Transient Isotopic Experiments Research Articles

Improving Copper Active Site Speciation on Cu-Ce/SSZ-13 for Ammonia Oxidation via Si/Al Ratio Modulation.

Catalytic oxidation is a promising purification technique for ammonia (NH3) emission. However, high ignition temperatures and NOx peroxide generation limit its effectiveness due to a lack of active sites. Herein, the effects of Si/Al ratio (SAR) modulation on the speciation of copper active sites and the reaction mechanism at different acidic sites were investigated by loading CuO-CeO2 onto SSZ-13 with different SARs (Cu-Ce/SAR15, 20, and 30). Among them, Cu-Ce/SAR20 exhibits the lowest induction temperature (T20 = 180 °C) and the highest nitrogen selectivity (above 95%), attributing to a higher number of Cu2+ exchange sites. In situ IR spectroscopy and isotopic (18O2) transient response experiments indicate that more active Cu2+ in Cu-Ce/SAR20 provides sufficient Lewis acidic sites for NH3 adsorption and favors the stability of Si-OH-Al structures (Brønsted acid sites). NH3 adsorbed at Lewis acidic sites tends to form peroxide byproducts (NOx), while the NH4+ adsorbed at Brønsted acidic sites generates the key intermediate NH4NO2, which decomposes to N2 at high temperatures, thus enhancing nitrogen selectivity. The whole process mainly follows the Mars-van Krevelen (M-K) mechanism, with the Langmuir-Hinshelwood (L-H) mechanism playing a supporting role. Z2Cu2+ coordinates with adjacent Al atoms within the six-membered ring (6MR) and undergoes a slight deformation at high temperatures, facilitating the migration of the lattice oxygen. SAR plays a crucial role in local environmental speciation of reactive Cu2+, where the sufficient isolated Al provided in SAR20 pulls Cu2+ into the eight-membered ring (8MR), allowing it to come into contact with NH3 more readily.

Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N2 Activation.

In this work, a relatively new class of materials, rare earth (RE) based high entropy oxides (HEO) are discussed in terms of the evolution of the oxygen vacant sites (Ov) content in their structure as the composition changes from binary to HEO using both experimental and computational tools; the composition of HEO under focus is the CeLaPrSmGdO due to the importance of ceria-related (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO2, the latter as representative nonreducible oxide, were used and compared as supports for Ru (6 wt % loading). The role of the Ov in the HEO is highlighted for the ammonia production with particular emphasis on the N2 dissociation step (N2(ads) → Nads) over a HEO; the latter step is considered the rate controlling one in the ammonia production. Density functional theory (DFT) calculations and 18O2 transient isotopic experiments were used to probe the energy of formation, the population, and the easiness of formation for the Ov at 650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPS probed the Ce-O chemical environment at different length scales. In particular, it was found that the particular HEO composition eases the Ov formation in bulk, in medium (Raman), and in short (localized) order (EPR); more Ov population was found on the surface of the HEO compared to the binary reference oxide (CePrO). Additionally, HEO gives rise to smaller and less sharp faceted Ru particles, yet in stronger interaction with the HEO support and abundance of Ru-O-Ce entities (Raman and XPS). Ammonia production reaction at 400 °C and in the 10-50 bar range was performed over Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO2 catalysts; the Ru/HEO had superior performance at 10 bar compared to the rest of catalysts. The best performing Ru/HEO catalyst was activated under different temperatures (650 vs 800 °C) so to adjust the Ov population with the lower temperature maintaining better performance for the catalyst. DFT calculations showed that the HEO active site for N adsorption involves the Ov site adjacent to the adsorption event.

Distinct structure-activity relationship and reaction mechanism over BaCoO3/CeO2 catalysts for NO direct decomposition

The dependency on morphology is crucial for achieving highly efficient direct decomposition of NO. Herein, a BaCoO3/CeO2 catalyst is synthesized using CeO2 small particles (p), spheres (s) and rods (r) as supports. The NO conversion to N2 (NTN2) at 800 °C follows the order BaCoO3/CeO2-r (78.8 %) > BaCoO3/CeO2-s (75.9 %) > BaCoO3/CeO2-p (56.9 %) > BaCoO3 (8.6 %) at a space velocity 1 g s/cm3. BaCoO3/CeO2-r exhibts high tolerance to O2 and stability with conversion decreasing from 78.8 % to 74.6 %, 60.0 % and 50.0 % at 800 °C with 1, 5 and 10 vol% O2, respectively. The high redox activity, higher active oxygen mobility and NO adsorption capability ensures its superior performance, while the high surface area (31.29 m2/g) and uniform distribution of active sites on the surface further promote the activity. The mechanism of NO direct decomposition is elucidated by in situ Diffuse reflectance infrared Fourier transform spectroscopy, 18O2 isotopic transient exchange experiments and density functional theory (DFT) calculation.

Structure Sensitivity of CO2 Hydrogenation on Ni Revisited.

Despite the large number of studies on the catalytic hydrogenation of CO2 to CO and hydrocarbons by metal nanoparticles, the nature of the active sites and the reaction mechanism have remained unresolved. This hampers the development of effective catalysts relevant to energy storage. By investigating the structure sensitivity of CO2 hydrogenation on a set of silica-supported Ni nanoparticle catalysts (2-12 nm), we found that the active sites responsible for the conversion of CO2 to CO are different from those for the subsequent hydrogenation of CO to CH4. While the former reaction step is weakly dependent on the nanoparticle size, the latter is strongly structure sensitive with particles below 5 nm losing their methanation activity. Operando X-ray diffraction and X-ray absorption spectroscopy results showed that significant oxidation or restructuring, which could be responsible for the observed differences in CO2 hydrogenation rates, was absent. Instead, the decreased methanation activity and the related higher CO selectivity on small nanoparticles was linked to a lower availability of step edges that are active for CO dissociation. Operando infrared spectroscopy coupled with (isotopic) transient experiments revealed the dynamics of surface species on the Ni surface during CO2 hydrogenation and demonstrated that direct dissociation of CO2 to CO is followed by the conversion of strongly bonded carbonyls to CH4. These findings provide essential insights into the much debated structure sensitivity of CO2 hydrogenation reactions and are key for the knowledge-driven design of highly active and selective catalysts.

Selective Photocatalytic Dehydrogenation of Formic Acid by an In Situ-Restructured Copper-Postmetalated Metal-Organic Framework under Visible Light.

Formic acid is considered as one of the most promising liquid organic hydrogen carriers. Its catalytic dehydrogenation process generally suffers from low activity, low reaction selectivity, low stability of the catalysts, and/or the use of noble-metal-based catalysts. Herein we report a highly selective, efficient, and noble-metal-free photocatalyst for the dehydrogenation of formic acid. This catalyst, UiO-66(COOH)2-Cu, is built by postmetalation of a carboxylic-functionalized Zr-MOF with copper. The visible-light-driven photocatalytic dehydrogenation process through the release of hydrogen and carbon dioxide has been monitored in real-time viaoperando Fourier transform infrared spectroscopy, which revealed almost 100% selectivity with high stability (over 3 days) and a conversion yield exceeding 60% (around 5 mmol·gcat–1·h–1) under ambient conditions. These performance indicators make UiO-66(COOH)2-Cu among the top photocatalysts for formic acid dehydrogenation. Interestingly, the as-prepared UiO-66(COOH)2-Cu hetero-nanostructure was found to be moderately active under solar irradiation during an induction phase, whereupon it undergoes an in-situ restructuring process through intraframework cross-linking with the formation of the anhydride analogue structure UiO-66(COO)2-Cu and nanoclustering of highly active and stable copper sites, as evidenced by the operando studies coupled with steady-state isotopic transient kinetic experiments, transmission electron microscopy and X-ray photoelectron spectroscopy analyses, and Density Functional Theory calculations. Beyond revealing outstanding catalytic performance for UiO-66(COO)2-Cu, this work delivers an in-depth understanding of the photocatalytic reaction mechanism, which involves evolutive behavior of the postmetalated copper as well as the MOF framework over the reaction. These key findings pave the way toward the engineering of new and efficient catalysts for photocatalytic dehydrogenation of formic acid.

Evolution behavior and active oxygen quantification of reaction mechanism on cube Cu2O for CO self-sustained catalytic combustion and chemical-looping combustion

Catalytic combustion (CC) and chemical looping combustion (CLC) are promising technologies for energy saving and emission reduction of CO2 in treatment of steelmaking off-gas. This work firstly reports and compares the evolution behavior and quantitative reaction mechanisms of cube Cu2O model catalyst for CC and CLC reactions. The Cu2O-CC exhibited the higher activity and stability than Cu2O-CLC. The typical characterization results suggested that the only surface unstable Cu2O was oxidized to CuO, and the excellent synergistic effect of metal-oxide interface (100) between Cu+/Cu2+ and active lattice oxygen species for Cu2O-CC reaction. But, for CLC reaction, Cu2O structure was collapsed, which caused the agglomeration of CuOx species and gradual decrease of reaction stability. Three different active oxygen species (surface cycle lattice oxygen, bulk lattice oxygen, and adsorbed oxygen) and the detailed reaction pathways were proposed by the in situ IR spectroscopy, isotopic (18O2) transient exchange experiments and DFT simulation. The intrinsic activity of surface cycle lattice oxygen was higher in terms of TOF (13.5 × 10−3 s−1) and facile formation of C16O18O on the cubic interface of Cu2O-CC through adsorbed CO during CC process. The contribution degrees of Mars-van-Krevelen (M-K) and Langmuir–Hinshelwood (L-H) mechanisms for CC and CLC reactions were 76.6% and 23.4% for CC, and 89.7% and 10.3% for CLC on Cu2O catalyst, respectively.

Role of surface diffusion in formation of unique reactivity for graphite oxidation: Time-resolved measurements in a pulsed diffusion reactor

Quantification of oxidation kinetics is essential to develop graphitic materials for diverse applications: from refractories found in gas-cooled nuclear reactors to catalysts needed for chemical manufacturing. Using well-defined highly oriented pyrolytic graphite, low-pressure isotopic transient experiments combined with controlled annealing periods are used to resolve the role of surface diffusion and quantify oxidation kinetics with nanomole-precision. We observe an unexpected increase in reactivity following annealing which is explained by the role of surface diffusion increasing the probability for trapping mobile oxygen at more reactive edge sites. Here, the locus of adsorption and spillover to the basal plane is distinct from the trapping location creating a more active oxygen species. Isotopic products reflect the population dynamics of oxygen added at the edge and surface diffusion that relocates basal plane oxygen to more reactive edge sites. Since this process proceeds in parallel with direct oxidation reactions, it is not likely to be observed using steady-state or conventional ‘bulk’ characterization techniques. Our unique time-resolved non-equilibrium measurement in a well-defined transport regime, enables observation of three distinct behaviors: short-term deactivation due to the balance of rates in oxygen supply/product formation, reactivity increases due to surface diffusion and longer-term reactivity increase with oxygen accumulation.

Deactivation of Co/γ-Al2O3 in CO methanation studied by transient isotopic experiments: The effect of Co particle size

SSITKA coupled with transient isothermal (TIH) and temperature-programmed hydrogenation (TPH) were used to investigate the influence of Co particle size (dCo = 11–21 nm) and time-on-stream, TOS (up to 25 h) on important kinetic parameters of CO methanation at 230 °C on Co/γ-Al2O3. TOFCO and TOFCH4 were independent of dCo and decreased with TOS. The θCO and θCHx (active CO-s and CHx-s) slightly increased with dCo as opposed to θCxHy (inactive species), which remained practically constant. On the contrary, θCHx and θCxHy largely decreased and increased, respectively, while θCO stayed practically constant with TOS. An inverse D-KIE independent of dCo was estimated by a novel 13CO/D2-SSITKA experiment, reported for the first time to the best of our knowledge. The reason of catalyst deactivation was the decrease of kCHx (hydrogenation of -CHx) likely because of the influence of -CxHy on the bonding of active species and on θH (a decrease was determined).

Insights into the Reaction Mechanism of Ethanol Conversion into Hydrocarbons on H-ZSM-5.

Ethanol dehydration to ethene is mechanistically decoupled from the production of higher hydrocarbons due to complete surface coverage by adsorbed ethanol and diethyl ether (DEE). The production of C3+ hydrocarbons was found to be unaffected by water present in the reaction mixture. Three routes for the production of C3+ hydrocarbons are identified: the dimerization of ethene to butene and two routes involving two different types of surface species categorized as aliphatic and aromatic. Evidence for the different types of species involved in the production of higher hydrocarbons is obtained via isotopic labeling, continuous flow and transient experiments complemented by UV/Vis characterization of the catalyst and ab initio microkinetic modeling.

Insights into the Reaction Mechanism of Ethanol Conversion into Hydrocarbons on H‐ZSM‐5

AbstractEthanol dehydration to ethene is mechanistically decoupled from the production of higher hydrocarbons due to complete surface coverage by adsorbed ethanol and diethyl ether (DEE). The production of C3+ hydrocarbons was found to be unaffected by water present in the reaction mixture. Three routes for the production of C3+ hydrocarbons are identified: the dimerization of ethene to butene and two routes involving two different types of surface species categorized as aliphatic and aromatic. Evidence for the different types of species involved in the production of higher hydrocarbons is obtained via isotopic labeling, continuous flow and transient experiments complemented by UV/Vis characterization of the catalyst and ab initio microkinetic modeling.

Isotopic transient studies of sodium promotion of Pt/Al2O3 for the water–gas shift reaction

The promotional effect of sodium on Pt/Al2O3 catalysts for the water–gas shift (WGS) reaction was investigated with operando FTIR during steady-state and isotopic transient experiments. The highest turnover frequency (TOF) promotion per surface Pt occurred on a 0.82wt.% Pt/Al2O3 catalyst with a 30:1molar ratio of Na:Pt. This catalyst exhibited a TOF of 0.35s−1 at 250°C and 7% CO, 11% H2O, 9% CO2, and 37% H2 compared to 0.43×10−2s−1 on Pt/Al2O3. Operando IR revealed that Na addition modifies CO adsorption on Pt to create more strongly bound, multiply-bonded CO species at 1768cm−1 and 1697cm−1 compared to predominantly linear CO on Na-free Pt/Al2O3 at 2060cm−1. Transient experiments with 12CO/13CO isotope switches showed that the number of carbon-containing active intermediates increased from less than 1% of the surface Pt for Na-free Pt/Al2O3 to nearly 100% of the surface Pt for 20Na:Pt/Al2O3, which indicates that only a small fraction of the Pt surface on the Na-free samples participates in the WGS reaction. From time-resolved IR spectra during transient WGS with 12CO/13CO and 12CO2/13CO2 isotope switches, we propose that surface formate species are spectators for all Na-promoted Pt/Al2O3 catalysts under these WGS conditions. The results suggest that Na promotes Pt/Al2O3 for WGS by modifying the local electronic properties of Pt and creating new H2O activation sites, which provide greater availability of surface OH/H species to react with nearly all CO on the metallic Pt surface through a non-formate pathway.

The effect of La3+-doping of CeO2 support on the water-gas shift reaction mechanism and kinetics over Pt/Ce1−xLaxO2−δ

Platinum nanoparticles (dPt=1.0–1.2nm) supported on single CeO2 and La2O3 metal oxides and Ce0.8La0.2O2−δ solid solution were prepared to investigate for the first time the effect of La3+-doping of ceria on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS at 250 and 300°C, (ii) the chemical nature of inactive species formed during WGS, and (iii) the prevailing mechanistic path among “redox” and “associative” both proposed in the literature. For this, steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry were performed to follow the H-path (use of D2O) and C-path (use of 13CO) of the WGS. In addition, other transient isotopic experiments using operando methodology (use of DRFTS and mass spectrometry) were designed to follow with time on stream the reactivity toward water of the various adsorbed species formed under WGS. It is proposed that on Pt/Ce1−xLaxO2−δ (x=0.0, 0.2 and 1.0) the WGS reaction follows both the “redox” and “associative” mechanisms but the extent of participation of each mechanism to the overall WGS reaction rate depends on the support chemical composition. The WGS kinetic rate (μmol COg−1s−1) increased by a factor of 2.0 and 2.8 at 300°C on 0.5wt% Pt supported on Ce0.8La0.2O2−δ compared to CeO2 and La2O3, respectively. This was explained by (i) the larger concentration of active surface intermediates formed around each Pt nanoparticle (larger extent of reactive zone) and (ii) the higher reactivity of sites (k, s−1) responsible for CO2 and H2 formation on Pt/Ce0.8La0.2O2−δ compared to Pt/CeO2 and Pt/La2O3. Active OH groups is suggested to be formed on defect sites (Ce3+□s) of Ce0.8La0.2O2−δ as a consequence of the introduction of La3+ into the ceria lattice, the latter enhancing the concentration of labile oxygen and its surface mobility, important characteristics of the “redox” mechanism.

Determination of the Au active site and surface active species via operando transmission FTIR and isotopic transient experiments on 2.3 wt.% Au/TiO2 for the WGS reaction

The kinetics and the active site for the water gas shift (WGS) reaction were investigated by operando FTIR during steady state and isotopic transient experiments on a 2.3wt.% Au/TiO2 catalyst. The WGS rate at 120°C and activation energy measured in the operando IR reactor for the WGS reaction (7% CO, 11% H2O, 9% CO2, and 37% H2) on the Au/TiO2 catalyst were 1.1±0.1×10−2 (molH2) (mol−1Au) s−1 and 56±3kJmol−1, respectively. We have shown that the kinetics measured over a pressed wafer in a transmission FTIR were similar to those measured in a regular plug flow reactor (PFR). Operando IR data were used to follow the catalyst deactivation as a function of time, and the amount of CO adsorbed on metallic Au species (Au0, 2100cm−1) correlated with the WGS rates, while CO adsorbed on partially negatively charged Au species (Auδ−, 2040cm−1) did not. The rate loss did not correlate with the amounts of formate- and carbonate-like species measured by IR but was consistent with a loss of active sites by sintering.Measurements by isotopic transient experiments (13CO–12CO) on the Au/TiO2 catalyst with an average Au particle size of 3nm showed that less than 3% of the total surface Au atoms on the Au nanoparticles were responsible for the majority of the catalytic activity. Time-resolved IR spectra collected during the isotopic transient experiments showed that CO adsorbed on Au0 species were involved in the reaction pathway, while the surface formate and carbonate species were spectators under these WGS conditions. Based on these results, we conclude that metallic corner Au atoms are the dominant active sites for WGS on Au/TiO2 catalysts and that CO adsorbed on Au0 is an active intermediate. In addition, we present a detailed description of the design, construction, and operation of a transmission FTIR cell capable of performing operando kinetic measurements and isotope switching experiments with a signal decay time of about 95% in 1.5s.

Effects of Reaction Temperature and Support Composition on the Mechanism of Water–Gas Shift Reaction over Supported-Pt Catalysts

The present work reports on a detailed study of the effect of support chemical composition and reaction temperature on the mechanism of the water–gas shift (WGS) reaction over supported-Pt catalysts. The effect of the same parameters on the chemical composition and surface concentration of active reaction intermediates was also determined for the first time, information that allowed elucidating also the site location of these intermediates, e.g., support versus metal or support–metal interface. The above-mentioned mechanistic information was rigorously provided by the application of steady-state isotopic transient kinetic analysis (SSTIKA) experiments coupled with mass spectrometry (MS) and DRIFTS techniques, and by other transient isotopic experiments designed. It was found that on Pt/CeO2 a switch of the WGS reaction mechanism from “redox” to a combination of redox and “associative formate with −OH group regeneration” is obtained after increasing the reaction temperature from 473 to 573 K. On the other ...

The role of support oxygen in the epoxidation of propene over gold–titania catalysts investigated by isotopic transient kinetics

Transient kinetic experiments were performed on gold–titania catalysts for the epoxidation of propene using hydrogen and oxygen to investigate the reaction mechanism. A ‘classical’ 1 wt% gold on titania catalyst was studied, as well as a 1 wt% gold on Ti-SBA-15 catalyst. Steady-State Isotopic Transient Kinetic experiments using oxygen-18 were performed, which provide information on the types and quantities of species present on the catalyst during reaction, as well as on possible reaction pathways. The isotopic transients of the reaction products take place on a time scale of minutes, while the isotopic switch of the unreacted feed gases proceeds in a matter of seconds. This indicates that the oxygen pool on the catalysts, either in the form of adsorbed products or in the form of oxygen available for the reaction, is large in comparison to the product formation rates. The results indicate that support oxygen is playing a role in the reaction mechanism, but this could not be determined conclusively. For the water produced as a side product, it could be determined that support oxygen is ending up in the water; however, this can be explained by an exchange after its formation. The size of the oxygen pool in the neighborhood of gold, calculated based on the size of the TEM visible gold particles, is smaller than the amount of oxygen that ends up in the carbon containing oxidation products, which indicates that most likely part of the gold is present in sub-nanometer particles, not visible by TEM.

In-Situ UV/vis and Transient Isotopic Analysis of the Role of Oxidizing Agent in the Oxidative Dehydrogenation of Propane over Silica-Supported Vanadia Catalysts

In-situ UV/vis spectroscopy in combination with transient isotopic analysis was applied for elucidating the governing factors, which determine the superior performance of N2O as compared to O2 in the oxidative dehydrogenation of propane (ODP) over VOx/MCM-41, VOx/MCM-48, and VOx/SiO2 catalytic materials. Characterization studies applying in-situ steady-state UV/vis spectroscopy evidenced higher reduction degree of surface VOx species at 773 K in a C3H8−N2O (C3H8/N2O = 1/1) feed than in a C3H8−O2 (C3H8/O2 = 2/1) flow at controlled conversions of propane (<5%) and oxidizing agent (<10%). The in-situ UV/vis method in a transient mode enabled to distinguish the ability of O2 and N2O for reoxidation of reduced VOx species; N2O reoxidizes reduced VOx species slower than O2. This property of N2O is very essential for suppressing nonselective consecutive propene oxidation to COx (mainly to CO). Additionally, the results of transient isotopic experiments in the temporal analysis of products reactor revealed that adsorbed (nonlattice) oxygen species are formed upon O2 activation over reduced VOx species and participate in CO2 and CO formation. It is suggested that these nonselective species do not originate from N2O.

Spillover of labile OH, H, and O species in the H 2 production by steam reforming of phenol over supported-Rh catalysts

The aim of this work was to probe the back-spillover of labile O, OH and H species during steam reforming of phenol towards H 2 production in the 773–1028 K range over Rh supported on MgO and Mg-Ce-Zr-O mixed-metal oxides. This was made possible through steady state isotopic transient kinetic analysis (SSITKA) and other transient isotopic experiments (use of D 2O and 18O 2) combined with temperature-programmed desorption ones. The size of the active pool of H-containing intermediate species (H and/or OH) present in the H-path of reaction as a function of reaction T was measured over the 0.1 wt% Rh/50Mg-25Ce-25Zr-O catalyst by SSITKA experiments. It was found that the size of this pool decreases by 18% in the 773–928 K range ( θ = 68.9–56.3), whereas a significant decrease (by a factor of 10) was observed between 928 and 1028 K. These results demonstrate that most of the active H and OH species reside on the support and metal-support interface. Two kinds of H-containing intermediate species are likely to participate in the H-path of reaction. An inverse D kinetic isotopic effect was measured for the H 2 formation in the phenol steam reforming reaction in the 773–1028 K range.

The role of oxygen and hydroxyl support species on the mechanism of H 2 production in the steam reforming of phenol over metal oxide-supported-Rh and -Fe catalysts

The aim of this work was to probe for the first time the back-spillover of labile O and OH species during steam reforming of phenol towards H 2 production over Rh and Fe supported on MgO, Mg-Ce-O and Mg-Ce-Zr-O metal oxides. This was made possible through SSITKA and other transient isotopic experiments (use of D 2O and 18O 2). The size of the active pool of H-containing species (H and/or OH) present under reaction conditions as a function of reaction T and catalyst composition, and deuterium kinetic isotope effects were measured.

Understanding the Synergistic Catalytic Effect between La2O3 and CaO for the CH4 Lean De-NOx Reaction: Kinetic and Mechanistic Studies

Doping of La(2)O(3) crystallites with Ca(2+) ions significantly enhances the intrinsic rate of NO reduction by CH(4) in the presence of 5% O(2) at 550 degrees C compared to pure La(2)O(3) and CaO solids, while the opposite is true after doping of CaO with La(3+) ions. It was found that the 5 wt % La(2)O(3)-95 wt % CaO system has one of the highest intrinsic site reactivities (TOF = 8.5 x 10(-3) s(-1)) reported at 550 degrees C for the NO/CH(4)/O(2) reaction among metal oxide surfaces. The doping process occurred after first dispersing La(2)O(3) and CaO crystallites in deionized water heated to 60 degrees C for 90 min, while the dried material was then ground and heated slowly in air to 800 degrees C and kept at this temperature for 5 h. The doping process had the effect of creating surface oxygen vacant sites (F-type defects) in the oxide lattices the concentration of which is a function of the wt % La(2)O(3) used in the mixed oxide system as revealed by photoluminescence and O(2) chemisorption studies. According to DRIFTS (15)NO transient isotopic experiments (SSITKA), oxygen vacant sites in Ca(2+)-doped La(2)O(3) promote the formation of a more active chemisorbed NO(x) species (NO(2)(-)) that contributes to the enhancement of reaction rate as compared to pure lanthana, calcium oxide, and La(3+)-doped CaO. These results were supported by the kinetic orders of the reaction with respect to NO and O(2) obtained as a function of wt % La(2)O(3) content in the mixed oxide system. Carbon dioxide (a reaction product) competes for the same oxygen vacant sites to form stable adsorbed carbonate-like species, thus lowering the reduction rate of NO. The dependence of the reaction TOF on the wt % La(2)O(3) loading at 550 degrees C was found to follow the trend of the dependence of photoluminescence intensity on the wt % La(2)O(3) content in the La(2)O(3)-CaO oxide system.

Transient isotopic studies on 1-butene oxidation over a VO x-TiO 2 catalyst in presence of water vapor

The catalytic oxidation of 1-butene was studied in the temperature range of 390–553 K in presence and absence of O 2 and water vapor. An increase in acetic acid yield was observed with increasing temperature up to a maximum at 520 K. The addition of water promoted the formation of acetic acid and suppressed total oxidation reactions. Transient isotopic experiments with 18O 2/ 16O 2 and H 2 18O/H 2 16O were carried out to investigate the role of gas oxygen and water in the formation of acetic acid. In absence of water the isotopic exchange was slow and complete at high temperatures. On the contrary, a fast but incomplete isotopic exchange in acetic acid was observed in presence of water. Isotopic transient experiments with 18O 2 and H 2 18O revealed that 18O from H 2 18O was more readily incorporated into acetic acid than 18O from 18O 2 pointing to a role of water as a reactant.