Supported Metal Catalysts: Some Interesting New Leads In An Old Field

Soled, S.; Malek, Andrzej; Miseo, S.; Baumgartner, Joseph E.; Kliewer, Chris E.; Afeworki, Mobae; Stevens, Paul A.

doi:10.1016/s0167-2991(06)80896-7

Cited by 21 publications

(19 citation statements)

References 2 publications

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“…Ru/SiO 2 catalysts were prepared using ligand-assisted dispersion methods via incipient wetness impregnation of SiO 2 powders with aqueous Ru(NO)(NO 3 ) 3 and triethanolamine (TEA) solutions . SiO 2 (Davisil 646, 300 m 2 g –1 ) was treated in flowing dry air (Praxair, 99.99%, 10.0 cm 3 g –1 s –1 ) by heating to 898 K at 0.033 K s –1 and holding for 5 h before impregnation.…”

Section: Experimental Methodsmentioning

confidence: 99%

Mechanistic Connections between CO₂ and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Mansour

Iglesia

2021

J. Am. Chem. Soc.

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Catalytic routes for upgrading CO2 to CO and hydrocarbons have been studied for decades, and yet the mechanistic details and structure–function relationships that control catalytic performance have remained unresolved. This study elucidates the elementary steps that mediate these reactions and examines them within the context of the established mechanism for CO hydrogenation to resolve the persistent discrepancies and to demonstrate inextricable links between CO2 and CO hydrogenation on dispersed Ru nanoparticles (6–12 nm mean diameter, 573 K). The formation of CH4 from both CO2–H2 and CO–H2 reactants requires the cleavage of strong CO bonds in chemisorbed CO, formed as an intermediate in both reactions, via hydrogen-assisted activation pathways. The CO bonds in CO2 are cleaved via direct interactions with exposed Ru atoms in elementary steps that are shown to be facile by fast isotopic scrambling of C16O2–C18O2–H2 mixtures. Such CO2 activation steps form bound CO molecules and O atoms; the latter are removed via H-addition steps to form H2O. The kinetic hurdles in forming CH4 from CO2 do not reflect the inertness of CO bonds in CO2 but instead reflect the intermediate formation of CO molecules, which contain stronger CO bonds than CO2 and are present at near-saturation coverages during CO2 and CO hydrogenation catalysis. The conclusions presented herein are informed by a combination of spectroscopic, isotopic, and kinetic measurements coupled with the use of analysis methods that account for strong rate inhibition by chemisorbed CO. Such methods enable the assessment of intrinsic reaction rates and are essential to accurately determine the effects of nanoparticle structure and composition on reactivity and selectivity for CO2–H2 reactions.

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Section: Experimental Methodsmentioning

confidence: 99%

Mechanistic Connections between CO₂ and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Mansour

Iglesia

2021

J. Am. Chem. Soc.

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“…The impregnating solution contained Ru(NO)(NO 3 ) 3 (Alfa-Aesar, 32% wt. Ru) and triethanolamine (TEA; Sigma-Aldrich, 97%) (1:10 Ru:TEA molal ratio) in deionized H 2 O (17.9 MΩ resistivity) . After impregnation, powders were treated sequentially (i) in stagnant ambient air by heating to 373 at 0.017 K s –1 and holding for 8 h; (ii) in flowing dry air (Praxair, 99.999%, 0.83 cm 3 s –1 g –1 ) by heating to 673 at 0.033 K s –1 and holding for 3 h before cooling to ambient temperature; (iii) in 10% H 2 /He (Praxair, 99.999%, 0.83 cm 3 s –1 g –1 ) by heating to 723 at 0.033 K s –1 and holding for 3 h before cooling to ambient temperature in flowing He (Praxair, 99.999%, 0.83 cm 3 s –1 g –1 ); and (iv) in 1% O 2 /He (Praxair, 99.999%, 0.83 cm 3 s –1 g –1 ) flow at ambient temperature for 1 h before exposure to ambient air.…”

Section: Methodsmentioning

confidence: 99%

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

2017

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High CO* coverages lead to rates much higher than Langmuirian treatments predict because co-adsorbate interactions destabilize relevant transition states less than their bound precursors. This is shown here by kinetic and spectroscopic data-interpreted by rate equations modified for thermodynamically nonideal surfaces-and by DFT treatments of CO-covered Ru clusters and lattice models that mimic adlayer densification. At conditions (0.01-1 kPa CO; 500-600 K) which create low CO* coverages (0.3-0.8 ML from in situ infrared spectra), turnover rates are accurately described by Langmuirian models. Infrared bands indicate that adlayers nearly saturate and then gradually densify as pressure increases above 1 kPa CO, and rates become increasingly larger than those predicted from Langmuir treatments (15-fold at 25 kPa and 70-fold at 1 MPa CO). These strong rate enhancements are described here by adapting formalisms for reactions in nonideal and nearly incompressible media (liquids, ultrahigh-pressure gases) to handle the strong co-adsorbate interactions within the nearly incompressible CO* adlayer. These approaches show that rates are enhanced by densifying CO* adlayers because CO hydrogenation has a negative activation area (calculated by DFT), analogous to how increasing pressure enhances rates for liquid-phase reactions with negative activation volumes. Without these co-adsorbate effects and the negative activation area of CO activation, Fischer-Tropsch synthesis would not occur at practical rates. These findings and conceptual frameworks accurately treat dense surface adlayers and are relevant in the general treatment of surface catalysis as it is typically practiced at conditions leading to saturation coverages of reactants or products.

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“…The impregnated samples were then heated to 393 K at 0.017 K s −1 in flowing dry air and held for 12 h, after which the powders were heated to 573 K at 0.017 K s −1 in flowing dry air and held for 1 h with the intent to cause condensation reactions between the TEA complexes and surface silanol groups. 19 These samples were then heated to 673 K at 0.033 K s −1 in flowing 50% H 2 /He (Praxair, 99.999%, 1.0 cm 3 g −1 s −1 ) and held for 3 h to form supported metallic Ir clusters. The materials were then cooled to ambient temperature and passivated in flowing 0.5% O 2 /He (Praxair, 99.99%, 1.0 cm 3 g −1 s −1 ) for 6 h before exposure to ambient air.…”

Section: Introductionmentioning

confidence: 99%

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

2015

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Rates and locations of C−C cleavage during the hydrogenolysis of alkyl-cyclohexanes determine the isomeric products of ring opening and the yield losses from dealkylation. Kinetically relevant transition states for C−C rupture form by sequential quasi-equilibrated dehydrogenation steps that break C−H bonds, form C−metal bonds, and desorb chemisorbed H atoms (H*) from H*-covered surfaces. Activation enthalpies (ΔH ⧧ ), entropies (ΔS ⧧ ), and the number of H 2 (g) formed with transition states are larger for 3 C− x C rupture than for 2 C− 2 C or 2 C− 1 C cleavage for all cycloalkane reactants and Ir cluster sizes. 3 C− x C rupture transition states bind to surfaces through three or more C atoms, whereas those for less-substituted 2 C− 2 C bonds cleave via α,β species bound by two C atoms. 3 C− x C rupture involves larger ΔH ⧧ than 2 C− 2 C and 2 C− 1 C because the former requires that more C−H bonds cleave and H* desorb than for the latter two. These endothermic steps are partially compensated by C−metal bond formation, whereas the formation of additional H 2 (g) gives larger ΔS ⧧ . C−C rupture transition states for cycloalkanes have less entropy than those for C−C bonds in acyclic alkanes of similar size because C 6 rings decrease the rotational and conformational freedom. ΔH ⧧ values for all C−C bonds in a given reactant decrease with increasing Ir cluster size because the coordination of exposed metal atoms influences the stabilities of the H* atoms that desorb more than those of the transition states. ΔH ⧧ for 3 C− x C cleavage is more sensitive to cluster size because their transition states displace more H* than those for 2 C− 2 C or 2 C− 1 C bonds. These data and their mechanistic interpretation provide guidance for how surface coordination, reaction temperatures, and H 2 pressures can be used to control ring-opening selectivities toward desirable products while minimizing yield losses. These findings are consistent with trends for the hydrogenolysis of acyclic isoalkanes and seem likely to extend to C−X bond cleavage (where X = O, S, and N atoms) reactions during hydrotreating processes.

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Supported Metal Catalysts: Some Interesting New Leads In An Old Field

Cited by 21 publications

References 2 publications

Mechanistic Connections between CO₂ and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Mechanistic Connections between CO₂ and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

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Supported Metal Catalysts: Some Interesting New Leads In An Old Field

Cited by 21 publications

References 2 publications

Mechanistic Connections between CO2 and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Mechanistic Connections between CO2 and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

Contact Info

Product

Resources

About

Mechanistic Connections between CO₂ and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Mechanistic Connections between CO₂ and CO Hydrogenation on Dispersed Ruthenium Nanoparticles