Probing surface defects of ZnO using formaldehyde

Photothermal direct methane conversion to formaldehyde at the gas-solid interface under ambient pressure

Photocatalytic direct oxidation of methane to C1 oxygenates offers a green alternative to conventional energy-intensive and high-carbon-footprint multi-step processes. However, current batch-type gas-liquid-solid reaction systems under high-pressure conditions face critical challenges in real-time product separation and concentration for industrial implementation. Here, we demonstrate a continuous-flow gas-solid photothermal catalytic route for methane conversion to formaldehyde under ambient pressure, where the generated gas-phase formaldehyde can be easily collected by water absorption. The Ag single-atom modified ZnO photocatalyst achieves a formaldehyde production rate of 117.8 ± 1.7 μmol h-1 with 71.2 ± 0.8% selectivity. Meanwhile, a highly concentrated formaldehyde solution (514.2 ± 33.7 µmol mL-1, 1.54 ± 0.10 wt.%) is obtained through 12-hour water absorption, effectively overcoming the product enrichment barrier that plagues conventional batch reaction route. This study establishes a robust technological foundation for sustainable industrial-scale conversion of methane to value-added chemicals.

Ion Irradiation-Induced Coordinatively Unsaturated Zn Sites for Enhanced CO Hydrogenation

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.

Theoretical insights into the generation and reactivity of hydride on the ZnO(101̄0) surface

ZnO is an important catalytic material for CO/CO2 hydrogenation. In this work, the pristine ZnO(101̄0) and the surfaces with Zn-O dimer vacancies (ZnO(101̄0)-(Zn-O)DiV) and oxygen vacancies are calculated. We find that the hydride (H-) species can be generated via heterolytic H2 dissociation on these surfaces, and that ZnO(101̄0)-(Zn-O)DiV only needs to overcome the energy barrier of ∼0.10 eV. This is because the ZnO system has flexible orbitals for electron storage and release and the low-coordinated Zn3c atoms at the defect sites can form stable Zn-H- covalent bonds with high symmetry. Flexible Zn orbitals also impart the unique feature of activating multiple electrophilic adsorbates simultaneously as excess electrons exist. Moreover, we show that the covalent Zn-H- species can regulate the catalytic activity and selectivity for CO2 hydrogenation by preferentially producing *HCOO intermediates at Zn-O dimer vacancies. These results may help in the design of efficient Zn-based hydrogenation catalysts.

Unravelling the Impact of Metal Dopants and Oxygen Vacancies on Syngas Conversion over Oxides: A Machine Learning-Accelerated Study of CO Activation on Cr-Doped ZnO Surfaces

As a critical component of the OX-ZEO composite catalysts toward syngas conversion, the Cr-doped ZnO ternary system can be considered as a model system for understanding oxide catalysts. However, due to the complexity of its structures, traditional approaches, both experimental and theoretical, encounter significant challenges. Herein, we employ machine learning-accelerated methods, including grand canonical Monte Carlo and genetic algorithm, to explore the ZnO(1010) surface with various Cr and oxygen vacancy (OV) concentrations. Stable surfaces with varied Cr and OV concentrations were then systematically investigated to examine their influence on the CO activation via density functional theory calculations. We observe that Cr tends to preferentially appear on the surface of ZnO(1010) rather than in its interior regions and Cr-doped structures incline to form rectangular islands along the [0001] direction at high Cr and OV concentrations. Additionally, detailed calculations of CO reactivity unveil an inverse relationship between the reaction barrier (Ea) for C-O bond dissociation and the Cr and OV concentrations, and a linear relationship is observed between OV formation energy and Ea for CO activation. Further analyses indicate that the C-O bond dissociation is much more favored when the adjacent OVs are geometrically aligned in the [1210] direction, and Cr is doped around the reactive sites. These findings provide a deeper insight into CO activation over the Cr-doped ZnO surface and offer valuable guidance for the rational design of effective catalysts for syngas conversion.

Atomic-Scale Visualization of Heterolytic H2 Dissociation and COx Hydrogenation on ZnO under Ambient Conditions

Studying catalytic hydrogenation reactions on oxide surfaces at the atomic scale has been challenging because of the typical occurrence of these processes at ambient or elevated pressures, rendering them less accessible to atomic-scale techniques. Here, we report an atomic-scale study on H2 dissociation and the hydrogenation of CO and CO2 on ZnO using ambient pressure scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. We directly visualized the heterolytic dissociation of H2 on ZnO(101̅0) under ambient pressure and found that dissociation reaction does not require the assistance of surface defects. The presence of CO or CO2 on ZnO at 300 K does not impede the availability of surface sites for H2 dissociation; instead, CO can even enhance the stability of coadsorbed hydride species, thereby facilitating their dissociative adsorption. Our results show that hydride is the active species for hydrogenation, while hydroxyl cannot hydrogenate CO/CO2 on ZnO. Both AP studies and DFT calculations showed that the hydrogenation of CO2 on ZnO is thermodynamically and kinetically more favorable compared to that of CO hydrogenation. Our results point toward a two-step mechanism for CO hydrogenation, involving initial oxidation to CO2 at step sites on ZnO followed by reaction with hydride to form formate. These findings provide molecular insights into the hydrogenation of CO/CO2 on ZnO and deepen our understanding of syngas conversion and oxide catalysis in general.

Comprehensive Study of Oxygen Vacancies on the Catalytic Performance of ZnO for CO/H2 Activation Using Machine Learning-Accelerated First-Principles Simulations

Oxygen vacancies (OVs) play important roles on any oxide catalysts. In this work, using an investigation of the OV effects on ZnO(101̅0) for CO and H₂ activation as an example, we demonstrate, via machine learning potentials (MLPs), genetic algorithm (GA)-based global optimization, and density functional theory (DFT) validations, that the ZnO(101̅0) surface with 0.33 ML OVs is the most likely surface configuration under experimental conditions (673 K and 2.5 MPa syngas (H₂:CO = 1.5)). It is found that a surface reconstruction from the wurtzite structure to a body-centered-tetragonal one would occur in the presence of OVs. We show that the OVs create a Zn₃ cluster site, allowing H₂ homolysis and C-O bond cleavage to occur. Furthermore, the activity of intrinsic sites (Zn_3c and O_3c sites) is almost invariable, while the activity of the generated OV sites is strongly dependent on the concentration of the OVs. It is also found that OV distributions on the surface can considerably affect the reactions; the barrier of C-O bond dissociation is significantly reduced when the OVs are aligned along the [12̅10] direction. These findings may be general in the systems with metal oxides in heterogeneous catalysis and may have significant impacts on the field of catalyst design by regulating the concentration and distribution of the OVs.