Weaver details a study focusing on the surface chemistry of late transition metal oxides. He discusses oxygen desorption energetics and kinetics, and chemisorption properties of late transition metal ...oxides. Techniques useful in the characterization of oxides include surface x-ray diffraction, scanning tunneling microscopy and x-ray photoelectron spectroscopy.
Methane undergoes highly facile C–H bond cleavage on the stoichiometric IrO₂(110) surface. From temperature-programmed reaction spectroscopy experiments, we found that methane molecularly adsorbed as ...a strongly bound σ complex on IrO₂(110) and that a large fraction of the adsorbed complexes underwent C–H bond cleavage at temperatures as low as 150 kelvin (K). The initial dissociation probability of methane on IrO₂(110) decreased from 80 to 20% with increasing surface temperature from 175 to 300 K. We estimate that the activation energy for methane C–H bond cleavage is 9.5 kilojoule per mole (kJ/mol) lower than the binding energy of the adsorbed precursor on IrO₂(110), and equal to a value of ~28.5 kJ/mol. Low-temperature activation may avoid unwanted side reactions in the development of catalytic processes to selectively convert methane to value-added products.
Conspectus The abundance of cheap, natural gas has transformed the energy landscape, whereby revealing new possibilities for sustainable chemical technologies or impacting those that have relied on ...traditional fossil fuels. The primary component, methane, is underutilized and wastefully exhausted, leading to anthropogenic global warming. Historically, the manipulation of methane remained “clavis aurea,” an insurmountable yet rewarding challenge and thus the focus of intense research. This is primarily due to an inability to dissociate C–H bonds in methane selectively, which requires a high energy penalty and is an essential prerequisite for the direct conversion of methane into a large set of value-added products. The discovery of such processes would promise an energy gainful use of natural gas benefiting several essential chemical processes associated with C1 chemistry. This first C–H bond dissociation step of the methane molecule appears in numerous catalytic mechanisms as the rate-determining step or most essential barrier sequence for all subsequent steps that follow in the production of C–C, C–O, or C x –H y –O z bonds found in value added products. A main goal is to catalytically reduce the energy barrier for the first C–H bond dissociation to be able to achieve the activation of methane at low or moderate temperatures. As such there is great value in understanding the fundamental nature of the active sites responsible for bond breaking or formation and thus be able to facilitate better control of this chemistry, leading to the development of new technologies for fuel production and chemical conversion. Surface science studies offer enhanced perspectives for a careful manipulation of bonds over the last layer atoms of catalyst surfaces, an essential factor for the design of atomically precise catalysts and unravelling of the reaction mechanism. With the advent of new surface imaging, spectroscopy, and in situ tools, it has been possible to decipher the surface chemistry of complex materials systems and further our understanding of atomic active sites on the surfaces of metals, oxides, and carbides or metal–oxide and metal–carbide interfaces. The once considered near impossible step of C–H bond activation is now observed at low temperatures with high propensity over a collection of oxide, metal–oxide, and metal–carbide systems in a conventional or inverse configuration (oxide or carbide on metal). The enabling of C–H activation at low temperature has opened interesting possibilities for the specific production of chemicals such as methanol directly from methane, a step toward facile synthesis of liquid fuels. We highlight the most recent of these results and present the key aspects of active site configurations engineered from surface science studies which enable such a simple reactive event through careful manipulation of the last surface layer of atoms found in the catalyst structure. New concepts which help in the activation and conversion of methane are discussed.
Advances in the fundamental understanding of alkane activation on oxide surfaces are essential for developing new catalysts that efficiently and selectively promote chemical transformations of ...alkanes. In this tutorial review, we discuss the current understanding of alkane activation on crystalline metal oxide surfaces, and focus mainly on summarizing our findings on alkane adsorption and C-H bond cleavage on the PdO(101) surface as determined from model ultrahigh vacuum experiments and theoretical calculations. These studies show that alkanes form strongly-bound σ-complexes on PdO(101) by datively bonding with coordinatively-unsaturated Pd atoms and that these molecularly adsorbed species serve as precursors for C-H bond activation on the oxide surface. In addition to discussing the binding and properties of alkane σ-complexes on PdO(101), we also summarize recent advances in kinetic models to predict alkane dissociation rates on solid surfaces. Lastly, we highlight computations which predict that the formation and facile C-H bond activation of alkane σ-complexes also occurs on RuO
2
and IrO
2
surfaces.
Late transition-metal oxide surfaces that expose coordinatively-unsaturated metal atoms promote the formation and bond activation of alkane σ-complexes.
Late-transition-metal oxides have emerged as promising materials for enabling direct catalytic conversions of light alkanes to value-added products due to the ability of certain facets of these ...oxides to promote alkane C–H activation at low temperature. This review discusses the current understanding of alkane activation and oxidation on crystalline surfaces of late-transition-metal oxides, determined mainly from ultrahigh-vacuum (UHV) surface science experiments and density functional theory (DFT) calculations of alkane chemistry on the PdO(101), RuO2(110), and IrO2(110) surfaces. These studies show that chemically adsorbed alkane σ-complexes serve as precursors for initial C–H activation and that the formation of these adsorbed intermediates is critical to the high C–H activity of late-transition-metal oxides. The article discusses relationships between the surface structure and alkane C–H activity of these oxides. The binding and initial activation of alkane σ-complexes is also discussed, with an emphasis placed on establishing the underlying factors that produce differences in alkane activity among different oxides and alkanes. Thereafter, the current understanding of the mechanisms for C1–C3 alkane oxidation on the IrO2(110) surface is presented. This work reveals key reaction steps that govern the oxidation activity and selectivity of IrO2(110) and thereby provides insights into properties that may be manipulated to promote partial oxidation pathways, resulting in enhanced selectivity toward the conversion of alkanes to value-added products. On the basis of the current knowledge developed from fundamental studies, general challenges and opportunities for utilizing late-transition-metal oxides in catalysts for selective alkane oxidation are identified.
Realizing the efficient and selective conversion of ethane to ethylene is important for improving the utilization of hydrocarbon resources, yet remains a major challenge in catalysis. Herein, ethane ...dehydrogenation on the IrO2(110) surface is investigated using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. The results show that ethane forms strongly bound σ-complexes on IrO2(110) and that a large fraction of the complexes undergo C–H bond cleavage during TPRS at temperatures below 200 K. Continued heating causes as much as 40% of the dissociated ethane to dehydrogenate and desorb as ethylene near 350 K, with the remainder oxidizing to CO x species. Both TPRS and DFT show that ethylene desorption is the rate-controlling step in the conversion of ethane to ethylene on IrO2(110) during TPRS. Partial hydrogenation of the IrO2(110) surface is found to enhance ethylene production from ethane while suppressing oxidation to CO x species. DFT predicts that hydrogenation of reactive oxygen atoms of the IrO2(110) surface effectively deactivates these sites as H atom acceptors, and causes ethylene desorption to become favored over further dehydrogenation and oxidation of ethane-derived species. The study reveals that IrO2(110) exhibits an exceptional ability to promote ethane dehydrogenation to ethylene near room temperature, and provides molecular-level insights for understanding how surface properties influence selectivity toward ethylene production.
We used low energy electron diffraction (LEED) and temperature programmed desorption (TPD) to investigate the structure and reactivity of iridium oxide layers prepared by oxidizing Ir(1 0 0) at 765 K ...and O2 pressures ranging from 0.05 to 5 Torr. Our LEED results provide evidence that Ir(1 0 0) oxidation at O2 pressures up to 0.20 Torr produces a mixture of Ir oxide structures present as small domains, including a commensurate IrO2(1 0 1) structure that coexists with other structures. Oxidizing from 0.50 to 1 Torr causes formation of a commensurate IrO2(1 1 0)R27° structure and a sharp rise in the oxygen uptake from ~8 to 20 ML (monolayer) as the films exhibit signs of roughening. Further increasing the O2 pressure from 1 to 5 Torr causes the IrO2(1 1 0)R27° structure to be replaced with a so-called IrO2(1 1 0)-aligned structure, for which the IrO2(1 1 0) lattice vectors align with those of the Ir(1 0 0) substrate. We find that the oxidized Ir(1 0 0) surfaces become increasingly reactive toward the dissociation and oxidation of CH4 as IrO2(1 1 0) develops on the surface, and observe that the IrO2(1 1 0)-aligned structure is more reactive than the IrO2(1 1 0)R27° phase. Our findings demonstrate that the oxide phase evolution on Ir(1 0 0) is sensitive to the O2 pressure in the range from 0.05 to 5 Torr, and that the development of reactive IrO2(1 1 0) structures requires elevated O2 pressures (>0.5 Torr) and temperature.
The catalytic oxidation of CH4 over IrO2(110) films grown on Ir(100) was investigated using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at total pressures near 1 Torr. The IrO2(110) ...films undergo negligible reduction during catalytic CH4 oxidation in reactant mixtures with as much as 95% CH4 and temperatures from ca. 500–650 K, demonstrating that IrO2(110) can catalyze the oxidation of CH4 over a wide range of temperatures and mixture compositions. High coverages of OH groups and oxidized C-containing species formed on the IrO2(110) surfaces during CH4 oxidation, including excess OH groups bound directly to the initially, coordinatively unsaturated Ir atoms. The formation of excess OH groups demonstrates that O-rich IrO2(110) surfaces were maintained even under highly CH4-rich conditions and provides evidence that the dissociative adsorption of O2 is more facile than CH4 activation and conversion to adsorbed intermediates on IrO2(110). Extensively oxidized surface species with a CH y O2 stoichiometry preferentially formed under all reaction conditions studied. The conversion of CH4 to the CH y O2 surface species became optimal at an intermediate composition of the reactant mixture (∼90% CH4), consistent with a site competition between CH4 and O2 during their initial adsorption as well as a high oxidation activity of chemisorbed O atoms on IrO2(110). These results provide quantitative information about the identities and coverages of adsorbed species that form during the catalytic oxidation of CH4 on IrO2(110). Such knowledge is essential for validating first-principles models of the reaction kinetics for this system and ultimately gaining insights needed to optimize the performance of IrO2 catalysts for the oxidation of light alkanes.
Dilute Ti-Cu(111) alloys are found to be highly selective for converting ethanol to ethylene. Temperature-programmed reaction spectroscopy (TPRS) shows that adsorbed ethanol deoxygenates on ...Ti-Cu(111) surfaces with ∼10% Ti to produce gaseous C2H4 and H2 at temperatures near 400 K. Scanning tunneling microscopy and vibrational spectroscopy of adsorbed CO demonstrate that Ti surface sites are oxidized to TiO x by reaction with ethanol, causing the reaction selectivity to change from C2H4 to acetaldehyde production during repeated TPRS experiments with ethanol. TPRS simulations derived from density functional theory (DFT) calculations confirm that Ti ensembles within the Cu(111) surface layer promote ethanol deoxygenation at moderate temperature and reveal a significant enhancement in the activity and selectivity for gaseous C2H4 and H2 production as the Ti ensemble size is increased from monomer to trimer. DFT shows that increasing the Ti n ensemble size from n = 1 to 3 increases the stability of the adsorbed O atom released during C–O bond cleavage, thus facilitating ethanol deoxygenation. The calculations show that the O atom bound to Ti further enhances C2H4 production by destabilizing the adsorbed C2H4 and its dehydrogenation product and that this destabilization effect becomes more pronounced as the Ti ensemble size is increased from monomer to trimer. Our results demonstrate that dilute Ti-Cu(111) alloys promote the conversion of ethanol to ethylene at moderate temperature and reveal that this surface chemistry is strongly influenced by the Ti ensemble size and the adsorbed O atom released during C–O cleavage.
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