Reactive Molecular Beam Scattering Research Articles

Activity of Pdn (n = 1–5) Clusters on Alumina Film on Ni3Al(111) for CO Oxidation: A Molecular Beam Study

Single atom catalyst (SAC) is a vivid new area of research in catalysis. However, the activity in CO oxidation of isolated Pt or Pd atoms, generally supported on an oxide powder, is still controversial. Furthermore, the steady state activity of few atoms clusters is still not yet quantitatively known. In this work we study, by molecular beam reactive scattering (MBRS), the activity of Pd$_{\rm n}$ ($n$ = 1-5) clusters, grown on a nanostructured alumina films on a Ni$_3$Al(111) surface. It is shown that the single atoms are not active at 473 K but they diffuse and coalesce at 533K forming larger clusters. The activity of a cluster is proportional to the number of atoms it contains (n=2-5). At 533 K, the activity per (surface-) atom, which is the turnover frequency (TOF), is constant. Its value is close to those obtained for large clusters of 181$\pm$13 atoms and Pd (111) extended surfaces, in the same experimental conditions.

The Effects of P-Atoms on the Selective Dehydrogenation of C6H10 over Model Ru Surfaces

The incorporation of phosphorus into the Ru(0001) surface increases the selectivity of cyclohexene (C6H10) dehydrogenation to benzene (C6H6) by 100-fold when compared to Ru(0001) under steady-state conditions. We propose a series of elementary steps for the reactions of C6H10 over Ru(0001) and P0.4-Ru(0001) based on temperature-programmed reaction (TPR) of C6H10, 1,3-cyclohexadiene and reactive molecular beam scattering (RMBS) of C6H10 on Ru(0001) and P0.4-Ru(0001). TPR of 1,3-cyclohexadiene shows that P atoms alter the kinetically relevant step for C6H10 dehydrogenation from C–H activation in 1,3-cyclohexadiene on Ru(0001) to C–H activation in 2-cyclohexenyl on P0.4-Ru(0001). During TPR of C6H10, C6H6 forms over P0.4-Ru(0001) with an intrinsic activation energy that is 40 kJ mol–1 lower than that for Ru(0001). In addition, the presence of P atoms increases the apparent activation energy for deactivation by 21 kJ mol–1 during RMBS of C6H10. The increase in the barrier for deactivation, presumably by C–C bond rupture steps, significantly reduces the quantity of coke formed by consecutive TPR of C6H10 and contributes to greater selectivities for C6H6 formation. These observations suggest that the addition of P atoms to Ru(0001) introduces both electronic and geometric effects that alter the metal–adsorbate interactions. These findings indicate that transition-metal phosphides may be useful for selective dehydrogenation reactions important for reforming light hydrocarbons (e.g., ethane, propane, and cycloalkanes) to increase the yield of valuable alkenes and arenes.

Mechanistic Study of Formic Acid Decomposition over Ru(0001) and Px-Ru(0001): Effects of Phosphorus on C–H and C–O Bond Rupture

Transition metal phosphide (TMP) catalysts are selective and active toward C–O bond rupture in hydrodeoxygenation reactions; however, the manner in which C–O bond rupture mechanisms and intrinsic energy barriers differ between transition metals and TMP is not well understood. In this study, we characterize the chemical and structural properties of phosphorus (P) modified Ru(0001) surface using Auger electron spectroscopy, low-energy electron diffraction, and temperature-programmed desorption (TPD) of CO and NH3. The decomposition pathways for formic acid and the differences between the associated barriers were studied using temperature-programmed reaction (TPR) and reactive molecular beam scattering (RMBS) of DCOOH on pristine and P-modified Ru(0001) surfaces. TPR and TPD results suggest that P atoms introduce an electronic (and perhaps a geometric) effect that decreases the extent of electron exchange between Ru atoms and adsorbates, which decreases desorption energies and increases barriers for C–O and ...

Oxygen Activation and Reaction on Pd–Au Bimetallic Surfaces

Pd–Au bimetallic catalysts have shown promising performance for a number of oxidative reactions. The present study utilizes reactive molecular beam scattering (RMBS), reflection–absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), and density functional theory (DFT) techniques in an attempt to enhance the fundamental understanding of oxygen activation and reaction with CO on Pd–Au surfaces. Our results reveal that the presence of contiguous Pd sites is crucial for adsorption of oxygen molecules on Pd/Au(111) surfaces at 77 K. Upon heating, oxygen admolecules desorbed molecularly without detectable dissociation in O2-TPD measurements. CO-RMBS experiments indicate that at lower temperatures (77–150 K) oxygen admolecules were readily displaced by CO due to competitive adsorption. Oxygen admolecules can be thermally activated at higher temperatures (180–250 K) to react with CO to form CO2. DFT calculations show that the Pd–Au surface containing larger Pd ensembles favors dissociative CO oxidation, whereas associative CO oxidation and O2 desorption are the two main competing processes for the Pd–Au surface containing small Pd ensembles. An associative CO oxidation pathway was not experimentally observed, which is likely due to facile CO-induced O2 desorption. These results provide mechanistic insights into the interaction of oxygen with Pd–Au surfaces, which may prove informative for the rational design of Pd–Au catalysts for associated reactions involving O2 as a reactant.

Control of selectivity in allylic alcohol oxidation on gold surfaces: the role of oxygen adatoms and hydroxyl species.

Gold catalysts display high activity and good selectivity for partial oxidation of a number of alcohol species. In this work, we discuss the effects of oxygen adatoms and surface hydroxyls on the selectivity for oxidation of allylic alcohols (allyl alcohol and crotyl alcohol) on gold surfaces. Utilizing temperature programmed desorption (TPD), reactive molecular beam scattering (RMBS), and density functional theory (DFT) techniques, we provide evidence to suggest that the selectivity displayed towards partial oxidation versus combustion pathways is dependent on the type of oxidant species present on the gold surface. TPD and RMBS results suggest that surface hydroxyls promote partial oxidation of allylic alcohols to their corresponding aldehydes with very high selectivity, while oxygen adatoms promote both partial oxidation and combustion pathways. DFT calculations indicate that oxygen adatoms can react with acrolein to promote the formation of a bidentate surface intermediate, similar to structures that have been shown to decompose to generate combustion products over other transition metal surfaces. Surface hydroxyls do not readily promote such a process. Our results help explain phenomena observed in previous studies and may prove useful in the design of future catalysts for partial oxidation of alcohols.

Selective hydrogen production from formic acid decomposition on Pd-Au bimetallic surfaces.

Pd-Au catalysts have shown exceptional performance for selective hydrogen production via HCOOH decomposition, a promising alternative to solve issues associated with hydrogen storage and distribution. In this study, we utilized temperature-programmed desorption (TPD) and reactive molecular beam scattering (RMBS) in an attempt to unravel the factors governing the catalytic properties of Pd-Au bimetallic surfaces for HCOOH decomposition. Our results show that Pd atoms at the Pd-Au surface are responsible for activating HCOOH molecules; however, the selectivity of the reaction is dictated by the identity of the surface metal atoms adjacent to the Pd atoms. Pd atoms that reside at Pd-Au interface sites tend to favor dehydrogenation of HCOOH, whereas Pd atoms in Pd(111)-like sites, which lack neighboring Au atoms, favor dehydration of HCOOH. These observations suggest that the reactivity and selectivity of HCOOH decomposition on Pd-Au catalysts can be tailored by controlling the arrangement of surface Pd and Au atoms. The findings in this study may prove informative for rational design of Pd-Au catalysts for associated reactions including selective HCOOH decomposition for hydrogen production and electro-oxidation of HCOOH in the direct formic acid fuel cell.

Interaction of water with the clean and oxygen pre-covered Ir(1 1 1) surface

Adsorption and reaction of water on the clean and oxygen modified Ir(1 1 1) single crystal surfaces have been studied using temperature programmed desorption (TPD) and molecular beam reactive scattering (MBRS) techniques under ultrahigh vacuum (UHV) conditions. Water dissociates on the clean Ir(1 1 1) surface with a probability (estimated based on production of hydrogen) which decreases from ∼0.016 to 0.004 ± 0.0015 with increasing water coverages from 0.34 to 2.59 monolayer. Scattering experiments performed at various surface temperatures in the limit of zero coverage yield water dissociation probabilities in the range of ∼0.0005–0.012 (300–900 K) with an uncertainty expressed as ±20% of the dissociation probability. The apparent activation energy for water dissociation on clean Ir(1 1 1) is estimated to be ∼170 ± 5 kJ/mol employing MBRS techniques, which probably cannot be applied to TPD measurements with higher water coverages. We speculate that water dissociation occurs on the defects of the Ir(1 1 1) surface. Using isotopically labeled reactants, a strong interaction between adsorbed water and oxygen was found on Ir(1 1 1), indicated by a new water desorption feature at 235 K and scrambled oxygen and water desorption products.

Selective decomposition of formic acid on molybdenum carbide: A new reaction pathway

Selective decomposition of formic acid is important as a prototype to study selective bond cleavage of oxygenates. We demonstrate that carbon-modified Mo(1 1 0), C–Mo(1 1 0), is up to 15 times more selective for the dehydrogenation of formic acid than Mo(1 1 0). Reflection absorption infrared spectroscopy (RAIRS) indicates that carbidic carbon blocks active sites for C–O bond cleavage, decreasing the rate of dehydration. Steady-state reactive molecular beam scattering (RMBS) shows that dehydration is the dominant reaction pathway on clean Mo(1 1 0), while C–Mo(1 1 0) selectively promotes dehydrogenation. Kinetic analysis of RMBS data reveals that formic acid dehydrogenation on Mo(1 1 0) has an activation energy of 34.4 ± 3.3 kJ mol −1 while the C–Mo(1 1 0) surface promotes distinct pathways for dehydrogenation with an activation energy of only 12.8 ± 1.0 kJ mol −1. RAIRS spectra suggest the new pathways include the formation of monodentate formate, and at temperatures of 500 K and greater, direct activation of the C–H bond to form carboxyl, both of which decompose via a CO 2 δ - intermediate to evolve CO 2 and H 2.

Oxygen Exchange in the Selective Oxidation of 2-Butanol on Oxygen Precovered Au(111)

Direct evidence for C-O bond cleavage in the partial oxidation of 2-butanol on oxygen precovered Au(111) is provided using temperature programmed desorption (TPD) and molecular beam reactive scattering (MBRS) under ultrahigh vacuum (UHV) conditions. The oxygen precovered Au(111) surface can promote the partial oxidation of 2-butanol into 2-butanone with near 100% selectivity at low oxygen coverages, while 2-butanol adsorbs and desorbs molecularly on the clean Au(111) surface. Both C(2)H(5)C(16)OCH(3) and C(2)H(5)C(18)OCH(3) are observed in TPD after 2-butanol (C(2)H(5)CH(16)OHCH(3)) was dosed onto Au(111) precovered with (18)O(a). This oxygen exchange phenomenon serves as strong evidence for the C-O bond cleavage in 2-butanol partial oxidation to 2-butanone. Two surface intermediates are proposed for the selective oxidation of 2-butanol: 2-butoxide and eta(2)-aldehyde. As oxygen coverage increases, full oxidation is activated in addition to selective partial oxidation.

Surface Science Investigations of Oxidative Chemistry on Gold

Because of gold's resistance to oxidation and corrosion, historically chemists have considered this metal inert. However, decades ago, researchers discovered that highly dispersed gold particles on metal oxides are highly chemically active, particularly in low-temperature CO oxidations. These seminal findings spurred considerable interest in investigations and applications of gold-based materials. Since the discovery of gold's chemical activity at the nanoscale, researchers found that bulk gold also has interesting catalytic properties. Thus, it is important to understand and contrast the intrinsic chemical properties of bulk gold with those of nanoparticle Au. Despite numerous studies, the structure and active site of supported Au nanoclusters and the active oxygen species remain elusive, and model studies under well-controlled conditions could help identify these species. The {111} facet has the lowest surface energy and is the most stable and prevalent configuration of most supported gold nanoparticles. Therefore, a molecular-level understanding of the physical properties and surface chemistry of Au(111) could provide mechanistic details regarding the nature of Au-based catalysts and lead to improved catalytic processes. This Account focuses on our current understanding of oxidative chemistry on well-defined gold single crystals, predominantly from recent investigations on Au(111) that we have performed using modern surface science techniques. Our model system strategy allows us to control reaction conditions, which assists in the identification of reaction intermediates, the determination of the elementary reaction steps, and the evaluation of reaction energetics for rate-limiting steps. We have employed temperature-programmed desorption (TPD), molecular beam reactive scattering (MBRS), and Auger electron spectroscopy (AES) to evaluate surface oxidative chemistry. In some cases, we have combined these results with density functional theory (DFT) calculations. By controlling the reaction parameters that determine product selectivity, we have examined the chemical properties of bulk gold. Based on our investigations, the surface-bound oxygen atoms are metastable at low temperature. We also demonstrate that the oxygen atoms and formed hydroxyls are responsible for some of the distinct chemical behavior of gold and participate in surface reactions either as a Brønsted base or a nucleophilic base. We observe similar reaction patterns on gold surfaces to those on copper and silver surfaces, suggesting that the acid-base reactions that have been observed on copper and silver may also occur on gold. Our model chemical studies on gold surfaces have provided intrinsic fundamental insights into high surface area gold-based catalysts and the origin of the reactive oxygen species.

Annealing Effect on Reactivity of Oxygen-Covered Au(111)

We present results of an investigation into the effect of annealing on the reactivity of atomic oxygen adsorbed on Au(111) employing reactive molecular beam scattering (RMBS) and temperature-programmed desorption (TPD) techniques. Isotopically labeled water (e.g., H218O and D216O), carbon monoxide (CO), and oxygen-labeled carbon dioxide (C18O2) were used as probe molecules to investigate the reactivity of adsorbed oxygen. Our results show that the reactivity of atomic oxygen-precovered Au(111) is significantly altered by annealing. The annealed surfaces were prepared by depositing atomic oxygen (16O or 18O) at 77 K followed by annealing to temperatures ranging from 100−420 K before dosing probe molecules (H218O, CO, or C18O2) at 77 K. Without exception, annealing dramatically diminishes the reactivity of oxygen for all three probe reactions. In the case of the oxygen−water interactions, TPD indicates that annealing decreases the amount of oxygen isotope scrambling between oxygen and water. Additionally, t...

Selective Oxidation of Ethanol to Acetaldehyde on Gold

We present results from an investigation of the oxidative conversion of ethanol into acetaldehyde on Au(111) employing temperature-programmed desorption (TPD) and molecular beam reactive scattering (MBRS). Results from isotopic experiments show that ethanol initially undergoes O−H bond cleavage (producing ethoxide) followed by selective β−C-H bond (α to the oxygen) activation to form acetaldehyde and water on atomic oxygen precovered Au(111). Our study on bulk gold with atomic oxygen provides insight into the mechanistic details for high-surface-area gold-based catalysts and their application in the oxidation chemistry of alcohols employing molecular oxygen or air.

Surface Chemistry of Methanol on Clean and Atomic Oxygen Pre-Covered Au(111)

Desorption kinetics of methanol on Au(111) and the oxidation of methanol on atomic oxygen pre-covered Au(111) have been investigated employing temperature programmed desorption (TPD) and molecular beam reactive scattering (MBRS). On clean Au(111), methanol is weakly adsorbed and desorbs molecularly. Three adsorption states, which are assigned to methanol molecules desorbing from the monolayer (β phase), amorphous multilayers (α2 phase), and crystallized multilayers (α1 phase), are observed. For the β peak, detailed numerical analysis, based on mathematical inversion of the Polanyi-Wigner equation, leads to an optimized pre-exponential factor (log ν = 11.3 ± 1.1 s-1) and coverage-dependent desorption energies, which are further used to accurately simulate TPD spectra for submonolayer initial methanol coverages. For methanol coverages of 0.2 and 1.0 ML, desorption energies of 40.6 ± 1.6 kJ/mol and 34.8 ± 1.4 kJ/mol are extracted. In the presence of atomic oxygen, TPD of adsorbed methanol exhibits desorption of only CH3OH, H2O, CO, and CO2. No other partial oxidation products or derivatives such as hydrogen, formaldehyde, formic acid, or methyl formate are detected during TPD or MBRS measurements. On the basis of our experimental results and previous similar studies on other coinage metals, we suggest that abstraction of hydrogen is the initial step in the surface decomposition of methanol and that the adsorbed/surface bound methoxy group [CH3O(a)] is the primary intermediate in the oxidation of methanol.

Reactivity of Oxide Precursor States on Ru(0001)

Smooth and defect-rich Ru(0001) surfaces prepared under ultrahigh-vacuum (UHV) conditions have been loaded with oxygen under high-pressure (p </= 1 bar) and low-temperature (T < 550 K) conditions. On these surfaces the CO oxidation reaction has been investigated by means of thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS) and reactive molecular beam scattering (RMBS). Both surfaces are oxide-free and exhibit a high reactivity. The maximum CO/CO(2) conversion probability observed for a defect-rich Ru(0001) surface amounts to 6 x 10(-3) and is comparable to that of a surface covered with rutile RuO(2)(110) domains. RMBS experiments led to the identification of three different reaction channels. The first and second channel is related to CO adsorbing at oxygen-free defect sites and follow the Langmuir-Hinshelwood mechanism. Whereas the first reaction channel is already observed at room temperature, the second is thermally activated, contributing to the CO(2) yield only for reaction temperatures above 400 K. The third channel is due to the recombination of CO molecules with oxygen atoms located in smooth areas of the surface undisturbed by defects. This reaction channel is thermally activated as well.

Crossed Molecular Beam Reactive Scattering: Towards Universal Product Detection by Soft Electron‐Impact Ionization

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Adsorption and decomposition of CH3NO2 on a polycrystalline Pt foil using TPD, MBRS, XPS and FT-RAIRS

The adsorption and decomposition of nitromethane (CH 3 NO 2 ) on a polycrystalline Pt foil has been studied using a combination of temperature-programmed desorption (TPD), molecular beam reactive scattering (MBRS), and X-ray photoelectron (XPS) and Fourier-transform-reflection/absorption IR (FT-RAIRS) spectroscopies. Adsorption at 100 K is predominantly molecular, with multilayers of physisorbed CH 3 NO 2 desorbing at 140 K and the first monolayer at 170 K. Adsorption at 300 K leads to complete dissociation of nitromethane to yield CO, NO, and H 2 as major desorption products, C 2 N 2 , and CH 4 , H 2 O and CO 2 as minor desorption products. MBRS experiments reveal that some molecular H 2 O is also formed following adsorption at 300 K, but the surface lifetime of H 2 O at 300 K is relatively short and spontaneous desorption occurs at this temperature. The possible involvement of a nitrite intermediate in the thermal decomposition at 300 K and also under XPS acquisition conditions at lower temperatures is discussed.

Adsorption and decomposition of HCN on a polycrystalline Pt foil studied by TPD, MBRS, XPS and FT-RAIRS

The adsorption and decomposition of HCN on a polycrystalline Pt foil have been studied by temperature-programmed desorption (TPD), molecular beam reactive scattering (MBRS), X-ray photoelectron spectroscopy (XPS) and FT reflection–absorption IR spectroscopy (FT-RAIRS). Adsorption of HCN at 100 K yields two distinct CN-containing species for sub-monolayer coverages. The first is identified as a weakly adsorbed molecular HCN species whilst the second species is identified as CN(ads), formed by the dissociative chemisorption of HCN. Higher exposures of HCN at 100 K result in the additional formation of physisorbed multilayers of HCN, which are observed to desorb below 200 K. Adsorption of HCN at 300 K still yields CN(ads) and H(ads), with some of the latter desorbing as H2 at around 400 K, but also produces a new surface intermediate. The most likely structure for this intermediate is identified by FT-RAIRS as a trans-HCNH surface species. The near coincident desorption of HCN and H2 observed in the temperature range 400–500 K is then attributed to the decomposition of this HCNH intermediate and subsequent rapid recombination reactions. Above 650 K the desorption of C2N2 is observed owing to the surface combination of CN(ads) species.

Molecular Beam Reactive Scattering Apparatus with Electron Bombardment Detector

A crossed beams apparatus for study of thermal energy neutral-neutral reactions is described. The detector comprises a high efficiency (∼0.1%) electron bombardment ionizer, quadrupole mass filter, scintillation ion counter, and gated scalers synchronized with the beam modulation. The ionizer is nested within three chambers, each pumped by a separate ion pump and the innermost attached to a liquid nitrogen trap. The design is such that molecules which pass through the ionizer without being ionized fly on into another differentially pumped region before hitting a surface. The entire detector unit, including pumps and trap, is mounted on a rotatable platform (angular range ∼140°) which forms the lid of the scattering chamber. The beam sources also comprise modular units, mounted in differentially pumped side chambers which insert into ports in the scattering chamber. Angular distributions of reaction products have been measured for several reactions of Cl, Br, or D atoms with halogen molecules or hydrogen halides. Product velocity distributions have also been measured by time of flight for some of these reactions. In most cases, the partial pressure of interfering background species in the ionization region was ⪝ 10−15 Torr, and satisfactory data could be obtained with reactive scattering signals of a few counts per second.