Pt/TiH2 Catalyst for Ionic Hydrogenation via Stored Hydrides in the Presence of Gaseous H2

Article abstract of DOI:10.1021/acscatal.9b00917

A Pt/TiH₂ catalyst having hydrogen storage and release capability was investigated for selective hydrogenation of trans-cinnamaldehyde (CAL) to cinnamyl alcohol (COL) with gaseous dihydrogen. The catalytic behavior of this catalyst was significantly different from that of a reference Pt/TiO₂ catalyst with respect to the product selectivity and the hydrogenation mechanism. The Pt/TiH₂ catalyst showed a COL selectivity of 97% at a CAL conversion of 98%, which was ascribed to the function of a Pt crystallite-support boundary layer that caused the preferential adsorption of CAL with its carbonyl group. Furthermore, the carbonyl group was hydrogenated by hydride species (H⁺, H^-) supplied from the support and the hydride species consumed were compensated from gaseous dihydrogen; hydrogen atoms were formed by ordinary homolytic cleavage on Pt and then these hydrogen atoms moved onto the surface of TiH₂ and diffused into the bulk of the support, during which those simultaneously changed to hydride species (H⁺, H^-) via electron transfer with titanium species and hydride vacancies therein. The surface and bulk diffusion of the hydrogen atoms from Pt to TiH₂ support should be the dominant step rather than their addition to the carbonyl group of CAL (ordinary hydrogenation). That is, ionic hydrogenation occurs with Pt/TiH₂ in the presence of gaseous dihydrogen.

Full text of DOI:10.1021/acscatal.9b00917

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Article
A Pt/TiH2 catalyst for ionic hydrogenation via
stored hydrides in the presence of gaseous H2
QiFan Wu, Chao Zhang, Masahiko Arai, Bin Michael Zhang, Ruhui Shi, Peixuan Wu,
Zhuangqing Wang, Qiang Liu, Ke Liu, Weiwei Lin, Haiyang Cheng, and Fengyu Zhao
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00917 • Publication Date (Web): 14 Jun 2019
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A Pt/TiH2 Catalyst for Ionic Hydrogenation
via Stored Hydrides in the Presence of
Gaseous H2
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Qifan Wu,a,b,c Chao Zhang,*a,c Masahiko Arai,a,c Bin Zhang,a,b,c Ruhui Shi,a,b,c
Peixuan Wu,a,b,c Zhuangqing Wang,a,b,c Qiang Liu,a,c Ke Liu,a,c Weiwei Lin,a,c Haiyang
Cheng, a,c and Fengyu Zhao*a,c
aState Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
bSchool of Applied Chemistry and Engineering, University of Science and
Technology of China, Hefei, Anhui 230026, PR China
cJilin Province Key Laboratory of Green Chemistry and Process, Changchun Institute
of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
*Corresponding author, Tel: +86-431-85262410; Fax: +86-431-85262410;
E-mail address: czhang@ciac.ac.cn, zhaofy@ciac.ac.cn
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ABSTRACT
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A Pt/TiH2 catalyst having hydrogen storage and release capability was investigated
for selective hydrogenation of trans-cinnamaldehyde (CAL) to cinnamyl alcohol
(COL) with gaseous dihydrogen. The catalytic behavior of this catalyst was
significantly different from that of a reference Pt/TiO2 catalyst with respect to the
product selectivity and the hydrogenation mechanism. The Pt/TiH2 catalyst showed a
COL selectivity of 97% at a CAL conversion of 98%, which was ascribed to the
function of Pt crystallite - support boundary layer that caused the preferential
adsorption of CAL with its carbonyl group. Furthermore, the carbonyl group was
hydrogenated by hydride species (H+, H-) supplied from the support and the hydride
species consumed were compensated from gaseous dihydrogen; hydrogen atoms were
formed by ordinary homolytic cleavage on Pt and then these hydrogen atoms moved
onto the surface of TiH2 and diffused into the bulk of the support, during which those
simultaneously changed to hydride species (H+, H-) via electron transfer with titanium
species and hydride vacancies therein. The surface and bulk diffusion of the hydrogen
atoms from Pt to TiH2 support should be dominant step rather than their addition to
the carbonyl group of CAL (ordinary hydrogenation). That is, ionic hydrogenation
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occurs with Pt/TiH2 in the presence of gaseous dihydrogen.
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KEYWORDS: Hydrogenation, TiH2, hydrogen storage materials, H-D exchange,
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heterolytic, trans-cinnamaldehyde
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1. INTRODUCTION
Hydrogenation reactions are a major contributor to current chemical processes
(6%), for the upgrading of crude oils and the production of commodity, fine
chemicals, pharmaceuticals, jet fuel and so on.1 Gaseous hydrogen is a general
reducing agent in practical hydrogenation processes and its activation can occur
through homolytic or heterolytic cleavage. Homolytic cleavage of H2 always takes
place easily over a metal catalyst, which has been extensively studied and used in
hydrogenation reactions.2 Heterolytic activation of H2 is highly favored and desired
for the selective hydrogenation of polar groups (for example, carbonyl groups) than
non-polar groups (for example, carbon-carbon multiple bonds) and in some
semi-hydrogenation reactions.3 Heterolytic cleavage of H2 has been achieved with
homogeneous catalysts such as Frustrated Lewis Pairs; however, it is still a big
challenge for heterogeneous catalysts1e, 3b, 4. Although heterolytic activation of H2
requires more energy (ΔH0 = 17.35 eV) to produce H+ and H-, as compared to the
homolytic activation of H2 (ΔH0 = 4.5 eV) to produce 2H·, it is not impossible with
some heterogeneous catalysts.4c, 4d, 5 Previous studies have obtained some interesting
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and useful information on heterolytic cleavage of H2 on metal oxide catalysts.3a, 6
However, there are still important, open questions worthy of further investigation that
have been attracting interests of researchers in the related fields: How are heterolytic
hydrogen species generated? What are exact active centers? Does the heterolytic
hydrogenation occur with conventional catalysts? It is of scientific significance,
therefore, to explore new heterogeneous catalysts that can efficiently generate
heterolytically cleaved hydrogen and promote the desired selective hydrogenation and
to investigate reaction mechanisms of these processes/steps.
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In the past few decades, hydrogen storage materials have received much attention
due to the green and safe generation of hydrogen.7 Hydrogen storage materials show
unique properties of storage and release of hydrogen. In most cases, the stored
hydrogen is not electronically neutral but negatively charged and the released
hydrogen could be highly active. Hydrogenation reactions would occur in the absence
of additional hydrogen sources if the stored hydrogen could be released and available
for the reactions, for which the effective hydrogen storage and reusability of these
materials should carefully be considered. It is interesting to know and examine
difference in the catalytic actions between hydrogen species (hydrogen radicals)
formed via homolysis of dihydrogen and hydrogen species (charged hydrogen
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species) released from the hydrogen storage materials. Unfortunately, there are very
few reports concerned with such an important and interesting issue of heterogeneous
catalysis in the literature.
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Considering the above-mentioned circumstances and taking notice of its hydrogen
storage properties, the authors have selected a material of TiH2 as a catalyst support
and studied the activity of TiH2 - supported Pt catalysts for a model reaction of
liquid-phase selective hydrogenation of trans-cinnamaldehyde (CAL) to cinnamyl
alcohol (COL). TiH2 is one of interesting hydrogen storage materials, which can
uptake hydrogen of more than 4% in weight (about 20 mmol dihydrogen per 1 g). In
the present work, various Pt/TiH2 catalysts were prepared under different reduction
conditions and tested for the selective hydrogenation of CAL. It should be noted here
that a Pt/TiH2 catalyst reduced at a medium temperature (150 ºC) is highly selective to
the hydrogenation of C=O bond with an excellent selectivity of 97% to COL, in
contrast to low COL selectivity values with other Pt/TiH2 catalysts reduced at higher
temperatures and a reference Pt/TiO2 catalyst. In order to examine the functions of
TiH2 and Pt, the structures and properties of the catalysts prepared were examined by
different methods including TPR, TPD, XPS, in-situ DRIFT, TEM, and GC-MS.
Furthermore, isotope experiments were also conducted, in which CAL hydrogenation
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was tested with D2 over Pt/TiH2 catalysts containing no and some amount of D
species included by reduction in D2. On the basis of those reaction and
characterization results, it is confirmed that 1) the perimeter of Pt nanoparticles and
TiH2 support is the active center; 2) the stored hydrogen species contribute to the
heterolytic hydrogenation with H+ and H-; 3) the TiH2 support plays a role like
“Frustrated Lewis Pair” to stabilize and separate the H+ and H-. To our best
knowledge, it is the first report on heterolytic hydrogenation reactions over supported
Pt catalysts with hydrogen storage TiH2 as a carrier. It is expected that this study can
provide useful knowledge for the design of a new type of active supported metal
catalysts featuring the hydrogen storage capability and the production of heterolytic
hydrogen species for selective hydrogenation and other reactions.
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2. EXPERIMENTTAL
2.1 Materials. Commercially available reagents, trans-cinnamaldehyde (CAL)
(Beijing JK chemical; >99.0%), H2PtCl6·6H2O (Jilin Haodi; ≥ 99.9%), TiH2
(Aladdin, >99.0%), titanium (IV) oxide, TiO2 (P25) (Acros), isopropanol, toluene and
cyclohexane (Beijing chemicals; ≥ 99.9 %) were used as received.
2.2 Catalysts Preparation. Onto TiH2 supports, Pt was loaded in different contents
of Pt (0.1, 1.0 and 5.0 wt.-%) by incipient wetness method. H2PtCl6·6H2O was
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dissolved in water and then TiH2 support was added to form a slurry mixture. The
slurry was dried at 50 ºC in a water bath while stirring and then kept at 50 ºC in an
oven overnight. Prior to usage, the Pt-loaded samples were reduced under pure H2
flow at temperatures, T, of 150 ºC, 350 ºC and 450 ºC for 2 h. The catalysts are
referred to as (Pt/TiH2)-TH2 in the following. To vary the structural properties of
support, the following treatments were made for parent TiH2 sample. It was treated
with H2 at 150 ºC or 450 ºC for 2 h and then with N2 at decreasing temperatures for
0.5 h. The treated support is referred to as TiH2-150H2 or TiH2-450H2. For
comparison, a commercial TiO2 (P25) was also used as a support for Pt.
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2.3 Catalyst Characterization. The TiH2 supports and Pt loaded catalysts were
characterized by the following methods. X-ray photoelectron spectroscopy (XPS)
measurements without sputtering were carried on VG Microtech 3000 Multilab and
those with sputtering on ESCALAB 250Xi, ThermoScientific, in which the sample
was sputtered by Ar to a depth of 3 nm. X-ray powder diffraction (XRD)
measurements were carried on a Bruker D8 ADVANCE diffractometer, using Cu Kα
radiation (k = 0.154 nm). Transmission electron microscopy (TEM) images were
collected on a JEOL JEM-2010 operated at an accelerating voltage of 200 kV. The
size and metal dispersion of Pt nanoparticles were also examined by CO pulse
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chemisorption at 50 ºC with a pulse of CO (10%) in He on Micromeritics AutoChem
II 2920. Temperature programmed reduction (TPR) profiles were measured on
Micromeritics AutoChem II 2920. For TPR, a sample (100 mg) was treated with Ar at
100 ºC for 0.5 h to remove H2O adsorbed and cooled to 50 ºC. Then, the sample was
heated at 5 ºC/min to 900 ºC in a stream of H2 (10%) in Ar mixture at a flow rate of
50 mL/min. The amount of H2 consumed was measured by a thermal conductivity
detector (TCD). H2 release profiles were collected on Micromeritics AutoChem II
2920. A sample (100 mg) was reduced with H2 (10%) in Ar at a certain temperature
for 0.5 h and cooled to 50 ºC. Then, the sample was heated at 50 ºC/min to 900 ºC in a
stream of Ar at a flow rate of 50 mL/min and the amounts of species evolved were
measure by TCD. H2 release profiles was also made on QIC-20, Hiden, UK, with a
mass spectrometer using a sample of 50 mg, which was pretreated in the same manner
as mentioned above. The adsorption of the substrate, CAL, was examined by in situ
diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) on Thermo
Scientific Nicolet 6700 Fourier transform infrared spectroscopy. A catalyst sample
was reduced at 150 ºC for 30 min in a DRIFT cell with 10% H2/Ar mixture (30
mL/min) and then cooled to 30 ºC. The cell was purged with He (30 mL/min) for 30
min and the background spectrum was recorded. After 40 μL substrate (0.5 M in
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isopropanol) was introduced to the sample, the temperature of the cell was raised to
150 ºC slowly while passing He (30 mL/min). The spectrum was recorded at 150 ºC
every 30 min until all the physically adsorbed species were removed.
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2.4 Selective Hydrogenation. The catalytic performance of Pt on TiH2 supports
was tested for the selective hydrogenation of CAL using a 50 mL stainless steel
autoclave with an inner Teflon coating. For a typical catalytic reaction, 1.0 mmol
CAL and 50 mg catalyst were mixed in 5 mL of an organic solvent in the reactor. It
was purged with 1 MPa H2 three times and then sealed. The reaction was conducted at
4 MPa H2 and at 100 ºC. The products were analyzed by a gas chromatograph
(Shimadzu, 2010) equipped with a capillary column (Restek-50 30 m × 0.25 mm ×
0.25 μm) and a flame ionization detector (FID). Conversion was determined by
dividing the amount of CAL consumed by the initial amount of CAL; product
selectivity was calculated by dividing the amount of a certain product by the amount
of CAL consumed. Site-time-yield was calculated by dividing the amount of a certain
product by the amount of exposed Pt atoms determined by CO pulse chemisorption
and reaction time. The recycling test was performed as following: The reacted catalyst
was separated by centrifugations and washed three times by isopropanol. The catalyst
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was then used in the next recycling test, and the reaction conditions were the same
each time.
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3. RESULTS AND DISCUSSION
The catalytic performance of Pt/TiH2 materials was examined using a model
reaction of selective hydrogenation of trans-cinnamaldehyde (CAL) to cinnamly
alcohol (COL) (Scheme 1), in which COL and hydrocinnamaldehyde (HCAL) are
produced via hydrogenation of C=O and C=C, respectively, with hydrocinnamyl
alcohol (HCOL) as the final hydrogenation product. In some cases, condensation
products of organic solvents and CAL are formed. The performance of Pt/TiH2
catalysts was tested in CAL hydrogenation with H2. Further, the CAL hydrogenation
was also conducted with D2 over Pt/TiH2 catalysts to examine their catalytic features.
Then the physicochemical properties of these catalysts were examined by several
techniques to discuss the catalysis of selective CAL hydrogenation to COL on a new
kind of Pt/TiH2 catalysts.
Scheme 1. Reaction pathways for hydrogenation of CAL.
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3.1 Selective Hydrogenation with H2. Table 1 shows the results of CAL
hydrogenation with TiH2 and Pt/TiH2 samples. The supports alone, untreated and
treated in H2 at 150 ºC, were marginally active and the COL selectivity was small
with appreciable amounts of other condensation products from CAL and the solvent,
isopropanol (entries 1, 2). The loading of Pt in 1.0 wt.-% to TiH2 increased the
conversion to 67% from 6% in 3 h and the COL selectivity to 52% from 34% but the
condensation products were also produced (entry 3). When the Pt/TiH2 sample was
reduced by H2 at 150 ºC, the conversion was further increased to 83% and, more
interestingly, a high COL selectivity of 97% was obtained with even smaller amounts
of HCAL, HCOL, and the condensation products (entry 4). When the reaction was
conducted in N2 instead of H2, only a small amount of CAL was consumed and a
(relatively) large amount of byproducts was detected. That is, almost no
hydrogenation reaction occurred in N2 (entries 5, 6). The time profiles of total
conversion and product selectivity with the highly active (1%Pt/TiH2)-150H2 catalyst
is given in Supporting Information (SI, Figure S1).
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The influence of Pt loading on the performance of Pt/TiH2 catalyst was examined
with 0.1, 1.0, and 5.0 wt.-% Pt samples reduced at 150 ºC. As shown in Table S1, the
CAL conversion was found to decrease with the Pt loading, in particular, with 5.0
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wt.-% Pt. However, the high COL selectivity remained unchanged, being > 90% for
those catalysts examined.
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Table 1. Results of CAL hydrogenation over different catalysts.
Entry
Catalyst
Time Conversion
Selectivity (%)
(h)
(%)
COL HCAL HCOL Othersa
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2
3
4
5
6
TiH2
3
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3
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6
3
34
22
52
97
2
5
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1
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0
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6
1
5
7
54
65
35
1
TiH2-150H2
1.0%Pt/TiH2
67
83
3
(1.0%Pt/TiH2)-150H2
b
TiH2
92
92
b
(1.0%Pt/TiH2)-150H2
3
1
Reaction conditions: 50 mg catalyst, 1 mmol CAL, 5 mL isopropanol, 100 ºC, 4
MPa H2.
a. Products from the condensation of CAL and isopropanol.
b. 4 MPa N2 was used instead of H2.
Table 2 shows the results of 1.0 wt.-% Pt/TiH2 reduced at different temperatures.
The catalytic activity markedly decreased with an increase in the reduction
temperature; the CAL conversion was 98% in 4 h, 11% in 6 h, and 18% in 24 h for
the catalysts reduced at 150 ºC, 350 ºC, and 450 ºC, respectively (entries 1-3). The
catalyst reduced at 350 ºC still maintained a high COL selectivity of 88% but that at
450 ºC had a lower selectivity of 78% with the formation of condensation products in
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20%. In addition, the influence of TiH2 pretreatment with H2 was examined, in which
the reduction of Pt was conducted at 150 ºC. The TiH2 pretreatment at a higher
temperature of 450 ºC decreased the CAL conversion but little altered the high COL
selectivity (entry 5). All the Pt/TiH2 catalysts reduced at 150, 350 and 450 ºC showed
large C=O hydrogenation selectivity values. There is a difference in the COL
selectivity between the catalysts reduced at 350 ºC and 450 ºC (entries 2, 3), which is
due to the production of more condensation products for the latter catalyst. The sizes
of supported Pt particles were different, being 2.0, 4.4, 5.1 nm for those reduced at
150, 350 and 450 ºC, respectively (Table S2), and there were only slight differences in
the XPS Pt 4f binding energy (Figure S2). Therefore, the surface properties of Pt
particles should not be crucial for the high selective hydrogenation of C=O bond of
CAL over the Pt/TiH2 catalysts. For comparison, the activity of 1.0 wt.-% Pt/TiO2
(P25) was tested under the same conditions. The catalyst reduced at 150 ºC was more
active than the Pt/TiH2 reduced at the same temperature (Table 1 entry 4, Table 2
entry 6) but it was less selective to the COL formation (22% selectivity) with a larger
amount of condensation products. When the reduction was conducted at a higher
temperature of 450 ºC, the Pt/TiO2 catalyst became more selective to the
hydrogenation of C=O giving a COL selectivity of 77%, which was still smaller than
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97% of the Pt/TiH2 catalyst. The relatively high COL selectivity of the 450 ºC -
reduced catalyst can be interpreted by strong metal - support interaction (SMSI) via
electron transfer from the support to the Pt particles. Such a (partially) negatively
charged Pt surface would facilitate the adsorption of C=O bond rather than C=C bond,
promoting the selective hydrogenation of the former.8 For the two Pt catalysts on TiH2
and TiO2 supports reduced at a lower temperature of 150 ºC, in which the sizes of Pt
particles were comparable (about 2 nm) as seen in Figure S3, the configuration of
CAL adsorption should be different between the two catalysts.
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Table 2. Results of hydrogenation of CAL over Pt/TiH2 and Pt/TiO2 catalysts reduced
at different temperatures.
Entry
Catalyst
Time
(h)
Conversion
(%)
Selectivity (%)
COL
97
HCAL HCOL Othersa
1
2
3
4
5
6
7
(1.0%Pt/TiH2)-150H2
(1.0%Pt/TiH2)-350H2
4
6
98
11
18
99
67
93
95
0
4
2
0
3
6
2
1
0
2
8
88
(1.0%Pt/TiH2)-450H2
24
4
78
0
20
0
(1.0%Pt/(TiH2-150H2))-150H2
(1.0%Pt/(TiH2-450H2))-150H2
(1.0%Pt/TiO2)-150H2
97
3
4
94
2
1
3
22
6
66
0
(1.0%Pt/TiO2)-450H2
3
77
21
Reaction conditions: 50 mg catalyst, 1 mmol CAL, 5 mL isopropanol, 100 ºC, 4
MPa H2.
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a. Products from the condensation of CAL and isopropanol.
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Figure 1. H2 pressure dependence in hydrogenation of CAL over (Pt/TiH2)-150H2.
Reaction conditions: 25 mg catalyst, 1mmol CAL, 5 mL isopropanol, 100 ºC, 2, 4, 6
and 8 MPa H2. The TOF was calculated at the conversion values of 6%, 8%, 8% and
8%, at different H2 pressures.
Further, the influence of H2 pressure was examined for CAL hydrogenation using
the most active catalyst of (1.0%Pt/TiH2)-150H2. Figure 1 shows the results collected
at H2 pressures of 2 - 8 MPa. A linear relationship was found to exist in the
logarithmic plot of turnover frequency (TOF) against H2 pressure and, interestingly,
its slope was 1.18. This value was significantly different from the result with the
reference catalyst of (1.0%Pt/TiO2)-150H2, for which a slope of 0.61 was observed
(Figure S4) under the same reaction conditions. As indicated by Tomishige et al. in
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the hydrogenation of crotonaldehyde,3a dicarboxylic acids9 and ethers10, a reaction
order of around 1 means that hydride (H-) is more likely involved in the
rate-determining step. Thus, Figure 1 suggests that the present hydrogenation of CAL
over Pt/TiH2 catalyst is unlikely to occur via ordinary reaction processes that H2 is
dissociatively adsorbed on Pt followed by the addition of H species to the unsaturated
bonds of the adsorbed CAL molecules. Detailed mechanisms of the catalysis over the
Pt/TiH2 will be discussed later.
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3.2 Selective Hydrogenation with D2. To obtain further information about the
catalysis of a new type of catalyst of Pt/TiH2, D2 was used, instead of H2, for the CAL
hydrogenation and the catalyst reduction (Table 3 and Table S3). The average sizes of
Pt crystallites in the catalysts reduced in H2 and D2 were observed to be comparable
(both were about 2.0 nm, Table S2). The two catalysts showed comparable initial
TOF values in H2, but quite different ones in D2 (Table S3). Under the reaction
conditions used, the GC-MS analysis detected the formation of four kinds of COL
molecules different in the molecular weight (MW = 134, 135, or 136), indicating
COL formed via the addition of two H atoms to CAL, one D atom and one H atom, or
two D atoms, respectively. Noteworthy, a relative higher conversion is necessary for
the accurate analysis of the molecular weight of COL, and it was impossible to
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estimate the distribution of COL molecules (MW = 134, 135, 136) by the present
analysis. Table 3 gives important and interesting pieces of information: (1) the COL
selectivity is high in D2 as well as in H2 (entries 1 - 5), (2) COL molecule including D
atom is formed in H2 even when the catalyst is pre-reduced in D2 (entry 3), similar to
the reaction in D2 with the catalyst pre-reduced in H2 (entry 2), (3) the rate of COL
formation (STYCOL and TOF) is smaller for the reaction in D2 with the catalyst
reduced in H2 (entry 2) compared with the other reactions. The STYCOL(TOF) values
are comparable for the reactions with the (Pt/TiH2)-150H2 in H2 (entry 1) and those
with the (Pt/TiH2)-150D2 in H2 (entry 3) and in D2 (entry 4). Note again that the D
species in the support were included in the COL molecules and the influence of
gaseous D2 appeared on the rate of reaction only in the case of the catalyst reduced in
H2, in which D species are absent in the support. Those results indicate that the
present CAL hydrogenation over the Pt/TiH2 catalysts occurs via ionic hydrogenation.
Besides, the diffusion of H/D species in TiH2 is crucial for the reaction rate. In
addition, an additional interesting fact was observed: (4) when cyclohexane was used
as a solvent instead of isopropanol, COL (MW = 136, product D) and COL (MW =
135, product C) were formed but these were not detected in the reactions in
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isopropanol. It is probable that there occurred fast H-D exchange between isopropanol
and COL molecules, resulting in no COL products C and D detected in isopropanol.
Table 3. Results of CAL hydrogenation with 1%Pt/TiH2 catalysts using H2 and D2 for
CAL hydrogenation and catalyst reduction.
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H
D
H
D
OH
OH
OD
OD
O
H2 or D2
A
C
D
B
(MW = 134)
(MW = 135)
(MW = 136)
(MW = 135)
c
Entry
1
Catalyst
Conversion
(%)
Time
(h)
1
Reaction
gas
COLa
Ab
Bb
X
Cb Db STYCOL
(h-1)
(%)
(1.0%Pt/TiH2)
-150H2
49
H2
93.3
○
X
X
343
2
3
68
30
4
D2
H2
86.6
88.7
○
○
○
○
X
X
X
X
110
399
(1.0%Pt/TiH2)
-150D2
0.5
4
55
31
1
2
D2
D2
95.2
96.5
○
○
○
○
X
X
392
112
5d
○
○
Reaction conditions: 50 mg catalyst, 1 mmol CAL, 5 mL isopropanol, 100 ºC, 4
MPa H2 or D2.
a. Total COL selectivity.
b. “○” and “X” represent that the product was and was not detected, respectively.
c. STY = mole of COL produced per mole of exposed Pt atoms per 1 h. (The
amount of exposed Pt atoms was determined by CO pulse chemisorption)
d. Cyclohexane was used instead of isopropanol. Reaction conditions: 50 mg
catalyst, 1 mmol CAL, 10 mL cyclohexane, 100 ºC, 4 MPa D2.
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3.3 Catalyst Characterization. The structure of TiH2 was examined by XRD. The
XRD patterns of TiH2 treated in H2 at different temperatures of 150, 350, 450 ºC are
given in SI (Figure S5). The diffraction peaks at 2θ = 35.0, 40.6, 58.8, 70.2 and 73.8º
are respectively indexed to the (111), (200), (220), (311), and (222) planes of cubic
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TiH2 (JCPDF:25-0982). The structure did not change by the treatment in H2 at 350
ºC or below but that at a higher temperature of 450 ºC caused the phase change to
some extent from cubic (TiH1.92) to tetragonal phase (TiH1.92). The XRD patterns for
1.0 wt.-% Pt loaded samples were identical with those of the supports alone,
indicating that the support structure did not change on the loading of Pt by incipient
wetness and the subsequent reduction (Figure S6). Pt was dispersed on the supports in
the form of small nanoparticles with very weak XRD diffraction peaks.
Then, the H2 reduction behavior of as-prepared samples with and without Pt was
examined. Figure 2 shows two H2 consumption peaks at 450 ºC and 576 ºC for both
TiH2 and 1.0 wt.-% Pt/TiH2 samples. The former and latter H2 consumption peaks
should correspond to the hydrogen storage by -TiHx and -Ti phases, respectively.12
Either the amount of H2 consumed in the low-temperature region or that in the
high-temperature region was the same between the two samples. It is interesting to
note, therefore, that no H2 consumption due to the reduction of Pt precursors to Pt was
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observed. The Pt reduction should be already completed during the catalyst
preparation, for which the support is likely to contribute to the reduction.
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Figure 2. H2-TPR profiles of Pt/TiH2 catalyst and TiH2 support.
In addition, the thermal behavior of TiH2 and Pt/TiH2 samples reduced at different
temperatures was examined, in which a sample was heated in Ar at 50 mL/min and at
a heating rate of 10 ºC/min and the amount of H2 released was measured. Figure 3
shows that H2 is released at two temperature regions, 350 - 450 ºC and 450 - 550 ºC,
which should correspond to the dehydrogenation of TiH2 to -TiHx phase and -TiHx
to α-Ti phase, respectively. That is, hydrogen species stored in the support are
releasing out under the heat treatment. It should be noted that the hydrogen release in
the low-temperature region moved to much lower temperature in the presence of Pt
for the catalyst reduced at 150 ºC; the metal facilitates the release of hydrogen from
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the support.13 For the samples reduced at higher temperature (350 and 450 ºC),
however, such an effect of Pt was not observed.
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Figure 3. H2 release profiles of Pt/TiH2 catalysts and TiH2 supports reduced at
different temperatures during heating in an inert atmosphere measured by TCD.
The thermal gas releasing behavior was also examined for 1.0 wt.-% Pt loaded
catalyst that was reduced in H2 and then used in CAL hydrogenation in D2. Figure 4
gives the results of fresh and used catalysts, in which H2, H2O, and/or D2O were
detected. The release of H2 was observed to occur in two temperature regions, which
were maximized at 430 ºC and 510 ºC. The former is due to the hydrogen release
from -TiHx while the latter from -Ti, as described above.12 For H2O, it was released
in a small quantity at about 110 ºC, due to desorption of adsorbed water, and in a
larger quantity at higher temperatures with two peaks at about 435 ºC and 515 ºC. The
H2O releasing at high temperatures should be related to the decomposition of TiOxHy
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composite on the surface of the TiH2 support. Note that although the Pt/TiH2 sample
did not contain D species, two D2O peaks (merged) at around 90 ºC and 350 ºC were
also detected after the hydrogenation in D2, during which D species were therefore
taken in the catalyst (support). For the catalyst used in the CAL hydrogenation in H2,
H species are also likely to be included. These results indicate that the storage and
release of hydrogen species occur for the Pt/TiH2 catalyst; the results of Figure 4
demonstrate that hydrogen species move between the gas (H2, D2) and solid (catalyst)
phases. From the results of CAL hydrogenation using D2 and H2 (Table 3 and Table
S3) and catalyst characterization (Figures 3 and 4), it can be assumed that, for the
present three-phase reaction system, the transfer of hydrogen species takes place
between the gas and liquid (solvent) phases (dissolution) and between the liquid and
solid (catalyst) phases (storage and release). This hydrogen transfer will further be
discussed later.
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Figure 4. H2, H2O and D2O release from (1%Pt/TiH2)-150H2: (a) fresh and (b) used
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in CAL hydrogenation in D2 during heating in an inert atmosphere measured by MS.
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The surface of Pt/TiH2 was further examined by XPS with respect to the chemical
state of Ti and O species. Figure S7 shows the results of XPS Ti 2p of the TiH2
support and a series of Pt/TiH2 catalysts. Various Ti species such as Ti2+, Ti3+, and
Ti4+ were detected; the peaks at 455, 457, 458 and 458.8 eV are attributable to the Ti2+,
Ti3+, Ti3O5 and Ti4+, respectively.11c, 14 Ti4+ species were dominant on Pt/TiH2 (Figure
S7b) surface and no Ti2+ was detected. When the sample was Ar sputtered by a depth
of 3 nm, the relative amounts of Ti3+ and Ti3O5 increased, meaning that the chemical
state of Ti species was quite different between the surface thin layer and the inner
bulk phase, as shown in Figure S7d.14b Figure 5 gives the results of XPS O 1s,
indicating the presence of three types of O species. It is assumed that one is related to
the oxygen in Ti-O-Ti lattice (labeled as type A), one to either that near defect sites or
Ti-OH (labeled as type B), and the other to chemically adsorbed oxygen-containing
species (such as H2O, labeled as type C).14a Note that the percentage of the “type B”
O species (19%) was larger for Pt/TiH2 catalysts, as compared with the reference
Pt/TiO2 catalyst (9%).
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Figure 5. O1s XPS profiles of (1% Pt/TiH2)-150H2 (a) and (1%Pt/TiO2)-150H2 (b).
Finally, the adsorption of CAL substrate on TiH2 alone and the most active catalyst
of (1.0% Pt/TiH2)-150H2 was examined by in-situ DRIFT measurements. Figure 6
gives the DRIFT spectra collected (the assignment of absorption bands observed are
given in Table S4). The absorption band at 1673 cm-1 can be indexed to stretching
vibration of C=O group adsorbed physically on the samples, which was detected for
all the four samples before the treatment with He. When the supports were treated by
He gas at 150 ºC for 200 min, the absorption band at 1673 cm-1 almost disappeared
for TiO2 (P25) but two absorption bands remained at 1647 cm-1 and 1614 cm-1 for
TiH2, which were due to chemisorption of C=O and C=C bonds, respectively, and the
latter was stronger than the former. Namely, the support of TiH2 itself is likely to
adsorb CAL molecules strongly in contrast to TiO2. On the treatment with He at a
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higher temperature of 250 ºC, the two chemical absorption bands disappeared over
TiH2, indicating all the CAL molecules were removed from the TiH2. When Pt was
loaded onto TiO2, the absorption bands in the range of 1700 - 1600 cm-1 did not
change significantly on the He treatment; from the presence of the chemisorption
bands due to C=O and C=C groups, CAL was adsorbed with its C=C and C=O groups.
For the He treated (Pt/TiH2)-150H2 catalyst, however, the absorption band at 1614
cm-1 was observed but not that at 1647 cm-1. Those results indicate interesting
differences in the adsorption of the substrate CAL between Pt/TiH2 and TiH2 and
between Pt/TiH2 and Pt/TiO2 catalysts. For Pt/TiH2, the coexistence of TiH2 and Pt
crystallites is significant for the CAL adsorption and should produce adsorption sites
for CAL molecules at TiH2 - Pt crystallite boundary. Probably, the adsorption of CAL
occurs with its C=O group on the boundary (discussed later) and the unique
adsorption is so strong (C=O bond becomes weakened) that the absorption band due
to C=O cannot be detected. When the treatment with He was conducted at a higher
temperature of 250 ºC, two absorption bands were again observed at 1647 cm-1 and
1614 cm-1 (Figure 6b). This should result from a change of the unique adsorption of
CAL on the TiH2 - Pt crystallite boundary at higher temperature.
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Figure 6. In-situ DRIFT absorption spectra of CAL over TiH2 (a),
(1.0%Pt/TiH2)-150H2 (b), TiO2 (c), and (1.0%Pt/TiO2)-150H2 (d) before and after the
treatment with He at 150 ºC. For (a) and (b), the data after the He treatment at 250 ºC
is also presented.
3.4 Mechanism Consideration. As reported previously, various Ti species such as
TiO2, Ti2O3, TiO, Ti, etc. can exist on the surface of TiH2.14 Besides, oxyhydrides of
TiOxHy could form on heating, and some of them had the same crystal structure and
lattice parameters as TiH2.15 On the basis of the above-mentioned characterization
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results, the TiH2 support should be in the structure of a thin layer of TiOxHy covering
the bulk TiH2 as illustrated in Figure 7. The TiOxHy layer should be formed on the
surface of TiH2 during reduction at 150 ºC, in which various types of Ti species
coexisted. When Pt was loaded onto such a support and reduced, it was dispersed in
the form of nanoparticles of about 2 nm in size. When the reduced Pt/TiH2 catalyst is
exposed to gaseous H2, hydrogen may be stored by the support; ordinary homolytic
cleavage of H2 takes place over the surface of Pt crystallites (Eq. 1). Then, resulting H
species spillover on the TiOxHy surface via electron transfer with Tiδ+ and Ti(δ+1)+
species (Eq. 2 - 4). At the same time, the H species can also penetrate into the vacancy
of TiH2 lattices and be stored thereby in the form of H-. Once H enters into TiH2
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+ 16
lattice, it will form an H- and an H vacancy with positive charge (VH ) (Eq. 5). Both
H+ and H- in the support have migration characteristics: The H+ can spillover on and
diffuse within the TiOxHy layer and the H- can diffuse in the TiH2 by contact with
+
adjacent VH , regenerating H (Eq. 6, 7). Also, the H- can migrate to the surface of
TiOxHy layer.
On Pt:
H2→ 2H
(1)
On/within thin TiOxHy layer:
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H + Ti(δ+1)+→ H+ + Tiδ+
Tiδ+→ Ti(δ+1)++ e-
(2)
(3)
(4)
9
H++ e-→ H
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Within bulk TiH2:
TiH2 + H → TiH2(H-) + VH
(5)
(6)
(7)
+
TiH2(H-) → TiH2 + H-
+
H- + VH → H
Figure 7. Possible reaction mechanism of hydrogenation of CAL over
(Pt/TiH2)-150H2 catalyst. This is a simplified illustration: H+ and H- species are
formed and stored in the thin TiOxHy layer and the TiH2 bulk, respectively. H+ and H-
will attack O and C atoms of the carbonyl group of CAL adsorbed on the Pt crystallite
- support boundary.
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ACS Catalysis
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The FTIR results suggest that CAL molecule is adsorbed on the boundary of TiH2
support and Pt particles with its C=O bond (Figure 6). The surface of TiH2 exposes
the type B oxygen, oxygen species close to defects (oxygen vacancy) and of Ti-OH,
as indicated by XPS (Figure 5). We believe that oxygen-free defect sites and/or
Ti-OH sites serve as sites for the adsorption of the substrate molecule; O atom of a
CAL molecule interacts with the defect site or H atom, which causes the CAL
adsorption with its C=O bond preferentially than C=C bond. The strength of C=O
bond is weak as demonstrated by FTIR and so the O atom of CAL should more likely
interact with the defect sites than Ti-OH. The surface of Pt particles might interact
with the C atom of CAL molecule and contribute the configuration of CAL molecules
adsorbed on the catalyst. Thus, such preferential adsorption of CAL with its C=O
bond is crucial for its selective hydrogenation to COL observed with the Pt/TiH2
catalyst (Figure 7).
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On the basis of the results of hydrogenation and catalyst characterization as
above-mentioned, the present CAL hydrogenation over Pt/TiH2 catalyst is likely to
occur via unique processes different from ordinary ones through the dissociative
adsorption (homolytic cleavage) of H2 followed by the attack of the H atoms formed
to a substrate adsorbed on a catalyst. In the present case, the hydrogen species to be
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