H2-induced CO adsorption and dissociation over Co/Al2O3 catalyst

The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissociation. The H2 was adsorbed first, and activated to form H＊ over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH^＊, which reacted with adsorbed CO subsequently via CO^＊＋OH^＊ → CO2^＊＋H^＊; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO on the surface via CO^＊＋2H^＊ → CH^＊＋OH^＊. The formation of the surface species （CH） from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.     

Ethanol synthesis from syngas on Mn- and Fe-promoted Rh/γ-Al2O3

The catalytic hydrogenation of CO was studied over Mn- and/or Fe-promoted Rh/γ-Al₂O₃ catalysts. The catalysts were characterized by means of XRD, BET, H₂-TPR·H₂-TPD, XPS and DRIFTS. CO hydrogenation results showed that the doubly Mn- and Fe-promoted Rh/γ-Al₂O₃ catalysts exhibited superior catalytic activity and better ethanol selectivity. The DRIFTS results showed that Mn promoter stabilized the adsorbed CO on Rh⁺ and Fe stabilized adsorbed CO on Rh⁺ and Rh⁰, especially Rh⁰. The fact that doubly Mn- and Fe-promoted Rh/γ-Al₂O₃ owned more (Rhx⁰–Rhy⁺)–O–Fe³⁺·(Fe²⁺) active species was proposed to be a crucial factor accounting for its higher ethanol selectivity.     

Metal-Support interaction in Silica-Supported Cobalt Catalyst

The variations in catalytic properties for CO hydrogenation and adsorptive properties for H₂ and CO of silica-supported cobalt catalyst as a function of reduction temperature (400 to 700 °C) were investigated. A mild sintering of cobalt metal from 130 Å to 145 Å occurred only when the reduction temperature was further increased from 600 to 700 °C. However, a monotone decrease in the activity (per gram Co) for CO hydrogenation was observed with increasing reduction temperature. This is accompanied with a decrease in methane and an increase in olefin formation. The decrease in activity can not be accounted for by the variation of crystallite size of cobalt metal. A concurrent suppression in the amount of hydrogen and CO adsorptions was observed after reduction temperature was raised to 700 °C. The turnover frequencies, based on cobalt dispersion estimated from H₂ and CO chemisorption, remained, more or less, similar with increasing reduction temperature from 400 to 600 °C, but increased ca. 3 and 1.5 times, respectively, at 700 °C. The relative amount of subcarbonyl species (2062 cm^?1) with respect to the linear adsorbed CO (2010 cm^?1) was found to increase as well. Furthermore, reduction of silica surface at 700 °C was indicated by the presence of ?Si-H, which was detected by infrared spectroscopy at 2294 cm^?1. This shows the presence of metal-support interaction for silica-supported cobalt systems after reduction treatment at 700 °C.     

H2-induced CO adsorption and dissociation over Co/Al2O3 catalyst

The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissocia-tion. The H2 was adsorbed first, and activated to form H* over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH*, which reacted with adsorbed CO subsequently via CO*+OH* → CO2*+H*; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO onthe surface via CO*+2H* → CH*+OH*. The formation of the surface species (CH) from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.     

Effects of Titania on the Adsorptive and Catalytic Properties of Silica-Supported Cobalt Catalyst

A series of Ti-promoted (6 wt%) Co/SiO₂ catalysts with titania content of 0 to 10 wt% were sequentially prepared by incipient wetness impregnation, and characterized with X-ray diffraction, thermogravimetric analysis, chemisorption, temperature-programmed desorption and infrared spectroscopy. The influences of Ti addition and reduction temperature (400–700 °C) on the adsorptive behavior and the catalytic properties for CO hydrogenation were investigated. The presence of Ti decreases the adsorption capacity of the cobalt surface for H₂, but enhances activity per gram cobalt. In addition, the turnover frequency increases 2–4 times upon Ti addition at reduction temperatures of 400–700 °C. The promotion in activity is accompanied by an enhanced selectivity for higher hydrocarbons and olefins. These modifications can be rationalized by the creation of active sites for CO dissociation. The desorption of CO₂ at 100 °C during temperature-programmed desorption of CO indicates the formation of active sites for CO disproportionation. Infrared spectroscopy indicates an increase in the relative absorbance of 2060–2075 cm^?1 bands upon Ti addition, which are attributed to CO adsorbed on the defect sites of the cobalt surface. Therefore, the promotion effect of Ti may be directly related to the formation of defect sites on the cobalt surface induced by the decorated titania moieties.     

Effects of Ethanol Impregnation on the Catalytic Properties of Silica-Supported Cobalt Catalysts

Silca-supported Co₃O₄ (6 wt% as Co) catalysts were prepared by pore volume impregnation of ethanol or aqueous cobalt nitrate solutions, and calcined in vacuo to 300 °C. The catalytic performances of these catalysts for oxidation and hydrogenation of CO were examined. All Co₃O₄/SiO₂ catalysts were found to be very active in catalyzing oxidation of CO to CO₂ as compared to a commercial 1 wt% Pt/Al₂O₃. The ethanol-prepared catalysts exhibited higher activity than those of the aqua-prepared catalysts. Pre-calcination of the ethanol-prepared catalysts in oxygen at 600 °C resulted in a dramatic decrease in the activity. Temperature programmed oxidation indicated the presence of carbon deposits on the surface of used catalysts. Infrared spectra showed the continuous generation of CO₂ when these catalysts were exposed to CO. These indicate the primary role of CO disproportionation in catalytic oxidation of CO on Co₃O₄ at low temperature and explain the sharp decrease in activity in the initial period. After reduction at 400 °C, the ethanol-prepared catalysts were also found to be more active in catalyzing hydrogenation of CO, and produced less methane and olefin (C2-C4) fraction. Higher turnover frequencies were observed after high temperature reduction (600 °C) as well, at which ethoxyl groups were removed from silica surface. In both reactions, the enhanced activity for the ethanol-prepared catalysts can not be fully accounted for by the increase in the dispersion of Co₃O₄ or CO metal. This suggests that the surface structures of Co₃O₄ or CO were further modified by the carbonaceous species derived from ethanol.     

Hydrogenation du naphtalene et de l'anthracene lors de la complexation aux greffons cationiques C5H5Fe+ et C5Me5Fe+.

In order to investigate the mechanism of hydrogenation of polyaromatics during ligand exchange with a cyclopentadienyl ring of ferrocene, the influences of temperature, presence of aluminium powder, water, amount and quality of aluminium chloride have been examined for the reaction: ferrocene + naphthalene → CpFe⁺ naphthalene/CpFe⁺ tetralin. The reaction of CpFe(CO)₂Cl is analogous to that of ferrocene. The data are discussed in terms of a possible electron-transfer mechanism generating CpFe^I polyaromatic and further H atom abstraction. The new pentamethylcyclopentadienyliron analogues have been synthesized and the extent of hydrogenation corresponds with the parent series. However, with anthracene, the major product of the reaction with C₅Me₅Fe(CO)₂Br in the presence of AlCl₃, η⁵-C₅Me₅Fe⁺-η⁶-1,2,3,4-tetrahydroanthracene results in the change of hydrogeneration site.     

The influence of preadsorbed hydrogen and CO on adsorption of ethylene on the Pd(111) surface

The coadsorption of C₂H₄ with H₂ and CO on Pd(111) has been investigated at 300 and 330 K At 300 K two forms of adsorbed ethylene coexist on the surface in the presence of ethylene gas: a molecular form desorbing as C₂H₄ at 330 K and a dissociatively adsorbed form (giving only hydrogen in desorption spectra) which is stable both in vacuum and in hydrogen at 10^?8 Torr. The molecular form seems to be a precursor state for hydrogenation and for dissociative adsorption. Both processes are controlled by the amount of coadsorbed hydrogen which in turn is controlled by CO coverage.     

Surface species and gas phase products in the preferential oxidation of CO on TiO2-supported Au-Rh bimetallic catalysts

The oxidation of CO in the presence of hydrogen (PROX process) was investigated on bimetallic Au-Rh catalysts at 300–373 K by Fourier transform infrared spectroscopy and mass spectroscopy. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied. The IR studies revealed the formation of bi- and monodentate carbonates, bicarbonates and hydrocarbonates on the catalysts surfaces; these surface species proved to be not involved in the surface reactions. The formation of adsorbed formaldehyde was observed on all surfaces, except 1% (0.25Au+0.75Rh)/TiO₂. Adsorbed CO₂ (as the surface product of CO oxidation) was not detected on any surface. The presence of both O₂ and H₂ reduced the surface concentration of CO adsorbed on the metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO₂ was formed in the highest amount in the CO+O₂ mixture, the presence of H₂ suppressed the amount of CO₂ produced. This negative effect of H₂ was the lowest on the 1% Rh/TiO₂ and 1% (0.25Au+0.75Rh)/TiO₂ catalysts.     

Highly selective catalysts for liquid-phase hydrogenation of substituted alkynes based on Pd—Cu bimetallic nanoparticles

Catalytic properties of Pd—Cu bimetallic catalysts supported on SiO₂ and Al₂O₃ were studied in a model reaction of selective hydrogenation of diphenylacetylene. Application of PdCu₂(AcO)₆ heterobimetallic acetate complex as a precursor made it possible to obtain homogeneous Pd—Cu bimetallic nanoparticles. This result was supported by the data of IR spectroscopy of adsorbed CO. The Pd-Cu catalysts showed considerably higher selectivity than monometallic samples. Moreover, the introduction of copper decreases the hydrogenation rate of diphenylethylene (DPE) to diphenylethane. As a result, the maximum yield of the target product, DPE, increased from 78 to 93% in the presence of Pd—Cu catalysts.     

萘在贵金属Pd、Pt及Pd-Pt催化剂上的加氢活性及耐硫性能

采用等体积浸渍法制备了SiO2-Al2O3负载的Pd、Pt单金属催化剂及Pd/Pt摩尔比分别为1∶1、1∶4、4∶1的双金属催化剂(Pd1Pt1、Pd1Pt4、Pd4Pt1),对其进行X射线衍射(XRD)、透射电镜(TEM)、CO化学吸附和X射线光电子能谱(XPS)表征,并详细考察了各催化剂的萘加氢活性和耐硫性能.结果表明,在实验考察范围内,Pd4Pt1催化剂上的萘转化率最高可达98.2%,全饱和产物十氢萘选择性最高可达93.6%,十氢萘反/顺生成率之比最高可达7.8,均高于单金属Pd(97.5%,59.1%,4.3)和Pt(96.8%,39.9%,2.9)催化剂的值.萘在三种催化剂上的加氢速率顺序为vPd4Pt1vPdvPt.添加二苯并噻吩(DBT)后Pd4Pt1上的萘转化率和十氢萘选择性仍然最高,十氢萘反/顺比在Pt催化剂上不受影响,在Pd4Pt1催化剂上稍有降低,而在Pd催化剂上降低明显.在三种不同Pd/Pt摩尔比的双金属催化剂中,Pd4Pt1催化剂上的萘转化率和十氢萘选择性在添加DBT前后都是最佳的.     

PROBING STUDY OF Rh CATALYSTS ON DIFFERENT SUPPORTS IN CO HYDROGENATION

Rh catalysts on SiO₂ and CeO₂ were studied in CO hydrogenation by adding probing molecules during the reaction. The results demonstrate that alkanes were not formed through the oxygen-containing intermediates, e.g. methanol, ethanol or acetaldehyde. Ethanol and acetaldehyde were not produced through the formation of methanol. Acetaldehyde was not formed through ethanol dehydrogenation, however, ethanol was formed through the hydrogenation of acetaldehyde and CeO₂ itself can catalyze the hydrogenation of acetaldehyde.     

Electrocatalytic oxidation of carbon monoxide and methanol at Pt nanoparticles confined in SBA-15: voltammetric and in situ infrared spectroscopic studies

Electrocatalytic oxidation of carbon monoxide and methanol at Pt nanoparticles confined in mesoporous molecular sieve SBA-15 was studied by using cyclic voltammetry and in situ FTIR spectroscopy. Cyclic voltammetric studies revealed that the Pt nanoparticles confined in SBA-15 exhibit a high activity in the presence of hydrated phase consisting of SiO₂ in the SBA-15. In situ FTIR spectroscopy results discovered that IR absorption of CO adsorbed on Pt nanoparticles confined in SBA-15 has been enhanced 11-fold, and the full-width at half-maximum of the CO band is significantly increased, in comparison with IR feature of CO adsorbed on a bulk Pt electrode. The linearly adsorbed CO species is the only intermediate derived from dissociative adsorption of methanol, which is more readily oxidized to form CO₂ in the aid of the active oxide in SBA-15.This paper is dedicated to Professor G. Horanyi on the occasion of his 70th birthday and in recognition of his outstanding contribution to electrochemistry     

Electrochemical behavior of Pd–Rh alloys

Pd–Rh alloys were prepared by electrochemical codeposition. Bulk compositions of the alloys were determined by the energy dispersive X-ray analysis method, while surface compositions were determined from the potential of the surface oxide reduction peak. Cyclic voltammograms, recorded in 0.5 M H₂SO₄ for Pd–Rh alloys of different bulk and surface compositions, are intermediate between curves characteristic of Pd and Rh. The influence of potential cycling on electrochemical properties and surface morphologies of the alloys was studied. Due to electrochemical dissolution of metals, both alloy surface and bulk become enriched with Pd. Carbon oxides were adsorbed at a constant potential from the range of hydrogen adsorption. The presence of adsorbed CO₂ causes remarkable diminution of hydrogen adsorption but it does not significantly influence hydrogen insertion into the alloy bulk. On the other hand, in the presence of adsorbed CO, both hydrogen absorption and adsorption are strongly suppressed. Oxidative removal of the adsorbates results in a characteristic voltammetric peak, whose potential increases with the decrease in Rh surface content. Electron per site (eps) values calculated for the oxidation of the adsorbates change with alloy surface composition, more for CO₂ than CO adsorption, indicating the variation of the structure and composition of CO₂ and CO adsorption products. The course of the dependence of eps values on surface composition suggests that the products of CO₂ and CO adsorption on Pd–Rh alloys are similar but not totally identical.     

Effect of Halide Anions on the Electroreduction of CO2 to C2H4: A Density Functional Theory Study**

The halide anions present in the electrolyte improve the Faradaic efficiencies (FEs) of the multi-hydrocarbon (C₂₊) products for the electrochemical reduction of CO₂ over copper (Cu) catalysts. However, the mechanism behind the increased yield of C₂₊ products with the addition of halide anions remains indistinct. In this study, we analysed the mechanism by investigating the electronic structures and computing the relative free energies of intermediates formed from CO₂ to C₂H₄ on the Cu (100) facet based on density functional theory (DFT) calculations. The results show that formyl *CHO from the hydrogenation reaction of the adsorbed *CO acts as the key intermediate, and the C−C coupling reaction occurs preferentially between *CHO and *CO with the formation of a *CHO-CO intermediate. We then propose a free-energy pathway of C₂H₄ formation. We find that the presence of halide anions significantly decreases the free energy of the *CHOCH intermediate, and enhances desorption of C₂H₄ in the order of I⁻>Cl⁻>Br⁻>F⁻. Lastly, the obtained results are rationalized through Bader charge analysis.     

Efficient Ni-based catalysts for low-temperature reverse water-gas shift (RWGS) reaction

CO₂ hydrogenation for syngas can alleviate the pressure of un-controlled emissions of CO₂ and bring enormous economic benefits. Advantageous Ni-catalysts have good CO₂ hydrogenation activity and high CO selectivity merely over 700 °C. Herein, we introduced Cu into Ni catalysts, which were evaluated by H₂-TPR, XRD, BET, in-situ XPS and CO₂-TPD, and their CO₂ hydrogenation activity and CO selectivity were significantly affected by the Ni/Cu ratios, which was rationalized by the synergistic effect of bimetallic catalysts. In addition, the reduction temperatures of studied catalysts apparently affected the CO₂ hydrogenation, which were caused by the number and dispersion of the active species. It's found that the Ni₁Cu₁-400 had good stability, high CO selectivity (up to 90%), and fast formation rate (1.81×10⁻⁵ mol/g_cat/s) at 400 °C, which demonstrated a good potential as a superior catalyst for reverse water-gas shift (RWGS) reaction.     

Study on carbon dioxide reduction with water over metal oxide photocatalysts

Various metal oxides with 0.1 wt% Ag loaded as a cocatalyst were prepared by an impregnation method and examined their photocatalytic activity for CO₂ reduction with water. Among all the prepared Ag-loaded metal oxides, Ga₂O₃, ZrO₂, Y₂O₃, MgO, and La₂O₃ showed activities for CO and H₂ productions under ultraviolet light irradiation. Thus, metal oxides involving metal cations with closed shell electronic structures such as d⁰, d¹⁰, and s⁰ had the potential for CO₂ reduction with water. In situ Fourier transform infrared measurement revealed that the photocatalytic activity and selectivity for CO production are controlled by the amount and chemical states of CO₂ adsorbed on the catalyst surface and by the surface basicity, as summarized as follows: Ag/ZrO₂ enhanced H₂ production rather than CO production due to very little CO₂ adsorption. Ag/Ga₂O₃ exhibited the highest activity for CO production, because adsorbed monodentate bicarbonate was effectively converted to bidentate formate being the reaction intermediates for CO production owing to its weak surface basicity. Ag/La₂O₃, Ag/Y₂O₃, and Ag/MgO having both weak and strong basic sites adsorbed larger amount of carbonate species including their ions and suppressed H₂ production. However, the adsorbed carbonate species were hardly converted to the bidentate formate.     

Effect of La2O3 on Catalytic Performance of Au/TiO2 for CO Oxidation

Pure TiO₂ and La-doped TiO₂ were prepared by the sol-gel method. Au was supported on TiO₂ by the deposition-precipitation (DP) method, and its catalytic activity for CO oxidation was tested. The results showed that doping La in Au/TiO₂ could improve its catalytic activity obviously for CO oxidation. The analyses of X-ray diffraction (XRD), temperature-programmed desorption (TPD), and Brunauer-Emmett-Teller (BET) surface area further showed that the presence of La in TiO₂ not only increased its surface area and restrained the growth of TiO₂ crystallites, but could also enhance the microstrain of TiO₂. In terms of O₂-TPD, a new adsorbed species O⁻ appeared on the surface of La-doped TiO₂. The results of in-situ Fourier transform-infrared (FT-IR) spectroscopy illustrated that the high activity of Au/La₂O₃-TiO₂ was attributed to the presence of La promoting the reactivity of CO adsorbed on the Au site and the formation of the second active site on the surface of TiO₂     

Unravelling the Promotional Effect of La2O3 in Pt/La-TiO2 Catalysts for CO2 Hydrogenation

This work reports the preparation of a La₂O₃-modified Pt/TiO₂ (Pt/La-TiO₂) hybrid through an excess-solution impregnation method and its application for CO₂ hydrogenation catalysis. The Pt/La-TiO₂ catalyst is characterized by XRD, H₂ temperature-programmed reduction (TPR), TEM, X-ray photoelectron spectroscopy (XPS), Raman, EPR, and N₂ sorption measurements. The Pt/La-TiO₂ composite starts to catalyze the CO₂ conversion reaction at 220 °C, which is 30 °C lower than the Pt/TiO₂ catalyst. The generation of CH₄ and CO of Pt/La-TiO₂ is 1.6 and 1.4 times greater than that of Pt/TiO₂. The CO₂ temperature-programmed desorption (TPD) analysis confirms the strengthened CO₂ adsorption on Pt/La-TiO₂. Moreover, the in situ FTIR experiments demonstrate that the enhanced CO₂ adsorption of Pt/La-TiO₂ facilitates the formation of the active Pt–CO intermediate and subsequently boosts the evolution of CH₄ and CO. The cycling tests reveal that Pt/La-TiO₂ shows reinforced stability for the CO₂ hydrogenation reaction because the La species can prevent Pt nanoparticles (NPs) from sintering. This work may provide some guidance on the development new rare-metal-modified hybrid catalysts for CO₂ fixation.     

Surface and structural properties of pt/ceo2catalyst under preferential co oxidation in hydrogen (PROX)

Summary Preferential oxidation of CO in the presence of excess hydrogen was studied on Pt/CeO₂with 5% metal loading. Catalytic data were similar to those observed on 1% Pt/CeO₂earlier [16]. The optimum temperature region is T￡373 K; conversion and selectivity of CO oxidation strongly decreased at higher temperatures. High-pressure XPS indicated CO adsorbed on platinum particles and significant amount of water on the ceria surface. The top-most ceria surface re-oxidized as small amount of oxygen (3%) was introduced into the H₂/CO feed. Despite this surface re-oxidation, high-resolution TEM after reaction indicated oxygen deficient ceria bulk structure, in which the defects formed a super-cell, with CeO_1.695structure. The defective ceria is suggested to play an important role stabilizing the hydrogen bonded surface water, which (i) suppresses further hydrogen oxidation and (ii) reacts at the metal/support interface with linearly adsorbed CO in a low temperature water-gas-shift type reaction to produce CO₂.</o:p>