Control of copper particles deposition in mesoporous SBA-15 silica by modified CVD method

Article abstract of DOI:10.1016/j.ica.2014.08.008

Copper supported SBA-15 catalysts were prepared by modified Organometallic Chemical Vapor Deposition (OMCVD) technique. The materials were characterized by nitrogen physisorption, XRD, TEM, UV-Vis, XPS and temperature programmed reduction. Catalytic reductions of NO with CO and with a mixture of CO and methane as well as methanol decomposition were used as catalytic tests. The modified OMCVD procedure favors the formation of very finely dispersed nanoparticles located deeply into the micropores and the small mesopores of the SBA-15 silica matrix, while larger and predominantly located on the external surface copper species are detected for the materials prepared by conventional wet impregnation procedure.

Full text of DOI:10.1016/j.ica.2014.08.008

Inorganica Chimica Acta 423 (2014) 145–151
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Control of copper particles deposition in mesoporous SBA-15 silica by
modified CVD method
T. Tsoncheva a, , A. Gallo b,1, I. Genova a, I. Spassova c, M. Marelli b, M. Dimitrov a, M. Khristova c,
⇑
G. Atanasova c, D. Kovacheva c, D. Nihtyanova c,d, V. Dal Santo b
a Institute of Organic Chemistry with Centre of Phytochemistry, BAS, Sofia, Bulgaria
b CNR – Istituto di Scienze e Tecnologie Molecolari, via C. Golgi 19, 20133 Milano, Italy
c Institute of General and Inorganic Chemistry, BAS, 1113 Sofia, Bulgaria
d Institute of Mineralogy and Crystallography, BAS, 1113 Sofia, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2014
Received in revised form 29 July 2014
Accepted 4 August 2014
Available online 16 August 2014
Copper supported SBA-15 catalysts were prepared by modified Organometallic Chemical Vapor Deposi-
tion (OMCVD) technique. The materials were characterized by nitrogen physisorption, XRD, TEM, UV–Vis,
XPS and temperature programmed reduction. Catalytic reductions of NO with CO and with a mixture of
CO and methane as well as methanol decomposition were used as catalytic tests. The modified OMCVD
procedure favors the formation of very finely dispersed nanoparticles located deeply into the micropores
and the small mesopores of the SBA-15 silica matrix, while larger and predominantly located on the
external surface copper species are detected for the materials prepared by conventional wet impregna-
tion procedure.
Keywords:
Modified OMCVD
Copper deposition on SBA-15
Particles size control
Methanol decomposition
NO reduction
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
particles size was achieved by multi-step deposition of the precur-
sor (bis-(hexafluoroacetylacetonate) copper (II) hydrate
–
Many metallic properties, such as lattice constant, heat capacity,
melting point, optical, magnetic, electric, adsorption and catalytic
ones significantly change with decreasing of particles size in the
nanoscale [1–4]. Mesoporous silicas are considered as good host
matrices for nanosized metal particles due to their high surface
area, well developed pore volume and tunable pore size, shape
and topology [5], but the fine control of metal particles dispersion
is still a matter of challenge. Among the various preparation tech-
niques, the Organometallic Chemical Vapor Deposition is exten-
sively used as solvent free method for the preparation of
supported, homogeneously dispersed, metal nanosized particles
[6–10]. OMCVD technique is based on the adsorption of a metal pre-
cursor from the gas phase on the support followed by its decompo-
sition. The main problem with this technique is reaching the
desired metal loading due to the undesirable decomposition of pre-
cursor on the apparatus walls. In our previous study we proposed a
modified OMCVD method for tailored preparation of supported on
silica copper nanoparticles [11]. The control of copper loading and
Cu(hfac)2ꢀxH2O, x < 1 or x = 1) starting from parent copper material,
obtained by simple incipient wetness impregnation technique (WI).
This strategy [12,13] allowed copper particles growth by interac-
tion of copper precursor with metallic surface according the Eqs.
(1) and (2):
Hðads: on Cu NPsÞ þ CuðhfacÞ2ðgÞ ! CuðhfacÞðadsÞ þ hfacH "
ð1Þ
CuðhfacÞðadsÞ þ 0:5H2=Hðads:on Cu NPsÞ ! Cuð0Þ þ hfacH ";
ð2Þ
where Cu NPs stay for copper nanoparticles.
Alternatively, decomposition of Cu(hfac)2 on pure silica surface
according to Eqs. (3) and (4) was proved [14,15].
CuðhfacÞ2ðgÞþ2SiOHðsurfÞ ! SiOðSiOHÞꢁCuðhfacÞðadsÞþhfacHðgÞ "
ð3Þ
SiOðSiOHÞꢁCuðhfacÞðadsÞþH2 ! 2SiOHðsurf:ÞꢁCuð0ÞþhfacHðgÞ "
ð4Þ
We also demonstrated that facilitated mass transfer into
3D-ordered mesoporous structure of KIT-6 provides production
⇑
Corresponding author. Tel.: +359 029796640.
E-mail address: tsoncheva@orgchm.bas.bg (T. Tsoncheva).
Current address: Department of Chemical Engineering, University of California,
1
Santa Barbara 93106-5080, USA.
http://dx.doi.org/10.1016/j.ica.2014.08.008
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

146
T. Tsoncheva et al. / Inorganica Chimica Acta 423 (2014) 145–151
Table 1
Spectrometer using air-acetylene flame. Specific surface area and
pore size distribution were measured through nitrogen adsorp-
tion–desorption isotherms at 77 K using a Quantachrome NOVA
1200 apparatus. The powder X-ray diffraction patterns were
Samples composition, BET surface area (S), total pore volume (V), micropores pore
volume (Vmic) and average pore diameter (D).
Sample
SBA-15
Cu, wt.%
–
S, m2/g
924
V, cm3/g
1.14
Vmic, cm3/g
0.060
D, nm
7.8
recorded on Bruker D8 Advance diffractometer with Cu K
a radia-
tion and LynxEye detector. TEM micrographs of the samples from
Series A were collected by a ZEISS LIBRA 200 FE HRTEM. The
TEM micrographs of the samples from Series B were performed
by TEM JEOL 2100. The UV–Vis spectra of the powder samples
were recorded using a Jasco V-650 spectrophotometer equipped
with a diffuse reflectance unit. The XPS measurements were done
in the UHV chamber of a ESCALAB-Mk II (VG Scientific) electron
3Cu/SBA-15(A)
5Cu/SBA-15(A)
6Cu/SBA-15(A)
7Cu/SBA-15(A)
3.30
4.95
6.12
7.14
470
550
500
477
0.70
0.94
0.92
0.88
0.020
0.008
0.009
0.007
6.8
7.3
7.8
7.3
5Cu/SBA-15(B)
6Cu/SBA-15(B)
7Cu/SBA-15(B)
4.95
6.12
7.14
558
586
551
0.56
0.50
0.50
0.015
0.029
0.034
6.0
6.1
7.0
ꢀ
spectrometer. Modified Auger parameter aCu was calculated as
a
of uniform and very finely dispersed copper particles even at rela-
tively high copper loading [16]. The aim of current paper is to
expand our knowledge of the pore topology effect on the copper
particles growth in the 2D-SBA-15 mesoporous silica host matrix
during the modified OMCVD procedure. Nitrogen physisorption,
XRD, TEM, UV–Vis, XPS and Temperature Programmed Reduc-
tion-Thermo Gravimetric (TPR-TG) analyses were used for samples
characterization. Methanol decomposition to CO and H2 and Selec-
tive Catalytic Reduction (SCR) of NO with CO (NO–CO) or with a
mixture of CO and CH4 (NO–CO + CH4) were used as catalytic tests.
As was previously reported methanol decomposition and SCR of
NO with CO [11,16] could be successfully used as structure sensi-
tive reactions. Further, the increased interest to these reactions is
closely related to our project aimed at the development of simple
integrated system for vehicles application where decomposed
methanol is used as alternative fuel with a simultaneous removal
of exhausted toxic NO emissions by reduction with the products
of methanol decomposition (CO and methane).
ꢀ
Cu = KE(CuL3M4,5M4,5) + BE(Cu2p3/2), where KE(CuL3M4,5M4,5)
was the kinetic energy of the CuL3M4,5M4,5 and BE(Cu2p3/2) is the
binding energy of the Cu2p3/2 peak. The TPR-TG analyses were per-
formed with a Setaram TG 92 instrument. Typically, 40 mg of the
sample were placed in a microbalance crucible and heated in a
flow of 100 cm3 minꢁ1 H2 in Ar (1:1) up to 773 K at 5 K minꢁ1
.
2.3. Catalytic tests
Methanol conversion was carried out in a fixed bed flow reactor
(0.055 g of catalyst) and argon was used as a carrier gas (50 cm3
minꢁ1). The methanol partial pressure was 1.57 kPa. The catalysts
were tested under conditions of
a temperature-programmed
regime within the range of 350–770 K with heating rate of
1 K minꢁ1. On-line gas chromatographic analyses were performed
on HP apparatus equipped with flame ionization and thermo-
conductivity detectors, on a PLOT Q column, using an absolute
calibration method and a carbon based material balance. Catalytic
activity in SCR of NO–CO and NO–CO + CH4 was measured using an
integrated quartz micro-reactor and mass spectrometer system
(CATLAB, Hiden Analytical). The system features a fast-response,
low thermal mass furnace with integrated air-cooling, a precision
Quadrupole Mass Spectrometer, and a quartz inert capillary with
‘‘hot zone’’ inlet for continuous close-coupled catalyst sampling
with minimal dead volume and memory effects. Before the mea-
surements the catalysts were pretreated under Ar flow at 773 K.
The catalytic tests were performed from 313 to 773 K, with a gas
mixtures of Ar:CO:NO:CH4 (99.7:0.15:0.10:0.05 vol.%) and Ar:
2. Experimental
2.1. Materials
Mesoporous SBA-15 silica was prepared according to the proce-
dure described elsewhere [17]. A series of materials, denoted as
nCu/SBA-15(A), where n is the approximate amount of copper in
wt.% (the corresponding accurate values determined by AAS are
presented in Table 1), were obtained using the following proce-
dure. The initial 3Cu/SBA-15(A) sample was prepared by WI of
the silica support with 0.4 M aqueous solution of Cu(NO3)2ꢀ6H2O
(Sigma Aldrich) and further treatments in air at 620 K and in
hydrogen at 773 K for 1 h. The 5Cu/SBA-15(A) sample was obtained
from 3Cu/SBA-15(A) by OMCVD of Cu(hfac)2 as described earlier
[11]. In details, the catalyst 3Cu/SBA-15(A) was placed in a round
bottom flask with an open holder on the bottom where Cu(hfac)2
precursor was loaded to avoid any direct contact with the catalyst
before sublimation. The powder was gently mixed in rotary evap-
orator (hot oil bath) under H2 (800 mbar) for three hours at 403 K.
After deposition, the catalyst was slowly heated (1.5 K minꢁ1) up to
623 K in H2. The 6Cu/SBA-15(A) and 7Cu/SBA-15(A) samples were
prepared reiterating the deposition/decomposition steps using as a
starting material the previously obtained. Alternatively, a series of
three samples denoted as 5Cu/SBA-15(B), 6Cu/SBA-15(B) and 7Cu/
SBA-15(B) with similar to the OMCVD samples copper content
(Table 1) was obtained by direct WI of SBA-15 support with water
solution of copper nitrate. After the decomposition of nitrate pre-
cursor in air at 773 K for 2 h the samples were treated in hydrogen
at 523 K for 2 h.
CO:NO (99.7:0.15:0.15 vol.%) kept at
a
constant flow of
400 cm3 minꢁ1
.
3. Results and discussion
3.1. Structural characterization
3.1.1. Nitrogen physisorption data
The nitrogen physisorption isotherms for parent SBA-15 silica
and its copper modifications (Fig. 1a) are of type IV with H1
hysteresis loop and well defined step due to capillary condensation
in the 0.6–0.8 P/P0 region. According to IUPAC classification these
features are typical of mesoporous materials with cylindrical mes-
opores and narrow pore size distribution (Fig. 1b). The preserva-
tion of isotherms for the modified materials indicates absence of
significant structural changes with the silica host matrix. The
observed decrease in the BET surface area and total pore volume
after the modification reveals pore blocking due to deposition of
copper phase (Table 1) and this is more pronounced for the mate-
rials obtained by incipient wet impregnation (WI) procedure
(Table 1, samples denoted as nCu/SBA-15(B)). The deposition of
copper by OMCVD procedure (samples denoted as nCu/SBA-
15(A), where n = 5, 6 and 7) provides an increase in the specific
surface area and total pore volume as compared to the starting
2.2. Catalyst characterization
Copper content in the samples was determined by Atomic
Absorption Spectroscopy (AAS) with
a Perkin Elmer-3100

T. Tsoncheva et al. / Inorganica Chimica Acta 423 (2014) 145–151
147
3Cu/SBA-15 (A) material, obtained by simple WI procedure. A
simultaneous disappearance of micropores and creation of larger
mesopores are also observed (Fig. 1b and Table 1). We assign this
phenomenon to migration of the loaded during the starting step
copper particles with a simultaneous liberation of mesopores and
location of copper phase into the micropores. The observed
increase in the BET surface area could be also due to the deposition
of very finely dispersed copper particles (not excluded on the
external surface of silica matrix), which is accompanied with the
formation of additional interparticle mesoporosity (Fig. 1b and
Table 1).
the selected samples prepared by different methods are shown in
Fig. 2. The arrays of hexagonal two dimensional ordered layers of
pores are clearly identified on the images of SBA-15 loaded mate-
rials, supporting the data from the nitrogen physisorption and XRD
analyses for the preservation of silica structure during the copper
loading steps. Copper nanoparticles with almost spherical shape
are distinguished for all materials. The copper phase in the samples
obtained by (B) method predominantly consists of 10–20 nm par-
ticles (Fig. 2). According to nitrogen physisorption data, they are
located on the external surface and block the pore openings (see
above). Presence of more finely dispersed particles is not excluded
as well, but they seem to be in much lower amount and we could
not quantify them precisely. For the materials obtained by (A)
method TEM/STEM micrographs show a poor contrast mainly
due to the complex SBA-15 structure. Thus in order to isolate the
copper nanoparticles and better elucidate their size distribution
we dissolved the silica matrix with a mild procedure in diluted
HF at room temperature. The drawback effect of this treatment is
the loss of spatial information about the particles location (inside
or outside the pores), but reasonably the Cu nanoparticles struc-
ture was not affected. For these samples, copper particles about
40–100 nm are detected (Fig. 2) but in comparison with the WI
materials, here these particles appear much rarely as individual
species and many finely dispersed ones were also detected. In
Fig. 3 is presented particles size distribution for the samples
obtained by method (A). Particles between 1 and 3.5 nm dominate
in the sample 3Cu/SBA-15. The first OMCVD step (sample 5Cu/SBA-
15) leads to strong increase in the relative part of the particles
about 2.5–3 nm with a simultaneous about 3 times decrease in
the relative part of the smaller ones. The number of particles which
dimension is close or above the pore diameter of the silica support
is negligible. We could expect that during the first OMCVD step the
particles growth occurs into the pores of the support according to
Eqs. (1) and (2). However the significant increase in the pore vol-
ume for this sample as compared to 3Cu/SBA-15 (Table 1) urged
us to assume that this process also takes place on the external sur-
face. The observed strong decrease in the fraction of particles
below 2 nm supports our assumption that the precursor decompo-
sition occurs with higher probability on the metallic surface of the
already loaded copper nanoparticles (Eqs. (1) and (2)) and in much
less extent on the silica surface (Eqs. (3) and (4)). This assumption
is well supported with the particles size distribution during the
3.1.2. XRD measurements
The easily resolved three diffraction peaks in the XRD patterns
of parent SBA-15 silica and supported catalysts could be indexed
as (100), (110) and (200) planes which are typical of ordered hex-
agonal lattice (p6mm) of SBA-15 (support information, Fig.S1) [18].
The preservation of the characteristic reflections for all copper con-
taining materials confirms the nitrogen physisorption data (see
above) for the absence of significant structural collapse of the silica
host matrix during the modification. In the wide-angle region (sup-
port information, Fig. S1b) the strong diffraction peak at 2h of 43.3°
along with two weak peaks at 50.4° and 74.1° indicates presence of
well crystallized fcc Cu (JCPDS 04-0836). These reflections are not
well resolved only in the pattern of 3Cu/SBA-15(A), which could be
due to the presence of very finely dispersed amorphous copper
phase or to detection limitation. For the other materials, the inten-
sity of reflections increases along with the copper loading, which is
more evident for the samples from series (A). The average particle
size, calculated by the Scherrer’s formulae, increases from 20 to
40 nm and from 20 to 30 nm for the samples from series (A) and
(B), respectively with copper loading increase in the range of 5–
7 wt.% (Table 2). In addition, the weak and broad diffraction peak
at 2h of 36.8o attributed to finely dispersed Cu2O phase (JCPDS
78-2076) is also observed for all WI modifications, probably due
to partial re-oxidation of smaller copper particles after the expo-
sure of the samples to air.
3.1.3. TEM
More information for the support structure as well as for the
morphology and dispersion of the loaded copper phase was
obtained by TEM measurements. Representative TEM images for
SBA-15
SBA-15
a
b
3Cu/SBA-15(A)
5Cu/SBA-15(A)
6Cu/SBA-15(A)
5Cu/SBA-15(A)
6Cu/SBA-15(A)
7Cu/SBA-15(B)
7Cu/SBA-15(A)
7Cu/SBA-15(B)
7Cu/SBA-15(A)
3Cu/SBA-15(A)
6Cu/SBA-15(B)
0,0
0,2
0,4
0,8
1,0
relative press0u,r6e, p/p0
0
4
6
10 12
2pore diameter8, nm
Fig. 1. Nitrogen physisorption isotherms (a) and pore size distribution (b) of parent SBA-15 and selected copper modifications. The isotherms are shifted in y direction in
order to prevent overlapping.

148
T. Tsoncheva et al. / Inorganica Chimica Acta 423 (2014) 145–151
Table 2
Average copper particles diameter determined by XRD (dCuXRD) and TEM (dCuTEM) and specific activity (SA) for various samples in SCR of NO with CO (SANO–CO) or with mixture of
CO + CH4 (SANO–CO+CH4) and methanol decomposition (SAmethanol) at 630 K.
Sample
dCuXRD, nm
dCuTEM, nm
SANO–CO
SANO–CO+CH4
SAmethanol
3Cu/SBA-15(A)
5Cu/SBA-15(A)
6Cu/SBA-15(A)
7Cu/SBA-15(A)
5Cu/SBA-15(B)
6Cu/SBA-15(B)
7Cu/SBA-15(B)
Small
17
33
38
22
2.6
3.3
3.7
12.3
8.0
5.3
4.6
4.0
3.4
6.5
18.0
7.6
4.3
7.6
2.0
3.1
2.5
16.6
7.6
5.3
2.0
4.8
7.4
6.5
5.4
17.5
43.0
30.7
25
30
3Cu/SBA-15(A)
7Cu/SBA-15(A)
5Cu/SBA-15(B)
7Cu/SBA-15(B)
Fig. 2. STEM (up) and TEM (down) images of selected copper modifications.
second and third step of OMCVD procedure (Fig. 3, samples, 6Cu/
SBA-15 and 7Cu/SBA-15). Here the copper particles growth is well
established with a tendency of increase in the fraction of particles
with similar or larger diameter than the pores size. The observed
broadening of particles size distribution along with copper loading
also evidences that the regular growth of copper particles accord-
ing to Eqs. (1) and (2) is strongly restricted by 2D-pore structure of
SBA-15.
pronounced for 3Cu/SBA-15(A), does not exclude presence of
Cu2+ ions, which is probably a result of partial re-oxidation of finely
dispersed Cu particles.
In order to obtain more information for the surface oxidative
state of loaded copper particles XPS analyses were also performed.
The XPS spectra (Fig. support information, S3) for all copper mod-
ifications prepared by method (A) consist of major peak at BE
(Cu2p3/2) = 932.3 eV which is typical of reduced copper (Cu+ or
Cu0) species [19]. The slight asymmetry of the main peak towards
higher BE for the samples with higher copper content and the shift
of the maximum to BE = 933.1 eV for the samples with lower one
combined with the appearance of weak satellite peak at about
BE = 943.0 eV could be assigned to the presence of Cu2+ ions as well
[20,21] and refs. therein. Similar co-existence of copper in different
oxidation state is observed for the materials obtained by method
3.2. Spectroscopic data
UV–Vis spectra (support information, S2) for all copper modifi-
cations represent absorption feature at about 550–600 nm, which
is assigned to the plasmon resonance of metallic copper. The
appearance of absorption peaks about 240 and 760 nm, well

T. Tsoncheva et al. / Inorganica Chimica Acta 423 (2014) 145–151
149
30
20
10
0
are usually complex for interpretation, due to the existence of
interaction with the support surface and the location of particles
into the porous structure. As was illustrated, both preparation
techniques lead to the predominant formation of reduced copper
particles. That is why, before the TPR experiments, all the samples
were subjected to a standard treatment procedure consisting of
oxidation in air at 623 K for 1 h. The TPR-TG and TPR-DTG profiles
during the reduction of thus oxidized materials are compared in
Fig. 4. The main reduction effect is observed in the range of 430–
510 K. This reduction effect is smoother and shifted to higher tem-
perature for the (B) samples and indicates that more than 90% of
the copper is reduced in one step (Cu2+ ? Cu0) and it is probably
present in the form of well crystallized copper oxide particles with
different dispersion. On the contrary, the reduction of the samples
from series (A) starts at about 20 K lower temperature. The DTG
profiles are narrow and correspond to c.a. 80–90% reduction degree
of CuO to metallic copper. These features indicate a more uniform
dispersion of relatively smaller copper particles in the samples pre-
pared by method (A), in agreement with the TEM measurements. A
second well pronounced TPR-TG effect is also observed for all
materials above 650 K. It is better pronounced for the samples
from series (A) and could be related to the reduction of copper
oxide species, strongly interacted with the silica support which
are probably produced during the oxidation of very finely dis-
persed copper nanoparticles.
3Cu/SBA-15(A)
0
0
0
0
2
2
2
2
4
4
4
4
6
6
6
8
8
8
8
10
10
10
12
12
12
14
16
18
20
20
30
20
10
0
5Cu/SBA-15(A)
14
16
18
30
20
10
0
6Cu/SBA-15(A)
14
16
18
20
30
20
10
0
7Cu/SBA-15(A)
10 12 14
18 20
Me6an Diameter (nm)16
Fig. 3. Particles size distribution for the samples obtained by method (A).
(B). Due to the low intensity of Auger CuLMM spectra we calcu-
lated the modified Auger parameters (Table 3). This parameter
usually depends on the oxidation state of copper and according
ꢀ
to Kohler et. al. [22], aCu values of 1850.2 and 1847.3 eV belong
to Cu0 and Cu+, respectively. Here the aCu values for all materials
4. Catalytic tests
ꢀ
are between 1847.2 and 1848.8 eV which is closer to that one of
Cu1+. Thus co-existence of copper species in different oxidative
state is confirmed. Taking into account the data from the UV–Vis
and XRD measurements (see above), formation of a shell of amor-
phous oxide layer around the metal core after exposing the sam-
ples to air is assumed [23]. For each series of materials the
The application of various catalytic tests is a well-known strat-
egy to obtain valuable information for the dispersion and accessi-
bility of different metal particles hosted in porous matrix. Here
we used NO, CO, methanol and methane as probe molecules
because their effective diameter is much below the average pore
size of SBA-15 silica support. However we should take into consid-
eration that the location of particles in the porous structure con-
trols the catalytic process not only by the diffusion of reactants
and products to and from the catalytic sites, respectively, but also
by the formation of different intermediates. As reported [26],
methanol decomposes over copper catalysts through methyl for-
mate as an intermediate. For the SCR of NO with CH4 or CO, decom-
position of NO with the formation of surface O species and
generation of methoxy- or nitrate intermediates, respectively, are
proposed [27,28]. In Fig. 5 are presented temperature profiles of
SCR of NO with CO (Fig. 5a) or mixture of CO and CH4 (Fig. 5b)
and methanol decomposition to CO and hydrogen (Fig. 5c), respec-
tively. During the SCR of NO with CO or with a mixture of CO and
CH4, the materials prepared by (A) procedure exhibit higher cata-
lytic activity as compared to their analogues obtained by method
(B). The dependence is more complicated during methanol decom-
position test, where the samples obtained by (A) procedure exhibit
better catalytic activity only up to 5% Cu. In order to elucidate the
impact of the state of copper phase more precisely, we calculated
the specific activity (SA) as NO or methanol conversion (wt.%)
per unit copper loading (wt.%) (Table 2). For all catalytic tests the
extremely high SA is registered for 3Cu/SBA-15, which gradually
relative amount of copper ions on the surface (Cusurf/Cubulk
)
decreases with the increase in copper loading (Table 3). According
to TEM and XRD data this could be assigned to a decrease in copper
dispersion. We should also stress on the higher Cu exposure to the
surface for the materials obtained by method (B) as compared to
method (A). This observation well supports our assumption for dif-
ferent distribution of copper particles depending on the prepara-
tion procedure. Copper species seems to be larger and located in
a high extent on the external surface for the materials prepared
by (B) procedure, while the modified OMCVD method favors the
formation of very finely dispersed nanoparticles located deeply
into the micropores and the small mesopores of the silica matrix
together with a low fraction of isolated and large nanoparticles
on the external surface.
3.3. TPR-TG measurements of oxidized samples
The TPR method is a powerful strategy to obtain more informa-
tion for the dispersion of supported copper particles. Generally, in
the case of bulk metal oxides smaller particles are more easily
reduced [24,25]. However, for the supported materials, the results
Table 3
ꢀ
Surface concentrations and a values for copper samples obtained by various methods.
Mass ratio, Cu/Si, 102
Mass ratio, Cusurf/Cubulk
ꢀ
aCu
Sample
O, wt.%
Si, wt.%
Cu, wt.%
3Cu/SBA-15(A)
5Cu/SBA-15(A)
6Cu/SBA-15(A)
7Cu/SBA-15(A)
45.7
45.8
48.9
48.2
52.2
51.8
49.4
50.4
2.1
2.4
1.7
1.4
4.02
4.63
3.44
2.78
0.64
0.48
0.28
0.20
1847.6
1848.1
1847.6
1847.2
5Cu/SBA-15(B)
6Cu/SBA-15(B)
7Cu/SBA-15(B)
48.9
44.1
46.1
46.4
50.9
49.1
4.7
5.0
4.8
10.1
11.3
9.8
0.95
0.81
0.67
1848.8
1848.6
1848.2

150
T. Tsoncheva et al. / Inorganica Chimica Acta 423 (2014) 145–151
method (A)
method (B)
b
a
3Cu/SBA-15
3Cu/SBA-15
5Cu/SBA-15
6Cu/SBA-15
7Cu/SBA-15
5Cu/SBA-15
6Cu/SBA-15
7Cu/SBA-15
400
500
600
700
800
400
500
600
700
800
Temperature, K
Temperature, K
Fig. 4. TPR-TG (a) and TPR-DTG (b) profiles of copper modifications obtained by method (A-solid line) and (B-dash line). The profiles are shifted in y direction in order to
prevent overlapping.
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
a
b
3Cu/SBA-15(A)
5Cu/SBA-15(A)
6Cu/SBA-15(A)
7Cu/SBA-15(A)
5Cu/SBA-15(B)
6/SBA-15(B)
c
3Cu/SBA-15(A)
5Cu/SBA-15(A)
6Cu/SBA-15(A)
7Cu/SBA-15(A)
5Cu/SBA-15(B)
6Cu/SBA-15(B)
7Cu/SBA-15(B)
5Cu/SBA-15(A)
6Cu/SBA-15(A)
7Cu/SBA-15(A)
3Cu/SBA-15(A)
5Cu/SBA-15(B)
7Cu/SBA-15(B)
6Cu/SBA-15(B)
7/SBA-15(B)
300 400 500 600 700 800
300 400 500 600 700 800
500
600
700
800
Temperature, K
Temperature, K
Temperature, K
Fig. 5. Temperature dependencies of conversion in various catalytic tests: SCR of NO with CO (a), SCR of NO with CO and methane (b) and methanol decomposition to CO and
hydrogen (c).
decreases with copper loading increase, despite the preparation
procedure used. Taking into account TEM (Fig. 3 and Table 2),
nitrogen physisorption (Table 1) and XPS (Table 3) data we assign
this phenomenon to the extremely high activity of copper particles
below 2 nm. We assume that the particles growth above this size
results in a strong decrease in SA even at 5 wt.% copper loading.
Obviously, the higher dispersion in 5Cu/SBA-15(A) as compared
to 5Cu/SBA-15(B) ensures higher SA for the former material in all
catalytic tests. We can understand the observed results, taking into
account that for many reactions, named structure sensitive, the
activity per surface atom varies with particle size and morphology
[3,29]. It was found that the geometry of transition metal surface
sites can have a pronounced impact on the dissociation of diatomic
reaction structure sensitive [33]. We have recently reported size
dependent effect of methanol decomposition on Ni/SiO2 and Cu/
SiO2 [11,34]. Note, that the relation between the copper loading
and SA for the samples prepared by different procedures becomes
more complicated with the increase in copper loading above 5 wt.%
(Table 2). This could be due to the superposition of the size depen-
dent effect with the effect of accessibility of highly active copper
particles into the pore matrix. The physicochemical data (see
above) clearly indicate growth of copper particles with the copper
loading increase, which is better controlled using the procedure
(A). However, this procedure provides simultaneous blocking of
copper particles into the micro- and mesopores and deposition of
particles with different dispersion on the external surface.
molecules containing
p-bonds, such as NO and CO [30]. This
process occurs over ensembles consisting of 5 or 6 metal atoms
[31], which number is considered to be maximal for metal nano-
particles in the range 1.8–2.5 nm [32]. The formation of methyl for-
mate intermediate during the methanol decomposition was also
related to the activity of a specific geometric array (ensemble)
which is only achieved by a critical copper diameter, making this
5. Conclusions
Copper deposition on SBA-15 by modified OMCVD procedure,
which combines preparation of starting material with low copper
loading by simple incipient wetness impregnation technique and

T. Tsoncheva et al. / Inorganica Chimica Acta 423 (2014) 145–151
[5] A. Taguchi, F. Schüth, Microporous Mesoporous Mater. 77 (2005) 1.
151
further step-wise OMCVD of Cu(hfac)2 in presence of hydrogen,
results in controlled growth of finely dispersed copper particles
in relatively narrow size distribution. The restricted mass transfer
of the precursor in 2D-silica structure provides its decomposition
on the external surface as well, leading to the formation of isolated
relatively large copper particles. The regular growth of copper par-
ticles with the increase of OMCVD steps and the blocking of more
finely dispersed ones into the pores leads to a decrease in the spe-
cific catalytic activity for various catalytic tests (SCR of NO with CO,
SCR of NO with a mixture of CO and CH4, methanol decomposition
to CO and H2). Nevertheless, during the SCR of NO with CO or with
a mixture of CO and CH4, the materials prepared by OMCVD proce-
dure exhibit higher catalytic activity as compared to their ana-
logues obtained by simple impregnation technique. Just the
opposite, WI method seems to be more preferable procedure than
the modified OMCVD for the preparation of catalysts for methanol
decomposition with relatively high (above 5 wt.%) copper loading.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2014.08.008.
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