Photon Factory Activity Report 2018 #36 (2019)

BL-9C/2017G190

EXAFS analysis for Pt/CeO2 catalysts active for partial oxidation of methane

Rajaram Bal1,* and Takehiko Sasaki2
Refinery Technology Division, CSIR- Indian Institute of Petroleum,
Dehradun-248005, India
2
Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kaswhianoha, Kashiwa,Chiba, 277-8561, Japan
11 Introduction
Natural gas or mainly methane is considered to be analternative source of future fuel due to its very highabundance on earth. After decades of research methanesteam reforming (SRM) is established as the one and only
large scale commercial process for syngas production.
Partial oxidation of methane (POM) is considered to be
an alternative for SRM from energy and economic pointof view, produces synthesis gas with H2/CO ratio 2.
Because of exothermic nature of POM reaction, theprocess is capable of synthesis gas production at much
lower temperature.
Here, we are reporting the EXAFS characterization ofof Pt-nanoparticles supported CeO2 mesoporous spheres
for POM. The catalyst was highly active and stable formore than 60 h at 800 °C. Bimodal nature of pores of thecatalyst was found to be the reason for superior activity of
the catalyst [1].
2 Experiment
Pt-CeO2 mesoporous spheres were prepared by two step
synthesis technique. First, CeO2 mesoporous spheres withbimodal pore structure were prepared by solvo-thermal
method and then Pt-nanoparticles were deposited overprepared CeO2 surface by following a newly developed
method as reported elsewhere [1]. EXAFS measurementsof Pt-LIII edge were carried out at KEK-IMMS-PF BL9C.
3 Results and Discussion
Recently we have reported Pt-nanoparticles supported
CeO2 nanocrystals (2Pt-CeO2PS) with very high POM
activity [2]. The catalyst was prepared by a two-step
synthesis method and controlled deposition of Ptparticles led to formation of very small (1.54 nm, frommetal dispersion) and highly selective Pt-nanoparticles
formation. Here, we have prepared Pt-CeO2MS catalyst
with similar two-step method but the difference wassynthesis of support and its properties. Previous report
was about 20-60 nm of CeO2 crystals (Surface area 88.7m2/g) but here in this report we will be discussing about150 nm mesoporous spheres comprised of about 5-15 nmof CeO2 particles (Surface area 182 m2/g). The best andmost advantageous feature of the support (CeO2MS) andthe catalyst (2Pt-CeO2MS) was its bimodal pore structure
and very high surface area, which led to even muchsmaller Pt-particles (1.27 nm) formation and much more
selective Pt-CeO2MS catalyst for POM reaction [3].

EXAFS analysis was done for Pt LIII-edge to analyze
the local environment of 2Pt-CeO2MS-F and 2Pt-CeO2MSS catalyst. F and S represent fresh and spent, respectively.
Figure 1a and 1b shows the EXAFS Fourier transforms of2Pt-CeO2MS-F and 2Pt-CeO2MS-S catalysts. Table 1shows structural parameters for 2Pt-CeO2MS-F catalyst
and 2Pt-CeO2MS-S catalyst. The parameters wereobtained by curve fitting of Pt LIII -edge spectra of boththe catalysts. EXAFS spectra of 2Pt-CeO2MS-F catalyst
showed presence of Pt-oxide with Pt-O bond distance
0.1995 nm and co-ordination number 2.4. 2Pt-CeO2MS-F
catalyst also showed presence of metallic Pt with Pt-Pt
bond distance 0.2787 nm and co-ordination number 5.9.
So, 2Pt-CeO2F catalyst contains both Pt-oxide andmetallic Pt species. Presence of metallic Pt-species wasunusual but our recent study revealed that use ofhydrazine and CTAB could have been the reason forsuch observation [2]. Use of hydrazine reduced Pt-ions tometallic Pt during Pt-deposition and during calcination
although all the particles are supposed to be oxidized but
presence of CTAB which produced hydrogen species
could have reduced some of the oxidized Pt-species backto its metallic form. 2Pt-CeO2MS-S catalyst also showedpresence of both Pt-oxide and metallic Pt species. In thiscase, Pt-O bond distance was 0.2002 nm and coordination number 1.1, whereas Pt-Pt bond distance ofmetallic Pt particles were 0.2765 nm and co-ordination
number 5.8. Presence of Pt-oxide in 2Pt-CeO2MS-S
catalyst was due to re-oxidation of metallic Pt-species
during catalysis.
Table 1: Summary of the EXAFS fitting results for thefresh and spent Pt catalystsa
CNDW (10-5nm2)∆E0(eV)

Rf(%)

Pt-O 1.995±0.011

2.4±0.6

2.5±1.910.6±1.7

1.5

Pt-Pt 2.787±0.012

5.9±0.4

5.5±1.7

Spent Pt-O 2.002±0.013

1.1±0.4

0.42±2.1

9.1±2.0

1.6

Pt-Pt 2.765±0.008

5.8±1.1

5.5±1.0

PathFresh

R (10-1 nm)

Fitting was conducted in the range Δk: 3–12.5 (10−1 nm) andΔR: 1.2–3.0 (10−1 nm). Amplitude reducing factor S02 was set as 0.95
aPhoton Factory Activity Report 2018 #36 (2019)

3FT[k χ(k)]activity and stability of the catalyst. Different
characterization techniques revealed high metal-support
interaction, which highly enhanced the metal-support
interfacial surface area and improved activity and stabilityof the catalyst during POM. Superior activity and stabilityof the catalyst was due to catalysts bimodal nature, smaller
catalyst particles, high OSC and high metal-support
interactions.

1234-1r / 10 nm

563

FT[k χ(k)]0References[1] Singha, R. K.; Shukla, A.; Yadav, A.; Sasaki, T.;
Sandupatla, A. ; Deo, G.; Bal, R.; Catal. Sci. Technol.
2017, 7, 4720-4735.
[2] Singha, R. K.; Ghosh, S.; Acharyya, S. S.; Yadav, A.;
Shukla, A.; Sasaki, T.; Venezia, A. M.; Pendem, C.; Bal,R.; Catal. Sci. Technol. 2016, 6, 4601-4615.
[3] Kondratenko, V. A.; Karin, C. B.; Kondratenko, E. V.;
ACS Catal. 2014, 4, 3136-3144.

* raja@iip.res.in
0

1

2

4
3-1r / 10 nm

56

Figure 1: (a) (Upper panel) k3-weighted Fourier transformof Pt-LIII edge EXAFS for the fresh 2Pt-CeO2MS catalyst.
Amplitude and imaginary part are traced by thick curve
and thin curve, respectively. Observed data are shownwith solid lines and fitting data are shown with dotted lines.
(b) (Lower panel) k3-weighted Fourier transform of Pt-LIIIedge EXAFS for the spent 2Pt-CeO2MS catalyst.
Amplitude and imaginary part are traced by thick curve
and thin curve, respectively. Observed data are shownwith solid lines and fitting data are shown with dotted lines.

In summary, CeO2 mesoporous spheres prepared by solvothermal method showed bimodal nature of pore size
distribution. Bimodal nature of CeO2 mesoporous spheres
provided very high surface area. Pt-nanoparticles weredeposited over the prepared CeO2 mesoporous spheres bycontrolled deposition method, which showed very high Ptdispersion. Very high Pt-dispersion of prepared Pt-CeO2
catalyst was the result of Pt-deposition method in presenceof morphology controlling agent CTAB and capping agent
hydrazine. In addition, smaller pores of support CeO2
mesoporous spheres, which provided high surface area
improved Pt-dispersion. High OSC and bimodal nature of
the catalyst highly improved catalytic activity by lowering
the ignition temperature and protecting Pt-particles
residing inside the smaller pores of the catalyst. Ptparticles residing inside the smaller pores of the catalyst
were protected from sintering and rapid moleculartransportation through larger pores highly influenced POM