Interfacial Chemistry and Catalysis of Inorganic Materials.

Heterogeneous catalysis is essential to most industrial chemical processes. To achieve a better sustainability of these processes we need highly efficient and highly selective catalysts that are based on earth-abundant materials rather than the more conventional noble metals. Here, we discuss the potential of inorganic materials as catalysts for chemical transformations focusing in particular on the promising transition metal phosphides and sulfides. We describe our recent and current efforts to understand the interfacial chemistry of these materials that governs catalysis, and to tune catalytic reactivity by controlled chemical modification of the material surfaces and by use of interfacial electric fields.

Controlled Surface Modification of Cobalt Phosphide with Sulfur Tunes Hydrogenation Catalysis.

Transition metal phosphides have shown promise as catalysts for water splitting and hydrotreating, especially when a small amount of sulfur is incorporated into the phosphides. However, the effect of sulfur on catalysis is not well understood. In part, this is because conventional preparation methods of sulfur-doped transition metal phosphides lead to sulfur both inside and at the surface of the material. Here, we present an alternative method of modifying cobalt phosphide (CoP) with sulfur using molecular S-transfer reagents, namely, phosphine sulfides (SPR3). SPR3 added sulfur to the surface of CoP and using a series of SPR3 reagents having different P═S bond strengths enabled control over the amount and type of sulfur transferred. Our results show that there is a distribution of different sulfur sites possible on the CoP surface with S-binding strengths in the range of 69 to 84 kcal/mol. This provides fundamental information on how sulfur binds to an amorphous CoP surface and provides a basis to assess how number and type of sulfur on CoP influences catalysis. For the catalytic hydrogenation of cinnamaldehyde, intermediate amounts of sulfur with intermediate binding strengths at the surface of CoP were optimal. With some but not too much sulfur, CoP exhibited a higher hydrogenation productivity and a decreased formation of secondary reaction products. Our work provides important insight into the S-effect on the catalysis by transition metal phosphides and opens new avenues for catalyst design.

Interfacial Acid-Base Equilibria and Electric Fields Concurrently Probed by In Situ Surface-Enhanced Infrared Spectroscopy.

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 pKa by one unit. Since this is ca. 10× the Nernstian value of 59 mV/pKa, ∼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 pKa 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.

Two-Electron-Two-Proton Transfer from Colloidal ZnO and TiO2 Nanoparticles to Molecular Substrates.

Transfers of multiple electrons and protons are challenging yet central to many energy-conversion processes and other chemical and biochemical reactions. Semiconducting oxides can hold multiple redox equivalents. This study describes the 2e-/2H+ transfer reactivity of photoreduced ZnO and TiO2 nanoparticle (NP) colloids with molecular 2e-/2H+ acceptors, to form new O-H, N-H, and C-H bonds. The reaction stoichiometries were monitored by NMR and optical spectroscopies. Faster 2e-/2H+ transfer rates were observed for substrates forming O-H or N-H bonds, presumably due to initial hydrogen bonding at the oxide surface. Chemically reduced ZnO NPs stabilized by Na+ or Ca2+ also engage in 2e-/2H+ transfer reactivity, showing that protons transferred in these processes are inherent to the oxide nanoparticles and do not exclusively stem from photoreduction. These results highlight the potential of ZnO and TiO2 for multiple proton-coupled electron transfer (PCET) reactions.

Hydrogen on Cobalt Phosphide.

Cobalt phosphide (CoP) is one of the most promising earth-abundant replacements for noble metal catalysts for the hydrogen evolution reaction (HER). Critical to HER is the binding of H atoms. While theoretical studies have computed preferred sites and energetics of hydrogen bound to transition metal phosphide surfaces, direct experimental studies are scarce. Herein, we describe measurements of stoichiometry and thermochemistry for hydrogen bound to CoP. We studied both mesoscale CoP particles, exhibiting phosphide surfaces after an acidic pretreatment, and colloidal CoP nanoparticles. Treatment with H2 introduced large amounts of reactive hydrogen to CoP, ca. 0.2 H per CoP unit, and on the order of one H per Co or P surface atom. This was quantified using alkyne hydrogenation and H-atom transfer reactions with phenoxy radicals. Reactive H atoms were even present on the as-prepared materials. On the basis of the reactivity of CoP with various molecular hydrogen donating and accepting reagents, the distribution of binding free energies for H atoms on CoP was estimated to be roughly 51-66 kcal mol-1 (ΔG°H ≅ 0 to -0.7 eV vs H2). Operando X-ray absorption spectroscopy gave preliminary indications about the structure of hydrogenated CoP, showing a slight lattice expansion and no significant change of the effective nuclear charge of Co under H2-flow. These results provide a new picture of catalytically active CoP, with a substantial amount of reactive H atoms. This is likely of fundamental relevance for its catalytic and electrocatalytic properties. Additionally, the approach developed here provides a roadmap to examine hydrogen on other materials.

Bulk-to-Surface Proton-Coupled Electron Transfer Reactivity of the Metal-Organic Framework MIL-125.

Stoichiometric proton-coupled electron transfer (PCET) reactions of the metal-organic framework (MOF) MIL-125, Ti8O8(OH)4(bdc)6 (bdc = terephthalate), are described. In the presence of UV light and 2-propanol, MIL-125 was photoreduced to a maximum of 2( e-/H+) per Ti8 node. This stoichiometry was shown by subsequent titration of the photoreduced material with the 2,4,6-tri- tert-butylphenoxyl radical. This reaction occurred by PCET to give the corresponding phenol and the original, oxidized MOF. The high level of charging, and the independence of charging amount with particle size of the MOF samples, shows that the MOF was photocharged throughout the bulk and not only at the surface. NMR studies showed that the product phenol is too large to fit in the pores, so the phenoxyl reaction must have occurred at the surface. Attempts to oxidize photoreduced MIL-125 with pure electron acceptors resulted in multiple products, underscoring the importance of removing e- and H+ together. Our results require that the e- and H+ stored within the MOF architecture must both be mobile to transfer to the surface for reaction. Analogous studies on the soluble cluster Ti8O8(OOC tBu)16 support the notion that reduction occurs at the Ti8 MOF nodes and furthermore that this reduction occurs via e-/H+ (H-atom) equivalents. The soluble cluster also suggests degradation pathways for the MOFs under extended irradiation. The methods described are a facile characterization technique to study redox-active materials and should be broadly applicable to, for example, porous materials like MOFs.

Cation Effects on the Reduction of Colloidal ZnO Nanocrystals.

The effects of a variety of monatomic cations (H+, Li+, Na+, K+, Mg2+, and Ca2+) 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*2 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.

Bridging the Gap between Industrial and Well-Defined Supported Catalysts.

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.

Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies.

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(SiMe3)2)3 vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2 or HOSi(OtBu)3 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 89Y 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.

A Molecular Approach to Well-defined Metal Sites Supported on Oxides with Oxidation State and Nuclearity Control.

A molecular understanding of the catalytically active site is essential to rationally develop metal-containing heterogeneous catalysts. The controlled grafting of molecular precursors on pre-treated supports, often referred to as surface organometallic chemistry, is an approach to prepare well-defined heterogeneous catalysts with complex organic functionalities. However, many heterogeneous catalysts do not contain organic ligands coordinated to their active sites. To model such sites, the principles of surface organometallic chemistry therefore have to be adapted. Here, we describe a method, which provides access to molecularly-defined metal sites supported on oxides, which do not contain organic functionalities and are uniform in oxidation state and nuclearity. By consecutive grafting of suitable molecular precursors and controlled thermal treatment, we prepared and characterized well-defined dinuclear Cr(II) and Cr(III) species and mononuclear Cr(III) species supported on silica. We also investigated the polymerization activity of these materials in view of the well-known ethylene polymerization catalyst based on CrO(x)/SiO2, the so-called Phillips catalyst. This study led to new insights on the catalytically active sites in ethylene polymerization, which are based on Cr(III), not Cr(II).

Heterolytic Activation of C-H Bonds on Cr(III)-O Surface Sites Is a Key Step in Catalytic Polymerization of Ethylene and Dehydrogenation of Propane.

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.

Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates.

Mononuclear Cr(III) surface sites were synthesized from grafting [Cr(OSi(O(t)Bu)3)3(tetrahydrofurano)2] on silica partially dehydroxylated at 700 °C, followed by a thermal treatment under vacuum, and characterized by infrared, ultraviolet-visible, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy (XAS). These sites are highly active in ethylene polymerization to yield polyethylene with a broad molecular weight distribution, similar to that typically obtained from the Phillips catalyst. CO binding, EPR spectroscopy, and poisoning studies indicate that two different types of Cr(III) sites are present on the surface, one of which is active in polymerization. Density functional theory (DFT) calculations using cluster models show that active sites are tricoordinated Cr(III) centers and that the presence of an additional siloxane bridge coordinated to Cr leads to inactive species. From IR spectroscopy and DFT calculations, these tricoordinated Cr(III) sites initiate and regulate the polymer chain length via unique proton transfer steps in polymerization catalysis.

Polymerization of ethylene by silica-supported dinuclear Cr(III) sites through an initiation step involving C-H bond activation.

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.