Submonolayer Amounts Research Articles

(Invited) 4-NP Reduction As a Reaction-Based Indicator for Quantitatively Assessing the Detrimental Influences of Leaching on Catalysts

The fundamental understanding of liquid-phase catalytic reactions is unavoidably complicated when the catalyst is prone to leaching since questions inevitably arise as to the true nature of the catalyst. While the catalytic reduction of 4-nitrophenol by borohydride is widely accepted as a trusted model reaction, it has faced little scrutiny concerning the potential impact of leached species or the appropriateness of assigning catalytic activity to the inserted nanostructures without rigorous experimental verification. Here, we present results from a spectroscopically monitored split test in which supported silver catalysts are physically separated from the reactants midway through the reaction. It is unambiguously demonstrated that the influence of leaching is far from benign, instead acting to extinguish the catalytic activity of the inserted nanostructures while giving rise to an unsupported species that is the true catalytic entity. With only sub-monolayer quantities of silver leached from the supported structures, the unsupported species must be exceedingly catalytic. Moreover, it is shown that leaching is inherent to aqueous media containing dissolved oxygen, without which the supported nanostructures remain catalytically active. With the same nanomaterial being able to act as either a heterogeneous catalyst or as a reservoir from which leached metal is derived, such influences have undoubtedly compromised prior studies. We, nevertheless, capitalize on the sensitivity of 4-nitrophenol reduction to leached species by using it as a reaction-based indicator able to quantitatively determine the time-dependence of the leaching process and enhancements to oxidative etching when silver, copper, palladium, platinum, and gold are exposed to chloride ions.

Thermally Induced Crossover from 2D to 1D Behavior in an Array of Atomic Wires: Silicon Dangling-Bond Solitons in Si(553)-Au.

The self-assembly of submonolayer amounts of Au on the densely stepped Si(553) surface creates an array of closely spaced "atomic wires" separated by 1.5nm. At low temperature, charge transfer between the terraces and the row of silicon dangling bonds at the step edges leads to a charge-ordered state within the row of dangling bonds with ×3 periodicity. Interactions between the dangling bonds lead to their ordering into a fully two-dimensional (2D) array with centered registry between adjacent steps. We show that as the temperature is raised, soliton defects are created within each step edge. The concentration of solitons rises with increasing temperature and eventually destroys the 2D order by decoupling the step edges, reducing the effective dimensionality of the system to 1D. This crossover from higher to lower dimensionality is unexpected and, indeed, opposite to the behavior in other systems.

Elucidating the composition of PtAg surface alloys with atomic-scale imaging and spectroscopy.

Silver-based heterogeneous catalysts, modified with a range of elements, have found industrial application in several reactions in which selectivity is a challenge. Alloying small amounts of Pt into Ag has the potential to greatly enhance the somewhat low reactivity of Ag while maintaining high selectivity and resilience to poisoning. This single-atom alloy approach has had many successes for other alloy combinations but has yet to be investigated for PtAg. Using scanning tunneling microscopy (STM) and STM-based spectroscopy, we characterized the atomic-scale surface structure of a range of submonolayer amounts of Pt deposited on and in Ag(111) as a function of temperature. Near room temperature, intermixing of PtAg results in multiple metastable structures on the surface. Increasing the alloying temperature results in a higher concentration of isolated Pt atoms in the regions near Ag step edges as well as direct exchange of Pt atoms into Ag terraces. Furthermore, STM-based work function measurements allow us to identify Pt rich areas of the samples. We use CO temperature programmed desorption to confirm our STM assignments and quantify CO binding strengths that are compared with theory. Importantly, we find that CO, a common catalyst poison, binds more weakly to Pt atoms in the Ag surface than extended Pt ensembles. Taken together, this atomic-scale characterization of model PtAg surface alloys provides a starting point to investigate how the size and structure of Pt ensembles affect reaction pathways on the alloy and can inform the design of alloy catalysts with improved catalytic properties and resilience to poisoning.

Enhancing Oxygen Exchange Activity by Tailoring Perovskite Surfaces.

A detailed understanding of the effects of surface chemical and geometric composition is essential for understanding the electrochemical performance of the perovskite (ABO3) oxides commonly used as electrocatalysts in the cathodes of ceramic fuel cells. Herein, we report how the addition of submonolayer quantities of A- and B-site cations affects the rate of the oxygen reduction reaction (ORR) of Sr-doped LaFeO3 (LSF), LaMnO3 (LSM), and LaCoO3 (LSCo). Density functional theory calculations were performed to determine the stability of different active sites on a collection of surfaces. With LSF and LSM, rates for the ORR are significantly higher on the A-site terminated surface, while surface termination is less important for LSCo. Our findings highlight the importance of tailoring the surface termination of the perovskite to obtain its ultimate ORR performance.

Structures and Dynamics of Secondary and Tertiary Alkylphosphine Oxides Adsorbed on Silica.

The three secondary phosphine oxides [CH2 =CH(CH2 )4 ]2 HPO (1), [CH2 =CH(CH2 )5 ]2 HPO (2), and [CH2 =CH(CH2 )6 ]2 HPO (3), and two diphosphine dioxides, {[CH2 =CH(CH2 )6 ]2 PO(CH2 )7 }2 (4) and {[CH2 =CH(CH2 )6 ]2 PO(CH2 )4 }2 (5), incorporating long methylene chains, are described. The single crystal X-ray structures of 1, 2, and 5 have been determined. The phosphine oxides 3, 4, and 5 have been adsorbed on silica in submonolayer quantities to give 3 a-5 a. The 1 H, 13 C, and 31 P solid-state NMR spectra of polycrystalline 3-5 have been analyzed and compared with those of 3 a-5 a. The changes of the solid-state NMR characteristics upon adsorption and the surface mobilities of the phosphine oxides are discussed.

Scanning tunneling microscopy study of Cu-induced surface restructuring of Si(100)-(2 × 1)

We used scanning tunneling microscopy (STM) to study the Cu-induced restructuring of Si(100)-(2 × 1) surface at the atomic scale. Copper was deposited onto silicon substrates kept at room temperature (RT) or 500 °C. Submonolayer amounts of Cu were found to be sufficient to induce a visible restructuring of the Si(100)-(2 × 1) surface manifested by the disappearance of the vacancy line defects (VLDs), as well as dimer rows disordering (for the RT deposition) or the formation of a c(4 × 4) surface reconstruction (for the 500 °C deposition) not reported for the Cu/Si(100) system so far. Higher amounts of Cu deposited onto the RT-disordered surface at 500 °C led to the formation of 3D CuSi crystallites, accompanied by a significant surface roughening. In contrast, deposition of large amounts of Cu onto a pristine Si(100) at 500 °C resulted in the formation of a near-(1 × 1) local surface atoms ordering.

Suppression of Competing Reaction Channels by Pb Adatom Decoration of Catalytically Active Cu Surfaces During CO2 Electroreduction

The direct electrochemical conversion of carbon dioxide to chemicals and fuels is of fundamental scientific and technological interest. The control of the product selectivity, expressed in terms of the Faradaic efficiency, has remained a great challenge. Herein, we describe a surface-electrochemical synthetic strategy to tune the electrochemical CO2 reduction selectivity and yield by controlled suppression of the hydrogen evolution reaction (HER) reaction channel, resulting in increased Faradaic efficiencies for fuels and chemicals. We demonstrate that bimetallic catalysts consisting of only minute submonolayer amounts of Pb adatoms deposited on Cu surfaces exhibit and maintain unusually high selectivities for formate (HCOO–) over a large range of overpotentials. The bimetallic adatom electrodes were prepared using underpotential electrodeposition (UPD), which is able to precisely control the adatom coverage. While as little as 0.16 ML Pb surface adatoms on a polycrystalline Cu surface boosted the observed Faradaic HCOO– product selectivity 15 times, the 0.78 ML Pb/Cu catalyst showed the most favorable ratio of HCOO–/H2 production rate thanks to the effective suppression of the HER combined with a partial (−1.0 to −1.1 V vs RHE) enhancement of the HCOO– production. We argue that the favorable product efficiency is caused by selective adatom poisoning on the strongest binding hydrogen adsorption sites; in addition, electronic effects of Pb adatoms change the chemisorption of reactive intermediates. Our study reveals synthetic access to tailored selective bimetallic copper catalysts for the electrochemical CO2 reduction and demonstrates the enormous effect of even minute amounts of surface adatoms on the product spectrum.

Identifying the True Catalyst in the Reduction of 4-Nitrophenol: A Case Study Showing the Effect of Leaching and Oxidative Etching Using Ag Catalysts

The fundamental understanding of liquid-phase catalytic reactions is unavoidably complicated when the catalyst is prone to leaching since questions inevitably arise as to the true nature of the catalyst. While the catalytic reduction of 4-nitrophenol by borohydride is widely accepted as a trusted model reaction, it has faced little scrutiny concerning the potential impact of leached species or the appropriateness of assigning catalytic activity to the inserted nanostructures without rigorous experimental verification. Here, we present results from a spectroscopically monitored split test in which supported silver catalysts are physically separated from the reactants midway through the reaction. It is unambiguously demonstrated that the influence of leaching is far from benign, instead acting to extinguish the catalytic activity of the inserted nanostructures while giving rise to an unsupported heterogeneous catalyst that is the true catalytic entity. With only submonolayer quantities of silver leached fro...

Acceleration of hydrogen absorption by palladium through surface alloying with gold

Enhancement of hydrogen (H) absorption kinetics improves the performance of hydrogen-purifying membranes and hydrogen-storage materials, which is necessary for utilizing hydrogen as a carbon-free energy carrier. Pd-Au alloys are known to show higher hydrogen solubility than pure Pd. However, the effect of Au on the hydrogen penetration from the surface into the subsurface region has not been clarified so far. Here, we investigate the hydrogen absorption at Pd-Au surface alloys on Pd(110) by means of thermal desorption spectroscopy (TDS) and hydrogen depth profiling with nuclear reaction analysis (NRA). We demonstrate that alloying the Pd(110) surface with submonolayer amounts of Au dramatically accelerates the hydrogen absorption. The degree of acceleration shows a volcano-shaped form against Au coverage. This kinetic enhancement is explained by a reduced penetration barrier mainly caused by a destabilization of chemisorbed surface hydrogen, which is supported by density-functional-theory (DFT) calculations. The destabilization of chemisorbed surface hydrogen is attributed to the change of the surface electronic states as observed by angle-resolved photoemission spectroscopy (ARPES). If generalized, these discoveries may lead to improving and controlling the hydrogen transport across the surfaces of hydrogen-absorbing materials.

Stabilization of Mesoporous Iron Oxide Films against Sintering and Phase Transformations via Atomic Layer Deposition of Alumina and Silica

AbstractThe stabilization of crystal phases and nanostructured morphologies is an essential topic in application‐driven design of mesoporous materials. Many applications, e.g. catalysis, require high temperature and humidity. Typical metal oxides transform under such conditions from a metastable, low crystalline material into a thermodynamically more favorable form, i.e. from ferrihydrite into hematite in the case of iron oxide. The harsh conditions induce also a growth of the crystallites constituting pore walls, which results in sintering and finally collapse of the porous network. Herein, a new method to stabilize mesoporous templated metal oxides against sintering and pore collapse is reported. The method employs atomic layer deposition (ALD) to coat the internal mesopore surface with thin layers of either alumina or silica. The authors demonstrate that silica exerts a very strong influence: It shifts hematite formation from 400 to 600 °C and sintering of hematite from 600 to 900 °C. Differences between the stabilization via alumina and silica are rationalized by a different interaction strength between the ALD material and the ferrihydrite film. The presented approach allows to stabilize mesoporous thin films that require a high crystallization temperature, with submonolayer quantity of an ALD material, and to apply mesoporous materials for high temperature applications.

(Energy Technology Division Supramaniam Srinivasan Young Investigator Award Address) Enhanced Oxygen Electrocatalysis By Means of Electronic and Geometric Effects

The slow kinetics of the oxygen reduction and evolution reactions (ORR and OER) impede the widespread uptake of polymer electrolyte membrane (PEM) fuel cells and electrolysers. In order to improve the kinetics of the ORR and reduce the Pt loading at the fuel-cell cathode, we need to enhance the activity, stability and selectivity of the catalyst. This can be achieved by alteration of the electronic properties of the Pt surface atoms (electronic effects [1,2]) and/or modification of the geometric structure (atomic ensemble effects [3]). First, I will address atomic ensemble effects in electrocatalysis. We used a self-ordered molecular pattern of cyanide on Pt(111) surfaces to study the role of atomic ensembles for the ORR [3]. Cyanide groups form an ordered structure that blocks all the three-fold hollow sites, effectively blocking the adsorption of spectator anions in the electrolyte. Nonetheless, the holes in this structure are sufficiently large to allow the adsorption of reaction oxygen molecules. As a consequence, cyanide-modified Pt(111) presents a 25-fold enhancement over Pt(111) [3]. Secondly, I will focus on electronic effects by alloying Pt with other metals. In order to reduce the Pt loading, researchers have intensively studied alloys of Pt with late transition metals such as Ni and Co. However, these compounds typically degrade under fuel-cell conditions, due to dealloying. In contrast, Pt-lanthanide alloys present a very negative alloying energy, which should increase their resistance to degradation. We have studied novel Pt-lanthanide and Pt-alkaline earth electrocatalysts for the ORR. Pt-lanthanide and Pt-alkaline earth alloys are amongst the most active polycrystalline Pt-based catalysts ever reported [1,2], presenting up to a 6-fold increase in ORR activity, relative to Pt. The active phase consists of a compressed Pt overlayer of few Pt layers formed onto the bulk alloys by acid leaching. Notably, the oxygen reduction activity versus the bulk lattice parameter follows a volcano relation [1]. We use the lanthanide contraction to control strain effects and tailor the activity, stability and reactivity of Pt alloys. Finally, I will present new approaches to design and develop more efficient and stable OER electrocatalysts. We have explored the OER activity and stability of RuO2 nanoparticulate catalysts. Sub-monolayer amounts of IrOx on top of RuO2 thin films minimise Ru dissolution [4]. The strategy of tailoring the surface of the catalyst with sub-monolayer amounts of a stable oxide may allow us to tune the stability of active OER catalysts for PEM electrolysers.

The Au modified Ge(1 1 0) surface

The pristine Ge(1 1 0) surface is composed of Ge pentagons, which are arranged in relatively large (16 × 2) and c(8 × 10) unit cells. The deposition of sub-monolayer amounts of Au and mild annealing results into de-reconstructed Ge(1 1 0) regions completely free of Ge pentagons and regions composed of nanowires that are aligned along the high symmetry [11¯0] direction of the Ge(1 1 0) surface. The de-reconstructed Ge(1 1 0) regions consist of atomic rows that are aligned along the [11¯0] direction. A substantial fraction of these substrate rows are straight and resemble the atom rows of the unreconstructed, i.e. bulk terminated, Ge(1 1 0) surface, whereas the other substrate rows have a meandering appearance. These meandering atom rows are comprised of two types of atoms, one type that appears dim, whereas the other type appears bright in filled-state scanning tunneling microscopy images. Using density functional theory calculations, we have tested more than 20 different atomic models for the meandering atom rows. The density functional theory calculations reveal that it is energetically favorable for the deposited Au atoms to exchange position with Ge atoms in the first layer. Based on these findings we conclude that the bright atoms are Ge atoms, whereas the dim atoms are Au atoms.

Evolution of a liquid-like fluid phase on Ge/Au(111) at room temperature: A direct observation by STM

The evolution of a Au(111) surface after deposition of a submonolayer amount of Ge atoms has been investigated by STM. Diffusion and incorporation of Ge atoms into Au are active at room temperature where only negligible solubility is expected at equilibrium. It turned out that this surface phase is fluid, where the migration of step edges and successive appearance and disappearance of monolayer islands on the surface are confirmed. These phenomena seem to imply the weakening of the bonding strength between Au atoms by Ge incorporation and could be playing an important role in the low temperature crystallization process of Ge nanowires and thin films using Au as a catalyst.

Importance of Surface IrOx in Stabilizing RuO2 for Oxygen Evolution.

The high precious metal loading and high overpotential of the oxygen evolution reaction (OER) prevents the widespread utilization of polymer electrolyte membrane (PEM) water electrolyzers. Herein we explore the OER activity and stability in acidic electrolyte of a combined IrOx/RuO2 system consisting of RuO2 thin films with submonolayer (1, 2, and 4 Å) amounts of IrOx deposited on top. Operando extended X-ray absorption fine structure (EXAFS) on the Ir L-3 edge revealed a rutile type IrO2 structure with some Ir sites occupied by Ru, IrOx being at the surface of the RuO2 thin film. We monitor corrosion on IrOx/RuO2 thin films by combining electrochemical quartz crystal microbalance (EQCM) with inductively coupled mass spectrometry (ICP-MS). We elucidate the importance of submonolayer surface IrOx in minimizing Ru dissolution. Our work shows that we can tune the surface properties of active OER catalysts, such as RuO2, aiming to achieve higher electrocatalytic stability in PEM electrolyzers.

Intercalation of Si between MoS2 layers.

We report a combined experimental and theoretical study of the growth of sub-monolayer amounts of silicon (Si) on molybdenum disulfide (MoS2). At room temperature and low deposition rates we have found compelling evidence that the deposited Si atoms intercalate between the MoS2 layers. Our evidence relies on several experimental observations: (1) Upon the deposition of Si on pristine MoS2 the morphology of the surface transforms from a smooth surface to a hill-and-valley surface. The lattice constant of the hill-and-valley structure amounts to 3.16 Å, which is exactly the lattice constant of pristine MoS2. (2) The transitions from hills to valleys are not abrupt, as one would expect for epitaxial islands growing on-top of a substrate, but very gradual. (3) I(V) scanning tunneling spectroscopy spectra recorded at the hills and valleys reveal no noteworthy differences. (4) Spatial maps of dI/dz reveal that the surface exhibits a uniform work function and a lattice constant of 3.16 Å. (5) X-ray photo-electron spectroscopy measurements reveal that sputtering of the MoS2/Si substrate does not lead to a decrease, but an increase of the relative Si signal. Based on these experimental observations we have to conclude that deposited Si atoms do not reside on the MoS2 surface, but rather intercalate between the MoS2 layers. Our conclusion that Si intercalates upon the deposition on MoS2 is at variance with the interpretation by Chiappe et al. (Adv. Mater. 2014, 26, 2096–2101) that silicon forms a highly strained epitaxial layer on MoS2. Finally, density functional theory calculations indicate that silicene clusters encapsulated by MoS2 are stable.

Effects of K adsorption on the CO-induced restructuring of Co(11-20)

The location of potassium (K) on Cobalt (Co) and its effect on adsorption and adsorption-induced surface restructuring is important for understanding the deactivation of Co Fischer-Tropsch catalysts and the nature of the active surface. Co(11-20) restructures by anisotropic migration of Co atoms upon CO exposure. Deposition of sub-monolayer amounts of K on Co(11-20) and the effect on the CO-induced restructuring were therefore investigated using scanning tunneling microscopy (STM), high resolution photoemission spectroscopy (HR-PES), and density functional theory calculations (DFT). The combined STM and DFT results suggest that the preferred adsorption site for K at low coverage is in the vicinity of step edges. DFT also found that diffusion of K along the [0001] direction, in between the zigzag rows of the topmost Co layer is facile. The restructuring under CO exposure with K pre-adsorbed proceeded on the terraces rather than from the step edges, in a slower and more disordered manner. HR-PES showed that the amount of CO adsorbed at saturation significantly decreased with predeposited K. The obstructed migration of Co atoms across the surface may be important in understanding why very low amounts of K on supported Co catalysts significantly reduces the activity towards hydrogenation of CO.

Elimination and Quantification of Oxidation Induced Interstitial Injection via Ge Implants

The presence of Silicon-Germanium (SiGe) alloys at the Si/SiO2 interface during an oxidation is known to suppress the injection of silicon self interstitials that normally accompanies silicon oxidation and leads to observed effects such as Oxidation Enhanced Diffusion (OED) and stacking fault growth. To measure interstitial injection, a layer of implantation induced dislocation loops was introduced and the interstitial fluxes measured via quantitative plan-view TEM. Germanium was introduced via a second implant at 3keV over a range of doses between 1.7 x1014 cm-2 and 1.4 x1015 cm-2. We show that partial suppression can be observed for sub-monolayer quantities of germanium, and that more than three monolayers of SiGe are necessary to suppress interstitial injection below the detection limit during oxidation. This study shows that low energy implantation of germanium can be used to eliminate or modulate injection of interstitials. Additional results suggest the mechanism is related to stopping interstitial formation, not migration. Figure 1

(Invited) Switchable White Graphene: Electrochemistry of the Boron Nitride Nanomesh

On Rh(111), a monolayer of hexagonal boron nitride (h-BN, isoelectronic with graphene) forms a so-called nanomesh superstructure [1], characterized by a 3.2-nm lattice constant and strong electronic corrugation, which can be used for trapping atoms and molecules [2,3]. Here, we show that hydrogen underpotential deposition (H upd) is easier on h-BN/Rh(111) than on the naked substrate [4], and leads to submonolayer quantities of hydrogen intercalated between the h-BN overlayer and Rh(111), as demonstrated by electrolyte-to-vacuum transfer experiments and thermal desorption spectroscopy. In situ STM measurements reveal that the intercalation lifts the corrugation of the nanomesh, and that this process is fully reversible under potential control. Copper upd is used to quantify the defect density in the nanomesh, and to increase the understanding of the electrochemical window where the nanomesh is stable. By measuring dynamic contact angles of an electrolyte drop (see Figure), we show that the microscopic change within the 2-dimensional material leads to a macroscopic effect related to a 10% change in adsorption energy [4]. The static friction on the other hand, which can be extracted by extending the Young equation for non-equilibrium effects, remains unchanged for the surface in the two states. [1] M. Corso, W. Auwärter, M. Muntwiler, A. Tamai, T. Greber, J. Osterwalder, Science 303 (2004) 217–220.[2] S. Berner, M. Corso, R. Widmer, O. Groening, R. Laskowski, et al. Angew. Chem., Int. Ed. 46 (2007) 5115–5119. [3] H. Dil, J. Lobo-Checa, R. Laskowski, P. Blaha, S. Berner, J. Osterwalder, T. Greber, Science 319 (2008) 1824–1826. [4] S.F.L. Mertens, A. Hemmi, S. Muff, O. Gröning, S. De Feyter, J. Osterwalder, T. Greber, Nature 534 (2016) 676–679. Figure 1

Surface Chemistry of Aromatic Reactants on Pt- and Mo-Modified Pt Catalysts

Supported catalysts containing an oxophilic metal such as Mo and a noble metal such as Pt have shown promising activity and selectivity for deoxygenation of biomass-derived compounds. Here, we report that PtMo catalysts also promote hydrogenolysis of the model compound benzyl alcohol, while decarbonylation is most prevalent over unmodified Pt. A combination of single crystal surface science studies, density functional theory (DFT) calculations, and vapor phase upgrading experiments using supported catalysts was carried out to better understand the mechanism by which Mo promotes deoxygenation. Molybdenum was deposited in submonolayer quantities on a Pt(111) surface and reduced at high temperature. Temperature-programmed desorption (TPD) experiments using benzyl alcohol as a reactant showed greatly enhanced yields of the deoxygenation product toluene at moderate Mo coverages. To understand how the interaction of the aromatic group with the surface influenced this reactivity, we investigated the adsorption o...

Calorimetric measurement of adsorption and adhesion energies of Cu on Pt(111)

The adsorption energies of submonolayer amounts of one metal on the surface of another metal have been measured for decades by temperature programmed desorption. However, that method fails for metals that alloy. We report here the first measurement of the adsorption energy for any such metal-on-metal combination that forms a bulk alloy. The adsorption and interfacial energetics of vapor deposited Cu onto Pt(111) at 300K has been studied using single crystal adsorption calorimetry (SCAC) and X-ray photoelectron spectroscopy (XPS). The Cu grows as 2D pseudomorphic islands in the first layer and its heat of adsorption decreased linearly from 358 to 339kJ/mol. This is attributed to increasing lattice strain with island size, associated with the small lattice mismatch (8%). It adsorbs ~2kJ/mol more weakly in the 2nd layer than above 3ML, where it reaches the bulk heat of sublimation of Cu(solid), 337kJ/mol. The adhesion energy of multilayer Cu onto Pt(111) is 3.76J/m2. The extra stability of the first Cu monolayer compared to bulk Cu measured here is ~12kJ/mol, compared to a difference of ~83kJ/mol for underpotential deposition of Cu on a Pt(111) electrode, with the difference attributed to stronger bonding of Cu to the solvent and double layer compared to Pt.