for 1 h in the dark. For EPR measurements, 0.1 g of the
sample was transferred into Suprasil quartz tubes (2 mm
i.d. ] 3 mm o.d.) which were sealed at one end. For prelimi-
nary studies, the samples were evacuated below 1 Torr for D3
h and Ñame sealed at the other end. It was observed that the
photoyield of the samples subjected to evacuation was similar
to the photoyield for samples that were not evacuated. Hence,
further photoionization experiments were carried out without
evacuation. For di†use reÑectance spectroscopy (DRS) the
samples were loaded into a cylindrical quartz sample cell (22
mm diameter ] 20 mm path length).
Characterization
X-Ray di†raction powder patterns were recorded on a
Siemens 5000 X-ray di†ractometer using Cu Ka radiation of
wavelength 1.541 A in the range 10¡ \ 2h \ 50¡. Chemical
analysis was performed by electron probe micro-analysis on a
JEOL JXA-8600 spectrometer. The composition of the CrH-
SAPO-n and CrAPSO-n materials was determined by cali-
bration with known standards and by averaging over several
defocused areas to give the bulk composition. EPR spectra
were recorded at room temperature at 9.5 GHz using a
Bruker ESP 300 spectrometer with a 100 kHz Ðeld modula-
tion and low micropower to avoid power saturation. Photo-
produced TMB radical cation (TMB`) yields were determined
by double integration of the EPR spectra using the ESP 300
software. Each photoyield is an average of six determinations.
The di†use reÑectance spectra were recorded at room tem-
Fig. 1 Di†use reÑectance spectra of CrAPSO-5 samples (a) Cr/
Si \ 6.2 ] 10~3, (b) Cr/Si \ 9.5 ] 10~3, (c) Cr/Si \ 1.7 ] 10~2 and
(d) Cr/Si \ 7.7 ] 10~2.
perature using
a Perkin-Elmer 330 spectrophotometer
equipped with an integrating sphere.
Ag CrO was formed. However, dÈd optical transitions of
2
4
Photoirradiation
Cr(V) occur in the same region.24 Hence, we assign these
optical bands to Cr(V) formed by the oxidation of Cr(III)
during calcination. The EPR studies described later also
suggest that Cr(V) is present in the calcined samples and hence
we assign these optical bands to Cr(V). In the case of a Cr(III)
liquid state ion-exchanged sample, typically three bands at
270 nm, 430 nm and 629 nm are observed. The bands at 430
nm and 630 nm are typical of octahedral Cr(III) in accordance
with the literature.25
Thus, the four bands obtained in calcined, hydrated
CrAPSO-5 materials can be assigned to Cr(V) in either square
pyramidal or distorted octahedral coordination, where the
additional coordination is due to water molecules. In the case
of liquid state ion-exchanged samples, Cr exists in the ]3 oxi-
dation state. The Cr(V) assignments in CrAPSO-5 and Cr(III)
assignments in Cr-SAPO-n(l) (n \ 5, 8, 11) agree well with
EPR results.
The TMB impregnated SAPO materials were irradiated using
a 300 W Cermax xenon lamp (ILC-LX 300 UV) at room tem-
perature. The light was passed through a 10 cm water Ðlter to
prevent infrared radiation and through a Corning No. 7-51
Ðlter to give light of 300 ^ 20 nm. The samples, sealed in
quartz tubes were placed in a quartz dewar and rotated at a
speed of 4 rpm to ensure even irradiation. The photoproduced
TMB cation radicals were identiÐed by EPR spectroscopy and
di†use reÑectance spectroscopy.
Results
The pore sizes of the SAPO materials employed in the present
study range from 6.3 A in SAPO-11, 7.3 A in SAPO-5 to 8.7 A
in SAPO-8. These are bigger than the molecular dimensions
of the TMB molecule which are 5 A by 13 A and hence it is
possible to incorporate TMB into these SAPO materials. Pre-
liminary experiments were carried out in SAPO-5 materials.
The XRD patterns of the SAPO materials synthesized showed
the samples to be highly crystalline and the X-ray di†raction
patterns agree well with literature data. However for
SAPO-11 samples, the sample crystallinity was found to
decrease by D20% and D30% after calcination and ion-
exchange respectively.
EPR spectra
The EPR spectra of Cr ion liquid state ion-exchanged SAPO
material (CrH-SAPO) show a very weak and broad signal at
g \ 1.97 assigned to Cr(III). On impregnation of TMB (prior
to irradiation) a new signal at g \ 2.00 assigned to TMB` is
seen. The intensity of the signal at g \ 1.97 is much weaker in
comparison with the signal at g \ 2.00 assigned to TMB` and
hence can be considered to be negligible. This shows that
some TMB` cation radicals are produced during the sample
preparation. After being irradiated at room temperature for 5
min, the samples show strong EPR signals. With further
increase in irradiation time the intensity of the EPR signal
increases further and then reaches a plateau in D30 min. On
further irradiation, the EPR intensity remains the same or
decreases. An irradiation time of 30 min was hence selected for
comparative photoyield and stability studies. The weak EPR
signals arising before irradiation have the same line shape as
those observed after irradiation. Also a visual change in the
color of the SAPO/TMB samples is easily seen; the samples
prior to irradiation are pale green in color but turn dark green
UV-Vis spectra
The spectra of calcined CrAPSO-5 samples show typically
four bands at 270 nm, 350 nm, 450 nm and a broad band
centered around 620 nm as shown in Fig. 1. The intensities of
the bands increase monotonically with increase in Cr content.
The higher energy bands (270 nm and 350 nm) are typically
assigned to charge-transfer transitions associated with
Cr(VI).23 However, we did not detect any Cr(VI) in our cal-
cined samples. To test for extraframework Cr(VI), 1 g of
CrAPSO-5 was stirred in 20 ml of 1 M Ca(NO ) solution
overnight. The Ðltrate was collected and 1 M AgNO was
3 2
3
added dropwise to test for CrO 2~, but no precipitate due to
4
2922
Phys. Chem. Chem. Phys., 2001, 3, 2921È2927
Ranjit
