SPM Systems

Despite tremendous importance in catalysis, the design and improvement of the oxide- metal interface has been hampered by the limited understanding on the nature of interfacial sites, as well as the oxide-metal interaction (OMI). Through the construction of well-defined Cu₂O-Pt, Cu₂O-Ag, Cu₂O-Au interfaces, we found that Cu₂O Nanostructures (NSs) on Pt exhibit much lower thermal stability than on Ag and Au, although they show the same surface and edge structures, as identified by element-specific scanning tunneling microscopy (ES-STM) images. The activities of the Cu₂O-Pt and Cu₂O-Au interfaces for CO oxidation were further compared at the atomic scale and showed in general that the interface with Cu₂O NSs could annihilate the CO-poisoning problem suffered by Pt group metals and enhance the interaction with O₂, which is a limiting step for CO oxidation catalysis on group IB metals. While both interfaces could react with CO at room temperature, the OMI was found to determine the reactivity of supported Cu₂O NSs by 1) tuning the activity of interfacial oxygen atoms and 2) stabilizing oxygen vacancies or vice versa, the dissociated oxygen atoms at the interface. Our study provides new insight for OMI and for the development of Cu-based catalysts for low temperature oxidation reactions.

W. Huang, Q. Liu, Z. Zhou, Y. Li, Y. Wang, Y. Tu, D. Deng, F. Yang, and X. Bao
ChemRxiv. 2019. Preprint

The formation of interfacial metal–oxide structures on the Pt₃Ni(111) bimetallic surface was investigated using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) techniques at near-ambient pressure (NAP). Direct observation of surface images clearly shows the occurrence of surface segregation of the sub-surface Ni depending on the surrounding gas-phase conditions. Especially, the prepared topmost Pt-skin layer of the Pt₃Ni(111) is altered by Ni oxide segregation that makes an interfacial Pt-NiO_1−x nanostructure with dissociated oxygen. This metal–oxide interface could provide active sites for more-efficient carbon monoxide (CO) conversion processes under mixed CO/O₂ gas environments; the associated specific chemical binding energy was identified using NAP-XPS. The combined operando observations from the NAP-STM and NAP-XPS on the Pt₃Ni(111) surface reveal that the interfacial metal–oxide structure is strongly correlated with the origin of the enhanced catalytic activity at thermodynamic equilibrium.

J. Kim, W. H. Doh, H. Kondoh, K. Mase, J.J. Gallet, F. Bournel, B. S. Mun, and J. Y. Park
J. Electron Spectroscopy and Related Phenomena, 2019

We report on strategies for characterizing hexagonal coincidence phases by analyzing the involved spatial moiré beating frequencies of the pattern. We derive general properties of the moiré regarding its symmetry and construct the spatial beating frequency  as the difference between two reciprocal lattice vectors  of the two coinciding lattices. Considering reciprocal lattice vectors , with lengths of up to n times the respective (1, 0) beams of the two lattices, readily increases the number of beating frequencies of the nth-order moiré pattern. We predict how many beating frequencies occur in nth-order moirés and show that for one hexagonal lattice rotating above another the involved beating frequencies follow circular trajectories in reciprocal-space. The radius and lateral displacement of such circles are defined by the order n and the ratio x of the two lattice constants. The question of whether the moiré pattern is commensurate or not is addressed by using our derived concept of commensurability plots. When searching potential commensurate phases we introduce a method, which we call cell augmentation, and which avoids the need to consider high-order beating frequencies as discussed using the reported  moiré of graphene on SiC(0001). We also show how to apply our model for the characterization of hexagonal moiré phases, found for transition metal-supported graphene and related systems. We explicitly treat surface x-ray diffraction-, scanning tunneling microscopy- and low-energy electron diffraction data to extract the unit cell of commensurate phases or to find evidence for incommensurability. For each data type, analysis strategies are outlined and avoidable pitfalls are discussed. We also point out the close relation of spatial beating frequencies in a moiré and multiple scattering in electron diffraction data and show how this fact can be explicitly used to extract high-precision data.

P. Zeller, X. Ma, and S. Günther
New J. Phys. 19 (2017) 013015

Chemical reactions that convert sp2 to sp3 hybridization have been demonstrated to be a fascinating yet challenging route
to functionalize graphene. So far it has not been possible to precisely control the reaction sites nor their lateral order at the
atomic/molecular scale. The application prospects have been limited for reactions that require long soaking, heating, electric
pulses or probe-tip press. Here we demonstrate a spatially selective photocycloaddition reaction of a two-dimensional
molecular network with defect-free basal plane of single-layer graphene. Directly visualized at the submolecular level, the
cycloaddition is triggered by ultraviolet irradiation in ultrahigh vacuum, requiring no aid of the graphene Moiré pattern. The
reaction involves both [2+2] and [2+4] cycloadditions, with the reaction sites aligned into a two-dimensional extended and
well-ordered array, inducing a bandgap for the reacted graphene layer. This work provides a solid base for designing and engineering
graphene-based optoelectronic and microelectronic devices.

M. Yu, C. Chen, Q. Liu, C. Mattioli, H. Sang, G.Shi,
W. Huang, K. Shen, Z. Li, P.Ding, P. Guan, S. Wang,
Y. Sun, J. Hu, A. Gourdon, L. Kantorovich, F. Besenbacher,
M. Chen, F. Song and F.Rosei
Nature Chemistry volume 12, pages1035–1041 (2020)

We present a mechanistic study on the formation and dynamic changes of a ligand-based heterogeneous Pd catalyst for chemoselective hydrogenation of α,β-unsaturated aldehyde acrolein. Deposition of allyl cyanide as a precursor of a ligand layer renders Pd highly active and close to 100 % selective toward propenol formation by promoting acrolein adsorption in a desired configuration via the C=O end. Employing a combination of real-space microscopic and in-operando spectroscopic surface-sensitive techniques, we show that an ordered active ligand layer is formed under operational conditions, consisting of stable N-butylimine species. In a competing process, unstable amine species evolve on the surface, which desorb in the course of the reaction. Obtained atomistic-level insights into the formation and dynamic evolution of the active ligand layer under operational conditions provide important input required for controlling chemoselectivity by purposeful surface functionalization.

C. Schröder, M. C. Schmidt, P. A. Haugg, A. Baumann, J. Smyczek, and Prof. Dr. S. Schauermann
Angewandte Chemie, Volume60, Issue30
July 19, 2021
Pages 16349-16354

Metal adatoms play a key role in surface diffusion, adsorption conformation, and self-assembly of porphyrin molecules on metal surfaces. Herein, we study the specific influence of coadsorption of Fe, Co, and Pd atoms on the behavior of 2H-tetrakis(p-cyano)phenylporphyrin (2H-TCNPP) on Cu(111) using scanning tunneling microscopy. Upon co-deposition of Fe and Co, the molecules form one-dimensional (1D) linear chains after mild annealing on Cu(111) driven by the interaction of its cyano groups with metal adatoms. A similar behavior has been observed previously on Cu(111), mediated by Cu adatoms, where the functional CN groups were also found to lower the reaction rate of the so-called porphyrin self-metalation reaction with Cu atoms significantly, in comparison to the non-cyano-functionalized porphyrin. Upon co-deposition of Pd and mild annealing, we find a remarkably different behavior, that is, a massive reorganization from 1D molecular chains to a peculiar rectangular 2D (two-dimensional) network. The molecular appearance changes to a clover shape, which is attributed to a Pd-induced dehydrogenation and subsequent ring closure reaction of the phenyl and pyrrole groups.
 

A. Ceccatto dos Santos, R. C. de Campos Ferreira, J. C. Moreno-López, L. Barreto, M. Lepper, R. Landers, H.-P. Steinrück, H. Marbach, und A. de Siervo*
Chem. Mater. 2020, 32, 5, 2114–2122

Monolayer hexagonal boron–nitrogen–carbon (h-BNC) is considered a prominent candidate for the next generation of semiconductor electronic devices. Nevertheless, experimental evidence of h-BNC formation is limited, including a detailed study of its morphological and electronic properties. Here, successful growth of h-BNC from an unexplored single molecular precursor (hexamethyl borazine, C₆H₁₈B₃N₃) using a conventional CVD approach on Ir(111) is reported. The conformation structure of the monolayer and its correlation with the local electronic properties are discussed based on scanning tunneling microscopy/spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) results. The results show an h-BNC structure that can be described as BN-doped graphene since the moiré lattice parameter is preserved along with the alloy. This BN-doped cluster, renamed as h-BN “nanodonuts” according to the electronic density exhibited in STM images, have a tendency to place specific positions within the moiré superstructure, and it is constituted by at least (BN)₈ units arranged in a 6-fold BN rings conformation, as evidenced by simulation of STM images based on density functional theory (DFT). For a BN concentration of about 17%, a band gap between 1.4 and 1.6 eV was determined. The versatility of the novel molecular precursor is proven by the growth of a high-quality h-BN monolayer on Rh(111).
 

N. Herrera-Reinoza, A. Ceccatto dos Santos, L. H. de Lima, R. Landers, and A. de Siervo
Chem. Mater. 2021, 33, 8, 2871–2882

On-surface coupling reactions and molecular conformation are essential processes for building tailored functional molecular nanostructures. Here, we study the thermal debromination and reactivity of free-base tetra(4-bromophenyl)porphyrin (H₂TBrPP) on Cu(111) as a function of the substrate temperature. It has been previously reported in the literature that C–Br bonds remain intact at room temperature (RT) and that the Br···Cu(111) interaction induces a drastic surface reconstruction around the molecule periphery and a distortion in the adsorbate itself. However, based on a combination of STM and XPS experiments, supported by density functional theory (DFT) calculations, we instead demonstrate that debromination readily occurs at RT, leading to a new interpretation of both the adsorption behavior and the molecular coupling of H₂TBrPP on Cu(111). For the molecules deposited on the metallic substrate held above RT, our STM measurements show the growth of ordered 2D metal–organic frameworks (MOFs).
 

A. Ceccatto dos Santos, N. Herrera-Reinoza, A. Pérez Paz, D. John Mowbray, und A. de Siervo*
J. Phys. Chem. C 2021, 125, 31, 17164

We investigated the competitive coadsorption of CO and O₂ molecules on a P_t model surface using a catalytic reactor integrated with a scanning tunneling microscope at elevated pressure. CO-poisoned incommensurate atom-resolved structures are observed on the terrace sites of the P_t(111) surface under gaseous mixtures of CO and O₂. However, in situ surface measurements revealed that segmented local structures were influenced by the CO/O₂ partial pressures in the catalytic reactor at a total pressure of a few Torr. This could be related to the expected formation of the theoretical oxygen precursor intermediates during dissociation of O₂ on the surface before the chemical reaction. These findings provide microscopic insights into the early steps of the catalytic reaction pathways on the P_t surface during CO oxidation in an industrial chemical reactor.

J. Kim, M. C. Noh, W. H. Doh, and J. Y. Park
J. Phys. Chem. C 2018, 122, 6246-6254

NiO cluster formation with strictly controlled O₂ exposure on a Ni(1 1 1) surface has been investigated extensively for decades under ultra‐high vacuum (UHV) conditions. The classical model of three‐stage Ni oxidation refers to the relationship between NiO cluster evolution and the kinetics of O₂ exposure; however, this information has a critical inherent limitation because of the “pressure gap” between UHV and real reaction conditions. Here, we report reversible NiO phase transitions on the Ni(1 1 1) surface at near‐ambient pressure by using scanning tunneling microscopy at room temperature. The restricted kinetic growth of NiO cluster evolution expands unexpectedly to oxide multi‐layer formation at 100 mTorr of O₂. Furthermore, metastable NiO islands can be manipulated by varying the partial CO pressure of the gas mixture. The interplay between the CO and O₂ molecules on the Ni(1 1 1) is correlated definitely to either surface oxide formation or competitive CO adsorption on the defect‐laden multi‐layered NiO interface.

M. C. Noh, J. Kim, Dr. W. H. Doh, Dr. K. J. Kim, and Prof. Dr. J. Y. Park
ChemCatChem 2018, 10, 2046-2050

The initial oxidation of Ni₃Al(111) was imaged by in situ scanning tunneling microscopy (STM) at 700–750 K. At 740 K ± 10 K a moiré structure is formed as the major surface phase: high resolution STM data atomically resolve a top hexagonal lattice with a lattice constant of 2.93 ± 0.01 Å aligned or slightly rotated with respect to the substrate. Auger electron spectra acquired from the surface phase identify Al atoms in an oxidic environment together with Ni atoms unaffected by the oxidation of the Ni₃Al(111) surface. A special mass balance analysis applied to STM images recorded during formation of the moiré structure allowed to extract the metal content of the surface phase. The moiré phase can be attributed to a single O/Al double layer of α-Al₂O₃ ontop of the Ni₃Al(111) crystal. The surface double layer is laterally expanded by ∼7% with respect to α-Al₂O₃ and, relating to the next nearest neighbor distance of the substrate of 2.52 Å, it contains 0.73 ML oxygen and 0.49 ML aluminium atoms. The building principle of the surface phase is almost identical to the one of the reported O_i/Al_i interface layer of the so called (√67 x √67)RI 2.2° surface oxide, except for its rotational alignment with respect to the substrate as shown in a careful moiré analysis. It could be shown that this thinnest possible surface aluminum oxide layer is formed due to kinetic restrictions: the oxide grows within the first layer of the Ni₃Al(111) surface ejecting 0.5 ML surface metal atoms, which are then converted into the surface oxide laterally separated at the ascending step edge of the same terrace. While the formation of the surface oxide is kinetically hindered most likely by the availability of Al adatoms, all rearrangement processes required for the surface oxide formation on each terrace are not rate limiting as identified by in situ STM. Instead, the local oxide growth rather follows the kinetics driven by the adsorption probability of the impinging oxygen molecules and provides the possibility to entirely cover whole Ni₃Al(111) surface.

X. Ma, S. Günther
Phys. Chem. Chem. Phys. 2018, 20, 21844-21855

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of in situ microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu2O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution in situ imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces.

A. E. Baber, F. Xu, F. Dvorak, K. Mudiyanselage, M. Soldemo, J. Weissenrieder, S. D. Senanayake, J. T. Sadowski, J. A. Rodriguez, V. Matolín, M. G. White, and D. J. Stacchiola
The Journal of Physical Chemistry, 135, 45, 16781-16784, 2013

Copper has excellent initial activity for the oxidation of CO, yet it rapidly deactivates under reaction conditions. In an effort to obtain a full picture of the dynamic morphological and chemical changes occurring on the surface of catalysts under CO oxidation conditions, a complementary set of in situ ambient pressure (AP) techniques that include scanning tunneling microscopy, infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy were conducted. Herein, we report in situ AP CO oxidation experiments over Cu(111) model catalysts at room temperature. Depending on the CO:O2 ratio, Cu presents different oxidation states, leading to the coexistence of several phases. During CO oxidation, a redox cycle is observed on the substrate’s surface, in which Cu atoms are oxidized and pulled from terraces and step edges and then are reduced and rejoin nearby step edges. IRRAS results confirm the presence of under-coordinated Cu atoms during the reaction. By using control experiments to isolate individual phases, it is shown that the rate for CO oxidation decreases systematically as metallic copper is fully oxidized.

F. Xu, K. Mudiyanselage, A. E. Baber, M. Soldemo,
J. Weissenrieder, M. G. White, and D. J. Stacchiola
The Journal of Physical Chemistry, 118, 29, 15902-15909, 2014

The reducibility of metal oxides is of great importance to their catalytic behavior. Herein, we combined ambient‐pressure scanning tunneling microscopy (AP–STM), X‐ray photoemission spectroscopy (AP–XPS), and DFT calculations to study the CO titration of CuxO thin films supported on Cu(1 1 1) (CuxO/Cu(1 1 1)) aiming to gain a better understanding of the roles that the Cu(1 1 1) support and surface defects play in tuning catalytic performances. Different conformations have been observed during the reduction, namely, the 44 structure and a recently identified (5–7–7–5) Stone–Wales defects (5–7 structure). The DFT calculations revealed that the Cu(1 1 1) support is important to the reducibility of supported CuxO thin films. Compared with the case for the Cu2O(1 1 1) bulk surface, at the initial stage CO titration is less favorable on both the 44 and 5–7 structures. The strong CuxO↔Cu interaction accompanied with the charge transfer from Cu to CuxO is able to stabilize the oxide film and hinder the removal of O. However, with the formation of more oxygen vacancies, the binding between CuxO and Cu(1 1 1) is weakened and the oxide film is destabilized, and Cu2O(1 1 1) is likely to become the most stable system under the reaction conditions. In addition, the surface defects also play an essential role. With the proceeding of the CO titration reaction, the 5–7 structure displays the highest activity among all three systems. Stone–Wales defects on the surface of the 5–7 structure exhibit a large difference from the 44 structure and Cu2O(1 1 1) in CO binding energy, stability of lattice oxygen, and, therefore, the reduction activity. The DFT results agree well with the experimental measurements, demonstrating that by adopting the unique conformation, the 5–7 structure is the active phase of CuxO, which is able to facilitate the redox reaction and the Cu2O/Cu(1 1 1)↔Cu transition.

Dr. W. An, Dr. A. E. Baber, F. Xu, Dr. M. Soldemo, Prof. Dr. J. Weissenrieder, Dr. D. Stacchiola, and Dr. P. Liu
Chem.Cat.Chem, 6, 2364-2372, 2014

The transformation of CO₂ into alcohols or other hydrocarbon compounds is challenging because of the difficulties associated with the chemical activation of CO₂ by heterogeneous catalysts. Pure metals and bimetallic systems used for this task usually have low catalytic activity. Here we present experimental and theoretical evidence for a completely different type of site for CO₂ activation: a copper-ceria interface that is highly efficient for the synthesis of methanol. The combination of metal and oxide sites in the copper-ceria interface affords complementary chemical properties that lead to special reaction pathways for the CO₂→CH₃OH conversion.

J. Graciani, K. Mudiyanselage, F. Xu, A. E. Baber, J. Evans, S. D. Senanayake, D. J. Stacchiola, P. Liu, J. Hrbek, J. F. Sanz, and J. A. Rodriguez,
Science, 345, 547-550, 2014

Adsorbate-driven morphological changes of pitted-Cu(111) surfaces have been investigated following the adsorption and desorption of CO and H. The morphology of the pitted-Cu(111) surfaces, prepared by Ar⁺ sputtering, exposed a few atomic layers deep nested hexagonal pits of diameters from 8 to 38 nm with steep step bundles. The roughness of pitted-Cu(111) surfaces can be healed by heating to 450–500 K in vacuum. Adsorption of CO on the pitted-Cu(111) surface leads to two infrared peaks at 2089–2090 and 2101–2105 cm^-1 for CO adsorbed on under-coordinated sites in addition to the peak at 2071 cm^-1 for CO adsorbed on atop sites of the close-packed Cu(111) surface. CO adsorbed on under-coordinated sites is thermally more stable than that of atop Cu(111) sites. Annealing of the CO-covered surface from 100 to 300 K leads to minor changes of the surface morphology. In contrast, annealing of a H covered surface to 300 K creates a smooth Cu(111) surface as deduced from infrared data of adsorbed CO and scanning tunnelling microscopy (STM) imaging. The observation of significant adsorbate-driven morphological changes with H is attributed to its stronger modification of the Cu(111) surface by the formation of a sub-surface hydride with a hexagonal structure, which relaxes into the healed Cu(111) surface upon hydrogen desorption. These morphological changes occur ∼150 K below the temperature required for healing of the pitted-Cu(111) surface by annealing in vacuum. In contrast, the adsorption of CO, which only interacts with the top-most Cu layer and desorbs by 200 K, does not significantly change the morphology of the pitted-Cu(111) surface.

K. Mudiyanselage, F. Xu, F. M. Hoffmann, J. Hrbek, I. Waluyo, J. A. Boscoboinikd, and D. J. Stacchiola
Phys.Chem.Chem.Phys. 2015, 17, 3032

In situ X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and scanning tunneling microscopy (STM) were used to study the reduction of potassium-modified Cu₂O/Cu(111) by CO. By following the time evolution of the O _1s peak of Cu₂O, we determined that the apparent activation energy for Cu₂O reduction by 2 × 10^–4 Torr CO is decreased by ∼30% in the presence of K. On the K-modified surface, both XPS and IRRAS data show the formation of a surface species identified by IRRAS as carbonate (CO₃^2–), likely forming a K⁺-CO₃^2– complex, which is stable up to 500 K. STM images show that K+-CO₃^2– complexes form chains around reduced Cu islands, thereby hindering the mass transfer of Cu atoms and preventing the reconstruction of the surface. Theoretical calculations show that the formation of carbonate on the K-modified “44” Cu₂O structure is thermodynamically favorable compared to the formation of CO₂ on either the bare or K-modified surfaces.

I. Waluyo, K. Mudiyanselage, F. Xu, W. An, P. Liu, J. A. Boscoboinik, J. A. Rodriguez, and D. J. Stacchiola
J.Phys.Chem.C 2019, 123, 8057-8066

Here, we present the design of a new reactor-like high-temperature near ambient pressure scanning tunneling microscope (HT-NAP-STM) for catalysis studies. This HT-NAP-STM was designed for exploration of structures of catalyst surfaces at atomic scale during catalysis or under reaction conditions. In this HT-NAP-STM, the minimized reactor with a volume of reactant gases of ∼10 ml is thermally isolated from the STM room through a shielding dome installed between the reactor and STM room. An aperture on the dome was made to allow tip to approach to or retract from a catalyst surface in the reactor. This dome minimizes thermal diffusion from hot gas of the reactor to the STM room and thus remains STM head at a constant temperature near to room temperature, allowing observation of surface structures at atomic scale under reaction conditions or during catalysis with minimized thermal drift. The integrated quadrupole mass spectrometer can simultaneously measure products during visualization of surface structure of a catalyst. This synergy allows building an intrinsic correlation between surface structure and its catalytic performance. This correlation offers important insights for understanding of catalysis. Tests were done on graphite in ambient environment, Pt(111) in CO, graphene on Ru(0001) in UHV at high temperature and gaseous environment at high temperature. Atom-resolved surface structure of graphene on Ru(0001) at 500 K in a gaseous environment of 25 Torr was identified.
ACKNOWL

F. Taoa, L. Nguyen, and S. Zhang
Rev. Sci. Instrum. 84, 034101 (2013)

Catalysis science has emerged as one of the crucial fields in energy science responsible for a sustainable energy world. Fundamental study of surface structures of catalysts at atomic scale under reaction conditions or during catalysis is critical in understanding catalytic mechanisms because a single catalysis event is performed on a specific site comprising one or several atoms with appropriate geometric and electronic structures. Ambient pressure high temperature scanning tunneling microscopy (APHT-STM) can identify structural details of catalyst surfaces at nano or atomic level when catalysts are under reaction conditions or during catalysis. By using APHT-STM, structures of the step edge of Pt(111) and the surface of Ni(557) were studied under reaction conditions. For Pt(111) model catalyst in a CO environment at a pressure of 0.1 Torr or higher, Pt atoms at step edges exhibit a dynamic restructuring. They form kink sites at 0.1 Torr and create nanoclusters near the step edges at 1–10 Torr within 1–2 min. A pressure-dependent restructuring of step edges of Pt(111) was revealed. In in situ studies of the vicinal surface, Ni(557) shows a pressure-dependent restructuring in CO, resulting from reorganization of all surface atoms. Nickel nanoclusters are formed on the whole surface, consistent with the increased coverage of CO chemisorbed on the Ni surface at a relatively higher pressure. Restructuring of atoms at a step edge of terraces of a flat surface and all atoms of a vicinal surface suggest the dynamic nature of model metal catalyst surfaces. Essentially, the surface structure of a metal catalyst in a reactive environment is determined by its reaction or catalysis condition.

L. Nguyen, F. Cheng, S. Zhang, and F. Tao
J. Phys. Chem. C 2013, 117, 971-977

Near surface alloy (NSA) model catalyst Pt/Cu/Pt(1 1 1) was prepared on Pt(1 1 1) through a controlled vapor deposition of Cu atoms. Different coordination environments of Pt atoms of the topmost Pt layer with the underneath Cu atoms in the subsurface result in different local electronic structures of surface Pt atoms. Surface structure and chemistry of the NAS model catalyst in Torr pressure of CO were studied with high pressure scanning tunneling microscopy (HP-STM) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In Torr pressure of CO, the topmost Pt layer of Pt/Cu/Pt(1 1 1) is restructured to thin nanoclusters with size of about 1 nm. Photoemission feature of O 1s of CO on Pt/Cu/Pt(1 1 1) suggests CO adsorbed on both edge and surface of these formed nanoclusters. This surface is active for CO oxidation. Atomic layers of carbon are formed on Pt/Cu/Pt(1 1 1) at 573 K in 2 Torr of CO.

S. Zeng, L. Nguyen, F. Cheng, L. Liu, Y. Yu, and F.Tao
Appl. Surf. Sci. 320, 225-230 (2014)

We report direct observation at the atomic scale of the pressure- and temperature-dependent evolution of a model Rh(110) catalyst surface during transient and steady-state CO oxidation, using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) correlated against density functional theory (DFT) calculations. Rh(110) is susceptible to the well-known missing row (MR) reconstruction. O2 dosing produces a MR structure and an O coverage of 1/2 monolayer (ML), the latter limited by the kinetics of O2 dissociation. In contrast, CO dosing retains the (1 × 1) structure and a CO coverage of 1 ML. We show that CO dosing titrates O from the (2 × 1) structure and that the final surface state is a strong function of temperature. Adsorbed CO accelerates and O inhibits the (2 × 1) to (1 × 1) transition, an effect that can be traced to the influence of the adsorbates on the energy landscape for moving metal atoms from filled to empty rows. During simultaneous dosing of CO and O2, we observed steady-state CO oxidation as well as a transition to the (1 × 1) structure at temperatures more modest than in the titration experiments. This difference may reflect surface heating generated during CO oxidation. At more elevated temperatures the metallic surface transforms to a surface oxide, also active for CO oxidation. Being one of the first examples, these results demonstrate how operando experiment exploration in terms of correlation between surface structure dominated by reaction conditions and activity of a catalytic material and first-principles models can be integrated to disentangle the underlying thermodynamic and kinetic factors that influence the dependence of catalytic activity on surface structure at nano and atomic scales.

S. Zhang, L. Nguyen, Y. Zhu, S. Zhan, C. Tsung, and F. Tao
ACS Catal. 2017, 7, 663-674

Heterogeneous catalysis is a chemical process performed at a solid–gas or solid–liquid interface. Direct participation of catalyst atoms in this chemical process determines the significance of the surface structure of a catalyst in a fundamental understanding of such a chemical process at a molecular level. High-pressure scanning tunneling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM) have been used to observe catalyst structure in the last few decades. In this review, instrumentation for the two in situ/operando techniques and scientific findings on catalyst structures under reaction conditions and during catalysis are discussed with the following objectives: (1) to present the fundamental aspects of in situ/operando studies of catalysts; (2) to interpret the observed restructurings of catalyst and evolution of catalyst structures; (3) to explore how HP-STM and ETEM can be synergistically used to reveal structural details under reaction conditions and during catalysis; and (4) to discuss the future challenges and prospects of atomic-scale observation of catalysts in understanding of heterogeneous catalysis. This Review focuses on the development of HP-STM and ETEM, the in situ/operando characterizations of catalyst structures with them, and the integration of the two structural analytical techniques for fundamentally understanding catalysis.

F. Tao, and P. A. Crozier
Chem. Rev. 2016, 116, 3487-3539

Carbon monoxide (CO) is one of the most-studied molecules among the many modern industrial chemical reactions available. Following the Langmuir–Hinshelwood mechanism, CO conversion starts with adsorption on a catalyst surface, which is a crucially important stage in the kinetics of the catalytic reaction. Stepped surfaces show enhanced catalytic activity because they, by nature, have dense active sites. Recently, it was found that surface-sensitive adsorption of CO is strongly related to surface restructuring via roughening of a stepped surface. In this scanning tunneling microscopy study, we observed the thermal evolution of surface restructuring on a representative stepped platinum catalyst, P_t(557). CO adsorption at 1.4 mbar CO causes the formation of a broken-step morphology, as well as CO-induced triangular P_t clusters that exhibit a reversible disordered–ordered transition. Thermal instability of the CO-induced platinum clusters on the stepped surface was observed, which is associated with the reorganization of the repulsive CO–CO interactions at elevated temperature.

J. Kim, M. C. Noh, W. H. Doh, and J. Y. Park
J.Am.Chem.Soc. 2016, 138, 1110-1113

Polyethylene production through catalytic ethylene polymerization is one of the most common processes
in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be
directly inserted into the metal–carbon bond during chain growth, which has been awaiting microscopic
and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene
polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface. We
observed that ethylene polymerization proceeds on a specific triangular iron site at the boundary
between two carbide domains. Without an activator, an intermediate, attributed to surface-anchored
ethylidene (CHCH3), serves as the chain initiator (self-initiation), which subsequently grows by
ethylene insertion. Our finding provides direct experimental evidence of the ethylene polymerization
pathway at the molecular level.

W. Guo, J. Yin, Z. Xu, W. Li, Z. Peng, C. J. Weststrate, X. Yu,
Y. He, Z. Cao, X. Wen, Y. Yang, K. Wu, Y. Li,
J. W. Niemantsverdriet, und X. Zhou
Science 375, 1188–1191 (2022)