Influence of Support Acidity of Pt/Nb2O5 Catalysts on Selectivity of CO2 Hydrogenation

Operando Mobile Catalysis for Reverse Water Gas Shift Reaction

Metal atoms on the support serve as active sites for many heterogeneous catalysts. However, the active metal sites on the support are conventionally described as static, and the intermediates adsorbed on the support far away from the active metal sites cannot be transformed. Herein, we report the first example of operando mobile catalysis to promote catalytic efficiency by enhancing the collision probability between active sites and reactants or reaction intermediates. Specifically, ligand-coordinated Pt single atoms (isolated MeCpPt- species) are bonded on CeO2 and transformed into mobile MeCpPt(H)CO complexes during the reverse water gas shift reaction for operando mobile catalysis. This strategy enables the conversion of inert carbonate intermediates on the CeO2 support. A turnover frequency (TOF) of 6358 mol CO2  molPt -1  ⋅ h-1 and 99 % CO selectivity at 300 °C is obtained for reverse water gas shift reaction, dramatically higher than those of Pt catalysts reported in the literature. Operando mobile catalysis presents a promising strategy for designing high-efficiency heterogeneous catalysts for various chemical reactions and applications.

Probing the Gold/Water Interface with Surface-Specific Spectroscopy

Water is an integral component in electrochemistry, in the generation of the electric double layer, and in the propagation of the interfacial electric fields into the solution; however, probing the molecular-level structure of interfacial water near functioning electrode surfaces remains challenging. Due to the surface-specificity, sum-frequency-generation (SFG) spectroscopy offers an opportunity to investigate the structure of water near working electrochemical interfaces but probing the hydrogen-bonded structure of water at this buried electrode-electrolyte interface was thought to be impossible. Propagating the laser beams through the solvent leads to a large attenuation of the infrared light due to the absorption of water, and interrogating the interface by sending the laser beams through the electrode normally obscures the SFG spectra due to the large nonlinear response of conduction band electrons. Here, we show that the latter limitation is removed when the gold layer is thin. To demonstrate this, we prepared Au gradient films on CaF₂ with a thickness between 0 and 8 nm. SFG spectra of the Au gradient films in contact with H₂O and D₂O demonstrate that resonant water SFG spectra can be obtained using Au films with a thickness of ∼2 nm or less. The measured spectra are distinctively different from the frequency-dependent Fresnel factors of the interface, suggesting that the features we observe in the OH stretching region indeed do not arise from the nonresonant response of the Au films. With the newfound ability to probe interfacial solvent structure at electrode/aqueous interfaces, we hope to provide insights into more efficient electrolyte composition and electrode design.

Design of Binary Nb2O5-SiO2 Self-Standing Monoliths Bearing Hierarchical Porosity and Their Efficient Friedel-Crafts Alkylation/Acylation Catalytic Properties

Alkylation of aromatic hydrocarbons is among the most industrially important reactions, employing acid catalysts such as AlCl₃, H₂SO₄, HF, or H₃PO₄. However, these catalysts present severe drawbacks, such as low selectivity and high corrosiveness. Taking advantage of the intrinsic high acid strength and Lewis and Brønsted acidity of niobium oxide, we have designed the first series of Nb₂O₅-SiO₂(HIPE) monolithic catalysts bearing multiscale porosity through the integration of a sol-gel process and the physical chemistry of complex fluids. The MUB-105 series offers efficient solvent-free heterogeneous catalysis toward Friedel-Crafts monoalkylation and -acylation reactions, where 100% conversion has been reached at 140 °C while cycling. Alkylation reactions employing the MUB-105(1) catalyst have a maximum turnover number (TON) of 104 and a turnover frequency (TOF) of 9 h^-1, whereas for acylation, MUB-105(1) and MUB-105(2) yield maximum TON and TOF values of 107 and 11 h^-1, respectively. Moreover, the catalysts are selective, producing equal amounts of ortho- and para-substituted alkylated products and greater than 90% of the para-substituted acylated product. The highest catalytic efficiencies are obtained for the MUB-105(1) catalyst, bearing the smallest Nb₂O₅ particle sizes, lowest Nb₂O₅ content, and the highest amorphous character. The catalysts presented here are in a monolithic self-standing state, offering easy handling, reusability, and separation from the final products.

Hydrogenation of Carbon Dioxide to Methanol over Non-Cu-based Heterogeneous Catalysts

The increasing atmospheric CO₂ level makes CO₂ reduction an urgent challenge facing the world. Catalytic transformation of CO₂ into chemicals and fuels utilizing renewable energy is one of the promising approaches toward alleviating CO₂ emissions. In particular, the selective hydrogenation of CO₂ to methanol utilizing renewable hydrogen potentially enables large scale transformation of CO₂ . The Cu-based catalysts have been extensively investigated in CO₂ hydrogenation. However, it is not only limited by long-term instability but also displays unsatisfactory catalytic performance. The supported metal-based catalysts (Pd, Pt, Au, and Ag) can achieve high methanol selectivity at low temperatures. The mixed oxide catalysts represented by M_a ZrO_x (M_a =Zn, Ga, and Cd) solid solution catalysts present high methanol selectivity and catalytic activity as well as excellent stability. This Review focuses on the recent advances in developing Non-Cu-based heterogeneous catalysts and current understandings of catalyst design and catalytic performance. First, the thermodynamics of CO₂ hydrogenation to methanol is discussed. Then, the progress in supported metal-based catalysts, bimetallic alloys or intermetallic compounds catalysts, and mixed oxide catalysts is discussed. Finally, a summary and a perspective are presented.

Lanthanum-Modified MCF-Derived Nickel Phyllosilicate Catalyst for Enhanced CO2 Methanation: A Comprehensive Study

For the traditional preparation method, it is challenging to fabricate a supported nickel catalyst with fine size at high loading. In this work, a group of La-modified mesostructured cellular foam (MCF)-derived nickel phyllosilicates was designed and synthetized by a hydrothermal method followed by an impregnation-modification of La₂O₃, whose Ni contents varied from 25.3 to 32.2 wt %. Both the special property of phyllosilicate and the addition of a La₂O₃ modifier played significant roles in achieving high Ni dispersion and excellent catalytic performance. The formed nickel phyllosilicate was beneficial to obtain small Ni nanoparticles (<5 nm) due to its strong metal-support interaction and high specific surface area; the addition of the La₂O₃ modifier could further reduce the Ni particle size and decrease the reduction difficulty of the fabricated samples. On the contrary, a large Ni particle size of 13.0 nm was observed on the impregnated Ni/MCF (N/M-Im) catalyst with a Ni content of 31.7 wt %. As a result, the nickel phyllosilicate catalyst showed higher catalytic activity than the impregnated one, and the La modifier could further improve the catalytic activity especially at low temperature (<400 °C). Among all catalysts, the modified phyllosilicate catalyst N/M-P-32-5L with 180 °C-32 h-hydrothermal treatment and La₂O₃ content of 5 wt % was the best owing to its small-sized Ni particles, high H₂ and CO₂ chemisorption capacity, large turnover frequency (TOF) value, and low activation energy of 69.83 kJ mol^-1. In addition, the intermediates of formate and CO were detected through in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis. In a 100 h-lifetime test under harsh conditions and 600 °C-steam treatment, N/M-P-32-5L showed both high sintering resistance of Ni particles and high thermal stability without the collapse of pores as well as decrease of catalytic activity, which was attributed to the special physical and chemical properties of MCF-derived nickel phyllosilicate, strong metal-support interaction over the catalyst, and the promotion of the La₂O₃ modifier.

Insight into the effects of electronegativity on the H2 catalytic activation for CO2 hydrogenation: four transition metal cases from a DFT study

Electronegativity of transition metal dominates the type of H species, which has an important effect on the path choice of CO₂ hydrogenation.