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Al2O3
Applied Surface Science 259 (2012) 831–839
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DRIFTS study of CO adsorption on praseodymium modified Pt/Al2 O3 I. Tankov a , W.H. Cassinelli b , J.M.C. Bueno b , K. Arishtirova a , S. Damyanova a,∗ a b
Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Departamento de Engenharia Quimica, Universidade Federal de São Carlos, C.P. 676, 13565-905 São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 7 February 2012 Received in revised form 7 June 2012 Accepted 25 July 2012 Available online 3 August 2012 Keywords: DRIFT spectroscopy CO adsorption Platinum catalysts Praseodymium oxide
a b s t r a c t The effect of PrO2 content (1–20 wt.%) and temperature pretreatment on the structure and surface properties of PrO2 –Al2 O3 -supported Pt catalysts was studied by XRD, XPS and DRIFTS of carbon monoxide adsorption. XRD analysis showed that platinum particle size decreases with the increase of PrO2 content for samples calcined at high temperature of 1023 K. The intensity and position of the infrared bands were strongly dependent on the praseodymium oxide content and reduction temperature. Two kinds of Pt sites (Pt0 and Pt␦+ ) were recorded in reduced PrO2 -containing samples. A better thermal stability of the Pt CO bond in PrO2 -containing samples compared to Pt/Al2 O3 was observed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The world dependence on petroleum oil has generated interest in the use of natural gas. Reforming processes to transform natural gas into synthesis gas or hydrogen rich synthesis gas have been extensively studied in the recent years. The thermodynamic properties of methane reforming reactions still require a high temperature to obtain high methane conversion. Another problem is the carbon formation on the catalyst surface. The operating conditions require the use of stable and effective catalysts, resistant to coke. Therefore, the investigations should be focused on: (i) the catalyst resistance to coke formation; (ii) the type of supported metal phase and (ii) the kind of the used carrier that improve the catalyst efficiency. Much attention has been focused on supported noble metal (Pt, Pd, Rh) catalysts for methane reforming processes [1,2]. It was found that the most promising support is ␥-Al2 O3 thanks to its high thermal stability and high specific surface area [3]. Previous studies [4–7] have shown that the use of rare earth oxides (La2 O3 or CeO2 ) as carriers or promoters for alumina support leads to the increase of catalyst activity and stability. Praseodymium oxide is another rare earth oxide which could be successfully used as carrier or
∗ 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). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.138
promoter for catalysts in methane reforming processes due to the high oxygen storage capacity (OSC) caused by the presence of the couple ions Pr3+ /Pr4+ [8,9]. For selection of the appropriate reaction conditions, as well as for clarification of the mechanism of methane reforming reactions it is necessary to examine the structure and surface properties of the active metal phase. Fourier transformed infrared (FTIR) spectroscopy is an effective method for characterization of the surface chemistry of heterogeneous catalysts [10,11]. The most frequently used probe molecule is carbon monoxide (CO) [12]. It was found that the CO adsorption depends on the kind of supported metal, its dispersion and oxidation state, as well as on the kind of the carrier. FTIR spectroscopy analysis of platinum supported on CeO2 –La2 O3 –Al2 O3 has demonstrated that the presence of the both promoters (CeO2 and La2 O3 ) leads to a distribution of well dispersed Pt particles with different electron density on the catalyst surface in the reactions of partial oxidation and steam reforming of methane [13]. The study of spent ZrO2 –Al2 O3 and ZrO2 –Al2 O3 -supported Pt catalysts for reforming of methane with CO2 (dry reforming) by FTIR of CO adsorption has shown the suppression of carbon deposition over zirconium-containing samples due to the Pt–Zrn+ interaction [14]. The purpose of the present work is to investigate the effect of praseodymium oxide content and temperature pretreatment on the structure and surface properties of alumina-supported Pt catalysts modified with praseodymium. N2 adsorption–desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier transform
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spectroscopy of CO adsorption (DRIFTS) were used for sample characterization. 2. Experimental 2.1. Sample preparation The Pr6 O11 and PrO2 –Al2 O3 carriers with different PrO2 content (1–20 wt.%) were obtained by a procedure described in Ref. [15]. Platinum catalysts supported on Al2 O3 , Pr6 O11 and PrO2 –Al2 O3 oxides were prepared by wet impregnation of the carriers (6 g) with 100 ml 0.003 M solution of H2 PtCl6 ·6H2 O in ethanol. The mixture was stirred at room temperature for 2 h. The ethanol was evaporated at 298 K under vacuum. The solids were dried at 383 K overnight and calcined at 823 and 1023 K for 2 h. Pt content was about 1 wt.%. The samples were denoted as Pt/xPr–Al, where x is the theoretical PrO2 content. 2.2. Sample characterization Specific surface areas (SBET ) and pore volumes (Vp ) of Pt/xPr–Al samples were evaluated from the N2 adsorption–desorption isotherms, determined at 77 K with a Micromeritics TriStar 3000 apparatus. The samples were previously degassed at 423 K for 24 h under vacuum (10−4 mbar) to ensure a relatively clean surface. The specific areas of the samples were determined according to the standard BET procedure using nitrogen adsorption data taken in the relative equilibrium pressure interval of 0.03 < P/P0 < 0.3. XRD profiles of calcined and reduced samples were registered on 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 between 10◦ and 90◦ . Step size of 0.033 and step scan of 10 s was used to identify the structure of the samples. Phase identification was carried out by comparison with JCPDF database cards. The average particle size of Pt (DXRD ) was calculated using Sherrer’s equation. XPS spectra of the calcined and reduced samples were recorded on Escalab-MkII (VG Scientific) electron spectrometer with an unmonochromatized Mg 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. The binding energy of the C 1s at 284.9 eV was taken as an internal standard. The accuracy of the binding energy (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.
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 cell has a microporous ␣-Al2 O3 microreactor with a K-type thermocouple (Crommel Alummel) for measuring the temperature directly into the sample. The ␣-Al2 O3 micro-reactor is coupled to an array of stainless steel with CaF2 window and connectors for inlet and outlet gas. The spectrum resolution was 4 cm−1 and 64 scans were collected. To execute the experiments, approximately 50 mg of catalyst in powder form was used. The catalysts were reduced in 10% H2 /N2 gas mixture flow from ambient temperature to 303 K at heating rate of 5 K/min and from 303 K to 773 K (or 973 K) at heating rate of 10 K/min. The catalysts remained at 773 K (or 973 K) for 1 h. After reduction, the samples were cooled under gas mixture flow until 673 K. Then the cooling to 298 K was conducted in N2 gas. After stabilizing the cell temperature, CO adsorption was carried with CO pulses at partial pressure (PCO ) of 10 Torr under N2 flow. After each pulse, the spectrum was collected after 5 min in order to achieve equilibrium. The data pulses were necessary to achieve complete saturation of catalytic surface. After the adsorption process, the samples were submitted to temperature-programmed desorption of CO (TPD-CO). In these experiments, the catalysts were submitted to the cycle of heating and cooling in N2 flow to CO desorption. Initially, the samples were heated from ambient temperature to 323 K at a rate of 10 K/min and held at that temperature for an additional time (5 min). After cooling to ambient temperature, the spectra were collected and the samples were submitted to successive cycle of heating at different temperatures and cooling.
3. Results and discussion 3.1. Characterization The values of SBET and Vp of Pt/xPr–Al samples calcined at 823 K are summarized in Table 1. The values of DXRD of the samples calcined at 1023 K are listed in Table 1. The BE of Pt 4d5/2 and Pr 3d5/2 , as well as the atomic XPS ratios of calcined and reduced samples at 823 and 773 K, respectively, are also included in Table 1. The surface area of Pt/1Pr–Al sample is slightly lower than that of nonpromoted alumina with Pr. Decrease of the values of SBET and Vp with increasing the PrO2 content is observed. The sample with the highest PrO2 content (20 wt.%), as well as Pt/Pr6 O11 exhibit the lowest surface area. The results would be connected with some plugging of the pores of alumina with praseodymium oxide species and/or some redispersion during the impregnation procedure of the mixed oxides with Pt salt [16]. XRD patterns of Pt/Al, Pt/xPr–Al and Pt/Pr6 O11 samples calcined at 823 and 1023 K are shown in Fig. 1A and B, respectively.
Table 1 Textural and XPS characteristics of Pt/xPr–Al samples. Sample
SBET (m2 /g)
Vp (m3 /g)
Da (XRD) nm
Binding energy, eV
Pt 4d5/2
Pt/Al Pt/1Pr–Al Pt/6Pr–Al Pt/12Pr–Al Pt/20Pr–Al Pt/Pr6 O11 a
230 226 209 186 139 0.66
After calcination at 1023 K.
0.83 0.82 0.77 0.72 0.62 0.005
24 12 9 8 5 –
XPS ratios
Pr 3d5/2
Pt/Al
Pt/Pr
Pt/Pr + Al
Calc.
Red.
Calc.
Red.
Calc.
Red.
Calc.
Red.
Calc.
Red.
314.7 316.2 316.5 316.1 316.3 317.2
314.1 315.1 315.1 314.8 315.1 316.0
934.0 934.1 934.0 933.9 932.2
– 933.8 933.8 933.9 934.0 933.0
0.0017 0.0020 0.0026 0.0038 0.0127 –
0.0013 0.0009 0.0011 0.0012 0.0012 –
– 1.04 0.32 0.26 0.13 0.08
– 1.08 0.85 0.17 0.05 0.09
– 0.0019 0.0026 0.0036 0.0039 –
– 0.0009 0.0011 0.0012 0.0012 –
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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).
The peaks at 2 = 33.4◦ , 37.6◦ , 39.2◦ , 45.8◦ and 66.8◦ characteristic of the structure of ␥-Al2 O3 (JCPDS 10-425) are appeared in the XRD of alumina-supported samples. XRD of Pt/Pr6 O11 shows patterns at 2 = 28.1◦ , 46.7◦ , 55.6◦ , 75.7◦ and 78.2◦ , related to PrO2 (JCPDS 24-1006) and peaks at 2 = 32.7◦ and 68.5◦ due to Pr2 O3 (JCPDS 22-880). No peaks of praseodymium oxide species are detected in the XRD of Pt/xPr–Al samples. XRD patterns of Pt/Al and PrO2 -containing samples calcined at 823 K did not show peaks, corresponding to metallic platinum and/or platinum oxides (Fig. 1A). This means that the noble metal species are too small and cannot be detected by XRD. However, the calcination of the samples at higher temperature of 1023 K leads to an agglomeration of Pt particles at 2 = 39.7◦ and 81◦ [16], especially, for Pt/Al and Pt/xPr–Al with low PrO2 content (Fig. 1B). According to Lietz et al. [17] the agglomeration of Pt is due to the decomposition of surface [PtIV (OH)x Cly ]s or [PtOx Cly ]s species after calcination at high temperature (>773 K). Addition of a small amount of praseodymium oxide (1 wt.%) to Pt/Al sample causes significantly decrease of the average particle size of Pt (from 24 to 12 nm for Pt/Al and Pt/1Pr–Al, respectively, Table 1). DXRD of Pt decreases with increasing the praseodymium oxide content (to 5 nm for Pt/20Pr–Al, Table 1). Oxide Pt/Pr6 O11 sample did not show any peak assigned to agglomerated Pt (Fig. 1B). It should be noted that with exception of Pt/Al, all PrO2 -containing samples reduced at 773 K, previously calcined at 823 did not show XRD patterns of Pt. It suggests that Pt species are well dispersed on praseodymium modified alumina. These results are in accordance with previous observations [18,19] where the use of ␥-Al2 O3 modified with rare earth oxides such as CeO2 or La2 O3 prevents the strong agglomeration of noble metal due to the formation of a surface complex involving a Pt–O–Ce(La, Pr) interaction. The calcined Pt/Al sample has BE of Pt 4d5/2 core electron levels at 314.7 eV characterizing Pt oxide species [20] (Table 1). It is seen that the addition of Pr to Pt/Al leads to an increase of the values of BEs of PrO2 -modified samples (to 316.1–316.5 eV). The reduction of Pt/Al leads to a decrease of the BE of Pt 4d5/2 to 314.1 eV, which is related to metallic platinum (Pt0 ) [21]. It should be noted that the BEs values of reduced PrO2 -containig samples (314.8–315.1 eV)
are higher compared to that of alumina-supported Pt. The highest BE values are observed for Pt/Pr6 O11 (Table 1). The results suggest a presence of positively charged platinum atoms (Pt␦+ ) caused by the interaction between Pr-modified support and noble metal at high temperature treatment. The changes in the oxidation state of Pt are in agreement with results for lanthanide oxide-containing noble metal catalysts [4–6]. For calcined Pt samples supported on the mixed PrO2 –Al2 O3 oxides the BE of Pr 3d5/2 core level at ca. 934.0 eV can be assigned to Pr4+ [15]. The BEs values of Pr 3d5/2 remain unchanged after reduction (Table 1). Deposition of Pt on Pr6 O11 leads to a significant decrease of the BE values of Pr for calcined sample as well as for reduced one (932.2 and 933.0 eV, respectively) which means a change in the oxidation state of Pr. The higher BEs for Pt/xPr–Al samples compared to those for Pt/Pr6 O11 can be related to the strong interaction between Al and Pr as was observed for mixed PrO2 –Al2 O3 oxides in our previous work [15]. The atomic XPS Pt/Pr + Al ratios for calcined samples increase with increasing the PrO2 content which indicates an enrichment of the support surface by platinum oxide species (Table 1). However, for reduced samples there is an increase of the Pt/Pr + Al ratio up to 12 wt.% and after that it remains unchanged. The higher values of Pt/Pr compared to those of Pt/Al means that Pt is preferentially connected with praseodymium oxide species. All atomic XPS ratios are decreased for reduced samples that would be an indication for some agglomeration or re-dispersion of platinum species. 3.2. Surface properties of Pt/xPr–Al samples Examination of the surface properties of the samples by DRIFTS is carried out in the region 2150–1200 cm−1 . The IR bands at 2150–1750 cm−1 are associated with the adsorbed CO species on Pt sites and/or with adsorbed species the formation of which require the existence of both active metal and supporting material, i.e. initially formed on the metal sites and then migrated to the support [19,22]. These bands can be divided in two regions. The first region at 2150–1900 cm−1 is due to linearly adsorbed CO species [23].
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The second one at 1900–1750 cm−1 is related to bridge bonded or multi-coordinated adsorbed CO [24]. The bands in the region 1750–1200 cm−1 are due to the formation of a large number of different carbonate and/or formate species [25].
3.2.1. DRIFT spectra of CO adsorption in the region 2150–1750 cm−1 The DRIFT spectra of CO adsorbed on Pt/xPr–Al samples previously calcined at 823 K and after that reduced at 773 and 973 K in the region 2150–1750 cm−1 are shown in Fig. 2A and B, respectively. It is observed that the kind of support, as well as the reduction temperature influences the position and intensity of the IR bands. Adsorption of CO on alumina-supported Pt (Fig. 2A) produces a band at 2060 cm−1 with a significant broadening on the low-frequency side, characteristic of CO linearly bonded to surfaceexposed metal Pt atoms (Pt0 ) [26,27]. This indicates that the Pt component is predominantly in its fully reduced form. Changes in the spectrum of platinum deposited on pure praseodymium oxide (Fig. 2A) are revealed. The main band in the spectrum of Pt/Al sample is shifted towards a higher wavenumber of 2070 cm−1 in the spectrum of Pt/Pr6 O11 (blue shift [28]). The 2070 cm−1 band is accompanied by three low-intensive shoulders at 2045, 2027 and 2008 cm−1 assigned to on-top carbonyls on Pt0 species [29–31]. The observed shift as well as the broadening of the IR bands should be connected with: (i) the modified electron density of Pt due to the different nature of Pt–Al and Pt–Pr interactions depending on the support kind and/or (ii) the morphology change of the metal particles on the surface [26,27]. The platinum–praseodymium interaction is electronic, concerning the charge transfer between the noble metal and neighboring praseodymium oxide species being seen in Table 1. As was mentioned above, the BE values of Pt 4d5/2 electrons for reduced PrO2 -containing samples are higher compared to that for Pt/Al that is related to electron deficiency due to a strong interaction between Pt and Pr and formation of platininum species with positive charge, Pt␦+ . These results are similar to the observations for noble metals supported on CeO2 or La2 O3 [4,27]. Considering the highest intensity of 2070 cm−1 band in the spectrum of Pt/Pr6 O11 it can be concluded that platinum component exists mainly as Pt with low electron density (Fig. 2A). The addition of 1 wt.% PrO2 to Pt/Al causes a shift of the main 2060 cm−1 band assigned to linearly bonded Pt0 to 2068 cm−1 and appearance of poorly-resolved shoulder at about 2081 cm−1 in the spectrum of Pt/1Pr–Al sample (Fig. 2A). These observations propose again a change in the electronic state of platinum as was confirmed by the change of BE of Pt 4d5/2 (from 314.1 to 315.1 eV for Pt/Al and Pt/1Pr–Al, respectively, Table 1). In addition, it is well known [10,11] 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 [4,13,32]. Further increase of the praseodymium oxide content (6, 12 and 20 wt.%) in Pt/xPr–Al samples (Fig. 2A) shifts the adsorption bands, corresponding to Pt␦+ and Pt0 , towards lower wavenumbers (from 2081 and 2068 cm−1 to 2076 and 2062 cm−1 , respectively, (redshift [28]). Considering the Blyholder’s model [33] for bonding the CO molecules to metal surface the average number of Pt Pt bonds per platinum atom for small Pt particles is less than that for larger particles and a higher metal electron density is available for the back-donation from 5d electrons of Pt to 2* antibonding orbital of the adsorbed CO molecule. This leads to a decrease of the CO bond strength, shifting the CO stretching frequency to lower values. The presence of metal particles with high electron density is
supported by the appearance of bridge-bonded CO species at 1781 and 1830 cm−1 in the IR region of 1900–1750 cm−1 (Fig. 2A). The observed red-shift could be connected with the presence of smaller Pt particle size. The higher values of Pt/Pr + Al ratios of the samples with PrO2 content ≥6 wt.% compared to that of Pt/1PrAl (Table 1) suggest a high dispersion of Pt. It is clear that the intensity and position of CO adsorption bands for PrO2 -containing samples is influenced by the PrO2 content of support: (i) the band intensity increases with rising the praseodymium oxide content up to 12 wt.% and (ii) the intensity of high-frequency shoulder at ca. 2076 cm−1 compared to that of the main band at 2062 cm−1 is the highest for Pt/20Pr–Al sample (Fig. 2A). These observations can be connected with the presence of metal particles with different electron density. However, the surface sites for CO adsorption over Pt/Pr–Al samples with PrO2 content of 6, 12 and 20 wt.% are similar, since the position of the bands is practically the same (Fig. 1A). The lower bands intensity of Pt/20Pr–Al as well as of Pt/Pr6 O11 compared to those of supported Pt samples with PrO2 content of 6–12 wt.% can be related to a decoration phenomenon caused by the strong metal-support interaction (SMSI) [27,34]. The latter leads to a decrease of the number of exposed Pt sites for CO adsorption which can be supported by the unchanged value of atomic XPS Pt/Pr + Al ratio at 20 wt.% PrO2 (Table 1). Similar to CeO2 - and La2 O3 supported samples [7,13,27] the electronic effect, produced by the SMSI between Pt and Pr oxide species under reduction conditions leads to the formation of reduced support species (PrOx ), which migrate onto the Pt particles, causing a loss of the chemisorption capacity of metal. On the other hand, it cannot be excluded some agglomeration of Pt in Pt/Pr6 O11 under reductive atmosphere due to the low surface area of support in spite of the absence of XRD patterns. Submitting the samples Pt/6Pr–Al and Pt/20/Pr–Al previously calcined at 823 K to a higher reduction temperature of 973 K leads to a shift of the main band to higher frequency (from 2062 to 2068 cm−1 at 773 and 973 K, respectively, Fig. 2B), which could be connected with the change of the shape of Pt particles. In addition, the high-frequency shoulder for PrO2 -containing samples is also shifted to higher value (from 2076 to 2082 cm−1 at 773 and 973 K, respectively) due to the presence of more coordinatively unsaturated Pt, caused by the more intimate contact between Pt- and Pr-modified support at higher reduction temperature. Comparing the spectra of the samples reduced at both temperatures it can be concluded that the increase of reduction temperature up to 973 K leads to a more uniform electron distribution of Pt, revealed by the narrowness of the bands (Fig. 2B). 3.2.2. DRIFT spectra of CO adsorption in the region 1750–1200 cm−1 The DRIFT spectra of CO adsorption on Pt/xPr–Al samples in the 1750–1200 cm−1 region, previously reduced at 773 and 973 K, are shown in Fig. 2C and D, respectively. It was reported [35,36] that the IR bands in this region are mainly due to the formation of a large number of different carbonate species on the catalyst surface. It is well known [30] that the formate species (HCOO− ) also give adsorption bands at 1700–1200 cm−1 , which seriously hinders the unequivocal identification of the surface carbonate species. It was found [37] that the formate species can be formed at enhanced adsorption temperatures (500 K) at the periphery of platinum–alumina by spillover of adsorbed CO molecules from Pt to OH groups of alumina support. In the present work the CO adsorption on Pt/xPr–Al samples is carried out at room temperature, so the presence of formate species should not be expected. Moreover, the appearance of IR bands in the region 3100–2700 cm−1 (spectra not shown), corresponding to C–H vibrations of formates is not revealed. Based on these observations we can conclude that the
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Fig. 2. DRIFT spectra of CO adsorption in the 2150–1750 cm−1 and 1750–1200 cm−1 regions for Pt/xPr–Al samples reduced at 773 K (A and C, respectively) and 973 K (B and D, respectively).
presence of formate species on the surface of Pt/xPr–Al samples can be excluded. The IR spectra of Pt/Al and Pt/xPr–Al samples with PrO2 content up to 12 wt.% at 773 K exhibit a strong band centered at 1648 cm−1 and weaker ones at 1593, 1441, 1345 and 1229 cm−1 (Fig. 2C). The existence of bands in the frequency region 1600–1300 cm−1 can be taken as an indication of the geometry change of the adsorbed carbonate species (CO3 2− ). According to Ref. [38], these species do not lie flat on the surface but are oriented perpendicular to it. The adsorption band at 1593 cm−1 is due to the formation of bidentate carbonate species (b-CO3 2− ) [27], whereas the bands at 1441 and 1345 cm−1 are assigned to the monodentate carbonates species (mCO3 2− ) [36]. The formation of surface carbonate complexes involves adsorption of CO molecules on coordinatively unsaturated Lewis acid–base pair sites (Prn+ –O2− ), similar to the results reported for CO adsorption on ZrO2 [39]. The adsorption bands at 1648 and 1229 cm−1 are related to the presence of asymmetric bicarbonate species (HCO3 − ) [35]. It should be noted that in the IR spectrum of Pt/20Pr–Al sample the strong band at 1648 cm−1 now is appeared at lower wavenumber, 1629 cm−1 . The latter is assigned to symmetric stretching mode of O C O of surface bicarbonate [40]. In addition, a band at 1529 cm−1
due to b-CO3 2− is also observed. Considering the literature data [40] the formation of bicarbonates includes hydrogen bond between the protons in the Brönsted acid–base centers (OH groups) of alumina with an O atom originating in the carbonates already existing on the PrO2 surface. It should be mentioned that the amount of adsorbed carbonate and bicarbonate species is not much pronounced, which is represented by the low intensity of their bands. The IR bands in the spectra of Pt/Al, Pt/6Pr–Al and Pt/20Pr–Al samples become sharper and more intensive after reduction at higher temperature of 973 K (Fig. 2D). It could be suggested that the praseodymium oxide undergoes some redispersion, which leads to the formation of more uniform particles distribution during the high temperature pretreatment. It is seen that the intensity of the IR bands related to the bicarbonate and carbonates species at both reduction temperatures increases with increasing the praseodymium oxide content, which would be assigned to the formation of a large number of centers for CO adsorption in PrO2 -rich samples (Fig. 2C and D). 3.2.3. DRIFT spectra of CO desorption The CO desorption profiles in the 2150–1750 cm−1 region of Pt/Al and Pt/xPr–Al samples with different PrO2 content, reduced at
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Fig. 3. DRIFT spectra of CO desorption in the 2150–1750 cm−1 region for Pt/xPr–Al samples reduced at 773 K.
773 K, are shown in Fig. 3. It is clear from the figures that the intensity and position of the IR bands are changed with the increase of desorption temperature. There is a decrease of the total integrated band intensity due to a gradually decrease of the amount of
adsorbed CO molecules over Pt sites. The IR spectrum of aluminasupported Pt sample (Fig. 3) shows that the peak at ca. 2060 cm−1 with a significant broadening on the low-frequency side, previously correlated to CO molecules linearly adsorbed on Pt particles with
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Fig. 4. DRIFT spectra of CO desorption in the 1750–1200 cm−1 region for Pt/xPr–Al samples reduced at 773 K.
a high electron density, is shifted to ca. 2040 cm−1 with increase of the temperature up to 573 K. For PrO2 -containing samples the main band at 2068–2060 cm−1 , characterizing the CO adsorption on Pt0 , and the shoulder at ca. 2076 cm−1 assigned to platinum in a
close contact with the rare earth oxide component, Pt␦+ , (Fig. 3) are shifted to 2050–2030 cm−1 and to ca. 2064 cm−1 , respectively, with increasing the temperature up to 573 K. The band shifting would be explained with: (i) disappearance of the dipole–dipole coupling
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effect between neighboring CO molecules during the desorption process [41] and (ii) occurrence of the process CO(ad) → C(ad) + O(ad) [42]. The latter means that increasing the desorption temperature leads to the increase of the concentration of adsorbed carbon and formation of a strong Pt C bond. The DRIFTS results demonstrate that a complete CO desorption for the samples with PrO2 content up to 12 wt.% occurs at the same temperature, above 573 K. However, comparing the desorption spectra of these samples with those of Pt/20Pr–Al it can be seen that a complete CO desorption for the last one takes place at higher temperature, above 623 K. This is an indication that the Pt CO bond in the highest PrO2 -loaded sample possesses better thermal stability. The latter is related to the change of the electronic state of platinum species due to the Pt–Pr interaction, which results in a considerably higher dispersion of the noble metal. It was found [43] that the high-temperature desorption state of CO is larger in population on the smaller Pt particles than on the larger ones. Therefore, the small Pt particle size can enhance the charge transfer from the metal d orbital to antibonding 2* orbital of CO molecule, which is revealed by the formation of a strong Pt CO bond for Pt/20Pr–Al sample. There was no difference in the complete CO desorption temperature for the sample previously reduced at 973 K. The CO desorption profiles in the 1750–1200 cm−1 region for Pt/Al and Pt/xPr–Al samples with different PrO2 content, reduced at 773 K, are shown in Fig. 4. It is observed that the intensity and the shape of the IR bands of carbonate species depend on PrO2 content and temperature of reduction. It can be summarized that the HCO3 − species are first decomposed to CO2 (at 523 K) followed by decomposition of b-CO3 2− and m-CO3 2− species at higher temperature of 573 K. It means that the thermal stability of carbonates species is greater than that of bicarbonate ones. The latter could be assigned to the stronger Lewis acidity of Prn+ cations and to the stronger Lewis basicity of O2− anions of coordinatively unsaturated Prn+ –O2− sites over the surface of Pt/xPr–Al samples. The highest thermal stability of carbonate species is observed for Pt/Pr–Al sample with the highest PrO2 content (at 623 K, Fig. 4). 4. Conclusions The spectral characterization of Pt supported on Al2 O3 , Pr6 O11 and mixed PrO2 –Al2 O3 oxides with different PrO2 content (1–20 wt.%) show the influence of the type of support and temperature treatment on the surface state of Pt. Pt is well dispersed over the surface of all samples calcined at 823 K, whereas increasing the calcination temperature up to 1023 K leads to agglomeration of Pt. Two kinds of Pt sites are detected after reduction of the samples at 773 and 973 K by DRIFTS of CO adsorption: platinum sites with high electron density (Pt0 ) and platinum with some positive character (Pt␦+ ) due to the more intimate contact between Pt and Pr, revealed also by XPS. Acknowledgements The authors acknowledge financial support by project No. DTK-02/36 from National Science Fund at Bulgarian Ministry of Education, Youth and Science and FAPESP. References [1] L.V. Mattos, E. Rodino, D.E. Resasco, F.B. Passos, F.B. Noronha, Partial oxidation and CO2 reforming of methane on Pt/Al2 O3 , Pt/ZrO2 , and Pt/Ce-ZrO2 catalysts, Fuel Processing Technology 83 (2003) 147–161. [2] L.S.F. Feio, C.E. Hori, S. Damyanova, F.B. Noronha, W.H. Cassinelli, C.M.P. Marques, J.M.C. Bueno, The effect of ceria content on the properties of Pd/CeO2 /Al2 O3 catalysts for steam reforming of methane, Applied Catalysis A: General 316 (2007) 107–116.
[3] J. Xu, W. Zhou, Z. Li, J. Wang, J. Ma, Biogas reforming for hydrogen production over nickel and cobalt bimetallic catalysts, International Journal of Hydrogen Energy 34 (2009) 6646–6654. [4] R.M. Navarro, M.C. Alvarez-Galvan, M.C. Sanchez-Sanchez, F. Rosa, J.L.G. Fierro, Production of hydrogen by oxidative reforming of ethanol over Pt catalysts supported on Al2 O3 modified with Ce and La, Applied Catalysis B: Environmental 55 (2005) 229–241. [5] A.C.S.F. Santos, S. Damyanova, G.N.R. Teixeira, L.V. Mattos, F.B. Noronha, F.B. Passos, J.M.C. Bueno, The effect of ceria content on the performance of Pt/CeO2 /Al2 O3 catalysts in the partial oxidation of methane, Applied Catalysis A: General 290 (2005) 123–132. [6] Y. Nagai, T. Hirabayashi, K. Dohmae, N. Takagi, T. Minami, H. Shinjoh, S. Matsumoto, Sintering inhibition mechanism of platinum supported on ceriabased oxide and Pt-oxide-support interaction, Journal of Catalysis 242 (2006) 103–109. [7] J.C.S. Araujo, D. Zanchet, R. Rinaldi, U. Schuchardt, C.E. Hori, J.L.G. Fierro, J.M.C. Bueno, The effects of La2 O3 on the structural properties of La2 O3 –Al2 O3 prepared by the sol–gel method and on the catalytic performance of Pt/La2 O3 –Al2 O3 towards steam reforming and partial oxidation of methane, Applied Catalysis B: Environmental 84 (2008) 552–562. [8] K. Asami, K. Kusakabe, N. Ashi, Y. Ohtsuka, Synthesis of ethane and ethylene from methane and carbon dioxide over praseodymium oxide catalysts, Applied Catalysis A: General 156 (1997) 43–56. [9] S. Bernal, G. Blanco, J.J. Calvino, J.A.P. Omil, J.M. Pintado, Some major aspects of the chemical behavior of rare earth oxides: an overview, Journal of Alloys and Compounds 408–412 (2006) 496–502. [10] J. Mikulova, S. Rossignol, J. Barbier Jr., D. Duprez, C. Kappenstein, Characterizations of platinum catalysts supported on Ce, Zr, Pr-oxides and formation of carbonate species in catalytic wet air oxidation of acetic acid, Catalysis Today 124 (2007) 185–190. [11] V. Matsouka, M. Konsolakis, R.M. Lambert, I.V. Yentekakis, In situ DRIFTS study of the effect of structure (CeO2 –La2 O3 ) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/␥-Al2 O3 catalyst under simulated exhaust conditions, Applied Catalysis B: Environmental 84 (2008) 715–722. [12] R. Barth, R. Pitchai, R.L. Anderson, X.E. Verykios, Thermal desorption-infrared study of carbon monoxide adsorption by alumina-supported platinum, Journal of Catalysis 116 (1989) 61–70. [13] V.B. Mortola, S. Damyanova, D. Zanchet, J.M.C. Bueno, Surface and structural features of Pt/CeO2 –La2 O3 –Al2 O3 catalysts for partial oxidation and steam reforming of methane, Applied Catalysis B: Environmental 107 (2011) 221–236. [14] M.M.V.M. Souza, D.A.G. Aranda, M. Schmal, Reforming of methane with carbon dioxide over Pt/ZrO2 /Al2 O3 catalysts, Journal of Catalysis 204 (2001) 498–511. [15] I. Tankov, B. Pawelec, K. Arishtirova, S. Damyanova, Structure and surface properties of praseodymium modified alumina, Applied Surface Science 258 (2011) 278–284. [16] P. Sulcova, M. Trojan, Study of Ce1−x Prx O2 pigments, Thermochimica Acta 395 (2002) 251–255. [17] G. Lietz, H. Lieske, H. Spindler, W. Hanke, J. Volter, Reactions of platinum in oxygen- and hydrogen-treated Pt␥-Al2 O3 catalysts: II. Ultraviolet–visible studies, sintering of platinum, and soluble platinum, Journal of Catalysis 81 (1983) 17–25. [18] D.S. Brandão, R.M. Galvão, M. Grac¸a, M.C. Rocha, P. Bargiela, E.A. Sales, Pt and Pd catalysts supported on Al2 O3 modified with rare earth oxides in the hydrogenation of tetralin, in the presence of thiophene, Catalysis Today 133–135 (2008) 324–330. [19] S. Damyanova, J.M.C. Bueno, Effect of CeO2 loading on the surface and catalytic behaviors of CeO2 –Al2 O3 -supported Pt catalysts, Applied Catalysis A: General 253 (2003) 135–150. [20] G. Corro, C. Cano, J.L.G. Fierro, A study of Pt–Pd/␥-Al2 O3 catalysts for methane oxidation resistant to deactivation by sulfur poisoning, Journal of Molecular Catalysis A: Chemical 315 (2010) 35–42. [21] M. García-Dieguez, I.S. Pieta, M.C. Herrera, M.A. Larrubia, I. Malpartida, L.J. Alemany, Transient study of the dry reforming of methane over Pt supported on different ␥-Al2 O3 , Catalysis Today 149 (2010) 380–387. [22] G. Jacobs, F.G. Ghadiali, A. Pisanu, A. Borgna, W.E. Alvarez, D.E. Resasco, Characterization of the morphology of Pt clusters incorporated in a KL zeolite by vapor phase and incipient wetness impregnation. Influence of Pt particle morphology on aromatization activity and deactivation, Applied Catalysis 188 (1999) 79–98. [23] R.F. Hicks, Q.-J. Yen, A.T. Bell, Effect of metal-support interactions on the chemisorption of H2 and CO on PdSiO2 and PdLa2 O3 , Journal of Catalysis 89 (1984) 498–510. [24] P.V. Menacherry, G.L. Haller, Electronic effects and effects of particle morphology in n-hexane conversion over zeolite-supported platinum catalysts, Journal of Catalysis 177 (1998) 175–188. [25] W. Schießer, H. Vinek, A. Jentys, Surface species during catalytic reduction of NO by propene studied by in situ IR-spectroscopy over Pt supported on mesoporous Al2 O3 with MCM-41 type structure, Applied Catalysis B: Environmental 31 (2001) 263–274. [26] H. Chen, H. Yu, Y. Tang, M. Pan, G. Yang, F. Peng, H. Wang, J. Yang, Hydrogen production via autothermal reforming of ethanol over noble metal catalysts supported on oxides, Journal of Natural Gas Chemistry 18 (2009) 191–198. [27] B.A. Riguetto, S. Damyanova, G. Gouliev, C.M.P. Marques, L. Petrov, J.M.C. Bueno, Surface behavior of alumina-supported Pt catalysts modified with cerium as
I. Tankov et al. / Applied Surface Science 259 (2012) 831–839
[28]
[29]
[30] [31]
[32]
[33] [34]
revealed by X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy of CO adsorption, Journal of Physical Chemistry B 108 (2004) 5349–5358. G. Zhang, W. Wang, D. Chen, Chemical origin of red shift of C O stretching vibration in acetone complexes with various metal cations, Chemical Physics 359 (2009) 40–44. H. Hanawa, K. Kunimatsu, H. Uchida, M. Watanabe, In situ ATR-FTIR study of bulk CO oxidation on a polycrystalline Pt electrode, Electrochimica Acta 54 (2009) 6276–6285. D. Liu, G.-H. Que, Z.X. Wang, Z.F. Yan, In situ FT-IR study of CO and H2 adsorption on a Pt/Al2 O3 catalyst, Catalysis Today 68 (2001) 155–160. C. Vignatti, M.S. Avila, C.R. Apesteguia, T.F. Garetto, Catalytic and DRIFTS study of the WGS reaction on Pt-based catalysts, International Journal of Hydrogen Energy 35 (2010) 7302–7312. ˜ M.A. Ridao, J.L.G. Fierro, M.C. Alvarez-Galvan, R.M. Navarro, F. Rosa, Y. Briceno, Hydrogen production for fuel cell by oxidative reforming of diesel surrogate: influence of ceria and/or lanthana over the activity of Pt/Al2 O3 catalysts, Fuel 87 (2008) 2502–2511. G. Blyholder, Molecular orbitals view of chemisorbed carbon monoxide, Journal of Physical Chemistry 68 (1964) 2773–2777. S. Bernal, G. Blanco, J.M. Gatica, C. Larese, H. Vidal, Effect of mild Re-oxidation treatments with CO2 on the chemisorption capability of a Pt/CeO2 catalyst reduced at 500 ◦ C, Journal of Catalysis 200 (2001) 411–415.
839
[35] M. Paulis, H. Peyrard, M. Montes, Influence of chlorine on the activity and stability of Pt/Al2 O3 catalysts in the complete oxidation of toluene, Journal of Catalysis 199 (2001) 30–40. [36] A. Holmgren, B. Andersson, D. Duprez, Interactions of CO with Pt/ceria catalysts, Applied Catalysis B: Environmental 22 (1999) 215–230. [37] G.G. Olympiou, C.M. Kalamaras, C.D. Zeinalipour-Yazdi, A.M. Efstathiou, Mechanistic aspects of the water-gas shift reaction on alumina-supported noble metal catalysts: in situ DRIFTS and SSITKA-mass spectrometry studies, Catalysis Today 127 (2007) 304–318. [38] T. Iwasita, F.C. Nart, A. Rodes, E. Pastor, M. Weber, Vibrational spectroscopy at the electrochemical interface, Electrochimica Acta 40 (1995) 53–59. [39] K. Pokrovski, K.T. Jung, A.T. Bell, Investigation of CO and CO2 adsorption on tetragonal and monoclinic Zirconia, Langmuir 17 (2001) 4297–4303. [40] R. Philipp, K. Fujimoto, FTIR spectroscopic study of CO2 adsorption/desorption on MgO/CaO catalysts, Journal of Physical Chemistry 96 (1992) 9035–9038. [41] T. Montanari, R. Matarrese, N. Artioli, G. Busca, FT-IR study of the surface redox states on platinum–potassium–alumina catalysts, Applied Catalysis B: Environmental 105 (2011) 15–23. [42] M.W. Mcquire, C.H. Rochester, In situ FTIR study of CO/H2 reactions over Ru/SiO2 at high pressure and temperature, Journal of Catalysis 141 (1993) 355–367. [43] C.M. Kalamaras, S. Americanou, A.M. Efstathiou, “Redox” vs. “associative formate with OH group regeneration” WGS reaction mechanism on Pt/CeO2 : effect of platinum particle size, Journal of Catalysis 279 (2011) 287–300.
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DRIFTS study of CO adsorption on praseodymium modified Pt/Al2 O3 I. Tankov a , W.H. Cassinelli b , J.M.C. Bueno b , K. Arishtirova a , S. Damyanova a,∗ a b
Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Departamento de Engenharia Quimica, Universidade Federal de São Carlos, C.P. 676, 13565-905 São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 7 February 2012 Received in revised form 7 June 2012 Accepted 25 July 2012 Available online 3 August 2012 Keywords: DRIFT spectroscopy CO adsorption Platinum catalysts Praseodymium oxide
a b s t r a c t The effect of PrO2 content (1–20 wt.%) and temperature pretreatment on the structure and surface properties of PrO2 –Al2 O3 -supported Pt catalysts was studied by XRD, XPS and DRIFTS of carbon monoxide adsorption. XRD analysis showed that platinum particle size decreases with the increase of PrO2 content for samples calcined at high temperature of 1023 K. The intensity and position of the infrared bands were strongly dependent on the praseodymium oxide content and reduction temperature. Two kinds of Pt sites (Pt0 and Pt␦+ ) were recorded in reduced PrO2 -containing samples. A better thermal stability of the Pt CO bond in PrO2 -containing samples compared to Pt/Al2 O3 was observed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The world dependence on petroleum oil has generated interest in the use of natural gas. Reforming processes to transform natural gas into synthesis gas or hydrogen rich synthesis gas have been extensively studied in the recent years. The thermodynamic properties of methane reforming reactions still require a high temperature to obtain high methane conversion. Another problem is the carbon formation on the catalyst surface. The operating conditions require the use of stable and effective catalysts, resistant to coke. Therefore, the investigations should be focused on: (i) the catalyst resistance to coke formation; (ii) the type of supported metal phase and (ii) the kind of the used carrier that improve the catalyst efficiency. Much attention has been focused on supported noble metal (Pt, Pd, Rh) catalysts for methane reforming processes [1,2]. It was found that the most promising support is ␥-Al2 O3 thanks to its high thermal stability and high specific surface area [3]. Previous studies [4–7] have shown that the use of rare earth oxides (La2 O3 or CeO2 ) as carriers or promoters for alumina support leads to the increase of catalyst activity and stability. Praseodymium oxide is another rare earth oxide which could be successfully used as carrier or
∗ 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). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.138
promoter for catalysts in methane reforming processes due to the high oxygen storage capacity (OSC) caused by the presence of the couple ions Pr3+ /Pr4+ [8,9]. For selection of the appropriate reaction conditions, as well as for clarification of the mechanism of methane reforming reactions it is necessary to examine the structure and surface properties of the active metal phase. Fourier transformed infrared (FTIR) spectroscopy is an effective method for characterization of the surface chemistry of heterogeneous catalysts [10,11]. The most frequently used probe molecule is carbon monoxide (CO) [12]. It was found that the CO adsorption depends on the kind of supported metal, its dispersion and oxidation state, as well as on the kind of the carrier. FTIR spectroscopy analysis of platinum supported on CeO2 –La2 O3 –Al2 O3 has demonstrated that the presence of the both promoters (CeO2 and La2 O3 ) leads to a distribution of well dispersed Pt particles with different electron density on the catalyst surface in the reactions of partial oxidation and steam reforming of methane [13]. The study of spent ZrO2 –Al2 O3 and ZrO2 –Al2 O3 -supported Pt catalysts for reforming of methane with CO2 (dry reforming) by FTIR of CO adsorption has shown the suppression of carbon deposition over zirconium-containing samples due to the Pt–Zrn+ interaction [14]. The purpose of the present work is to investigate the effect of praseodymium oxide content and temperature pretreatment on the structure and surface properties of alumina-supported Pt catalysts modified with praseodymium. N2 adsorption–desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier transform
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spectroscopy of CO adsorption (DRIFTS) were used for sample characterization. 2. Experimental 2.1. Sample preparation The Pr6 O11 and PrO2 –Al2 O3 carriers with different PrO2 content (1–20 wt.%) were obtained by a procedure described in Ref. [15]. Platinum catalysts supported on Al2 O3 , Pr6 O11 and PrO2 –Al2 O3 oxides were prepared by wet impregnation of the carriers (6 g) with 100 ml 0.003 M solution of H2 PtCl6 ·6H2 O in ethanol. The mixture was stirred at room temperature for 2 h. The ethanol was evaporated at 298 K under vacuum. The solids were dried at 383 K overnight and calcined at 823 and 1023 K for 2 h. Pt content was about 1 wt.%. The samples were denoted as Pt/xPr–Al, where x is the theoretical PrO2 content. 2.2. Sample characterization Specific surface areas (SBET ) and pore volumes (Vp ) of Pt/xPr–Al samples were evaluated from the N2 adsorption–desorption isotherms, determined at 77 K with a Micromeritics TriStar 3000 apparatus. The samples were previously degassed at 423 K for 24 h under vacuum (10−4 mbar) to ensure a relatively clean surface. The specific areas of the samples were determined according to the standard BET procedure using nitrogen adsorption data taken in the relative equilibrium pressure interval of 0.03 < P/P0 < 0.3. XRD profiles of calcined and reduced samples were registered on 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 between 10◦ and 90◦ . Step size of 0.033 and step scan of 10 s was used to identify the structure of the samples. Phase identification was carried out by comparison with JCPDF database cards. The average particle size of Pt (DXRD ) was calculated using Sherrer’s equation. XPS spectra of the calcined and reduced samples were recorded on Escalab-MkII (VG Scientific) electron spectrometer with an unmonochromatized Mg 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. The binding energy of the C 1s at 284.9 eV was taken as an internal standard. The accuracy of the binding energy (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.
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 cell has a microporous ␣-Al2 O3 microreactor with a K-type thermocouple (Crommel Alummel) for measuring the temperature directly into the sample. The ␣-Al2 O3 micro-reactor is coupled to an array of stainless steel with CaF2 window and connectors for inlet and outlet gas. The spectrum resolution was 4 cm−1 and 64 scans were collected. To execute the experiments, approximately 50 mg of catalyst in powder form was used. The catalysts were reduced in 10% H2 /N2 gas mixture flow from ambient temperature to 303 K at heating rate of 5 K/min and from 303 K to 773 K (or 973 K) at heating rate of 10 K/min. The catalysts remained at 773 K (or 973 K) for 1 h. After reduction, the samples were cooled under gas mixture flow until 673 K. Then the cooling to 298 K was conducted in N2 gas. After stabilizing the cell temperature, CO adsorption was carried with CO pulses at partial pressure (PCO ) of 10 Torr under N2 flow. After each pulse, the spectrum was collected after 5 min in order to achieve equilibrium. The data pulses were necessary to achieve complete saturation of catalytic surface. After the adsorption process, the samples were submitted to temperature-programmed desorption of CO (TPD-CO). In these experiments, the catalysts were submitted to the cycle of heating and cooling in N2 flow to CO desorption. Initially, the samples were heated from ambient temperature to 323 K at a rate of 10 K/min and held at that temperature for an additional time (5 min). After cooling to ambient temperature, the spectra were collected and the samples were submitted to successive cycle of heating at different temperatures and cooling.
3. Results and discussion 3.1. Characterization The values of SBET and Vp of Pt/xPr–Al samples calcined at 823 K are summarized in Table 1. The values of DXRD of the samples calcined at 1023 K are listed in Table 1. The BE of Pt 4d5/2 and Pr 3d5/2 , as well as the atomic XPS ratios of calcined and reduced samples at 823 and 773 K, respectively, are also included in Table 1. The surface area of Pt/1Pr–Al sample is slightly lower than that of nonpromoted alumina with Pr. Decrease of the values of SBET and Vp with increasing the PrO2 content is observed. The sample with the highest PrO2 content (20 wt.%), as well as Pt/Pr6 O11 exhibit the lowest surface area. The results would be connected with some plugging of the pores of alumina with praseodymium oxide species and/or some redispersion during the impregnation procedure of the mixed oxides with Pt salt [16]. XRD patterns of Pt/Al, Pt/xPr–Al and Pt/Pr6 O11 samples calcined at 823 and 1023 K are shown in Fig. 1A and B, respectively.
Table 1 Textural and XPS characteristics of Pt/xPr–Al samples. Sample
SBET (m2 /g)
Vp (m3 /g)
Da (XRD) nm
Binding energy, eV
Pt 4d5/2
Pt/Al Pt/1Pr–Al Pt/6Pr–Al Pt/12Pr–Al Pt/20Pr–Al Pt/Pr6 O11 a
230 226 209 186 139 0.66
After calcination at 1023 K.
0.83 0.82 0.77 0.72 0.62 0.005
24 12 9 8 5 –
XPS ratios
Pr 3d5/2
Pt/Al
Pt/Pr
Pt/Pr + Al
Calc.
Red.
Calc.
Red.
Calc.
Red.
Calc.
Red.
Calc.
Red.
314.7 316.2 316.5 316.1 316.3 317.2
314.1 315.1 315.1 314.8 315.1 316.0
934.0 934.1 934.0 933.9 932.2
– 933.8 933.8 933.9 934.0 933.0
0.0017 0.0020 0.0026 0.0038 0.0127 –
0.0013 0.0009 0.0011 0.0012 0.0012 –
– 1.04 0.32 0.26 0.13 0.08
– 1.08 0.85 0.17 0.05 0.09
– 0.0019 0.0026 0.0036 0.0039 –
– 0.0009 0.0011 0.0012 0.0012 –
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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).
The peaks at 2 = 33.4◦ , 37.6◦ , 39.2◦ , 45.8◦ and 66.8◦ characteristic of the structure of ␥-Al2 O3 (JCPDS 10-425) are appeared in the XRD of alumina-supported samples. XRD of Pt/Pr6 O11 shows patterns at 2 = 28.1◦ , 46.7◦ , 55.6◦ , 75.7◦ and 78.2◦ , related to PrO2 (JCPDS 24-1006) and peaks at 2 = 32.7◦ and 68.5◦ due to Pr2 O3 (JCPDS 22-880). No peaks of praseodymium oxide species are detected in the XRD of Pt/xPr–Al samples. XRD patterns of Pt/Al and PrO2 -containing samples calcined at 823 K did not show peaks, corresponding to metallic platinum and/or platinum oxides (Fig. 1A). This means that the noble metal species are too small and cannot be detected by XRD. However, the calcination of the samples at higher temperature of 1023 K leads to an agglomeration of Pt particles at 2 = 39.7◦ and 81◦ [16], especially, for Pt/Al and Pt/xPr–Al with low PrO2 content (Fig. 1B). According to Lietz et al. [17] the agglomeration of Pt is due to the decomposition of surface [PtIV (OH)x Cly ]s or [PtOx Cly ]s species after calcination at high temperature (>773 K). Addition of a small amount of praseodymium oxide (1 wt.%) to Pt/Al sample causes significantly decrease of the average particle size of Pt (from 24 to 12 nm for Pt/Al and Pt/1Pr–Al, respectively, Table 1). DXRD of Pt decreases with increasing the praseodymium oxide content (to 5 nm for Pt/20Pr–Al, Table 1). Oxide Pt/Pr6 O11 sample did not show any peak assigned to agglomerated Pt (Fig. 1B). It should be noted that with exception of Pt/Al, all PrO2 -containing samples reduced at 773 K, previously calcined at 823 did not show XRD patterns of Pt. It suggests that Pt species are well dispersed on praseodymium modified alumina. These results are in accordance with previous observations [18,19] where the use of ␥-Al2 O3 modified with rare earth oxides such as CeO2 or La2 O3 prevents the strong agglomeration of noble metal due to the formation of a surface complex involving a Pt–O–Ce(La, Pr) interaction. The calcined Pt/Al sample has BE of Pt 4d5/2 core electron levels at 314.7 eV characterizing Pt oxide species [20] (Table 1). It is seen that the addition of Pr to Pt/Al leads to an increase of the values of BEs of PrO2 -modified samples (to 316.1–316.5 eV). The reduction of Pt/Al leads to a decrease of the BE of Pt 4d5/2 to 314.1 eV, which is related to metallic platinum (Pt0 ) [21]. It should be noted that the BEs values of reduced PrO2 -containig samples (314.8–315.1 eV)
are higher compared to that of alumina-supported Pt. The highest BE values are observed for Pt/Pr6 O11 (Table 1). The results suggest a presence of positively charged platinum atoms (Pt␦+ ) caused by the interaction between Pr-modified support and noble metal at high temperature treatment. The changes in the oxidation state of Pt are in agreement with results for lanthanide oxide-containing noble metal catalysts [4–6]. For calcined Pt samples supported on the mixed PrO2 –Al2 O3 oxides the BE of Pr 3d5/2 core level at ca. 934.0 eV can be assigned to Pr4+ [15]. The BEs values of Pr 3d5/2 remain unchanged after reduction (Table 1). Deposition of Pt on Pr6 O11 leads to a significant decrease of the BE values of Pr for calcined sample as well as for reduced one (932.2 and 933.0 eV, respectively) which means a change in the oxidation state of Pr. The higher BEs for Pt/xPr–Al samples compared to those for Pt/Pr6 O11 can be related to the strong interaction between Al and Pr as was observed for mixed PrO2 –Al2 O3 oxides in our previous work [15]. The atomic XPS Pt/Pr + Al ratios for calcined samples increase with increasing the PrO2 content which indicates an enrichment of the support surface by platinum oxide species (Table 1). However, for reduced samples there is an increase of the Pt/Pr + Al ratio up to 12 wt.% and after that it remains unchanged. The higher values of Pt/Pr compared to those of Pt/Al means that Pt is preferentially connected with praseodymium oxide species. All atomic XPS ratios are decreased for reduced samples that would be an indication for some agglomeration or re-dispersion of platinum species. 3.2. Surface properties of Pt/xPr–Al samples Examination of the surface properties of the samples by DRIFTS is carried out in the region 2150–1200 cm−1 . The IR bands at 2150–1750 cm−1 are associated with the adsorbed CO species on Pt sites and/or with adsorbed species the formation of which require the existence of both active metal and supporting material, i.e. initially formed on the metal sites and then migrated to the support [19,22]. These bands can be divided in two regions. The first region at 2150–1900 cm−1 is due to linearly adsorbed CO species [23].
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The second one at 1900–1750 cm−1 is related to bridge bonded or multi-coordinated adsorbed CO [24]. The bands in the region 1750–1200 cm−1 are due to the formation of a large number of different carbonate and/or formate species [25].
3.2.1. DRIFT spectra of CO adsorption in the region 2150–1750 cm−1 The DRIFT spectra of CO adsorbed on Pt/xPr–Al samples previously calcined at 823 K and after that reduced at 773 and 973 K in the region 2150–1750 cm−1 are shown in Fig. 2A and B, respectively. It is observed that the kind of support, as well as the reduction temperature influences the position and intensity of the IR bands. Adsorption of CO on alumina-supported Pt (Fig. 2A) produces a band at 2060 cm−1 with a significant broadening on the low-frequency side, characteristic of CO linearly bonded to surfaceexposed metal Pt atoms (Pt0 ) [26,27]. This indicates that the Pt component is predominantly in its fully reduced form. Changes in the spectrum of platinum deposited on pure praseodymium oxide (Fig. 2A) are revealed. The main band in the spectrum of Pt/Al sample is shifted towards a higher wavenumber of 2070 cm−1 in the spectrum of Pt/Pr6 O11 (blue shift [28]). The 2070 cm−1 band is accompanied by three low-intensive shoulders at 2045, 2027 and 2008 cm−1 assigned to on-top carbonyls on Pt0 species [29–31]. The observed shift as well as the broadening of the IR bands should be connected with: (i) the modified electron density of Pt due to the different nature of Pt–Al and Pt–Pr interactions depending on the support kind and/or (ii) the morphology change of the metal particles on the surface [26,27]. The platinum–praseodymium interaction is electronic, concerning the charge transfer between the noble metal and neighboring praseodymium oxide species being seen in Table 1. As was mentioned above, the BE values of Pt 4d5/2 electrons for reduced PrO2 -containing samples are higher compared to that for Pt/Al that is related to electron deficiency due to a strong interaction between Pt and Pr and formation of platininum species with positive charge, Pt␦+ . These results are similar to the observations for noble metals supported on CeO2 or La2 O3 [4,27]. Considering the highest intensity of 2070 cm−1 band in the spectrum of Pt/Pr6 O11 it can be concluded that platinum component exists mainly as Pt with low electron density (Fig. 2A). The addition of 1 wt.% PrO2 to Pt/Al causes a shift of the main 2060 cm−1 band assigned to linearly bonded Pt0 to 2068 cm−1 and appearance of poorly-resolved shoulder at about 2081 cm−1 in the spectrum of Pt/1Pr–Al sample (Fig. 2A). These observations propose again a change in the electronic state of platinum as was confirmed by the change of BE of Pt 4d5/2 (from 314.1 to 315.1 eV for Pt/Al and Pt/1Pr–Al, respectively, Table 1). In addition, it is well known [10,11] 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 [4,13,32]. Further increase of the praseodymium oxide content (6, 12 and 20 wt.%) in Pt/xPr–Al samples (Fig. 2A) shifts the adsorption bands, corresponding to Pt␦+ and Pt0 , towards lower wavenumbers (from 2081 and 2068 cm−1 to 2076 and 2062 cm−1 , respectively, (redshift [28]). Considering the Blyholder’s model [33] for bonding the CO molecules to metal surface the average number of Pt Pt bonds per platinum atom for small Pt particles is less than that for larger particles and a higher metal electron density is available for the back-donation from 5d electrons of Pt to 2* antibonding orbital of the adsorbed CO molecule. This leads to a decrease of the CO bond strength, shifting the CO stretching frequency to lower values. The presence of metal particles with high electron density is
supported by the appearance of bridge-bonded CO species at 1781 and 1830 cm−1 in the IR region of 1900–1750 cm−1 (Fig. 2A). The observed red-shift could be connected with the presence of smaller Pt particle size. The higher values of Pt/Pr + Al ratios of the samples with PrO2 content ≥6 wt.% compared to that of Pt/1PrAl (Table 1) suggest a high dispersion of Pt. It is clear that the intensity and position of CO adsorption bands for PrO2 -containing samples is influenced by the PrO2 content of support: (i) the band intensity increases with rising the praseodymium oxide content up to 12 wt.% and (ii) the intensity of high-frequency shoulder at ca. 2076 cm−1 compared to that of the main band at 2062 cm−1 is the highest for Pt/20Pr–Al sample (Fig. 2A). These observations can be connected with the presence of metal particles with different electron density. However, the surface sites for CO adsorption over Pt/Pr–Al samples with PrO2 content of 6, 12 and 20 wt.% are similar, since the position of the bands is practically the same (Fig. 1A). The lower bands intensity of Pt/20Pr–Al as well as of Pt/Pr6 O11 compared to those of supported Pt samples with PrO2 content of 6–12 wt.% can be related to a decoration phenomenon caused by the strong metal-support interaction (SMSI) [27,34]. The latter leads to a decrease of the number of exposed Pt sites for CO adsorption which can be supported by the unchanged value of atomic XPS Pt/Pr + Al ratio at 20 wt.% PrO2 (Table 1). Similar to CeO2 - and La2 O3 supported samples [7,13,27] the electronic effect, produced by the SMSI between Pt and Pr oxide species under reduction conditions leads to the formation of reduced support species (PrOx ), which migrate onto the Pt particles, causing a loss of the chemisorption capacity of metal. On the other hand, it cannot be excluded some agglomeration of Pt in Pt/Pr6 O11 under reductive atmosphere due to the low surface area of support in spite of the absence of XRD patterns. Submitting the samples Pt/6Pr–Al and Pt/20/Pr–Al previously calcined at 823 K to a higher reduction temperature of 973 K leads to a shift of the main band to higher frequency (from 2062 to 2068 cm−1 at 773 and 973 K, respectively, Fig. 2B), which could be connected with the change of the shape of Pt particles. In addition, the high-frequency shoulder for PrO2 -containing samples is also shifted to higher value (from 2076 to 2082 cm−1 at 773 and 973 K, respectively) due to the presence of more coordinatively unsaturated Pt, caused by the more intimate contact between Pt- and Pr-modified support at higher reduction temperature. Comparing the spectra of the samples reduced at both temperatures it can be concluded that the increase of reduction temperature up to 973 K leads to a more uniform electron distribution of Pt, revealed by the narrowness of the bands (Fig. 2B). 3.2.2. DRIFT spectra of CO adsorption in the region 1750–1200 cm−1 The DRIFT spectra of CO adsorption on Pt/xPr–Al samples in the 1750–1200 cm−1 region, previously reduced at 773 and 973 K, are shown in Fig. 2C and D, respectively. It was reported [35,36] that the IR bands in this region are mainly due to the formation of a large number of different carbonate species on the catalyst surface. It is well known [30] that the formate species (HCOO− ) also give adsorption bands at 1700–1200 cm−1 , which seriously hinders the unequivocal identification of the surface carbonate species. It was found [37] that the formate species can be formed at enhanced adsorption temperatures (500 K) at the periphery of platinum–alumina by spillover of adsorbed CO molecules from Pt to OH groups of alumina support. In the present work the CO adsorption on Pt/xPr–Al samples is carried out at room temperature, so the presence of formate species should not be expected. Moreover, the appearance of IR bands in the region 3100–2700 cm−1 (spectra not shown), corresponding to C–H vibrations of formates is not revealed. Based on these observations we can conclude that the
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Fig. 2. DRIFT spectra of CO adsorption in the 2150–1750 cm−1 and 1750–1200 cm−1 regions for Pt/xPr–Al samples reduced at 773 K (A and C, respectively) and 973 K (B and D, respectively).
presence of formate species on the surface of Pt/xPr–Al samples can be excluded. The IR spectra of Pt/Al and Pt/xPr–Al samples with PrO2 content up to 12 wt.% at 773 K exhibit a strong band centered at 1648 cm−1 and weaker ones at 1593, 1441, 1345 and 1229 cm−1 (Fig. 2C). The existence of bands in the frequency region 1600–1300 cm−1 can be taken as an indication of the geometry change of the adsorbed carbonate species (CO3 2− ). According to Ref. [38], these species do not lie flat on the surface but are oriented perpendicular to it. The adsorption band at 1593 cm−1 is due to the formation of bidentate carbonate species (b-CO3 2− ) [27], whereas the bands at 1441 and 1345 cm−1 are assigned to the monodentate carbonates species (mCO3 2− ) [36]. The formation of surface carbonate complexes involves adsorption of CO molecules on coordinatively unsaturated Lewis acid–base pair sites (Prn+ –O2− ), similar to the results reported for CO adsorption on ZrO2 [39]. The adsorption bands at 1648 and 1229 cm−1 are related to the presence of asymmetric bicarbonate species (HCO3 − ) [35]. It should be noted that in the IR spectrum of Pt/20Pr–Al sample the strong band at 1648 cm−1 now is appeared at lower wavenumber, 1629 cm−1 . The latter is assigned to symmetric stretching mode of O C O of surface bicarbonate [40]. In addition, a band at 1529 cm−1
due to b-CO3 2− is also observed. Considering the literature data [40] the formation of bicarbonates includes hydrogen bond between the protons in the Brönsted acid–base centers (OH groups) of alumina with an O atom originating in the carbonates already existing on the PrO2 surface. It should be mentioned that the amount of adsorbed carbonate and bicarbonate species is not much pronounced, which is represented by the low intensity of their bands. The IR bands in the spectra of Pt/Al, Pt/6Pr–Al and Pt/20Pr–Al samples become sharper and more intensive after reduction at higher temperature of 973 K (Fig. 2D). It could be suggested that the praseodymium oxide undergoes some redispersion, which leads to the formation of more uniform particles distribution during the high temperature pretreatment. It is seen that the intensity of the IR bands related to the bicarbonate and carbonates species at both reduction temperatures increases with increasing the praseodymium oxide content, which would be assigned to the formation of a large number of centers for CO adsorption in PrO2 -rich samples (Fig. 2C and D). 3.2.3. DRIFT spectra of CO desorption The CO desorption profiles in the 2150–1750 cm−1 region of Pt/Al and Pt/xPr–Al samples with different PrO2 content, reduced at
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Fig. 3. DRIFT spectra of CO desorption in the 2150–1750 cm−1 region for Pt/xPr–Al samples reduced at 773 K.
773 K, are shown in Fig. 3. It is clear from the figures that the intensity and position of the IR bands are changed with the increase of desorption temperature. There is a decrease of the total integrated band intensity due to a gradually decrease of the amount of
adsorbed CO molecules over Pt sites. The IR spectrum of aluminasupported Pt sample (Fig. 3) shows that the peak at ca. 2060 cm−1 with a significant broadening on the low-frequency side, previously correlated to CO molecules linearly adsorbed on Pt particles with
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Fig. 4. DRIFT spectra of CO desorption in the 1750–1200 cm−1 region for Pt/xPr–Al samples reduced at 773 K.
a high electron density, is shifted to ca. 2040 cm−1 with increase of the temperature up to 573 K. For PrO2 -containing samples the main band at 2068–2060 cm−1 , characterizing the CO adsorption on Pt0 , and the shoulder at ca. 2076 cm−1 assigned to platinum in a
close contact with the rare earth oxide component, Pt␦+ , (Fig. 3) are shifted to 2050–2030 cm−1 and to ca. 2064 cm−1 , respectively, with increasing the temperature up to 573 K. The band shifting would be explained with: (i) disappearance of the dipole–dipole coupling
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effect between neighboring CO molecules during the desorption process [41] and (ii) occurrence of the process CO(ad) → C(ad) + O(ad) [42]. The latter means that increasing the desorption temperature leads to the increase of the concentration of adsorbed carbon and formation of a strong Pt C bond. The DRIFTS results demonstrate that a complete CO desorption for the samples with PrO2 content up to 12 wt.% occurs at the same temperature, above 573 K. However, comparing the desorption spectra of these samples with those of Pt/20Pr–Al it can be seen that a complete CO desorption for the last one takes place at higher temperature, above 623 K. This is an indication that the Pt CO bond in the highest PrO2 -loaded sample possesses better thermal stability. The latter is related to the change of the electronic state of platinum species due to the Pt–Pr interaction, which results in a considerably higher dispersion of the noble metal. It was found [43] that the high-temperature desorption state of CO is larger in population on the smaller Pt particles than on the larger ones. Therefore, the small Pt particle size can enhance the charge transfer from the metal d orbital to antibonding 2* orbital of CO molecule, which is revealed by the formation of a strong Pt CO bond for Pt/20Pr–Al sample. There was no difference in the complete CO desorption temperature for the sample previously reduced at 973 K. The CO desorption profiles in the 1750–1200 cm−1 region for Pt/Al and Pt/xPr–Al samples with different PrO2 content, reduced at 773 K, are shown in Fig. 4. It is observed that the intensity and the shape of the IR bands of carbonate species depend on PrO2 content and temperature of reduction. It can be summarized that the HCO3 − species are first decomposed to CO2 (at 523 K) followed by decomposition of b-CO3 2− and m-CO3 2− species at higher temperature of 573 K. It means that the thermal stability of carbonates species is greater than that of bicarbonate ones. The latter could be assigned to the stronger Lewis acidity of Prn+ cations and to the stronger Lewis basicity of O2− anions of coordinatively unsaturated Prn+ –O2− sites over the surface of Pt/xPr–Al samples. The highest thermal stability of carbonate species is observed for Pt/Pr–Al sample with the highest PrO2 content (at 623 K, Fig. 4). 4. Conclusions The spectral characterization of Pt supported on Al2 O3 , Pr6 O11 and mixed PrO2 –Al2 O3 oxides with different PrO2 content (1–20 wt.%) show the influence of the type of support and temperature treatment on the surface state of Pt. Pt is well dispersed over the surface of all samples calcined at 823 K, whereas increasing the calcination temperature up to 1023 K leads to agglomeration of Pt. Two kinds of Pt sites are detected after reduction of the samples at 773 and 973 K by DRIFTS of CO adsorption: platinum sites with high electron density (Pt0 ) and platinum with some positive character (Pt␦+ ) due to the more intimate contact between Pt and Pr, revealed also by XPS. Acknowledgements The authors acknowledge financial support by project No. DTK-02/36 from National Science Fund at Bulgarian Ministry of Education, Youth and Science and FAPESP. References [1] L.V. Mattos, E. Rodino, D.E. Resasco, F.B. Passos, F.B. Noronha, Partial oxidation and CO2 reforming of methane on Pt/Al2 O3 , Pt/ZrO2 , and Pt/Ce-ZrO2 catalysts, Fuel Processing Technology 83 (2003) 147–161. [2] L.S.F. Feio, C.E. Hori, S. Damyanova, F.B. Noronha, W.H. Cassinelli, C.M.P. Marques, J.M.C. Bueno, The effect of ceria content on the properties of Pd/CeO2 /Al2 O3 catalysts for steam reforming of methane, Applied Catalysis A: General 316 (2007) 107–116.
[3] J. Xu, W. Zhou, Z. Li, J. Wang, J. Ma, Biogas reforming for hydrogen production over nickel and cobalt bimetallic catalysts, International Journal of Hydrogen Energy 34 (2009) 6646–6654. [4] R.M. Navarro, M.C. Alvarez-Galvan, M.C. Sanchez-Sanchez, F. Rosa, J.L.G. Fierro, Production of hydrogen by oxidative reforming of ethanol over Pt catalysts supported on Al2 O3 modified with Ce and La, Applied Catalysis B: Environmental 55 (2005) 229–241. [5] A.C.S.F. Santos, S. Damyanova, G.N.R. Teixeira, L.V. Mattos, F.B. Noronha, F.B. Passos, J.M.C. Bueno, The effect of ceria content on the performance of Pt/CeO2 /Al2 O3 catalysts in the partial oxidation of methane, Applied Catalysis A: General 290 (2005) 123–132. [6] Y. Nagai, T. Hirabayashi, K. Dohmae, N. Takagi, T. Minami, H. Shinjoh, S. Matsumoto, Sintering inhibition mechanism of platinum supported on ceriabased oxide and Pt-oxide-support interaction, Journal of Catalysis 242 (2006) 103–109. [7] J.C.S. Araujo, D. Zanchet, R. Rinaldi, U. Schuchardt, C.E. Hori, J.L.G. Fierro, J.M.C. Bueno, The effects of La2 O3 on the structural properties of La2 O3 –Al2 O3 prepared by the sol–gel method and on the catalytic performance of Pt/La2 O3 –Al2 O3 towards steam reforming and partial oxidation of methane, Applied Catalysis B: Environmental 84 (2008) 552–562. [8] K. Asami, K. Kusakabe, N. Ashi, Y. Ohtsuka, Synthesis of ethane and ethylene from methane and carbon dioxide over praseodymium oxide catalysts, Applied Catalysis A: General 156 (1997) 43–56. [9] S. Bernal, G. Blanco, J.J. Calvino, J.A.P. Omil, J.M. Pintado, Some major aspects of the chemical behavior of rare earth oxides: an overview, Journal of Alloys and Compounds 408–412 (2006) 496–502. [10] J. Mikulova, S. Rossignol, J. Barbier Jr., D. Duprez, C. Kappenstein, Characterizations of platinum catalysts supported on Ce, Zr, Pr-oxides and formation of carbonate species in catalytic wet air oxidation of acetic acid, Catalysis Today 124 (2007) 185–190. [11] V. Matsouka, M. Konsolakis, R.M. Lambert, I.V. Yentekakis, In situ DRIFTS study of the effect of structure (CeO2 –La2 O3 ) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/␥-Al2 O3 catalyst under simulated exhaust conditions, Applied Catalysis B: Environmental 84 (2008) 715–722. [12] R. Barth, R. Pitchai, R.L. Anderson, X.E. Verykios, Thermal desorption-infrared study of carbon monoxide adsorption by alumina-supported platinum, Journal of Catalysis 116 (1989) 61–70. [13] V.B. Mortola, S. Damyanova, D. Zanchet, J.M.C. Bueno, Surface and structural features of Pt/CeO2 –La2 O3 –Al2 O3 catalysts for partial oxidation and steam reforming of methane, Applied Catalysis B: Environmental 107 (2011) 221–236. [14] M.M.V.M. Souza, D.A.G. Aranda, M. Schmal, Reforming of methane with carbon dioxide over Pt/ZrO2 /Al2 O3 catalysts, Journal of Catalysis 204 (2001) 498–511. [15] I. Tankov, B. Pawelec, K. Arishtirova, S. Damyanova, Structure and surface properties of praseodymium modified alumina, Applied Surface Science 258 (2011) 278–284. [16] P. Sulcova, M. Trojan, Study of Ce1−x Prx O2 pigments, Thermochimica Acta 395 (2002) 251–255. [17] G. Lietz, H. Lieske, H. Spindler, W. Hanke, J. Volter, Reactions of platinum in oxygen- and hydrogen-treated Pt␥-Al2 O3 catalysts: II. Ultraviolet–visible studies, sintering of platinum, and soluble platinum, Journal of Catalysis 81 (1983) 17–25. [18] D.S. Brandão, R.M. Galvão, M. Grac¸a, M.C. Rocha, P. Bargiela, E.A. Sales, Pt and Pd catalysts supported on Al2 O3 modified with rare earth oxides in the hydrogenation of tetralin, in the presence of thiophene, Catalysis Today 133–135 (2008) 324–330. [19] S. Damyanova, J.M.C. Bueno, Effect of CeO2 loading on the surface and catalytic behaviors of CeO2 –Al2 O3 -supported Pt catalysts, Applied Catalysis A: General 253 (2003) 135–150. [20] G. Corro, C. Cano, J.L.G. Fierro, A study of Pt–Pd/␥-Al2 O3 catalysts for methane oxidation resistant to deactivation by sulfur poisoning, Journal of Molecular Catalysis A: Chemical 315 (2010) 35–42. [21] M. García-Dieguez, I.S. Pieta, M.C. Herrera, M.A. Larrubia, I. Malpartida, L.J. Alemany, Transient study of the dry reforming of methane over Pt supported on different ␥-Al2 O3 , Catalysis Today 149 (2010) 380–387. [22] G. Jacobs, F.G. Ghadiali, A. Pisanu, A. Borgna, W.E. Alvarez, D.E. Resasco, Characterization of the morphology of Pt clusters incorporated in a KL zeolite by vapor phase and incipient wetness impregnation. Influence of Pt particle morphology on aromatization activity and deactivation, Applied Catalysis 188 (1999) 79–98. [23] R.F. Hicks, Q.-J. Yen, A.T. Bell, Effect of metal-support interactions on the chemisorption of H2 and CO on PdSiO2 and PdLa2 O3 , Journal of Catalysis 89 (1984) 498–510. [24] P.V. Menacherry, G.L. Haller, Electronic effects and effects of particle morphology in n-hexane conversion over zeolite-supported platinum catalysts, Journal of Catalysis 177 (1998) 175–188. [25] W. Schießer, H. Vinek, A. Jentys, Surface species during catalytic reduction of NO by propene studied by in situ IR-spectroscopy over Pt supported on mesoporous Al2 O3 with MCM-41 type structure, Applied Catalysis B: Environmental 31 (2001) 263–274. [26] H. Chen, H. Yu, Y. Tang, M. Pan, G. Yang, F. Peng, H. Wang, J. Yang, Hydrogen production via autothermal reforming of ethanol over noble metal catalysts supported on oxides, Journal of Natural Gas Chemistry 18 (2009) 191–198. [27] B.A. Riguetto, S. Damyanova, G. Gouliev, C.M.P. Marques, L. Petrov, J.M.C. Bueno, Surface behavior of alumina-supported Pt catalysts modified with cerium as
I. Tankov et al. / Applied Surface Science 259 (2012) 831–839
[28]
[29]
[30] [31]
[32]
[33] [34]
revealed by X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy of CO adsorption, Journal of Physical Chemistry B 108 (2004) 5349–5358. G. Zhang, W. Wang, D. Chen, Chemical origin of red shift of C O stretching vibration in acetone complexes with various metal cations, Chemical Physics 359 (2009) 40–44. H. Hanawa, K. Kunimatsu, H. Uchida, M. Watanabe, In situ ATR-FTIR study of bulk CO oxidation on a polycrystalline Pt electrode, Electrochimica Acta 54 (2009) 6276–6285. D. Liu, G.-H. Que, Z.X. Wang, Z.F. Yan, In situ FT-IR study of CO and H2 adsorption on a Pt/Al2 O3 catalyst, Catalysis Today 68 (2001) 155–160. C. Vignatti, M.S. Avila, C.R. Apesteguia, T.F. Garetto, Catalytic and DRIFTS study of the WGS reaction on Pt-based catalysts, International Journal of Hydrogen Energy 35 (2010) 7302–7312. ˜ M.A. Ridao, J.L.G. Fierro, M.C. Alvarez-Galvan, R.M. Navarro, F. Rosa, Y. Briceno, Hydrogen production for fuel cell by oxidative reforming of diesel surrogate: influence of ceria and/or lanthana over the activity of Pt/Al2 O3 catalysts, Fuel 87 (2008) 2502–2511. G. Blyholder, Molecular orbitals view of chemisorbed carbon monoxide, Journal of Physical Chemistry 68 (1964) 2773–2777. S. Bernal, G. Blanco, J.M. Gatica, C. Larese, H. Vidal, Effect of mild Re-oxidation treatments with CO2 on the chemisorption capability of a Pt/CeO2 catalyst reduced at 500 ◦ C, Journal of Catalysis 200 (2001) 411–415.
839
[35] M. Paulis, H. Peyrard, M. Montes, Influence of chlorine on the activity and stability of Pt/Al2 O3 catalysts in the complete oxidation of toluene, Journal of Catalysis 199 (2001) 30–40. [36] A. Holmgren, B. Andersson, D. Duprez, Interactions of CO with Pt/ceria catalysts, Applied Catalysis B: Environmental 22 (1999) 215–230. [37] G.G. Olympiou, C.M. Kalamaras, C.D. Zeinalipour-Yazdi, A.M. Efstathiou, Mechanistic aspects of the water-gas shift reaction on alumina-supported noble metal catalysts: in situ DRIFTS and SSITKA-mass spectrometry studies, Catalysis Today 127 (2007) 304–318. [38] T. Iwasita, F.C. Nart, A. Rodes, E. Pastor, M. Weber, Vibrational spectroscopy at the electrochemical interface, Electrochimica Acta 40 (1995) 53–59. [39] K. Pokrovski, K.T. Jung, A.T. Bell, Investigation of CO and CO2 adsorption on tetragonal and monoclinic Zirconia, Langmuir 17 (2001) 4297–4303. [40] R. Philipp, K. Fujimoto, FTIR spectroscopic study of CO2 adsorption/desorption on MgO/CaO catalysts, Journal of Physical Chemistry 96 (1992) 9035–9038. [41] T. Montanari, R. Matarrese, N. Artioli, G. Busca, FT-IR study of the surface redox states on platinum–potassium–alumina catalysts, Applied Catalysis B: Environmental 105 (2011) 15–23. [42] M.W. Mcquire, C.H. Rochester, In situ FTIR study of CO/H2 reactions over Ru/SiO2 at high pressure and temperature, Journal of Catalysis 141 (1993) 355–367. [43] C.M. Kalamaras, S. Americanou, A.M. Efstathiou, “Redox” vs. “associative formate with OH group regeneration” WGS reaction mechanism on Pt/CeO2 : effect of platinum particle size, Journal of Catalysis 279 (2011) 287–300.