PdO Surface Research Articles

Methane combustion reactivity during the metal→metallic oxide transformation of Pd-Pt catalysts: Effect of oxygen pressure

Density functional theory combined with kinetic models were used to probe different kinetics consequences by which methane activation on different oxygen chemical potential surfaces as oxygen pressure increased. The metallic oxide → metal transformation temperature of Pd-Pt catalysts increased with the increase of the Pd content or/and O2 pressure. The methane conversion rate on Pt catalyst increased and then decreased to a constant value when increasing the O2 pressure, and Pd catalyst showed a poor activity performance in the case of low O2 pressure. Moreover, its activity increased as the oxygen chemical potential for O2 pressure increased in the range of 2.5–10 KPa. For metal clusters, the CH bond and OO bond activation steps occurred predominantly on *-* site pairs. The methane conversion rate was determined by O2 pressure because the adsorbed O atoms were rapidly consumed by other adsorbed species in this kinetic regime. As the O2 pressure increased, the metallic active sites for methane activation were decreased and there was no longer lack of adsorbed O atoms, resulting in the decrease of the methane conversion rate. Furthermore, when the metallic surfaces were completely covered by adsorbed oxygen atoms at higher oxygen chemical potentials, Pt catalyst showed a poor activity due to a high CH bond activation barrier on O*O*. In the case of high O2 pressure, Pd atoms preferred to segregate to the active surface of Pd-Pt catalysts, leading to the formation of PdO surfaces. The increase of Pd segregation promoted a subsequent increase in active sites and methane conversion rate. The PdO was much more active than metallic and O* saturated surfaces for methane activation, inferred from the theory and experimental study. Pd-rich bimetallic catalyst (75% molar Pd) showed a dual high methane combustion activity on O2-poor and O2-rich conditions.

Mechanism of Low‐Temperature Carbon Combustion Over Ag‐Pd/Alumina Catalysts

AbstractThe mechanism of carbon combustion over Ag−Pd/alumina catalysts has been studied with the aim of further improving the performance of Ag‐based diesel particulate filter catalysts. Alumina‐supported Ag−Pd, Ag, and Pd catalysts were prepared and compared for their carbon combustion ability. Over the Ag−Pd, Ag, and Pd catalysts, peaks of CO2 emission were observed at 425, 510, and 650 °C, respectively. Details of the catalyst structure and carbon oxidation process were investigated using energy dispersive X‐ray (EDX) mapping and temperature‐dependent Ag and Pd K‐edge X‐ray absorption fine structure analysis during carbon combustion. The EDX mapping and extended X‐ray absorption fine structure analyses suggested that Ag−Pd and PdO particles stacked together in the Ag−Pd/alumina catalyst. With this structure, oxygen atoms could migrate effectively to the Ag−Pd particles from the PdO surfaces, allowing low‐temperature combustion over the Ag−Pd/alumina catalyst.

Adsorbate Pairing on Oxide Surfaces: Influence on Reactivity and Dependence on Oxide, Adsorbate Pair, and Density Functional

Open-shell molecules on metal oxide surfaces frequently display cooperative adsorption mechanisms, where pairs of adsorbates are significantly more stable than the isolated species. In this work, density functional theory is used to investigate the cooperative adsorption of OH—H and NO2—NO on rocksalt BaO(100), rutile TiO2(110), fluorite CeO2(111), and tetragonal PdO(101) surfaces. The OH and NO2 adsorbates are considered to be located at the metal sites, whereas H and NO are situated on the oxygen sites. Despite differences in adsorption mechanisms, the pairing is found to be consistently exothermic. The pairing is most pronounced on BaO(100) and weakest on PdO(101). OH and H form more stable pairs than do NO2 and NO. In all cases except CeO2(111), the hybrid HSE06 functional predicts a stronger pairing than the semilocal PBE functional. The absolute pairing energy is a convoluted measure of charge transfer energies, electrostatic interactions, and ionic relaxations, which is analyzed in detail using a thermodynamic cycle. Adsorbate pairing has marked effects on the reactivity, which is exemplified by studying the reaction of CO with OH on CeO2(111).

Study of methane oxidation over alumina supported Pd–Pt catalysts usingoperandoDRIFTS/MS andin situXAS techniques

Methane oxidation over Pd–Pt/Al2O3 model catalysts calcined at three different conditions is investigated using operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, and in situ X-ray absorption spectroscopy while cycling the feed gas stoichiometry between lean (net-oxidising) and rich (net-reducing) conditions. When calcined in air, alloy Pd–Pt nanoparticles are present only on catalysts subjected to elevated temperature (800 ◦C) whereas calcination at lower temperature (500◦C) leads to segregated Pt and Pd nanoparticles on the support. Here, we show that the alloy Pd–Pt nanoparticles undergo reversible changes in surface structure and composition during transient methane oxidation exposing a PdO surface during lean conditions and a metallic Pd–Pt surface (Pd enriched) under rich conditions. Alloyed particles seem more active for methane oxidation than their monometallic counterparts and, furthermore, an increased activity for methane oxidation is clearly observed under lean conditions when PdO has developed on the surface, analogous to monometallic Pd catalysts. Upon introducing rich conditions, partial oxidation of methane dominates over total oxidation forming adsorbed carbonyls on the noble metal particles. The carbonyl spectra for the three samples show clear differences originating from different surfaces exposed by alloyed vs. non-alloyed particles. The kinetics of the noble metal oxidation and reduction processes as well as carbonyl formation during transient methane oxidation are discussed.

The influence of incommensurability on the long-range periodicity of the Pd(100)-([formula omitted])R27°-PdO(101)

The structural model of the (5×5)R27°-PdO(101) surface oxide grown on Pd(100) has been proposed and refined by a number of authors over more than a decade. In the current contribution we discuss the long-range periodicity of this structure arising along one of the crystallographic directions due to its incommensurability with the substrate. Analyzing the results of surface sensitive diffraction studies, we determined a slight distortion of the previously reported perfect (5×5)R27° surface oxide unit cell. Considering it, we were able to achieve both qualitatively and quantitatively better fit to the experimental diffraction data than it was possible for the perfect structure. Further, taking into account the experimentally obtained scanning tunneling microscopy data and closely examining high-resolution patterns recorded by means of high-energy surface X-ray diffraction, we developed a qualitative structural model based on a larger non-orthogonal surface unit cell to shed more light on the long-range order of the PdO(101) surface oxide. The model comprises a shift of the atoms of the PdO perpendicularly to the direction of the incommensurability to correct for it. This structural model reproduces the fine details of the high-resolution diffraction patterns and qualitatively explains the periodic stripes of structural distortion observed in the images recorded by a scanning tunneling microscope.

Optimization of Pd catalysts supported on Co3O4 for low-temperature lean combustion of residual methane

A series of Pd/Co3O4 catalysts with increasing palladium loading in the range of 0.5–5wt.% was prepared by incipient wetness impregnation of Co3O4. Solution combustion synthesis with urea as a fuel was used to optimize the Co3O4 reactive support for palladium in the combustion of methane in lean conditions. The obtained catalysts were thoroughly examined by XRD, XPS, XRF, RS, FESEM, H2-TPR, TGA, and N2-BET techniques. The catalytic tests of CH4 combustion were performed for 0.5, 1, and 2vol.% CH4, with constant lambda value. The obtained results revealed that a sub-stoichiometric fuel-to-oxidizer ratio, 0.75, results in the most catalytically active Co3O4 phase. The important differences in the catalysts’ activity were apparent for the highest CH4 concentration, with the 3% Pd/Co3O4 being the most active catalyst. The observed activity was explained considering the physicochemical, spectroscopic, and microscopic characterization of the catalysts with the PdO nanocrystals surface distribution being the determining factor for the catalysts’ reactivity. A simple model accounting for the observed dispersion effect is proposed. The model is based on a two types of the interaction of the surface PdO active phase with the Co3O4 support. Firstly, cobalt oxide helps to remove hydroxyl species from the PdO surface, thus making the active sites more available for methane activation. Secondly, cobalt spinel provides lattice oxygen to the PdO phase, again helping to recreate active sites thereon.

Oxygen reduction on zirconium-stabilized-PdO surfaces: a first-principles study

The oxygen reduction reaction properties on PdO and Zr-doped PdO surfaces, and the relative stability of the concerned surfaces, have been studied by first-principles calculations.

CO oxidation on PdO catalysts with perfect and defective rutile-TiO2 as supports: Elucidating the role of oxygen vacancy in support by DFT calculations

To explore metal oxide–oxide support interactions and their effects, the mechanism of CO oxidation on PdO catalysts with rutile TiO2 or TiO2-x (TiO2 with a bridging oxygen vacancy) as the support, was studied by density functional theory calculations, compared with that on pure PdO surface. For TiO2 as the support, support effect leads to the change of the preferential CO adsorption sites from the coordinatively unsaturated Pd (Pdcus) site on pure PdO surface to the bridging site of coordinatively unsaturated Pd and O atoms (Pdcus and Ocus), thus altering the reaction pathway of CO oxidation, whereas the support effect has little influence on the energy barrier. However, for TiO2-x as the support, the presence of the oxygen vacancy leads to the energy barrier remarkably decreased compared with that on pure or TiO2-supported PdO surface. The change of Bader charges indicates the oxygen vacancy in the support can tune the oxidizability of PdO surface active oxygen Ocus, thus adjusting the CO adsorption strength at the bridging site of Pdcus and Ocus to be favorable for the extraction process of Ocus. Tuning oxygen vacancies in supports can be used as a new perspective to design improved supported oxide catalysts.

Investigation of Methane Oxidation Reactions Over a Dual-Bed Catalyst System using 18 O Labelled DRIFTS coupling.

Low loading Pd-supported (0.2 wt % Pd) Y-stabilized zirconia (YSZ) and LaMnO3 (LM) perovskite were associated to study the partial oxidation of methane using labelled 18 O2 in the gas phase. Synthesis gas production was demonstrated to occur through an indirect reaction in which oxygen is first consumed in the total methane combustion. A Mars-van Krevelen mechanism was observed over Pd/YSZ at 425 °C to yield C16 O2 and C16 O. A significant enhancement of the Pd/YSZ catalyst activity was achieved by the association of LM-Pd/YSZ in a dual catalyst bed, resulting in a significant increase of the oxidation rate. Vibration bands of adsorbed formate species, assumed to be intermediates to the gas production, were observed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupling experiments. It was proposed that LM enables the generation of highly active singlet O2 , which is activated on the YSZ oxygen vacancies to assist a rapid recovery of surface PdO and increase formate decomposition into CO and H2 in Pd-supported catalyst.

First Principles Study of Molecular O2 Adsorption on the PdO(101) Surface

Interactions of O2 with the PdO(101) surface were studied using spin-dependent density-functional theory (DFT) with both the PBE and the non-local hybrid HSE exchange–correlation functional. The adsorption energies are strongly overestimated (by 40–60 kJ/mol) with PBE, whereas HSE predicts adsorption energies that are within ~5 kJ/mol of values derived from temperature programmed desorption (TPD) experiments. A detailed partial density of states analysis indicates that the band gap between the PdO d-band center and the LUMO of O2 plays an important role in determining the adsorption strength. This gap is larger for the HSE functional and leads to a decrease in the back donation of the metal d-states to the O2 LUMO orbital resulting in weaker adsorption. Based on the DFT–HSE calculations, three adsorption minima are found to be stable. The most favored configuration, with an adsorption energy of −67 kJ/mol, consists of an O2 molecule lying flat and interacting with two coordinatively unsaturated Pd (Pdcus) surface atoms. The other two configurations have weaker adsorption energies of about −25 kJ/mol and bind to a single Pdcus atom with the O2 molecule oriented away from the surface. The HSE results can be correlated with the observed TPD spectra, which shows only one type of O2 configuration at low coverages with a subsequently lower temperature (more weakly bound) peak evolving at higher coverages associated with the singly coordinated O2 adsorption configurations that start to populate when two adjacent Pdcus sites start to become unavailable.

H2 adsorption and dissociation on PdO(101) films supported on rutile TiO2 (110) facet: elucidating the support effect by DFT calculations.

To explore metal oxide-support interactions and their effect, H2 adsorption and dissociation on PdO(101)/TiO2(110) films with different film thicknesses, in comparison with that on pure PdO(101) surface without TiO2(110) support, were studied by density functional theory calculation. A monolayer PdO(101) film supported on TiO2 facet shows different properties to a pure PdO(101) surface. On the monolayer PdO(101)/TiO2(110) film, TiO2 support leads to stronger molecular adsorption of H2 on coordinatively unsaturated Pd top sites than that on a pure PdO surface. H2 dissociation with the formation of OH was preferred thermodynamically but slightly unfavorable kinetically on the monolayer PdO film due to the TiO2 support effect. Graphical abstract On the monolayer PdO(101)/TiO2(110) film, the TiO2 support effect leads to stronger H2 molecular adsorption on coordinatively unsaturated Pd top sites than on pure PdO surface. H2 dissociation with the formation of OH is preferred thermodynamically but slightly unfavorable kinetically on the film due to the TiO2 support effect.

A New Design Strategy for Observing Lithium Oxide Growth-Evolution Interactions Using Geometric Catalyst Positioning.

Understanding the catalyzed formation and evolution of lithium-oxide products in Li-O2 batteries is central to the development of next-generation energy storage technology. Catalytic sites, while effective in lowering reaction barriers, often become deactivated when placed on the surface of an oxygen electrode due to passivation by solid products. Here we investigate a mechanism for alleviating catalyst deactivation by dispersing Pd catalytic sites away from the oxygen electrode surface in a well-structured anodic aluminum oxide (AAO) porous membrane interlayer. We observe the cross-sectional product growth and evolution in Li-O2 cells by characterizing products that grow from the electrode surface. Morphological and structural details of the products in both catalyzed and uncatalyzed cells are investigated independently from the influence of the oxygen electrode. We find that the geometric decoration of catalysts far from the conductive electrode surface significantly improves the reaction reversibility by chemically facilitating the oxidation reaction through local coordination with PdO surfaces. The influence of the catalyst position on product composition is further verified by ex situ X-ray photoelectron spectroscopy and Raman spectroscopy in addition to morphological studies.

In‐Situ TEM on Epitaxial and Non‐Epitaxial Oxidation of Pd and Reduction of PdO at P = 0.2–0.7 bar and T = 20–650 °C

The oxidation of Pd particles and subsequent reduction of PdO were studied in 0.2–0.7 bar O2 and H2 gas at 20–650 °C by in‐situ transmission electron microscopy. The 15 nm thick Pd particles were 50–300 nm wide, and (111) oriented on the SiN support. Two oxidation mechanisms were observed: 1) the slow formation of epitaxial PdO at the Pd particle edges with orientations OR(1): PdO[201]//Pd[111] and PdO(020)//Pd{022}, 5.8° from OR(2): PdO[110]//Pd[100] and PdO[001]//Pd[001]. 2) the fast formation of randomly oriented PdO nucleating at the Pd particle edges. During the reduction process we observed epitaxial formation [OR(1) and OR(2)] as well as nonepitaxial formation of Pd (and Pd hydride). The formation of randomly oriented (5–10 nm) Pd particles on a (001) surface of PdO was much slower than of the epitaxial Pd grains on other PdO surfaces. A systematic study of the electron beam irradiation effect on the phase transformations showed that this effect can be avoided.

Enhanced methane combustion performance over NiAl2O4-interface-promoted Pd/γ-Al2O3

Poor durability and low Pd utilization efficiency are the main drawbacks of supported Pd catalysts for low-temperature methane combustion. Here, we present a facile way to stabilize the performance of Pd/Al2O3 by introducing a spinel NiAl2O4 interface as a promoter at no cost to the activity. The NiAl2O4 interface is formed by impregnation of alumina supports in the nickel-containing solution, followed by high-temperature calcination. A series of 0.4wt.% Pd/xNiO/γ-Al2O3 with NiO loading varying among 0.0, 0.5, 1.0, and 9.0wt.% is investigated. The formation of a NiAl2O4 interface promotes the distribution and crystallization of PdO, suppresses PdO particles aggregation and surface OH− species accumulation during the reaction, and finally leads to the excellent performance of Pd/0.5NiO/γ-Al2O3. Pd/0.5NiO/γ-Al2O3 demonstrates higher Pd utilization efficiency than the state-of-the-art Pd-based catalysts and better stability than Pd/Al2O3. Such a spinel interface promotion strategy may bring new insight into the design of highly efficient Pd-based catalysts and their potential technological applications.

Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary CH Bonds

Achieving selective C-H bond cleavage is critical for developing catalytic processes that transform small alkanes to value-added products. The present study clarifies the molecular-level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C-H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ-complexes preferentially adopt geometries on PdO(101) in which only primary C-H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H-Pd dative bond forms at each CH3 group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C-H bonds.

Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary CH Bonds

AbstractAchieving selective CH bond cleavage is critical for developing catalytic processes that transform small alkanes to value‐added products. The present study clarifies the molecular‐level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary CH bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ‐complexes preferentially adopt geometries on PdO(101) in which only primary CH bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a HPd dative bond forms at each CH3 group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane CH bonds.

Promotion of CO oxidation on PdO(101) by adsorbed H2O

We investigated the influence of adsorbed H2O on the oxidation of CO on PdO(101) using temperature programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations. We find that water inhibits CO adsorption on PdO(101) by site blocking, but also provides a more facile pathway for CO oxidation compared with the bare oxide surface. In the presence of adsorbed H2O, the oxidation of CO on PdO(101) produces a CO2 TPRS peak that is centered at a temperature ~50K lower than the main CO2 TPRS peak arising from CO oxidation on clean PdO(101) (~330 vs. 380K). RAIRS shows that CO continues to adsorb on atop-Pd sites of PdO(101) when H2O is co-adsorbed, and provides no evidence of other reactive intermediates. DFT calculations predict that the CO oxidation mechanism follows the same steps for CO adsorbed on PdO(101) with and without co-adsorbed H2O, wherein an atop-CO species recombines with an oxygen atom from the oxide surface lattice. According to DFT, hydrogen bonding interactions with adsorbed H2O species stabilize the carboxyl-like transition structure and intermediate that result from the initial recombination of CO and O on the PdO(101) surface. This stabilization lowers the energy barrier for CO oxidation on PdO(101) by ~10kJ/mol, in good agreement with our experimental estimate.

Methane Oxidation over PdO(101) Revealed by First-Principles Kinetic Modeling.

The catalytic oxidation of methane to carbon dioxide and water over PdO(101) is investigated with first-principles based microkinetic modeling. Extensive exploration of the reaction landscape allows for determination of preferred pathways at different reaction conditions. The predicted kinetic behavior is in good agreement with a range of experimental findings including reaction orders in methane, water, and oxygen as well as apparent activation energies. The results consolidate the role of the PdO(101) surface in the activity of PdO catalysts and offer starting points for computational design of materials with improved catalytic activity. Moreover, the study demonstrates the predictive power of first-principles based kinetic modeling for oxide surfaces when hybrid functionals are applied in conjugation with kinetic models that go beyond the mean-field approximation.

Transient Structures of PdO during CO Oxidation over Pd(100)

In situ high-energy surface X-ray diffraction was employed to determine the surface structure dynamics of a Pd(100) single crystal surface acting as a model catalyst to promote CO oxidation. The measurements were performed under semirealistic conditions, i.e., 100 mbar total gas pressure and 600 K sample temperature. The surface structure was studied in detail both in a steady gas flow and in a gradually changing gas composition with a time resolution of 0.5 s. The experimental technique allows for rapid reciprocal space mapping providing the complete information on structural changes of a surface with unprecedented time resolution in harsh conditions. Our results show that the (√5 × √5)R27°-PdO(101) surface oxide forms in a close to stoichiometric O2 and CO gas mixture as the mass spectrometry indicates a transition to a highly active state with the reaction rate limited by the CO mass transfer to the Pd(100) surface. Using a low excess of O2 in the gas stoichiometry, islands of bulk oxide grow epitaxial...

Vacancy-Mediated Processes in the Oxidation of CO on PdO(101).

Metal oxide films can form on late transition-metal catalysts under sufficiently oxygen-rich conditions, and typically cause significant changes in the catalytic performance of these materials. Several investigations using sensitive in situ surface characterization techniques reveal that the CO oxidation activity of Pd and other late transition-metal catalysts increases abruptly under conditions at which metal oxide structures begin to develop. Findings such as these provide strong motivation for developing atomic-scale descriptions of oxidation catalysis over oxide films of the late transition-metals. Surface oxygen vacancies can play a central role in mediating oxidation catalysis promoted by metal oxides. In general, adsorbed reactants abstract oxygen atoms from the lattice of the oxide surface, thereby creating oxygen vacancies, while gaseous O2 replenishes the reactive surface oxygen atoms and eliminates oxygen vacancies. Oxygen vacancies also represent a distinct type of surface site on which the binding and reactivity of adsorbed species can differ compared with sites on the pristine oxide surface. Detailed characterization of vacancy-mediated rate processes is thus essential for developing reliable mechanistic descriptions of oxidation catalysis over reducible metal oxide films. Careful measurements performed in ultrahigh vacuum (UHV) using well-defined oxide surfaces in combination with molecular simulations afford the capability to isolate and characterize such reaction steps, and thus provide information that is needed for developing mechanistic models of oxidation catalysis over metal oxides. In this Account, we discuss vacancy-mediated processes that are involved in the oxidation of CO on the PdO(101) surface as determined from UHV surface science experiments and density functional theory (DFT) calculations. These studies show that CO binds strongly on Pd atoms that are located next to surface oxygen vacancies, and diffuses rapidly to these sites during reduction of the oxide surface by CO. The enhanced binding also raises the energy barriers for desorption and oxidation of CO, but the difference in these barriers remains nearly identical to that for CO adsorbed on the pristine PdO(101) surface. These recent studies also show that oxygen from the subsurface efficiently eliminates surface oxygen vacancies during CO oxidation at temperatures as low as 400 K, and thereby reveal a facile pathway by which PdO(101) surface domains can be maintained during oxide reduction.