Identification of catalytic active sites is pivotal in the design of highly effective heterogeneous metal catalysts, especially for structure-sensitive reactions. Downsizing the dimension of the metal species on the catalyst increases the dispersion, which is maximized when the metal exists as single atoms, namely, single-atom catalysts (SACs). SACs have been reported to be efficient for various catalytic reactions. We show here that the Pt SACs, although with the highest metal atom utilization efficiency, are totally inactive in the cyclohexane (C 6 H 12 ) dehydrogenation reaction, an important reaction that could enable efficient hydrogen transportation. Instead, catalysts enriched with fully exposed few-atom Pt ensembles, with a Pt−Pt coordination number of around 2, achieve the optimal catalytic performance. The superior performance of a fully exposed few-atom ensemble catalyst is attributed to its high d-band center, multiple neighboring metal sites, and weak binding of the product.

The capture of radioactive I 2 vapor from nuclear waste under industrial operating conditions remains a challenging task, as the practical industrial conditions of high temperature (≥150 °C) and low I 2 concentration (∼150 ppmv) are unfavorable for I 2 adsorption. We report a novel guanidinium-based covalent organic framework (COF), termed TGDM, which can efficiently capture I 2 under industrial operating conditions. At 150 °C and 150 ppmv I 2 , TGDM exhibits an I 2 uptake of ∼30 wt %, which is significantly higher than that of the industrial silver-based adsorbents such as Ag@MOR (17 wt %) currently used in the nuclear fuel reprocessing industry. Characterization and theoretical calculations indicate that among the multiple types of adsorption sites in TGDM, only ionic sites can bond to I 2 through strong Coulomb interactions under harsh conditions. The abundant ionic groups of TGDM account for its superior I 2 capture performance compared to various benchmark adsorbents. In addition, TGDM exhibits exceptionally high chemical and thermal stabilities that fully meet the requirements of practical radioactive I 2 capture (high-temperature, humid, and acidic environment) and differentiate it from other ionic COFs. Furthermore, TGDM has excellent recyclability and low cost, which are unavailable for the current industrial silver-based adsorbents. These advantages make TGDM a promising candidate for capturing I 2 vapor during nuclear fuel reprocessing. This strategy of incorporating chemically stable ionic guanidine moieties in COF would stimulate the development of new adsorbents for I 2 capture and related applications.

Among them, great efforts have been made to elucidate the correlations between the reaction performance and the specific physical properties of the catalyst. During this period, some secondary concepts were introduced and widely discussed such as strong metal-support interaction (SMSI), [8][9][10][11][12] hydrogen spillover, [13,14] and the confinement effect, [15][16][17][18] etc.The "confinement effect" is a vital factor affecting the reaction, focusing prominently on porous materials. It mainly includes the physical constraint effect, [19,20] the electronic effect, [21] and the molecular enrichment effect. [22][23][24] The confinement effect was introduced for the first time on zeolite catalysts by constructing a spherical confined model to estimate van der Waals adsorption energies. [25] Since then, some other zeolites, [18,26] carbon nanotubes (CNTs), [27][28][29][30] and metal-organic frameworks [31,32] (MOFs) have been stepwisely involved in this specific topic to explain the correlation between the properties of a specific local environment and the reaction activity. The most prominent feature in confinement effect is the separation of catalyst on substrate molecules based on the size of the molecules, which is also called the physical constraint effect. The catalytic performance depends on a great extent on the precise match between the size/shape of confined space and substrate, while the other effects exist in confined space due to the nature of catalysts. The structural, physical, and chemical properties of zeolites, CNTs, and MOFs are summarized in Table 1. Albeit they all have some characteristics such as well crystallinity, high surface area, and uniform pores/channels, they differ in many significant aspects as discussed below.The confinement effect in the solid acid-base catalytic system has been substantially described and summarized by Iglesia [33][34][35][36][37][38] and Lercher et al. [39,40] Zeolite, as the most representative crystalline solid acid catalyst with periodically designed pore structure, has been widely applied in both fundamental and applied catalytic research. It exhibits diverse frameworks and tunable pore structures, [41,42] and features exceptional chemical and thermal stability, as well as unique Lewis and Brønsted acid sites. [43] Iglesia et al. suggested that the confined location of Brønsted acid sites in zeolites played an essential role in the catalytic consequences. [23] Furthermore, through extensive works, using zeolite as a host, they concluded

Selective hydrogenation of CO2 to methanol is a “two birds, one stone” technology to mitigate the greenhouse effect and solve the energy demand–supply deficit. Cu-based catalysts can effectively catalyze this reaction but suffer from low catalytic stability caused by the sintering of Cu species. Here, we report a series of zeolite-fixed catalysts Cu/ZnO x (Y)@Na-ZSM-5 (Y is the mass ratios of Cu/Zn in the catalysts) with core–shell structures to overcome this issue and strengthen the transformation. Fascinatingly, in this work, we first employed bimetallic metal–organic framework, CuZn-HKUST-1, nanoparticles (NPs) as a sacrificial agent to introduce ultrasmall Cu/ZnO x NPs (∼2 nm) into the crystalline particles of the Na-ZSM-5 zeolite via a hydrothermal synthesis method. The catalytic results showed that the optimized zeolite-encapsulated Cu/ZnO x (1.38)@Na-ZSM-5 catalyst exhibited the space time yield of methanol (STYMeOH) of 44.88 gMeOH·gCu –1·h–1, much more efficient than the supported Cu/ZnO x /Na-ZSM-5 catalyst (13.32 gMeOH·gCu –1·h–1) and industrial Cu/ZnO/Al2O3 catalyst (8.46 gMeOH·gCu –1·h–1) under identical conditions. Multiple studies demonstrated that the confinement in the zeolite formwork affords an intimate surrounding for the active phase to create synergies and avoid the separation of Cu–ZnO x interfaces, which results in an improved performance. More importantly, in the long-term test, the Cu/ZnO x (1.38)@Na-ZSM-5 catalyst exhibited constant STYMeOH with superior durability benefitted from its fixed structure. The current findings demonstrate the importance of confinement effects in designing highly efficient and stable methanol synthesis catalysts.

Heterogeneous catalytic processes produce the majority of the fuels and chemicals in the chemical industry and have kept improving the welfare of human beings for centuries. Although most of the heterogeneous catalytic reactions occur at the gas–solid interface, numerous cases have demonstrated that the condensed water near the active site and/or the aqueous phase merging the catalysts play positive roles in enhancing the performance of heterogeneous catalysts and creating novel catalytic conversion routes. We enumerate the traditional heterogeneous catalytic reactions that enable significant rate/selectivity promotion in the aqueous phase or adsorbed micro water environment and discuss the role of water in specific systems. Some of the novel heterogeneous reactions achieved with only the assistance of the aqueous phase have been summarized. The development of reactions with the participation of the aqueous phase/water and the investigation of the role of water in the heterogeneous catalytic reactions will open new horizons for catalysts with better activity, improved selectivity, and novel processes.

Aldol condensation and esterification reactions provide paths to upgrade ethanol and acetaldehyde to highervalue molecules useful as fuels or intermediates for the synthesis of polymers. Transition-metal-substituted BEA zeolites (M-BEA) catalyze these reactions; however, the mechanisms for these processes in M-BEA and the effects of incidental or purposefully included silanol groups are not reported. Here, we combine kinetic and spectroscopic measurements obtained during catalytic reactions of acetaldehyde (CH 3 CHO), ethanol (C 2 H 5 OH), and hydrogen (H 2 ) mixtures over a series of Ti-BEA catalysts that possess a known range of silanol group densities to examine the kinetic relevance of intervening steps and the impact of silanol groups on catalytic rates. Across all Ti-BEA, rates for aldol condensation and esterification increase with the pressure of CH 3 CHO; however, C 2 H 5 OH and H 2 O weakly inhibit the rates of these reactions. The substitution of CD 3 CDO for CH 3 CHO decreases aldol condensation rates slightly (∼10%) but leads to greater esterification rates (2-to 5-fold). The kinetic isotope effects together with the measured dependence of rates on reactant pressures suggest that aldol condensation and esterification occur on unoccupied Ti sites and involve multiple kinetically relevant steps. CH 3 CHO deprotonates irreversibly, and the kinetically relevant nucleophilic attack of the enolate to CH 3 CHO* (i.e., adsorbed CH 3 CHO on Ti sites) leads to aldol products, while the nucleophilic attack of the enolate to C 2 H 5 OH* gives esters. Selectivities toward aldol condensation increase with the ratio of CH 3 CHO to C 2 H 5 OH pressure and with increases in the silanol density of the as-synthesized Ti-BEA. During catalysis, in situ infrared spectroscopy demonstrates that these silanol groups react with C 2 H 5 OH to form ethoxysilane groups (i.e., SiOC 2 H 5 ) that modify the polarity of the environment near Ti active sites. As initial silanol densities increase, steady-state turnover rates for aldol condensation and esterification increase by factors of 5 and 2, respectively. The changes in rates and selectivities among Ti-BEA catalysts likely reflect changes in excess free energies of transition states for enolization and nucleophilic attack of the enolate to adsorbed coreactants. The differences in excess stability report on the interactions among reactive intermediates at framework Ti atoms and the ethoxysilane and remaining silanol groups present. The in situ modification of these pore environments confers changes in the stability of reactive species in a manner that contradicts intuition when considering the initial state of the catalyst but can be reconciled after accounting for the formation of persistent alkoxy surface moieties in the pores.

Reforming of methanol is one of the most favorable chemical processes for on-board H 2 production, which alleviates the limitation of H 2 storage and transportation. The most important catalytic systems for methanol reacting with water are interfacial catalysts including metal/metal oxide and metal/carbide. Nevertheless, the assessment on the reaction mechanism and active sites of these interfacial catalysts are still controversial. In this work, by spectroscopic, kinetic, and isotopic investigations, we established a compact cascade reaction model (ca. the Langmuir− Hinshelwood model) to describe the methanol and water activation over Pt/NiAl 2 O 4 . We show here that reforming of methanol experiences methanol dehydrogenation followed by water−gas shift reaction (WGS), in which two separated kinetically relevant steps have been identified, that is, C−H bond rupture within methoxyl adsorbed on interface sites and O−H bond rupture within O l H (O l : oxygen-filled surface vacancy), respectively. In addition, these two reactions were primarily determined by the most abundant surface intermediates, which were methoxyl and CO species adsorbed on NiAl 2 O 4 and Pt, respectively. More importantly, the excellent reaction performance benefits from the following bidirectional spillover of methoxyl and CO species since the interface and the vacancies on the support were considered as the real active component in methanol dehydrogenation and the WGS reaction, respectively. These findings provide deep insight into the reaction process as well as the active component during catalysis, which may guide the design of new catalytic systems.

Together photo‐ and thermal energy promote catalytic reactions in a synergetic way. However, how light cooperates with thermal energy is still unclear. Here, C−H bond rupture within HCOOH* was determined to be the rate‐determining step, with adsorbed CO* as the most abundant surface intermediate under both thermal and photothermal reaction conditions, as confirmed by kinetic isotopic effects and in‐situ FTIR characterizations. Clear evidence of kinetically relevant consistency was found under both thermal and photothermal HCOOH decomposition reactions over a Pd/LaCrO3/C3N4 composite. More information can be found in the Research Article by H. Zhang et al. (DOI: 10.1002/chem.202104623).