S.E. Collins, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
(838 K) leading to the formation of Pd5Ga3 after reduction. They stu-
died the methanol synthesis at 523 K from hydrogenation of CO (H2/CO
= 1.7, 2 MPa) and CO2 (H2/CO2 = 3, 3 MPa) mixtures over Pd2Ga/α-
Ga2O3. CO hydrogenation did not occur, whereas considerable me-
thanol yield from H2/CO2 was observed. The methanol synthesis during
CO2 hydrogenation was attributed to the formation of a PdGa inter-
metallic. Recently, a strong metal − support interaction was found
specifically between the polar (002) Ga2O3 surface and supported Pd
nanoparticles, which facilitates electron transfer and the formation of
PdxGay nanoparticles [11].
Fiordaliso et al. [6] prepared a GaPd2/SiO2 catalyst (intermetallic
particle sizes around 12 nm) by simple incipient wetness of nitrates
(Pd:Ga = 1:2 atom/atom), and reduction with hydrogen at 823 K. They
tested the CO2 hydrogenation at atmospheric pressure, that is, at very
low conversion. TOF values were calculated on the basis of GaPd2
surface, estimated from mean particle size. A similar catalyst was syn-
thesized by Manrique et al. [7] and tested up to 0.8 MPa at several
temperatures. Both research groups claimed that the catalyst perfor-
mance was entirely due to a reaction pathway occurring over the sur-
face of the bimetallic crystallites.
the catalytic activity for CO2 conversion can be greatly promoted at the
metal/oxide and metal/carbide interfaces. Additionally, electronic ef-
fects create unique electronic properties at the (bi)metal/oxide inter-
face, which can improve the activation of CO2 and its subsequent
trasnformation.
However, it is not straightforward to isolate these factors, that is,
Pd-Ga intermetallic surface vs. (bi)metal-oxide interface and/or (bi)
metal + oxide dual sites, which can occur simultaneously.
In this work, a series of palladium-supported on silica and promoted
with gallium catalysts were synthesized, characterized and tested in the
CO2 hydrogenation reaction. The palladium loading was kept constant
and the loading of the gallium promoter was changed from ca. 2–8 wt.
% to rationalize the effect of the PdGa alloying and the metal-oxide
interface. CO2+H2 reaction experiments at 0.1 and 0.7 MPa were
monitored by in situ infrared spectroscopy, while in-situ X-ray ab-
sorption (EXAFS) and high resolution transmission electron microscopy
(HRTEM) were used for the characterization.
2. Experimental
Some of us investigated the reaction mechanism of CO2 hydro-
genation to methanol on a Pd-Ga/Ga2O3 catalyst by means of in situ
FTIR [2,3]. The formation of Pd-Ga nanoparticles under reductive
condition was characterized by transmission electron microscopy. In
situ FTIR reaction experiments performed at low (0.1 MPa) and
medium (0.7 MPa) pressures suggested that the reaction intermediates,
chemisorbed on the gallia surface, were successively hydrogenated
from (bi)carbonates to formate (mono- and bidentate and bridged),
methylenebisoxy, and methoxy species to give methanol, while hy-
drogen supply was accomplished via spillover from the Pd-Ga metal
particles. The increase in the selectivity to methanol, as a result of a
2.1. Catalysts
A series of catalysts was prepared as follows: Davison Grade 59 si-
lica gel (BET specific surface = 301 m2 g−1, pore size = 160 Å, particle
size =52 μm) was first loaded (2 wt.% Pd) via ion exchange of palla-
dium acetate, at pH = 11 in NH4OH(aq). The supported tetrammine
palladium complex was then decomposed to diammine palladium on
silica by drying at 423 K under flowing air, in a glass reactor.
Subsequently, different amounts of Ga(NO3)3.xH2O, in aqueous solu-
tion, were added to the [Pd(NH3)2]2+/SiO2 by incipient wetness im-
pregnation to obtain Ga to Pd atomic ratios equal to 2, 4, 8 atom/atom.
The gallium oxide loadings are equivalent to ca. 3.5, 7 and 14 % of a
theoretical monolayer on the SiO2 support [28]. The resulting materials
were calcined in air flow at 673 K (2 h), and reduced under flow of 5%
H2/Ar at 723 K (2 h). Finally, the reduced catalysts were passivated at
298 K under a dilute O2/Ar flowing mixture and stored under Ar. These
Ga-Pd/SiO2 catalysts were labeled PdGa20, PdGa40 and PdGa80, re-
spectively. A 2 wt.% Pd/SiO2 catalyst and two reference Ga2O3/SiO2
catalysts, that is, 4.9 and 9.2 wt.% Ga, were prepared following the
same procedure, and labeled PdGa00, Ga40 and Ga80, respectively.
Palladium dispersion of the reduced catalysts was measured before and
after reaction by CO pulse chemisorption [29] and transmission elec-
tron microscopy. The main characteristics of the investigated materials
partial inhibition of the reverse water gas shift reaction (RWGS: CO2
+
H
2 → CO + H2O), was proposed to be associated with the formation of
intermetallic Pd2Ga nanoparticles. In addition, a more practical mate-
rial, palladium-supported in silica and promoted with gallium oxide
catalyst, Ga2O3–Pd/SiO2 (2 wt% Pd and 4.9 wt% Ga), was synthesized
with the strategy of maximizing both the dispersion of the expensive,
active metals and the intimate contact between Pd and Ga [4,5]. These
results indicated that the intimacy between metal (Pd or PdGa) and
gallium oxide must be optimal to accomplish the desired formate/H2
ratio necessary to achieve improved catalytic activity, via H spillover,
and subsequent increase in methanol formation rate. Further work
using physical mixtures of Ga2O3/SiO2 and Pd/SiO2 (without intimate
contact between Pd and Ga) suggested that atomic hydrogen was gen-
erated on the silica-supported Pd particles, which spillovers onto the
silica-supported gallia particles, where the carbonaceous species were
then hydrogenated. Overall, these reaction results showed (as it was
also evidenced by in situ FTIR) that the Pd(Ga)-Ga2O3 catalysts worked
as a true bifunctional system.
More recently, García-Trenco et al. [9] prepared colloidal Pd2Ga-
based nanoparticle catalysts by thermally decomposing Pd(II) acetate in
the presence of Ga(III) stearate followed by H2 reduction at 483−563
K. The methanol synthesis was studied by liquid-phase hydrogenation
of carbon dioxide, at high pressure (5 MPa). They concluded that Ga2O3
and bimetallic Pd2Ga alloy played the role of forming active sites,
showing a good correlation between the activity and the content of
Ga2O3 surrounding the Pd2Ga nanoparticles, and suggesting that me-
thanol is produced via a bifunctional mechanism involving both phases.
The importance of the activation and conversion of CO2 on multi-
functional catalytic sites, available at the (bi)metal/oxide interface by
taking advantage of the synergy between the metal nanoparticles and
oxide support and the possibility of tuning the activity and selectivity
was recently reviewed by Kattel et al. [25] and recently explored by
Choi et al. [26] using Ni/gallia as model catalysts. In this regard, Ro-
dríguez et al. [27] highlighted the importance of the metal/oxide in-
terface for the catalytic CO2 transformation to methanol, showing that
2.2. Catalytic activity
The catalytic performance for carbon dioxide hydrogenation (H2/
CO2 = 3) was evaluated over the set of materials in a glass-lined SS
microreactor for at least 24 h at 3 MPa, from 493 to 523 K. The reaction
products were analyzed by gas chromatography, in two Shimadzu GC-
9A units arranged in parallel (Carbosieve S-II 60/80 mesh and Porapak-
QS 80/100 mesh, TCD and FID). Further details about the reaction
setup, catalysts pretreatment and operational procedures can be found
2.3. In situ infrared spectroscopy
In situ transmission infrared spectroscopy was performed using
approximately 23 mg cm−2 samples pressed into self-supporting wafers
at 5 ton/cm2. The wafers were located in a heated Pyrex cell (with
water-cooled NaCl windows), which was attached to a conventional
high-vacuum system (base pressure = 1.33 × 10−4 Pa) and a manifold
for gas flow. Infrared transmission spectra were acquired with a Nicolet
8700 FTIR spectrometer using a MCT-A detector (4 cm−1 resolution,
100 scans). Further processing of the spectra was carried out with
2