The adsorption of CO,O2, and H2 on Pt: I. Thermal desorption spectroscopy studies

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O₂, and h₂ on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O₂ adsorption at steps or defects than on terraces.     

Effects of bimetallic modification on the decomposition of CH3OH and H2O on Pt/W(110) bimetallic surfaces

The decomposition pathways of methanol and water on Pt-modified W(110) bimetallic surfaces have been investigated using density functional theory (DFT), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). The reaction of methanol on submonolayer and monolayer Pt-modified W(110) surfaces is compared to that on clean Pt(111) and W(110). Similar to clean W(110), the Pt/W(110) bimetallic surfaces remain active toward the dissociation of methanol, although the reaction pathway leading to the production of CH₄ is reduced on the bimetallic surfaces. The Pt/W(110) surfaces are also active toward the decomposition of water. These results are compared with previous studies of the reactions of H₂ and ethylene on Pt/W(110) bimetallic surfaces to reveal the different Pt-modification effects for the dissociation of oxygen-containing molecules.     

The adsorption of CO,O2, and H2 on Pt: II. Ultraviolet photoelectron spectroscopy studies

Ultraviolet photoelectron spectroscopy (UPS) has been used to study the chemisorption of CO, O₂, and H₂ on platinum. Three single crystal surfaces ((111), 6(111) × (100), and 6(111) × (111)) and two polycrystalline surfaces were studied. These studies yielded three important results. First, the most dominant change in the Pt valence band upon gas adsorption was a decrease in the height of the peak immediately below the Fermi level. This decrease was nearly identical for all three gases studied. Second, CO adsorption resulted in the formation of a resonance state ～8 eV below the Fermi level which was attributed to CO molecular orbitals. In contrast, no dominant resonance states were observed for adsorbed O or H. The lack of an O resonance state on platinum is in contrast to the results observed for O adsorbed on Fe and Ni and suggests important differences between the OPt chemisorption bond and the OFe and ONi chemisorption bonds. Finally, adsorption of CO at steps or defects led to a decrease in work function while its adsorption on terraces led to an increase in work function. For H, adsorption at steps led to an increase in work function while adsorption on terraces led to a decrease in work function. The adsorption of O led to an increase in work function on all of the surfaces studied.     

Biomass-derived oxygenate reforming on Pt(111): A demonstration of surface science using d-glucose and its model surrogate glycolaldehyde

Molecules derived from cellulosic biomass, such as glucose, represent an important renewable feedstock for the production of hydrogen and hydrocarbon-based fuels and chemicals. Development of efficient catalysts for their reformation into useful products is needed; however, this requires a detailed understanding of their adsorption and reaction on catalytically active transition metal surfaces. In this paper we demonstrate that the standard surface science techniques routinely used to characterize the reaction of small molecules on metals are also amenable for use in studying the adsorption and reaction of complex biomass-derivatives on single crystal metal surfaces. In particular, Temperature Programmed Desorption (TPD) and High Resolution Electron Energy Loss Spectroscopy (HREELS) combined with Density Functional Theory (DFT) calculations were used to elucidate the adsorption configuration of d-glucose and glycolaldehye on Pt(111). Both molecules were found to adsorb in an η₁ aldehyde configuration partially validating the use of simple, functionally-equivalent model compounds for surface studies of cellulosic oxygenates.     

Ethylene adsorption on thin films of Ni,Pd, Pt,Cu, Au and Al; Work function measurements

The changes in work function φ upon adsorption of C₂H₄ on clean film surfaces of six fcc metals (Ni, Pd, Pt, Cu, Au and Al) have been followed by means of photoelectron emission at 293 K. A marked difference was observed in the behaviour between Ni, Pd and Al on the one side and on Cu, Au and Pt on the other side: while with Ni, Pd and Al, φ as a function of coverage goes through a maximum, with Cu, Au and Pt, φ only decreases. In the discussion, the data obtained by work function measurements are related to other literature data. Several films covered with C₂H₄species were also submitted to a heat treatment while in other experiments H₂ was admitted to the surface covered by C₂H₄ species. In some experiments C₂h₄ was admitted to surfaces covered by H₂. In all cases φ was measured. The experiments reveal that C₂h₄is absorbed only reversibly on Cu and Au. On Ni, Pd and Pt, C₂H₄ is adsorbed initially with dissociation and this leads to an increase in φ on Ni and Pd and a decrease on Pt. Hydrogenated reactive species contribute to the lowering of φ observed with Ni, Pd and Pt. As with Cu and Au also on Ni, Pd and Pt a weakly bound C₂h₄is observed which leads to a decrease in φ as well. The behaviour of φ indicates that upon Al, C₂h₄ adsorbs first dissociatively to a small extent, while the weakly bound C₂H₄species act as intermediates for strongly adsorbed species which were observed after some time.     

NH3在Ni/Pt(111)和Ni/WC(001)表面上分解的第一性原理研究

采用密度泛函理论和slab模型,研究NH₃在Ni单原子层覆盖的Pt（111）和WC（001）表面上的物理与化学行为,计算了Ni单原子覆盖表面的电子结构以及NH₃的吸附与分解.表面覆盖的单原子层中,Ni原子的性质与Ni（111）面上的Ni原子明显不同.与Ni（111）相比,Ni/Pt（111）和Ni/WC（001）表面上Ni原子dz²轨道上的电子更多地转移到了其它位置,该轨道上电荷密度降低有利于NH₃吸附.在Ni/Pt（111）和Ni/WC（001）面上NH₃吸附能均大于Ni（111）,NH₃分子第一个N-H键断裂的活化能则明显比Ni（111）面上低,有利于NH₃的分解,吸附能增大使NH₃在Ni/Pt（111）和Ni/WC（001）面上更倾向于分解,而不是脱附.N₂分子的生成是NH₃分解的速控步骤,该反应能垒较高,说明N₂分子只有在较高温度下才能生成.WC与Pt性质相似,但Ni/Pt（111）和Ni/WC（001）的电子结构还是有差异的,与Ni（111）表面相比,NH₃在Ni/Pt（111）表面上分解速控步骤的能垒降低,而在Ni/WC（001）上却升高.要获得活性好且便宜的催化剂,需要对Ni/WC（001）表面做进一步改进,降低N₂分子生成步骤的活化能.     

Similarities in the decomposition and dehydrogenation of cyclohexene on (4 × 4)-CMo(110) and Pt(111)

The decomposition and dehydrogenation of cyclohexene have been used as probing reactions to directly compare the surface reactivities of carbon-modified Mo(110) and Pt(111). By using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS), we find that the dominant decomposition pathway of cyclohexene can be converted from complete decomposition to atomic carbon and hydrogen on clean Mo(110) to partial dehydrogenation to benzene and hydrogen on (4 × 4)-CMo(110). The latter reaction mechanism is very similar to that observed on Pt(111). Furthermore, in contrast to the near identical catalytic properties that are commonly observed for bulk molybdenum carbide and nitride powder materials, we find that the reactivities of carbon-modified and nitrogen-modified Mo(110) surfaces are qualitatively different.     

Improved resolution in HREELS using maximum-entropy deconvolution: CO on PtxNi1−x(111)

High-resolution electron energy loss spectroscopy (HREELS) has been used to study stretch vibrations of CO chemisorbed at low coverage on Pt_xNi_1−x(111). Bayesian probability theory along with the entropic prior (Maxent) has been employed to deconvolve the apparatus function and to improve the apparent energy resolution. Maxent has proven very successful in a wide range of inversion problems. Here the resolution enhancement enables the positions of CO on the PtNi surface to be identified. It appears that CO is predominantly on top of Ni with the Ni atoms coordinated threefold laterally and with Ni in linear chains or on top of Pt. Furthermore, the ratio of the Pt to the Ni peak is used to study the dependence of the Pt concentration in the first layer on the annealing temperature.     

Surface characterization of supported Pt,Rh, and alloy particles by CO chemisorption

Temperature programmed desorption (TPD) of CO is used to determine surface areas, binding states, and changes upon oxidation for 10–1000 Å particles of Pt, Rh, and Pt-Rh alloy on amorphous SiO₂. A low area sample configuration is used to obtain rapid and uniform heating and cooling in an ultra-high vacuum system. It is shown that both metals exhibit a higher CO binding state for small particles, but, as particle size increases, this state disappears and is replaced by a more weakly bound state. These states are suggested to be associated with (111) and higher surface free energy planes on these surfaces, heating Rh above 700 K in O₂ at 10^?6 Torr produces an oxide on which the CO saturation coverage is at least a factor of 10 lower than on the reduced surface. For Pt, oxidation produces only a small decrease in CO coverage, although the binding energy of CO increases on the oxygen treated surface. The difference in desorption temperatures for CO on Pt and Rh is consistent with previous experiments which show that an oxidation-reduction cycle produces a surface layer which is enriched in Rh and that the oxidized alloy contains no Pt atoms.     

Vibration spectroscopy of benzene adsorbed on Pt(111) and Ni(111)

High resolution electron energy loss spectroscopy has been applied to study the adsorption of benzene (C₆H₆ and C₆D₆) on Pt(111) and Ni(111) single crystal surfaces between 140 and 320 K. The vibrational spectra provide evidence that benzene is chemisorbed with its ring parallel to the surface, predominantly π bonded to the platinum and nickel surface respectively. A significant frequency increase of the CH-out-of-plane bending mode, largest in the case of platinum, is observed compared to the free molecule. On both metals two phases of benzene exist simultaneously, characterized by a different frequency shift. The shifts are explained by electronic interaction between the metal d-orbitals and molecules adsorbed in on top and threefold hollow sites respectively. The vibrational spectra of the multilayer condensed phase of benzene exhibit the infrared active modes of the gasphase molecule as expected.     

TPD, HREELS and UPS study of the adsorption and reaction of methyl nitrite (CH3ONO) on Pt(111)

The adsorption and reaction of methyl nitrite (CH₃ONO, CD₃ONO) on Pt(111) was studied using HREELS, UPS, TPD, AES, and LEED. Adsorption of methyl nitrite on Pt(111) at 105 K forms a chemisorbed monolayer with a coverage of 0.25 ML, a physisorbed second layer with the same coverage that desorbs at 134 K, and a condensed multilayer that desorbs at 117 K. The Pt(111) surface is very reactive towards chemisorbed methyl nitrite; adsorption in the monolayer is completely irreversible. CH₃ONO dissociates to form NO and an intermediate which subsequently decomposes to yield CO and H₂ at low coverages and methanol for CH₃ONO coverages above one-half monolayer. We propose that a methoxy intermediate is formed. At least some C–O bond breaking occurs during decomposition to leave carbon on the surface after TPD. UPS and HREELS show that some methyl nitrite decomposition occurs below 110 K and all of the methyl nitrite in the monolayer is decomposed by 165 K. Intermediates from methyl nitrite decomposition are also relatively unstable on the Pt(111) surface since coadsorbed NO, CO and H are formed below 225 K.     

Adsorption and decomposition of γ-butyrolactone on Pd(1 1 1) and Pt(1 1 1)

The adsorption and thermal chemistry of γ-butyrolactone (GBL) on the (1 1 1) surface of Pd and Pt has been investigated using a combination of high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). HREELS results indicate that GBL adsorbs at 160 K on both surfaces through its oxygenate functionality. On Pd(1 1 1), adsorbed GBL undergoes ring-opening and decarbonylation by 273 K to produce adsorbed CO and surface hydrocarbon species. On Pt(1 1 1), very little dissociation is observed using HREELS, with almost all of the GBL simply desorbing. TPD results are consistent with decarbonylation and subsequent dehydrogenation reactions on Pd(1 1 1), although small amounts of CO₂ are also detected. TPD results from Pt(1 1 1) indicate that a small proportion of adsorbed GBL (perhaps on defect sites) does undergo ring-opening to produce CO, CO₂, and H₂. These results suggest that the primary dissociation pathway for GBL on Pd(1 1 1) is through O-C scission at the carbonyl position. Through comparisons with previously published studies of cyclic oxygenates, these results also demonstrate how ring strain and functionality affect the ring-opening rate and mechanism.     

The hydrogenation of CN on Pd(111) and Pd(100)

Adsorbed CN may be produced on Pd(111) and Pd(100) surfaces at RT by dissociative adsorption of cyanogen. HREELS measurements show that adsorbed CN forms adsorbed HCN or DCN on these Pd surfaces by reaction with H adsorbed from the residual gas or by dosing with H₂ or D₂. The reaction temperature is slighly lower for Pd(100) than for Pd(111), and the range of temperatures over which the reaction takes place much narrower. The reaction occurs on a time scale easily monitored with HREELS.     

Low sputter damage of metal single crystalline surfaces investigated with medium energy ion scattering spectroscopy

It was observed clearly that the sputter damage due to Ar⁺ ion bombardment on metal single crystalline surfaces is extremely low and the local surface atomic structure is preserved, which is totally different from semiconductor single crystalline surfaces. Medium energy ion scattering spectroscopy (MEIS) shows that there is little irradiation damage on the metal single crystalline surfaces such as Pt(111), Pt(100), and Cu(111), in contrast to the semiconductor Si(100) surfaces, for the ion energy of 3–7 keV even above 10¹⁶–10¹⁷ ions/cm² ion doses at room temperature. However, low energy electron diffraction (LEED) spots became blurred after bombardment. Transmission Electron Microscopy (TEM) studies of a Pt polycrystalline thin film showed formation of dislocations after sputtering. Complementary MEIS, LEED and TEM data show that on sputtered single-crystal metal surfaces, metal atoms recrystallize at room temperature after each ion impact. After repeated ion impacts, local defects accumulate to degrade long range orders.     

A LEED, TPD and HREELS investigation of NO adsorption on Sn/Pt(111) surface alloys

The adsorption of NO on Pt(111), and the (2 × 2)Sn/Pt(111) and (√3 × √3)R30°Sn/Pt(111) surface alloys has been studied using LEED, TPD and HREELS. NO adsorption produces a (2 × 2) LEED pattern on Pt(111) and a (2√3 × 2√3)R30° LEED pattern on the (2 × 2)Sn/Pt(111) surface. The initial sticking coefficient of NO on the (2 × 2)Sn/Pt(111) surface alloy at 100 K is the same as that on Pt(111), S₀ = 0.9, while the initial sticking coefficient of NO on the (√3 × √3)R30°Sn/Pt(111) surface decreases to 0.6. The presence of Sn in the surface layer of Pt(111) strongly reduces the binding energy of NO in contrast to the minor effect it has on CO. The binding energy of β-state NO is reduced by 8–10 kcal/mol on the Sn/Pt(111) surface alloys compared to Pt(111). HREELS data for saturation NO coverage on both surface alloys show two vibrational frequencies at 285 and 478 cm⁻¹ in the low frequency range and only one N-O stretching frequency at 1698 cm⁻¹. We assign this NO species as atop, bent-bonded NO. At small NO coverage, a species with a loss at 1455 cm⁻¹ was also observed on the (2 × 2)Sn/ Pt(111) surface alloy, similar to that observed on the Pt(111) surface. However, the atop, bent-bonded NO is the only species observed on the (√3 × √3)R30°Sn/Pt(111) surface alloy at any NO coverage studied.     

The adsorption and reaction of ethylene glycol and 1,2-propanediol on Pd(111): A TPD and HREELS study

The reactions of ethylene glycol and 1,2-propanediol have been studied on Pd(111) using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Both molecules initially decompose through O–H activation, forming ethylenedioxy (–OCH₂CH₂O–) and 1,2-propanedioxy (–OCH₂CH(CH₃)O–) surface intermediates. For ethylene glycol, increases in thermal energy lead to dehydrogenation and formation of carbonyl species at both oxygen atoms. The resulting glyoxal (O═CHCH═O) either desorbs molecularly or reacts through one of two competing pathways. The favored pathway proceeds via C–C bond scission, dehydrogenation, and decarbonylation to form carbon monoxide and hydrogen. In a minor pathway, small amounts of glyoxal undergo C–O bond scission and recombination with surface hydrogen to form ethylene and water. The same reaction mechanism occurs for 1,2-propanediol after methyl elimination and formation of glyoxal. However, this is accompanied by a minor pathway involving a methylglyoxal (O=CHC(CH₃)=O) intermediate. The prevalence of the dehydrogenation/decarbonylation pathway in the current work is consistent with the high selectivity for C–C scission in the aqueous phase reforming of polyols on supported Pd catalysts.     

The decomposition of ammonia on the flat (111) and stepped (557) platinum crystal surfaces

Ammonia adsorption, desorption and decomposition to H₂ and N₂ has been studied on the flat (111) and stepped (557) single crystal faces of platinum using molecular beam surface scattering techniques. Both surfaces show significant adsorption with sticking coefficients on the order of unity. The stepped (557) surface is 16 times more reactive for decomposition of ammonia to N₂ and H₂ than the flat (111) surface. Kinetic parameters have been determined for the ammonia desorption process from the Pt(111) surface. The mechanism of ammonia decomposition on the (557) face of platinum has been investigated.     

Vibrational EELS,XPS and UPS study of CO chemisorbed on Pt10Ni90(111): Evidence for the existence of different sites

CO adsorption on the (111) face of a Pt₁₀Ni₉₀ alloy single crystal has been investigated at room temperature by vibrational electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (XPS and UPS). Two well separated CO stretching modes develop at 2070 and 1820 ± 10 cm^?1, with their intensities reaching 64 and 36% respectively of the total intensity at saturation coverage. They are attributed to CO adspecies in terminal and bridge bonded configuration respectively. The UPS spectra of 4σ, 5σ and 1π molecular orbitais of adsorbed CO show complex features which may be resolved into two components having the main characteristics of CO adsorbed on pure Pt(111) and Ni(111) respectively. Such behaviour is also observed by XPS on C 1s on O 1s peaks. Their respective contributions, in both XPS and UPS spectra are about 64 and 36% of the whole spectrum. Finally compared to Ni(111) — on which CO adsorbs mainly in bridge configuration — the alloying with 10% Pt has generated the appearance of a large number of new sites for CO chemisorption associated with the presence of Pt atoms at the surface. The large amount of terminal CO adspecies is interpreted in terms of considerable surface enrichment of the alloy in platinum.     

Studying the effect of hydrogen on diamond growth by adding C10H10Fe under high pressures and high temperatures

In this paper, hydrogen-doped industrial diamonds and gem diamonds were synthesized in the Fe–Ni–C system with C₁₀H₁₀Fe additive, high pressures and high temperatures range of 5.2–6.2?GPa and 1250–1460°C. Experimental results indicate similar effect of hydrogen on these two types of diamonds: with the increasing content of C₁₀H₁₀Fe added in diamond growth environment, temperature is a crucial factor that sensitively affects the hydrogen-doped diamond crystallization. The temperature region for high-quality diamond growth becomes higher and the morphology of diamond crystal changes from cube-octahedral to octahedral. The defects on the {100} surfaces of diamond are more than those on the {111} surfaces. Fourier transform infrared spectroscopy (FTIR) results indicate that the hydrogen atoms enter into the diamond crystal lattice from {100} faces more easily. Most interestingly, under low temperature, nitrogen atoms can also easily enter into the diamond crystal lattice from {100} faces cooperated with hydrogen atoms.     

Low energy electron diffraction studies of the surfaces of molecular crystals (ice,ammonia, naphthalene,benzene)

Low Energy Electron Diffraction (LEED) has been used to study the surface structures of thin films of molecular crystals. The samples were grown epitaxially on metal single crystal substrates at low temperatures. Both Pt(111) and Ag(111) surfaces were used as substrates in order to identify the influence of the substrate on molecular film structure. Previous observations of ice (0001) and naphthalene (001) surfaces on films grown on Pt(111) substrates [Surface Sci. 55 (1976) 413], were confirmed using the Ag(111) substrate. The NH₃(111) and benzene (111) surfaces were also studied on films grown on either substrate. All observed molecular crystal surfaces showed no evidence of surface reconstruction. To minimize sample charging and electron beam induced damage, LEED experiments were performed on samples of thickness less than 10?10² nm, with low energy electron exposures less than 1 C cm^?2. The maximum thickness and exposure values were characteristic of the particular molecular crystal. The relationship between the structure of the initial adsorbed monolayer and the molecular crystal orientation is discussed.