Selective Hydrogenolysis of Glycerol to 1,3-Propanediol over Rhenium-Oxide-Modified Iridium Nanoparticles Coating Rutile Titania Support

Article abstract of DOI:10.1021/acscatal.9b03824

The effect of support in Ir-ReO_x catalysts for glycerol hydrogenolysis to 1,3-propanediol was investigated. Rutile TiO₂ support showed high activity, even higher than previously reported SiO₂ support. Anatase TiO₂, C, ZrO₂, CeO₂, Al₂O₃, and MgO supports showed very low activity of supported Ir-ReO_x pairs. Higher Ir-based 1,3-propanediol productivity of Ir-ReO_x/rutile catalyst was obtained at the initial stage even with lower Re/Ir ratio (typical Ir loading amount, 4 wt %, nominal ratio of 0.25; actual ratio of 0.24) without addition of H₂SO₄ than that of Ir-ReO_x/SiO₂. The 1,3-propanediol productivity over Ir-ReO_x catalysts showed dependency on catalyst compositions (metal loading amount), and the relationship between catalyst structure and activity was further established over Ir-ReO_x/rutile. Relatively high Ir loading amount in comparison with small surface area (6 wt %, on 6 m² g^-1 rutile TiO₂) showed the highest activity (Ir-based activity). From combined characterization results altogether (TPR, TEM, XPS, XAS, CO adsorption, CO FT-IR) with a kinetics study, the Ir metal particles interacted with the partially oxidized ReO_x cluster (average valence of Re: +3) almost totally covering the surface of rutile TiO₂ particles, and the active site was the Ir-ReO_x interface. Small amounts of Ir species were incompletely reduced; however, such IrO_x species as well as rutile TiO₂ support were not directly involved in glycerol hydrogenolysis. The role of rutile support was regarded as providing a unique environment for stabilization of uniform and small Ir-ReO_x particles with very high surface density on rutile TiO₂, which increased the number of active sites per Re amount.

Full text of DOI:10.1021/acscatal.9b03824

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Article
Selective hydrogenolysis of glycerol to 1,3-propanediol over rhenium
oxide-modified iridium nanoparticles coating rutile titania support
Lujie Liu, Takehiro Asano, Yoshinao Nakagawa, Masazumi Tamura, Kazu Okumura, and Keiichi Tomishige
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b03824 • Publication Date (Web): 21 Oct 2019
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Selective hydrogenolysis of glycerol to 1,3-propanediol over rhenium oxide-modified iridium
nanoparticles coating rutile titania support
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a
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a,b,*
Masazumi Tamura,a,b
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Lujie Liu, Takehiro Asano, Yoshinao Nakagawa,
c
a,b,*
Kazu Okumura and Keiichi Tomishige
a
Department of Applied Chemistry, School of Engineering, Tohoku University,
6
-6-07 Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
b
Research Center for Rare Metal and Green Innovation, Tohoku University,
468-1, Aoba, Aramaki, Aoba-ku, Sendai 980-0845, Japan
c
Department of Applied Chemistry, Faculty of Engineering, Kogakuin University,
665-1 Nakano-machi, Hachioji, Tokyo 192-0015, Japan
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*
Corresponding authors.
School of Engineering, Tohoku University,
6-6-07, Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
E-mail: yoshinao@erec.che.tohoku.ac.jp, Tel/Fax: +81-22-795-7215 (Y. N.);
tomi@erec.che.tohoku.ac.jp, Tel/Fax: +81-22-795-7214 (K. T.)
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Abstract
The effect of support in Ir-ReO catalysts for glycerol hydrogenolysis to 1,3-propanediol was
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investigated. Rutile TiO support showed high activity, even higher than previously reported SiO
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support. Anatase TiO , C, ZrO , CeO , Al O and MgO supports showed very low activity of supported
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Ir-ReO pairs. Higher Ir-based 1,3-propanediol productivity of Ir-ReO /Rutile catalyst was obtained at
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initial stage even with lower Re/Ir ratio (typical Ir loading amount: 4 wt%, nominal ratio of 0.25; actual
ratio of 0.24) without addition of H SO than that of Ir-ReO /SiO . The 1,3-propanediol productivity
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over Ir-ReO catalysts showed dependency on catalyst compositions (metal loading amount), and the
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relationship between catalyst structure and activity was further established over Ir-ReO /Rutile.
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-1
Relatively high Ir loading amount in comparison with small surface area (6 wt%, on 6 m g rutile
TiO ) showed the highest activity (Ir-based activity). Combined characterization results altogether
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(TPR, TEM, XPS, XAS, CO adsorption, CO FT-IR) with kinetics study, the Ir metal particles
interacted with partially oxidized ReO cluster (average valence of Re: +3) almost totally covered the
x
surface of rutile TiO particles, and the active site was the Ir-ReO interface. Small amount of Ir species
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x
(20%) were incompletely reduced; however such IrO species as well as rutile TiO support were not
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directly involved in glycerol hydrogenolysis. The role of rutile support was regarded as providing
unique environment for stabilization of uniform and small Ir-ReO particles with very high surface
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density on rutile TiO , which increased the number of active site per Re amount.
2
Keywords
Iridium, Rhenium, Glycerol hydrogenolysis, 1,3-Propanediol, Rutile titania, Support effect
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1
. Introduction
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Biomass is regarded as an alternative to fossil resources in fine chemicals synthesis, which has
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been extensively investigated by using heterogeneous catalyst systems in the past decade. Glycerol,
as a by-product in manufacturing biodiesel fuel by transesterification of triglycerides to fatty acid esters,
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is a platform compound for the production of value-added biomass-derived chemicals. Many effective
routes, such as dehydration,2b, 3 oxidation, and selective hydrogenolysis, have been proposed in
glycerol chemoselective conversion. The catalytic selective hydrogenolysis of glycerol to 1,3-
propanediol (1,3-PrD) has attracted much research interest because 1,3-PrD can be used as a monomer
to produce various polymers, including polytrimethylene terephthalate.2a
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To date, multicomponent-multifunctional noble metal catalysts have been examined for glycerol
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hydrogenolysis to 1,3-PrD, and Pt-WO based and Ir-ReO based
catalysts exhibit the most
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promising performance to obtain high 1,3-PrD yield in water solvent. Glycerol hydrogenolysis over
these typical catalysts proceeds under different reaction conditions. Recent reports have shown that
higher reaction temperature (413453 K) at H pressure of 18 MPa was required for improving 1,3-
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a, 6c-e, 8
PrD production over Pt-WO based catalysts,
in contrast to lower reaction temperature (393 K)
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at typical H pressure of 8 MPa over Ir-ReO /SiO catalyst.
Although the highest 1,3-PrD yield
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was obtained with Pt-WO based catalyst, the 1,3-PrD productivity (P = (Moles of glycerol ×
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Yield1,3-PrD × 76 g mol ) / (noble metal amount (g) × reaction time (h))) over Pt-WO based catalysts
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was clearly lower than that of SiO supported Ir-ReO based catalyst even when they are compared at
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different temperature optimized for each catalyst (Table S1). In the open literature, the highest 1,3-
PrD yield was 6669% over Pt/WO /AlOOH with P
of 2 g gPt-1 h-1,6a and the highest P1,3-PrD
1,3-PrD
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obtained among Pt-WO based catalysts was 5 g gPt-1 h over Pt-WO /t-ZrO with 49% 1,3-PrD yield
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catalyst gave the highest 1,3-PrD yield of 38% (81% conversion) over 4 wt%-Ir Ir-ReO /SiO (Re/Ir
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=
0.83, actual ratio) + H SO system at 393 K, whereas the highest P
was accomplished by high
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loading amount Ir-ReO /SiO (20 wt% Ir, Re/Ir = 0.34, actual molar ratio) catalyst with 22 (initial
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stage, 23% conversion) and 7 g gIr-1 h-1 at highest yield (32%, 69% conversion) without addition of
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sulfuric acid in our recent work. In the hydrogenolysis over Ir-ReO /SiO catalyst, we have proposed
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the direct hydrogenolysis mechanism,7a, 7b, 9b where Re-OH site binds the terminal OH group of
substrate and active hydrogen species with hydride like nature produced at the Ir atoms neighboring to
Re species attack the 2-position of the bound substrate. The role of sulfuric acid was serving as
-
+
stabilizing the adsorption site (Re-O + H  Re-OH) and suppressing the leaching of Ir and Re
metals7a-c or enhancing the interaction between glycerol and Ir-ReO /SiO catalyst rather than
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altering the reaction pathways. Noble metals are very expensive, and therefore enhancing the activity
based on noble metal is of great significance for practical application. Rhenium is also an expensive
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element, and the usage amount should be also decreased.
One of promising applications of Ir-ReO /SiO is selective conversion of a wide range of biomass-
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derived chemicals.7a-c, 11 We recently found that severe loss of Re species occurred on some Ir-
ReO /SiO catalysts during the calcination process for the preparation at high Re loading amount on
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silica support. Only about one third of total amount of Re precursor was actually loaded on SiO after
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calcination (Re: 6.7 wt%, actual amount; 19.4 wt%, assumed amount) over highly active 20 wt%-Ir Ir-
ReO /SiO (0.34, actual Re/Ir molar ratio; 1, nominal ratio) due to the sublimation of rhenium oxide
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at 773 K. Large amount of Re species was wasted, which results in economic issues by the use of
expensive Re precursor. Decreasing the calcination temperature definitely increased the actual loading
amount of Re on silica support, while the incomplete decomposition of Ir precursor decreased the
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activity significantly (ca. half). Another approach is to lower Re loading amount on silica support to
suppress the Re O sublimation. The 20 wt%-Ir Ir-ReO /SiO (Re/Ir = 0.25, nominal) gave almost the
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same actual ratio as the nominal one. However, the activity was much decreased by decrease of Re
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amount, especially for catalyst with lower Ir loading. To gain insight into the enhanced activity at
higher Ir loading amount, the structure of 20 wt%-Ir Ir-ReO /SiO (Re/Ir = 0.34, actual) was further
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clarified. It was suggested that the catalyst structure with ReO clusters attached to neighboring Ir
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nanoparticles facilitated the Ir-ReO interface (active site) formation per Re amount.
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The metalsupport interaction over supported metal catalysts is a hot topic to reveal how the
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catalytic performance was influenced by such interaction. The fundamental understanding of support
effects refers to stabilizing nanoparticles,13 promoting electron transfer,14 encapsulating metal
nanoparticles by metal-oxide support14b, 15 or support directly participating in reaction. Both IrO and
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RuO have a rutile structure, and the strategy for dispersion of Ir has been employed using rutile as
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a TiO support.1 Smaller particles (12 nm) have been observed for Ir and Ir-Au highly dispered on
2b
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rutile TiO in N O decomposition, dehydrogenative synthesis of organics and CO oxidation. On the
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other hand, in many cases of bimetallic catalysts, the reaction proceeds at the interface of
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metalpromoter on the support, and the strong metalpromoter interaction can enhance the activity.
However, such interaction might be weakened when metalsupport and promotersupport interactions
become stronger because metal and promoter become individually dispersed on the support. Therefore,
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it remains a challenge for development of bimetallic catalysts by the assistance of support properties.
In fact, we have developed various bimetallic catalysts for conversion of biomass-related compounds
using SiO or C support which are relatively inert supports.11b-f, 21
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In the present research, the influence of support for Ir-ReO catalysts for glycerol hydrogenolysis
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was investigated in detail, and we found that very high Ir loading amount per surface area of rutile
TiO support shows high activity. The required Re amount was relatively low (Re/Ir 0.30, actual).
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The interface of Ir-ReO site and how it stabilizes on rutile TiO will be discussed based on activity
x
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tests of various related catalysts and characterization results.
2
. Experimental Section
Catalysts preparation. Ir-ReO /S (S represents support: SiO , MgO, CeO , -Al O , ZrO ,
x
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activated carbon, H-ZSM-5, anatase TiO (denoted as “Anatase”), rutile TiO (denoted as “Rutile”),
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and P25 TiO (denoted as “P25”) as listed in Table S2) catalysts were prepared by sequential
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impregnation method according to previous reports:7a, 7b Support was firstly impregnated with H IrCl
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aqueous solution. When noted, Ir(NO ) was used instead of H IrCl . After drying overnight at 383 K,
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the sample was subsequently impregnated with NH ReO solution. Catalyst was eventually obtained
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after calcined at 773 K for 3 h. Our previous work indicated that calcination at relatively higher
temperature was necessary for complete decomposition of H IrCl (773 K), otherwise a significant
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b
decline of activity was observed. In the present work, the calcination temperature was set at 773 K
despite the sublimation of rhenium species at this temperature. The loading amount of Ir after the
calcination was almost the same considering experimental errors and it is verified that Ir amount is
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assumed to be constant during the preparation procedure. Herein, the nominal Re/Ir molar ratio
typically 0.25) was based on the assumed Ir loading amount whereas the actual metal loading amount
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of both Ir and Re was determined by characterization as described later. The value with a mark (*) of
Re/Ir refers to the actual ratio which is calculated by actual Ir and Re amount in this paper while the
nominal ratio is shown as “Re/Ir = x, nominal”.
Activity test. Glycerol hydrogenolysis was carried out in a stainless steel autoclave (190 ml).
Before conducting the reaction, two methods were applied to the reduction of catalyst. One method
was the reduction of catalyst in water solvent. Appropriate amount of catalyst and water solvent was
introduced to the glass inner vessel, and then it was transferred to the autoclave. The reactor was purged
with 1 MPa H for three times to replace the air. Afterwards, the system was heated to 473 K and
2
pressurized to 8 MPa H for reduction treatment for 1 h. Those catalysts that were reduced in the
2
presence of water were called as Ir-ReO (L, 473). Then, a certain amount of glycerol was added to
x
the reactor under air at ambient temperature. After removing the air, the system was rapidly increased
to reaction temperature (393 K), and then the autoclave was pressurized with H to 8 MPa (defined as
2
0 h reaction time). We also measured glycerol conversion at 0 h ( 1% conv.) to obtain reliable kinetics
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b
results, which was in accordance with our recent work. The stirring rate was fixed at 500 rpm to
eliminate mass transfer effects. The other method refers to the catalysts reduced by H flow (100% H ,
2
2
-1
30 mL min ) at various temperatures for 1 h using a fixed-bed reactor. These samples after gas-phase
reduction were denoted as (G, xxx), similarly to the case of catalysts reduced by liquid-phase reduction
(
L, xxx). For example, Ir-ReO (G, 473) means catalysts reduced by H flow at 473 K for 1 h. All the
x
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gas-phase reduced catalysts were introduced to the reactor under N atmosphere to avoid oxidation of
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catalyst by the contact with air unless noted; in some cases, catalyst after reduction was passivated by
% O /He for 15 min to examine the catalyst stability under air. Subsequently, same operation was
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performed: purging the N with H , heating to 393 K, and pressurizing to 8 MPa to start the reaction.
2 2
For both methods, the standard reaction conditions were the same and described as follows: 4 g (43
mmol) of glycerol, 2 g of water, and 150 mg of 4 wt%-Ir Ir-ReO /TiO (6 mg, 31 mol Ir), 8.0 MPa
x
2
H , reaction temperature at 393 K for 4 h. Some of the parameters were sometimes changed and the
2
details can be found below the table or figure. In regard to reuse test, Ir-ReO /Rutile catalyst was firstly
x
reduced in H flow (G, 573) and introduced into the reactor under N . After the first run, decantation
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method was used to easily obtain the wet form catalyst under N atmosphere after reaction due to the
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7b
hydrophobic property of rutile TiO support. The inductively-coupled plasma atomic emission
2
spectrometry (ICP-AES, Thermo Fisher iCAP6500) was employed to measure the leached Ir and Re
amount in these solutions. In the following runs, no further reduction was performed, and water
solvents with and without being degassed were both selected for comparison. The weight of glycerol
and water in each run was adjusted to keep at a constant ratio of glycerol/water/catalyst.
Product analysis. The products in the liquid phase were quantified by gas chromatograph
(Shimadzu GC-2025) equipped with a TC-WAX capillary column (diameter 0.25 mm, 30 m) using 1-
butanol as an internal standard, while the gas phase products were quantified by gas chromatograph
with a Rtx-1-PONA capillary column (diameter 0.25 mm, 100 m) using standard gases (methane,
ethane and propane) to obtain calibration curve. These products were also identified by GC–MS
(QP5050, Shimadzu). The propane (main gas-phase product) and C–C cracking products are defined
as “Others” in this work. The conversion and selectivity were determined on the carbon basis as
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b
described in our recent work. The mass balance was examined and the difference of each result was
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1
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1
1
1
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1
1
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within the experimental error of ±10%. The productivity of 1,3-PrD (P1,3-PrD) was calculated as
9
b
-1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
previously reported: P
1
= (Moles of glycerol × Yield1,3-PrD × 76 g mol ) / (Ir amount (g) × t (h)).
,3-PrD
The glycerol conversion rate (v ) was determined from converted glycerol (mmol) amount per catalyst
g
amount (g) per hour (h).
Catalyst characterization. The surface areas of various supports were obtained on an instrument
(
Micromeritics Gemini) by N adsorption (BET method). A wavelength dispersive X-ray fluorescence
2
(XRF) instrument (Bruker, S8 Tiger) was used to measure the actual loading amount of iridium and
rhenium on TiO support under He atmosphere. The calcined catalyst (0.8 g) and internal standard
2
ZrO (0.1 g) were well mixed prior to measurement, and calibration curve was produced by using
2
9b
commercially available IrO and ReO as previously reported in our recent work. The amount of Ir
2
2
and Re over typical samples (4 wt%-Ir, Re/Ir = 0.25 and 1, nominal) after gas-phase reduction or
reaction was also examined by XRF, and the values were almost the same to those after calcination.
The measurements were repeated three times for typical samples, and the experimental error of actual
*
Re/Ir ratio was within ± 0.04. Temperature-programmed reduction (TPR) patterns were obtained with
a home-made fixed-bed flow reaction system. The catalyst (100 mg) in a glass tube was heated under
-1
-1
the 5% H /Ar (30 mL min ) at 10 K min from 300 to 1000 K. The consumed H amount was
2
2
calculated based on the signal intensity area of thermal conductivity detector (TCD), which corrected
by the consumed H amount of calcined 4 wt% Ir/SiO .
2
2
The transmission electron microscope (TEM) imaging of samples was performed on HITACHI
HF 2000EDX apparatus. The samples after reduction were suspended in ethanol solvent, and dropped
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3
2
on Cu grids. Average particle size was calculated by n d /n d (d : particle size; n : number of
i
i
i
i
i
i
particle with size d ). CO chemisorption measurement employed a volumetric method at ambient
i
0
1
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3
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9
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0
temperature by using high-vacuum system. Prior to measurement, catalysts were reduced at 573 K by
H flow. The pure CO pressure after adsorption was about 1.1 kPa. The acquired CO adsorption amount
2
was assumed to be equal to that of the surface Ir atoms, which further determined the Ir dispersion by
CO/Ir (D ). FT-IR spectra of adsorbed CO on catalysts were performed in an in-situ cell by diffuse
CO
reflectance mode on Nicolet 6700 FT-IR. Samples were put into a cup and reduced with H at 573 K
2
in the cell, which corresponds to (G, 573) pre-reduction for the activity test. CO pulses were introduced
into the system at ambient temperature using N as a carrier gas until the adsorption reached saturation,
2
and the intensity of each spectrum was normalized by adjusting the relative peak area to corresponding
D .
CO
X-ray photoelectron spectra (XPS) were obtained on the Shimadzu AXIS-ULTRA DLD
spectrometer under high vacuum at ambient temperature. The binding energy was calibrated with C 1s
peak of sample-loaded adhesive tape at 284.6 eV. Samples after pre-reduction at 573 K by H flow
2
were directly introduced to the analysis chamber under N atmosphere without exposure to air. For
2
catalyst after passivation, the reduced catalyst was treated with 1% O /He for 15 min at ambient
2
temperature, and then loaded into the chamber. The curve fitting of XPS results was performed with
the software CasaXPS, ver. 2.3.19 (Casa software Ltd.). In the case of Ir 4f pattern, the two signals for
Ir 4f and Ir 4f were separated by 3.0 eV at a fixed relative peak ratio of 4:3 and full width at half-
7
/2
5/2
maximum (fwhm) ratio of 1:1. For Re 4f pattern, the difference of peak separation by 2.4 eV for Re
4f and Re 4f , and the relative peak ratio and peak widths were similarly set as the theoretical values.
7
/2
5/2
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The patterns were fitted by Gaussian functions and Lorentzian asymmetric functions for Ir and Re,
respectively. In addition, the curve fitting for Ti 2p was also employed at fwhm ratio of 2:1 for Ti 2p1/2
to Ti 2p using Gaussian functions.
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1
1
1
1
1
1
1
1
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3/2
The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure
EXAFS) spectroscopy was measured with the approval of Japan Synchrotron Radiation Research
(
Institute (JASRI; Proposal No. 2019A1369 and 2019A1716), and details of measurement and analysis
are shown in Supporting Information. The sample preparation method was as follows: In the case of
catalyst after calcination at 773 K, the powder was directly filled into a plastic bag for measurement.
For the reduced catalysts, samples were pre-reduced at 573 K for 1 h by H flow. After cooling, they
2
were transferred to a plastic bag under N atmosphere. The catalyst after reaction (same reaction
2
conditions to the reuse test) was also collected under N atmosphere, dried after washing with methanol,
2
and then placed into a plastic bag. The thickness of all the samples was about 2 mm, which gave the
edge jump for Ir L -edge at 0.20.7 (transmission mode) and Re L -edge at 0.0010.005 (fluorescence
3
3
mode), respectively.
3
. Results and discussion
Optimization of Ir-ReO catalysts in hydrogenolysis of glycerol to 1,3-propanediol. The Ir-
x
ReO pairs with fixed Ir amount (4 wt%) at a ratio of Re/Ir = 0.25 (nominal) were loaded on various
x
supports, and their activities in glycerol hydrogenolysis were measured (Figure 1A). The reduction
temperature was increased from 473 K which was the typical temperature for complete reduction of
7a-c, 9b
Ir-ReO /SiO
to 573 K, since higher temperature is generally necessary to reduce metal oxide
x
2
2
2
species which are more strongly interacted with reducible TiO relative to SiO support. The Ir-
2
2
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ReO /Rutile demonstrated the highest activity with high selectivity to 1,3-PrD in comparison with
x
other screened supports including SiO which has been used in both our works and other researchers’
2
5
b, 7a, 7b, 7d, 9b
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works for Ir-ReO -catalyzed hydrogenolysis.
x
Interestingly, the performance of TiO2-
supported catalysts much depended on the crystalline phase of TiO , and glycerol hydrogenolysis
2
hardly proceeded over Ir-ReO /Anatase. Ir-ReO catalyst supported on P25 TiO which is a mixture
x
x
2
of rutile and anatase showed an activity between those of Ir-ReO /Anatase and Ir-ReO /Rutile. The
x
x
activity of Ir-ReO /H-ZSM-5 was also higher than that of Ir-ReO /SiO , although it was lower than
x
x
2
that of Ir-ReO /Rutile. The good activity of Ir-ReO /H-ZSM-5 can be related to the small external
x
x
surface area of H-ZSM-5 zeolite: Ir particles might be densely located on the external surface of H-
9
b
ZSM-5 and Re species, similarly to the case of high-loading Ir-ReO /SiO catalyst. Moreover, the
x
2
acidity of H-ZSM-5 in hot water solvent7c, 11d may also lead to the higher activity. Carbon, ZrO , CeO ,
2
2
Al O and MgO supports were much less effective than SiO for activity of supported Ir-ReO . In
2
3
2
x
terms of selectivity, all Ir-ReO catalysts except those on basic supports (CeO and MgO) showed good
x
2
selectivity to 1,3-PrD. Next, the influence of Ir amount with constant ratio of nominal Re/Ir (0.25) was
investigated for Ir-ReO /Rutile catalyst (Figure 1B). A volcano-type dependency of glycerol
x
conversion on Ir loading amount (28 wt%) was observed over Ir-ReO /Rutile, and the activity reached
x
the maximum at Ir amout of 6 wt%. Similar volcano-type conversion tendency was also found in the
9
b
optimization of Ir-ReO /SiO . However, such Ir-ReO /SiO catalyst at Ir amount of 20 wt% gave
x
2
x
2
higher activity, which is clearly larger than the optimized one (6 wt%-Ir) on rutile TiO , probably
2
2
-1
2
-1
because of the higher SBET of SiO (485 m g ) than rutile TiO (6 m g ). The reaction results over
2
2
Ir-ReO /SiO catalysts (4 wt%-Ir, Re/Ir = 1, and 20 wt%-Ir, Re/Ir = 1, nominal) which have been
x
2
1
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reported to be very active are also shown in Figure 1B for comparison. In the case of SiO supported
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Ir-ReO catalysts, our recent work suggested that higher Ir loading amount facilitated the formation of
x
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direct contact of neighboring Ir-ReO particles that possess more active site per Re amount. Here,
x
the total Ir amount was set as the same (31 mol) for all catalysts. The activity of Ir-ReO /Rutile (48
x
wt% Ir, Re/Ir = 0.25, nominal) was significantly higher than that of these Ir-ReO /SiO catalysts. In
x
2
addition to Ir-based activity, the amount of Re used in the preparation of Ir-ReO /Rutile catalysts was
x
reduced to a quarter compared to those for Ir-ReO /SiO (Ir: 4 or 20 wt%, Re/Ir = 1, nominal). To gain
x
2
insight into the structure difference of Ir-ReO pairs on rutile TiO and SiO support, the Ir loading
x
2
2
amount was fixed at 4 wt% in the following discussion to compare with other Ir-ReO catalysts
x
7
a-c, 9a
including the previously reported 4 wt%-Ir Ir-ReO /SiO (Re/Ir = 1, nominal).
The activity over
x
2
Ir-ReO /Rutile (4 wt%-Ir, Re/Ir = 0.25) was close to that of Ir-ReO /Rutile (6 wt%-Ir, Re/Ir = 0.25)
x
x
which was the most active one at Ir amount from 2 to 8 wt% as shown in Figure 1B. As investigation
of the effect of precursor of Ir, Ir-ReO /Rutile catalyst (4 wt%-Ir, Re/Ir = 0.25) was also prepared by
x
using Ir(NO ) as precursor of Ir instead of H IrCl . The activity of the catalyst prepared with Ir(NO )
3 4
3
4
2
6
was much lower than that standard Ir-ReO /Rutile catalyst prepared with H IrCl , which can be due to
x
2
6
the formation of larger particles using Ir(NO ) as discussed in the later section. This trend is different
3
4
from that of Ir-ReO /SiO catalysts where the catalytic performance was not changed by the use of
x
2
different Ir precursors on catalyst preparation.7c
Next, the effect of nominal Re/Ir ratio over 4 wt%-Ir Ir-ReO /Rutile catalysts on glycerol
x
5b,
hydrogenolysis was further examined as summarized in Table 1. Similarly to Ir/SiO and ReO /SiO ,
2
x
2
5
e, 9b
both monometallic Ir/Rutile and ReO /Rutile cannot catalyze glycerol hydrogenolysis. The
x
1
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glycerol conversion increased linearly with Re/Ir ratio between 0.06 and 0.25, and gradually saturated
at higher Re/Ir ratio with a moderate decrease of 1,3-PrD selectivity. The promoting effect of sulfuric
0
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acid addition was also investigated over 4 wt%-Ir Ir-ReO /Rutile (Re/Ir = 0.25). Although the activity
x
was slightly enhanced, it was different from the case of 4wt%-Ir Ir-ReO /SiO (Re/Ir = 1), where the
x
2
5b, 7b
activity decreased significantly (ca. half) without addition of H SO .
We recently found that the
2
4
Re O was prone to sublimate at relatively high loading amount on SiO support during the calcination
2
7
2
9
b
2
-1
at 773 K. Given the much smaller S
of TiO support, particularly for rutile TiO (6 m g ), the
2 2
BET
actual Ir and Re amounts were measured by XRF (Table 2). Overall, the actual amount of Ir on TiO2
was almost the same as the nominal value. For ReO /Rutile catalyst, the actual Re amount was slightly
x
lower than the nominal one (Table 2, entry 2). For those of Ir-ReO /Rutile catalysts (entries 37), the
x
actual Re/Ir ratio was almost the same as the nominal one at the ratio between 0.06 and 0.25; however,
it drastically stopped increasing at around 0.3 when the nominal ratio of Re to Ir exceeded 0.25. The
loss of Re can be assigned to the Re O sublimation on rutile TiO . The 4 wt%-Ir Ir-ReO /Rutile (Re/Ir
2
7
2
x
=
0.25) prepared with Ir(NO ) precursor had also similar actual Re/Ir ratio (entry 8). For the catalyst
3 4
with different Ir amount at fixed nominal Re/Ir ratio (0.25), sublimation of Re species was more likely
to occur at higher Ir loading amount above 4 wt% (entries 911). As regard to Ir-ReO pairs supported
x
on anatase and P25, it seems that Re species were easier to be sublimated on anatase than on rutile or
P25 TiO (entries 5, 13 and 15).
2
In the case of SiO supported noble metals (Ir, Rh and Pt) with ReO modification catalysts, the
2
x
iridium oxide was easier to reduce.9a, 23 In addition, for noble metal catalysts on reducible supports
2
4
including TiO , reduction temperature for pre-activation with H is generally an important parameter.
2
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High reduction temperature favors the formation of strong metalsupport interaction (SMSI), where
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suboxide (TiO ) species may migrate and cover the surface of metal nanoparticles.15b, 25 In some cases,
x
0
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SMSI much improves the catalytic performance, while in some cases the overlayer of these suboxide
species blocks the access of reactants to the active site, giving negative effect on catalytic
performance.12a, 12b Therefore, the effect of reduction temperature was investigated on a series of TiO2
supported Ir-ReO catalysts. Table 3 lists the catalytic performance of Ir-ReO /TiO reduced at various
x
x
2
reduction conditions. The tested reduction conditions were liquid-phase reduction at 473 K, as the
7b, 9b
optimum reaction condition for Ir-ReO /SiO ,
and gas-phase reduction at 473, 573, 673 and 773
x
2
K (G, 473773). The reduction at 773 K is the typical condition to induce SMSI for TiO -supported
2
catalysts.15a Ir-ReO /Rutile and Ir-ReO /P25 catalysts showed similar trends for the change of
x
x
reduction conditions. Gas-phase reduced catalysts (G, 473) showed higher activity than those reduced
in liquid phase (L, 473). The activity was further increased when the reduction temperature in gas
phase was increased to 573 K. After that, the activity was rather decreased with increase of reduction
temperature. The selectivity was almost unchanged by the change of reduction conditions. These trends
can be explained by the difference of reduction degree among the reduction temperature range of 573
K and the aggregation of metal particles and/or the covering of metal surface with TiO by SMSI above
x
5
73 K. The reduction behavior of Ir-ReO /Rutile will be explained in later section. In the case of Ir-
x
ReO /Anatase, the activity was much lower than Ir-ReO /Rutile for all the tested reduction conditions.
x
x
The (G, 473) catalyst showed the highest activity among Ir-ReO /Anatase catalysts, and the activity
x
began to decrease at higher temperature (573 K). The 1,3-PrD selectivity over Ir-ReO /Anatase varied
x
by the increase of reduction temperature: selectivity to 1,3-PrD decreased and that to 1,2-PrD increased
1
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with increase of reduction temperature. These trends for activity and selectivity for Ir-ReO /Anatase
x
were rather similar to those for Ir-ReO /SiO . The effects of reduction conditions imply that more
x
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active sites are available over rutile TiO support for glycerol hydrogenolysis to 1,3-PrD, and these
2
active sites can be stabilized over rutile TiO other than SiO or anatase supports. We selected Ir-
2
2
*
ReO /Rutile (4 wt%-Ir, Re/Ir = 0.24 , (G, 573)) catalyst as the standard one because of better
x
performance and almost no loss of Re species during calcinations (i.e. similar Re/Ir value of nominal
*
*
(
0.25) and actual ones (0.24 )). Although 4 wt%-Ir Ir-ReO /Rutile (Re/Ir = 0.30 , (G, 573)) catalysts
x
showed higher activity, significant loss of Re was observed during calcination on this catalyst. We also
*
tested the effect of the exposure to air after reduction. The 4 wt%-Ir Ir-ReO /Rutile (Re/Ir = 0.24 ) was
x
first reduced in the condition of (G, 573), and then passivated with 1 % O /He for 15 min at ambient
2
temperature. This catalyst showed clearly lower activity than that without passivation (Table 3, entry
6), indicating that some species on rutile TiO were easily oxidized even at low oxygen concentration.
2
Performance of Ir-ReO /Rutile catalyst. Table 4 gives the dependency of glycerol
x
*
hydrogenolysis on reaction time over 4 wt%-Ir Ir-ReO /Rutile (Re/Ir = 0.24 ). With reaction time
x
prolonged, the conversion was steadily increased while 1,3-PrD selectivity decreased due to the
overhydrogenolysis of 1,3-PrD to 1-propanol. The highest 1,3-PrD yield achieved by 4 wt%-Ir Ir-
*
ReO /Rutile (Re/Ir = 0.24 ) was 36% at 12 h with glycerol conversion of 69% and 52% selectivity to
x
1,3-PrD. This is a similar value to the one obtained over 4 wt%-Ir Ir-ReO /SiO (Re/Ir = 1) + H SO
4
x
2
2
7
a
(
38%). The Ir-ReO /Rutile is highly active in glycerol hydrogenolysis. The turnover frequency based
x
0
on surface Ir ((TOF = converted glycerol amount (mol) / (total Ir amount (mol)) × 0.33 (CO
s
0
adsorption CO/Ir, Table 5) × 0.66 (Ir ratio on the basis of FT-IR of adsorbed CO), Table S7) ×
1
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*
reaction time (h) was 830 h over 4 wt%-Ir Ir-ReO /Rutile (Re/Ir = 0.24 ), which was clearly higher
x
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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-1
than that achieved by 4 wt%-Ir Ir-ReO /SiO (Re/Ir = 1) + H SO (TOF : 370 h ) or 20 wt%-Ir Ir-
x
2
2
4
s
-1 9b
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ReO /SiO (Re/Ir = 1) (450 h ). The initial productivity of 1,3-PrD of 4 wt%-Ir Ir-ReO /Rutile (Re/Ir
x 2 x
*
=
0.24 ) catalyst was 52 g gIr-1 h-1, and the P1,3-PrD at highest yield was 17 g gIr-1 h-1 (Table 4). In the
open literature as summarized in Table S1, the P1,3-PrD of SiO supported Ir-Re alloy catalyst (Re/Ir =
2
1) was 7 g gIr-1 h (40% conv. and 37% 1,3-PrD selectivity); Under similar reaction conditions, the
-1
7d
P1,3-PrD was 18 g gIr-1 h-1 (initial one, at glycerol conversion of 2025%) and 6 g gIr-1 h-1 (highest 1,3-
7a, 7b
and 22 g gIr-1 h-1 (initial one) and 7 g
PrD yield) over 4 wt%-Ir Ir-ReO /SiO (Re/Ir = 1) + H SO ,
x
2
2
4
gIr-1 h (highest yield) over 20 wt%-Ir Ir-ReO /SiO (Re/Ir = 1). Moreover, Yang’s group has
-1
9b
x
2
proposed the monometallic Ir/H-ZSM-5 as an effective catalyst for 1,3-PrD production in the absence
26
0
3+
of acid very recently. The Ir -Ir ratio as well as the Brønsted acid on the catalyst was correlated
with the activity. However, harsh reaction conditions (453 K, 8 MPa H ) were required to reach a
2
maximum P1,3-PrD of 2 g gIr-1 h-1 at initial stage (29% conversion, 57% 1,3-PrD selectivity), which is
much lower than Ir-ReO /Rutile catalysts. As far as we are concerned, the Ir-ReO /Rutile is the most
x
x
active catalyst for glycerol hydrogenolysis to 1,3-PrD among the reported catalysts even with such a
*
low Re/Ir ratio (0.24 ) (Table S1).
*
The standard Ir-ReO /Rutile (Ir: 4 wt%, Re/Ir = 0.24 ) was selected for kinetic study. The reaction
x
order of H was 0.9 (Figure 2A) which was close to the case of Ir-ReO /SiO (about +1), suggesting
2
x
2
+
−
the heterolytic dissociation of H to produce proton (H ) and hydride (H ), and the hydride can be
2
regarded as one active species in glycerol hydrogenolysis.7b, 9b, 11b Additionally, higher selectivity to
1,3-PrD (54%  73%) was obtained with increasing H pressure from 2 to 8 MPa at a similar glycerol
2
1
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conversion level (1116% conversion, Table S3). Furthermore, the influence of glycerol concentration
on hydrogenolysis was investigated (Figure 2B and Table S4). The glycerol conversion rate (vg)
gradually increased with increasing glycerol concentrations from 5 to 50 wt%, and it became almost
saturated at higher glycerol concentration ( 50 wt%). However, for Ir-ReO /SiO catalysts (4 wt%-Ir
0
1
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9
0
1
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0
x
2
and 20 wt%-Ir), the saturation was found at lower glycerol concentration (20 wt% glycerol).5b, 7b, 9b
The glycerol concentration effect was influenced by the interaction between glycerol and catalyst,
5
b
rather than changing the reaction mechanisms. Probably, the interaction between glycerol and the
active site on Ir-ReO /Rutile is weaker than that on Ir-ReO /SiO , and high glycerol concentration is
x
x
2
required to reach saturation. The reaction order on glycerol concentration around standard reaction
conditions (67 wt% glycerol solution) was about 0.2 (Figure S1), suggesting that the coverage of
glycerol adsorption on the catalyst surface is almost saturated under the standard reaction conditions.
Another important point was the high 1,3-PrD selectivity (about 70%), which showed independency
of glycerol concentrations (1080 wt%). Similar behavior was also observed over 20 wt%-Ir Ir-
9b
ReO /SiO catalyst. It is noteworthy that an obvious decline of activity was found when no water
x
2
solvent (100 wt% glycerol solution) was added (Figure 2B). We have discussed the plausible formation
way of ReOH site that was regarded as the adsorption site for glycerol rather than serving as acid,
and this site was formed by activation of water molecules on ReO cluster with low valence attached
x
9
b
to Ir metal. Previous Ir-ReO /SiO system was optimized by liquid-phase reduction (L, 473), and the
x
2
presence of water during pre-reduction treatment makes it difficult to provide more evidences for this
assumption. On the contrary, the Ir-ReO /Rutile was optimized by gas-phase reduction at higher
x
temperature (G, 573). The lower activity without addition of water can be explained by the absence of
1
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water molecules available for activation on ReO cluster to form ReOH site for glycerol adsorption.
x
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9
1
1
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1
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2
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Since the glycerol hydrogenolysis also produces water, further work is required to clarify the role of
water in glycerol hydrogenolysis over metal oxide modified bimetallic catalysts. Overall, the reaction
mechanism around the standard conditions is essentially the same to the cases of 4 wt%-Ir Ir-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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7
8
9
0
1
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8
9
0
1
2
3
4
5
6
7
8
9
0
*
*
ReO /SiO (Re/Ir = 0.83 ) + H SO and 20 wt%-Ir Ir-ReO /SiO (Re/Ir = 0.34 ) system: Glycerol was
x
2
2
4
x
2
adsorbed on ReO cluster at the 1-position to form terminal alkoxide. Meanwhile the H heterolytically
x
2
dissociated on the Ir metal surface producing the hydride to attack the alkoxide at 2-position, cleave
the CO bond, and release the 1,3-PrD by hydrolysis of the reduced alkoxide.7b, 9b
Next, the reusability of 4 wt%-Ir Ir-ReO /Rutile was investigated. First, the catalysts were
x
collected under N atmosphere, washed with deionized water which was stored under air, and then
2
used for the next run. The activity of both catalysts was gradually decreased with repeated usage (Table
S5). However, the Ir and Re amount over both used catalysts was the same to the fresh catalyst as
*
*
*
*
determined by XRF (0.24  0.24 ; 0.30  0.31 after first run use) as well as very low leached Ir
and Re amount over these catalysts (Ir: 0.2% and Re: 0.3%, based on actual amount of metal over
catalyst), indicating that metal leaching was not the main cause for activity decrease. Then another test
was carried in which the water solvent for washing was further degassed to remove any residual oxygen.
After four times usage, both the high activity and 1,3-PrD selectivity was maintained, giving good
*
reusability of Ir-ReO /Rutile (4 wt%-Ir, Re/Ir = 0.24 ) under this strict O -free environment (Table 6).
x
2
*
This behavior is different from high loading Ir-ReO /SiO (Ir: 20 wt%, Re/Ir = 0.34 ) since the SiO
2
x
2
9b
supported Ir-ReO pairs could be reused when only the gas phase was O -free. It indicates that the
x
2
Ir-ReO active species are less stable to oxidation on rutile TiO rather than those on SiO support,
x
2
2
1
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which was further examined by XPS measurement in the later discussion.
Characterization of Ir-ReO /Rutile and related catalysts. H -TPR was performed to determine
x
2
0
1
2
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4
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7
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9
0
1
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0
1
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4
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8
9
0
the reducibility of those catalysts prepared with different Ir precursors, Re/Ir ratios, and crystalline
phases of TiO (Figures 3 and S2). The ReO /Rutile (0.7 wt%) showed a small single reduction peak
2
x
between 400500 K, which temperature range was clearly lower than that (550700 K) on
7
b, 9b
ReO /SiO .
The total H consumption corresponded to the partially reduced ReO species on rutile
2 x
x
2
TiO to the valence of +2 (Table S6). Two main peaks were observed for both Ir/Rutile and Ir-
2
ReO /Rutile. The Ir/Rutile was gradually reduced at a wide range of temperature (350800 K),
x
suggesting the incomplete reduction for Ir/Rutile at 573 K. The reduction was promoted significantly
as the reduction peaks became narrow and shifted to lower temperature with increasing the Re addition
amount. The reduction was almost completed at 573 K with Re/Ir ratio ≥ 0.24. It agrees well with the
*
*
optimization reduction temperature (573 K) of Ir-ReO /Rutile (Re/Ir = 0.24 and 0.3 ) for catalytic
x
activity as discussed in earlier section. The average valence of Re was calculated to be +1.5+3.5 on
Ir-ReO /Rutile catalysts (Table S6). The range of reduction temperature of Ir-ReO /Rutile by the use
x
x
of Ir(NO ) as Ir precursor was similar to the standard (H IrCl precursor), with a small difference of
3
4
2
6
curve shape (Figure 3). In addition to rutile TiO supported Ir-ReO catalysts, the reducibility of
2
x
anatase and P25 TiO based catalysts is also shown in Figure S2. The pure TiO supports presented no
2
2
obvious reduction peak, and the promotion effect of reduction of Ir with small amount of Re addition
was also observed on Ir-ReO /Anatase and Ir-ReO /P25. The average valence of Re on Ir-ReO /TiO
2
x
x
x
(
4wt%, Re/Ir = 0.25) calculated with the assumption of complete IrO reduction to Ir and no TiO2
2
reduction, decreased in the following order: Ir-ReO /Rutile (+6.2, precursor of Ir(NO ) )  Ir-
x
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ReO /P25 (+4.3)  Ir-ReO /Rutile (+2.5)  Ir-ReO /Anatase (+1.0) (Table S6). However, there was
x
x
x
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the existence of partially reduced rutile TiO support. For the Ir/Anatase and Ir/P25, the calculated H2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
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9
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3
4
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7
8
9
0
consumption amount based on actual Ir amount (IrO + 2H  Ir + 2H O) was consistent with that
2
2
2
determined by TPR. However, the value of consumed H amount by TPR measurement (44.15 mol)
2
on Ir/Rutile was slightly larger than the theoretical one (20.81 mol Ir, 41.62 mol), indicating the
presence of Ir promoted the reduction of TiO (Table S6). Therefore, it can be deduced that some
2
amount of rutile TiO also consumed H in TPR measurement for those Ir-ReO /Rutile catalysts, and
2
2
x
the Re valence determined by TPR cannot accurately correspond to the actual Re oxidized state.
The particle size of various Ir/TiO and Ir-ReO /TiO catalysts after reduction (G, 573) was
2
x
2
measured by TEM (Figure 4). The average particle size of monometallic iridium (4 wt%) on TiO2
decreased in order: Ir/Anatase (6.6 nm)  Ir/P25 (2.8 nm)  Ir/Rutile (2.6 nm). The high dispersion of
Ir on rutile TiO can be explained by the interfacial lattice matching of IrO (rutile structure) and rutile
2
2
TiO (lattice constant of IrO : a = b = 0.450 nm, c = 0.315 nm and rutile TiO : a = b = 0.459 nm, c =
2
2
2
0
.296 nm),17a which favors the formation of Ir-O-Ti bond for Ir species anchored during impregnation
and calcination. With addition of small amount of Re (Re/Ir = 0.25), the promotion effect on the
dispersion of particles was observed over all the Ir-ReO /TiO catalysts. The Ir-ReO pairs
x
2
x
demonstrated higher dispersion on rutile TiO (1.9 nm) than that on P25 (2.5 nm) or anatase (5.3 nm).
2
Additionally, those particles were preferentially to locate on the rutile TiO rather than anatase surface
2
because P25 was a mixture of anatase and rutile TiO (weight ratio of 4:1) and the particles were
2
countable on only a few regions on bulk TiO (Figure 4F). Combined with catalytic performance on
2
Ir-ReO supported over rutile TiO , anatase and P25 (Figure 1A), we suggested that both dispersion of
x
2
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Ir particles and interaction between Ir-ReO should be taken into consideration to understand the
x
activity difference over TiO supported Ir-ReO catalysts. The location of Ir particles and Re species
2
x
0
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on TiO support may be more important. Some Re species on anatase TiO were probably mainly
2 2
located distant from Ir particles on the surface of support, which results in no activity for glycerol
hydrogenolysis. For P25 support, some Re species were present on the crystallites of anatase that
cannot be interacted with Ir particles since the activity over Ir-ReO /P25 was clearly lower than that
x
over Ir-ReO /Rutile and the average Ir particle size of Ir-ReO supported over P25 (2.5 nm) and rutile
x
x
TiO (1.9 nm) was comparable (Figures 4E and F). In addition, small particle size with narrow
2
*
distribution was also observed on 4 wt%-Ir Ir-ReO /Rutile (Re/Ir = 0.30 ) (1.8 nm, Figure S3). The
x
particle size increased with higher Ir loading amount at fixed Re/Ir (0.25) (Figure S4). Another
important point is the surface density of Ir particles on Ir/Rutile and Ir-ReO /Rutile catalysts: the
x
surface of rutile TiO particles was almost totally covered with Ir particles. In the case of 4 wt%-Ir Ir-
2
ReO /Rutile (Re/Ir = 0.25) prepared by Ir(NO ) precursor, large particles were observed with wide
x
3 4
*
size distribution (3.7 nm, Figure S5). The standard catalyst of 4 wt%-Ir Ir-ReO /Rutile (Re/Ir = 0.24 )
x
reduced at high temperature (773 K) showed slightly larger particle size (2.1 nm) (Figure S6) compared
to that reduced at 573 K (1.9 nm).
*
Figure 5 presents the XPS results for Ir/Rutitle, Ir-ReO /Rutile (Re/Ir = 0.24 ), Ir-ReO /Rutile
x
x
*
*
(Re/Ir = 0.30 ) catalysts after reduction, and Ir-ReO /Rutile (Re/Ir = 0.24 ) after reduction and
x
passivation. The surface valence of Ir and Re and the fitting results are summarized in Tables 6 and 7,
0
3+
respectively. The Ir 4f signals over these catalysts were deconvoluted into a complex of Ir , Ir , and
Ir species. The main peak of Ir 4f binding energy (BE) value at 60.661.1 eV was assigned to Ir0
4
+
7
/2
2
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species (Figure 5A, B, C, and D, and Table 7), with Ir species at peak position 61.762.2 eV, and
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8
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
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4+
27
Ir species at peak position 62.763.2 eV. The fitting validity can be confirmed by the TPR results
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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2
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4
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7
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9
0
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2
3
4
5
6
7
8
9
0
for Ir/Rutile that the Ir species were not fully reduced at 573 K. For SiO supported Ir-ReO catalysts,
2
x
a typical reduction temperature at 473 K was sufficient for all the Ir species reduced to metallic state,
3+
4+
5b, 11h
and no signals for Ir and Ir species can be detected in XPS measurement.
The presence of

+
surface Ir species can be due to the small amount of oxidized Ir species strongly bonded to rutile
TiO . In addition, the reduction of surface Ir species was promoted by Re modifier since the proportion
2
0
3+
4+
of Ir increased from about 70% to 80% with decreasing the amount of Ir and Ir complex (Table
7). A positive shift of Ir peak position (about 0.5 eV) to higher BE value was found on Ir-ReO /Rutile
x
in comparison with Ir/Rutile. Such shift was not ascribed to the energy calibration error because the
Ti4+ 2p3/2 showed similar peak position (458.8 eV,28 Figure S7) over these samples. It can be
explained by the electronic interaction between Ir and ReO on rutile TiO surface, where electrons
x
2
n+
were transferred from Ir species to Re to stabilize ReO at intermediate valence (mainly +3 as shown
x
later). It can be also supported by the fact that FT-IR spectra of adsorbed CO over Ir-ReO /Rutile
x
appeared at higher wavenumber than that over Ir/Rutile as discussed in the section below. With respect
to Re 4f signals over related catalysts, it also demonstrates different surface oxidized state of Re species
than that over Ir-ReO /SiO as discussed below. Additional two peaks of Ti 3p at BE value of 38.0
x
2
2
9
30
eV and Ir 5p of 48.5 eV were detected on all the Ir-ReO /Rutile samples. In the case of Ir-
3
/2
x
*
*
ReO /Rutile catalysts (G, 573) at Re/Ir ratio of 0.24 and 0.30 , two peaks at BE value of 41.7 and 42.5
x
3+
eV were set for raw spectra of Re 4f7/2 fitting. The first main peak can be assigned to Re species,
which was rarely observed over SiO supported Ir-ReO catalysts (typically a variety of Re species at
2
x
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b, 11g, 11h
0
, +2, +4, +6, and +7 over Ir-ReO /SiO ),
but available over other bimetallic catalysts such as
x
2
Pd-Re,29, 31 Ru-Re and Rh-Re catalyst. The second smaller peak was ascribed to Re species
32
33
4+
0
1
2
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0
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Figure 5E and F, and Table 7). On both catalysts, about 80% of Re species were found to be at the
oxidized state of +3 with the rest of Re species at +4 (Table 7). Those different surface states of Ir
(
slightly oxidized) and ReO (mostly stabilized at +3) stabilized on rutile TiO indicates the unique
x
2
advantage of using rutile TiO as a support than inert SiO support. Another important point is the
2
2
oxidized state of Re after passivation. Although the surface composition of Ir over Ir-ReO /Rutile
x
*
(
Re/Ir = 0.24 ) after passivation was similar to the corresponding catalyst after reduction, about half
3
+
4+
7+
amount of Re species (Re and Re ) were oxidized to Re after exposure to carrier gas with low
3
+
4+
oxygen concentration (1% O /He) (Table 7). Therefore the Re species (Re and Re ) were sensitive
2
to oxygen, and can be easily oxidized even in the presence of small amount of oxygen. The possible
n+
deactivation mechanism for Ir-ReO /SiO was assigned to the coverage of Re species on Ir metal
x
2
9
b
n+
7+
surface after exposure to oxygen atmosphere. Some Re species were prone to be oxidized to Re ,
which has high solubility in water, and readsorbed on the surface of Ir particles. It can explain the
observed decrease of activity both in the repeated usage when the O in water solvent was not totally
2
removed (Table S5), and for catalyst after passivation (Table 3, entry 6). The surface atom ratio of
Re /Ir calculated by XPS measurement is also presented in Table 7. These values were close to the
s
s
actual ratio determined by XRF. Therefore, the Ir and Re valence can be determined from the XPS
fitting results on the basis of the assumption that all the Ir and Re species are detectable in XPS
3+
measurement. Moreover, the accurate amount of Ti species was particularly difficult to calculate
4
+
28
when combined with rich Ti species. In our analyses of XPS results, the surface Ti species were
2
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mainly Ti on rutile TiO with only about 4% of Ti species over 4 wt%-Ir catalysts at Re/Ir ratio of
2
5
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8
9
1
1
1
1
1
1
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1
1
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6
*
*
0
.24 and 0.30 after reduction (Figure S7).
0
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9
0
1
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0
1
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9
0
In order to further determine the surface state of Ir and Re, FT-IR of adsorbed CO combined with
CO adsorption was performed (Figure 6). No peak of adsorbed CO on ReO /Rutile was detected (data
x
-1
not shown here). A strong peak at 20832089 cm was observed on Ir-ReO /Rutile catalysts (Figure
x
7
b
6A). Generally, ReO cannot absorb CO molecules, and the peak position of CO molecule adsorbed
x
-1
7b, 7d, 34
on Ir metal was about 20662069 cm (linear adsorption of CO) for Ir-ReO /SiO
and that on
x
2
-1 35
Re metal surface was 20302050 cm . Therefore, the shift of the former peak to higher wavenumber
+
3+
4+
can be explained by the presence of Ir (Ir and Ir ) species on Ir metal surface. As a result, the Ir
atoms became cationic, which was consistent with the XPS results. We further separated the peak at
-1
wavenumber range of 20302110 cm by two peaks (Figure 6B and Table S7). The former peak at
-1
+
higher wavenumber (20832089 cm ) was assigned to the linearly adsorbed CO on Ir species, which
-1
+
also has been observed with strong intensity (2090 cm ) on partially oxidized Ir species over Ir-
36
Au/P25, Ir-Au/Al O and Ir-Ag/Al O . Particularly, for those Au-Ir/Al O and Ag-Ir/Al O samples,
2
3
2
3
2
3
2
3
0
3+
4+
the partially oxidized Ir species (Ir with Ir and Ir complex as confirmed by XPS) on these catalysts
+
surface after reduction showed similar vibrations position of linearly adsorbed CO on Ir as the case
36c, 36d
-1
of that on Ir-ReO /Rutile.
The latter peak at lower position (20702076 cm ) was linearly
x

+
adsorbed CO on Ir metal (Table S7). The area ratio of CO adsorbed on Ir to Ir metal was close to the
XPS fitting results (Table 7). It seems that the Ir-ReO /Rutile catalyst with lower Ir loading amount (2
x
+
wt%) is prone to be oxidized because higher proportion of Ir species was suggested on 2 wt%-Ir
catalysts (Table S7). Moreover, the less active catalyst with lower Ir loading amount (2 wt%-Ir) showed
2
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-
1
same peak position (2083 cm ) as the case of monometallic Ir/Rutile catalyst. This result suggests that
the local structure of catalyst with lower loading amount (2 wt%-Ir) was different from those catalysts
with higher Ir loading amount.
0
1
2
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0
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0
XAS analyses were performed with Ir/Rutile and Ir-ReO /Rutile catalysts after reduction or
x
catalytic use. The valence of Ir was determined from Ir L -XANES spectra using the intensity of white
3
+
line, and the results also supported the presence of Ir species for all the examined catalysts after
reduction (G, 573) or reaction (Figures S8(І)), which was consistent with TPR, XPS, and FT-IR of
*
adsorbed CO results. The calcined Ir-ReO /Rutile (Ir: 4 wt%, Re/Ir = 0.24 ) showed similar spectrum
x
to that of IrO . The Ir L -edge EXAFS analysis results are shown in Table 8 (the raw EXAFS spectra
2
3
are shown in Figure S9). The presence of Ir-O shell indicates the incomplete reduction of Ir species
over rutile TiO supported catalysts (Table 8, entries 112) in contrast to the fully reduced Ir metal on
2
Ir-ReO /SiO after reaction (entry 13). For the Ir/Rutile and Ir-ReO /Rutile catalysts after reduction or
x
2
x
reaction, similar fitting results including the number of shells, coordination number (CN), and bond
distance were obtained (entries 16), suggesting that the local structure was similar over catalysts after
reduction or reaction. The CN of Ir-Ir (or -Re) bond was about 9 for the 4 wt%-Ir Ir-ReO /Rutile
x
catalysts at various Re/Ir ratio (entries 39) with bond distance of 0.2740.275 nm. The CN of Ir-Ir
(
or -Re) bond was increased from about 8 to 10 when Ir loading amount increased from 2 wt% to 8
wt% at nominal Re/Ir ratio of 0.25 (entries 3, and 1012) with same bond distance of 0.274 nm. This
increasing of CN tendency can be explained by larger particles on catalysts with higher Ir loading
amount which was verified by TEM results. The Ir-ReO pairs on rutile TiO even with lower Re/Ir
x
2
ratio (Ir: 4 wt%) were in higher dispersion than SiO supported Ir-ReO pairs with higher Re/Ir ratio
2
x
2
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since the lower CN of Ir-Ir (or -Re) was suggested by curve fitting results. An interesting point was
the decrease of Ir-O bond distance after monometallic Ir/Rutile modified by Re. The bond distance of
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1
1
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0
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0
Ir-O decreased from about 0.212 nm on Ir/Rutile to 0.203 nm on Ir-ReO /Rutile, suggesting the
x
addition of Re increased the affinity between Ir and oxygen in a local region.
The Re L -XANES spectra are shown in Figure S8(ІІ) and (ІІІ), and the results of EXAFS analysis
3
are shown in Table 9 (the raw EXAFS spectra are shown in Figures S10). Similar valence of Re over
*
4
wt%-Ir Ir-ReO /Rutile (Re/Ir = 0.24 ) after calcination to NH ReO (Re precursor) confirmed the
x
4
4
accuracy of white line intensity method. The average valence of Re was determined to be about +3 for
all the examined Ir-ReO /Rutile catalysts, typically shown in Figure S8(ІІІ). This valence was similar
x
7
b
to the case of Ir-ReO /SiO + H SO (Ir: 4 wt%) after reaction, but higher than the high loading
x
2
2
4
*
amount Ir-ReO /SiO (Ir: 20 wt%, Re/Ir = 0.34 ) catalyst in the absence of H SO after reaction
x
2
2
4
9
b
(+1.1). There is no clear relationship between the average Re valence and catalytic performance.
Based on analyses of Re L -edge EXAFS in Table 9 (entries 110), the presence of Re-O bond with
3
CN from 0.5 to 1.1 was suggested over the tested catalysts, indicating that the Re species are partially
oxidized on catalysts after reduction or reaction. The bond distance of Re-O (0.2110.212 nm) over
rutile TiO supported catalysts was shorter than that (0.2020.203 nm) of SiO supported 4 wt%-Ir Ir-
2
2
7a, 7b
*
ReO catalyst,
which has also been observed over 20 wt%-Ir Ir-ReO /SiO (Re/Ir = 0.34 )
x 2
x
9
b
catalyst. It is likely that the single bond of Re-O is more abundant on both Ir-ReO /Rutile and 20 wt%-
x
Ir Ir-ReO /SiO than that on 4 wt%-Ir Ir-ReO /SiO . With respect to the bond distance of Re-Re (or -
x
2
x
2
Ir), it was similar for those catalysts regardless of support (rutile TiO and SiO ) or composition of
2
2
catalysts (0.2630.268 nm). Compared to the bond distance of Ir-Ir (or -Re) (0.2740.275 nm), the
2
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shorter bond distance (0.2630.268 nm) suggests the absence of Ir-Re alloy, which is in agreement
with other characterization results (CO FT-IR and XPS). An important point was the CN of Re-Re (or
0
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3
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2
3
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0
-Ir), which to some extent can reflect the strength of Ir-ReO interaction. Our very recent work
x
suggested that the Ir-ReO interaction can be enhanced by increasing the Ir loading amount up to 20
x
9b
wt% on SiO . In that case, slight larger ReO clusters were stabilized and attached to neighboring Ir
2
x
*
metal surface with an increase of CN of Re-Re (or -Ir) from 6.2 (4 wt%-Ir Ir-ReO /SiO , Re/Ir = 0.83 )
x
2
9
b
to 8.6 (20 wt%-Ir Ir-ReO /SiO , Re/Ir = 0.34). Higher CN of Re-Re (or -Ir) was also obtained on
x
2
rutile TiO supported catalysts except 2 wt%-Ir Ir-ReO /Rutile. The CN of Re-Re (or -Ir) over 2 wt%-Ir
2
x
Ir-ReO /Rutile catalyst was similar to the 4 wt%-Ir Ir-ReO /SiO (entries 8 and 11), but lower than that
x
x
2
over other Ir-ReO /Rutile catalysts. In addition, the fitting results of corresponding catalysts after
x
reduction or reaction (entries 14) were similar, indicating that such stable structure can be maintained
after catalytic use.
Relationship between activity and catalyst structure in glycerol hydrogenolysis to 1,3-PrD
over Ir-ReO /Rutile. On the basis of the discussion of activity and characterization results above, the
x
relationship between the catalytic performance and structure was further clarified. The average valence
of both Ir and Re determined by different method is summarized in Table 5, and those results are
consistent except the Re valence calculated by TPR (Table S6). The disagreement can be due to the
incomplete reduction of Ir species, and partially reduced TiO which was catalyzed by noble metal as
2
we discussed in earlier section. The results of XPS and XANES analyses for Re valence were similar,
and we think that XPS and XANES reflected the exact Re valence. Here the glycerol conversion was
fixed at 2025% to compare the 1,3-PrD productivity (P1,3-PrD) over various Ir-ReO /Rutile catalysts
x
2
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with different compositions (Figure 7 and Table 5). The increasing tendency of P1,3-PrD correlated to
the Ir loading amount with similar trend of CN of Re-Re (or -Ir) change in the range of 26 wt%, and
then it decreased slightly at higher Ir loading amount with higher CN of Re-Re (or -Ir) (Figure 7A).
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0
+
+
0
Then we considered the relationship between the activity and surface ratio of Ir /(Ir +Ir ) which was
+
determined from fitting results of CO FT-IR (Table S7). If the Ir species were directly involved in
the glycerol hydrogenolysis, the activity over 2 wt%-Ir Ir-ReO /Rutile should be higher than those
x
catalysts with higher Ir loading amount since smaller particles (TEM and EXAFS) and more Ir
+
species (CO FT-IR) were suggested on 2 wt%-Ir catalyst. The lower activity over Ir-ReO /Rutile at
x
+
lower Ir loading amount excludes the direct participation of Ir species during the hydrogenolysis
reaction. We further calculated the coverage ratio of ReO on Ir metal surface (Table 5), and coverage
x
0
ratio (6267%) was almost independent of Ir loading amount on rutile TiO . The density of surface Ir
2
0
on rutile TiO (Ir loading amount / 192.2  N  D  Ir ratio (determined from CO FT-IR) / S of
2
A
CO
BET
2
2
rutile TiO ) decreased in order: 8 wt%-Ir Ir-ReO /Rutile (5.4/nm )  6 wt%-Ir (4.7/nm )  4 wt%-Ir
2
x
2
2
(
4.4/nm )  2 wt%-Ir (2.4/nm ), which correlated with P
value over catalysts at Ir amount from
,3-PrD
1
2 to 6 wt% as plotted in Figure 7A. The lower P1,3-PrD over 8 wt%-Ir Ir-ReO /Rutile was attributed to
x
the larger particle size (3.0 nm, Figure S4). On the other hand, similar kinetic results on 4 wt%-Ir Ir-
*
ReO /Rutile (Re/Ir = 0.24 ) and SiO supported Ir-ReO catalysts (reaction order on H about 1 and
x
2
x
2
reaction order on glycerol concentration about 0 under standard reaction conditions) suggest that rutile
TiO is not directly involved in this reaction. Therefore the active site on Ir-ReO /Rutile is the Ir-ReO
2
x
x
interface and the activity showed dependency on the structure of Ir and Re species. In the case of 4
wt%-Ir catalysts with different Re/Ir ratio (Figure 7B), slightly higher ratio gives higher surface ratio
2
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