Single-atom catalysts (SACs) enable greener and more economically sustainable chemical production by significantly improving thermocatalysis efficiency and selectivity through maximized atom utilization and highly homogeneous metal coordination environments. Unfortunately, SACs are fundamentally constrained by the stability owing to the severe aggregation of single atoms, especially under the high-temperature thermocatalysis operations, which compromises the overall catalytic performance. Here, we report a synthetic strategy to realize the highly thermal-stable SACs resistance to sintering at harsh conditions through harnessing the inherent metal affinity and fluidity of liquid metal. A stable liquid metal-active metal interaction is formed, profiting from the superior metal affinity of liquid metal. Combined with the fluidity of liquid metal, active metal atoms can move but remain confined to the liquid metal as the metallic single-atom state at high temperatures. This catalyst exhibits outstanding thermal durability for ethane dehydrogenation, sustaining stable operation for over 100 h at 650 °C with an impressive ethylene selectivity of 98%. The strategy of constructing stable metal-metal interactions by utilizing the inherent metal affinity and dynamic fluidity of liquid metal will pave a practical way for the design of highly thermal-stable SACs.

Hydrogen is a promising clean energy carrier to address global energy and environmental challenges. Although platinum (Pt)-based catalysts are the benchmark for the hydrogen evolution reaction (HER), their high cost and scarcity limit their widespread application. Two-dimensional transition metal dichalcogenides (TMDs), particularly with the unconventional 1T' phase, have emerged as promising alternatives, yet synthesizing them with high phase purity and stability remains challenging. Here, by using amorphous phosphorus (P)-doped Pd nanoparticles (a-PdP NPs) as templates, we develop a facile and general wet-chemical method to synthesize high-phase-purity and stable 1T'-TMD monolayers (MLs), including MoS2, WS2, and MoWS2, to construct a-PdP@1T'-TMD core-shell NPs. Experimental and theoretical analyses reveal that the formation and stabilization of 1T'-MoS2 MLs are attributed to the strong Pd-S interaction, electron donation from oleylamine, and amorphous nature of the template. The resulting a-PdP@1T'-MoS2 catalyst exhibits superior HER performance, requiring an overpotential of only -182.3 mV to achieve 1,000 mA·cm-2 and maintaining high stability for over 500 h at 500 mA·cm-2, outperforming the commercial Pt/C and placing it among the best reported MoS2-based catalysts. Impressively, the synthesized a-PdP@1T'-MoS2 can also be used as an efficient and stable support to grow single-atomically dispersed Pt with further enhanced HER activity, indicating its promise as a versatile platform for the design and preparation of advanced electrocatalysts.

The selective formation of C-C bonds, coupled with effective removal of oxygen, plays a crucial role in the process of upgrading biomass-derived oxygenates into fuels and chemicals. However, co-feeding reactants with water is sometimes necessary to assist binding sites in catalytic reactions, thereby achieving desirable performance. Here, we report the design of a CeSnBeta catalyst featuring dual Lewis acidic sites for the efficient production of isobutene from acetone via C-C coupling followed by deoxygenation. By incorporating Ce species onto SnBeta, which was synthesized through liquid-phase grafting of dealuminated Beta, we created confined dual Lewis acidic centers within Beta zeolites. The cooperative action of Ce species and framework Sn sites within this confined environment enabled selective catalysis of the acetone-to-isobutene cascade reactions, showcasing enhanced stability even without the presence of water. Dual Lewis acidic centers, CeOx and framework Sn, confined in Beta zeolite, catalyze the acetone C-C coupling followed by deoxygenation to isobutene, achieving a desirable product distribution and stability in the absence of water.

Acrylonitrile (ACN) is one of the most important industrial intermediates for the production of a variety of commodities that are intertwined with our daily life. Currently, ACN is mainly produced using the commercial propylene-based process (i.e., the SOHIO process). Driven by the climbing cost of propylene, alternative feedstock such as propane and biomass-derived compounds have been studied. In this review, we summarize the advances in the catalytic production of ACN with the above-mentioned feedstocks. In particular, we briefly summarize the well-established catalysts (i.e., BiMo-based and Sb-based ones) for propylene ammoxidation. Then, we focus on the development of catalysts for the ammoxidation of propane (e.g., Mo-V-Nb-Te-based, Sb-V-based, and other novel catalytic systems) as well as in the conversion of biomass-derived compounds. Finally, perspectives on catalyst design and engineering control for the next generation of ACN production processes will be presented.

Proton transfer is critically important to many electrocatalytic reactions, and directed proton delivery could open new avenues for the design of electrocatalysts. However, although this approach has been successful in molecular electrocatalysis, proton transfer has not received the same attention in heterogeneous electrocatalyst design. Here, we report that a metal oxide proton relay can be built within heterogeneous electrocatalyst architectures and improves the kinetics of electrochemical hydrogen evolution and oxidation reactions. The volcano-type relationship between activity enhancement and pKa of amine additives confirms this improvement; we observe maximum rate enhancement when the pKa of a proton relay matches the pH of the electrolyte solution. Density-functional-theory-based reactivity studies reveal a decreased proton transfer energy barrier with a metal oxide proton relay. These findings demonstrate the possibility of controlling the proton delivery and enhancing the reaction kinetics by tuning the chemical properties and structures at heterogeneous interfaces.

Noble metals have been extensively employed in a variety of hydrotreating catalyst systems for their featured functionality of hydrogen activation but may also bring side reactions such as undesired deep hydrogenation. It is crucial to develop a viable approach to selectively inhibit side reactions while preserving beneficial functionalities. Herein, we present modifying Pd with alkenyl-type ligands that forms homogeneous-like Pd-alkene metallacycle structure on the heterogeneous Pd catalyst to achieve the selective hydrogenolysis and hydrogenation. Particularly, a doped alkenyl-type carbon ligand on Pd-Fe catalyst is demonstrated to donate electrons to Pd, creating an electron-rich environment that elongates the distance and weakens the electronic interaction between Pd and unsaturated C of the reactants/products to control the hydrogenation chemistry. Moreover, high H 2 activation capability is maintained over Pd and the activated H is transferred to Fe to facilitate C-O bond cleavage or directly participate in the reaction on Pd. The modified Pd-Fe catalyst displays comparable C-O bond cleavage rate but much higher selectivity (>90%) than the bare Pd-Fe (<50%) in hydrotreating of diphenyl ether (DPE, modelling the strongest C-O linkage in lignin) and enhanced ethene selectivity (>90%) in acetylene hydrogenation. This work sheds light on the controlled synthesis of selective hydrotreating catalysts via mimicking homogeneous analogues.

Vanadia supported on well-defined ceria nanocubes are synthesized at various loadings using a liquid-phase chemical grafting technique and are compared to those prepared using traditional incipient wetness impregnation. Raman and IR characterization reveal that, as vanadia loading is increased to near monolayer coverage, vanadia deposited using grafting shows greatly enhanced dispersion (i.e., improved VOx monomer/dimer distribution). Methanol is used as a probe molecule to explore the redox behavior of the catalysts. IR and temperature programmed desorption of methanol show increased CO2 formation occurs on the bare ceria support and increasing the dispersion of vanadia promotes dehydrogenation to formaldehyde due to the inhibited oxygen vacancy formation on VOx/CeO2. Hydrogen temperature programmed reduction demonstrates that the catalyst reducibility and formation of surface oxygen vacancy are directly related to the degree of vanadia oligomerization. The samples prepared using grafting exhibit superior catalytic performance for methanol oxidative dehydrogenation to formaldehyde compared to the impregnated samples, due to the presence of highly dispersed VOx species (i.e., monomers and dimers).

Liquid-phase chemical grafting method was used to graft Zn onto TiO2 with preferentially exposed (101) or (001) facet. The obtained ZnxTiyOz materials were characterized using various techniques (e.g., XRD, Raman, DRIFTS etc.) and evaluated for the acetone-to-isobutene reaction. It was found that over TiO2 (001), both terminal and bridging hydroxyls were readily titrated by Zn deposition, whereas a substantial amount of bridging hydroxyls on TiO2 (101) remained. Although dominant Zn-O-terminated surface was obtained on two ZnxTiyOz samples, bridging hydroxyls with high H-D exchange reactivity were observed on ZnxTiyOz (101) compared with ZnxTiyOz (001). The bridging hydroxyls showing rapid proton transfer efficiently stabilizes a transition state of diacetone alcohol intramolecular rearrangement for isobutene production as opposed to the diacetone alcohol dehydration.

While C-O bond cleavage is pivotal in the depolymerization/valorization of lignin, it is still challenging to control the reaction selectivity under high activity due to the higher dissociation energy of aromatic C-O bonds relative to other reactions such as direct ring hydrogenation. Herein, we report the activation of Al2O3-supported earth-abundant MnO with embedded Ru to enhance the selective hydrogenolysis of aromatic C-O bonds in both a model compound and real lignin. Complementary characterizations demonstrate that the embedment of Ru into the MnO phase generates vacancy-enriched MnO under a hydrogen atmosphere, and such abundant active sites enable about threefold enhancement of the specific reaction rate for C-O bond hydrogenolysis. Moreover, the defective MnO overlayer on Ru nanoparticles has a stronger interaction with the O in diphenyl ether with preferential vertical adsorption, which inhibits the activation and hydrogenation of the aromatic ring, leading to higher selectivity for direct C-O bond cleavage. In the depolymerization of real lignin, the bimetallic Ru-MnO shows significantly higher (fivefold) activity than monometallic Ru under the tested condition. This work provides a general framework for the rational design of highly efficient catalysts for selective C-O bond cleavage.

Selective hydrodeoxygenation (HDO)/hydrogenolysis of phenolics to produce arene in liquid-phase conditions is a key challenge in biomass valorization. Cost-effective base-metal catalysts are intriguing for this application, yet they typically suffer from inferior performance compared to precious metals. Herein, we report a Cs-G@CoFe catalyst with a bimetallic alloy core and graphene overlayers doped with Cs, on which HDO of phenol with >90% benzene selectivity was achieved under liquid-phase conditions. More importantly, the bimetallic catalyst showed a significantly enhanced activity (>threefold) compared with the monometallic Fe-based catalyst. A combination of characterizations confirmed the formation of a CoFe alloy core wrapped by several graphene overlayers in the Cs-G@CoFe catalyst. In contrast to the inhibited C-O cleavage product by the addition of Cs in the Cs-G@Fe catalyst, the addition of Cs on Cs-G@ CoFe improved the activity for selective C- O bond cleavage. The surface chemistry has been probed using H2 chemisorption to identify the enhanced activity in the selective hydrogenolysis of phenolics.

Iron-carbide-based catalysts have been explored in the selective hydrodeoxygenation (HDO) of phenol, aiming at elucidating the role of active site and alkali metal. Complementary characterization such as X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and scanning transmission electron microscopy coupled with electron energy loss spectroscopy, together with catalytic evaluations revealed a rapid structural reconstruction of iron carbide (Fe3 C) catalysts, leading to a stable defective graphene-covered metallic Fe active phase (G@Fe) under reaction conditions. Further studies using different alkali metals (i. e., Na, K, and Cs) revealed that alkali metals showed negligible effect on the phase transformation of Fe3 C. However, the reconstructed G@Fe doped with alkali metals inhibited the tautomerization, a facile reaction pathway to saturation of the aromatic ring, leading to enhanced selectivity to arene. The extent of inhibition of tautomerization or selectivity to arene was closely related to the degree of electron donation of alkali metal to Fe.

CeO2 nanoshapes, cubes with dominant (100) facets and octahedra with dominant (111) facets, were synthesized to investigate the influence of surface structure on acid-base properties. An optimization of calcination temperatures, coupled with Raman and TEM, was employed to minimize the intrinsic (Frenkel-type) defect sites and their potential complications on the facet studies involved in this work. The acid-base properties of these CeO2 nanoshapes were characterized with in situ pyridine and CO2 adsorption using infrared spectroscopy and quantified using pyridine, ammonia, and CO2 temperature programmed desorption (TPD). The (100) facet displayed weaker acid sites with lower site density than the (111) facet which had a similar density of base sites but those on the (100) facet were stronger. A strong correlation was observed between 2-propanol conversion and each facet's acid-base properties. CeO2 cubes exhibited greater base-site catalyzed dehydrogenation to acetone while the more acidic CeO2 octahedra were more active in dehydration to propene. (C) 2021 Published by Elsevier Inc.

Once confined in zeolites, carbides of inexpensive transition metals, such as molybdenum (Mo) and tungsten (W), exhibit similar catalytic activity as platinum group noble metals. Thus far, the intrinsic thermodynamic properties and their relations with the local interfacial phenomena of such carbide−zeolite heterogeneous catalytic materials have rarely been explored. Here, employing high temperature oxide melt solution calorimetry, for the first time, we determined the energetics of molybdenum carbide (Mo2C) formation under confinement in zeolite Y (Mo2C/FAU) as a function of Si/Al ratio experimentally. As Si/Al ratio increases, the formation enthalpies of Mo2C/FAU from constituent oxides and carbides become less endothermic, spanning within a narrow range, between ∼0 and 10 kJ/mol TO2 (tetrahedron unit). Confinement of refractory Mo2C in zeolite Y is energetically more favorable than encapsulation of MoO3 in the same host, by more than ∼30 kJ/mol TO2. Such significant energetic differences between Mo2C/FAU and MoO3/FAU in formation enthalpies and the highly exothermic Mo2C−FAU guest−host interaction energies, highlight robust bonding at the carbide−zeolite interfaces that harnesses the refractory nature of Mo2C guest species, and compensate the energetic deficiency for achieving subnano-sized Mo2C particles. ■ INTRODUCTION Zeolite-based heterogeneous catalytic materials play critical roles in multiple fields of modern chemical industry. Examples include selective conversion of fossil fuels, biomass conversion, and biorefinery. In these applications, noble metal nanoor subnano-particles encapsulated in zeolites show high performance, yet development of stable alternative catalytic materials using earth-abundant elements is necessary for a more sustainable future. It has been reported that once properly supported, refractory carbides of inexpensive major industrial metals, such as molybdenum (Mo) and tungsten (W), present comparable activity as platinum (Pt) group noble metals. For instance, encapsulation of Mo and W carbides in zeolites, such as ZSM-5 and zeolite Y, leads to catalysts with high stability and comparable performance as the noble metal− zeolite systems in biomass and crude oil upgrading, and methane dehydroaromatization. Encapsulation in zeolite frameworks enables “sinter-proof”, dispersed transition metal carbide (TMC) particles with uniform size, and high catalytic activity, selectivity, and stability. Moreover, the crystalline zeolite framework with well-defined porosity can tolerate the tough carbide synthesis conditions. Current research focusing on TMC/zeolite catalysts is rich in reaction kinetics and pathway design, and activity/selectivity boost. Several computational thermodynamic predictions have been documented. However, there is few experimental study on the formation energetics of TMC/zeolite systems, which governs the bonding specifics of guest−host interfaces and stabilizes the highly metastable subnano-sized TMC particles/clusters. Such quantitative energetic insights will enable benchmarking data for machine learning and simulation for predictive design, synthesis, and process of TMC−zeolite catalytic materials. The thermochemistry of zeolites has been pioneered by Prof. Alexandra Navrotsky and collaborators over the past a few decades, in which formation of zeolite pure phases, and small molecule−zeolite binding (water, CO2, and small organics) have been systematically studied. A comprehensive review can be found in the references. However, as mentioned earlier, we realize that the potentially rich energetic landscape, and guest−host interactions for zeolite-encapsulation of TM-based solid-state materials with much higher melting points, such as oxides (TMOs), carbides (TMCs) and nitrides (TMNs), have not been systematically explored. Our recent research on confinement/encapsulation of heterocore particles is primarily on the TMO−aluminosilicate zeolite Received: July 15, 2021 Published: September 17, 2021 Article pubs.acs.org/IECR © 2021 American Chemical Society 13991 https://doi.org/10.1021/acs.iecr.1c02822 Ind. Eng. Chem. Res. 2021, 60, 13991−14003 D ow nl oa de d vi a U N IV O F C A L IF O R N IA D A V IS o n Se pt em be r 29 , 2 02 1 at 0 8: 20 :2 4 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. systems. Owing to their negatively charged microporous frameworks constructed by corner-sharing tetrahedra (TO4, T = Si or Al), aluminosilicate zeolites are friendly hosts welcoming guest species with proper sizes, ranging from water to small organics and cations. Such an open system with tunable surfaces and compositions enables intricate local chemistry within the pores which defines the nature of the guest species, and determines the strength of guest−host interactions. Recently, using adsorption calorimetry, we determined the formation energetics of copper oxo clusters (CuOx) under tight confinement of mordenite (MOR) with water as the gentle oxidant in real-time. Meanwhile, hydration thermodynamics of other TM-MOR samples, including Coand Fe-MOR, was investigated. Our results highlighted the high thermal stability of CuOx in MOR, up to 915 °C in nitrogen flow, compared with other guest species such as iron carbonates and/or bicarbonates detected in FeMOR. Meanwhile, we elucidated the critical roles of small molecular species, such as water and CO2, in minimizing the overall energy of the TMO−zeolite systems by effective stabilization of ionic species. Very recently, we investigated the thermodynamics of molybdenum trioxide (MoO3) particles encapsulated in zeolite Y (faujasite or FAU) with varied Si/Al ratio. Interestingly, we found that encapsulation of MoO3 only slightly neutralized the metastability of zeolite Y, which has an endothermic formation enthalpy compared with constituent oxides (SiO2, Al2O3, and H2O). Additionally, at the same MoO3 loading, higher framework Si/Al ratio resulted in less endothermic MoO3/FAU formation enthalpies, ranging from 61.1 ± 1.8 kJ/mol TO2 at Si/Al = 2.9 to 32.8 ± 1.4 kJ/ mol TO2 at Si/Al = 45.6. These studies strongly suggest that the encapsulation thermodynamics of TMOs in zeolites is closely governed by the Si/Al ratio, and the chemical natures of the TMO guest species. Moreover, we also noticed that once present as the guests in zeolite frameworks, TMOs appear to be “flexible” taking advantage of the confinement chemistry and other guest species, such as water and CO2 to alter their chemical identity, phases, oxidation states and/or local structures, seeking for the lowest possible energetic states under encapsulation. Here, we report the first experimental thermodynamic (calorimetric) study on the confinement of molybdenum carbide in zeolite Y (Mo2C/FAU) with a wide range of Si/Al ratios, from 2.9 to 45.6. Zeolite Y is selected as the host candidate to sustain the harsh synthesis conditions of refractory TMCs involving a carburization process at temperatures about 700 °C. Employing the unique calorimetric capabilities in the Alexandra Navrotsky Institute for Experimental Thermodynamics (AlexInstitute) at Washington State University (WSU) with supports from structural, morphological, in situ spectroscopic, and integrated thermal analyses, we revealed the formation energy landscape of Mo2C/FAU as Si/Al ratio varies, elucidated the energetically favorable Mo2C−FAU guest−host interactions, and compared the distinctively different thermodynamics of Mo2C encapsulation with MoO3 confinement in FAU. ■ EXPERIMENTAL METHODS Material Synthesis. Zeolite HY samples with different Si/ Al ratios, from 2.9 to 45.6, were obtained from Alfa Aesar. These samples were also used in our earlier study on thermodynamics of MoO3 encapsulated in zeolite Y. 33 Synthesis of materials with molybdate carbides confined in zeolite HY (Mo2C/FAU) has three steps, including (1) Mo precursor introduction via impregnation, (2) Mo oxide formation in FAU (MoO3/FAU) by calcination in air, and (3) carburization of MoO3 to form Mo2C in FAU. Specifically, by incipient wetness impregnation (IWI) of ammonium molybdate tetrahydrate (AMT, Sigma-Aldrich, 99%), the same amount of Mo precursor, was loaded into the void space of each FAU to ensure all samples have the same Mo content of 5 wt %. After pretreatment at 80 °C under vacuum for at least 4 h, the FAU sample (1.0 g) was impregnated with aqueous solution of AMT (1.5 mL, 0.05 mol/L). Subsequently, the impregnated FAU was sonicated for 1 h in ambient conditions followed by oven-drying at 120 °C for at least 12 h before oxidative calcination at 600 °C in air for 10 h in a furnace leading to the formation of MoO3/FAU. Eventually, MoO3/FAU was carburized under 15 vol % CH4/H2 flow (85 mL/min) for the final product, Mo2C/ FAU. More specifically, according to Iida et al., under continuous 15 vol % CH4/H2 flow of 85 mL/min, the furnace temperature was increased stepwise, to 300 °C at 5 °C/min first, and to 700 °C at 1 °C/min followed by isothermal treatment at 700 °C for at least 2 h for complete carburization. The dominant molybdenum carbide phase in FAU is Mo2C according to previous reports. The sample labeled as Mo2C/2.9FAU represents Mo2C confined/encapsulated in zeolite Y (FAU) with a Si/Al ratio of 2.9. Sample Characterizations. The composition of each sample was determined by coupling inductively coupled plasma mass spectrometry (ICP-MS, Agilent 770) and thermal analysis using an integrated thermogravimetry−differential scanning calorimetry−mass spectrometry system (TG-DSCMS, Netzsch STA 449 F5 Jupiter integrated QMS 403 D Aeölos). In a measurement, the sample powder was placed in a platinum (Pt) crucible prior to analysis from 30 to 1000 °C (10 °C/min) in N2 flow (50 mL/min). The evolved species in TG-DSC were simultaneously analyzed by a mass spectrometer (MS) coupled with a heated (200 °C) capillary to identify the chemic

In continuation of the authors’ previous discussions on upgrading the aqueous-phase compounds in bio-oil, the state of the art for upgrading compounds in the oil phase and real bio-oil are presented in this chapter. Specifically, they focus on hydrotreating/cracking the phenolics, furanics, or heavier oligomers, as well as the approaches discussed in the last chapter (e.g. steam reforming), to produce valuable fuels/chemicals. In addition, the advances of catalyst development and a reaction mechanism are also discussed, in an attempt to correlate the catalyst structure with performances (e.g. oxophilicity vs hydrodeoxygenation), serving as a basis of design of selective and durable catalysts for bio-oil upgrading.

Ti3C2Tx MXene is a member of the recently discovered two-dimensional early transition metal carbide and nitride family of MXenes with potential applications in energy storage and heterogeneous catalysis at elevated temperatures. Here, we apply a suite of in situ techniques to probe Ti3C2Tx MXene's thermal evolutions, including in situ X-ray diffraction (XRD), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and integrated thermogravimetry-differential scanning calorimetry-mass spectrometry (TG-DSC-MS). In light of this set of in situ investigations, we find heterogeneity in the layering of Ti3C2Tx MXene revealed only at higher temperatures. Our findings present behavior up to 600 °C, particularly interlayer water and -OH surface end-capping groups. In one group of layers, their interlayer spacing shrinks as water deintercalates, but the other group of layers unexpectedly shows no change in the interlayer spacing. This is strong evidence that intercalants act as guest pillaring agents in the latter layering group, which stabilize these layers at higher temperatures while keeping the interlayer space accessible.