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 experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na
2
WO
4
catalytic active sites for CH
4
...heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO
4
catalytic active sites resulting in surface WO
5
sites with retarded kinetics for C-H scission. On the other hand, dimeric Mn
2
O
5
surface sites, identified and studied
via ab initio
molecular dynamics and thermodynamics, were found to be more efficient in activating CH
4
than the poisoned surface WO
5
sites or the original WO
4
sites. However, the surface reaction intermediates formed from CH
4
activation over the Mn
2
O
5
surface sites are more stable than those formed over the Na
2
WO
4
surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO
x
, in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na
2
WO
4
surface sites towards CH
4
activation but can yield MnO
x
surface sites that activate CH
4
faster than Na
2
WO
4
surface sites, but unselectively.
The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not necessarily enhance the activity of the surface Na
2
WO
4
catalytic active sites for CH
4
heterolytic dissociation during OCM.
Ca, Ni, Co, and Ge promoters were examined as potential candidates to substitute for the current toxic Cr in Cu-promoted Fe oxide-based catalysts for the HT-WGS reaction. The Ca and Ni promoters were ...found to improve catalyst performance relative to promotion with Cr. The HS-LEIS surface analysis data demonstrate that Ca and Ge tend to segregate on the surface, while Ni, Co, and Cr form solid solutions in the Fe3O4 bulk lattice. The corresponding number of catalytic active sites, redox, and WGS activity values of the catalysts were determined with CO-TPR, CO+H2O-TPSR, and SS-WGS studies, respectively. The poorer HT-WGS performances of the Ge and Co promoters are related to the presence of surface Ge and Co that inhibits catalyst redox ability, with the Co also not stabilizing the surface area of the Fe3O4 support. The Ni promoter uniformly disperses the Cu nanoparticles on the catalyst surface and increases the number of FeOx-Cu interfacial redox sites. The Ca promoter on the catalyst surface, however, enhances the activity of the FeOx-Cu interfacial redox sites. The CO+H2O TPSR results reveal that the redox ability of the active sites follows the SS-WGS performance of the catalysts and show the following trend: 3Cu8CaFe > 3Cu8NiFe ≥ 3Cu8CrFe > 3Cu8CoFe >> 3Cu8GeFe. Furthermore, all the catalysts followed a redox-type reaction mechanism for the HT-WGS reaction.
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 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.
The structure of supported MoO x /Al2O3 catalysts is investigated using ab initio molecular dynamics (AIMD) and density-functional theory (DFT). Phase diagrams were computed to understand the ...hydroxylation coverage as a function of the temperature, H2O partial pressure, and MoO x loading. We relate the shifts in experimental Raman vibrational frequencies under hydrated conditions to hydrogen bonding interactions and the MoO x anchoring location. We showcase that the use of AIMD as a benchmark for DFT can provide insight into how, under certain hydrated conditions, DFT excels as an inexpensive method for computing vibrational frequencies, while, under dehydrated conditions, it is susceptible to the largest errors. Additionally, to facilitate the analysis of the hydroxyl region of experimental infrared spectra, we compute the power spectra of individual hydroxyls and see a strong relationship between OH stretching frequencies and the surrounding coordination environment, which are also impacted by anchored MoO x . We compare computational and experimental IR and Raman spectra of catalysts synthesized herein under the same conditions.
The structure of supported MoOx/Al2O3 catalysts is investigated using ab initio molecular dynamics (AIMD) and density-functional theory (DFT). Phase diagrams were computed to understand the ...hydroxylation coverage as a function of the temperature, H2O partial pressure, and MoOx loading. We relate the shifts in experimental Raman vibrational frequencies under hydrated conditions to hydrogen bonding interactions and the MoOx anchoring location. We showcase that the use of AIMD as a benchmark for DFT can provide insight into how, under certain hydrated conditions, DFT excels as an inexpensive method for computing vibrational frequencies, while, under dehydrated conditions, it is susceptible to the largest errors. Additionally, to facilitate the analysis of the hydroxyl region of experimental infrared spectra, we compute the power spectra of individual hydroxyls and see a strong relationship between OH stretching frequencies and the surrounding coordination environment, which are also impacted by anchored MoOx. Here, we compare computational and experimental IR and Raman spectra of catalysts synthesized herein under the same conditions.
•First time application of dynamic heating in competing exothermic & endothermic reactions.•Demonstrates selectivity shift and rate enhancement via dynamic electrification.•Mechanistic insights into ...dynamic catalytic processes via Operando characterization.•Improves the understanding of dynamic operation of competing reactions .
The performance of electrified reactors under dynamic operation can surpass that of conventional steady-state operation. Here, we demonstrate that rapid pulse heating (RPH) operation of the CO2 hydrogenation reaction at 1 bar over a Ni/Al2O3 catalyst increases the reaction rate and shifts the product selectivity toward CO over CH4 at low reaction temperatures (<500 °C). Operando FTIR characterization and kinetics experiments reveal a consecutive redox pathway, with *CO being a key surface intermediate that either desorbs or is further hydrogenated to CH4. We propose that the selectivity change is due to the transient coverages of *CO and *H over the Ni surface during temperature pulsing, which facilitates CO desorption and suppresses the subsequent deep deoxygenation and hydrogenation to CH4.
The effect of Ce addition to the Cr-free Al-promoted Cu–Fe oxide-based catalysts is investigated. Catalyst characterization (X-ray diffraction (XRD), in situ Raman spectroscopy, high-sensitivity ...low-energy ion scattering (HS-LEIS), Brunauer–Emmett–Teller (BET) analysis), CO-temperature-programmed reduction chemical probing, and steady-state WGS activity reveal that (i) in the absence of Al, Ce addition via coprecipitation has a detrimental effect on the catalytic activity related to the poor thermostability and formation of less active Ce–Cu–O NPs, (ii) the addition of Ce via coprecipitation also does not improve the performance of the CuAlFe catalyst because of the formation of a thick CeO x overlayer on the active Cu–FeO x interface, and (iii) impregnation of Ce onto the CuAlFe catalyst exhibits significant improvement in catalytic performance due to the formation of a highly active CeO x –FeO x –Cu interfacial area. In summary, Al does not surface-segregate and serves as a structural promoter, while Ce and Cu surface-segregate and act as functional promoters in Ce/CuAlFe mixed oxide catalysts.
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