The real active sites over Zn-Cr catalysts for direct synthesis of isobutanol from syngas: Structure-activity relationship

Article abstract of DOI:10.1039/c5ra17289f

A series of Zn-Cr oxides nanoparticles were prepared by a coprecipitation procedure. The structure of different catalysts was investigated by X-ray Absorption Fine Structure (XAFS), X-ray photoelectron spectroscopy (XPS), temperature programed reduction of hydrogen (H₂-TPR) and in situ infrared spectrum (in situ IR). Both EXAFS and XANES demonstrated the cation disorder distribution became more serious with decreasing annealing temperature and increasing Zn/Cr molar ratios. The cation distribution also affected the oxygen state on the surface over Zn-Cr spinel. The population of surface hydroxyl species increased with more serious cation disorder distribution and they facilitated the formate formation which was a significant intermediate C1 species for alcohol synthesis. This study was the first time to investigate the situation of cation distribution in Zn-Cr spinel by XAFS and related it to catalyst performance. The results revealed that the isobutanol productivity presented a linear relationship to the level of cation disorder distribution in Zn-Cr spinel, unambiguously revealing the real active sites and structure-activity relationship.

Full text of DOI:10.1039/c5ra17289f

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The real active sites over Zn-Cr catalysts for direct synthesis
of isobutanol from syngas: Structure-Activity relationship
Shaopeng Tian, a,b Sichen Wang, c Yingquan Wu, a,b Junwen Gao, c Hongjuan Xie, a
Xiaoli Li, a,b Guohui Yang, a Yizhuo Han a and Yisheng Tan*a
(a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of
Sciences, Taiyuan, Shanxi 030001, China
bUniversity of the Chinese Academy of Sciences, Beijing 100039, China
c Shaanxi Yanchang Petroleum (Group) Corp. Ltd., Xi′an 710075, P. R. China)
*Tel.: +86 351 4044287. Eꢀmail address: tan@sxicc.ac.cn (Y. Tan)
Abstract
A series of ZnꢀCr oxides nanoparticles were prepared by a coprecipitation
procedure. The structure of different catalysts was investigated by Xꢀray Absorption
Fine Structure (XAFS), Xꢀray photoelectron spectroscopy (XPS), Temperature
programed reduction of hydrogen (H2ꢀTPR) and in situ infrared spectrum (in situ IR).
Both EXAFS and XANES demonstrated the cation disorder distribution became more
serious with decreasing annealing temperature and increasing Zn/Cr molar ratios. The
cation distribution also affected the oxygen state on the surface over ZnꢀCr spinel.
The population of surface hydroxyl species increased with more serious caiton
disorder distribution and they facilitated the formate formation which was a
significant intermediate C1 species for alcohol synthesis. This study was the first time
to investigate the situation of cation distribution in ZnꢀCr spinel by XAFS and related
it to catalyst performance. The results revealed that the isobutanol productivity

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presented a linear relationship to the level of cation disorder distribution in ZnꢀCr
spinel, unambiguously revealing the real active sites and structureꢀactivity
relationship.
Keywords: CO hydrogenation; isobutanol; ZnꢀCr catalyst; cation distribution.
1. Introduction
With the increasing price of petroleum, environmental concerns and gasoline
additive octane demands, the need for petroleumꢀfree routes to value added chemicals
will continue to intensify in the future.1ꢀ6 The synthesis of higher alcohols from
syngas (a mixture of CO and H2) has roused considerable interest due to their
potential application as good gasoline additives or alternative motor fuel for reducing
7, 8
the exhaust emission.
Among the higher alcohols, isobutanol is considered as one
of the most important species among coalꢀbased clean synthetic fuels and chemical
intermedium. It has been confirmed to be a better and cleaner automobile fuel with
high octane number and can be used to produce plasticizer, medicine and antiager,
4, 5, 9
etc.
Furthermore, isobutanol had been approved as gasoline additives by U.S.
Environmental Protection Agency (EPA) in 2010. The rapid increase of isobutanol
consumption requires a low cost manufacturing approach. A particularly promising
nonꢀnoble metal family of catalysts, ZnCr2O4ꢀbased catalysts modified with
potassium has been widely studied due to its longer time for retaining high activity,
less severe coke deposition, and ability to form higher alcohols with a high selectivity
to isobutanol. 10ꢀ13 The selectivity of methanol plus isobutanol is equal or greater than
94% in alcohol phase product for ZnꢀCr oxides catalyst. That will simplify the process

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of separation in isobutanol production and contributes greatly to the industrialization
of isobutanol. Additionally, the origin of this abnormal high selectivity for isobutanol
is also the motivation for understanding this catalytic process and real active sites for
isobutanol in detail.
Since 1980s, ZnꢀCr based oxides have been extensively studied and some
pioneering work and excellent reviews have been published on this subject.14, 15 Many
studies focused on the mechanism of the reaction and the true active sites for
3, 16
isobutanol formation.
However, there is still debate about the real active sites for
11, 12
isobutanol formation on ZnꢀCr oxides catalyst. William S. Epling
believed that
the excess ZnO on Zn/Cr spinel was the active site while Zn/Cr spinel acted only as a
17ꢀ19
20
highꢀsurfaceꢀarea support while Massimo Bertoldi
and Noritastsu
held the
point that the nonꢀstoichiometry phase led the catalytic activity to form alcohol. In
fact, our group also made much effort to study the pathway and the real active sites of
isobutanol formation from syngas. Wu 3, 4, 9 believed that the mechanism of isobutanol
formation is that C1 intermediate (formyl) addition at αꢀcarbon on methanol to form
C2+ alcohols and then C2+ alcohols underwent an aldolꢀcondensation to form
6, 20
isobutanol. Kou
held the point that the nonꢀstoichiometry phase was the real
active site for isobutanol formation on ZnꢀCr spinel catalyst.
In recent years, it has been shown that ZnCr2O4 spinel can adopt normal and
partially cation disorder distribution spinel structures. As a result, the physical and
chemical properties can be tailored through the variation of cations on the tetrahedral
and octahedral coordination sites.21ꢀ24 On another front, Xꢀray absorption structure

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(XAS) spectroscopy is a powerful tool to investigate the local structure of metal
oxides. Both EXAFS and XANES provide valuable information.25 Recently, they
have been used to investigate the cation distribution in spinels, such as CuFe2O4,
24, 26ꢀ30
ZnFe2O4, MnFe2O4, etc.
Surprisingly, merely articles deal with the structure
and the situation of cation distribution in ZnꢀCr spinel.
According to this, we present an Xꢀray absorption structure (XAS) study on the
Zn and Cr distribution in ZnCr2O4 nanoparticles and focus on the relationship
between the isobutanol formation and the cation distribution in ZnꢀCr spinel catalyst
in this manuscript. A series of ZnꢀCr oxides nanocatalysts with different degrees of
cation disorder distribution were obtained by a typical coprecipitation method.
Catalysts were then characterized by XAFS, XPS, in situ IR and H2ꢀTPR to clarify the
real active sites over ZnꢀCr catalyst for isobutanol formation from syngas and confirm
our conjecture: the structureꢀactive relationship.
2. Experimental methods
2.1 Preparation of catalysts
ZnꢀCr oxides samples were prepared by a typical coprecipitation procedure.
Appropriate amounts of starting materials Zn(NO3)2·6H2O mixed Cr(NO3)3·9H2O
and (NH4)2CO3·CH3NO2 were dissolved in 800mL of deionized water, respectively.
Then the two kinds of solutions were pumped together by peristaltic pumps in a
o
wellꢀstirred thermostated container with pH of 9 at 60 C. Next, the dark green
suspension was aged for 15h, then filtered, washed thoroughly by deionized water at
o
60 C, and dried at 100 oC overnight. Dried samples was milled into powders and

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o
annealed at 350 C for 6h to remove carbonates, etc. Then, the dark powders were
impregnated with 3wt% K2O as a promotor using potassium carbonate and the
incipient wetness method. Finally, the catalysts were dried at 100 oC for 12h and
o
calcined at from 400 to 700 C for 6h in order to form the corresponding ZnꢀCr
spinelꢀlike or spinel oxides. Different ratios of Zn/Cr catalysts would be got by
controlling the amounts of starting materials zinc nitrate and chromium(Ⅲ) nitrate.
Catalysts could be labeled as ZCꢀ0.5ꢀ700, ZCꢀ0.5ꢀ550, ZCꢀ0.5ꢀ400, ZCꢀ0.65ꢀ400 and
ZCꢀ0.8ꢀ400, respectively. For the ZCꢀ0.5ꢀ700, the “0.5” and “700” represented molar
ratio of Zn to Cr and calcination temperature respectively, the rest of catalysts are
similar.
2.2 Characterization of catalysts
2.21 XAFS Data acquisition
The Zn and Cr Kꢀedge XAFS spectra of all samples were measured in the
transmission mode at the beamline BL14W1 of the Shanghai Synchrotron Radiation
Facility (SSRF). The storage ring was operated at 3.5 GeV under “topꢀup” mode with
a constant current of 250 mA. A double crystal Si (111) monochromator was used for
the experiments. The powder samples were homogeneously smeared on the Scotch
adhesive tape, and several layers were folded to reach the optimum absorption
thickness ( Δμ d ≈ 1,Δμ is the absorption edge jump and d is the physical
thickness of the sample)
2.22 XAFS Data Analysis

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XAFS data were analyzed with the ATHENA and ARTEMIS code of Ravel. The
energy thresholds were determined as the maxima of the first derivative. Absorption
curves were normalized to 1, and the EXAFS signals χ(k) were obtained after the
removal of the preedge and postꢀedge background. The Fourier transform (FT) spectra
were obtained as k3χ(k) with a Hanning window in the range 3 −14.5 Å−1 for the Zn
Kꢀedge and 3 −13.5 Å−1 for the Cr Kꢀedge. Theoretical amplitude and phaseꢀshift
functions were calculated with the FEFF8.6 code using the structure parameters of the
normal spinel structure of ZnCr2O4. Due to the transferability of amplitude and phase
shifts, the coordination parameters of unknown samples can be obtained by fitting the
experimental curves with theoretical amplitudes and phase shifts.
2.23 XANES Calculation
Zn Kꢀedge XANES simulations based on selfꢀconsistent multipleꢀscattering (MS)
theory were performed using FEFF 8.6 code with the Hedin –Lundqvist (H −L)
exchange potential. A 57ꢀatom cluster (radius 0.53 nm) was used to calculate the
selfꢀconsistent field (SCF) muffinꢀtin atomic potential, and a 221ꢀatom cluster (radius
0.80 nm) for fullꢀmultiple scattering (FMS) XANES calculations was considered. The
normal spinel ZnCr2O4 was used as the model structure.
2.24 XPS, IR and H2-TPR analysis
The XPS patterns were recorded using an AXIS ULTRA DLD Xꢀray
photoelectron spectrometer, equipped with a multichannel detector. Charge
referencing was done against adventitious carbon (C 1s, 284.8 eV). A Shirleyꢀtype
background was subtracted from the signals. The recorded spectra were fitted using

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Gauss–Lorentz curves to determine the surface composition of different samples. IR
Spectra of CO adsorption and desorption were obtained with a TENSORꢀ27 in the
range from 4000 to 1000 cm−1 with 4 cm−1 resolution. Before CO adsorption, all of
o
catalysts were reduced with a 10% hydrogenꢀinꢀnitrogen mixture for 0.5 h at 400 C
on line. CO adsorption was taken at 400 oC and after 0.5 h of pure Ar flow at the same
temperature and the adsorbates existing on the surface of different ZnꢀCr spinel
catalysts would be measured.
2.25 Activity measurements
Each of these catalysts was reduced with a 10% hydrogenꢀinꢀnitrogen mixture
o
for 4 h at 400 C on line. Then, these catalysts were tested for isobutanol catalytic
activity in a fixed bed using a feed gas of 2.6:1 H2 and CO mixture at a space velocity
of 3,000 hꢀ1. The reactions were run at temperature 400 oC and pressure 10 MPa. The
products were analyzed by four onꢀline GC during the reaction. Inorganic gas
products consisted of CO, H2, H2O and CO2 were detected online by thermal
conductivity measurements using a GC4000A (carbon molecular sieves column). The
organic gas products consisted of hydrocarbons and methanol were detected online by
flame ionization measurements using a GC4000A (GDXꢀ403 column). The alcohol
products in liquid phase were detected by flame ionization measurements using a
GCꢀ7AG (Chromsorb101). The H2O and methanol products in liquid phase were
detected by thermal conductivity measurements using
(GDXꢀ401column).
a
GC4000A

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3 Results and discussion
3.1 XANES Analysis
Normalized Zn Kꢀedge XANES spectra of samples are shown in Fig. 1. There
are three resolved peaks and a significant shoulder at the white line region, which
reflects the electronic transition from zinc 1s core level to unoccupied states of p
type.31 The most striking features due to cation exchange in ZnCr2O4 at the ZnꢀK edge
appeared around 9668 eV and 9680 eV named feature A and feature B. Feature A
increases while feature B decreases in amplitude with the decreasing annealing
temperature and increasing Zn/Cr molar ratios. In a similar report for the spinel
31
structure of ZnFe2O4, Stewart et al.
have shown that feature A increases while
feature B decreases for the Zn Kꢀedge XANES spectrum when the degree of cation
disorder distribution increases. It is highly probable that these changes have the same
32
origin for ZnCr2O4. To support this hypothesis, S. Chen et al. also performed Zn
Kꢀedge XANES simulations for the normal ZnCr2O4 structure and for isolated Zn
atom substitutionally replacing Cr at octahedral sites and the calculated spectra
showed a similar change as we observed here. Hence, a conclusion could be taken
cautiously that the change trend of feature A and feature B shows an important local
structural change in ZnCr2O4 spinel which results from considerable zinc cations
transference from tetrahedral to octahedral sites. A close look of absorptionꢀedge (see
inset in Fig. 1, upper left side) revealed that it moved to higher energy, implying these
samples owned higher valence and more oxide vacancies, which should also due to
the more serious cation disorder distribution.

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Fig.1 The normalized XANES spectra for the ZnꢀKꢀedge.
Fig. 2 showed the normalized Cr Kꢀedge XANES spectra. The preꢀedge peak
(see inset in Fig. 2, upper left corner) increaseed while the absorptionꢀedge was
shifted to higher energy (see inset in Fig. 2, bottom right corner) with decreasing
annealing temperature and increasing Zn/Cr molar ratios. As we know, the preꢀedge
peak is due to the 1s to 3d quadrupole transitions (formally electric dipole forbidden),
though the intensity of the quadrupole transition is generally very low. The increase in
the intensity of the preꢀedge peak can be attributed to the enhancement of the orbital
pꢀd mixing, allowed in a tetrahedral symmetry but forbidden in the octahedral one.
Thus, the increase of the preꢀedge peak intensity reflects the increase of the
occupation number of Cr atom in tetrahedral sites (the degree of cation disorder
distribution) shown as follows: ZCꢀ0.8ꢀ400 > ZCꢀ0.65ꢀ400 >ZCꢀ0.5ꢀ400 >
ZCꢀ0.5ꢀ550 > ZCꢀ0.5ꢀ700. In addition, absorptionꢀedge (inset in Fig. 2，bottom right
side) moving to higher energy also points out the increase of the degree of cation
disorder distribution. All data are well consistent with the analysis results of the Zn

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Kꢀedge.
Fig.2 The normalized XANES spectra for the CrꢀKꢀedge
3.2 EXAFS Analysis
The corresponding Fourier transforms of Zn Kꢀedges are compared in Fig. 3. In
general, in a normal structure of ZnCr2O4, the Zn atoms are tetraꢀcoordinated by four
oxygens at 1.97 Å and 12 Cr atoms are at 3.45 Å as second neighbors. On the other
hand, Cr atoms are octalꢀcoordinated by 6 O at 1.99 Å and 6 Cr atoms at 2.94 Å as
second neighbors. Obviously, compared to the first coordination peak, the second
coordination peak is more sensitive to the cation distribution. We may point out that,
except for the two normal intensive peaks centered on around 1.6 and 3.01 Å, an
additional relatively weaker peak appears at about 2.6 Å, which clearly come from the
cation disorder distribution. Stewart et al.31 have observed a similar change in the Zn
Kꢀedge of ZnFe2O4. For our samples, depending on the corresponding atomic distance
mentioned above, when Zn atoms substitute to octahedral sites, a new shell would
appear at about 2.94 Å (in theory) from central Zn, resulting in a new peak. In fact,

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the additional peak appears at about 2.6 Å instead of at about 2.94 Å because of the
instrumental error. Not surprisingly, the location of this additional peak is happening
to be the second coordination distance of Cr (seen in Fig. S2). It further clearly
confirmed our conclusion that some part of Zn cations would enter into the octahedral
sites, signifying the cation disorder distribution appears. The new peak increased in
amplitude with decreasing annealing temperature and increasing Zn/Cr molar ratios,
demonstrating the trend of more serious cation disorder distribution. All analysis is
consist Cr Kꢀedge of ZnFe2O4 (seen in Fig S2).
Fig. 3 The Fourier transform EXAFS signals for the Zn Kꢀedge
In order to estimate the degree of cation disorder distribution accurately, a
leastꢀsquares fitting was performed over the rꢀspace range 0.8−3.6 Å for the Fourier
33
transform of the Zn Kꢀedge, following the method as described in ref 29.
All
parameters are calculated by means of the FEFF 8.6 code. The results of fitting
EXAFS data, such as lattice parameter (a), firstꢀshell, outerꢀshell mean–square
disorder (σ2) and the degree of cation disorder distribution (fractional occupancy of

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Zn atoms in octahedral sites), were showed in Table 1. In our computational process,
all of Rꢀfacter ranged from 2.3% to 3.6% which confirmed the reliability and
goodness of fitting results. As an example, the best fitting curve of the Zn Kꢀedge for
the sample ZCꢀ0.5ꢀ700 was shown in Fig. 4 and the rests were similar.
Fig. 4. Zn Kꢀedge EXAFS spectra of ZCꢀ0.5ꢀ700，compared with the best fit curve
The lattice parameter of ZCꢀ0.5ꢀ700 is similar to the JCPDS standard normal
spinel phase (a=b=c=8.3275). This implies that the ZCꢀ0.5ꢀ700 sample has a normal
spinel structure with rarely cation disorder distribution. However, the lattice
parameter of ZnꢀCr spinel phase increases continuously with decreasing calcination
temperature and increasing the Zn/Cr molar ratio. In general, the lattice parameter of
the spinel structure varies in accordance with the average radii of the cations in the
tetrahedral and octahedral sites. That means the entire spinel framework and lattice
parameter could adjust themselves by swelling or contracting to accommodate the
size of the cations when the metal atoms were conversion between tetrahedral and
octahedral sites [28]. Therefore, the mismatch in lattice parameter should be result

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from the more serious cation disorder distribution between tetrahedral and octahedral
sites. Both firstꢀshell and outerꢀshell MSD (meanꢀsquare disorder) increased with
decreasing the annealed temperature and increasing the Zn/Cr molar ratio, further
implying the more serious cation disorder distribution.
Table. 1. Results of fitting EXAFS data to a theoretical standard. Uncertainties in the least
significant digit are given in parentheses.
Fitting results of different catalyst
ZCꢀ0.5ꢀ700
8.32(3)
0.0093(2)
0.0047(3)
0(2)
ZCꢀ0.5ꢀ550 ZCꢀ0.5ꢀ400 ZCꢀ0.65ꢀ400 ZCꢀ0.8ꢀ400
lattice parameter a: (Å)
Firstꢀshell MSD (Å2)
Outerꢀshell MSD (Å2)
ZnBsite (%)
8.34(2)
0.0101(3)
0.0051(3)
6(2)
8.36(3)
0.0102(2)
0.0054(2)
16.5(4)
ꢀ6.40(2)
2.1
8.44(3)
0.0108(2)
0.0054(2)
23.5(6)
ꢀ5.97(2)
3.6
8.53(3)
0.0114(3)
0.0085(2)
27(7)
ΔE0
ꢀ6.89(2)
2.3
ꢀ6.31(2)
2.4
ꢀ6.13(4)
3.3
Rꢀfacter (%)
‘a’ is the lattice parameter of ZnꢀCr spinel, MSD means meanꢀsquare disorder including firstꢀshell
and outer–shell, ZnBsite is the percentage of Zn in octahedral site (the degree of cation disorder
distribution), ΔE0 is such that the energy origins lay on the rapidly rising portion of the
absorption spectrum and the Rꢀfactor is a measure of the mismatch between data and fit
normalized by the amplitude of the data.
Previous studies have shown that, despite the strong affinity of zinc cations for
tetrahedral sites, attempts to synthesize spinel by various routes may yield significant
33
amounts of zinc in the octahedral sites.
For our samples, in fact, the degree of
cation disorder distribution is about 0, 6, 16.5, 23.5 and 27% for samples of
ZCꢀ0.5ꢀ700, ZCꢀ0.5ꢀ550, ZCꢀ0.5ꢀ400, ZCꢀ0.65ꢀ400 and ZCꢀ0.8ꢀ400 respectively. It
clearly reveals the variation trend of cation distribution in ZnCr2O4 spinel structure.
As to the sample ZCꢀ0.5ꢀ700, all of the Zn2+ cations locate in tetrahedral vacancies,
demonstrating that ZCꢀ0.5ꢀ700 sample has a normal spinel structure. Conversely,
approximately 27% of Zn2+ ions substitute to octahedral sites for the sample annealed
at 400 °C and Zn/Cr molar ratio is 0.8 (ZCꢀ0.8ꢀ400). Furthermore, the percentage of

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disordered Zn2+ cations (in octahedral vacancies) increases from 0% to 16.5% with
decreasing the annealed temperature and increases continuously from 16.5% to 27%
with increasing the Zn/Cr molar ratio.
We may draw a conclusion cautiously from EXAFS and XANES analysis that
the cation disorder distribution becomes more serious with decreasing annealing
temperature and increasing Zn/Cr molar ratios. Specifically, higher annealing
temperatures lead to significant crystallite growth and the particle size affects the
cation distribution dramatically. In general, bigger particle size means most of metals
are located in the bulk, so that the metals are situated in the thermodynamically most
stable sites and form normal spinel structure. As to smaller particle size, a large
fraction of the metals is located on the surface. Due to surface truncation, the large
number of dangling bonds will affect the bonding environment of the metals and the
results indicate that the lowestꢀenergy configurations of the metals are satisfied by
27
parts of cation disorder distribution. Camilla Nordhei also held the point that the
surface metal coordination would be distorted by adsorbed oxygen and water or as a
consequence of hydroxide and that lead to cation disorder distribution. This
hypothesis could also be further confirmed by XPS, H2ꢀTPR and in situ DRIFT
analysis below. In the case of Zn2+ꢀrich samples, the extra smallꢀsized Zn cations will
enter the spinel lattice to generate the nonꢀstoichiometric spinel. Because of the great
difference of the crystal lattice energy of ZnO and Cr2O3 (being 4107.25 kJ/mol and
15186 kJ/mol, respectively which mean cations located at octahedral sites are more
stable than tetrahedral sites), the extra smallꢀsized Zn cations prefer into the

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octahedral sites to exacerbate further the level of disordered distribution. Z. V.
Marinkovic 34 also held the same point.
In a word, annealing temperatures affects cation distribution by tailoring particle
size and in the case of Zn2+ꢀrich samples the extra smallꢀsized Zn cations will enter
the spinel lattice to exacerbate further the level of cation disordered distribution. As
we all know, the spinel with more serious disorder distribution have better catalytic
activity since it owns more defects and oxygen vacancies which should activates the
reactants easier. The physical and chemical properties also could be tailored through
the variation of cations on the tetrahedral and octahedral sites and then effect
dramatically on isobutanol formation.
O-XPS analysis of catalysts
O 1s XPS spectra of catalysts were shown in Fig. 5. Three oxygen signals
located at 529.4, 531.1, and 532.7 eV attributed to lattice oxygen Olatt, surface
absorbed oxygen Oads and hydroxyl species OOHꢀ on the surface, respectively.35 The
molar ratio of Oads/(Olatt + OOHꢀ +Oads) and OOHꢀ/(Olatt + OOHꢀ +Oads)was obtained
thanks to quantitative calculation of the corresponding peaks areas and the result was
shown in Table S1. It revealed that the ratio of (OOHꢀ + Oads) /(Olatt + OOHꢀ +Oads)
became higher following the more serious cation disorder distribution. This result
could be further confirmed by the H2ꢀTPR study (seen in Fig. 6). As we discussed
above, the spinel with cation disorder distribution is not situated in the
thermodynamically most stable sites and the “metastable state” could exist steadily is
due to the large number of dangling bonds (surface absorbed oxygen Oads and

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hydroxyl species OOHꢀ). The population of these dangling bonds increases with
decreasing annealing temperature and increasing Zn/Cr molar ratios, indicating the
trend of more serious cation disorder distribution.
lattice oxygen
surface absorbed oxygen
surface hydroxyl grooups
e
d
c
b
a
523 524 525 526 527 528 529 530 531 532 533 534 535 536 537
EB/eV
Fig.5. O 1s XPS spectra of catalysts, a: ZCꢀ0.5ꢀ700, b: ZCꢀ0.5ꢀ550, c: ZCꢀ0.5ꢀ400, d:
ZCꢀ0.65ꢀ400 and e: ZCꢀ0.8ꢀ400
H2-TPR analysis of catalysts
Redox properties of different catalysts were evaluated by H2ꢀTPR and the results
were displayed in Fig. 6. For each catalyst, just one main hydrogen consumption peak
o
was observed in the temperature range of 300–400 C and the area was becoming
larger with more serious cation disorder distribution. It was reported that Zn2+ and
Cr3+ were very difficult to be reduced in the ZnꢀCr spinel. However, our catalysts are
situated in the “metastable state” due to the cation disorder distribution and that
makes our catalysts could be reduced much easier. Besides, In Fig.1 and Fig. 2, the
absorptionꢀedge of both Zn and Cr move to higher energy which means the valence of
them become higher than normal spinel. The higher valence should make them could
be reduced much easier.

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In a word, because of the more serious cation disorder distribution, the valence
of both Zn and Cr become higher and that “metastable state” needs more dangling
bonds to stay steadily. It makes our catalysts could be reduced much easier and the
larger hydrogen consumption should be attributed to the increase of
oxygenꢀcontaining species on the surface. For our catalysts, considering the inꢀsitu
DRIFT analysis (seen in Fig. 7ꢀ1 and 7ꢀ2) and catalytic performance (seen in Table.
2), it seems that the surface hydroxyl species is not just stabilize the “metastable
state” but also facilitate the alcohol synthesis.
Fig. 6. H2ꢀTPR profiles of catalysts, a: ZCꢀ0.5ꢀ700, b: ZCꢀ0.5ꢀ550, c: ZCꢀ0.5ꢀ400, d: ZCꢀ0.65ꢀ400
and e: ZCꢀ0.8ꢀ400
In situ DRIFTS analysis of catalysts
Additional information about the identity and population of the species was
provided by an in situ DRIFTS study after adsorbing sufficient CO molecule at the
reaction temperature. Fig. 7ꢀ1 showed the consumption of hydroxyl species (3400
cmꢀ1) after reacting with adsorbed CO molecule of different catalysts. The amount of
hydroxyl consumption become larger with more serious cation disorder distribution,
demonstrating that these catalysts owned more hydroxyl on the catalyst surface and

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that was in agreement with the XPS and H2ꢀTPR analysis.
Fig. 7-1. DRIFTS spectra of CO adsorbed on catalysts upon desorption at 400 oC, a: ZCꢀ0.5ꢀ700,
b: ZCꢀ0.5ꢀ550, c: ZCꢀ0.5ꢀ400, d: ZCꢀ0.65ꢀ400 and e: ZCꢀ0.8ꢀ400
Fig. 7ꢀ2 showed the corresponding C1 species after adsorbed CO reaction with
surface hydroxyl species. Two mainly C1 species were observed, namely: formate
ꢀ
species [solid line: 1600 cmꢀ1, υas(CO2 ); 1375 cmꢀ1, σ(CH)] and carbonates species
(dot line: 1560 cmꢀ1, 1330 cmꢀ1). For catalyst ZCꢀ0.5ꢀ700, the spectrum showed
mostly adsorbed carbonates with a small formate signal. The population of the
formate species increased with more serious cation disorder distribution and got
maximised for ZCꢀ0.8ꢀ400 catalyst. As to the carbonate species, however, the
changing trend showed in an opposite direction.
36
Alexis T. Bell and coꢀworkers believed that the formation of formate species
was an activated process, very likely involving the transfer of a proton from a surface
hydroxyl group to adsorbed CO and the formation of carbonate species is attributed to
the interactions of CO with both O2ꢀ and OHꢀ sites on the surface of ZrO2. By the use

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of temperature programmed surface reaction (TPSR), Wachs and Madix 37 discovered
that the stable intermediate on the reaction from CO/CO2 to CH3OH was a formate
while the carbonate species could be easily converted to CO2 which is useless for
alcohol formation. K. C. Waugh 37 drew an important conclusion that formate species
was a true intermediate and it was the most stable and longꢀlived intermediate in
methanol synthesis. From discuss above, some conclusions could be drawn cautiously
that formate species was a very significant intermediate for methanol synthesis and
formate formation involved hydroxyl group and adsorbed CO on catalyst surface.
Fig. 7-2. DRIFTS spectra of CO adsorbed on catalysts upon desorption at 400 oC, a: ZCꢀ0.5ꢀ700,
b: ZCꢀ0.5ꢀ550, c: ZCꢀ0.5ꢀ400, d: ZCꢀ0.65ꢀ400 and e: ZCꢀ0.8ꢀ400
Catalysts active measurement
The information of typical catalytic performance including reaction condition
was shown in Table 2. The conversion of CO using ZCꢀ0.5ꢀ700 catalyst is extremely
small, only ∼10% CO being conversed, and the selectivity of isobutanol was tiny
(4.44%). After lowering calcination temperature，the conversion was enhanced, ∼15%
and ∼30% conversion of CO was obtained by ZCꢀ0.5ꢀ550 and ZCꢀ0.5ꢀ400 samples

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and the selectivity of isobutanol was promoted from 4.44% to 10.78%. Compared the
o
samples with different Zn/Cr molar ratios calcined at 400 C, the conversion of CO
kept ~30%, while the selectivity of isobutanol monotonously increased from 10.78%
to 15.99% with increasing the Zn/Cr molar ratios.
The catalysts annealed at lower temperature showed much higher activity and
much better selectivity of isobutanol. When lowing the annealing temperature from
700 oC to 400 oC, about 3 times CO conversion (from 10.04% to 30.72%) and 2 times
isobutanol selectivity (from 4.44% to 10.78%) were observed. As to the catalysts with
different Zn/Cr molar ratios, the catalysts present similar CO conversion (~30%) and
much better isobutanol selectivity (from 10.78% to 15.99%). The outstanding
capability of the catalysts should be attributed to the more serious cation disorder
distribution. It is worth noting that the selectivity of methanol plus isobutanol is
closely 95% in alcohol pahse which is a typical character of ZnꢀCr catalysts. This
should simplify the process of products separation and then contribute greatly to the
industrialization of isobutanol.
The present ZnꢀCr catalysts presented a lower selectivity to isobutanol than that
of the ZnꢀCr samples reported by William S. Epling and coꢀworkers, 11, 12 possibly due
to the differences in the reaction conditions and/or sample composition and texture. It
must be stressed that the point of our present study is not to improve on the highest
reported selectivity to isobutanol, but to study the active sites of isobutanol over
ZnꢀCr spinel catalysts.

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Table. 2. Typical catalytic performance of ZnꢀCr based catalysts
Alcohol distribution /wt%
Methanol Ethanol Propanol Isobutanol
CO
conversion
(%)
Total
alcohol rate
(g/ml h)
0.019
catalysts
C5+
alcohol
0.63
ZCꢀ0.5ꢀ700
ZCꢀ0.5ꢀ550
ZCꢀ0.5ꢀ400
ZCꢀ0.65ꢀ400
ZCꢀ0.8ꢀ400
10.04
87.92
85.23
83.63
81.45
79.77
3.05
3.26
1.75
1.68
1.09
3.78
1.49
1.06
2.16
2.43
4.44
8.34
15.81
0.034
0.58
30.72
0.067
10.78
13.19
15.99
0.31
30.43
0.075
0.51
30.68
0.084
0.70
o
Reaction conditions: Temperature=400 C, Pressure=10 Mpa, GHSV=3000 hꢀ1
Because catalysis is a surface phenomenon, to better understand the
structureꢀactivity relationship, we set the unit of isobutanol productivity as “mg of
isobutanol per m2 of catalyst surface area per hour” instead of “mg of isobutanol per
ml of catalyst per hour”. The texture parameters of catalysts used here can be seen in
Table. S2. Another fact what we should notice here is that the cation disorder
distribution in our samples mainly involve the surface atoms because these atoms
could exist in the disorder state due to large number of dangling bonds while the bulk
atoms in spinel are situated in the thermodynamically most stable sites: normal spinel
structure. Fig. 8 showed that the isobutanol productivity was approximately linear to
the level of cation disorder distribution, confirming that the cation distribution
effected dramatically on the isobutanol formation. This is the first time revealing
that the productivity of isobutanol is strongly related to the cation distribution in
ZnꢀCr spinel investigated by XAFS. To be specific, with decrease annealing
temperature and increase Zn/Cr molar ratio of ZnꢀCr catalysts, the level of cation
disorder distribution becomes higher. It leads less perfect crystal, more oxygen

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vacancies and more defects which should lower the activation energy for isobutanol.
Furthermore, the defects and vacancies affect drastically the oxygen state on catalyst
surface which plays a significant role in the process of isobutanol formation.
What we must say is the conclusion of structureꢀactivity relationship in this
article is just suitable for a certain limited range. To be specific, higher annealed
temperatures than 700 oC would have little influence on the structure of ZnꢀCr spinel.
On the other hand, it will be meaningless to prepare our catalysts at lower temperature
o
than our reaction condition (400 C). In the case of Zn/Cr different ratios samples, if
the Zn/Cr ratio is less than 0.5，the redundant Cr cations will form Cr2O3 to disturb
our research and if the Zn/Cr ratio is more than 0.8, some of parts of Zn cations
wouldn’t enter the spinel lattice but formed wurtzite crystal structure which will
disturb our experience.
Fig. 8 The curve of isobutanol productivity with the level of cation disorder distribution
Conclusion
A series of ZnꢀCr oxides nanoparticles with different degrees of cation disorder
distribution were obtained by a coprecipitation and postcalcined method. Both

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EXAFS and XANES were used to investigate the change of cation disorder
distribution. ZnꢀCr nanoparticles with a disorder distribution parameter of 27% were
o
obtained when annealing temperature was 400 C and the Zn/Cr ratio was 0.8. The
detailed analysis demonstrates that the disorder distribution strongly depends on the
annealing temperature and the Zn/Cr molar ratio. It becomes more serious with
decreasing calcination temperature and increasing the Zn/Cr molar ratio.
The cation distribution will tailor the oxygen state on the surface of catalysts and
affect dramatically isobutanol formation. The population of absorptionꢀoxygen
species increases with more serious caiton disorder distribution in ZnꢀCr spinel. In
addition, it seems that the surface hydroxyl species is not just stabilize the “metastable
state” but also facilitate the formate formation species which is a very significant
intermediate C1 species for alcohol synthesis.
The isobutanol productivity is closely related to the cation disorder distribution.
This clearly demonstrates that the disorder cation especially Zn cations in octahedral
vacancy should be the active sites for isobutanol formation. In heterogeneous catalysis,
a catalyst having a finely designed structure usually exhibits better catalytic
performance. As to our research specifically, more disorder cation distribution leads
more defects which effects drastically on catalysts characters and makes isobutanol
formation more effectively. A better understanding of real active sites over ZnꢀCr
based catalysts for isobutanol formation from syngas should play a significant role to
enhance the catalyst performance or design a more effective new catalyst for higher
alcohol synthesis.

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Acknowledgements
The authors thank the Shanghai Synchrotron Radiation Facility for providing the
beamline BL14W1 for the beam time measurement. This work was supported by the
National Natural Science Foundation of China (21573269), The Prospective Project
of Institute of Coal Chemistry, The Chinese Academy of Sciences (No.
2011SQZBJ13) and the Cooperative Project of Shaanxi Yanchang Petroleum (Group)
Corp. Ltd., China (JT1014SKF0003).
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