SPM Aarhus 150 NAP

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

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

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

Pt–Fe bimetallic alloys are important model catalysts for a number of catalytic reactions. Combining scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we have studied the structures of Pt–Fe surface alloys prepared on Pt(111) under a variety of conditions. Although the surface and subsurface structures of the Pt–Fe surface alloy could be varied with the deposition amount of Fe atoms and the annealing temperature, a characteristic alloy surface with a bright striped pattern could be identified, which consists of a Pt-dominant surface layer with a small percentage of Fe atoms in the form of isolated atoms or clusters in the surface lattice and a subsurface layer with an ordered Pt₃Fe alloy structure. The bright stripes observed in STM were surface dislocations caused by stress relaxation owing to the lattice mismatch between the surface and subsurface layers. This characteristic alloy surface could be prepared on Pt(111) by depositing sub-monolayer Fe at ∼460 K to facilitate Fe diffusion in the near-surface region, or annealing multilayer Fe at ∼700 K, to enhance bulk diffusion of Fe atoms. The synthesis of this Pt–Fe alloy surface with well-defined structures could allow for further model catalytic studies.

H. Chen, R. Wang, R. Huang, C. Zhao, Y. Li, Z. Gong, Y. Yao, Y. Cui, F. Yang, and X. Bao
J. Phys. Chem. C 2019, 123, 17225-17231

Fundamental understanding of chemistry confined to nanospace remains a challenge since molecules encapsulated in confined microenvironments are difficult to be characterized. Here, we show that CO adsorption on Pt(111) confined under monolayer hexagonal boron nitride (h-BN) can be dynamically imaged using near ambient pressure scanning tunneling microscope (NAP-STM) and thanks to tunneling transparency of the top h-BN layer. The observed CO superstructures on Pt(111) in different CO atmospheres allow to derive surface coverages of CO adlayers, which are higher in the confined nanospace between h-BN and Pt(111) than those on the open Pt surface under the same conditions. Dynamic NAP-STM imaging data together with theoretical calculations confirm confinement-induced molecule enrichment effect within the 2D nanospace, which reveals new chemistry aroused by the confined nanoreactor.
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H. Wu, P. Ren, P. Zhao, Z. Gong, X. Wen, Y. Cui, Q. Fu, and X. Bao
NanoResearch, 2019, 12(1):85-90

Graphene-like ZnO (g-ZnO) nanostructures (NSs) and thin films were prepared on Au(111), and their reactivities toward CO and H₂ were compared with that of wurtzite ZnO (w-ZnO) (0001) single crystals. The interaction and reaction between CO/H₂ and the different types of ZnO surfaces were studied using near-ambient-pressure scanning tunneling microscopy (NAP-STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The reactivity of the w-ZnO(0001) surface toward CO and H₂ was found to be more prominent than those on the surfaces of g-ZnO/Au(111). CO oxidation took place primarily at the edge sites of w-ZnO(0001) and the interface between g-ZnO NSs and Au(111), while g-ZnO thin films on Au(111) appeared to be inert below 600 K. Similarly, the w-ZnO(0001) surface could dissociate H₂ at 300 K, accompanied by a substantial surface reconstruction, while g-ZnO on Au(111) appeared inert for H₂ activation at 300 K. DFT calculations showed that the reactivities of ZnO surfaces toward CO could be related to the formation energy of oxygen vacancy (EOvf), which could be related to the charge transfer to lattice oxygen atoms or surface polarity.

H. Chen, L. Lin, Y. Li, R. Wang, Z. Gong, Y. Cui, Y. Li, Y. Liu, X. Zhao, W. Huang, Q. Fu, F. Yang, and X. Bao
ACS Catal. 2019, 9, 1373-1382

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

The growth and structural properties of ZnO thin films on both Au(111) and Cu(111) surfaces were studied using either NO₂ or O₂ as oxidizing agent. The results indicate that NO₂ promotes the formation of well-ordered ZnO thin films on both Au(111) and Cu(111). The stoichiometric ZnO thin films obtained on these two surfaces exhibit a flattened and non-polar ZnO(0001) structure. It is shown that on Au(111), the growth of bilayer ZnO nanostructures (NSs) is favored during the deposition of Zn in presence of NO₂ at 300 K, whereas both monolayer and bilayer ZnO NSs could be observed when Zn is deposited at elevated temperatures under a NO₂ atmosphere. The growth of bilayer ZnO NSs is caused by the stronger interaction between two ZnO layers than between ZnO and Au(111) surface. In contrast, the growth of monolayer ZnO NSs involves a kinetically controlled process. ZnO thin films covering the Au(111) surface exhibits a multilayer thickness, which is consistent with the growth kinetics of ZnO NSs. Besides, the use of O₂ as oxidizing agent could lead to the formation of sub-stoichiometric ZnOx structures. The growth of full layers of ZnO on Cu(111) has been a difficult task, mainly because of the interdiffusion of Zn promoted by the strong interaction between Cu and Zn and the formation of Cu surface oxides by the oxidation of Cu(111). We overcome this problem by using NO2 as oxidizing agent to form well-ordered ZnO thin films covering the Cu(111) surface. The surface of the well-ordered ZnO thin films on Cu(111) displays mainly a moiré pattern, which suggests a (3 × 3) ZnO superlattice supported on a (4 × 4) supercell of Cu(111). The observation of this superstructure provides a direct experimental evidence for the recently proposed structural model of ZnO on Cu(111), which suggests that this superstructure exhibits the minimal strain. Our studies suggested that the surface structures of ZnO thin films could change depending on the oxidation level or the oxidant used. The oxidation of Cu(111) could also become a key factor for the growth of ZnO. When Cu(111) is pre-oxidized to form copper surface oxides, the growth mode of ZnO_x is altered and single-site Zn could be confined into the lattice of copper surface oxides. Our studies show that the growth of ZnO is promoted by inhibiting the diffusion of Zn into metal substrates and preventing the formation of sub-stoichiometric ZnO_x. In short, the use of an atomic oxygen source is advantageous to the growth of ZnO thin films on Au(111) and Cu(111) surfaces.

X. ZHAO, H. CHEN, H. WU, R. WANG, Y. CUI, Q. FU, F. YANG, and X. BAO
Acta Phys.-Chim.Sin 2018, 34(12), 1373-1380

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

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

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

The interface between metal and reducible oxide has attracted increasing interest in catalysis. The FeO_x–Pt interface has been a typical example, which showed remarkable activity for the preferential oxidation of CO (PROX) at low temperatures. However, model catalytic studies under vacuum conditions or in high-pressure O-rich environment at 450 K have reported two different active phases with iron in two different valence states, invoking a possible pressure gap. To identify the active phase for low-temperature CO oxidation and PROX, it is necessary to investigate the stability and activity of FeO/Pt(111) under the realistic reaction conditions. We thus conducted an in situ study on FeO/Pt(111) from ultrahigh vacuum to the atmospheric pressure of reactant gases. Our study shows FeO islands were easily oxidized in 1 Torr O₂ to form the trilayer FeO₂ islands. However, the presence of 2 Torr CO could prevent the oxidation of FeO islands and lead to CO oxidation at the FeO/Pt(111) interface. The FeO/Pt(111) surface exhibits an excellent activity for CO₂ production with an initial reaction rate measured to be ∼1 × 10¹⁴ molecules·cm^–2·s^–1 at 300 K. FeO islands supported on Pt(111) were further investigated in the PROX gas, i.e., the mixture of 98.5% H₂, 1% CO, and 0.5% O₂, at elevated pressures up to 1 bar. Our results thus bridged the pressure gap and identified the bilayer FeO islands on Pt(111) as the active phase for PROX under the realistic reaction conditions.

H. Chen, Y. Liu, F. Yang, M. Wei, X. Zhao, Y. Ning, Q. Liu, Y. Zhang, Q. Fu, and X. Bao
J. Phys. Chem. C 2017, 121, 10398-10405

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

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

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

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)

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

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

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

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

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)

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

The ordering transition of an amorphous carbon layer into graphene was investigated by high-temperature scanning tunneling microscopy. A disordered C layer was prepared on a Ru(0001) surface by chemical vapor deposition of ethylene molecules at ∼660 K. The carbon layer grows in the form of dendritic islands that have almost the same density as graphene. Upon annealing of the fully covered surface, residual hydrogen desorbs and a coherent but still disordered carbon layer forms, with almost the same carbon coverage as in graphene. The ordering of this layer into graphene at 920 to 950 K was monitored as a function of time. A unique mechanism was observed that involves small topographic holes in the carbon layer. The holes are mobile, and on the trajectories of the holes the disordered carbon layer is transformed into graphene. The transport of C atoms across the holes or along the hole edges provides a low-energy pathway for the ordering transition. This mechanism is prohibited in a dense graphene layer, which offers an explanation for the difficulty of removing defects from graphene synthesized by chemical methods.

S. Günther, S. Dänhardt, M. Ehrensperger, P. Zeller, S. Schmitt, J. Wintterlin
ACSNano, 7, N.1, 154-164 (2013)

Deposition of a porphyrin onto metallic copper followed by heating leads to an unprecedented type of linking of the molecules giving a mixture of covalent multiporphyrin nanostructures at the surface.

M. In't Veld, P. Iavicoli, S. Haq, D. B. Amabilino, R. Raval
Chem. Commun. 13, 1536