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Journal of Chemistry
Volume 2013 (2013), Article ID 502014, 8 pages
Sulfur Resistance of Pt-W Catalysts
1Instituto de Investigaciones en Catálisis y Petroquímica (INCAPE), FIQ, UNL-CONICET, Santiago del Estero 2654, S3000AOJ Santa Fe, Argentina
2Facultad de Ciencias, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain
3Facultad de Ingeniería Química, Universidad Nacional del Litoral, Santiago del Estero 2829, S3000AOJ Santa Fe, Argentina
Received 28 June 2012; Accepted 26 November 2012
Academic Editor: Ana M. Garcia-Campana
Copyright © 2013 Carolina P. Betti et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The sulfur resistance of low-loaded monometallic Pt catalysts and bimetallic Pt-W catalysts during the partial selective hydrogenation of styrene, a model compound of Pygas streams, was studied. The effect of metal impregnation sequence on the activity and selectivity was also evaluated. Catalysts were characterized by ICP, TPR, XRD, and XPS techniques. Catalytic tests with sulfur-free and sulfur-doped feeds were performed. All catalysts showed high selectivities (>98%) to ethylbenzene. Activity differences between the catalysts were mainly attributed to electronic effects due to the presence of different electron-rich species of Pt0 and electron-deficient species of . Pt0 promotes the cleavage of H2 while the adsorption of styrene. The catalyst successively impregnated with W and Pt (WPt/Al) was more active and sulfur resistant than the catalyst prepared with an inverse impregnation order (PtW/Al). The higher poison resistance of WPt/Al was attributed to both steric and electronic effects.
Catalytic selective hydrogenation reactions are of vital importance in the petrochemical and petroleum refining industries. In refining operations one such important reaction is the selective hydrogenation of pyrolysis gasoline (Pygas). Pygas is a naphtha-range product with a high content of aromatics and olefins that is produced as a byproduct of the production of olefins in the steam cracking units . Due to its high content of aromatics and olefins, pyrolysis gasoline is used as high octane compounds for blending in the gasoline pool or as a highly unsaturated feedstock for the extraction of BTX aromatics. Up to 15% of Pygas compounds are highly reactive, mono- and diolefins that tend to polymerize into gums in vessels, pipelines, and catalysts must therefore be eliminated from the feedstock of the refinery units before its processing . The most widespread method of purification is the selective hydrogenation of vinyllic compounds, keeping the aromatic rings unaltered [3–5]. Styrene is routinely used as a useful laboratory model compound of Pygas to investigate the catalytic performance of metal catalysts. It has two types of unsaturated groups thus needing selective hydrogenation , and it is one of the most refractory compounds of these streams . Styrene hydrogenation is also used in the industrial practice as a test reaction for assessing the performance of the hydrogenation catalysts under real conditions .
Pygas selective hydrogenation catalysts must have a great resistance to poisoning because Pygas usually contains variable degrees of sulfur and nitrogen compounds. Deactivation by sulfur poisoning of noble metal supported catalysts is especially serious and has been widely studied . Catalyst deactivation can be caused by a decrease in the number of active sites or a change in their nature. As it is well known, sulfur compounds are the most effective poisons for metal catalysts extensively used in petrochemical industries. For naphtha range feedstocks thiophenic compounds are the most poisonous and therefore thiophene is usually taken as a model compound for sulfur resistance studies .
In a previous paper we have evaluated the activity and poisoning of different monometallic alumina supported catalysts . Pd and Pt low-loaded catalysts were the most sulfur resistant and the most active for the selective hydrogenation of styrene to ethylbenzene. Besides it was found that the formation of oxychlorinated species, refractory to reduction, contributed to the sulfur resistance by forming electron deficient metal species forming weaker sulfur-metal bonds. It was also rationalized that chloride species could additionally contribute to sulfur resistance by a steric factor.
Pt catalysts have proved to be active for many selective hydrogenation reactions. The increase in the demand for Pt in many industries such as the petroleum refining and fuel cell ones has led to a drastic increase in the price of the noble metal. This situation is expected to become worse in the next decades. For this reason it is convenient to decrease the load of Pt in industrial catalysts, partly replacing it with cheaper components. This additional components are usually nonnoble metals and the so-called multimetallic not only can have a lower cost but also enhanced properties of selectivity and poison resistance [11, 12]. One family of bimetallic catalysts drawing especial attention is that in which the second component is a transition metal with available electron in the nd level . Pt bimetallic catalysts are used in many processes, the most commonly known being the catalytic reforming of virgin naphtha for the production of high octane gasoline and BTX. Some authors suppose that Pt bonds with the other metals, thus forming alloys of enhanced catalytic properties [14, 15]. It is also stated that the increase in activity and selectivity of the bimetallic systems is a product of the high dispersion of Pt, which is achieved by the dilution effect of the second metal . Other authors claimed that the selectivity of bimetallic catalysts is mainly produced by the “ensemble” effect, the promoter producing the physical isolation of Pt in smaller clusters on the main particle . The word “ensemble” refers to the number of active sites on the surface that are required by a specific reaction. Reactions demanding big ensembles (big number of active sites) can be selectively suppressed by poisoning or when a fraction of the active sites is blocked. Electronic effects on the other hand are related to a redistribution of the charge, as a consequence of the metal-metal bonds. Depending on the nature of the metal-metal and metal-sulfur interactions many phenomena can occur when sulfur reacts over the surface of a bimetallic particle: (a) the formation of bimetallic sulfides is reported for some systems, and these bimetallic sulfides having properties much different from those of the pure monometal sulfides; (b) in other systems the interaction between sulfur and one of the metal is a repulsion type and leads to a weak bimetallic bond and a reduction of the degree of mixing of the metals; (c) bimetallic systems can also be gotten in which one of the metals promotes the reactivity of the second towards sulfur; (d) the case of the formation of bimetallic alloys that produces, as a consequence of an electronic effect, a decrease of the affinity for sulfur of both metals. This last situation is particularly interesting and useful for the design of thiotolerant catalysts. Thus the understanding of the interaction of sulfur with bimetallic particles is critical for the design of catalysts and process conditions that prevent or decrease the deactivation of the catalyst.
In a recent work  we found that the order of impregnation and of the kind of metal precursor salts had a great influence on the activity and sulfur resistance of Pt-Ni catalysts for the gas phase selective hydrogenation of styrene. In this work our attention is now focused on the sulfur resistance of low-loaded mono- and bimetallic catalysts with Pt as the active phase and W as a promoter. The reaction is carried out in the liquid phase in order to obtain a higher reactor throughput. The influence of the metals impregnation sequence on the activity, selectivity, and sulfur resistance is especially studied.
2.1. Catalysts Preparation
γ-Al2O3 Ketjen CK300 was used as support. This was previously calcined in air at 823 K to stabilize its texture (: 224 m2 g−1). Platinum and tungsten mono- and bimetallic catalysts supported over alumina were prepared by the incipient wetness technique. Volume and concentration of the impregnating solutions were adjusted to get 1 wt% Pt and 3.5 wt% W on the final mono- and bimetallic catalysts.
An aqueous acid solution of H2PtCl6 (Strem Chemicals, Cat N°: 78-0200 purity > 99.9%) at pH 1 (with HCl) was used to prepare the platinum monometallic catalyst (sample Pt/Al). The tungsten monometallic catalyst (sample W/Al) was prepared using a phosphotungstic acid solution H3PO4·12WO3 (Fluka, Cat N°: 79690 purity > 99.9%). After impregnation the monometallic catalysts were dried 24 h at 373 K and calcined in an air flow at 823 K during 3 or 7 h. They were named Pt/Al and W/Al, respectively. The calcination conditions used for preparing W/Al assured the complete elimination of phosphorus from the catalyst [19, 20].
A portion of W/Al was impregnated with an aqueous acid solution of H2PtCl6 (pH 1 with HCl) to prepare the bimetallic catalyst (WPt/Al). Part of the Pt/Al catalyst batch was impregnated with an acid solution of H3PO4·12WO3 to obtain the bimetallic catalyst, PtW/Al. WPt/Al and PtW/Al bimetallic catalysts were both dried and calcined in an air flow at 823 K during 3 (WPt/Al) or 7 h (PtW/Al). Immediately before the catalytic test the catalysts were reduced in a hydrogen stream during 1 h at 673 K.
2.2. Catalyst Characterization
The contents of platinum, phosphorus, and tungsten of the catalysts were determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES) using a Perkin Elmer ICP OPTIMA 2100 instrument.
The dispersion of the metal particles was measured by means of the chemisorption of CO in a pulse apparatus. The samples of the catalysts were first reduced 1 h in hydrogen at 673 K. Then the samples were flushed in nitrogen for 1 h and cooled down to room temperature. Then 0.25 cm3 pulses of a reducing gas mixture were successively sent to the reactor. This mixture is comprised of 3.5% (v/v) CO in N2. Part of the CO is chemisorbed, and the rest flows from the adsorption cell to a methanator containing a Ni/kieselguhr catalyst. The methanator is fed with a hydrogen stream, and CO is quantitatively transformed into CH4. This methane is detected in a flame ionization detector downstream. The amount of CO adsorbed is measured by calculating the amount of methane not reaching the detector.
The electronic state of the surface species on the catalyst was determined by X-ray Photoelectron Spectroscopy (XPS) in a VG-Microtech Multilab equipment, equipped with a (hν: 1253.6 eV) radiation and a pass energy of 50 eV. The analysis pressure during data acquisition was 5 × 10−7 Pa. Samples were treated in situ in the presence of a H2 stream during 1 h at 673 K. A careful deconvolution of the spectra was made, and the areas under the peaks were estimated by calculating the integral of each peak after subtracting a Shirley background and fitting the experimental peak to a combination of Lorentzian/Gaussian lines of 30–70% proportions. The reference binding energy (BE) was C 1 s peak at 284.6 eV. For the quantification, the areas were normalized using the sensitivity factors of each element.
X-ray diffraction (XRD) measurements of powdered samples were obtained using a Shimadzu XD-D1 instrument with a CuK radiation (λ = 1.5405 Å) in the 21 < 2θ < 49° at a scan rate of 0.25° min−1. Samples were powdered and reduced under a hydrogen flow, the samples were put into the chamber of the equipment, and then the spectrum was acquired.
The reducibility of the surface species on the catalysts was assessed by temperature programmed reduction (TPR), using a Micromeritics AutoChem II instrument equipped with a TCD. Samples were pretreated under an oxygen flow stream at 673 K during 30 min, and then they were cooled down up to room temperature in argon. TPR profiles were obtained increasing the temperature up to 1223 K at a heating rate of 10 K min−1 in a 5% (v/v) hydrogen : argon stream.
2.3. Catalytic Tests
Activity, selectivity, and sulfur resistance of the prepared catalysts were assessed with the test reaction of styrene hydrogenation in the presence of small amounts of thiophene. The reaction was performed in a stainless steel, PTFE-coated stirred tank reactor, operated in batch mode, at 353 K, 2 MPa hydrogen pressure, and 1200 rpm. In some blank tests the reaction was performed in the absence of thiophene. In these poison-free tests the reactor was loaded with 0.3 g of catalyst and 200 mL of a solution of 5% (v/v) styrene (Aldrich, Cat. N° S497-2, purity > 99%) in toluene (Merck, Cat. N° TX0735-44, purity > 99%) using n-decane (Fluka, Cat. N° 30550, purity > 98%) as an internal chromatographic standard. In the case of the catalytic tests in the presence of sulfur, 300 ppm of thiophene was added to the feed. Catalytic tests were carried out in duplicates. The average experimental error was lower than 5%. Reactants and products were analyzed by gas chromatography using a 30 m, J&W INNOWax 19091N-213 capillary column.
3. Results and Discussion
3.1. Catalysts Characterization
The values of mass concentration determined by ICP technique were 1 wt% of Pt and 3.5 wt% of W. The W/Pt atomic ratio was equal to 3. On the other hand, no phosphorus was detected by ICP chemical analysis after the calcination and reduction of the prepared catalysts, suggesting that all phosphorus was eliminated from the catalysts. Table 1 shows the dispersion of the samples, as calculated from the CO chemisorption data. As W does not chemisorb CO at room temperature, in the case of the bimetallic catalysts we considered that only the platinum sites were responsible for CO chemisorption, and we assumed a chemisorption stoichiometry H : Pt = 1. The results show that the dispersion of monometallic Pt/Al is 1.4 times higher than that of WPt/Al and twice as high as that of PtW/Al. This indicates that Pt in the monometallic system is more accessible to CO chemisorption. The lower CO adsorption on PtW/Al could be due to an effect of decoration of Pt particles by W species, the latter being not active for CO adsorption [21, 22].
The X-ray diffractograms of all the catalysts presented only the typical signals of the gamma phase of alumina: 37.7°, 45.98°, and 66.98° [23, 24]. Because of the low concentration of platinum, no peak at 39.9° due to the reflections of Pt0, neither those corresponding to the planes of PtO2 or Pt0 could be found [25–27]. For the bimetallic systems the absence of peaks in the region 20°< 2θ < 30° indicates that there is no tungsten oxide present as a crystalline segregated phase. Segregated monoclinic WO3 crystallites can be detected by XRD for W surface concentrations higher than the corresponding surface monolayer saturation value (4.3 Watoms nm−2) . In our case, the W surface density was lower than the corresponding monolayer value (0.64 Watoms nm−2). Because of the small particle size and the low sensitivity of the XRD method, Pt and WOx crystalline phases cannot be detected in our case.
Figure 1 shows the TPR traces of the calcined Pt and W mono- and bimetallic catalysts. It can be seen that the TPR trace of the Pt monometallic catalyst has a main hydrogen consumption peak at 504 K that can be assigned to the reduction of oxychlorinated Ptδ+OxCly species [29–33]. These particles are agglomerated over the support during the calcination of the catalyst.
For the monometallic W/Al sample the TPR trace shows that the main reduction peak starts at 900 K. The maximum hydrogen consumption occurs at temperatures higher than 1100 K. Several authors have reported that highly dispersed WOx, bound to alumina by strong bonds, had reduction peaks at high temperatures (950–1300 K) [34–36]. Therefore the observed reduction peak can be assigned to the reduction of amorphous WOx species.
Up to 800 K the reduction traces of WPt/Al and PtW/Al bimetallic catalysts (Figure 1) are very similar to the trace of Pt/Al, with a main reduction peak at 501.4 and 526.9 K, respectively, corresponding to the reduction of Ptδ+OxCly species. Comparing these profiles, there is a shift of the oxychlorinated species reduction peak of −2.6 and 22.9 K, for WPt/Al and PtW/Al, respectively, indicating that the presence of W modifies the reducibility of Pt. These shifts point to the presence of different active sites on the catalysts surface and suggest an interaction or electronic effect between Pt, W, and Al2O3. Besides, the slight shift to lower temperatures of the main reduction peak of WPt/Al suggests that the reduction of oxychlorinated species is more easily accomplished on WPt/Al: when platinum is impregnated over the monometallic W/Al catalyst. On the other hand the marked shift of the main peak of reduction of the PtW/Al catalyst to higher temperatures suggests that the Pt particles are harder to reduce (in comparison to the monometallic catalyst) when tungsten is impregnated after Pt. This could be due to an effect of decoration of Pt particles by W species as reported by Sica and Gigola  for Pd-WOx/Al2O3 catalysts with low metal content. The possible decoration of Ptδ+OxCly particles by WOx species is an example of metal-support interaction related to the mobility of support surface species. Surface species would migrate onto Pt particles during heat treatments, mobility being only important at high temperatures.
It can also be seen that the high temperature peak (>950 K) of the TPR traces of the bimetallic catalysts is similar to that of the W/Al catalyst and would therefore be related to the reduction of amorphous WOx species bound to alumina through strong bonds. The second reduction peak of PtW/Al is fairly similar to that of the W/Al catalyst, though it is shifted to higher temperatures in the case of WPt/Al. This indicates than tungsten species on WPt/Al are more refractory to reduction, a property likely related to their double calcination and reduction treatment.
According to the TPR results, the reduction temperature adopted for platinum mono- and bimetallic catalysts (673 K) during the synthesis of the catalysts should ensure, at least partly, the presence of Pt0 metal atoms on the surface.
The XPS spectra of the catalysts had a peak next to 198.5 eV that would correspond to Cl 2p3/2, attributed to chloride species that were not eliminated during the thermal treatment stages . Figures 2 and 3 contain the XPS spectra of the Pt 4d5/2 and W 4f7/2 regions for all catalysts. The maximum binding energy (BE) of Pt 4d5/2 and W 4f7/2 and Cl/Al, W/Al, and Pt/Al atomic ratios are also listed in Table 1.
As it can be seen in Figure 2 all catalysts had low intensity values of the Pt 4d5/2 peak in the 313.5 ± 0.2 eV range. From the literature reports this peak can be attributed to Pt0 [37, 38]. In Figure 2 a second peak at 315.2–316.7 eV can be found that can be attributed to the presence of complex electrodeficient Ptδ+ species (0 < δ < 2) stabilized by the presence of remaining chloride as oxychloride compounds Ptδ+OxCly  formed during the calcination or the following reduction process . When a comparison is made between the Pt high BE peak of the bimetallic catalyst against that of the Pt/Al one, it can be seen that there is a shift towards lower BE values of 1.5 for PtW/Al and 0.8 eV WPt/Al. This might indicate the presence of Ptδ+ species with a higher availability of electrons than those present on the monometallic catalyst.
The spectra of W 4f7/2 for W/Al, WPt, and PtW catalysts are plotted in Figure 3. They have a peak at 35.8 ± 0.2 eV and a 4f5/2 doublet peak at 38.0 ±0.2 eV, suggesting the presence of electron-deficient species Wρ+ (with ρ < 6) [20, 37, 39]. The BE of W 4f7/2 reported in Table 1 indicates that tungsten is present on the alumina surface of WPt/Al and PtW/Al catalysts with slightly higher values of BE than on the tungsten monometallic catalyst. This pattern and the presence of Ptδ+ electron deficient species suggest the formation of different types of intermediate surface species: Ptδ+OxCly-WOx-Al2O3 or WOx-Ptδ+OxCly-Al2O3. This could be active sites participating in the reaction on the bimetallic catalyst. The formation of intermediate Pd-WOx-Al2O3 on Pd-W bimetallic catalysts has been earlier reported by L’Argentiѐre and Fígoli  and Lederhos et al. .
An inspection of the Cl/Al atomic ratios of Table 1 reveals that WPt/Al has 1.7 and 2.5 times more surface chlorine than Pt/Al and PtW/Al, respectively. Table 1 also shows that the Pt/Al atomic surface ratio is 1.2 and 2.3 times higher than the same ratio on the WPt/Al and PtW/Al catalysts. This indicates that there are more available superficial platinum species on the monometallic platinum catalyst. The value of the W/Al atomic ratio of WPt/Al is quite lower than that of the PtW/Al or W/Al catalysts, because of a possible decoration effect of WOx species by Ptδ+OxCly oxychloride species.
3.2. Catalytic Tests
According to the internal standard method, the selectivity during the tests was ca. 98% for all catalysts. No hydrogenolysis of the poison was detected.
Figure 4 shows the results of total conversion of styrene as a function of time for the mono- and bimetallic catalysts. It can be seen that Pt/Al is the most active material. The order of activity found was Pt/Al > WPt/Al > PtW/Al. It can be noted that WPt/Al has a lower activity than Pt/Al at small values of time-on-stream but the activity becomes similar at TOS values higher than 320 min.
Figure 5 shows the catalytic test results obtained with the thiophene-doped feed. It can be seen that in comparison to the tests with the sulfur-free feed there is a decrease of the total conversion of styrene for all samples. The catalyst ordering according to activity level is however the same.
For both the sulfur-free and sulfur-contaminated feedstocks, the most active catalyst was the Pt/Al one. In this sense it is well known that electron-rich Pt metallic centers can cleave the bonds in H2 by means of the interaction of a filled d metal orbital with the empty sigma antibonding molecular orbital of hydrogen . The Pt/Al atomic ratio found by XPS was Pt/Al > WPt/Al > PtW/Al; thus the activity order found during the tests with sulfur-doped and sulfur-free feeds could be related to the amount of accessible surface Pt sites. Pt/Al would have the highest amount of Pt0 on the surface, Pt0 in turn having the highest electronic population in the 4d orbital that favors the cleavage of hydrogen. Also, as observed by XPS, Pt/Al has the most electrodeficient Ptδ+ species (BE = 316.7 eV) thus favoring the adsorption of electron-rich styrene. In summary the differences in activity between the reported catalysts could be attributed to electronic effects. It must be also remarked that two types of sites are necessary to increase the catalytic activity for hydrogenation of styrene.
During the tests with the thiophene spiked feed the activities of all catalysts were decreased. According to the most accepted model for sulfur poisoning of group VIII metals, poisoning occurs by a donation of electrons from the metals to the sulfur atom. Particularly thiophene interacts with the surface of metals in a planar way through the electrons of the aromatic nucleus (weak η5 bond) [33, 41, 42]. Therefore during the poisoning tests some of the electrons of the 5d Pt orbital would be shared with the S atom and for this reason the metal would have a lower amount of available electrons for cleaving the hydrogen bond. The catalytic activity is thus reduced. In this sense the differences in sulfur resistance between the catalysts can be partly attributed to electronic effects.
The least active catalyst during both kinds of tests (with and without thiophene) was the PtW/Al one. The low activity could be associated to the decoration effect detected during the catalyst characterization. The low dispersion, the shift to high temperatures of the Ptδ+OxCly reduction peak, and the high surface W/Al atomic ratio would support this supposition. Therefore another possible source of deactivation would be the geometrical blocking of active sites by WOx deposited over the surface of the Pt particles on the PtW/Al catalyst.
Initial styrene reaction rates were determined for all catalysts during the tests with both sulfur-free () and sulfur-doped () feeds. A zero order with respect to styrene was supposed, in accordance with the conversion patterns measured. From the initial reaction values and considering a simple linear deactivation model (zero order for deactivation) , the fraction of poisoned sites () can be calculated as . Table 2 shows the values of initial hydrogenation rate as obtained by least squares fitting with the Scientist program. The mean deviation of the initial reaction rates ( and ) was about 2–4% indicating that the obtained fit was good.
Table 2 shows that WPt/Al had a lower fraction of poisoned sites than Pt/Al. The values of PtW/Al and Pt/Al were however similar. The higher sulfur resistance of the bimetallic WPt/Al catalyst could be related to the high surface Cl/Al ratio, indicating that Pt oxychlorinated species prevent the adsorption of thiophene by means of an steric effect (big size of species) and/or an electronic effect (high electronegativity of chlorine). PtW/Al and Pt/Al turned out to be the least sulfur resistant catalysts, probably because of their low chloride content (as detected by XPS). Chloride is supposed to inhibit the adsorption of thiophene. A high concentration of Pt0 electron-rich sites with low BE values could also contribute to the poisoning. These would act as electron donor sites, providing available electrons to the bond with the S atom of thiophene. Both effects would promote a strong adsorption of thiophene and thus an enhanced blocking of the catalyst active sites.
The effect of the sequence of metal impregnation in bimetallic Pt-W catalysts was evaluated. The properties assessed were the catalytic activity and the sulfur resistance, during the selective hydrogenation of styrene.
The XPS results indicated the presence of different Pt0 and Ptδ+OxCly species. The mono- and bimetallic catalysts thus showed different Pt/Al and Cl/Al surface atomic ratios.
During the tests of styrene hydrogenation with pure and sulfur contaminated feeds, the order of activity found was: Pt/Al > WPt/Al > PtW/Al. The differences in activity between the reported catalysts were mainly attributed to electronic effects. This was related to the fact that two types of sites might participate synergically during the hydrogenation of styrene: (a) electron-rich platinum active sites cleaving the H2 molecule and (b) electrodeficient Ptδ+ species, acting as Lewis acid sites for adsorption of styrene.
It was found that the order of impregnation of W and Pt in W-Pt/Al catalysts is important for determining the sulfur resistance of the final material.
The higher sulfur resistance of WPt/Al would come from the higher amount of surface chlorine (that produces both electronic and geometrical blocking effects). PtW/Al and Pt/Al had lower chlorine contents and higher concentrations of Pt0. They thus had an enhanced capacity for the strong adsorption of thiophene and deactivated more rapidly.
The experimental assistance of C. A. Mázzaro and D. A. Coria is greatly acknowledged. The authors are also gratefully indebted to UNL, CONICET, and ANPCyT for financing this work.
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