CO oxidation on a Cu(100) catalyst

The oxidation of carbon monoxide by molecular oxygen on a single crystal Cu(100) catalyst was studied at 458 K using reactant gas mixtures with CO/O₂ ratios of 2/1, 10/1 and 25/1 at a total pressure of 10 Torr. The catalytic activities were found to be strongly dependent upon the CO/O₂ ratio. Under stoichiometric reaction conditions (CO/O₂ = 2), the initial CO oxidation activity decreased sharply; with a highly reducing reaction mixture (CO/O₂ = 25), the initial activity gradually increased. These changes in catalytic activities with reactant gas mixture composition correlate with changes in surface composition, namely an increase in the surface oxygen coverage. Post-reaction TPD revealed the presence of a carbonate-like species which decomposed at ca. 630 K.     

Methanol synthesis on a Cu(100) catalyst

The synthesis of methanol on a Cu(100) single crystal surface was studied between 500–550 K and at pressures between 44–102 kPa using a gas mixture of CO₂/CO/H₂ = 1/2/12. The specific reaction rates found for methanol synthesis were approximately an order of magnitude lower than those rates previously reported for silica supported, Cu-based catalysts. Furthermore the rates observed for the Cu(100) catalyst are estimated to be several orders of magnitude smaller than those rates found for ZnO supported Cu catalysts at comparable reaction conditions. The very low concentration of ionic copper species on the surface is thought to be responsible for the low activity of the Cu(100) catalyst.     

The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H₂ was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H₂ on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.     

The mechanism of CO oxidation over Cu(110): Effect of CO gas energy

The effect of the temperature of gas phase CO upon the kinetics of the oxygen titration reaction: CO_g +O_a CO_2,g, has been studied. It is found that the reaction's rate is independant of CO gas temperature between 300 and 623 K. The activation energy (6.5 kcal/mole), dependence upon CO pressure (first-order), and independence upon oxygen coverage for 0.1 _o 0.4 are all independant of the CO gas phase temperature. This result rules out any Eley-Rideal type mechanism whereby CO reacts directly from the gas phase with an oxygen adatom without first being accommodated to the surface temperature in an absorbed state. The result is instead interpretable in terms of a Langmuir-Hinshelwood mechanism.Camille and Henry Dreyfus Foundation Teacher-Scholar Fellowship.     

Infrared spectroscopic detection of Rh(1) by CO in supported Rh catalyst

The detection limit of Rh(1) in the Rh/Al₂O₃ catalyst in a form of Rh¹(CO)₂ was determined by FTIR spectroscopy. It was demonstrated that at least 0.5 g Rh, corresponding to 0.005 wt% of Rh, can be identified in this way. During synthesis gas conversion the predominant surface species is Rh _x -CO, but a detectable amount of Rh(1) exists on the catalyst up to 473 K.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Mechanism of low-temperature CO oxidation on a model Pd/Fe2O3 catalyst

A model Pd/Fe₂O₃ catalyst prepared by the vacuum technique has been studied in the carbon monoxide oxidation in the temperature range of 300–550 K at reagent pressures P(CO)=16 Torr, P(O₂)= 4 Torr. It has been shown that the activity of the fresh catalysts is determined by palladium. According to the XPS data, the reduction with carbon monoxide results in the formation of Fe²⁺ (formally Fe₃O₄) and appearance of the catalytic activity in this reaction at low temperatures (350 K). High low-temperature activity of the catalyst is supposed to be connected with the reaction between oxygen adsorbed on the reduced sites of the support (Fe²⁺) and CO adsorbed on palladium (CO_ads) at the metal–oxide interface.     

Catalytic performance of K-promoted Rh/USY catalysts in preferential oxidation of CO in rich hydrogen

K-promoted Rh/USY (molar ratio: K/Rh=3) catalyst was found to exhibit high performance in preferential oxidation of CO in rich hydrogen. Such high performance was maintained in the presence of steam and CO₂. The CO oxidation activity of the K-Rh/USY catalyst was independent of the partial pressure of H₂, while the activity of the unpromoted Rh/USY catalyst was decreased significantly in hydrogen-rich stream. The effect of potassium addition on the catalyst structure was investigated and is discussed in terms of the differences in the catalytic performance.     

Methanol synthesis on Cu(100) from a binary gas mixture of CO2 and H2

The rate of methanol synthesis over a Cu(100) single crystal from a 1 1 mixture of CO₂ and H₂ has been measured at a total pressure of 2 bar and a temperature range of 483–563 K. At these conditions the apparent activation energy is determined to be 69 kJ mol^–1, and at 543 K the turnover rate is 2.7 × 10^–4 (site s)^–1. A kinetic model for the methanol synthesis is presented. Predictions from this model are in good agreement with the rates of methanol synthesis observed on real catalysts at industrial conditions.     

Pt-Fe催化剂次表层Fe结构表面的CO氧化反应性能

利用紫外光电子能谱等表面科学方法,研究了Pt-Fe模型催化剂的次表层Fe结构[即Pt/Fe/Pt(111)结构]在不同条件下CO的吸附及其氧化反应。结果表明,Pt/Fe/Pt(111)结构在H_2气氛或者超高真空中是种稳定结构,最外层的原子与Pt(111)相同,是密排的铂原子面;但次表层的原子中有约0.5单层的铁原子,使费米边附近(0~2.0)e V的电子态密度明显低于Pt(111)表面,从而改变表面的CO和O_2吸附以及反应性能。程序升温的紫外光电子能谱结果显示,Pt/Fe/Pt(111)表面在(100~300)K,CO的吸附受温度的影响不明显,且O_2能够吸附、活化并使共吸附的CO发生氧化反应;当温度为300 K时,O_2无法在Pt/Fe/Pt(111)表面吸附、活化,所以CO氧化反应无法进行。Pt/Fe/Pt(111)结构虽然能有效地减弱CO的吸附从而避免CO毒化的问题,但O_2的吸附和活化也受到显著抑制并影响到一定条件下CO的氧化反应。     

Supported Rh catalysts for methane partial oxidation prepared by OM-CVD of Rh(acac)(CO)2

The organometallics chemical vapour deposition (OM-CVD) technique, using Rh(acac)(CO)₂ as a precursor, was employed for the preparation of heterogeneous Rh catalysts supported on low surface area refractory oxides (α-Al₂O₃, ZrO₂, MgO and La₂O₃). Prepared systems were tested in the methane catalytic partial oxidation (CH₄-CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh₄(CO)₁₂.Catalysts supported on Al₂O₃, ZrO₂ and MgO show better or comparable performances with respect to the reference system.Complete decomposition of Rh precursor during formation of the metal phase under reductive conditions was investigated by TPRD and confirmed by infrared and mass spectrometry data.Supported Rh phase was characterized by CO and H₂ chemisorption, CO-DRIFT spectroscopy and HRTEM microscopy in fresh and aged selected samples. Rh(I) isolated sites and Rh(0) metal particles were found on fresh catalysts; after ageing an extensive reconstruction occurs mainly consisting in a sintering of Rh isolate sites to metal particles but without large increase in mean particles size.Catalytic performances and Rh species balance were found to be dependent on the support material.     

The oxidation of carbon monoxide on Rh(100) under steady state conditions: An FT-IR study

The steady-state oxidation of CO on clean Rh(100) at low pressures has been investigated using in-situ infrared spectroscopy and mass spectrometry. The results show that at a fixed CO pressure, the temperature at which the CO₂ formation rate maximizes decreases as a function of increasing O₂ pressure. Vibrational data indicate that this maximum rate coincides with a CO coverage of less than 0.01 monolayers.     

Structure sensitivity of CO oxidation over model Au/TiO2 2 catalysts

Model catalysts of Au clusters supported on TiO₂ thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from ~2.5 to ~6.0 nm. The reactivities of these Au/TiO₂ catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5–6.0 nm. The specific rates of reaction ((product molecules) × (surface site)^-1 × s^-1 were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO₂(001)/Mo(100) is structure sensitive. This revised version was published online in July 2006 with corrections to the Cover Date.     

Au–Cu alloy nanoparticles supported on silica gel as catalyst for CO oxidation: Effects of Au/Cu ratios

Au–Cu bimetallic catalysts with Au/Cu ratios ranging from 3/1 to 20/1 were prepared on silica gel support by a two-step method. The catalysts were characterized by ICP, XRD and TEM. The results showed that, irrespective of Au/Cu ratios, all the bimetallic nanoparticles had significantly reduced particle sizes (3.0–3.6 nm) in comparison with monometallic gold catalysts (5.7 nm). Both CO oxidation and PROX reactions were employed to evaluate the catalytic activities of Au–Cu bimetallic catalysts. For CO oxidation, the alloy catalysts show non-monotonic temperature dependence showing a valley in the intermediate temperature range. The catalyst with Au/Cu ratio of 20/1 gave the highest activity at room temperature, but its activity showed the deepest valley with increasing the reaction temperature. On the other hand, the catalyst with Au/Cu ratio of 3/1 exhibited the best performance for PROX reaction. For the Au/Cu ratios investigated, the bimetallic catalysts showed superior performance to monometallic gold catalysts, demonstrating the synergy between gold and copper.     

Oxygen induced direct hydrogenation of CO on Ni(100) surface

A trace amount of oxygen in H₂ promotes a new type of direct hydrogenation reaction of adsorbed CO on Ni(100) surface. The formation of H_xCO_y was suggested by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). HREEL spectra showed the formation of surface hydroxyl (OH) and the C-H bonds of H_iCO_y species but no carbonyl (C=O) loss peak was detected although thermal desorption yielded large amount of CO. The H _x CO _y undergoes the decomposition at 400–450 K on the hex-OH Ni(100) surface, which yielded CO, CO₂, H₂ and H₂CO. It was confirmed that no C-H bond formation occurs on c(2 × 2)-O, p(2 × 2)-O Ni(100) and hex-OH Ni(100) as well as on clean Ni(100) surfaces. This fact indicates that the gas phase oxygen may induce the direct hydrogenation of CO to form H _x CO _y , which is analogous to the hydrogenation of O to form hex-OH onNi(100).     

A comparison of CO oxidation on ceria-supported Pt,Pd, and Rh

Steady-state, CO-oxidation kinetics have been studied at differential conversions on model, ceria-supported, Pt, Pd, and Rh catalysts, from 467 to 573 K, and the results compared to the alumina-supported metals. On each of the ceria-supported metals, there is a second mechanism for CO oxidation under reducing conditions which involves oxygen from ceria reacting with CO on the metals. The rates of this second process are independent of which metal is used. The process has a significantly lower activation energy (14±1 kJ/mol compared to 26±2 kJ/mol on alumina-supported catalysts) and different reaction orders for both CO (zeroth-order compared to –1) and 02 (0.40 to 0.46 compared to first-order). This second process leads to significant rate enhancements over alumina-supported catalysts at low temperatures, especially for Pt. The implications of these results for automotive catalysis are discussed.     

Methanol Synthesis on Potassium-Modified Cu(100) from CO + H2 and CO + CO2 + H2

Methanol cannot be produced from CO + H₂ on a clean copper surface, but a promotional effect of potassium on methanol synthesis from mixtures of CO + H₂ and CO + CO₂ + H₂ at a total pressure of 1.5 bar on a Cu(100) surface is shown in this work. The experiments are performed in a UHV chamber connected with a high-pressure cell (HPC). The methanol produced is measured with a gas chromatograph and the surface is characterized with surface science techniques. The results show that potassium is a promoter for the methanol synthesis from CO + H₂, and that the influence of CO₂ is negligible. Investigation of the post-reaction surface with TPD indicates that potassium carbonate is present and plays an important role. The activation energy is determined as 42 ± 3 kJ/mol for methanol synthesis on K/Cu(100) from CO + H₂.     

DYNAMIC KINETICS OF THE COMMON INTERMEDIATES FORMED IN CO AND CO2 HYDROGENATIONS ON Rh/MgO

Stable intermediates are formed during both CO and CO₂ hydrogenations on Rh/MgO at 190-220^°C. The intermediates consist of at least four species, each of which is consecutively and irreversibly converted at a similar rate, forming the final product of CH₄ by reaction with H₂     

N_2O在Rh(100)表面吸附机理的量子化学研究

在对三效催化剂(TWC)表面N2O的催化反应实验及理论研究的基础上,选用了在过渡金属催化体系量子化学理论研究中常用的密度泛函方法,采用CASTEP计算程序,交换相关函数采用PW91函数,计算了N2O在催化剂Rh(100)面上的化学吸附。计算结果显示:通过对N端吸附位和O端吸附位的N-N、N-O、Rh-N、Rh-O等键长和吸附能的比较,说明NO最有可能发生的是N端顶位吸附。     

Local forcing of a nonlinear surface reaction: CO oxidation on Pt(100)

A novel spatiotemporal perturbation method for nonlinear surface reactions is reported, thus allowing the creation of new spatially localized structures. Forcing was achieved by dosing reactant gases through a capillary positioned near the catalyst surface, providing control over the local surface coverage and reaction rate. The emergence of localized concentration patterns and oscillations in an otherwise stable system is attributed to a local modification of the catalytic properties of the surface due to external forcing. Based on the spatial orientation, the temporal and thermal stability of the modified surface, as well as the affinity of CO toward the perturbed surface, subsurface O is proposed as a possible source of the observed localized patterning and surface memory effect. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

CO and H2 oxidation on a platinum monolith diesel oxidation catalyst

This paper presents experimental and modelling results for the oxidation of mixtures of hydrogen and carbon monoxide in a lean atmosphere. Transient light-off experiments over a platinum catalyst (80 g/ft³ loading) supported on a washcoated ceramic monolith were performed with a slow inlet temperature ramp. Results for CO alone agree with earlier results that predict self-inhibition of CO; that is an increasing light-off temperature with increasing CO concentration. Addition of hydrogen to the feed causes a reduction in light-off temperature for all concentrations of CO studied. The most significant shift in light-off temperature occurs with the addition of small amounts of hydrogen (500 ppm, v/v) with only minor marginal enhancement occurring at higher hydrogen concentrations. Hydrogen alone in a lean atmosphere will oxidise at room temperature. In mixtures of hydrogen and CO, the CO was observed to react first until a conversion of about 50% was observed, at which point the conversion of hydrogen rapidly went from 0 to 100%.

Simulations performed using literature mechanistic models for the oxidation of these mixtures predicted that hydrogen ignites first, followed by CO, a direct contradiction of the experimental evidence. Upon changing the activation energy between adsorbed hydrogen and oxygen, the CO was observed to oxidise first, however, no enhancement of light-off was predicted. The effect cannot be explained by the mechanistic model currently under discussion.