The extensive emission of greenhouse gases, primarily CO2 and CH4, has contributed to intensified global warming. Dry reforming of methane (DRM, CH4 + CO2 → 2CO + 2H2) offers a pathway for the synergistic utilization of these two major greenhouse gases, presenting important implications for both environmental protection and energy sustainability. However, the catalysts still face the challenges such as carbon deposition and sintering of active metals, which adversely affect the catalytic performance and long-term stability. Oxygen vacancies, which are common lattice defects in metal oxides, have been demonstrated to improve the DRM performance by modulating the surface and interfacial properties of the catalysts. This review systematically summarizes research progresses in DRM over the past decade, outlines the major challenges and emphasizes the critical roles of oxygen vacancies in suppressing carbon deposition and inhibiting metal sintering. Furthermore, the mechanisms through which oxygen vacancies influence DRM reactions are discussed, combined with their formation pathways and regulation strategies. These insights provide essential theoretical foundations for the design and synthesis of highly efficient and stable DRM catalysts.

Defect engineering critically influences metal oxide catalysis, yet controlling coordinatively unsaturated metal sites remains challenging due to their inherent instability under reaction conditions. Here, we demonstrate that high-flux argon ion (Ar+) irradiation above recrystallization temperatures generated well-defined coordinatively unsaturated Zn (CUZ) sites on ZnO(101̅0) surfaces that exhibited enhanced stability and activity for CO hydrogenation. Combining low-temperature scanning probe microscopy, ambient pressure X-ray photoelectron spectroscopy, and surface-ligand infrared spectroscopy with density functional theory calculations, we identified <12̅10> step edges exposing CUZ sites as the dominant active sites. These sites facilitate hydrogen-assisted CO dissociation through a mechanism distinct from formate-mediated pathways on stoichiometric ZnO. The ion-irradiation approach effectively addressed instability of Zn species, a major problem in ZnO catalysis, enabling stable performance in syngas conversion when combined with zeolites. Our atomic scale investigation provided spectroscopic fingerprints for active sites on the ZnO catalyst and insights into the structure-activity relationships of ZnO for CO hydrogenation. Our approach for engineering thermally stable defect sites in oxide catalysts provided opportunities for rational catalyst design beyond traditional preparation methods.

Developing photocatalysts with both high efficiency and reaction pathway selectivity is essential for achieving efficient and sustainable CO2 conversion. By incorporating sulphur vacancies into MoS2, an S-scheme heterojunction photocatalyst (MoS2-SVs/g-C3N4) was developed, achieving efficient and selective CO2 photoreduction to CH3OH. The structural and photoelectronic characterisation of the system shows that the heterogeneous interface between MoS2 and g-C3N4 is in close contact. The introduction of SVs effectively modulates the electronic structure and surface activity of MoS2, which in turn enhances the CO2 reduction performance. Optical and electronic structure analyses reveal that the heterojunction promotes favourable band alignment and interfacial electric potential gradients, which together suppress charge recombination and enhance directional carrier separation. Under irradiation, the MoS2-SVs/g-C3N4 photocatalyst exhibited outstanding photocatalytic CH3OH production with a yield of 10.06 μmol·h−1·g−1, significantly surpassing the performance of control samples while demonstrating excellent product selectivity and remarkable stability. Mechanistic studies further verify that vacancy-induced energy band modulation with Fermi energy level enhancement significantly reduces the multi-electron transfer barrier, thus preferentially driving the CH3OH generation pathway. This work proposes a universal structural design strategy that synergistically coordinates vacancy engineering with band structure modulation, establishing both theoretical principles and practical methodologies for developing selective multi-electron CO2 reduction systems.

Despite wide studies demonstrating the versatility of the metal oxide-zeolite (OXZEO) catalyst concept to tackle the selectivity challenge in syngas chemistry, the active sites of metal oxides and the mechanism of CO/H2 activation remain to be elucidated. Herein, we demonstrate experimentally the role of Cr in zinc-chromium oxides and unveil visually, for the first time, the active sites for CO activation employing scanning transmission electron microscopy-electron energy loss spectroscopy using the volumetric density of surface carbon species as a descriptor. The ZnCr2O4 spinel surface with atomic ZnOx overlayer is the most active site for C-O bond dissociation, particularly at the narrow ZnCr2O4(110) facets constrained between the (311) and (111) facets, followed by the Cr-doped wurtzite ZnO surface. In comparison, the surfaces of ZnCr2O4 with aggregated ZnOx overlayers, pure ZnO, and the stoichiometric ZnCr2O4 exhibit a significantly lower activity. In situ synchrotron-based vacuum ultraviolet photoionization mass spectrometric study on different temperature programmed surface reactions with isotopes of C18O, 13CO, and D2 validates direct CO dissociation over ZnCrn oxides in CO, forming CH2 and further to hydrocarbons if H2 is present and CH2CO intermediates in syngas. The activity of CO dissociation and hydrogenation over ZnCrn oxides correlates well with the syngas-to-light-olefins activity of ZnCrn-SAPO-18 composite catalysts as a function of the Cr/Zn ratio.

NMR experiments reveal a mechanism of syngas conversion in which CO reacts with OCH3 on the oxide surface, generating ketene intermediates, which can either form acetate or diffuse into zeolite.

Although ketene has been proposed to be an active intermediate in a number of reactions including OXZEO (metal oxide-zeolite)-catalyzed syngas conversion, dimethyl ether carbonylation, methanol to hydrocarbons, and CO2 hydrogenation, its chemistry and reaction pathway over zeolites are not well understood. Herein, we study the pathway of ketene transformation to gasoline range hydrocarbons over the molecular sieve H-SAPO-11 by kinetic analysis, in situ infrared spectroscopy, and solid-state nuclear magnetic resonance spectroscopy. It is demonstrated that butene is the reaction intermediate on the paths toward gasoline products. Ketene transforms to butene on the acid sites via either acetyl species following an acetic acid ketonization pathway or acetoacetyl species with keto-enol tautomerism following an acetoacetic acid decarboxylation pathway when in the presence of water. This study reveals experimentally for the first time insights into ketene chemistry in zeolite catalysis.

Sn/ZSM-5 catalyzes direct syngas conversion into ethane with ethane selectivity 84% via an ethylene intermediate.

Formation and evolution of active hydrogen species from H-2 on a metal oxide is of great importance in many hydrogenation reactions, while understanding the nature of surface hydrogen species remains a great challenge. Herein, high-vacuum-based transmission infrared spectroscopy coupled with temperature-programmed desorption has been applied to study surface hydrogen species over ZnO and ZnCrOx catalysts. We show that D-2 dissociates readily to form Zn-D and O-D on ZnO at 153 K, and simultaneous Zn-D and O-D desorption in the form of D-2 occurs at 223 K. This corroborates reversible dissociation and recombination of D-2 on ZnO. ( Cr-)Zn-D and (Cr-)O-D species appear on ZnCr2O4 in D-2 at 273 K, indicating a higher barrier for D-2 activation than that on ZnO. (Cr-)Zn-D can diffuse to form (Cr-)O-D at 573 K, and then desorption of surface O-D groups requires a higher temperature of 943 K. Theoretical studies demonstrate that surface geometry plays a crucial role in the evolution of the surface hydrogen species. The polar ZnCr2O4(111) surface can stabilize metal-H species due to the high barrier for its surface diffusion and the thermodynamic obstruction for its association with O-H, in contrast with the nonpolar ZnO(10 (1) over bar0) surface. These results provide insights into the tuning of the stability of surface metal-H species on metal oxide catalysts, which will contribute to tailoring the hydrogenation processes in the H-involved catalytic reactions.

Despite significant progress achieved in Fischer–Tropsch synthesis (FTS) technology, control of product selectivity remains a challenge in syngas conversion. Herein, we demonstrate that Zn 2+ -ion exchanged ZSM-5 zeolite steers syngas conversion selectively to ethane with its selectivity reaching as high as 86 % among hydrocarbons (excluding CO 2 ) at 20 % CO conversion. NMR spectroscopy, X-ray absorption spectroscopy, and X-ray fluorescence indicate that this is likely attributed to the highly dispersed Zn sites grafted on ZSM-5. Quasi-in-situ solid-state NMR, obtained by quenching the reaction in liquid N 2 , detects C 2 species such as acetyl (-COCH 3 ) bonding with an oxygen, ethyl (-CH 2 CH 3 ) bonding with a Zn site, and epoxyethane molecules adsorbing on a Zn site and a Brønsted acid site of the catalyst, respectively. These species could provide insight into C−C bond formation during ethane formation. Interestingly, this selective reaction pathway toward ethane appears to be general because a series of other Zn 2+ -ion exchanged aluminosilicate zeolites with different topologies (for example, SSZ-13, MCM-22, and ZSM-12) all give ethane predominantly. By contrast, a physical mixture of ZnO-ZSM-5 favors formation of hydrocarbons beyond C 3+ . These results provide an important guide for tuning the product selectivity in syngas conversion.

AbstractDespite significant progress achieved in Fischer–Tropsch synthesis (FTS) technology, control of product selectivity remains a challenge in syngas conversion. Herein, we demonstrate that Zn2+‐ion exchanged ZSM‐5 zeolite steers syngas conversion selectively to ethane with its selectivity reaching as high as 86 % among hydrocarbons (excluding CO2) at 20 % CO conversion. NMR spectroscopy, X‐ray absorption spectroscopy, and X‐ray fluorescence indicate that this is likely attributed to the highly dispersed Zn sites grafted on ZSM‐5. Quasi‐in‐situ solid‐state NMR, obtained by quenching the reaction in liquid N2, detects C2 species such as acetyl (‐COCH3) bonding with an oxygen, ethyl (‐CH2CH3) bonding with a Zn site, and epoxyethane molecules adsorbing on a Zn site and a Brønsted acid site of the catalyst, respectively. These species could provide insight into C−C bond formation during ethane formation. Interestingly, this selective reaction pathway toward ethane appears to be general because a series of other Zn2+‐ion exchanged aluminosilicate zeolites with different topologies (for example, SSZ‐13, MCM‐22, and ZSM‐12) all give ethane predominantly. By contrast, a physical mixture of ZnO‐ZSM‐5 favors formation of hydrocarbons beyond C3+. These results provide an important guide for tuning the product selectivity in syngas conversion.

Domoic acid (DA) is a naturally occurring neurotoxin known to bioaccumulate in marine products. Despite its hypertoxicity, the enrichment and analysis of trace DA in complex marine organisms remains a challenge. We describe herein the fabrication of a postsynthetic-modified magnetic zeolite imidazolate framework-8 (Fe3O4 SPs@ZIF-8/Zn2+), based on Fe3O4 superparticles, for the adsorption of DA from complex biological matrices. The adsorption of DA is rapid (∼5 min) and occurs through strong electrostatic interactions and chelation with coordinatively unsaturated zinc sites on the surface of Fe3O4 SPs@ZIF-8/Zn2+. Employing our Fe3O4 SPs@ZIF-8/Zn2+ sorbent in a magnetic solid-phase extraction, followed by liquid chromatographic separation and tandem mass spectrometric detection, resulted in a facile, rapid, efficient, and sensitive method for the enrichment and detection of trace DA in marine products. After optimization, this method yielded satisfactory precision (relative standard deviation ≤3.4%; n = 5) with a high degree of linearity from 1.0 to 1000.0 pg mL-1 ( r2 = 0.9997) and a detection limit of 0.2 pg mL-1 (S/N = 3). Recoveries of 93.1-102.3% were obtained in spiked aquatic products. In addition, trace levels of DA (49.2 pg mL-1) were found in shellfish samples, confirming the applicability of our Fe3O4 SPs@ZIF-8/Zn2+ adsorbent for the detection of DA in seafood.

Despite significant efforts towards the direct conversion of syngas into liquid fuels, the selectivity remains a challenge, particularly with regard to high-quality gasoline with a high octane number and a low content of aromatic compounds. Herein, we show that zeolites with 1D ten-membered-ring (10-MR) channel structures such as SAPO-11 and ZSM-22 in combination with zinc- and manganese-based metal oxides (Zna Mnb Ox ) enable the selective synthesis of gasoline-range hydrocarbons C5 -C11 directly from syngas. The gasoline selectivity reached 76.7 % among hydrocarbons, with only 2.3 % CH4 at 20.3 % CO conversion. The ratio of isoparaffins to n-paraffins was as high as 15, and the research octane number was estimated to be 92. Furthermore, the content of aromatic compounds in the gasoline was as low as 16 %. The composition and structure of Zna Mnb Ox play an important role in determining the overall activity. This process constitutes a potential technology for the one-step synthesis of environmentally friendly gasoline with a high octane number from a variety of carbon resources via syngas.

Mordenite (MOR) zeolite is known to possess a 12-membered ring (12 MR) and 8 MR channels, which are interconnected by 8 MR side pockets. Hyperpolarization (HP) Xe-129 NMR reveals a new Xe adsorption site in addition to the usual 12 MR main channels and 8 MR side pockets. Different guest molecules exhibit explicitly site-selective adsorption on different sites of MOR, as demonstrated by in situ continuous-flow HP Xe-129 NMR studies. For example, methanol prefers to adsorb on 12 MR main channels while water prefers to adsorb on this new site connecting the 12 MR main channel and 8 MR pocket. Furthermore, the adsorption of methanol reduces the pore size of main channels and the free mean path of Xe there, while water reduces the interaction of Xe molecules with MOR.

Syngas conversion by Fischer-Tropsch synthesis (FTS) is characterized by a wide distribution of hydrocarbon products ranging from one to a few carbon atoms. Reported here is that the product selectivity is effectively steered toward ethylene by employing the oxide-zeolite (OX-ZEO) catalyst concept with ZnCrOx-mordenite (MOR). The selectivity of ethylene alone reaches as high as 73% among other hydrocarbons at a 26% CO conversion. This selectivity is significantly higher than those obtained in any other direct syngas conversion or the multistep process methanol-to-olefin conversion. This highly selective pathway is realized over the catalytic sites within the 8-membered ring (8MR) side pockets of MOR via a ketene intermediate rather than methanol in the 8MR or 12MR channels. This study provides substantive evidence for a new type of syngas chemistry with ketene as the key reaction intermediate and enables extraordinary ethylene selectivity within the OX-ZEO catalyst framework.