Temperature-programmed Reaction Measurements Research Articles

Electrocatalytic Conversion of Methane to Ethylene Utilizing Highly Durable Barium Niobate Perovskites

Pursuing efficient conversion of methane to value-added products such as olefins and aromatics is a prospective use of natural methane feedstocks. The demand for on-site methane conversion has further increased due to the recent discoveries of shale gas reserves. Electrochemical methane conversion provides a means to control the surface oxide ion concentration, aiming to eliminate the over-oxidation of methane while also aiding in methane activation via applied potential. High temperature electrolysis processes provide improved reaction kinetics. Unfortunately, high temperature operation also leads to material degradation via sintering, crystal structure disproportion to thermodynamically more stable phases, and interfacial reactions that reduce performance. We demonstrated that the Sr2Fe1.5Mo0.5O6- d (SFMO) perovskite which was reported with desirable methane activation properties had poor chemical and electrochemical stability as it formed carbonates and coke under methane rich environments.[1][2] Here we present an exciting class of barium niobate perovskite materials with varying levels of Mg and Fe co-doping that show promising catalytic activity towards methane activation in the electrochemical and conventional heterogeneous oxidative coupling environment. The perovskite material was prepared by conventional solid-state methods and by microwave synthesis methods to compare the role of surface area. Thermogravimetric analysis and temperature programmed reaction measurements under methane show superior chemical stability in comparison to SFMO perovskites. These catalysts’ electrochemical properties are evaluated by cyclic voltammetry, impedance spectroscopy, and chronoamperometric measurements coupled with mass spectra analysis that demonstrate durable operation for five continuous days. A maximum ethylene production of 277 mmolcm-2h-1 with a faradaic efficiency of 20% at 1V was obtained. Stability determinations of our perovskite oxide electrocatalysts for EC-OCM offer an excellent example of our approach towards evaluation of materials durability under the highly carburizing environments. These perovskite materials could also serve as a support for a wide variety of catalyst materials for high temperature applications thus opening new possibilities.

Effects of phosphorus on C[sbnd]C, C[sbnd]O, and C[sbnd]H bond rupture during acetic acid decomposition over Ru(0001) and Px-Ru(0001)

Investigation of the conversion of acetic acid (a model oxygenate) on well-defined metal and metal phosphide surfaces provides insight to the reasons for the high selectivities of transition metal phosphide (TMP) catalysts for hydrodeoxygenation of bio-oils. Temperature programmed reaction (TPR) of acetic acid isotopologues on pristine Ru(0001) and a P modified Ru(0001) surface, representative of the Ru2P(111) facet, show an identical sequence of elementary steps for acetic acid decomposition. The inclusion of P atoms in the surface does, however, lead to electronic changes of the Ru atoms and increase activation barriers for CO, CC, and CH bond scission by values of ∼5–10kJmol−1. The differences increase the prevalence of CC bond rupture over CO bond rupture by a factor of two (determined in TPR measurements). Based on the changes in product selectivity and the relative changes in activation energies for CC and CO bond rupture, these results indicate that the addition of P atoms will increase selectivities toward CO bond rupture during high temperature, hydrotreating catalysis, which matches reported HDO selectivities that are greater on TMP catalysts than on the corresponding metallic nanoparticles. These results provide fundamental insight to the reasons for the greater selectivity for CO bond rupture.

Synthesis, Structural and Optical Properties of Mn-Doped Ceria Nanoparticles: A Promising Catalytic Material

The structure, the morphology and the thermal, optical and the surface properties of nanocrystalline CeO2 doped with Mn have been studied by X-ray diffraction (XRD), field-emission transmission electron microscopy (FE-TEM), energy-dispersive X-ray analysis, thermogravimetric analysis, UV–Vis absorption spectroscopy and Fourier transform infrared spectroscopy. The XRD results confirmed the successful incorporation of Mn into the CeO2 lattice through the formation of nanoscale face-centered cubic solid solution. The FE-TEM observations supported the nanocrystalline nature of the solid solutions. The presence of structural defects and their role on the band gap have been discussed on the basis of absorption spectral studies. The structural differences correlate with results from temperature-programmed reaction (TPR) experiments with H2 consumption. The TPR measurements showed an enhanced bulk reduction at much lower temperatures, indicating increased oxygen mobility in the samples, which enable to enhanced oxygen diffusion at lower temperatures.

Oxidation State and Symmetry of Magnesia-Supported Pd13OxNanocatalysts Influence Activation Barriers of CO Oxidation

Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd(13) clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd(13) cluster, leading to facile partial oxidation to form Pd(13)O(4) clusters with C(4v) symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd(13)O(6) clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd(13)O(4) cluster. Subsequent reactions on the resulting lower-symmetry Pd(13)O(x) (x < 4) clusters entail lower activation energies. The nonsymmetric Pd(13)O(6) clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.

Sonogashira Coupling on an Extended Gold Surface in Vacuo: Reaction of Phenylacetylene with Iodobenzene on Au(111)

Temperature-programmed reaction measurements supported by scanning tunneling microscopy have shown that phenylacetylene and iodobenzene react on smooth Au(111) under vacuum conditions to yield biphenyl and diphenyldiacetylene, the result of homocoupling of the reactant molecules. They also produce diphenylacetylene, the result of Sonogashira cross-coupling, prototypical of a class of reactions that are of paramount importance in synthetic organic chemistry and whose mechanism remains controversial. Roughened Au(111) is completely inert toward all three reactions, indicating that the availability of crystallographically well-defined adsorption sites is crucially important. High-resolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy show that the reactants are initially present as intact, essentially flat-lying molecules and that the temperature threshold for Sonogashira coupling coincides with that for C-I bond scission in the iodobenzene reactant. The fractional-order kinetics and low temperature associated with desorption of the Sonogashira product suggest that the reaction occurs at the boundaries of islands of adsorbed reactants and that its appearance in the gas phase is rate-limited by the surface reaction. These findings demonstrate unambiguously and for the first time that this heterogeneous cross-coupling chemistry is an intrinsic property of extended, metallic pure gold surfaces: no other species, including solvent molecules, basic or charged (ionic) species are necessary to mediate the process.

Ultrathin magnesia films as support for molecules and metal clusters: Tuning reactivity by thickness and composition

AbstractUltrathin metal oxide films have attracted considerable interest in recent years as versatile substrate for the design of nanocatalytic model systems. In particular, it has been proposed theoretically and confirmed experimentally that the electronic structure of adsorbates can be influenced by the layer thickness and the stoichiometry, i.e., the type and number of defects, of the oxide film. This has important consequences on the chemical reactivity of the oxide surface itself and of oxide supported metal clusters. It also opens new possibilities to influence and to control chemical reactions occurring at the surface of these systems. The present feature focuses on very recent experiments that illustrate the effects of a proper adjustment of layer thickness and composition of ultrathin MgO(100) films on chemical transformations. On the magnesia surface itself, the photodissociation dynamics of methyl iodide molecules is investigated via femtosecond‐laser pump–probe mass spectrometry. Furthermore, the catalytic oxidation of carbon monoxide at mass‐selected Au20 clusters deposited on magnesia is explored through temperature programmed reaction measurements. In the latter case, detailed first principles calculations are able to correlate the experimentally observed reactivity with structural dimensionality changes that are induced by the changing thickness and composition of the magnesia support. magnified image

Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO 2 Surfaces

The catalytic activity of metal clusters of different sizes adsorbed on oxide surfaces can be explored systematically by using model catalysts. We studied the temperature-programmed reaction of CO with O2 catalyzed by Pd clusters (Pd(n), for n = 1, 2, 4, 7, 10, 16, 20, and 25) that were size-selected in the gas phase and deposited on rutile TiO2(110). X-ray photoemission spectroscopy revealed that the Pd 3d binding energy varied nonmonotonically with cluster size and that the changes correlated with strong size variations in CO oxidation activity. Taking final-state effects into account, low activity was correlated with higher-than-expected Pd 3d binding energy, which is attributed to a particularly stable valence electronic structure; electron transfer from the TiO2 support to the Pd clusters also occurs. Ion scattering shows that small clusters form single-layer islands on the surface and that formation of a second layer begins to occur for clusters larger than Pd10.

Control and Manipulation of Gold Nanocatalysis: Effects of Metal Oxide Support Thickness and Composition

Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au(20) was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O(2) molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O(2) molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au(20) nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects.

Principle and Mechanism of Direct Porphyrin Metalation: Joint Experimental and Theoretical Investigation

The direct metalation of tetraphenylporphyrin with bare metal atoms (Co and Zn) was studied with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction measurements on ordered monolayer films of the molecules adsorbed on a Ag(111) surface. The mechanism of this novel type of surface reaction was investigated using density functional theory (DFT) calculations for the related gas-phase reactions of the unsubstituted porphyrin with the metals Fe, Co, Ni, Cu, and Zn. The reaction starts with the formation of an initial complex, in which the metal atom is coordinated by the intact unreduced porphyrin. This complex resembles the sitting-atop complex proposed for porphyrin metalation with metal ions in solution. In two subsequent steps, the pyrrolic hydrogen atoms are transferred to the metal atom, forming H2, which is eventually released. The activation barriers of the H-transfer steps vary for the different metal atoms. DFT calculations suggest that metalations with Fe, Co, and Ni show two-state reactivity, while those with Cu and Zn proceed on a single potential energy surface. For metalation with Zn, we calculated a barrier of the first hydrogen transfer step of 32.6 kcal mol(-1), in good agreement with the overall experimental activation energy of 31 kcal mol(-1).

Selective NH 3 oxidation on (110) and (111) iridium surfaces

Ammonia oxidation was studied in situ on Ir(110) and Ir(111) under low-pressure ( ∼ 10 −7 mbar ) conditions. NH 3 does not dissociate on a flat Ir(111) surface, but surface defects and O ad facilitate NH 3 ad decomposition. High-energy resolution fast XPS measurements were used to monitor the surface coverage during reaction on Ir(110), and temperature-programmed reaction measurements were applied to reveal the gas phase reaction products during reaction. The steady-state NH 3 oxidation reaction starts between 350 and 500 K on both surfaces, which also show similar selectivity. Below 600 K, N 2 and H 2O are the principal reaction products. Above 600 K, the selectivity changes to NO and H 2O, with the exact temperature depending on the NH 3:O 2 pressure ratio. The surface population changes from NH ad/N ad to O ad around 500 K, about 200 K lower than the selectivity change from N 2 to NO observed in the gas phase. This behavior can be explained by considering the activation energies for N 2 and NO ad formation. We present a model to explain why Ir is more selective toward N 2 than Pt.

The Adsorption and Decomposition of C2H4 on Ru(001): A Combined TPR and Work Function Change Study

Work function change (ΔΦ) measurements during adsorption and surface heating in a ΔΦ−temperature programmed reaction (TPR) mode combined with TPD are demonstrated to provide new information on the interaction and chemistry of ethylene on Ru(001). Rearrangement of second layer ethylene molecules has been observed between 82 and 120 K. This is a competing process with molecular desorption, interpreted as a result of migration of second layer molecules toward the surface. Our results are consistent with and support previous studies that suggested the formation of a surface intermediate (η2(C,C)CHCH2) during ethylene dehydrogenation to ethylidine. Employing a derivative mode with respect to temperature −d(ΔΦ)/dT, we find an early onset for ethylidine decomposition near 265 K. ΔΦ−TPR measurements in the range 560−720 K reveal three distinct CH decomposition peaks, reflecting different activation energies for the decomposition reaction sites. The dipole moment of an adsorbed CH has been determined to be μ = 0.43 D, suggesting a rather polarized Ru−CH complex; its structure is independent of the adsorption site. Finally, carbide polymerization to form graphite has been detected above Ts = 560 K for the first time using work function change measurements. Good agreement was found between the contribution to ΔΦ by the graphite layer formed on the Ru(001) surface and ab initio calculations performed previously on this system.

Alkali promotion of Ni/MgO catalysts

The effects of the alkali addition (e.g., Li, Na, K) on the physico-chemical properties of a 19% Ni/MgO catalyst (alk at/Ni at = 0.122 ÷ 0.150) have been evaluated by TPR, TPD of H 2 (H-TPD) and CO (CO-TPD), TEM and temperature-programmed reaction measurements in toluene hydrogenation (TPTH). Lithium promotes the reduction negatively affecting the morphology and dispersion of the Ni/MgO catalyst, whereas both Na and K enhance the development of the active surface area. Alkali exert a physical effect on the active phase which lowers both the CO chemisorption stoichiometry and the sticking coefficient of toluene hydrogenation. Alkalis also induce an electronic effect on the active Ni phase resulting both in the inhibition of the Bouduard reaction (2 CO (ads) → C (ads) + CO 2) and in a drop (≈20 kJ mol −1) of E app in toluene hydrogenation.

Neutron studies of nanostructured CuOAl 2O 3 NO x removal catalysts

Nanostructured powders of automotive catalytic system CuOAl 2O 3, targeted for nitrogen oxides (NO x ) removal under lean-burn engine conditions, were investigated using neutron diffraction and small-angle neutron scattering. The crystal phases, structural transformations and microstructure oof 10 mol% CuAl 2O 3 powders are characterized according to the heat-treatment conditions. These properties are correlated with the pore structure and NO x removal efficiency determined by nitrogen adsorption isotherm, electron spin resonance, and temperature-programmed reaction measurements. The γ-(Cu, Al) 2O 3 phase and the mass-fractal-like aggregate of particles (size ≈ 26 nm) at annealing temperatures below 900°C were found to be crucial to the high NO x removal performance. The transformation to bulk crystalline phases of α-Al 2O 3 + CuAl 2O 4 spinel above ∼ 1050°C corresponds to a drastic drop of NO x removal efficiency. The usefulness of neutron-scattering techniques as well as their complementarity with other traditional methods of catalytic research are discussed.

Kinetic studies on the mechanism of the catalytic ethene oxidation

The kinetics of ethene oxidation catalyzed by an oxygen-preconditioned silver foil has been measured with an ultrahigh-vacuum-compatible flow reactor, using mass spectrometric gas analysis. The gradient-free reactor consisted of two concentric quartz tubes, forming a jacket of small width, covered with the catalyst. Additional temperature-programmed desorption studies and experiments with selected different types of oxygen species confirmed that only adsorbed atomic oxygen is reactive and that an oxidic surface layer is required for stable and high selectivity. Isothermal (typically 250 °C, 26 mbar; 1 mbar = 100 N m −2) batch and flow as well as temperature-programmed reaction measurements were evaluated by means of the Horiuti-Temkin procedure. The selectivity behavior is correctly described by the kinetic model. Rate constants and the estimated overall apparent activation energy are discussed with respect to the suggested reaction mechanism.

Alkali metal, chlorine and other promoters in the silver-catalysed selective oxidation of ethylene

Ethylene oxidation over well characterised Ag(111) surfaces has been investigated by temperature-programmed reaction measurements and by differential batch reactor studies at pressures up to 50 Torr. The influence of chlorine predosing on catalytic activity indicates that a chemisorbed atomic oxygen species is responsible for both partial oxidation and complete oxidation to CO2+ H2O. This tends to be confirmed by experiments using N2O as the oxidant, both with the single crystal speciemen and with a practical Ag–αAl2O3 catalyst in a flow microreactor. Dissolved oxygen, like adsorbed chlorine, is found to be a selectivity promoter. Adsorbed Cs also increases the rate of ethylene oxide production but can also positively influence the overall activity of the system. The results suggest that chlorine and dissolved oxygen promoters principally affect the primary chemistry, whereas the main effect of Cs is on the secondary chemistry (further oxidation of ethylene oxide). This view tends to be confirmed by temperature programmed reaction measurements and by direct experiments on the influence of CI and Cs on the isomerisation and combustion of ethylene oxide itself. It is suggested that these effects are primarily electronic in origin and a mechanism based on this view is presented. In the presence of Cs, both CO2 and NOx can act as selectivity promoters for the formation of ethylene oxide.