Adsorption Of Carbon Monoxide Research Articles

Selective CO adsorption using sulfur-doped Ni supported by petroleum-based activated carbon

Carbon monoxide (CO) is an important platform compound that can be transformed into various fine chemicals. However, CO manufactured by the conventional methods contains many other gases such as hydrogen, methane, nitrogen and carbon dioxide, and additional separation processes are required to utilize CO as a raw material. The current state-of-the-art techniques for separating CO have problems of high energy consumptions. There are needs for an alternate separation process. In this work, we developed a Ni-based adsorbent supported by petroleum-based activated carbon (PAC, BET > 1300 m2/g). The affinity and capacity for CO were evaluated by CO isotherm and CO temperature-programmed desorption. The selectivity to CO was evaluated by the breakthrough test of multicomponent gases (10% H2, 10% CO, 1% CH4, and 1% CO2 with He balance). Ni/PAC was doped with sulfur to increase the CO adsorption activity, and the sulfur-doped Ni/PAC (Ni/PACS) showed outstanding CO adsorption capacity, which is 9 times higher than that of the sulfur-free Ni/carbon adsorbent (Ni/PAC). The Ni/PACS could recover the CO to over 99% purity from the multicomponent gases. The sulfur-doped Ni/carbon adsorbent showed high affinity, high capacity, and high selectivity for CO separation.

Transformation of the Electronic Characteristics of Ytterbium Nanofilms, Induced by Carbon-Monoxide Molecules Adsorbed on Their Surface

The adsorption of carbon monoxide (CO) molecules on ytterbium nanofilms composed of 16–200 single layers (6.1–76 nm-thick) is studied. The films are grown on grain-oriented tungsten ribbons with the preferential surface orientation (100). It is shown that, prior to the adsorption of CO molecules, ytterbium is divalent, possessing the [Xe]4f146s2 electron configuration. The adsorption of gas molecules in the surface region of the film led to the formation of a layer with trivalent ytterbium (with the [Xe]4f135d16s2 electron configuration). The minimum thickness of the ytterbium layer modified by adsorbed CO molecules is found to be composed of 16 single layers, corresponding to a thickness of 6.1 nm, which is anomalously higher than the values reported in publications.

Adsorbent screening for CO2/CO separation for applications in syngas production

Single gas carbon dioxide (CO2) and carbon monoxide (CO) adsorption behaviour is investigated in detail with different adsorbents for the potential separation of syngas. In this study, 20 zeolites, 2 activated aluminas, 3 activated carbons, and 2 silica gels were investigated by determining pure gas isotherms of CO2 and CO at 30 °C. The results were compared to literature to determine which adsorbents would be the most promising for CO2 separation from syngas at this temperature. Since CO2 concentrations vary in syngas based on the method of production, different adsorbents were selected for purification and bulk separation of CO2 from a syngas mixture.The individual adsorption capacities of CO2 and CO were analysed separately using a gravimetric system, modelled, and then compared for all of the adsorbents studied. CO2 is shown to be more favourably adsorbed compared to CO, with CO2 having higher adsorption capacities and heats of adsorption for all adsorbent samples studied. High-density silica gel and H-Y type zeolites were found to be promising adsorbents for the bulk separation of CO2 from syngas due to the shape of their isotherms, large CO2 adsorption capacities, their favourable adsorption capacity ratios for CO2 over CO. For the purification of syngas, Na+ exchanged FAU structured zeolites with their high CO2 adsorption capacity at low pressures, and favourable CO2 over CO adsorption capacity ratios was found to be the most promising adsorbent.

Carbon Monoxide(II) Purification in the Plenum Valves of the Building Ventilation System

The results of field studies on the selection of the best way to clean the supply air from carbon monoxide(II) in urban buildings are presented. A pilot plant has been developed to clean the supply air in the outer wall of the building using various sorbents, chemisorbent. Various methods for purifying supply air have been investigated. The isotherms of adsorption of carbon monoxide(II) on activated carbon, shungite, zeolite for calculating the volume of purified gas at constant air temperature were constructed according to the Freundlich equation, and empirical dependences of the concentration of carbon monoxide(II) on time were obtained using various options for purification of the supply air. The research results showed that the best sorbent for cleaning the outside air of carbon monoxide(II) is shungite + silica gel + manganese dioxide + zeolite, the cleaning efficiency was 95.35 %, and the second most effective sorbent for cleaning was the shungite from Karelian field – 86.5 %. The results can be used to clean the internal air of buildings with plenum valves along the height of the facade.

Adsorption and dissociation of carbon monoxide on rhodium surface: AB INITIO study

Ab initio molecular dynamics simulations of carbon monoxide adsorption and dissociation on Rh(100), Rh(110), and Rh(111) surfaces have been performed using projector augmented wave (PAW) pseudopotential and Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. The results reveal the C-downward configuration is the most preferable configuration for CO molecules on rhodium surface. CO dissociation energies at Rh(100) hollow site, Rh(110) bridge site, and Rh(111) hollow site are 1.64 eV, 0.05 eV, and 0.91 eV, respectively, which are much lower than that in vacuum (13.19 eV). The minimum energy barrier calculated by nudged elastic band (NEB) method indicates Rh(110) surface provides the highest activity by lowering the energy barrier and dissociation energy by ∼13eV.

Adsorptions of Diatomic Gaseous Molecules (H2, N2 and CO) on the Surface of Li+@C16B8P8 Fullerene-Like Nanostructure: Computational Studies

Density functional theory (DFT) calculations have been performed to investigate the adsorption of hydrogen (H2), nitrogen (N2) and carbon monoxide (CO) diatomic gaseous molecules at the surface of Li+ contained C16B8P8 fullerene-like nanostructure (Li+@C16B8P8). The evaluated results from the optimized structures indicated that the adsorption processes could be taken placed for the interacting gas and fullerene systems. Moreover, the electronic properties indicated that the electrical conductivities of Nano Clusters systems are changed after the adsorption processes, in which it could be a signal for detection or sensing of the existence of the gas in the environment. These changes lead to declining the HOMO/LUMO gap of the Fullerene-Like Nano Cage to its original value. As a finding of this work, it could be mentioned that the Li+@C16B8P8 fullerene-like nano cage could be considered as a suitable adsorbent for the CO, N2 and H2 gaseous. It means that the utilized Li+@C16B8P8 Fullerene-Like Nano Cage can detect the existence of gas in the environment.

Electrode stimulation: Redox reactions induced by modulating the electrostatic potential in solution

The rates of model heterogeneous redox reactions have been modulated by changing the electrostatic potential in the solution immediately adjacent to a working electrode, (WE), kept at a fixed value with respect to a reference electrode. The approach involves passing current between a stimulating electrode (SE) placed in front and at a short distance from the WE and a distant counter electrode. Conditions were selected so that the potential distribution in solution closely approximates that prescribed by the primary current distribution and thus maintain the electrolyte composition in the region between SE and WE virtually unperturbed. Illustrations are provided for the formation of gold oxide and the oxidation of adsorbed carbon monoxide on Pt in acidic aqueous electrolytes.

A study of properties of palladium metal as a component of fuel cells

Adsorptive studies focusing on palladium metal can model a situation in which some parts of platinum are replaced with palladium in a fuel cell, or some kind of PdPt alloy is used. Correspondingly thus, adsorptions of small molecules such as carbon monoxide and hydrogen are studied using density functional theory and the neural network method of generating higher dimensional potential energy surfaces. Various lower index surfaces of palladium metal are considered, and the adsorption energies appear to increase according to: Pd(211) ≃ Pd(110) >Pd(100) > Pd(111) with the adsorption of both molecules. The numerical values of these energies are in the range 0.52 to 1.86 eV with the adsorption of carbon monoxide while the values are between 0.28 to 1.76 eV with the adsorption of hydrogen. Exploring possible adsorption sites gives insight on better ways in which charge transfers are maximized while at the same time forming of the by-products, water and carbon dioxide, becomes efficient. Charge transfers give some insight into the study of electrification process in the system. These values are computed to be about 1.06e with the adsorption of hydrogen and upto 0.90e with the adsorption of carbon monoxide. It appears that (211) and (110) surfaces offer better catalytic role for the electrification process than (100) or (111) surfaces. Barriers of dissociations and combinations are calculated to confirm a possible reality of the assumed processes within the system. Electronic structure calculation using band structure and projected density of states reveals the d electrons of the surface palladium atoms are most affected by the adsorptions. Training of the system potential energies using the neural network method shows promising opportunity to study further complex problems in such systems.

A Synthetic Route for the Preparation of PtRu-Ni Core-Shell Nanoparticles Using Polydopmaine Protective Coating for a CO Tolerant Electrocatalyst

Carbon supported PtRu-Ni/C catalysts with a Ni-rich core structure are prepared through high temperature treatment of as-prepared polyol PtRu/C catalysts. It is well acknowledged that in conventional PtRu alloys, Ru prevents the adsorption of carbon monoxide on Pt surface via bifunctional mechanism. Also, the ligand effect of Ni is known to alter the electron structure of Pt for increased methanol oxidation reaction (MOR). In this work, the optimal Ni amount is determined for enhanced MOR and high resistance to carbon monoxide poisoning. The diffusion of Ni into the core of the catalysts is conducted by controlling the temperature during heat treatment. The as-prepared polyol PtRu/C catalysts have carbonized polydopamine layer on the surface, which prevents particle segregation during high temperature heat treatment. The carbon layer also partakes the role of transferring Ni particles into the core of the catalyst. When compared to using the polyol method to directly fabricate PtRu-Ni/C catalysts, this method is far more superior. Because Ni has a very low reducing power, most of it is lost during polyol process, and the final product has an extremely low Ni content. In this study, Energy-dispersive X-ray spectroscopic line scan profile and X-ray photoelectron spectroscopic analysis confirm that the PtRu-Ni/C catalyst has a Ni-rich core and PtRu-rich shell structure. From Cyclic voltammograms for methanol oxidation and polymer electrolyte membrane fuel cell tests using CO containing H2 gas, the core-shell structured PtRu-Ni/C alloy formed by high-temperature annealing is demonstrated to have higher tolerance to CO poisoning than PtRu/C catalysts synthesized by co-deposition via the polyol method.

Kinetic Study of the Reaction of Adsorbed CO with Methanol and Ethanol on Pt Electrode

Direct alcohol fuel cells (DAFCs), which mostly use low-molecular weight alcohols such as methanol and ethanol as fuel, have attracted considerable interest in recent years. Methanol is recognized as a promising fuel for DAFCs, and thus the anode reaction in direct methanol fuel cells (DMFCs), namely, the electrooxidation of methanol, has been extensively studied in order to improve performance of DMFCs and to achieve cost reduction. Among various electrocatalysts for the oxidation, Pt-based anodes show excellent electrocatalytic activities. Complete electrooxidation of a methanol molecule, which is necessary for efficient oxidation, gives six electrons: CH3OH + H2O → CO2 + 6H+ + 6e-. (1) It is well accepted that the oxidation reaction proceeds on Pt electrodes via a dual path mechanism consisting of indirect and direct paths. The indirect path involves adsorbed carbon monoxide (COad) as a poisoning intermediate, and the direct path involves a non-CO intermediate as a reactive one. Water produces oxygenated spices such as hydroxide and oxide, and it plays an important role in removal of COad. Thus, it seems that the methanol oxidation does not proceed without water. However, as we have reported previously [1, 2], the oxidation produces current without water (panel a1 of Figure 1), and the current is of the same order of magnitude as that observed in the presence of water, e.g., 0.1 and 1 M methanol aqueous solutions. Detailed studies using surface-enhanced infrared absorption spectroscopy and high-performance liquid chromatography have revealed that COad reacts with methanol to form methyl formate (HCOOCH3): COad + CH3OH → HCOOCH3, (2) and also that the current is produced in the absence of water partly by the formation of formaldehyde (HCHO) and mainly by that of methyl formate via non-CO pathway: CH3OH → HCHO + 2H+ + 2e- (3) and 2CH3OH → HCOOCH3 + 4H+ + 4e-. (4) Methanol is toxic for human and is mostly produced from natural gas, which is a major disadvantage of using methanol as fuel. On the other hand, ethanol is non-toxic and can be sustainably produced on a large scale from biomass. In these regards, direct ethanol fuel cells have a strong advantage over DMFCs. Moreover, ethanol has a higher energy density than methanol, that is, its complete electrooxidation with water gives twelve electrons: C2H5OH + 3H2O → 2CO2 + 12H+ + 12e-. (5) The cleavage of the C–C bond, which is involved in the complete oxidation, requires a high activation energy. Thus, even when Pt is used as the anode, most ethanol is incompletely oxidized to acetaldehyde (CH3CHO) which may be further oxizied to acetic acid (CH3COOH): C2H5OH → CH3CHO + 2H+ + 2e- (6) and CH3CHO + H2O →CH3COOH + 2H+ + 2e-. (7) We have recently found that ethanol is also oxidized without water (panel b1). From the analogy of reaction 2, it has been deduced that COad reacts with ethanol to form ethyl formate (HCOOC2H5): COad + C2H5OH → HCOOC2H5. (8) As the electrode potential (E) increases, the coverage of COad gradually decreases (panel b2) and the current increases in the potential region above 0.4 V vs. SHE (panel b1). This indicates that the number of vacant sites on Pt surface increases gradually due to reaction 8, and reaction 6 proceeds at vacant sites at E > 0.4 V. To verify the occurrence of reactions 2 and 8, this present work conducts a kinetic study on the reaction of COad with the alcohols: the time course of the coverage of COad can be well interpreted by Langmuir-Hinshelwood mechanism, and the rate constants for the reactions 2 and 8 at 0.8 V are estimated to be 0.0042 and 0.0051 s-1, respectively (panels a3 and b3). Furthermore, the dependence of the rate constants on the E is studied, which will be discussed in the presentation. REFERENCES [1] H. Okamoto, T. Gojuki, N. Okano, T. Kuge, M. Morita, A. Maruyama, Y. Mukouyama, Electrochim . Acta, 136 (2014) 385. [2] Y. Mukouyama, S. Yamaguchi, K. Iida, T. Kuge, M. Kikuchi, S. Nakanishi, ECS Trans., 80 (2017) 1471. FIGURE CAPTION Figure 1 (top) Cyclic voltammograms for the oxidation of (left) methanol and (right) ethanol at a sweep rate of 0.1 Vs-1. The concentration of methanol was 24 M, and that of ethanol was 17 M. The high-concentration alcohol solutions contained 0.5 M H2SO4 as an electrolyte. (middle) The dependence of the coverage of COad on E for (left) the 24 M metanol oxidation and (right) the 17 M ethanol oxidation. (bottom) Time courses of the coverage of COad at 0.8 V. Figure 1

(Invited) Modification, Activation and Stabilization of Copper(I) Oxide Photocathodes: Competition between Catalytic and Inhibiting Effects during Light-Induced Electoreduction of CO2

Among representative examples of semiconducting materials for the selective reduction of carbon dioxide to desired small organic molecules (fuels, utility chemicals), there are transition metal oxides (p-type semiconductors), in particular Cu-based oxides, such as Cu2O, CuFeO2, CuBi2O4, CuNb2O6, CuO and Cu2O) that are capable of generating electrons with use of solar visible light. Selectivity of the catalytic systems largely depends on the activing adsorptive (CO2) phenomena and the affinity of catalytic centers to the adsorbed carbon monoxide intermediates leading to their protonation or hydrogenation. Here application of aqueous or water-containing electrolytes is generally preferred. Furthermore, a compromise must be reached between the activity of a catalyst toward hydrogen evolution (water reduction) and its ability to promote reductive adsorption of CO2 and to generate moderate coverages of H atoms capable of stabilizing subsequent reduced intermediates. Our interest concentrates on copper(I) oxides but their practical use requires means of stabilization without poisoning or deactivation. To improve stability against photocorrosion, to assure the sufficient electron-hole pair separation, as well as to increase population of electrons in the conduction band, mixed oxide systems combining the p-type semiconductor (e.g. Cu2O) with n-type semiconductors such as TiO2, SrTiO3 or KTaO3 have been proposed. To meet requirements for practical Cu2O-based photocathodes, not only the ability to absorb efficiently visible light and to assure fast interfacial electron transfers but also the stability issue needs to be addressed. Contrary to the poor performance of the pristine (bare) copper(I) oxide photocathode, the visible-light-illuminated photoelectrochemical reduction of carbon dioxide has been successfully performed using the p-type Cu2O-semiconductor (deposited onto the transparent fluorine-doped conducting glass electrode) over-coated with the ultra-thin films of various polymer (poly(4-vinyl pyridine, oligoaniline) or polynuclear type materials (titanium, zirconium and tungsten oxides).. In this respect, ultra-thin films of functionalized carbon nanostructures, supramolecular complexes (with nitrogen containing ligands and certain transition metal sites) or robust bacterial biofilms could also be advantageous. For example, the oligoaniline or tungsten oxide over-layers [1,2] have been generated on copper(I) oxide surfaces. Under such conditions, the copper(I) oxide photoelectrode does not change the oxidation state during photoelectrochemical diagnostic experiments. Thus certain robust (not necessarily thin) could exhibit the effect of stabilization of Cu2O against photocorrosion. Among important issues is the ability of CO2 to undergo adsorption (and activation toward reduction to CO) at both bare and modified surfaces of Cu2O. The proposed bi-layered photocathode has been demonstrated to produce such simple organic fuels as alcohols (mainly methanol and some ethanol). [1] Szaniawska E., Rutkowska I.A., Frik M., Wadas A., Seta E., Krogul-Sobczak A., Rajeshwar K., Kulesza P.J., Electrochim. Acta, 265, 400 (2018). [2] Rutkowska I.A., Szaniawska E., Taniewicz J., Wadas A., Seta E., Kowalski D., Kulesza P.J., J. Electrochem. Soc., 166, H3271 (2019).

Analysis and optimization of a methanol reactor with the adsorption of carbon monoxide and water

Methanol is generally produced in adiabatic or Lurgi type catalytic reactors from syngas or pure carbon dioxide and hydrogen. In this research, an innovative methanol reactor is analyzed and optimized, because sorbents for the capture of carbon monoxide and water are used. In particular, zeolite molecular sieves having high SiO2/Al2O3 are used to capture carbon monoxide while zeolites 4A are implemented to adsorb water molecules. No sorbents for the capture of carbon monoxide are suggested before. In this system, the two reactions in methanol production are both favored, then it is possible to increase the methanol yield, reducing the outlet reaction temperature, compared to a traditional adiabatic reactor. An ANOVA analysis and a response surface methodology are also developed. Results show that the capture fraction is the most important factor with the aim to improve the methanol yield and to reduce the reaction temperature. Optimal operating conditions are found in order to have a nearly-isothermal system (493 K) maximizing the methanol yield (37%): the capture fraction, recycle of gases, inlet temperature and reaction pressure must be respectively equal to 80%, 79.7%, 493.32 K and 55 bar. In the future work, an experimental reactor can be realized to verify the obtained results.

Highly selective adsorption of CO over N2 on CuCl-loaded SAPO-34 adsorbent

Abstract Carbon monoxide (CO)/N2 separation is of importance for current chemical industry. However, CO/N2 separation remains a challenge due to the similar molecular size and the small variance of volatility of CO and N2. In this work, molecular sieve SAPO-34 was loaded with CuCl by monolayer dispersion method for the preparation of Cu(I) containing adsorbents. The resulted adsorbents were characterized via nitrogen adsorption/desorption at 77 K, X-ray fluorescence (XRF) and X-ray diffraction (XRD). The results indicated that CuCl was successful loaded into the molecular sieve and well-dispersed. CO and N2 single component adsorption isotherms were recorded under 298 K, 308 K and 318 K by using volumetric method. One of the CuCl-loaded SAPO-34 adsorbent exhibited a very high CO adsorption capacity of 1.84 mmol/g at 100 kPa, 298 K and high CO/N2 selectivity.

Rethinking CO adsorption on transition-metal surfaces: Effect of density-driven self-interaction errors

Adsorption of the molecule CO on metallic surfaces is an important unsolved problem in Kohn-Sham density functional theory (KS-DFT). We present a detailed study of carbon monoxide adsorption on fcc (111) surfaces of 3d, 4d and 5d metals using nonempirical semilocal density functionals for the exchange-correlation energy: the local-density approximation (LDA), two generalized gradient approximations or GGAs (PBE and PBEsol), and a meta-GGA (SCAN). The typical error pattern (as found earlier for free molecules and for free transition metal surfaces), in which results improve from LDA to PBE or PBEsol to SCAN, due to the satisfaction of more exact constraints, is not found here. Instead, for CO adsorption on transition metal surfaces, we find that, while SCAN overbinds much less than LDA, it overbinds slightly more than PBE. Moreover, the tested functionals often predict the wrong adsorption site, as first pointed out for LDA and GGA in the ``CO/Pt (111) puzzle". This abnormal pattern leads us to suspect that the errors of PBE and SCAN for this problem are density-driven self-interaction errors associated with incorrect charge transfer between molecule and metal surface. We point out that, by the variational principle, overbinding by an approximate functional would be reduced if that functional were applied not to its selfconsistent density for the adsorbed system but to an exact or more correct density for that system. Finally, we show for CO on Pt(111) that the site preference is corrected and the adsorption energy is improved for the PBE functional by using not the selfconsistent PBE density but a PBE+U density. The resulting correction to the PBE total energy is much larger for the adsorbed system than for its desorbed components, showing that the error is in the density of the adsorbed system. This seems to solve the ``CO/Pt (111) puzzle", in principle if not fully in practice.

Computational and experimental demonstrations of one-pot tandem catalysis for electrochemical carbon dioxide reduction to methane

Electroreduction of carbon dioxide to hydrocarbons and oxygenates on copper involves reduction to a carbon monoxide adsorbate followed by further transformation to hydrocarbons and oxygenates. Simultaneous improvement of these processes over a single reactive site is challenging due to the linear scaling relationship of the binding strength of key intermediates. Herein, we report improved electroreduction of carbon dioxide by exploiting a one-pot tandem catalysis mechanism based on computational and electrochemical investigations. By constructing a well-defined copper-modified silver surface, adsorbed carbon monoxide generated on the silver sites is proposed to migrate to surface copper sites for the subsequent reduction to methane, which is consistent with insights gained from operando attenuated total reflectance surface enhanced infrared absorption spectroscopic investigations. Our results provide a promising approach for designing carbon dioxide electroreduction catalysts to enable one-pot reduction of products beyond carbon monoxide and formate.

A Study of Platinum Metal Surface as a Component of Fuel Cells

Adsorption energies of small molecules carbon monoxide and hydrogen are studied using density functional theory and the neural network method of generating higher dimensional potential energy surfaces. Various lower index surfaces of platinum metal are considered, and the adsorption energies appear to increase according to: P t(211) > P t(100) > P t(110) > P t(111) with the adsorption of both molecules. The large adsorption energies mean only energetic interactions with the ions from air such as oxygen ion can detach the adsorbates from the surface thereby forming water and carbon monoxide as by-products. Exploring possible adsorption sites gives insight on better ways in which charge exchanges are maximized while at the same time forming of the by-products becomes efficient. The adsorption energies range between 0.85 eV to 2.08 eV with the adsorption of CO while the values are between 1.30 eV to 1.9 eV with the adsorption of hydrogen. Charge transfers give some insight into the study of electrification process in the system. These values are computed to be about 0.37e with the adsorption of hydrogen and upto 0.24e with the adsorption of carbon monoxide. Training of the system potential energies using the neural network method shows promising opportunity to study further complex problems in such systems.

Influence of CO Admolecules on the Electronic State of Ytterbium Nanofilms Grown on Silicon Substrates

The adsorption of carbon monoxide (CO) molecules on ytterbium nanofilms from 16 to 200 monolayers thick (61–76 nm) has been studied. The films have been grown on single-crystal Si(111) substrates. It has been shown that before CO molecule adsorption, ytterbium is in the divalent state with electronic configuration [Xe]4f 146s2. After gas molecule adsorption, a layer of trivalent ytterbium (electronic configuration [Xe]4f 135d16s2) forms in a part of the film that is adjacent to the surface. The thickness of the layer modified by CO admolecules has been estimated. It has been found that the thickness is within 9–22 monolayers (3.4–8.4 nm).

Catalytic thermodynamic model for nanocluster adsorbates

We present an approach to study nanocatalysis using density functional theory (DFT), statistical mechanics, and thermodynamics. The analysis starts by using a coordination style approach, which is key to producing a mesoscale model free of arbitrary parameters for sizes of ∼3 nm < D < 100 nm. We apply DFT in a coordination type calculation of the nanocrystal binding energy and the adatom adsorption energy to give us a Hamiltonian of the nanocluster-adatom system. This is followed by informational statistical mechanical principles and thermodynamics to complete the model. Carbon monoxide adsorbates are studied on gold clusters, hydrogen molecules on palladium clusters, and oxygen radicals on platinum clusters. The data exhibits size effects for the measured thermodynamic properties with cluster diameters between 3 and 9 nm. In spite of modeling three different systems, we find only small differences in the large-scale entropy (∼−70 J/K-mol) and enthalpy (∼−20 kJ/mol) of the nanoclusters. Shape effects are predicted to be greatest for nanoclusters of size ∼10 nm. For the Pd-H2 system, we use a cubic model which has close agreement with experimental data for Pd cubes. The computationally efficient procedure we derive provides a theoretical scheme to determine the size and shape dependence of the entropy and enthalpy of nanocluster-adsorbate systems from sequential single molecule adsorption.

Structural Evolution of α-Fe2O3(0001) Surfaces Under Reduction Conditions Monitored by Infrared Spectroscopy.

The precise determination of the surface structure of iron oxides (hematite and magnetite) is a vital prerequisite to understand their unique chemical and physical properties under different conditions. Here, the atomic structure evolution of the hematite (0001) surface under reducing conditions was tracked by polarization-resolved infrared reflection absorption spectroscopy (IRRAS) using carbon monoxide (CO) as a probe molecule. The frequency and intensity of the CO stretch vibration is extremely sensitive to the valence state and electronic environments of surface iron cations. Our comprehensive IRRAS results provide direct evidence that the monocrystalline, stoichiometric α-Fe2O3(0001) surface is single Fe-terminated. The initial reduction induced by annealing at elevated temperatures produces surface oxygen vacancies, where the excess electrons are localized at adjacent subsurface iron ions (5-fold coordinated). A massive surface restructuring occurs upon further reduction by exposing to atomic hydrogen followed by Ar+-sputtering and annealing under oxygen poor conditions. The restructured surface is identified as a Fe3O4(111)/Fe1−xO(111)-biphase exposing both, Fe3+ and Fe2+ surface species. Here the well-defined surface domains of Fe3O4(111) exhibit a Feoct2-termination, while the reduced Fe1−xO(111) is Fe2+(oct)-terminated. These findings are supported by reference IRRAS data acquired for CO adsorption on magnetite (111) and (001) monocrystalline surfaces.

Unravelling Platinum-Zirconia Interfacial Sites Using CO Adsorption.

Understanding platinum (Pt) speciation on catalysts is crucial for the design of atom-efficient materials and optimized formulations. The adsorption of carbon monoxide (CO) as a probe molecule is widely used to reveal Pt dispersion and structures, yet the assignment of IR bands is not straightforward, hindering determination of the nature of the surface sites or ensemble involved. CO adsorption was studied here over a zirconia-supported Pt catalyst. Specific sites at the interface between Pt and the support were highlighted, giving rise to an unusual band around 1660 cm-1 that could be confidently assigned to a Pt2-CO bridging carbonyl interacting head-on with a support surface hydroxyl. This adduct was yet unstable in the present conditions and was converted into a linear and bridged carbonyl bound only to Pt. Such sites are potentially important for bifunctional reactions requiring both metal and acid/base properties, particularly those occurring at the metal-support perimeter. Such adducts have probably been mistaken for carbonate-type species in many past contributions and could potentially represent crucial reaction intermediates for CO oxidation and the water-gas shift reaction.