On the mechanism and energetics of Boudouard reaction at FeO(1 0 0) surface: 2CO→C+CO2

We present a density functional study on the mechanism of metal dusting at surfaces of transition metal oxides. The present study focuses on one of the major metal dusting processes: the Boudouard reaction on FeO(1 0 0) surface. Cluster models are used to represent the surface and the effects of cluster sizes and relaxation upon CO adsorption are carefully examined. It is found that the CO adsorption on a well-aligned transition metal oxide surface is very weak and does not lead to CO-bond dissociation. A minimum energy path of a gas-phase CO attacking an adsorbed CO from the surface normal is calculated and the structural change of the reaction species along the reaction path is examined. The results suggest that the Boudouard reaction is extremely unfavorable energetically at the transition metal oxide surfaces and that a pitting mechanism dictates the dusting process, in agreement with practical observations.     

Support‐dependent activity of noble metal substituted oxide catalysts for the water gas shift reaction

The water gas shift reaction was carried out over noble metal ion substituted nanocrystalline oxide catalysts with different supports. Spectroscopic studies of the catalysts before and after the reaction showed different surface phenomena occurring over the catalysts. Reaction mechanisms were proposed based upon the surface processes and intermediates formed. The dual site mechanism utilizing the oxide ion vacancies for water dissociation and metal ions for CO adsorption was proposed to describe the kinetics of the reaction over the reducible oxides like CeO₂. A mechanism based on the interaction of adsorbed CO and the hydroxyl group was proposed for the reaction over ZrO₂. A hybrid mechanism based on oxide ion vacancies and surface hydroxyl groups was proposed for the reaction over TiO₂. The deactivation of the catalysts was also found to be support dependent. Kinetic models for both activation and deactivation were proposed. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

CaSO4氧载体与CO反应中Boudouard反应的热力学分析及其抑制研究

在反应温度500～1200 ℃、CO浓度0.5%～99.0%、CO2浓度0～70%和SO2浓度0～10%条件下,利用化学平衡常数,对化学链燃烧 CaSO4与CO反应过程中Boudouard反应的可行性展开热力学研究,获得了抑制Boudouard反应及积碳生成的方法。由于CaSO4竞争还原反应和Boudouard反应平衡常数的表达形式不同,通过比较各个反应的CO平衡转化率以判断各个反应进行程度的高低。在化学链燃烧燃料反应器适宜温度范围（850～1050 ℃）,CO浓度越高,Boudouard反应越容易进行,SO2浓度对Boudouard反应进行的程度没有影响。而提高反应温度和反应气中CO2浓度,则有利于抑制Boudouard反应的进行。当气体反应物中CO2浓度超过3.7%,即使CO浓度达到95.0%,Boudouard反应不能进行。     

Surface structure of Ni/MgO catalysts: Effects of carbon and hydrogen on the reactivity towards CO. HRTEM and FTIR studies

CO adsorption on high loading Ni/MgO samples treated at 800 and 900°C has been studied by using IR spectroscopy. Linear and bridged monocarbonyls are produced under low CO pressure and the Boudouard reaction occurs producing C, adsorbed on the metal particles, and CO₂, stabilized on the MgO matrix. Effects of hydrogen and carbon on surface reactivity have been studied. The morphology of the catalysts and of carbonaceous deposits produced by heating in CO at high temperature are examined by HRTEM.     

Transition metal/fluorite-type oxides as active catalysts for reduction of sulfur dioxide to elemental sulfur by carbon monoxide

Reduction of SO₂ by CO to elemental sulfur on fluorite oxide and transition metal/fluorite oxide composite catalysts is discussed within the redox framework. The oxygen vacancies on the fluorite oxide are active catalyst sites and their creation is a key step to initiate the redox reaction. Transition metal/fluorite oxide composites demonstrate a strong synergism for the reaction. A reduction kinetics study of the copper/ceria system as a model composite catalyst has found that surface oxygen is highly active but its activity is inhibited by SO₂ adsorption. The TPR profile of the copper/ceria catalyst shows a significantly lower reduction peak than the individual components.     

Effect of palladium and nickel on the temperature programmed reduction of metal oxides and metal oxide layers

A new binary oxide support was suggested as being useful in many commercial reactions. Our study was focused on the reduction effect of metal oxide layer on alumina during reaction. Hence temperature programmed reduction of both bulk metal oxide and metal oxide layer on alumina was studied first and the effect of palladium and nickel on the reduction of the oxide support was also investigated. Vanadium oxide was mainly studied and niobium oxide, tantalum oxide, titanium oxide and zirconium oxide were also compared. Some metal oxides were reduced in a hydrogen stream at elevated temperature. In these cases both the bulk metal oxide and metal oxide layer were reduced. A tiny amount of palladium or nickel affected the reduction by decreasing the reduction temperature. The decrease of the reduction temperature was explained by means of increased adsorption of hydrogen on the transition metal and ability of the metal to spillover of the hydrogen to the oxides.     

Role of support in lean DeNO x catalysis

This article reviews the accumulated theoretical results, in particular density functional theory calculations, on two catalytic processes, CO oxidation and NO reduction on metal surfaces. Owing to their importance in automotive emission control, these two reactions have generated a lot of interest in the last 20 years. Here the pathways and energetics of the involved elementary reactions under different catalytic conditions are described in detail and the understanding of the reactions is generalized. It is concluded that density functional theory calculations can be applied to catalysis to elucidate mechanisms of complex surface reactions and to understand the electronic structure of chemical processes in general. The achieved molecular knowledge of chemical reactions is certainly beneficial to new catalyst design.     

Fabrication of an Effusive Molecular Beam Instrument for Surface Reaction Kinetics—CO Oxidation and NO Reduction on Pd(111) Surfaces

A simple molecular beam instrument (MBI) was fabricated for measuring the fundamental parameters in catalysis such as, sticking coefficient, transient and steady state kinetics and reaction mechanism of gas/vapor phase reactions on metal surfaces. Important aspects of MBI fabrication are given in detail. Nitric oxide (NO) decomposition and NO reduction with carbon monoxide (CO) on Pd(111) surfaces were studied. Interesting results were observed for the above reactions and they support the efficiency of the MBI to derive the fundamental parameters of adsorption and catalysis. Sustenance of CO oxidation at 400 K is dependent mostly on the absence of CO-poisoning; apparently, CO + O recombination is the rate determining step ≤400 K. NO adsorption measurements on Pd(111) surface clearly indicating a typical precursor kinetics. Displacement of the chemisorbed CO by NO on Pd(111) surfaces was observed directly with NO + CO beams in the transient kinetics. It is also relatively easy to identify the rate-determining step directly from the MBI data and the same was demonstrated for the above reactions.     

金属氧化物在OX-ZEO催化剂中催化CO x 加氢制低碳烯烃研究进展

金属氧化物-分子筛（OX-ZEO）双功能催化剂可实现CO _x 加氢制低碳烯烃的高选择性转化。本文概述了OX-ZEO催化CO _x 加氢制低碳烯烃反应中金属氧化物的研究进展,通过对CO _x 加氢制甲醇/乙烯反应热力学分析指出了“接力催化”的优势,重点讨论了金属氧化物的种类和组成、制备方法及金属氧化物和分子筛的“亲密度”对催化性能的影响,探讨了催化反应机理、氧空位的作用及抑制副反应的策略。分析了OX-ZEO催化反应面临的问题和挑战,展望了OX-ZEO催化体系的发展趋势,认为通过元素掺杂、助剂修饰、优化制备条件等可提高金属氧化物的氧空位含量,进而可提高催化活性,也可通过对金属氧化物进行表面疏水改性抑制副产物CO₂,提高C原子利用率。     

Theoretical studies of surface reactions on metals.: I. Ethyl to ethylene conversion on Ni(1 0 0). II. Photodissociation of methane on platinum

The goal of this research is the development and application of theoretical techniques that will provide a molecular level understanding of surface processes, especially reaction mechanisms and energetics. Electronic structures are described by an ab initio embedding formalism that permits an accurate determination of energies and adsorbate structure. An overview of the theoretical method is presented and applications to catalytic and photochemical reactions on nickel and platinum surfaces are discussed. Preliminary studies of ethylene production from ethyl adsorbed on nickel are reported. A second study concerns the photodissociation of methane physisorbed on platinum. Results are reported for a methane molecule interacting with a Pt ring model of the surface. Configuration interaction theory is used to sort out states resulting from electron attachment to methane from lower energy states that correspond to metal excitations. A mechanism is proposed for the photodissociation process.     

Mechanism of Iron Catalysis of Carbon Monoxide Decomposition in Refractories

The catalytic effects of selected iron phases (metals, oxides, sulfides, and carbides) on the Boudouard reaction (2CO ⇄ CO₂+ C) studied in an effort to more fully understand the disintegration of refractories when exposed to CO for long periods of time. It was found that active Fe atoms generated from the reduction of the iron oxides, especially α-Fe₂O₃, are the actual catalysts for the Boudouard reaction. The catalytic process, confirmed by thermodynamic calculations, kinetic data, and X-ray diffraction data, consists of adsorption and decomposition of CO simultaneously forming carbides of iron. The chemisorption and subsequent decomposition of the iron carbides, rather than diffusion, constitute the rate-controlling process for carbon deposition.     

A feasibility study of preparing carbon nanotubes by using a metal dusting process

Metal dusting (MD), often occurs in strongly carburizing atmospheres, is a catastrophic form of corrosion which leads to disintegration of materials into a powdery mixture of graphite and metal dusts. These tiny metal dusts can catalyze the deposition of nano-sized carbon materials from the atmosphere. In this paper, we demonstrate that both muti-wall CNTs and single-wall CNTs can be prepared by exploiting the MD process under proper reaction conditions. Since a steel coupon and a CO–CO₂ mixed gas are the only reactants needed, the MD process is low-cost, simple, and easily scaled up. Most importantly, fresh catalyst nano-particles are constantly provided during the process; therefore, a high yield of CNTs is anticipated. It has been confirmed herein that the amount of the CNTs remarkably increases when the reaction time is extended from 30 to 100h, indicating a possibility of the mass production. However, according to TEM observation, the deposited amorphous carbon and metal dusts adhered on surface of the CNTs also gradually increase with prolonging the reaction period.     

Infrared studies of the reactive adsorption of organic molecules over metal oxides and of the mechanisms of their heterogeneously-catalyzed oxidation

The use of IR spectroscopic techniques to provide information on the mechanisms of catalytic oxidation over metal oxide catalysts is briefly discussed. The data published on studies of the catalytic oxidation of methanol, of linear C₄ hydrocarbons and of methylaromatics over different metal oxide surfaces are reviewed and discussed. Lattice oxygen appears to act as the active oxygen species in both selective and total oxidation. Generalized mechanisms of these complex oxidation reactions are proposed and the catalyst features affecting selectivities in these reactions are discussed. The reaction network is apparently essentially governed by the organic chemistry of the reacting molecule (thus being substantially the same over the different oxide catalysts). However, the catalyst surface governs the rate of the different steps, favoring some paths over others. Thus, selectivity is determined by the catalyst chemical behavior and by the reaction variables (contact time, temperature, gas-phase composition, presence of steam, etc.). IR studies, if performed under conditions where some intermediates are actually detectable and jointly with other techniques, can give valuable information on the catalysis mechanisms. On the other hand, it has been concluded that in situ studies frequently do not give reliable information on reaction mechanisms, because under reaction conditions spectators rather than intermediates are detected.     

Surface relaxation at the metal/electrolyte interface

X-ray diffraction is an ideal technique for the in situ study of single crystal metal surfaces in an electrochemical environment. In this paper, measurements of the low-index surfaces of Au and Pt are described, in particular with reference to surface expansion effects. Surface expansion can be probed potentiodynamically to correlate expansion with the adsorption/desorption of solution species. In general, the results are in good agreement with recent theoretical calculations. The X-ray technique can also give insight into electrocatalytic reactions as shown by the results for the adsorption and oxidation of CO on Pt(111).     

金属氧化物吸附剂脱硫过程中羰基硫的生成

金属氧化物脱硫剂脱硫过程中产生羰基硫的现象缩小了脱硫剂的使用范围。本文从气体中存在一氧化碳、二氧化碳、单质硫和硫化氢出发给出了均相、非均相生成羰基硫的途径,特别综述了生成羰基硫的催化作用。在总结氧化铁基、氧化锌基、氧化锰基及氧化铜基主流金属氧化物脱硫剂脱硫过程中产生羰基硫研究结果的基础上,发现吸附在金属硫化物表面上的一氧化碳与活性硫化物的相互作用是生成羰基硫的主要反应步骤,活性硫化物可能是硫化氢热解成蒸气硫或硫氢根离子及硫化氢与金属氧化物反应生成的金属硫化物。     

Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules

Infrared spectra analysis of species formed by acid probe adsorption on divided metal oxides and alkaline zeolites can lead to information on their surface basicity, particularly on the nature and strength of basic sites. Results obtained from carbon monoxide, carbon dioxide, sulfur dioxide, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, ammonia and pyridine are critically reviewed. It is concluded that no probe can be universally used. Pyrrole in the case of alkaline zeolites, CO₂ for weakly basic metal oxides and for basic OH groups and CO for the characterization of highly basic structural defects on metal oxides activated at high temperature appear quite suitable probes. When other methods are used (TPD, microcalorimetry, volumetry, etc.) IR spectroscopy is a complementary method necessary for the knowledge of the type of sites involved; in particular, data obtained from CO₂ and SO₂ adsorption have to be carefully used since both probes can lead on highly basic metal oxides to polydentate (bulk) species which do not result from their adsorption but from their reaction with the given oxide. Ammonia and pyridine, generally used as probes for the measure of the acidity of catalysts, also adsorb on basic oxides through a dissociative chemisorption.     

Density functional study on the reaction of CO molecules with MgO surfaces

Adsorption of CO to the MgO surface modeled by Mg_nO_n (n = 4, 6, 9, 10) clusters was investigated employing the density functional method, and modes of bonding, adsorption energies, and CO exchange mechanisms were discussed. The atoms at the low coordination sites possess the small amount of charges consistent with the crystal field theory. The adsorption to such sites results in large stabilization though the magnitude is less remarkable than in the case of hydrogen adsorption. A possible mechanism for the CO exchange reaction, observed experimentally, is presented based on the energetics calculated. The CO's adsorbed and in the gas phase are exchangeable through the two-molecule adsorption state, which is realized at the edge site or the O-atom defect site. For the latter, the structure of the intermediate is more consistent with the IR measurement.     

Adsorption and reaction of CO on vanadium oxide–Pd(111) “inverse” model catalysts: an HREELS study

The room temperature adsorption and reaction of CO on Pd(111) surfaces decorated with submonolayer coverages of vanadium oxide – so-called inverse model catalysts – have been studied by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The HREELS surface phonon spectra of the V oxide phases have been measured and used to monitor the changes in the oxide as a result of the interaction with CO. The intramolecular C–O stretching frequency of CO adsorbed on the V-oxide/Pd(111) surfaces displays two vibrational loss components as a function of CO coverage as it has been observed on the clean Pd(111) surface. The relative intensities of the two vibrational features as a function of V oxide coverage however suggest that the balance of CO adsorption sites is modified as compared to clean Pd(111) by the presence of the V oxide–Pd phase boundary. Preferential population of high coordination adsorption sites by CO in the vicinity of the oxide–metal interface is proposed. The analysis of the V oxide phonon spectra indicates that adsorbed CO partially reduces the V oxide at the boundaries of the oxide islands to the Pd metal. The reduction of V oxide by CO is dependent on the oxygen content of the V oxide phase. The reduction of V oxide is confirmed by the XPS V 2p core level shifts.     

Insights from Theory on the Relationship Between Surface Reactivity and Gold Atom Release

Density functional theory, informed by experimental studies, is used to investigate the interplay of surface morphology, the adsorption site of reactants, the nature of the interaction between adsorbates and the surface, the potential energy landscape for adsorbates on the surface, adsorbate coverage, temperature, and the dynamic evolution of these factors during adsorption and reaction. We summarize our current understanding of Au atom release on the (111) surface and the corresponding effects on adsorption and reactivity. Gold was selected for these investigations because of the recent intense interest in the activity of gold nanoparticles for several important catalytic reactions. Fundamental experimental studies on Au single-crystal surfaces have established that atomic O is extremely active for oxidation of CO and olefins, that the local bonding of O is an important factor in determining the reactivity and selectivity for oxidation, and that Au atom release is induced by electronegative adsorbates, such as O, Cl, and S. These experimental results guided our theoretical studies. Density functional theory is an extremely useful tool since it evaluates the energetics associated with the incorporation of gold into the adsorbate layer, while providing fundamental physical insight into the underlying cause of gold incorporation. We use our results from static DFT calculations along with ab initio molecular dynamics simulations to understand the effect of surface morphology on the activity of gold for CO oxidation. Our investigation of Au atom release and incorporation induced by electronegative atoms clearly illustrates the importance of using experiments in combination with theory to establish the importance of and the underlying reasons for metal atom release and the affect on bonding and reactivity.