Linear CO Research Articles

Effect of Pt Particle Size and Valence State on the Performance of Pt/TiO2 Catalysts for CO Oxidation at Room Temperature

A series of Pt/TiO2 catalysts with various Pt particle sizes and valence states were prepared and tested for CO oxidation at room temperature. Field-emission transmission electron microscopy and X-ray photoelectron spectroscopy analyses confirmed that the activity of the Pt/TiO2 catalyst was influenced by the particle size and valence state of the catalyst particles. Excellent CO oxidation activity was observed at room temperature using highly dispersed, small metallic Pt particles. Increasing the Ptmetallic/Pttotal ratio resulted in an increase of turnover frequencies. According to the Fourier-transform infrared spectroscopy results, the linear CO species that was adsorbed on metallic Pt sites at 25 °C reacted with atmospheric O2 and was easily desorbed. However, linear CO species adsorbed on PtOx (x ≥ 2) sites was only desorbed at temperatures ≥100 °C, confirming the lack of CO oxidation activity at room temperature with ionic Pt catalysts.

Isotopic transient studies of sodium promotion of Pt/Al2O3 for the water–gas shift reaction

The promotional effect of sodium on Pt/Al2O3 catalysts for the water–gas shift (WGS) reaction was investigated with operando FTIR during steady-state and isotopic transient experiments. The highest turnover frequency (TOF) promotion per surface Pt occurred on a 0.82wt.% Pt/Al2O3 catalyst with a 30:1molar ratio of Na:Pt. This catalyst exhibited a TOF of 0.35s−1 at 250°C and 7% CO, 11% H2O, 9% CO2, and 37% H2 compared to 0.43×10−2s−1 on Pt/Al2O3. Operando IR revealed that Na addition modifies CO adsorption on Pt to create more strongly bound, multiply-bonded CO species at 1768cm−1 and 1697cm−1 compared to predominantly linear CO on Na-free Pt/Al2O3 at 2060cm−1. Transient experiments with 12CO/13CO isotope switches showed that the number of carbon-containing active intermediates increased from less than 1% of the surface Pt for Na-free Pt/Al2O3 to nearly 100% of the surface Pt for 20Na:Pt/Al2O3, which indicates that only a small fraction of the Pt surface on the Na-free samples participates in the WGS reaction. From time-resolved IR spectra during transient WGS with 12CO/13CO and 12CO2/13CO2 isotope switches, we propose that surface formate species are spectators for all Na-promoted Pt/Al2O3 catalysts under these WGS conditions. The results suggest that Na promotes Pt/Al2O3 for WGS by modifying the local electronic properties of Pt and creating new H2O activation sites, which provide greater availability of surface OH/H species to react with nearly all CO on the metallic Pt surface through a non-formate pathway.

Individual Heat of Adsorption of Adsorbed CO Species on Palladium and Pd–Sn Nanoparticles Supported on Al2O3 by Using Temperature-Programmed Adsorption Equilibrium Methods

The present article is dedicated to the adsorption of CO on reduced 2% Pd/Al2O3 and 2% Pd-x% Sn/Al2O3 (weight %, x = 2 or 5 wt %) in the 300–713 K temperature range to study the geometric and electronic effects of Sn on the palladium adsorption sites. Using Fourier transform infrared (FTIR) spectroscopy, it is shown that the insertion of Sn leads to (a) the total disappearance of the Pd sites forming bridged CO species (denoted as “B”), which are the dominant species on Pd0 particles and (b) a significant increase in the Pd sites forming linear CO species (denoted as “L”). This is ascribed to a geometric effect of Sn that dilutes the superficial palladium sites. The measurement of the individual heats of adsorption of the different adsorbed CO species by using two original temperature-programmed adsorption equilibrium methods (denoted AEIR and TPAE) allows the estimation of the electronic effect of Sn on the Pd sites. On 2% Pd/Al2O3, in parallel to the formation of two strongly adsorbed B CO species, two linear L1Pd0 and L2Pd0 CO species are formed, which exhibit different heats of adsorption. For the dominant L1Pd0 CO species, the heat of adsorption decreases linearly, from 92 kJ/mol to 54 kJ/mol, as its coverage increases at coverage 0 and 1, respectively, while that of the L2Pd0 species is >165 kJ/mol at coverage 1. On the two Pd–Sn containing particles, two linear CO species are formed denoted L12Pd–xSn and L22Pd–xSn with x = 2 or 5. The L12Pd–xSn species dominates the CO adsorption on the two solids. It is shown that its heat of adsorption (which is slightly dependent on x) linearly varies with its coverage: ∼90 kJ/mol and ∼50 kJ/mol at low and high coverage, respectively. The comparison with the heat of adsorption of L1Pd0 indicates that the electronic effect of tin is very modest, compared to its geometric effect. This conclusion is consistent with literature data dedicated to DFT calculation. Moreover, (a) XRD and TEM/EDX analysis suggest that bimetallic particles such as Pd3Sn and Pd2Sn are present and (b) the impact of tin on the H2 chemisorption on Pd0 sites are presented.

Heat of adsorption of CO on EUROPT-1 using the AEIR method: Effect of analysis parameters, water and sample mode

Fourier transform infrared spectroscopy (FTIR) in transmission and in diffuse reflectance (DRIFT) modes is used to characterize the adsorbed species formed during the adsorption of CO on EUROPT-1 (a 6.3% Pt/SiO2 catalyst) as a function of different experimental parameters such as the duration ta of the adsorption at 300K, the adsorption temperature Ta (300–673K range) and pressure Pa (1–4kPa range), the reduction temperature TR in hydrogen (423, 523, 673K) and the presence of H2O. Whatever the experimental conditions the adsorption of CO on EUROPT-1 is dominated by linear CO species (denoted L CO) characterized by IR bands in the 2090–2040cm−1 range with a small contribution of bridged CO species (denoted B CO) with IR bands in the 1880–1830cm−1 range. It is shown that the intensity, shape and positions of the IR bands of the L CO are strongly dependent on the experimental conditions. Increasing the duration of adsorption ta at 300K leads to an increase of the IR band intensity due to an activated reconstruction process. The presence of H2O leads to a shift of the L CO species IR band towards lower wavenumbers. Transmission and DRIFT modes provide similar qualitative data. However, the measurement of the heats of adsorption of the L CO species through the adsorption equilibrium infrared spectroscopy (AEIR) method, which is based on the quantification of the IR bands of adsorbed species, reveals the limitations of the DRIFT technique at high temperatures. Using transmission mode, it is shown that the heats of adsorption of the L CO species linearly vary which its coverage θL from 90kJ/mol to 250kJ/mol at θL=1 and 0 respectively whatever (a) the reduction temperature and (b) the absence or the presence of H2O. These values are consistent with previous measurements on others Pt° particles supported on different metal oxides. However the value at θL=0 is higher by about 30kJ/mol, possibly due to the high Pt dispersion (D≈0.9) of the reduced-reconstructed EUROPT-1. This study emphasizes that the use of CO to characterize supported Pt° particles is not straightforward considering the different experimental parameters that may affect the Pt structure and IR band features of the adsorbed species.

Catalytic Deoxygenation of Octanoic Acid over Supported Palladium: Effects of Particle Size and Alloying with Gold

Catalytic deoxygenation of octanoic acid (OA) to n-heptane was investigated over silica-supported Pd and PdAu catalysts at 260 °C and 1 atm in a fixed-bed microreactor. Pd/SiO2 catalysts were prepared by incipient wetness (IW) and ion exchange (IE). Bimetallic catalysts were prepared using an IE procedure that is known to produce supported PdAu nanoparticles. The Pd nanoparticles (7.5 nm average size) in the Pd/SiO2 (IW) catalyst exhibit well-defined (100) and (111) facets, as evidenced by high-resolution electron microscopy (HREM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. As expected, the smaller nanoparticles (1.5 nm average size) in the Pd/SiO2 (IE) catalyst display strong linear and bridging CO DRIFTS bands. The PdAu/SiO2 (1/1 atomic ratio) catalyst contains 5 nm alloy nanoparticles with Pd-rich surfaces, as evidenced by HREM with energy-dispersive X-ray (EDX) analysis and in situ EXAFS spectroscopy. DRIFTS thermal desorption experiments demonstrated that...

Time-resolved, in situ DRIFTS/EDE/MS studies on alumina-supported rhodium catalysts: effects of ceriation and zirconiation on rhodium-CO interactions.

The effects of ceria and zirconia on the structure–function properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al2O3) during CO exposure are described. Ceria and zirconia are introduced through two preparation methods: 1) ceria is deposited on γ-Al2O3 from [Ce(acac)3] and rhodium metal is subsequently added, and 2) through the controlled surface modification (CSM) technique, which involves the decomposition of [M(acac)x] (M=Ce, x=3; M=Zr, x=4) on Rh/γ-Al2O3. The structure–function correlations of ceria and/or zirconia-doped rhodium catalysts are investigated by diffuse reflectance infrared Fourier-transform spectroscopy/energy-dispersive extended X-ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time-resolved, in situ conditions. CeOx and ZrO2 facilitate the protection of Rh particles against extensive oxidation in air and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts prepared by CSM are observed and compared with Rh/γ-Al2O3 samples, whereas supported Rh particles are easily disrupted by CO forming mononuclear Rh geminal dicarbonyl species. DRIFTS results indicate that, through the interaction of CO with ceriated Rh particles, a significantly larger amount of linear CO species form; this suggests the predominance of a metallic Rh phase.

EPR study of methyl radical in van-der-Waals solids

High-resolution EPR spectra of CH3 radicals trapped in solid N2 matrix were obtained in a temperature range 7–28K. The analysis of the spectrum suggests that, in the whole temperature range, the radical performs both fast rotational motion around the C3-symmetry axis and libration motion about the C2-symmetry axes. A comparison study of the methyl libration motion in gas matrices of linear molecules, N2, CO, N2O and CO2, was carried out using a model of the infinite potential well with flat bottom. As a result, averaged angular deviations of the radical orientation and dimensions of the potential wells were estimated. We assessed contributions of both the classical motion and the quantum-mechanical correction to the radical reorientation. A possibility of utilizing the trapped CH3 radical as a spin probe to study order–disorder structural phase transition at the microscopic level was discussed. An empirical formula obtained earlier for the matrix shift of the isotropic hyperfine coupling (hfc) constant of CH3 in matrices of spherical particles was shown to hold true for matrices of linear molecules as well. The experimental results and their treatment made it possible to report an estimate of the hfc tensor anisotropy for the free methyl radical. This information is of special interest to the theory of atoms and molecules.

Electrocatalysis of carbon black- or poly(diallyldimethylammonium chloride)-functionalized activated carbon nanotubes-supported Pd–Tb towards methanol oxidation in alkaline media

The Pd–Tb/C catalysts with different Pd/Tb ratios were synthesized by a simple simultaneous reduction reaction with sodium borohydride in aqueous solution. The structure and morphology of those catalysts had been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrocatalytic performance of those catalysts for methanol oxidation in alkaline media was investigated using cyclic voltammetry (CV), linear sweep voltammetry (LSV) and CO stripping experiments. It is found that the 20%Pd–1%Tb/C catalyst has a higher catalytic activity than the 20%Pd/C catalyst, but the effect of Tb cannot be explained by a bi-functional mechanism. According to the X-Ray photoelectron spectroscopy (XPS) analyses, it is suggested that the higher content of metallic Pd caused by the addition of Tb contributes to the better catalytic activity of 20%Pd–1%Tb/C. Based on the good electrocatalytic performance of 20%Pd–1%Tb/C, the 20%Pd–1%Tb catalyst supported on poly(diallyldimethylammonium chloride) (PDDA)-functionalized activated carbon nanotubes was prepared, and it exhibits a better catalytic activity. The improvement mainly results from the further increase of metallic Pd due to the presence of PDDA.

Co-adsorption of hydrogen and CO on Pt film: An in-situ ATR-IR study combined with DFT calculations

The co-adsorption of hydrogen and CO on Pt film was studied by in-situ attenuated total reflection Infrared (ATR-IR) Spectroscopy. It was found that the adsorption of CO on Pt is much stronger than that of hydrogen, and the co-adsorbed hydrogen species lead to dramatic red shifts of both linear and bridged-adsorbed CO peaks. The more hydrogen species adsorbed on Pt surface, the more red shifts of CO frequencies were observed. Based on the results obtained from the well-designed experiments, it is deduced that H may act as an electron donor on Pt surface and negative charge is transferred from H to Pt. Thus, the surface charge properties of Pt film were changed, which enhances the d–π∗ backdonation effect between Pt and CO and consequently leads to dramatic red shifts of CO peaks. The density functional theory (DFT) calculations based on simple models confirmed the charge transfer effect between hydrogen and Pt. This study also shows that ATR-IR technique is a powerful tool to investigate the gas–solid interface.

Experimental Microkinetic Approach of the Surface Reconstruction of Cobalt Particles in Relationship with the CO/H2 Reaction on a Reduced 10% Co/Al2O3 Catalyst

The adsorption of x% CO/He (x = 1 and 2) on a reduced 10 wt % Co/Al2O3 catalyst at an adsorption temperature of 560 K > Ta > 420 K leads to the formation of a linear CO species on Co° sites (denoted LCo° species) with an infrared (IR) band at ≈2030 cm–1 at full coverage. This adsorption is associated with the formation of carbon that is implicated in the reconstruction of the surface of the cobalt particles, leading to the progressive transformation of the LCo° species into a new linear CO species on Co°C sites of the reconstructed surface with an IR band at ≈2060 cm–1. An experimental microkinetic approach of the reconstruction process via the LCo° → LCo°C transformation reveals the impact of different kinetic parameters such as the reaction temperature, the CO partial pressure, and the presence of hydrogen. During the reconstruction the coverages of the LCo° and LCo°C species remain very high (>0.95) due to their high heats of adsorption determined by the Adsorption Equilibrium Infra Red spectroscopy method. It is shown that the rate of the reconstruction process is controlled by that of the disappearance of the LCo° species via its dissociation into adsorbed elemental carbon and oxygen species according to a pseudo first kinetic order elementary step involving a small amount of free cobalt sites (≈4 × 1012 sites/cm2 of cobalt particles). The impact of the reaction temperature on the LCo° → LCo°C transformation indicates that (a) the activation energy of the LCo° dissociation which controls the reconstruction process is ≈125 kJ/mol and (b) the formation of elemental carbon via the disproportionation of the LCo° species seems unlikely. These two conclusions are consistent with literature data on density functional theory (DFT) calculations. For adsorption temperature >560 K, the formation of superficial graphitic like species overlaps the reconstruction process. In the presence of hydrogen, the reconstruction is observed for H2/CO ratios <3 but not for higher values (i.e., H2/CO = 10).

Imazethapyr Enantioselectively Affects Chlorophyll Synthesis and Photosynthesis in Arabidopsis thaliana

Imazethapyr (IM) is a chiral herbicide with reported enantioselective biological activities between its enantiomers. This report investigated the effect of enantioselectivity between R- and S-IM in Arabidopsis thaliana on chlorophyll synthesis and photosynthesis. The results suggest that R-IM inhibited the transcription of chlM to a greater extent than S-IM, which reduced chlorophyll synthesis. R-IM also showed a stronger inhibitory effect than S-IM on the transcription of photosynthesis-related genes, affecting linear electron transport and CO(2) fixation. IM stress enantioselectively induced transcriptional upregulation of the ndhH gene, a representative of the NDH complex. In contrast, the expression of pgr5 was downregulated, which demonstrated that IM stress enhanced adenosine 5'-triphosphate (ATP) synthesis by stimulating an NDH-dependent and not ferredoxin (FD)-independent route. This study suggested that R-IM has a greater toxic effect on photosynthesis than S-IM, affecting plant growth through chlorophyll synthesis.

Heats of adsorption of linearly adsorbed CO species on Co2+ and Co° sites of reduced Co/Al2O3 catalysts in relationship with the CO/H2 reaction

IR spectra in transmission mode are used to measure the heats of adsorption at different coverages θ:E(θ), of two linearly adsorbed CO species formed on Co2+ and Co° sites (noted LCo2+ and LCo°) of reduced x% Co/Al2O3 (wt%, x≤10) catalysts according to the adsorption equilibrium infrared spectroscopy procedure developed previously. For LCo2+ species characterized by an IR band at 2151cm−1, ELCo2+(θ) varies linearly with θ from ELCo2+(1)=45 kJ/mol to ELCo2+(0)=52 kJ/mol. These values are modified by the presence of neither Co° particles nor carbon deposition (from CO disproportionation reaction) nor H2 using CO/H2 gas mixtures. During the CO adsorption at high temperatures (i.e. 538K), C deposition on the surface and in the bulk of the cobalt particles modifies the Co° adsorption sites. This leads to a transformation LCo° (IR band at 2020cm−1)→LCo°C (IR band at 2060cm−1) where LCo°C denotes a linear CO species formed on Co° sites modified by the C deposition. The heat of adsorption of the LCo°C species varies linearly with its coverage from ELCo°C(1)=93 kJ/mol to ELCo°C(0)=165 kJ/mol. In the presence of H2 with a ratio H2/CO=10, the C deposition is strongly decreased and the LCo° species dominates the surface of the cobalt particles. However, in these conditions its hydrogenation into CH4 disturbs its adsorption equilibrium in a large coverage range and only ELCo°(1)=108 kJ/mol has been determined. For a ratio H2/CO=3, the carbon deposition cannot be prevented leading to the formation of the LCo°C species. It is shown that the ELCo°C(θ) values are not modified by the presence of adsorbed hydrogen. The heats of adsorption of the different adsorbed CO species on Co° sites are consistent with some literature data on DFT calculations and experimental values obtained on model cobalt surfaces (i.e. single crystals). In particular, they confirm DFT calculations which indicate that the C deposition on reduced cobalt particles decreases slightly the heats of adsorption of the linear CO species adsorbed on Co° sites.

Cutting Properties of Austenitic Stainless Steel by Using Linear Polarized CO&lt;sub&gt;2&lt;/sub&gt; Laser without Assist Gas

Recently, laser cutting is used in many industries. Generally, in laser cutting of metallic materials, assist gas and its nozzle are needed to remove the molten metal. However, because of the gas nozzle should be moved closer to the position about 1 mm from the surface of a workpiece, it is thought that existence of the nozzle causes lack of flexibility of laser cutting. Therefore, the new cutting process, Assist Gas Free laser cutting (hereafter, called as AGF laser cutting), has been developed and investigated about cutting properties in our laboratory. In this process, the pressure at the bottom side of a workpiece is reduced by a vacuum pump, and the molten metal can be removed by the air flow caused by the pressure difference between both sides of the specimen. On the other hand, when cutting of metallic materials with a linear polarized laser is performed, it is known that the cutting kerf might slant. This phenomenon is also observed in AGF laser cutting. In the present study, cutting properties of austenitic stainless steel by using liner polarized CO2 laser in AGF laser cutting was investigated. Cutting speed and direction were varied in order to study the effect of these parameters on cutting properties. As a result, when the angle formed by the cutting direction and the polarized direction of laser was parallel, the kerf slant could be depressed, and the critical cutting speed could be the fastest in any other cutting direction.

Theoretical and Experimental Studies of CO2 and H2 Separation Using the 1-Ethyl-3-methylimidazolium Acetate ([emim][CH3COO]) Ionic Liquid

The performance of [emim][CH(3)COO] ionic liquid (IL) to separate mixtures of CO(2) and H(2) is studied using both classical and ab initio simulation methods and experiments. Simulations show that H(2) solubility and permeability in [emim][CH(3)COO] are quite low with Henry's law constants about 1 × 10(4) bar and permeabilities in the range 29-79 barrer at 313-373 K. In the case of CO(2) absorption in [emim][CH(3)COO], ab initio molecular dynamics simulations predict two types of CO(2) absorption states. In type I state, CO(2) molecules interact with the [CH(3)COO](-) anion through strong complexation leading to high CO(2) solubility. The C atom of CO(2) is located close to the O atoms of the [CH(3)COO](-) anion with an average distance of about 1.61 Å. The CO(2) bond angle (θ(OCO)) is about 138°, significantly perturbed from that of an isolated linear CO(2). In type II state, the CO(2) molecule maintains a linear configuration and is located at larger separations (>2.2 Å) from the [CH(3)COO](-) anion. The weaker interaction of CO(2) with the [CH(3)COO](-) anion in type II state is similar to the one observed when CO(2) absorbs in [bmim][PF(6)]. Simulations further demonstrate that the [emim](+) cation competes with CO(2) to interact with the [CH(3)COO](-) anion. The predicted high CO(2) permeability and low H(2) permeability in [emim][CH(3)COO] are also verified by our experiments. The experimental CO(2) permeability in [emim][CH(3)COO] is in the range of 1325-3701 barrer, and high experimental CO(2)/H(2) permeability selectivities of 21-37 at 313-373 K are observed. We propose that by replacing [emim](+) cation with 1-butyl-1-methylpyrrolidinium ([PY(14)](+)) further enhancement of CO(2) solubility in [PY(14)][CH(3)COO] IL will be obtained as well as good performance to separate CO(2) and H(2).

Heats of adsorption of linear CO species adsorbed on reduced Fe/Al 2O 3 catalysts using the AEIR method in diffuse reflectance mode

IR spectra in diffuse reflectance mode are quantitatively exploited to determine the heats of adsorption of two linearly adsorbed CO species formed on Fe 2+ and Fe° sites (denoted L Fe 2 + and L Fe° CO species) of reduced x% Fe/Al 2O 3 (wt%, x = 1 and 5) catalysts according to the AEIR procedure developed previously using the IR transmission mode. The IR transmission properties of iron containing catalysts are limited particularly for high iron loadings favoring the use of the diffuse reflectance mode. The heats of adsorption of the L Fe 2 + CO species linearly vary with the coverage of the sites from E L Fe 2 + ( 1 ) = 45 kJ / mol to E L Fe 2 + ( 0 ) = 66 kJ / mol at coverage 1 and 0, respectively. These values are modified by the presence of neither Fe° sites nor carbonaceous adsorbed species. The heats of adsorption of the L Fe° CO species on a C-free iron surface (for adsorption temperature T a < 456 K), linearly vary with its coverage from E L Fe ° ( 1 ) = 79 kJ / mol to E L Fe ° ( 0 ) = 105 kJ / mol at high and low coverages. For T a > 456 K, the CO dissociation overlaps the CO adsorption equilibrium leading to a C-containing iron surface. The presence of carbonaceous species has no significant impact on the heat of adsorption of the L Fe° species at high coverages whereas it increases significantly that at low coverages: E L Fe ° ( 0 ) = 120 kJ / mol. Considering reduced iron supported catalysts, the AEIR method is particularly useful because it allows the determination of the individual heats of adsorption of two adsorbed CO species that can be simultaneously present on the surface for high iron loadings. The study confirms that quantitative exploitations of DRIFT spectra are available for well designed experimental conditions offering an alternative for solid catalysts with low IR transmission properties.

Mechanisms and Kinetics for Sorption of CO2 on Bicontinuous Mesoporous Silica Modified with n-Propylamine

We studied equilibrium adsorption and uptake kinetics and identified molecular species that formed during sorption of carbon dioxide on amine-modified silica. Bicontinuous silicas (AMS-6 and MCM-48) were postsynthetically modified with (3-aminopropyl)triethoxysilane or (3-aminopropyl)methyldiethoxysilane, and amine-modified AMS-6 adsorbed more CO2 than did amine-modified MCM-48. By in situ FTIR spectroscopy, we showed that the amine groups reacted with CO2 and formed ammonium carbamate ion pairs as well as carbamic acids under both dry and moist conditions. The carbamic acid was stabilized by hydrogen bonds, and ammonium carbamate ion pairs formed preferably on sorbents with high densities of amine groups. Under dry conditions, silylpropylcarbamate formed, slowly, by condensing carbamic acid and silanol groups. The ratio of ammonium carbamate ion pairs to silylpropylcarbamate was higher for samples with high amine contents than samples with low amine contents. Bicarbonates or carbonates did not form under dry or moist conditions. The uptake of CO2 was enhanced in the presence of water, which was rationalized by the observed release of additional amine groups under these conditions and related formation of ammonium carbamate ion pairs. Distinct evidence for a fourth and irreversibly formed moiety was observed under sorption of CO2 under dry conditions. Significant amounts of physisorbed, linear CO2 were detected at relatively high partial pressures of CO2, such that they could adsorb only after the reactive amine groups were consumed.

Structure of the Au–Rh bimetallic system formed on titanate nanowires and nanotubes

Au, Rh and Au–Rh nanoclusters were studied on one-dimensional titania nanostructures by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and infrared spectroscopy (FT–IR). On titanate nanowire and tube supports the gold 4f 7/2 XP emission appeared after reduction at 83.6 eV and 85.6 eV indicating two different sizes or chemical environments of gold nanoclusters. Small clusters also developed in rhodium containing samples besides the pure metallic state. Upon CO adsorption on the 1% Rh/titanate nanostructures the IR stretching frequencies characteristic of the twin (geminal) form were dominant, whereas bimetallic nanosystems featured a pronounced linear CO stretching vibration. At the same time the highest binding energy state disappeared almost completely indicating the enlargement of nanoclusters which was in agreement with SEM results. Very likely “core-shell” bimetallic clusters form, where gold covers the rhodium.

Adsorption and Oxidation of Carbon Monoxide on Pt/C, Pt3Co/C, and PtRu/C Catalysts Studied by In-Situ Attenuated Total Reflection Fourier-Transform Infrared

The adsorption and oxidation of CO on commercial nanoparticle catalysts supported on carbon black (Pt/C, Pt3Co/C, PtRu/C) were examined at 23, 40, and 60 degrees C in 0.1 M HClO4 by use of in situ ATR-FTIR (attenuated total reflection Fourier-transform infrared) spectroscopy. Absorption bands for the adsorbed CO assigned to linear (atop) CO (CO(L)) and bridge CO (CO(B)) were observed around 2040 cm(-1) and 1850 cm(-1), respectively, at high CO coverage theta(CO) close to 0.8 on all three types of catalysts. The adsorption rates of both CO(L) and CO(B) at the initial stage were found to decrease in the order Pt/C > Pt3Co/C > PtRu/C, indicating that the interaction of CO with PtRu is weakest. The adsorption of CO on these catalysts resulted in the growth of a sharp O-H stretching band around 3630 to 3640 cm(-1), which was assigned to non-hydrogen-bonded water molecules (isolated H2O) co-adsorbed with CO. For the electrooxidation reaction of CO, PtRu/C exhibited the highest activity at all temperatures. It was confirmed that the dominant factor for determining CO oxidation activity was the onset potential for the oxidation of isolated H2O, E(onset)(H2O), to provide an oxygen species that is consumed in either a Langmuir-Hinshelwood mechanism (Pt/C, Pt3Co/C) or the bi-functional mechanism (PtRu/C). In addition, PtRu/C exhibited the weakest Pt-CO interaction. The values of E(onset)(H2O) at PtRu/C were lowest among the three catalysts from 23 to 60 degrees C. With increasing temperature, the E(onset)(H2O) at Pt/C and Pt3Co/C shifted to less positive potential, resulting in increased CO oxidation activity, while the shift in E(onset)(H2O) at PtRu/C was relatively small.

New insights into the role of the electronic properties of oxide promoters in Rh-catalyzed selective synthesis of oxygenates from synthesis gas

A series of 2.5% Rh/M@Al 2O 3 model catalysts were prepared by supporting Rh on high-area γ-Al 2O 3, resulting in a surface covered by a monolayer (4.5–7 atoms/nm 2) of MO x promoter oxides (M = Fe, V, Nb, Ta, Ti, Y, Pr, Nd, Sm). The catalysts were extensively characterized and evaluated for the conversion of synthesis gas to oxygenates at 553 K, 5.0 MPa, H 2/CO = 1, and space velocity adjusted to attain CO conversion around 15%. The broad range of products formed depending on the specific promoter were, for the first time, quantitatively described using the selectivity parameter ( Φ) defined here, which indicates, for a given reaction product, the contribution of carbon atoms derived from dissociative ( C dis) and nondissociative ( C ins) activation of CO. Both the catalytic activity and, more interestingly, the selectivity pattern given by the Φ parameter were correlated with the electronic properties of the MO x promoters (i.e., electron-donating/electron-withdrawing capacity) for an extensive series of catalysts. Low-temperature and at-work CO-FTIR experiments suggested that the high activity and hydrocarbon selectivity displayed by catalysts promoted by more electron-withdrawing (acidic) oxide promoters (e.g., TaO x ) were related to a higher proportion of bridged Rh 2(CO) B adsorption sites and to a higher electron density (i.e., a higher electron back-donation ability) of the Rh 0 surface sites, both factors promoting CO dissociation events. In contrast, linear CO adsorption on Rh 0 sites displaying decreased electron back-donation in catalysts promoted by electron-donating (basic) oxides (e.g., PrO x , SmO x ) was likely related to nondissociative CO activation and thus to the selective formation of oxygenates. TEM, XPS, and CO-FTIR results pointed to differences in morphology, rather than size or partial electronic charge, of the nanosized Rh 0 crystallites as the likely cause for the different proportions of CO adsorption sites. The Rh 0 NP morphology, both as-reduced and at-work, is a function of the electronic properties of the underlying promoter oxide.

Surface Properties of Supported, Colloid-Derived Gold/Palladium Mono- and Bimetallic Nanoparticles

Mono- and bimetallic Au/Pd nanoparticles deposited on different supports (Al2O3, TiO2, SiO2) have been prepared using a colloidal route. Electron microscopy revealed a narrow particle size distribution of as-prepared nanoparticles with a mean diameter in the range of 3−4 nm. CO adsorption combined with DRIFTS measurements was employed to gain information about the available adsorption sites on these nanoparticles. On Pd nanoparticles, linear, μ2-briged, and μ3-bridged CO was observed. Upon reductive treatment of the particles, the signal for the μ2-bridged species on Pd(111) faces decreased significantly. The gold particles revealed a surprisingly strong red shift of the signal at around 2110 cm−1 after reduction, indicating a morphological change of the particles upon treatment in hydrogen. In contrast, treatment in oxygen at the same temperature did not alter the IR spectrum notably. EXAFS and DRIFTS indicated a redistribution of the constituents of the bimetallic nanoparticles upon reduction: the core−shell structure of the as-prepared particles changed to a structure with more evenly distributed constituents.