I.C. Freitas et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 26–37
29
Table 1
Textural properties of Cu/ZrO2 samples.
2
3
2
Sample
ZrO2
SBET (m /g)
Vp (cm /g)
SACu (m /gcat)
DCu (%)
dCu (nm)
212
186
175
146
137
0.9
0.8
0.7
0.6
0.6
–
–
34
23
14
8
–
5
1
2
3
Cu/ZrO2
10.4
13.8
14.7
12.3
3.1
4.4
7.6
12.6
0Cu/ZrO2
0Cu/ZrO2
0Cu/ZrO2
corresponds to the highest metal particle size of Cu0 in Cu/ZrO2
with the highest copper loading of 30 wt.% (Table 1).
Some textural characteristics of t-ZrO2 and calcined Cu/ZrO2
samples with different Cu content are listed in Table 1. The metal-
of metallic copper species, to a higher kinetic energy of 919.3 eV
for 30Cu/ZrO2 sample (Fig. 2B and Table 2). The values of modified
Auger parameter, ˛Cu, are 1850.8–1851.1 eV and 1846.8–1847.4 eV,
which correspond to Cu0 and Cu species, respectively (Table 2). It
+
+
lic Cu surface area, SACu, the apparent metallic Cu dispersion, DCu
,
can be concluded that the Cu species is dominant on the surface of
and the metal particle size measured by N O are also given in the
Table 1. The BET specific surface areas and pore volume of Cu/ZrO2
Cu/ZrO2 sample with the lowest Cu content (5Cu/ZrO2), whereas
metallic copper is prevailing for samples with higher Cu content
2
0
samples are lower compared to those of t-ZrO2 support. SBET of
(20 and 30 wt.%). There is some equilibrium between the Cu and
2
+
pure zirconia is 212 m /g. The increase of Cu content leads to a
Cu species for 10Cu/ZrO2, being seen in Fig. 2B.
decrease of the values of SBET and Vp caused by: (i) consumption
of surface hydroxyl groups of the support by reaction with the
active oxide precursor up to 10 wt.% Cu and (ii) agglomeration of
CuO at Cu content ≥20 wt.% and coverage of support pores. The
latter is supported by the XRD spectra of calcined 20Cu/ZrO2 and
All Cu/ZrO2 samples exhibit a spin–orbit doublet of the Zr 3d
core level into 3d5/2 and 3d3/2 levels with energy gap of 2.4 eV
between them and a relative intensity ratio (I 3d5/2/I 3d3/2) of 1.5
(Fig. 2C). This indicates the existence of ZrO2-like species according
to the literature data [28]. Decomposition of the spectra produces
peaks attributed to the existence of two kinds of zirconium species,
assigned to species I with low BE at ca.182.3 eV (ZrI), characteristic
3
0Cu/ZrO2 samples, where there is an increase of the peak inten-
sity of supported Cu oxide species (Fig. 1A). The highest apparent
0
4+
Cu dispersion measured by N O titration is observed for Cu/t-ZrO
of Zr ions in pure ZrO2 and species II with higher BE at ca.183.7 eV
2
2
sample with the lowest Cu content (5 wt.%), to which corresponds
the smallest metallic Cu particle size of 3.1 nm (Table 1). The low-
est apparent Cu dispersion is detected for 30Cu/ZrO2 caused by
(ZrII) (Table 2) due to the formation of zirconium species bound to a
more electron-attractive species and formation of partially reduced
Zr sites [28]. The fraction of ZrI species for all samples is larger
0
␦+
the copper agglomeration. On the other hand, the 5Cu/ZrO2 sam-
ple is characterized by the lowest surface metallic area, probably,
due to a strong interaction between the copper oxide species and
surface of zirconia carrier.
compared to that of species ZrII.
The value of atomic XPS Cu/Zr ratio of reduced Cu/ZrO2 sam-
ples increases in the following order: 5Cu/ZrO2 > 10Cu/ZrO2 >
20Cu/ZrO2 > 30Cu/ZrO2 (Table 2). A strong increase of the XPS
intensity Cu/Zr ratio is observed up to 20 wt.% Cu and no too strong
with further increase of the Cu content (at 30 wt.%). The highest
Cu/Zr ratio value of 1.46 for 30Cu/ZrO2 means a Cu enrichment on
the ZrO2 surface.
The XPS of O 1s core electrons (Fig. 2D) are broad due to over-
lapping contributions of lattice oxygen from zirconia and supported
copper species. The broad O 1s peaks are decomposed in two peaks
at the corresponding position using XPS peak splitting program
(XPS Casa Software). According to the literature [29,30] there might
be two types of oxygen species in the Cu/ZrO2 system after reduc-
tion: oxygen species of ZrO2 and/or Cu O (O ) and oxygen species
3.2. Surface properties
3.2.1. XPS analysis
The catalyst surface composition and oxidation state of the com-
ponents are investigated by XPS. XPS of Cu 2p and Auger, Zr 3d and
O 1s core electron levels for reduced Cu/ZrO2 samples are shown
in Fig. 2A–D, respectively. The XPS parameters are summarized in
Table 2. The values of BEs of Cu 2p3/2, Zr 3d5/2 and of O 1s core elec-
tron as well as the values of the width at half maximum (FWHM)
of Cu 2p3/2 are reported in Table 2. All reduced samples exhibit
symmetric Cu 2p3/2 and Cu 2p1/2 main peaks with BEs values at ca.
2
I
of Zr OH (O ), whose binding energy are at ca. 530.3 eV and at ca.
II
532.2 eV, respectively (Table 2).
9
32.5–931.9 eV and ca. 952.4 eV, respectively, and with a spin–orbit
coupling energy of about 20 eV (Fig. 2A). A shakeup satellite at ca.
42 eV is not detected, which suggests the absence of Cu2 species
+
3.2.2. DRIFT spectra of CO adsorption
9
[
22]. It should be noted, that there is some increase of the value of BE
of Cu 2p3/2 with decrease of Cu content (from 931.9 to 932.5 eV for
0Cu/ZrO2 and 5Cu/ZrO , respectively, Table 2), probably, caused
DRIFT spectra of CO adsorption on reduced Cu/ZrO2 samples
−
1
in the high frequency (HF) region of 2200–2000 cm
and in
−
1
3
the low frequency (LF) region of 1800–1200 cm
are shown in
2
by an electron transfer from Cu to ZrO . The FWHM of Cu 2p3/2 is
Figs. 3A–D and 4 A–D, respectively. The figures also display the
spectra of temperature-programmed desorption (TPD) of CO at
373–773 K. A first inspection of the figures reveals that the distri-
bution of the bands for all samples is quite similar. A description of
the bands is shown in Table S1 (see Supplementary Material).
According to the literature data [31], in the HF spectral region
the CO adsorption on copper surface exhibits IR bands associated
with the linear or bridge-bonded CO species interacting with CuO,
2
2
.1–2.4 eV, which means the presence of at least two kinds of sur-
face copper species in different chemical environments. Therefore,
+
traces of other species, like Cu , cannot be excluded, since the BEs
of Cu+ generally overlap with those of Cu in the Cu 2p core level.
0
0
+
To distinguish Cu from Cu species, having similar BE, the X-ray
induced Auger electron spectra of reduced Cu/ZrO2 samples in the
kinetic region of 928–902 eV are presented in Fig. 2B. A clear differ-
0
+
ence is observed in the shapes of Cu L M45M45 Auger photoelectron
Cu O or Cu sites [31]. In general, the bands related to Cu CO car-
3
2
−
1
spectra of the samples due to the specific bonding interactions
at the interface between the both metal and oxide species. Peaks
bonyls are revealed at 2116–2111 cm , while the bands assigned
to CO adsorbed on Cu0 species are detected at ≤ 2100 cm [31].
Practically, there is no difference in the copper spectral region
of the IR spectra of reduced Cu/ZrO2 samples being seen in the
−1
at 918.8–919.3 eV and at 914.6 eV (Table 2) are revealed that are
0
assigned to Cu and Cu O, respectively [27]. The increase of Cu con-
2
−
Fig. 3A–D. Well defined band with a maximum at ca. 2097 cm
1
tent in Cu/ZrO2 samples leads to a shift of the peak, characteristic