Temperature-programmed Reaction Spectroscopy Research Articles

CO oxidation on IrO2(110) surfaces

We investigated the oxidation of CO on stoichiometric and O-rich IrO2(110) surfaces using temperature programmed reaction spectroscopy (TPRS), density functional theory (DFT) calculations and microkinetic simulations. Adsorbed CO on the s-IrO2(110) surface generates CO and CO2 peaks near 545 K during TPRS, and only about 38 % of the CO oxidized to CO2 when the initial CO layer was saturated. Pre-adsorbed O-atoms, so-called on-top oxygen (Ot), promote the oxidation of CO adsorbed on IrO2(110). On the Ot-covered surface, CO oxidation by Ot atoms produces a CO2 TPRS peak at ∼370 K, and all of the initially adsorbed CO oxidizes to CO2 when the initial Ot coverage is greater than the CO coverage. In agreement with the TPRS results, DFT calculations predict that the barrier is about 100 kJ/mol lower for CO oxidation by an Ot atom than a lattice O-atom of IrO2(110). A microkinetic model, parameterized with energy barriers computed using DFT, accurately reproduces the CO and CO2 TPRS traces only after CO binding energies are lowered to values determined using a hybrid exchange-correlation functional and the barrier for CO molecules to fill bridging O-vacancies is lowered. The simulations predict that O-vacancies play an important role in mediating the CO oxidation kinetics on s-IrO2(110), and thereby demonstrate the importance of future spectroscopic studies aimed at characterizing the nature of the surface CO and O species involved in reaction. This study provides new insights for understanding CO oxidation on IrO2(110), and provides evidence that several elementary steps can be involved in governing this chemistry.

Surface chlorination of IrO2(110) by HCl.

The ability to controllably chlorinate metal-oxide surfaces can provide opportunities for designing selective oxidation catalysts. In the present study, we investigated the surface chlorination of IrO2(110) by HCl using temperature programmed reaction spectroscopy (TPRS), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. We find that exposing IrO2(110) to HCl, followed by heating to 650K in ultrahigh vacuum, produces nearly equal quantities of on-top and bridging Cl atoms on the surface, Clt and Clbr, where the Clbr atoms replace O-atoms that are removed from the surface by H2O formation. After HCl adsorption at 85K, only H2O desorbs at low Cl coverages during TPRS, but HCl begins to desorb in increasing yields as the Cl coverage is increased above about 0.5 monolayer (ML). The desorption of Cl2 was not observed under any conditions, in good agreement with the high barrier for this reaction predicted by DFT. A maximum Cl coverage of 1 ML, with nearly equal coverages of Clt and Clbr atoms, could be generated by reacting HCl with IrO2(110) in UHV. Our results suggest that a kinetic competition between recombinative HCl and H2O desorption under the conditions studied limits the saturation Cl coverage to a value less than the 2 ML maximum predicted by thermodynamics. XPS further shows that the partitioning of Cl between the Clt and Clbr states can be altered by subjecting partially chlorinated IrO2(110) to reductive or oxidative treatments, demonstrating that the Cl site population can change dynamically in response to the gas environment. Our results provide insights for understanding the chlorination of IrO2(110) by HCl and can enable future experimental studies to determine how Cl-modification alters the surface chemical reactivity of IrO2(110) and potentially enhances selectivity toward partial oxidation chemistry.

Achieving Ultra-High Selectivity to Hydrogen Production from Formic Acid on Pd-Ag Alloys.

Palladium-silver-based alloy catalysts have a great potential for CO-free hydrogen production from formic acid for fuel cell applications. However, the structural factors affecting the selectivity of formic acid decomposition are still debated. Herein, the decomposition pathways of formic acid on Pd-Ag alloys with different atomic configurations have been investigated to identify the alloy structures yielding high H2 selectively. Several PdxAg1-x surface alloys with various compositions were generated on a Pd(111) single crystal; their atomic distribution and electronic structure were determined by a combination of infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). It was established that the Ag atoms with Pd neighbors are electronically altered, and the degree of alteration correlates with the number of nearest Pd. Temperature-programmed reaction spectroscopy (TPRS) and DFT demonstrated that the electronically altered Ag domains create a new reaction pathway that selectively dehydrogenates formic acid. In contrast, Pd monomers surrounded by Ag are demonstrated to have a similar reactivity compared to pristine Pd(111), yielding CO and H2O in addition to the dehydrogenation products. However, they bind to the produced CO weaker than pristine Pd, demonstrating an enhancement in resistance to CO poisoning. This work therefore shows that surface Ag domains modified by interaction with subsurface Pd are the key active sites for selective decomposition of formic acid, while surface Pd atoms are detrimental to selectivity. Hence, the decomposition pathways can be tailored for CO-free H2 production on Pd-Ag alloy systems.

Co3+–O Bond Elongation Unlocks Co3O4 for Methane Activation under Ambient Conditions

The pursuit of low-temperature methane activation is a paramount challenge for heterogeneous catalysis because of the high stability of the C–H bonds (435 kJ mol–1). Very few catalysts fulfill this through either constructing complex interfaces or using special noble metal oxides. Bond length, which is highly temperature-sensitive, has long been ignored in catalyst studies. Here, we exploit a bond-length elongation strategy to unlock a nonprecious metal oxide for dissociating methane at normal temperature and pressure. Mesoporous bubblelike Co3O4 with elongated Co3+–O bonds, synthesized through a unique “melting–foaming–solidifying” route, is found to activate methane at room temperature to form COx species on the very surface revealed by temperature-programmed reaction spectroscopy experiments. A combination of experimental and theoretical methods suggests that the elongated Co3+–O bonds contribute to the greatly enhanced methane adsorption, in contrast to the weak adsorption of methane on conventional nonprecious metal oxides. Accordingly, the activation energy for the C–H bond cleavage is smaller than that for methane desorption, which finally leads to the low-temperature methane activation. The simultaneous enhancement in intrinsic activity and mass transfer endows mesoporous bubblelike Co3O4 with excellent catalytic activity for methane combustion, where the complete conversion temperature is much below that over the conventional Co3O4 nanocatalysts.

Kinetics and selectivity of methane oxidation on an IrO2(110) film

Undercoordinated, bridging O-atoms (Obr) are highly active as H-acceptors in alkane dehydrogenation on IrO2(110) surfaces but transform to HObr groups that are inactive toward hydrocarbons. The low C–H activity and high stability of the HObr groups cause the kinetics and product selectivity during CH4 oxidation on IrO2(110) to depend sensitively on the availability of Obr atoms prior to the onset of product desorption. From temperature programmed reaction spectroscopy (TPRS) and kinetic simulations, we identified two Obr-coverage regimes that distinguish the kinetics and product formation during CH4 oxidation on IrO2(110). Under excess Obr conditions, when the initial Obr coverage is greater than that needed to oxidize all the CH4 to CO2 and HObr groups, complete CH4 oxidation is dominant and produces CO2 in a single TPRS peak between 450 and 500 K. However, under Obr-limited conditions, nearly all the initial Obr atoms are deactivated by conversion to HObr or abstracted after only a fraction of the initially adsorbed CH4 oxidizes to CO2 and CO below 500 K. Thereafter, some of the excess CH x groups abstract H and desorb as CH4 above ∼500 K while the remainder oxidize to CO2 and CO at a rate that is controlled by the rate at which Obr atoms are regenerated from HObr during the formation of CH4 and H2O products. We also show that chemisorbed O-atoms (‘on-top O’) on IrO2(110) enhance CO2 production below 500 K by efficiently abstracting H from Obr atoms and thereby increasing the coverage of Obr atoms available to completely oxidize CH x groups at low temperature. Our results provide new insights for understanding factors which govern the kinetics and selectivity during CH4 oxidation on IrO2(110) surfaces.

Conversion of methanol on rutile TiO2 (110) and tungsten oxide clusters: 1. population of defect-dependent thermal reaction pathways.

Tungsten oxide clusters deposited on rutile TiO2 (110) single crystals were used as a model system for heterogenous oxide-oxide bifunctional catalysts. The population of different thermal reaction routes in methanol conversion in the presence of preadsorbed oxygen was probed under UHV conditions. By temperature programmed reaction spectroscopy, we have identified three thermal reaction channels, namely the deoxygenation under formation of methane, the partial oxidation forming formaldehyde and the condensation route under desorption of ethane and dimethyl ether. The specific local reaction environment at the oxidic surface was found to be key for the population of the different reaction channels as exhibited by the introduction of Lewis acidic and basic sites (especially (WO3)n clusters) and available charge carriers such as Ti3+. Especially the amount of bulk Ti3+ interstitials, that can partially transfer charge towards the tungsten oxide clusters at the TiO2 surface, was found to be a key parameter that enables a relatively high methanol conversion in thermal reactions. It turned out that the deoxygenation is by far the most dominant reaction followed by the partial oxidation. The condensation is observed only in low amounts under special conditions, but is an interesting example for reactivity at defect sites.

Oxophilicity Drives Oxygen Transfer at a Palladium–Silver Interface for Increased CO Oxidation Activity

A single-layer AgOx phase grown on Ag(111) efficiently transfers oxygen to Pd domains at room temperature, rendering the Pd-decorated surface highly reactive toward CO oxidation. Oxygen transfer from AgOx to Pd and the surface reactivity toward CO were investigated as a function of the Pd coverage using X-ray photoelectron spectroscopy, surface infrared spectroscopy of adsorbed CO, temperature-programmed reaction spectroscopy, and density functional theory (DFT) calculations. Our results show that all of the oxygen from the AgOx layer (∼0.375 monolayer) migrates to the surface of Pd during formation of a nearly complete Pd bilayer at 300 K and that the oxygen coverages generated on Pd increase as the Pd cluster size decreases, reaching values that exceed the oxygen concentration in the AgOx layer by as much as a factor of 2. Experimental measurements and DFT calculations show that preferential binding of oxygen on the edges of the Pd clusters enhances the oxygen coverage on Pd clusters of decreasing size and produces a heterogeneous spatial distribution of oxygen. CO adsorbs in high coverages at 100 K by binding on both the terraces and O-rich edges of the Pd clusters. During subsequent heating, oxidation of the adsorbed CO consumes nearly all of the oxygen that transferred from AgOx to the Pd domains; in contrast, the pure AgOx layer exhibits limited reactivity toward CO adsorbed at 100 K. These results demonstrate that differences in oxophilicity drive facile oxygen transfer from Ag to the edges of Pd nanoclusters and thereby give rise to an efficient pathway for CO oxidation on bimetallic PdAg surfaces. The cooperation between the Pd and Ag domains results in near-interfacial chemistry that may be broadly important in catalysis by bimetallic alloys.

Selective Hydrogenation of Acrolein on a Pd/Ag(111) Single-Atom Alloy Surface

The hydrogenation of acrolein on the Ag(111) and Pd/Ag(111) single-atom alloy (SAA) surfaces was studied using temperature-programmed reaction spectroscopy. On Ag(111), the percent conversion of acrolein increased with increasing atomic hydrogen coverage. At an acrolein coverage of 0.17 ML followed by a low exposure to atomic hydrogen, the highest selectivity to 2-propenol was reached at 19%. However, increasing the acrolein coverage caused the selectivity to 2-propenol to decrease and the selectivity to propanal to increase to a maximum of 92%. In all cases, increasing atomic hydrogen exposure decreased the selectivity to 2-propenol. The selectivity to 1-propanol was constant at only 1%. Activity and conversion on the Ag(111) and Pd/Ag(111) SAA surfaces were compared for Pd coverages of 0.2, 0.5, and 0.9%. With increasing Pd coverage, the percent conversion increased while the selectivity to 2-propenol decreased. The selectivity toward 1-propanol only increased from 1 to 3% for the 0.9% Pd/Ag(111) SAA surface.

Reactivity of C3Hx Adsorbates in Presence of Co-adsorbed CO and Hydrogen: Testing Fischer–Tropsch Chain Growth Mechanisms

The identity of the surface intermediates involved in chain growth during Fischer–Tropsch synthesis remains a topic of ongoing debate. In the present work we use a combination of temperature programmed reaction spectroscopy and high resolution X-ray photoemission spectroscopy to study the reactivity of C3Hx adsorbates on a Co(0001) single crystal surface in order to explore the stabilities of the different C3Hx surface intermediates and to study elementary reaction steps relevant to chain growth and chain termination. Propene (H3C–CH=CH2) and propyl (H3C–CH2–CH2–) adsorbates react below 200 K already, either by desorption of propene or by dehydrogenation to adsorbed propyne (H3C–C≡CH). Co-adsorbed Had and COad do not affect the temperature at which propyl and propene react, but they do suppress the dehydrogenation pathway in favour of propene desorption. Their high reactivity under simulated FTS conditions disqualifies them as feasible intermediates for FTS, which requires long-lived intermediates to match the low monomer formation rate. Propyne, the most stable C3Hx adsorbate in the absence of COad, is hydrogenated to propylidyne (H3C–CH2–C≡) > 230 K when both COad and Had are present. Propylidyne dimerization occurs around 313 K and produces a 3-hexyne (H5C2–C≡C–C2H5) surface intermediate which is hydrogenated to 3-hexene (H5C2–CH=CH–C2H5) above 350 K. These findings are of direct relevance to FTS: they show that the high coverage of COad and Had present during the reaction influence the reactivity of CxHy adsorbates involved in chain growth and ultimately steer product selectivity. The findings provide further experimental support for the previously proposed alkylidyne chain growth mechanism on close-packed cobalt terraces: CO stabilizes CxHy growth intermediates in the alkylidyne form and growth proceeds via coupling of a long chain alkylidyne and methylidyne (CH).

Simultaneously accelerating the regeneration of FeII and the selectivity of 2e- oxygen reduction over sulfide iron-based carbon aerogel in electro-Fenton system

The regeneration of surface FeII and the electro-formation of H2O2 play crucial roles for determining the degradation efficiency of heterogeneous electro-Fenton process. The sulfide iron-based carbon aerogel was synthesized directly as cathode. The main composition of iron compounds were Fe3O4 and FeS∙FeS2, which were embedded in the matrix of carbon aerogels. Most of sulfur was evenly distributed on carbon, favoring the formation of −COOH groups. The nature of existed sulfur were investigated by temperature-programmed reaction spectroscopy respectively under the CO2 and air. The S-doping promoted the regeneration of FeII and improved the selectivity of 2e− oxygen reduction with iron-based carbon materials. As high as 99 % removal of dimethyl phthalate and 3-chlorophenol were obtained in 240 min. Meanwhile, the EPR results demonstrated that HO∙ was formed by on site decomposing H2O2 with FeSCA, which exhibited high stability with low Fe leaching (< 2 ppm) and high recyclability even in acidic medium.

Enhancement of Formic Acid Dehydrogenation Selectivity of Pd(111) Single Crystal Model Catalyst Surface via Brønsted Bases

The influence of ammonia (NH3) on the doubly deuterated formic acid (DCOOD, FA) dehydrogenation selectivity for a Pd(111) single crystal model catalyst surface was investigated under ultrahigh vacuum conditions using temperature-programmed desorption and temperature-programmed reaction spectroscopy techniques. NH3 adsorption on Pd(111) revealed reversible, molecular desorption without any significant decomposition products, while DCOOD adsorption on Pd(111) yielded D2, D2O, CO, and CO2 as a result of dehydration and dehydrogenation pathways. Functionalizing the Pd(111) surface with ammonia suppressed the FA dehydration and enhanced the dehydrogenation pathway. The boost in the FA dehydrogenation of Pd(111) in the presence of NH3 can be linked to the ease of FA deprotonation as well as the stabilization of the decomposition intermediate (i.e., formate) due to the presence of ammonium counterions on the surface. In addition, the presence of a H-bonded ammonia network on the Pd(111) surface increased the hydrogen atom mobility and decreased the activation energy for molecular hydrogen desorption. In the presence of NH3, catalytic FA decomposition on Pd(111) also yielded amidation reactions, which further suppressed CO liberation and prevented poisoning of the Pd(111) active sites due to strongly bound CO species.

Adsorption and Oxidation of CH4 on Oxygen-Rich IrO2(110)

We investigated the oxidation of CH4 on oxygen-pre-covered IrO2(110) surfaces using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT). Our TPRS results show th...

Activation of Small Organic Molecules on Ti2+-Rich TiO2 Surfaces: Deoxygenation vs. C–C Coupling

Abstract Rutile TiO2 is an important model system for understanding the adsorption and conversion of molecules on transition metal oxide catalysts. In the last decades, point defects, such as oxygen vacancies and Ti3+ interstitials, exhibited an important influence on the reaction of oxygen and oxygen-containing molecules on titania surfaces. In brief, partially reduced TiO2 containing a significant amount of Ti3+ is often more active for the conversion of such molecules. In this study, we investigate an even higher reduced surface prepared by argon ion bombardment of a rutile TiO2 (110) single crystal. By X-ray photoelectron spectroscopy we show that, besides Ti4+, this surface is almost equally dominated by Ti3+ and Ti2+. To probe the reactivity of these highly reduced surfaces, we have adsorbed two different classes of oxygen-containing molecules and utilized temperature programmed reaction spectroscopy to investigate the conversion. While alcohols (in this case methanol) already show a defect-dependent partial conversion in a deoxygenation reaction on the (stochiometric or slightly reduced) rutile TiO2 (110) surface, ketones (e.g. acetone) are usually not converted on the rutile TiO2 (110) surface independent on the bulk defect density. Here, we present a nearly full conversion for both molecules via deoxygenation reactions and reductive C–C coupling, forming different hydrocarbons at different temperatures between 375 K and 640 K on the sputtered Ti2+ rich surface.

Kinetics and Mechanism of Aspartic Acid Adsorption and Its Explosive Decomposition on Cu(100).

The mechanism and kinetics of aspartic acid (Asp, HO2CCH(NH2)CH2CO2H) decomposition on Cu(100) have been studied using X-ray photoemission spectroscopy and temperature-programmed reaction spectroscopy. We investigate the Asp decomposition mechanism in detail using unlabeled d-Asp and isotopically labeled l-Asp-4-13C (HO2CCH(NH2)CH213CO2H), l-Asp- d7 (DO2CCD(ND2)CD2CO2D), l-Asp-2,3,3- d3 (HO2CCD(NH2)CD2CO2H), and l-Asp-15N-2,3,3- d3 (HO2CCD(15NH2)CD2CO2H). The monolayer of Asp adsorbed on the Cu(100) surface is in a doubly deprotonated bi-aspartate form (-O2CCH(NH2)CH2CO2-). During heating, Asp decomposes on Cu(100) with kinetics consistent with a vacancy-mediated explosion mechanism. The mechanistic steps yield CO2 by sequential cleavage of the C3-C4 and C1-C2 bonds, and N≡CCH3 and H2 via decomposition of the remaining CH(NH2)CH2 intermediate. Deuterium labeling has been used to demonstrate that scrambling of H(D) occurs during the decomposition to acetonitrile of the CD(NH2)CD2 intermediate formed by decarboxylation of l-Asp-2,3,3- d3 and l-Asp-15N-2,3,3- d3.

The fate of bromine after temperature-induced dehydrogenation of on-surface synthesized bisheptahelicene.

The on-surface synthesis of bisheptahelicene by Ullmann coupling of 9-bromoheptahelicene on Au(111) and its temperature-induced dehydrogenation is studied using temperature-programmed reaction spectroscopy and time-of-flight secondary ion mass spectrometry. Specific dehydrogenation products of bisheptahelicene after loss of 6, 8 and 10 hydrogen atoms are identified, corresponding to molecules having undergone Diels-Alder transformations and intramolecular C-C coupling reactions. By combining with atomic hydrogen produced by dehydrogenation, the Ullmann coupling side-product bromine desorbs as HBr. H2 desorption emerges only after all Br has desorbed. Such characteristic behavior is explained by a kinetic model which explicitly considers the coverage of transient atomic H on the surface. Heating experiments performed with saturated layers of different Br-containing molecules reveal that the onset of HBr desorption depends strictly on the dehydrogenation step and therefore on the structure of the molecules.

Bulk Defect Dependence of Low-Temperature Partial Oxidation of Methanol and High-Temperature Hydrocarbon Formation on Rutile TiO2 (110)

Titanium dioxide (TiO2) is among the most studied model (photo)catalyst materials. The influence of surface point defects, like oxygen vacancies and particularly bulk defects such as Ti3+ interstitials, is usually underestimated or even ignored. We present a systematic study under well-defined UHV conditions illustrating the importance of such defects for the thermal reaction of methanol at the rutile TiO2 (110) single crystal surface by using temperature-programmed reaction spectroscopy (TPRS) and Fourier-transform infrared reflection–absorption spectroscopy (FT-IRRAS). It will be shown that the population of different reaction pathways, namely, the partial oxidation of methanol to formaldehyde and the deoxygenation forming hydrocarbons, especially methane, depends on the bulk defect density and the presence or absence of oxygen adsorbates. While at elevated temperatures molecular desorption is pronounced for the less defective substrates, for higher reduction grades the high-temperature deoxygenation ch...

CO as a Promoting Spectator Species of CxHy Conversions Relevant for Fischer–Tropsch Chain Growth on Cobalt: Evidence from Temperature-Programmed Reaction and Reflection Absorption Infrared Spectroscopy

Cobalt-catalyzed low temperature Fischer–Tropsch synthesis is a prime example of an industrially relevant reaction in which CxHy intermediates involved in chain growth react in the presence of a large quantity of COad. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside CxHy adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the CxHy intermediates formed at different temperatures, and infrared absorption spectroscopy was used to obtain more specific information on the chemical identity of the various reaction intermediates formed. Ethene, propene, and but-1-ene precursors decompose below 200 K. The 1-alkyne adsorbate is identified as a major product, and some alkylidyne species form as well when the initial alkene coverage is high. The surface hydrogen atoms produced in the low temperature decomposition step start leaving the surface >300 K. When an alkyne/Had-covered surface i...

Hydrogen oxidation on oxygen-rich IrO2(110)

We investigated the adsorption and oxidation of H2 on O-rich IrO2(110) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. Our results show that H2 dissociation occurs efficiently on O-rich IrO2(110) at low temperature and initiates from an adsorbed H2 σ-complex on the coordinatively-unsaturated Ir atoms (Ircus). We find that on-top oxygen atoms (Oot), adsorbed on the Ircus sites, promote the desorption-limited evolution of H2O during subsequent oxidation of the adsorbed hydrogen on IrO2(110) while suppressing reaction-limited production of H2O via the recombination of bridging HO groups (HObr) (~500 to 750 K) during TPRS. The desorption-limited TPRS peak of H2O shifts from ~490 to 550 K with increasing Oot coverage, demonstrating that Oot atoms stabilize adsorbed OH and H2O species. DFT predicts that molecularly-adsorbed H2 dissociates on O-rich IrO2(110) at low temperature and that the resulting H-atoms redistribute to produce a mixture of HObr and HOot groups, with equilibrium favouring HOot groups. Our calculations further predict that subsequent H2O evolution occurs through the recombination of HObr/HOot and HOot/HOot pairs, and that these reactions represent desorption-limited pathways because the dissociative chemisorption of H2O is favoured over molecular adsorption on IrO2(110). The higher stability of HOot groups and their preferred formation causes the higher-barrier HOot/HOot recombination reaction to become the dominant pathway for H2O formation with increasing Oot coverage, consistent with the experimentally-observed upshift in the H2O TPRS peak temperature.

Oxygen adsorption on spontaneously reconstructed Au(511)

The four-fold site on {100} facets, which are potential catalytic sites on high-curvature gold nanoparticles, is difficult to prepare on a gold single crystal due to surface reconstruction. Here, the Au(511) surface with a high density of step edges and a well-maintained {100} local structure was studied by scanning tunneling microscopy (STM), temperature programmed reaction spectroscopy (TPRS), and density functional theory (DFT) calculations. Annealing at 550 K and 870 K induces reconstruction on the Au(511) surface to a mixture of (311) and (711) micro-terraces. At room temperature and low oxygen coverage, oxygen adsorbs at both three- and four-fold sites on the surface, leading to a well-ordered zig-zag structure. Oxygen atoms cause significant orbital hybridization and a slight displacement of the gold atoms to which they bind. High oxygen coverages induce the formation of clusters, which are not active towards oxidation of isopropanol at room temperature and a pressure of 10−10 mbar. Oxygen adsorption saturates at 1 monolayer (ML), and two separate oxygen recombinative desorption peaks were observed at coverages above 0.11 ML. Our characterization of this surface and the O adsorption structure allows future studies of {100} micro-terraces for oxidation reactions, which contributes to studies on gold-based catalysts with an irregular shape.

Understanding the Intrinsic Surface Reactivity of Single-Layer and Multilayer PdO(101) on Pd(100)

We investigated the intrinsic reactivity of CO on single-layer and multilayer PdO(101) grown on Pd(100) using temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) experiments, as well as density functional theory (DFT) calculations. We find that CO binds more strongly on multilayer than single-layer PdO(101) (∼119 kJ/mol vs 43 kJ/mol), and that CO oxidizes negligibly on single-layer PdO(101), whereas nearly 90% of a saturated layer of CO oxidizes on multilayer PdO(101) during TPRS experiments. RAIRS further shows that CO molecules adsorb on both bridge-Pdcus and atop-Pdcus sites (coordinatively unsaturated Pd sites) of single-layer PdO(101)/Pd(100), while CO binds exclusively on atop-Pdcus sites of multilayer PdO(101). The DFT calculations reproduce the much stronger binding of CO on multilayer PdO(101), as well as the observed binding site preferences, and reveal that the stronger binding is entirely responsible for the higher CO oxidation activity o...