Maximum Noble‐Metal Efficiency in Catalytic Materials: Atomically Dispersed Surface Platinum

Platinum is the most versatile element in catalysis, but it is rare and its high price limits large‐scale applications, for example in fuel‐cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst’s surface. To maximize the noble‐metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria “nanopocket”, which binds Pt²⁺ so strongly that it withstands sintering and bulk diffusion. On model catalysts we experimentally confirm the theoretically predicted stability, and on real Pt‐CeO₂ nanocomposites showing high Pt efficiency in fuel‐cell catalysis we also identify these anchoring sites.     

Generation of Aryl Radicals through Reduction of Hypervalent Iodine(III) Compounds with TEMPONa: Radical Alkene Oxyarylation

A novel method for selective generation of aryl radicals from diaryliodonium salts and iodanylidene malonates with sodium 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPONa) as a single‐electron transfer (SET) reducing reagent is described. In the presence of various alkenes, aryl radicals formed after SET‐reduction of hypervalent iodine compounds undergo alkene addition and the adduct radicals that are thus generated are efficiently trapped by the concomitantly generated TEMPO radical to eventually afford oxyarylated products in moderate to very good yields. The efficiency of aryl radical generation of various iodine(III) reagents is studied and the generation of an iodanylidene malonate aryl radical is also investigated by computational methods.     

A Theoretical Study on the Mechanism of C2H4 Oxidation over a Neutral V3O8 Cluster

Density functional theory (DFT) calculations are used to investigate the reaction mechanism of V₃O₈+C₂H₄. The reaction of V₃O₈ with C₂H₄ produces V₃O₇CH₂+HCHO or V₃O₇+CH₂OCH₂ overall barrierlessly at room temperature, whereas formation of hydrogen‐transfer products V₃O₇+CH₃CHO is subject to a tiny overall free energy barrier (0.03 eV), although the formation of the last‐named pair of products is thermodynamically more favorable than that of the first two. These DFT results are in agreement with recent experimental observations. The (O_b)₂V(O_tO_t)^. (b=bridging, t=terminal) moiety containing the oxygen radical in V₃O₈ is the active site in the reaction with C₂H₄. Similarities and differences between the reactivities of (O_b)₂V(O_tO_t)^. in V₃O₈ and the small VO₃ cluster [(O_t)₂VO_t^.] are discussed. Moreover, the effect of the support on the reactivity of the (O_b)₂V(O_tO_t)^. active site is evaluated by investigating the reactivity of the cluster VX₂O₈, which is obtained by replacing the V atoms in the (O_b)₃VO_t support moieties of V₃O₈ with X atoms (X=P, As, Sb, Nb, Ta, Si, and Ti). Support X atoms with different electronegativities influence the oxidative reactivity of the (O_b)₂V(O_tO_t)^. active site through changing the net charge of the active site. These theoretical predictions of the mechanism of V₃O₈+C₂H₄ and the effect of the support on the active site may be helpful for understanding the reactivity and selectivity of reactive O^. species over condensed‐phase catalysts.     

Differences and Commonalities in the Gas‐Phase Reactions of Closed‐Shell Metal Dioxide Clusters [MO2]+ (M=V,Nb, and Ta) with Methane

High‐level electronic structure calculations, in combination with Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometric studies, permit the mechanism by which closed‐shell, “naked” [TaO₂]⁺ brings about C?H bond activation of methane to be revealed. These studies also help to understand why the lighter congeners of [MO₂]⁺ (M=V, Nb) are unreactive under ambient conditions.     

Reducing the Cost and Preserving the Reactivity in Noble‐Metal‐Based Catalysts: Oxidation of CO by Pt and Al–Pt Alloy Clusters Supported on Graphene

The oxidation mechanisms of CO to CO₂ on graphene‐supported Pt and Pt‐Al alloy clusters are elucidated by reactive dynamical simulations. The general mechanism evidenced is a Langmuir–Hinshelwood (LH) pathway in which O₂ is adsorbed on the cluster prior to the CO oxidation. The adsorbed O₂ dissociates into two atomic oxygen atoms thus promoting the CO oxidation. Auxiliary simulations on alloy clusters in which other metals (Al, Co, Cr, Cu, Fe, Ni) replace a Pt atom have pointed to the aluminum doped cluster as a special case. In the nanoalloy, the reaction mechanism for CO oxidation is still a LH pathway with an activation barrier sufficiently low to be overcome at room temperature, thus preserving the catalyst efficiency. This provides a generalizable strategy for the design of efficient, yet sustainable, Pt‐based catalysts at reduced cost.     

Fabrication of Ruthenium Nanoparticles in Porous Organic Polymers: Towards Advanced Heterogeneous Catalytic Nanoreactors

A novel strategy has been adopted for the construction of a copolymer of benzene–benzylamine‐1 (BBA‐1), which is a porous organic polymer (POP) with a high BET surface area, through Friedel–Crafts alkylation of benzylamine and benzene by using formaldehyde dimethyl acetal as a cross‐linker and anhydrous FeCl₃ as a promoter. Ruthenium nanoparticles (Ru NPs) were successfully distributed in the interior cavities of polymers through NaBH₄, ethylene glycol, and hydrothermal reduction routes, which delivered Ru‐A, Ru‐B, and Ru‐C materials, respectively, and avoided aggregation of metal NPs. Homogeneous dispersion, the nanoconfinement effect of the polymer, and the oxidation state of Ru NPs were verified by employing TEM, energy‐dispersive X‐ray spectroscopy mapping, cross polarization magic‐angle spinning ¹³C NMR spectroscopy, and X‐ray photoelectron spectroscopy analytical tools. These three new Ru‐based POP materials exhibited excellent catalytic performance in the hydrogenation of nitroarenes at RT (with a reaction time of only ≈30 min), with high conversion, selectivity, stability, and recyclability for several catalytic cycles, compared with other traditional materials, such as Ru@C, Ru@SiO₂, and Ru@TiO₂, but no clear agglomeration or loss of catalytic activity was observed. The high catalytic performance of the ruthenium‐based POP materials is due to the synergetic effect of nanoconfinement and electron donation offered by the 3D POP network. DFT calculations showed that hydrogenation of nitrobenzene over the Ru (0001) catalyst surface through a direct reaction pathway is more favorable than that through an indirect reaction pathway.     

Direct Observation of Oxygen Dissociation on Non‐Stoichiometric Metal Oxide Catalysts

Oxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi‐step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ and (La_0.8Sr_0.2)_0.95MnO_3±δ, using gas‐phase isotope‐exchange with a 1:1 ¹⁶O₂:¹⁸O₂ ratio. Oxygen transport mechanisms between gas–surface reactions and surface–bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ is better at oxygen dissociation than (La_0.8Sr_0.2)_0.95MnO_3±δ. Heteroexchange, involving lattice oxygen, dominates on La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ. In contrast, (La_0.8Sr_0.2)_0.95MnO_3±δ shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.     

The Site‐Assembly Determines Catalytic Activity of Nanoparticles

Heterogeneous catalysts are often designed as metal nanoparticles supported on oxide surfaces. Here, the relation between particle morphology and reaction kinetics is investigated by scaling relation kinetic Monte Carlo simulations using CO oxidation over Pt nanoparticles as a model reaction. We find that different particle morphologies result in vastly different catalytic activities. The activity is strongly affected by kinetic couplings between sites, and a wide site distribution generally enhances the activity. The present study highlights the role of site‐assemblies as a concept that, in addition to isolated active sites, can be used to understand catalytic reactions over nanoparticles.     

How Absorbed Hydrogen Affects the Catalytic Activity of Transition Metals

Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show that a sufficient concentration of subsurface hydrogen, H^sub, may either significantly increase or decrease the bond energy and the reactivity of the adsorbed hydrogen, H^ad, depending on the metal. We predict a representative reaction, ethyl hydrogenation, to speed up on Pd and Pt, but to slow down on Ni and Rh in the presence of H^sub, especially on metal nanoparticles. The identified effects of subsurface H on surface reactivity are indispensable for an atomistic understanding of hydrogenation processes on transition metals and interactions of hydrogen with metals in general.     

Lowering Energy Barriers in Surface Reactions through Concerted Reaction Mechanisms

Any technologically important chemical reaction typically involves a number of different elementary reaction steps consisting of bond‐breaking and bond‐making processes. Usually, one assumes that such complex chemical reactions occur in a step‐wise fashion where one single bond is made or broken at a time. Using first‐principles calculations based on density functional theory we show that the barriers of rate‐limiting steps for technologically relevant surface reactions are significantly reduced if concerted reaction mechanisms are taken into account.     

The Geometric Structure of Silver‐Doped Silicon Clusters

Cationic silver‐doped silicon clusters, Si_nAg⁺ (n=6–15), are studied using infrared multiple photon dissociation in combination with density functional theory computations. Candidate structures are identified using a basin‐hopping global optimizations method. Based on the comparison of experimental and calculated IR spectra for the identified low‐energy isomers, structures are assigned. It is found that all investigated clusters have exohedral structures, that is, the Ag atom is located at the surface. This is a surprising result because many transition‐metal dopant atoms have been shown to induce the formation of endohedral silicon clusters. The silicon framework of Si_nAg⁺ (n=7–9) has a pentagonal bipyramidal building block, whereas the larger Si_nAg⁺ (n=10–12, 14, 15) clusters have trigonal prism‐based structures. On comparing the structures of Si_nAg⁺ with those of Si_nCu⁺ (for n=6–11) it is found that both Cu and Ag adsorb on a surface site of bare Si_n⁺ clusters. However, the Ag dopant atom takes a lower coordinated site and is more weakly bound to the Si_n⁺ framework than the Cu dopant atom.