Mechanism of the Yb<Superscript>2+</Superscript> → Yb<Superscript>3+</Superscript> valence transition in ytterbium nanofilms upon chemisorption of CO and O<Subscript>2</Subscript> molecules on their surface

The dependences of the work function of ytterbium nanofilms with a thickness ranging from 1 to 32 monolayers on the amount of CO or O₂ molecules chemisorbed on their surface have been investigated experimentally. It has been found that these dependences have a pronounced nonmonotonic character. The mechanism of the Yb²⁺ → Yb³⁺ valence transition, which occurs upon the chemisorption of CO or O₂ molecules on the surface of ytterbium nanofilms, has been developed using the results of this study together with the previously obtained data.     

Theoretical study on the catalytic properties of single-atom catalyst stabilised on silicon-doped graphene sheets

ABSTRACT

The stable configurations, electronic structures and catalytic activities of single-atom metal catalyst anchored silicon-doped graphene sheets (3Si-graphene-M, M?=?Ni and Pd) are investigated by using density functional theory calculations. Firstly, the adsorption stability and electronic property of different gas reactants (O₂, CO, 2CO, CO/O₂) on 3Si-graphene-M substrates are comparably analysed. It is found that the coadsorption of O₂/CO or 2CO molecules is more stable than that of the isolated O₂ or CO molecule. Meanwhile, the adsorbed species on 3Si-graphene-Ni sheet are more stable than those on the 3Si-graphene-Pd sheet. Secondly, the possible CO oxidation reactions on the 3Si-graphene-M are investigated through Eley–Rideal (ER), Langmuir–Hinshelwood (LH) and new termolecular Eley–Rideal (TER) mechanisms. Compared with the LH and TER mechanisms, the interaction between 2CO and O₂ molecules (O₂?+?CO → CO₃, CO₃?+?CO → 2CO₂) through ER reactions (< 0.2?eV) are an energetically more favourable. These results provide important reference for understanding the catalytic mechanism for CO oxidation on graphene-based catalyst.     

Chemisorption and decomposition reactions of oxygen-containing organic molecules on clean Pd surfaces studied by UV photoemission

We have used uv photoeinission (primarily at a photon energy hv = 40.8 eV) to study chemisorption and decomposition reactions of small oxygen-containing organic molecules on clean polycrystalline Pd surfaces at 120 and 300 K. These molecules include methanol (CH₃OH), dimethyl ether (CH₃OCH₃) formaldehyde (H₂CO), acetaldehyde [H(CH₃)CO], and acetone [(CH₃)₂CO]. Chemisorption bonding of these molecules to the Pd surface occurs primarily through the lone-pair orbitais associated with the oxygen atoms, as evidenced by chemical bonding shifts of these orbitais toward larger electron binding energy relative to the other adsorbate valence orbitals. At 300 K all the molecules studied decompose on the surface, resulting in chemisorbed CO. Since chemisorbed (as well as condensed) phases of some of these molecules (CH₃OH and H(CH₃)CO) are observed at low temperature, the decomposition to CO is a thermally-activated reaction. The observed orbital shifts associated with chemisorption bonding are used to make rough estimates of interaction strengths and chemisorption bond energies (within the framework of Mulliken's theory of electron donor-acceptor complexes as applied to chemisorption by Grimley). The resulting heats of chemisorption are consistent with the observed surface reactions.     

The detection by SERS of resonantly excited desorption of pyridine from silver island films by IR laser absorption

We have shown that when a very thin film of pyridine about two or three monolayers thick on a silver island film is exposed to a pulsed CO₂ laser line whose frequency corresponds to that of a pyridine vibrational mode the physisorbed molecules within the pyridine film can be desorbed even at liquid He temperatures. It is interesting that this observation was first made using surface enhanced Raman scattering. Experimental results are presented from which it is concluded that the phenomenon can be described as resonantly excited desorption. The absorbed IR energy seems to be localized within the pyridine film and the silver film and thermallized to some degree causing some of the physisorbed molecules to desorb. Analysis of the SERS spectra before and after the resonantly excited desorption has enabled us to separate out the SERS spectra due to the physisorbed pyridine and the chemisorbed pyridine.     

A photoelectron spectroscopic study of the adsorption of CO and CO2 on platinum

The adsorption of CO and CO₂ on platinum has been studied by UV photoelectron spectroscopy using both He I and He II radiation. The modulation of the intensity of the spectral features observed for adsorbed CO as the photon energy is changed is used to assign the observed levels. The results are in reasonable agreement with recent theoretical and experimental work. The levels are observed to shift by different amounts compared to gas phase CO because of chemical binding effects. The adsorption of CO₂ produces spectral features that are shifted by the same amount compared to gas phase CO₂. This, together with the absence of any localized attenuation of the platinum valence band and the low heat of adsorption, indicates that CO₂ is physisorbed on platinum.     

Catalytic Reaction Mechanism Testing

Experimental tests for determining the mechanism of catalytic reactions were suggested. Quantitative relations were obtained that allowed the mechanism of formation of product molecules in an arbitrary catalytic reaction to be determined by isothermic relaxation methods. The relations found were compared with the literature data on the 2CO + O₂ → 2CO₂ reaction on the (111) surface of Pd. The experimental data were shown to be insufficient for the unambiguous determination of the mechanism of this reaction. The results available corresponded to the participation of physically adsorbed CO molecules in the formation of CO₂. The temperature dependence of the reaction rate was determined by the transition of strongly adsorbed oxygen atoms into the mobile reactive state.     

A transient study of the co-adsorption and reaction of CO and oxygen on Ir(110)

The heterogeneously catalyzed reaction of CO and O₂ to form CO₂ over Ir(110) has been studied through measurements of the transient kinetics of the various elementary reactions that may limit the steady state rate. Rate expressions for these elementary reactions — the desorption of CO, the oxidation of CO via the Langmuir-Hinshelwood mechanism, the adsorption of CO and the adsorption of oxygen — were developed using thermal desorption mass spectrometry. Several phenomena were observed: (1) the activation energies for CO desorption and CO oxidation depend markedly upon the composition of the adlayer; (2) diffusion in the adlayer may limit the rates of CO desorption and CO oxidation; (3) the formation of a surface oxide modifies these four rate processes; and (4) chemisorbed CO blocks sites for oxygen adsorption, but chemisorbed oxygen does not block sites for CO adsorption.     

Nitric oxide reduction by CO ON Rh(111): Temperature programmed reaction

The reaction of NO with CO on Rh(111) has been studied with temperature programmed reaction (TPR). Comparisons are made with the reaction of O₂ with CO and the reaction of NO with H₂. The rate-determining step for both CO oxidation reactions is CO(a) + O(a) → CO₂(g). Repulsive interactions between adsorbed CO and adsorbed nitrogen atoms lead to desorption of CO in a peak at 415 K which is in the temperature range where the reaction between CO(a) and O(a) produces CO₂(g). Thus the extent of reaction of CO(a) with NO(a) is less than that between CO(a) and O(a) due to the lower coverage of CO caused by adsorbed N atoms and NO. A similar repulsive interaction between NO(a) and H(a) suppresses the NO + H₂ reaction. CO + NO reaction behavior on Rh(111) is compared to that observed on Pt(111).     

Alternate interpretation of final state relaxation for chemisorbed molecules

Ab initio Xα electronic spectra have been calculated for CO before and after chemisorption onto Ni(001) in the observed c(2 × 2) symmetry. The electronic spectra of a monolayer of C₂H₂ has also been calculated and referenced to E_F of Ni. The theoretical eigenvalue spectra of these two molecules agrees colosely with ionization potentials measured by UPS when these molecules are chemisorbed onto Ni. Charge density calculations for CO indicate that the molecular orbitais are delocalized by the COCO and CONi bonds. These results suggest that final state relaxation effects are small and possibly zero for the chemisorbed molecule and that the relaxation energy of chemisorbed molecules is approximately the correlation energy.     

Normal Unenhanced Raman Spectra of CO and CH4 Adsorbed on Cobalt(poly)

Abstract

Normal unenhanced Raman spectra (NURS) of low-polarizability CO molecules were observed for the first time on cobalt at R. T. and residual gas pressure. We assign five bands observed between 2030–2130 cm^?1 to linear chemisorbed CO species, while those observed between 1840–2010 cm^?1 have been ascribed to the 2-fold chemisorbed species. The three bands observed between 1740–1830 cm^?1 we believe are due to the 3-fold species. The corresponding fourteen Co-C stretches were observed and assigned. A model based upon electron backdonation is proposed for each of the three structures. NURS were also observed at R. T. for physisorbed CH₄ and assignments are made to the four frequencies of CH_4.     

Angular distribution of the desorption of CO2 produced on Pt(111) surface at low temperatures

A new CO₂ formation process was observed in the CO oxidation over Pt(111) surface below 200 K. The desorption flux of the product CO₂, which is formed from the interaction between chemisorbed CO and adsorbed oxygen molecules O₂^2? (a), showed a very sharp angular distribution along the surface normal.     

Bond making and breaking on transition-metal surfaces: Theoretical projections based on bond-order conservation

Our analytic Morse-potential model of chemisorption based on bond-order conservation [Surface Sci. 150 (1985) L115; 163 (1985) L645, L730] has been used to calculate the heats of chemisorption of various diatomic AB and polyatomic AB_x species (coordinated via A) and to estimate the activation barriers for their dissociation and transformations. Examples include adspecies such as CH_x, NH_x, OH_x, and possible intermediates and elementary steps of reactions such as CO + O → CO₂, NO + N → N₂ + O, N₂ + H₂ → NH₃, H₂ + O₂ → H₂O, and CO + H₂ → CH₄. Both the qualitative projections and numerical estimates are in good agreement with experiment. In particular, it is shown that (1) the most reactive adspecies should be the most weakly bound, and (2) the recombination activation barrier should primarily depend on (and may even be close to) the heat of chemisorption of the weaker bound partner.     

Activation mechanisms of silver toward CO oxidation by tungsten carbide substrate: A density functional theory study

The development of efficient catalysts for low temperature CO oxidation is important to the application of fuel cells. In this work, we report that the Ag monolayer on WC (0001) surface (Ag_ML/WC) could effectively catalyze CO oxidation through the L-H mechanism (CO + O₂ → OOCO → CO₂ + O). The most sluggish reaction step is suggested to be CO + O → CO₂ with a barrier of 0.48 eV, which is 1.21 eV lower than the barrier of O₂ dissociation. The electronic structure and d-band center analyses demonstrate that the promoted activity may originate from the synergistic effect between Ag monolayer and WC. The present study is conducive to design new efficient and cost-effective catalysts for CO oxidation without using of the noble platinum.     

The reactivity and auger chemical shift of oxygen adsorbed on platinum

The reaction of carbon monoxide with oxygen chemisorbcd on polycrystalline platinum has been studied using Auger spectroscopy. Two types of chemisorbed oxygen are distinguished on the basis of Auger electron chemical shifts and reactivity towards carbon monoxide. When the substrate is below 800 K, a single very reactive type of chemisorbed oxygen is formed. Above 800 K a new species begins to form which is characterized by an Auger chemical shift of about 6 eV and by low reactivity. The decay of the oxygen Auger signal using several fixed pressures of carbon monoxide was measured. The reaction is first order in carbon monoxide pressure but no clear decision can be made about the order with respect to oxygen coverage. With the reaction $\text{CO +}\text{1}\text{20}{}_2\text{→ CO}{}_2$ operating at steady-state, the oxygen coverage was measured as a function of CO pressure. In the region 363–600 K, the steady state oxygen coverage began to decline measurably when reached 0.1. When p_CO>p_O2the oxygen coverage became immeasurably small. A simple model is used to relate these phenomena to observed carbon dioxide production rates.     

Analysis of Dissociation—Recombination Processes for the CO<Subscript>2</Subscript> Molecule with the Spin—Orbit Coupling Taken into Account

The dissociation CO₂(X¹Σ) + M → CO(X¹Σ) + O(³P) + M and recombination CO(X¹Σ) + O(³P) + M → CO₂(X¹Σ) + M processes are considered with the spin—orbit coupling taken into account in the ground and several excited states of the CO₂ molecule. Because of the specific features of mutual position of potential energy surfaces of the CO₂ molecule in the ground and several excited states and the large values of spin—orbit interaction matrix elements, which causes the quantum nonadiabatic transition of the molecule from one state to another, these processes become effectively spin-allowed and the rate constants for the nonadiabatic reactions have large values. The proposed dissociation and recombination mechanisms include reactions involving singlet—triplet crossings.     

ESR study of pyrrhotite iron vacancies and the adsorption of CO and H2S

The ESR spectra were determined for pyrrhotite in vacuum, under CO, H₂S and a CO/H₂S mixture. The singlet signal observed in vacuum at room temperature is split into a doublet in H₂S and when heated in vacuum or CO. The g-values are dependent upon the nature of the adsorbing gas. The catalytic effect in H₂S-CO mixture is explained by a two step reaction: (i) H₂S⁺ and CO^- adsorption, (ii) interaction of chemisorbed H₂S⁺ with chemisorbed CO^- on the surface of catalyst to form active hydrogen.     

Selective effects of homogeneous and heterogeneous reactions in the system N21-CO

In the system N₂¹-CO, effects arising only from the reactions in the homogeneous phase have been isolated in a reactor with inert walls. After an induction period of 0.13 s, they involve a stationary rate of enhancement of N₂(B³Π_g) and N(⁴S) concentrations according to the reactions N₂(X¹Σ_g⁺)_v¹+CO→ CO(X¹Σ⁺)_v1 + N₂(X¹Σ_g⁺ and CO(X¹Σ⁺)_v1 + N₂(A³Σ_u⁺) → N (⁴S) + N(⁴S) + CO(X¹Σ⁺).In a reactor with active walls, both the above reactions in the homogeneous phase and heterogeneous reactions due to CO adsorbed on the walls are involved according to CO_ads + N₂(X¹Σ_g⁺)_{v1 ads or not} → CO(X¹Σ⁺)_v1 + N₂(X¹Σ_g⁺ and N(⁴S) + N(⁴S) + CO_ads → N₂(X¹Σ_g⁺) + CO(X¹Σ⁺)N(⁴S) + CO_ads → N₂(X¹Σ_g⁺) + CO(X¹Σ⁺). In this case, the rate of enhancement is not stationary. Furthermore, for cylindrical reactors with large diameters, the two types of reaction do not interact and their effects are additive.     

Reactive ion scattering study of physisorbed adsorbates: experiment and theory

We have studied reactive ion scattering (RIS) of hyperthermal (1–100 eV) Cs⁺ projectiles from physisorbed surfaces. RIS experiments from physisorbed water on Pt(1 1 1) reveal scattering products of Cs(H₂O)_n⁺ (n=1–3) cluster ions. The yields for RIS products are extremely high compared to those with chemisorbed species. Classical molecular dynamics simulations provide a new mechanism that explains the enhanced RIS yields with physisorbed species. Slow Cs⁺ projectiles pick up physisorbed molecules via an ion–surface abstraction reaction, preferably without direct collisions between projectile and adsorbate. This RIS process is very efficient and mechanistically different from the RIS process responsible for chemisorbed species that occurs through direct collision-induced desorption.     

Chemical effects on vibrational properties of adsorbed molecules on metal surfaces

Vibrational properties of chemisorbed molecules on metal surfaces are studied with a focus on the coverage (θ) dependent chemical shift of the frequencies. The electronic properties of an incomplete monolayer of adsorbates are calculated by means of the coherent potential approximation, in which the electron hopping between the adsorbates (band formation effect) and the depolarization effect due to the proximity of ionized adsorbed molecules are taken into account consideration. It is found that in a weakly chemisorbed system, such as CO/Cu, the negative shift in chemical origin amounts to the positive dipole shift at low coverage. It is also shown that the variation of the back-donated charge with θ gives rise to the coverage dependent polarizability, which in turn influences the frequency shift estimated by the previous dipole coupling theory. The coverage dependent back-donation also plays a significant role in the work function change Δϕ of the substrate. The polarity of a weakly chemisorbed CO remains unchanged compared to a free CO (⁻CO⁺) so that Δϕ exhibits the initial lowering in the presence of the positive dipoles. The increase in the back-donated charge with θ causes the decrease in the effective dipole moment towards a compensation of the positive hole due to 5σ donation. A simple explanation is offered to clarify the characteristic difference of the work function change between the strongly and weakly chemisorbed CO molecules on metal surfaces. In particular, a possible origin of the work function minimum observed for CO/Cu systems is discussed in terms of the coverage dependent back-donation.     

Theory of oscillatory oxidation of carbon monoxide over platinum

It is shown that the finite rate of decay for temperature fluctuations of the catalytic surface in the Langmuir-Hinshelwood reaction: 2 CO + O₂ → 2 CO₂ over Pt can give rise to a self-sustained oscillatory regime. The waveforms and periods are in good qualitative agreement with experiments by Dauchot and Van Cakenberghe.