C. Dörfelt, M. Hammerton, D. Martin et al.
Journal of Catalysis 395 (2021) 80–90
octahedrally coordinated CrIII cations in B-sites, following the gen-
eral formula:
for hydrogenation would result in catalytic activity correlating
with larger metal surface area, which is affected by both particle
size and shape.
AIðItetÞB2IIðIoctÞO4
The third mechanism, described by Yurieva et al. for the hydro-
genation of acetone by a reduced copper chromite spinel catalyst,
depends on both the surface of Cu0 nanoparticles and the bulk spi-
nel lattice [28]. According to the authors, acetone is adsorbed on
Copper aluminate spinel (CuAl2O4) is partially inverse, with
CuII and AlIII each found in both tetrahedral and octahedral sites
[12]. In manganese aluminate spinel (MnAl2O4), the degree of
inversion is dependent on the method of preparation and oxidative
transfer between sites can occur, with MnII in tetrahedral sites con-
verting to MnIII in octahedral sites and the formation of an Al2O3
phase with the displaced AlIII [13,14]. Fast redox processes within
manganese oxides are considered key for their catalytic activity
and Mn is therefore often used as an oxophilic redox promotor
and electron scavenger to improve selectivity or activity of metal
oxide catalysts [15].
The oxidic precursors require an activation step to become
hydrogenation catalysts, involving reduction of the copper species
under hydrogen flow at elevated temperature. Hydrogen is thought
to penetrate the mixed metal oxide bulk to react with CuII ions,
yielding H+ and Cu0 [16,17]. Copper atoms migrate to the surface
of the catalyst particle and form hemispherical copper nanoparti-
cles in close contact with the residual bulk spinel [18,19]. The pro-
tons remain sequestered in the resulting cation deficient lattice in
tetrahedral sites previously occupied by Cu2+, covalently bonded to
one lattice oxygen and stabilising the structure in the active state
[18,20]. XPS measurements combined with XRD showed CuII in
tetrahedral sites are reduced to CuI (at 150 °C), which migrate to
octahedral sites, and to Cu0 (at 250 °C), whereas CuII in octahedral
sites are reduced at higher temperature (300 °C) to Cu0 nanoparti-
cles and CuI, which remain stabilised in the octahedral sites of the
spinel [18,21]. In-situ XANES investigations of CuAl2O4 (formed by
impregnation of Al2O3 with 5 wt% Cu) showed the final copper oxi-
dation states as 70% Cu0 and 30% CuI, in a spinel-like environment
[22].
The role of the reduced spinel and the copper nanoparticles in
the catalytic mechanism of hydrogenation reactions of C@C and
C@O double bonds is the subject of some debate. Three possible
mechanisms are described in previous research, in which the dif-
ferent copper species are assigned different roles. The first possible
mechanism was championed by Bechara et al. in 1985, where they
found the catalytic activity of isoprene and 1,3-pentadiene hydro-
genation to correlate with the amount of CuI in octahedral sites of a
copper chromite spinel, as well as with H species occluded in the
spinel [23]. Bechara et al. concluded that the oxidisable part of
the reduced spinel (surface) was therefore more important than
the surface area of the metallic copper and identified the active site
as a CuI-H pair [23]. Hubaut et al. extended this research to the
the surface of the Cu0, which supplies two electrons to the
p* orbi-
tal of the carbonyl group, giving the carbon a negative charge.
Simultaneously, a proton from the spinel lattice transfers to the
oxygen to form an alcohol group. This mechanism then describes
the migration of the resulting oxidised Cu2+ back into the spinel
lattice to occupy a previously vacated cation site, whilst a second
proton migrates in the reverse direction to the anionic carbon,
allowing the alcohol to desorb from the nanoparticle surface. In
this way, the reduced spinel lattice behaves as a Brønsted acid,
e-
supplying protons, and the nanoparticle acts as a CuII $ Cu0 switch,
supplying electrons. However, the migration of copper between
the spinel and nanoparticle is likely mass transport limiting to
the catalytic rate of reaction, making this aspect of the mechanism
debatable.
In order to determine which, if any, of these three mechanisms
proposed for the traditional chromate system is most likely to be
correct in the contemporary copper aluminate spinel-based cata-
lyst (CuOꢀCuAl2O4) the structure of the activated copper aluminate
was investigated. Detailed insights were obtained by characteris-
ing the bulk catalysts using XRD, TPR, XANES and EXAFS after syn-
thesis by co-precipitation and calcination to give the spinel
structure, and during and after activation in hydrogen forming
the final catalyst. The catalytic performance in the reduction of
model substrate butyraldehyde by copper aluminate catalysts with
varying copper metal surface area and a pure copper spinel model
CuAl2O3 was studied to deduce structure–activity relationships. In
particular, the effect of manganese on the structure and redox
properties of copper aluminate spinel-based catalysts, which has
not been previously investigated, was used as a tool to probe the
active structure. Using a combination of techniques, this work
attempts to investigate the role of manganese in structure forma-
tion, activation and catalytic behaviour of these industrially rele-
vant catalysts and contribute to the discussion on the reaction
mechanism.
2. Materials and methods
2.1. Catalyst synthesis
Copper aluminate (CuOꢀCuAl2O4) catalysts were synthesised via
co-precipitation of the metal nitrates with sodium carbonate. A
metal nitrate feed solution (0.6 M Cu(NO3)2ꢀ3H2O, 0.6 M Al(NO3)3-
ꢀ9H2O and 0.1 M Mn(NO3)2ꢀ4H2O in Mn including catalyst synthe-
sis) and Na2CO3 (2 M) were co-fed into a precipitation vessel
containing warm water (50 °C, stirring at 400 rpm) at a rate of
5 mL/min (0.2 M final Cu concentration). The pH was kept at 6.5
by small adjustments to the rate of addition and the precipitates
were aged for 1 h (50 °C). The precipitates were subsequently fil-
tered and washed by re-suspending in deionised water until the
spent wash fluid had a conductivity ꢁ 0.5 mS. The obtained solids
were dried at 120 °C overnight and subsequently calcined at 750 °C
for 2 h (2 K/min). Catalysts were then activated under H2 flow,
heating at 1 K/min to 300 °C. The final temperature was then held
for 1 h before flushing with argon and allowing to cool.
selective 1,2-hydrogenation of a,b-unsaturated aldehyde or ketone
to the allylic alcohol [24]. This mechanism requires the catalytic
activity to be controlled by the amount of CuI present in the acti-
vated spinel.
The second proposed mechanism identified Cu0 nanoparticles
as the site for hydrogen activation under reaction conditions, sim-
ilar to Group VIII (Pt, Pd or Ni) metal catalysts, but with higher acti-
vation energy [25]. Gudkov et al. reported the dependency of the
rate equation for the hydrogenation of butyraldehyde on atomic
hydrogen. Dissociatively adsorbed hydrogen was confirmed to par-
ticipate in the reaction mechanism by isotope studies of irre-
versible butyraldehyde hydrogenation using adsorbed deuterium,
where deuterium/hydrogen exchange rates increased with increas-
ing copper content [25]. The capability of metallic copper to disso-
ciatively adsorb hydrogen was reported to be dependent on
particle size and the presence of high index faces (211), (311)
and (755) [26,27]. It is expected that a mechanism where the cop-
per nanoparticles supply atomic hydrogen and are the active site
Pure copper aluminate spinel (CuAl2O4) was prepared in a sim-
ilar co-precipitation method as described above, using a modified
molar ratio of Cu:Al = 1:2. The calcination step was also modified
to 800 °C for 2 h (5 K/min). Residual CuO was removed using sat-
81