CHEMCATCHEM
FULL PAPERS
(220) [JCPDS file no. 85-1326] corresponding to metallic Cu
phase were more intense than those for the 5% H2 and 100%
H2 reduced Cu–Al-7 samples at 2008C. The medium- and low-
intensity peaks at 2q=36.48 (111), 61.78 (220) and 38.88 (111)
indicate the presence of Cu2O and a very small amount of un-
reduced CuO phases. In spite of a higher extent of metallic Cu
in Cu–Al-7 reduced at high temperature, its activity was much
lower than that of the low-temperature-reduced samples.
In the case of used catalyst (100% H2 reduced at 2008C),
high-intensity peaks at 2q=43.38 (111), 50.48 (200), 74.18
(220) and 36.68 (111), 61.78 (220), 29.48 (110) confirmed the
progressive reduction of CuO to Cu0 through Cu2O under
WGSR conditions (Figure 3A). However, complete reduction of
Cu2O to Cu0 is still not possible as water is known to oxidize
Cu to Cu2O but not further to CuO.[33] The presence of stable
CuO species (peaks at 2q=35.58 (002), 38.88 (111), 48.88
(202)) was owing to the inaccessibility by surrounding Alx–Oy.
Thus, coexistence of various reduced and oxidized Cu species
contribute to the activity of the developed Cu–Al-7 catalyst.
Used Cu–Al-7 also displayed diffraction peaks at 2q=29.68
(040), 49.38 (200), 67.68 (171) [JCPDS file no. 72-0359] corre-
sponding to boehmite (AlOOH) phase formed under WGS con-
ditions under which steam is one of the prevailing compo-
nents.[34] Thus, not only Cu aggregation is prevented by the
formation of boehmite but also the Cu particle size becomes
stabilized as a result of the presence of Cu2O phase in 100%
H2 reduced and used samples, which was also evidenced from
the high-resolution TEM images (see Figure 5).[35]
tion, because high-temperature reduction was mainly responsi-
ble for the formation of reduced Cu species, which is also evi-
dent from the XRD studies (Figure 3B). If the potential scan
was reversed, the reduction of anode current peak AIII occur-
ring as CIII at 0.4 V and CI peak at ꢀ0.350 V could be related to
metallic Cu species. The maximum reduction peak potential
(Ic,max) for the sample reduced with 100% H2 at 2008C was
higher than those of the samples reduced with 5% H2 at 200
and 3408C owing to the presence of electrochemically active
sites in the former sample, which had maximum WGSR activity.
The effect of reduction conditions on morphology and Cu
aggregation was studied by field-emission SEM and the results
are shown in the Supporting Information, Figure S1. Spherical
Cu particles were observed in a sample reduced under 5% H2
at 2008C (Figure S1A). In contrast, the sample reduced at the
same temperature but with 100% H2 exhibited an appreciable
aggregation of Cu particles (Figure S1B). The spherical mor-
phology was completely transformed to a dendrite-like struc-
ture (Figure S1C) probably owing to a still higher extent of ag-
gregation of smaller Cu particles at 3408C reduction tempera-
tures. A similar observation was made by a comparative study
of HRTEM images as shown in Figure S2. These structural
changes contributed to the difference in activity of the rele-
vant samples.
The effect of different conditions on the particle size, mor-
phology, and fringe patterns of Cu–Al-7 catalyst was investigat-
ed in more detail by HRTEM characterization (Figure 5). As
shown in Figure 5A, Cu–Al-7 catalyst reduced under 5% H2
consists of bigger size (14–16 nm) particles than the sample re-
duced under 100% H2 (5–7 nm, Figure 5B). It is most likely
that 100% H2 reduction led to the formation of smaller partial-
ly and completely reduced Cu2O and metallic Cu particles, re-
spectively, as confirmed by XRD analysis. Under 5% H2 reduc-
tion conditions, the catalyst mainly exists as CuO confirmed by
the fringe pattern in Figure 5C that matches very well with the
XRD results discussed above. Reduction under 100% H2
formed the catalyst with a majority of smaller size Cu2O and
Cu0 particles, as seen from the fringe pattern in Figure 5D. In
addition, the needlelike typical morphology of boehmite was
also observed (Figure S3B–D).[37] Even after the reaction time
of 24 and 100 h, particle size was stable in the range of 4–
7 nm as shown in Figure S4A. The fringe pattern (Figure S4B)
of 24 h used sample indicates that metallic Cu phase was
formed during the reaction. In the case of the 100 h used
sample, a number of small Cu metal particles (5 nm) were seen
on the boehmite (Figures 5E, S3D), the latter helped to pre-
vent the aggregation of metal particles. The fringe pattern of
this sample shown in Figure 5F confirmed the existence of
metallic Cu, Cu2O, and boehmite (AlO(OH)) phases.[29] The for-
mation of the boehmite phase was also evident from the SEM
images without any change in needlelike morphology in the
used samples of 24 and 100 h reaction time, as shown in
Figure 6. Cu metal aggregation was visible for the particles
separated from the boehmite phase.
The active sites present in Cu–Al-7 catalyst reduced under
various conditions were also estimated by cyclic voltammetry.
In Figure 4, cyclic voltammograms of 100% and 5% H2 re-
duced Cu–Al-7 catalysts are shown at 200 and 3408C. The sam-
ples reduced with 100% H2 at 2008C and 5% H2 at 3408C ex-
hibited three anodic current peaks, namely, AI, AII, and AIII at
ꢀ0.46, ꢀ0.20, and 0.157 V related to the formation of Cu2O,
CuCl salt layer, and CuO, respectively.[36] In contrast, the sample
reduced with 5% H2 at 2008C exhibited a single shallow
anodic current peak AIII at ꢀ0.20 V. The current density and the
potential of anodic peak AI increased with the increase in re-
duction temperature from 200 to 3408C even for 5% H2 condi-
The inhibition of Cu metal particle aggregation by in situ for-
mation of boehmite leads to high stability of the catalysts, as
discussed below.
Figure 4. Cyclic voltammograms of Cu–Al-7 catalysts reduced at 200 and
3408C.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
4
&
ÞÞ
These are not the final page numbers!