Physicochemical Stabilization of Pt against Sintering for a Dehydrogenation Catalyst with High Activity, Selectivity, and Durability

Suppressing irreversible catalyst deactivation is critical in heterogeneous catalysis. In particular, deactivation via sintering of active sites is a significant issue for reactions involving harsh reaction/regeneration conditions. In this work, we developed a PtGa/γ-Al₂O₃ alkane dehydrogenation catalyst with exceptionally high activity, selectivity, and long-term stability by markedly suppressing Pt sintering under harsh conditions (reaction/regeneration at >823 K). To stabilize Pt, physical and chemical stabilization strategies were synergistically combined. For the former, Pt was introduced during the synthesis of γ-Al₂O₃ via sol–gel chemistry, which can increase the interfacial contact between Pt and γ-Al₂O₃ due to the partial entrapment of Pt in γ-Al₂O₃. For the latter, atomically dispersed Ce was doped on γ-Al₂O₃, which can stabilize Pt via strong Pt–O–Ce interactions. Because of effective Pt stabilization, the catalyst showed remarkably steady activity and selectivity behaviors over the repeated reaction cycles, although the catalyst is regenerated via simple oxidation rather than industrially used oxychlorination. The Pt stabilization strategies reported in this work can be applied to other metal-catalyzed reactions that involve severe reaction/regeneration conditions

Synergistic Integration of Ion-Exchange and Catalytic Reduction for Complete Decomposition of Perchlorate in Waste Water

Ion-exchange has been frequently used for the treatment of perchlorate (ClO₄^–), but disposal or regeneration of the spent resins has been the major hurdle for field application. Here we demonstrate a synergistic integration of ion-exchange and catalytic decomposition by using Pd-supported ion-exchange resin as an adsorption/catalysis bifunctional material. The ion-exchange capability of the resin did not change after generation of the Pd clusters via mild ethanol reduction, and thus showed very high ion-exchange selectivity and capacity toward ClO₄^–. After the resin was saturated with ClO₄^– in an adsorption mode, it was possible to fully decompose the adsorbed ClO₄^– into nontoxic Cl^– by the catalytic function of the Pd catalysts under H₂ atmosphere. It was demonstrated that prewetting the ion-exchange resin with ethanol significantly accelerate the decomposition of ClO₄^– due to the weaker association of ClO₄^– with the ion-exchange sites of the resin, which allows more facile access of ClO₄^– to the catalytically active Pd-resin interface. In the presence of ethanol, >90% of the adsorbed ClO₄^– could be decomposed within 24 h at 10 bar H₂ and 373 K. The ClO₄^– adsorption-catalytic decomposition cycle could be repeated up to five times without loss of ClO₄^– adsorption capacity and selectivity

2,6-Di-<i>tert-</i>butylpyridine Sorption Approach to Quantify the External Acidity in Hierarchical Zeolites

This work was aimed to evidence that substituted pyridine, 2,6-di-<i>tert</i>-butylpyridine, is a suitable probe for the quantitative investigation of the external acidity in hierarchically structured zeolites. The 2,6-di-<i>tert</i>-butylpyridine was too large to enter the micropores, even in wide pore zeolites, and nearly no sites in nonmesoporous zeolites were available. Accessibility studies of acid sites in zeolites TNU-9 and BEA involving quantitative IR measurements with hindered 2,6-di-<i>tert</i>-butylpyridine as a probe were performed. The extinction coefficients of the 1615 cm^–1 diagnostic bands of 2,6-di-<i>tert</i>-butylpyridine interacting with Brønsted acid sites were determined. Lewis acid sites were not detected with the probe. The accessibility factor (AF) for the 2,6-di-<i>tert</i>-butylpyridine probe molecule was defined as the number of sites detected by adsorption of the dTBPy (external sites) divided by the total amount of acid sites in the studied zeolites as quantified by pyridine sorption. Upon desilication resulting in the fabrication of the secondary mesopores, the enhanced accessibility of the protonic sites was observed. In comparison to the mesoporous zeolites with the secondary system of mesopores generated by alkaline leaching, considerably higher accessibility of protonic sites was evidenced in both ultrathin ZSM-5 and delaminated ITQ-2 zeolite

Rational Design of the Polymeric Amines in Solid Adsorbents for Postcombustion Carbon Dioxide Capture

Substantial efforts have been made to increase the CO₂ working capacity of amine adsorbents for an efficient CO₂ capture. However, the more important metric for assessing adsorbents is the regeneration heat required for capturing a fixed amount of CO₂. In this work, we synthesized polyethyleneimine (PEI)/SiO₂ adsorbents functionalized with various epoxides. This provided adsorbents with six different amine structures showing various CO₂/H₂O adsorption properties. Our studies revealed that the CO₂ working capacity was not a decisive factor in determining the regeneration heat required for CO₂ capture. This is because the benefit of large CO₂ working capacity was canceled out by the difficulty of CO₂ desorption. Instead, the suppression of H₂O co-adsorption was critical for reducing the regeneration heat because substantial latent heat is required for H₂O desorption. Consequently, the PEI/SiO₂ functionalized with 1,2-epoxybutane required a much lower regeneration heat (2.66 GJ tCO₂^–1) than the conventional PEI/SiO₂ (4.03 GJ tCO₂^–1) because of suppressed H₂O co-adsorption as well as moderately high CO₂ working capacity

Comprehensive Understanding of the Effects of Carbon Nanostructures on Redox Catalytic Properties and Stability in Oxidative Dehydrogenation

The intrinsic redox catalytic properties of metal-free carbons have been widely investigated due to their fundamental interest as well as potential practical applications. Although a large variety of nanostructured carbons are now available, the effects of carbon nanostructures on redox properties have not been comprehensively understood. In this work, the redox catalytic properties and thermochemical stabilities of 16 different types of carbons, including activated carbon, carbon nanotubes, onion-like carbons, and microporous/mesoporous templated carbons were systematically investigated using <i>n</i>-butane oxidative dehydrogenation as a model reaction. The results demonstrate that the overall catalytic activity increases with increasing content of CO active sites. However, with increasing CO content, the activity per site (i.e., turnover frequency) gradually decreases, while the alkene selectivity increases due to the decreased reducibility of each CO site. Since more CO sites are present in a thermochemically less stable amorphous framework, the carbons generally exhibit a trade-off relationship between catalytic activity and stability. However, a graphitic carbon with “coin-stacking” carbon layers showed exceptionally high activity and stability simultaneously. This is attributed to its unique carbon structure that simultaneously provides high graphitic order and abundant carbon edge sites where CO active sites are grafted

Optimum Utilization of Biochemical Components in <i>Chlorella</i> sp. KR1 via Subcritical Hydrothermal Liquefaction

Product distributions in bio-crude, aqueous phase, and solid residue were rigorously analyzed during the hydrothermal liquefaction (HTL) of <i>Chlorella</i> sp. KR1 in order to optimize utilization of energy and chemicals. A non-asphaltene (paraffinic) fraction in the bio-crude, which can be readily upgraded to high-quality fuels via a subsequent catalytic process, was mainly produced due to lipid extraction. Above 170 °C, lipid extraction was almost complete, and hence, the non-asphaltene content did not increase further with increasing temperature. Carbohydrates could be extracted, mainly as polysaccharides, in the aqueous phase at mild temperatures (<200 °C). At high temperatures (>200 °C), they decompose and react with proteins via the Maillard reaction to form asphaltene (polycyclic aromatics), which contains large amounts of heteroatoms such as N and S. Although high-temperature carbohydrate conversion could yield more bio-crude with high energy values, it dominantly contributed to formation of the asphaltene fraction, which is difficult to upgrade catalytically. As high-temperature HTL requires a large energy input, the recovery and utilization of intact carbohydrates and proteins at mild temperatures (<200 °C) appears to be more promising. Energy Return on Investment (EROI) analysis also showed that 170 °C is the optimum HTL temperature for maximizing the energy production

Significant Roles of Carbon Pore and Surface Structure in AuPd/C Catalyst for Achieving High Chemoselectivity in Direct Hydrogen Peroxide Synthesis

Direct synthesis of hydrogen peroxide (H₂O₂) from hydrogen (H₂) and oxygen (O₂) has been widely investigated as an attractive way for small-scale/on-site H₂O₂ production. Among various catalysts, carbon-supported AuPd catalysts have been reported to exhibit the most promising H₂O₂ productivity and selectivity. In this work, to better understand the catalytic role of the surface properties and porous structures of the carbon supports, we systematically investigated AuPd catalysts supported on various nanostructured carbons including activated carbon, carbon nanotube, carbon black, and ordered mesoporous carbons. The results showed that a high density of oxygen functional groups on the carbon surface was essential for synthesizing highly dispersed bimetallic catalysts with effective AuPd alloying, which is a prerequisite for achieving high H₂O₂ selectivity. Regarding porous structure, a solely mesoporous carbon support was superior to microporous ones. Microporous carbons such as activated carbon suffered from diffusion limitation, leading to significantly slower H₂ conversion than mesoporous catalysts. Furthermore, H₂O₂ produced from AuPd catalyst in the micropores was more prone to subsequent disproportionation/hydrogenation into H₂O due to retarded diffusion of the H₂O₂ out of the microporous structure, which led to decreased H₂O₂ selectivity. The present study showed that solely mesoporous carbons with high surface oxygen content are most desirable as a support for AuPd catalyst in order to achieve high H₂O₂ productivity and selectivity

Hydrogen Peroxide Synthesis via Enhanced Two-Electron Oxygen Reduction Pathway on Carbon-Coated Pt Surface

Continuous on-site electrochemical production of hydrogen peroxide (H₂O₂) can provide an attractive alternative to the present anthraquinone-based H₂O₂ production technology. A major challenge in the electrocatalyst design for H₂O₂ production is that O₂ adsorption on the Pt surface thermodynamically favors “side-on” configuration over “end-on” configuration, which leads to a dissociation of O–O bond via dominant 4-electron pathway. This prefers H₂O production rather than H₂O₂ production during the electrochemical oxygen reduction reaction (ORR). In the present work, we demonstrate that controlled coating of Pt catalysts with amorphous carbon layers can induce selective end-on adsorption of O₂ on the Pt surface by eliminating accessible Pt ensemble sites, which allows significantly enhanced H₂O₂ production selectivity in the ORR. Experimental results and theoretical modeling reveal that 4-electron pathway is strongly suppressed in the course of ORR due to a thermodynamically unfavored end-on adsorption of O₂ (the first electron transfer step) with 0.54 V overpotential. As a result, the carbon-coated Pt catalysts show an onset potential of ∼0.7 V for ORR and remarkably enhanced H₂O₂ selectivity up to 41%. Notably, the produced H₂O₂ cannot access the Pt surface due to the steric hindrance of the coated carbon layers, and thus no significant H₂O₂ decomposition via disproportionation/reduction reactions is observed. Furthermore, the catalyst shows superior stability without considerable performance degradation because the amorphous carbon layers protect Pt catalysts against the leaching and ripening in acidic operating conditions

Maximizing Biojet Fuel Production from Triglyceride: Importance of the Hydrocracking Catalyst and Separate Deoxygenation/Hydrocracking Steps

Various parameters in the catalytic hydroconversion of triglycerides (palm oil) were carefully investigated for maximizing the production of biojet fuel. The results showed that the deoxygenation of triglyceride via hydrotreatment should be carried out in a separate reactor prior to the hydrocracking step (i.e., two-step reaction process). Otherwise, the CO generated during deoxygenation can poison the metal components in the metal/acid bifunctional catalysts (Pt/zeolites), which can cause significant imbalance between the metal and acid functions in hydrocracking. This leads to fast catalyst deactivation via coke formation, heavy formation of aromatics, and overcracking of hydrocarbons, resulting in the reduction of final biojet fuel yield. In the two-step process, the second hydrocracking step mainly determines the final biojet fuel yield, and thus, a rational design of the hydrocracking catalysts that can suppress overcracking is essential. The diffusion characteristics of the multibranched hydrocarbon (e.g., 2,2,4-trimethylpentane) in the hydrocracking catalysts could be correlated with the yields of the jet fuel-range C8–C16 hydrocarbons and the <i>iso</i>/<i>n</i>-paraffin ratios. The result indicates that the facile diffusion of multibranched isomers out of catalysts before excessive cracking is important for the suppression of the formation of light hydrocarbons (≤C7). Consequently, Pt supported on nanocrystalline large-pore BEA zeolite showed the largest biojet fuel yield with the highest <i>iso</i>-paraffin content. Under the optimized conditions, 55 wt % of biojet fuel with respect to palm oil was achieved after final distillation, which satisfied all the required fuel specifications

Cross-Linked “Poisonous” Polymer: Thermochemically Stable Catalyst Support for Tuning Chemoselectivity

Designed catalyst poisons can be deliberately added in various reactions for tuning chemoselectivity. In general, the poisons are “transient” selectivity modifiers that are readily leached out during reactions and thus should be continuously fed to maintain the selectivity. In this work, we supported Pd catalysts on a thermochemically stable cross-linked polymer containing diphenyl sulfide linkages, which can simultaneously act as a catalyst support and a “permanent” selectivity modifier. The entire surfaces of the Pd clusters were ligated (or poisoned) by sulfide groups of the polymer support. The sulfide groups capping the Pd surface behaved like a “molecular gate” that enabled exceptionally discriminative adsorption of alkynes over alkenes. H₂/D₂ isotope exchange revealed that the capped Pd surface alone is inactive for H₂ (or D₂) dissociation, but in the presence of coflowing acetylene (alkyne), it becomes active for H₂ dissociation as well as acetylene hydrogenation. The results indicated that acetylene adsorbs on the Pd surface and enables cooperative adsorption of H₂. In contrast, ethylene (alkene) did not facilitate H₂–D₂ exchange, and hydrogenation of ethylene was not observed. The results indicated that alkynes can induce decapping of the sulfide groups from the Pd surface, while alkenes with weaker adsorption strength cannot. The discriminative adsorption of alkynes over alkenes led to highly chemoselective hydrogenation of various alkynes to alkenes with minimal overhydrogenation and the conversion of side functional groups. The catalytic functions can be retained over a long reaction period due to the high thermochemical stability of the polymer