Identifying Descriptors for Promoted Rhodium-Based Catalysts for Higher Alcohol Synthesis <i>via</i> Machine Learning

Rhodium-based catalysts offer remarkable selectivities toward higher alcohols, specifically ethanol, via syngas conversion. However, the addition of metal promoters is required to increase reactivity, augmenting the complexity of the system. Herein, we present an interpretable machine learning (ML) approach to predict and rationalize the performance of Rh-Mn-P/SiO2 catalysts (P = 19 promoters) using the open-source dataset on Rh-catalyzed higher alcohol synthesis (HAS) from Pacific Northwest National Laboratory (PNNL). A random forest model trained on this dataset comprising 19 alkali, transition, post-transition metals, and metalloid promoters, using catalytic descriptors and reaction conditions, predicts the higher alcohols space-time yield (STYHA) with an accuracy of R2 = 0.76. The promoter’s cohesive energy and alloy formation energy with Rh are revealed as significant descriptors during posterior feature-importance analysis. Their interplay is captured as a dimensionless property, coined promoter affinity index (PAI), which exhibits volcano correlations for space-time yield. Based on this descriptor, we develop guidelines for the rational selection of promoters in designing improved Rh-Mn-P/SiO2 catalysts. This study highlights ML as a tool for computational screening and performance prediction of unseen catalysts and simultaneously draws insights into the property–performance relations of complex catalytic systems

Building Blocks for High Performance in Electrocatalytic CO₂ Reduction: Materials, Optimization Strategies, and Device Engineering

In recent years, screening of materials has yielded large gains in catalytic performance for the electroreduction of CO₂. However, the diversity of approaches and a still immature mechanistic understanding make it challenging to assess the real potential of each concept. In addition, achieving high performance in CO₂ (photo)­electrolyzers requires not only favorable electrokinetics but also precise device engineering. In this Perspective, we analyze a broad set of literature reports to construct a set of design–performance maps that suggest patterns between performance figures and different classes of materials and optimization strategies. These maps facilitate the screening of different approaches to electrocatalyst design and the identification of promising avenues for future developments. At the device level, analysis of the network of limiting phenomena in (photo)­electrochemical cells leads us to propose a straightforward performance metric based on the concepts of maximum energy efficiency and maximum product formation rate, enabling the comparison of different technologies

Catalytic Oxychlorination versus Oxybromination for Methane Functionalization

The catalytic oxyhalogenation is an attractive route for the functionalization of methane in a single step. This study investigates methane oxychlorination (MOC) and oxybromination (MOB) under a wide range of conditions over various materials having different oxidation properties to assess the effect of hydrogen halide (HX, X = Cl, Br) on the catalyst performance. The oxyhalogenation activity of the catalysts, ranked as RuO₂ > Cu–K–La–X > CeO₂ > VPO > TiO₂ > FePO₄, is correlated with their ability to oxidize the hydrogen halide and the gas-phase reactivity of the halogen with methane. The product distribution is found to be strongly dependent on the nature of the catalyst and the type of halogen. The least reducible FePO₄ exhibits a marked propensity to halomethanes (CH₃X, CH₂X₂), and the strongly oxidizing RuO₂ favors combustion in both reactions, while other systems reveal stark selectivity differences between MOC and MOB. VPO and TiO₂ lead to a selective CH₃Br production in MOB and pronounced CO formation in MOC, whereby product distribution was only slightly affected by the variation of the HX concentration. In contrast, CeO₂ and Cu-based catalysts provide a high selectivity to CH₃Cl but give rise to a marked CO₂ formation when HBr is used as a halogen source. The behavior of the latter systems is explained by the higher energy of the metal–Cl bond in comparison to the metal–Br bond, enabling more suppression of the unwanted CO and CO₂ formation when HCl is used, as also inferred from the more pronounced performance dependence on the HX content in the feed. Extrapolating this result, the highest reported yields of chloromethanes (28% at >82% selectivity) and bromomethanes (20% at >98% selectivity) are attained over CeO₂, by adjusting the feed HX content to curb the CO₂ generation. A vis-à-vis comparison of MOC and MOB presented for the first time in this study deepens the understanding of halogen-mediated methane functionalization as a key step toward the design of an oxyhalogenation process

Design of Base Zeolite Catalysts by Alkali-Metal Grafting in Alcoholic Media

This study investigates the synthesis of base catalysts through the postsynthetic grafting of alkali cations (Li, Na, K, Rb, or Cs) onto USY zeolites in alcoholic solutions of the corresponding metal hydroxides. In contrast to previous studies conducted in aqueous media, the utilization of alcohols (MeOH, EtOH, or <i>i</i>PrOH) offers increased control over the metalation process while simultaneously averting degradation and dissolution of the crystalline framework. The achievement of close to an atomic dispersion of the alkali metals in the zeolite is confirmed by in-depth characterization combining ²³Na MAS NMR, microscopy, elemental mapping, and CO₂ chemisorption. Both the size of the alkali cation and the carbon number of the alcohol influence the incorporation efficiency, which is found to correlate with the expected basic strength of the corresponding metal alkoxide. The presence of framework aluminum results in higher sodium loadings due to the parallel incorporation by both ion exchange and metalation. However, cations exchanged to the Al sites do not provide the distinct basic properties of the grafted metals, and thus, high-silica zeolites attain the highest basicity. Catalytic testing in the self-condensation of propanal, a model reaction for the deoxygenation of bio-oil, demonstrates excellent activity, with a > 90% selectivity to the aldol reaction and a stable performance. The selective character arising from the distinctive strength coupled with the isolated nature and high accessibility of the grafted basic sites holds a large potential for the development of superior zeolite catalysts for base-catalyzed applications

Design of Hierarchical Zeolite Catalysts for the Manufacture of Polyurethane Intermediates

This study undertakes the design of hierarchically structured zeolites for the synthesis of methylenedianiline (MDA) mixtures via the liquid-phase condensation of aniline with formaldehyde. Affordable acid and base treatments enabled the controlled generation of an auxiliary network of intracrystalline mesopores within commercial zeolites of different frameworks, among which the FAU topology is identified as superior. As the micropore structure of the zeolite can be preserved, the unique shape selectivity is retained while improved mass transport in the hierarchical crystals leads to a 7-fold increased activity and a 3-fold prolonged lifetime before thermal regeneration is necessary. The linear correlation between the MDA yield and the product of the Brønsted acidity and the external surface area emphasizes the need to balance acid site and pore quality in the design of an optimal catalyst. Hierarchical USY zeolites outperform the state-of-the-art delaminated zeolites and mesoporous aluminosilicates. These results offer a viable solution for the replacement of the industrially applied homogeneous catalyst (HCl), resulting in an efficient and environmental friendly technology for the production of polyurethanes

Selective Methane Oxybromination over Nanostructured Ceria Catalysts

This article studies the impact of the carrier type (MgO, SiO₂, SiC, Al₂O₃, and ZrO₂) and synthesis method (dry impregnation, coprecipitation, hydrothermal synthesis, and mechanochemical synthesis) on the structure, redox properties, and performance of supported CeO₂ catalysts for methane oxybromination. Major distinctions are evidenced in the product distribution with respect to bulk CeO₂, and the selectivity to methyl bromide (CH₃Br) varies in the following order: CeO₂/MgO (61–81%) > CeO₂/SiC (56–73%) ≈ CeO₂/SiO₂ (52–71%) > bulk CeO₂ (40%) ≈ CeO₂/ZrO₂ > CeO₂/Al₂O₃ (28–35%) at 6–40% methane conversion. The selectivity is primarily governed by the catalyst propensity to combust CH₃Br and byproduct dibromomethane (CH₂Br₂), which is strongly affected by the choice of the carrier. Specifically, the formation of carbon oxides is substantially suppressed over CeO₂ nanoparticles stabilized on basic MgO (CO_<i>x</i> selectivity <10%) with respect to bulk CeO₂ (CO_<i>x</i> selectivity ≥50%), whereas it is promoted by near atomic dispersions of CeO₂ on acidic ZrO₂ and Al₂O₃ carriers. Changes in the size and shape of CeO₂ nanoparticles on MgO had little impact on the selectivity, as they exhibit similar oxidation properties after exposure to the reaction environment. The synthesis–structure–performance relationships developed demonstrate the great potential of supporting CeO₂ to enhance the oxybromination performance

Molecular-Level Understanding of CeO₂ as a Catalyst for Partial Alkyne Hydrogenation

The unique catalytic properties of ceria for the partial hydrogenation of alkynes are examined for acetylene hydrogenation. Catalytic tests over polycrystalline CeO₂ at different temperatures and H₂/C₂H₂ ratios reveal ethylene selectivities in the range of 75–85% at high degrees of acetylene conversion and hint at the crucial role of hydrogen dissociation on the overall process. Density-functional theory is applied to CeO₂(111) in order to investigate reaction intermediates and to calculate the enthalpy and energy barrier for each elementary step, taking into account different adsorption geometries and the presence of potential isomers of the intermediates. At a high hydrogen coverage, β-C₂H₂ radicals adsorbed on-top of surface oxygen atoms are the initial reactive species forming C₂H₃ species effectively barrierless. The high alkene selectivity is owed to the lower activation barrier for subsequent hydrogenation leading to gas-phase C₂H₄ compared to that for the formation of β-C₂H₄ radical species. Moreover, hydrogenation of C₂H₅ species, if formed, must overcome significantly large barriers. Oligomers are the most important byproduct of the reaction and they result from the recombination of chemisorbed C₂H_<i>x</i> species. These findings rationalize for the first time the applicability of CeO₂ as a catalyst for olefin production and potentially broaden its use for the hydrogenation of polyunsaturated and polyfunctionalized substrates containing triple bonds

Hierarchy Brings Function: Mesoporous Clinoptilolite and L Zeolite Catalysts Synthesized by Tandem Acid–Base Treatments

Hierarchical clinoptilolite and L zeolites are prepared using optimized tandem dealumination–desilication treatments. The main challenge in the postsynthetic modification of these zeolites is the high Al content, requiring a tailored dealumination prior to the desilication step. For natural clinoptilolite, sequential acid treatments using aqueous HCl solutions were applied, while for L a controlled dealumination using ammonium hexafluorosilicate is required. Subsequent desilication by NaOH treatment yields mesopore surfaces of up to 4-fold (clinoptilolite, 64 m² g^–1; L, 135 m² g^–1) relative to the parent zeolite (clinoptilolite, 15 m² g^–1; L, 45 m² g^–1). A thorough characterization sheds light on the composition, crystallinity, porosity, morphology, coordination, and acidity of the modified clinoptilolite and L zeolites. It is elaborated that, besides the degree of dealumination, the resulting Al distribution is a critical precondition for the following mesopore formation by desilication. Adsorption experiments of Cu²⁺ and methylene blue from aqueous solutions and the catalytic evaluation in alkylations and Knoevenagel condensation evidence the superiority of the hierarchical zeolites, as compared to their purely microporous counterparts. Finally, the postsynthetic routes for clinoptilolite and L are generalized with other recently reported modification strategies, and presented in a comprehensive overview

Solid-State Chemistry of Cuprous Delafossites: Synthesis and Stability Aspects

Cuprous delafossites exhibit exceptional electrical, magnetic, optical, and catalytic properties. Through the application of a battery of <i>in situ</i> and <i>ex situ</i> characterization methods complemented by density functional theory (DFT) calculations, we gathered an in-depth understanding of the synthesis of CuMO₂ (M = Al, Cr, Fe, Ga, Mn) by the solid-state reaction of Cu₂O and M₂O₃ and of their stability against oxidative disproportionation to CuM₂O₄ and CuO. TGA-DTA and XRD studies of the synthesis revealed that the nature of the M³⁺ cation strongly impacts (<i>i</i>) the formation temperature of the delafossite phase, which occurred at a much lower temperature for CuCrO₂ than for the other metals (1073 versus 1273–1423 K), (<i>ii</i>) the mechanism of formation of the CuMO₂ in different atmospheres, which was found to comprise up to four steps in air and a single step in N₂, and (<i>iii</i>) the kinetics of the process, which could be significantly accelerated upon mechanochemical activation of the precursors by ball milling. The identification of unstable intermediate phases and, thus, a proper description of the synthesis mechanism was only possible by the application of <i>in situ</i> XRD. Electron microscopy, nitrogen sorption, and mercury porosimetry analyses of the precursor oxide mixtures at different stages of the synthesis in air revealed that particle agglomeration took place prior to the solid-state reactions forming the intermediate spinel phase and the delafossite, respectively, and that these led to a substantial drop in porosity and specific surface area. On the basis of XRD and He pycnometry, the resulting CuMO₂ samples exhibit pure delafossite phase with rhombohedral structure (<i>R</i>3̅<i>m</i>), except for CuMnO₂ which features a monoclinic structure (<i>C</i>2/<i>m</i>). Upon heating in air, CuCrO₂ retained its structure up to 1373 K, while all other delafossites decomposed, CuAlO₂ at 1073 K, CuGaO₂ at 873 K, CuFeO₂ at 773 K, and CuMnO₂ at 673 K. The DFT-calculated surface phase diagram of CuCrO₂ and CuAlO₂ indicated that, at elevated oxygen pressures, the terminations with 1/2 and 0 ML of Cu are the most stable for the (0001) facet. The formation enthalpy for interstitial oxygen species in the bulk is endothermic for both delafossites, while that for oxygen insertion in subsurface layers of these terminations is still endothermic for CuCrO₂ but slightly exothermic for CuAlO₂. These results provide an improved understanding of the chemistry of these mixed oxides, enabling their optimization for specific applications

Enhanced Reduction of CO₂ to CO over Cu–In Electrocatalysts: Catalyst Evolution Is the Key

Copper–indium catalysts have recently shown promising performance for the selective electrochemical reduction of CO₂ to CO. In this work, we prepared Cu–In nanoalloys by the in situ reduction of CuInO₂ and In₂O₃-supported Cu nanoparticles and found that the structure of these nanoalloys evolves substantially over several electrocatalytic cycles, in parallel with an increase in the activity and selectivity for CO evolution. By combining electrochemical measurements with ex situ characterization techniques, such as XRD, STEM, elemental mapping, and XPS, we show that this behavior is caused by the segregation of copper and indium in these materials, resulting in the formation of a heterogeneous nanostructure of Cu-rich cores embedded within an In­(OH)₃ shell-like matrix. The evolved catalysts show high electrocatalytic performance at moderate overpotential (i.e., <i>j</i>_CO > 1.5 mA cm^–2 at −0.6 V vs RHE). We found that the removal of In­(OH)₃ from these heterogeneous nanostructures decreases the performance of the evolved catalysts, particularly in terms of the selectivity toward CO, which then recovers with the reappearance of the hydroxide following the re-equilibration of the material. On the other hand, an In­(OH)₃-supported Cu catalyst exhibits a current efficiency for CO comparable to that of the evolved nanoalloys without the need for an equilibration stage, indicating that In­(OH)₃ plays a crucial role in favoring the production of CO over Cu–In electrocatalysts. These findings shed light on the link between the architecture of these materials and their performance and underscore the potential of nonreducible hydroxides to act as promoters in CO₂ reduction electrocatalysis