Single-Atom Alloy Pd–Ag Catalyst for Selective Hydrogenation of Acrolein

Pd–Ag alloy catalysts with very dilute amounts of Pd were synthesized. EXAFS results demonstrated that when the concentration of Pd was as low as 0.01 wt %, Pd was completely dispersed as isolated single atoms in Ag nanoparticles. The activity for the hydrogenation of acrolein was improved by the presence of these isolated Pd atoms due to the creation of sites with lower activation energy for H₂ dissociation. In addition, for the same particle size, the 0.01% Pd/8% Ag alloy nanoparticles exhibited higher selectivity than their monometallic counterparts, suggesting that the Pd atom may act as a site for the favorable bonding of the acrolein molecule for facile hydrogenation of the aldehyde functionality

Direct Synthesis of Low-Coordinate Pd Catalysts Supported on SiO₂ via Surface Organometallic Chemistry

Highly dispersed low-coordinate Pd sites on SiO₂ are fabricated by grafting the Pd^II PCP-pincer complex (^tBuPCP)­Pd–OH (^tBuPCP = 2,6-C₆H₃(CH₂P^tBu₂)₂) on SiO₂, followed by calcination with ozone (100 °C) and reduction with H₂ (300 °C). The chemisorption process and structure of this organometallic complex on SiO₂ is established by solution-phase ¹H and ³¹P NMR and solid-state ³¹P CPMAS NMR spectroscopy, XPS, DRIFTS, and AC-HAADF-STEM. The CO adsorption properties of the Pd centers reveal a surprisingly high fraction of adsorption sites where CO is bound in a linear fashion, indicative of low-coordinate Pd. Furthermore, enhanced selectivity of these catalyst centers in aerobic alcohol oxidation versus a control catalyst argues that these low-coordinate sites are the catalytically active sites

Isolated Fe^II on Silica As a Selective Propane Dehydrogenation Catalyst

We report a comparative study of isolated Fe^II, iron oxide particles, and metallic nanoparticles on silica for non-oxidative propane dehydrogenation. It was found that the most selective catalyst was an isolated Fe^II species on silica prepared by grafting the open cyclopentadienide iron complex, bis­(2,4-dimethyl-1,3-pentadienide) iron­(II) or Fe­(<i>o</i>Cp)₂. The grafting and evolution of the surface species was elucidated by ¹H NMR, diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopies. The oxidation state and local structure of surface Fe were characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure. The initial grafting of iron proceeds by one surface hydroxyl Si–OH reacting with Fe­(<i>o</i>Cp)₂ to release one diene ligand (<i>o</i>CpH), generating a SiO₂-bound Fe^II(<i>o</i>Cp) species, <b>1-Fe</b><i><b>o</b></i><b>Cp</b>. Subsequent treatment with H₂ at 400 °C leads to loss of the remaining diene ligand and formation of nanosized iron oxide clusters, <b>1-C</b>. Dispersion of these Fe oxide clusters occurs at 650 °C, forming an isolated, ligand-free Fe^II on silica, <b>1-Fe</b>^<b>II</b>, which is catalytically active and highly selective (∼99%) for propane dehydrogenation to propene. Under reaction conditions, there is no evidence of metallic Fe by in situ XANES. For comparison, metallic Fe nanoparticles, <b>2-NP-Fe</b>^<b>0</b>, were independently prepared by grafting Fe­[N­(SiMe₃)₂]₂ onto silica, <b>2-FeN*</b>, and reducing it at 650 °C in H₂. The Fe NPs were highly active for propane conversion but showed poor selectivity (∼14%) to propene. Independently prepared Fe oxide clusters on silica display a low activity. The sum of these results suggests that selective propane dehydrogenation occurs at isolated Fe^II sites

Pushing the Limits on Metal–Organic Frameworks as a Catalyst Support: NU-1000 Supported Tungsten Catalysts for <i>o</i>‑Xylene Isomerization and Disproportionation

Acid-catalyzed skeletal C–C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, <i>o</i>-xylene isomerization/disproportionation is a probe reaction for strong Brønsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal–organic framework (MOF), as a catalyst for <i>o</i>-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), ³¹P NMR, N₂ physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WO_<i>x</i> clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000-supported catalyst were comparable to a control WO_<i>x</i>-ZrO₂, but the NU-1000 composite material was unusually active toward the transmethylation pathway that requires two adjacent active sites in a confined pore, as created when PTA is confined in NU-1000. This work shows the promise of metal–organic framework topologies in giving access to unique reactivity, even for aggressive reactions such as hydrocarbon isomerization

Stabilizing Single-Atom and Small-Domain Platinum via Combining Organometallic Chemisorption and Atomic Layer Deposition

Oxide-supported single-atom Pt materials are prepared by combining surface organometallic chemisorption with atomic layer deposition (ALD). Here Pt is supported as a discrete monatomic “pincer” complex, stabilized by an atomic layer deposition (ALD) derived oxide overcoat, and then calcined at 400 °C under O₂. ALD-derived Al₂O₃, TiO₂, and ZnO overlayers are effective in suppressing Pt sintering and significantly stabilizing single Pt atoms. Furthermore, this procedure decreases the overall Pt nuclearity (∼1 nm average particle diameter) versus bare Pt (∼3.8 nm average diameter), as assayed by aberration corrected HAADF-STEM. The TiO₂ and ZnO overcoats are significantly more effective at stabilizing single-atom Pt species and decreasing the overall Pt nuclearity than Al₂O₃ overcoats. Vibrational spectroscopy of adsorbed CO also shows that oxidized Pt species commonly thought to be single Pt atoms are inactive for catalytic oxidation of adsorbed CO. CO chemisorption measurements show site blockage by the ALD overcoats

Correction to “Computationally Guided Discovery of Catalytic Cobalt-Decorated Metal–Organic Framework for Ethylene Dimerization”

Correction to “Computationally Guided Discovery of Catalytic Cobalt-Decorated Metal–Organic Framework for Ethylene Dimerization

General Method for Determination of the Surface Composition in Bimetallic Nanoparticle Catalysts from the L Edge X‑ray Absorption Near-Edge Spectra

Bimetallic PtPd on silica nanoparticle catalysts have been synthesized, and their average structure has been determined by Pt L₃ and Pd K edge extended X-ray absorption fine-structure spectroscopy. The bimetallic structure is confirmed from elemental line scans by scanning transmission electron microscopy of the individual 2-nm-sized particles. A general method is described to determine the surface composition of bimetallic nanoparticles even when both metals adsorb; for example, CO, by combining the quantitative characterization by X-ray absorption near-edge structure spectra at L edges with CO adsorption with the adsorption stoichiometry determined by Fourier transform infrared spectroscopy. Determination of the surface composition leads to a better understanding of the changes in catalytic chemistry that accompany alloy formation. Although monometallic Pt and Pd have similar turnover rates for neopentane hydrogenolysis and isomerization, on the basis of the surface composition, it appears that in the bimetallic PtPd catalysts, the rate and products are determined predominantly by Pt with little contribution from surface Pd. Density functional theory calculations indicate that the center of the Pt d-band density of states shifts to higher energy, or closer to the Fermi level, whereas that in Pd shifts to lower energy away from the Fermi level. Similarly, the calculated enthalpy of CO adsorption increases on Pt, but decreases on Pd. It is speculated that because of the very low surface coverage of the neopentane reaction intermediates, only surface atoms that form the strongest bonds are catalytically activethat is, Ptrather than all surface atoms. The dominant role of Pd, therefore, appears to be to (slightly) modify Pt rather than to contribute to the catalytic conversion

Fine-Tuning the Activity of Metal–Organic Framework-Supported Cobalt Catalysts for the Oxidative Dehydrogenation of Propane

Few-atom cobalt-oxide clusters, when dispersed on a Zr-based metal–organic framework (MOF) NU-1000, have been shown to be active for the oxidative dehydrogenation (ODH) of propane at low temperatures (<230 °C), affording a selective and stable propene production catalyst. In our current work, a series of promoter ions with varying Lewis acidity, including Ni­(II), Zn­(II), Al­(III), Ti­(IV) and Mo­(VI), are anchored as metal-oxide,hydroxide clusters to NU-1000 followed by Co­(II) ion deposition, yielding a series of NU-1000-supported bimetallic-oxo,hydroxo,aqua clusters. Using difference envelope density (DED) analyses, the spatial locations of the promoter ions and catalytic cobalt ions are determined. For all samples, the promoter ions are sited between pairs of Zr₆ nodes along the MOF <i>c</i>-axis, whereas the location of the cobalt ions varies with the promoter ions. These NU-1000-supported bimetallic-oxide clusters are active for propane ODH after thermal activation under O₂ to open a cobalt coordination site and to oxidize Co­(II) to Co­(III), as evidenced by operando X-ray absorption spectroscopy at the Co K-edge. In accord with the decreasing Lewis acidity of the promoter ion, catalytic activity increases in the following order: Mo­(VI) < Ti­(IV) < Al­(III) < Zn­(II) < Ni­(II). The finding is attributed to increasing ease of formation of Co­(III)–O^• species and stabilization of a cobalt­(III)-oxyl/propane transition state as the Lewis acidity of the promoter ions decreases. The results point to an increasing ability to fine-tune the structure-dependent activity of MOF-supported heterogeneous catalysts. Coupled with mechanistic studiescomputational or experimentalthis ability may translate into informed prediction of improved catalysts for propane ODH and other chemical reactions

Metal–Organic Framework Supported Cobalt Catalysts for the Oxidative Dehydrogenation of Propane at Low Temperature

Zr-based metal–organic frameworks (MOFs) have been shown to be excellent catalyst supports in heterogeneous catalysis due to their exceptional stability. Additionally, their crystalline nature affords the opportunity for molecular level characterization of both the support and the catalytically active site, facilitating mechanistic investigations of the catalytic process. We describe herein the installation of Co­(II) ions to the Zr₆ nodes of the mesoporous MOF, NU-1000, via two distinct routes, namely, solvothermal deposition in a MOF (SIM) and atomic layer deposition in a MOF (AIM), denoted as Co-SIM+NU-1000 and Co-AIM+NU-1000, respectively. The location of the deposited Co species in the two materials is determined via difference envelope density (DED) analysis. Upon activation in a flow of O₂ at 230 °C, both materials catalyze the oxidative dehydrogenation (ODH) of propane to propene under mild conditions. Catalytic activity as well as propene selectivity of these two catalysts, however, is different under the same experimental conditions due to differences in the Co species generated in these two materials upon activation as observed by <i>in situ</i> X-ray absorption spectroscopy. A potential reaction mechanism for the propane ODH process catalyzed by Co-SIM+NU-1000 is proposed, yielding a low activation energy barrier which is in accord with the observed catalytic activity at low temperature

A Hafnium-Based Metal–Organic Framework as a Nature-Inspired Tandem Reaction Catalyst

Tandem catalytic systems, often inspired by biological systems, offer many advantages in the formation of highly functionalized small molecules. Herein, a new metal–organic framework (MOF) with porphyrinic struts and Hf₆ nodes is reported. This MOF demonstrates catalytic efficacy in the tandem oxidation and functionalization of styrene utilizing molecular oxygen as a terminal oxidant. The product, a protected 1,2-aminoalcohol, is formed selectively and with high efficiency using this recyclable heterogeneous catalyst. Significantly, the unusual regioselective transformation occurs only when an Fe-decorated Hf₆ node and the Fe–porphyrin strut work in concert. This report is an example of concurrent orthogonal tandem catalysis