Thermodynamic complexity of sulfated zirconia catalysts

Article abstract of DOI:10.1016/j.jcat.2016.08.001

A series of sulfated zirconia (SZ) catalysts were synthesized by immersion of amorphous zirconium hydroxide in sulfuric acid of various concentrations (1–5?N). These samples were fully characterized by X-ray diffraction (XRD), thermogravimetric analysis and mass spectrometry (TGA-MS), and aqueous sulfuric acid immersion and high temperature oxide melt solution calorimetry. We investigated the enthalpies of the complex interactions between sulfur species and the zirconia surface (ΔH_SZ) for the sulfated zirconia precursor (SZP), ranging from ?109.46?±?7.33 (1?N) to ?42.50?±?0.89 (4?N) kJ/mol S. ΔH_SZ appears to be a roughly exponential function of sulfuric acid concentration. On the other hand, the enthalpy of SZ formation (ΔH_f), becomes more exothermic linearly as sulfur surface coverage increases, from ?147.90?±?4.16 (2.14?nm^?2) to ?317.03?±?4.20 (2.29?nm^?2) kJ/mol S, indicating formation of energetically more stable polysulfate ?species.

Full text of DOI:10.1016/j.jcat.2016.08.001

Journal of Catalysis 342 (2016) 158–163
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Thermodynamic complexity of sulfated zirconia catalysts
a,
d,
Naiwang Liu a,b, Xiaofeng Guo c, Alexandra Navrotsky b, , Li Shi , Di Wu
⇑
⇑
⇑
a State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
b Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, One Shields Avenue, Davis, CA 95616, United States
c Earth System Observations, Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States
d The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99163, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of sulfated zirconia (SZ) catalysts were synthesized by immersion of amorphous zirconium
hydroxide in sulfuric acid of various concentrations (1–5 N). These samples were fully characterized by
X-ray diffraction (XRD), thermogravimetric analysis and mass spectrometry (TGA-MS), and aqueous sul-
furic acid immersion and high temperature oxide melt solution calorimetry. We investigated the enthal-
Received 24 June 2016
Revised 2 August 2016
Accepted 3 August 2016
pies of the complex interactions between sulfur species and the zirconia surface (
zirconia precursor (SZP), ranging from À109.46 7.33 (1 N) to À42.50 0.89 (4 N) kJ/mol S.
to be a roughly exponential function of sulfuric acid concentration. On the other hand, the enthalpy of SZ
D
HSZ) for the sulfated
DHSZ appears
Keywords:
Sulfated zirconia
Enthalpy of formation
Heterogeneous catalysis
Thermodynamics
Calorimetry
formation
(DHf), becomes more exothermic linearly as sulfur surface coverage increases, from
À147.90 4.16 (2.14 nmÀ2) to À317.03 4.20 (2.29 nmÀ2) kJ/mol S, indicating formation of energetically
more stable polysulfate species.
Ó 2016 Elsevier Inc. All rights reserved.
Olefins conversion
Stability and activity
1. Introduction
interactions in aqueous solutions during catalyst precursor (SZP)
synthesis (enthalpy of immersion of zirconia in sulfuric acid), the
Heterogeneous catalysis plays a critical role in modern chemical
industry. Unless the catalytic active components are stabilized
through subtle, thermodynamically favorable interactions on a
substrate they are generally not robust for industrial applications
[1]. Well-known examples include supported metal nanoclusters
[2] and dispersed super-acidic moieties [3]. Thus, knowing the
strength of interactions and bonding between molecular-level
active catalytic species and support materials is of great impor-
tance for understanding and optimizing parameters for successful
catalyst synthesis and operation.
Because of its very strong surface acidity, sulfated zirconia (SZ)
has been applied extensively in the chemical industry as an acid
catalyst [4]. Applications include isomerization [5], hydrocracking
[6], alkylation [7,8] and oligomerization of hydrocarbons [9].
Numerous experimental (spectroscopy, diffraction and micro-
scopy) [10–14] and computational (density functional theory and
molecular dynamics) [15,16] studies have been performed to
explore the specific binding, surface assembly and/or structure,
and catalytic activity of SZ. However, few studies have investigated
the energetics of SZ materials, including sulfur species – zirconia
material phase evolution during calcination, and the energetics of
formation of the as-made SZ catalyst.
In this context, calorimetry is a powerful tool to probe the inter-
actions and bonding energetics on surfaces or at interfaces.
Recently, we have performed a series of systematic thermochemi-
cal studies on small molecule – surface interactions using immer-
sion, solution and/or gas adsorption calorimetry [17–21]. In
addition, the bonding energetics of CO2 [22,23], CH4 [24], water,
ethanol [25], N,N-dimethylformamide (DMF) and N,N-
diethylformamide (DEF) [26] was also investigated for various
metal–organic frameworks (MOFs). Most of these bonding or inter-
action enthalpies were found to be functions of adsorbate molecule
coverage.
Here, we take a similar approach to evaluate interactions in the
SZ catalyst. Combining sulfuric acid immersion calorimetry, high
temperature oxide melt solution calorimetry, and thermogravi-
metric analysis and mass spectrometry (TGA-MS), we determined
the magnitude of sulfur species – zirconia surface interactions
(enthalpy of immersion) under SZP synthesis conditions at room
temperature. Additionally, we identified two groups of energeti-
cally distinct sulfur species adsorbed on the SZP surface. Further,
we quantified the enthalpies of SZ formation (DHf) from con-
stituent oxides. These energetics were determined, interpreted
and discussed as functions of sample treatment temperature, sul-
⇑
Corresponding authors.
E-mail addresses: anavrotsky@ucdavis.edu (A. Navrotsky), yyshi@ecust.edu.cn
(L. Shi), d.wu@wsu.edu (D. Wu).
http://dx.doi.org/10.1016/j.jcat.2016.08.001
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

N. Liu et al. / Journal of Catalysis 342 (2016) 158–163
159
furic acid concentration, and sulfur species coverage. These mea-
surements provide thermodynamic insights for sulfation of zirco-
nia surfaces in both catalytic material synthesis and applications.
deviations of the mean) in the figures. Prior to immersion
calorimetry, samples were outgassed at 200 °C overnight to maxi-
mize removal of adsorbed species.
2. Materials and methods
2.5. High temperature oxide melt solution calorimetry
2.1. Synthesis of sulfated zirconia (SZ)
The drop solution enthalpies of all samples in molten sodium
molybdate (3Na2OÁ4MoO3) at 703 °C were measured using a
custom-made Tian-Calvet twin microcalorimeter. The detailed
methodology was described elsewhere [27]. Sample pellets
(ꢀ5 mg) were hand-pressed and dropped from ambient conditions
into the solvent in a platinum crucible in the hot calorimeter. The
calorimetric assembly was flushed using oxygen (50 mL/min) to
maintain oxidizing environment and the solvent was bubbled with
oxygen (0.5 mL/min) to promote sample dissolution and to propel
evolved gases. All samples were degassed at 200 °C overnight
before calorimetric measurement. The calorimeter was calibrated
using the known heat content of high purity alpha alumina (Alfa
Aesar, 99.997%). The thermodynamic cycles used to derive the
Zirconium hydroxide (Z) was synthesized via a sol-gel method.
Ammonium hydroxide solution (ꢀ25% v/v) was dripped into zirco-
nium oxychloride (ZrOCl2Á8H2O) aqueous solution kept at 60 °C
under vigorous stirring until the pH reached 8.0. This solution
was aged at room temperature for 12 h followed by filtration.
The precipitation, zirconium hydroxide, was collected and washed
5 times to maximize chloride ion removal. The dried zirconium
hydroxide powder was then subjected to sulfation immersion/
impregnation for 1 h, in which sulfuric acid solutions of different
concentration from 1 to 5 N were used. After another round of fil-
tration and drying, we obtained the sulfated zirconia precursor
(SZP). The final product, sulfated zirconia (SZ) was achieved
through calcination of SZP at 650 °C in air for 2 h. For clarity, the
final treatment temperature is marked after each sample name in
the tables. Hence, the zirconium hydroxide, sulfated zirconia pre-
cursor and sulfated zirconia were denoted with temperature
labeled as Z-25, SZP-25 and SZ-650.
enthalpy of SZ formation (DHf) are listed in Table 1.
2.6. Catalytic activity test
An alkylation reaction in which olefins react with aromatics
leading to reduced olefin concentration in aromatic liquids
(Scheme 1) [31] was employed to test the catalytic activity of SZ
treated in different conditions. Alkylation was carried out in a fixed
bed tubular micro-reactor equipped with a constant flow pump to
enable precise flow rate and temperature control. Catalyst (SZ,
2 mL) was loaded into the micro-reactor, the spare space of which
was filled with quartz sand (space filler). The tests were performed
at 1.0 MPa with a liquid hourly space velocity (LHSV) of 30 hÀ1
[32]. Bromine Index is an indicator of olefin content in aromatics.
According to ASTM standard D 2710-92, it is defined as the amount
of bromine (mg) consumed by 100 g of hydrocarbon. The inlet and
effluent liquids of the fixed bed tubular micro-reactor were moni-
tored by a Bromine Index Analyzer (OSC 971KK 101, Ogawa Seiki
Co., Ltd., Tokyo, Japan). The olefin conversion rate, X, was calcu-
lated as X = (Ni À No)/Ni  100%, in which Ni and No correspond to
the bromine index of the inlet and effluent liquids, respectively.
The specific olefin and aromatic compounds involving in the alky-
lation reactions and their contents are listed in Table 2 [32].
2.2. Characterization
Powder X-ray diffraction (XRD) patterns were collected using a
Bruker-AXS-D8 Advance diffractometer (Bruker, AXS, Inc., Madi-
son, WI) operated at 40 kV and 40 mA (Cu Ka, k = 0.15406 nm).
Data were acquired in the 2h range of 20–70° (step size 0.02°
and 1 s per step). The sample phase distribution and crystallite size
were quantified by Rietveld refinement using JADE 6.1 software.
The Brunauer-Emmett-Teller (BET) surface areas were mea-
sured using a Gemini 2360 instrument (Micromeritics, Norcross,
GA). Prior to BET analysis, samples were degassed at 200 °C under
vacuum for 12 h. All the measurements were triplicated to ensure
reproducibility.
2.3. Thermogravimetric analysis and mass spectrometry (TGA-MS)
Thermogravimetric analysis (TGA) was carried out using a Net-
zsch STA 449 instrument (Netzsch, Selb, Germany). About 20 mg
sample was pressed into a pellet and heated from 30 to 1000 °C
(10 °C/min) under argon flow (40 mL/min). The evolved gas was
introduced into a mass spectrometer (Cirrus-2, MKS Instruments
UK Ltd., Crewe, UK) through a capillary tube heated at 100 °C. Each
TGA-MS analysis was corrected by recording a baseline with empty
platinum crucible. The mass spectrometer was calibrated using
calcium oxalate monohydrate (CaC2O4ÁH2O), which has known
water content.
Table 1
Thermodynamic cycles for enthalpies of SZ formation,
DHf.
Reaction formulaa
Reaction
enthalpy
ZrO2ÁxSO3ÁyH2O (tetragonal/ads/ads, 25 °C) ? ZrO2 (sol,
703 °C) + yH2O (g, 703 °C) + xSO3 (sol, 703 °C)b
ZrO2 (tetragonal, 25 °C) ? ZrO2 (sol, 703 °C)
H2O (ads, 25 °C) ? H2O (g, 25 °C)
H2O (g, 25 °C) ? H2O (g, 703 °C)
SO3 (g, 25 °C) ? SO3 (sol, 703 °C)
D
Hds
c
D
D
D
D
D
H1
d
H2
e
H3
f
H4
2.4. Immersion calorimetry
SO3 (ads, 25 °C) ? SO3 (g, 25 °C)
Hf
D
Hf =
D
Hds
À
D
H1
À
D
H2
À
D
H3
À
DH4
The immersion enthalpies of zirconia in sulfuric acid solutions
(1–5 N) were measured using a Setaram C80 microcalorimeter
a
‘‘g”, ‘‘ads” and ‘‘sol” denote ‘‘gas”, ‘‘adsorption” and ‘‘in sodium molybdate
solution”, respectively.
Ref. Majzlan et al. [28] and Drouet and Navrotsky [29].
D
enthalpies of sulfated zirconia (SZ) and nanophase tetragonal zirconia obtained
using high temperature oxide melt solution calorimetry, respectively.
(Setaram Instrumentation, Caluire, France) equipped with
a
b
c
custom-made dropping tube. Each sample was hand-pressed into
a pellet (about 10 mg), weighed on a microbalance, and dropped
from room temperature into the solution at 25 °C. The drop
resulted in a heat effect (calorimetric peak) associated with the
reaction between the solid sample and the solution. Immersion
calorimetry on each sample was repeated at least eight times to
ensure reproducibility, which is shown as error bars (2 standard
Hds and DH1 (DH1 = 27.06 0.71 kJ/mol ZrO2 [30]) are drop solution
d
D
D
D
H2 is enthalpy of H2O adsorption on sulfated zirconia surface.
H3 = 25.65 kJ/mol H2O [30].
H4 is drop solution enthalpy for SO3, and it cannot be determined directly
e
f
experimentally but has been calculated from the formation enthalpies of SO2 and
O2. According to previous work,
H4 = À217.02 4.17 kJ/mol [28].
D

160
N. Liu et al. / Journal of Catalysis 342 (2016) 158–163
CH3
CH2 CH R2
R1
C
H
R2
R1
Scheme 1. Alkylation between aromatics and olefins.
Table 2
The specific olefin and aromatic compounds (BI = 1035)
involving in the alkylation reactions and their contents.
Compound
Content (wt%)
Nonaromatics
Benzene
Toluene
2.08
0.06
0.05
Ethylbenzene
p-Xylene
m-Xylene
o-Xylene
C9
6.85
8.92
18.98
12.25
37.12
13.69
Fig. 2. Mass spectrometry (MS) curves of H2O, SO2 and O2 for SZP-25 (4 N) sample.
C10 and C10
+
3. Results
3.1. Characterization
The TGA and DTG traces and corresponding MS curves of SZP-25
(4 N) are presented in Figs. 1 and 2. MS signals were observed for
M/Z = 18, 32 and 64, which correspond to H2O, O2 and SO2, respec-
tively. Thermal analysis of SZP-25 in argon results in a series of
reactions, which can be categorized into two groups. H2O is des-
orbed from the SZP surface first. There appears to be two types
of energetically distinct H2O species. Physisorbed H2O is released
at about 120 °C, while liberation of chemisorbed H2O takes place
between 300 and 500 °C. This is followed by decomposition of sul-
fur species from 600 to 850 °C, leading to evolution of SO2 and O2
(SO2: O2 ꢀ 2:1). The position, shape and area of MS peaks for H2O,
O2 and SO2 correspond very well with the TGA and DTG curves
(Figs. 1 and 2).
The SO2 MS signal of SZP-25 (4 N) was further interpreted in
detail to reveal the energetic insights into sulfur species – zirconia
surface interactions in SZ catalyst synthesis (see Fig. 3). SZP-25
(4 N) presents two overlapped SO2 MS peaks upon heating to
1000 °C. Applying Gaussian multi-peak fitting, we were able to
split this signal into two peaks (Fig. 3A). The first peak is sharp,
centered at approximately 650 °C, while the second peak is broad,
Fig. 3. Separation of SO2 mass spectrometry (MS) signals for SZP-25 (4 N) sample.
spanning 600 to 850 °C. We also attempted to separate these two
peaks by ‘‘quenching” the SZ sample at about 650 °C, followed by
another round of TGA-MS examination using the same experimen-
tal conditions (see Fig. 3B and C). These results clearly suggest two
types of energetically distinct sulfuric components on ZrO2.
The XRD patterns of Z-25, SZP-25, Z-650, SZ-650, SZ-720 and
SZ-1000 are plotted in Fig. 4. These SZP and SZ samples had been
prepared using 4 N sulfuric acid. At 25 °C, both Z-25 and SZP-25
are amorphous. Upon calcination at 650 °C in air, Z-650 crystal-
lized to monoclinic zirconia, while SZ-650 became a mixture of
monoclinic and tetragonal phases. SZ-720 became completely
tetragonal as temperature increased to 720 °C. Ultimately, all sam-
ples became monoclinic after calcination at 1000 °C. Sample com-
positions are listed in Tables 3 and 4. BET surface areas for all
samples are listed in Table 4. The sample surface area tends to
increase as sulfuric acid concentration increases, ranging from
56.19 0.18 (1 N) to 84.85 0.38 (5 N) m2/g.
The MS traces, collected from room temperature to 1000 °C, of
SZP-25 and SZ-650 prepared with sulfuric acid of different concen-
tration (Cs = 1–5 N) are presented in Fig. 5. The MS profiles of SZP-
25 and SZ-650 exhibit distinctly different behavior as a function of
Cs. Generally, the loading of sulfur species on the SZP-25 surface
(Csurface) is proportional to the sulfuric acid concentration Cs. There
Fig. 1. Thermogravimetric analysis (TGA) and differential thermogravimetric
analysis (DTG) profiles for SZP-25 (4 N) sample.

N. Liu et al. / Journal of Catalysis 342 (2016) 158–163
161
filled, while as coverage increases, states more favorable in energy,
or representing stronger interactions among sulfate moieties
themselves rather than interactions with the surface, become
dominant.
3.4. Catalytic activity test
Figs. 8 and 9 show the olefins conversion as a function of the
time on stream (TOS). Without anchored sulfur species, the bare
zirconia surface does not have catalytic activity. All SZ samples
show decreasing olefin conversion as a function of TOS. For a par-
ticular TOS, the olefin conversion increases as the calcination tem-
perature increases from 500 to 650 °C. Beyond 700 °C, the SZ
catalysts suffer from significant deactivation due to loss of sulfur
species (Fig. 8). The SZ samples treated by sulfuric acid with higher
concentration show better olefin conversion at the same TOS
(Fig. 9). The catalytic activity enhancement reaches a maximum
at Cs = 4 N. Further increase of Cs does not result in better
performance.
Fig. 4. Powder X-ray diffraction (XRD) patterns of (A) pure zirconia at 25 °C (Z-25),
(B) sulfated zirconia precursor (SZP-25) 25 °C, (C) pure zirconia at 650 °C (Z-650),
(D) sulfated zirconia at 650 °C (SZ-650), (E) sulfated zirconia at 720 °C (SZ-720), and
(F) sulfated zirconia at 1000 °C (SZ-1000).
4. Discussion
4.1. Surface – structure – activity relations: calcination of SZP-25 (4 N)
SZP-25 (4 N) was reported to have optimized performance for
conversion of olefins (%) [8]. TGA-MS analysis was carried out on
this sample to mimic the calcination process in SZ catalyst synthe-
sis. Our results highlight the interplay and competition between
surface chemistry and structural evolution as a function of temper-
ature. The desorption/decomposition of the sample occurs in two
major temperature ranges (Figs. 1 and 2). The first step takes place
between 100 and 300 °C due to slow yet noticeable desorption of
water. In this temperature range, dehydration does not result in
phase transition (see Fig. 4). Although it is difficult to differentiate
physisorbed and chemisorbed water on the TGA trace, clear signal
splitting can be identified from the MS curve of water (Fig. 2).
In sharp contrast, in the second step (600–850 °C) both signifi-
cant weight loss on TGA curve, and well resolved SO2 and O2 MS
traces corresponding to detachment of sulfur species are observed.
In this range, the XRD patterns of SZ at 650 °C (monoclinic and
tetragonal) and 720 °C (tetragonal) strongly suggest that sulfur
removal not only leads to sulfur species decomposition on the sur-
face, but also significantly modifies the bulk zirconia structure
(Fig. 4). For pure zirconia heated at 650 °C (Z-650), tetragonal
phase is not observed. This strongly suggests that the surface
bonded sulfur species play a crucial role in zirconia phase selection
at high temperature (600–850 °C) through significant modification
of the structure and energetic states of the surface. Indeed, various
studies report that the tetragonal phase is necessary for SZ cat-
alytic activity [33], especially for isomerization of hydrocarbons,
for which sulfated monoclinic zirconia exhibits undetectable activ-
ity. Similar behavior, with little catalytic activity for SZ-500 (mon-
oclinic), was observed in the alkylation reactions (olefins removal
from aromatics) carried out in the present study (Fig. 8). Moreover,
the well-separated sulfur MS peaks of SZP-25 (4 N) indicate that
are two peaks for SZP-25 samples. As Cs increases, the height or
intensity of the first peak centered at about 650 °C increases. In
sharp contrast, there is only one peak for SZ-650 samples. The
MS intensity and peak area for SZ-650 samples do not change for
Cs higher than 2 N.
3.2. Immersion calorimetry
The immersion calorimetry data are plotted in Fig. 6. All immer-
sion enthalpies are exothermic, confirming stabilizing surface
interactions between zirconia and sulfate species. The enthalpy
of immersion of pure zirconia (Z-25) tends to become less exother-
mic as the concentration of sulfuric acid increases from 1 to 5 N,
ranging from À109.46 7.33 (1 N) to À42.50 0.89 (4 N) kJ per
mole of sulfur unit (kJ/mol S). This plot levels at about À42 kJ/mol
S and further increase of sulfuric acid concentration above 3 N does
not lead to less exothermic enthalpy of immersion.
3.3. High temperature oxide melt solution calorimetry
The enthalpy of drop solution (DHds, kJ/mol ZrO2) tends to be
more endothermic, values ranging from 18.34 3.04 to
23.47 1.85 kJ/mol ZrO2, as Cs increases from 1 to 5 N. Considering
both sulfation and hydration factors and using the thermodynamic
cycle listed in Table 1, we were able to calculate the enthalpy of SZ
formation from constituent oxides (DHf, kJ/mol S). The SZ forma-
tion enthalpy becomes more exothermic as the sulfur coverage
increases, ranging from À147.90 4.16 to À317.03 4.20 kJ/mol S
(Fig. 7). From a thermodynamic standpoint, this is reasonable in
that, at lower sulfur coverage, only the zirconia surface sites are
Table 3
Post immersion calorimetry SZP sample compositions and enthalpies of sulfur species – zirconia interactions (
from immersion calorimetry.
DHSZ, with different units). These enthalpies data come directly
Sample
Composition
D
HSZ (kJ/mol ZrO2)
D
HSZ (J/g ZrO2)
DHSZ (kJ/mol S)
SZP-25 (1 N)
SZP-25 (2 N)
SZP-25 (3 N)
SZP-25 (4 N)
SZP-25 (5 N)
ZrO2Á0.07SO3Á0.49H2O
ZrO2Á0.09SO3Á0.56H2O
ZrO2Á0.15SO3Á1.03H2O
ZrO2Á0.20SO3Á1.09H2O
ZrO2Á0.20SO3Á1.11H2O
À7.94 0.53
À7.79 0.60
À8.09 0.53
À8.55 0.18
À8.64 0.28
À57.54 3.85
À55.50 4.29
À52.35 3.41
À53.78 1.13
À54.17 1.78
À109.46 7.33
À87.64 6.78
À51.31 3.35
À42.50 0.89
À42.55 1.40

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N. Liu et al. / Journal of Catalysis 342 (2016) 158–163
Table 4
Sample characterization and thermodynamic data for high temperature oxide melt solution calorimetry, including enthalpies of dissolution directly measured by calorimetry,
Hds and enthalpies of SZ formation, Hf, calculated using thermodynamic cycle listed in Table 1.
D
D
Sample
Composition
Surface Area (m2/g)
S coverage (nmÀ2
)
D
Hds (kJ/mol ZrO2)
DHf (kJ/mol S)
SZ-650 (1 N)
SZ-650 (2 N)
SZ-650 (3 N)
SZ-650 (4 N)
SZ-650 (5 N)
ZrO2Á0.02SO3Á0.07H2O
ZrO2Á0.03SO3Á0.07H2O
ZrO2Á0.03SO3Á0.09H2O
ZrO2Á0.04SO3Á0.13H2O
ZrO2Á0.04SO3Á0.13H2O
56.19 0.18
67.18 0.19
73.68 0.23
83.19 0.47
84.85 0.38
2.29
2.28
2.20
2.16
2.14
18.34 0.56
16.79 0.43
19.66 0.11
22.26 0.84
23.47 0.43
À317.03 4.20
À305.30 4.18
À230.61 4.16
À187.81 4.24
À147.90 4.16
Fig. 5. SO2 mass spectrometry (MS) signals for (A) SZP-25 and (B) SZ-650 (1–5 N)
samples.
Fig. 7. Enthalpy of SZ formation from constituent oxides for SZ-650 as a function of
sulfur species surface coverage.
Fig. 6. Enthalpy change for sulfur species – zirconia surface interactions for SZP-25
as a function of sulfuric acid concentration.
Fig. 8. Olefin conversion rate of SZ catalyst prepared at different temperatures,
from 500 to 700 °C.
there are two groups of energetically distinct sulfate species on zir-
conia surface as a function of sulfur loading (Fig. 2). These can be
identified as the sulfate moieties directly adsorbed through
chemisorption before sulfate monolayer formation, and the subse-
quent, presumably attached through hydrogen bonding, sulfur spe-
cies beyond the monolayer. The molar ratio of monolayer sulfur to
loosely attached sulfur is approximately 1:1 (Fig. 3).
of the amorphous zirconia surface and the large variety of possible
sulfur species. The thermochemical approach offers a unique angle
evaluating the general behavior of a population of molecular spe-
cies on complex, amorphous surfaces.
Our results are consistent with the simulation and spec-
troscopy. Using periodic plane wave pseudopotential calculations
based on density functional theory (DFT), Haase and Sauer [15]
predicted that at ultra-low coverage there would be three most
stable configurations of sulfur species on different surfaces of
tetragonal zirconia: a tridentate sulfate ion on the (101) surface
4.2. Surface energetics of SZ: a function of sulfur species coverage
Despite numerous investigations, our understanding of the sur-
face chemistry of SZ is still limited due to the intrinsic complexity

N. Liu et al. / Journal of Catalysis 342 (2016) 158–163
163
tration (Cs). The enthalpy of immersion becomes less exothermic
as Cs increases, while the enthalpy of SZ formation tends to be
more exothermic linearly as sulfur species coverage increases.
These thermochemical insights are tightly correlated to the surface
coverage and configuration of sulfur species. In a broader context,
knowing such energetic insights is critical to understand catalyst
synthesis and active compounds – catalyst support interactions
for heterogeneous catalysts. The thermodynamic insights gained
here into competitive interactions of sulfur species on zirconia sur-
faces may enrich our understanding of sulfated zirconia catalysts
and their analogues.
Conflict of interest
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the United States Department of
Energy, Office of Basic Energy Sciences, Grant DE-FG02-
05ER15667. N.L. thanks the China Scholarship Council for the State
Scholarship Fund (No. 201506740047). D.W. thanks the institu-
tional funds from the Gene and Linda Voiland School of Chemical
Engineering and Bioengineering at Washington State University.
Fig. 9. Olefin conversion rate of SZ catalyst prepared under different sulfuric acid
concentration, from 1 to 6 N.
(À322 kJ/mol S), a bidentate sulfate complex on the (001) surface
(À408 kJ/mol S), and
a SO3 complex on the (001) surface
(À467 kJ/mol S). In this case, the site-isolated monosulfates (SO3,
HSOÀ4 , SO24À) are well dispersed and directly anchored at the most
strongly interacting zirconia surface sites with minimum self-
interactions. Subsequently, as the surface coverage increases, sul-
fur species may react among themselves forming complexes and/
or polysulfate species upon calcination. Employing DFT with peri-
odic boundary conditions, Hofmann et al. [16] predicted that at rel-
atively high coverage, as the sulfate loading reached 2H2SO4 per
two surface unit cells (four Zr sites), transformation from monosul-
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Using XRD, TGA-MS, immersion and high temperature oxide
melt drop solution calorimetry, we performed a detailed study
on the thermodynamics of two critical topics for sulfated zirconia
catalysts, sulfuric acid immersion (catalyst precursor preparation)
and sulfate – zirconia bonding as a function of sulfuric acid concen-