STRUCTURE, THERMAL STABILITY, AND CO ADSORPTION PROPERTIES OF PD NANOPARTICLES SUPPORTED ON AN ULTRA-THIN SiO2 FILM

2007 ◽  
Vol 14 (05) ◽  
pp. 927-934 ◽  
Author(s):  
J.-L. LU ◽  
J. WEISSENRIEDER ◽  
S. KAYA ◽  
H.-J. GAO ◽  
S. SHAIKHUTDINOV ◽  
...  
Keyword(s):  
Thermal Stability ◽  
Photoelectron Spectroscopy ◽  
Elevated Temperatures ◽  
Morphological Changes ◽  
Co Adsorption ◽  
Pd Nanoparticles ◽  
Silica Film ◽  
Adsorption Properties ◽  
Scanning Tunneling ◽  
Tunneling Microscopy

Nucleation, growth, and thermal stability of Pd particles vapor-deposited on an ultra-thin crystalline silica film grown on Mo (112) have been studied by scanning tunneling microscopy, X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and temperature-programmed desorption of CO . No preferential nucleation of Pd on the silica film is found at room temperature deposition: the hemispherical Pd nanoparticles are homogenously dispersed on the support at all coverages studied (0.01 - 1 ML (mono layer)). The Pd particles are resistant toward sintering up to 700 K as judged by STM; however, CO adsorption studies have revealed surface chemical modification at temperatures as low as 550 K. Strong morphological changes are observed above 800 K (ultimately resulting in elongated rectangular islands at ~1000 K), which is accompanied by strong alterations of CO adsorption properties. The results are rationalized in terms of Pd and Mo substrate interdiffusion at elevated temperatures, while the silica film basically preserves its structure.

Surface Structure of Pd3Fe(111) and Effects of Oxygen Adsorption

2009 ◽  
Vol 1217 ◽  
Author(s):  
Xiaofang Yang ◽  
Lindsey A. Welch ◽  
Jie Fu ◽  
Bruce E. Koel
Keyword(s):  
Photoelectron Spectroscopy ◽  
Elevated Temperatures ◽  
High Vacuum ◽  
Low Energy ◽  
Ultra High Vacuum ◽  
Single Crystal Sample ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Crystal Sample ◽  
Orr Kinetics

AbstractPd-Fe alloys have attracted attention in PEM fuel cell research because they were found to be comparable to Pt electrocatalysts in oxygen reduction reaction (ORR) kinetics at the cathode. In this study, the surface morphology of a Pd3Fe(111) single-crystal sample and oxygen reaction on the surface were investigated by low energy electron diffraction (LEED), low energy ion scattering (LEIS), x-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions. Strong segregation of Pd atoms was observed after annealing in UHV. Particularly, Pd single adatoms and dimers were found on the surface after high temperature annealing, which differs from most other well-studied binary alloy systems. Low free energy of Pd, strain relaxation, and interaction between Pd and Fe, are potentially responsible for the formation of this unusual surface. Adsorption of oxygen reversed the segregation trend and oxidized surface Fe. Ordered surface phases were observed after oxygen exposures at elevated temperatures. The reducing activity of Fe atoms in the alloy inhibited Pd oxidation, and weakened Pd-O interactions on Pd3Fe(111) are consistent with enhanced ORR kinetics.

scholarly journals On-Surface Porphyrin Transmetalation by Pb/Cu Redox Exchange

2020 ◽  
Author(s):  
Jan Herritsch ◽  
Stefan R. Kachel ◽  
Qitang Fan ◽  
Mark Hutter ◽  
Lukas Heuplick ◽  
...  
Keyword(s):  
Metal Complexes ◽  
Photoelectron Spectroscopy ◽  
Structural Changes ◽  
Elevated Temperatures ◽  
Scanning Tunneling ◽  
Side Reactions ◽  
Metal Exchange ◽  
Exchange Reactions ◽  
Tunneling Microscopy ◽  
Operation Conditions

Metal complexes at surfaces and interfaces play a prominant role in many areas of modern technology, including catalysis, sensors, and organic electronics. An important aspect of these interfaces is the possible exchange of the metal center, because this reaction can drastically alter the properties of the metal complex and thus of the interface. Here, we demonstrate that such metal exchange reactions are indeed possible and can proceed already at moderate temperatures even in the absence of solvents. Specifically, we studied the redox transmetalation of a monolayer of lead(II)-tetraphenylporphyrin (PbTPP) with copper from a Cu(111) surface under ultrahigh-vacuum (UHV) conditions using multiple surface-sensitive techniques. Temperature dependent X-ray photoelectron spectroscopy (XPS) reveals that the Pb/Cu exchange starts already below 380 K and is complete at 600 K. The identity of the reaction product, CuTPP, is confirmed by mass spectrometric detection in a temperature-programmed reaction (TPR) experiment. Scanning tunneling microscopy (STM) sheds light on the adsorbate structure of PbTPP at 300 K and uncovers the structural changes accompanying the transmetalation and side-reactions of the phenyl substituents. Moreover, individual free Pb atoms are observed as a product of the metal exchange. Our study suggests that surfaces functionalized with metal complexes may consist of other species than intended under operation conditions, which often involve elevated temperatures.

scholarly journals On-Surface Porphyrin Transmetalation by Pb/Cu Redox Exchange

2020 ◽  
Author(s):  
Jan Herritsch ◽  
Stefan R. Kachel ◽  
Qitang Fan ◽  
Mark Hutter ◽  
Lukas Heuplick ◽  
...  
Keyword(s):  
Metal Complexes ◽  
Photoelectron Spectroscopy ◽  
Structural Changes ◽  
Elevated Temperatures ◽  
Scanning Tunneling ◽  
Side Reactions ◽  
Metal Exchange ◽  
Exchange Reactions ◽  
Tunneling Microscopy ◽  
Operation Conditions

Metal complexes at surfaces and interfaces play a prominant role in many areas of modern technology, including catalysis, sensors, and organic electronics. An important aspect of these interfaces is the possible exchange of the metal center, because this reaction can drastically alter the properties of the metal complex and thus of the interface. Here, we demonstrate that such metal exchange reactions are indeed possible and can proceed already at moderate temperatures even in the absence of solvents. Specifically, we studied the redox transmetalation of a monolayer of lead(II)-tetraphenylporphyrin (PbTPP) with copper from a Cu(111) surface under ultrahigh-vacuum (UHV) conditions using multiple surface-sensitive techniques. Temperature dependent X-ray photoelectron spectroscopy (XPS) reveals that the Pb/Cu exchange starts already below 380 K and is complete at 600 K. The identity of the reaction product, CuTPP, is confirmed by mass spectrometric detection in a temperature-programmed reaction (TPR) experiment. Scanning tunneling microscopy (STM) sheds light on the adsorbate structure of PbTPP at 300 K and uncovers the structural changes accompanying the transmetalation and side-reactions of the phenyl substituents. Moreover, individual free Pb atoms are observed as a product of the metal exchange. Our study suggests that surfaces functionalized with metal complexes may consist of other species than intended under operation conditions, which often involve elevated temperatures.

scholarly journals Effect of CO Molecule Orientation on the Reduction of Cu-Based Nanoparticles

Nanomaterials ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 279
Author(s):  
Sergey Y. Sarvadii ◽  
Andrey K. Gatin ◽  
Vasiliy A. Kharitonov ◽  
Nadezhda V. Dokhlikova ◽  
Sergey A. Ozerin ◽  
...  
Keyword(s):  
Electric Field ◽  
External Electric Field ◽  
Adsorption Process ◽  
Co Adsorption ◽  
Reduction Rate ◽  
Sample Surface ◽  
Potential Difference ◽  
Precipitation Method ◽  
Scanning Tunneling ◽  
Tunneling Microscopy

The adsorption of CO on the surface of Cu-based nanoparticles was studied in the presence of an external electric field by means of scanning tunneling microscopy (STM) and spectroscopy (STS). Nanoparticles were synthesized on the surface of a graphite support by the impregnation–precipitation method. The chemical composition of the surface of the nanoparticles was determined as a mixture of Cu2O, Cu4O3 and CuO oxides. CO was adsorbed from the gas phase onto the surface of the nanoparticles. During the adsorption process, the potential differences ΔV = +1 or −1 V were applied to the vacuum gap between the sample and the grounded tip. Thus, the system of the STM tip and sample surface formed an asymmetric capacitor, inside which an inhomogeneous electric field existed. The CO adsorption process is accompanied by the partial reduction of nanoparticles. Due to the orientation of the CO molecule in the electric field, the reduction was weak in the case of a positive potential difference, while in the case of a negative potential difference, the reduction rate increased significantly. The ability to control the adsorption process of CO by means of an external electric field was demonstrated. The size of the nanoparticle was shown to be the key factor affecting the adsorption process, and particularly, the strength of the local electric field close to the nanoparticle surface.

scholarly journals Dual role of CO in the stability of subnano Pt clusters at the Fe3O4(001) surface

2016 ◽  
Vol 113 (32) ◽  
pp. 8921-8926 ◽  
Author(s):  
Roland Bliem ◽  
Jessi E. S. van der Hoeven ◽  
Jan Hulva ◽  
Jiri Pavelec ◽  
Oscar Gamba ◽  
...  
Keyword(s):  
Density Functional ◽  
Elevated Temperatures ◽  
Time Lapse ◽  
Dual Role ◽  
Oxidation Catalyst ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Active Metal ◽  
Catalytically Active ◽  
The Stability

Interactions between catalytically active metal particles and reactant gases depend strongly on the particle size, particularly in the subnanometer regime where the addition of just one atom can induce substantial changes in stability, morphology, and reactivity. Here, time-lapse scanning tunneling microscopy (STM) and density functional theory (DFT)-based calculations are used to study how CO exposure affects the stability of Pt adatoms and subnano clusters at the Fe3O4(001) surface, a model CO oxidation catalyst. The results reveal that CO plays a dual role: first, it induces mobility among otherwise stable Pt adatoms through the formation of Pt carbonyls (Pt1–CO), leading to agglomeration into subnano clusters. Second, the presence of the CO stabilizes the smallest clusters against decay at room temperature, significantly modifying the growth kinetics. At elevated temperatures, CO desorption results in a partial redispersion and recovery of the Pt adatom phase.

Scanning Tunneling Microscope Study of Etch Pits on Highly Oriented Pyrolytic Graphite Heated in an Atomic Absorption Electrothermal Analyzer

1997 ◽  
Vol 51 (12) ◽  
pp. 1896-1904 ◽  
Author(s):  
Kurt G. Vandervoort ◽  
Kristin N. McLain ◽  
David J. Butcher
Keyword(s):  
Elevated Temperatures ◽  
Pyrolytic Graphite ◽  
Reaction Rates ◽  
Surface Reactivity ◽  
Scanning Tunneling ◽  
Highly Oriented Pyrolytic Graphite ◽  
Tunneling Microscopy ◽  
Etch Pits ◽  
Pit Formation ◽  
Second Period

Scanning tunneling microscopy (STM) was used to elucidate monolayer etch pits that form on highly oriented pyrolytic graphite (HOPG) heated in an electrothermal analyzer. Pits form at elevated temperatures due to reactions between oxygen and exposed carbon edge atoms (defects) and additionally with intraplanar carbon atoms (through abstraction). Samples of HOPG without analyte or matrix modifier were placed in the depression of a pure pyrolytic graphite platform and heated by using standard analysis furnace programs. Under argon stop-flow conditions, pits form in less than a second at atomization temperatures equal to and above 1200 °C. With low argon flow rates (40 mL/min), pits formed at atomization temperatures equal to and greater than 1750 °C in less than a second. Quantitative pit formation rates were used to indicate oxygen partial pressure, which may be as high as ∼ 10−3 atm at 1200 °C. Reaction rates were used to predict surface degradation due to oxygen attack and determine that 1-μm depth normal to the surface would be removed by 200 successive 5-second-period furnace firings at 1200 °C. Implications for increases in surface reactivity and analyte intercalation are discussed.

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