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Y. Chen et al. / Journal of Catalysis xxx (2016) xxx–xxx
[27], using Cu or Au promoted TiC (001) to produce CH3OH from
CO2 hydrogenation at 250–320 °C. The Cu/TiC (001) material
exhibited a CH3OH production rate that was an order of magnitude
higher than that for the pure TiC (001) material. Xu et al. [28]
molybdate (AM) precursor, (NH4)6MO7O24ꢃ4H2O (Alfa Aesar).
Approximately 1.3 g of AM was sieved to 125–250 m and then
l
loaded into a quartz tube reactor. The AM was treated in H2 flow-
ing at 400 mL/min for 70 min, as the temperature was increased
from 25 to 350 °C and held at 350 °C for 12 h. The reaction gas
was then switched to 15% CH4/H2 (400 mL/min) while the temper-
ature was increased to 590 °C in 1.5 h and maintained at 590 °C for
2 h; the reactor was then immediately quenched to room temper-
ature. The Cu/Mo2C and Pd/Mo2C catalysts were prepared using a
wet impregnation method described elsewhere [37]. Briefly, the
freshly-synthesized Mo2C was transferred under 15% CH4/H2 gas
into a beaker containing 70 mL deaerated water (to avoid the oxida-
tion of Mo2C) with target amounts of Cu(NO3)2 and Pd(NO3)2ꢃ4NH3
and allowed to interact for 20 h to achieve the 5 wt% nominal
metal loading. Argon was continuously purged through the solu-
tions during the wet impregnation process to deaerate and agitate
the solution. This method enabled the metal precursor to directly
interact with the native Mo2C surface (as opposed to a passivated
material). A recently study by Wyvratt et al. reported that deposi-
tion of active metal onto the native Mo2C produced nanoscale
metal domains that were better-dispersed than those deposited
onto a passivated Mo2C surface, resulting in superior catalytic per-
formance for the WGS reaction [38]. It has also been suggested by
Schaidle et al. that Cu and Pd are deposited onto the Mo2C via elec-
trostatic adsorption and both metals are reduced in-situ by the
Mo2C support during the wet impregnation process [37]. The
resulting catalyst slurry was dried at 110 °C for 2 h and reduced
in flowing H2 (400 mL/min) at 300 °C for 4 h to decompose the
nitrate and produce the Cu or Pd domains. The Co/Mo2C and
Fe/Mo2C catalysts were synthesized using the incipient wetness
impregnation. The impregnation was performed using an aqueous
solution containing target amount of Co(NO3)2 or Fe(NO3)3, on the
Mo2C support with a pore volume of 0.13 cm3/g (measured by N2
physisorption). The incipient wetness was applied because only
small amounts of Co and Fe (<2 wt%) could be deposited via elec-
trostatic adsorption [37]. The freshly synthesized Mo2C was trans-
ferred under Argon to a water-tolerant, oxygen free glove box filled
with N2 to avoid any bulk or surface oxidation of Mo2C. The result-
ing catalysts were dried in the glove box on a heating plate at
110 °C for 2 h and then transferred under Argon into a quartz reac-
tor where they were reduced in flowing H2 (400 mL/min) for 4 h at
450 °C to produce the Fe and Co domains.
described the use of Mo carbides, including
a-MoC1ꢁx (x < 0.5)
and b-MoCy (y ꢂ 0.5), for CO2 hydrogenation at 300 °C. They con-
cluded that the reaction activity and selectivity were strong func-
tions of the Mo/C ratio; however, CO and CH4 accounted for
ꢂ70% of the products. Porosoff et al. [29] recently reported that a
bulk Mo2C catalyst outperformed CeO2 supported Pt or Pd catalysts
for CO2 hydrogenation, producing primarily CO (ꢂ93% selectivity)
at 300 °C and 1 atm. The addition of Co onto Mo2C resulted in a
slight increase in the CO selectivity.
Investigations of CO2 hydrogenation over TMC-based and other
types of catalysts have most often been carried out at relatively
high temperatures (250–300 °C), where desirable products like
CH3OH are not thermodynamically favored. Fan et al. [30] reported
results for CO2 hydrogenation over Cu/ZnO and Cu/Cr2O3 catalysts
at 200 °C with 7.5 bar of CO2, 22.5 bar of H2 and ethanol as the sol-
vent. This lower temperature not only enhanced the selectivities to
CH3OH but decreased selectivities to CO (product of an endother-
mic reaction) [31]. The best-performing catalyst, Cu/ZnO, afforded
a rate of 0.02 l
molCO2 mꢁ2 sꢁ1 with selectivities to CH3OH, CO, and
ethyl formate of 73%, 20% and 6%, respectively. The results sug-
gested that ethyl formate was an intermediate for the production
of CH3OH. Huff and Sanford [32] developed a cascade system incor-
porating homogeneous catalysts for CO2 hydrogenation through
formic acid and formate intermediates, although incompatibility
among the catalyst components inhibited overall performance.
Yu and Tsang [33] reported the production of methyl formate from
CO2 in liquid methanol over a Cu/ZnO/Al2O3 catalyst. A rate of
0.05 l
molCO2 mꢁ2 sꢁ1 was achieved with 79% selectivity to methyl
formate (balance CO) at 150 °C, 140 bar CO2 and 20 bar H2. They
suggested that surface formate was a key intermediate. Inspired
by these results, we recently explored the use of Mo2C supported
Cu catalyst for CO2 hydrogenation at 135 °C, with 10 bar CO2 and
30 bar H2 in 1,4-dioxane [34]. The addition of Cu onto Mo2C signif-
icantly enhanced the CH3OH TOF (2.0 ꢀ 10ꢁ4 sꢁ1). Introducing
ethanol reagent improved the CH3OH production (by ꢂ40%) as a
consequence of enhanced formation of the formate intermediate.
Research described in this paper extends our investigation of
CO2 hydrogenation over nanostructured Mo2C supported metal
catalysts in liquid solvents to include other metals and tempera-
tures higher than 135 °C. We also interrogated the reaction path-
ways by probing the systems with possible intermediates,
including CO and CH3OH. The use of liquid solvents, as opposed
to reactions in gas phase, can impact catalyst stability [35]. For
example, Verhoef et al. [36] reported that MCM-41 supported het-
eropoly acid catalysts were susceptible to severe deactivation dur-
ing liquid phase esterifications compared to carrying out these
reactions in the gas phase. This deactivation was primarily due to
the presence of water which enhanced the mobility of the hetero-
poly acid species and caused catalyst sintering [36]. Therefore, we
also investigated stabilities of the M/Mo2C catalysts by comparing
the surface and bulk, physical and chemical properties before and
after reaction. The findings will enhance our understanding of
Mo2C-supported metal catalysts for low temperature CO2 hydro-
genation and provide a scientific basis for their rational design
for other related applications.
2.2. Catalyst characterization
Surface areas of the materials were determined from N2
physisorption based on the BET method using a Micromeritics
ASAP 2010 analyzer. The Horvath–Kawazoe method was used to
determine the pore size distributions. All of the Mo2C-based cata-
lysts were degassed (<5 mmHg) at 350 °C for 4 h prior to the sur-
face area measurements. The bulk crystalline structures were
characterized using X-ray diffraction (Rigaku Miniflex 600) with
2h ranging from 10° to 90° and a scan rate of 5°/min. Crystallite
sizes were estimated via line broadening analysis using the Scher-
rer equation [39]. Scanning electron microscopy (SEM) for select
catalysts was performed using FEI Nova Nanolab Dualbeam (FIB/
SEM). To enhance the conductivity, the materials were gold sputter
coated prior to imaging. Elemental analyses were carried out using
Energy Dispersive X-ray Spectroscopy (EDX). All the materials
were passivated in 1% O2/He for 5 h before performing SEM in
order to be loaded into the sample chamber without bulk oxida-
tion. Metal compositions for the M/Mo2C catalysts were deter-
mined by inductively coupled plasma (ICP-OES) using a Varian
710-ES analyzer.
2. Materials and methods
2.1. Catalyst preparation
The bulk Mo2C catalyst was prepared using a temperature pro-
grammed reaction (TPR) technique starting from an ammonium
The surface site densities for Mo2C-based catalysts were deter-
mined via CO chemisorption using a Micromeritics AutoChem II
Please cite this article in press as: Y. Chen et al., Low temperature CO2 hydrogenation to alcohols and hydrocarbons over Mo2C supported metal catalysts, J.