CNx stabilized Ni-Ga nanoparticles for CO2 hydrogenation: Role of preparation methods

Article abstract of DOI:10.1016/j.cattod.2019.03.031

Carbon based framework are believed to be adsorbed large amount of hydrogen reversibly on its surface and this adsorbed active H-species can increase the rate of surface hydrogenation reactions. The basic nitrogen functionality if present can provides additional sites for adsorption of CO₂ molecule. On this regard, CO₂ hydrogenation has been investigated on multifunctional nitrogen doped carbon (CNx) consisting of Ni-Ga nanoparticles. Alloy nanoparticles (2–10 nm) of Ni₃Ga and Ni₃Ga₅ has been identified from HR-TEM in combination of d value calculation form XRD. Whereas, mix Ni and Ni-Ga phase appears in case of higher loading catalyst i.e. NG-CNx-3. Under experimental conditions (2 MPa, 280 °C), the CO₂ hydrogenation products are CH₄, CO (major) and methanol. The activity of the synthesised catalysts strictly depends on the synthesis method and support. The catalyst consisting 14 and 9 (wt %) of Ni and Ga on CNx (NG-CNx-2) shows highest catalytic activity in terms of CO₂ conversion. While NGBCNx-2 with 14 and 9 (wt %) of Ni and Ga on BCNx is found to be active towards methanol selectivity which may be due to the finely dispersed Ni-Ga alloy nanoparticles with size distribution between 2–5 nm.

Full text of DOI:10.1016/j.cattod.2019.03.031

Accepted Manuscript
Title: CNx stabilized Ni-Ga nanoparticles for CO2
hydrogenation: Role of preparation methods
Authors: Reena Goyal, Woo Jin Lee, Siddharth Sameer, Bipul
Sarkar, Ken Chiang, Ankur Bordoloi
PII:
S0920-5861(18)30541-8
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.03.031
CATTOD 12053
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
29 April 2018
3 March 2019
15 March 2019
Please cite this article as: Goyal R, Jin Lee W, Sameer S, Sarkar B, Chiang K, Bordoloi
A, CNx stabilized Ni-Ga nanoparticles for CO hydrogenation: Role of preparation
2
methods, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.03.031
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CNx stabilized Ni-Ga nanoparticles for CO hydrogenation: Role of
2
preparation methods
a
b
a
a, c
d
a
Reena Goyal, Woo Jin Lee, Siddharth Sameer, Bipul Sarkar, Ken Chiang, * and Ankur Bordoloi*
a
Catalyst and Conversion Division, CSIR- Indian Institute of Petroleum, Dehradun-248005, Uttarakhand, India
School of Engineering, RMIT University, Melbourne VIC 3000, Australia
Chemical Science Divison, CSIR-Indian Institute of Petroleum, Dehradun-248005, Uttarakhand, India
b
c
d
Energy Flagship, Commonwealth Scientific and Industrial Research Organization (CSIRO), Clayton, VIC 3168,
Australia
AUTHOR INFORMATION
Corresponding Author
*
E-mail addresses: reenagoyal1988@gmail.com
Tel.: +91 135 2525898; Fax: +91 135 2660202
Graphical Abstract
Highlight
(
1) Freeze drying approach likely gives suitable local environment for the deposition of metal on CNx
matrix.
(
2) Increase in particle sizes with the upsurge in the metal concentration, seems to control the CO
conversion as well as methanol selectivity.
2
(
(
3) Ni
3
Ga alloy particles seems less active for methanol synthesis when compare to Ni
5
Ga
3
.
4) Ga supported on CNx appears to be methanol selective whereas, Ni appears as hydrocarbon specific.
Abstract
1

Carbon based framework are believed to be adsorbed large amount of hydrogen reversibly on its surface
and this adsorbed active H-species can increase the rate of surface hydrogenation reactions. The basic
nitrogen functionality if present can provides additional sites for adsorption of CO
regard, CO hydrogenation has been investigated on multifunctional nitrogen doped carbon (CNx)
consisting of Ni-Ga nanoparticles. Alloy nanoparticles (2-10 nm) of Ni Ga and Ni Ga has been identified
from HR-TEM in combination of d value calculation form XRD. Whereas, mix Ni and Ni-Ga phase appears
in case of higher loading catalyst i.e. NG-CNx-3. Under experimental conditions (2 MPa, 280°C), the CO
hydrogenation products are CH , CO (major) and methanol. The activity of the synthesised catalysts strictly
depends on the synthesis method and support. The catalyst consisting 14 and 9 (wt %) of Ni and Ga on
CNx (NG-CNx-2) shows highest catalytic activity in terms of CO conversion. While NGBCNx-2 with 14
2
molecule. On this
2
3
3
5
2
4
2
and 9 (wt %) of Ni and Ga on BCNx is found to be active towards methanol selectivity which may be due
to the finely dispersed Ni-Ga alloy nanoparticles with size distribution between 2-5 nm.
Keywords: Interrelation studies, CO hydrogenation, NiGa alloy, carbon based support, role of dopant
2
1
. Introduction
To relieve our dependence on fossil fuels and consequently reduce the risk of global warming,
scientist seeks carbon neutral alternative source of energy and chemicals.[1-3] As a result, the
sustainable use of CO
alternative fuels. Currently, the vast majority of CO
chemicals, materials and carbohydrates.[4] Significant research effort has been devoted to develop
technologies that will hydrogenate the chemically stable CO into new fuels or chemicals.[5]
Thermodynamically, CO can be reversibly reduced to methane, methanol, CO and formic acid.[6]
Whereas, the selective production of CO from CO hydrogenation known as the reverse water gas
2
is envisaged as one of the feedstock for the manufacture of chemicals and
2
usage involves the production of organic
2
2
2
shift (rWGS) reaction is a key process in many industry.[7, 8] The formed CO is an important
starting material that could be utilized to synthesize higher hydrocarbons through processes such
as the Fischer-Tropsch reaction.[9-11] Among these options, Ni/SiO
found to be active for the selective formation of methane. While, the most active materials reported
for selective CO formation are mainly precious metals catalysts like Pt/CeO [8, 14] and
Au/TiO [15]. Previous studies showed that Ni-Ga with particular combinations was highly active
for low temperature (190-250°C) CO hydrogenation at an ambient pressure.[16] On a contrary,
during space exploration to lowering the risk of hypercapnia caused by excessive CO in the cabin
air, the CO level needs to be reduced.[17] The combination of these two processes reverse water
gas shift (rWGS) reaction and water electrolysis can overcome the risk of hypercapnia as well as
provide O required for the cabin air (Figure S1).
2
[12] and Ru/AL
2
3
O [13] were
2
2
2
2
2
2
The current commercial methanol synthesis catalyst is exclusively based on Cu and Zn.[18, 19]
Since Cu is very sensitive to poison like sulphur species, halides, Fe and Ni and lead, as well as to
thermal deactivation caused by sintering, efforts have been made to find alternative materials
which have higher resistance against poisoning and deactivation. Metals including Ni, Ru, Ga, and
Pt supported on Al
2
O
3
, SiO
2
, CeO
2
, TiO
2
, Mo C have been assessed.[20] More likely it has been
2
explained Ni rich sites activate CO formation and Ga rich sites facilitate methanol production.[16]
2

Further study suggests that, support plays an important role in CO
reported that when CuZnO used with different support such as ZrO
followed a distinct CO hydrogenation rate and carbon utilization factor.[21] In recent times,
Mo C-supported metal catalyst shows good methanol selectivity in liquid phase CO
hydrogenation.[22] Therefore, metal supported carbon based material plays an important role in
CO hydrogenation.
2
hydrogenation;[21] Liu et al.
2
, TiO , and CeO ; they
2
2
2
2
2
2
On another front, carbon based framework were believed to adsorb large amount of hydrogen
reversibly on its surface.[23] This micro-environment with adsorbed active H-species at the
surface increases the rate of surface hydrogenation reactions. Further investigation also suggested
that beside the H-adsorption at the surface, CO
CO hydrogenation.[24, 25] Carbon based catalyst like Fe-AC[26] and Fe-Co[27, 28] supported
on activated carbon already proven to be highly active for Fischer-Tropsch synthesis. It has also
been stated that presence of some basic metals promoter such as (Na, K, Mg) enhances the CO
2
adsorption is the most important parameter for
2
2
reduction capabilities and provides additional stability to the catalyst framework.[29, 30] Thereby,
carbon based materials may prove better support instead of earlier reported acidic supports.
Additionally, if any heteroatom like nitrogen (like nitrogen rich carbon materials) is present insides
its framework can provide some basic sites on the catalyst surface which enhances the hydrophilic
nature and improve the resistance against deactivation by coke formation.[31] Basic nature of CNx
frame can also facilitate CO adsorption on surface.[32] Additionally, mesoporous CNx due to its
2
high surface area help to homogeneous dispersion of different metal nanoparticles.
Therefore this study aims to explore Ni-Ga alloy nanoparticles dispersed on nitrogen doped
mesoporous carbon material for CO hydrogenation. The size anisotropy as well as the activities
2
of Ni-Ga sites can be altered by changing the synthesis/treatment protocols. Therefore, in current
report, we used a simple freeze drying approach to synthesize NiGa metal nanoclusters with
control size and dispersion on CNx support and tested for CO conversion.
2
Experimental
2
.1 Materials Synthesis
All the chemicals and materials purchased were of high purity and analytical grade. NG-CNx
catalyst having Nickel/Gallium mole ratio of 5/3 were prepared by freeze drying approach on CNx.
The mole ratio of metal salt were kept constant named as NGCNx1, 2, 3 and NGBCNx1, 2, 3
(
metal loading 7/3, 14/6 and 28/12) Mesoporous CNx is prepared by nano-casting approach on
SBA-15. The synthetic procedure for NGCNx-1 is as follows:
.1.1 SBA-15: The hard template mesoporous silica was prepared by reported method in the
2
literature.[33] Firstly, 2.0g of P123 was dissolved in 40 ml deionized water. After continuous
stirring of 4h, 2M HCl solution was added into it followed by raising the temperature 40°C. 4.5 g
of TEOS as silica source was mixed to above solution. By keeping the temperature constant, the
mixed solution was left under continuous stirring for 24h. Further, the obtained mixture was heated
under hydrothermal conditions in air oven at 100°C for 48 hr. Finally, the obtained gel like
composition was filter, washed, and calcined at 550°C for 6h in static air.
3

2
.1.2 MCNx: To synthesise MCNx, 1g of the as-prepared SBA-15 was added to a round bottom
flask together with 11g CCl and 4.4g ethylene diamine. This mixture was heated to 90°C under
reflux in an oil bath to allow polymerization take place. After drying the material overnight at
00°C, the obtained solid was carbonised by calcination in a tubular furnace at 600°C for 6h under
flow. The silica template remained of the material was removed by refluxing the material in a
M NaOH with water: ethanol 1:1 solution for 4h at 100°C. The solution was filtered and the
4
1
N
2
1
resultant material was washed with 3-4 liter of water to remove all hydroxide ions. The process
was repeated for 3-4 times. The final material was dried at 100°C and denoted as MCNx.
2
.1.3 NG-CNx: For the impregnation of Nickel and Gallium nanoparticles on MCNx. Nickel
and gallium nitrate in 7:3 mole ratio was dissolved in adequate amount of water. The resulting
mixture was dispersed to above synthesized 0.5g MCNx. The obtained solution was placed in a
lyophilizers under continuous rotation (controlled at ca. 5 rpm) at temperature -10°C under
vacuum. When the sample was dried, the sample containing nickel gallium nanoparticles was
slowly reduced at 400°C in a flow 9% N O He at 20 ml/min. Similarly, NG-CNx-2, 3 was prepared
2
with nickel/gallium mole ratio of 14/6 and 28/12 respectively.
NG-BCNx: To synthesize BCNx, 4.4g ethylene diamine and 8.5g CCl
5. In a separate beaker 0.2-0.8g dimethylaminoborane was dissolved in 4g CCl
4
were added to 1g SBA-
. The two
solutions were mixed and the temperature was kept at 80-120°C under refluxing condition for 7-
h. The obtained material was carbonized at 600°C under N atmosphere for 6-8h. Silica template
1
4
8
2
was removed with 0.5M NaOH at 100°C for 2-3h. Finally, the obtained material obtained was
filtered, washed repeatedly with water (until pH becomes 7) and dried in an air oven at 100°C. The
impregnation of nickel and gallium is same as described above.
NG-CNx-2urea: The 1g of above prepared CNx was taken into 1litre of water and left for
continuous stirring. 14 mole % of nickel and 6 mole% gallium nitrate salt was added to it. After
1
h of vigorous stirring thrice of urea to metal salt (nickel and gallium) were mixed into it and kept
for slow reduction metal particles at 60℃for 48h. Later on this mixture was filtered and dried.
.2 CO Hydrogenation
2
2
The Catalyst testing of this newly synthesised material was carried out in a fixed bed continuous
flow reactor. The reaction temperature and pressure were varied in the range of 180-350°C and 1-
2
0 bar, respectively. The catalyst was activated in 25 ml/min flow of 3% H
prior to the reaction and left overnight at 200°C in 15 ml/min flow of 3% H
hydrogenation of CO was performed at 1 to 25 bar using the feed composition of CO
1:3:0.25:4.1 and the total flow rate of 100 ml/min.
2
/He, at 550 °C for 3h
/He. The catalytic
:H : He: Ar
2
2
2
2
=
The reactant and product were separated and analysed using four different capillary column
mounted on an online Agilent 490 micro-GC equipped with thermal conductivity detector (TCD).
In each experiment, 0.5 to 1g catalyst was loaded to an Inconel tubular reactor. All catalysts were
evaluated at a CO conversion 3-8%. Gas hourly space velocity was kept constant at 6,000-12,000
2
-1
h . The reaction was carried out for more than 12 h, and the turnover frequency and the yield were
calculated after the steady state was accomplished. The conversion and selectivity of the catalyst
was calculated as:
4

Molesof CO2 reacted(C%)
Conversion (C%) 
Selectivi yt (C%) 
x100
Molesof CO initially used (C%)
2
Molesof products(C%)
x100
Molesof CO reacted(C%)
2
2
.3 Catalyst characterization
The X-ray diffraction (XRD) patterns were collected using a Bruker D8 advance x-ray
diffractometer fitted with a Lynx eye high-speed strip detector and a Cu Kα (λ = 0.15418 nm)
radiation source. Diffraction patterns in the 2θ scan range of 5°-80° were recorded at a rate of 0.5
degrees per minute. The morphology and size of the synthesized mesoporous carbon nitride
samples were observed by field-emission-type scanning electron microscopy (FE-SEM) (FEI
Quanta 200 F), at an acceleration voltage of 30 kV, equipped with an energy dispersive X-ray
spectrometer (EDS). Transmission electron microscopy (TEM) images were recorded using a
JEOL (JEM 2100) microscope. All the samples were prepared by mounting an ethanol-dispersed
sample on a lacey carbon Formvar coated Cu grid. The specific surface area (SBET) was obtained
by using the Brunauer, Emmett, and Teller (BET) method from N
2
gas adsorption-desorption
isotherm at -196°C (BELSORP-mini, BEL Japan, Inc.). Prior to the measurements, the samples
−
5
were degassed under vacuum (1×10 Torr) for 2 h at 200°C. The BET specific surface areas were
determined from the desorption data in the relative pressure (P/P ) range from 0.06 to 0.2. The
o
pore size distributions (PSDs) were calculated from the nitrogen adsorption branch using the
Barrett–Joyner–Halenda (BJH) method and the maximum of the PSD was considered as the
average pore size. The pore volume was considered as the volume of liquid nitrogen adsorbed at
P/P = ca. 1.
o
Temperature programmed reduction (TPR) experiments were carried out in a Micromeritics,
Auto Chem II 2920 instrument connected with a thermal conductivity detector (TCD). Prior to
TPR, the catalysts were also heated at 350°C for 2 h in helium and then placed in 10% H /Ar with
2
–
1
a flow rate of 30 mL min in the temperature range of 40-950 °C with an increment of 10 °C/min.
The surface compositions of carbon nitride samples were analysed by X-ray photoelectron
spectroscopy (K-Alpha, Thermo Scientific Corp.) using a non-monochromatic Mg Kα X-ray
source. The XPS profiles were fitted using a Gaussian–Lorentzian function, and the peak positions
were normalized by positioning the C 1s peak at 284.5 eV.
2
. Results and discussion
The bulk structures of the Ni-Ga on supported catalyst were investigated by powder X-ray
diffractometer. All the unreduced sample shows very broad and diffuse pattern typical of very
small crystallite of metal nanoparticles mixed with amorphous CNx (Figure S2; in supporting
information). Peaks identified at 2θ values of 37.3°, 43.3°, 62.8° and 75.6° correspond to the (111),
(200), (220) and (311) planes of NiO and match very well with hexagonal primitive lattice (JCPDF
card no. 14-0117).[34] There is a significant increase in the peak intensity on reduction, and this
5

is due to the better exposed crystal phases. In the XRD patterns of the reduced NG-CNx-1, -2 and
3 catalysts are shown in Figure 1, the diffractions is observed at 2θ value of 43.7°, 50.9° and 74.7°
corresponds to the (111), (200) and (220) planes of Ni Ga alloy crystals (JCPDS card no. 65-0141).
The inter-planar distances (Figure 1) for the (111) and (200) planes of Ni Ga are 2.04 and 1.74Å
which is in agreement with the JCPDS card no. 65-0141. When the support is changed from CNx
to BCNx, crystalline phases related to Ga Ni alloy is detected. In case of the material prepared
-
3
3
3
5
from urea deposition method (Figure 1), there is no phase responsible for alloy formation but the
peaks for the separated two metallic phases can be seen. Based on the result, it is concluded that,
varying the support and the synthesis method, observable change in the crystallinity of material is
possible. Which may responsible for the change in catalytic activity of the material.[16]
The SEM micrographs of the unreduced NGCNx materials displayed spiral type of material, as
shown Figure S3. However no metal particles are clearly visible from SEM images. TEM images
(Figure 2) of the NGCNx-2 reduced catalyst shows metal nanoparticles with an average particle
size of 2-5 nm, homogeneously distributed over the entire surface. The same is confirmed by the
elemental mapping shown in Figure S4. Clear crystalline fringes can be seen under HRTEM. On
calculating the interplanar (shown in Figure S5) distance between fringes (2.01 and 1.85 Å), it is
assumed that these particles are mainly related to the Ni Ga nano alloy formation which is also
3
consistent to the finding from XRD analysis. By increasing the concentration of metals i.e. NG-
CNx-3; the average particle size increases. Formation of irregular size particles (in Figure S6) in
the range of 5-10 nm is observed. E-mapping associated with HR-TEM instrument shows the
homogeneous distribution of different element over the surface of NGCNx-3 catalyst (Figure S7).
By changing the support from CNx to BxCN, two different morphologies namely doughnut and
rounded shape particles (as shown in Figure S8) are seen. From the HR-TEM images, d values
evidence the formation of Ni
5
Ga alloy particles which also matches with XRD data. By changing
3
the synthesis method from freeze drying to urea deposition of metals; larger metal nanoparticles
with random shape on surface of support are formed (Figure S9). These results suggest that freeze
drying provides a better control over the urea deposition in obtaining uniformly distributed metal
nanoparticles with an average size of 2-5 nm.
The N physisorption experiment results are shown in Figure S10 and tabulated in Table 1. It is
2
clear that increasing metal concentration leads to decrease in the BET surface areas of the
urea
materials. On the contrary, surface areas of NG-CNx-1 catalysts as prepared by urea mediated
metal deposition method doesn’t decrease. This can be explained by the fact that TEM image
shows Ni and Ga particles are not distributed on support surface but they are separately
agglomerated inside the catalyst, hence surface area of material doesn’t decreases drastically.
(
Figure S9). Spent NG-CNx-2 catalyst also doesn’t show much decrease in surface area (Table 1).
However N adsorption-desorption isotherms of all NG-CNx catalyst reveals the type IV isotherm
mesoporous nature of materials) with H1 hysteresis but there is change in shape of isotherm i.e,
2
(
Urea
both NG-CNx-2 & NG-BCNx-2 and NG-CNx-2
& NGCNx-1 has similar isotherm while NG-
CNx-3 shows less porosity . This can be explained by the fact that both NG-CNx-2 & NG-BCNx-
2
has similar metal loading while NGCNx-1 has low loading and NG-CNx-2Urea has no metal
6

particles on surface which doesn’t change inherent nature of support while due to high metals
loading in NG-CNx-3 catalyst, the pores on surface got blocked. In order to judge the size of
mesopore from BJH data, we found that in all NG-CNx catalyst prepared by freeze drying shows
a bimodal pore system with an average pore diameter of 38 and 54Å (Figure S11) but in case of
Ng-BCNx-2 the additional mesopore are very small in size (30 Å) While the sample prepared
using urea deposition shows only one type of pores with an average pore diameter 42 Å. Thereby,
one can assume metals particles of average size below 5 nm are goes inside pores and gives the
apparent pore size to increases.
The H -TPR was carried out to study the reduction capacity of catalysts, as the catalytic activity
2
largely depends on the reduction behaviour of the materials. Figure S12 exhibits the TPR of CNx,
Ni-CNx and Ga-CNx; TPR of pure CNx shows broad band in the high temperature region that can
be due to surface nitrogen reduction. Reduction of pure Ni-CNx and Ga-CNx have two and three
reduction peaks between 400-750°C respectively. This peaks corresponding to the reduction of
different Ni-species on CNx matrix.[35] Whereas for Ga-CNx catalyst the peaks at 529 °C and
7
12 °C can be ascribed to small Ga
O
2 3
particles and isolated bulk Ga
2
O ;[36] while the additional
3
peak at 667°C may be due to the Ga attached with the support nitrogen species. In NG-CNx-2
catalyst (Figure 3) the reduction peak may be ascribe to the reduction of Ni-species. Which may
be due to two types i) Ni
of unreduced catalyst) present in the catalyst. The relatively lower T
be due to trifling interaction between support and metal or smaller particle size.[2] The peak at
600°C is due to reduction of free gallium species bounded with surface nitrogen. Reduction
3
Ga alloy nanoparticles and ii) pure NiO (it will support our XRD data
m
compare to Ni-CNx might
≥
profile of NG-BCNx2 is also similar to NGCNx2; where all the reduction temperature shifted to
higher temperature. Increase in particle size (as experienced form TEM) may be the causes for
such a shift.[37, 38] After comparative analysis we conclude the formation of nickel-gallium alloy
in our catalyst. Thereby, the T is lower than the general reduction temperature of free NiO and
m
Ga-oxide nanoparticle. Whereas in case of NGCNx-3, no separate peaks are visible except a broad
reduction band at 499°C. Which can be deconvoluted into two peaks; the former peak (around
4
62°C) relates to NiO, and the latter peak relates to Ga-oxide.
In general, the agglomeration of metal particles restricts the exposure of core metal species for
reduction leading to a longer reduction time.[39] The material prepared using urea deposition
urea
method NG-CNx-2 has two reduction temperatures below 500°C, which may be ascribed to
NiO in two local atmosphere and the reduction peak for Ga
2
O appears at somewhat higher
3
temperature. The same have been experienced from HR-TEM images. Moreover, reduction
behaviour due to metal-support interaction seems to be decreases with increasing metal loading,
due to the limited availability of surface nitrogen functionality. But at the same time the effect of
agglomeration and larger particle size shift the reduction temperature to higher values; as seen for
NG-CNx-3.
To further confirm surface structure and chemical composition; especially for nickel and
gallium, catalyst were studied by X-ray photoelectron spectroscopy. Ni 2p XPS spectra of NG-
CNx-2 (Figure 4) catalyst shows two peaks at binding energy 856.4 eV and 873.9 eV with an
7

energy difference of 17.5 ev. In addition, two small satellites peaks are also present due to shake
up B. E. The peak at 856.4 eV is at higher binding energy than metallic nickel and NiO, although
it is in close proximity with sheltie-type ABO
4
.[40] Nevertheless the energy gap between 2p3/2 and
2
p
1/2 peaks (i.e. 17.5 eV for all the catalyst except NG-CNx-2urea) related to metallic or no-oxygen
containing species. This result indicates the possible alloy formation for the catalyst prepared by
freeze drying method followed by calcination at 400°C. Whereas, the catalyst prepared using urea
deposition method has binding energy corresponding to 856.0 eV and 874.3 eV, confirm the
presence of Ni-OH in the catalyst rather than metallic nickel. In case of NG-BCNx-2, the binding
energy related to gallium (Figure 5) also shows a positive shift; which may be due to the formation
of nickel gallium alloy and thus binding energy of both the entity increases.[41] In case of NG-
urea
CNx-2 catalyst, the binding energy 20.1 eV correspond to Ga
2
O species. The C1s spectrum of
3
all prepared catalyst can be deconvoluted into five peaks (Figure S13). The two highly intense
3
2
peaks at low binding energy may be assigned to sp type graphitic carbon and sp diamond type
carbon structural moieties. One peak at 286.7 eV is due to CN type structural moieties inside
catalyst. The other two peaks at high binding energy is assigned to C-OH and –C=O type surface
functionalities of support. N1s spectra (in Figure S14) of all NiGa catalyst can be deconvoluted
into three peaks related to pyridinic, pyrolic and quaternary C-N structure. In case of NG-BCNx-
2
catalyst, one additional boron element is present in survey scan. And after deconvolution of B1s
spectra (Figure S15) it is noticed the peak is corresponding to C-B-N type structural moieties of
catalyst.
3
.1 Catalytic Activity
The catalytic activities of NG-CNx catalyst prepared by freeze drying method are listed in Table
. In general CO hydrogenation can be described by a reaction involving four equilibrium steps.
퐶푂 + 퐻 → 퐶푂 + 퐻 푂
2
2
(1)
(2)
(3)
(4)
2
2
2
퐶푂 + 6퐻 → 퐶퐻 + 퐻 푂
2
4
2
퐶푂 + ꢀ퐻 → 퐶퐻 푂퐻
2
3
ꢀ
퐶푂 + ꢀ퐻 → 퐶퐻 + 퐶푂
2 4 2
From the data in Table 2, we found that Ga-CNx is more selective toward methanol compare to
Ni-CNx, although the conversion of CO is lower than Ni-CNx. It is also worth to notice that Ni-
CNx produce more CH and less CO compared to Ga-CNx. This implies that Ni is particularly
active for the hydrogenation of CO into light hydrocarbon. As a combined sample, NG-CNx-2
led the CO conversion as well as methanol yield to almost double improvement compared to the
2
4
2
2
case either Ga or Ni was solely included. This result is in close proximity to the work done by
Studt et al. [16]; as the appearance of NiGa alloy improved the methanol formation rate (0.05
mol/molmetal/ h) than the individual counterpart.
Influence of reaction temperature and pressure
Influence of the reaction temperature is shown in Figure 6 & 7, as the reaction temperature rise
from 250 to 330°C, CO conversion is found to increase with the decrease in methanol selectivity.
2
Inversely, the production rates of CO increased with the temperature due to endothermic character
8

of RWGS.[42] Under the high concentration of CO, CH
possibly due to the successive hydrogenation of the adsorbed CO via hydrogen absorbed by the
surface carbon.[12] At 330°C and 20 bar, CO conversion reaches up to 7.9 % and turnover
frequency number improved almost double compared to that obtained from the reference [20]
4
production also increases. This is
2
-
1
-5
-5
(
TOF, sec : 6.91 x 10 from Ni
5
Ga
3
/SiO2[16] vs 13.12 x 10 from NG-CNx-2). Pressure has also
conversion increases
played an important role in product selectivity. With increasing pressure, CO
2
with enhancement in methanol and methane selectivity whereas CO selectivity decreased (in
Figure 7 and Table 2). According to the Reaction 3, more methanol is produced with increasing
reaction pressure. As the reaction (1 to4) are dependent appositively to temperature and pressure,
the production ratio of methanol to CO changed accordingly. The CO can react further with H
and can form CO and CH in parallel. Reaction 1 and 4 may be dominant with increasing metal
loading (discussed latter). While, Reaction 2 and 3 needs less H , which may account for the
smaller H conversion with small CO and water production too.
2
2
4
2
2
3
.2 Influence of metal loading
Concentration of metal loading was also investigated. As shown in Figure 8, the conversions of
CO and H were increased with the increase in metal loading and then decreased for NG-CNx-3.
2
2
This may be due to the degree of metal dispersion which required a certain amount of metal to
achieve the maxima, followed by metal-metal agglomeration causes larger particle size. While CO
and H
obtained from NG-CNx-2. In addition, when NG-CNx-3 was used, the production of CO and H
are inhibited significantly. Instead, methane became a main product. This is clearly indicating that
high Ni content is in favour of the methanation of CO . The ratio of C-selectivity CO to MeOH
shows the lowest value of 12.3 with NG-CNx-2, indicating the highest activity for methanol
synthesis. This is further evident from that of O-selectivity. Formation of crystalline Ni Ga phase
2
O selectivity is decreased with metal concentration, the highest methanol selectivity is
2
O
2
3
for all NG-CNx-1, -2 and -3 additionally NiO phase in NG-CNx-3 was confirmed by XRD analysis
in Figure 1. In addition, NG-CNx-2 sample shows the lowest reduction peak at 401 °C, indicating
the least interaction between metal and support and/or smaller particle size. Combining these
aspects, it is believed that the abundancy of smaller Ni
high activity of methanol synthesis. As discussed earlier, higher metal loaded catalyst (i.e. NG-
CNx-3) shows formation of Ni Ga and NiO particles on the surface; which seems less active for
3
Ga species on the surface is related to the
3
methanol synthesis. As considerably larger amount of methane is formed from NG-CNx-3, it might
be due to NiO which is active for methanation reaction.
3
.3 Role of NiGa alloy on CO
The effect of promotor addition on the activity of CO
have explained earlier the Ni-Ga nanoparticles synthesized on CNx support lead to formation of
different crystal structure (alloy like Ni Ga, Ni Ga /or metallic Ni / Ga or their oxides) on CNx
2
conversion and methanol selectivity
2
conversion is shown in Figure 9. As we
3
3
4
support. While formation of NiGa alloy nanoparticles is also observed on BCNx support; but the
morphology of nanoparticles changes in case of NG-BCNx catalyst. Furthermore, among the
different planes of Ni-Ga alloy nanoparticles, (111) plane of Ni
3
Ga in NG-CNx and (100) plane of
9

NiGa alloy in NG-BCNx catalyst are found active for CO
conversion of CO is observed over NG-BCNx (Figure 9) but maximum methanol selectivity of
3.5% was achieved in this series of catalysts. Boron better known as an electron-deficient
promoter, thereby BCNx surface tends to improve the dissociative adsorption of CO to enhance
2
hydrogenation. Although, the least
2
1
2
the methanol selectivity.[35] In our previous work with BCNx support we calculated the absolute
electronegativity of the support (0.2145), which is higher than the CNx (0.1806) support.[35]
Thereby, the boron-doped catalyst (i.e. NG-BCNx) has greater oxidation strength than the bare
CNx catalyst.
Similarly, we have observed that NG-CNx-3 has less exposed (111) plane of Na
compared to NG-CNx-2 and thereby shows less conversion and methanol selectivity. Comparing
all the results, we have concluded that the (111) plane of Ni Ga alloy is the most reactive phase
for CO conversion while the (100) plane NiGa alloy is most active phase for methanol selectivity.
.4 Role of deposition method and support on product selectivity
On other hand, the method of deposition imparts a significant effect on CO
3
Ga plane
3
2
3
2
reduction activity.
As discussed earlier catalyst prepared through urea deposition method did not lead to formation of
nickel gallium nano alloy, which is reported to be active for methanol formation. In place of Ni-
Ga alloy, isolated nickel and gallium or corresponding oxides are found to form over the catalyst.
Consequently, the catalyst is found to be indolent for methanol selectivity though it achieve 7.1%
CO
and the formation of NiGa alloy nanoparticles of average particle size 2-5 nm (for NG-CNx-2)
with large exposed (111) face of Na Ga. This results proves that freeze drying technique is very
2
conversion (Figure 9). The catalyst prepared via freeze drying shows lower agglomeration
3
effective to minimize metal-metal agglomeration compared to the conventional deposition
techniques.[43, 44] Moreover, the method provides suitable local atmosphere for the formation of
Ni-Ga alloy with exposed (111) face of Na
discussed above).
3
Ga upon calcination (as per HRTEM and XRD data
With the increase in the metal concentration, visible difference in particle sizes has been
observed within the catalysts prepared via freeze drying. This means higher metal loading
enhances the chance of agglomeration as examined by Maillard et al.[45] and the average particle
size increases from 1-4 to 5-10 nm with the increase in metal loading (Table 1).
These catalyst with large particles shows less activity and lower methanol selectivity (Figure 8
and Table 2). Lower loadings show better distribution of NiGa nanoalloy, but conversion of CO
remains low. On the other side, CO selectivity decreases with metal loading while the equilibrium
yield of C increases with metal loading. This can be due to the more amount of free Ni on the
2
1
catalyst surface.[46] Although, a minimum amount of metal loading is necessary to achieve the
minimum residence time for the reaction,[47] it is concluded that on the basis of current finding,
metal nanoparticles synthesized through freeze drying method with 14% Ni and 6% Ga (NG-CNx-
2
) is the most active catalyst for CO conversion.
2
3
. Conclusions
In this work, CO
2
hydrogenation over NiGa supported mesoporous nitrogen rich carbon catalyst
were studied. The following conclusion can be drown from the finding of the present study.
1
0

1
2
) Deposition of metal on CNx matrix has been closely evaluated. The catalyst prepared by
conventional urea deposition method appears to have the higher extent of metal-metal
agglomeration (Table 1) with average particle size between 10-30 nm. In compare,
catalyst prepared by freeze drying method shows uniform particle distribution. These
results suggest, freeze drying provides a better control over the conventional urea
deposition to obtain uniformly distributed metal nanoparticles with an average size of 2-
5
nm.
) Freeze drying approach likely gives suitable local environment for the formation of NiGa
alloy nanoparticle. d values evidence the formation of Ni Ga alloy particles in case of
3
NG-CNx catalyst prepared via freeze drying method. Whereas, urea deposition found to
form random individual metal nanoparticle.
3
4
) Increase in particle sizes with the upsurge in the metal concentration, seems to control
the CO
) Both, Ni
Whereas, Ni
to Ni Ga
2
conversion as well as methanol selectivity.
Ga and Ni Ga alloy nanoparticle found to be active for CO
Ga alloy particles seems less active for methanol synthesis when compare
3
5
3
2
hydrogenation.
3
5
3
.
5
6
) Ga supported on CNx appears to be methanol selective whereas, Ni appears as
hydrocarbon specific. The hydrogenation property of Ni leads more hydrocarbon
formation compare to oxidation to methanol.
) Boron promoted CNx surface tends to improve the dissociative adsorption of CO to
2
enhance the methanol selectivity. A maximum methanol selectivity of 13.5% was noted
with NG-BCNx-2 catalyst. Higher absolute electronegativity of BCNx support (0.2145)
(
which is higher than the CNx (0.1806) support) provide higher oxidation strength for
higher methanol selectivity.
ASSOCIATED CONTENT
Supporting Information.
Physicochemical characterization data like XRD, SEM & HR-TEM images, elemental mapping,
TPR, XP spectra, etc.; this material is available free of charge via the Internet at http://pubs.acs.org.
Notes
The authors declare no competing financial interest.
Acknowledgements
We are thankful to CSIR for the financial support in terms of the OLP-0912 grant. R. G.
acknowledges the CSIO, Melbourne for research fellowship and the catalyst testing facility. R.G.
also acknowledge Council for Scientific and Industrial Research (CSIR), India for doctoral
fellowship. We also acknowledge DIIP for their support. The authors thank the Analytical Science
Division, Indian Institute of Petroleum for analytical services.
1
1

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1
3

Figure 1. X-ray diffraction pattern of different Ni-Ga catalyst supported on mesoporous CNx
reduced in 5% H balance He for 1h.
2
Figure 2. HR-TEM images of NG-CNx-2 catalyst. Inset: particle distribution histogram; X-axis
represents particle size in nm.
1
4

Figure 3. H -TPR pattern of CNx and Ni-Ga catalyst supported on CNx support obtained with
2
different preparation method.
Figure 4. XP spectra of Ni 2p3/2 for Ni-Ga catalyst supported on different CNx support and
different preparation method.
1
5

Figure 5. XP spectra of Ga 3p3/2 for Ni-Ga catalyst supported on different CNx support and
different preparation method.
Figure 6. Effect of reaction temperature in CO
2
hydrogenation over NG-CNx-2 catalyst. Reaction
conditions: CO /H /He/Ar=1/3/0.25/4.1, space velocity of 6000 ml/gcat·h, 280 °C and 20 bar.
2
2
1
6

Figure 7. Influence of reaction pressure on CO
2
conversion and the selectivity of products.
/He/Ar=1/3/0.25/4.1, space velocity of 6000
Reaction conditions: Catalyst=NG-CNx-1, CO
2
/H
2
ml/gcat·h, 280 °C.
Figure 8. Influence of metal concentration in NG-CNx catalysts on the CO
Reaction conditions:CO /H /He/Ar=1/3/0.25/4.1, space velocity of 6000 ml/gcat·h, 280 °C and 20
b. (a) CO conversion and C-Selectivity, (b) H conversion and H-Selectivity and (c) O-Selectivity.
2
hydrogenation.
2
2
2
2
1
7

Figure 9. Influence of boron addition and synthesis method. Comparison of NG-CNx-2 with
boron-added NG-CNx-2 and NG-CNx-2 prepared by urea deposition. Reaction
conditions:CO /H /He/Ar=1/3/0.25/4.1, space velocity of 6000 ml/gcat·h (280 °C and 15 BAR).
a) CO conversion and C-Selectivity, (b) H conversion and H-Selectivity and (c) O-Selectivity.
: Combined selectivity of MeOH and DME.
2
2
(
2
2

Table 1. Physicochemical properties of supported CNx catalysts
a
Catalyst
Metal (mole %)
S
(
BET
2
V
T
D
p
P
(nm)
size
CN
(in wt%)
−1
3
−1
m g ) (cm g ) (Å)
Ni
-
-
14
7
14
28
14
14
14
Ga
-
6
-
3
6
12
6
6
6
C
N
CNx
Ga-CNx
Ni-CNx
NG-CNx-1
NG-CNx-2
NG-CNx-3
NG-BCNx
NG-CNx-2
492
359
384
425
305
224
351
415
285
0.41
0.25
0.27
0.36
0.28
0.17
0.26
0.29
0.22
41
44
48
44
49
30
33
40
47
78
18
17
13
15
18
14
15
16
14
76
74
75
73
69
1-4
2-5
5-10
12-20* 73
urea
10-30
2-7
74
70
b
NG-CNx-2
a
2
-1 b
Measured by N adsorption using standard BET methods in m g ; spent catalyst. 0.1g sample
2
was taken for CN analysis and the H2 required to remove all the N site has been calculated on the
basis of 0.1g catalyst. The same wt. of the material was also been taken during the TPR experiment.
Table 2. The catalytic performance of the catalysts prepared by different methods for the CO
2
hydrogenation.
Catalyst
R
xn cond.a
Conv.
(%)
-
1.2
2.7
Selec. (%)
Ref.
MPa
°C
MeOH
-
5.2
CO
-
80.0
52.3
C
-
12.9
44.2
1
¤
-
-
rest
CNx
Ga-CNx
Ni-CNx
2
2
2
280
280
280
⌂
⌂
⌂
3.2
≤1.4
1
8

NG-CNx-2
NG-CNx-2
1.5
2
0.1
0.5
5
280
280
200
302
250
250
4.1
5.2
-
3.8
6.8
0.25
1.3
10.1
60.0
94.0
83.3
2.1
9.8
≤0.1
≤0.1
⌂
⌂
[16]
[48]
[49]
[20]
b
Ni-Ga-SiO
Cu-CeO
Pd-Ga
2
c
2
-
2
O
3
19.6
10.7
Pd-ZnO
2
39.0
-
-
a
Condition-0.5 g catalyst at 280°C with 2MPa feed consisting CO
2
:H
2
:He:Ar =1:3:0.25:4.1 with
b
c
2
a flowrate of 100ml/min. in mol/molmetal h; TOF of methanol in mol/cm S calculates as
TON/time of reaction, whereas, TON = number of moles of reactant consumed/(mole of catalyst);
¤
rest – it consist C + DME; ⌂- in our reaction.
2
1
9