Helena E. Hagelin-Weaver

Parahydrogen-induced polarization by pairwise replacement catalysis on Pt and Ir nanoparticles.

Pairwise and random addition processes are ordinarily indistinguishable in hydrogenation reactions. The distinction becomes important only when the fate of spin correlation matters, such as in parahydrogen-induced polarization (PHIP). Supported metal catalysts were not expected to yield PHIP signals given the rapid diffusion of H atoms on the catalyst surface and in view of the sequential stepwise nature of the H atom addition in the Horiuti-Polanyi mechanism. Thus, it seems surprising that supported metal hydrogenation catalysts can yield detectable PHIP NMR signals. Even more remarkably, supported Pt and Ir nanoparticles are shown herein to catalyze pairwise replacement on propene and 3,3,3-trifluoropropene. By simply flowing a mixture of parahydrogen and alkene over the catalyst, the scalar symmetrization order of the former is incorporated into the latter without a change in molecular structure, producing intense PHIP NMR signals on the alkene. An important indicator of the mechanism of the pairwise replacement is its stereoselectivity, which is revealed with the aid of density matrix spectral simulations. PHIP by pairwise replacement has the potential to significantly diversify the substrates that can be hyperpolarized by PHIP for biomedical utilization.

Shaped Ceria Nanocrystals Catalyze Efficient and Selective Para-Hydrogen-Enhanced Polarization.

Intense para-hydrogen-enhanced NMR signals are observed in the hydrogenation of propene and propyne over ceria nanocubes, nano-octahedra, and nanorods. The well-defined ceria shapes, synthesized by a hydrothermal method, expose different crystalline facets with various oxygen vacancy densities, which are known to play a role in hydrogenation and oxidation catalysis. While the catalytic activity of the hydrogenation of propene over ceria is strongly facet-dependent, the pairwise selectivity is low (2.4% at 375 °C), which is consistent with stepwise H atom transfer, and it is the same for all three nanocrystal shapes. Selective semi-hydrogenation of propyne over ceria nanocubes yields hyperpolarized propene with a similar pairwise selectivity of (2.7% at 300 °C), indicating product formation predominantly by a non-pairwise addition. Ceria is also shown to be an efficient pairwise replacement catalyst for propene.

Enhanced catalytic methane coupling using novel ceramic foams with bimodal porosity

Monolithic reactor concepts are currently intensively discussed in the literature. For the oxidative coupling of methane relying on a balance of surface and gas phase reactions, such concepts have previously been claimed to have beneficial effects with respect to obtainable C2 yields. In order to verify the superior performance in the case of a foam catalyst, ceria and samaria foams were fabricated by a direct foaming process. In both cases, mechanically stable, homogeneous open-cell foams were obtained as revealed by 3D magnetic resonance imaging, Hg-porosimetry, and scanning electron microscopy. As a characteristic feature of the introduced foaming methodology the process resulted in bimodal pore size distributions, ensuring low pressure drops on the one hand and sufficiently large surface areas on the other hand. Oxidative coupling of methane was carried out over the samaria foams. It was possible to obtain C2 yields that were indeed higher than those obtained with the samaria powder, in contrast to honeycomb monoliths previously studied in the literature.

Silica‐Encapsulated Pt‐Sn Intermetallic Nanoparticles: A Robust Catalytic Platform for Parahydrogen‐Induced Polarization of Gases and Liquids

Recently, a facile method for the synthesis of size-monodisperse Pt, Pt3 Sn, and PtSn intermetallic nanoparticles (iNPs) that are confined within a thermally robust mesoporous silica (mSiO2 ) shell was introduced. These nanomaterials offer improved selectivity, activity, and stability for large-scale catalytic applications. Here we present the first study of parahydrogen-induced polarization NMR on these Pt-Sn catalysts. A 3000-fold increase in the pairwise selectivity, relative to the monometallic Pt, was observed using the PtSn@mSiO2 catalyst. The results are explained by the elimination of the three-fold Pt sites on the Pt(111) surface. Furthermore, Pt-Sn iNPs are shown to be a robust catalytic platform for parahydrogen-induced polarization for in vivo magnetic resonance imaging.

Semihydrogenation of Propyne over Cerium Oxide Nanorods, Nanocubes, and Nano‐Octahedra: Facet‐Dependent Parahydrogen‐Induced Polarization

The pairwise selectivity of hydrogenation is a fundamental quantity in hydrogenation catalysis that underpins the NMR signal enhancement achievable by parahydrogen‐induced polarization (PHIP). Herein, we show how the crystal facet dependence of the pairwise selectivity of the semihydrogenation of propyne, when interpreted in the context of recent DFT calculations, can reveal new details about the hydrogenation mechanism. The pairwise selectivity of propyne hydrogenation is strongly shape dependent, which reflects the surface atom arrangements exposed on the different facets of CeO2 nanocrystals. In this first demonstration of a catalyst shape dependence of PHIP, an unprecedented pairwise selectivity of 8.1 % is observed over oxygen‐deficient facets of rods, whereas oxygen‐rich octahedra facets deliver only 1.6 % selectivity. The PHIP data are consistent with a concerted addition pathway through a six‐membered‐ring transition state, as predicted in the DFT study.

Frontispiece: Shaped Ceria Nanocrystals Catalyze Efficient and Selective Para‐Hydrogen‐Enhanced Polarization

NMR Spectroscopy. C. R. Bowers, H. E. Hagelin-Weaver et al. describe in their Communication on page 14270 ff. how intense NMR signals are induced by incorporating para-H2 into propene and propane over octahedron-, cube-, or rod-shaped ceria nanocrystals.

Oxygen Generation from Carbon Dioxide for Advanced Life Support

The partial electrochemical reduction of CO2 using ceramic oxygen generators (COGs) is well known and has been studied. Conventional COGs use yttria-stabilized zirconia (YSZ) electrolytes and operate at temperatures greater than 700 C (1, 2). Operating at a lower temperature has the advantage of reducing the mass of the ancillary components such as insulation. Moreover, complete reduction of metabolically produced CO2 (into carbon and oxygen) has the potential of reducing oxygen storage weight if the oxygen can be recovered. Recently, the University of Florida developed ceramic oxygen generators employing a bilayer electrolyte of gadolinia-doped ceria and erbia-stabilized bismuth oxide (ESB) for NASA s future exploration of Mars (3). The results showed that oxygen could be reliably produced from CO2 at temperatures as low as 400 C. These results indicate that this technology could be adapted to CO2 removal from a spacesuit and other applications in which CO2 removal is an issue. This strategy for CO2 removal in advanced life support systems employs a catalytic layer combined with a COG so that the CO2 is reduced completely to solid carbon and oxygen. First, to reduce the COG operating temperature, a thin, bilayer electrolyte was employed. Second, to promote full CO2 reduction while avoiding the problem of carbon deposition on the COG cathode, a catalytic carbon deposition layer was designed and the cathode utilized materials shown to be coke resistant. Third, a composite anode was used consisting of bismuth ruthenate (BRO) and ESB that has been shown to have high performance (4). The inset of figure 1 shows the conceptual design of the tubular COG and the rest of the figure shows schematically the test apparatus. Figure 2 shows the microstructure of a COG tube prior to testing. During testing, current is applied across the cell and initially CuO is reduced to copper metal by electrochemical pumping. Then the oxygen source becomes the CO/CO2. This presentation details the results of testing the COG.

Surface Science: Foundations of Catalysis and Nanoscience

Price £39.95 The first edition of Kurt W. Kolasinski’s book Surface Science: Foundations of Catalysis and Nanoscience was an almost immediate success and has been described as a ‘classic of its time’. It bridged the gap between primer-type textbooks that just cover what is absolutely necessary for an advanced undergraduate course in surface science and those heavy textbooks targeting mainly the research community, which are suited only for the most ambitious undergraduate students. The second, fully revised edition is substantially enlarged with two completely new chapters and new worked examples throughout the book, which increased its volume by more than 150 pages to 486 in total.