C (PM-1, Figure 3a). A small amount of cubic PrO2 or hexagonal Pr2O2CO3 (ICSD, 37-0805) occurred, as well. The presence of hexagonal carbonates Pr2O2CO3 depends on the ratio between the reactants: in the syntheses of the samples PM-2 and PM-3 where hexagonal carbonates were observed, more organics were used than for the other samples. It is noteworthy that in order to obtain carbonate-free Pr6O11 by the Pechini method, temperatures higher than those in the

case of the sol-gel method are necessary. Here again, the ratio between the reactants was found to play an important role. High amounts of ethylenediamine required higher temperatures for the formation of Pr6O11 (see Table 1 and 2). This is not surprising since a more complex organic matrix needs to be decomposed in these cases. The citrate method leads to crystalline samples at 400 °C, but only in the form of monoclinic Pr2O2CO3 with small fractions of praseodymium oxide (Table 2 and Figure 3b). The data agree with DTA (Figure 1) and literature results.11 The further calcination at 540 and 590 °C resulted in the formation of Pr6O11 with a small amount of cubic PrO2 or with residues of hexagonal Pr2O2CO3 (Figure 3). When compared to the other synthesis methods, the citric method required the highest temperatures to obtain cubic Pr6O11. In summary, by all synthesis methods applied in this study, praseodymium oxide Pr6O11 with cubic fluorite-like structure was obtained after calcination at sufficiently high temperature (Figure 3a). The sol-gel method already allows its formation

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Figure 2. TEM image of praseodymium oxide obtained by the solgel method and calcined at 440 °C (SG-4 (440 °C)).

Figure 3. (a) XRD pattern of the samples obtained by the different synthesis methods and calcined at the indicated temperatures. (b) The influence of the calcination temperature on the composition of the samples, shown for the example of the sample (CM-4) obtained by the citrate method and subsequently calcined at 350 and 540 °C.

at 420-450 °C, but a small monoclinic phase of Pr2O2CO3 remained as well. By the Pechini method, crystalline Pr6O11 formed at temperatures above 500 °C. For samples obtained by the citrate method, higher temperatures (550-600 °C) were necessary. The formation of praseodymium oxide probably occurred at higher temperatures in the latter two cases, because in these syntheses large organic matrices need to be decomposed. All methods allow the obtainment of nanostructured praseody-

Borchert et al.

Figure 4. IR spectrum of the sample SG-4 (440 °C) obtained by the sol-gel method and subsequently calcined at 440 °C. The spectrum was recorded at 250 °C after reduction with H2 at 400 °C.

mium oxide with crystalline domains in the size range of some 10 nm. 3.1.4. FTIRS. Figure 4 shows a representative IR spectrum (intensity converted with the Kubelka-Munk function) of a sample prepared by the sol-gel method and calcined at 440 °C. The spectrum taken at 250 °C after reduction with H2 at 400 °C clearly reveals that the surface of the sample is covered by carbonate species to a large extent (intense absorption bands in the range of 1000-1800 cm-1). CH groups (2850-2950 cm-1)31 were found as well in some cases. All three bands visible in Figure 4 at 1050, 1380, and 1460 cm-1 can be assigned to monodentate carbonates (literature values: 1040-1080, 1300-1370, and 1470-1530 cm-1 32,33 (named “unidentate carbonates” in ref 33); see Table 3 and Figure 4). Of course, the observation of carbonates is in agreement with XRD results. It has to be noted, however, that their presence on the surface can also be due to CO2 adsorption from air as a consequence of the basic properties, which are ascribed to all lanthanide oxides. As Rosynek pointed out in his review,34 this behavior results from the low electronegativities of the lanthanides. These are the basis for the strong ionic character of the lanthanide oxides, the basicity of which is thus considerably higher compared with that of other oxides (e.g., alumina). Especially when oxygen terminated, a high tendency to adsorb CO2 is consequently expected. Monodentate carbonate species were found to be present on the surface of all samples, independent of the synthesis method and the pretreatment (without pretreatment, after pretreatment with hydrogen at 400 °C, or with oxygen at 400 °C). 3.2. Surface and Catalytic Properties in the Interaction with CO and CO + O2. 3.2.1. CO Adsorption (FTIRS). The adsorption of carbon monoxide on the surface of the samples from all synthesis methods was investigated by FTIR spectroscopy. In the following, the results for the reduced sample SG-4 (440 °C) obtained by the sol-gel synthesis and calcined at 440 °C are discussed as an example. Figure 5 shows the IR spectra of adsorbed CO, which were recorded at 25, 100, 200, 300, and 400 °C while continuously exposing the sample to 4 vol % CO. The spectra show that CO does not adsorb on praseodymium cations (Pr3+ or Pr4+). In comparison, the adsorption of CO on Ce4+ cations at room temperature results in bands at 2156 and 2177 cm-1.33 Assuming that the Lewis acidity of Pr and Ce cations is similar, signals corresponding to CO adsorption on

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TABLE 3: Position (cm-1) and Assignment of IR Bands after CO Adsorption IR bands our work

literature

species

ref

1050, 1380, 1460

1040-1080, 1300-1370, 1470-1530

32, 33

1298, 1602

980-1020, 1220-1270, 1620-1670

1567

1020-1030, 1250-1270, 1530-1620

1770 2350 1582, 2836, 2945 2709 2850, 2920, 2955

1132, 1219, 1396, 1728 2285-2410 1329-1369, 1558-1587, 2852, 2945 2706, 2796 1375-1465, 2850-2960

monodentate carbonate unidentate carbonate bidentate carbonate bounded with two Pr cations (COads) (type II) bidentate carbonate bounded with one Pr cation (COads) (type I) bridged carbonate (from CO2ads) CO2, gas formate formyl intermediate CH group

Pr cations should accordingly be expected in the region of 2150-2200 cm-1, but are obviously absent (Figure 5). This indicates that the surface is oxygen terminated. At 25 °C, the spectrum shows bands at 1298, 1567, and 1602 cm-1 upon CO adsorption which can be attributed to bidentate carbonates (Figure 5), bonded to one (type I) or two (type II) praseodymium cations (see Table 3). In more detail, the band at 1567 cm-1 is assignable to type I, and the band at 1602 cm-1 is assignable to type II. The broad band at ∼1298 cm-1 contains two unresolved bands of both species. The bidentate carbonates of type I and II are schematically shown in Table 3. According to the intensity ratio, the amount of bidentate carbonates bound with 1 Pr cation is higher. A weak band at 1770 cm-1 suggests that additionally bridged carbonates were formed (Table 3 and Figure 5). The bridged carbonates may be generated via weakly adsorbed CO2 and differ from bidentate carbonates in their thermal stability.33 In the OH region, two broad and intensive bands at 3221 and 3471 cm-1 are present (Figure 5). At 100 °C, the intensity of the bands belonging to bidentate carbonates is slightly decreased. In the OH region, a band at 3388 cm-1 appears (Figure 5). At 200 °C, the OH groups disappear and CH groups are apparently generated (Figure 5).

Figure 5. IR spectra of adsorbed CO on the reduced sample SG-4 (440 °C) recorded at the indicated temperatures under a continuous CO feed (4 vol %). All spectra are referenced to background spectra recorded before exposure to CO at the corresponding temperature.

32 32 33 38 37 37 31

In the case of CeO2, the disappearance of OH groups from the surface at comparable temperatures was reported to result in the formation of H2.35 The hydrogen is incorporated into bulk ceria by formation of bronze-like species (CeO2Hx).36 A similar process may also occur in the case of praseodymium oxide. The stored hydrogen can then lead to CH species, the formation of which apparently starts at 200 °C. At 300 °C, new bands appear in the CH stretching region at 2709 and 2945 cm-1 connected with an increase of the band at 2836 cm-1, indicating further reduction of the surface carbonates or CO with the hydrogen stored in the bulk. The latter two absorption peaks are in accordance with formate species formed. This interpretation is also supported by the fact that the intensity of the band at 1582 cm-1 (CdO) is very high. On the basis of the intensity of the absorption at 1307 cm-1 (COO-), a lower intensity would be expected if only the bidentate carbonates accounted for the feature at 1582 cm-1. In principle, formates should result in an additional band at ∼1370 cm-1 (symmetric COO stretch mode). Why this vibration is not clearly observable in the spectrum is presently unclear. The third band at 2709 cm-1 is indicative of formyl species, which can be an intermediate in the formation of formates.37 Upon increasing the temperature to 400 °C (Figure 5), the intensity of the CH bands strongly decreases. Simultaneously, the bands of bidentate carbonates have the normal intensity ratio again. This indicates that formate and formyl species are not stable on praseodymium oxide at 400 °C. The gas phase was analyzed parallel with the IR experiments at 100, 200, and 300 °C with an NDIR photometer. However, CO2 was not observed as a product. Similar results were obtained for samples from all other synthesis methods. After exposures to 4 vol % CO, the two types of bidentate carbonates were observed on the surface in all cases. Indications for CO adsorption on praseodymium cations were never found. 3.2.2. Coadsorption of CO and Oxygen (FTIRS). In the next step, IR spectroscopy was used to study the interaction of CO and oxygen with the praseodymium oxide samples. Figure 6a shows IR spectra after exposure to CO and oxygen at various temperatures for the sample SG-4 (440 °C) obtained by the solgel synthesis and calcined at 440 °C. Prior to CO/O2 exposure, the samples were first reduced with H2 and then shortly pretreated with oxygen (see Section 2.3.).

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Figure 6. IR spectra (a) and gas phase analysis (b) of CO oxidation at different temperatures on the sample SG-4 (440 °C) which was first reduced with H2 and subsequently shortly pretreated with oxygen (1 min 1% O2 in argon). All IR spectra are referenced to background spectra recorded before exposure to CO/O2 at the corresponding temperature.

Spectra were recorded at 250, 300, 350, and 400 °C while exposing the sample to a continuous gas feed of 4 vol % CO and 2 vol % O2. Small amounts of gaseous carbon dioxide are observable in the IR spectrum (band at 2350 cm-1)38 and were already detected at the exit of the IR reaction cell at ∼250 °C. It has to be noted, however, that similar amounts of CO2 were also measured at 250 °C for the empty reaction cell made of steel. Besides the gas-phase signals, two groups of bands appear in the carbonate region at 1284 cm-1 (broad band consisting of two unresolved bands) and 1567/1602 cm-1. As in the case of CO adsorption (Figure 5), they correspond to bidentate carbonates bonded to one (type I) and two (type II) praseodymium cations (Table 3). Note that the amount of carbonates of type I slightly predominates. Raising the temperature stepwise to 400 °C results in an increase of intensity of all bands in the series of spectra in Figure 6a. From 300 °C on, small amounts of CO2 were photometrically detected in the gas phase (Figure 6b) at a level that clearly exceeded the activity of the empty reaction cell. This shows that the praseodymium oxide starts to be active for CO oxidation at ∼300 °C. Sazonov39 studied other rare-earth oxides and found that CO and O2 already react at room temperature on the surface, but up to 250 °C, CO2 was not observed in the gas phase. This suggests that the desorption of carbon dioxide is limiting the reaction of CO oxidation. Our investigations yield similar results for praseodymium oxide. The bidentate carbonates already form at 25 °C upon exposure to carbon monoxide and oxygen (data

Borchert et al. not shown), but CO2 is released into the gas phase only at temperatures from ∼ 300 °C on. Comparing the results for exposing the samples to CO and to CO + O2, it can be concluded that bidentate carbonates form on the surface of the praseodymium oxides under both conditions. However, the release of CO2 into the gas phase at ∼300 °C occurred only in the presence of oxygen. Under these reaction conditions (low temperatures around 300 °C), the different synthesis methods revealed essentially no differences. 3.2.3. TPR with CO (Laboratory Reactor). In order to analyze the role of lattice oxygen and its mobility, as a potentially important factor for the use of praseodymium oxide as an oxidation catalyst, selected samples were studied by temperatureprogrammed reduction (TPR). For this purpose, unpretreated samples were heated in a laboratory reactor under a continuous flow of CO (4 vol %) up to 900 °C (3 K/min). In these experiments, CO2 formation took place at 450 °C (large amount) and 820 °C (small amount). In accordance with the FTIR results, no CO2 was released into the gas phase below 450 °C in the absence of oxygen. The appearance of two peaks (450 and 820 °C) in the TPR experiment points to two types of oxygen in praseodymium oxide with different reactivities. Such a twostep process was also reported for the oxidation of methane by Pr-doped cerium oxide obtained via the Pechini method.40 In this study, TPR with methane revealed a more reactive oxygen species to be responsible for the oxidation of methane to carbon dioxide at 490-510 °C and a less reactive oxygen species accountable for the oxidation to carbon monoxide at 880 °C.40 Focusing on the more reactive species in the present case, the TPR data prove that at 450 °C the oxygen ion mobility of the praseodymium oxide reaches a sufficiently high level for the oxidation of CO by removable lattice oxygen. 3.2.4. CO Oxidation (Laboratory Reactor). Next, CO oxidation was investigated in a fixed-bed laboratory reactor at ambient pressure conditions in the temperature range of 200-600 °C. The influence of the catalyst pretreatment, synthesis methods, and synthesis parameters on the catalytic activity with respect to the oxidation of carbon monoxide is discussed in the following subsections. 3.2.4.1. Influence of the Catalyst Pretreatment. Figure 7a shows the influence of the catalyst pretreatment (without pretreatment and after pretreatment with hydrogen and oxygen at 600 °C) on the catalytic activity taking CM-1 (590 °C) as an example. The activity of the empty reactor without any sample is shown for comparison. CO oxidation starts at 300-350 °C on the praseodymium oxide catalysts, in agreement with the IR investigations. In the temperature range between 350 and 450 °C, the conversion of CO strongly increases. This correlates with the temperaturedependent development of the lattice oxygen mobility. The TPR experiments revealed that CO can be oxidized by lattice oxygen at ∼450 °C (see Section 3.2.3.). Moreover, praseodymium oxide is often used as a component in solid solutions of rare-earth oxides, because due to the variable valence state (Pr3+/Pr4+) it strongly enhances the number of oxygen vacancies and therefore the lattice oxygen mobility.24,40-42 Temperature-dependent isotopic exchange experiments revealed that the lattice oxygen mobility in solid solutions of praseodymium oxide and cerium oxide or zirconium oxide starts to increase strongly at temperatures around ∼400 °C.24,42 The correlation between the increase of the lattice oxygen mobility and the increase of activity suggests that the lattice oxygen plays a major role in the oxidation of CO. This is also in agreement with a study by Takasu et al. where CO oxidation experiments with isotopically

Nanostructured Praseodymium Oxide

Figure 7. (a) Influence of the catalyst pretreatment on the catalytic activities in the CO oxidation for CM-1 (590 °C) as an example. Results for the empty reactor without any sample are shown as well. (b) Influence of the synthesis method of the catalysts on the catalytic activities in the CO oxidation for the samples with the maximum activity. A commercial Pr6O11 sample from Aldrich is included for comparison. (c) Influence of the synthesis parameters (as listed in Table 1) on the catalytic activities in the CO oxidation for the samples obtained via the sol-gel method.

marked oxygen revealed that the product contained lattice oxygen atoms already at temperatures of 320 °C.19 The degree of the increase of activity was found to depend on the catalyst pretreatment. The catalysts pretreated with hydrogen and oxygen at 600 °C show higher conversion of CO in this intermediate temperature range (350-450 °C) than the pristine catalyst. More quantitatively, a given CO2 yield is obtained at a temperature ∼40 °C lower. This suggests the

J. Phys. Chem. C, Vol. 112, No. 8, 2008 3061 generation of more oxygen vacancies by both pretreatments, leading to a higher oxygen ion mobility and thus catalytic activity. In the case of hydrogen, this effect was temperature dependent and became more prominent when the pretreatment temperature was increased from 400 to 500 to 600 °C. For ceria, it is known that the oxide surface is stable in a hydrogen atmosphere at temperatures at and below 400 °C.36,43 Only at temperatures above 400 °C, the formation of OH groups, desorption of water by reaction of two OH groups, the reduction of Ce4+ to Ce3+, and the production of oxygen vacancies occur.44,45 In our experiments, an increase of catalytic activity was already observed at a pretreatment temperature of 400 °C, indicating that the reduction of praseodymium oxide occurs at lower temperatures than those in the case of ceria. With respect to the pretreatment with oxygen, Hyde et al. showed that heating Pr6O11 in an oxygen atmosphere led to a reduction as well, because of thermally induced phase transitions.29 So, in essence, both pretreatments result in a loss of oxygen. If the reaction is conducted at higher temperature (above 500 °C), the catalyst pretreatment no longer plays a role, probably because at these high temperatures the oxygen ion mobility is already high enough so that this factor no longer limits the reaction. The effect of the hydrogen pretreatment on the structure of the praseodymium oxides was studied in more detail by XRD. For this purpose, the sample SG-4 (440 °C) was heated in hydrogen at 600 °C for 2 h. Before reduction, the sample consisted of Pr6O11, with cubic structure and with a very small amount of monoclinic Pr2O2CO3 (Table 2 and Figure 3a). The XRD investigation showed that the structure after the pretreatment with hydrogen was unchanged and still cubic (data not shown for brevity). The reflections, however, were slightly shifted to smaller angles 2θ. In agreement with the literature, this points to a lower oxygen content.46,47 Thus, the oxide retains the structure after the reduction with hydrogen at 600 °C, but has a composition PrOx with x < 1.83 (Pr6O11). In summary, at low temperatures (∼300-350 °C) the desorption of carbon dioxide limits the reaction of CO oxidation. At intermediate temperature (between 350 and 450 °C), the degree of reduction of praseodymium oxide plays an important role, because it governs the oxygen ion mobility which probably limits the conversion in this temperature range. Above 500 °C, the oxygen transport becomes very rapid in any case. The maximum CO conversion is reached at 550 °C with 9596% (Figure 7a). Long-time stability was tested at 500 °C over 190 h, and no signs of deactivation were found. 3.2.4.3. Influence of the Synthesis Methods. Figure 7b shows the influence of the synthesis method on the catalytic activity for the samples with the maximum activity in each case. Data obtained for commercial Pr6O11 (99.9%, Aldrich, 3 m2/g) is included for comparison. All samples synthesized in the present study exhibit catalytic activity in the temperature range between 300 and 600 °C. For commercial Pr6O11, significantly higher temperatures are necessary to reach comparable conversion rates. Thus, the nanostructured catalysts investigated here are distinctly more active than the commercial material in the temperature regime up to 700 °C. Among the praseodymium oxide catalysts prepared by the different synthesis methods, only small differences with respect to CO oxidation can be discerned. In the temperature window between 350 to 500 °C, the conversion of CO was found to increase at a given temperature in the following order: Pechini

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Borchert et al. 4. Summary and Conclusion

Figure 8. Suggested mechanism of CO oxidation on the praseodymium oxide catalysts.

method, PM-2 (490 °C) < sol-gel method, SG-4 (440 °C) < calcination of praseodymium nitrate, C-1 (500 °C) < citrate method, CM-4 (540 °C). According to the XRD data (Table 2), two of these four Pr6O11 samples have Pr2O2CO3 with monoclinic (SG-4 (440 °C)) or hexagonal (PM-2(490 °C)) structure as an admixture, whereas the other two Pr6O11 samples have small amounts of PrO2 (CM-4 (540 °C)) or an amorphous phase (C-1(500 °C)) as admixtures. The presence of bulk carbonates may cause lower CO conversion, because the oxygen transport from the bulk to the surface may be inhibited. At ∼500 °C, the oxygen transport is very rapid in any case so the phase composition (synthesis method) no longer leads to differences in the activity of the samples. 3.2.4.4. Influence of Synthesis Parameters. Not only the synthesis methods but also the synthesis parameters can play a role in the catalytic activity. For catalysts, obtained via the solgel method, Figure 7c allows one to judge the influence of the praseodymium-to-propylene oxide ratio and of the concentration of the praseodymium cations in the ethanol solution on the catalytic properties in the CO oxidation. As discussed before, the synthesis parameters influence the particle size. According to the XRD data (Table 2), the sample SG-4 (440 °C) prepared with Pr/C3H6O ) 1:15 and 1 M Pr in ethanol exhibits the smallest crystallites. It is noteworthy that this sample also shows the highest activity in the temperature region from 350 to 450 °C. The CO conversion in this temperature region decreases with increasing crystallite size: XCO(SG-4(440 °C)) > XCO(SG-1(420 °C)) > XCO(SG-3(540 °C)) > XCO(SG-5(460 °C)) > XCO(SG-2(540 °C)). The correlation between these trends can be explained by more favorable oxygen ion mobility in the case of small crystallites due to faster transport mechanisms along the numerous grain boundaries. Efficient transport of oxygen along grain boundaries was already reported in studies of other nanostructured rare-earth oxides such as Gd-doped ceria.48 At high temperature (above 500 °C), the crystallite size apparently does not play an important role, because the oxygen ion mobility is now sufficiently high in all cases. For the other synthesis methods, similar trends were observed. In conclusion, the synthesis methods and parameters play no role for the maximum activity of praseodymium oxide, which is obtained at high temperature for the test reaction of carbon monoxide oxidation. However, at intermediate temperatures small but significant differences were observed. On the one hand, the presence of bulk carbonates seems to inhibit the reaction. On the other hand, the activity for the conversion of CO to CO2 is apparently better the smaller the crystallite size of the material.

Nanostructured praseodymium oxides were successfully prepared via four different synthesis methods: calcination of praseodymium nitrate, sol-gel method with propylene oxide, citrate method, and modified Pechini method. According to XRD, all synthesis methods yield Pr6O11 with a cubic fluoritelike structure at sufficiently high calcination temperatures. The size of the nanocrystalline domains was in the range of a few tens of nanometers. The surface and catalytic properties of the nanostructured oxides were studied taking CO adsorption as well as CO oxidation as a first reaction. More complex reactions such as methane oxidation and the selective reduction of NO with methane will be considered in a forthcoming publication.49 CO oxidation starts at 300 °C and is limited by the desorption of carbon dioxide. The maximum conversion of CO (∼9596%) was reached at 550 °C independent of the synthesis. However, at moderate temperatures (∼350-500 °C), the activity was found to depend slightly on the synthesis method and parameters, probably because resulting structural differences affect the oxygen ion mobility. In particular, small crystallite sizes turned out to be beneficial. The possibility to govern the oxygen ion mobility in this temperature regime, e.g., by reductive pretreatments, may also be of importance for more complex oxidation reactions. In the temperature range up to ∼700 °C, the catalysts prepared in the present work were more active than commercial Pr6O11. Comparing the calcination of the nitrate with the more sophisticated synthesis methods tried out in this work, our study reveals that the latter routes do not provide clear advantages, at least for the oxidation of CO. With properly chosen synthesis parameters and using low calcination temperatures, the sophisticated methods were certainly successful in obtaining crystalline praseodymium oxide with significantly lower domain sizes than those in the case of the simple calcination of the nitrate. But they also imply the usage of large organic matrices which decompose completely only at high calcination temperature. By consequence, a positive effect of small crystallite sizes turned out to be accompanied and compensated by a negative effect of synthesis byproducts such as bulk carbonates. As far as the reaction mechanism is concerned, different scenarios have been proposed in the literature for the oxidation of CO on rare-earth oxides. Otsuka et al.5 studied CO oxidation on praseodymium oxide and described the reaction with a simple Langmuir-Hinshelwood-like mechanism, where adsorbed CO reacts on the surface with surface lattice oxygen. In contrast, a Mars van Krevelen mechanism involving diffusion of lattice oxygen to the surface was suggested in other studies of rareearth oxides.19 On the basis of the investigations performed in the present work, we are able to contribute to a more detailed mechanistic understanding of the reaction. The IR investigations reveal that CO does not adsorb on praseodymium cations (Pr3+ or Pr4+), but leads to the formation of bidentate carbonates on the surface. This indicates that the surface of the oxide is oxygen terminated and therefore directly active for CO oxidation (see Section 3.2.1.). Li et al. showed that bidentate carbonates on ceria can be transformed to monodentate carbonates, especially at high temperature.33 In our study, the surface was initially covered only by monodentate carbonates (see Section 3.1.4.). During CO or CO/O2 exposures bidentate carbonates form, but a transformation into monodentate carbonates could not be observed spectroscopically under reaction conditions (see Sections 3.2.1. and 3.2.2.). However, after switching off the reaction gases, the system came always back to a state with only

Nanostructured Praseodymium Oxide monodentate carbonates. This provides an indication that the transformation of bidentate to monodentate species also occurred in our case. The transformation of a bidentate carbonate (which can be considered as CO bound to two surface oxygen ions) into a monodentate carbonate (which can be considered as CO2 bound to one surface oxygen ion) creates an oxygen Vacancy in the vicinity of the monodentate carbonate, which, in case of a sufficiently high oxygen mobility, can be immediately refilled by lattice oxygen. If the subsequent decomposition of the adjacent monodentate carbonate into CO2 is rapid, this would explain why the monodentate carbonates are not observable by IR spectroscopy as intermediate species on the surface under reaction conditions. Dissociative adsorption of oxygen at other surface sites finally leads to refilling of the lattice with oxygen. The whole mechanism as proposed here is summed up in Figure 8, where the various steps of the catalytic cycle are schematically depicted. Acknowledgment. We are very grateful to Dr. J. Birkenstock and Prof. Dr. R. Fischer (University of Bremen) for the access to the XRD facilities and the extensive support during the measurements. Furthermore, we thank O. Oppermann and Prof. Dr. A. Rosenauer (University of Bremen) for the TEM investigations. Yulia Borchert is grateful for support by a fellowship within the postdoc program of the Alexander von Humboldt Foundation. Financial help of the Fonds der Chemischen Industrie is gratefully acknowledged. We thank Dr. Th. Schro¨der (IHP, Frankfurt/Oder) for valuable discussions. References and Notes (1) Fathi, M.; Bjorgum, E.; Viig, T.; Rokstad, O. A. Catal. Today 2000, 63, 489. (2) Sadykov, V. A.; Kuznetsova, T. G.; Alikina, G. M.; Frolova, Y. V.; Lukashevich, A. I.; Potapova, Y. V.; Muzykantov, V. S.; Rogov, V. A.; Kriventsov, V. V.; Kochubei, D. I.; Moroz, E. M.; Zyuzin, D. I.; Zaikovskii, V. I.; Kolomiichuk, V. N.; Paukshtis, E. A.; Burgina, E. B.; Zyryanov, V. V.; Uvarov, N. F.; Neophytides, S.; Kemnitz, E. Catal. Today 2004, 93-95, 45. (3) Lacombe, S.; Geantet, C.; Mirodatos, C. J. Catal. 1994, 151, 439. (4) Choudhary, V. R.; Mulla, S. A. R.; Uphade, B. S. Ind. Eng. Chem. Res. 1997, 36, 2096. (5) Otsuka, K.; Kunitomi, M. J. Catal. 1987, 105, 525. (6) Zhang, X.; Walters, A. B.; Vannice, M. A. Catal. Today 1996, 27, 41. (7) Chi, Y.; Chuang, S. S. C. J. Phys. Chem. B 2000, 104, 4673. (8) Wang, W.; Lin, P.; Fu, Y.; Cao, G. Catal. Lett. 2002, 82, 19. (9) Rajendran, M.; Mallick, K. K.; Bhattacharya, A. K. Mater. Lett. 1998, 37, 10. (10) Thangadurai, V.; Huggins, R. A.; Weppnar, W. J. Solid State Electrochem. 2001, 5, 531. (11) Popa, M.; Kakihana, M. Solid State Ionics 2001, 141-142, 265. (12) Hussein, G. A. M. J. Anal. Appl. Pyrol. 1996, 37, 111. (13) Antoshin, G.; Minachev, K.; Dmitriev, R. Russ. Chem. Bull. 1967, 16, 1793. (14) Hussein, G. A. M.; Balboul, B. A. A.; A-Warith, M. A.; Othman, A. G. M. Thermochim. Acta 2001, 369, 59. (15) Hussein, G. A. M. J. Anal. Appl. Pyrolysis 1994, 29, 89. (16) Moosath, S. S.; Abraham, J.; Swaminathan, T. V. Z. Anorg. Allg. Chem. 1963, 324, 90.

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