Regioselective hydroformylation of propene catalysed by rhodium-zeolite.

Hydroformylation is an industrial process for the production of aldehydes from alkenes1,2. Regioselective hydroformylation of propene to high-value n-butanal is particularly important, owing to a wide range of bulk applications of n-butanal in the manufacture of various necessities in human daily life3. Supported rhodium (Rh) hydroformylation catalysts, which often excel in catalyst recyclability, ease of separation and adaptability for continuous-flow processes, have been greatly exploited4. Nonetheless, they usually consist of rotationally flexible and sterically unconstrained Rh hydride dicarbonyl centres, only affording limited regioselectivity to n-butanal5-8. Here we show that proper encapsulation of Rh species comprising Rh(I)-gem-dicarbonyl centres within a MEL zeolite framework allows the breaking of the above model. The optimized catalyst exhibits more than 99% regioselectivity to n-butanal and more than 99% selectivity to aldehydes at a product formation turnover frequency (TOF) of 6,500 h-1, surpassing the performance of all heterogeneous and most homogeneous catalysts developed so far. Our comprehensive studies show that the zeolite framework can act as a scaffold to steer the reaction pathway of the intermediates confined in the space between the zeolite framework and Rh centres towards the exclusive formation of n-butanal.

Potential Dominates Structural Recombination of Single Atom Mn Sites for Promoting Oxygen Reduction Reaction.

Single atom sites (SAS) often undergo structural recombination in oxygen reduction reaction (ORR), while the effect of valence state and reconstruction on active centers needs to be investigated thoroughly. Herein, the Mn-SAS catalyst with uniform and precise Mn-N4 configuration is rationally designed. We utilize operando synchrotron radiation to track the dynamic evolution of active centers during ORR. Under the applied potential, the structural evolution of Mn-N4 into Mn-N3 C and further into Mn-N2 C2 configurations is clarified. Simultaneously, the valence states of Mn are increased from +3.0 to +3.8 and then decreased to +3.2. When the potential is removed, the catalyst returned to its initial Mn+3.0 -N4 configuration. Such successive evolutions optimize the electronic and geometric structures of active centers as evidenced by theory calculations. The evolved Mn+3.8 -N3 C and Mn+3.2 -N2 C2 configurations respectively adjust the O2 adsorption and reduce the energy barrier of rate-determining step. Thus, it can achieve an onset potential of 0.99 V, superior stability over 10,000 cycles, and a high turnover frequency of 1.59 s-1 at 0.85 VRHE. Our present work provides new insights into the construction of well-defined SAS catalysts by regulating the valence states and configurations of active centers.

Controlled Partial Linker Thermolysis in Metal-Organic Framework UiO-66-NH2 to Give a Single-Site Copper Photocatalyst for the Functionalization of Terminal Alkynes.

Metal-organic frameworks (MOFs) with expanding porosity and tailored pore environments are intriguing for catalytic applications. We report herein a straightforward method of controlled partial linker thermolysis to introduce desirable mesopores into mono-ligand MOFs, which is different from the classical thermolyzing method that starts from mixed-linker MOFs. UiO-66-NH2 , after partial ligand thermolysis, exhibits significant mesoporosity, retained crystal structure, improved charge photogeneration and abundant anchoring sites, which is ideal to explore single-site photocatalysis. Atomically dispersed Cu is then accommodated in the tailored pore. The resulting single-site Cu catalyst exhibits excellent performance for photocatalytic alkylation and oxidation coupling for the functionalization of terminal alkynes. The study highlights the advantage of controlled partial linker thermolysis to synthesize hierarchical MOFs to achieve the advanced single-site photocatalysis.

Palladium and Ruthenium Dual-Single-Atom Sites on Porous Ionic Polymers for Acetylene Dialkoxycarbonylation: Synergetic Effects Stabilize the Active Site and Increase CO Adsorption.

Heterogeneous single-metal-site catalysts usually suffer from poor stability, thereby limiting industrial applications. Dual Pd1 -Ru1 single-atom-sites supported on porous ionic polymers (Pd1 -Ru1 /PIPs) were constructed using a wetness impregnation method. The two isolated metal species in the form of a binuclear complex were immobilized on the cationic framework of PIPs through ionic bonds. Compared to the single Pd- or Ru-site catalyst, the dual single-atom system exhibits higher activity with 98 % acetylene conversion and near 100 % selectivity to dialkoxycarbonylation products, as well as better cycling stability for ten cycles without obvious decay. Based on DFT calculations, it was found that the single-Ru site exhibited a strong CO adsorption energy of -1.6 eV, leading to an increase in the local CO concentration of the catalyst. Notably, the Pd1 -Ru1 /PIPs catalyst had a much lower energy barrier of 2.49 eV compared to 3.87 eV of Pd1 /PIPs for the rate-determining step. The synergetic effect between neighboring single sites Pd1 and Ru1 not only enhanced the overall activity, but also stabilized PdII active sites. The discovery of synergetic effects between single sites can deepen our understanding of single-site catalysts at the molecular level.

Monolayer NiIr-Layered Double Hydroxide as a Long-Lived Efficient Oxygen Evolution Catalyst for Seawater Splitting.

Promoting the oxygen evolution reaction (OER) with saline water is highly desired to realize seawater splitting. This requires OER catalysts to resist serious corrosion and undesirable chloride oxidation. We introduce a 5d transition metal, Ir, to develop a monolayer NiIr-layered double hydroxide (NiIr-LDH) as the catalyst with enhanced OER performance for seawater splitting. The NiIr-LDH catalyst delivers 500 mA/cm2 at only 361 mV overpotential with ∼99% O2 Faradaic efficiency in alkaline seawater, which is more active than commercial IrO2 (763 mV, 23%) and the best known OER catalyst NiFe-LDH (530 mV, 92%). Moreover, it shows negligible activity loss at up to 650 h chronopotentiometry measurements at an industrial level (500 mA/cm2), while commercial IrO2 and NiFe-LDH rapidly deactivated within 0.2 and 10 h, respectively. The incorporation of Ir into the Ni(OH)2 layer greatly altered the electron density of Ir and Ni sites, which was revealed by X-ray absorption fine structure and density functional theory (DFT) calculations. Coupling the electrochemical measurements and in situ Raman spectrum with DFT calculations, we further confirm that the generation of rate-limiting intermediate *O and *OOH species was accelerated on Ni and Ir sites, respectively, which is responsible for the high seawater splitting performance. Our results also provide an opportunity to fabricate LDH materials containing 5d metals for applications beyond seawater splitting.

Iron Single Atoms-Assisted Cobalt Nitride Nanoparticles to Strengthen the Cycle Life of Rechargeable Zn-Air Battery.

The development of nonprecious metal catalysts with both oxygen reduction and evolution reactions (ORR/OER) is very important for Zn-air batteries (ZABs). Herein, a Co5.47 N particles and Fe single atoms co-doped hollow carbon nanofiber self-supporting membrane (H-CoFe@NCNF) is synthesized by a coaxial electrospinning strategy combined with pyrolysis. X-ray absorption fine spectroscopy analyses confirm the state of the cobalt nitride and Fe single atoms. As a result, H-CoFe@NCNF exhibits a superior bifunctional performance of Eonset  = 0.96 V for ORR, and Ej = 10 = 1.68 V for OER. Density functional theory calculations show that H-CoFe@NCNF has a moderate binding strength to oxygen due to the coexistence of nanoparticle and single atoms. Meanwhile, the Co site is more favorable to the OER, while the Fe site facilitates the ORR, and the proton and charge transfer between N and metal atoms further lower the reaction barriers. The liquid ZAB composed of H-CoFe@NCNF has a charge-discharge performance of ≈1100 h and a peak power density of 205 mW cm-2 . The quasi-solid-state ZAB assembled by the self-supporting membrane of H-CoFe@NCNF is proven to operate stably in any bending condition.

Tandem Catalysis for Selective Oxidation of Methane to Oxygenates Using Oxygen over PdCu/Zeolite.

Selective oxidation of methane to oxygenates with O2 under mild conditions remains a great challenge. Here we report a ZSM-5 (Z-5) supported PdCu bimetallic catalyst (PdCu/Z-5) for methane conversion to oxygenates by reacting with O2 in the presence of H2 at low temperature (120 °C). Benefiting from the co-existence of PdO nanoparticles and Cu single atoms via tandem catalysis, the PdCu/Z-5 catalyst exhibited a high oxygenates yield of 1178 mmol g-1 Pd h-1 (mmol of oxygenates per gram Pd per hour) and at the same time high oxygenates selectivity of up to 95 %. Control experiments and mechanistic studies revealed that PdO nanoparticles promoted the in situ generation of H2 O2 from O2 and H2 , while Cu single atoms not only accelerated the activation of H2 O2 for the generation of abundant hydroxyl radicals (⋅OH) from H2 O2 decomposition, but also enabled the homolytic cleavage of CH4 by ⋅OH to methyl radicals (⋅CH3 ). Subsequently, the ⋅OH reacted quickly with the ⋅CH3 to form CH3 OH with high selectivity.

In Situ Spectroscopic Characterization and Theoretical Calculations Identify Partially Reduced ZnO1-x /Cu Interfaces for Methanol Synthesis from CO2.

The active site of the industrial Cu/ZnO/Al2 O3 catalyst used in CO2 hydrogenation to methanol has been debated for decades. Grand challenges remain in the characterization of structure, composition, and chemical state, both microscopically and spectroscopically, and complete theoretical calculations are limited when it comes to describing the intrinsic activity of the catalyst over the diverse range of structures that emerge under realistic conditions. Here a series of inverse model catalysts of ZnO on copper hydroxide were prepared where the size of ZnO was precisely tuned from atomically dispersed species to nanoparticles using atomic layer deposition. ZnO decoration boosted methanol formation to a rate of 877 gMeOH kgcat -1 h-1 with ≈80 % selectivity at 493 K. High pressure in situ X-ray absorption spectroscopy demonstrated that the atomically dispersed ZnO species are prone to aggregate at oxygen-deficient ZnO ensembles instead of forming CuZn metal alloys. By modeling various potential active structures, density functional theory calculations and microkinetic simulations revealed that ZnO/Cu interfaces with oxygen vacancies, rather than stoichiometric interfaces, Cu and CuZn alloys were essential to catalytic activation.

Significantly Enhanced Overall Water Splitting Performance by Partial Oxidation of Ir through Au Modification in Core-Shell Alloy Structure.

Developing efficient bifunctional electrocatalysts for overall water splitting in acidic conditions is the essential step for proton exchange membrane water electrolyzers (PEMWEs). We first report the synthesis of core-shell structure nanoparticles (NPs) with an Au core and an AuIr2 alloy shell (Au@AuIr2). Au@AuIr2 displayed 4.6 (5.6) times higher intrinsic (mass) activity toward the oxygen evolution reaction (OER) than a commercial Ir catalyst. Furthermore, it showed hydrogen evolution reaction (HER) catalytic properties comparable to those of commercial Pt/C. Significantly, when Au@AuIr2 was used as both the anode and cathode catalyst, the overall water splitting cell achieved 10 mA/cm2 with a low cell voltage of 1.55 V and maintained this activity for more than 40 h, which greatly outperformed the commercial couples (Ir/C||Pt/C, 1.63 V, activity decreased within minutes) and is among the most efficient bifunctional catalysts reported. Theoretical calculations coupled with X-ray-based structural analyses suggest that partially oxidized surfaces originating from the electronic interaction between Au and Ir provide a balance for different intermediates binding and realize significantly enhanced OER performance.

Operando Cooperated Catalytic Mechanism of Atomically Dispersed Cu-N4 and Zn-N4 for Promoting Oxygen Reduction Reaction.

Dual-metal single-atom catalysts exhibit superior performance for oxygen reduction reaction (ORR), however, the synergistic catalytic mechanism is not deeply understood. Herein, we report a dual-metal single-atom catalyst consisted of Cu-N4 and Zn-N4 on the N-doped carbon support (Cu/Zn-NC). It exhibits high-efficiency ORR activity with an Eonset of 0.98 V and an E1/2 of 0.83 V, excellent stability (no degradation after 10 000 cycles), surpassing state-of-the-art Pt/C and great mass of Pt-free single atom catalysts. Operando XANES demonstrates that the Cu-N4 as active center experiences the change from atomic dispersion to cluster with the cooperation of Zn-N4 during ORR process, and then turns to single atom state again after reaction. DFT calculation further indicates that the adjustment effect of Zn on the d-orbital electron distribution of Cu could benefit to the stretch and cleavage of O-O on Cu active center, speeding up the process of rate determining step of OOH*.

In situ XAFS study on the formation process of cobalt carbide by Fischer-Tropsch reaction.

Fischer-Tropsch (F-T) synthesis is an effective approach to convert the syngas of H2 and CO into lower olefin and other valuable products for the chemical industry. Cobalt carbide (Co2C), which was regarded as the sign of activity loss in the past, has recently been recognized as a highly-active phase for F-T synthesis. However, systematic study on the formation process of Co2C by F-T reaction is still lacking. Herein, for the first time, in situ XAFS (X-ray Absorption Fine Structure) experiments were conducted to elaborate the Co2C formation under operando conditions. F-T reaction processes starting from Co and CoO were analysed with the conclusion that Co2C could be formed under both conditions. For the CoO process, Co2C was transformed directly from CoO as a wavelet transform and EXAFS fitting results revealed that there was no sign of Co metal in the whole process. Thermodynamic analysis indicated that the ΔG value of the CoO process is much smaller than that of the Co process, which means that CoO is thermodynamically easier to transform to Co2C. Combining with the shorter reduction time from Co3O4 to CoO, it can be concluded that CoO is more favourable as the precursor to synthesize Co2C, which might be applied to the F-T industry. Besides, catalytic evaluation shows that the CO2 selectivity, CO conversion and the ratio of olefin/paraffin for the CoO process are different from those of the Co process. In addition, the reaction temperatures were also investigated wherein Co2C would be partially transformed to metallic Co when the temperature was increased up to 270 °C. This work provides fundamental and applicable guidance towards the synthesis of Co2C by F-T reaction.

High-Efficient, Stable Electrocatalytic Hydrogen Evolution in Acid Media by Amorphous Fex P Coating Fe2 N Supported on Reduced Graphene Oxide.

Development of efficient and durable non-Pt catalysts for hydrogen evolution reaction (HER) in acid media is highly desirable. Iron nitride has emerged as a promising catalyst for its cost-effective nature, but the corresponding acidic stability must be promoted. Herein, phosphorus-decorated Fe2 N and reduced graphene oxide (P-Fe2 N/rGO) composite are designed and synthesized. X-ray photoelectron spectroscopy and X-ray absorption fine structure (XAFS) show that a thin layer amorphous iron phosphide is coated on the surface of Fe2 N nanoparticles, which could be responsible for the well resistance of chemical corrosion in acidic media. Meanwhile, the P-decoration could tune the electronic state and coordination environment of iron atom as evidenced by XAFS, resulting in dramatically enhanced electrocatalytic activity of P-Fe2 N/rGO. Density functional theory calculations reveal that both the P-connected N atoms and the Fe atoms in P-Fe2 N/rGO catalyst are the main active sites for H* adsorption. The hydrogen-binding free energy |ΔGH* | value is close to zero for P-Fe2 N/rGO, suggesting a good balance between the Volmer and Heyrovsky/Tafel steps in HER kinetics. As expected, P-Fe2 N/rGO catalyst could achieve a low ηonset of 22.4 mV, a small Tafel plot of 48.7 mV dec-1 , and remarkable stability for HER in acid electrolyte.

A Stable Bifunctional Catalyst for Rechargeable Zinc-Air Batteries: Iron-Cobalt Nanoparticles Embedded in a Nitrogen-Doped 3D Carbon Matrix.

Low-cost, efficient bifunctional electrocatalysts are needed to mediate the oxygen reduction and oxygen evolution reactions (ORR/OER) in Zn-air batteries. Such catalysts should offer binary active sites and an ability to transfer oxygen-based species and electrons. A 3D catalyst, composed of nanoparticles of CoFe alloy embedded in N-doped carbon nanotubes tangled with reduced graphene oxide, was developed, which presents appreciable ORR/OER activity when applied in a Zn-air battery. A high open-circuit voltage of 1.43 V, a stable discharge voltage of 1.22 V, a high energy efficiency of 60.1 %, and excellent stability after 1 600 cycles at 10 mA cm-2 are demonstrated. An all-solid-state battery had an outstanding lifetime and high cell efficiency even upon bending. In situ X-ray absorption spectroscopy revealed that OOH* and O* intermediates induce variations in the Fe-Fe and Co-Co bond lengths, respectively, suggesting that Fe and Co species are crucial to the ORR/OER processes.

Heat treated carbon supported iron(ii)phthalocyanine oxygen reduction catalysts: elucidation of the structure-activity relationship using X-ray absorption spectroscopy.

This paper focuses on studying the influence of the heat treatment on the structure and activity of carbon supported Fe(ii)phthalocyanine (FePc/C) oxygen reduction reaction (ORR) catalysts under alkaline conditions. The FePc macrocycle was deposited onto ketjen black carbon and heated treated for 2 hours under inert atmosphere (Ar) at different temperatures (400, 500, 600, 700, 800, 900 and 1000 °C). The atomic structure of Fe in each sample has been determined by XAS and correlated to the activity and ORR mechanisms determined in electrochemical half cells and in a complete H2/O2 anion exchange membrane fuel cells (AEM-FC). The results show that the samples prepared at 600 and 700 °C have the highest electrochemical catalytic activity for the ORR, consistent with the findings that the FeN4 active sites are thermally stable up to 700 °C, confirmed by both XANES linear combination fittings and EXAFS fittings. Upon annealing at temperatures above 800 °C, the FeN4 structure partially decomposes to small iron nanoparticles. The transition from the FeN4 structure to metallic Fe results in a significant loss in ORR activity and an increase in the production of undesirable HO2- during catalysis.

Three-dimensional Fe2 N@C microspheres grown on reduced graphite oxide for lithium-ion batteries and the Li storage mechanism.

Nanostructured iron compounds as lithium-ion-battery anode material have attracted considerable attention with respect to improved electrochemical energy storage and excellent specific capacity, so lots of iron-based composites have been developed. Herein, a novel composite composed of three-dimensional Fe2 N@C microspheres grown on reduced graphite oxide (denoted as Fe2 N@C-RGO) has been synthesized through a simple and effective technique assisted by a hydrothermal and subsequent heating treatment process. As the anode material for lithium-ion batteries, the synthetic Fe2 N@C-RGO displayed excellent Li(+) -ion storage performance with a considerable initial capacity of 847 mAh g(-1) , a superior cycle stability (a specific discharge capacity of 760 mAh g(-1) remained after the 100th cycle), and an improved rate-capability performance compared with those of the pure Fe2 N and Fe2 N-RGO nanostructures. The good performance should be attributed to the existence of RGO layers that can facilitate to enhance the conductivity and shorten the lithium-ion diffusion path; in addition, the carbon layer on the surface of Fe2 N can avert the structure decay caused by the volume change during the lithiation/delithiation process. Moreover, in situ X-ray absorption fine-structure analysis demonstrated that the excellent performance can be attributed to the lack of any obvious change in the coordination geometry of Fe2 N@C-RGO during the charge/discharge processes.

Modulating the Electronic Structure of Cobalt-Vanadium Bimetal Catalysts for High-Stable Anion Exchange Membrane Water Electrolyzer.

Modulating the electronic structure of catalysts to effectively couple the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is essential for developing high-efficiency anion exchange membrane water electrolyzer (AEMWE). Herein, a coral-like nanoarray composed of nanosheets through the synergistic layering effect of cobalt and the 1D guiding of vanadium is synthesized, which promotes extensive contact between the active sites and electrolyte. The HER and OER activities can be enhanced by modulating the electronic structure through nitridation and phosphorization, respectively, enhancing the strength of metal-H bond to optimize hydrogen adsorption and facilitating the proton transfer to improve the transformation of oxygen-containing intermediates. Resultantly, the AEMWE achieves a current density of 500 mA cm-2 at 1.76 V for 1000 h in 1.0 M KOH at 70 °C. The energy consumption is 4.21 kWh Nm-3 with the producing hydrogen cost of $0.93 per kg H2. Operando synchrotron radiation and Bode phase angle analyses reveal that during the high-energy consumed OER, the dissolution of vanadium species transforms distorted Co-O octahedral into regular octahedral structures, accompanied by a shortening of the Co-Co bond length. This structural evolution facilitates the formation of oxygen intermediates, thus accelerating the reaction kinetics.

Sub-10-nm-sized Au@AuxIr1-x metal-core/alloy-shell nanoparticles as highly durable catalysts for acidic water splitting.

The absence of efficient and durable catalysts for oxygen evolution reaction (OER) is the main obstacle to hydrogen production through water splitting in an acidic electrolyte. Here, we report a controllable synthesis method of surface IrOx with changing Au/Ir compositions by constructing a range of sub-10-nm-sized core-shell nanocatalysts composed of an Au core and AuxIr1-x alloy shell. In particular, Au@Au0.43Ir0.57 exhibits 4.5 times higher intrinsic OER activity than that of the commercial Ir/C. Synchrotron X-ray-based spectroscopies, electron microscopy and density functional theory calculations revealed a balanced binding of reaction intermediates with enhanced activity. The water-splitting cell using a load of 0.02 mgIr/cm2 of Au@Au0.43Ir0.57 as both anode and cathode can reach 10 mA/cm2 at 1.52 V and maintain activity for at least 194 h, which is better than the cell using the commercial couple Ir/C‖Pt/C (1.63 V, 0.2 h).

Engineering ZrO2-Ru interface to boost Fischer-Tropsch synthesis to olefins.

Understanding the structures and reaction mechanisms of interfacial active sites in the Fisher-Tropsch synthesis reaction is highly desirable but challenging. Herein, we show that the ZrO2-Ru interface could be engineered by loading the ZrO2 promoter onto silica-supported Ru nanoparticles (ZrRu/SiO2), achieving 7.6 times higher intrinsic activity and ~45% reduction in the apparent activation energy compared with the unpromoted Ru/SiO2 catalyst. Various characterizations and theoretical calculations reveal that the highly dispersed ZrO2 promoter strongly binds the Ru nanoparticles to form the Zr-O-Ru interfacial structure, which strengthens the hydrogen spillover effect and serves as a reservoir for active H species by forming Zr-OH* species. In particular, the formation of the Zr-O-Ru interface and presence of the hydroxyl species alter the H-assisted CO dissociation route from the formyl (HCO*) pathway to the hydroxy-methylidyne (COH*) pathway, significantly lowering the energy barrier of rate-limiting CO dissociation step and greatly increasing the reactivity. This investigation deepens our understanding of the metal-promoter interaction, and provides an effective strategy to design efficient industrial Fisher-Tropsch synthesis catalysts.

Modulating Ni-S coordination in Ni3S2 to promote electrocatalytic oxidation of 5-hydroxymethylfurfural at ampere-level current density.

Electricity-driven oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a highly attractive strategy for biomass transformation. However, achieving industrial-grade current densities remains a great challenge. Herein, by modulating the water content in a solvothermal system, Ni3S2/NF with stabilized and shorter Ni-S bonds as well as a tunable coordination environment of Ni sites was fabricated. The prepared Ni3S2/NF was highly efficient for electrocatalytic oxidation of HMF to produce FDCA, and the FDCA yield and Faraday efficiency could reach 98.8% and 97.6% at the HMF complete conversion. More importantly, an industrial-grade current density of 1000 mA cm-2 could be achieved at a potential of only 1.45 V vs. RHE for HMFOR and the current density could exceed 500 mA cm-2 with other bio-based compounds as the reactants. The excellent performance of Ni3S2/NF originated from the shorter Ni-S bonds and its better electrochemical properties, which significantly promoted the dehydrogenation step of oxidizing HMF. Besides, the gram-scale FDCA production could be realized on Ni3S2/NF in a MEA reactor. This work provides a robust electrocatalyst with high potential for practical applications for the electrocatalytic oxidation of biomass-derived compounds.

Water-participated mild oxidation of ethane to acetaldehyde.

The direct conversion of low alkane such as ethane into high-value-added chemicals has remained a great challenge since the development of natural gas utilization. Herein, we achieve an efficient one-step conversion of ethane to C2 oxygenates on a Rh1/AC-SNI catalyst under a mild condition, which delivers a turnover frequency as high as 158.5 h-1. 18O isotope-GC-MS shows that the formation of ethanol and acetaldehyde follows two distinct pathways, where oxygen and water directly participate in the formation of ethanol and acetaldehyde, respectively. In situ formed intermediate species of oxygen radicals, hydroxyl radicals, vinyl groups, and ethyl groups are captured by laser desorption ionization/time of flight mass spectrometer. Density functional theory calculation shows that the activation barrier of the rate-determining step for acetaldehyde formation is much lower than that of ethanol, leading to the higher selectivity of acetaldehyde in all the products.