Neil M. Schweitzer

Identification of active sites in CO oxidation and water-gas shift over supported Pt catalysts

Comparing active site reactivity Noble metal nanoparticles often exhibit behaviors distinct from atomic and bulk versions of the same material. Gold and platinum dispersed on metal oxide supports, for example, show remarkable low-temperature reactivity for carbon monoxide (CO) oxidation by oxygen or water. Ding et al. used infrared spectroscopy to identify CO adsorbed on isolated platinum atoms or nanoparticles dispersed on zeolite and oxide supports. Temperature-programmed desorption studies showed that CO reacted at much lower temperatures when adsorbed on nanoparticles versus on isolated metal atoms. Science, this issue p. 189 Infrared spectroscopy reveals that carbon monoxide oxidizes more readily on supported noble metal nanoparticles than on isolated atoms. [Also see Perspective by Stephens et al.] Identification and characterization of catalytic active sites are the prerequisites for an atomic-level understanding of the catalytic mechanism and rational design of high-performance heterogeneous catalysts. Indirect evidence in recent reports suggests that platinum (Pt) single atoms are exceptionally active catalytic sites. We demonstrate that infrared spectroscopy can be a fast and convenient characterization method with which to directly distinguish and quantify Pt single atoms from nanoparticles. In addition, we directly observe that only Pt nanoparticles show activity for carbon monoxide (CO) oxidation and water-gas shift at low temperatures, whereas Pt single atoms behave as spectators. The lack of catalytic activity of Pt single atoms can be partly attributed to the strong binding of CO molecules.

High Activity Carbide Supported Catalysts for Water Gas Shift

Nanostructured carbides are refractory materials with high surface areas that could be used as alternatives to the oxide materials that are widely used as support materials for heterogeneous catalysts. Carbides are also catalytically active for a variety of reactions, offering additional opportunities to tune the overall performance of the catalyst. In this paper we describe the synthesis of molybdenum carbide supported platinum (Pt/Mo(2)C) catalysts and their rates for the water gas shift reaction. The synthesis method allowed interaction of the metal precursor with the native, unpassivated support. The resulting materials possessed very high WGS rates and atypical Pt particle morphologies. Under differential conditions, rates for these catalysts were higher than those for the most active oxide-supported Pt catalysts and a commercial Cu-Zn-Al catalyst. Experimental and computational results suggested that active sites on the Pt/Mo(2)C catalysts were located on the perimeter of the Pt particles and that strong interactions between Pt and the Mo(2)C surface gave rise to raft-like particles.

Sintering-resistant Single-Site Nickel Catalyst Supported by Metal-Organic Framework

Developing supported single-site catalysts is an important goal in heterogeneous catalysis since the well-defined active sites afford opportunities for detailed mechanistic studies, thereby facilitating the design of improved catalysts. We present herein a method for installing Ni ions uniformly and precisely on the node of a Zr-based metal-organic framework (MOF), NU-1000, in high density and large quantity (denoted as Ni-AIM) using atomic layer deposition (ALD) in a MOF (AIM). Ni-AIM is demonstrated to be an efficient gas-phase hydrogenation catalyst upon activation. The structure of the active sites in Ni-AIM is proposed, revealing its single-site nature. More importantly, due to the organic linker used to construct the MOF support, the Ni ions stay isolated throughout the hydrogenation catalysis, in accord with its long-term stability. A quantum chemical characterization of the catalyst and the catalytic process complements the experimental results. With validation of computational modeling protocols, we further targeted ethylene oligomerization catalysis by Ni-AIM guided by theoretical prediction. Given the generality of the AIM methodology, this emerging class of materials should prove ripe for the discovery of new catalysts for the transformation of volatile substrates.

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.

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 in situ 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.

Electronic structure engineering in heterogeneous catalysis: Identifying novel alloy catalysts based on rapid screening for materials with desired electronic properties

The immense phase space of multimetallic materials spanned by structural and compositional degrees of freedom precludes thorough screening for efficient alloy catalysts, even with combinatorial high-throughput experiments or quantum-chemical calculations. Based on X-ray absorption spectroscopy measurements and density functional theory calculations, we have identified critical electronic structure descriptors that govern local chemical reactivity of different sites in metal alloys. These descriptors were used to develop a model that allows us to predict variations in the adsorption energy of various adsorbates on alloy surfaces based on easily accessible physical characteristics of the constituent elements in alloys, mainly their electronegativity, atomic radius, and the spatial extent of valence orbitals. We show that this model, which is grounded on validated theories of chemisorption on metal surfaces, can be used to rapidly screen through a large phase space of alloy catalysts and identify optimal alloys for targeted catalytic transformations. We underline the potential of the electronic structure engineering, relating alloy geometry to its catalytic performance using simple electronic structure descriptors, in catalysis.

Volcano-shape glycerol oxidation activity of palladium-decorated gold nanoparticles

Bimetallic PdAu catalysts are more active than monometallic ones for the selective oxidation of alcohols, but the reasons for improvement remain insufficiently detailed. A metal-on-metal material can probe the structure–catalysis relationship more clearly than conventionally prepared bimetallics. In this study, Pd-on-Au nanoparticles with variable Pd surface coverages (sc%) ranging from 10 to 300 sc% were synthesized and immobilized onto carbon (Pd-on-Au/C). Tested for glycerol oxidation at 60 °C, pH 13.5, and 1 atm under flowing oxygen, the series of Pd-on-Au/C materials showed volcano-shape catalytic activity dependence on Pd surface coverage. Increasing surface coverage led to higher catalytic activity, such that initial turnover frequency (TOF) reached a maximum of ∼6000 h−1 at 80 sc%. Activity decreased above 80 sc% mostly due to catalyst deactivation. Pd-on-Au/C at 80 sc% was >10 times more active than monometallic Au/C and Pd/C, with both exhibiting TOF values less than ∼500 h−1. Glyceric acid was the dominant primary reaction product for all compositions, with its zero-conversion selectivity varying monotonically as a function of Pd surface coverage. Glyceric acid yield from Pd-on-Au/C (80 sc%) was 42%, almost double the yields from Au/C and Pd/C (16% and 22%, respectively). Ex situ X-ray absorption near edge structure analysis of two Pd-on-Au/C materials with comparable activities (60 sc% and 150 sc%) showed that the former had less oxidized Pd ensembles than the latter, and that both catalysts were less oxidized compared to Pd/C. That Au stabilizes the metallic state of surface Pd atoms may be responsible for activity enhancement observed in other PdAu-catalyzed oxidation reactions. Decorating a Au surface with Pd generates a catalyst that has the deactivation resistance of Au, the higher glyceric acid selectivity of Pd, and the synergistically higher activities that neither metal has.

Bridging Zirconia Nodes within a Metal–Organic Framework via Catalytic Ni-Hydroxo Clusters to Form Heterobimetallic Nanowires

Metal-organic frameworks (MOFs), with their well-ordered pore networks and tunable surface chemistries, offer a versatile platform for preparing well-defined nanostructures wherein functionality such as catalysis can be incorporated. Notably, atomic layer deposition (ALD) in MOFs has recently emerged as a versatile approach to functionalize MOF surfaces with a wide variety of catalytic metal-oxo species. Understanding the structure of newly deposited species and how they are tethered within the MOF is critical to understanding how these components couple to govern the active material properties. By combining local and long-range structure probes, including X-ray absorption spectroscopy, pair distribution function analysis, and difference envelope density analysis, with electron microscopy imaging and computational modeling, we resolve the precise atomic structure of metal-oxo species deposited in the MOF NU-1000 through ALD. These analyses demonstrate that deposition of NiOxHy clusters occurs selectively within the smallest pores of NU-1000, between the zirconia nodes, serving to connect these nodes along the c-direction to yield heterobimetallic metal-oxo nanowires. This bridging motif perturbs the NU-1000 framework structure, drawing the zirconia nodes closer together, and also underlies the sintering resistance of these clusters during the hydrogenation of light olefins.

Size effect of the active sites in UiO-66-supported nickel catalysts synthesized: Via atomic layer deposition for ethylene hydrogenation

Ni(II) ions have been deposited on the Zr6 nodes of a metal-organic framework (MOF), UiO-66, via an ALD-like process (ALD = atomic layer deposition). By varying the number of ALD cycles, three Ni-decorated UiO-66 materials were synthesized. A suite of physical methods has been used to characterize these materials, indicating structural and high-surface-area features of the parent MOF are retained. Elemental analysis via X-ray photoelectron spectroscopy (XPS) indicates that the anchored Ni ions are mainly on surface and near-surface MOF defect sites. Upon activation, all three materials are catalytic for ethylene hydrogenation, but their catalytic activities significantly vary, with the largest clusters displaying the highest per-nickel-atom activity. The study highlights the ease and effectiveness ALD in MOFs (AIM) for synthesizing, specifically, UiO-66-supported NiyOx catalysts.

Evidence for geometric effects in neopentane conversion on PdAu catalysts

Silica-supported Pd and shell/core PdAu nanoparticles of a similar size were evaluated for neopentane conversion. Monometallic Pd exhibited poor neopentane isomerization selectivity in favor of high selectivity to primary and secondary hydrogenolysis products. Similarly sized PdAu catalysts of increasing Pd weight loading were synthesized to evaluate the effect of increasing Pd monolayers on neopentane conversion. All PdAu catalysts had neopentane conversion selectivity within the range of monometallic Pd catalysts from previous work (~5–30%). However, there was an inverse relationship between Pd weight loading and neopentane isomerization selectivity. The increase in isomerization selectivity did not correlate to a decrease in heats of adsorption as seen with monometallic Pd catalysts, but was correlated with the catalyst surface structure which suggests a geometric effect as the cause for changes in catalytic performance rather than an electronic effect.