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.
Complex spectral envelopes of transition metal photo-excitations obtained using X-ray Photoelectron Spectroscopy (XPS) contain extensive information on the oxidation states and chemical bonding but ...pose multiple challenges for extracting reliable data due to the presence of multiple closely lying binding energy peaks. In this work, we outlined a procedure for graphite supported copper nanoparticles (Cu NP/graphite) XPS data interpretation that involves constructing spectral envelopes of the potential copper components (Cu2O, CuO and Cu(OH)2) extracted from the diverse set of Cu NP/graphite samples and using Linear Least Squares (LLS) fitting to reconstruct the exact surface composition of Cu NP/graphite samples. We utilized Informed Amorphous Sample Model (IASM) to calculate spectral envelopes using a physical process affecting the series of Cu NP/graphite samples, namely their synthesis procedure, to construct an informed line shape necessary to complete data reproduction by the model. The method described herein can be used to interpret crucial XPS data obtained in many science and engineering disciplines, including chemistry, fundamental and applied surface science, catalysis, semiconductors and many others. A brief discussion is also provided on the opportunities and pitfalls of deriving standard model line shapes from user sourced online databases.
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•We analyzed and modeled spectral envelopes of complex graphite supported Cu nanoparticles.•Copper nanoparticles of varying composition were synthesized.•Cu2O, CuO and Cu(OH)2 line shapes from experimental data were created.•Informed amorphous sample model(IASM) utilized•Complex Cu NP XPS envelopes were interpreted.
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•Cobalt amine molecular complexes were immobilized on an activated carbon support.•Pseudo-tetrahedral Co(NH3)x2+, where x = 4 or 5, exist on Co/N-C catalyst.•Thermal treatment reduces ...Co3+to Co2+, excess NH3 ligands oxidize to form N2O.•Pseudo-tetrahedral cobalt amine sites most likely serve as the catalytic active sites for ethylene dimerization
Catalytic dimerization of ethylene (C2H4) to butene (C4H8) utilizes homogenous catalysts composed of transition metal molecular complexes in the presence of a co-catalyst or Ni, Rh or Cr supported heterogeneous catalysts. Recently, a heterogeneous cobalt catalyst supported on NH4OH-treated carbon (Co/NC) has gained attention. However, the structure of the active site, the identity of the active phase, the role of NH4OH treatment and the effect of thermal activation have remained elusive. Herein, in situ Raman spectroscopy showed the absence of crystalline cobalt oxide phases, such as CoO or Co3O4, in the working catalyst. An intense Raman band at 810 cm−1 was detected in the as-synthesized catalyst, which corresponds to μCo-peroxo species formed via auto-oxidation of the Co(NH3)x2+ sites during the drying step. UV–vis DRS absorbance bands at ∼286, 340 and 560 nm indicated that a mixture of octahedral Co3+ and Co2+ is present in the as-synthesized catalyst. During thermal treatment in inert atmosphere, the 265 and 810 cm−1 Raman bands diminished signifying the reduction of μCo-peroxo sites, with the oxidation of some NH3 ligands to N2O(g), indicated by an IR doublet at ∼2225 cm−1. UV–vis DRS during thermal treatment corroborated the reduction of Co3+ to Co2+, with Co2+ sites coordinated in a tetrahedral fashion, as indicated by a new absorbance band at ∼600−700 nm. The presence of the immobilized pseudo-tetrahedral Co(NH3)x2+ molecular complexes active for dimerization is proposed as an alternative to previous reports that attribute olefin dimerization activity to crystalline cobalt oxide phase in this catalyst. Steady-state catalytic test results at 1 bar, 80 °C, 5.0 % C2H4 at 10 sccm, 0.3 g catalyst show that model catalyst containing crystalline cobalt oxides (Co/C) exhibited similar initial butene productivity to the Co(NH3)x2+ containing Co/NC catalyst, but suffered sharper deactivation due to the irreversible olefin deposition on Co3+ sites.
Understanding reaction kinetics is crucial for designing and applying heterogeneous catalytic processes in chemical and energy conversion. Here, we revisit the Langmuir–Hinshelwood (L-H) kinetic ...model for bimolecular surface reactions, originally formulated for metal catalysts, assuming immobile adsorbates on neighboring pair sites, with the rate varying linearly with the density of surface sites (sites per unit area); r ∝ *o 1. Supported metal oxide catalysts, however, offer systematic control over *o through variation of the active two-dimensional metal oxide loading in the submonolayer region. Various reactions catalyzed by supported metal oxides are analyzed, such as supported VO x catalysts, including methanol oxidation, oxidative dehydrogenation of propane and ethane, SO2 oxidation to SO3, propene oxidation to acrolein, n-butane oxidation to maleic anhydride, and selective catalytic reduction of nitric oxide with ammonia. The analysis reveals diverse dependencies of reaction rate on *o for these surface reactions, with r ∝ *o n , where n equals 1 for reactions with a unimolecular rate-determining step and 2 for those with a bimolecular rate-limiting step or exchange of more than 2 electrons. We propose refraining from a priori assumptions about the nature and density of surface sites or adsorbate behavior, advocating instead for data-driven elucidation of kinetics based on the density of surface sites, adsorbate coverage, etc. Additionally, recent studies on catalytic surface mechanisms have shed light on nonadjacent catalytic sites catalyzing surface reactions in contrast to the traditional requirement of adjacent/pair sites. These findings underscore the need for a more nuanced approach in modeling heterogeneous catalysis, especially supported metal oxide catalysts, encouraging reliance on experimental data over idealized assumptions that are often difficult to justify.
Supported‐phosphate‐catalysts (SPCs) are a versatile class of heterogeneous materials used in various industrially relevant processes including Fischer‐Tropsch synthesis (FTS), selective catalytic ...reduction (SCR) of NOx with NH3, and Guerbet reaction of C2H5OH to n‐C4H9OH. Herein, model SPCs synthesized by impregnating various supports with phosphoric acid were studied using in‐situ chemical probe temperature‐programmed‐desorption infrared spectroscopy. Hydroxylation of the catalyst surface was observed during the temperature ramp on all SPCs. This behavior was not observed on the bare supports during identical experiments. The results indicate that hydroxylation of the surface occurs via sacrificial hydrogen‐transfer reaction, where the chemical probe (NH3 or C2H5OH) can be utilized as the hydrogen‐donor. Results herein also show that the extent and the rate of hydrogen‐transfer reaction depend on the identity of the support, phosphate loading, and identity of the hydrogen‐donor molecule. Surface Brønsted acid sites (P−OH) do not contribute to the reaction and P=O surface sites are proposed as the active sites for the observed hydrogen‐transfer. Based on the current work, it is suggested that any catalytic reaction that involves hydrogenation over phosphate‐promoted or phosphate‐based catalysts needs to account for the surface‐mediated hydrogen‐transfer capability of the phosphate sites in the overall reaction mechanism.
We show that surface phosphate sites (P=O) in supported‐phosphate‐catalysts (SPCs) are involved in the surface‐mediated hydrogen‐transfer reaction, spectroscopically evidenced by new −OH peaks evolving in the IR spectra during chemical‐probe‐TPD‐DRIFTS experiments. The hydrogen‐transfer activity of various SPCs studied herein was ranked in the order of the support cation electronegativity, with higher activity for low cation electronegativity (Zr4+(OH)4) and low activity for high cation electronegativity (Si4+O2). Other factors that influence observed hydrogen‐transfer activity in SPCs are also studied herein.
The literature for the oxidative coupling of methane (OCM) on supported Mn/Na2WO4/SiO2 catalysts is systematically and critically reviewed. The influence of the precursors, starting SiO2 support ...crystallinity, synthesis method, calcination temperature, and OCM reaction conditions on the catalyst structure is examined. The supported Mn/Na2WO4/SiO2 catalyst system is found to be dynamic with the catalyst structure quite dependent on the set of variables. Although almost all of the reported studies have determined the catalyst crystalline structures under ambient conditions (room temperature and air exposed), recent in situ/operando characterization study under OCM reaction conditions revealed that all previously detected crystalline phases of the active Mn–Na–W–O components are not present because the reaction temperature is above the melting points of their oxides. The presence of Na also induces the crystallization of the silica support to SiO2 (cristobalite) at elevated temperatures. The nature of the surface active sites under OCM reaction conditions is still not known because of the absence of in situ/operando surface spectroscopy characterization studies under relevant reaction conditions. Consequently, the proposed structure–activity models in the literature are highly speculative since they are lacking supporting data. The rate-determining-step involves activation of the methane C–H bond by atomic surface O* as demonstrated by a kinetic isotope effect (KIE) between CH4 and CD4. Although the reaction kinetics follow a Langmuir–Hinshelwood type mechanism, r = CH41O21/2, isotopic 18O2–16O2 studies have shown that the catalyst lattice also provides O* for the OCM reaction suggesting involvement of a Mars–van Krevelen mechanism. Recommendations are given regarding the experimental investigations that could establish the fundamental reaction aspects of OCM by supported Mn/Na2WO4/SiO2 catalysts that would allow for the rational design of improved catalysts.
Low-solubility Mg2+-containing mineral surfaces, including periclase (MgO) and magnesite (MgCO3), can act as sustainable sources of Mg2+ ions to yield green fertilizer struvite (MgNH4PO4) via a ...two-dimensional surface-facilitated growth. This work studied the kinetics, mechanism, and single-particle speciation of the natural mineral dolomite (CaMg(CO3)2) during struvite formation utilizing NH4 + and PO4 3– ions from simulated wastewater. A range of Mg2+/NH4 +/PO4 3– ratios were tested and 0.69:1:1 500/600 ppm of dolomite/monoammonium phosphate (MAP) yielded the highest NH4 + removal efficiency of 37%. The highest PO4 3– removal of 94% was obtained using ratio 1.37:1:1 (1000/600 ppm dolomite/MAP) but resulted in an amorphous solid phosphate phase rather than struvite. Moreover, significant compositional heterogeneities were observed in the spatially resolved ex situ single-crystal Raman spectroscopy with CaCO3, MgCO3, MHPO4, and M(H2PO4)2 (M: Mg or Ca) detected. Temporally resolved in situ Raman spectroscopy and spectrokinetic analysis provided mechanistic insights into the onset of the struvite crystal formation and growth. In particular, hydroxyl (OH) functional groups on the dolomite surface were consumed during CO3 2– ion adsorption. The adsorbed CO3 2– further served as adsorption sites for a combination of T d symmetry PO4 3–, C 3v HPO4 2–, and C 2v H2PO4 – in early stages of reaction. In the later stages, for example, during the onset of struvite crystal formation, adsorbed dimeric/polymeric H2PO4 – units formed on the dolomite surface, which likely provide structural synergy at the solid–solution boundary for crystalline struvite nucleation and growth.
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.
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