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The molecularly dispersed TiO 2 /SiO 2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of titanium isopropoxide. Experimental monolayer dispersion of surface titanium oxide species on SiO 2 was reached at ∼4 Ti atoms/nm 2 with a two-step impregnation procedure. The surface structures of the molecularly dispersed TiO 2 /SiO 2 under various environments were extensively investigated by in-situ spectroscopic techniques (e.g., Raman, UV-vis-NIR DRS, and XANES) as well as XPS. The combined characterization techniques revealed the consumption of surface Si-OH groups and the formation of Ti-O-Si bridging bonds. In the dehydrated state, the surface Ti atoms in the 1% TiO 2 /SiO 2 sample (0.24 Ti atoms/nm 2 ) are predominantly found to be isolated TiO 4 units, whereas at maximum surface coverage (∼4 Ti atoms/nm 2 ), two-dimensional polymerized TiO 5 units are dominant on the silica surface. The in-situ spectroscopic studies demonstrated that the coordination and ligands of the surface Ti cations change upon hydration/dehydration as well as during methanol oxidation. Methanol oxidation showed that the molecularly dispersed surface titanium oxide species exhibit completely different catalytic behavior (predominantly redox products) compared to bulk titanium oxide (predominantly dehydration products). Furthermore, the TOF of the surface titanium oxide species is strongly dependent on their local structures and varies by 1 order of magnitude (isolated TiO 4 . polymerized TiO 5 ). These new results provide fundamental insights about molecular structure-reactivity/selectivity relationships of the molecularly dispersed TiO 2 /SiO 2 supported oxides.

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Mixed metal catalysts containing Pt, Ir, Ru, Os, and Rh were synthesized on three different conductive oxide supports, Ebonex, which is a mixture of Ti 4 O 7 and other phases, phase-pure microcrystalline Ti 4 O 7 , and Ti 0.9 Nb 0.1 O 2 , a doped rutile compound. Ebonex-supported catalysts were prepared as arrays and screened combinatorially for activity and stability as bifunctional oxygen reduction/water oxidation catalysts. The highest activity and stability was found in the Pt-Ru-Ir ternary region at compositions near Pt 4 Ru 4 Ir 1. X-ray near edge absorption spectra indicated a significant electronic interaction between the catalyst and the support, and a substantial increase in catalyst utilization was observed, even though the support surface areas were relatively low. Both Ebonex and Ti 4 O 7 have short-lived electrochemical stability under conditions of oxygen evolution at ϩ1.6 V vs. RHE in 0.5 M H 2 SO 4. Current at these supported catalysts gradually decreases, and the decrease is attributed to loss of electronic conductivity. Ebonex and Ti 4 O 7 are also thermally oxidized in air at temperatures above 400°C. In contrast, Ti 0.9 Nb 0.1 O 2 , which has a nondefective oxygen lattice, is quite resistant to electrochemical and thermal oxidation. Conditioning of Ti 0.9 Nb 0.1 O 2-supported Pt 4 Ru 4 Ir 1 at positive potentials had little effect on the activity of the catalyst.

CO2 reduction to higher value products is a promising way to produce fuels and key chemical building blocks while reducing CO2 emissions. The reaction at atmospheric pressure mainly yields CH4 via methanation and CO via the reverse water-gas shift (RWGS) reaction. Describing catalyst features that control the selectivity of these two pathways is important to determine the formation of specific products. At the same time, identification of morphological changes occurring to catalysts under reaction conditions can be crucial to tune their catalytic performance. In this contribution we investigate the dependency of selectivity for CO2 reduction on the size of Ru nanoparticles (NPs) and on support. We find that even at rather low temperatures (210 °C), oxidative pretreatment induces redispersion of Ru NPs supported on CeO2 and leads to a complete switch in the performance of this material from a well-known selective methanation catalyst to an active and selective RWGS catalyst. By utilizing in situ X-ray absorption spectroscopy, we demonstrate that the low-temperature redispersion process occurs via decomposition of the metal oxide phase with size-dependent kinetics, producing stable single-site RuO x /CeO2 species strongly bound to the CeO2 support that are remarkably selective for CO production. These results show that reaction selectivity can be heavily dependent on catalyst structure and that structural changes of the catalyst can occur even at low temperatures and can go unseen in materials with less defined structures.

Catalysts consisting of atomically dispersed Pt (Ptiso) species on CeO2 supports have received recent interest due to their potential for efficient metal utilization in catalytic convertors. However, discrepancies exist between the behavior (reducibility, interaction strength with adsorbates) of high surface area Ptiso/CeO2 systems and of well-defined surface science and computational model systems, suggesting differences in Pt local coordination in the two classes of materials. Here, we reconcile these differences by demonstrating that high surface area Ptiso/CeO2 synthesized at low Pt loadings (<0.1% weight) exhibit resistance to reduction and sintering up to 500 °C in 0.05 bar H2 and minimal interactions with COproperties previously seen only for model system studies. Alternatively, Pt loadings >0.1 weight % produce a distribution of sub-nanometer Pt structures, which are difficult to distinguish using common characterization techniques, and exhibit strong interactions with CO and weak resistance to sintering, even in 0.05 bar H2 at 50 °Cproperties previously seen for high surface area materials. This work demonstrates that low metal loadings can be used to selectively populate the most thermodynamically stable adsorption sites on high surface area supports with atomically dispersed metals. Further, the site uniformity afforded by this synthetic approach is critical for the development of relationships between atomic scale local coordination and functional properties. Comparisons to recent studies of Ptiso/TiO2 suggest a general compromise between the stability of atomically dispersed metal catalysts and their ability to interact with and activate molecular species.

In the high-temperature environments needed to perform catalytic processes, supported precious metal catalysts severely lose their activity over time. Even brief exposure to high temperatures can lead to significant losses in activity, which forces manufacturers to use large amounts of noble metals to ensure effective catalyst function for a required lifetime. Generally, loss of catalytic activity is attributed to nanoparticle sintering, or processes by which larger particles grow at the expense of smaller ones. Here, by independently controlling particle size and particle loading using colloidal nanocrystals, we reveal the opposite process as a novel deactivation mechanism: nanoparticles rapidly lose activity by high-temperature nanoparticle decomposition into inactive single atoms. This deactivation route is remarkably fast, leading to severe loss of activity in as little as ten minutes. Importantly, this deactivation pathway is strongly dependent on particle density and concentration of support defect sites. A quantitative statistical model explains how for certain reactions, higher particle densities can lead to more stable catalysts.Increased catalyst stability, especially in automotive emissions control applications, is of paramount importance in order to decrease the loading of rare and precious noble metals 1,2 .

Metal organic frameworks (MOFs), with their crystalline, porous structures, can be synthesized to incorporate a wide range of catalytically active metals in tailored surroundings. These materials have potential as catalysts for conversion of light alkanes, feedstocks available in large quantities from shale gas that are changing the economics of manufacturing commodity chemicals. Mononuclear high-spin (S = 2) Fe(II) sites situated in the nodes of the MOF MIL-100(Fe) convert propane via dehydrogenation, hydroxylation, and overoxidation pathways in reactions with the atomic oxidant N 2 O. Pair distribution function analysis, N 2 adsorption isotherms, X-ray diffraction patterns, and infrared and Raman spectra confirm the single-phase crystallinity and stability of MIL-100(Fe) under reaction conditions (523 K in vacuo, 378−408 K C 3 H 8 + N 2 O). Density functional theory (DFT) calculations illustrate a reaction mechanism for the formation of 2-propanol, propylene, and 1-propanol involving the oxidation of Fe(II) to Fe(III) via a high-spin Fe(IV)O intermediate. The speciation of Fe(II) and Fe(III) in the nodes and their dynamic interchange was characterized by in situ X-ray absorption spectroscopy and ex situ Mossbauer spectroscopy. The catalytic relevance of Fe(II) sites and the number of such sites were determined using in situ chemical titrations with NO. N 2 and C 3 H 6 production rates were found to be first-order in N 2 O partial pressure and zero-order in C 3 H 8 partial pressure, consistent with DFT calculations that predict the reaction of Fe(II) with N 2 O to be rate determining. DFT calculations using a broken symmetry method show that Fe-trimer nodes affecting reaction contain antiferromagnetically coupled iron species, and highlight the importance of stabilizing high-spin (S = 2) Fe(II) species for effecting alkane oxidation at low temperatures (<408 K).