Xiaoyan Liu

The active phase for the partial oxidation of methane (POM) to syngas on Ni catalysts has been attributed to metallic nanoparticles, with high Ni loadings (~10 wt%) considered essential. Here we show, on NiO surfaces, that in situ-generated [Ni ₁ O ₁ Ni ₄ ] motifs, where one Ni atom sits on an oxygen atom that bridges four surrounding Ni atoms, drive POM with promising efficiency. A Ni/Al ₂ O ₃ -ME catalyst with Ni loading of 0.8 wt% fabricated via deposition of preformed Ni nanoparticles onto Al ₂ O ₃ achieves 92.0% CH ₄ conversion and 87.0% CO/H ₂ selectivity at 650 °C for POM, maintaining a stable H ₂ /CO ratio of 2.0. Operando spectroscopy and environmental microscopy confirm the oxidation of metallic Ni to NiO during POM. The results of operando high-resolution annular bright-field imaging, combined with density functional theory calculations, suggest that the [Ni ₁ O ₁ Ni ₄ ] structure formed on the NiO(100) surface reduces the C–H activation barrier to 12.5 kcal mol ⁻¹ . Thus, metallic Ni nanoparticles with high metal loadings may not be strictly required for efficient POM.

Site isolation is an effective strategy for achieving high selectivity for acetylene semihydrogenation, yet often at the cost of reduced activity. Moreover, most of the site isolation approaches involve single-atom catalysts (SACs), less is known about the active site ensemble that can behave even superior to SACs in both activity and selectivity. Herein, by decoration of partially reduced GaO_x on the Pd particles in Pd/Al₂O₃ and followed by in situ treatment in reaction gas, we are able to construct isolated Pd₃ sites and identify the structure by using in situ CO-DRIFTS and DFT calculations. Reaction tests for acetylene semihydrogenation under simulated industrial feed gas conditions show that the isolated Pd₃ sites offer ∼99% ethylene selectivity at acetylene conversion of 95% at very mild reaction conditions (50 °C and 1 bar), with a turnover frequency of 13.5-fold that of Pd₂Ga intermetallic compound, outperforming most of the state-of-the-art Pd-based catalysts. The combined TPD experiments and theoretical calculations reveal that the partially reduced GaO_x on the Pd surface not only isolates and stabilizes the Pd₃ geometry but also alters the electronic properties of Pd via electron transfer from Ga to Pd atoms, thereby enhancing acetylene adsorption while weakening ethylene adsorption, leading to greatly increased activity and selectivity. This work opens up a distinct avenue from the prevalent SAC strategy toward site-isolation, highlighting the great potential of manipulating the active site ensemble in overcoming the activity-selectivity trade-off in selective hydrogenation reactions.

Single-atom catalysts (SACs) have shown exceptional promise in a variety of selective hydrogenations due to their uniform and isolated active sites, yet the intrinsic nature of the active sites and reaction mechanism remain elusive. Herein, by a rapid thermal treatment (RTT) method, we are able to finely tune the coordination structure of Pt₁/CeO₂ SAC and establish a linear correlation between the Pt–O coordination number, electronic structure, and catalytic activity for furfural/3-nitrostyrene hydrogenations. Integrated quasi in situ spectroscopic characterizations and DFT calculations reveal the structural evolution of Pt₁–O_v–Ce active sites and the redox mechanism. RTT at 600 °C results in the formation of Pt₁–O_v–Ce, which allows for the preferential end-on adsorption of ─C═O/–NO₂ groups into O_v and the charge transfer from the Pt₁ single atom to the reactant. On the other hand, hydrogen is dissociated on the Pt single atoms and then reacts with the adsorbed ─C═O/–NO₂ group to accomplish the selective hydrogenation, accompanied by the regeneration of O_v. Moreover, the interfacial Pt₁–O_v–Ce is found to be 2.4–15.7-fold more active than Pt₁–––O_v (peripheral O_v). These findings highlight the dictating role of the coordination structure of SACs and elucidate the cooperative mechanism between Pt single atoms and interfacial oxygen vacancies, thus offering design principles for overcoming the activity-selectivity trade-off in other selective hydrogenations.

ConspectusSingle-atom catalysis has become one of the most active frontiers in catalysis in the past decade. This concept not only gives birth to a new kind of heterogeneous catalysts featuring well-defined isolated active sites and strong covalent (or electronic) metal-support interaction, which deliver unique catalytic activity, selectivity, and stability distinct from their nanoparticulate counterparts, but also together with the principles and concepts in history, reshapes our understanding of heterogeneous catalysis and drives the catalysis research from the nanoscale and subnanoscale to the more precise atomic scale.Due to the extremely high free energy, the isolated gaseous metal atoms cannot survive alone but are stabilized on the support materials through strong chemical binding, forming metal-centered coordination moieties resembling organometallics in homogeneous catalysis. The coordination environment, including the inner shell and outer shell comprised of the support, reactants, and environmental molecules, determines the electronic and geometric properties of central atoms and, in turn, the catalytic performance of SACs. In some cases, the neighboring atoms on the support can be directly involved in the catalysis in a metal-support concerted catalysis manner. Therefore, the coordination environment not only serves as an effective descriptor of the structure-performance relationship but also provides great opportunities to fabricate better-performed SACs through modulation of the coordination atoms, number, promoter, and so on.In this Account, recent advances in our group in the identification and modulation of the coordination environment of SACs are highlighted. First, the heterogeneity of the coordination environment of SACs is discussed regarding the nonuniform structure and composition of the support. Then, various approaches to tune the inner-shell and outer-shell coordination environments are shown for the effective improvement of the catalytic performance of SACs. Finally, the structure evolution of SACs driven by external stimuli and reactant/products is discussed. The atomic understanding of the coordination environment of SACs will help to elucidate the nature of single-atom catalysis and enrich the theoretical aspects of heterogeneous catalysis.

Propane dehydrogenation is an energy-intensive industrial reaction that requires high temperatures (550–750 °C) to overcome thermodynamic barriers. Here we overcome these limits and demonstrate that near-ambient propane dehydrogenation can be achieved through photo-thermo-catalysis in a water-vapour environment. We reduce the reaction temperature to 50–80 °C using a single-atom catalyst of copper supported on TiO₂ and a continuous-flow fixed-bed reactor. The mechanism differs from conventional propane dehydrogenation in that hydrogen is produced from the photocatalytic splitting of water vapour, surface-bound hydroxyl radicals extract propane hydrogen atoms to form propylene without over-oxidation, and water serves as a catalyst. This route also works for the dehydrogenation of other small alkanes. Moreover, we demonstrate sunlight-driven water-catalysed propane dehydrogenation operating at reaction temperatures as low as 10 °C. We anticipate that this work will be a starting point for integrating solar energy usage into a wide range of high-temperature industrial reactions.

Transition metal carbide shows excellent performance in selective hydrogenation of acetylene, however, the carburization of Pd-based intermetallic compounds remains infeasible. Here we report the successful synthesis of an unprecedented Pd₃ZnC_x intermetallic carbide, via co-infiltration of zinc and carbon in one-step carburization by syngas. Utilizing state-of-the-art in situ characterizations and theoretical calculation, we unveil the dynamic evolution of Pd₃ZnC_x during carburization, forming a Pd₃Zn like cubic phase carbide structure. A unique transitional state (Pd_t) with low content of Zn/C co-infiltration is clearly identified facilitating phase transition and sustain incorporation of carbon and zinc at elevated temperatures. The Pd₃ZnC_x carbide shows by far the best catalytic performance in the selective hydrogenation of acetylene with a high selectivity (>90%) even at a high H₂/C₂H₂ ratio. Our results therefore provide a co-infiltration strategy and dynamic insights for the one-step synthesis of Pd based intermetallic carbides, towards high-performance intermetallic compound for selective hydrogenation of acetylene.