Determination of the chemical nature of active surface sites present on bulk mixed metal oxide catalysts

CH3OH temperature programmed surface reaction (TPSR) spectroscopy was employed to determine the chemical nature of active surface sites for bulk mixed metal oxide catalysts. The CH3OH-TPSR spectra peak temperature, Tp, for model supported metal oxides and bulk, pure metal oxides was found to be sensitive to the specific surface metal oxide as well as its oxidation state. The catalytic activity of the surface metal oxide sites was found to decrease upon reduction of these sites and the most active surface sites were the fully oxidized surface cations. The surface V5+ sites were found to be more active than the surface Mo6+ sites, which in turn were significantly more active than the surface Nb5+ and Te4+ sites. Furthermore, the reaction products formed also reflected the chemical nature of surface active sites. Surface redox sites are able to liberate oxygen and yield H2CO, while surface acidic sites are not able to liberate oxygen, contain either H+ or oxygen vacancies, and produce CH3OCH3. Surface V5+, Mo6+, and Te4+ sites behave as redox sites, and surface Nb5+ sites are Lewis acid sites. This experimental information was used to determine the chemical nature of the different surface cations in bulk Mo-V-Te-Nb-Ox mixed oxide catalysts (Mo(0.6)V(1.5)Ox, Mo(1.0)V(0.5)Te(0.16)Ox, Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox). The bulk Mo(0.6)V(1.5)Ox and Mo(1.0)V(0.5)Te(0.16)Ox mixed oxide catalytic characteristics were dominated by the catalytic properties of the surface V5+ redox sites. The surface enrichment of these bulk mixed oxide by surface V5+ is related to its high mobility, V5+ possesses the lowest Tammann temperature among the different oxide cations, and the lower surface free energy associated with the surface termination of V=O bonds. The quaternary bulk Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox mixed oxide possessed both surface redox and acidic sites. The surface redox sites reflect the characteristics of surface V5+ and the surface acidic sites reflect the properties normally associated with supported Mo6+. The major roles of Nb5+ and Te4+ appear to be that of ligand promoters for the more active surface V and Mo sites. These reactivity trends for CH3OH ODH parallel the reactivity trends of propane ODH because of their similar rate-determining step involving cleavage of a C-H bond. This novel CH3OH-TPSR spectroscopic method is a universal method that has also been successfully applied to other bulk mixed metal oxide systems to determine the chemical nature of the active surface sites.     

Nanocarbons: Opening New Possibilities for Nano-engineered Novel Catalysts and Catalytic Electrodes

Nanocarbons find increasing relevance for the development of advanced, sometimes radically new, catalysts and catalytic electrodes. This perspective contribution discusses the potential of nanocarbons as a new class of catalytic materials, even if carbons (in the form mainly of active carbon materials) are already extensively applied as supports for catalysts. The control of nano-dimension and the improved understanding in tailoring the surface reactivity open new possibilities for their nano-engineering and the development of novel catalytic materials. With focus on the nature of the active sites in nanocarbon catalysts, we discuss here some of the novel possibilities opened by these materials to address the new challenges for catalysis deriving from moving to a more sustainable chemical and energy production.     

B-MWW Zeolite: The Case Against Single-Site Catalysis

Boron-containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron-containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B₂(OH)_xO_(3−x/2) (x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM-22 isomorphously substituted with boron (B-MWW). Using ¹¹B solid-state NMR spectroscopy, we show that the majority of boron species in B-MWW exist as isolated BO₃ units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B-MWW for ODH of propane falsifies the hypothesis that site-isolated BO₃ units are the active site in boron-based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium-based catalysts and provides an important piece of the mechanistic puzzle.     

B‐MWW Zeolite: The Case Against Single‐Site Catalysis

Boron‐containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron‐containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B₂(OH)_xO_(3?x/2) (x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM‐22 isomorphously substituted with boron (B‐MWW). Using ¹¹B solid‐state NMR spectroscopy, we show that the majority of boron species in B‐MWW exist as isolated BO₃ units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B‐MWW for ODH of propane falsifies the hypothesis that site‐isolated BO₃ units are the active site in boron‐based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium‐based catalysts and provides an important piece of the mechanistic puzzle.     

静电纺丝法制备硼掺杂氧化硅纳米纤维用于低碳烷烃氧化脱氢制烯烃

乙烯和丙烯等低碳烯烃是重要的基础有机化工产品,广泛应用于化工生产的各个领域.相比于其他工艺,低碳烷烃氧化脱氢制烯烃工艺具有不受热力学平衡限制、无积炭等特点而被广泛研究.近年发现六方氮化硼(h-BN)、硼化硅(SiB6)和磷酸硼(BPO4)等非金属硼基催化剂能够高效催化烷烃氧化脱氢反应,并抑制产物烯烃的过度氧化,表现出高的催化活性和烯烃选择性.大量的研究表明,硼基催化剂活性起源于催化剂表面的"BO"物种(如B-O和B-OH等基团).氧化硼(B2O3)作为一种氧化气氛中化学性质稳定的含硼化合物,兼具丰富的"BO"位点,在反应条件下可形成多种结构以适用不同的化学环境,为制备高效的烷烃氧化脱氢催化剂提供了可能.在之前的研究中,多将B2O3浸渍在常规的TiO2,SiO2,A12O3等三维多孔载体上用于氧化脱氢反应.考虑到B2O3结构的灵活性和易于成键特性,需开发更为有效的合成策略,以提升B2O3催化剂在氧化脱氢反应中的活性和稳定性.本文采用静电纺丝技术合成了直径为100～150 nm的多孔掺硼二氧化硅纳米纤维(PBSN)用于低碳烷烃氧化脱氢反应.静电纺丝法合成的催化剂中硼物种在开放的氧化硅纤维骨架上均匀分散且稳定固载.一维纳米纤维结构不仅有利于扩散,且赋予催化剂在高重时空速(WHSV)条件下优异的烷烃氧化脱氢反应活性.在乙烷氧化脱氢反应中,当乙烷的转化率达到44.3％时,乙烯的选择性和产率分别为84％和44.2 μmol gcat-1 s-1.而在丙烷脱氢反应中,当丙烷转化率为19.2％时,总烯烃选择性及丙烯产率分别为90％和76.6 μmol gcat-1 s-1.在温度为545 ℃,丙烷WHSV高达84.6 h-1的条件下,催化剂保持长时间稳定.与其他负载型氧化硼催化剂相比,PBSN催化剂具有更高的烯烃选择性和稳定性.研究表明,在氧化硅负载B2O3催化剂催化丙烷氧化脱氢反应中,载体中Si-OH基团的存在可能会降低丙烯的选择性.瞬态分析和动力学实验表明,硼基催化剂催化烷烃氧化脱氢反应过程中O2的活化受到烷烃的影响.本文不仅为高效硼基催化剂的合成提供了新思路,也为深入理解该类催化剂上烷烃氧化脱氢反应过程提供了实验支撑.     

纳米碳材料非金属催化的研究进展

纳米碳材料直接作为催化剂的非金属碳催化是目前材料科学与催化领域的前沿方向之一.相对于传统金属催化剂,纳米碳材料催化剂具有高效环保、低能耗、耐腐蚀等优点.在烃类转化、化学品合成、能源催化等领域表现出优异的催化性能和发展潜力.综述了近年来纳米碳非金属催化研究的最新进展,主要包括新型纳米碳材料的表面性质、催化特性、反应机理和宏观制备等关键问题,并对纳米碳催化存在的挑战和前景进行了展望.     

Phosphate modified carbon nanotubes for oxidative dehydrogenation of n-butane

Catalytic performance of phosphate-modified carbon nanotube(PoCNT) catalysts for oxidative dehydrogenation(ODH) of n-butane has been systematically investigated. The Po CNT catalysts are characterized by SEM, TEM, XPS and TG techniques. We set the products selectivity as a function of butane conversion over various phosphate loading, and it is found that the PoCNT catalyst with the 0.8% phosphate weight loading(0.8PoCNT) exhibits the best catalytic performance. When the phosphate loading is higher than 0.8 wt%, the difference of catalytic activity among the PoCNT catalysts is neglectable. Consequently, the ODH of n-butane over the 0.8PoCNT catalyst is particularly discussed via changing the reaction conditions including reaction temperatures, residence time and n-butane/O_2 ratios. The interacting mechanism of phosphate with the oxygen functional groups on the CNT surface is also proposed.     

In situ UV-vis-NIR diffuse reflectance and Raman spectroscopy and catalytic activity studies of propane oxidative dehydrogenation over supported CrO3/ZrO2 catalysts

The molecular structures, oxidation states, and reactivity of 3 and 6% CrO3/ZrO2 catalysts prepared by incipient wetness impregnation were examined under different conditions. The in situ Raman spectroscopic studies under dehydrated conditions reveal that the 3 and 6% CrO3/ZrO2 catalysts possess equal amounts of monochromate and polychromate species. Consequently, monolayer coverage on this ZrO2 support is about 3% CrO3. The 6% CrO3/ZrO2 possesses an additional Raman band due to Cr2O3 crystals corresponding to the remaining 3% CrO3. Furthermore, during reaction conditions the polychromate species is preferentially reduced, the monochromate species are slightly affected, and the Cr2O3 crystals are not affected. The in situ UV-vis-NIR diffuse reflectance spectroscopy results reveal that under steady-state reaction conditions the extent of reduction and edge energy position of surface Cr6+ cations increase with an increase in reduction environment for the 3 and 6% CrO3/ZrO2 samples. Propane oxidative dehydrogenation (ODH) studies reveal that the catalytic activity expressed in moles of propane converted per gram catalyst per second is similar for the two catalysts, which is consistent with equal amounts of molecularly dispersed chromia present. The turnover frequency for the 6% CrO3/ZrO2 catalyst is, however, smaller than that for the 3% CrO3/ZrO2 sample due to the presence of Cr2O3 crystals, which are relatively inactive for propane ODH. For this catalytic system and for the experimental conditions used, propene, CO, and CO2 are primary products. Furthermore, the 33-39% propene selectivity is not affected by the C3H8/O2 ratio for both catalysts. Structure-reactivity studies suggest that the molecularly dispersed species are present in equal amounts in the 3 and 6% CrO3/ZrO2 samples as Cr6+ monochromate and polychromate species are the most effective catalytic active sites taking part in the propane ODH reaction.     

Intrinsic activity modulation and structural design of NiFe alloy catalysts for an efficient oxygen evolution reaction

NiFe alloy catalysts have received increasing attention due to their low cost, easy availability, and excellent oxygen evolution reaction (OER) catalytic activity. Although it is considered that the co-existence of Ni and Fe is essential for the high catalytic activity, the identification of active sites and the mechanism of OER in NiFe alloy catalysts have been controversial for a long time. This review focuses on the catalytic centers of NiFe alloys and the related mechanism in the alkaline water oxidation process from the perspective of crystal structure/composition modulation and structural design. Briefly, amorphous structures, metastable phases, heteroatom doping and in situ formation of oxyhydroxides are encouraged to optimize the chemical configurations of active sites toward intrinsically boosted OER kinetics. Furthermore, the construction of dual-metal single atoms, specific nanostructures, carbon material supports and composite structures are introduced to increase the abundance of active sites and promote mass transportation. Finally, a perspective on the future development of NiFe alloy electrocatalysts is offered. The overall aim of this review is to shed light on the exploration of novel electrocatalysts in the field of energy.

Effective strategies to increase the intrinsic activity by electronic modulation and to increase the number of active sites by structural design are discussed for improving the oxygen evolution activities of NiFe alloys.     

Hydrogenation of Furfural to Furfuryl Alcohol over Co-B Amorphous Catalysts Prepared by Chemical Reduction in Variable Media

Five Co-B amorphous alloy catalysts were prepared by chemical reduction in different media, including pure water and pure ethanol as well as the mixture of ethanol and water with variable ethanol content, Their catalytic properties were evaluated using liquid phase furfural hydrogenation to furfuryl alcohol as the probe reaction. It was found that the reaction media had no significant influence on either the amorphous structure of the Co-B catalyst or the electronic interaction between metallic Co and alloying B. This could successfully account for the fact that all the as-prepared Co-B catalysts exhibited almost the same selectivity to furfuryl alcohol and the same activity per surface area （ Rs ）, which could be considered as the intrinsic activity, since the nature of active sites remained unchanged. However, the activity per gram of Co （ R^mH ） of the as-prepared Co-B catalysts increased rapidly when the ethanol content in the water-ethanol mixture used as the reaction medium for catalyst preparation increased. This could be attributed to the rapid increase in the surface area possibly owing to the presence of more oxidized boron species which could serve as a support for dispersing the Co-B amorphous alloy particles.     

A Bait-and-Switch Method for the Construction of Artificial Esterases for Substrate-Selective Hydrolysis

Outcomes of chemical reactions are generally dominated by the intrinsic reactivities of reaction partners, but enzymes frequently override such constraints to transform less reactive molecules in the presence of more reactive ones. Despite the attractiveness of such catalysis, it is difficult to build synthetic catalysts with these features. Micellar imprinting is a powerful method to create template-complementary binding sites inside protein-sized water-soluble nanoparticles. When a photocleavable functional monomer was used to bind two phosphonate/phosphate templates as transition-state analogues, active sites with predetermined size and shape were formed inside doubly cross-linked micelles through molecular imprinting. Postmodification replaced the binding group with a catalytic pyridyl group, forming highly selective artificial esterases. The catalysts displayed enzyme-like kinetics and turnover numbers that were in the hundreds. The selectivity of the catalysts, derived from the substrate-complementary imprinted active sites, enabled transformation of less reactive esters in the presence of more reactive ones.     

Role of support-nanoalloy interactions in the atomic-scale structural and chemical ordering for tuning catalytic sites

The understanding of the atomic-scale structural and chemical ordering in supported nanosized alloy particles is fundamental for achieving active catalysts by design. This report shows how such knowledge can be obtained by a combination of techniques including X-ray photoelectron spectroscopy and synchrotron radiation based X-ray fine structure absorption spectroscopy and high-energy X-ray diffraction coupled to atomic pair distribution function analysis, and how the support-nanoalloy interaction influences the catalytic activity of ternary nanoalloy (platinum-nickel-cobalt) particles on three different supports: carbon, silica, and titania. The reaction of carbon monoxide with oxygen is employed as a probe to the catalytic activity. The thermochemical processing of this ternary composition, in combination with the different support materials, is demonstrated to be capable of fine-tuning the catalytic activity and stability. The support-nanoalloy interaction is shown to influence structural and chemical ordering in the nanoparticles, leading to support-tunable active sites on the nanoalloys for oxygen activation in the catalytic oxidation of carbon monoxide. A nickel/cobalt-tuned catalytic site on the surface of nanoalloy is revealed for oxygen activation, which differs from the traditional oxygen-activation sites known for oxide-supported noble metal catalysts. The discovery of such support-nanoalloy interaction-enabled oxygen-activation sites introduces a very promising strategy for designing active catalysts in heterogeneous catalysis.     

Carbon-Doped BN Nanosheets for the Oxidative Dehydrogenation of Ethylbenzene

Carbon-based catalysts have demonstrated great potential for the aerobic oxidative dehydrogenation reaction (ODH). However, its widespread application is retarded by the unavoidable deactivation owing to the appearance of coking or combustion under ODH conditions. The synthesis and characterization of porous structure of BCN nanosheets as well as their application as a novel catalyst for ODH is reported. Such BCN nanosheets consist of hybridized, randomly distributed domains of h-BN and C phases, where C, B, and N were confirmed to covalent bond in the graphene-like layers. Our studies reveal that BCN exhibits both high activity and selectivity in oxidative dehydrogenation of ethylbenzene to styrene, as well as excellent oxidation resistance. The discovery of such a simple chemical process to synthesize highly active BCN allows the possibility of carbocatalysis to be explored.     

Heterogeneous partial oxidation catalysis on metal oxides

This review paper presents an overview of heterogeneous selective ammoxidation and oxidative dehydrogenation (ODH) of light alkanes, particularly of ethane. The conversion of ethane to ethene is in great demand in the domestic and worldwide chemical industry. The review has been voluntarily restricted to metal oxide-type catalysts, as it is devoted to the special issue honouring Edmond Payen and is based on 30 years of experience and discussions with pioneering scientists in the field. The main key factors, designated by Grasselli as the “7 pillars”, have been emphasised: isolation of active sites, M–O bond strength, crystalline structure, redox features, phase cooperation, multifunctionality and the nature of the surface oxygen species. The main features and physical and chemical properties of solid catalysts for selective oxidation compared to total oxidation have also been emphasised. Several case studies have been presented to illustrate the concept and importance of the key factors of catalyst preparation and activation and of the catalytic atmosphere. Based on such analysis and recent discoveries and process developments perspective views are also given.     

Copolymer-Induced Intermolecular Charge Transfer: Enhancing the Activity of Metal-Free Catalysts for Oxygen Reduction

Breaking the electroneutrality of sp² carbon lattice is a viable way for nanocarbon material to modulate the charge delocalization and to further alter the electrocatalytic activity. Positive charge spreadsheeting is preferable for catalyzing the oxygen reduction reaction (ORR) and other electrochemical reactions. Analogously to the case of intramolecular charge transfer by heteroatom doping, electrons in the conjugated carbon lattice can be redistributed by the intermolecular charge transfer from the nanocarbon material to the polyelectrolyte. A copolymeric electrolyte, epichlorohydrin-dimethylamine copolymer (EDC) was synthesized. The EDC-modified carbon nanotube (CNT) hybrid was subsequently fabricated by sonication treatment and served as a metal-free carbonaceous electrocatalyst with remarkable catalytic activity and stability. The resultant hybrid presents positive charge spreadsheeting on CNT as a result of the interfacial electron transfer from CNT to EDC. DFT calculations were further carried out to reveal that the enhancement of the wrapped EDC polyelectrolyte originates from the synergetic effect of the quaternary ammonium-hydroxyl covalently bonded structure. The CNT-EDC hybrid not only provides an atomically precise regulation to modulate nanocarbon materials from inactive carbonaceous materials into efficient metal-free catalysts, but it also opens new avenues to develop metal-free catalysts with well-defined and highly active sites.     

Carbon‐Doped BN Nanosheets for the Oxidative Dehydrogenation of Ethylbenzene

Carbon‐based catalysts have demonstrated great potential for the aerobic oxidative dehydrogenation reaction (ODH). However, its widespread application is retarded by the unavoidable deactivation owing to the appearance of coking or combustion under ODH conditions. The synthesis and characterization of porous structure of BCN nanosheets as well as their application as a novel catalyst for ODH is reported. Such BCN nanosheets consist of hybridized, randomly distributed domains of h‐BN and C phases, where C, B, and N were confirmed to covalent bond in the graphene‐like layers. Our studies reveal that BCN exhibits both high activity and selectivity in oxidative dehydrogenation of ethylbenzene to styrene, as well as excellent oxidation resistance. The discovery of such a simple chemical process to synthesize highly active BCN allows the possibility of carbocatalysis to be explored.     

钴/氮掺杂碳催化剂及其氧还原催化机理研究

以尿素做氮源、醋酸钴做金属源,用湿法合并高温热处理法合成了钴/氮共掺杂碳的非贵金属氧还原催化剂Co-N/C-T. 采用循环伏安（CV）法和线性扫描法（LSV）探究了氮源和金属源用量以及热处理温度对氧还原反应电催化活性的影响,活性最好的催化剂Co_0.13-N_0.3/C-800的峰电位达到0.829 V（vs.RHE）,接近商用Pt/C的活性,但比商用Pt/C有更好的耐甲醇性和稳定性. 同时,采用SEM,TEM,BET,XRD和XPS方法表征了催化剂结构和组分特征,并提出催化剂可能的电催化活性氧还原反应机理.     

Bridging the Surface Charge and Catalytic Activity of a Defective Carbon Electrocatalyst

Electrocatalysis is dominated by reaction at the solid–liquid–gas interface; surface properties of electrocatalysts determine the electrochemical behavior. The surface charge of active sites on catalysts modulate adsorption and desorption of intermediates. However, there is no direct evidence to bridge surface charge and catalytic activity of active sites. Defects (active sites) were created on a HOPG (highly oriented pyrolytic graphite) surface that broke the intrinsic sp²‐hybridization of graphite by plasma, inducing localization of surface charge onto defective active sites, as shown by scanning ion conductance microscopy (SICM) and Kelvin probe force microscopy (KPFM). An electrochemical test revealed enhanced intrinsic activity by the localized surface charge. DFT calculations confirmed the relationship between surface charge and catalytic activity. This work correlates surface charge and catalytic activity, providing insights into electrocatalytic behavior and guiding the design of advanced electrocatalysts.     

应用于氧还原反应的非贵金属原子分散级金属-氮-碳催化剂的设计

为了进一步实现质子交换膜燃料电池(PEMFC)能量转化技术的大规模开发和应用, 提高催化剂的成本效益是先决条件. 目前, 与铂族等贵金属基催化剂相比, 原子分散的金属-氮-碳(M-N-C)催化剂也在提高活性位点密度、 原子利用率和催化活性等方面表现出巨大的潜力, 是最有望代替铂基催化剂的首选材料. 在原子分散M-N-C催化剂的制备过程中, 获得活性位点均匀分散且结构体系最优化是挑战性问题. 基于此, 我们重点研究了各种有利于原子分散的M-N-C催化剂的制备方法, 以及不同催化剂中原子的化学环境调控对催化位点的影响. 本文从M-N-C催化剂的合成与表征、 反应机理、 密度泛函理论计算等方面进行了深入的探讨, 着重讨论了双金属位点、 原子簇结构和杂原子对催化位点的化学环境调控. 最后, 提出了原子分散M-N-C催化剂大规模应用存在的问题及进一步优化的发展方向.     

Nanocarbons for Catalytic Desulfurization

Nanocarbon catalysts are green and sustainable alternatives to metal‐based catalysts for numerous catalytic transformations. The application of nanocarbons for environmental catalysis is an emerging research discipline and has undergone rapid development in recent years. In this focus review, we provide a critical analysis of state‐of‐the‐art nanocarbon catalysts for three different catalytic desulfurization processes. In particular, we focus on the advantages and limitations as well as the reaction mechanisms of the nanocarbon catalysts at the molecular level.