Performing CO Oxidation Research Articles

Enabling real-time detection of photocatalytic reactions by a re-useable micro-reactor

We present a highly sensitive and versatile Si-based μ-reactor for photocatalytic experiments. The μ-reactor can be operated in front side (through a borosilicate window) as well as backside (through the catalyst support) illumination. The μ-reactor is sealed by compressing the Si-based microchip, a parafilm gasket and the supported catalyst by four screws. This design allows for fast and reliable assembly of the μ-reactor and the microchip is re-usable for several independent experiments, making the presented μ-reactor ideal for catalyst screening studies. We analyze volatile reactants on-line by connecting the μ-reactor to a mass-spectrometer. The limiting capillary of the μ-reactor allows for nearly 100% collection efficiency (i.e. every produced molecule will be collected) by limiting the gas flow to the mass-spectrometer—enabling highly sensitive measurements and an absolute calibration. Finally, the small height of the reactor volume results in very fast response times, which make the reactor suitable for studies of transient activity phenomena—including catalyst activation and deactivation. We demonstrate these capabilities of the μ-reactor by performing CO oxidation experiments on a TiO2 thin film with subsequent calibration of the chip. Based on a mathematical model of the μ-reactor we additionally present a method for determining the residence time, the reaction volume and the working distance.

Effect of Surface Temperature on the Distribution and Reactivity of Rh Active Sites for CO Oxidation

AbstractSingle‐atom catalysts are a fast‐emerging area in which late‐transition metal atoms are supported on oxides, metals, and carbonaceous supports. They show great promise for selective chemical reactions due to their well‐defined active sites, and significantly reduce precious metal loading. However, oxide‐supported single‐atom catalysts have drawbacks such as deactivation due to sintering, and there are on‐going debates about the exact nature of the active sites. Herein we report a model system where the surface temperature during Rh deposition on a copper oxide support dictates the Rh size distribution. Our temperature‐programmed desorption experiments reveal that Rh clusters perform CO oxidation at lower temperature than single atoms, and isotopic labelling demonstrates differences in the mechanism in that the C−O bond is broken and reformed on clusters, as opposed to a pure Mars van Krevelen mechanism for CO oxidation at the single‐atom Rh sites. We further examine the cluster size distributions with scanning tunneling microscopy and X‐ray photoelectron spectroscopy which confirm the presence of both clusters and single atoms for the lower temperature deposition and only single atoms for the higher temperature deposition. Together, these results provide a fundamental understanding of the differences in the CO oxidation mechanism on Rh atoms and clusters.

Infrared surface spectroscopy and surface optical reflectance for operando catalyst surface characterization

Using a new sample environment, the two operando surface diagnostic techniques two-dimensional surface optical reflection (2D-SOR) and polarization-modulated infrared reflection–absorption spectroscopy (PM-IRRAS) have been combined with mass spectrometry (MS) to characterize a catalytic reaction. With both techniques integrated in a custom-built setup, we can correlate molecular information of the adsorbed surface species from PM-IRRAS with information about oxide formation on the sample from 2D-SOR. The new setup was evaluated by performing CO oxidation over a Palladium single crystal Pd(100) surface. The results reveal that the macroscopic and the microscopic molecular behavior correlate well. When the CO adsorption peak disappears in the PM-IRRAS spectrum, the formation of a well-defined ultra-thin surface oxide is observed in the 2D-SOR trend. We discuss the benefits and limitations of the two techniques as well as their potential for further studies of catalytic reactions at both gas–solid and liquid–solid interfaces.

Biomimetic CO oxidation below \u2212100\u2009\xb0C by a nitrate-containing metal-free microporous system

CO oxidation is of importance both for inorganic and living systems. Transition and precious metals supported on various materials can oxidize CO to CO2. Among them, few systems, such as Au/TiO2, can perform CO oxidation at temperatures as low as −70 °C. Living (an)aerobic organisms perform CO oxidation with nitrate using complex enzymes under ambient temperatures representing an essential pathway for life, which enables respiration in the absence of oxygen and leads to carbonate mineral formation. Herein, we report that CO can be oxidized to CO2 by nitrate at −140 °C within an inorganic, nonmetallic zeolitic system. The transformation of NOx and CO species in zeolite as well as the origin of this unique activity is clarified using a joint spectroscopic and computational approach.

Chemical kinetics for operando electron microscopy of catalysts: 3D modeling of gas and temperature distributions during catalytic reactions

In situ environmental transmission electron microscopy (ETEM) is a powerful tool for observing structural modifications taking place in heterogeneous catalysts under reaction conditions. However, to strengthen the link between catalyst structure and functionality, an operando measurement must be performed in which reaction kinetics and catalyst structure are simultaneously determined. To determine chemical kinetics for gas-phase catalysis, it is necessary to develop a reliable chemical engineering model to describe catalysis as well as heat and mass transport processes within the ETEM cell. Here, we establish a finite element model to determine the gas and temperature profiles during catalysis in an open-cell operando ETEM experiment. The model is applied to a SiO2-supported Ru catalyst performing CO oxidation. Good agreement is achieved between simulated compositions and those measured experimentally across a temperature range of 25 – 350 °C. In general, for lower conversions, the simulations show that the temperature and gas are relatively homogeneous within the hot zone of the TEM holder where the catalyst is located. The uniformity of gas and temperature indicates that the ETEM reactor system behavior approximates that of a continuously stirred tank reactor (CSTR). The large degree of gas-phase uniformity also allows one to estimate the catalytic conversion of reactants in the cell to within 10% using electron energy-loss spectroscopy. Moreover, the findings indicate that for reactant conversions below 35%, one can reliably evaluate the steady-state reaction rate of catalyst nanoparticles that are imaged on the TEM grid.

Dynamic charge and oxidation state of Pt/CeO2 single-atom catalysts.

The catalytic activity of metals supported on oxides depends on their charge and oxidation state. Yet, the determination of the degree of charge transfer at the interface remains elusive. Here, by combining density functional theory and first-principles molecular dynamics on Pt single atoms deposited on the CeO2 (100) surface, we show that the common representation of a static metal charge is oversimplified. Instead, we identify several well-defined charge states that are dynamically interconnected and thus coexist. The origin of this new class of strong metal-support interactions is the relative position of the Ce(4f) levels with respect to those of the noble metal, allowing electron injection to (or recovery from) the support. This process is phonon-assisted, as the Ce(4f) levels adjust by surface atom displacement, and appears for other metals (Ni) and supports (TiO2). Our dynamic model explains the unique reactivity found for activated single Pt atoms on ceria able to perform CO oxidation, meeting the Department of Energy 150 °C challenge for emissions.

An atomic-scale view of single-site Pt catalysis for low-temperature CO oxidation

Single-atom catalysts have attracted great attention in recent years due to their high efficiencies and cost savings. However, there is debate concerning the nature of the active site, interaction with the support, and mechanism by which single-atom catalysts operate. Here, using a combined surface science and theory approach, we designed a model system in which we unambiguously show that individual Pt atoms on a well-defined Cu2O film are able to perform CO oxidation at low temperatures. Isotopic labelling studies reveal that oxygen is supplied by the support. Density functional theory rationalizes the reaction mechanism and confirms X-ray photoelectron spectroscopy measurements of the neutral charge state of Pt. Scanning tunnelling microscopy enables visualization of the active site as the reaction progresses, and infrared measurements of the CO stretch frequency are consistent with atomically dispersed Pt atoms. These results serve as a benchmark for characterizing, understanding and designing other single-atom catalysts. Single-atom catalysts are of growing importance, but the nature of their structure and reactivity remains under debate. Here, Sykes and co-workers show that single Pt atoms on a well-defined Cu2O surface are capable of performing low-temperature CO oxidation, and provide data on the binding site and electronic structure of the Pt atoms.

Atomic-Resolution Characterization of Surface Structures and Metal-Support Interfaces on Nanostructured Pt/CeO2 Catalysts Performing CO Oxidation

Atomic-Resolution Characterization of Surface Structures and Metal-Support Interfaces on Nanostructured Pt/CeO2 Catalysts Performing CO Oxidation

Catalytic oxidation of solid carbon and carbon monoxide over cerium‐zirconium mixed oxides

A series of cerium‐zirconium mixed oxides was prepared and evaluated in the catalytic oxidation of solid coke with 10 vol % O2 in He at 673 K using a thermogravimetric analyzer. The measured first order rate constant for coke oxidation was proportional to the catalyst loading when the mass ratio of catalyst to coke was low, which enabled the calculation of a surface area specific reaction rate. The validity of the normalization method was confirmed by performing CO oxidation over the cerium‐zirconium mixed oxides in a fixed bed reactor at 573 K. Although there was no correlation between the coke oxidation rate and the oxygen storage capacity or the reducibility of the catalysts, there was an excellent correlation to the CO oxidation rate. Kinetic studies of both coke and CO oxidation suggested an important role of surface lattice oxygen from the catalyst in the two reactions. © 2016 American Institute of Chemical Engineers AIChE J, 63: 725–738, 2017

Gold-TiO2-Nickel catalysts for low temperature-driven CO oxidation reaction

Nickel-doped-TiO2 catalysts were prepared by the sol–gel method and surface modified with gold nanoparticles (AuNPs) by the urea-deposition-precipitation technique. The as-synthesized catalysts were characterized by X-ray diffraction, Raman and XPS spectroscopies, N2 physisorption, STEM-HAADF microscopy and TPR hydrogen consumption. The Au/TiO2-Ni catalysts were evaluated catalytically performing CO oxidation reactions. The catalyst with nickel content of 1wt. % (Au/TiO2-Ni 1) showed the highest CO conversion with respect to the high-nickel-content or bare/commercial TiO2 at 0°C. In situ DRIFTS showed a strong participation of both nickel due to the presence of surface-nickel-metallic nanoparticles formed during the CO adsorption process at reaction temperatures above 200°C, and surface-bridged-nickel-CO species. A minor deactivation rate was observed for the Au/TiO2-Ni 1 catalyst in comparison with the Au/TiO2 one. The oxygen vacancies that were created on the sol–gel-doped TiO2 improved the catalytic behavior during the performance of CO oxidation reactions, and inhibited the AuNP sintering.

The Important Role of Hydroxyl on Oxidation Catalysis by Gold Nanoparticles

Although gold is generally considered to be a relatively inert metal, supported gold nanoparticles have demonstrated exceptionally high catalytic activity for the oxidation of carbon monoxide and alcohols at modest temperatures. In both cases, the presence of hydroxyl groups substantially promotes the reaction rate, presumably by participating in the reaction. Direct comparisons of CO oxidation to alcohol oxidation over gold catalysts have been difficult for scientists to explain. The former reaction is usually performed with gas phase reagents, whereas the latter reaction is often performed in the condensed phase. In this Account, we discuss the important role of hydroxyl for these two oxidation reactions catalyzed by gold, in terms of its influence on the turnover frequency. During CO oxidation over gold, a hydroxyl can directly react with CO to form COOH, which eventually decomposes to CO2. The gas phase CO oxidation reaction likely occurs at the gold-support interface, where adsorbed hydroxyl groups can be found after the addition of water to the feed. When we perform CO oxidation in liquid water, increasing the pH substantially promotes the reaction rate by providing an external source of hydroxyl. Likewise, we can also promote alcohol oxidations over gold catalysts in aqueous media by increasing the pH of the system. Since the hydroxyl groups are supplied through the reaction medium instead of on the support surface, the gold-support interface is much less important in the aqueous phase reactions. Even bulk gold powder becomes an active oxidation catalyst in alkaline water. The role of O2 in both CO and alcohol oxidation in aqueous media is to remove electrons from the gold surface that are deposited during oxidation, maintaining electroneutrality. Thus, the oxidation of CO and alcohols in water at high pH is analogous to the electrochemical oxidation reactions performed on gold electrodes. As the field of chemistry continues to encourage the development of sustainable chemical processes utilizing environmentally benign reaction conditions, the use of water as a "green" solvent becomes an attractive choice. In general, however, heterogeneous catalysts that scientists have developed over the last century for the petrochemical industry have not been optimized for use in aqueous media. Given the active role of water in oxidation reactions catalyzed by gold, additional research is needed to understand how water affects other catalytic transformations on traditional transition metal catalysts.

Synthesis of nanosized Ce0.85M0.1Ru0.05O2−δ (M = Si, Fe) solid solution exhibiting high CO oxidation and water gas shift activity

Nanosized Ce0.85M0.1Ru0.05O2−δ (M=Si, Fe) has been synthesized using a low temperature sonication method and characterized using XRD, TEM, XPS and H2-TPR. The potential application of both the solid solutions has been explored as exhaust catalysts by performing CO oxidation. The addition of Si- and Fe- in Ce0.95Ru0.05O2−δ greatly enhanced the reducibility of Ce0.85M0.1Ru0.05O2−δ (M=Si, Fe), as indicated by the H2-TPR study. The oxygen storage capacity has been used to correlate surface oxygen reactivity to the CO oxidation activity. Both the compounds reversibly release lattice oxygen and exhibit excellent CO oxidation activity with 99% conversion below 200°C. A bifunctional reaction mechanism involving CO oxidation by the extraction of lattice oxygen and rejuvenation of oxide vacancy with gas feed O2 has been used to correlate experimental data. The performance of both the solid solutions has also been investigated for energy application by performing the water gas shift reaction. The present catalysts are highly active and selective towards the hydrogen production and a lack of methanation activity is an important finding of present study.

Kinetics of carbon monoxide oxidation with Sn0.95M0.05O2−δ(M = Cu, Fe, Mn, Co) catalysts

Base metal substituted Sn0.95M0.05O2−δ (M = Cu, Fe, Mn, Co) catalysts were synthesized by the solution combustion method and characterized by XRD, XPS, TEM and BET surface area analysis. The catalytic activities of these materials were investigated by performing CO oxidation. The rates and the apparent activation energies of the reaction for CO oxidation were determined for each catalyst. All the substituted catalysts showed high rates and lower activation energies for the oxidation of CO as compared to unsubstituted SnO2. The rate was found to be much higher over copper substituted SnO2 as compared to other studied catalysts. 100% CO conversion was obtained below 225 °C over this catalyst. A bifunctional reaction mechanism was developed that accounts for CO adsorption on base metal and support ions and O2 dissociation on the oxide ion vacancy. The kinetic parameters were determined by fitting the model to the experimental data. The high rates of the CO oxidation reactions at low temperatures were rationalized by the high dissociative chemisorption of adsorbed O2 over these catalysts.

High rates of CO and hydrocarbon oxidation and NO reduction by CO over Ti0.99Pd0.01O1.99

This study aims at synthesizing a new by substituting 1atom% Pd2+ in ionic state in TiO2 in the form of Ti0.99Pd0.01O1.99 with oxide-ion vacancy. The catalyst was synthesized by solution combustion method and was characterized by XRD and XPS. The catalytic activity was investigated by performing CO oxidation, hydrocarbon oxidation and NO reduction. A reaction mechanism for CO oxidation by O2 and NO reduction by CO was proposed. The model based on CO adsorption on Pd2+ and dissociative chemisorption of O2 in the oxide-ion vacancy for CO oxidation reaction fitted the experimental for CO oxidation. For NO reduction in presence of CO, the model based on competitive adsorption of NO and CO on Pd2+, NO chemisorption and dissociation on oxide-ion vacancy fitted the experimental data. The rate parameters obtained from the model indicated that the reactions were much faster over this catalyst compared to other catalysts reported in the literature. The selectivity of N2, defined as the ratio of the formation of N2 and formation of N2 and N2O, was very high compared to other catalysts and 100% selectivity was reached at temperature of 350°C and above. As the N2O+CO reaction is an intermediate reaction for NO+CO reaction, it was also studied as an isolated reaction and the rate of the isolated reaction was less than that of intermediate reaction.

Effect of multifractalities of catalyst surface: a Monte Carlo study of catalytic CO oxidation

For modeling disordered catalyst surfaces, we propose a method for generating multifractal surfaces with varying fractal dimensionality profiles. For three surfaces generated by the method, Monte Carlo simulations of catalytic CO oxidation are carried out to investigate the effect of morphology of the catalyst surface on its catalytic activity and kinetic phase transitions. The results show that multifractal surfaces with the more symmetric and less dispersive fractal dimensionality profile perform CO oxidation more efficiently. The first-order kinetic phase transition occurs at the higher partial pressure of CO for the surface with the less dispersive dimensionality profile.