Design of micro-shell Cu-Al porous ceramometals as catalysts for the water-gas shift reaction

Article abstract of DOI:10.1039/c7ra06672d

XRD, NMR, SEM with mapping, and XPS were used to examine the structure and microstructure of porous ceramometals, CuAlO/CuAl, synthesized by mechanochemical alloying of a mixture of Cu + Al (Cu:Al = 87:13 wt%) followed by hydrothermal treatment and heat treatment in air. The egg-shell nature of the microstructure was revealed: the metallic cores consisting of copper and aluminum alloys are surrounded by the oxide matrix containing copper oxides and X-ray amorphous mixed oxides of copper and aluminum. Catalytic activity of ceramometals in the water-gas shift reaction (WGSR) estimated for their fine fractions at 240 °C and expressed as efficient first-order rate constants is lower than that of a CuZnAl oxide catalyst due to the lower (approximately by an order of magnitude) specific surface area of the ceramometals. The specific activity of ceramometals expressed as the specific rate constant related to the unit surface area of metallic copper exceeds that for the CuZnAl oxide catalyst and correlates with the content of crystallized intermetallics. Mechanical, textural and thermophysical properties of ceramometals were assessed. The presence of ultramacropores with sizes up to tens of microns was shown to be typical for ceramometals. As a result, the activity of granulated ceramometal catalysts in the WGSR, due to a high diffusion permeability, is comparable with that of granulated CuZnAl oxide. A mild leaching substantially increases the activity of cermet granules.

Full text of DOI:10.1039/c7ra06672d

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Design of micro-shell Cu–Al porous ceramometals
as catalysts for the water–gas shift reaction
Cite this: RSC Adv., 2017, 7, 42443
a
T. Minyukova,a K. Valeev,a S. Cherepanova,a A. Salanov,a
*
S. Tikhov,
V. Kaichev,
ab
a
A. Saraev,ab A. Andreev,
O. Lapinaab and V. Sadykovab
XRD, NMR, SEM with mapping, and XPS were used to examine the structure and microstructure of porous
ceramometals, CuAlO/CuAl, synthesized by mechanochemical alloying of a mixture of Cu + Al (Cu : Al ¼
87 : 13 wt%) followed by hydrothermal treatment and heat treatment in air. The egg-shell nature of the
microstructure was revealed: the metallic cores consisting of copper and aluminum alloys are
surrounded by the oxide matrix containing copper oxides and X-ray amorphous mixed oxides of copper
and aluminum. Catalytic activity of ceramometals in the water–gas shift reaction (WGSR) estimated for
their fine fractions at 240 ꢀC and expressed as efficient first-order rate constants is lower than that of
a CuZnAl oxide catalyst due to the lower (approximately by an order of magnitude) specific surface area
of the ceramometals. The specific activity of ceramometals expressed as the specific rate constant
related to the unit surface area of metallic copper exceeds that for the CuZnAl oxide catalyst and
correlates with the content of crystallized intermetallics. Mechanical, textural and thermophysical
properties of ceramometals were assessed. The presence of ultramacropores with sizes up to tens of
microns was shown to be typical for ceramometals. As a result, the activity of granulated ceramometal
catalysts in the WGSR, due to a high diffusion permeability, is comparable with that of granulated CuZnAl
oxide. A mild leaching substantially increases the activity of cermet granules.
Received 15th June 2017
Accepted 24th August 2017
DOI: 10.1039/c7ra06672d
rsc.li/rsc-advances
and mesopores. Such catalysts could also prevent emergence of
temperature gradients within the catalytic layer due to a high
1 Introduction
thermal conductivity provided by the presence of metal parti-
cles in their bulk. Cermet catalysts synthesized from Al–Cu–Fe
alloys were applied earlier for the steam reforming of meth-
anol.11,12 These catalysts possess a complex microstructure
comprised of the metal cores surrounded by the oxidic shell
with segregated nanoparticles of metallic Cu. The specic
activity of copper was found to change non-monotonously with
its dispersion.11 The aim of the present work was development
and detailed characterization of a new ceramometal CuAlO/
CuAl catalyst prepared through the step of mechanical alloy-
ing of Cu–Al powders. Main characteristics of these catalysts
precursors were reported in the previous work.13
The water–gas shi reaction (WGSR) is one of the primary
industrial steps of hydrogen production. In recent decades,
many works devoted to kinetics and properties of catalysts for
this process have been published1–5 and summarized in a series
of reviews.6–9 These catalysts are usually comprised of Cu, Zn
and Al oxide composites prepared by coprecipitation or
impregnation. Among the key properties of a catalyst (support)
for WGSR is its ability to ensure a high dispersion of copper.
According to data reported in ref. 2, specic activity of metallic
copper in the WGS remains nearly constant in a broad range of
dispersions, while structure sensitivity is claimed in other
works.1 Hence, variation of conditions of catalytic testing makes
it difficult to compare the results. Another main problem of the
low temperature WGSR reaction is a low activity of the
conventional granulated CuZnAl catalyst per the unit volume of
the catalyst bed8–10 explained by a low loading density and poor
access of reagents into the bulk of granules with a low porosity.
A possible way to increase such a volume activity is the use of
ceramometal catalysts with enhanced real and loading density
possessing also a developed system of transport macropores
2 Experimental
2.1. Preparation
Aluminum powder (PAP-2, GOST 5494-95, Russia) with platelet-
shaped particles (typical sizes 10/40 microns) and electrolytic
copper powder (PMS-1, GOST 4960-75, Russia) of dendritic
shape were used as starting materials.
(a) Mechanical activation (MA) of the mixture containing
(wt%) 87Cu + 13Al was used to synthesize the powdered
precursor. MA was carried out in a high-power planetary ball
mill APF (the diameter of milling steel balls 5 mm; ball-to-
aBoreskov Institute of Catalysis SB RAS, Lavrentieva Ave., 5, 630090 Novosibirsk,
Russia. E-mail: tikhov@catalysis.ru
bNovosibirsk State University, Pirogova Str., 2, 630090 Novosibirsk, Russia
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powder mass ratio 8 : 1; powder weight 100 g; balls acceleration particles in an organic binder followed by polishing. The latter
65 g). The activation time was 3–12 min. The increase of MA technique allowed to study the microstructure of the powder
time decreases the average particle size and increases the particles. Transmission electron microscopy (TEM) studies of
alloying degree. The vials and milling balls were made of steel. powders were carried out using JEM-2010 and JEM-2200FS
The volume of the milling vial was 1 L. The content of Fe (JEOL Ltd.) microscopes; High-Angle Annular Dark Field Scan-
incorporated into the powder mixtures as contamination was ning Transmission Electron Mode (HAADF STEM) and Energy
very low (0.14 ry0.02 wt%). To prevent sticking of the powder to Dispersive X-ray Spectroscopy (EDX) were utilized. The EDX in
balls and vial walls, ethanol was used as a process control agent. the STEM mode was used for the elemental mapping of samples
It was added into the vial at 30 s before the start of milling.
by Al K (E ¼ 1.486 keV) and Cu K (E ¼ 8.040 keV) radiation and
(b) The product of mechanical activation was loaded into semi-quantitative analysis of areas as small as 1 nm in diameter.
a stainless steel die specially designed to ensure a free access of To prepare samples for TEM studies, powders were dispersed in
water and hydrogen release. Variation of the internal shape of ethanol and suspensions were subjected to ultrasonic treatment
the die where a powder was loaded provides a broad variation of at 35 kHz. The suspension was poured onto a holey carbon
ꢀ
the shape of monoliths.
substrate on a Mo grid and dried at 20 C.
(c) For the hydrothermal treatment (HTT), the loaded die was
Al NMR experiments were carried out on a Bruker Avance 400
placed into the vessel with boiling water and kept there for ve MHz spectrometer with a magnetic eld of 9.4 T. Spectra were
hours. This step provides formation of strong monoliths due to recorded at resonance frequencies of 27Al and 63Cu in this
conjugation of the red-ox reaction of aluminum oxidation and magnetic eld, i.e. 104.31 and 106.12 MHz, respectively. Due to
creation of contacts between the particles due to generation of rather broad resonance lines and experimental dead time, the
aluminum hydroxides. Similar to the MA step, oxidation of Al at spectra were acquired by spin echo technique with two equiv-
the HTT step diminishes the extent of the exothermic reaction alent pulses p1 – s – p1 – s. The pulse duration p1 of 27Al NMR
of Me–Al interaction with air at the calcination step.
experiments was 0.5 ms (p/8), and for 63Cu NMR spectra p1 was 1
(d) The monolithic product of HTT was removed from the ms (p/6). The interpulse delay s was 15 ms for static experiments,
die, dried for 1 hour at 120 ꢀC and calcined under air at 550 ꢀC. and it was synchronized with the sample spinning rate in MAS
This yields a mechanically strong material and develops the (magic angle spinning) experiments. The experiments were
mesoporous structure of cermets. The density of granules carried out with different MAS probes of 4, 2.5 and 1.3 mm outer
increased with decreasing the time of mechanical activation rotor diameter providing sample rotation at 15, 30, and 60 kHz.
from 3.87 (12 min) to 4.24 g cmꢁ3 (3 min).
The static spectra were obtained with the 4 mm probe. The
The reference sample was represented by the CuZnAl bulk spectral width used in experiments was 5 MHz, and the
oxide catalyst (similar to the commercial one) synthesized by sequence repetition delay varied from 0.1 to 2 s since all
the heat treatment of mixed Cu–Zn–Al hydroxide (CZA). The samples contained intermetallic and paramagnetic compounds
hydroxide was obtained via coprecipitation from 10% aqueous which decrease T1 relaxation times. The chemical shis were
3+
solutions of metal nitrates by a sodium carbonate solution with externally referenced to diluted Al(OH)6 at 0 ppm in 27Al
subsequent washing and drying according to the procedure experiments, and to copper wire at 2396 ppm.15–17 The decom-
reported in a patent.14
position of static spectra was done on Lorentzian and Gaussian
lines to obtain the best t.
The chemical analysis of the catalyst surface was carried out
using X-ray photoelectron spectroscopy (XPS). The XPS
2.2. Characteristics of samples
X-ray diffraction (XRD) patterns of powders were recorded by measurements were performed on a SPECS's Surface Nano
means of a D8 Advance powder diffractometer (Bruker AXS) Analysis GmbH (Germany) photoelectron spectrometer equip-
equipped with a one-dimensional Lynx-Eye detector using Cu ped with a PHOIBOS-150-MCD-9 hemispherical electron energy
Ka radiation. XRD patterns were recorded in the 2q range analyzer, a FOCUS-500 X-ray monochromator, and an XR-50M
15–95ꢀ with a step of 0.05ꢀ and a counting time of 3 s per step. X-ray source with a double Al/Ag anode. The spectrometer was
The quantitative analysis was carried out using Rietveld also equipped with a high-pressure cell (HPC), which enables
renement in TOPAS soware (Bruker AXS GmbH). The average heating samples before analysis in gaseous mixtures at pres-
size of the crystallites was also determined using TOPAS taking sures up to 0.5 MPa.18 The core-level spectra were obtained
into account the instrumental peak broadening. The instru- using the monochromatic AlKa radiation (hn ¼ 1486.74 eV)
mental peak broadening was determined from the XRD pattern under ultrahigh vacuum conditions. The charge correction was
of a reference sample (SRM676, corundum) by the approxima- performed by setting the Al 2p peak at 74.5 eV corresponding to
tion method using a convolution of Lorentzian and Gaussian aluminum in Al2O3. Before the XPS analysis, the catalysts were
functions. The aluminum content in Cu1ꢁxAlx solid solutions treated in 1 bar O2 at 270 ꢀC for 1 h in HPC. Aer treatment of
was calculated using Vegard's law.
samples in the HPC the intensity of C 1s peak was very low,
Scanning electron microscopy (SEM) studies of powders which did not allow this peak to be used for calibration of the
were performed on a JSM 6460SV microscope (JEOL Ltd.) binding energy scale. Relative element concentrations were
equipped with an energy-dispersive spectroscopy unit EDX determined from the integral intensities of XPS peaks using the
INCA (Oxford Inst.). Samples for these studies were prepared cross-sections according to Scoeld.19 For detailed analysis, the
either by spreading the powders over a substrate or by xing spectra were tted into several peaks aer the background
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subtraction by the Shirley method. All the spectra were analyzed dimensions 3.2 ꢂ 3.2 ꢂ 5 mm3 was also studied in the special
using the CasaXPS soware.18 The line shape used for tting the Temkin single-row reactor.23 Due to a considerable variation of
Al 2p and Cu 3p spectra was the product of Lorentzian and the bulk density at close weights, contact time also changed
Gaussian functions. An asymmetric, so-called “LF” line shape signicantly, from 0.02 to 0.05 s. Thermal stability (kstab) was
was used for approximation of the Cu 2p3/2 spectra of copper.20 estimated from the ratio of k at 240 ꢀC aer 20 h of operation to
Note that the asymptotic form of the LF line shape is equivalent the initial k (without overheat).
to the asymptotic form of the theoretical Doniach-Sunjic
asymmetric line shape. However, this new line shape
3 Results and discussion
3.1. Phase composition according to XRD and NMR data
describes the asymmetric XPS peak more correctly than the
conventional Doniach-Sunjic function does because the LF
function is nite. Aer the XPS analysis of the oxidized samples,
the catalysts were treated in 1 bar H2 at 270 ꢀC for 1 h, and the
3.1.1. XRD. According to XRD (Fig. 1), ceramometals
contain a solid solution of aluminum in copper and an inter-
XPS analysis of the reduced catalysts was carried out. XPS
metallic, with the solid solution being the dominant phase, as
data were used to estimate the surface concentration of copper
seen from the intensities. The copper oxide phase was observed
(QCu ¼ Cu/(Cu + Al)) and the surface area of metallic copper SCu
in all tested samples while separate phases of aluminum or
aluminum oxide were not detected. Traces of a well crystallized
phase with the interplanar distances of 3.37, 2.75, 2.46, 2.23 and
z SBET$QCu
.
The macropore volume was assessed from the difference
between the volumes of micro/mesopores and the total pore
volume. The total pore volume of the monolith was estimated
from the values of true and apparent densities. True density was
measured using a helium pycnometer – Autopycnometer 1320
(Micromeritics). The fraction of micropores and mesopores
(called for brevity the “mesopores) as well as the specic surface
area (SSA) were determined from adsorption isotherms of
nitrogen recorded at 77 K using an ASAP-2400 Micromeritics
instrument. The macropore volume was estimated from the
difference of the total and mesopore volume.
˚
2.06 A not found in the PDF database (see an inset in Fig. 1)
were detected. The most likely this phase belongs to an
unknown intermetallic structure.
Table 1 lists qualitative and quantitative results of XRD
phase analysis of ceramometals corresponding to diffraction
patterns in Fig. 1. The new phase is not presented in Table 1
because it has a low (ca. 1 wt%) concentration.
As seen in Table 1, the concentration of Al4Cu9 intermetallic
goes through the maximum with increasing the time of
preliminary MA of the copper–aluminum powder. Such changes
in the intermetallic concentration were observed earlier by XRD
Mechanical properties (compressive strength) were esti-
mated as the limiting pressure that can be applied to the circle
side of cylindrical pellets or granules before its destruction.21,22
Thermal expansion coefficient was measured on a DIL-402C
(Netzsch) horizontal dilatometer. Samples were examined as
prisms with a section of 6 ꢂ 6 mm2 and a length of 10 mm in
the range from 50 to 500 ꢀC in air at a heating rate of 2ꢀ minꢁ1
.
Catalytic properties in WGSR were studied in a laboratory
ow setup with the gas chromatographic analysis of the reac-
tion mixture under the same conditions as earlier used for
studies of the commercial Cu–Zn–Al–O catalyst.9 The catalysts
were activated in the mixture of 5% H2 in He at 3000–4000 hꢁ1
with a temperature ramp of 2ꢀ minꢁ1 up to 270 ꢀC for 2 h.
Activity was measured at a pressure of 1 bar in the mixture of
CO : H2O : H2 ¼ 8 : 42 : 50 at a steam/gas ratio of 0.6–0.7;
a catalyst (0.14–0.25 mm fraction) mixed with quartz sand of the
same particle size was used. The height of the catalyst bed
diluted with quartz was 3 cm, and the reactor diameter was
20 mm. The temperature was continuously controlled with the
aid of a thermocouple placed in a pocket of the reactor at the
center of the catalyst bed. Catalyst was characterized by the
apparent rate constant of the forward reaction, which was
determined taking into account the reversibility of the reaction
in an approximation of the rst order with respect to CO. The
reaction rate constant was calculated under the assumption
that the reactor operated in the plug ow regime. All catalytic
data are given for the catalysts that reached a steady state
activity (aer continuous operation for at least 25 h). The bulk
density of the fraction of catalysts without quartz varied from 1
Fig. 1 Diffraction patterns of ceramometals at different MA times.
Symbols denote peaks corresponding to the following phases: * –
a solid solution of aluminum in copper, AlxCu1, > – Al4Cu9 interme-
tallic [24-0003], C – CuO [05-0661]. The inset displays a fragment of
to 2.3 g cmꢁ3. Activity of granules having a prismatic shape and the diffraction pattern of the new phase.
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Table 1 XRD phase analysis of ceramometals, wt%
3 min
6 min
9 min
12 min
Cu1ꢁxAlx solid solution
Al4Cu9 intermetallic
CuO
49% (x ¼ 0.14)
12%
39%
62% (x ¼ 0.10)
20%
18%
48% (x ¼ 0.12)
27%
25%
53.0% (x ¼ 0.06)
0%
47%
3.1.2. 27Al NMR. Static spin echo 27Al NMR spectra of
cermets are displayed in Fig. 2a. An example of the spectrum
deconvolution is shown in Fig. 3. The static 27Al NMR spectra
exhibit four main lines corresponding to different Al environ-
ments. The “aluminum oxide” (Al3+) is centered at ꢃ0 ppm. The
rest spectral lines are shied due to the so-called Knight shi,16
which is caused by a local eld created by the conduction
electrons on the nuclei.
As follows from our previous study of ball milled source
precursors,13 lines at ꢃ870 and ꢃ590 ppm can be related to
Al4Cu9 intermetallic compound, which for cermet are located at
1000 and 600 ppm (Fig. 2a). The line at ꢃ1400 ppm is assigned
to Cu1ꢁxAlx solid solutions with various stoichiometry.13,17
Therefore, these results are in the qualitative agreement with
XRD data, while aluminum oxide is detected only by NMR. Due
for the MA products13 that were further used as precursors for
the synthesis of cermets. A similar trend is typical also for other
MA conditions for CuAl-containing systems.24,25 The concen-
tration of the solid solution is nearly constant, except for the
sample activated 6 min. The concentration of aluminum in the
solid solution decreases monotonically with the time of acti-
vation (excluding 6 min point), while CuO concentration goes
through the minimum. Remarkable is the absence of crystal-
lized forms of mixed copper–aluminum oxides of the spinel
type. It should be noted that XRD could not detect X-ray
amorphous components in cermets as well as in MA prod-
ucts.12,13 Hence, variation of the content of aluminum-
containing components in the X-ray amorphous state could
not be followed by this method.
Fig. 2 (a) Static 27Al NMR spectra of Cu–Al cermets (85 : 13 wt%) in dependence on the time of mechanical activation (down-top): 3, 6, 9 and
12 min. The intensity is normalized to maxima in the spectra. (b) Static 63Cu NMR spectra of Cu–Al cermets (85 : 13 wt%) in dependence on MA
time (down-top): 3, 6, 9 and 12 min. The intensity is normalized to maxima in the spectra.
Fig. 3 (a) An example of 27Al spectrum decomposition for the Cu–Al cermet (85 : 13 wt%) with 3 min activation time; (b) an example of 63Cu
spectrum decomposition for the Cu–Al cermet (85 : 15 wt%) with 9 min activation time.
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to the skin effect, not all aluminum atoms are excited associated with the additive line shi caused by a gradual
equivalently. change of copper environment from pure Cu (Cu metal) to
MAS spectra in Fig. 4 show a better resolution of the lines Cu(xAl, yCu), which exhibits lower values of the chemical shi.
displayed in Fig. 2a. The shoulder at 1400 ppm disappears since Moreover, an obvious correlation can be drawn between narrow
its width is not averaged under MAS conditions. First, the Al lines in the 9 min sample, which emerge in 63Cu and 27Al MAS
oxide part is resolved to ensure the separation of octahedral NMR spectra (Fig. 2a and b). These lines are attributed to a well
(ꢃ0 ppm) and tetrahedral (ꢃ60 ppm) Al sites. Another feature of crystallized AlCu** compound having a certain stoichiometry
15 kHz 27Al spin echo MAS spectra is the emergence of a rather with one non-equivalent Al site and two non-equivalent Cu sites.
narrow line near ꢃ900 ppm. Such a small width may correspond However, the amount of this compound is low and does not
to a well-crystallized Cu–Al** compound containing less than exceed 1 wt%. This phase may be related to the phase observed
1 at% of the observed Al. It can be concluded from the data in XRD (Fig. 1).
shown in Fig. 4 that concentration of the Cu–Al** compound
increases with MA time up to 9 min and completely disappears Cu–Al phases (Al4Cu9, CuAl**, etc.) passes through the
aer 12 min of activation. maximum with increasing the MA time, whereas CuO oxide
In general, according to XRD and NMR data, the content of
The simulation of AlT and AlO sites of 15 kHz echo MAS 27Al shows an opposite behavior. Ceramometals prepared at all
NMR spectra by simple Czjeck model gives AlO/AlT ratios from times of mechanical activation contain a noticeable amount of
1.1 to 0.6. Neither of the ratios corresponds to a certain alumina oxides, most likely in the form of mixed Cu–Al oxides. However,
that generally has AlO/AlT in the range between 2 and 37.26,27 these compounds are very dispersed and XRD amorphous.
This observation coupled with a broad basement under main
AlOx peaks and abnormally intensive satellite lines suggests
that samples composition could be close to CuAl2O4 which is
3.2. Microstructure and space distribution of oxide and
metal phases in the bulk of Cu–Al–O/CuAl cermets
a partially inversed spinel; moreover, samples may contain
delafossite or CuxAlyOz structures.28,29
Fig. 5 displays the microstructure of cermets as revealed by
3.1.3. 63Cu NMR. Static spin echo 63Cu spectra are shown SEM. As seen in this Figure and conrmed by EDX data (Fig. 6),
in Fig. 2b. It is important to mention that paramagnetic CuO all cermets consist of metallic cores surrounded with the oxide
does not contribute to 63Cu NMR spectra, therefore, it is not layer, thus forming
a microshell structure. Macropores
considered here. Nevertheless, the spectra are highly informa- observed as dark patches in Fig. 5 are situated between stacked
tive and display different Cu environments. First of all, cermet particles. The oxide layer forms a framework that inte-
a distinct line of Cu metal is found at ꢃ2390 ppm. Despite grates all the components into a monolith. The composition of
hydrothermal treatment and calcination, Cu metal is still the metallic core is close to that of the initial blend. The aver-
present in the sample. Since Cu metal phase was not detected by aged composition of the oxide layer, according to the mapping
XRD in these samples (Fig. 1), this means that Cu particles size data presented as a diagram (Fig. 6), is close to Cu2AlO3–4. Such
is less than 1–2 nm and/or their amount is small. The rest part a composition has an excess of copper in comparison with
of spectra corresponds to Cu–Al intermetallic compounds. copper–aluminum spinel. Oxide fragments with the stoichi-
Unfortunately, several non-equivalent Cu sites of the compound ometry CuAlO2+x (corresponds to the structure of rhombohedral
are not resolved in 63Cu NMR due to their large width or hexagonal modications of delafossite)30 and CuO were oen
(ꢃ1790 and ꢃ790 ppm for all samples), therefore no certain line observed in samples activated for 9 and 12 min. The thickness
can be attributed to Al4Cu9 or CuxAl1ꢁx etc., which is designated of the oxide layer decreases with increasing the MA time.
as “CuAl” (Fig. 2b). The broad character of these lines is also Sample activated for 9 min has oxide “streaks” which may result
from cracking of brittle copper–aluminum alloys. Sample acti-
vated for 12 min has many ne particles which are completely
oxidized upon calcination in air. Black spots correspond to
epoxy resin penetrating through ultramacropores of cermets to
a depth of 500 mm. Thus, the oxide part of ceramometals aer
calcination in air concentrates mostly at the external surface of
cermet macropores, sometimes as “streaks” due to cracking of
metallic cores.
The ratio of the oxide and metal parts of ceramometals can
be qualitatively estimated from the phase diagrams obtained by
the elemental mapping. As seen in Fig. 6, the intensity of the
metallic part (Fig. 6a, a spot near the Cu–Al line with the oxygen
content close to zero) in the sample activated for 3 min is much
higher as compared to the oxide one. The 12 min sample
(Fig. 6d) shows an opposite pattern, in spite of a lower thickness
of the oxide layer. The latter may be caused by the dispersion of
the Cu–Al precursor during its mechanochemical synthesis.13,24
Fig. 4 15 kHz spin echo 27Al MAS spectra of Cu–Al cermets for the
sample activated for 3, 6, 9 and 12 min.
This increases the total Me/air interfacial area, thus facilitating
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Fig. 5 Electron microscopy images of Cu–Al–O cermet sections obtained from MA products at different alloying time: (a) 3 min, (b) 6 min,
(c) 9 min, (d) 12 min.
Fig. 6 Phase diagram according to EDX mapping of Cu–Al–O cermet sections obtained from MA products at different alloying time: (a) 3 min,
(b) 6 min, (c) 9 min, (d) 12 min.
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an increase in the oxide amount even at a smaller thickness of shake-up satellites typical of Cu2+ disappear from the Cu 2p
the oxide layer. spectrum, and the Auger parameter becomes equal to 1851.3,
The presence of aluminum oxide in the surface layer of which indicates a complete reduction of copper to the metallic
ceramometals was additionally conrmed by XPS analysis. X-ray state.
photoelectron spectroscopy was used to examine a series of
Hence, CuAlO/CuAl ceramometal catalysts have a complex
catalysts treated in oxygen and hydrogen at 270 ꢀC. Their typical arrangement. Their metallic cores are comprised of the solid
spectra are displayed in Fig. 7. One can see that the spectrum solution of aluminum in copper and the intermetallic, which
recorded aer oxidative treatment has the Al 2p peak with the prevent a complete oxidation of the metal component upon
binding energy of 74.5 eV (Fig. 7a). This position of the peak is calcination in air. The oxide component, which has a thickness
typical for Al3+ ions in Al2O3 oxide (74.2–74.6 eV).31–34 Additional of several microns, is X-ray amorphous and forms a joint
peaks in the region of high binding energies correspond to the framework. This component includes at least the mixed oxides of
Cu 3p3/2–Cu 3p1/2 doublet. A shi of the Cu 3p3/2–Cu 3p1/2 copper and aluminum and, possibly, copper–oxygen clusters. As
doublet to lower binding energies and increase in their relative the time of preliminary MA of the Cu–Al precursor is increased,
intensity indicate reduction of copper cations on the surface.
concentration of the solid solution goes through the maximum,
The analysis of Cu 2p spectra revealed that the chemical and that of CuO – through the minimum for sample 6 min of
(and, probably, structural) state of the surface layers of ceram- MA, while concentration of intermetallics (Al4Cu9 and unknown
ometals is very complicated (Fig. 7b). On the one hand, phase) goes through the maximum for sample with 9 min of MA
according to XPS data, the Cu : Al atomic ratio is ꢃ1 : 1.4/1.5 (3 (Table 1, Fig. 7). An increase in the concentration of the oxide
and 12 min of MA) and 1 : 1.0/1.1 (9 and 6 min of MA). It means component with time of MA is related also to deagglomeration of
that the cationic stoichiometry in the surface layers is closer to the brittle Cu(Al) solid solution, which forms already at the MA
that of CuAlO2 delafossite30 or other structures of a mixed oxide step,13 thus increasing the metal/gas interfacial area.
in which the oxidation state of copper is 1+. On the other hand,
copper in the surface layers of the oxidized catalyst is mostly in
the Cu2+ state. This statement is evidenced by the binding
energy of Cu 2p3/2 equal to 935.9 eV, the intensity ratio of the
main peak to shake-up satellite (0.46), and the Auger parameter
equal to 1851.4 eV.33–35 CuO oxide is usually characterized by
3.3. Specic activity in WGSR and the nature of active
surface sites
Temperature dependences of CO conversion for ceramometal
a lower binding energy of Cu 2p3/2, in the range of 933.6–
934.6 eV.34,36–43 This phenomenon can be attributed to the
presence of subnanosized Cu–O clusters in the surface layers of
ceramometals. In such clusters, the initial oxide state is close to
Cu2+; however, they can easily loose oxygen even in a weakly
reducing medium up to the Cu0 state which is active in the
WGSR.34 Indeed, aer treatment in hydrogen, the peaks of
catalysts are given in Fig. 8 in comparison with known
commercial analog of ceramic catalyst (CZA).10 At close space
velocities (4200–4400 hꢁ1 for CuAl ceramometals and
ꢃ5000 hꢁ1 for CZA catalyst) CO conversions are apparently
higher for CZA catalyst. Conversions clearly increase with the
time of MA of Cu–Al mixture, even exceeding those for CZA
catalyst in the case of sample activated for 12 min.
Fig. 7 Core-level XPS spectra obtained after treatment of the catalyst in 1 bar O2 at 270 ꢀC (O2–Ox) and in 1 bar H2 at 270 ꢀC (H2-red) directly in
the high pressure cell of the photoelectron spectrometer.
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In principle, this observation is supported by XPS data,
according to which the concentration of copper on the surface
of all ceramometal catalysts aer catalytic testing is higher in
comparison with ceramic CZA one (Table 2). It should be noted
that reduction of these catalysts decreases the surface copper
concentration, but the general tendency is preserved. In
a similar way, the surface concentration of copper in various
oxides with the perovskite structure was found to be approxi-
mately twofold higher as compared to copper catalysts for
WGSR.44
A more detailed analysis showed that changes in the copper
concentration on the surface (ꢃ30%) are not so pronounced as
changes in specic activity (ꢃ200%) (Table 2). Thus, it can be
assumed that differences between the catalysts are caused not
only by the concentration of active sites but also by a consider-
Fig. 8 Temperature dependence of CO conversion in WGSR for able difference in specic activity of these sites. This is
studied catalysts. The weights of samples were varied due to differ-
ence of loading density.
conrmed also by the values of constants referred to the unit
surface area of metallic copper, which were estimated form XPS
data and the total specic surface area of the catalysts. For most
of ceramometal catalysts, the indicated values are higher than
Due to different specic densities of ceramometals a proper
for the oxide catalyst (Table 2). Thus, ceramometal catalysts
comparison of their catalytic activity requires estimation of
have sites with a higher specic activity as compared to the
specic rate constants listed in Table 2. When the rate constant
is estimated using the equation for the plug-ow reactor, an
conventional catalyst containing mixed oxides. The concentra-
tion of such sites on the surface of ceramometals goes through
important parameter is the contact time, which at the same
the maximum as the time of preliminary MA is increased
weight hourly space velocity depends on the volume of the
(Table 2). An essential difference in the activity per the unit
catalyst bed. In the case of ceramometal catalysts, their much
higher density due to presence of metallic particles should be
taken into account. This results in a substantial decrease in the
surface area of metallic copper was observed in earlier works as
well.1,44,45
As shown by XRD, samples of ceramometal catalysts aer
catalytic testing have nearly the same phase composition as the
volume of the catalytic layer at comparable catalysꢀt weights
(Table 2). The estimated rate constant (k, sꢁ1) at 240 C mono-
initial samples, except for CuO, which turns into metallic
tonically increases with the time of preliminary MA approxi-
copper (Fig. 9). With the increase of the time of preliminary
mately by a factor of 2.5, but remains lower as compared to that
mechanical alloying of Cu–Al powder the average crystallite size
for the porous oxide catalyst (Table 2). Stability of the catalysts
changes in a similar way, while stability of CZA is better. Obvi-
ously, the key factor determining the catalyst activity is the
specic surface area; on the average, its value for the ceramic
analog exceeds that for ceramometal catalysts by an order of
magnitude (Table 2).
However, the specic (per the unit surface area) activity of
ceramometal catalysts is 2–3 times higher as compared to
of the solid solution decreases from 25–34 nm to ꢃ11 nm, and
that of the Al4Cu9 intermetallic – from 33–46 to 14–19 nm. The
size of metallic copper particles changes non-monotonically.
First it decreases from 17–24 (3 min MA) to 7–10 nm for the
6 min sample, and then increases to 28–40 nm for the 12 min
sample. However, we should stress that XRD is not able to
distinguish particles of metallic copper formed on the surface
of oxide microshell envelope and its crystallites located within
the metal core of ceramometals. Moreover, XRD is not able to
detect nanoparticles with the size below 2 nm. Thus, according
to NMR data, in the initial ceramometals metallic copper is
ceramic copper–zinc–aluminum one (see Table 2: k/(m ꢂ S)). It
means that the concentration of active sites (known to be
copper atoms)1,2 on the surface of ceramometal catalysts is
substantially higher.
Table 2 Catalytic properties of CuAl cermets (240 ꢀC) (0.14–0.25 mm fraction)a
Stability,
Sample
m, g dgran, g cmꢁ3 dcat.bed, g cmꢁ3 k, sꢁ1 arb.un.
SBET, m2 gꢁ1 k/(m ꢂ SBET), sꢁ1 mꢁ2 Cu/(Cu + Al) k/(m ꢂ SCu), sꢁ1 mꢁ2 Cu
CuAl(3)
CuAl(6)
CuAl(9)
CuAl(12)
1.24 4.24
1.79 4.22
1.71 4.00
2.00 3.87
ꢃ2.3
ꢃ2.2
ꢃ2.1
ꢃ2.0
ꢃ1.0
2.8
3.3
4.8
7.8
9.1
0.6
0.8
0.8
0.9
>0.9
14.4
9.9
9.0
15.7
121
0.2
0.2
0.3
0.2
0.1
0.59
0.51
0.52
0.59
0.39
ꢃ0.3
ꢃ0.4
ꢃ0.6
ꢃ0.3
ꢃ0.3
CZA comm 0.70 2.00
a
dcat.bed – catalyst bed density; Cu/(Cu + Al) – from XPS data.
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Fig. 9 Diffraction patterns of cermets at different MA times after
reduction in hydrogen (270 ꢀC) and catalytic experiments. Symbols
denote peaks corresponding to the following phases: * – solid solution
of aluminum in copper, AlxCu1, > – Al4Cu9 intermetallic [24-0003],
B – Cu [04–0836], , – Al2O3.
Fig. 10 The effect of the time of preliminary MA of the Cu–Al
precursor on specific catalytic activity of surface copper (a) and
a relative content of Al4Cu9 intermetallic according to XRD and NMR
data (the content of aluminum in cermets with respect to that in other
phases) (b).
present, while it is not revealed by XRD (vide supra Fig. 1–3).
Hence, analysis of X-ray particle sizes for metallic copper in
CuAlO/CuAl ceramometals is apparently not informative as
compared with XPS data.
stabilization of subnanosizes but also by a partial distortion of
the structure and variation of the chemical properties due to
a strong interaction with support.35 The surface structure of the
Al4Cu9 intermetallic, which was described in detail in,46 can be
used as an example to analyze the effect of such alloys on the
formation of subnanodispersed structures of metallic copper on
their surface. Thus, for the (110) face, aluminum actually
‘decorates’ the surface of metallic copper, separating it into the
aggregates including from two to some tens of copper atoms. It
is well known that the bulk oxidation of aluminum alloys with
transition metals is hindered by the formation of a dense oxide
layer. The formation of the subsurface layer during HTT and
calcination in air may be accompanied by the emergence of
oxide subnanostructures differing from the structures of bulk
oxides observed by structural methods. In the course of reduc-
tive treatments, copper will change its oxidized state to the
metallic one, whereas aluminum will not do this. Overall, this
should stabilize copper subnanoparticles (clusters), which
possess an increased catalytic activity. Similar stable nano-
composites Cu/Al2O3, which are formed upon reduction of
copper and aluminum mixed oxides with the delafossite struc-
ture by hydrogen are reported in the literature.30
A comparison of the values of constants referred to the unit
surface area of metallic copper by XPS data with the content of
the Al4Cu9 intermetallic in cermets revealed quite a good
correlation of these values (Fig. 10). Note that the unknown
intermetallic phase also has a maximum concentration in the
sample activated for 9 min (Fig. 4) with the highest activity per
the surface site. This suggests that the highly active sites of
metallic copper are located on the surface of various copper and
aluminum alloys, or more exactly, on the surface of the oxide
substrate that forms a microshell envelope on the surface of
these cores aer HTT and calcination in air. The activity of such
sites is much higher as compared to the known sites, so their
structure should have some distinctive features. Most likely,
these sites are associated with copper in a high dispersion
having a clustered subnanosized structure. According to the
literature, the specic surface area of metallic copper particles
tested in WGSR does not exceed 35 m2 gꢁ2; so in a spherical
approximation, this gives the particle size not smaller than
19 nm. In our case, copper clusters on the surface of ceram-
ometals seem to have a much smaller size, which could make
them much more active.
In an earlier study on steam reforming of methanol over the
catalysts synthesized from Cu–Al–Fe alloys, a conclusion was
made on the presence of “dispersed copper particles which
cannot be observed by XRD”.11 Later, the presence of dispersed
particles was associated with the formation of a thin oxide layer
on the surface of Cu–Al–Fe alloy, which stabilizes the dispersed
copper particles.12 The effect of a substrate on the properties of
such subnanoparticles (clusters) is caused not only by
More coarse particles of metallic copper observed by XRD in
oxidized cermets are formed most likely from copper oxide.12
The absence of a strict correlation between concentration of
intermetallic or copper on the surface and specic activity may
be caused by changes in the ratio of nanodispersed and more
active subnanodispersed copper stabilized on the surface of
intermetallic in dependence on the properties of the precursor.
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Table 3 Textural and mechanic properties CuAl cermets and CZA catalyst
Sample
Time MA, min
SSA, m2 gꢁ1
Vmeso, cm3 gꢁ1
Vmacro, cm3 gꢁ1
Compress. strength, MPa
CuAl(3)
CuAl(6)
CuAl(9)
CuAl(12)
CZA
3
6
9
12
—
14.4
9.9
9.0
15.7
122
0.02
0.01
0.02
0.04
0.25
0.05
0.06
0.07
0.04
10.5
9.9
7.7
9.7
8.0
<0.001
high activity per the mass unit. These factors are important
arguments demonstrating that ceramometal catalysts are
promising for WGSR.
3.4. Pore structure and its effect on the strength and activity
The volume of micro- and mesopores in ceramometal catalysts
is not high (0.01–0.04 cm3 gꢁ1) (Table 3). It is much lower as
compared to the bulk oxide analog (0.25 cm3 gꢁ1). This deter-
mines a much higher specic surface of the latter (Table 3).
However, the macropore volume (>0.1 mm) of ceramic catalyst
estimated from the difference between the total pore volume
and mesopore volume is close to zero, whereas for ceramometal
catalysts it is equal to 0.04–0.07 cm3 gꢁ1 (Table 3). Macropores
of ceramometals with the size up to tens of microns are clearly
seen as black spots on the electron microscopy images (Fig. 6).
These macropores are located between cermet particles
comprised of the metal cores with the oxide shell, while mes-
opores are situated in the oxide shells. The macropore volume
changes non-monotonically and contrariwise to the strength of
cermets. Sample with 9 min of MA has a minimum strength
(Table 3). However, irrespective of the developed macropore
structure, which usually results in the decrease of the catalyst
strength, the total strength of ceramometals is comparable with
the strength of ceramic catalyst (8 MPa).
Leaching that promotes formation of additional nano-
structures based on copper oxide can be an additional factor
increasing the concentration of surface active sites.47 In steam
reforming of methanol over the catalysts synthesized from Cu–
Al–Fe alloys, a mild leaching results in the substantial increase
of activity.11,12 In our study, a similar treatment of cermet
granules also gave more than a twofold increase in the catalyst
activity (Fig. 11).
This effect can be also related to the presence of aluminum-
enriched layers on the surface of Cu–Al alloys.46 Aer oxidative
treatments, such a layer will be oxidized and block the surface of
metallic copper. Upon leaching of aluminum, blocking of the
surface will decrease and activity will increase. It cannot be
ruled out that the decreased activity for 3 and 6 min samples,
irrespective of a relatively high concentration of the interme-
tallic (Fig. 10), is related exactly to the high concentration of
aluminum in the surface layer, which decreases with increasing
the MA time to 9 and 12 min.
The developed macropore structure should enhance the
diffusion permeability of the granules which is a traditional
shape of industrial catalysis.44 A high diffusion permeability will
equalize the activity of the coarse fraction of ceramometal
catalysts and ceramic catalyst. Indeed, the activity of a small
fraction of cermet (12 min MA) was lower as compared to that of
the oxide catalyst, whereas an opposite trend was observed for
the coarse fraction (Fig. 11): activity of the 3 ꢂ 5 mm2 cermet
granules (k ¼ 5.1 sꢁ1) exceeded by more than 20% the activity of
granulated oxide catalyst (k ¼ 4.4 sꢁ1). Thus, when going from
a ne to a coarse fraction, activity of the cermet decreases by
35%, and activity of the oxide catalyst by 52%. Note also that the
density of ceramometals (ꢃ4 g cmꢁ3) is substantially higher
than that of CZA (ꢃ2 g cmꢁ3), while for known oxide catalysts
the density is even lower (ꢃ1 g cmꢁ3),10 which compensate their
For the most active ceramometal CuAlO/CuAl (MA time
12 min), the effective linear thermal expansion coefficient fꢁo6r
ꢀ
the temperature range of 25–500 C was equal to ꢃ10 ꢂ 10
(degꢁ1). For other cermets, it varied within a range from 9 to
28 ꢂ 10ꢁ6 (degꢁ1) in dependence on the MA time for a powdered
precursor, affecting both the metal/oxide ratio and the pore
volume of ceramometals. Overall, the variation range of thermal
expansion coefficient makes it possible to select such condi-
tions of the synthesis which would protect ceramometal coat-
ings against cracking and peeling from the metal tubes or other
metallic substrates (microchannel plates, foils, etc.). Thus,
ceramometal catalysts are promising for use in compact devices
for the production of hydrogen.
4 Conclusions
The evolution of the structure and microstructure of CuAlO/
CuAl ceramometals synthesized by mechanochemical activa-
tion of the Cu + Al mixture (Cu : Al ¼ 87 : 13 wt%) with subse-
quent hydrothermal treatment leading to integration of the
powder into a monolith, drying and calcination was studied in
dependence on the MA time. It was shown that such ceram-
ometals have the egg-shell microstructure where the metallic
cores containing aluminum and copper alloys are surrounded
by the oxide matrix containing copper oxides and X-ray
Fig. 11 Activity of small and coarse fractions of different catalysts in
WGSR (240 ꢀC, CO : H2O : H2 ¼ 8 : 42 : 50).
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amorphous mixed oxides of copper and aluminum. The study
revealed non-monotonic changes in the concentration of Al4Cu9
intermetallic in cermets to occur with variation of the MA time
from 3 to 12 min and a similar change in the activity referred to
the unit surface area. A high concentration as well as a high
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Conflicts of interest
23 O. A. Kungurova, A. A. Khassin, S. V. Cherepanova,
A. A. Saraev, V. V. Kaichev, N. V. Shtertser,
G. K. Chermashentseva, E. Yu. Gerasimov, E. A. Paukshtis,
There are no conict to declare.
´
Acknowledgements
O. V. Vodyankina, T. P. Minyukova and G. Abou-Jaoude,
Appl. Catal., A, 2017, 539, 48–58.
This work was in part supported by FP7 Project BIOGO.
24 K. Wieczorek-Ciurowa, K. Gamrat and Z. Sawłowicz, J. Therm.
Anal. Calorim., 2005, 80, 619–623.
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25 R. Besson, M.-N. Avettand-Fenoel, L. Thuinet, J. Kwon,
A. Addad, P. Roussel and A. Legris, Acta Mater., 2015, 87,
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