Structure of the ammonia synthesis catalyst

Catalyst-Free Activation and Fixation of Nitrogen by Laser-Induced Conversion

Ultrafast N2 fixation reactions are quite challenging. Currently used methods for N2 fixation are limited, and strong dinitrogen bonds usually need to be activated via extreme temperature or pressure or by the use of an energy-consuming process with sophisticated catalysts. Herein, we report a novel laser-based chemical method for N2 fixation under ambient conditions without catalysts, this method is called laser bubbling in liquids (LBL), and it directly activates N2 in water (H2O) and efficiently converts N2 into valuable NH3 (max: 4.2 mmol h-1) and NO3- (0.17 mmol h-1). Remarkably, the highest yields of NH3 and NO3- are 4 orders of magnitude greater than the best values for electrocatalysis reported to date. Notably, we further validate the experimental mechanism by using optical emission spectroscopy to detect the production of intermediate plasma and by employing isotope tracing. We also establish that an extremely high-temperature environment far from thermodynamic equilibrium inside a laser-induced bubble and the kinetic process of rapid quenching of bubbles is crucial for N2 activation and fixation to generate NH3 and NOx via LBL. Based on these results, it is shown that LBL is a simple, safe, efficient, green, and sustainable technology that enables the rapid conversion of the renewable feedstocks H2O and N2 to NH3 and NO3-, facilitating new prospects for chemical N2 fixation.

Photocatalytic nitrogen fixation under an ambient atmosphere using a porous coordination polymer with bridging dinitrogen anions

The design of highly electron-active and stable heterogeneous catalysts for the ambient nitrogen reduction reaction is challenging due to the inertness of the N₂ molecule. Here, we report the synthesis of a zinc-based coordination polymer that features bridging dinitrogen anionic ligands, {[Zn(L)(N₂)_0.5(TCNQ-TCNQ)_0.5]·(TCNQ)_0.5}_n (L is tetra(isoquinolin-6-yl)tetrathiafulvalene and TCNQ is tetracyanoquinodimethane), and show that it is an efficient photocatalyst for nitrogen fixation under an ambient atmosphere. It exhibits an ammonia conversion rate of 140 μmol g^-1 h^-1 and functions well also with unpurified air as the feeding gas. Experimental and theoretical studies show that the active [Zn²⁺-(N≡N)^--Zn²⁺] sites can promote the formation of NH₃ and the detachment of the NH₃ formed creates unsaturated [Zn²⁺···Zn⁺] intermediates, which in turn can be refilled by external N₂ sequestration and fast intermolecular electron migration. The [Zn²⁺···Zn⁺] intermediates stabilized by the sandwiched cage-like donor-acceptor-donor framework can sustain continuous catalytic cycles. This work presents an example of a molecular active site embedded within a coordination polymer for nitrogen fixation under mild conditions.

Theoretical Exploration on the Role of Magnetic States to the N2 Fixation behaviors of 2D Transition Metal Tri-borides (TMB3 s)

Fixing nitrogen (N₂ ) by electrosynthesis method has become a promising way to ammonia (NH₃ ) production, nevertheless, developing electrocatalysts combining long-term stable and low-cost feathers are still a great challenge to date. Using comprehensive first-principles calculations, we herein investigate the potential of a new class of two-dimensional (2D) transition metal tri-borides (TMB₃ s) as nitrogen reduction reaction (NRR) electrocatalysts, and explore the effect of magnetic orders on the NRR. Our results show that the TMB₃ s can sufficiently activate N₂ and convert it to NH₃ . Particularly, TiB₃ is identified as a high-efficiency catalyst for NRR because of its low limiting potential (-0.24 V) and good suppression of the competitive hydrogen evolution reaction (HER). For the first time, we present that these TMB₃ s with various magnetic states exhibit different performances in the adsorption of N₂ and NRR intermediates, and minor effect on activation of N₂ . Besides, VB₃ , CrB₃ , MnB₃ , and FeB₃ monolayers possess the superior capacity to suppress surface oxidation via the self-activating process, which reduces ^* O/^* OH into ^* H₂ O under NRR electrochemical conditions, thus favoring the N₂ electroreduction. This work paves the way for finding high-performance NRR catalysts for transition metal borides and pioneering the research of magnetic states effects in NRR.

High-Efficiency Photocatalytic Ammonia Synthesis by Facet Orientation-Supported Heterojunction Cu2O@BiOCl[100] Boosted by Double Built-In Electric Fields

In this work, the advantages of in situ loading, heterojunction construction, and facet regulation were integrated based on the poly-facet-exposed BiOCl single crystal, and a facet-oriented supported heterojunction of Cu₂O and BiOCl was fabricated (Cu₂O@BiOCl[100]). The photocatalytic nitrogen reduction reaction (pNRR) activity of Cu₂O@BiOCl[100] was as high as 181.9 μmol·g^-1·h^-1, which is 4.09, 7.13, and 1.83 times that of Cu₂O, BiOCl, and Cu₂O@BiOCl-ran (Cu₂O randomly supported on BiOCl). Combined with the results of the photodeposition experiment, X-ray photoelectron spectroscopy characterization, and DFT calculation, the mechanism of Cu₂O@BiOCl[100] for pNRR was discussed. When Cu₂O directionally loaded on the [100] facet of BiOCl, electrons generated by Cu₂O will be transmitted to the [100] facet of BiOCl through Z-scheme electron transmission. Due to the directional separation characteristics of charge in BiOCl, the electrons transmitted from Cu₂O are enriched on the [001] facet of BiOCl, which will together with the original electrons generated by pristine BiOCl act on pNRR, thus greatly improving the activity of photocatalytic ammonia synthesis. Thus, a new construction scheme of biphasic semiconductor heterojunction was proposed, which provides a reference research idea for designing and synthesizing high-performance photocatalysts for nitrogen reduction.

Visible-light-driven photocatalytic N2 fixation to nitrates by 2D/2D ultrathin BiVO4 nanosheet/rGO nanocomposites

Photocatalytic nitrogen fixation is a promising approach owing to its environmental friendliness and cost-effectiveness. The 2D/2D BiVO₄/rGO hybrid developed in this study exhibits a high nitrate-production rate of 1.45 mg h^-1 g^-1 and an apparent quantum efficiency (QE) of 0.64% at 420 nm, which represents one of the most highly active photocatalysts reported thus far.

Dual Active Centers Bridged by Oxygen Vacancies of Ru Single Atoms Hybrids Supported on Molybdenum Oxide for Photocatalytic Ammonia Synthesis

Photocatalytic synthesis of ammonia (NH 3 ) holds significant potential compared with the Haber-Bosch process. However, the reported photocatalysts suffered from low efficiency owing to localized electrons deficiency. Here, Ru-SA (single atoms)/H x MoO 3-y hybrids with abundant of Mo n+ (n < 6) species neighboring oxygen vacancies (O V ) are synthesized via a H-spillover process. Detailed characterizations demonstrate that Ru-SA/H x MoO 3 y hybrids can quantitatively produce NH 3 from N 2 and H 2 by the synergetic effect of dual active centers (Ru SA and Mo n+ ). That is, Ru SA boost the activation and migration of H 2 , and Mo n+ species act as the trapping sites of localized electrons and the adsorption and dissociation sites of N 2 , finally leading to NH 3 synthesis on Mo n+ -OH. The NH 3 generation rate is as high as 4.0 mmol h -1 g -1 , accompanied by an apparent quantum efficiency over 6.0% at 650 nm. Our finding may open up a new strategy for acquiring a better NH 3 synthesis approach under mild conditions.

Large-scale stationary hydrogen storage via liquid organic hydrogen carriers

Large-scale stationary hydrogen storage is critical if hydrogen is to fulfill its promise as a global energy carrier. While densified storage via compressed gas and liquid hydrogen is currently the dominant approach, liquid organic molecules have emerged as a favorable storage medium because of their desirable properties, such as low cost and compatibility with existing fuel transport infrastructure. This perspective article analytically investigates hydrogenation systems' technical and economic prospects using liquid organic hydrogen carriers (LOHCs) to store hydrogen at a large scale compared to densified storage technologies and circular hydrogen carriers (mainly ammonia and methanol). Our analysis of major system components indicates that the capital cost for liquid hydrogen storage is more than two times that for the gaseous approach and four times that for the LOHC approach. Ammonia and methanol could be attractive options as hydrogen carriers at a large scale because of their compatibility with existing liquid fuel infrastructure. However, their synthesis and decomposition are energy and capital intensive compared to LOHCs. Together with other properties such as safety, these factors make LOHCs a possible option for large-scale stationary hydrogen storage. In addition, hydrogen transportation via various approaches is briefly discussed. We end our discussions by identifying important directions for future research on LOHCs.

3D hollow Bi2O3@CoAl-LDHs direct Z-scheme heterostructure for visible-light-driven photocatalytic ammonia synthesis

In this paper, the novel 3D hollow Z-scheme heterojunction photocatalysts based on Bi₂O₃ and CoAl layered double hydroxides (Bi₂O₃@CoAl-LDHs) were prepared for efficient visible-light-driven photocatalytic ammonia synthesis. The synthesized nanohybrid exhibits excellent photocatalytic ammonia synthesis performance (48.7 μmol·L^-1·h^-1) and structural stability, which is primarily attributed to the fact that Z-scheme heterojunction significantly enhanced lifetime of photogenerated carriers (6.22 ns) and transfer efficiency of surface photogenerated electrons (72.5%). Strict control experiments and nitrogen isotope labeling results show that nitrogen and hydrogen in the produced ammonia come from nitrogen and water in the reactant respectively. Electron paramagnetic resonance (EPR) experiments and density functional theory (DFT) calculations further reveal that the built-in electric field due to the difference between Bi₂O₃ and CoAl-LDHs is the key to constructing the Z-scheme heterojunction. In addition, results of partial density of states (PDOS) show that Co in Bi₂O₃@CoAl-LDHs composite is the active site for photocatalytic N₂ fixation.

Exploring the N2 Adsorption and Activation Mechanisms over the 2H/1T Mixed-Phase Ultrathin Mo1-xWxS2 Nanosheets for Boosting N2 Photosynthesis

Solar-driven conversion of nitrogen (N₂) to ammonia (NH₃) is highly appealing, yet in its infancy, the low photocatalytic efficiency and unclear adsorption and activation mechanisms of N₂ are still issues to be addressed. In this study, ultrathin alloyed Mo_1-xW_xS₂ nanosheets with tunable hexagonal (2H)/trigonal (1T) phase ratios were proposed to boost photoreduction N₂ efficiency, while the mechanisms of N₂ adsorption and activation were explored simultaneously. The alloyed Mo_1-xW_xS₂ nanosheets for the 1T phase concentration of 33.6% and Mo/W = 0.68:0.32 were proven to reach about 111 μmol g_cat^-1 h^-1 under visible light, which is 3.7 (or 3)-fold higher than that of pristine MoS₂ (or WS₂). With the aid of density functional theory calculations and in situ N₂ adsorption X-ray absorption near-edge fine structure techniques, the adsorption and activation behaviors of N₂ over the interface of Mo_1-xW_xS₂ nanosheets were investigated during the N₂ reduction process. The results show that the W doping causes a higher electron density state in W 5d orbitals, which can further polarize the adsorbed N₂ molecules for adsorption and activation. This work provides a new insight into the adsorption and activation mechanisms for the NH₃ synthesis.

Controlling the Shapes of Nanoparticles by Dopant-Induced Enhancement of Chemisorption and Catalytic Activity: Application to Fe-Based Ammonia Synthesis

We showed recently that the catalytic efficiency of ammonia synthesis on Fe-based nanoparticles (NP) for Haber-Bosch (HB) reduction of N₂ to ammonia depends very dramatically on the crystal surface exposed and on the doping. In turn, the stability of each surface depends on the stable intermediates present during the catalysis. Thus, under reaction conditions, the shape of the NP is expected to evolve to optimize surface energies. In this paper, we propose to manipulate the shape of the nanoparticles through doping combined with chemisorption and catalysis. To do this, we consider the relationships between the catalyst composition (adding dopant elements) and on how the distribution of the dopant atoms on the bulk and facet sites affects the shape of the particles and therefore the number of active sites on the catalyst surfaces. We use our hierarchical, high-throughput catalyst screening (HHTCS) approach but extend the scope of HHTCS to select dopants that can increase the catalytically active surface orientations, such as Fe-bcc(111), at the expense of catalytically inactive facets, such as Fe-bcc(100). Then, for the most promising dopants, we predict the resulting shape and activity of doped Fe-based nanoparticles under reaction conditions. We examined 34 possible dopants across the periodic table and found 16 dopants that can potentially increase the fraction of active Fe-bcc(111) vs inactive Fe-bcc(100) facets. Combining this reshaping criterion with our HHTCS estimate of the resulting catalytic performance, we show that Si and Ni are the most promising elements for improving the rates of catalysis by optimizing the shape to decrease reaction barriers. Then, using Si dopant as a working example, we build a steady-state dynamical Wulff construction of Si-doped Fe bcc nanoparticles. We use nanoparticles with a diameter of ∼10 nm, typical of industrial catalysts. We predict that doping Si into such Fe nanoparticles at the optimal atomic content of ∼0.3% leads to rate enhancements by a factor of 56 per nanoparticle under target HB conditions.

Ammonia Decomposition Enhancement by Cs-Promoted Fe/Al2O3 Catalysts

Abstract

A range of Cs-doped Fe/Al2O3 catalysts were prepared for the ammonia decomposition reaction. Through time on-line studies it was shown that at all loadings of Cs investigated the activity of the Fe/Al2O3 catalysts was enhanced, with the optimum Cs:Fe being ca. 1. Initially, the rate of NH3 decomposition was low, typically < 10% equilibrium conversion (99.7%@500°C) recorded after 1 h. All catalysts exhibited an induction period (typically ca. 10 h) with the conversion reaching a high of 67% equilibrium conversion for Cs:Fe = 0.5 and 1. The highest rate of decomposition observed was attributed to the balance between increasing the concentration of Cs without blocking the active site. Analysis of H2-TPR and XPS measurements indicated that Cs acts as an electronic promoter. Previously, Cs has been shown to act as a promoter for Ru, where Cs alters the electron density of the active site, thereby facilitating the recombination of N2 which is considered the rate determining step. In addition, XRD and N2 adsorption measurements suggest that with higher Cs loadings deactivation of the catalytic activity is due to a layer of CsOH that forms on the surface and blocks active sites.

Graphic Abstract

Investigation of N2 adsorption on Fe3O4(001) using ambient pressure X-ray photoelectron spectroscopy and density functional theory

Reactions on iron oxide surfaces are prevalent in various chemical processes from heterogeneous catalysts to minerals. Nitrogen (N₂) is known to dissociate on iron surfaces, a precursor for ammonia production in the Haber-Bosch process, where the dissociation of N₂ is the limiting step in the reaction under equilibrium conditions. However, little is known about N₂ adsorption on other iron-based materials, such as iron oxide surfaces that are ubiquitous in soils, steel pipelines, and other industrial materials. An atomistic description is reported for the binding of N₂ on the Fe₃O₄(001) surface using first principles calculations with ambient pressure X-ray photoelectron spectroscopy. Two primary adsorption sites are experimentally identified from N₂ dissociation on Fe₃O₄(001). The electronic signatures associated with the valence band region unambiguously show how the electronic structure of magnetite transforms near ambient pressures due to the binding of atomic nitrogen to different surface sites. Overall, the experimental and theoretical results of our study bridge the gap between ultra-high vacuum studies and reaction conditions to provide insight into other nitrogen-based chemistry on iron oxide surfaces that impact the agriculture and energy industries.

Pothole-rich Ultrathin WO3 Nanosheets that Trigger N≡N Bond Activation of Nitrogen for Direct Nitrate Photosynthesis

Nitrate is a raw ingredient for the production of fertilizer, gunpowder, and explosives. Developing an alternative approach to activate the N≡N bond of naturally abundant nitrogen to form nitrate under ambient conditions will be of importance. Herein, pothole-rich WO₃ was used to catalyse the activation of N≡N covalent triple bonds for the direct nitrate synthesis at room temperature. The pothole-rich structure endues the WO₃ nanosheet more dangling bonds and more easily excited high momentum electrons, which overcome the two major bottlenecks in N≡N bond activation, that is, poor binding of N₂ to catalytic materials and the high energy involved in this reaction. The average rate of nitrate production is as high as 1.92 mg g^-1  h^-1 under ambient conditions, without any sacrificial agent or precious-metal co-catalysts. More generally, the concepts will initiate a new pathway for triggering inert catalytic reactions.

Activity enhancement of cobalt catalysts by tuning metal-support interactions

Interactions between metal nanoparticles and support materials can strongly influence the performance of catalysts. In particular, reducible oxidic supports can form suboxides that can decorate metal nanoparticles and enhance catalytic performance or block active sites. Therefore, tuning this metal-support interaction is essential for catalyst design. Here, we investigate reduction-oxidation-reduction (ROR) treatments as a method to affect metal-support interactions and related catalytic performance. Controlled oxidation of pre-reduced cobalt on reducible (TiO₂ and Nb₂O₅) and irreducible (α-Al₂O₃) supports leads to the formation of hollow cobalt oxide particles. The second reduction results in a twofold increase in cobalt surface area only on reducible oxides and proportionally enhances the cobalt-based catalytic activity during Fischer-Tropsch synthesis at industrially relevant conditions. Such activities are usually only obtained by noble metal promotion of cobalt catalysts. ROR proves an effective approach to tune the interaction between metallic nanoparticles and reducible oxidic supports, leading to improved catalytic performance.

Tuning metal-support interaction can strongly influence the performance of a catalyst, and is thus essential for catalyst design. Here, the authors investigate reduction-oxidation-reduction treatments as a method to affect metal-support interactions of cobalt-based catalysts in Fischer-Tropsch synthesis.

Kinetics in the real world: linking molecules, processes, and systems

Unravelling elementary steps, reaction pathways, and kinetic mechanisms is key to understanding the behaviour of many real-world chemical systems that span from the troposphere or even interstellar media to engines and process reactors. Recent work in chemical kinetics provides detailed information on the reactive changes occurring in chemical systems, often on the atomic or molecular scale. The optimisation of practical processes, for instance in combustion, catalysis, battery technology, polymerisation, and nanoparticle production, can profit from a sound knowledge of the underlying fundamental chemical kinetics. Reaction mechanisms can combine information gained from theory and experiments to enable the predictive simulation and optimisation of the crucial process variables and influences on the system's behaviour that may be exploited for both monitoring and control. Chemical kinetics, as one of the pillars of Physical Chemistry, thus contributes importantly to understanding and describing natural environments and technical processes and is becoming increasingly relevant for interactions in and with the real world.

Heterogeneous Fe3 single-cluster catalyst for ammonia synthesis via an associative mechanism

The current industrial ammonia synthesis relies on Haber–Bosch process that is initiated by the dissociative mechanism, in which the adsorbed N₂ dissociates directly, and thus is limited by Brønsted–Evans–Polanyi (BEP) relation. Here we propose a new strategy that an anchored Fe₃ cluster on the θ-Al₂O₃(010) surface as a heterogeneous catalyst for ammonia synthesis from first-principles theoretical study and microkinetic analysis. We have studied the whole catalytic mechanism for conversion of N₂ to NH₃ on Fe₃/θ-Al₂O₃(010), and find that an associative mechanism, in which the adsorbed N₂ is first hydrogenated to NNH, dominates over the dissociative mechanism, which we attribute to the large spin polarization, low oxidation state of iron, and multi-step redox capability of Fe₃ cluster. The associative mechanism liberates the turnover frequency (TOF) for ammonia production from the limitation due to the BEP relation, and the calculated TOF on Fe₃/θ-Al₂O₃(010) is comparable to Ru B5 site.

The current industrial ammonia synthesis relies on the Haber-Bosch process that is limited by the Brønsted–Evans–Polanyi relation. Here, the authors propose a new strategy that an anchored Fe₃ on θ-Al₂O₃(010) surface serves as a heterogeneous single cluster catalyst for ammonia synthesis from first-principles calculations and microkinetic analysis.

Hydrogen abstraction from metal surfaces: when electron-hole pair excitations strongly affect hot-atom recombination

Using molecular dynamics simulations, we predict that the inclusion of nonadiabatic electronic excitations influences the dynamics of preadsorbed hydrogen abstraction from the W(110) surface by hydrogen scattering. The hot-atom recombination, which involves hyperthermal diffusion of the impinging atom on the surface, is significantly affected by the dissipation of energy mediated by electron-hole pair excitations at low coverage and low incidence energy. This issue is of importance as this abstraction mechanism is thought to largely contribute to molecular hydrogen formation from metal surfaces.

Light-Switchable Oxygen Vacancies in Ultrafine Bi5 O7 Br Nanotubes for Boosting Solar-Driven Nitrogen Fixation in Pure Water

Solar-driven reduction of dinitrogen (N₂ ) to ammonia (NH₃ ) is severely hampered by the kinetically complex and energetically challenging multielectron reaction. Oxygen vacancies (OVs) with abundant localized electrons on the surface of bismuth oxybromide-based semiconductors are demonstrated to have the ability to capture and activate N₂ , providing an alternative pathway to overcome such limitations. However, bismuth oxybromide materials are susceptible to photocorrosion, and the surface OVs are easily oxidized and therefore lose their activities. For realistic photocatalytic N₂ fixation, fabricating and enhancing the stability of sustainable OVs on semiconductors is indispensable. This study shows the first synthesis of self-assembled 5 nm diameter Bi₅ O₇ Br nanotubes with strong nanotube structure, suitable absorption edge, and many exposed surface sites, which are favorable for furnishing sufficient visible light-induced OVs to realize excellent and stable photoreduction of atmospheric N₂ into NH₃ in pure water. The NH₃ generation rate is as high as 1.38 mmol h^-1 g^-1 , accompanied by an apparent quantum efficiency over 2.3% at 420 nm. The results presented herein provide new insights into rational design and engineering for the creation of highly active catalysts with light-switchable OVs toward efficient, stable, and sustainable visible light N₂ fixation in mild conditions.

Photo-illuminated diamond as a solid-state source of solvated electrons in water for nitrogen reduction

The photocatalytic reduction of N₂ to NH₃ is typically hampered by poor binding of N₂ to catalytic materials and by the very high energy of the intermediates involved in this reaction. Solvated electrons directly introduced into the reactant solution can provide an alternative pathway to overcome such limitations. Here we demonstrate that illuminated hydrogen-terminated diamond yields facile electron emission into water, thus inducing reduction of N₂ to NH₃ at ambient temperature and pressure. Transient absorption measurements at 632 nm reveal the presence of solvated electrons adjacent to the diamond after photoexcitation. Experiments using inexpensive synthetic diamond samples and diamond powder show that photocatalytic activity is strongly dependent on the surface termination and correlates with the production of solvated electrons. The use of diamond to eject electrons into a reactant liquid represents a new paradigm for photocatalytic reduction, bringing electrons directly to reactants without requiring molecular adsorption to the surface.

Surface temperature effects on the dynamics of N2 Eley-Rideal recombination on W(100)

Quasiclassical trajectories simulations are performed to study the influence of surface temperature on the dynamics of a N atom colliding a N-preadsorbed W(100) surface under normal incidence. A generalized Langevin surface oscillator scheme is used to allow energy transfer between the nitrogen atoms and the surface. The influence of the surface temperature on the N(2) formed molecules via Eley-Rideal recombination is analyzed at T = 300, 800, and 1500 K. Ro-vibrational distributions of the N(2) molecules are only slightly affected by the presence of the thermal bath whereas kinetic energy is rather strongly decreased when going from a static surface model to a moving surface one. In terms of reactivity, the moving surface model leads to an increase of atomic trapping cross section yielding to an increase of the so-called hot atoms population and a decrease of the direct Eley-Rideal cross section. The energy exchange between the surface and the nitrogen atoms is semi-quantitatively interpreted by a simple binary collision model.

Dynamics simulation of N(2) scattering onto W(100,110) surfaces: A stringent test for the recently developed flexible periodic London-Eyring-Polanyi-Sato potential energy surface

An efficient method to construct the six dimensional global potential energy surface (PES) for two atoms interacting with a periodic rigid surface, the flexible periodic London-Eyring-Polanyi-Sato model, has been proposed recently. The main advantages of this model, compared to state-of-the-art interpolated ab initio PESs developed in the past, reside in its global nature along with the small number of electronic structure calculations required for its construction. In this work, we investigate to which extent this global representation is able to reproduce the fine details of the scattering dynamics of N(2) onto W(100,110) surfaces reported in previous dynamics simulations based on locally interpolated PESs. The N(2)/W(100) and N(2)/W(110) systems are chosen as benchmarks as they exhibit very unusual and distinct dissociative adsorption dynamics although chemically similar. The reaction pathways as well as the role of dynamic trapping are scrutinized. Besides, elastic/inelastic scattering dynamics including internal state and angular distributions of reflected molecules are also investigated. The results are shown to be in fair agreement with previous theoretical predictions.

High-pressure scanning tunneling microscopy: tip reactions

Scanning tunneling microscopy (STM) is an ideal tool to image conducting and semiconducting surfaces with atomic resolution. The technique provides high-resolution images in vacuum or even high-pressure environments. Since STM can be operated at elevated pressures and temperatures, images can be collected in situ under catalytic conditions. In this work, we demonstrate that artifacts can be observed when imaging in situ since reactions can occur on the tip, and care should be taken when analyzing the data obtained.