Photon Factory Activity Report 2014 #32 (2015) B

BL-9C/2012G701

XAFS Study of Stability of Pt on Pt/Nb-AlMCM-41 in a Redox Atmosphere
Masashi Tanaka, Masaru Komatsubara, and Masakazu Iwamoto*Research and Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 1128551, Japan

13 Results and Discussion
Fig. 1a shows the Pt L3-edge XANES spectra ofPt/AlM41 and Pt/Nb16-AlM41 samples. Both spectra
were similar to platinum foil. The coordination numbersof platinum for Pt/AlM41 and Pt/Nb16-AlM41, whichwere calculated from the EXAFS data using the leastsquare method, were 9.8 and 10.0, respectively. These
results indicate that platinum on M41 was not influenced
by the co-loaded niobium.
The oxidation state of platinum under reducing oroxidizing atmospheres was measured using the Pt L3-edge
XANES spectrum (a white line) at 11.564 keV, which isattributed to the electron transition from the 2p to 5dorbitals, and is a good indicator of the oxidation state ofplatinum [5,6]. Fig. 1b summarizes the change in whiteline intensity during redox cycles for Pt/AlM41 andPt/Nb16-AlM41. Two findings are apparent. Peakheights were low upon reduction and high upon oxidation

1.52ab

Pt/Nb16-AlM41
1.5
Height at 11.564 keV

2 Experiment
M41 was synthesized using colloidal silica Snowtex-20(Nissan Chemical Industries, Ltd.) and C12H25N(CH3)3Br(Tokyo Kasei Kogyo Co., Ltd.). Niobium ion was loadedby the template ion-exchange (TIE) method [4]. The
resulting powder was calcined at 873 K for 6 h in air toremove the templates. Platinum was then impregnatedonto calcined and Nb-loaded M41 using aqueous H2PtCl6.
Platinum loading was 2 wt% metal. The sample wasagain calcined at 873 K for 6 h in air. The catalyst wasabbreviated as Pt/NbX-M41, where X is the Si/Nb atomicratio. The XAFS spectrum of Pt L3-edge was recorded on
BL-9C of PF-KEK.

of both samples. Pt species supported on oxides havebeen reported to give peak heights of 1.8-2.0 and 1.2-1.4,
respectively, upon oxidation and reduction [6-8]. The
values in Fig. 1b, 1.42-1.50, indicate that Pt supported on
AlM41 was usually in a reduced state. In addition, therange for Pt/Nb16-AlM41 was smaller than for Pt/AlM41,
revealing that Pt loaded on the former remained metallic.
This finding is in good agreement with reports thatplatinum remained metallic on acidic supports [5,9].
Both findings indicate that the two active temperature
ranges observed in H2-SCR did not result fromdifferences between Pt and PtO (or PtO2).

Normalized absorption / a.u.

1 Introduction
The selective catalytic reduction of NO with hydrogen(H2-SCR) in the presence of excess oxygen has attracted
attentions due to the activity at temperatures as low as
373 K. Several studies on Pt- or Pd-supported catalysts
report two active temperature ranges in H2-SCR [1-3].
Two explanations were suggested for this: different statesof active metals [2,3] and change in reaction mechanism[1]. Recently, we found that Pt-loaded AlMCM-41(aluminum-containing mesoporous silica MCM-41,Pt/AlM41) also shows two active temperature ranges(approximately 373 and 500 K) in H2-SCR and that
addition of niobium to Pt/AlM41 (Pt/Nb-AlM41) greatly
enhances catalytic activity at 373-650 K. In this study,
the possibility that differences between reduced andoxidized metals might result in two active temperature
ranges on H2-SCR over Pt/Nb-AlM41 was investigatedusing the XAFS spectroscopy.

Pt/AlM41

Pt foil

0.5

11.55

11.56

11.57

11.58

1.48
1.46
1.44
1.42
1.4

Photon energy / keV

123
45Cycle number

6Fig. 1: (a) Pt L3-edge XANES spectra of Pt/AlM41 andPt/Nb16-AlM41 evacuated at 773 K. (b) Change inheight of white line at 11.564 keV in Pt L3-edge XANES
spectra of (circle) Pt/AlM41 and (triangle) Pt/Nb16AlM41 with redox treatments. The sample was reducedby H2 at cycle numbers 1, 3, and 5, and oxidized by O2 at
2, 4, and 6. Each reduction or oxidation was performed at
773 K (PH2 13.3 kPa or PO2 13.3 kPa) for 10 min.
References[1] A. Ueda et al., Catal. Today 45, 135 (1998).
[2] T. Nanba et al., Appl. Catal. B 46, 353 (2003).
[3] F. J. P. Schott et al., Appl. Catal. B 87, 18 (2009).
[4] M. Iwamoto and Y. Tanaka, Catal. Surv. Jpn. 5, 25(2001).
[5] J. Shibata et al., J. Phys. Chem. B 108, 18327 (2004).
[6] H. Hirata et al., Catal. Today 164, 467 (2011).
[7] A. P. Ferreira et al., J. Catal. 263, 335 (2009).
[8] K. O. Rocha et al., Appl. Catal. A 431-432, 79 (2012).
[9] H. Yoshida et al., Catal. Today 87, 19 (2003).
* iwamotom@tamacc.chuo-u.ac.jp