Assessing Density Functionals for Describing Methane Dissociative Chemisorption on Pt(110)-(2×1) Surface

In this work, we explore the suitability of several density functionals with the generalized gradient approximation (GGA) and beyond for describing the dissociative chemisorption of methane on the reconstructed Pt(110)-(2\begin{document}$ \times $\end{document}1) surface. The bulk and surface structures of the metal, methane adsorption energy, and dissociation barrier are used to assess the functionals. A van der Waals corrected GGA functional (optPBE-vdW) and a meta-GGA functional with van der Waals correction (MS PBEl-rVV10) are selected for ab initio molecular dynamics calculations of the sticking probability. Our results suggest that the use of these two functionals may lead to a better agreement with existing experimental results, thus serving as a good starting point for future development of reliable machine-learned potential energy surfaces for the dissociation of methane on the Pt(110)-(2\begin{document}$ \times $\end{document}1) surface.     

Formation of Isolated Single-Atom Pd1 Sites on the Surface of Pd–Ag/Al2O3 Bimetallic Catalysts

Kinetics and Catalysis - The structure of Pd–Ag/Al2O3 samples with different Ag/Pd ratios has been studied by a complex of physicochemical methods (H2-TPR, XRD, H2-TPD, TEM). Catalysts have...     

HREELS Studies on Adsorption and Decomposition of ~(15)NO on Mo(100)-c(2×2)N Surface

MolybdenumnitridesascatalystsexhibituniquecataIyticactivitiesforawidevarietyofreactions,particularlyforthehydrodenitrogenationintherefineryprocessingofcrudesl.However,thefundamentalunderstandingconcerningthereactionmechanismsovermolybdenumnitrideisnotasweIlestablishedasthatontheMosurface.APplicationofpowerfulsurfacesciencespectroscopiesonwell-characterizednitrideoverlayersshouldprovideimPortantinsightsintothesurfacestrllctureandreactivityofsuchimportantcatalyticmaterials.Inthepresentwork,ahi…     

The Study of Nitric Oxide Adsorption and the Mechanism of Surface “Explosions” in the Reaction of CO + NO on Pt(100) and Pd(110) Single Crystal Surfaces

High resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) were used to study NO adsorption and the reactivity of CO_ads and NO_ads molecules on Pd(110) and Pt(100) single crystal surfaces. Compared to the Pt(100)-(1 × 1) surface, the unreconstructed Pt(100)-hex surface is chemically inert toward NO dissociation into N_ads and O_ads atoms. When a mixed adsorbed CO_ads + NO_ads layer is heated, a so-called surface explosion is observed when the reaction products (N₂, CO₂, and N₂O) synchronously desorb in the form of sharp peaks with a half-width of 7-20 K. The shape specificity of TPR spectra suggests that the vacancy mechanism consists of the autocatalytic character of the reaction initiated by the formation an initial concentration of active sites due to partial desorption of molecules from the CO_ads + NO_ads layer upon heating to high temperatures. Kinetic experiments carried out on the Pd(110) surface at a constant reaction pressure and a linear increase in the temperature confirm the explosive mechanism of the reaction NO + CO.     

NO2在Ag/Pt(110)双金属表面上的吸附和分解

利用俄歇电子能谱(AES)和程序升温脱附谱(TDS)研究了NO₂在Ag/Pt(110)双金属表面的吸附和分解.室温下NO₂ 在Ag/Pt(110)双金属表面发生解离吸附, 生成NO(ads)和O(ads)表面吸附物种. 在升温过程中NO(ads)物种发生脱附或者进一步分解. 500 K时NO₂在Ag/Pt(110)双金属表面发生解离吸附生成O(ads)表面吸附物种. Pt 向Ag传递电子, 从而削弱Pt－O键的强度, 降低O(ads)从Pt 表面的并合脱附温度. 发现能够形成具有稳定组成的Ag/Pt(110)合金结构, 其表现出与Pt(110)-(1×2)相似的解离吸附NO₂能力, 但与O(ads)的结合明显弱于Pt(110)-(1×2). 该AgPt(110)合金结构是可能的低温催化直接分解氮氧化物活性结构.     

Unexpected Formation of α -(N-Methyl)-aminoalkanephosphonate

Abstract

In contrary to a known² reaction of 2H-azirines with triethyl phosphite in the presence of ethanol, prolonged heating of 5,5-dimethyl-1-pyrroline 1 (or its 1-oxide) with trimethyl phosphite in methanolic medium yields a new ester 4 δ_H(ccl₄) 0.86 and 1.12 (2×Me, 2×s), 2.35 (NMe,s), 3.68, and 3.75 (2×OMe,2×d,J=10.3 and 9.9 Hz)     

Adsorption and Reaction of CO on Mo(100)-c(2×2)-N Surface Studied by HREELS

Molybdenum nitrides have attracted significant attention recently due to their uniquephysical and chemical properties. Many studies have shown that Mo,N is an activecatalyst for CO hydrogenation. The fundamental understanding concerning the reactionmechanisms over Mo=N. however, is not well established. The powerful surface sciencetechniques may provide an insight into the surface structure and properties of such animportant catalytic material. In previous paper', we described the results o…     

High resolution photoemission and x-ray absorption spectroscopy of a lepidocrocite-like TiO2 nanosheet on Pt(110) (1 × 2)

The electronic structure of TiO(2) nanosheets on the Pt(110)-(1 × 2) surface has been investigated by using high resolution photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Ti 2p XAS spectra of the deposited TiO(2) films have been theoretically evaluated and, from the comparison with the experimental data, the assignment to a lepidocrocite-like structure is confirmed. Coexistence of TiO(2) islands with PtO(2) stripes for incomplete nanosheets is confirmed by high resolution photoemission data. The location of the valence and conduction band edges of the nanosheet has been experimentally determined allowing us to describe in details subtle electronic effects due to the interface with the substrate. The locations of the valence band maximum and the leading peak in the O 1s XAS spectrum indicate a band gap similar to anatase but with the Fermi level closer to mid-gap than found for bulk, n-type TiO(2).     

Theoretical Calculations on Amphoteric Adsorptions of the Hydrogenated C(100)-2×1 Surface

In this work we study the adsorptions of some small molecules or group on the hydrogenated C(100)-2×1 surface using density functional theory method. The calculated results show that the ionization potential (IP) of the hydrogenated C(100)-2×1 surfaces after adsorption has amphoteric characteristics. From the weak basic NH3 molecule with small IP and negative electron affinity (EA), through the neutral H2O molecule, to the weak acid HF molecule and the OH group with large EA and IP, the IP values of the adsorbed diamond surfaces vary from decrease, through invariability, to slight increase for HF and obvious increase for OH. In all adsorption species, only the OH group makes the hydrogenated C(100)-2×1 surface change to the metal from the semiconductor with a wide-band gap, while the others only introduce impurity states into the electronic structures of the hydrogenated C(100)-2×1 surfaces.     

H atom adsorption and diffusion on Si(110)-(1×1) and (2×1) surfaces

We present a periodic density-functional study of hydrogen adsorption and diffusion on the Si(110)-(1×1) and (2×1) surfaces, and identify a local reconstruction that stabilizes the clean Si(110)-(1×1) by 0.51 eV. Hydrogen saturates the dangling bonds of surface Si atoms on both reconstructions and the different structures can be identified from their simulated scanning tunneling microscopy/current image tunneling spectroscopy (STM/CITS) images. Hydrogen diffusion on both reconstructions will proceed preferentially along zigzag rows, in between two adjacent rows. The mobility of the hydrogen atom is higher on the (2×1) reconstruction. Diffusion of a hydrogen vacancy on a monohydride Si(110) surface will proceed along one zigzag row and is slightly more difficult (0.2 eV and 0.6 eV on (1×1) and (2×1), respectively) than hydrogen atom diffusion on the clean surface.     

Study of Hydrogen Adsorption on Pt/WO3-ZrO2 through Pt Sites

The rate determining step and the energy barrier involved in hydrogen adsorption on Pt/WO_3- ZrO_2 were studied based on the assumption that the hydrogen adsorption occurs only through Pt sites. The rate of hydrogen adsorption on Pt/WO_3-ZrO_2 was measured in the adsorption temperature range of 323-573 K and an initial hydrogen pressure of 50 Torr.The rates of hydrogen uptake were very high for the initial few minutes and the adsorption continued for more than 5 h below 523 K.The hydrogen uptake far exceeded the H/Pt ratio of unity for all adsorption temperatures,indicating that the adsorption of hydrogen involved the dissociative adsorption of hydrogen on Pt sites to form hydrogen atoms,the spillover of hydrogen atoms onto the surface of the WO_3-ZrO2 catalyst,the diffusion of spiltover hydrogen atom over the surface of the WO_3-ZrO_2 catalyst,and the formation of protonic acid site originated from hydrogen atom by releasing an electron in which the electron may react with a second hydrogen atom to form a hydride near the Lewis acid site.The rate determining step was the spillover with the activation energy of 12.3 kJ/mol.The rate of hydrogen adsorption cannot be expressed by the rate equation based on the assumption that the rate determining step is the surface diffusion.The activity of Pt/WO_3-ZrO_2 was examined on n-heptane isomerization in which the increase of hydrogen partial pressure provided positive-effect on the conversion of n-heptane and negative-effect on the selectivity towards iso-heptane.     

Theoretical Studies of H＿2O Adsorption on Pure and Pt Loading TiO＿2（Rutile）（110） Surfaces

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DFT Investigations About Pyrazine Molecules on Si(100)-2×1 Surface

It is important to understand the interface of aromatic molecules on semiconductor surfaces because of the rich functionality of such molecules on semiconductor surfaces.The chemisorption of pyrazine molecules on the Si(100)-2×1 surface has been investigated using the B3LYP density functional theory with Si9H12 one-dimer and Si15H16 two-dimer cluster models.The calculated results predict that N-dative bonded-state,C2-C5 [4 2] and the tight-bridge1,2,5,6 products may coexist on the Si(100)-2×1 surface.     

Observation of photocatalytic dissociation of water on terminal Ti sites of TiO2(110)-1 × 1 surface

The water splitting reaction based on the promising TiO(2) photocatalyst is one of the fundamental processes that bears significant implication in hydrogen energy technology and has been extensively studied. However, a long-standing puzzling question in understanding the reaction sequence of the water splitting is whether the initial reaction step is a photocatalytic process and how it happens. Here, using the low temperature scanning tunneling microscopy (STM) performed at 80 K, we observed the dissociation of individually adsorbed water molecules at the 5-fold coordinated Ti (Ti(5c)) sites of the reduced TiO(2) (110)-1 × 1 surface under the irradiation of UV lights with the wavelength shorter than 400 nm, or to say its energy larger than the band gap of 3.1 eV for the rutile TiO(2). This finding thus clearly suggests the involvement of a photocatalytic dissociation process that produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, that is, OH(br), and the other either occurs as OH(t) at Ti(5c) sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of the water can only produce OH(t) or oxygen adatoms exactly at the original Ti(5c) sites, without the trace of OH(br). Such a difference clearly indicates that the photocatalytic dissociation of the water undergoes a process that differs significantly from the attachment of electrons injected by the tip. Our results imply that the initial step of the water dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.     

Oxidation of Anatase TiO2(001) (1×4) Surface

Anatase TiO₂(001) surface arouses lots of research interests since it is believed to be the most reactive surface. However, recent STM measurements showed that except the defect sites, anatase TiO₂(001) (1×4) reconstructed surface is inert to H₂O adsorption. It was indicated that oxidation could be the reason which induces the inert surface reactivity. Therefore, it is strongly motivated to understand the oxidation structures as well as the oxidation process on this surface. In this work, based on first principles calculations, we investigated the oxidized structures and processes of TiO₂ anatase (001) surface with (1×4) reconstruction. We have discovered two kinds of oxidized structures through the molecular adsorption and dissociated adsorption with different oxidation ratio. To understand the oxidation process, we studied the reaction barrier of oxidation process. We conclude the stability of different oxidized structures with different oxidation ratio by comparing the free energy of the system as a function of oxygen chemical potential. Based on that, a first-principles-based phase diagram of the low-energy oxidized surface structures is provided. The effects of the lattice stress are also studied. Results show that the oxidized structure and oxidation ratio strongly depend on the temperature and pressure. The lattice stress also plays an important role.     

Stranski-Krastanov growth of para-sexiphenyl on Cu(110)-(2×1)O revealed by optical spectroscopy

We have studied the growth of para-sexiphenyl (p-6P) on the Cu(110)-(2×1)O surface using reflectance difference spectroscopy (RDS) in combination with scanning tunneling microscopy (STM). The evolution of the optical anisotropy reveals that the growth of p-6P on the Cu(110)-(2×1)O surface at room temperature follows the Stranski-Krastanov growth mode with a two monolayer thick wetting layer. During all stages of growth, the p-6P molecules are well orientated with their long molecular axis aligned parallel to the Cu-O rows along the [001] direction of the Cu(110) substrate. The high packing density of the p-6P molecules in the first and second monolayer evidenced by RDS and STM is believed to be responsible for the switch from layer-by-layer to three-dimensional island growth.     

Role of Reduced Defects for Coupling Reactions of Acetaldehyde on Anatase TiO2(001)-(1×4) Surface

The chemistry of acetaldehyde (CH₃CHO) adsorbed on the anatase TiO₂(001)-(1×4) surface has been investigated by temperature-programmed desorption (TPD) method. Our experimental results provide the direct evidence that the perfect lattice sites on the anatase TiO₂(001)-(1×4) surface are quite inert for the reaction of CH₃CHO, but the reduced defect sites on the surface are active for the thermally driven reductive carbon-carbon coupling reactions of CH₃CHO to produce 2-butanone and butene. We propose that the coupling reactions of CH₃CHO on the anatase TiO₂(001)-(1×4) surface should undergo through the adsorption of paired CH3CHO molecules at the reduced defect sites, since the existing reduced Ti pairs provide the suitable adsorption sites.     

Adsorption and decomposition of cyclohexanone (C6H10O) on Pt(111) and the (2 × 2) and (√3 × √3)r30°-Sn/Pt(111) surface alloys

Adsorption and decomposition of cyclohexanone (C(6)H(10)O) on Pt(111) and on two ordered Pt-Sn surface alloys, (2 × 2)-Sn/Pt(111) and (√3 × √3)R30°-Sn/Pt(111), formed by vapor deposition of Sn on the Pt(111) single crystal surface were studied with TPD, HREELS, AES, LEED, and DFT calculations with vibrational analyses. Saturation coverage of C(6)H(10)O was found to be 0.25 ML, independent of the Sn surface concentration. The Pt(111) surface was reactive toward cyclohexanone, with the adsorption in the monolayer being about 70% irreversible. C(6)H(10)O decomposed to yield CO, H(2)O, H(2), and CH(4). Some C-O bond breaking occurred, yielding H(2)O and leaving some carbon on the surface after TPD. HREELS data showed that cyclohexanone decomposition in the monolayer began by 200 K. Intermediates from cyclohexanone decomposition were also relatively unstable on Pt(111), since coadsorbed CO and H were formed below 250 K. Surface Sn allowed for some cyclohexanone to adsorb reversibly. C(6)H(10)O dissociated on the (2 × 2) surface to form CO and H(2)O at low coverages, and methane and H(2) in smaller amounts than on Pt(111). Adsorption of cyclohexanone on (√3 × √3)R30°-Sn/Pt(111) at 90 K was mostly reversible. DFT calculations suggest that C(6)H(10)O adsorbs on Pt(111) in two configurations: by bonding weakly through oxygen to an atop Pt site and more strongly through simultaneously oxygen and carbon of the carbonyl to a bridged Pt-Pt site. In contrast, on alloy surfaces, C(6)H(10)O bonds preferentially to Sn. The presence of Sn, furthermore, is predicted to make the formation of the strongly bound C(6)H(10)O species bonding through O and C, which is a likely decomposition precursor, thermodynamically unfavorable. Alloying with Sn, thus, is shown to moderate adsorptive and reactive activity of Pt(111).     

The Adsorption and Photocatalytic Decomposition of Citric Acid on Pt/TiO_2

The adsorption and photocatalutic decomposition of citric acid on both Pt/TiO2 powder and n-TiO2 single crystal electrode were studied in aqueous solutions of various pH. It was found that citrate ions were chemisorbed on TiO2, which could increase the interfacial capacity and the filling factor of photocurrent-potential curves. The quantity of adsorption, slope of mott-Shottky plot and the rate of photocatalytic decomposition of citric acid were found to depend strongly on pH of solution. The phot ocatalytic decomposition of citric acid was discussed in light of its adsorption on TiO2.     

Formation of sulfhydryl (SH) species on the clean and (2 × 2)-S covered Pt(111) surfaces by H2S decomposition

H₂S decomposition on the clean and (2 × 2)-S covered Pt(111) surfaces has been characterized using high-resolution electron energy loss (HREELS) and temperature-programmed desorption (TPD) spectroscopies. On the Pt(111)-(2 × 2)-S surface, a mixture of molecular H₂S and sulfhydryl (SH) species forms following H₂S adsorption at 110 K. The molecular H₂S desorbs at 140 K leaving a (2 × 2)-S overlayer saturated with SH; the SH is stable up to 190 K. On the clean Pt(111) surface, a mixture of atomic sulfur, SH and chemisorbed molecular H₂S is formed following H₂S adsorption at 110 K. On the clean surface, adsorbed SH decomposes near 150 K. We report here the first definitive observation of an adsorbed sulfhydryl species on a metal surface.