Reaction Rate Of CO Oxidation Research Articles

Effect of cobalt on CeO2 nanorod supported Pt catalyst: Structure, performance, kinetics and reaction mechanism in CO oxidation

Catalytic oxidation represents a highly efficient and extensively employed approach for the elimination of CO in exhaust gas. Rationally designing and preparing catalysts with exceptional activity and stability in the presence of H2O and SO2 at low temperatures represent crucial avenues for future advancements. In this work, a series of Pt catalysts supported on CeO2 nanorods incorporated with varying amounts of Cobalt (Co/CeO2 ratio ranging from 0 to10) were prepared and evaluated for their catalytic activity in CO oxidation under both dry and wet conditions. The ultrahigh dispersion of Pt, outstanding redox properties, and abundant oxygen vacancies on the surface of Co-doped CeO2 nanorod supported Pt catalysts were revealed by XRD, HAADF-STEM, Raman spectroscopy and H2-TPR techniques. The catalytic activity for CO oxidation strongly depends on the Co content and reaction conditions. Among them, Pt/Co-CeO2-5 % exhibits the highest CO oxidation performance with a T90 (the temperature to achieve 90 % conversion of CO) of 170 °C under dry conditions, which is significantly lower by 140 °C compared to T90 of Pt/CeO2. Pt/Co-CeO2-1 % demonstrates superior CO oxidation performance with a T90 of 195 °C under wet conditions, exhibiting a reduction in temperature by 65 °C compared to the T90 of Pt/CeO2. Moreover, Pt/Co-CeO2 exhibits excellent resistance to SO2 compared to Pt/CeO2. Kinetic studies, in situ DRIFT analysis and isotopic experiments demonstrated direct participation of H2O in the reaction. On the catalysts with a low Co/Ce ratio (Co/Ce < 0.03), CO readily reacts with hydroxyl groups dissociated from H2O to form carboxyl intermediates, which subsequently undergo rapid decomposition into CO2, resulting in an enhanced reaction rate of CO oxidation at low temperature.

Influence of effusion cooling air on the CO production in the primary zone of a pressurized single sector model gas turbine combustor

In this study, the influence of effusion cooling air on the CO production in the primary zone of a pressurized single sector model gas turbine combustor is investigated in a parametric study using a combination of laser-based diagnostics. Three species (OH, CO, CH2O) are probed using laser-induced fluorescence (LIF). In principle, this combination of probed species allows to derive the forward reaction rate of CO oxidation and the local heat release rate from the LIF signals. Based on simulations and measurements in a laminar flame, we show that the limited optical resolution of the intensified cameras severely limits the applicability of the method for pressurized flames. However, by employing a conditioning strategy, mainly using CH2O-LIF as a marker for low temperature chemistry, it was shown that effusion cooling air affects the structure of the premixed flame at large distances from the effusion cooled liner. At higher blowing ratios, samples measured in the primary zone during the CO production phase exhibit a higher CO concentration at a given level of OH concentration. Most likely, this is caused by the strong non-linear sensitivity of OH concentration to temperature, shifting the local equilibrium towards higher CO concentrations, when fractions of the effusion cooling air are recirculated towards the inlet nozzle. In the exhaust phase, dilution effects seem to dominate, as a higher effusion cooling air mass flow systematically lowers both measured OH and CO signals.

Solid Fe Resources Separated from Rolling Oil Sludge for CO Oxidation.

The efficient recycling of valuable resources from rolling oil sludge (ROS) to gain new uses remains a formidable challenge. In this study, we reported the recycling of solid Fe resources from ROS by a catalytic hydrogenation technique and its catalytic performance for CO oxidation. The solid Fe resources, after calcination in air (Fe2O3-H), exhibited comparable activity to those prepared by the calcinations of ferric nitrate (Fe2O3-C), suggesting that the solid resources have excellent recycling value when used as raw materials for CO oxidation catalyst preparation. Further studies to improve the catalytic performance by supporting the materials on high surface area 13X zeolite and by pretreating the materials with CO atmosphere, showed that the CO pretreatment greatly improved the CO oxidation activity and the best activity was achieved on the 20 wt.%Fe2O3-H/13X sample with complete CO conversion at 250 °C. CO pretreatment could produce more oxygen vacancies, facilitating O2 activation, and thus accelerate the CO oxidation reaction rate. The excellent reducibility and sufficient O2 adsorption amount were also favorable for its performance. The recycling of solid Fe resources from ROS is quite promising for CO oxidation applications.

Pt Supported on Plasma-Chemical Titanium Nitride for Efficient Room-Temperature CO Oxidation

Catalysts of carbon monoxide oxidation were synthesized by deposition of platinum on titanium nitride (TiN). Two substrates with an average particle size of 18 and 36 nm were obtained by hydrogen reduction of titanium tetrachloride in a stream of microwave plasma of nitrogen. The surface of the catalysts was studied by X-ray photoelectron spectroscopy (XPS). The data obtained by us in the present work indicate the presence of oxynitride as a transition layer between nitride and oxide. It was found that the CO oxidation rate on the 9 - 15 wt.% Pt loaded TiN catalysts is 120 times higher than that on the platinum black with a specific surface of 30 m2/g. Increase in the reaction rate of CO oxidation on Pt/TiN catalysts as compared to platinum black can be associated with both an increase in the concentration of CO molecules adsorbed and a decrease in the activation energy of the reaction. Catalysts are promising for use in catalytic air purification systems.

Enhanced catalytic performance of atomically dispersed Pd on Pr-doped CeO2 nanorod in CO oxidation

Single-atom noble metal catalysts have been widely studied for catalytic oxidation of CO. Regulating the coordination environment of single metal atom site is an effective strategy to improve the intrinsic catalytic activity of single atom catalyst. In this work, single atom Pd catalyst supported on Pr-doped CeO2 nanorods was prepared, and the performance and nature of Pr-coordinated atomic Pd site in CO catalytic oxidation are systematically investigated. The structure characterization using AC-HAADF-STEM, EXAFS, XRD and Raman spectroscopy demonstrate the formation of single atom Pd site and abundant surface oxygen vacancies on the surface of Pr-doped CeO2 nanorod. With the combination of the XPS characterization and DFT calculations, the oxidation state of Pd on Pr-doped CeO2 nanorod is determined lower than that on CeO2 nanorod. The turnover frequency of CO oxidation is markedly increased from 8.4 × 10−3 to 31.9 × 10−3 s with Pr-doping at 130 ºC and GHSV of 70,000 h−1. Combined with kinetic studies, DRIFT and DFT calculations, the doped-Pr atoms reduced the formation energy of oxygen vacancies and generate more oxygen vacancies around the atomically dispersed Pd sites on the surface of cerium oxide, which reduces the dissociation energy of oxygen, thereby accelerating the reaction rate of CO oxidation.

Synthesis and Properties of a Carbon Monoxide Oxidation Catalyst Based on Plasma-Chemical Titanium Carbonitride, Titanium Dioxide, and Palladium

A CO oxidation catalyst based on plasma-chemical titanium carbonitride TiC0.2N0.8 and TiO2 with a palladium content of 10 wt % has been synthesized. The dependence of the reaction rate of CO oxidation at room temperature and low CO concentrations (<100 mg/m3) on the TiC0.2N0.8 content has been studied. It has been found that, at TiC0.2N0.8 concentrations from 8 to 10 wt %, the catalyst exhibited a maximum rate of the CO oxidation reaction, which was 2.5 times higher than the rate of reaction on a catalyst based on pure TiO2 that included palladium clusters. The catalysts are promising for use in catalytic and photocatalytic systems for air purification.

Synthesis and properties of Pt/TiN catalyst for low-temperature air purification from carbon monoxide

Catalysts of carbon monoxide oxidation were synthesized by deposition of platinum on titanium nitride (TiN). Two substrates with an average particle size of 18 and 36 nm were obtained by hydrogen reduction of titanium tetrachloride in a stream of microwave plasma of nitrogen. The surface of the catalysts was studied by X-ray photoelectron spectroscopy (XPS). The data obtained by us in the present work indicate the presence of oxynitride as a transition layer between nitride and oxide. It was found that the CO oxidation rate on the 9–15 wt. % Pt loaded TiN catalysts is 120 times higher than that on the platinum black with a specific surface of 30 m2g–1. Increase in the reaction rate of CO oxidation on Pt/TiN catalysts as compared to platinum black can be associated with both an increase in the concentration of CO molecules adsorbed and a decrease in the activation energy of the reaction. Catalysts are promising for use in catalytic air purification systems.

A computational study of CO oxidation on IrO2 (1 1 0) surface

Though, oxygen-rich iridium oxide (O-IrO2 (110)) surface exhibits excellent catalytic activity towards methane activation and oxygen evolution reactions, the CO oxidation mechanism on this surface remains elusive. Here, we used density functional theory calculations, ab initio molecular dynamics (AIMD), and microkinetic simulations to explore the CO oxidation reactions via Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R), and Mars-Van Krevelen (MvK) mechanisms on O-IrO2 (110) surface. We find that the C-O coupling and CO2 desorption are the rate-determining steps in L-H and E-R mechanisms, respectively. The CO oxidation via the MvK mechanism on the IrO2 (110) surface is more difficult to proceed. Both AIMD and microkinetic simulation results demonstrate that CO2 can desorb from the O-IrO2 (110) surface at 400 K. The production temperature of CO2 on the O-rich IrO2 (110) surface via E-R mechanism is lower than that via the L-H mechanism. Therefore, we predict that the O-rich IrO2 (110) surface can accelerate the CO oxidation reaction rate.

Operando Near-Ambient Pressure X-ray Photoelectron Spectroscopy Study of the CO Oxidation Reaction on the Oxide/Metal Model Catalyst ZnO/Pt(111)

The identification of the active sites in heterogeneous catalysis is important for a mechanistic understanding of the structure–reactivity relationship. Among others, the oxide/metal boundaries are expected to contain the active sites in various catalytic reactions. To reveal their nature and their chemical evolution under reaction conditions, the catalytic role of an oxide/metal system consisting of well-ordered ZnO nanoislands grown on Pt(111) in low-temperature CO oxidation was studied by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in operando conditions, and additionally by ultra-high vacuum scanning tunneling microscopy. To illustrate the special role played by the oxide/metal boundaries, a systematic comparative study of ZnO/Pt(111) with the pristine Pt(111) surface was undertaken. The regimes where mass transfer limitation starts to occur were identified using NAP-XPS and mass spectrometry measurements in combination, allowing a sound discussion on the relation between steady-state molar fractions of reactants/product and surface reactivity. Via the measurement of the steady-state CO2 molar fraction, we observed that the CO oxidation reaction rate over the ZnO/Pt(111) system is superior to that over Pt(111) in a temperature range extending to 410 K. The pivotal, albeit unexpected, role of ZnO-bound hydroxyls was clearly highlighted by the observation of the chemical signature of the CO + OH associative reaction at the ZnO/Pt boundaries. The carboxyl formed at low temperature (410 K) can be the intermediate species in the CO oxidation reaction, the OHs at the Pt/ZnO boundary being the cocatalyst, which explains the synergistic effect of ZnO and Pt. However, the species formed at higher temperature (from 445 K) are formates that would essentially be spectators.

Evidence of the Encapsulation Model for Strong Metal–Support Interaction under Oxidized Conditions: A Case Study on TiOx/Pt(111) for CO Oxidation by in Situ Wide Spectral Range Infrared Reflection Adsorption Spectroscopy

Strong metal–support interaction (SMSI) has been found to significantly modify catalytic performance in the last 4 decades. The origins of the promotion effects of SMSI still reamin unclear. In this work, fully covered TiOx/Pt(111) model surfaces were prepared to mimic the proposed encapsulation model. The catalytic activities for CO oxidation on such model surfaces as a function of the TiOx coverage were examined in a model catalysis reaction cell. The TiOx ordered thin films significantly enhanced the reaction rate for CO oxidation on Pt(111). The apparent activation energy was obtained to be 51 kJ/mol for CO oxidation on the TiOx/Pt(111), which is much lower than that of 97 kJ/mol on a clean Pt(111) surface. The stability and catalytic efficiency of such SMSI model surfaces under reaction conditions were confirmed by our home-built in situ wide spectral range infrared reflection adsorption spectroscope, which is capable of measuring both the surface species and the changes of a catalyst in one spectrum...

Fluxing template-assisted synthesis of sponge-like Fe2O3 microspheres toward efficient catalysis for CO oxidation

Constructing a porous architecture is a considerable strategy to enhance the catalytic activity of metal oxides catalysts for CO oxidation. In this work, we have developed porous sponge-like Fe2O3 microspheres by employing a facile aerosol spray pyrolysis. The NaNO3 salt in the spray solution plays a crucial role as a fluxing sacrifice template in the formation of the sponge-like structure, in which a high surface area of 216.2 m2 g−1 and an average pore size of 4 nm are obtained. This novel Fe2O3 catalyst exhibits an improved catalytic activity compared to usual iron oxides catalysts. Nearly 50% CO conversion at a relatively low temperature of 200 °C and 100% CO conversion at 300 °C at a space velocity of 60 000 ml h−1 g−1 are achieved. Furthermore, it displays an outstanding catalytic stability without distinct decay for 1000 min in a continuous stream at 300 °C. In addition to the effect of plentiful adsorption sites for the gas reactant, the promoted catalytic performance is also attributed to the function of abundant OH groups rooted in the large surface of the sponge-like structure, which induces faster reaction rate of CO oxidation via a bicarbonate route.

Utilizing Quantitative in Situ FTIR Spectroscopy To Identify Well-Coordinated Pt Atoms as the Active Site for CO Oxidation on Al2O3-Supported Pt Catalysts

Relationships between geometric structures of active metallic sites and areal rates of reaction (structure sensitivity) are extensively studied for supported metal catalysts. For CO oxidation on irreducible oxide-supported Pt catalysts, there still exists a discrepancy regarding structure sensitivity. Theoretical and single-crystal analyses suggest the CO oxidation reaction rate should be highly structure sensitive, whereas measurements on supported Pt catalysts show only minimal structure sensitivity. Here, we used quantitative in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) to investigate the influence of CO oxidation reaction conditions on the fraction of well-coordinated (WC) and under-coordinated (UC) Pt active sites on a series of four α-Al2O3-supported Pt catalysts with average Pt sizes ranging from ∼1.4 to 19 nm. Pt nanoparticle surfaces were observed to restructure under CO oxidation reaction conditions, increasing the fraction of UC Pt sites. Reconstruction rendered ...

Density Functional Theory (DFT) Studies of CO Oxidation over Nanoporous Gold: Effects of Residual Ag and CO Self-Promoting Oxidation

We report a systematic study of CO oxidation mechanism over nanoporous (NPG) using the density functional theory (DFT). In the study, the (111) and (100) flat planes that were identified as the most abundant in the nanoporous gold are mimicked by Agx@Au-(111) and Agx@Au-(100) slabs (x = 1 – 3). A total of 50 reaction pathways are examined at different active sites. A simplified microkinetics model termed the Sabatier analysis, which is built on the adsorption energies and activation barriers, is used to evaluate the reaction rate of different reaction pathways. Our theoretical results indicate that the Au-kink sites joining the (111) and (100) flat planes are the major active sites. The residual Ag atoms in the Au-kink site promote the adsorption of O2 species and hence increase the reaction rate of CO oxidation. Besides the discussion of the Ag-impurity effect, we also propose that the nearby coadsorbed CO at Au steps can promote the dissociation of OCOO* reaction intermediate significantly via an electr...

Preparation, Characterization, and Kinetic Study of a Core–Shell Mn3O4@SiO2 Nanostructure Catalyst for CO Oxidation

Mn3O4@SiO2 prepared via a Stober route showed a steady reaction rate for CO oxidation at 250 °C after 12 h. The reaction orders, with respect to CO (0.49–0.12) and O2 (0.4–0.6), were obtained for a Mn3O4@SiO2 catalyst (dMn3O4 = 7.6 nm) in the temperature range of 220–280 °C. Via Operando Raman spectroscopy, a COad-induced change in the symmetric stretch of Mn–O–Mn (from 636 cm–1 to 642 cm–1) was observed for Mn3O4@SiO2, indicating a tiny structure modification of core Mn3O4 by the adsorption of CO. With the combination of the catalyst structure and the kinetics of CO oxidation, we assume that this reaction proceeds mainly through the Langmuir–Hinshelwood mechanism at or below 280 °C. In comparison with the reference α-Mn2O3 catalyst, the presence of shell SiO2 does not change the reaction behavior but improves the catalyst stability.

Characteristic of Low Calorific Fuel Gas Combustion in Porous Burner by Preheating Air

The combustion characteristic of low calorific fuel gas was numerically investigated in porous burner by preheating air. Two-dimensional temperature profile, flame propagation precess, and CO reaction rate were analyzed detailly by preheating air, and compared with that of room air. The results showed that when the air is preheated, the combustion flame location locates to upstream, the maximum combustion temperature is higher than that of room air, and flame propagation velocity decreases.The CO oxidation reaction rate increases gradually with the radius distance increaing, but reaction region decreases. CO oxidation region guradually decreases and locates to the upstream with air preheating temperature increasing. Peaks of CO oxidation rate gradually change from two to one.

Kinetics and Active Surfaces for CO Oxidation on Pt-Group Metals Under Oxygen Rich Conditions

This mini review summaries recent works on identifying the active surfaces for CO oxidation on Pd, Pt, and Rh under oxygen rich conditions. A significantly high reaction rate for CO oxidation under oxygen rich conditions has been observed. Results using in situ characterization methods of ambient scanning tunneling microscope, surface X-ray diffraction, ambient pressure X-ray photoemission spectroscopy, X-ray absorption spectroscopy, and infrared reflection adsorption spectroscopy (IRAS), were included. Most X-ray related methods reveal that the achievements of the high reaction rates for CO oxidation on Pd, Pt, and Rh under oxygen rich conditions are accompanied with the appearance of oxides on the surface, leading to that the oxide phase is considered to be the active surface. In contrast, recent in situ IRAS results conclude that a chemisorbed oxygen covered metallic surface is the active surface. Kinetic data support that the reaction on the metallic surfaces can reach the high rate, e.g. a mass-transfer limit turnover frequency, without the necessity of the presence of oxide. Therefore, we point out that the appearance of oxides on Pt-group metals during CO oxidation is possibly due to the transfer-limit of CO gas, resulting in exposing the catalyst surface to an ambient atmosphere much richer in oxygen and thus building-up the oxide. Moreover, photons in X-ray related experiments may aid to overcome the formation barrier of oxide on a chemisorbed oxygen covered metallic surface. The formation of oxide is also affected by the mass-transfer properties of the in situ reaction cells. If the amount of incoming CO molecules under the mass-transfer limit of CO is high enough, the build-up of oxide may be precluded being consumed by reacting with CO.

Evidence of Nonelectrochemical Shift Reaction on a CO-Tolerant High-Entropy State Pt–Ru Anode Catalyst for Reliable and Efficient Residential Fuel Cell Systems

A randomly mixed monodispersed nanosized Pt-Ru catalyst, an ultimate catalyst for CO oxidation reaction, was prepared by the rapid quenching method. The mechanism of CO oxidation reaction on the Pt-Ru anode catalyst was elucidated by investigating the relation between the rate of CO oxidation reaction and the current density. The rate of CO oxidation reaction increased with an increase in unoccupied sites kinetically formed by hydrogen oxidation reaction, and the rate was independent of anode potential. Results of extended X-ray absorption fine structure spectroscopy showed the combination of N(Pt-Ru)/(N(Pt-Ru) + N(Pt-Pt)) ≑ M(Ru)/(M(Pt) + M(Ru)) and N(Ru-Pt)/(N(Ru-Pt) + N(Ru-Ru)) ≑ M(Pt)/(M(Ru) + M(Pt)), where N(Pt-Ru)(N(Ru-Pt)), N(Pt-Pt)(N(Ru-Ru)), M(Pt), and M(Ru) are the coordination numbers from Pt(Ru) to Ru(Pt) and Pt (Ru) to Pt (Ru) and the molar ratios of Pt and Ru, respectively. This indicates that Pt and Ru were mixed with a completely random distribution. A high-entropy state of dispersion of Pt and Ru could be maintained by rapid quenching from a high temperature. It is concluded that a nonelectrochemical shift reaction on a randomly mixed Pt-Ru catalyst is important to enhance the efficiency of residential fuel cell systems under operation conditions.

Inhibition at Perimeter Sites of Au/TiO2 Oxidation Catalyst by Reactant Oxygen

TiO(2)-supported gold nanoparticles exhibit surprising catalytic activity for oxidation reactions compared to noble bulk gold which is inactive. The catalytic activity is localized at the perimeter of the Au nanoparticles where Au atoms are atomically adjacent to the TiO(2) support. At these dual-catalytic sites an oxygen molecule is efficiently activated through chemical bonding to both Au and Ti(4+) sites. A significant inhibition by a factor of 22 in the CO oxidation reaction rate is observed at 120 K when the Au is preoxidized, caused by the oxygen-induced positive charge produced on the perimeter Au atoms. Theoretical calculations indicate that induced positive charge occurs in the Au atoms which are adjacent to chemisorbed oxygen atoms, almost doubling the activation energy for CO oxidation at the dual-catalytic sites in agreement with experiments. This is an example of self-inhibition in catalysis by a reactant species.

Design of a Highly Active Ir/Fe(OH)x Catalyst: Versatile Application of Pt‐Group Metals for the Preferential Oxidation of Carbon Monoxide

The proton-exchange membrane fuel cell (PEMFC) has been regarded as one of the most promising candidates for the efficient use of hydrogen energy. However, small amounts of CO (0.3–1%) in the H2 stream from reforming processes must be selectively removed because CO is highly poisonous to the Pt anode of a PEMFC. The preferential oxidation of CO in a H2-rich gas (PROX) is presently the most effective approach to address this problem. Oxide-supported Au catalysts are highly active for the PROX reaction even at room temperature, but the lower stability and sensitivity to CO2 constrain their practical applications. Supported Pt catalysts, on the other hand, are less active and only a few have shown reasonable activity for conversion of CO at temperatures lower than 60 8C. Therefore, it is highly desirable to develop improved catalysts with better catalytic performance for the PROX reaction at lower temperatures. Ir has a higher melting point and surface energy than other metals with 5f orbitals, such as Pt and Au, and Ir can be well-dispersed on and strongly interact with the support. However, compared to Ptand Au-based catalysts, Ir-based catalysts have limited applications in heterogeneous catalysis and are rarely investigated for the PROX reaction, most probably because of its inferior activity. Although much effort has been made to improve the activity of Ir-based catalysts and remarkable progress has been achieved, their activities for the PROX reaction are still low at low temperatures. In fact, there is no report so far claiming that Ir-based catalysts can show high activity at temperatures below 80 8C; thus it remains a formidable challenge to utilize Ir-based catalysts for the PROX reaction at ambient temperatures. One basic task of modern catalysis is to rationally design catalysts based on the fundamental understanding of their reaction mechanisms. Especially, the contribution of support materials to the performance of the final catalysts should be taken into account. For the PROX reaction, the strong binding of CO to Ir poisons the surface so that O2 cannot competitively adsorb on the Ir surface and be activated at low temperatures, thereby prohibiting the conversion of CO to CO2. Therefore, weakening the adsorption strength of CO and/or promoting the activation of O2 at lower temperatures have become the crucial steps. Ferric oxide has proven effective for O2 activation and has been used extensively as an additive to Pt-based catalysts. Recently, we have designed a bimetallic catalyst by adding FeOx to a supported Ir catalyst, and the activity for the PROX reaction was improved. Further study of the catalytic reactions showed that the reaction rate of CO oxidation correlated well with the presence and amount of Fe, suggesting that Fe sites were indeed the active sites for O2 activation. [13] The coordinatively unsaturated Fe center was also recently identified as the site to activate O2, which helped the design of a highly active FeOx/Pt/SiO2 catalyst to totally convert CO at room temperature. All of these studies suggest that the presence of low-valent Fe (Fe) played a decisive role in improving the PROX activity, thus providing a clue for obtaining a highly effective Ir-based catalyst by incorporating materials containing, or easily forming, Fe species. Ferric hydroxide (Fe(OH)x) is a novel support material which has recently been adopted to stabilize various types of metal species for CO oxidation. It possesses a large surface area and a large amount of OH groups; these unique properties make Fe(OH)x a good candidate for generating highly dispersed metal clusters or even single-atom catalysts. Furthermore, the longer Fe O bonds in Fe(OH)x (compared to those in Fe2O3) make it easier to form Fe 2+

Kinetic analysis of reduction process of supported Rh/Al2O3 catalysts by time resolved in-situ UV–vis spectroscopy

Kinetic analysis of reduction of Rh/Al2O3 with low Rh-loadings (0.01–0.1wt%) has been carried out by time resolved in-situ UV–vis spectroscopy. Based on XAFS and UV–vis spectra, it was indicated that Rh was present as surface isolated Rh3+ on catalyst calcined at 773K in air. The rate of reduction of Rh3+ to metallic Rh0 was determined by in-situ UV–vis measurements under a flow of 1%H2/He at 523–823K. The reduction rate increased with Rh-loading but decreased as the calcination temperature elevated. The decrease of metallic Rh0 formation with the calcination temperature was in harmony with the reaction rate of CO oxidation. The apparent activation energy (Ea) for the reduction of Rh3+ to Rh0 was found to be constant around 60–62kJmol−1 and independent of the calcination temperature and the Rh-loading, indicating the equivalent intrinsic activity of supported Rh species on Rh/Al2O3 with the low Rh-loadings.