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Role of Different Active Sites in Heterogeneous Alkene Hydrogenation on Platinum Catalysts Revealed by Means of Parahydrogen-Induced Polarization Vladimir V. Zhivonitko,*,† Kirill V. Kovtunov,†,‡ Irene E. Beck,§,^ Artem B. Ayupov,§,|| Valerii I. Bukhtiyarov,§,# and Igor V. Koptyug†,r †
International Tomography Center, Siberian Branch of the Russian Academy of Sciences (SB RAS), Institutskaya Street 3A, 630090 Novosibirsk, Russia § Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences (SB RAS), Lavrentieva Avenue 5, 630090 Novosibirsk, Russia ABSTRACT: Substantial NMR signal enhancements provided by parahydrogen-induced polarization (PHIP) are associated with the ability of a catalyst to incorporate both H atoms of a dihydrogen molecule into the same product molecule. Therefore, PHIP can provide valuable information about the mechanisms and kinetics of catalytic hydrogenation reactions as well as produce hyperpolarized molecules for sensitivity enhancement in NMR. In this work, the PHIP technique was applied to study the structure sensitivity and the support effects on the degree of pairwise H2 addition in propene hydrogenation over supported platinum catalysts. Four series of Pt catalysts supported on Al2O3, SiO2, ZrO2, and TiO2 were examined. A nontrivial dependence of the selectivity toward pairwise H2 addition on the Pt particle size was found. Its analysis indicates that at least three types of different active sites coexist on the catalysts surface. Among them, the major one is responsible for the nonpairwise H2 addition to the double bond, whereas pairwise addition can proceed on the other two minor active sites. An explanation of the nature of these active sites is proposed. A substantial increase in the pairwise addition selectivity was found for Pt/TiO2 catalysts as compared to other catalyst series, possibly due to a strong metalsupport interaction taking place even after low temperature catalyst reduction.
1. INTRODUCTION From its early days, NMR has been proved to be efficient in numerous fields of scientific interest. It can provide much information about the structure of chemical and biological species and about the dynamic characteristics of media such as diffusion and flow phenomena. It was successfully employed for noninvasive imaging of the inner structure of various objects, and new applications are constantly emerging. At the same time, NMR has quite a low sensitivity, which is dictated by the weakness of the interaction between nuclear spins and the applied magnetic fields. As a consequence, the population differences between the spin energy levels are small and the polarization of nuclear spins is rather weak (e.g., ∼104 for protons at a 14 T magnetic field). To date, several hyperpolarization techniques providing different approaches for increasing the population differences have been developed. Among them, only parahydrogen-induced polarization (PHIP) has a direct relation to catalysis. PHIP exploits the high spin order of parahydrogen molecule, which is one of the nuclear spin isomers of molecular hydrogen.1 The high molecular symmetry of parahydrogen has to be broken in order to induce nuclear spin alignment and consequently the hyperpolarization resulting in the strong NMR signal enhancements.1,2 Commonly, the symmetry breaking is achieved by involving the r 2011 American Chemical Society
parahydrogen molecule in a chemical reaction resulting in an addition of two atoms of parahydrogen to a nonsymmetrical substrate,3 although different approaches can be utilized as well.4 Originally, homogeneous hydrogenation reaction was employed, as this chemical process ensures the preservation of the correlation between two proton nuclei of a parahydrogen molecule and at the same time provides the required initial symmetry breaking. A substantial signal enhancement can be achieved with the use of homogeneous catalysis, but the presence of the catalyst dissolved in the reaction mixture sets a limit on the potential extension of PHIP to a wide range of novel NMR applications. Recently, it has been shown that PHIP can be produced in a heterogeneous reaction as well,5 and during the past few years various heterogeneous catalytic systems that allow the observation of PHIP have been found.610 Moreover, the successful use and the advantages of heterogeneous catalysts utilization for boosting the sensitivity in magnetic resonance imaging applications were demonstrated as well.1113 Among other things, PHIP provides a unique type of reaction product labeling as the polarization can be observed only in the Received: April 12, 2011 Published: June 20, 2011 13386
dx.doi.org/10.1021/jp203398j | J. Phys. Chem. C 2011, 115, 13386–13391
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Table 1. Particle Sizes and H2 Chemisorption Measurements for Pt Catalysts Studied in This Work mean particle size, nm Pts, μmol/g support
(H2 chemisorption)
TEM
H2 chemisorption
Al2O3
44.7
0.8
Al2O3 Al2O3
35.3 22.6
1.3 ( 0.3 2.2 ( 0.5
1.3 2.1
Al2O3
9.5
3.8 ( 0.9
4.5
Al2O3
8.0
6.5 ( 1.7
5.9
Al2O3
3.5
11.5 ( 3.7
12.4
SiO2
21.4
1.8 ( 0.4
2.1
SiO2
15.7
2.6 ( 0.5
3.0
SiO2
9.5
3.8 ( 1.7
6.0
TiO2 TiO2
8.9 a (29.3 b) 5.3 a (22.9 b)
0.7 ( 0.15 2.0 ( 0.4
5.4a (1.6b) 11.5a (2.7b)
TiO2
4.2 a (7.4 b)
9.4 ( 2.4
15.5a (9.0b)
ZrO2
38.4
1.2 ( 0.3
1.1
ZrO2
30.2
1.6 ( 0.5
1.8
3 nm, we can reveal yet another type of active sites (III) with reaction dimension DR ≈ 3.8 that also promote the pairwise addition of dihydrogen (see Figure 3). Such a high reaction dimension and a strong structure sensitivity can be explained only by a multiatomic nature of the participating active sites and/or by a high coverage of the metal surface with stable byproduct, for example propylidyne surface species, which can isolate the active metal sites localized in-between. Consequently, the large DR values observed are suggested to originate from both (i) the statistics of the face atoms in Pt crystallites and (ii) the surface site statistics of the crystallite faces carrying carbonaceous deposits. It is well-known that carbon deposition on a metal surface is a structurally sensitive process which is much more significant for larger particles,27 because dehydrogenative processes leading to multiply bonded residues occur most readily on flat surfaces comprising atoms with a high coordination number, while atoms at the edges or corners remain clean. At the same time, even the maximal coverage of Pt(111) faces with strongly absorbed spectator species does not slow down the hydrogenation reaction occurring via the conventional H2 dissociative mechanism, because of the significant mobility of such species driven by surface diffusion, which is necessary to free up active sites. On the other hand, dehydrogenative CH bond activation occurs even more readily on less dense (such as (100) or (110)) crystal faces, where carbon deposition is more facile as compared to the close-packed Pt(111),28 while the decrease in atom density of a crystal face tends to restrict the surface diffusion of adsorbed atoms and molecules. Taking into account all these considerations, we can suppose that the active sites of type III represent a small group of Pt atoms located in the close vicinity of each other or even an isolated platinum atom on the less dense crystal faces (100) or (110) localized between the strongly adsorbed multiply bonded carbonaceous residues, where the hydrogenation process proceeds via a nonconventional concerted mechanism involving weakly coordinated molecular dihydrogen. A similar mechanism was suggested for olefin hydrogenation over Pt/TiO2 catalysts reduced at high temperature.29 In that case, the chemisorption of dihydrogen was almost suppressed due to SMSI state, while the activity of the catalyst toward alkene hydrogenation was preserved. Only a weak adsorption of propene takes place on these isolated active sites leading to the formation of a π-complex, which in turn can activate a dihydrogen molecule coordinated to the same platinum atom. Then the weakly held molecular hydrogen attacks the activated olefinic double bond forming 13389
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Figure 4. Influence of the support type on the particle size dependence of the selectivity toward pairwise H2 addition in propene hydrogenation over supported Pt catalysts.
the coordinated propyl and the hydride ligands, from which the reductive elimination produces a propane molecule and leads to the regeneration of the active site thereafter. It should be noted that a small charge transfer toward Pt increasing the Pauli repulsion with the π orbitals of the unsaturated molecule and hence weakening the adsorption should favor the latter weak-coordination mechanism. Such charge transfer could be induced by a high coverage of the surface with the unsaturated hydrocarbons30 and/ or via electronic interaction of the metal with reduced titania.31,32 Though a smaller number of different Pt particle sizes were available on supports other than Al2O3, a similar trend was nevertheless found in the variation of selectivity toward pairwise addition with the variation of particle size (Figure 4). Furthermore, the selectivity values evaluated for Pt catalysts supported on Al2O3, ZrO2, and SiO2 were almost the same for similar dispersions and could be described by a single curve, in contrast to Pt/TiO2 catalysts which exhibited a much higher activity in the pairwise H2 addition. Such a pronounced difference seems to be a general trend for the entire range of Pt particle sizes, indicating an important role of the support in the pairwise route of propene hydrogenation. One can obviously point out that the existence of the strong metalsupport interaction (SMSI) for TiO2 support is a particular property of Pt/TiO2,33 which distinguishes this catalyst series from all other catalysts tested. The conventional reduction temperature that results in the most pronounced SMSI for Pt/TiO2 is ∼775 K,33,34 which is much higher than that used in this work (575 K). Nevertheless, the following arguments can be provided to support the validity of this hypothesis in relation to our experimental results. The accepted criterion for SMSI is the decrease in the CO or H2 adsorption capacity of the supported Pt, which is usually more pronounced when the metal particles are smaller.35 A significant decrease in the chemisorption capacity for hydrogen (2.54-fold) and CO (1.62.4-fold) was observed for our Pt/TiO2 samples as the reduction temperature was increased from 475 to 575 K, evidencing the appearance of the SMSI state after reduction at a higher temperature. Actually, there are several specific interactions between noble metal particles and oxide supports that can result in SMSI. They have been summarized by Bernal et al. in the order of their appearance depending on the sample reduction temperature (Tr).36 The first interaction type is the existence of epitaxial relationships between titania supports and Pt particles, namely, the occurrence of a parallel alignment of metal and support
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[h k l] axes with the same Miller indices. This interaction is not strictly correlated to a specific Tr value. The other interactions are electronic interactions involving charge transfer at lower Tr values, decoration and encapsulation for higher Tr, and alloying at the highest temperatures. In addition, the increase in the metalsupport interaction energy can affect the shape of metal nanoparticles and thus change the ratio of different surface planes. It is doubtful that a significant part of the platinum surface in our catalyst samples is covered with the reduced forms of TiOx type, because of the relatively low reduction temperature used. Most likely, the observed increase in the pairwise addition degree could be induced by a charge transfer between platinum and titania and/or by the alteration of the supported particles morphology due to SMSI with a re-distribution of crystal faces toward the less densely packed surface faces containing numerous defects. Obviously, such face redistribution should lead to an increase in the density of both active site types II and III, which in turn favors the pairwise addition of dihydrogen. Some experimental facts in evidence of such TiO2 influence can be found in the literature. They include a preferential decrease in the number of platinum close-packed (terrace) sites as compared to the more open (step) sites induced by a higher reduction temperature according to the IR spectroscopy of adsorbed CO on Pt/TiO2,37 an increase in the amount of strongly adsorbed olefins on Pt/TiO2 catalyst with increasing reduction temperature in contrast to Pt/SiO2 and Pt/Al2O3 catalysts,38 and the difference in the reaction order with respect to hydrogen in propene hydrogenation on Pt/SiO2 (n ∼ 1) and on Pt/TiO2 catalysts (n ∼ 0.6) under the same conditions.39 All of these facts indicate that in the case of TiO2-supported platinum, a noncompetitive hydrogenation mechanism with H2 adsorption on isolated sites unattackable by hydrocarbon molecules prevails up to much higher reaction temperatures.
4. CONCLUSIONS Particle size effects play an important role in modern heterogeneous catalysis as the activity and selectivity of a catalyst often crucially depend on whether small or large metal particles are supported. On the other hand, the properties of the support can also significantly affect the catalyst efficiency. In this work, parahydrogen-induced polarization (PHIP) was used to study the influence of both the metal particle size and the support properties on the propene hydrogenation activity and the contribution of the pairwise H2 addition to the overall alkene hydrogenation process. It has been shown that the selectivity toward pairwise H2 addition and hence the magnitude of the NMR signal enhancement has an unusual dependence on the platinum particle size; i.e., the sympathetic structure sensitivity observed for Pt particles smaller than 3 nm switches to an antipathetic sensitivity for larger particles. The analysis of the Pt particle size dependence of TOFs for the overall propene hydrogenation and for the pairwise H2 addition allowed us to conclude that there are different surface active sites responsible for the major nonpairwise (site I) and the minor pairwise (sites II and III) hydrogen addition to a double bond on Pt/Al2O3 catalysts. The reaction dimensions estimated from these results clarify the nature of these active sites. They indicate that the sites I responsible for the dominating nonpairwise H2 addition are located on the closely packed terraces, whereas the pairwise H2 addition occurs either on the most coordinatively unsaturated corner platinum atoms and/or zero-dimensional defects of smaller Pt particles (sites II) or on the isolated active sites III 13390
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The Journal of Physical Chemistry C localized between the strongly adsorbed hydrocarbon residues on less dense crystal faces (or steps) of larger Pt particles. The selectivity toward pairwise H2 addition on similar-sized platinum particles was the same for Pt/Al2O3, Pt/ZrO2, and Pt/SiO2 catalysts but was several times higher for Pt/TiO2 catalysts. This Ptanatase synergism was supposed to be due to a low-temperature SMSI that can induce a charge transfer as well as particle morphology modification associated with the redistribution of surface crystal faces toward the less densely packed faces and defect sites, therefore providing an increase in the number of both types of active sites II and III that sustain pairwise H2 addition. Finally, it should be noted that in spite of the pairwise dihydrogen addition to a carboncarbon double bond on supported platinum being the minor process for all studied catalysts, the observed polarization effects provide the NMR signal enhancement factors of up to 150. This means that heterogeneous supported metal catalysts become potentially suitable for producing hyperpolarized gases that can be used, for instance, to substantially shorten the acquisition times in MRI applications. Moreover supported metal catalysts can be expected to provide even higher levels of hyperpolarization, if a suitable catalyst is purposely designed via a thorough choice of the support and a certain optimization of the active component particle size as well as the catalyst preparation procedure.
’ AUTHOR INFORMATION Corresponding Author
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’ ACKNOWLEDGMENT This work was partially supported by grants from RAS (5.1.1), RFBR (11-03-00248-a, 11-03-93995-CSIC-a), SB RAS (Integration Grants 9, 67, 88), the program of support of leading scientific schools (NSh-7643.2010.3), the Russian Ministry of Education and Science (State Contract 02.740.11.0262), and the Council on Grants of the President of the Russian Federation (MK-1284.2010.3). ’ REFERENCES (1) Bowers, C. R. Sensitivity enhancement utilizing parahydrogen. In Encyclopedia of Nuclear Magnetic Resonance; Gant, D. M., Harris, R. K., Eds.; Wiley: Chichester, U.K., 2002; Vol. 9, pp 750769. (2) Bowers, C. R.; Weitekamp, D. P. Phys. Rev. Lett. 1986, 57, 2645–2648. (3) Duckett, S. B.; Colebrooke, S. A. Parahydrogen enhanced NMR spectroscopic methods: A chemical perspective. In Encyclopedia of Nuclear Magnetic Resonance; Gant, D. M., Harris, R. K., Eds.; Wiley: Chichester, U. K., 2002; Vol. 9, pp 598620. (4) Adams, R. W.; Aguilar, J. A.; Atkinson, K. D.; Cowley, M. J.; Elliott, P. I.; Duckett, S. B.; Green, G. G.; Khazal, I. G.; Lopez-Serrano, J.; Williamson, D. C. Science 2009, 323, 1708–1711. (5) Koptyug, I. V.; Kovtunov, K. V.; Burt, S. R.; Anwar, M. S.; Hilty, C.; Han, S. I.; Pines, A.; Sagdeev, R. Z. J. Am. Chem. Soc. 2007, 129, 5580–5586. (6) Kovtunov, K. V.; Beck, I. E.; Bukhtiyarov, V. I.; Koptyug, I. V. Angew. Chem., Int. Ed. 2008, 47, 1492–1495.
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