Evaluation of Rh/CexTi1-xO2 catalysts for synthesis of oxygenates from syngas using XPS and TPR techniques

Article abstract of DOI:10.1016/j.cattod.2015.07.050

Rh/Ce_xTi_1-xO₂ catalysts have been investigated for the synthesis of ethanol from syngas. For this purpose various Ce_xTi_1-xO₂ (x = 0, 0.25, 0.5, 0.75, and 1) solid solution supports were synthesized using co-precipitation, which is an inexpensive and environmental friendly method. 2 wt% Rh has been deposited over these supports by using wet impregnation method. We have evaluated these Rh catalysts for synthesis of ethanol from syngas at atmospheric pressure and 350 psi. Among, the various catalysts, Rh supported on Ce_0.75Ti_0.25O₂ catalyst exhibit better selectivity toward oxygenates compared to other catalysts. Interestingly, Ti rich samples exhibit higher selectivity toward hydrocarbons while Ce rich samples exhibit higher selectivity toward oxygenated products. The catalytic results at atmospheric pressure show that titania rich catalysts exhibit higher H₂ conversion and ceria rich catalysts exhibit higher CO-conversion at 250 °C at atmospheric pressure. Temperature programmed reduction measurements suggest that Rh promotes the cerium surface reduction. X-ray photoelectron measurements show that all the catalysts exhibit peaks due to Ce⁴⁺ and Ce³⁺ oxidations states in the Ce 3d spectra. O1s spectra of activated Rh/TiO₂ and Rh/Ce_0.5Ti_0.5O₂ catalysts show two peaks after reduction in hydrogen. One peak is due to the oxygen atoms from the individual oxides and the other peak is from the compound formation between Rh and the support. X-ray photoelectron measurements of activated catalysts also show that Rh/Ce_0.5Ti_0.5O₂ catalyst exhibit higher Rh¹⁺ ions on the surface compared to Rh/TiO₂ and exhibits better selectivity toward oxygenated products.

Full text of DOI:10.1016/j.cattod.2015.07.050

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Evaluation of Rh/CexTi1−xO2 catalysts for synthesis of oxygenates from
syngas using XPS and TPR techniques
Ephraim Sheerin, Gunugunuri K. Reddy, Panagiotis Smirniotis∗
Chemical Engineering Program, Biomedical, Chemical & Environmental Engineering Department, University of Cincinnati, Cincinnati, OH 45221-0012,
United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Rh/CexTi1−xO2 catalysts have been investigated for the synthesis of ethanol from syngas. For this purpose
various CexTi1−xO2 (x = 0, 0.25, 0.5, 0.75, and 1) solid solution supports were synthesized using co-
precipitation, which is an inexpensive and environmental friendly method. 2 wt% Rh has been deposited
over these supports by using wet impregnation method. We have evaluated these Rh catalysts for synthe-
sis of ethanol from syngas at atmospheric pressure and 350 psi. Among, the various catalysts, Rh supported
on Ce0.75Ti0.25O2 catalyst exhibit better selectivity toward oxygenates compared to other catalysts. Inter-
estingly, Ti rich samples exhibit higher selectivity toward hydrocarbons while Ce rich samples exhibit
higher selectivity toward oxygenated products. The catalytic results at atmospheric pressure show that
titania rich catalysts exhibit higher H2 conversion and ceria rich catalysts exhibit higher CO-conversion
at 250 ◦C at atmospheric pressure. Temperature programmed reduction measurements suggest that Rh
promotes the cerium surface reduction. X-ray photoelectron measurements show that all the catalysts
exhibit peaks due to Ce4+ and Ce3+ oxidations states in the Ce 3d spectra. O1s spectra of activated Rh/TiO2
and Rh/Ce0.5Ti0.5O2 catalysts show two peaks after reduction in hydrogen. One peak is due to the oxygen
atoms from the individual oxides and the other peak is from the compound formation between Rh and
the support. X-ray photoelectron measurements of activated catalysts also show that Rh/Ce0.5Ti0.5O2 cat-
alyst exhibit higher Rh1+ ions on the surface compared to Rh/TiO2 and exhibits better selectivity toward
oxygenated products.
Received 13 April 2015
Received in revised form 16 July 2015
Accepted 30 July 2015
Available online xxx
Keywords:
Syngas
Ethanol
Methanol
Rhodium
Ce–Ti
Solid solutions
XPS
TPR
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
gallons of ethanol were produced in the US in 2002, mostly for
use as a fuel additive [6]. Although this is small fraction of the US
Oxygenated carbon products like ethanol and methanol are very
promising fuel alternatives and have attracted significant attention
in recent years. Ethanol and Methanol are also one of the important
starting feedstock for other chemicals production [1]. Methanol
is produced in quantities over 50 million tons per year [2] and
is present in many industrial sectors. It is used as a raw mate-
rial for the synthesis of formaldehyde, one of the most important
organic molecules (around 5 × 107 tons of formaldehyde produced
per year) [3], for the synthesis of olefins [4,5], such as propylene
and ethylene (bio polymer precursors). Ethanol is widely blended
into the unleaded gasoline pool with varied percentages to signif-
icantly decreasing the dependence of our society on crude oil, at
the same time reducing the harmful tailpipe emissions of CO, par-
ticulate matter, NOx, and other pollutants. In fact, over two billion
consumption of 134 billion gallons per year of gasoline, studies
show that there is a potential to increase ethanol production to
34–75 billion gallons per year [7]. In addition to its potential appli-
cation as a transportation fuel, bioethanol has been considered as a
feedstock for the synthesis of variety of chemicals, fuels, and poly-
mers. Methanol is also known as a fuel [4,8–10] either for fuel cells
[11] or mixed with gasoline, or indirectly as a raw material for the
synthesis of diesel, gasoline, dimethylether, hydrocarbons. Thus,
the synthesis of oxygenated products like methanol, and ethanol
allows getting stable and easily stored energy, as alternatives to
fossil fuels. As fuels, ethanol and methanol have several ideal prop-
erties: they are nontoxic, easy to store and also transportation fuel
results in lower net petroleum use and lower green house gas
emissions than gasoline per mile driven. Consequently, there is a
growing worldwide interest in the production of oxygenates from
biomass and possibly from other readily available carbonaceous
sources such as coal without CO2 emission, and its use as a fuel for
transportation, chemical feed stocks, and as an hydrogen carrier in
the future.
∗
Corresponding author.
E-mail addresses: smirnipp@ucmail.uc.edu, Panagiotis.Smirniotis@uc.edu
(P. Smirniotis).
http://dx.doi.org/10.1016/j.cattod.2015.07.050
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: E. Sheerin, et al., Evaluation of Rh/CexTi1−xO2 catalysts for synthesis of oxygenates from syngas using
XPS and TPR techniques, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.050

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The catalysts for the synthesis of oxygenates from syngas can
be classified into four categories: (a) Rh based catalysts [12], (b) Cu
based catalysts [13], (c) modified Fisher–Tropsch catalysts [14,15],
(d) modified Mo-based catalysts [16]. Among the various cata-
lysts, Rh-based catalysts received much attention because of its
high activity and its ability to adsorb reactive CO both associatively
and dissociatively, allowing it to form both hydrocarbons and oxy-
genates [12]. Because Rh is an expensive metal, the improvement of
activity and ethanol selectivity over Rh-based catalysts is necessary
for achieving a commercial available process.
The mechanism of the syngas to various products over Rh
based catalysts was proposed by the Arakawa et al. [16]. The main
products of the reaction are hydrocarbons, oxygenates, and CO2.
According to their mechanism, first CO and H2 adsorbs on the Rh0
site [16]. Then, hydrogen dissociates into adsorbed hydrogen (Had).
Some of the CO dissociates into carbon and atomic oxygen. Then,
Had combines with C and started to form CHx fragments. Combina-
tion of two or three CHx fragments or addition of more Had to the
CHx fragment leads to the formation of hydrocarbons like methane,
ethane, propane, etc. This reaction primarily happens on metallic
Rh sites. On the other hand insertion of CO into the CHx fragments
leads to the formation of CH2CO, CH3CO fragments which finally are
converted or rearranged into oxygenates. The insertion of CO into
CHx fragment occurs on Rh1+ site. Hence, the mechanism suggest
that metallic Rh is mainly responsible for the CO and H2 conver-
sion and Rh1+ is responsible for the oxygenated product formation.
Hence, it is important to have balance between Rh0 and Rh1+ ions
to have better oxygenated products yield.
was complete (pH ≈ 8.5). The obtained precipitate gels were fur-
ther aged overnight, and filtered off. The obtained cakes were oven
dried at 80 ◦C for 12 h, and finally calcined at 500 ◦C for 5 h. The rate
of heating as well as cooling was kept at 5 ◦C min−1
.
2.1.2. Preparation of Rh/CexTi1−xO2 catalysts
The Rh/CexTi1−xO2 catalysts are synthesized by using wet
impregnation method. In a typical synthesis, 50 mL of deionized
water was added to a 100 mL beaker containing 1.0 g of support.
The mixture was heated to 80 ◦C, under continuous stirring condi-
tions. A predetermined quantity of rhodium nitrate precursor was
then added, and the mixture was evaporated to dryness. The result-
ing material obtained was further dried overnight at 110 ◦C. Finally,
the catalyst was calcined at 450 ◦C for 5 h.
2.2. Catalyst characterization
2.2.1. BET surface area analysis
The BET surface areas were obtained by N2 adsorption on a
Micromeritics ASAP 2010 Instrument. Prior to the analysis, sam-
ples were oven dried at 120 ◦C for 12 h and flushed with Argon
for 2 h. The pore size distribution analyses were conducted by N2
physisorption at liquid N2 temperature using Micromeritics ASAP
2010 apparatus. All samples were degassed at 300 ◦C under vacuum
before analysis.
2.2.2. X-ray diffraction measurements
Powder X-ray diffraction (XRD) patterns were recorded on
The influence of various promoters on the catalytic activity of
Rh has been extensively studied in recent years. Metal catalysts
supported on reducible metal oxides exhibit remarkably enhanced
performance in many catalytic processes, including hydrogenation
[17], partial oxidation [18], or reforming reaction [19]. A strong
interaction between support and metal (SMSI) exists in the CeO2-
supported metal catalysts and gives the catalysts with high activity
for many catalytic reactions [20–22]. However, the main problem
associated with the ceria is poor thermal stability and low lattice
oxygen vacancies [23]. On the other hand, lattice oxygen of the
support is very important for the SMSI interactions. Introduction
of metal ions into the CeO2 lattices improves the oxygen vacan-
cies and catalytic performance of CeO2-supported metal catalysts
through changing the physical and chemical properties of the sup-
ports [24–26].
In the present study, we investigated the catalytic performance
of Rh/CexTi1−xO2 catalysts for the synthesis of oxygenates from syn-
gas. Our XPS measurements show that CexTi1−xO2 mixed oxides
exhibits higher Rh1+ ions on the surface and exhibit better selec-
tivity toward oxygenated products. Our catalytic measurements
at atmospheric pressure show that CeO2 and Ce0.75Ti0.25O2 cat-
alysts exhibits higher CO conversion and TiO2 and Ti0.75Ce0.25O2
catalysts exhibit higher CO conversion. Among the various cat-
alysts Rh/Ce0.75Ti0.25O2 catalyst exhibits best selectivity toward
oxygenated products.
a Phillips Xpert diffractometer using nickel-filtered Cu K␣
(0.154056 nm) radiation source. 0–80 ◦C were collected using 0.02
step size and a counting time of 1 s per point. Crystalline phases
were identified by comparing the observed reflections with the
reference ones from ICDD files [27].
2.2.3. TPR measurements
The temperature-programmed reduction (TPR) with hydro-
gen, of various catalyst samples were performed by means of an
automated catalyst characterization system (Micromeritics, model
AutoChem II 2920), which incorporates a thermal conductivity
detector (TCD). The experiments were carried out at a heating rate
of 5 ◦C min−1. The reactive gas composition was H2 (10 vol.%) in
nitrogen. The flow rate was fixed at 10 mL/min (STP). The TPR mea-
surements were carried out following activation after cooling the
sample in helium flow to 50 ◦C. The sample was then held at 50 ◦C
under flowing helium to remove the remaining adsorbed oxygen
until the TCD signal returned to the baseline. Subsequently, the
TPR experiments were performed up to a temperature of 800 ◦C.
The water formed during the reduction was removed by using an
ice trapper. The gas stream coming from the reactor was passed
through a trapper before the gas entered into the G.C. A mixture of
isopropanol and liquid nitrogen was used in the trapper to remove
the formed water during the TPR experiment.
2.2.4. Raman spectroscopy measurements
The Raman spectra of all the samples investigated in the present
study were obtained on HORIBA Jobin Yvon HR 800 equipped with a
liquid-nitrogen cooled CCD detector. The emission line at 514.5 nm
from Ar+ ion laser was focused on the sample under microscope and
the analyzed spot being ∼1 ␮m. The power of the incident beam on
the sample was 3 mW. Time of acquisition was adjusted according
to the intensity of the Raman scattering. The wavenumber values
2. Experimental
2.1. Catalyst synthesis
2.1.1. Preparation of CexTi1−xO2 supports
Various CexTi1−xO2 (x = 0–1) supports were prepared by using
co-precipitation method. In
a
typical preparation, calculated
reported from the spectra are accurate to within 2 cm−1
.
amounts of ammonium cerium nitrate and titanium chloride are
dissolved separately in deionized water and the aqueous solutions
mixed. Dilute aqueous ammonia was added gradually drop wise
to the mixture solutions, and vigorously stirred; until precipitation
2.2.5. XPS measurements
The X-ray photoelectron spectroscopy (XPS) measurements
were performed on a Pyris VG Thermo Scientific spectrometer using
Please cite this article in press as: E. Sheerin, et al., Evaluation of Rh/CexTi1−xO2 catalysts for synthesis of oxygenates from syngas using
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Al K␣ (1486.6 eV) radiation as the excitation radiation. Charging of
the catalyst samples was corrected by setting the binding energy
of the adventitious carbon (C 1s) at 284.6 eV. The XPS analysis was
performed at ambient temperature and at pressures typically on
the order of <10−8 Torr [28]. Prior to analysis, the samples were
out gassed under vacuum for 4 h. The Rh 3d spectra of activated
catalysts were deconvulated to deduce the Rh1+/Rh0 on the surface.
are presented in Figs. 1 and 2, respectively. Interestingly, hydro-
gen conversion increases with increasing the amount of titania
amount into the cerium lattice. The order of activity is Rh/TiO2 >
Rh/ i0.75Ce0.25O2 > Rh/Ti0.5Ce0.5O2 > Rh/Ti0.25Ce0.75O2 > Rh/CeO2.
On the other hand, the CO conversion decreases with increas-
ing the amount of titania into the cerium lattice at 250 ◦C. The
order of activity is Rh/Ti0.25Ce0.75O2 > Rh/Ti0.75Ce0.25O2 > Rh/Ti0.5
Ce0.5O2 ≫ Rh/CeO2 > Rh/TiO2. CeO2 favors CO dissociation and
the TiO2 favors hydrogen dissociation. These results clearly show
that hydrogen decomposes at the Rh–TiOx interface and CO
decomposes at Rh–CeOx interface. Interestingly, at atmospheric
pressure only products with single carbon like CH4, CO2, CH3OH
were observed. There is no formation of products with higher
number of carbons like ethanol, acetaldehyde, or ethane, etc. Since
hydrogen conversion is high for titania rich samples, these samples
also exhibited better yields toward CH4, CO2, CH3OH, and H2O.
Completely different behavior observed at reaction tempera-
ture 300 ◦C. Only Rh/TiO2 and Rh/Ti0.25Ce0.75O2 catalysts exhibited
higher catalytic activity. Rh/CeO2 and Rh/Ce0.5Ti0.5O2 catalysts
exhibited least activity. Even though the Rh/Ce0.25Ti0.75O2 catalyst
initially exhibited higher activity, however it deactivates with time.
Both Rh/TiO2 and Rh/Ti0.25Ce0.75O2 exhibited similar hydrogen
conversions of about 80–85%. The order of hydrogen conversion is
Rh/TiO2 > Rh/Ti0.25Ce0.75O2 > Rh/Ti0.75Ce0.25O2 > Rh/Ti0.5Ce0.5O2 >
Rh/CeO2. Interestingly, Rh/Ti0.25Ce0.75O2 catalyst exhibited
much higher CO conversion compared to Rh/TiO2 cata-
lyst. These results confirm that presence of ceria favors
the CO dissociation. Rh/Ti0.25Ce0.75O2 catalyst exhibited CO
conversion of 40% and Rh/TiO2 exhibited CO conversion
below 10%. The, Rh/TiO2 catalyst exhibited lesser CO con-
version than Rh/Ti0.25Ce0.75O2 and Rh/Ti0.5Ce0.5O2 catalysts.
The order of activity of CO conversion is Rh/Ti0.75Ce0.25O2 >
2.3. Catalytic activity
2.3.1. High pressure experiments
The syngas to ethanol reaction was carried out in a vertical down
flow fixed bed differential stainless steel reactor at 350 psi. In a
typical experiment, ca. required amount of powdered catalyst was
placed between two plugs of quartz wool. The reactor is filled with
glass beads below and above the catalyst to avoid the free space.
The reactor was placed vertically inside a programmable tubular
furnace (Lindberg), which was heated electrically. The pressure of
the reactor was maintained by the back pressure regulators. Before
the reaction the catalyst was reduced at 300 ◦C in the presence
of hydrogen for 2 h. After the reduction, the temperature of the
reactor was bought to the desired temperature and the H2/CO mix-
ture (H2/CO = 2) was introduced into the reactor. The products of
the reactions were analyzed using HP gas chromatograph. The CO
conversion was calculated by
COf − COi
XCO(%) =
× 100
COi
The selectivity of products were calculated by
niCi
ꢀ
SEtOH(%) =
× 100
niCi
Rh/Ti0.25Ce0.75O2 > Rh/Ti0.5Ce0.5O2 > Rh/TiO2 > Rh/CeO2.
Both
The yield of oxygenates was calculated by
Rh/TiO2 and Rh/Ti0.25Ce0.75O2 catalysts exhibited similar yield
toward CO2. However, Rh/Ti0.25Ce0.75O2 catalysts exhibited much
higher yield toward methanol compared to the Rh/TiO2 catalyst.
Moreover, Rh/TiO2 catalyst exhibited much higher CH4 yield
compared to the Rh/Ti0.25Ce0.75O2 catalyst. These results show
that CeO2 favors oxygenated product formation and TiO2 favors
hydrocarbon product formation at atmospheric pressure.
Yoxy = Xco(%) × Soxy(%)
10, 000
2.3.2. Experiments at atmospheric pressure
The syngas to ethanol reaction was carried out in a vertical down
flow fixed bed quartz reactor with 6 mm i.d. at atmospheric pres-
sure. In a typical experiment, ca. required amount of powdered
catalyst was placed between two plugs of quartz wool. The reac-
tor was placed vertically inside a programmable tubular furnace,
which was heated electrically. Before the reaction the catalyst was
reduced at 300 ◦C in the presence of hydrogen for 2 h. After the
reduction, the temperature of the rector bought to desired temper-
ature and the H2/CO mixture (H2/CO = 2) was introduced into the
reactor.
3.1.2. Experiments at 350 psi pressure
The five catalysts consist of 2% Rh (wt%) loaded on pure
ceria, pure titania, and mixed metal oxides, Rh/CeO2, Rh/TiO2,
Rh/CexTi1−xO2. Fig. 3 shows the CO conversion profiles of various
Rh/CexTi1−xO2 catalysts as a function of reaction temperature. The
catalysts Rh/TiO2, Rh/CeO2 and Rh/Ce0.5Ti0.5O2 exhibit 100% CO
conversion at 325 ◦C. Between Rh/CeO2 and Rh/TiO2 catalysts, the
titania supported catalyst exhibits higher CO conversion.
We divided all the products into three categories, namely
hydrocarbon, oxygenated and CO2 products. The selectivities of
hydrocarbon, oxygenated and CO2 products for various Rh/TiO2,
Rh/CeO2 and Rh/CexTi1−xO2 catalysts are presented in Table 1. Since
all the catalysts exhibit very low conversions at 250 and 275 ◦C, the
selectivites are presented for only for 300 ◦C and 325 ◦C reaction
temperatures. All tested catalysts formed carbon dioxide during
the reaction with atomic carbon selectivities between 1 and 16%
from 225 ◦C to 325 ◦C. As shown in Table 1, Ce rich samples shows
lower selectivity toward CO2 and Ti rich samples shows higher
selectivity toward CO2. Also, Ce rich compounds exhibit higher
selectivities toward oxygenated products and lower selectivity
toward hydrocarbon products. Ti rich catalysts exhibit lower
selectivities toward oxygenated products and higher selectivity
toward hydrocarbon products. Among the various catalysts, the
Rh/Ce0.75Ti0.25O2 exhibits better selectivity and yield toward oxy-
genated products.
3. Results and discussion
3.1. Catalytic activity
3.1.1. Experiments at atmospheric pressure
Initially, we have evaluated Rh/CexTi1−xO2 catalysts for syngas
to oxygenates synthesis at atmospheric pressure to investigates
the effect of the support on the catalytic properties of Rh. It is
known that the structural and catalytic properties of the CeO2,
TiO2 and Ce–Ti mixed oxides are completely different. It is very
important to investigate the adsorption behavior of CO and H2 over
these mixed oxides. At atmospheric pressure the reaction between
CO and H2 is very simple and methane and CO2 are the main prod-
ucts and hence, we can understand the catalysts behavior easily.
We have evaluated all the catalysts at two temperatures namely,
250 ◦C and 300 ◦C. The hydrogen conversion, and CO conversion
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60
25
CeO2
50
40
30
20
Ce0.75Ti0.25O2
Ce0.5Ti O2
20
15
10
5
Ce T0i.05.75O2
0.25
TiO2
CeO
Ce0.725Ti0.25O2
Ce0.5Ti0.5O2
Ce0.25Ti0.75O2
TiO2
10
0
0
2
4
6
8
0
2
4
6
8
Time (hours)
Time (hours)
Fig. 1. H2 and CO conversion over various Rh/CexTi1−xO2 (x = 0–1) catalysts (temperature: 250 ◦C, pressure: 1 atm, H2/CO = 2, space velocity 12,000 mL gcat−1 h−1).
100
45
90
40
80
CeO2
35
30
25
20
15
10
70
60
50
40
30
20
10
Ce0.75Ti0.25O2
Ce0.5Ti0.5O2
Ce0.25Ti0.75O2
TiO2
CeO2
Ce0.75Ti0.25O2
Ce0.5Ti O2
Ce T0i.05.75O2
0.25
TiO2
0
1
2
3
4
5
6
7
8
0
2
4
6
8
Time (hours)
Time (hours)
Fig. 2. H2 and CO conversion over various Rh/CexTi1−xO2 (x = 0–1) catalysts (temperature: 300 ◦C, pressure 1 atm, H2/CO = 2, space velocity 12,000 mL gcat−1 h−1).
Rh supported on Ce0.75Ti0.25O2 catalyst (0.60) exhibits better
oxygenated yields compared to the Rh/CeO2 (0.58) and Rh/TiO2
(0.51) catalysts at 300 ◦C. Rh catalysts loaded on pure oxide
supports result in less oxygenated yield than the mixed oxide
supported catalysts. Even though Rh/TiO2 exhibited higher CO
conversion compared to some of the mixed oxide supports, it
exhibits lowest selectivity toward ethanol compared to mixed
oxide supports. Interestingly, Ti rich supports exhibit higher
selectivity toward hydrocarbons and Ce rich supports exhibit
higher selectivity toward oxygenated products.
3.2. Catalyst characterization
3.2.1. BET specific surface area
BET surface areas are presented in Table 2. Pure titania has a
measured surface area of 94 m2/g, and pure ceria has a measured
Table 1
CO conversion, CO2 selectivity, oxygenated selectivity, hydrocarbon selectivities of various Rh/TiO2, Rh/CeO2 and Rh/CexTi1−xO2 catalysts (H2/CO = 2, pressure = 350 psi, 2 wt%
Rhodium, space velocity 6000 mL gcat−1 h−1, oxygenated selectivity: methanol + ethanol + propanol + acetaldehyde + formaldehyde + formic acid, hydro carbon selectivity:
methane + ethane + propane).
Catalyst
Rh/CeO2
CO2 selectivity
Ethanol selectivity
Oxygenated selectivity
Hydrocarbon selectivity
300 ◦
325 ◦C
300 ◦
325 ◦C
300 ◦
325 ◦C
300 ◦
325 ◦C
300 ◦
325 ◦C
C
52.9
59.1
33
48.9
56.1
62.4
65
67.3
77.7
80.8
10.3
6.8
15.1
4.4
10.8
4
44.4
35.2
54.3
35.3
34.9
19.7
13
10.1
15
10.7
52.9
59.1
33
48.9
56.1
62.4
65
67.3
77.7
80.8
C
Rh/Ce0.75Ti0.25O2
Rh/Ce0.5Ti0.5O2
Rh/Ce0.25Ti0.75O2
Rh/TiO2
C
C
1
0.3
3.2
2.1
C
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5
+
*
*
CeO2
Anatase TiO
2
TiO2
100
80
60
40
20
0
Ce0.25Ti0.75O2
Ce0.5Ti0.5O2
Ce0.75Ti0.25O2
CeO2
Ti
*
*
*
*
*
*
Ti/Ce=3
Ti/Ce=1
Ti/Ce=1/3
+
+
Ce
+
+
+
240 250 260 270 280 290 300 310 320 330
Temperature (oC)
20
30
40
50
Two theta (oC)
60
70
80
Fig. 3. CO conversion of Rh/CexTi1−xO2 (x = 0–1) catalysts at various temperatures
(H2/CO = 2, pressure = 350 psi, 2 wt% rhodium, space velocity 6000 mL gcat−1 h−1).
Fig. 4. X-ray diffraction profiles of various CexTi1−xO2 supports.
Table 2
+ CeO
*
Anatase TiO
BET surface areas of pure supports and Rh promoted catalysts.
2
2
*
SSAa
Rh loadingb
SSAa
*
*
*
CeO2
57
143
176
208
94
Rh/CeO2
67
122
134
173
67
Ti
*
*
TiO2–CeO2 (1:3)
TiO2–CeO2 (1:1)
TiO2–CeO2 (3:1)
TiO2
Rh/TiO2–CeO2 (1:3)
Rh/TiO2–CeO2 (1:1)
Rh/TiO2–CeO2 (3:1)
Rh/TiO2
*
Ti/Ce=3
a
Measured by N2 absorption method.
BET surface area after Rh loading.
b
Ti/Ce=1
Ti/Ce=1/3
Ce
surface area of 57 m2/g. The incorporation of titania into the ceria
lattice increases the support surface area by more than double.
Among the various supports, Ce0.25Ti0.75O2 catalyst exhibits the
highest surface area of 208 m2/g. The increase in the surface area
is due to incorporation of Ti into the ceria lattice and formation of
solid solution [25]. There is a slight surface area decrease for all
the supports after impregnating with Rh; this is possibly due to
pore-blocking of the support by Rh.
+
+
+
+
+
+
20
30
40
50
Two theta (oC)
60
70
80
Fig. 5. X-ray diffraction profiles of various Rh/CexTi1−xO2 supports.
3.2.2. X-ray diffraction (XRD)
The X-ray diffraction patterns of pure supports and Rh promoted
samples are presented in Figs. 4 and 5. Pure Ceria exhibits peaks at
28.4◦, 33.1◦, 47.3◦, 56.3◦, 69.5◦ 2ꢀ values. These peaks are due to the
cubic fluorite structure of ceria. Similarly the X-ray diffraction pat-
tern of the titania exhibits peaks due to the anatase phase which
is active for catalytic applications compared to the titania rutile
phase. Interestingly, there are no peaks corresponding to titania
rutile phase. All the Ce–Ti mixed oxides exhibit amorphous pat-
terns. This is mainly due to the formation of mixed metal oxides and
decrease in the crystallite size [27]. The X-ray diffraction patterns of
the Rh promoted ceria and titania catalysts also exhibit peaks due
to the cubic fluorite structure of ceria and titania anatase phase,
respectively. All the mixed oxide Rh catalysts exhibit amorphous
patterns due to the high specific surface area and the formation of
Ce–Ti solid solutions.
+
*
*
CeO2
Anatase TiO2
Ti
*
*
*
Ti/Ce=3
Ti/Ce=1
Ti/Ce=1/3
Ce
+
3.2.3. Raman spectroscopy
The Raman spectra of pure supports are shown in Fig. 6. Raman
spectroscopy results are consistent with X-ray diffraction measure-
200
400
600
800
ments. The Raman spectra of CeO2 show a peak around 465 cm−1
,
Wave Number (cm-1)
which is due to the cubic fluorite structure of ceria [23]. Titania
exhibits peaks due to the titania-anatase phase, and again there
Fig. 6. Raman spectra of various Rh/CexTi1−xO2 supports.
Please cite this article in press as: E. Sheerin, et al., Evaluation of Rh/CexTi1−xO2 catalysts for synthesis of oxygenates from syngas using
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Table 3
6
Atomic percentages of various components in Rh/CexTi1−xO2 catalysts determined
EDX.
Ti
Weight %
Atomic %
Ti/Ce=3
Rh
Ti
Ce
O
Rh
Ti
Ce
O
CeO2
5.2
2.7
4.8
5.5
6
80
72.4
5.7
15
18
21.4
21.4
37.7
3.54
1.5
2.3
2.1
1.6
32.2
24.5
16.3
8.4
64.3
65.7
65.1
65.3
65.6
TiO2–CeO2 (1:3)
TiO2–CeO2 (1:1)
TiO2–CeO2 (3:1)
TiO2
6.8
16
30
56.4
8.3
16.3
24.2
32.8
Ti/Ce=1
37.4
1+
0
Rh to Rh
Ti/Ce=1/3
Ce
O1s
CeO Surface
CeO2 Bulk
2
Rh3+ to Rh1+
50
Reduction
Reduction
Ti
0
100
150
700
800
Temperature (ºC)
Ti/Ce=3
Fig. 7. Temperature programmed reduction profiles of various Rh/CexTi1−xO2 sup-
ports.
Ti/Ce=1
are no peaks corresponding to the titania-rutile phase. The Raman
spectra of mixed oxide show amorphous patterns, which is consis-
tent with XRD patterns. There are no peaks corresponding to Rh
oxide or compounds between Rh oxide and support, which suggest
that Rh is in highly dispersed state over the ceria-titania supports
(not shown).
Ti/Ce=1/3
Ce
525
528
531
534
3.2.4. TPR
The TPR profiles of the various Rh promoted catalysts are pre-
sented in Fig. 7. The TPR profile of the Rh/CeO2 catalyst exhibits
a total of four reduction peaks. The peak at 25 ◦C corresponds to
the reduction of Rh3+ to Rh1+. The other two peaks at around
75 ◦C are due to the reduction of Rh1+ to Rh metal and ceria sur-
face reduction. The peak at higher temperature is due to the ceria
bulk reduction. As per literature reports, the surface ceria reduc-
tion occurs at 500 ◦C and bulk ceria reduction occurs at 800 ◦C in
the case of pristine ceria sample [29]. However, in the present
study, the ceria surface reduction occurs at 75 ◦C. This is due to
the strong metal support interaction (SMSI) between Rh and ceria,
which indicates high catalytic activity. Moreover, the deposition
of noble metals on ceria-based materials strongly enhances the
reducibility of Ce4+ due to hydrogen spillover on the surface. The
surface oxygen vacancies were introduced by hydrogen spillover
from the noble metal to the catalyst support, and then these vacan-
cies at the surface were subsequently filled up by the bulk oxygen
being transported to the surface due to ionic conductivity. The TPR
pattern of Rh/Ce0.75Ti0.25O2 sample exhibits different pattern com-
pared to Rh/CeO2 sample. In this case, Rh3+ to Rh1+ reduction and
the surface ceria reduction occurs simultaneously, and then reduc-
tion due to Rh1+ to Rh0. The reduction temperature of all the three
peaks shifted to lower temperature compared to Rh/CeO2. This is
due to the incorporation of titania, into the ceria lattice. The defect
throughout the crystal produced an increase in the oxygen mobil-
ity and diffusion in the lattices, which promotes the reduction of
both surface and bulk reductions in the CeO2–TiO2 supports. This
type of behavior was well reported in the literature. The reduc-
tion behavior of ceria depends on several parameters like synthesis
method, type of metal ion doped, crystallite size, etc. The TPR pat-
terns of Rh/Ce0.5Ti0.5O2 and Rh/Ce0.25Ti0.75O2 are similar. For both
catalysts exhibited peaks due to Rh3+ to Rh1+ reduction, Rh1+ to Rh0
reduction, and ceria surface reduction happened simultaneously.
Interestingly, in these supports the reduction temperatures
Binding energy (eV)
Fig. 8. O1s X-ray photoelectron spectra of various Rh/CexTi1−xO2 supports.
Table 4
XPS binding energies of various Rh/CexTi1−xO2 catalysts and Rh1+/Rh0 ratios of
Rh/TiO2 and Rh/Ce0.5Ti0.5O2 catalysts.
O 1s
Rh 3d
Ti 2p
Ce 3d
CeO2
530
310.3
309.5
310.2
310
916.7
918.4
918.6
918.5
TiO2–CeO2 (1:3)
TiO2–CeO2 (1:1)
TiO2–CeO2 (3:1)
TiO2
530.1
530.5
530.6
530.7
458.5
458.9
459.1
459.2
310.2
once again shifted to higher temperatures. EDX analysis was
performed on all the catalysts to find out the atomic composition
of the catalyst. The adjusted weight and atomic % of various com-
ponents in various catalysts are presented Table 3. All the catalysts
exhibit atomic percentages according to compositions within the
experimental error.
3.2.5. X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron measurements were performed over
Rh/CexTi1−xO2 catalysts to investigate the surface properties and
elemental oxidation states. Figs. 8–11 show the O1s, Rh 3d, Ti 2p,
and Ce 3d spectra of Rh/CexTi1−xO2 catalysts. The binding energy
values of O1s, Rh 3d, Ti 2p and Ce 3d of all the fresh catalysts are
presented in Table 4. The O1s peak is generally broad and com-
plicated, because of the nonequivalence of surface O ions from
different oxides. As per the literature, the O ions in pure CeO2
exhibit intense peaks at 529.6 eV. The O 1 s binding energy values
reported for TiO2 are 530.0 eV, respectively [28]. All the samples
exhibit only one peak around 530 eV. This peak is due to the ioniza-
tion of oxygen atom from support. As expected, the O1s peak shifted
Please cite this article in press as: E. Sheerin, et al., Evaluation of Rh/CexTi1−xO2 catalysts for synthesis of oxygenates from syngas using
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7
Rh3d
Ti
Ti2p
Ti/Ce=3
Ti/Ce=1
Ti
Ti/Ce=1/3
Ti/Ce=3
Ti/Ce=1
Ti/Ce=1/3
Ce
455
460
465
470
305
310
315
320
Binding energy (eV)
Binding energy (eV)
Fig. 10. Ti 2p X-ray photoelectron spectra of various Rh/CexTi1−xO2 supports.
Fig. 9. Rh 3d X-ray photoelectron spectra of various Rh/CexTi1−xO2 supports.
with a satellite at 315 eV. These peaks primarily belong to Rh3+ oxi-
dation state. These results confirm the formation of Rh2O3 during
the impregnation of Rh over ceria-titania mixed oxides. Fig. 10 also
shows Ti 2p XPS spectra of the various Ti samples investigated in the
present study. All the samples exhibit a peak at around 458 eV with
to higher binding energy side from Ce-rich samples to Ti-rich
samples.
Rh 3d XPS spectra of various Rh/CexTi1−xO2 catalysts are pre-
sented in Fig. 9. All the samples exhibit a peak around 310 eV along
Ti/Ce=1/3
Ce
lll
V
l
U
l
ll
V
V
lll
ll
U
U
V
U
920
910
900
890
880
870
920
910
900
890
880
870
Binding energy (eV)
Binding energy (eV)
Ti/Ce=3
Ti/Ce=1
920
910
900
890
880
870
920
910
900
890
880
870
Binding energy (eV)
Binding energy (eV)
Fig. 11. Ce 3d X-ray photoelectron spectra of various Rh/CexTi1−xO2 supports.
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a satellite at 465 eV. These results suggest that Ti is in 4+ oxidation
state.
O1s
The Ce 3d XPS spectra of Rh//CexTi1−xO2 catalysts are presented
in Fig. 11. Typically, the Ce 3d XPS core level spectra exhibit three
lobed envelopes (around 879–890, 895–910, and 916 eV) such as
those ionization, depicted in Fig. 11. From these envelopes, the
coexistence of both Ce3+ and Ce4+ oxidation states is distinguish-
able, although the 4+ oxidation state is predominant. The bands
labeled “v” collectively represent the Ce 3d5/2 ionization, while
bands labeled “u” represent the Ce 3d3/2 ionization [30]. The bands
with unprimed labels represent the primary Ce 3d5/2 and Ce 3d3/2
transitions, while the primed labels represent ionization satellite
features. Specifically, the band (u0, u) is the Ce 3d3/2 ionization
Ti/Ce=1
and the band (v0, v) is the Ce 3d5/2 ionization for Ce3+ and Ce4+
.
Ti
The bands labeled vꢀ, vꢀꢀ, vꢀꢀꢀ and uꢀ, uꢀꢀ, uꢀꢀꢀ are satellites arising
from the Ce 3d5/2 ionization and Ce 3d3/2, respectively. Accord-
ing to Burroughs et al. (1976), the peaks uꢀ and vꢀ arise from
Ce3+ oxidation state and remaining peaks arises from Ce4+ oxi-
dation state. Interestingly, all the samples exhibit peaks due to
both Ce3+ and Ce4+ oxidation states. Among the various samples
Rh/Ce0.25Ti0.75O2 catalysts sample exhibits higher intensity of Ce3+
peaks.
525
530
535
Binding Energy (eV)
Fig. 12. O1s X-ray photoelectron spectra of activated Rh/TiO2 and Rh/Ce0.5Ti0.5O2
((a) Rh/TiO2 (b) Rh/Ce0.5Ti0.5O2).
[32–34]. In the present study the peak at 532 eV is due to the
Oxygen atom from the Rh oxide and the peak at 528 eV is due
to the formations of partially reducible Rh1+–CeO2 − x compound.
Interestingly, the peak due to individual rhodium oxide is more
intense in the case of Rh/TiO2 sample and the peak due to partially
reducible Rh–CeO2 − x or Rh–TiO2 − x is more intense in the case of
Rh/Ce0.5Ti0.5O2. These results suggest that the formation of partially
reducible Rh–Ce0.5Ti0.5O2 − x is higher in the case of Rh/Ce0.5Ti0.5O2
compared to the Rh/TiO2 sample. This is due to the better reducibil-
ity of Ce–Ti mixed oxide compared to the titania alone. XPS results
show that the formation of partially reducible oxide is easy in case
of Rh/Ce0.5Ti0.5O2 compared to the Rh/TiO2 catalyst which favors
the formation of Rh1+ easily.
Rh 3d X-ray photoelectron spectra of activated Rh/TiO2 and
Rh/Ce0.5Ti0.5O2 catalysts are presented in Fig. 13. Interestingly, both
the catalysts exhibit good Rh1+/Rh0 ratios. This is due to the bet-
ter interaction between Rh and support and formation of partially
reducible oxides during the reduction. Between the Rh/TiO2 and
Rh/Ce0.5Ti0.5O2 samples, Rh/Ce0.5Ti0.5O2 exhibits higher Rh1+/Rh0
concentrations. This is due to the better reducibility of Ce0.5Ti0.5O2
support compared to the TiO2 alone, which stabilizes the formation
of partially reducible Rh1+–CeO2 − x oxide.
Our catalytic results show that the mixed oxides exhibit bet-
ter selectivity toward oxygenates compared to the titania alone.
We investigated by using XPS. Rh/TiO2 and Rh/Ce0.5Ti0.5O2 acti-
vated catalysts after reduction in hydrogen at 300 ◦C for 2 h The O1s,
Rh 3d X-ray photoelectron spectra of Rh/TiO2 and Rh/Ce0.5Ti0.5O2
activated catalysts after reduction in hydrogen at 300 ◦C for 2 h
are presented in Figs. 12 and 13, respectively. Interestingly, the
activated Rh/TiO2 and Rh/Ce0.5Ti0.5O2 exhibit two peaks in the
O1s XPS spectra. One peak is due to the oxygen atoms from the
individual oxides and the other peak is from the compound for-
mation between Rh and the support. During the reduction due
to the strong metal support interaction there will be formation
of partially reducible oxide compounds like Rh1+–CeO2 − x and
Rh1+–TiO2 − x. The formation of these compounds is very important
for the formations of Rh1+ species. Formation of these compounds
stabilizes the Rh1+ species during the reaction. These results are
in good agreement with the literature reports. Reddy et al. [31]
reported that V2O5/CeO2/Al2O3 exhibits an additional peak in the
O1s spectra when there is a formation CeVO4 compound along
with regular peak. Another study also Ca/Zr sorbents synthesized
by aerosol method exhibits two peaks in the O1s XPS spectra one
due to individual oxides and other is due to formation of CaZrO3
(b)
(a)
Rh0
Rh1+
Rh0
Rh1+
300
305
310
315
320
300
305
310
315
320
Binding energy (eV)
Binding energy (eV)
Fig. 13. Rh 3d X-ray photoelectron spectra of activated Rh/TiO2 and Rh/Ce0.5Ti0.5O2 ((a) Rh/TiO2 (b) Rh/Ce0.5Ti0.5O2).
Please cite this article in press as: E. Sheerin, et al., Evaluation of Rh/CexTi1−xO2 catalysts for synthesis of oxygenates from syngas using
XPS and TPR techniques, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.050

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Acknowledgements
9
4. Conclusions
High surface area and high active Rh/CexTi1−xO2 catalysts have
been synthesized by eco-friendly coprecipitation and impregna-
tion methods for ethanol synthesis from syngas. The catalytic
activity results show that ceria rich based catalysts exhibit bet-
ter selectivity toward oxygenates and titania rich based catalysts
exhibit better selectivity toward hydro carbons. Among the vari-
ous catalysts Rh/Ce0.75Ti0.25O2 catalyst gives the highest selectivity
for oxygenated compound(s). BET surface area of pure supports
increases when we go from TiO2 to Ce0.25Ti0.75O2 catalyst and fur-
ther increase in the Ce amount leads to a decrease in the surface
area. Peaks due to cubic fluorite structure of ceria and anatase tita-
nia was observed in X-ray diffraction patterns of the ceria and
titania alone. There are no peaks corresponding to either Rh or
compounds formation between Rh and support in the in the X-ray
diffraction pattern of the Rh based catalysts. Temperature pro-
grammed reduction measurements suggest that doping of titanium
into the ceria lattice decreases the reduction temperatures of ceria
and Rh reduction peaks up to 25% and further increase in the
Ti doping into the ceria lattice shift the reduction temperatures
toward higher temperature side. X-ray photoelectron measure-
ments show that O 1s peak shifted to higher binding energy side
from Ce-rich samples to Ti-rich samples. X-ray photoelectron mea-
surements also show that Rh/Ce0.5Ti0.5O2 exhibits more Rh1+ ions
compared to Rh/TiO2 sample. O1s spectra of activated catalysts
after pre-reduction also exhibit two peaks one is due to individual
The authors gratefully acknowledge the financial support from
the Ohio Coal Development Authority (Award Number: OCDO-11-
12-C-02).
References
[1] N. Kumar, M.L. Smith, J.J. Spivey, J. Catal. 289 (2012) 218–226.
[2] M. Gupta, J.J. Spivey, Catal. Today 147 (2009) 126–132.
[3] C. Maldonado, J.G. Fierro, G. Birke, P. Martinez, J. Chil. Chem. Soc. 55 (2010)
506.
[4] F.J. Keil, Microporous Mesoporous Mater. 29 (1999) 49.
[5] M. Aresta, A. Dibenedetto, A. Angelini, J. CO2 Util. 3 (2013) 65.
[6] M. Gupta, M.L. Smith, J.J. Spivey, ACS Catal. 1 (2011) 641–656.
[7] C. Riley, AIChE Spring Meeting, Louisiana, New Orleans, 2002.
[8] N.D. Subramanian, G. Balaji, C.S.S.R. Kumar, J.J. Spivey, Catal. Today 147 (2009)
100–106.
[9] J.E. Jackson, F.M. Bertsch, J. Am. Chem. Soc. 112 (1990) 9085.
[10] M. Bjørgen, F. Joensen, M. Spangsberg Holm, U. Olsbye, K.-P. Lillerud, S. Svelle,
Appl. Catal. A Gen. 345 (2008) 43.
[11] G.A. Olah, Angew. Chem. Int. Ed. Engl. 44 (2005) 2636.
[12] N.D. Subramanian, J. Gao, X. Mo, J.G. Goodwin Jr., W. Torres, J. Catal. 272
(2010) 204–209.
[13] J.J. Spivey, C. Zhang, C. Liu, W. Xing, Fuel 94 (2012) 401.
[14] M.A. Haider, M.R. Gogate, R.J. Davis, J. Catal. 261 (2009) 9.
[15] X. Sun, G.W. Robert, Appl. Catal. A: Gen. 247 (2003) 133.
[16] M. Pijolat, V. Perrichon, Appl. Catal. 13 (1985) 321.
[17] J.J. Spivey, A. Egbebi, Chem. Soc. Rev. 36 (2007) 1514.
[18] H. Arakawa, T. Fukushima, M. Ichikawa, S. Natsushita, K. Takeuchi, T.
Matsuzaki, Y. Sugi, Chem. Lett. (1985), 881.d.
[19] C.M. Li, S.T. Zhang, B.S. Zhang, D. Su, S. He, Y.F. Zhao, J. Liu, F. Wang, M. Wei,
D.G. Evans, X. Duan, J. Mater. Chem. 1 (2013) 2461.
[20] I.X. Green, W. Tang, M. Neurock Jr., J.T. Yates, J. Am. Chem. Soc. 134 (2012)
13569.
oxide and other one is due to Rh1+–CeO2 − x
.
[21] L.M. Martinez, T.M. Araque, J.C. Vargas, A.C. Roger, Appl. Catal. B: Environ. 132
(2013) 499.
Dedication to Eli Ruckenstein
[22] T. Hou, B. Yu, S. Zhang, T. Xu, D. Wang, W. Cai, Catal. Commun. 58 (2015) 137.
[23] A. Cervantes, G. Del Angel, G. Torres, G. Lafaye, J. Barbier Jr., J.N. Beltramini, J.G.
Caban˜as-Moreno, A. Espinosa de los Monteros, Catal. Today 212 (2013) 2–9.
[24] S.S. Kim, S.J. Lee, S.C. Hong, Chem. Eng. J. 169 (2011) 173–179.
[25] B.M. Reddy, A. Khan, Catal. Surv. Asia 9 (2005) 155.
[26] F. Giordano, A. Trovarelli, C. de Leitenburg, M. Giona, J. Catal. 193 (2000) 273.
[27] J. He, G.K. Reddy, P. Smirniotis, S.W. Thiel, N.G. Pinto, J. Phys. Chem. C 115
(2011) 24300–24309.
In August 1990 I met Professor Eli Ruckenstein and I decided to
work with him for my Ph.D. Very soon, I realized that I was working
with an exceptional scientist and engineer, somebody who had pas-
sion for new findings, a professor who dedicated all his life in trying
to understand and study a plethora of very interesting problems.
With him I learned how to do research, how to acquire new knowl-
edge and work in numerous catalysis problems and projects at the
same time. His life, dedication and appreciation for knowledge was
an ideal for me to exemplify throughout my research work and
shape my professional attitude for the career I was about to start.
Professor Ruckestein is a leaving legacy and his work and find-
ings will remain always an amazing contribution to benefit our
society. I dedicate this paper to him in appreciation of his endless
contributions in heterogeneous catalysis and studies of surfaces.
Panagiotis Smirniotis
[28] B.M. Reddy, K.N. Rao, G.K. Reddy, A. Khan, S.E. Park, J. Phys. Chem. C 111
(2007) 18751–18758.
[29] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and
748 Amorphous Materials, 2nd ed., Wiley, New York, 1974.
[30] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, in: G.E. Muilenberg (Ed.),
Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden
Prairie, MN, 1978.
[31] A. Khan, P. Chen, P. Boolchand, P.G. Simrnitios, J. Catal. 253 (2008) 91.
[32] A. Burroughs, A. Hamnett, A.F. Orchard, G. Thornton, J. Chem. Soc. Dalton
Trans. 1 (1976) 1686.
[33] B.M. Reddy, K.N. Rao, G.K. Reddy, A. Khan, J. Phys. Chem. C 111 (2007)
18751.
[34] R. Koirala, K.R. Gunugunuri, P.G. Smirniotis, J. Phys. Chem. C 115 (2011)
24084.
Chemical Engineering, University of Cincinnati.
Please cite this article in press as: E. Sheerin, et al., Evaluation of Rh/CexTi1−xO2 catalysts for synthesis of oxygenates from syngas using
XPS and TPR techniques, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.050