Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid.

Uncommon metal oxidation states in porphyrinoid cofactors are responsible for the activity of many enzymes. The F430 and P450nor co-factors, with their reduced NiI - and FeIII -containing tetrapyrrolic cores, are prototypical examples of biological systems involved in methane formation and in the reduction of nitric oxide, respectively. Herein, using a comprehensive range of experimental and theoretical methods, we raise evidence that nickel tetraphenyl porphyrins deposited in vacuo on a copper surface are reactive towards nitric oxide disproportionation at room temperature. The interpretation of the measurements is far from being straightforward due to the high reactivity of the different nitrogen oxides species (eventually present in the residual gas background) and of the possible reaction intermediates. The picture is detailed in order to disentangle the challenging complexity of the system, where even a small fraction of contamination can change the scenario.

Room-Temperature On-Spin-Switching and Tuning in a Porphyrin-Based Multifunctional Interface.

Molecular interfaces formed between metals and molecular compounds offer a great potential as building blocks for future opto-electronics and spintronics devices. Here, a combined theoretical and experimental spectro-microscopy approach is used to show that the charge transfer occurring at the interface between nickel tetraphenyl porphyrins and copper changes both spin and oxidation states of the Ni ion from [Ni(II), S = 0] to [Ni(I), S = 1/2]. The chemically active Ni(I), even in a buried multilayer system, can be functionalized with nitrogen dioxide, allowing a selective tuning of the electronic properties of the Ni center that is switched to a [Ni(II), S = 1] state. While Ni acts as a reversible spin switch, it is found that the electronic structure of the macrocycle backbone, where the frontier orbitals are mainly localized, remains unaffected. These findings pave the way for using the present porphyrin-based system as a platform for the realization of multifunctional devices where the magnetism and the optical/transport properties can be controlled simultaneously by independent stimuli.

Self-Metalation of Porphyrins at the Solid-Gas Interface.

Self-metalation is a promising route to include a single metal atom in a tetrapyrrolic macrocycle in organic frameworks supported by metal surfaces. The molecule-surface interaction may provide the charge transfer and the geometric distortion of the molecular plane necessary for metal inclusion. However, at a metal surface the presence of an activation barrier can represent an obstacle that cannot be compensated by a higher substrate temperature without affecting the layer integrity. The formation of the intermediate state can be facilitated in some cases by oxygen pre-adsorption at the supporting metal surface, like in the case of 2H-TPP/Pd(100). In such cases, the activation barrier can be overcome by mild annealing, yielding the formation of desorbing products and of the metalated tetrapyrrole. We show here that the self-metalation of 2H-TPP at the Pd(100) surface can be promoted already at room temperature by the presence of an oxygen gas phase at close-to-ambient conditions via an Eley-Rideal mechanism.

Tunability of the CO adsorption energy on a Ni/Cu surface: Site change and coverage effects.

The adsorption energy of carbon monoxide on Ni ad-islands and ultra-thin films grown on the Cu(110) surface can be finely tuned via a complex interplay among diffusion, site change mechanisms, and coverage effects. The observed features of CO desorption can be explained in terms of migration of CO molecules from Cu to Ni islands, competition between bridge and on-top adsorption sites, and repulsive lateral adsorbate-adsorbate interactions. While the CO adsorption energy on clean Cu(110) is of the order of 0.5 eV, Ni-alloying allows for its controlled, continuous tunability in the 0.98-1.15 eV range with Ni coverage. Since CO is a fundamental reactant and intermediate in many heterogeneous catalytic (electro)-conversion reactions, insight into these aspects with atomic level detail provides useful information to potentially drive applicative developments. The tunability range of the CO adsorption energy that we measure is compatible with the already observed tuning of conversion rates by Ni doping of Cu single crystal catalysts for methanol synthesis from a CO2, CO, and H2 stream under ambient pressure conditions.

Reverse Water-Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure.

The interaction of CO, CO2, CO + H2, CO2 + H2, and CO + CO2 + H2 with the nickel (110) single crystal termination has been investigated at 10(-1) mbar in situ as a function of the surface temperature in the 300-525 K range by means of infrared-visible sum frequency generation (IR-vis SFG) vibrational spectroscopy and by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Several stable surface species have been observed and identified. Besides atomic carbon and precursors for graphenic C phases, five nonequivalent CO species have been distinguished, evidencing the role of coadsorption effects with H and C atoms, of H-induced activation of CO, and of surface reconstruction. At low temperature, carbonate species produced by the interaction of CO2 with atomic oxygen, which stems from the dissociation of CO2 into CO + O, are found on the surface. A metastable activated CO2(-) species is also detected, being at the same time a precursor state toward dissociation into CO and O in the reverse water-gas shift mechanism and a reactive species that undergoes direct conversion in the Sabatier methanation process. Finally, the stability of ethylidyne is deduced on the basis of our spectroscopic observations.

Carbon dioxide reduction on Ir(111): stable hydrocarbon surface species at near-ambient pressure.

Stable hydrocarbon surface species in the carbon dioxide hydrogenation reaction on Ir(111) were identified by means of infrared-visible sum-frequency generation vibrational spectroscopy and X-ray photoelectron spectroscopy under near-ambient pressure conditions (0.1 mbar). By introducing gas phase binary and ternary mixtures of CO2, CO, and H2 into the reaction chamber, stable ethylidyne and ethynyl species were found at the metal surface above 425 K, in remarkable analogy with that observed during the ethylene decomposition process yielding graphene. In addition, upon increasing temperature (up to 600 K depending on the reaction conditions), vibrational and electronic spectroscopic fingerprints appeared that could be attributed to the nucleation of aromatic hydrocarbons at the edge of metastable graphenic clusters interacting with the metal surface.

Towards optimal seeding for the synthesis of ordered nanoparticle arrays on alumina/Ni3Al(111).

The adsorption and the nucleation of different transition metals (Fe, Co, Ni, Cu, Pd, Ag, and Au) on alumina/Ni3Al(111) have been studied to shed light on the first stages of the synthesis of supported nanoparticles, focusing in particular on the possibility of producing ordered arrays. Affinity for oxygen, atomic radii, electronic properties and kinetics have been taken into account to rationalize the different behavior. In agreement with empirical findings, Pd is confirmed to be the best choice for a highly ordered nucleation following the "dot" superstructure of the alumina, due to a remarkable preference for the corresponding adsorption sites (holes) with respect to others, and for a rather strong binding. Atom by atom nucleation of this material has been studied, for seeds up to 6 atoms that offer a stiff anchoring of nanoparticles to the support.

Self-seeded nucleation of Cu nanoclusters on Al2O3/Ni3Al(111): an ab initio investigation.

The mechanisms of seeding and nucleation of Cu nanoclusters onto an ultrathin alumina template supported on Ni3Al(111) has been investigated by means of ab initio calculations. Single Cu ad-atom diffusion on the oxide film is effective at room temperature, allowing preferential occupation of the defective sites of the so-called "dot" structure, where the adsorption is much stronger than in the "network" or any other surface site of the oxide. After the adsorption of the first Cu atom, further nucleation at the "dot" sites proceeds with the formation of multi-atomic seeds (with up to 6 atoms contained in the defect) that offer stiff anchoring for larger clusters. The whole process is thermodynamically favoured. We therefore clearly confirm and rationalize some experimental evidence showing that the ultrathin Al2O3/Ni3Al(111) is an efficient template for the growth of highly ordered arrays of small Cu nanoparticles.

Stabilization versus competing de-metalation, trans-metalation and (cyclo)-dehydrogenation of Pd porphyrins at a copper surface.

Metal-porphyrins are studied intensively due their potential applications, deriving from the variety of electronic and chemical properties, tunable by selecting metal centers and functional groups. Metalation, de- and trans-metalation processes are fundamental in this sense to investigate both the synthesis and the stability of these molecular building blocks. More specifically, Pd coordination in tetrapyrroles revealed to be potentially interesting in the fields of cancer therapy, drug delivery and light harvesting. Thus, we focused on the stability of palladium tetraphenyl porphyrins (PdTPPs) on a copper surface by means of combined spectroscopy and microscopy approaches. We find that PdTPPs undergo coverage-dependent trans-metalation accompanied by steric rearrangements already at room temperature, and fully trans-metalate to CuTPPs upon mild annealing. Side reactions such as (cyclo)-dehydrogenation and structural reorganization affect the molecular layer, with Pd-Cu alloying and segregation occurring at higher temperature. Instead, oxygen passivation of the Cu support prevents the metal-involving reactions, thus preserving the layer and increasing the chemical and temperature stability of the Pd porphyrins.

Growth and Redox Properties of Boron on Al(111): Competing Affinities in the Case of Honeycomb AlB2.

The complexity of the geometric and electronic structure of boron allotropes is associated with the multicentric bonding character and the consequent B polymorphism. When growth is limited to two-dimensions (2D), the structural and electronic confinement yields the borophenes family, where the interaction with the templating substrate actually determines the stability of inequivalent boron phases. We report here a detailed study of the growth of the honeycomb AlB2 phase on Al(111), followed by an investigation of its oxidation and reduction properties. By means of a combined experimental and theoretical approach, we show that the structure of the B/Al interface is affected by the complex interplay between B, Al, and common reactive agents like oxygen and hydrogen. While kinetic effects associated with diffusion and strain release influence the AlB2 growth in vacuo, Al, B, O, and H chemical affinities determine its redox behavior. Reduction with atomic hydrogen involves the B layer and yields an ordered honeycomb borophane H/AlB2 phase. Instead, oxidation takes place at the Al interface, giving origin to buried and 1D surface aluminum oxide phases.

Blue phosphorene on Au(111): theoretical, spectroscopic and diffraction analysis reveal the role of single Au adatoms.

In investigating the monoatomic layers of P, several stable two-dimensional (2D) allotropes have been theoretically predicted. Among them, single-layer blue phosphorus (BlueP) appears to deliver promising properties. After initial success, where the structure of BlueP triangular patches on Au(111) was conceived on the basis of scanning tunneling microscopy (STM) and density functional theory (DFT), the surface structure model was revisited multiple times with increasing accuracy and insight of theoretical calculations and experimental datasets. Interestingly, the quest for a reliable atomic structure model of BlueP on Au(111) turned out to be very contentious and challenging, particularly considering the possible incorporation of Au atoms in the 2D sheet of P. This article proposes an extended report on theoretical findings that can be extracted from DFT calculations of the orbital projected band structure and employed for an efficient comparison protocol between the calculations and experimental datasets obtained from angle-resolved photoemission spectroscopy (ARPES). The findings, together with experimental and simulated data from STM imaging and surface X-ray diffraction (SXRD), show a clear way to verify the presence and characterize the stabilizing effect of foreign atoms in 2D materials.

From borophene polymorphs towards a single honeycomb borophane phase: reduction of hexagonal boron layers on Al(111).

The recent interest in characterizing 2D boron polymorphs has led to claims of the first stabilization of a honeycomb phase with conical Dirac-like electron dispersion. However, the synthesis of chemically stable, single, and homogeneous 2D boron phases still represents a significant experimental challenge. This is ascribed to the intrinsic boron electronic configuration that, at variance with carbon, leads to the formation of multi-center covalent bonds. External charge compensation by substrate-induced doping can steer the geometry of the layer, both in the buckling and in the density of B vacancies, like in the case of the recently achieved stabilization of honeycomb boron layers on Al(111). The price to pay is however a strong boron-support interaction, resulting in general in a limiting kinetic hindrance with respect to the synthesis of homogenous single phases. In the specific case of Al(111) an AlB2 layer is known to form at the surface, quite far from a desirable quasi-freestanding borophene monolayer and at variance with graphene, which can be easily synthesized in an almost freestanding configuration e.g. on Ir(111). We provide here evidence for the (reversible) formation of well-ordered honeycomb borophane upon hydrogenation of the honeycomb boron phase on Al(111).

Tailoring bimetallic alloy surface properties by kinetic control of self-diffusion processes at the nanoscale.

Achieving control of the nanoscale structure of binary alloys is of paramount importance for the design of novel materials with specific properties, leading to, for example, improved reaction rates and selectivity in catalysis, tailored magnetic behavior in electronics, and controlled growth of nanostructured materials such as graphene. By means of a combined experimental and theoretical approach, we show that the complex self-diffusion mechanisms determining these key properties can be mostly defined by kinetic rather than energetic effects. We explain how in the Ni-Cu system nanoscale control of self-diffusion and segregation processes close to the surface can be achieved by finely tuning the relative concentration of the alloy constituents. This allows tailoring the material functionality and provides a clear explanation of previously observed effects involved, for example, in the growth of graphene films and in the catalytic reduction of carbon dioxide.

Black or red phosphorus yields the same blue phosphorus film.

After the discovery of graphene, many other 2D materials have been predicted theoretically and successfully prepared. In this context, single-sheet black phosphorus - phosphorene - is emerging as a viable contender in the field of (2D) semiconductors. Phosphorene offers high carrier mobility and an anisotropic structure that gives rise to a modulation of physical and chemical properties. This opens the way to many novel and fascinating applications related to field-effect transistors and optoelectronic devices. In previous studies, a single layer of blue phosphorene intermixed with Au atoms was grown using purified black phosphorus as a precursor. Starting from the observation that phosphorus vapor mainly consists of P clusters, in this work we aimed at obtaining blue phosphorus using much less expensive purified red phosphorus as an evaporant. By means of microscopy, spectroscopy and diffraction experiments, we show that black or red phosphorus deposition on Au(111) substrates yields the same blue phosphorus film.

Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid.

Uncommon metal oxidation states in porphyrinoid cofactors are responsible for the activity of many enzymes. The F430 and P450nor co-factors, with their reduced NiI- and FeIII-containing tetrapyrrolic cores, are prototypical examples of biological systems involved in methane formation and in the reduction of nitric oxide, respectively. Herein, using a comprehensive range of experimental and theoretical methods, we raise evidence that nickel tetraphenyl porphyrins deposited in vacuo on a copper surface are reactive towards nitric oxide disproportionation at room temperature. The interpretation of the measurements is far from being straightforward due to the high reactivity of the different nitrogen oxides species (eventually present in the residual gas background) and of the possible reaction intermediates. The picture is detailed in order to disentangle the challenging complexity of the system, where even a small fraction of contamination can change the scenario.

Stabilization and activation of molecular oxygen at biomimetic tetrapyrroles on surfaces: from UHV to near-ambient pressure.

Recent advances in the development of surface science methods have allowed bridging, at least partially, the pressure gap between the ultra-high vacuum environment and some applicative conditions. This step has been particularly critical for the characterization of heterogenous catalytic systems (solid-liquid, solid-gas interfaces) and, specifically, of the electronic, structural, and chemical properties of tetrapyrroles at surfaces when arranged in 2D networks. Within a biomimetic picture, in which 2D metalorganic frameworks are expected to model and reproduce in a tailored way the activity of their biochemical proteic counterparts, the fundamental investigation of the adsorption and activation of small ligands at the single-metal atom reaction sites has progressively gained increasing attention. Concerning oxygen, biology offers a variety of tetrapyrrole-based transport and reaction pockets, as e.g. in haemoglobin, myoglobin or cytochrome proteins. Binding and activation of O2 are accomplished thanks to complex charge transfer and spin realignment processes, sometimes requiring cooperative mechanisms. Within the framework of surface science at near-ambient pressure (towards and beyond the mbar regime), recent progress has unveiled novel and interesting properties of 2D metalorganic frameworks and heterostacks based on self-assembled tetrapyrroles, thus opening possible, effective applicative routes in the fields of light harvesting, heterogenous (electro-)catalysts, chemical sensing, and spintronics.

Interplay between CO Disproportionation and Oxidation: On the Origin of the CO Reaction Onset on Atomic Layer Deposition-Grown Pt/ZrO2 Model Catalysts.

Pt/ZrO2 model catalysts were prepared by atomic layer deposition (ALD) and examined at mbar pressure by operando sum frequency generation (SFG) spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) combined with differentially pumped mass spectrometry (MS). ALD enables creating model systems ranging from Pt nanoparticles to bulk-like thin films. Polarization-dependent SFG of CO adsorption reveals both the adsorption configuration and the Pt particle morphology. By combining experimental data with ab initio density functional theory (DFT) calculations, we show that the CO reaction onset is determined by a delicate balance between CO disproportionation (Boudouard reaction) and oxidation. CO disproportionation occurs on low-coordinated Pt sites, but only at high CO coverages and when the remaining C atom is stabilized by a favorable coordination. Thus, under the current conditions, initial CO oxidation is found to be strongly influenced by the removal of carbon deposits formed through disproportionation mechanisms rather than being determined by the CO and oxygen inherent activity. Accordingly, at variance with the general expectation, rough Pt nanoparticles are seemingly less active than smoother Pt films. The applied approach enables bridging both the "materials and pressure gaps".

Large interlayer relaxation at a metal-oxide interface: the case of a supported ultrathin alumina film.

The structure of the metal-oxide interface in the alumina/Ni{3}Al(111) system is investigated by comparing backscattering and forward-scattering photoelectron diffraction modulation functions of chemically nonequivalent aluminum and oxygen species with multiple-scattering simulations. We observe large relaxation effects at the metal-oxide interface layer: Al atoms of the Ni3Al alloy surface are lifted by more than 0.7 A above the ideal termination, thus creating a new, metallic layer between the oxide and the alloy. The effect of the interface atomic rearrangement on the properties of the supported ultrathin alumina oxide film is discussed.

The Rh oxide ultrathin film on Rh(100): an x-ray photoelectron diffraction study.

The surface and interface structure of the RhO(2) ultrathin film grown on Rh(100) is investigated by means of x-ray photoelectron diffraction. Experimental and simulated one- and two-dimensional angular distribution intensities of the O1s and Rh3d(5/2) chemically shifted core levels are quantitatively analyzed. The previously proposed O-Rh-O trilayer model is independently confirmed. A rippled buckling of the metal surface is observed at the oxide-metal interface, with a mean interfacial Rh-O distance which is 0.2 Å larger with respect to previous findings. The link between the local atomic rearrangement and the overall geometric and electronic properties of the oxide is discussed on the basis of a thorough comparison with the corresponding RhO(2) rutile structure.

Carbon dioxide hydrogenation on Ni(110).

We demonstrate that the key step for the reaction of CO 2 with hydrogen on Ni(110) is a change of the activated molecule coordination to the metal surface. At 90 K, CO 2 is negatively charged and chemically bonded via the carbon atom. When the temperature is increased and H approaches, the H-CO 2 complex flips and binds to the surface through the two oxygen atoms, while H binds to the carbon atom, thus yielding formate. We provide the atomic-level description of this process by means of conventional ultrahigh vacuum surface science techniques combined with density functional theory calculations and corroborated by high pressure reactivity tests. Knowledge about the details of the mechanisms involved in this reaction can yield a deeper comprehension of heterogeneous catalytic organic synthesis processes involving carbon dioxide as a reactant. We show why on Ni the CO 2 hydrogenation barrier is remarkably smaller than that on the common Cu metal-based catalyst. Our results provide a possible interpretation of the observed high catalytic activity of NiCu alloys.