PrO2–Al2O3 catalysts for dry methane reforming

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Surface and structural features of Pt/PrO2 –Al2 O3 catalysts for dry methane reforming I. Tankov a , K. Arishtirova a , J.M.C. Bueno b , S. Damyanova a,∗ a b

Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Departamento de Engenharia Química, Universidade Federal de São Carlos, C.P. 676, São Carlos, SP 13565-905, Brazil

a r t i c l e

i n f o

Article history: Received 25 January 2013 Received in revised form 20 June 2013 Accepted 16 August 2013 Available online xxx Keywords: Dry methane reforming Pt catalysts PrO2 –Al2 O3 oxides

a b s t r a c t Pt catalysts supported on mixed PrO2 –Al2 O3 oxides with different PrO2 loading (1–20 wt.%) were prepared by wetness impregnation method. The samples were characterized by XRD, XPS, UV–vis DRS, DRIFTS of CO adsorption, TPR and TEM. XRD analysis of the samples at high calcination temperature of 1023 K showed that the size of Pt particles decreases with increasing the PrO2 content. TPR results exhibited a promoted synergetic effect between Pt and Pr oxide expressed by decreasing the reduction temperatures of both species. XPS and DRIFTS data demonstrated a charge transfer from Pt to Pr oxide species, creating Pt sites with different electron density. It was shown that the addition of praseodymium oxide to aluminasupported Pt catalyst leads to improvement of the catalytic activity and stability. It was found that 6 wt.% is the optimal content of PrO2 to obtain high active Pt catalysts. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The reaction of reforming of CH4 with CO2 (CH4 + CO2 ↔ 2CO + 2H2 ) or dry methane reforming (DMR) has been extensively investigated in the last years because of the following interesting features: (i) it becomes industrially advantageous compared to steam reforming or partial oxidation in synthesis gas production since the H2 /CO product ratio is close to unit, suitable for further use in the production of oxygenated compounds, as well as in the Fischer–Tropsch synthesis [1–3]; (ii) the reaction allows the exploitation of natural gas resources with high CO2 content, avoiding the expensive and intricate gas separation process; (iii) it is an interesting way for the use of biogas, a renewable resource containing methane (60–70%) and carbon dioxide (30–40%) produced by anaerobic assimilation of biomass [4] and (iv) the reaction contributes to the depletion of two dangerous greenhouse gases (CO2 and CH4 ). The commonly used catalysts for DMR reaction are Ni-based catalysts since they are relatively cheap and available [5]. Unfortunately, the Ni catalysts deactivate very fast under high reforming temperatures due to the sintering of active phase and carbon deposition [6–8]. In recent years, the use of noble metals catalysts for

∗ Corresponding author at: Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. Block 10, 1113 Sofia, Bulgaria. Tel.: +359 2 9792588; fax: +359 2 9712967. E-mail addresses: [email protected], [email protected] (S. Damyanova).

DMR has acquired a great attention because they possess high activity and stability due to their lower sensitivity to the carbon formation in comparison with that of the non-noble metals. Catalytic performance of supported metallic catalysts for DMR is affected by several factors such as the nature of support, the introduction of modifiers as well as the conditions of catalysts preparation and pretreatment [9]. The most widely used carrier for DMR catalysts is ␥-Al2 O3 due to its high thermal stability (1023 K) and specific surface area (220–280 m2 /g). Lanthanide and lanthanide-based oxides are effective carriers or promoters for catalysts in methane reforming processes due to the multiple positive effects on the enhancement of catalytic performance. The use of lanthanide-promoted Ni/Al2 O3 catalysts in dry reforming of methane has been widely reported [10–12]. The effect of the addition of lanthanide oxides (La2 O3 and CeO2 ) and preparation procedure were investigated in the reforming of methane with carbon dioxide over Ni/Al2 O3 catalysts [11]. It was shown, that the addition of lanthanide prior to nickel leads to high nickel reducibility and to a decrease of the nickel particle size. However, the activity reforming was not affected to a large extent by the impregnation order of nickel and lanthanides [11]. The CeO2 presence (1–5 wt.%) in impregnated Ni/Al2 O3 catalysts was related to reducing the chemical interaction between the nickel and alumina support, which results in the increase of nickel reducibility and dispersion [12]. The catalytic activity and thermal stability were remarkably improved after adding La, Sm, and Yb into Ni–B/Al2 O3 catalysts. The influence of lanthanides on the noble metal catalysts behavior in dry reforming of methane has been also extensively

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investigated. Among them, ceria has been selected as a structural promoter due to its oxygen storage action that improves: (i) the reversibility of oxidation/reduction reactions of noble metals with changes in redox potential and (ii) the oxidation of coke deposits over active phases [13]. Creation of oxygen vacancies by ceria favors the adsorption of CO2 on the catalyst surface [14]. The knowledge on the interaction between rare earth oxides and precious metals will be useful in the design of efficient catalysts for reforming processes of hydrocarbons [15]. Trovarelli has summarized a considerable amount of data that demonstrate the strong interaction between precious metals with lanthanides at atomic level [16]. Other authors proposed that ceria may change the properties of the Group VIII metals through electronic interactions and prevents the sintering of the metals, stabilizing their dispersed state [17,18]. It was pointed [19] that CeO2 transfers oxygen to noble metal that is followed by oxygen is transfer to the reactants. Thus, the action of CeO2 strongly affects the state of precious metal and may act as oxygen carrier [19]. The improvement of the adsorption and dissociation of CO2 on the catalyst surface doped with ceria will lead to increase of the coke gasification. Praseodymium oxide, a material similar to ceria, also crystallizes in a cubic fluorite structure. However, it possesses lower stability, allowing oxygen exchange to be effected at lower temperatures than that of ceria. High-surface area forms of praseodymium-based mixed oxides, analogous to ceria–alumina, could provide new oxygen storage materials with easily accessible oxygen [20–24]. The aim of the work was to study the effect of PrO2 content on the structure, surface and catalytic properties of Pt catalysts supported on mixed PrO2 –Al2 O3 oxides in the reaction of CH4 reforming with CO2 . To our knowledge this is the first attempt to use praseodymium modified alumina as support for catalysts for DMR. A combination of different techniques such as N2 adsorption-desorption measurement, X-ray diffraction (XRD), temperature-programmed reduction with H2 (H2 -TPR), Xray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption and transmission electron microscopy (TEM) were used for physicochemical characterization of the samples. Reforming of CH4 with CO2 was used as a test reaction. 2. Experimental 2.1. Sample preparation The preparation of catalysts was consisted of: (i) modification of ␥-alumina with different PrO2 content (1–20 wt.%) described in Ref. [25] and (ii) deposition of Pt on praseodymium modified supports. The catalysts were obtained by impregnation of the mixed PrO2 –Al2 O3 oxides with a solution of H2 PtCl6 .6H2 O (40% Merck) in ethanol. The mixture was stirred at room temperature for 4 h. The ethanol was removed in a rotary evaporator at 298 K. The solids were dried at 383 K overnight and calcined at 823 and 1023 K for 4 h. The nominal content of Pt was 1 wt.%. The supports and catalysts were denoted as xPr–Al and Pt/xPr–Al, respectively, where x is the nominal weight of PrO2 . 2.2. Methods Specific surface area (SBET ), pore volume (Vp ) and average pore diameter (Dp ) of xPr–Al and Pt/xPr–Al samples were evaluated from the N2 adsorption-desorption isotherms, determined at 77 K with a Micromeritics TriStar 3000 apparatus in the relative equilibrium pressure interval of 0.03 < P/P0 < 0.3. The samples were previously degassed at 423 K for 24 h under vacuum (10−4 mbar).

XRD analysis was performed by a computerized Seifert 3000 XRD diffractometer using Cu K␣ (l = 0.15406 nm) radiation and a PW 2200 Bragg–Brentano /2 goniometer equipped with a bent graphite monochromator and an automatic slit. Diffraction peaks were recorded in a 2 range of 10–90◦ with step size of 0.033 and step scan of 10.0 s. Phase identification was carried out by comparison with JCPDF database cards. The average Pt particles size (DXRD ) of the samples was calculated by Scherrer’s equation. UV–vis DRS spectra of the samples were recorded in the range of 200–800 nm at room temperature using a Varian Cary 5000 UV–vis spectrometer equipped with an integration sphere. XPS spectra of the calcined, reduced and spent catalysts were recorded on Escalab-MkII (VG Scientific) electron spectrometer with an unmonochromatized Al K␣ radiation, operating at 12 kV and 20 mA. During data acquisition, the residual pressure inside the analysis chamber was kept below 5 × 10−9 Torr. Binding energies (BE) were determined through curve fitting of the spectra. BE of C 1s core level at 284.9 eV was taken as an internal standard. The accuracy of the BE values was ±0.2 eV. The intensities of the peaks were estimated by integrating each peak after subtraction of Shirley type background and fitting the experimental peak to a mixture of Lorentzian/Gaussian lines. The ratio of the corresponding peak intensities was calculated and corrected with theoretical sensitivity factors based on Scofield’s photoionization cross-sections [26]. TPR profiles were recorded on a Micromeritics Pulse Chemisorb 2705 fitted with a thermo conductivity detector (TCD). In order to remove surface contaminants, the sample (0.100 g) loaded in a quartz reactor was pretreated at 423 K in a He stream for 1 h. After cooling to r.t. a flow of 5% H2 /N2 (30 ml/min) was passed through the sample and the temperature was raised at a rate of 10 K/min up to 1273 K while the TCD signal was recorded. A cooling trap was placed between the sample and the detector retained the water formed during the reduction process. The samples were subjected to redox treatment at 773 K (R-O773 ) and at 1073 K (R-O1073 ), which consists of H2 -TPR to 1273 K followed by cooling to 773 K/1073 K under N2 and in situ oxidation with an oxygen flow (40 ml/min) at 773 K/1073 K for 2 h. After the oxidation the samples were cooled to r.t. and H2 -TPR to 1273 K was carried out. DRIFT spectra of adsorbed CO were recorded using a Thermo Nicolet 4700 Nexus FT-IR spectrophotometer with MCT detector and HTHV (high temperature and high vacuum) diffuse reflectance infrared Fourier transform Spectroscopy-reactor cell (DRIFTS HTHV cell-Spectra Tech). The ␣-Al2 O3 micro-reactor was coupled to an array of stainless steel with CaF2 window and connectors for inlet and outlet gas. The sample (m = 0.05 g) was reduced in a 10% H2 /N2 gas mixture flow from r.t. to 303 K at heating rate of 5 K/min and from 303 K to 773 K at heating rate of 10 K/min. The catalysts remained at 773 K for 1 h. After reduction, the sample was cooled under H2 /N2 gas mixture flow up to 673 K, followed by cooling to 298 K in N2 flow. After stabilizing the cell temperature, CO adsorption was carried out with CO pulses at partial pressure (PCO ) of 10 Torr under N2 flow. The spectrum resolution was 4 cm−1 and 64 scans were collected. TEM images of spent catalysts were recorded on a JEOL JEM3010 microscope (300 kV, 1.7 A˚ point resolution). The samples were prepared by dropping a suspension containing the catalyst previously reduced at 773 K in isopropanol onto amorphous carbon films supported on copper grids. At least ten representative images were taken for each sample. In order to obtain statistically reliable information, the length of ca. 100 particles was measured. The reaction of reforming of CH4 with CO2 was carried out in a vertical fixed-bed reactor made of quartz tube (6 mm i.d.) under atmospheric pressure and at temperatures of 723, 823 and 923 K. The catalyst sample (m = 0.05 g) with particle diameter of 0.2–0.25 mm was diluted with quartz (0.2–0.3 mm). Prior to each

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I. Tankov et al. / Applied Catalysis A: General xxx (2013) xxx–xxx Table 1 Textural properties of supports and Pt/xPrO2 –Al2 O3 samples calcined at 823 K. Sample a

Al2 O3 Pt/Al 1Pr–Al Pt/1Pr–Al 6Pr–Al Pt/6Pr–Al 12Pr–Al Pt/12Pr–Al 20Pr–Al Pt/20Pr–Al a b

SBET (m2 /g)

Vp (m3 /g)

Dp (nm)

DXRD (nm)b

248 230 245 226 223 209 199 186 157 139

0.890 0.830 0.890 0.820 0.820 0.770 0.750 0.720 0.500 0.620

10.0 14.5 14.5 14.5 14.6 14.8 14.9 15.6 14.4 15.4

– 24 – 12 – 9 – 8 – 5

Data for supports are taken from Ref. [25]. After calcination at 1023 K.

reaction test the catalyst was heated from r.t. to 573 K in a N2 flow (45 ml/min). The sample was reduced in situ by heating in a 10% H2 /N2 flow (50 ml/min) from 573 to 773 K at a rate of 10 K/min and maintaining the last temperature for 1 h. The catalyst was purged with flowing nitrogen (60 ml/min) for 40 min to remove the physically adsorbed hydrogen from the surface. The reactant gas mixture of CH4 , CO2 and N2 with ratio of 1:1:3 was fed into the reactor at the flow rate of 100 ml/min. GHSV = 48.103 cm3 /gh. The reaction products were analyzed in a gas chromatograph equipped with a 3 m HayeSep D 100/120 (SUPELCO) column and a TCD. 3. Results 3.1. Textural properties It is detected that the N2 adsorption-desorption isotherms of the mixed xPrO2 –Al2 O3 oxides with different PrO2 content [25] and those of supported Pt catalysts are similar. The isotherms of all samples have hysteresis loops belonging to H1 type according to IUPAC classification, characteristic of mesoporous materials [27]. The values of SBET , Vp and Dp of the samples are listed in Table 1. The SBET of Pt/1Pr–Al sample is slightly lower than that of non-promoted sample with Pr. The increase of PrO2 content causes a strong decrease of the SBET , probably, due to a plugging of the pores of support with praseodymium oxide species. Pt/Pr6 O11 sample has the lowest textural characteristics. Comparing the values of SBET and Vp of the Pt/xPr–Al samples with those of the corresponding supports, a decrease of the SBET of about 20 m2 /g and a small change of Vp are observed. This suggests that an additional blockage of the pores of the carrier during the impregnation with H2 PtCl6 can occur. 3.2. XRD XRD patterns of Pt/Pr6 O11 and Pt/хPr–Al samples with different PrO2 content, previously calcined at 823 and 1023 K, are shown in Fig. 1A and B, respectively. For comparison the XRD of Pr6 O11 and ␥-Al2 O3 are also included. It is well known that Pr6 O11 is a mixed-valence oxide composed of PrO2 and Pr2 O3 in a ratio of 4:1 [23,28]. XRD of Pr6 O11 at 2 = 28.1◦ , 46.7◦ , 55.6◦ , 75.7◦ and 78.2◦ can be related to PrO2 (JCPDS 24-1006), whereas the diffraction peaks at 2 = 32.7◦ and 68.5◦ are assigned to Pr2 O3 (JCPDS 22-880). XRD of Pt/xPr–Al samples exhibit peaks at 2 = 33.4◦ , 37.6◦ , 39.2◦ , 45.8◦ and 66.8◦ due to ␥-Al2 O3 (JCPDS 10-425). The absence of diffraction peaks corresponding to either PrO2 or Pr2 O3 in the XRD of Pt/xPr–Al after calcination at both temperatures suggests that the praseodymium oxide particles are quite small to be identified by XRD. It is seen that the increase of praseodymium oxide content (≥6 wt.%) causes a broadening and decrease of the intensity of the characteristic diffraction patterns of alumina support. According to Ref. [29], the impregnation of Al2 O3 with an acidic nitrate solution of ZrO(NO3 )2 (pH ≤3 at 10–20%) could erode the surface and break

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the Al O Al bond among the particles of Al2 O3 , leading to decrease of the crystallization degree of alumina. In the case of Pt/Al and Pt/xPr–Al samples calcined at 823 K metallic platinum (JCPDS 4-0802) is identified by the presence of very small peaks at 2 = 39.7◦ and 81.2◦ related to the reflection plans (1 1 1) and (3 1 1), respectively (Fig. 1A). It is well known [30] that platinum oxides (PtO and PtO2 ) are unstable and can be easily decomposed at temperatures above 773 and 823 K, respectively, and metallic Pt (Pt0 ) to be formed. Fig. 2B shows Pt agglomeration after calcination of Pt/Al and Pt/xPr–Al samples at 1023 K. There is a strong agglomeration of metallic Pt particles over pure Al2 O3 support. The average particle size, DXRD , of Pt crystallites decreases strongly with increasing the PrO2 content (from 24 nm to 5 nm for Pt/Al and Pt/20Pr–Al, respectively, Table 1). Similar observations have been observed in previously our works [21,22]. It could be concluded that the addition of praseodymium oxide to alumina leads to a decrease of the size of supported noble metal particles. It should be noted that here is no evidence for the presence of agglomerated Pt particles on the surface of Pr6 O11 at both calcination temperatures (Fig. 1A and B). The diffractions peaks of Pt/Pr6 O11 sample are identical to that of pure Pr6 O11 with fluorite structure. 3.3. UV–vis DRS UV–vis DRS of xPr–Al and Pt/xPr–Al samples calcined at 823 K are shown in Fig. 2A and B, respectively. The UV–vis spectra of xPr–Al carriers (Fig. 2A), previously cited in Ref. [25], present absorption band in the range 270–310 nm related to O2− → Pr4+ charge transfer [31]. The band is shifted to higher wavelengths with increasing the praseodymium oxide content as a result of some interaction between Al and Pr. All xPr–Al samples possess welldefined absorption bands at 445, 469, 482 and 593 nm, attributed to the 4f ↔ 4f transitions 3 H4 → 3 P2 , 3 H4 → 3 P1 , 3 H4 → 3 P0 and 3 H → 1 D (3 H – ground state; 3 P , 3 P , 3 P and 1 D – excited 4 2 4 2 1 0 2 states) of Pr3+ ions, respectively [32]. Since the 4f shell of trivalent lanthanide ions is well shielded by the filled 5s and 5p orbits, the energy levels of the 4f electrons can be slightly influenced by the environment of lanthanide ion. The deposition of platinum on the surface of xPr–Al supports causes: (i) a decrease of the intensity of adsorption bands, corresponding to O2− → Pr4+ charge transfer and (ii) an increase of the intensity of bands attributed to 4f ↔ 4f transitions of Pr3+ ions (Fig. 2B). The first phenomenon suggests that Pt interacts preferentially with the rare earth oxide component in the xPr–Al system through the formation of Pt–PrOx surface complex, hindering the O2− → Pr4+ charge transfer. The increased intensity of the bands, connected with the 4f ↔ 4f transitions, are due to the overlapping of additional reflections from the existence of Pt oxide species formed on the support surface [33]. 3.4. XPS 3.4.1. Pt 4d5/2 core electron level Since the Al 2p line overlaps with the Pt 4f one, the Pt 4d line was used in the study. The data in the literature [34,35] concerning the oxidation state of Pt are contradictory that was related to the different methods of preparation of supported Pt catalysts and to the type of XPS equipment. XPS of Pt 4d5/2 core electron level of calcined, reduced and spent supported Pt catalysts are shown in Fig. 3A, B and C, respectively. The BEs values of Pt 4d5/2 are summarized in Table 2. A broad and asymmetric Pt 4d5/2 peak accompanied by an intensive one at 306.9 eV is revealed in the XPS of all samples. The latter peak is a combination of carbon energy loss and photoemission from the Pr 4s core electron level [36]. The platinum oxidation state of each sample was obtained by deconvolution of the XPS Pt 4d5/2 line into three components with BEs

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Fig. 1. XRD spectra of Al2 O3 , Pr6 O11 and Pt/xPr–Al samples calcined at 823 K (A) and 1023 K (B).

at 313.2–314.1, 316.5–317.2 and 319.9–320.1 eV (Table 2). The first component can be assigned to platinum in its fully reduced state, Pt0 [37]. The BE at 316.5–317.2 eV is typical for PtO species [38]. The BE values at 319.9–320.1 eV are higher compared to that of pure PtO2 [39], which would be related to some incompletely decomposed platinum–chlorine-containing species ([PtIV Ox Cly ]s ) formed by the interaction between platinum and chloride precursor salt of platinum (H2 PtCl6 ). The carefully examination of Cl 2p core electron level showed traces of chloride ions, not completely removed during the calcination procedure at 823 K. Platinum in calcined Pt/Al exists mainly as Pt0 (Table 2). The oxidation state of platinum undergoes change after deposition on

Pr6 O11 (Fig. 4A). A noticeable decline of the BE value from 314.1 to 313.5 eV and of the amount of Pt0 component from 52.9 to 21.4% are revealed for calcined Pt/Al and Pt/Pr6 O11 samples, respectively. At the same time, an increase of the amount of platinum oxide species is detected (Table 2). These effects could be associated with a charge transfer from platinum to praseodymium, maintaining Pt in an electron deficient state. Similar changes in the oxidation state have been observed for Pt deposited on Al2 O3 and CeO2 [21]. It can be concluded that Pt on xPr–Al carriers maintains some positive character compared to that on pure alumina (Table 2). The BE values of platinum components for all samples remain unchanged after reduction but an alteration of the Pt proportion as

Fig. 2. UV–vis diffuse reflectance spectra of calcined xPr–Al (A) and Pt/xPr–Al (B) samples.

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Fig. 3. XPS of Pt 4d5/2 core levels of calcined (A), reduced (B) and spent (C) Pt/xPr–Al catalysts.

well as a decrease of Pt 4d5/2 peak intensities is observed (Fig. 3B and Table 2). The reduction treatment causes a noticeable increase of the amount of Pt0 component. However, Pt particles in an electron deficient state still exist after reduction at 773 K, being revealed in a greater extent for PrO2 -containing samples. XPS of spent catalysts (Fig. 3C and Table 2) show that both, the peak intensities and the BE values of Pt 4d5/2 of reduced Pt/xPr–Al samples have not changed after DMR reaction. In contrary to that, the intensity of the peaks corresponding to Pt of Pt/Al sample is

noticeable decreased. These observations propose that the presence of praseodymium oxide prevents the platinum agglomeration for Pt/xPr–Al, whereas a platinum sintering over pure alumina is occurred (Fig. 3C). 3.4.2. Pr 3d5/2 core electron level XPS of Pr 3d5/2 core electron level for calcined, reduced and spent Pt/Pr6 O11 and Pt/xPr–Al catalysts are shown in Fig. 4A, B and C, respectively. XPS of bulk Pr6 O11 is added as a reference. The Pr 3d5/2

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Table 2 XPS characteristics of calcined, reduced and spent Pt/xPrO2 –Al2 O3 catalysts. Sample Pt/Al Calcined Reduced Spent Pt/1Pr–Al Calcined Reduced Spent Pt/6Pr–Al Calcined Reduced Spent Pt/12Pr–Al Calcined Reduced Spent Pt/20Pr–Al Calcined Reduced Spent Pt/Pr6 O11 Calcined Reduced Spent Pr6 O11 Calcined Reduced a

Pt 4d5/2 (eV)

Pr 3d5/2 (eV)

O 1s (eV)

Pt/Al

Pt/Pr + Al

531.3 531.3 531.4

0.0017 0.0013 0.0004

– – –

314.1 (52.9)a 314.1 (54.8) 314.1 (36.1)

317.1 (28.1) 317.1 17.0) –

320.1 (19.0) 320.1 (28.2) 320.1 (63.9)

313.6 (30.1) 313.6 (45.5) 314.1 (59.7)

316.5 (41.1) 316.5 (32.7) –

320.1 (28.8) 320.1 (21.8) 320.1 (40.3)

933.9 (79.4) 933.5 (77.1) 933.4 (75.6)

531.6 531.0 531.4

0.0020 0.0009 0.0010

0.0019 0.0009 0.0010

313.2 (33.3) 313.2 (50.2) 313.1 (44.2)

316.5 (40.6) 316.5 (35.8) 316.6 (35.1)

319.9 (26.1) 319.9 (14.0) 320.1 (20.7)

933.9 (78.6) 934.1 (69.2) 933.9 (79.3)

531.2 531.5 531.5

0.0026 0.0011 0.0012

0.0026 0.0011 0.0012

313.6 (38.3) 313.6 (46.3) 313.2 (48.6)

316.6 (34.7) 316.6 (26.5) 316.5 (24.9)

319.9 (27.0) 319.9 (27.2) 319.9 (26.5)

933.9 (75.4) 933.8 (73.8) 933.9 (77.5)

531.4 531.1 531.3

0.0038 0.0012 0.0011

0.0036 0.0012 0.0011

313.7 (37.9) 313.6 (47.3) 313.6 (49.4)

316.6 (35.2) 316.6 (35.6) 316.6 (18.9)

319.9 (26.8) 319.9 (17.1) 319.9 (31.7)

933.8 (77.8) 934.1 (74.7) 933.7 (77.2)

531.2 531.4 531.3

0.0127 0.0012 0.0012

0.0039 0.0012 0.0011

313.5 (21.4) 313.9 (42.7) 313.7 (48.6)

317.2 (54.2) 316.9 (36.9) 316.6 (29.9)

320.1 (24.4) 320.1 (20.4) 319.9 (21.5)

932.1 (74.1) 932.9 (77.0) 932.6 (72.2)

528.0 529.2 529.1

– – –

– – –

933.1 (64.0) 933.1 (67.0)

528.4 529.2

– –

– –

– –

– –

– –

– – –

The percentage of component.

Fig. 4. XPS of Pr 3d5/2 core levels of calcined (A), reduced (B) and spent (C) Pt/xPr–Al catalysts.

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Fig. 5. XPS of O 1s core levels of calcined (A), reduced (B) and spent (C) Pt/xPr–Al catalysts.

line of Pr6 O11 contains two features centered at 928.4 eV (vI ) and 933.1 eV (vII ) corresponding to Pr2 O3 and PrO2 , respectively [40]. The complex shape of Pr 3d5/2 line originates from the simultaneous effect of covalency hybridization and interatomic electrostatic coupling between the 3d hole and outer unpaired 4f electrons [41]. Praseodymium ion in Pr2 O3 (Pr3+ ) is in 4f2 electron configuration in the ground state. However, in the Pr 3d XPS spectra the O 2p–Pr 4f covalence mixing in the initial state results in a complex final state interaction after emission of Pr 3d photoelectron. This final state interaction is characterized by the occurrence of a charge transfer from the O 2p valence band to empty Pr 4f levels. A bindingantibinding interaction between the two main final states 4f2 c (c: core hole) and 4f3 vc (v: valence band hole) takes place. In analogy to Pr2 O3 , the Pr 3d XPS spectrum of PrO2 is characterized by the final state interaction of 4f1 c and 4f2 vc state with O 2p valence band states, so that further final state charge transfer processes result in 4f2 vc and 4f3 v2c electron configuration, respectively [42]. The deposition of platinum on pure Pr6 O11 leads to a decrease of the BE value of Pr 3d5/2 (from 933.1 to 932.1 eV for calcined Pr6 O11 and Pt/Pr6 O11 , respectively), which indicates that praseodymium is reduced in some extent (Fig. 4A and Table 2). In the case of Pt/xPr–Al samples, the BE values of Pr 3d5/2 are about of 2 eV higher than that of Pt/Pr6 O11 . The latter suggests that the praseodymium ion of Pt/xPr–Al samples is in a more saturated coordination state (Table 2). The BE value of Pr 3d5/2 for calcined Pt/Pr6 O11 sample increase by 0.8 eV (from 932.1 to 932.9 eV) after reduction at 773 K, but it remains practically unchanged for Pr6 O11 and Pt/xPr–Al (Table 2). The shift to higher BE value can be connected with some redispersion of praseodymium oxide species after deposition of platinum on Pr6 O11 due to a Pt–O–Pr interaction. It should be noted that the Pr 3d5/2 peaks for all reduced samples are characterized with decreased intensity (Fig. 4B). This means that the agglomeration of praseodymium species cannot be excluded. The intensities and BE values of Pr 3d5/2 lines of reduced samples do not undergo alterations after DMR reaction (Fig. 4C and Table 2). 3.4.3. O 1s core electron level The XPS spectra of O 1s core electron levels for calcined, reduced and spent Pt/xPr–Al catalysts are shown in Fig. 5A, B and C, respectively. For comparison, the XPS of O 1s of Al2 O3 and Pr6 O11 are

included. The BEs of O 1s core levels are summarized in Table 2. The O 1s spectrum of alumina shows a single peak at 531.2 eV, characteristic value of the O Al bond in alumina [43]. Two different oxygen components can be found after examination of the O 1s photoemission line of Pr6 O11 sample, at 528.4 and 531.4 eV (Table 2). The first one can be assigned to the surface oxygen ions, surrounded by Pr ions in Pr6 O11 lattice that indicates a Pr O bond [41]. The second one may be related to adsorbed oxygen species − such as O− , O2− 2 or O2 . A formation of surface hydroxyl and/or carbonate species cannot be excluded [44]. Adsorption of H2 O or CO2 from the atmosphere can occur since the cation of PrO2 is strongly basic. The deposition of platinum on Pr6 O11 leads to a shifting of the peaks position of O 1s at 528.4 and 531.4 eV toward lower values of 528.0 and 530.9 eV, respectively (Table 2). In addition, an increase of the ratio between the intensities of the O 1s peaks at 528.0 and 530.9 eV is revealed. As was mentioned above, the charge transfer from platinum to praseodymium causes the platinum to be more oxidized. Therefore, the observed shift should be assigned to a weakening the Pr O bond strength in the presence of Pt, which leads to a high lattice oxygen mobility. Fig. 5A shows that both, the shape and the position of the XPS O 1s peaks for Pt/xPr–Al samples can be indexed for bare alumina. This means that the effect of platinum on the oxidation state of praseodymium and aluminum components in the mixed xPr–Al oxides is diminished. A decrease of the intensity of O 1s peaks is observed for reduced Pr6 O11 , Pt/Pr6 O11 and Pt/xPr–Al samples (Fig. 5B). A change of the BE values of the O 1s core levels for Pt/xPr–Al samples after reduction is not detected (Table 2). Contrary to that, the reductive treatment of Pr6 O11 and Pt/Pr6 O11 causes a shift of the position of O 1s peaks at 528.4 and 528.0 eV, respectively, assigned to lattice oxygen, toward a higher BE value of 529.2 eV. The latter would be connected with the generation of defective structures (oxygen vacancies) in Pr6 O11 under hydrogen atmosphere. In addition, the relative intensity of the O 1s peak, corresponding to adsorbed OH− (at 531.1 eV) is higher than that at 529.2 eV. It suggests a formation of praseodymium hydroxide in the presence of water formed under reduction process. The XPS of spent catalysts indicate no changes in the O 1s spectra of reduced samples after reaction (Fig. 5C). XPS Pt/Al and Pt/Pr + Al atomic ratios of calcined, reduced and spent Pt catalysts are listed in Table 2. The values of the ratios of

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Fig. 6. DRIFT spectra of CO adsorption on reduced Pt/xPr–Al samples.

calcined samples increase with increasing the PrO2 content, which indicates an enhancing of the Pt dispersion degree [28]. The values of Pt/Pr + Al ratios decrease under reductive atmosphere, but they, practically, remain unchanged after reaction (Table 2). However, there is a significant decrease of the Pt/Al ratio for spent Pt/Al catalyst that suggests a strong agglomeration of Pt species. 3.5. DRIFTS of CO adsorption The DRIFT spectra of CO adsorption on reduced Pt/xPr–Al catalysts in the higher frequency region of 2150–1900 cm−1 (HF) and in lower frequency region of 1900–1760 cm−1 (LF) are shown in Fig. 6. According to the literature data [35,45], the bands in HF and LF region can be related to CO linearly bonded to one Pt and CO bonded on two surface Pt atoms (bridged bonded species), respectively. Adsorption of CO on Pt/Al produces a band at 2060 cm−1 with a significant broadening on the low-frequency side, characteristic of CO linearly bonded to surface-exposed metal Pt atoms, Pt0 [35] (Fig. 6). It means that Pt on alumina dominates in its fully reduced state. DRIFT spectrum of Pt/Pr6 O11 shows a main band at 2070 cm−1 , accompanied by three shoulders at 2045, 2027 and 2008 cm−1 assigned to on-top carbonyls on Pt0 species. Addition of a small amount of PrO2 (1 wt.%) to Pt/Al leads to a shift of the main 2060 cm−1 band, assigned to Pt0 , to 2066 cm−1 and to appearance of a small shoulder at about 2081 cm−1 . The position of the last two bands are shifted to lower wavenumbers (to 2060 and 2076 cm−1 , respectively) with the increase of PrO2 content up to 20 wt.%. It can be concluded that the position and intensity of the bands in the IR spectra of supported Pt samples depend on the kind of support. The observed shift and broadening of the bands should be connected with the modified electron density of Pt due to the different nature of Pt–Al and Pt–Pr interactions depending on the support

kind, as well as with the morphology change of metal particles. It is clear the presence of metal particles with different electron density. It is known [46] that the frequency of adsorbed CO is higher when the state of the metal is more positive. It is reasonable to suggest that platinum preferentially interacts with praseodymium oxide species in the presence of a much larger alumina area through the Pt O Pr bond similar to the observations for Pt/Al2 O3 modified with CeO2 or La2 O3 [4,20]. The latter is confirmed by the higher BEs values of Pt 4d5/2 electrons for reduced PrO2 -containing samples compared to that of Pt/Al (Table 2), which means some electron deficiency due to a strong interaction between Pt and Pr and formation of Pt species with positive charge (Pt␦+ ). Therefore, the higher frequency bands at 2070 cm−1 (for Pt/Pr6 O11) and at 2081 and 2076 cm−1 (for Pt/xPr–Al) would be related to the CO linearly adsorbed on Pt atoms in a more saturated coordination state created by the strong metal-support interaction (SMSI) at high temperature of reduction. The observed frequency downshift of the bands related to P0 and Pt␦+ species for samples with PrO2 content ≥6 wt.% compared to those of Pt/1Pr–Al (from 2068 and 2081 cm−1 to 2060 and 2076 cm−1 , respectively) may be connected with the presence of small Pt particles with high electron density according to the model of Blyholder [47] for bonding the CO molecules to the metal surface. The appearance of the bands at 1879, 1838 and 1783 cm−1 for Pt samples supported on alumina and mixed xPr–Al oxides (Fig. 6) suggests the presence of metal particles with high electron density, accessible to CO adsorption [48]. The lower band intensity for Pt/Pr6 O11 and Pt/20Pr–Al compared to those of supported Pt samples with PrO2 content of 6–12 wt.% would be related to the some decoration effect caused by the strong metal-support interaction between Pt and Pr-oxide species [35]. Pt atoms can be covered by partially reduced support species (PrOx ) formed under reduction treatment that leads to a decrease of the accessibility of CO adsorption on reduced platinum, similar to the observations for Pt/La2 O3 (CeO2 )–Al2 O3 systems [35,49]. 3.6. TPR and redox properties The TPR profiles of calcined Pr6 O11 , Pt/Pr6 O11 , Pt/Al and Pt/xPr–Al samples and the effect of the subsequent reoxidation and reduction cycles are compared in Fig. 7. The TPR profile of Pt/Al exhibits series of well-defined temperature peaks at approximately 360, 466, 662, 793 and 928 K that means a presence of different Pt species. The peak at 360 K can be associated with the reduction of PtO2 [50]. Considering the XPS results it could be proposed that platinum presents thoroughly as Pt0 in Pt/Al sample after reduction process. It has been reported [51], that a significant part of PtO2 is reduced to Pt0 and a smaller fraction – to Pt2+ species. The TPR signal at 466 K corresponds to a reduction of oxychlorplatinum surface complex, [PtOx Cly ]s , formed by decomposition of the chloride precursor during the calcination procedure [52]. The peak at 662 K, corresponding to a maximum rate of reduction, is correlated to the reduction of Pt-oxide species weakly associated with alumina surface [50]. The temperature features of low intensity at 793 and 928 K would be connected with the reduction of: (i) clustered platinum oxide species and (ii) platinum oxide species in isolated patches, respectively [39]. The TPR profile of bulk Pr6 O11 exhibits a main peak at 809 K with small shoulders on the low temperature side: at 680 and 742 K (Fig. 7). The TPR shape of Pr6 O11 is very similar to that reported in the literature [53] and it can be interpreted as a stepwise reduction process. Therefore, the shoulders in the TPR curve of Pr6 O11 are related to the reduction of superficial oxygen (O2− or O− anions) of the crystalline complex, causing the partial reduction of PrO2 to Pr2 O3 in the complex oxide (Pr6 O11 ). The major H2 uptake at around 809 K is attributed to the bulk reduction of PrO2 by elimination of O2− anions of the lattice and formation of Pr2 O3 [54].

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Fig. 7. TPR profiles of Pr6 O11 Pt/Pr6 O11 and Pt/xPr–Al samples subjected to different treatments: TPR, R-O773 and R-O1073 .

The TPR profile of Pt/Pr6 O11 exhibits two main peaks at 444 and 550 K. The first one is likely due to a concomitant reduction of Pt oxide crystallites, highly dispersed on support, and of synergistically reduced Pr oxide species, since the hydrogen consumption is higher than the amount expected for the complete reduction of platinum precursor to Pt0 [21,55]. The second peak would be primarily associated with the reduction of carrier. It is worth noting that in the TPR profile of Pr6 O11 sample the temperature features attributed to superficial reduction, as well as to the bulk reduction of the carrier, are moved to a lower temperature after deposition of platinum. Comparing the TPR profiles of Pt/Al and Pt/Pr6 O11 samples it is seen that Pt deposited on praseodymium oxide has a lower reduction temperature (444 K) compared to that deposited on alumina surface (662 K). The TPR profiles of Pt/xPr–Al samples show that both the position and intensity of reduction peaks depend on the PrO2 content and therefore, on the type and strength of the interaction between supported Pt oxide species and Pr-modified support. To facilitate the description of the TPR profiles of Pt/xPr–Al samples, the

TPR patterns are divided in three temperature regions: 310–420 K, 430–530 K and 570–740 K. The reduction peaks in the first temperature region are identical to the peak at 360 K observed in the TPR profile of Pt/Al, which was previously assigned to the reduction of PtO2 species. The H2 patterns in the temperature region of 430–530 K can correlate to a reduction of the sum of dispersed Ptand Pr oxide species. The peaks in the highest temperature region of 570–740 K are related to the reduction of platinum particles in a more intimate contact with support and to the superficial Pr oxide reduction; with the main contribution of Pr component [56]. Fig. 7 shows that the peak at 662 K in the TPR profile of Pt/Al, corresponding to a maximum reduction rate of Pt oxide species, is shifted to a lower temperature of 472 K after addition of 6 wt.% PrO2 . It means that Pr significantly improves the noble metal reducibility [21]. In addition, the ratio between the intensities of the TPR peaks in the region 430–530 K and those at 570–740 K decreases with increasing the PrO2 content up to 20 wt.%. This occurrence, probably, is due to some partially coverage of Pt sites by reduced Pr oxide species hindering the hydrogen access [57]. For Pt/20Pr–Al sample

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Fig. 8. TEM images and particle size distribution for spent Pt/Al and Pt/20Pr–Al catalysts.

a small shoulder at about 574 K on the left side of the strong peak in the temperature region 570–740 K is appeared. Comparing the TPR profiles of Pt/xPr–Al samples and Pt/Pr6 O11 it can be concluded that the H2 peaks assigned to the reduction of Pt and Pr oxide species of Pt/xPr–Al are positioned at temperatures higher than those of Pt/Pr6 O11 . For bulk Pr6 O11 material the intensity of TPR peaks characteristic of the surface oxygen reduction is significantly increased after subjecting the sample to R-O773 , while the role of the bulk reduction is diminished (Fig. 7). The latter suggests some saturation of the support surface by oxygen species after R-O773 , which possesses a greater mobility than the lattice oxygen. The TPR profile of Pt/Pr6 O11 sample subjected to R-O773 differs from that of pure Pr6 O11 (Fig. 7). It is seen that the maximum of the TPR peak assigned to the reduction of Pr oxide species is shifted to a lower temperature (from 550 to 440 K) and completely coincides with the peak corresponding to Pt reduction. This phenomenon can be caused by an increase of the Pt-support interface during the redox aging treatment, which promoted strongly the lattice oxygen mobility of support.

Significant changes in the TPR profile of platinum sample supported on non-modified alumina are observed (Fig. 7). Two hydrogen peaks are revealed for Pt/Al subjected to R-O773 : no well resolved peak in the range 340–570 K and a small one at 650–830 K. The lower intensity of these TPR signals compared to those obtained after the first reductive run allow us to assume that there is a strong agglomeration of Pt oxide species during the redox cycle. This hypothesis is similar to the results reported in Ref. [58], which demonstrated that the platinum supported on bare alumina shows a great loss of the surface due to the sintering process under reduction. The behavior of Pt/xPr–Al samples subjected to R-O773 is similar to that of Pt/Al (Fig. 7). Two H2 uptake peaks are revealed: the first one at 310–520 K and the small one at 560–680 K, which have been already associated with the simultaneous reduction of platinum and praseodymium oxides, respectively. There is an alteration in the TPR profiles of Pt/xPr–Al samples with respect to those after the first reductive run, which can be summarized as follows: (i) the peaks resulted after R-O773 are characterized with lower intensity and decreased temperature maximum position and (ii)

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Table 3 Catalytic properties of Pt/Al, Pt/Pr6 O11 and Pt/xPr–Al catalysts in reforming of CH4 with CO2 at different reaction temperatures (CH4 /CO2 = 1; mcat = 0.05 g). The data are taken after 120 min of work. Catalyst Pt/Al

Pt/1Pr–Al

Pt/6Pr–Al

Pt/12Pr–Al

Pt/20Pr–Al

Pt/Pr6 O11

Treac. (K)

CH4 (%)

CO2 (%)

H2 yield (%)

CO yield (%)

H2 /CO

723 823 923 723 823 923 723 823 923 723 823 923 723 823 923 723 823 923

20.4 31.1 46.7 30.8 41.8 57.3 42.9 50.3 66.0 40.9 45.2 67.1 44.1 47.2 67.3 25.6 25.8 53.3

31.6 42.6 56.3 35.6 49.5 67.0 49.8 63.0 77.0 48.6 62.0 78.1 48.6 59.1 79.2 32.6 33.4 58.7

14.8 25.3 41.9 28.4 37.9 52.4 39.4 45.2 60.5 37.0 36.8 61.6 41.8 41.2 61.3 23.4 22.0 50.6

26.0 36.8 51.5 33.2 45.6 62.1 46.3 55.4 71.5 44.7 53.6 72.6 46.3 53.1 73.2 29.5 29.6 56.0

0.57 0.69 0.81 0.85 0.81 0.84 0.85 0.81 0.84 0.83 0.69 0.85 0.90 0.78 0.84 0.79 0.74 0.90

the intensity of the peak in the region 560-680 K is significantly lower than that in the region 310-520 K. The decreased intensities of the peaks after R-O773 suggest that some of the metal oxide species are agglomerated under reoxidation at 773 K. At the same time the value of the TPR peak maxima diminish with about 20 K after reoxidation that means a better reducibility of Pt and Pr oxide species. It should be noted that a considerable part of the Pr oxide species produced after R-O773 can be reduced at lower temperatures than Pr oxide species in the calcined sample, similar to the results revealed for Pt/Pr6 O11 . Comparing the TPR profiles of Pt/xPr–Al and Pt/Al samples after R-O773 (Fig. 7) it is clear that the TPR peaks of Pt/xPr–Al possess higher intensity than those of Pt/Al. It confirms the claim that the presence of praseodymium oxide can prevent the platinum sintering during the reduction treatment. The TPR profiles of Pt/Al and all Pt/xPr–Al samples subjected to R-O1073 exhibit a strong agglomeration of Pt and Pr oxide species, expressed by significantly decrease of the intensity of H2 uptakes (Fig. 7). However, at the same redox conditions Pr6 O11 and Pt/Pr6 O11 samples undergo almost completely recovery of their initial structure, presented by slightly lower intensities of the resulted peaks compared to those revealed after the first reductive run. This dissimilarity in the redox behavior of the samples could be explained by the different nature of Pt-support interaction in Pt/Pr6 O11 and in Al2 O3 -supported Pt samples modified with praseodymium oxide. In summary it can be concluded that there is a synergism between platinum and praseodymium oxide species, which leads to a better reducibility of the both species revealed by the decrease of their reduction temperature. The presence of PrO2 hinders the Pt agglomeration in Pt/xPr–Al samples during the reoxidation at 773 K, whereas platinum and praseodymium sintering process is occurred after reoxidation at 1073 K. 3.7. TEM TEM images of spent Pt/Al and Pt/20Pr–Al catalysts accompanied by the particle size distribution are plotted in Fig. 8. TEM analysis of Pt/Al catalyst after DMR reaction shows a heterogeneous distribution of Pt particles: (i) a significant part of small particles is concentrated in the range 4-16 nm and (ii) a small fraction of large particles of about 40-90 nm is present. In contrast to that, the particle size distribution is much narrower for spent Pt/20Pr–Al catalyst,

where the particle size is smaller than 6 nm. It can be concluded that the presence of praseodymium oxide leads to a homogeneous distribution of the metal Pt particles and hinders their agglomeration under reaction conditions. 3.8. Dry methane reforming The values of CH4 and CO2 conversions as well as of H2 and CO yields of Pt/Al, Pr6 O11 and Pt/xPr–Al catalysts in the reaction of CH4 with CO2 at 723, 823 and 923 K and ratio of CH4 :CO2 = 1:1 are listed in Table 3. The data are taken at 120 min of work. The change of CH4 and CO2 conversions as well as of CO and H2 yields at 823 K with time on stream is shown in Fig. 9. The catalytic performance of the catalysts is strongly dependent on the PrO2 content. CH4 and CO2 conversions of Pt/Al strongly decrease up to 200 min reaction time, but after that it continues to decrease slowly with time on stream, being seen in Fig. 9. The values of CH4 and CO2 conversions of Pt/Pr6 O11 become lower compared to those of alumina-supported Pt. Nevertheless, the H2 and CO yields of Pt/Pr6 O11 are higher than those obtained for Pt/Al. The addition of a small amount of PrO2 (1 wt.%) to Pt/Al catalyst leads to the increase of CH4 and CO2 conversions, as well as of H2 /CO ratio (Table 3). Further increase of the PrO2 content enhances the conversion values of CH4 and CO2 (Table 3). The catalyst with 6 wt.% PrO2 exhibits the highest values of CH4 and CO2 conversions. Contrary to Pt/Al and Pt/1Pr–Al, the catalysts with PrO2 loading ≥6 wt.% are relatively stable with time on stream (Fig. 9). The effect of reaction temperature on the behavior of Pt/6Pr–Al catalyst is plotted in Fig. 10. It is seen that the CH4 and CO2 conversions as well as the H2 and CO yields increase with the increase of reaction temperature. The same trend in the catalytic performance of the other samples is observed (Table 3). The values of H2 /CO ratio for all catalysts are less than unit (Table 3) meaning that the thermodynamic equilibrium is not reached [59]. It has been shown [6] that the reaction equilibrium for the production of hydrogen and/or synthesis gas via DMR is strongly influenced by the simultaneous occurrence of the reverse water-gas shift (RWGS) reaction (CO2 + H2 ⇔ CO + H2 O). According to the kinetic investigations of CH4 reforming with CO2 it has been shown that RWGS reaction operates very close to thermodynamic equilibrium over a wide range of temperatures [60]. The values of CO2 conversion for all samples are higher relative to those of CH4 conversions as a consequence of the presence of RWGS reaction.

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Fig. 9. Stability evolution of Pt/xPr–Al catalysts in dry methane reforming with time on stream at CH4 :CO2 ratio of 1:1 and 823 K.

4. Discussion The results show that the temperature treatment as well as the PrO2 content has a significant influence on the structure and electronic properties of Pt catalysts supported on xPr–Al carriers. Pt-support interaction can affect either the degree of Pt sintering during calcination or the ability of Pt to be reduced and reoxidized under reductive-oxidation atmosphere. The XRD analysis shows that the presence of PrO2 , as well as the increase of its content causes the Pt particle size to decrease on the surface of Pr-modified alumina after calcination at higher temperature of 1023 K (Fig. 1A). The calcination at 1023 K causes the thermal decomposition of [PtOx Cly ]s species that leads to the formation of Pt atoms or small Pt-containing clusters, which rapidly migrate on the surface of alumina and metallic crystallites are formed via nucleation [21,61]. XPS and DRIFTS results show the existence of a strong Pt–Pr interaction in Pt/xPr–Al samples that leads to the formation of Pt sites with different electron density (Pt0 and Pt␦+ ). There are

different factors, which may influence the electron density of noble metal: (i) the presence of traces of Cl ions from the metal precursor could change the electronic state of platinum by the interaction with the electronegative Cl atoms, which act as electron acceptors. This leads to a decrease of the electron density of the metal d orbital and to a shift of the CO bands to a higher frequency [49] and (ii) the morphology of the metal particles and the structure of the exposed metal surface as suggested by the model of Blyholder [47]. According to this model the average number of platinum bonds per platinum atom is lower than in the larger particles when the bonding of CO molecules on the metal surface of the small particles is occurred. A higher metal electron density is available for back-donation of 5d electrons of Pt onto the 2 antibonding orbital of the CO molecules, which leads to a decrease of the CO bond strength and lowering the CO adsorption frequency [47]. The latter results in (i) a higher CO bands intensity for Pt/6Pr–Al and Pt/12Pr–Al compared to those of Pt/1Pr–Al and Pt/Al and (ii) a shift of the CO bands for Pt/1Pr–Al to lower wavenumbers when the PrO2 content is raised up to

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Fig. 10. Evolution of the CH4 and CO2 conversions and H2 and CO yields of Pt/6PrO2 –Al catalyst in dry methane reforming as a function of reaction temperature.

20 wt.% (Fig. 7A). The lower CO band intensity in the IR spectrum of Pt/20Pr–Al sample compared to that of Pt/6Pr–Al and Pt/12Pr–Al would be related to some platinum coverage by PrOx species that leads to a loss of the exposed Pt surface to CO adsorption [35]. This conception is supported by the XPS and TPR studies, which showed: (i) a decrease of Pt 4d5/2 peak intensities for Pt/xPr–Al samples after reduction at 773 K and (ii) a decrease of the intensity of H2 uptake peak, assigned to platinum, with increasing the PrO2 content. XPS data show that praseodymium ion of Pt/Pr6 O11 is in a more reduced state compared to that of Pr6 O11 (Table 2). This is not surprising taking into account the fact that Pt on Pr6 O11 surface is in an electron deficient state due to the charge transfer from platinum to praseodymium. Another reason for this phenomenon should be the presence of oxychloride (PrOCl) and/or hydroxychloride (Pr(OH)2 Cl) surface complexes formed during impregnation process. These complexes are stable up to a high temperature of calcination [62] and stabilize the praseodymium in a lower oxidation state (3+). On the other hand, praseodymium of Pt/xPr–Al catalysts possesses a higher BE than that of Pt/Pr6 O11 , being seen in Table 2. In a previous our work [25] it was shown that (i) the praseodymium oxide species is well dispersed on alumina surface due to the formation of Pr O Al bond and (ii) the Pr 3d5/2 BE values of xPr–Al samples are in the range 933.9-934.1 eV. Moreover, it is generally accepted that the smaller particles possess higher BE than the larger ones due to a decreased extent of the relaxation energy [63]. The different nature of Pt-support interaction in Pt/Pr6 O11 , Pt/Al and Pt/xPr–Al catalysts causes a different reducibility of the both, platinum and praseodymium. Comparing the TPR profiles of Pt/Al and Pt/xPr-samples a better platinum reducibility over xPr–Al supports is revealed (Fig. 7). This phenomenon is due to the higher Pt dispersion as a consequence of the intimate contact between Pt and Pr oxide species. It has been reported [54,42] that the formation of Pt–PrOx complex is a result of the spill-over of dissociated H2 from platinum toward support. The presence of Pt–PrOx complex enhances the oxygen exchange capacity of Pr6 O1I , i.e. facilitates the oxygen transfer from the lattice to the surface. This is evident from the TPR and XPS O 1s core level for reduced Pt/Pr6 O11 sample (Figs. 5B and 7, respectively). However, comparing the TPR of Pt/Pr6 O11 and Pt/xPr–Al it can be proposed that the synergetic effect between Pt and Pr oxide species is much less pronounced in the case of Pt/xPr–Al samples, expressed by the higher reduction temperature of the Pt and Pr oxide species (Fig. 7). The latter is connected with the existence of other types of interactions, namely Pt–Al and Pr–Al. The different behavior of the catalysts in the DMR reaction can be mainly assigned to the support kind. Comparing the catalytic performance of Pt/Pr6 O11 and Pt/Al it is clear that Pt/Pr6 O11 catalyst possesses lower activity than that of Pt/Al (Fig. 9). As was discussed above, (i) the SBET of Pt/Pr6 O11 is much lower than that of

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Pt/Al (Table 1) and (ii) the DRIFTS results demonstrate a lower CO band intensity for Pt/Pr6 O11 sample compared to that of Pt/Al one (Fig. 6). Therefore, the lower values of CH4 and CO2 conversions on Pt/Pr6 O11 catalyst can be connected either with significantly lower specific surface area of Pr6 O11 or with a partially coverage of platinum active sites by reduced PrOx species. In addition, the H2 and CO yields, as well as the H2 /CO ratio value of Pt/Pr6 O11 catalyst are higher compared to those of Pt/Al. Several studies have been shown that hydroxyl groups may be involved in methane reforming reaction [64]. XPS data indicate that a great amount of OH− groups present on the surface of Pt/Pr6 O11 catalyst under reduction treatment (Fig. 5B). The hydroxyl groups can react with the carbon formed during the DMR reaction to yield H2 and CO (C + H2 O → CO + H2 ). It is clear that the presence of praseodymium oxide in Pt/xPr–Al catalysts improves their catalytic performances in the reaction of reforming of CH4 with CO2 (Table 3). The changes of activity can be related to the increased dispersion of Pt, being observed by XRD and TEM analyses. On the other hand, the coexistence of Pt sites with different electron density and Pr3+ /Pr4+ redox couples can facilitate the activation of CH4 and enhance the carbon resistance, respectively, similar to the observations for Rh/CeO2 –Al2 O3 catalysts [65]. It means that the activity and stability of the catalysts will depend on the support kind, similar to the previously reported results for supported Pt and Pd catalysts [48]. The higher values of CH4 and CO2 conversions for the catalysts with PrO2 ≥6 wt.% compared to that for Pt/Al and Pt/1Pr–Al (Fig. 9 and Table 3) is due to the increased number of the exposed active metal Pt sites available for methane adsorption. It is well known [66] that the catalysts deactivation during the DMR reaction is mainly attributed to the carbonaceous species growing over metal surface and/or to the agglomeration of the active metal phase. The carbon formation may occur through the both, methane decomposition (CH4 → C + 2H2 ) and carbon monoxide disproportionation (2CO → C + CO2 , Boudouard reaction). The active carbon (C* ) can be removed by CO2 or H2 O as co-reactants and/or oxygen from the support near the metal particles. Therefore, the equilibrium between the rate of methane decomposition over exposed metal particles and the rate of carbon removing will determine the overall stability of the catalyst. The results show that supported Pt catalysts with PrO2 content ≥6 wt.%, as well as Pt/Pr6 O11 sample deactivate very slowly with time on stream under reaction conditions. It means timely elimination of the carbon deposited on the catalyst surface, confirmed by the TEM results (Fig. 8). This phenomenon is correlated with the redox chemistry of praseodymium oxide. Barroso et al. [66] have been suggested that Pr2 O3 can become reoxidized by CO2 during the reforming reaction with subsequent formation of PrO2 and CO (Pr2 O3 + CO2 → 2PrO2 + CO). This reaction involves oxygen transport when CO2 is dissociated into CO and O, and the atomic oxygen is transferred to oxidize the praseodymium oxide. After that, PrO2 may react with carbon residues (4PrO2 + C → 2Pr2 O3 + CO2 ) produced by CH4 decomposition, regenerating again the reduced Pr2 O3 oxide and gasifying the carbon deposits. It is found that the Pt/Al catalyst deactivates gradually with time on stream (Fig. 9). This can be assigned to the sintering of Pt particles, being confirmed by the XPS and TEM analyses of spent catalyst. 5. Conclusions The results can be summarized in the following conclusions: (i) The presence of Pr oxide species in Pt/xPrO2 –Al2 O3 catalysts influences significantly the size of noble metal by providing the formation of Pt particles with smaller average particle size (<6 nm). The improved Pt dispersion for Pr-containing catalysts originates mostly from the interaction between the noble metal species and

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