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A catalytic process for the selective formation of cis olefins would help minimize the production of unhealthy trans fats during the partial hydrogenation of edible oils. Here we report on the design of such a process on the basis of studies with model systems. Temperature programmed desorption data on single crystals showed that the isomerization of trans olefins to their cis counterparts is promoted by (111) facets of platinum, and that such selectivity is reversed on more open surfaces. Quantum mechanics calculations suggested that the extra stability of cis olefins seen on hydrogen-saturated Pt(111) surfaces may be due to a lesser degree of surface reconstruction, a factor found to be significant in the adsorption on close-packed platinum surfaces. Kinetic data using catalysts made out of dispersed tetrahedral Pt nanoparticles corroborated the selective promotion of the trans -to- cis isomerization on the (111) facets of the metal. Our work provides an example for how catalytic selectivity may be controlled by controlling the shape of the catalytic particles. The selective formation of cis olefins would reduce the production of unhealthy trans fats during the partial hydrogenation of edible oils. Single-crystal surfaces with well-defined supported nanoparticle catalysts now show that platinum (111) surfaces can selectively promote the non-thermodynamic isomerization of trans olefins to their cis counterparts.

A Perspective is offered on the lessons learned from surface-science studies on enantioselective chemistry on solid surfaces performed by the author’s groups. Our emphasis is on studies on model systems, mainly metal single-crystal surfaces under controlled environments, but extension of such research to more realistic samples relevant to heterogeneous catalysis is also briefly discussed. Enantioselective chemistry on surfaces is here divided into three guiding modalities, depending on the underlying mechanism. First, enantioselective chemistry resulting from the use of intrinsically chiral surfaces, which can be made from achiral solids such as metals by exposing the appropriate planes, is discussed. Next, the imparting of enantioselectivity to achiral surfaces by modifying them with adsorbates is classified in terms of two operating mechanisms: first, via the formation of supramolecular surface ensembles with chiral adsorption sites, and second, by relying on the effect of the local chiral environment intrinsically provided by the chiral modifiers through a one-to-one interaction between the modifier and the reactant. A discussion is then provided on studies with more complex samples involving metal nanoparticles and high-surface-area porous oxides. Finally, the present state of our understanding of enantioselective surface chemistry and the prognosis for the future are provided. Graphical Abstract

Colloidal and sol-gel procedures have been used to prepare heterogeneous catalysts consisting of platinum metal particles with narrow size distributions and well defined shapes dispersed on high-surface-area silica supports. The overall procedure was developed in three stages. First, tetrahedral and cubic colloidal metal particles were prepared in solution by using a procedure derived from that reported by El-Sayed and coworkers [Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA (1996) Science 272:1924-1926]. This method allowed size and shape to be controlled independently. Next, the colloidal particles were dispersed onto high-surface-area solids. Three approaches were attempted: (i) in situ reduction of the colloidal mixture in the presence of the support, (ii) in situ sol-gel synthesis of the support in the presence of the colloidal particles, and (iii) direct impregnation of the particles onto the support. Finally, the resulting catalysts were activated and tested for the promotion of carbon-carbon double-bond cis-trans isomerization reactions in olefins. Our results indicate that the selectivity of the reaction may be controlled by using supported catalysts with appropriate metal particle shapes.

A new synthetic strategy has been developed to encapsulate supported Pt nanoparticles in heterogeneous catalysts to prevent their sintering. Model catalysts were first prepared by dispersing -3-nm Pt nanoparticles on -120-nm silica beads. These were then covered with a fresh layer of mesoporous silica, a few tens of nanometers thick, and etched to re-expose the metal surface to the reaction mixtures. TEM images were used to confirm the success of each of the synthesis steps, and both CO titrations and kinetic measurements for the catalytic conversion of cis- and trans-2-butenes with hydrogen were employed to test the degree of re-activation of the catalyst obtained after the etching treatment, which had to be tuned to give simultaneous maximum activity and maximum catalyst stability. The resulting encapsulated platinum nanoparticles were shown to resist sintering during calcination at temperatures as high as 1075 K, whereas the unprotected catalysts were seen to sinter by 875 K.

New advances in colloidal and other self-assembly synthetic methods have afforded the controlled growth of nanoparticles with well-defined sizes and shapes. Recently, the catalysis community has been trying to capitalize on this knowledge for the design of new catalytic processes. In particular, the use of metal nanoparticles with specific shapes has been explored in several instances as a way to control reaction selectivity. Here we review the results from our efforts to use platinum nanoparticles dispersed on high-surface-area supports to perform selective olefin conversions. Emphasis is given to the surface-science experiments and quantum-mechanics calculations that led us to identify potential variations in selectivity in carbon–carbon double-bond isomerization and hydrogenation reactions with the structure of the metal surface. Temperature programmed desorption (TPD) and reflection–absorption infrared spectroscopy data for 2-butenes adsorbed on Pt(111) single-crystal surfaces highlighted the relative higher stability of adsorbed cis-2-butene compared to trans-2-butene and the preference for the promotion of trans-to-cis conversions on that surface. It was also determined that coadsorbed hydrogen plays a key role in defining the relative stabilities of the adsorbates, favoring pi rather than di-sigma bonding and reversing the higher stability of the trans adsorbates seen on clean Pt(111). DFT calculations suggested that such unique results may be accounted for by the need for extensive surface reconstruction to accommodate the adsorbates on such flat planes, a requirement that appears to be less severe with the cis isomer. TPD experiments on stepped Pt(557) surfaces pointed to the minimal importance of steps in promoting these isomerization reactions, although they do seem to help with the full hydrogenation to the alkanes. More extensive olefin adsorption destabilization with hydrogen coadsorption and faster alkane production was seen on Pt(100), but selectivity towards the cis isomer was still identified. On the more open (2 × 1)-reconstructed Pt(110) surface, on the other hand, trans-2-butene is the most stable of the two isomers. It was finally shown that these surface-science results translate into changes in selectivity in real catalysts with platinum nanoparticle shape. Catalysts were prepared by using colloidal Pt nanoparticles with tetrahedral, cubic, and rounded shapes, and unique selectivity toward cis-2-butene formation was measured on the first of those samples. It appears that the (111) facets exposed by the tetrahedral Pt nanoparticles do show the same trans-to-cis conversion preference in catalysis seen in the surface-science studies carried out with single-crystal surfaces and under ultrahigh vacuum conditions.

The adsorption of crotonaldehyde on Cu-Pt alloy surfaces was characterized by density functional theory (DFT). Two surfaces were considered: Cu2Pt/Cu(111) and Cu3Pt/Cu(111). It was determined that the presence of Pt on the surface, even when isolated as single atoms fully surrounded by Cu, provides additional stability for the adsorbates, increasing the magnitude of the adsorption energy by as much as 40 kJ/mol. The preferred bonding on both surfaces is via multiple coordination, with the most stable configuration being a cis arrangement with di-σ bonding of the C=O bond across a Cu–Cu bridge and an additional π bonding to a Pt atom. The fact that Pt significantly affects the adsorption of unsaturated aldehydes such as crotonaldehyde explains why the kinetics of their hydrogenation using single-atom alloy (SAA) catalysts vary with alloy composition, as we previously reported, and brings into question the simple model in which the role of Pt is only to promote the dissociation of H2.

In this brief perspective we summarize the main results emanating from our research with gold-titania catalysts. We first discuss the development of new Au@Void@TiO2 yolk-shell nanostructures, describing the basic steps involved in their synthesis and analyzing their advantages in comparison with more standard supported catalysts. We then introduce the two catalytic processes that we have studied with the help of such nanostructures: low-temperature oxidations, and the photocatalytic splitting of water to produce molecular hydrogen. During our exploration of the use of Au@Void@TiO2 samples to increase catalyst stability against metal nanoparticle sintering, we discovered that the treatment of the amorphous titania in these Au/TiO2 systems with sodium hydroxide creates new titanate sites that promote carbon monoxide oxidation at cryogenic temperatures. The new catalysis was shown to involve at least two different adsorption sites on the titania phase. In connection with the photocatalytic generation of molecular hydrogen, we acquired several compelling pieces of evidence to challenge the accepted interpretation of the role of the metal in enhancing the photocatalytic activity of the semiconductor. Instead of trapping the electrons generated by photon absorption on the titania and directing the reduction steps, we argue that the main contribution of the gold (or platinum) nanoparticles is to promote the recombination of the hydrogen atoms formed by reduction of protons at the semiconductor surface. We finish our presentation with some brief suggestions for new designs of yolk-shell-based nanostructures and new applications for gold/titania catalysis.

The case is made here for the power of using atomic layer deposition (ALD) as a way to induce changes in the nature of the oxides used as supports in many catalytic processes. ALD provides a route to grow thin films in a conformal way and with submonolayer thickness control, affording the creation of unique mixed-oxide structures with new reaction sites. This approach is exemplified here for the case of the hydrogenation of unsaturated aldehydes with platinum-based catalysts. Silica-supported catalysts were modified with thin alumina films, grown by ALD using trimethylaluminum(III) (TMA) and water, and their performance contrasted with pure Pt/SiO2 and Pt/Al2O3 samples as well as with catalysts previously reported by us made by silica ALD on Pt/Al2O3. The quality of the alumina films grown on Pt/SiO2 was first evaluated by using N2 adsorption–desorption isotherms in conjunction with SBA-15 as the support, a mesoporous material with well-defined 1D cylindrical pores. An initial deposition of approximately 1.5 Å of the alumina film per ALD cycle was estimated from those measurements, with retention of the narrow distribution of pore diameters indicative of homogeneous coverage throughout the length of the pores. The catalytic hydrogenation of cinnamaldehyde was then determined to be slower but more selective with silica supports compared to alumina. Addition of a half of a monolayer of alumina to Pt/SiO2 reduces the total activity, but only marginally. In exchange, the new mixed-oxide catalysts exhibit a higher selectivity toward the production of the desirable unsaturated alcohol at high conversions, and a lower activity for its subsequent hydrogenation to the saturated alcohol. These trends were associated with the formation of new Brønsted and Lewis acidic sites, possibly based on mixed Si–O–Al surface structures.

In chemical catalysis, spillover is the process in which hydrogen atoms are made from hydrogen molecules at one site and then added to other atoms or molecules at another. A study reveals details of this effect. See Letter p.68 Hydrogen spillover in supported catalysts Hydrogen spillover—the surface migration of hydrogen atoms from the metal catalyst particle on which they are generated onto the catalyst support—was discovered in the early 1960s, but remains poorly understood. Waiz Karim and colleagues now use advanced nanofabrication to place multiple pairs of iron oxide and platinum nanoparticles on titanium oxide and aluminium oxide supports, with varying inter-particle distance, and observe the extent of the reduction of the iron oxide particles by hydrogen atoms generated on the platinum. The results reveal that hydrogen spillover is fast and efficient on titanium oxide, and extremely slow and short-ranged on aluminium oxide. The results should aid our understanding of hydrogen storage and catalytic hydrogenation reactions, and the approach to creating and probing model catalyst systems open up new avenues for studying fundamental processes in supported catalysts.