Role of Different Active Sites in Heterogeneous Alkene Hydrogenation on Platinum Catalysts Revealed by Means of Parahydrogen-Induced Polarization

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 H 2 addition in propene hydrogenation over supported platinum catalysts. Four series of Pt catalysts supported on Al 2 O 3 , SiO 2 , ZrO 2 , and TiO 2 were examined. A nontrivial dependence of the selectivity toward pairwise H 2 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 H 2 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/TiO 2 catalysts as compared to other catalyst series, possibly due to a strong metalÀsupport interaction taking place even after low temperature catalyst reduction.

Strong Metal–Support Interactions for Palladium Supported on TiO2 Catalysts in the Heterogeneous Hydrogenation with Parahydrogen

“…The major differences in the PHIP effects observed for metal nanoparticles supported on titania relative to those previously observed for metals on other supports [11][12][13] were tentatively attributed to low-temperature SMSI effects. Therefore, in this communication the behavior of titania-supported catalysts is …”

“…[9][10][11] Recently,i tw as shown that the nature of the catalyst support can have as ignificant impact on the rate of the pairwise addition of hydrogen. [12,13] Remarkably,t itania-supported metal catalysts were shown to exhibit much higher levels of the pairwise addition of hydrogen (and larger PHIP effects) relative to that exhibited by metals on other supports. [9,14] Therefore, examination of the nature of the active sites for titania-supported nanoparticles in the contexto ft he pairwise addition of hydrogen may open up new possibilities for the rational preparation of catalysts that are able to maximize the observed PHIP effects and for the production of hyperpolarizedm olecular contrast agents potentially suitable for in vivo magnetic resonance imaging investigations.…”

“…[19,20] Therefore, significant alteration in the selectivity of the pairwisea ddition of hydrogen for titania-supportedc atalysts described above is not surprising. [11][12][13] One plausible explanation for the nature of the SMSI effect is the electronic interaction between the metal and the support, which leads to the formation of an electron-rich metal. [21,22] Another possible explanation is migration of the support material and encapsulation of metal nanoparticles.I mportantly,t itania is able to rapidly migrate over considerable distances and to appear on initially pristine metal surfaces.…”

Kinetic Study of Propylene Hydrogenation over Pt/Al2O3 by Parahydrogen-Induced Polarization

“…Therefore, in principle, the PHIP effect should not be observed in a heterogeneous hydrogenation, but recently PHIP has indeed been observed for this type of reaction [26,27]. This means that pairwise hydrogen addition does take place in this reaction to some extent, but the origin of the PHIP effect is still unclear.…”

Development of new methods in modern selective organic synthesis: preparation of functionalized molecules with atomic precision

“…The influence of the platinum particle size on the PHIP has been studied in detail for propylene hydrogenation catalyzed by Pt/g-Al 2 O 3 with various metal particle sizes. 673 The dependence of the magnitude of PHIP on the particle size proved to be non-monotonic: the effect was least pronounced for particles of diameter 2 ± 4 nm and increased for both larger and smaller particles (Fig. 30).…”