The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported ...Na2WO4/SiO2 catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOx sites are responsible for selectively activating CH4 to C2Hx and over‐oxidizing CHy to CO and (ii) molten Na2WO4 phase is mainly responsible for over‐oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2 catalysts.
In the current study, with the aid of state‐of‐the‐art spectroscopic techniques, transient kinetic analysis, and implementation of robust experimental methodologies, we resolve the nature of catalytic active sites and reaction mechanism for oxidative coupling of methane over supported Na2WO4/SiO2 catalysts.
The involvement of lattice oxygen species is important toward oxidative coupling of the methane reaction (OCM) over supported Mn-Na2WO4/SiO2 catalysts, but there is no consensus regarding the types, ...role, and origin of lattice oxygen species present in supported Mn-Na2WO4/SiO2 catalysts, which hinders the understanding of the OCM reaction network. In the present study, by utilizing the temporal analysis of products technique, we show that supported Na2WO4/SiO2 catalysts possess two different types of oxygen species, dissolved O2 and atomic O, at an OCM-relevant temperature. The addition of Mn-oxide to this catalyst increases the total amount and release rate of dissolved O2 species and improves C2 selectivity of both dissolved O2 and atomic lattice O species.
Plane-wave density functional theory has been used to study oxygen adsorption on graphene, graphite, and (12,0) zigzag single-walled carbon nanotubes with and without Stone–Wales (SW) and ...single-vacancy (SV) defects to understand the role of defects on carbonaceous material reactivity. Atomic oxygen adsorption leads to the formation of an epoxide on defect-free graphene and graphite and an ether on the exterior wall of carbon nanotubes and SW-defected materials. O2 chemisorption is endothermic on defect-free graphene and graphite and slightly exothermic on defect-free nanotubes. O2 chemisorption energies are predicted to be −1.1 to −1.4 eV on an SW defect and −6.0 to −8.0 eV on an SV defect. An SW defect lowers the energy barriers by 0.90 and 0.50 eV for O2 chemisorption on graphene and nanotubes, respectively. The formation of a C–O–O–C group is important for O2 dissociation on defect-free and SW-defected materials. The energy barrier is less than 0.30 eV on an SV defect. The more reactive SW defect toward O adsorption on graphene is mostly due to the strained defective carbon atoms being able to donate more electrons to an O to form an ether. The larger 2s character in the hybrid orbitals in an ether than in an epoxide makes the ether C–O bond stronger. Stronger C–O binding on an SW-defective carbon nanotube than on a defect-free nanotube is in part due to more flexibility of the defect to release the epoxide ring strain to form an ether.
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.
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•A detailed mechanism for HOPG oxidation was described using time resolved transient experiments.•Pulsed oxygen titration was conducted at low coverage with nanomole precision.•O2 adsorption, spillover and surface diffusion steps were distinguished using isotopes.•Surface diffusion during annealing played a key role in increasing surface reactivity.•A unique mechanism for trapping mobile oxygen at reactive edge sites was derived.
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•Mass transport is investigated in highly dynamic pulsed micro reactors such as TAP systems.•Model covers Knudsen to well beyond Knudsen conditions (advection); captures slip flow ...conditions.•High-fidelity predictions over space and time (x,t) reveal transport behavior in critical regions.•Evaluation of packing material attributes informs how they impact exit flow profiles of a reactor.•Very large gas pulses produce a turbulent region near the reactor inlet described by eddy diffusion.
The objective of this work is to develop a high-fidelity transport model for pulsed reactor scenarios involving Knudsen and beyond-Knudsen (viscid flow) conditions. This is done to eventually support accurate analyses of flow regimes and micro-kinetic processes under dynamic conditions. The resulting model renders mass transport under mixed modes of diffusion, advection and eddy (turbulent) diffusion, allowing for variance of input conditions (pulse magnitudes, temperature, etc.), gaseous mixtures and various reactor designs. The model assumes a five-zone reactor, covering inlet, pre-catalyst, catalyst, post-catalyst and outlet zones. A chief attribute of the modeling approach herein is the detailed analysis it provides for quantification of multiple transport metrics over reactor space and time, a transport “map”. Key among these are the Knudsen number and the relative contribution of advection versus diffusion as the pulse intensity is increased. The ability to map the reactor transport is useful for determining an optimal loading configuration to observe desired kinetic quantities associated with different chemical reaction mechanisms.
We report a combined experimental/theoretical approach to studying heterogeneous gas/solid catalytic processes using low-pressure pulse response experiments achieving a controlled approach to ...equilibrium that combined with quantum mechanics (QM)-based computational analysis provides information needed to reconstruct the role of the different surface reaction steps. We demonstrate this approach using model catalysts for ammonia synthesis/decomposition. Polycrystalline iron and cobalt are studied via low-pressure TAP (temporal analysis of products) pulse response, with the results interpreted through reaction free energies calculated using QM on Fe-BCC(110), Fe-BCC(111), and Co-FCC(111) facets. In TAP experiments, simultaneous pulsing of ammonia and deuterium creates a condition where the participation of reactants and products can be distinguished in both forward and reverse reaction steps. This establishes a balance between competitive reactions for D* surface species that is used to observe the influence of steps leading to nitrogen formation as the nitrogen product remains far from equilibrium. The approach to equilibrium is further controlled by introducing delay timing between NH3 and D2 which allows time for surface reactions to evolve before being driven in the reverse direction from the gas phase. The resulting isotopic product distributions for NH2D, NHD2, and HD at different temperatures and delay times and NH3/D2 pulsing order reveal the role of the N2 formation barrier in controlling the surface concentration of NH x * species, as well as providing information on the surface lifetimes of key reaction intermediates. Conclusions derived for monometallic materials are used to interpret experimental results on a more complex and active CoFe bimetallic catalyst.
We recently showed that phase-pure molybdenum carbide nanotubes can be durable supports for platinum (Pt) nanoparticles in hydrogen evolution reaction (HER). In this paper we further characterize ...surface properties of the same Pt/β-Mo2C catalyst platform using carbon monoxide (CO)-Pt and CO-Mo2C bond strength of different Pt particle sizes in the <3 nm range. Results from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temporal analysis of products (TAP) revealed the existence of different active sites as Pt particle size increases. Correlation between the resultant catalyst activity and deposited Pt particle size was further investigated using water–gas-shift (WGS) as a probe reaction, suggesting that precise control of particle diameter and thickness is needed for optimized catalytic activity.
Light alkanes are abundant in shale gas resources. The bulk mixed metal oxide MoVTe(Sb)NbOx catalysts play a very important role in dehydrogenation and selective oxidation reactions of these short ...hydrocarbons to produce high-value chemicals. This catalyst system mainly consists of M1 and less-active M2 crystalline phases. Due to their ability to directly monitor the catalysts under the relevant industrial conditions, in situ/operando techniques can provide information about the nature of active sites, surface intermediates, and kinetics/mechanisms, and may help with the synthesis of new and better catalysts. Sophisticated catalyst design and understanding is necessary to achieve the desired performance (activity, selectivity, lifetime, etc.) at reasonable reaction conditions (temperature, pressure, etc.). This article critically reviews the progress made in research of these MoVTe(Sb)NbOx catalysts in oxidation reactions mainly through in situ/operando techniques and suggests the future direction needed to realize the industrialization of these catalysts.
The structure and promotional effect of Mn in supported Mn-Na2WO4/SiO2 catalysts for the oxidative coupling of methane (OCM) reaction has been debated for a longtime in the literature. In the current ...investigation, with the aid of multiple in-situ characterization studies, we show that the freshly calcined supported 1.2Mn-5Na2WO4/SiO2 catalyst possesses crystalline Na2WO4, Mn2O3 and SiO2 (cristobalite phase) along with surface MnOx and Na-WOx sites at low temperature and oxidizing environments. Under the OCM reaction environment (T > 800 °C), the crystalline Na2WO4 phase melts and Mn2O3 phase reduces. In contrast, the surface MnOx and Na-WOx sites exhibit excellent thermal and chemical stability. Exposure of the 1.2Mn-5Na2WO4/SiO2 catalyst to the OCM reaction environment redisperses the molten Na2WO4 phase on the SiO2 support to form new surface WOx sites. Interestingly, the stable MnOx species interacts with both molten Na2WO4 phase and surface Na-WOx sites during OCM reaction. Controlled transient kinetic experiments in TAP and detailed steady state OCM fixed-bed reaction studies reveal the role and promotional effect of Mn in the 1.2Mn-5Na2WO4/SiO2 catalyst. The W-oxides (both molten Na2WO4 and surface Na-WOx sites) are the active sites for the catalytic OCM reaction and the MnOx species only function as promoters. The promotion of MnOx strongly depends on the gas phase O2 partial pressure and the MnOx species act as mediators for oxygen exchange between the gas phase molecular O2 and catalyst lattice oxygen. The temperature dependent MnOx promotion reveals that the MnOx species selectively promote the molten Na2WO4 phase at lower reaction temperature and the surface Na-WOx sites at higher temperature.
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•Mn-oxide is present as surface MnOx sites during the OCM reaction.•MnOx sites interact with W-oxide centers (both molten Na2WO4 phase and surface Na-WOx sites) during the OCM reaction.•W-oxide centers are the active site for the OCM reaction.•MnOx species act only as promoters during the OCM reaction.•MnOx promotion is strongly influenced by gas-phase oxygen partial pressure.
The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported ...Na2WO4/SiO2 catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOx sites are responsible for selectively activating CH4 to C2Hx and over‐oxidizing CHy to CO and (ii) molten Na2WO4 phase is mainly responsible for over‐oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2 catalysts.
In the current study, with the aid of state‐of‐the‐art spectroscopic techniques, transient kinetic analysis, and implementation of robust experimental methodologies, we resolve the nature of catalytic active sites and reaction mechanism for oxidative coupling of methane over supported Na2WO4/SiO2 catalysts.
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