Many industrial catalysts contain isolated metal sites on the surface of oxide supports. Although such catalysts have been used in a broad range of processes for more than 40 years, there is often a very limited understanding about the structure of the catalytically active sites. This Review discusses how surface organometallic chemistry (SOMC) engineers surface sites with well-defined structures and provides insight into the nature of the active sites of industrial catalysts; the Review focuses in particular on olefin production and conversion processes.

Significance The Phillips catalyst—CrO x /SiO 2 —produces 40–50% of global high-density polyethylene, yet several fundamental mechanistic controversies surround this catalyst. What is the oxidation state and nuclearity of the active Cr sites? How is the first Cr–C bond formed? How does the polymer propagate and regulate its molecular weight? Here we show through combined experimental (infrared, ultraviolet-visible, X-ray near edge absorption spectroscopy, and extended X-ray absorption fine structures) and density functional theory modeling approaches that mononuclear tricoordinate Cr(III) sites immobilized on silica polymerize ethylene by the classical Cossee–Arlman mechanism. Initiation (C–H bond activation) and polymer molecular weight regulation (the microreverse of C–H activation) are controlled by proton transfer steps.

The insertion of an olefin into a preformed metal-carbon bond is a common mechanism for transition-metal-catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal-carbon bond is not present in the precatalyst. The Phillips catalyst, CrO3 dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first CrC bond is formed remain unknown. We synthesized well-defined dinuclear Cr(II) and Cr(III) sites on silica. Whereas the Cr(II) material was a poor polymerization catalyst, the Cr(III) material was active. Poisoning studies showed that about 65 % of the Cr(III) sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si-(μ-OH)-Cr(III) species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr(III) O bonds.

We describe the reactivity of well-defined chromium silicates toward ethylene and propane. The initial motivation for this study was to obtain a molecular understanding of the Phillips polymerization catalyst. The Phillips catalyst contains reduced chromium sites on silica and catalyzes the polymerization of ethylene without activators or a preformed Cr-C bond. Cr(II) sites are commonly proposed active sites in this catalyst. We synthesized and characterized well-defined chromium(II) silicates and found that these materials, slightly contaminated with a minor amount of Cr(III) sites, have poor polymerization activity and few active sites. In contrast, chromium(III) silicates have 1 order of magnitude higher activity. The chromium(III) silicates initiate polymerization by the activation of a C-H bond of ethylene. Density functional theory analysis of this process showed that the C-H bond activation step is heterolytic and corresponds to a σ-bond metathesis type process. The same well-defined chromium(III) silicate catalyzes the dehydrogenation of propane at elevated temperatures with activities similar to those of a related industrial chromium-based catalyst. This reaction also involves a key heterolytic C-H bond activation step similar to that described for ethylene but with a significantly higher energy barrier. The higher energy barrier is consistent with the higher pKa of the C-H bond in propane compared to the C-H bond in ethylene. In both cases, the rate-determining step is the heterolytic C-H bond activation.

Grafting molecular precursors on partially dehydroxylated silica followed by a thermal treatment yields silica-supported M(III) sites for a broad range of metals. They display unique properties such as high activity in olefin polymerization and alkane dehydrogenation (M = Cr) or efficient luminescence properties (M = Yb and Eu) essential for bioimaging. Here, we interrogate the local structure of the M(III) surface sites obtained from two molecular precursors, amides M(N(SiMe)) vs siloxides (M(OSi(OBu))·L with L = (THF) or HOSi(OBu) for M = Cr, Yb, Eu, and Y, by a combination of advanced spectroscopic techniques (EPR, IR, XAS, UV-vis, NMR, luminescence spectroscopies). For paramagnetic Cr(III), EPR (HYSCORE) spectroscopy shows hyperfine coupling to nitrogen only when the amide precursor is used, consistent with the presence of nitrogen neighbors. This changes their specific reactivity compared to Cr(III) sites in oxygen environments obtained from siloxide precursors: no coordination of CO and oligomer formation during the polymerization of ethylene due to the presence of a N-donor ligand. The presence of the N-ligand also affects the photophysical properties of Yb and Eu by decreasing their lifetime, probably due to nonradiative deactivation of excited states by N-H bonds. Both types of precursors lead to a distribution of surface sites according to reactivity for Cr, luminescence spectroscopy for Yb and Eu, and dynamic nuclear polarization surface-enhanced Y NMR spectroscopy (DNP SENS). In particular, DNP SENS provides molecular-level information about the structure of surface sites by evidencing the presence of tri-, tetra-, and pentacoordinated Y-surface sites. This study provides unprecedented evidence and tools to assess the local structure of metal surface sites in relation to their chemical and physical properties.

Understanding how applied potentials and electrolyte solution conditions affect interfacial proton (charge) transfers at electrode surfaces is critical for electrochemical technologies. Herein, we examine mixed self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid (4-MBA) and 4-mercaptobenzonitrile (4-MBN) on gold using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS). Measurements as a function of the applied potential, the electrolyte pD, and the electrolyte concentration determined both the relative surface populations of acidic and basic forms of 4-MBA, as well as the local electric fields at the SAM–solution interface by following the Stark shifts of 4-MBN. The effective acidity of the SAM varied with the applied potential, requiring a 600 mV change to move the pK a by one unit. Since this is ca. 10× the Nernstian value of 59 mV/pK a, ∼90% of the applied potential dropped across the SAM layer. This emphasizes the importance of distinguishing applied potentials from the potential experienced at the interface. We use the measured interfacial electric fields to estimate the experienced potential at the SAM edge. The SAM pK a showed a roughly Nernstian dependence on this estimated experienced potential. An analysis of the combined acid–base equilibria and Stark shifts reveals that the interfacial charge density has significant contributions from both SAM carboxylate headgroups and electrolyte components. Ion pairing and ion penetration into the SAM also influence the observed surface acidity. To our knowledge, this study is the first concurrent examination of both effective acidity and electric fields, and highlights the relevance of experienced potentials and specific ion effects at functionalized electrode surfaces.

We synthesized isolated Cr­(III) sites on SiO2−Al2O3 and Al2O3 by grafting and subsequent controlled thermal treatment of Cr­(OSi­(OtBu)3)3(THF)2 and Cr­(Al­(OiPr)4)3 on alumina. CrO x /Al2O3 was obtained from incipient wetness impregnation of Al2O3 with CrO3 in H2O followed by calcination, as carried out for the synthesis of industrial Cr-based dehydrogenation catalysts. These materials were characterized by IR, EPR, XAS, and by the adsorption of the probe molecules CO and pyridine, and the results were compared to previously reported isolated Cr­(III)/SiO2. All of these materials were active in propane dehydrogenation at 550 °C, where higher TOFs were obtained for Cr­(III)/SiO2-Al2O3 and Cr­(III)/Al2O3 than for CrO x /Al2O3 and Cr­(III)/SiO2. For mechanistic studies the reverse reaction, propene hydrogenation, was studied. Here, the order of reactivity was opposite that of dehydrogenation, with CrO x /Al2O3 and Cr­(III)/SiO2 being more active in hydrogenation than Cr­(III)/SiO2-Al2O3 and Cr­(III)/Al2O3. Kinetic analysis and labeling studies with D2 showed that the rate law is in all cases first order in H2 partial pressure but had different dependence on propene partial pressure from catalyst to catalyst. We found small normal kinetic isotope effects of 1 ≤ KIE ≤ 2, activation enthalpies up to 40 kJ mol–1, and large negative activation entropies between −100 and −194 J K–1 mol–1 for the different Cr catalysts. We also performed parahydrogen-induced polarization (PHIP) experiments, which showed that H2 addition to propene proceeds, at least in part, via a pairwise mechanism. The experimental data for propene hydrogenation suggests adsorption/desorption pre-equilibria of H2 (or D2) and propene followed by H2 activation and insertion of propene. DFT computations for propane dehydrogenation and propene hydrogenation on Cr­(III) on a periodic alumina model show that a mechanism involving X–H activation (X = R, H) is energetically feasible with activation enthalpies and entropies that are comparable to the experimentally determined values.

The effects of a variety of monatomic cations (H, Li, Na, K, Mg, and Ca) and larger cations (decamethylcobaltocenium and tetrabutylammonium) on the reduction of colloidal ZnO nanocrystals (NCs) are described. Suspensions of "TOPO"-capped ZnO NCs in toluene/THF were treated with controlled amounts of one-electron reductants (CoCp* or sodium benzophenone anion radical) and cations. Equilibria were quickly established and the extent of NC reduction was quantified via observation of the characteristic near-IR absorbance of conduction band electrons. Addition of excess reductant with or without added cations led to a maximum average number of electrons per ZnO NC, which was dependent on the NC volume and on the nature of the cation. Electrons are transferred to the ZnO NCs in a stoichiometric way, roughly one electron per monovalent cation and roughly two electrons per divalent cation. This shows that cations are charge-balancing the added electrons in ZnO NCs. Overall, our experiments provide insight into the thermodynamics of charge storage and relate the colloidal chemistry of ZnO with bulk oxide semiconductors. They indicate that the apparent band energies of colloidal ZnO are highly dependent on cation/electrolyte composition and concentration, as is known for bulk interfaces, and that electrons and cations are added stoichiometrically to balance charge, similar to the behavior of Li-batteries.