Download EXAFS investigations on silica-supported silver catalysts for the

EXAFS investigations on silica-supported silver
catalysts for the selective liquid phase oxidation of
alcohols to aldehydes
M.J. Beier, T.W. Hansen1 and J.-D. Grunwaldt
Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs.
Lyngby, Denmark
1Center for Electron Nanoscopy, Technical University of Denmark, Building 307, DK-2800 Kgs. Lyngby, Denmark
EXAFS is a strong technique to characterize supported metal nanoparticles useful e.g. in oxidation
catalysis. Supported noble metals such as Au, Ru and Pd can serve as catalyst materials in the
selective oxidation of alcohols in the liquid phase [1]. On the other hand, silver being catalytically
active in many gas phase oxidations has only rarely been used. Using a special screening approach
Ag/SiO2 together with CeO2 nanoparticles was
found to be catalytically active in the oxidation
of benzyl alcohol to benzaldehyde [2]. In order to
obtain an active silver catalyst, the calcination
temperature in air needed to be around 500 °C. In
order to shed more light into the oxidation state
of silver, ex situ and in situ X-ray absorption
spectroscopy studies were performed.
The current study was done at beamline X1 with
a beam size of typically 10 mm × 1 mm and a
Si(311) double crystal monochromator. The
experiments were carried out in an in-house-built Figure 1: Calcination of 10%Ag/SiO in air
2
in situ reaction cell [3]. XANES spectra at the Ag followed by in situ XANES at the Ag K-edge.
K-edge were recorded to monitor changes of the
Ag oxidation state upon calcination. XANES
spectra were processed by energy calibration,
background subtraction and normalization using
the WinXAS 3.1 software[4]. EXAFS spectra
were extracted from the XAS spectra after
analogous treatment, deglitching and Fouriertransformation between k = 3 A-1 and 13 A-1.
EXAFS fitting was performed in R-space based
on the silver lattice structure using the FEFF 7.0
code [5]. Particle sizes were calculated from the
fitted first shell coordination number as
described in [6] assuming spherical particles.
Figure 2: Ag K-edge EXAFS spectrum of fresh
In situ XANES analysis of the calcination process 10%Ag/SiO2 (a) and silver foil (b). Inset: Fourier
in an open capillary showed that silver became transformed EXAFS spectra of the silver catalyst
reduced upon prolonged heat treatment at 500 °C and silver foil (dashed line).
(Figure 1). At low temperatures silver is present
as the AgNO3 precursor used for catalyst synthesis. At temperatures close to 500 °C, an
intermediate silver species is formed. The XANES differed from that of Ag2O but still appeared to
be oxidized judging from the edge position. Since lower calcination temperatures afforded inactive
catalysts, metallic silver appears to be required for catalytic activity.
Accordingly, metallic silver was also found on freshly prepared 10%Ag/SiO2 (Figure 2).
The lower Ag-Ag backscattering amplitude seen in the FT-EXAFS spectrum of the silver catalyst
compared to silver foil suggested an average silver particle size below 5 nm which is in strong
contradiction to particles sizes obtained from TEM (volume weighted) and XRD (Table 1) which
agree well. Thus, the comparably small EXAFS amplitude likely did not originate from surface
silver atoms. Lattice disorder caused by metallic silver oxygen species might serve as an
explanation as metallic silver is known to strongly interact with oxygen also depending on the heat
treatment [7].
Table 1: Particle sizes obtained from different characterization techniques.
Characterization technique
Particle size (nm)
XRD
29
EXAFS
1.5
TEM
33
Investigating catalyst samples after different reactions times ex situ an increase with time was
found which correlates with the catalyst deactivation observed in previous experiments (Figure 3).
Thus, metallic silver oxygen species might be required for the formation of the active site. In
addition, the instability of the species might at
least partially account for catalyst deactivation.
In fact, used catalyst could be partially
reactivated by an additional calcination step. Of
course this could also be explained by removal of
carbon blocking the active site as often observed
for other noble metal catalysts.
In summary, silver catalysts interesting for the
selective oxidation of alcohols were investigated.
Metallic-like silver particles are required for
obtaining good catalytic activity. Discrepancies
in the silver particle size obtained from TEM,
XRD and EXAFS suggest that silver-oxygen
species are present in the investigated
Figure 3: Fourier transformed EXAFS spectra of 10%Ag/SiO2 catalyst and are beneficial for the
10%Ag/SiO2 after different reaction times; (a) 0 catalytic performance.
min, (b) 15 min, (c) 90 min, (d) 300 min, (e) silver
foil.
We wish to thank Adam Webb for support during the EXAFS measurements at beamline X1.
HASYLAB at DESY is acknowledged for granting beam time and the European Union for
financial support (Project no.: RII13-CT-2004-506008).
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