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Al2O3 nanosystem with metal-oxide active component
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 603 (2009) 108–110
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
XAFS study of Pt/Al2O3 nanosystem with metal-oxide active component I.E. Beck, V.V. Kriventsov , D.P. Ivanov, V.I. Zaikovsky, V.I. Bukhtiyarov Boreskov Institute of Catalysis SB RAS (BIC SB RAS), Pr. Akademika Lavrentyeva 5, 630090 Novosibirsk, Russia
a r t i c l e in f o
a b s t r a c t
Available online 3 January 2009
A study of monodisperse Pt/g-Al2O3 catalysts was carried by XAFS. Key factors of the particle size control were the composition of the precursor solutions and the pretreatment of the carrier. XAFS revealed that variation of preparation methods caused formation of three types of Pt particles (metal, oxide, metal-oxide) located on the g-Al2O3 surface directly affecting the catalytic activity in CH4 complete oxidation. A method of reliable estimation of the phase composition of the active component is developed taking into account the nano-size effects and Pt oxidation state. & 2009 Elsevier B.V. All rights reserved.
Keywords: EXAFS Nanoparticle Catalytic system Platinum oxide Local structure XANES
1. Introduction Recently, a great attention was focused on a new way of preparation of nano-structured catalysts with controlled sizes of active component nanoparticles supported on an oxide carrier for studying nano-size effects in complete oxidation reactions. The key factors for the particle size control were the composition of the original precursor solutions and the pretreatment of the carrier. These methods were shown not to affect the textural properties of the carrier. Obviously, development of methods for controlling both the particle size and active component structure along with elucidation of key factor contributions to the activity and stability of the catalysts allows to obtain high-effective catalysts and to diminish the noble metal loading as well. Structural studies present valuable information to achieve these aims, especially XAFS as a powerful tool to study the local structure and state of the supported nanoparticles of noble metals. Earlier, the catalytic activity of the size-controlled Pt nanoparticles with a narrow particle size distribution supported on g-Al2O3 was tested in CH4 complete oxidation [1]. The reaction studied was shown to be strongly size sensitive. This work is devoted to a structural study of the most active in CH4 complete oxidation Pt/Al2O3 nanosystem by XAFS.
2. Experiment Monodisperse 1%Pt/g-Al2O3 catalysts were prepared by incipient wetness impregnation of the dehydrated carrier (Sasol TKA-432) or the same after acid pretreatment. Aqueous solutions of oligomeric m-hydroxo the Pt4+complexes with different acidity Corresponding author. Tel.: +7 383 329 40 13; fax: +7 383 330 30 56.
E-mail address: [email protected] (V.V. Kriventsov). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.12.170
were used as active component precursors [1]. Freshly impregnated samples were dried and calcined in air. Then initial samples and ones after 2 h-aging (430 1C, 1%CH4 in air) or 3 h-reduction (300 1C, H2 flow) were studied by XAFS and TEM. All TEM measurements were performed on a JEM-2010 (200 kV, line resolution 0.14 nm). The size distribution of the Pt particles was determined by measuring 300 particles for each sample. XAFS (Pt-L3 edge) transmission spectra for all the samples studied were measured (EXAFS/XANES step 1.5/0.3 eV) and treated by the standard way [2,3] at SSRC, Novosibirsk. The radial distribution function (RDF) was calculated from the EXAFS spectra in k3w(k) as the modulus of Fourier transform at the k-intervals 3.5–12.5 A˚ 1. Curve fitting with EXCURV92 [4] was used to determine the distances—R and coordination numbers—CN using known XRD data. The Debye–Waller factors were fixed: 2s2=0.011–0.013 A˚2.
3. Results and discussion The normalized XANES spectra (Pt-L3 edge, *-white line) and RDFs describing the Pt local arrangements for the catalysts and reference samples are shown on Figs. 1 and 2, respectively. EXAFS data (distances—R and corresponding coordination numbers— CN) of the Pt local arrangement and TEM data for the studied samples are presented in Table 1. The typical feature, named as the white line [5], is observed on XANES (Pt-L3) spectra of all the samples studied (Fig. 1a–j), the height of white line amplitude being directly associated with the oxidic phase presence [6]. Indeed, it should be noted that in the analysed samples, from the Pt nitrate precursor solution (a) to the Pt-foil (j), a pronounced tendency is observed. In addition to decrease of the amplitudes of the white line in the series, some shift of the edge position of spectra to region of smaller energy
ARTICLE IN PRESS I.E. Beck et al. / Nuclear Instruments and Methods in Physics Research A 603 (2009) 108–110
Fig. 1. XANES spectra (Pt-L3, *-white line) of the (a–j) samples: Pt4+ nitrate solution, n1, n2, n3, n3-aged, n4, n5, n6, n5-red, Pt-foil.
Fig. 2. RDFs describing of Pt local arrangement of the (a–j) samples: Pt4+ nitrate solution, n1, n2, n3, n3-aged, n4, n5, n6, n5-red, Pt-foil.
109
values was found. The maximum shift value within few eV was established for the references (a,j). One could assume that for both the precursor solution (a) and for the n1 (b) and n2 (c) samples, the contribution of the oxidic phase is close to 100%. Indeed, both the highest amplitudes of the white line and the conditions of samples preparation (impregnation of un-pretreated carrier and mild oxidative calcination) provides a basis for this conclusion. For all other samples it is clearly observed (Fig. 1d–i) that the proportion of the oxidic phase seems to diminish in the series from (d) to (i) samples and in the last one the minimum was reached. On the RDFs of the Pt4+–nitrate precursor solution (Fig. 2a), several peaks are present. The first intense peak is attributed to the Pt–O distance 2.0 A˚ [7,11]. The set of fitting values is: RPt O=2.0 A˚, CN6.0, so practically non-distorted octahedral oxygen nearest surrounding is realised in this case. Unambiguously the next peaks within the region 2.3–4.0 A˚ having lower amplitudes can be attributed to several greater Pt–O–Pt distances, corresponding to the fitting values 3.05 and 3.6–3.8 A˚, respectively. The same few remarkable peaks are observed on the RDFs for the n1 and n2 samples (Fig. 2b and c) prepared on an un-pretreated carrier which seems to contain the most oxidized platinum among supported catalysts under study. The first high intensive peak is attributed to the Pt–O distance, corresponding to the fitting values RPt O=2.0 A˚, CN5.9, the second low intensive and slightly spread peak is attributed to the Pt–O–Pt distance, corresponding to the fitting value 3.10 A˚. The results obtained do not contradict to the known XRD data for Pt-oxide systems [7,11]. All the other supported catalysts under study, prepared on the acid-pretreated carrier, contained significant amount of the metallic platinum along with the oxidic one. The shape of the RDF (Fig. 2j) for the reference Pt-foil is typical for the fcc structure, because a pronounced set of metallic Pt–Pt peaks was observed. The main peak corresponds to the shortest Pt–Pt distance (RPt Pt=2.77 A˚, CN=12) [8], fitting gives a very close values (RPt Pt2.75 A˚, CN12.1) for this first peak. It should be noted that presence of similar main peaks (corresponding to the shortest first Pt–O (2.0 A˚) or/and shortest first Pt–Pt (2.75 A˚) distances, which are typical for oxidic and metallic phases) on the RDFs (Fig. 2b–i) was revealed for all the supported samples. However, the amplitudes and shapes of these peaks as well as their proportion strongly differ for various samples, seemingly owing to differences in phase composition. A procedure of rough estimation of the phase composition for these samples (Table 1) was developed. First, to estimate the metallic phase amount (%Pt0) in the sample studied, the ratio of the CN value calculated from fitting for the first shortest Pt–Pt distance to typical metallic CN 12 was considered. Second, to estimate the oxidic phase amount (%Ptox) in the sample studied, the ratio of the CN value calculated from fitting for the first shortest Pt–O distance to CN 6 typical for the pure PtO2 phase was considered. Then verification of this approach was carried out via comparison of the total calculated phase content Ptox+Pt0 with 100% concerning that the deviation should not exceed 10% from the method limitations. As seen from Table 1, these deviations do not exceed 10% for the most part of the samples studied, thus a direct application of the proposed approach is quite valid. The analysis of possible reasons for some discrepancy of the estimated phase composition reveals deviations 410% only for the high dispersed metal-containing samples n3 and n5 (Table 1, TEM data). It is well-known from literature [9], that for small nanoparticles having the noble metal fcc structure with small particle sizes 30 A˚, CN corresponding to the first shortest Pt–Pt distance is about 9 due to great surface atoms contribution in the of surface/bulk ratio. This value strongly differs from 12, that is typical for bulk metal. Calculation of the phase composition
ARTICLE IN PRESS 110
I.E. Beck et al. / Nuclear Instruments and Methods in Physics Research A 603 (2009) 108–110
Table 1 EXAFS and TEM data of the samples prepared via impregnation. Pt–Pt
Pt–O
Calculated phase composition
Size correction
Sample
d (nm) (TEM) R, A˚
CN
R, A˚
CN
%Ptox, PtO2
%Pt0
100% (Ptox+Pt0)%
n5-red
– – – 2.73 2.73 2.73 2.74 2.73 2.72
– – – 2.6 2.9 6.8 5.6 7.6 3.8
1.99 1.99 1.99 1.99 1.99 1.98 1.99 1.98 1.99
6.1 5.9 5.6 4.4 3.4 2.9 2.1 1.6 1.5
– – – 22 24 56.5 47 63 32
0 0 5 3 18 5 17 10 43
– 0.9 1.9 4.8 2.6 6.3 2.8 3.7 2.4
Pt-foil
2.75
12.1
–
–
100 100 95 75 58 48.5 36 27 25 37.5(PtO) 0
100
0
–
4+
Pt nitrate solution n1a n2a n3b n3-agedc n4b n5b n6b d
%Pt0
CN
32
9
62
9
42 63
9 6 12
a
un-pretreated carrier. acid-pretreated carrier. n3-2 h-aging, 430 1C, 1%CH4 in air. d n5-3 h-reduction, 300 1C, H2 flow. b c
taking into account the CN 9 instead of CN 12 for metallic phase gives in some cases a more reliable estimation presented in Table 1 (the penultimate column). Note that, taking into account the nano-sized effect, the calculated total sum of values (Ptox+Pt0)% is about 90% (58+32) for the n3-aged sample, 98% (36+62) for the n5 sample, but only 67% (25+42) for the n5-red sample. Evidently, for the two samples n3-aged and n5, correction for the nano-sized effect yielded more reliable estimations of the phase composition. However, the situation with the third preliminary reduced sample n5-red seems to be more complicated, because the obtained calculated sum is greatly inconsistent with the expected total value 100% even taking into account a rather smaller value of CN 6 for the metallic phase. In this case, the recalculated total sum of values (Ptox+Pt0)% is about 88% (25+63) for the n5-red sample. Obviously, the ambiguousness of the situation concerning this sample is determined not only by contribution of nano-sized effects in the metallic phase, but also by underestimation of the oxidic phase amount. Indeed, this sample was reduced in H2 flow and as a result very small nano-sized Pt0 particles were obtained. However, the Pt0 particles having great surface area were known [10] to be oxidized into PtO even at room temperature, but for transformation into PtO2 more harsh conditions are required. So in this case for the preliminary reduced sample contacting with air it is reasonable to calculate the oxidic phase amount taking into account the typical for the PtO oxide CN4 for the first shortest Pt–O distance, that is in a good agreement to XRD data [11]. In this case, the calculated value of oxidic phase amount for n5red sample is about 37.5%) and the corrected total sum of values (Ptox+Pt0)% is about 100.5% (37.5+63). It should be noted that this obtained estimation of the phase composition (calculated taking into account both nano-size effects (CNPt Pt=6) and Pt2+ oxidation state (CNPt O=4)) is very close to the expected value
100% and agrees well with the data of other methods without any contradictions.
4. Conclusion The local structures of nano-structured Pt/g-Al2O3 catalysts very active in CH4 complete oxidation were studied by XAFS. All possible structural models were discussed in detail. These catalysts were shown to be complex polyphase metal-oxide systems supported on the Al2O3 surface. A method for reliable estimation of the phase composition in the complex metal-oxide phase is developed, taking into account both nano-size effects and changes of Pt oxidation state.
Acknowledgments This research was supported by Russian Science and Innovation Agency (Contract no. 02.513.11.3203), RFBR06-03-33005a, RFBR-08-03-01150a, RFBR08-03-01016, RFBR06-03-32578a, RFBRCNRS08-03-92502a, RFBR-09-03-01012a, RFBR06-03-08173 Grants. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
I. E. Beck, et al., ISRHHC-13 Abstracts, USA, California, 16 July 2007, 91. D.I. Kochubey, EXAFS Spectroscopy of Catalysts, Nauka, Novosibirsk, 1992. K.V. Klementiev, VIPER freeware: www.desy.de/~klmn. N. Binsted, J. Campbell, et al., EXCURV92, Daresb. Laboratory, UK. J.H. Sinfelt, G.D. Meitzner, Accoun. Chem. Res. 26 (1) (1993) 16. H. Yoshida, S. Nonoyama, et al., Phys. Scr. T 115 (2005) 813. ICSD Codes: 24922, 24923-a-PtO2; 202407, 30443, 24925-b-PtO2. ICSD Codes: 76153-Pt0. R.E. Benfield, J. Chem. Soc. Faraday Trans. 88 (8) (1992) 1107. C.P. Hwang, C.-T. Yeh, J. Catal. 182 (1999) 48. ICSD Codes: 26599-PtO.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
XAFS study of Pt/Al2O3 nanosystem with metal-oxide active component I.E. Beck, V.V. Kriventsov , D.P. Ivanov, V.I. Zaikovsky, V.I. Bukhtiyarov Boreskov Institute of Catalysis SB RAS (BIC SB RAS), Pr. Akademika Lavrentyeva 5, 630090 Novosibirsk, Russia
a r t i c l e in f o
a b s t r a c t
Available online 3 January 2009
A study of monodisperse Pt/g-Al2O3 catalysts was carried by XAFS. Key factors of the particle size control were the composition of the precursor solutions and the pretreatment of the carrier. XAFS revealed that variation of preparation methods caused formation of three types of Pt particles (metal, oxide, metal-oxide) located on the g-Al2O3 surface directly affecting the catalytic activity in CH4 complete oxidation. A method of reliable estimation of the phase composition of the active component is developed taking into account the nano-size effects and Pt oxidation state. & 2009 Elsevier B.V. All rights reserved.
Keywords: EXAFS Nanoparticle Catalytic system Platinum oxide Local structure XANES
1. Introduction Recently, a great attention was focused on a new way of preparation of nano-structured catalysts with controlled sizes of active component nanoparticles supported on an oxide carrier for studying nano-size effects in complete oxidation reactions. The key factors for the particle size control were the composition of the original precursor solutions and the pretreatment of the carrier. These methods were shown not to affect the textural properties of the carrier. Obviously, development of methods for controlling both the particle size and active component structure along with elucidation of key factor contributions to the activity and stability of the catalysts allows to obtain high-effective catalysts and to diminish the noble metal loading as well. Structural studies present valuable information to achieve these aims, especially XAFS as a powerful tool to study the local structure and state of the supported nanoparticles of noble metals. Earlier, the catalytic activity of the size-controlled Pt nanoparticles with a narrow particle size distribution supported on g-Al2O3 was tested in CH4 complete oxidation [1]. The reaction studied was shown to be strongly size sensitive. This work is devoted to a structural study of the most active in CH4 complete oxidation Pt/Al2O3 nanosystem by XAFS.
2. Experiment Monodisperse 1%Pt/g-Al2O3 catalysts were prepared by incipient wetness impregnation of the dehydrated carrier (Sasol TKA-432) or the same after acid pretreatment. Aqueous solutions of oligomeric m-hydroxo the Pt4+complexes with different acidity Corresponding author. Tel.: +7 383 329 40 13; fax: +7 383 330 30 56.
E-mail address: [email protected] (V.V. Kriventsov). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.12.170
were used as active component precursors [1]. Freshly impregnated samples were dried and calcined in air. Then initial samples and ones after 2 h-aging (430 1C, 1%CH4 in air) or 3 h-reduction (300 1C, H2 flow) were studied by XAFS and TEM. All TEM measurements were performed on a JEM-2010 (200 kV, line resolution 0.14 nm). The size distribution of the Pt particles was determined by measuring 300 particles for each sample. XAFS (Pt-L3 edge) transmission spectra for all the samples studied were measured (EXAFS/XANES step 1.5/0.3 eV) and treated by the standard way [2,3] at SSRC, Novosibirsk. The radial distribution function (RDF) was calculated from the EXAFS spectra in k3w(k) as the modulus of Fourier transform at the k-intervals 3.5–12.5 A˚ 1. Curve fitting with EXCURV92 [4] was used to determine the distances—R and coordination numbers—CN using known XRD data. The Debye–Waller factors were fixed: 2s2=0.011–0.013 A˚2.
3. Results and discussion The normalized XANES spectra (Pt-L3 edge, *-white line) and RDFs describing the Pt local arrangements for the catalysts and reference samples are shown on Figs. 1 and 2, respectively. EXAFS data (distances—R and corresponding coordination numbers— CN) of the Pt local arrangement and TEM data for the studied samples are presented in Table 1. The typical feature, named as the white line [5], is observed on XANES (Pt-L3) spectra of all the samples studied (Fig. 1a–j), the height of white line amplitude being directly associated with the oxidic phase presence [6]. Indeed, it should be noted that in the analysed samples, from the Pt nitrate precursor solution (a) to the Pt-foil (j), a pronounced tendency is observed. In addition to decrease of the amplitudes of the white line in the series, some shift of the edge position of spectra to region of smaller energy
ARTICLE IN PRESS I.E. Beck et al. / Nuclear Instruments and Methods in Physics Research A 603 (2009) 108–110
Fig. 1. XANES spectra (Pt-L3, *-white line) of the (a–j) samples: Pt4+ nitrate solution, n1, n2, n3, n3-aged, n4, n5, n6, n5-red, Pt-foil.
Fig. 2. RDFs describing of Pt local arrangement of the (a–j) samples: Pt4+ nitrate solution, n1, n2, n3, n3-aged, n4, n5, n6, n5-red, Pt-foil.
109
values was found. The maximum shift value within few eV was established for the references (a,j). One could assume that for both the precursor solution (a) and for the n1 (b) and n2 (c) samples, the contribution of the oxidic phase is close to 100%. Indeed, both the highest amplitudes of the white line and the conditions of samples preparation (impregnation of un-pretreated carrier and mild oxidative calcination) provides a basis for this conclusion. For all other samples it is clearly observed (Fig. 1d–i) that the proportion of the oxidic phase seems to diminish in the series from (d) to (i) samples and in the last one the minimum was reached. On the RDFs of the Pt4+–nitrate precursor solution (Fig. 2a), several peaks are present. The first intense peak is attributed to the Pt–O distance 2.0 A˚ [7,11]. The set of fitting values is: RPt O=2.0 A˚, CN6.0, so practically non-distorted octahedral oxygen nearest surrounding is realised in this case. Unambiguously the next peaks within the region 2.3–4.0 A˚ having lower amplitudes can be attributed to several greater Pt–O–Pt distances, corresponding to the fitting values 3.05 and 3.6–3.8 A˚, respectively. The same few remarkable peaks are observed on the RDFs for the n1 and n2 samples (Fig. 2b and c) prepared on an un-pretreated carrier which seems to contain the most oxidized platinum among supported catalysts under study. The first high intensive peak is attributed to the Pt–O distance, corresponding to the fitting values RPt O=2.0 A˚, CN5.9, the second low intensive and slightly spread peak is attributed to the Pt–O–Pt distance, corresponding to the fitting value 3.10 A˚. The results obtained do not contradict to the known XRD data for Pt-oxide systems [7,11]. All the other supported catalysts under study, prepared on the acid-pretreated carrier, contained significant amount of the metallic platinum along with the oxidic one. The shape of the RDF (Fig. 2j) for the reference Pt-foil is typical for the fcc structure, because a pronounced set of metallic Pt–Pt peaks was observed. The main peak corresponds to the shortest Pt–Pt distance (RPt Pt=2.77 A˚, CN=12) [8], fitting gives a very close values (RPt Pt2.75 A˚, CN12.1) for this first peak. It should be noted that presence of similar main peaks (corresponding to the shortest first Pt–O (2.0 A˚) or/and shortest first Pt–Pt (2.75 A˚) distances, which are typical for oxidic and metallic phases) on the RDFs (Fig. 2b–i) was revealed for all the supported samples. However, the amplitudes and shapes of these peaks as well as their proportion strongly differ for various samples, seemingly owing to differences in phase composition. A procedure of rough estimation of the phase composition for these samples (Table 1) was developed. First, to estimate the metallic phase amount (%Pt0) in the sample studied, the ratio of the CN value calculated from fitting for the first shortest Pt–Pt distance to typical metallic CN 12 was considered. Second, to estimate the oxidic phase amount (%Ptox) in the sample studied, the ratio of the CN value calculated from fitting for the first shortest Pt–O distance to CN 6 typical for the pure PtO2 phase was considered. Then verification of this approach was carried out via comparison of the total calculated phase content Ptox+Pt0 with 100% concerning that the deviation should not exceed 10% from the method limitations. As seen from Table 1, these deviations do not exceed 10% for the most part of the samples studied, thus a direct application of the proposed approach is quite valid. The analysis of possible reasons for some discrepancy of the estimated phase composition reveals deviations 410% only for the high dispersed metal-containing samples n3 and n5 (Table 1, TEM data). It is well-known from literature [9], that for small nanoparticles having the noble metal fcc structure with small particle sizes 30 A˚, CN corresponding to the first shortest Pt–Pt distance is about 9 due to great surface atoms contribution in the of surface/bulk ratio. This value strongly differs from 12, that is typical for bulk metal. Calculation of the phase composition
ARTICLE IN PRESS 110
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Table 1 EXAFS and TEM data of the samples prepared via impregnation. Pt–Pt
Pt–O
Calculated phase composition
Size correction
Sample
d (nm) (TEM) R, A˚
CN
R, A˚
CN
%Ptox, PtO2
%Pt0
100% (Ptox+Pt0)%
n5-red
– – – 2.73 2.73 2.73 2.74 2.73 2.72
– – – 2.6 2.9 6.8 5.6 7.6 3.8
1.99 1.99 1.99 1.99 1.99 1.98 1.99 1.98 1.99
6.1 5.9 5.6 4.4 3.4 2.9 2.1 1.6 1.5
– – – 22 24 56.5 47 63 32
0 0 5 3 18 5 17 10 43
– 0.9 1.9 4.8 2.6 6.3 2.8 3.7 2.4
Pt-foil
2.75
12.1
–
–
100 100 95 75 58 48.5 36 27 25 37.5(PtO) 0
100
0
–
4+
Pt nitrate solution n1a n2a n3b n3-agedc n4b n5b n6b d
%Pt0
CN
32
9
62
9
42 63
9 6 12
a
un-pretreated carrier. acid-pretreated carrier. n3-2 h-aging, 430 1C, 1%CH4 in air. d n5-3 h-reduction, 300 1C, H2 flow. b c
taking into account the CN 9 instead of CN 12 for metallic phase gives in some cases a more reliable estimation presented in Table 1 (the penultimate column). Note that, taking into account the nano-sized effect, the calculated total sum of values (Ptox+Pt0)% is about 90% (58+32) for the n3-aged sample, 98% (36+62) for the n5 sample, but only 67% (25+42) for the n5-red sample. Evidently, for the two samples n3-aged and n5, correction for the nano-sized effect yielded more reliable estimations of the phase composition. However, the situation with the third preliminary reduced sample n5-red seems to be more complicated, because the obtained calculated sum is greatly inconsistent with the expected total value 100% even taking into account a rather smaller value of CN 6 for the metallic phase. In this case, the recalculated total sum of values (Ptox+Pt0)% is about 88% (25+63) for the n5-red sample. Obviously, the ambiguousness of the situation concerning this sample is determined not only by contribution of nano-sized effects in the metallic phase, but also by underestimation of the oxidic phase amount. Indeed, this sample was reduced in H2 flow and as a result very small nano-sized Pt0 particles were obtained. However, the Pt0 particles having great surface area were known [10] to be oxidized into PtO even at room temperature, but for transformation into PtO2 more harsh conditions are required. So in this case for the preliminary reduced sample contacting with air it is reasonable to calculate the oxidic phase amount taking into account the typical for the PtO oxide CN4 for the first shortest Pt–O distance, that is in a good agreement to XRD data [11]. In this case, the calculated value of oxidic phase amount for n5red sample is about 37.5%) and the corrected total sum of values (Ptox+Pt0)% is about 100.5% (37.5+63). It should be noted that this obtained estimation of the phase composition (calculated taking into account both nano-size effects (CNPt Pt=6) and Pt2+ oxidation state (CNPt O=4)) is very close to the expected value
100% and agrees well with the data of other methods without any contradictions.
4. Conclusion The local structures of nano-structured Pt/g-Al2O3 catalysts very active in CH4 complete oxidation were studied by XAFS. All possible structural models were discussed in detail. These catalysts were shown to be complex polyphase metal-oxide systems supported on the Al2O3 surface. A method for reliable estimation of the phase composition in the complex metal-oxide phase is developed, taking into account both nano-size effects and changes of Pt oxidation state.
Acknowledgments This research was supported by Russian Science and Innovation Agency (Contract no. 02.513.11.3203), RFBR06-03-33005a, RFBR-08-03-01150a, RFBR08-03-01016, RFBR06-03-32578a, RFBRCNRS08-03-92502a, RFBR-09-03-01012a, RFBR06-03-08173 Grants. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
I. E. Beck, et al., ISRHHC-13 Abstracts, USA, California, 16 July 2007, 91. D.I. Kochubey, EXAFS Spectroscopy of Catalysts, Nauka, Novosibirsk, 1992. K.V. Klementiev, VIPER freeware: www.desy.de/~klmn. N. Binsted, J. Campbell, et al., EXCURV92, Daresb. Laboratory, UK. J.H. Sinfelt, G.D. Meitzner, Accoun. Chem. Res. 26 (1) (1993) 16. H. Yoshida, S. Nonoyama, et al., Phys. Scr. T 115 (2005) 813. ICSD Codes: 24922, 24923-a-PtO2; 202407, 30443, 24925-b-PtO2. ICSD Codes: 76153-Pt0. R.E. Benfield, J. Chem. Soc. Faraday Trans. 88 (8) (1992) 1107. C.P. Hwang, C.-T. Yeh, J. Catal. 182 (1999) 48. ICSD Codes: 26599-PtO.