Methanol synthesis using CO2 and H2 on nano Silver-Ceria zirconia catalysts: Influence of preparation method

Article abstract of DOI:10.1166/jnn.2019.16611

Silver catalysts supported on ceria-zirconia (CZ) mechanically mixed oxide were synthesized by wet impregnation and chelating methods. Nominal loadings of 5 wt.% of Ag was deposited on the CZ support. These catalysts were tested for the CO₂ hydrogenation reaction to methanol with feed gas composition of CO₂-H₂ =3:1 at 250 °C, 20 bar total pressure and GHSV of 1800 hr¹. The calcined and reduced catalysts were characterized using XRD, BET, TPR, SEM-EDS, XPS and FTIR-DRIFTs techniques. Finely deposited silver crystallites sized in the range of 20-50 nm were observed through SEM and HR-TEM analysis. TPR and XRD studies demonstrated the presence of Ag₂O and metallic silver (Ag0) on CZ support. About 10% of CO formation was observed on chelating catalyst (5Ag/CZ CHE). However, only, 5% CO was observed on impregnated (5Ag/CZ IMP) catalyst. The greater CO formation was associated with ease reduction of Ag₂O to metallic silver in 5Ag/CZ CHE catalyst. Further, 70% of methanol selectivity was observed on 5Ag/CZ IMP due to the presence of Ag₂O on CZ. FTIR-DRIFTs results revealed the methanol formation via formate intermediates and CO formation via RWGS reaction on the studied catalysts.

Full text of DOI:10.1166/jnn.2019.16611

Article
Journal of
Nanoscience and Nanotechnology
Vol. 19, 3197–3204, 2019
Copyright © 2019 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Methanol Synthesis Using CO and H on
2
2
Nano Silver-Ceria Zirconia Catalysts:
Influence of Preparation Method
∗
Nagaraju Pasupulety , Hafedh Driss, Mohammed Raoof A. Rafiqui, Abdulrahim A. Al-Zahrani,
Muhammad A. Daous, Arshid M. Ali, Sharif F. Zaman, and Lachezar A. Petrov
Chemical and Materials Engineering Department, Faculty of Engineering, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Silver catalysts supported on ceria-zirconia (CZ) mechanically mixed oxide were synthesized by
wet impregnation and chelating methods. Nominal loadings of 5 wt.% of Ag was deposited on the
CZ support. These catalysts were tested for the CO hydrogenation reaction to methanol with feed
2
ꢀ
−1
gas composition of CO –H = 3:1 at 250 C, 20 bar total pressure and GHSV of 1800 h . The
2
2
calcined and reduced catalysts were characterized using XRD, BET, TPR, SEM-EDS, XPS and
FTIR-DRIFTs techniques. Finely deposited silver crystallites sized in the range of 20–50 nm were
observed through SEM and HR-TEM analysis. TPR and XRD studies demonstrated the presence
0
of Ag O and metallic silver (Ag ꢀ on CZ support. About 10% of CO formation was observed on
2
chelating catalyst (5Ag/CZ CHE). However, only, 5% CO was observed on impregnated (5Ag/CZ
IP: 109.94.175.86 On: Sat, 02 Mar 2019 23:07:59
IMP) catalyst. The greater CO formation was associated with ease reduction of Ag O to metallic
Copyright: American Scientific Publishers
2
silver in 5Ag/CZ CHE catalyst. Further, 70% of methanol selectivity was observed on 5Ag/CZ IMP
Delivered by Ingenta
due to the presence of Ag O on CZ. FTIR-DRIFTs results revealed the methanol formation via
2
formate intermediates and CO formation via RWGS reaction on the studied catalysts.
Keywords: Nano Silver Catalysts, CZ Support, CO Hydrogenation, Methanol, FTIR-DRIFTs.
2
1
. INTRODUCTION
of methanol.13–16 It is also widely accepted that the hydro-
genation of formate type complexes is the rate-limiting
Recently, catalytic hydrogenation of CO to methanol is
studied extensively (Eq. (1)). This process has the advan-
tage of mitigation of green house gas and also converts
2
1
6ꢀ17
step of the reaction.
On the other hand, reverse water
gas shift (RWGS, Eq. (2)) reaction is the parallel process
1
–3
16
less value CO feed-stock to valuable chemicals. Most
occurring on the surface of metallic Cu.
2
of studies are focused on copper and promoted copper cat-
4
ꢀ5
CO +3H = CH OH+H O
alysts for methanol synthesis from CO and H . How-
2
2
3
2
2
2
ever, there are few studies on silver based catalysts. Baiker
−1
ꢁ
H298 K = −49ꢂ58 kJ mol
(1)
6
7
et al. and Koppel et al. reported that the selectivities
to methanol in hydrogenation of CO on Ag/ZrO and
CO +H = CO+H O ꢁH298 K = +41ꢂ12 kJ mol−
1
2
2
2
2
2
8
Cu/ZrO2 catalysts are similar. Further, Frohlich et al.
(2)
reported that doping of Cu/ZrO with silver leads to the
2
increase of the selectivity to methanol without change of
No systematic information on the mechanism of hydro-
CO conversion.
Structure-activity and mechanistic aspects of the cop-
per catalysts have been extensively studied.
genation of CO to methanol over silver containing cat-
alysts has been available. Recently, Grabowski et al.
reported CO2 hydrogenation studies on Ag/ZrO2 and
2
2
1
8
9
–12
Adsorp-
tion of CO and hydrogen dissociation are reported on
Cu and Cu surface sites. Further, it was accepted that
formation of formate complexes leading to the preparation
Ag/ZrO /ZnO catalysts. The authors stated that metallic
silver contributes to RWGS and the presence of Ag is
leading to methanol formation on the co-precipitated cata-
lysts. However, this report lacks the information about the
mechanism of the formation of surface adsorbed species.
2
2
+
0
+
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 6
1533-4880/2019/19/3197/008
doi:10.1166/jnn.2019.16611
3197

Methanol Synthesis Using CO and H on Nano Silver-Ceria Zirconia Catalysts: Influence of Preparation Method Pasupulety et al.
2
2
The aim of this work is to understand the sur-
face species formation on ceria-zirconia mixed oxide
as 5Ag/CZ IMP CAL and reduced catalysts represented as
5Ag/CZ IMP RED.
(
CZ, Ce0ꢂ75Zr0ꢂ25O ꢃ support and silver catalysts supported
2
on ceria-zirconia mixed oxide (Ag/CZ) and their partic-
ipation in reaction mechanism by using FTIR-DRIFTs
studies. Silver (5 wt.%) was deposited on CZ by two dif-
ferent methods: (i) by wet impregnation; and (ii) by depo-
sition in the presence of citricacid used as a chelating
agent. Detailed characterization of CZ and Ag/CZ cata-
lysts were reported using BET, XRD, XPS and SEM-EDS
techniques. Finally, structure and activity correlation was
discussed as a part of the discussion of mechanism of CO2
hydrogenation to methanol.
2.3. Catalyst Characterization
The BET surface area and pore size analysis of Ag/CZ
samples using DFT method were performed by Nova Sta-
tion Quantachrome (USA) adsorption equipment at liq-
uid nitrogen temperature after out gassing the samples at
ꢀ
2
00 C under vacuum for 2 h.
The X-ray diffractograms of the Ag/CZ samples were
obtained on a EQUINOX 1000 Inel XRD instrument using
Co K = 1.7902 Å with X-ray source generator settings at
ꢄ
4
1
0 kV and 30 mA with real time acquisition over 2ꢅ from
0 to 110 .
ꢀ
XPS results of CZ and Ag/CZ samples were collected
2
. EXPERIMENTAL DETAILS
on SPECS GmbH high vacuum multi-technique surface
analysis system equipped with Mg Kꢄ 1253.6 eV X-ray
source. The reported binding energy values are corrected
using C 1s 284.6 eV signal.
Transmission electron microscopy (TEM), High resolu-
tion TEM images were collected on a Technai 200 kV
D1234 Super Twin microscope with camera length of
2
.1. Materials
Ceria and zirconia oxides were obtained Rhodia-Belgium.
To prepare the ceria-zirconia mixed oxide support (CZ),
these single oxides were mechanically mixed with ultra-
sound mixer in a ratio of CeO –ZrO = 3–1. The resul-
2
2
ꢀ
tant support was thermally treated at 450 C under static
air. Silver nitrate (AgNO , 99.8%) was supplied by BDH,
England and was used without any further purification.
3
9
7 cm. Sample morphology was investigated by means
of field emission scanning electron microscopy (FE-SEM,
Quanta FEG450, FEI) using an ETD Everhart Thornley
detector (HV mode) and a solid-state back scattering elec-
tron detector (VCD).
2
2
.2. Catalyst Preparation Methods
.2.1. Wet Impregnation Method
For preparation of 5 g catalyst batch of 5 wt.% of silver
IP: 109.94.175.86 On: Sat, 02 Mar 2019 23:07:59
In situ DRIFTs experiments were performed using
catalyst supported on CZ, approximat eCl yo p0 .y4 r 1i g gh to: fA Am g eN r Oi can Scientific Publishers
3
3
a Bruker Tensor-II FTIR spectrometer equipped with a
DLATGS detector at room temperature, Harrick praying
mantis with high temperature and high pressure reaction
chamber (4 mm thick ZnS windows (≥6 MPa)). Approxi-
mately, 0.1 g of catalyst was loaded and it was reduced at
Delivered by Ingenta
was dissolved in 50 cm of DI water in the bulb of rotava-
por. To this silver nitrate solution about 4.75 g of calcined
CZ were added and the resultant suspension was mixed for
2
h at room temperature. Then, the excess water was evap-
orated and the resultant powder was dried in a preheated
oven at 90 C for 12 h.
ꢀ
3
−1
ꢀ
250 C for 2 h under the flow of H (20 cm min ꢃ. Sub-
2
sequently, the chamber flushed with argon for 1 h. Then
the reaction mixture gas (CO –H = 1–3) was introduced
2
2
2
.2.2. Chelating Method for Catalyst Preparation
into the chamber with an applied pressure of 20b and col-
lected the sample data.
Citric acid to silver content mole ration was maintained
at 3–1. Silver nitrate solution with 0.41 g of silver nitrate
dissolved in 50 cm of deionized water was used to obtain
3
Temperature programmed reduction (TPR) of the cal-
cined Ag/CZ IMP and Ag/CZ CHE samples (0.1 g)
were studied using 4.95% H –N mixture gas with Quan-
5
wt.% nominal silver loading on CZ. The citric acid solu-
2
2
tion was added to silver nitrate solution under continu-
ous stirring and the resultant mixture was maintained at
5
4
uous stirring. Then, the water was evaporated quickly by
heating on hot plate. The obtained gel was cooked for 24 h
using a water bath with temperature at 90 C. The resultant
foam like solid material was crushed to powder. Finally,
the support and Ag/CZ catalysts were calcined at 450 C,
tachrome pulsar automated chemisorptions instrument at
ꢀ
−1
ꢀ
ramping rate of 10 C min .
0 C for 2 h for homogenization. To this mixture solution,
.75 g of CZ was added and aged for 1 h under contin-
2
.4. Catalytic Activity Tests
Catalytic activity studies were performed using PID Eng
and Tech system equipped with gas mass flow meters, tem-
perature controllers and micro flow reactor. Catalytic activ-
ꢀ
ꢀ
ꢀ
ity tests were carried out at 250 C, pressure of 20 bars
−1
3
h under static air. The calcined powders were pelletized,
and GHSV of 1800 h . A calcined catalyst sample of
0.50 g was loaded and subsequently treated with hydro-
grained and selected fraction of 0.1–0.3 mm was used for
reaction and characterization.
The prepared catalysts were denoted as 5Ag/CZ IMP is
for impregnated catalysts and 5Ag/CZ CHE is for chelat-
ing prepared catalyst. The calcined catalysts represented
3
−1
ꢀ
gen flow of 50 cm min at 250 C for 2 h. Further, the
CO –H gas mixture with 1–3 mole ratio was introduced
2
2
into the reactor to attain the required pressure. The reac-
tion stream composition directed to Agilent 7890A GC
3198
J. Nanosci. Nanotechnol. 19, 3197–3204, 2019

Pasupulety et al. Methanol Synthesis Using CO and H on Nano Silver-Ceria Zirconia Catalysts: Influence of Preparation Method
2
2
Table I. Rietveld analysis and BET results of CZ and Ag/CZ catalysts.
(
equipped with TCD detector with Restek Plot Q for CO2
and CO evaluation and FID detector with HP Plot Q for
methanol estimation) to record the activity data for 2 h
after accomplishing the steady state.
Nominal/
Rietveld
(Ag wt.%)
Rietveld
crystallite
size (nm) m g
BET
Pore volume
cm g
2
−1
3
−1
Catalyst
CZ
–
15 (CZ)
15 (CZ)
105
94
0.52
0.25
5Ag/CZIMP CAL
5/4.2
3
. RESULTS AND DISCUSSION
39 (Ag)
3
.1. XRD Analysis
5Ag/CZ IMP RED
5/4.6
5/4.1
5/4.8
15 (CZ)
45 (Ag)
77
70
62
0.22
0.21
0.19
Figure 1 depicts the X-ray diffraction patterns of pre-
ꢀ
pared catalysts. The support CZ calcined at 450
C
5Ag/CZ CHE CAL
5Ag/CZCHE RED
15 (CZ)
22 (Ag)
shows the presence of ceria-zirconia mixed oxide phase
of Ce0ꢂ75Zr0ꢂ25O . The calcined silver supported catalysts
15 (CZ)
2
26 (Ag)
also displayed identical ceria-zirconia mixed oxide phase,
which suggest the intact structure of CZ even after Ag
loading on it. Further, both the Ag/CZ (IMP and CHE)
calcined catalyst diffractogram contains the X-ray signals
expected values. The discrepancies are in the limit of
the experimental error of the implemented method. In the
impregnated catalyst, the Ag crystal sizes are significantly
larger than that of chelated catalyst. Furthermore, in the
reduced samples the silver crystal size is slightly bigger
than in the calcined samples, which suggests that the silver
aggregation during reduction process.
BET surface area and total pore volume of the support
and the Ag/CZ samples are presented in Table I. The sup-
port, CZ shows greater BET surface area (200 m g ꢃ
ꢀ
related to metallic silver at 2ꢅ = 44.2, 51.3 and 76 . On the
other hand, identical metallic silver bands with greater
relative intensity was observed for reduced Ag/CZ cata-
lysts over their calcined counter parts under similar XRD
conditions (20 mg, 2 h of XRD analysis). Metallic sil-
ver formation was reported for Ag/ZrO and Ag/ZrO /ZnO
2
2
catalysts under long-term pretreatment conditions with
ꢀ
18ꢀ19
heating above 400 C in static air atmosphere.
2
−1
However, no silver oxide phases were detected in all the
studied Ag/CZ catalysts. It could be due to the smaller size
−1
and pore volume (0.5 cm g ꢃ compared to Ag/CZ cata-
lysts. It is obvious from Table I results that the surface area
and pore volume have decreased after deposition of Ag on
of Ag O/AgO particles (<4 nm, detection limit of XRD
2
techniques) and or masked by theI Pb i: g1 c0 r9y .s9t a4 l .s1 7o f5 .C8 Z6 .On: Sat, 02 Mar 2019 23:07:59
CZ. It could be due to the support pores blocked by the
The Rietveld method was used for t Ch eo pd yi f rf irga hc tt i: o An mp ee ar ikc san Scientific Publishers
deposited silver particles. The support and Ag/CZ catalysts
Delivered by Ingenta
deconvolution and the content evaluation of silver and
shows ordered mesopores with unimodal size distribution
of about 8 nm. The mesopores structure was intact even
after the deposition of silver.
ceria-zirconia mixed oxide phases. The results of full
quantitative XRD analysis by the Rietveld method are
shown in Table I, which comprises the quantitative com-
parison between crystal sizes of the equivalent phases in
the calcined and reduced catalysts. As can be seen, the
contents of particular phases are close to their nominal
3
.2. TPR Studies
The TPR profiles of calcined CZ and Ag/CZ samples are
shown in Figure 2. No reduction profile was observed for
ꢀ
support CZ up to 350 C. However, a single reduction
peak was observed for Ag/CZ samples with temperature
ꢀ
maxima at 180 C. Silver oxide Ag O is reduced quanti-
2
tatively to metallic silver in the temperature range of 150–
ꢀ
19
2
00 C. The obtained hydrogen consumption capacity
for Ag/CZ IMP and Ag/CZ CHE samples were 90 and
−1
1
20 ꢆmol g respectively. The increased hydrogen con-
sumption for Ag/CZ CHE compared to Ag/CZ IMP sam-
ple suggests the abundance of Ag O in it.
2
Further, the ease reducibility of Ag O to metallic silver
2
suggests high dispersion of Ag O on CZ obtained by using
2
chelating agent. The results are in agreement with SEM
mapping analysis where, finely deposited silver particles
were observed in 5Ag/CZ CHE.
3
.3. XPS Studies
XPS analysis results of Ag 3d orbital for Ag/CZ calcined
and reduced samples are presented in Figure 3. After the
deconvolution of calcined samples, Ag 3d5/2 yielded three
Figure 1. XRD analysis of CZ and Ag/CZ catalysts.
J. Nanosci. Nanotechnol. 19, 3197–3204, 2019
3199

Methanol Synthesis Using CO and H on Nano Silver-Ceria Zirconia Catalysts: Influence of Preparation Method Pasupulety et al.
2
2
Ag/CZ IMP catalyst. In addition, greater silver content was
also observed for reduced Ag/CZ CHE catalyst through
Rietveld analysis. Hence, yielded high amounts of CO dur-
ing CO hydrogenation.
2
The XPS results of Ce and Zr 3d orbital shows the pres-
4
+
3+
4+
2+
ence of Ce /Ce and Zr /Zr oxidation states in all
4
+
3+
samples. The Ce /Ce correspond to CeO and Ce O
2
2
3
4
+
2+
phases. Further, Zr (major)/Zr (minor) correspond to
ZrO2 and non-equilibrium ZrO phases. Similar results
reported by Zheng et al. for ceria-zirconia mixed oxide
phase of Ce0ꢂ75Zr0ꢂ25O2. The O 1s orbital deconvolution
in all the studied samples produced three types of com-
ponents at binding energy 530, 532 and 534 eV. These
signals correspond to O-metal bond, surface –OH groups
and adsorbed water respectively.
2
0
3
.4. SEM/EDS Results
Figure 2. TPR profiles of calcined catalysts.
The SEM mapping-EDS measurements are carried out for
Ag/CZ samples to find out the silver deposition on CZ.
The images are presented in Figures 4 and 5 respectively
for calcined and reduced samples. The calcined support
signals at binding energy (BE) 367, 367.7 and 368.3 eV.
2
+
+
These signals, respectively corresponds to Ag , Ag and
0
18
(
CZ, Fig. 4(a)) showed spherical crystalline aggregates due
Ag oxidation states for AgO, Ag O and metallic silver.
2
to Ce–Zr–O links. The selected area EDS analysis of the
sample (Fig. 4(b)) revealed Ce, Zr and O elements. The
size of these spherical particles were measured through
HR-TEM analysis and their size ranged in 10–15 nm.
These results are in line with XRD Rietveld analysis.
The BE of these signals are very close, therefore the
quantitative results about the surface concentrations of the
corresponding silver species are not very precise. How-
ever, these results might be used for qualitative estima-
tions. On the other hand, only two signals obtained for
IP: 109.94.175.86 On: Sat, 02 Mar 2019 23:07:59
As observable from Figure 4(c), the polydispersive silver
reduced Ag/CZ samples at 367.7 and 368.3 eV, which
Copyright: American Scientific Publishers
+
0
crystals (white spots) were deposited all over on CZ sur-
are attributed to Ag and Ag oxidation states fDo re l Ai v ge r Oe d by Ingenta
2
face for calcined Ag/CZ IMP sample. EDS analysis of the
and metallic silver. It was reported that metallic silver
0
chosen fragment of Ag/CZ IMP (Figs. 4(d and e)) sample
shows that white spots majorly made up of silver micro-
crystallites. On the other hand, silver crystallites deposi-
tion is quite different for calcined Ag/CZ CHE. The silver
crystallites finely deposited on the surface of CZ due to
the usage of chelating agent (Figs. 4(f–h)) and stays in
close contact with CZ in Ag/CZ CHE. The silver crystal-
lites size was about 20 and 40 nm for Ag/CZ CHE and
Ag/CZ IMP catalysts respectively.
(
Ag ꢃ contributes to RWGS reaction to produce CO and
water. Further, methanol formation through formate com-
+
18
plexes was reported for Ag species (Ag O). In the
2
0
present study, TPR results revealed predominant Ag for-
mation through Ag O reduction in Ag/CZ CHE over
2
Figures 5(a) and (b) presents the reduced Ag/CZ IMP
and Ag/CZ CHE catalysts SEM-mapping. It is obvious
from the images that the silver crystallites distribution was
higher for chelating method than the impregnation method.
Further, the silver crystallites size in the reduced samples
was slightly greater than the silver crystallites present in
the calcined samples.
3
.5. FTIR-DRIFTs Studies
The DRIFT bands observed upon methanol, CO and CO
2
ꢀ
adsorption on CZ at 250 C are presented in Figure 6(a).
The methanol adsorption on CZ produced major bands
−
1
−1
in the region of 1000–1100 cm and 2800–3000 cm .
−1
The bands appeared at 1000, 1030 and 1060 cm corre-
sponds to stretching vibrations of O–C in –OCH groups
3
Figure 3. XPS analysis of Ag 3d orbital.
resulted due to on-top and bridging type co-ordinations on
3200
J. Nanosci. Nanotechnol. 19, 3197–3204, 2019

Pasupulety et al. Methanol Synthesis Using CO and H on Nano Silver-Ceria Zirconia Catalysts: Influence of Preparation Method
2
2
IP: 109.94.175.86 On: Sat, 02 Mar 2019 23:07:59
Copyright: American Scientific Publishers
Delivered by Ingenta
Figure 4. SEM mapping-EDS analysis of calcined catalysts (a) and (b) = CZ; (c–e) = 5Ag/CZ IMP CAL; (f–h) = 5Ag/CZ CHE CAL.
CZ.21 Whereas, the multiple bands appeared in the region
of 2800–2900 and 2900–3000 cm are attributed to C–H
symmetric and asymmetric stretching vibrations in –OCH3
groups. The CO adsorption on CZ resulted in two bands
at 2110 and 2180 cm , respectively, corresponds to CO
−1
−1
3+
complexes with Ce species and carbonyl complexes of
Zr species. On the other hand, pure CO adsorption
at 1 and 20 bar on CZ produced bands in two regions at
200–2400 and 1200–1700 cm . The bands in the later
4
+
22
2
−1
2
region will provide significant information with respect
to CO coordination on CZ. DRIFTs bands observed at
2
−1
1
345 and 1405 cm (B1 and B3) are assigned to stretch-
ing vibrations of bidentate carbonate species. The bands
of monodentate symmetric stretching carbonates appeared
−1
at 1375 and 1450 cm (B2 and B4) and monodentate
−
1
asymmetric stretching carbonates observed at 1520 cm
(
−1
B5). The bands appeared at 1560–1580 cm (B6, B7)
are attributed to symmetric and asymmetric stretching of
polydentate carbonate species respectively.2
3ꢀ24
In conclu-
sion, the order of observed surface carbonates with respect
ꢀ
to their relative intensities on CZ at 250 C is as follows:
polydentate carbonates > mono ꢁ bidentate carbonates.
The in situ FTIR-DRIFTs analysis on CZ and Ag/CZ
ꢀ
catalysts were performed at 250 C, 2 MPa with feed gas
composition of CO –H = 1–3. The results are presented
2
2
in Figures 6(b)–(d). The support CZ (Fig. 6(b)), shows
the surface species related to methane and CO from the
Figure 5. SEM mapping analysis of reduced catalysts (a) 5Ag/CZ IMP
RED and (b) 5Ag/CZ CHE RED.
J. Nanosci. Nanotechnol. 19, 3197–3204, 2019
3201

Methanol Synthesis Using CO and H on Nano Silver-Ceria Zirconia Catalysts: Influence of Preparation Method Pasupulety et al.
2
2
(
a)
(b)
(
c)
(d)
IP: 109.94.175.86 On: Sat, 02 Mar 2019 23:07:59
Copyright: American Scientific Publishers
5
Ag/CZ CHE, aDfteerlrievaecrtieondDbRyIFTIns genta
–
C-O , HCOO , 1575
as
–
(
e)
C-O , HCOO , 1390
s
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
60 min
30 min
–
–
–
–
–
–
0.2
0.4
0.6
0.8
1.0
1.2
1
5 min
1
5
0 min
min
0 min
3000 29002800
1800
1600
1400
1200
Wavenumber (cm–1
)
ꢀ
Figure 6. FTIR-DRIFTs studies (a) adsorption of CO, CO2 and methanol on CZ at 250 C and 1 bar or 20 bar; (b) In situ DRIFTs data on CZ at
ꢀ
ꢀ
ꢀ
2
50 C, 20 bar; (c) In situ DRIFTs data on Ag/CZ IMP at 250 C, 20 bar; (d) In situ DRIFTs data on 5Ag/CZ CHE at 250 C, 20 bar; (e) DRIFTs
ꢀ
data of 5Ag/CZ CHE after 4 h of reaction, followed by flushing with argon at 250 C and 1 bar.
H
H
H
H
H
H
C
O
C
C
C
O
O
bi-dentate
+
H
O
O
+H
OH
O
+2H
O
mono
CO2 + H
H COH
3
–
Formalin
Formaldehyde
Scheme 1. Reaction pathway through formates intermediate in CO hydrogenation.
2
3202
J. Nanosci. Nanotechnol. 19, 3197–3204, 2019

Pasupulety et al. Methanol Synthesis Using CO and H on Nano Silver-Ceria Zirconia Catalysts: Influence of Preparation Method
2
2
beginning of the reaction. The sharp bands appeared at
−1
3
010 and 1305 cm are respectively, assigned to C–H
stretching and bending of –CH functional group. Further,
3
C–H symmetric and asymmetric stretching vibrations of
−
CH functional group observed at 2910 and 3100 cm .
2
1
–
−1
The doublet at 2110 and 2180 cm suggests the formation
of CO through the RWGS reaction of CO and H . The
2
2
hydrocarbons and CO formation was increased with the
increase in reaction time indicates improved hydrogenation
on CZ. Further, new bands observed at 1050, 2830 and
−
960 cm corresponds to the vibrations of –OCH func-
3
1
2
tional groups. It is noteworthy that, all the CO adsorptions
2
(
bands B1–B7 for mono, bi and polydentate carbonates)
are resulted in the presence of hydrogen on CZ. However,
the order of observed surface carbonates with respect to
their relative intensities on CZ in the presence of hydrogen
at 250 C after 3 h of reaction is as follows: polydentate
carbonates > bidentate > mono carbonates.
ꢀ
ꢀ
−1
Figure 7. Catalytic activity studies at 250 C, 20 bar and 1800 h
GHSV.
On the other hand, Ag/CZ IMP (Fig. 6(c)) and Ag/CZ
CHE (Fig. 6(d)) showed identical bands for hydrocar-
3
.6. Catalytic Activity Studies
CO hydrogenation activity results on CZ and Ag/CZ cat-
alysts are presented in Figure 7. Activity tests were con-
bons, CO and methoxy (–OCH ꢃ functional groups. How-
2
3
ever, differences observed in the CO adsorptions in the
2
ꢀ
−1
−1
ducted at 250 C, 20 bar pressure and 1800 h GHSV.
The support, CZ showed only 55% of methanol selec-
tivity with 5% CO
methanol selectivity with 7 and 8% CO
obtained on Ag/CZ IMP and Ag/CZ CHE catalysts respec-
region of 1200–1700 cm on Ag/CZ catalysts compared
to CZ. For example, almost equal relative intensity of
mono/bidentate to polydentate carbonates was observed on
Ag/CZ IMP after 3 h of reaction. Whereas, bidentate car-
bonates were predominant over mono and polydentate car-
conversion. Whereas, 70% and 60%
2
conversion was
2
bonates on Ag/CZ CHE after 3 hI Po f: 1r e0 a9c .t 9i o4 n. 1. 7T 5h .e 8s 6e CO On : Sat,t i0v 2e l yM. a Tr h2 e0 1o 9t h 2e 3r : 0p 7r o: 5d u9 cts observed on these catalysts
2
adsorption differences influenced the RC Wo pG y Sr i gr he at :c tAi omn e ar ni cd an S wc i ee r ne ti mf i ce tPh au nb el i sa hn ed r sC O. About 5% of CO selectivity was
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thereby CO formation on these catalysts. The observed CO
observed on CZ and Ag/CZ IMP catalysts. The CO for-
mation was high (10%) on Ag/CZ CHE under the studied
reaction conditions. The results indicate that RWGS reac-
tion is favorable on Ag/CZ CHE over Ag/CZ IMP catalyst.
The activity results were supported by TPR data wherein,
greater metallic silver content was observed through the
−1
doublet bands at 2110 and 2180 cm for Ag/CZ CHE cat-
alyst in Figure 6(d), was more intense compared to Ag/CZ
IMP catalyst after 3 h of reaction under similar DRIFTs
conditions.
DRIFTs data on Ag/CZ CHE after reaction (4 h of
in situ study) followed by flushing with argon gas at
reduction of Ag O in Ag/CZ CHE over Ag/CZ IMP cat-
2
ꢀ
2
50 C and 1 bar pressure are presented in Figure 6(e).
alyst. The distributed metallic silver on the surface of
Ag/CZ CHE leading to the formation of bidentate carbon-
The time resolved spectral data yield information about
−1
the intermediate formed during the CO hydrogenation.
2
ates (1345 cm ꢃ selectively during in situ CO hydro-
2
The spectrum shows bands in the region of 1200–1600 and
genation studies. On the other hand, silver distribution in
Ag/CZ IMP favors the formation of equal proportions of
mono, bi and polydentate carbonates leading to greater
methanol selectivity. About 10% CO2 conversion with
−1
2
800–3000 cm due to the presence of adsorbed carbon-
ates, hydrocarbons and alkoxy species. All the three (mono
− −1
1
(
1375, 1450 and 1520 cm ꢃ, bi (1345 and 1405 cm ꢃ and
−1
poly dentate (1560–1580 cm ꢃ carbonates were detected
7
5% methanol selectivity was reported on bulk ZrZnAg10
along with hydrocarbons (C–H stretching at 2850 and
ꢀ
(
with 10% silver) catalyst at 250 C, 80 bar pressure and
−
1
2
915 cm of –CH ꢃ. Furthermore, methoxy species vibra-
−1
18
2
3600 h GHSV. Hence, the results can be improved on
Ag/CZ catalysts by changing the reaction parameters.
−1
tions were observed at 1050, 2830 and 2960 cm . After
careful review of after reaction DRIFTs data, new bands
identified at 1390, 1575 and 2880 cm corresponds to
formates (HCOO ꢃ surface intermediate compound. The
bands at 1390 and 1570 cm assigned to symmetric and
−1
−
4. CONCLUSIONS
Mono, bi and polydentate carbonates are resulted on CZ
−1
−
asymmetric stretching of C–O bond in HCOO group. The
and Ag/CZ catalysts by CO adsorptions. TPR and XRD
2
−1
−
band at 2880 cm attributed to C–H stretching in HCOO
studies established the presence of Ag O and metallic sil-
2
group.25 The results clearly demonstrates that silver sup-
ported catalysts are forming methanol through the formates
pathway (Scheme 1).
ver on CZ in Ag/CZ samples. The chelating agent (citric
acid) deposited the silver on the surface of CZ in Ag/CZ
CHE, whereas, silver deposition was all over the CZ in
J. Nanosci. Nanotechnol. 19, 3197–3204, 2019
3203

Methanol Synthesis Using CO and H on Nano Silver-Ceria Zirconia Catalysts: Influence of Preparation Method Pasupulety et al.
2
2
impregnation method. RWGS reaction was prominent on
chelating catalyst over impregnated catalyst. It is associ-
9. M. Saito, T. Fujitani, I. Takahara, T. Watanabe, M. Takeuchi,
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3
6, 577 (1995).
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38, 311 (1996).
1. K. Klier, Adv. Catal. 31, 243 (1982).
ated with ease reduction of Ag O to metallic silver in
2
1
chelating catalyst. Among the studied catalysts, impreg-
nated catalyst exhibited 70% of methanol selectivity due
1
1
to the presence of Ag O on CZ. FTIR-DRIFTs studies
revealed methanol formation through formates pathway.
12. Y. Okamoto, K. Fukino, T. Imanaka, and S. Teraniski, J. Phys. Chem.
7, 3747 (1983).
3. Y. W. Suh, S.-H. Moon, and H.-K. Rhee, Catal. Today 63, 447
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Catal. B 29, 207 (2001).
2
8
1
1
(
Acknowledgments: King Abudulaziz University-
Deanship of Scientific Research, Jeddah, KSA is greatly
acknowledged for their financial support to this work.
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Catal. B 34, 255 (2001).
1
6. J. B. Hansen, Handbook of Heterogenous Catalysis, VCH-Wiley,
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IP: 109.94.175.86 On: Sat, 02 Mar 2019 23:07:59
Copyright: American Scient Ri fi ec c Pe i uv eb dl i :s h1 5e r As pril 2018. Accepted: 1 October 2018.
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3204
J. Nanosci. Nanotechnol. 19, 3197–3204, 2019