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Effect of the Reduction Temperature on the Activity and Selectivity of Titania-Supported Iridium Nanoparticles for Methylcyclopentane Reaction Orlando Hernández-Cristóbal, Gabriela Díaz,* and Antonio Gómez-Cortés Instituto de Física, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica s/n, Ciudad Universitaria, México DF 04510, México ABSTRACT: The catalytic performance of Ir/TiO2 reduced at low (573 K, LTR) and high temperature (773 K, HTR) was studied in the methylcyclopentane (MCP) reaction at atmospheric pressure in a flow system. Ir/Al2O3 was used as the reference catalyst. Catalysts were prepared by deposition−precipitation with urea and characterized by ICP, H2-TPR, HAADF-STEM, CsSTEM, and DRIFTS. For LTR samples, the activity follows the order Ir/TiO2 > Ir/Al2O3, and at low conversion and temperature, ring opening (RO) of MCP is selective (2MP/3MP = 2.7; n-H/3MP = 0.05) with SRO = 100%. As the reaction temperature increased, C1−C5 products appeared but Ir/TiO2 was still more selective to RO products than Ir/Al2O3. Ir/TiO2 HTR was significantly less active, with SRO = 100% and a product distribution approaching a nonselective behavior with n-H/ 3MP = 1. This was not observed in the case of Ir/Al2O3 HTR. The structure of Ir/TiO2 HTR included nanoparticles, clusters of few atoms, and isolated iridium atoms. The results were analyzed in light of the metal−support interaction (SMSI) effect.

1. INTRODUCTION Supported metal catalysts are typically constituted of metal nanoparticles dispersed on the surface of an oxide support. They are key components in a great number of technological processes from oil refining to environmental protection. The study of the C−C bond cleavage of cyclic compounds at the surface of metal catalysts is of great fundamental and practical interest. For example, the actual demand of cleaner fuels requires the selective opening of naphthenic compounds to paraffins while avoiding the secondary reactions leading to light hydrocarbons. Among cyclic molecules, methylcyclopentane (MCP) has been extensively studied as a probe molecule on supported metal catalysts.1−8 The product distribution obtained from endocyclic C−C bond cleavage over metals is strongly dependent on the catalyst structure, which, in turn, can be modified by parameters such as the particle size or metal− support interactions. Also, experimental conditions and the presence of carbonaceous residues play a role in defining the reaction products.5−10 The products from the ring opening (RO) of MCP are n-hexane (n-H), 2-methylpentane (2MP), and 3-methylpentane (3MP), and the product distribution results from three possible mechanisms according to Gault et al.1 A nonselective mechanism leads to an equal probability of breaking the C−C bonds, producing a statistical distribution of products with 2MP/3MP and n-H/3MP ratios both equal to 2. A completely selective mechanism (dicarbene path) will produce only 2MP and 3MP with ratios n-H/3MP = 0 and 2MP/3MP = 2. The third type of mechanism is partially selective, yielding to a product distribution intermediate between those obtained with the nonselective mechanism and the selective one. While in platinum-based catalysts particle size effects characterize the behavior of this metal in the MCP reaction,1,11 these effects have not been seen in the case of iridium-based catalysts, where a selective mechanism prevails, leading to the formation of 2MP and 3MP8,12,13 for the MCP © 2014 American Chemical Society

reaction at atmospheric pressure. The same result was obtained more recently by studying the MCP reaction at high pressure.12,14 Changes in the mechanism to a nonselective one have been observed when the iridium particles supported on alumina become covered with carbonaceous residues,7,15,16 and by this, large ensembles of metal atoms are eliminated, suppressing the formation of dicarbene intermediates. Contrary to alumina support, the dicarbene mode (selective mechanism) was observed when silica was used as the support for hydrogenolysis of naphthenic C6 rings.17 Metal−oxide interactions are important factors determining the properties and performances of supported catalysts. In particular, titania has played a central role because the metal− support interactions (SMSI) developed after high-temperature reduction treatments have been among the strongest observed, leading to anomalous behavior of the catalyst.18,19 Contrary to the extensive reports in the literature concerning the use of refractory oxides such as Al2O3 and SiO2 as supports of metal nanoparticles for MCP conversion, less information is available in the case of titania. Most of the studies concern, for example, Pt/TiO2 and Rh/TiO2.20−22 In the case of Ir/TiO2,9,23 studies were found on the properties of this system in framework studies concerning bimetallic platinum−iridium-supported catalysts. The goal of this work was to study the MCP reaction at atmospheric pressure on Ir/TiO2 prepared by deposition− precipitation with urea (DPU). The objective focused on the properties of the iridium nanoparticles when the catalyst is submitted to high-temperature reduction where SMSI effects Received: Revised: Accepted: Published: 10097

March 26, 2014 May 20, 2014 May 23, 2014 May 23, 2014 dx.doi.org/10.1021/ie501283c | Ind. Eng. Chem. Res. 2014, 53, 10097−10104

Industrial & Engineering Chemistry Research

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packed directly in the sample holder and activated in situ at 573 or 773 K for 1 h under a 5% H2/He flow (30 mL/min). After the reduction treatment, the cell was purged with He and then cooled to 298 K. A reference spectrum of the solid was collected under a He flow before admittance of 5% CO/He (30 mL/min) for 2 min. Then the cell was purged again with He for 10 min, and a new DRIFT spectrum was collected. In all cases, reported spectra take into account the spectrum of the freshly reduced solid prior to CO adsorption. 2.3. Catalytic Activity. The gas-phase conversion of methylcyclopentane (MCP; Aldrich) was followed at atmospheric pressure in the temperature range 413−573 K in a RIG100 multitask unit. Before catalytic tests, 0.025 g of the catalyst sample was activated in situ with a H2 flow (30 mL/min) at 573 or 773 K. After activation, a feed of hydrogen (30 mL/min) saturated with MCP vapor obtained by bubbling H2 through a saturator containing MCP at 253 K was admitted through the system. Under these conditions, the molar ratio H2/MCP = 50 and the weight hourly space velocity (WHSV) was 1440 h−1. The reaction temperature was increased progressively, and catalytic measurements were taken at different temperatures. The reaction products were analyzed online using a gas chromatograph equipped with a flame ionization detector. MCP conversion as a function of the reaction temperature was calculated as follows:

are classically reported. We found that the Ir/TiO2 catalyst reduced at 573 K was more active compared with the Ir/Al2O3 catalyst taken as a reference, although both samples showed a selective RO of MCP with SRO = 100% at low reaction temperature and conversion. Also, at high reaction temperature and conversion, Ir/TiO2 was still more selective to MCP RO than Ir/Al2O3. The reduction at 773 K produced a less active Ir/TiO2 catalyst with SRO = 100% and a product distribution with 2MP/3MP = 2 and n-H/3MP = 1, indicating a less selective catalyst. Diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO pointed to a blockage of adsorption sites at the surface of Ir/TiO2 but not in the case of the alumina-supported catalyst.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Samples were prepared by DPU. Titania P-25 (50 m2/g, 80% anatase and 20% rutile) and Al2O3 (Degussa, 80 m2/g) were used as supports, and IrCl4· xH2O was obtained from Aldrich as the iridium precursor. Before preparation, the supports were dried at 373 K for 24 h. The iridium nominal content in the catalyst was 1 wt %. The preparation method was reported previously.24 The support was added to an aqueous solution containing IrCl4 and urea, and the suspension was maintained at 353 K under vigorous stirring for 16 h. After the deposition−precipitation procedure, the sample was washed with water, then centrifuged, dried under a vacuum, and finally calcined at 673 K for 2 h. Reduction treatments in hydrogen flow were done at 573 K (low-temperature reduction, LTR) or 773 K (high-temperature reduction, HTR). 2.2. Catalyst Characterization. The actual iridium content in the samples was determined by inductively coupled plasma (ICP) using an Optima 4300 DV (Perkin-Elmer) spectrophotometer. Hydrogen temperature-programmed reduction (H2-TPR) experiments were done in a RIG-100 unit under a 5% H2/Ar gas mixture (30 mL/min) flow and a heating rate of 10 K/min from room temperature to 773 K. The particle size distributions of the reduced catalysts were obtained from micrographs recorded on a JEM 2010 FasTem analytical microscope from JEOL equipped with a Z-contrast annular detector (HAADF). Samples of the catalysts were ground and ultrasonically dispersed in ethanol for 5 min. Drops of the dispersion were deposited on commercial holey-carbon/ lacey-carbon-coated copper grids for observation. A total of 250 particles were measured to obtain the particle size distribution and average particle size defined as ⟨ds⟩ =

α (%) =

[MCPin − MCPout ] × 100 (mol%) [MCPin]

where MCPin and MCPout are respectively the moles of MCP in the feed before and after reaction. Selectivity toward the different reaction products was obtained from the following expression: Si (%) =

Ci × 100 (mol%) n ∑i = 1 Ci

where Ci represents the moles of products with i = 1 to n = 6 carbon atoms.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The actual iridium content as determined by ICP was 0.80 and 0.86 wt % respectively for iridium supported on titania and alumina. The reduction properties of the catalysts were studied by hydrogen-programmed reduction (TPR). Figure 1 displays the

∑i ni(di)3 ∑i ni(di)2

where di is the diameter of the ni particle. In addition, the structure of the Ir/TiO2 catalyst reduced at high temperature was analyzed in a JEOL ARM200F (200 kV) scanning transmission electron microscopy (STEM)/transmission electron microscopy (TEM) microscope equipped with a CEOS Cs corrector on the illumination system. CO adsorption on the surface of the catalysts was studied by FTIR spectroscopy in a Nicolet Nexus 470 spectrophotometer using an environmentally controlled high-pressure/high-temperature Spectra Tech DRIFT cell with ZnSe windows. All spectra were collected from 128 scans with a resolution of 4 cm−1. For each experiment, 0.025 g of the reduced sample was

Figure 1. H2-TPR profiles of TiO2, Ir/TiO2, and Ir/Al2O3 catalysts calcined at 673 K. 10098

dx.doi.org/10.1021/ie501283c | Ind. Eng. Chem. Res. 2014, 53, 10097−10104


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Figure 2. Typical images and particle size distributions of catalysts reduced at 773 K: (a) Ir/TiO2; (b) Ir/Al2O3. (c) Structure of the Ir/TiO2 HTR catalyst as revealed from Cs-STEM observation.

H2-TPR profiles of TiO2, Ir/TiO2, and Ir/Al2O3 samples calcined at 673 K. TPR of the titania support is characterized by small hydrogen uptake at temperatures higher than 600 K. For Ir/TiO2, the TPR profile showed three clearly defined peaks with maximum hydrogen consumption at 366, 409, and 507 K. Small hydrogen uptake is also observed at temperatures higher

than 550 K. The peak around 370 K could be assigned to the reduction of some large iridium oxide particles,25,26 whereas a larger number of iridium oxide species are reduced around 510 K, which can be identified as well-dispersed iridium species.26,27 The high-temperature hydrogen uptake observed in the TPR profile could be associated with either reduction of the iridium 10099



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Figure 3. DRIFT spectra of CO adsorbed at 298 K on the surface of Ir/TiO2 and Ir/Al2O3 catalysts reduced at low (LTR, 573 K) and high temperature (HTR, 773 K): (a) Ir/TiO2; (b) Ir/Al2O3.

respectively. Ir0 and IrO2 were identified in the Ir/TiO2 LTR catalyst from high-resolution TEM images, and the presence of some remaining iridium oxide after reduction at 573 K is in good agreement with the TPR results. After reduction at 773 K, only iridium metal particles were identified. Observation of the structure of the Ir/TiO2 HTR catalyst using a Cs-HR-STEM/TEM-HAADF microscope evidenced that the iridium phase was present as nanoparticles but also as clusters of a few atoms and isolated iridium atoms on the surface of the support (Figure 2c). The existence of such a single-atom structure on the surface of a support has been reported in the case of supported mononuclear iridium complexes prepared by the reaction of Ir(Acac) with highly dehydroxylated MgO29 and in Ir/FeOx catalysts.30 The structure of the Ir/TiO2 HTR catalyst provides a high density of reactive sites located at faces, corners, and steps that characterize highly dispersed catalysts. DRIFT spectra of CO adsorbed at room temperature on the surface of catalysts reduced in situ at 573 or 773 K are shown in Figure 3. In Figure 3a, the Ir/TiO2 LTR and HTR catalysts are presented. For the LTR catalyst, a broad intense band in the spectral range 1950−2100 cm−1 is observed, indicating that CO is adsorbed on iridium sites of different coordination. Decomposition of the spectrum showed four contributions with maxima at 2076, 2061, 2052, and 1998 cm−1. On their side, the Ir/Al2O3 sample reduced at 573 K (Figure 3b) showed a much broader absorption band in the carbonyl region (1950− 2175 cm−1). Decomposition led to CO bands at 2122, 2077, 2043, 2000, and 1947 cm−1.

species not already reduced or eventually reduction of the support (Ti 4+ → Ti 3+). The observation of hydrogen uptake at these temperatures already in the bare titania supports the idea of some reduction of TiO2. It is well-known that TiO2 is partially reduced to TiO2‑x by hydrogen at high temperatures, and this process is promoted by the presence of dispersed metal crystallites.18 On their side, Ir/Al2O3 TPR is characterized by hydrogen consumptions at higher temperature, one broad peak around 500 K, and another at higher temperature (ca. 730 K). Quantitative data of hydrogen consumption associated only with reduction peaks below 573 K showed that the reduction of iridium oxide to iridium metal in the calcined samples is not complete assuming the Ir4+ → Ir0 reduction process; however, the reduction degree in the case of a titania-supported catalyst is higher than that in the case of an alumina-supported catalyst. Accordingly, the reduction at 773 K will improve the degree of reduction of the iridium phase in the sample. The average particle sizes of iridium nanoparticles supported on TiO2 after reduction at 573 and 773 K were respectively 1.4 and 1.7 nm. In the case of Ir/Al2O3 catalysts, the average particle size was slightly lower, 1.1 nm for the LTR sample and 1.3 nm for the HTR one. Iridium particles disperse well over both supports, and reduction at 773 K led to an increase in the average particle size of only about 20% in both samples. Analysis of the results in the literature has demonstrated the intrinsic tendency of iridium to be present in clusters with diameters of around 1 nm and to be resistant to sintering.28 Representative HAADF images of Ir/TiO2 and Ir/Al2O3 catalysts reduced at 773 K and their corresponding particle size distributions are shown in parts a and b of Figure 2, 10100



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Let us examine the second possibility, the blockage of adsorption sites, as the cause of the intensity diminution of the CO band when adsorbed on the surface of the Ir/TiO2 HTR catalyst. When metal nanoparticles are supported on reducible oxides and reduced at high temperature (773 K), a strong SMSI develops. The evidence of such a state is the loss of the adsorption capacity of H2 and CO, which cannot be explained in terms of metal dispersion.18 Several models, geometric or electronic in nature, have been proposed to explain this phenomenon. Among them, the migration of partially reduced support moieties to the surface of the nanoparticles, blocking in this way metal adsorption sites or reaction sites, was proposed. In the case of Ir/TiO2 catalysts, surface analysis by X-ray photoelectron spectroscopy of samples reduced at 773 K has shown that partial coverage of the iridium crystals by TiO2−x species occurs when progressively higher temperatures are employed to reduce the catalyst.36 Accordingly, diminution of the absorption band when the Ir/TiO2 catalyst is reduced at 773 K can most likely be explained by this phenomenon. On the other hand, the observed slight red shift of the band could be due to a diminution of the vibration coupling of adsorbed CO molecules when the surface coverage decreases. 3.2. Catalyst Performance. The results for MCP reaction are shown in Figure 4, where MCP conversion, as previously

When the Ir/TiO2 sample was reduced at 773 K, the intensity of the CO band substantially decreased, becoming more symmetrical, and a slight shift of the maximum to low wavenumbers was observed (Figure 3a; Ir/TiO2 HTR). This behavior is clearly related to the presence of the reducible TiO2 support because when iridium was deposited on a refractory oxide like Al2O3, this trend was not observed (Figure 3b; Ir/ Al2O3 HTR). The intensity of the band in the case of the Ir/ Al2O3 sample did not change significantly, but the relative contribution of the bands associated with specific CO adsorbed species changed compared to the LTR sample. Also, a shift to lower wavenumbers was observed. Assignment of the bands is somehow complicated because of the various assignments found in the literature even for the same band. Two types of adsorbed species have been generally recognized, linear (LCO) and gem dicarbonyl (GCO) adsorbed species. Solymosi et al.,31,32 studying the adsorption of CO over Ir/Al2O3 reduced at different temperatures, observed two bands at 2060−2070 and 2020 cm−1; the former was attributed to CO linearly bonded to larger iridium crystallites and the latter to CO linearly bonded to highly dispersed iridium atoms influenced strongly by the support. Toolenaar et al.33 found two peaks at 2050 and 2080 cm−1, which were assigned to CO adsorbed on low-coordination iridium sites (i.e., edges and corners) and CO adsorbed on high-coordination iridium sites (i.e., crystal planes), respectively. McVicker et al.34 concluded that the band at ca. 2078 cm−1 is associated with CO adsorbed on isolated iridium atoms that can accommodate two CO molecules each, i.e., iridium dicarbonyl species, whereas the peak at ca. 2009 cm−1 is attributable to iridium carbonyl species. Bourane et al.,35 studying the CO species adsorbed over 1 wt % Ir/γ-Al2O3, also found a pair of bands at ca. 2080 and 2000−2013 cm−1, and on the basis of their behavior, they assigned these bands to Ir(CO)2 species. These bands are noticed generally in highly dispersed iridium catalysts, where GCO species adsorb on small clusters or isolated iridium atoms. On the other hand, Irδ+CO species were identified by a CO band at ca. 2100 cm−1 on oxidized Ir/Al2O3 after CO adsorption at room temperature.32 On the basis of the high dispersion and structure of the catalysts in our study, as evidenced by electron microscopy and taking into account the reported literature, we may assign the peaks around 2050 cm −1 to LCO adsorbed on lowcoordination iridium sites and the bands at ca. 2076 and 2000 cm−1 to GCO species. The high-frequency contribution around 2120 cm−1 in the alumina catalyst could be due to the presence of remaining Irδ+ species. The higher reduction temperature will change the particle size distribution and therefore the topology of the iridium sites available for CO adsorption, which may account for differences in the types of CO adsorbed species. The diminution of the absorption intensity of the CO adsorbed on the surface of the iridium nanoparticles in the case of the Ir/TiO2 HTR sample would indicate less adsorption iridium sites when the sample is submitted to high-temperature reduction. This can be explained either by an increase in the particle size due to sintering or by a blockage of adsorption sites at the surface of the metal nanoparticles. Sintering can be discarded as the reason for an important diminution of the surface iridium sites because a significant increase in the particle size was not observed by STEM.

Figure 4. Conversion of MCP as a function of the reaction temperature over Ir/TiO2 and Ir/Al2O3 (LTR and HTR) catalysts.

defined, is plotted as a function of the reaction temperature for LTR and HTR Ir/TiO2 and Ir/Al2O3 catalysts. Titania and alumina supports showed no activity in the temperature range analyzed. At the experimental conditions of this work, Ir/TiO2 LTR was more active than Ir/Al2O3 LTR; in the former, about 10% conversion is obtained at 433 K, while in the latter, this conversion is attained at 493 K. This result could be due to a lower percentage of reduced iridium phases in the case of the alumina-supported catalyst, as the TPR experiments indicated. The reduction treatment performed at 773 K strongly affected the activity of the titania-supported catalyst, Ir/TiO2 HTR; total suppression of the activity was not observed, but a less active catalyst was generated; around 8% conversion of MCP was attained at 533 K, about 100 K higher than on Ir/ TiO2 LTR catalyst. For the Ir/Al2O3 HTR catalyst, on the basis that more reduced iridium sites will be available after hightemperature reduction, an increase in the activity of the catalyst was expected. At low reaction temperatures, the catalyst was more active than the Ir/Al2O3 LTR sample, but as the reaction took place at subsequently higher temperatures, the activity of 10101



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rapidly than 3MP) and n-H slightly increased. For example, the product distribution at 493 K is 2MP (52.0 mol %), 3MP (22.1 mol %), and n-H (1.8 mol %). At 573 K, the only reaction product was CH4. In the case of the Ir/Al2O3 LTR catalyst, about the same behavior was observed. On their side, Ir/TiO2 HTR is 100% selective for RO of MCP, with a product distribution approaching a nonselective behavior with a higher production of n-H (2MP/3MP = 2; nH/3MP = 1). The RO product distribution at similar conversion for the Ir/TiO2 catalyst reduced at 773 K was 2MP (50 mol %), 3MP (24 mol %), and n-H (26 mol %). As in the LTR sample, Ir/Al2O3 HTR was also 100% selective to RO products at low temperature and conversion. The 2MP/3MP and n-H/3MP ratios were 2.6 and 0.03, respectively. As the reaction temperature and conversion increased, the values of these ratios were around 2 and 0.4, which were about the same as those in the LTR catalyst, but as a consequence of the lower activity, the catalyst was more selective toward RO compared to the LTR sample. Accordingly, under our experimental conditions, Ir/TiO2 reduced at 573 K was more active and more selective to RO products at high temperature and conversion than the aluminasupported catalyst taken as a reference. In terms of the RO product distribution, as the reaction temperature increased, both catalysts behaved about the same way. Also, the 2MP/ 3MP and n-H/3MP values changed with the reaction temperature, showing a trend of progressively lower 2MP/ 3MP ratio and higher n-H/3MP ratio. This behavior was also observed by Djeddi et al.,9 although experimental conditions were not the same. The progressive appearance of cracking products was related to the presence of consecutive reactions as conversion increased. As the reaction temperature increased, the relative coverage of the catalyst surface by hydrocarbon molecules and hydrogen changed and multiple fragmentations took place as the product molecules remained strongly adsorbed at the catalyst surface.37 After the reduction at 773 K, the activity of both the titaniaand alumina-supported iridium catalysts changed although apparently not in the same way. The titania HTR catalyst was less active, with 100% selectivity to the RO products showing a product distribution approaching a nonselective behavior with higher n-H production, n-H/3MP = 1. The activity of the alumina HTR was higher in a first stage compared to the LTR sample but showed signs of deactivation as the reaction proceeded. The catalyst was 100% selective to RO of MCP, with about the same product distribution as shown by the LTR catalyst. Examples of SMSI for metals supported on nonreducible carriers such as Al2O3 have been reportedly connected with reduction at high temperatures. A strongly reduced adsorption capacity after reduction at temperatures above 773 K has been observed on alumina-supported catalysts. Formation of an alloy under these conditions has been suggested.38,39 Under another approach, using model systems, a gradual interdiffusion of the surface species in the alumina support has been shown.40 The influence of the calcination temperature has also been considered in the development of a thin layer of an aluminate phase decorating the surface of the metal particles.41 While this reduction temperature was enough in the case of the titania catalyst to provoke classical SMSI effects on the adsorption capacity of the catalyst, it is borderline in the case of the alumina catalyst. When Ir/Al2O3 HTR and LTR samples were compared, the DRIFT spectra showed that the CO probe

the Ir/Al2O3 HTR catalyst did not follow the same trend as the LTR catalyst. Moreover, the behavior of the MCP conversion with temperature indicated some loss of activity of the catalyst. It seems that HTR treatment is not leading to a solely more reduced catalyst. Concerning the selectivity displayed by the catalysts, two types of reaction products were observed as a function of the reaction temperature over Ir/TiO2 and Ir/Al2O3 catalysts, RO products coming from cleavage of a single C−C bond of MCP (2MP, 3MP, and n-H), and cracking products (C1−C5) coming from multiple C−C bond ruptures. In Table 1, the catalytic Table 1. Catalytic Performance of Ir/TiO2 and Ir/Al2O3 (LTR and HTR) for the MCP Conversion catalyst Ir/TiO2 LTR

Ir/Al2O3 LTR

Ir/TiO2 HTR Ir/Al2O3 HTR

T (K)

α (%)

SRO (%)

SCR (%)

2MP/ 3MP

n-H/ 3MP

433 453 473 493 513 553 573 473 493 513 533 553 573 533 593 473 493 573

10 18 45 78 88 94 100 4 10 24 54 83 94 8 15 12 18 70

100 97 95 76 50 10 0 100 100 92 5 2 0 100 100 100 100 34

0 3 5 24 50 90 100 0 0 8 95 98 100 0 0 0 0 66

2.7 2.6 2.5 2.4 2.2 1.8 2.7 2.7 2.3 2.3 2.3 2.1 2 2.6 2.5 2

0.05 0.03 0.05 0.1 0.1 0.2 0.02 0.01 0.1 0.2 0.2 1.1 1.0 0.03 0.04 0.4

α = MCP conversion; SRO = ring-opening product selectivity; SCR = cracking products (C1−C5) selectivity; LTR = low-temperature reduction (573 K); HTR = high-temperature reduction (773 K); 0.025 g of catalyst; H2/MCP = 50; WHSV = 1440 h−1.

properties displayed by Ir/TiO2 and Ir/Al2O3 LTR and HTR catalysts are shown, where the reaction temperature, total conversion, selectivity for ring-opening (SRO) and cracking (SCR) products, and the 2MP/3MP and n-H/3MP ratios that provide information about the types of mechanisms for RO (selective or nonselective) are tabulated. From inspection of the data in Table 1, it is confirmed that, as reported in the literature for iridium-based catalysts,9,11−13 Ir/TiO2 LTR and Ir/Al2O3 LTR catalysts are very selective for RO reactions; at low temperature and about 10% conversion, SRO is 100% with 2MP/3MP and n-H/3MP ratios of around 2.7 and 0.03, indicating a selective mechanism by dicarbene species where n-H is produced in very small quantities. As the reaction temperature and MCP conversion increased, SRO gradually decreased on both catalysts, although for similar conversion, titania-supported iridium is still more selective for RO than the alumina-supported catalyst; at MCP conversion around 80%, SRO is 76% for Ir/TiO2 LTR and only 2% for Ir/ Al2O3 LTR. At low temperature and conversion over Ir/TiO2 LTR, the RO products follow the order 2MP (72.2 mol %), 3MP (27.4 mol %), and n-H (1.3 mol %), and as the temperature increased, the 2MP and 3MP products decreased (2MP more 10102



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(6) Paál, Z. Hydrocarbon product selectivity: a tool for characterizing the active state of platinum catalysts. Catal. Today 1988, 2, 595. (7) Fulop, E.; Gnutzmann, V.; Paál, Z.; Vogel, W. EuroPt-1 after various pretreatments: structure, surface composition and catalytic activity in methylcyclopentane reactions. Appl. Catal 1990, 66, 319. (8) Van Senden, J. G.; Van Broekhoven, E. H.; Wreesman, C. T. J.; Ponec, V. Selectivity of iridium catalysts in reactions of C 6 hydrocarbons: The role of surface carbonaceous layers and metal particle size. J. Catal. 1984, 87, 468. (9) Djeddi, A.; Fechete, I.; Garin, F. Selective Ring-Opening of Methylcyclopentane over Titania-Supported Monometallic (Pt, Ir) and Bimetallic (Pt−Ir) Catalysts. Top Catal. 2012, 55, 700. (10) Djeddi, A.; Fechete, I.; Garin, F. Selective ring opening of methylcyclopentane over Pt/γ-Al2O3, Ir/γ-Al2O3 and Pt-Ir/γ-Al2O3 catalysts with hydrogen at atmospheric pressure. Appl. Catal., A 2012, 413− 414, 340. (11) Gault, F. G.; Amir-Ebrahimi, V.; Gain, F.; Parayre, P.; Weisang, F. Skeletal rearrangements of hydrocarbons on metals. Bull. Soc. Chim. Belg. 1979, 88, 475. (12) McVicker, G. B.; Daage, M.; Touvelle, M. S.; Hudson, C. W.; Klein, D. P.; Baird, W. C., Jr.; Cook, B. R.; Chen, J. G.; Hantzer, S.; Vaughan, D. E. W.; Ellis, E. S.; Feeley, O. C. Selective Ring Opening of Naphthenic Molecules. J. Catal. 2002, 210, 137. (13) Weisang, F.; Gault, F. G. Selective Isomerization of Methylpentanes on Iridium Catalysts. J. Chem. Soc., Chem. Commun. 1979, 11, 519. (14) Samoila, P.; Boutzeloit, M.; Especel, C.; Epron, F.; Marécot, P. Selective ring-opening of methylcyclopentane on platinum-based bimetallic catalysts. Appl. Catal., A 2009, 369, 104. (15) Chow, M.; McVicker, G. B. Conversion of methylcyclopentane and acyclic hexanes over supported platinum catalysts: II. Partial pressure and temperature effects on the formation of methylcyclopentane from the three acyclic hexanes. J. Catal. 1988, 112, 303. (16) Schepers, F. J.; Van Broekhoven, E. H.; Ponec, V. Apparent particle size sensitivity in hydrocarbon reactions. J. Catal. 1985, 96, 82. (17) Do, P. T.; Alvarez, W. E.; Resasco, D. E. Ring opening of 1,2and 1,3-dimethylcyclohexane on iridium catalysts. J. Catal. 2006, 238, 477. (18) Tauster, S. J.; Fung, S. C. Strong metal−support interactions: Occurrence among the binary oxides of groups IIA−VB. J. Catal. 1978, 55, 29. (19) Haller, G. L.; Resasco, D. E. Metal−Support Interaction: Group VIII Metals and Reducible Oxides. Adv. Catal. 1989, 36, 173. (20) Anderson, J. B. H.; Burch, R.; Cairns, J. A. The influence of metal−support interactions on the reaction of methylcyclopentane over supported Pt catalysts. J. Catal. 1987, 107, 351. (21) Clarke, J. K. A.; Dempsey, R. J.; Baird, T. Effects of Strong Metal−Support Interactions on Hydrocarbon Reactions over Titaniasupported Platinum. Mode of Action of a Near-selective Hydrogenolysis Catalyst. J. Chem. Soc., Faraday Trans. 1990, 86, 2789. (22) Fenoglio, R. J.; Nuñez, G. M.; Resasco, D. E. Effects of oxidation and reduction treatments on methylcyclopentane hydrogenolysis over Rh/TiO2 and Rh/SiO2 catalysts. J. Catal. 1990, 121, 77. (23) Vicerich, M. A.; Benitez, V. M.; Especel, C.; Epron, F.; Pieck, C. L. Influence of iridium content on the behavior of Pt-Ir/Al2O3 and PtIr/TiO2 catalysts for selective ring opening of naphthenes. Appl. Catal., A 2013, 453, 167. (24) Gómez-Cortés, A.; Díaz, G.; Zanella, R.; Ramírez, H.; Santiago, P.; Saniger, J. Au-Ir/TiO2 Prepared by Deposition Precipitation with Urea: Improved Activity and Stability in CO Oxidation. J. Phys. Chem., C 2009, 113, 9710. (25) Huang, Y.-J.; Xue, J.; Schwarz, J. A. Analysis of temperatureprogrammed reduction profiles from metal-supported catalysts. J. Catal. 1988, 111, 59. (26) Carnevillier, C.; Epron, F.; Marecot, P. Controlled preparation and characterization of plurimetallic Pt−Sn and Pt−Ir−Sn/Al2O3 reforming catalysts. Appl. Catal., A 2004, 275, 25.

is not adsorbing over significantly different iridium surfaces, as in the case of the titania catalyst. The activity behavior of the Ir/Al2O3 HTR catalyst deserves close attention, but it is beyond the scope of the present work. As a consequence of the SMSI state, after reduction at 773 K, particular sites may develop at the catalyst surface, which characterizes the ability of the Ir/TiO2 catalyst to cleave the secondary−tertiary C−C bonds in MCP. Also, active sites related with the catalyst structure, as revealed by Cs-STEM, could play a role in the observed catalytic performance.

4. CONCLUSIONS Highly dispersed iridium supported on TiO2 catalysts were prepared by DPU and reduced at low (LTR, 573 K) and high temperature (HTR, 773 K). The average particle size did not vary significantly after reduction at high temperature. The catalyst structure included iridium nanoparticles, clusters, and isolated iridium atoms. For MCP reaction, Ir/TiO2 LTR was more active than a reference Ir/Al2O3 catalyst reduced at 573 or 773 K. At low conversion and temperature, the only observed products were 2MP, 3MP, and n-H, with a distribution indicating a selective mechanism for RO of MCP. At high temperature and conversion, C1−C5 products appeared; however, even at these conditions, iridium supported on titania was more selective to RO products compared to the aluminasupported catalyst. When reduced at 773 K, the Ir/TiO2 catalyst suffered a loss of activity, showing SRO = 100% and a product distribution approaching nonselective ring cleavage of MCP. Particular sites developed at this state after reduction at 773 K along with the catalyst structure may be responsible for the behavior of the Ir/TiO2 catalyst.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: diaz@fisica.unam.mx. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS LCM-IF and the Kleberg Advanced Electron Microscopy Center, UTSA, is acknowledged for the electron microscopy images. M. Sc. Ciro Márquez from FQ-UNAM is acknowledged for ICP determinations. O.H.-C. is grateful to CONACYT and Red Nano-CONACYT for financial support.

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REFERENCES

(1) Maire, G.; Plouidy, G.; Prudhomme, J. C.; Gault, F. G. The mechanisms of hydrogenolysis and isomerization of hydrocarbons on metals: I. Hydrogenolysis of cyclic hydrocarbons. J. Catal. 1965, 4, 556. (2) Barron, Y.; Maire, G.; Muller, J. M.; Gault, F. G. The mechanisms of hydrogenolysis and isomerization of hydrocarbons on metals: II. Mechanisms of isomerization of hexanes on platinum catalysts. J. Catal. 1966, 5, 428. (3) Glassl, H.; Hayek, K.; Kramer, R. Electron microscopy of Pt/ Al2O3 model catalysts: III. The hydrogenolysis of methylcyclopentane as a function of particle size. J. Catal. 1981, 68, 397. (4) Török, B.; Kiss, J. T.; Bartók, M. Characteristics of adsorbed intermediates in the hydrogenative ring opening of methylcyclobutane and methylcyclopentane over silica-supported Pt and Pd catalysts. Catal. Lett. 1997, 46, 169. (5) Bragin, O. V.; Karpinski, Z.; Matusek, K.; Paál, Z.; Tétényi, P. Comparative study of various platinum catalysts in skeletal reactions of C6-hydrocarbons. J. Catal. 1979, 56, 219. 10103



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(27) Subramanian, S.; Schwartz, J. A. Structure and activity of composite oxide supported platinum−iridium catalysts. Appl. Catal. 1991, 74, 65. (28) Lu, J.; Aydin, C.; Browning, N. D.; Wang, L.; Gates, B. C. SinterResistant Catalysts: Supported Iridium Nanoclusters with Intrinsically Limited Sizes. Catal. Lett. 2012, 142, 1445. (29) Uzun, A.; Ortalan, V.; Browning, N. D.; Gates, B. C. A siteisolated mononuclear iridium complex catalyst supported on MgO: Characterization by spectroscopy and aberration-corrected scanning transmission electron microscopy. J. Catal. 2010, 269, 318. (30) Lin, J.; Wang, A.; Qiao, B.; Liu, X.; Yang, X.; Wang, X.; Liang, J.; Li, J.; Liu, J.; Zhang, T. Remarkable Performance of Ir1/FeOx SingleAtom Catalyst in Water Gas Shift Reaction. J. Am. Chem. Soc. 2013, 135, 15314. (31) Solymosi, F.; Novák, É.; Molnar, A. Infrared spectroscopic study on CO-induced structural changes of iridium on an alumina support. J. Phys. Chem. 1990, 94, 7250. (32) Solymosi, F.; Raskó, J. An infrared study of CO and NO adsorption on alumina-supported iridium catalyst. J. Catal. 1980, 62, 253. (33) Toolenaar, F. J. C. M.; Bastein, A. G. T. M.; Ponec, V. The effect of particle size in the adsorption of carbon monoxide on iridium: An infrared investigation. J. Catal. 1983, 82, 35. (34) McVicker, G. B.; Baker, R. T. K.; Garten, R. L.; Kugler, E. L. Chemisorption properties of iridium on alumina catalysts. J. Catal. 1980, 65, 207. (35) Bourane, A.; Nawdali, M.; Bianchi, D. Heats of Adsorption of the Linear CO Species Adsorbed on a Ir/Al2O3 Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium. J. Phys. Chem., B 2002, 106, 2665. (36) Reyes, P.; Salinas, D.; Campos, C.; Oportus, M.; Murcia, J.; Rojas, H.; Borda, G.; García-Fierro, J. L. Selective hydrogenation of furfural on Ir/TiO2 Catalysts. Quim. Nova 2010, 33, 777. (37) Oliviero, L.; Paal, Z. Hydrogenolysis of Hexane on Supported Rhodium Catalysts. React. Kinet. Catal. Lett. 2001, 74, 233. (38) Dautzenberg, F. M.; Wolters, H. B. M. State of dispersion of platinum in alumina-supported catalysts. J. Catal. 1978, 51, 26. (39) Den Otter, G. J.; Dautzenberg, F. M. Metal−support interaction in Pt/Al2O3 catalysts. J. Catal. 1978, 53, 116. (40) Linsmeier, Ch.; Taglauer, E. Strong metal−support interactions on rhodium model catalysts. Appl. Catal. A 2011, 391, 175. (41) Ivanova, A. S.; Slavinskaya, E. M.; Gulyaev, R. V.; Zaikovskii, V. I.; Stonkus, O. A.; Danilova, I. G.; Plyasova, L. M.; Polukhina, I. A.; Boronin, A. I. Metal−support interactions in Pt/Al2O3 and Pd/Al2O3 catalysts for CO oxidation. Appl. Catal. B 2010, 97, 57.

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