Bulk Catalysts Research Articles

Bulk hydrotreating MonW12-nS2 catalysts based on SiMonW12-n heteropolyacids prepared by alumina elimination method

A series of unsupported mono- and bimetallic MonW12-nS2 catalysts were synthesized by alumina elimination from supported MonW12-nS2/Al2O3 samples using acid etching. Alumina supported catalysts have been in turn prepared by using monometallic H4SiMo12O40 and H4SiW12O40 heteropolyacids (HPAs), their mixture with Mo/W atomic ratio equal to 1/11 and 3/9, and mixed bimetallic H4SiMo1W11O40 and H4SiMo3W9O40 HPAs. All catalysts were characterized by N2 adsorption, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), extended X-ray absorption fine structure (EXAFS) spectroscopy and powder X-ray diffraction (XRD) and their performance were evaluated in simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of naphthalene. The etching process led to a successful removal of all the support and of the partially sulfided species, with sulfidation degrees of both Mo and W above 90 % on the final bulk solids. The active phase also underwent a rearrangement, as higher average length and stacking were measured on the bulk catalysts than on the original supported ones. Mixed MoWS2 phase was evidenced in all solids, prepared from mixed HPAs (MonW12-nS2) or from the mixture of monometallic HPAs (RefMonW12-nS2), by EXAFS and ToF-SIMS, with however a larger quantity on the MoW solids. It seems that the mixed MoWS2 phase observed on the supported MoW catalysts is maintained through the etching process, while on RefMonW12-nS2 the mixed phase, observed in a much lesser extent in the corresponding supported catalyst, could result from the aggregation of the monometallic slabs. MonW12-nS2 catalysts were found more effective than the monometallic catalysts and than the corresponding RefMonW12-nS2, in both dibenzothiophene hydrodesulfurization and naphthalene hydrogenation, which was related to the presence of the mixed phase maintained through the etching of the support.

Controlling Selectivity and Stability in the Hydrocarbon Wet-Reforming Reaction Using Well-Defined Ni + Ga Intermetallic Compound Catalysts

In this study, we present the use of compositionally and structurally well-defined, oxide-supported nanoparticle Ni + Ga intermetallic compound (IMC) catalysts in the wet reforming of propane. The definition of the IMC catalysts allowed for more direct connections to be made between catalyst bulk and surface compositions and catalyst performance in wet reforming. We show that Ni + Ga catalysts exhibit comparable or better rates of reaction on a per-site basis and improved stability in comparison to other leading formulations. We also demonstrate excellent control over product selectivity as a function of Ni + Ga IMC bulk and surface compositions with nearly ideal selectivity toward CO2/H2 or CO/H2 achieved. Selectivity toward the production of smaller hydrocarbons could also be suppressed significantly due to uniquely limited rehydrogenation kinetics of the Ni + Ga IMCs. Our studies also shed light on the stability of Ni + Ga IMCs under reaction conditions and how high conversion in reactions that involve many strongly bound reaction intermediates can lead to IMC phase relaxation to the most stable phase with concomitant surface composition and catalytic performance changes. Correlations between surface chemistry and catalyst performance were afforded by both the well-defined nature of the IMCs and computational surface science studies.

Oxygen vacancy–engineered δ-MnOx/activated carbon for room-temperature catalytic oxidation of formaldehyde

Although oxygen vacancies (OVs) commonly act as adsorption/active sites in catalytic oxidation of formaldehyde (HCHO), thereby strongly influencing catalyst activity, their control and translation into scale-up products for practical application remain challenging. Herein, δ-MnOx/activated carbon was synthesized via in situ reduction coupled with ammonia modification, and the developed method was found to allow easy OV control for large-scale production. OV concentration was effectively regulated through adjustment of Mn3+ content, and OV roles in the catalytic reaction were probed by several techniques. The optimized catalyst featured superior HCHO removal efficiency and CO2 selectivity at room temperature, mainly due to oxygen activation by abundant OVs to form reactive oxygen species. The intermediates and pathways of HCHO removal were investigated. Thus, this work provides insights into the enhancement of active site exposure through OV control for a single bulk catalyst and demonstrates its applicability for efficient and commercially viable room-temperature oxidation of HCHO.

Recent Advances and Challenges in 2D Metal-Free Electrocatalysts for N2 Fixation.

The ever-growing requirement of ammonia for industry and energy supply motivates people to find clean and cost-effective alternatives to overcome the shortcomings of the century-old Haber-Bosch process. Electrocatalytic N2 reduction (NRR) is considered a prospective way for ammonia production at ambient conditions. Recently, two-dimensional (2D) metal-free materials are emerging as highly efficient and robust electrocatalysts for NRR owing to their advanced features: highly exposed surface, abundant active sites, tunable electronic states, and long-term stability compared to metal-based and bulk catalysts. In this minireview, we briefly summarize the latest edge reports on 2D metal-free materials catalyzed NRR. Also, we discuss the challenges and perspectives on the design and synthesis of novel 2D metal-free catalysts.

Deactivation of Ni spinel derived catalyst during the oxidative steam reforming of raw bio-oil

Deactivation of a bulk catalyst derived from NiAl2O4 spinel during the oxidative steam reforming (OSR) of raw bio-oil has been studied. The experiments were performed in a continuous system with two units in series: a thermal treatment unit at 500 °C for the controlled deposition of pyrolytic lignin, and a fluidized bed reactor (700 °C; S/C, 6; O/C, 0.34; space time, 0.15 gcatalysth·gbio-oil−1; time on stream, 1, 2, 4 and 6 h) for the OSR of the remaining oxygenates. The deactivation affects the reforming of bio-oil oxygenates according to their reactivity (from lower to higher), with the reforming of phenols being rapidly affected. The causes of deactivation are: i) coke deposition on the Ni0 sites and on the Al2O3 support (6 wt% of each coke type after 6 h on stream), and; ii) sintering of Ni0 crystals (with an increase in crystal size from 10.8 to 17.7 nm (measured by TEM)). The catalyst deactivation rate increases with time on stream, with the bio-oil oxygenates being the main coke precursors.

Tunable Fe3O4 nanoparticles assembled porous microspheres as catalysts for Fischer-Tropsch synthesis to lower olefins

Fe catalyzed Fischer-Tropsch synthesis to lower olefins (FTO) has been recognized as a structure-sensitive reaction. Nanostructure Fe-based catalysts can expose more active surface. But bulk nanostructure Fe catalysts without support interaction show easily sintered and agglomerated, which makes hard to investigate size effect. Therefore, the effect of particle size on bulk Fe catalysts have rarely been reported. Herein, nanoparticles assembled Fe3O4 bulk catalysts were synthesized via a PAA-mediated solvothermal method to solely investigate the size effect of Fe phase. By tuning preparation parameters, a series of porous-Fe3O4 microspheres assembled by different nanoparticle size (8.5–16.5 nm) was obtained, maintaining constant microspheres size. When Fe3O4 nanoparticles are smaller than 10 nm, catalysts show similar catalytic activity (FTY). Beyond 10 nm, the FTY obviously decreases with increasing of particle size. Particularly, Fe3O4 with nanoparticle size of 9.9 nm performs the highest activity as well as C2-C4= selectivity and O/P ratio. The smaller particles attribute to C5+ formation, while inhibit CH4 selectivity. By CO-TPD, TPH-MS and XRD analysis, we discuss size effect on CO adsorption, surface carbon species and carburization degree, building relationship between particle size and catalytic activity. It is found that the carburization degree of iron phase correlates positively with catalytic activity. These results deepen understanding of the structure-performance relationship for bulk iron catalysts in FTO.

Catalytic Behavior of Alkali Treated H-MOR in Selective Synthesis of Ethylenediamine via Condensation Amination of Monoethanolamine

Catalytic behavior of alkali treated mordenite (H-MOR) in selective synthesis of ethylenediamine (EDA) via condensation amination of monoethanolamine (MEA) was investigated. Changes in the structural and acidic properties of alkali treated H-MOR were systematically investigated by N2 adsorption/desorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), temperature programmed ammonia desorption (NH3-TPD), pyridine adsorption was followed by infrared spectroscopy (Py-IR), and X-ray fluorescence (XRF) analyses. The results show that alkali treatment produces more opening mesopores on the H-MOR crystal surfaces and leads to an increase in the number of B acid sites and the strength of the acid sites. The mesopores effectively enhance the rate of diffusion in the bulk catalyst. Moreover, the B acid sites are active sites in selective synthesis of EDA. Due to improvements in the diffusion conditions and reactivities, alkali treated H-MOR shows an excellent catalytic performance under mild reaction conditions. The conversion of MEA was 52.8% and selectivity to EDA increased to 93.6%, which is the highest selectivity achieved so far. Furthermore, possible mechanism for the formation of EDA is discussed.

Three-dimensionally macroporous MnZrOx catalysts for propane combustion: Synergistic structure and doping effects on physicochemical and catalytic properties

Three-dimensionally macroporous (3DM) MnZrOx catalysts were fabricated to reveal the structure and Zr-doping effects on both physicochemical properties and propane combustion behaviors. The increasing addition of zirconium is favorable for the formation of 3DM structure and amorphous Mn-Zr solid solution, leading to tunable physicochemical properties. The significant activity improvement after zirconium addition was originally attributable to the superior redox ability, higher oxygen mobility and more abundant oxygen vacancy. The excellent catalytic activity, cycling stability and water resistant ability over 3DM Mn0.6Zr0.4Ox make it a promising material for hydrocarbons elimination. The comparative TPSR, in situ DRIFTs and kinetics study over 3DM and bulk catalysts emphasize the advantageous function of 3DM architecture on promoting propane adsorption, oxidation and lattice oxygen mobility.

C–O bond activation using ultralow loading of noble metal catalysts on moderately reducible oxides

Selective C–O activation of multifunctional molecules is essential for many important chemical processes. Although reducible metal oxides are active and selective towards reductive C–O bond scission via the reverse Mars–van Krevelen mechanism, the most active oxides undergo bulk reduction during reaction. Here, motivated by the enhanced oxide reducibility by metals, we report a strategy for C–O bond activation by doping the surface of moderately reducible oxides with an ultralow loading of noble metals. We demonstrate the principle using highly dispersed Pt anchored onto TiO2 for furfuryl alcohol conversion to 2-methylfuran. A combination of density functional theory calculations, catalyst characterization (scanning transmission electron microscopy, electron paramagnetic resonance, Fourier-transform infrared spectroscopy and X-ray absorption spectroscopy), kinetic experiments and microkinetic modelling expose substantial C–O activation rate enhancement, without bulk catalyst reduction or unselective ring hydrogenation. A methodology is introduced to quantify various types of sites, revealing that the cationic redox Pt on the TiO2 surface is more active than metallic sites for C–O bond activation. Reducible metal oxides selectively catalyse the hydrodeoxygenation of C–O bonds in bio-based aromatic molecules, although they show limited performance. Now, using TiO2 as an example, a method is reported to enhance the activity of the oxide by surface doping with an ultralow loading of Pt.

Deactivation of hydrodesulfurization catalysts during reactor startup

AbstractMetal sulfide catalysts for ultra‐deep hydrodesulfurization of diesel generally lose a fraction of their catalytically active sites during reactor startup. The underlying mechanisms are discussed. A laboratory diagnostic tool consisting of three probe molecules is developed for testing metal sulfide catalysts' start‐of‐run (SOR) activity maintenance. It is found that a significant fraction of the active sites on a commercial supported catalyst are deactivated permanently, but this is not the case with a bulk metal sulfide catalyst. The SOR deactivation of the bulk catalyst is completely reversible, while that of the supported catalyst is partially reversible. The diagnostic tool may provide a basis for developing a high‐throughput approach for evaluating and enhancing catalyst SOR stability, thereby increasing plant productivity.

CO2 methanation and reverse water gas shift reaction. Kinetic study based on in situ spatially-resolved measurements

The reaction kinetics for the CO2 methanation and reverse water gas shift reaction over an ordered-mesoporous Ni/Al2O3 catalyst were determined. For the parameter estimation and model discrimination, the kinetic data were obtained by means of spatially-resolved measurement in a catalytic plate reactor. In detail, ~21,000 high-resolution gas composition data were gathered along the reactor axis using a movable sampling capillary connected to a mass spectrometer. Additionally, the catalyst surface temperature was determined via infrared thermography. The influence of reaction temperature (320–420 °C), total pressure (1.2–7.3 barabs), and GHSV, as well as possible inhibition of products such as CH4 and H2O, were investigated.A one-dimensional model of the reactor was developed describing the conservation of mass in the bulk gas and catalyst phase. The Bayesian approach was used to estimate the kinetic parameters of 20 proposed Langmuir-Hinshelwood rate expressions for the CO2 methanation that were derived based on three different mechanisms (i.e., direct dissociation, hydrogen assisted dissociation, and hybrid mechanism). Two kinetic models reflected the measured data very well. The most probable models suggest that the rate determining step includes the reaction of an oxygenated complex (COH* or HCOO*) with an active site (*) or an adsorbed hydrogen (H*). Furthermore, water was assumed to be adsorbed as a hydroxyl species (OH*), while methane did not influence the reaction. Temperature- and time-resolved Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements confirmed the presence of both adsorbed surface intermediates.

CO2 Methanation on Fe Catalysts Using Different Structural Concepts

AbstractThe hydrogenation of carbon dioxide towards methane and water is evaluated on different types of iron catalysts. The catalysts refer to different structural concepts implying a bare iron oxide, a silica‐supported and a core‐shell system. Highest CO2 conversion of about 20 % is achieved with the bulk catalysts and the supported material. However, although revealing reduced CH4 formation rate, the core‐shell catalyst exhibits pronounced resistance against coke formation as well as thermal sintering and particle attrition upon syngas reaction.

Application of Dolomite for tar mitigation in downdraft gasifiers: An experimental analysis

The biomass gasifiers have been adopted to generate producer gas widely across the globe. The producer gas finds application in automotive sectors in spark and combustion ignition engines. Furthermore the producer gas is used in power production and thermal applications. Inspite of the wide applications of biomass gasifiers it is not broadly employed due to its inherent disadvantages. The formation of tar along with the producer gas is a major bottleneck in applying producer gas as a viable alternative fuel. The tar mitigation methods viz., physical, thermal and catalytic tar removal methods has been employed to refine the producer gas. From published literature it is evident that many bulk and nano catalysts have been investigated for tar cracking. The dolomite mineral is a potential material for tar cracking in producer gas but has not been investigated widely. In this study the tar cracking capability of dolomite has been investigated in a downdraft gasifier. Two important tar cracking influencing parameters viz., dolomite tar cracking temperature and dolomite quantity required for tar cracking has been investigated. From the experimental study it is eviden that the optimum dolomite operating temperature is 700 °C and the optimum dolomite quantity is 200 g.

Three-dimensionally ordered macro-mesoporous CoMo bulk catalysts with superior performance in hydrodesulfurization of thiophene.

The introduction of surfactants during the fabrication of hydrodesulfurization catalysts could not only tune the microstructure but also promote the dispersion of active components. In this work, CoMo bulk catalysts with the hierarchical structure of three-dimensionally ordered macro–mesopores were successfully fabricated by using a colloidal crystal template with the addition of PEG 400 and/or F127 surfactants. The obtained samples were characterized by various techniques, and the possible mechanism of the structure formation was also discussed. The characterization and evaluation results reveal that the addition of surfactants can promote the formation of the mesopores (3–4 nm) inside the macroporous walls of these bulk catalysts, which is essential for the increase of catalyst surface area, and the active sites for reaction. The CoMo–PF-1 catalyst displayed superior catalytic performance for thiophene hydrodesulfurization with the thiophene conversion of 99.4% under 1 MPa at 360 °C, which is much higher than that (77.8%) at 0.1 MPa. This result is even comparable to our previous report with the thiophene conversion of 99.2% over the 3DOM CoMo catalyst under 3 MPa.

Highly Active Bulk Mo(W)S2 Hydrotreating Catalysts Synthesized by Etching out of the Carrier from Supported Mono- and Bimetallic Sulfides

A bulk MoWS2 catalyst has been synthesized by acid etching of the carrier from the supported MoWS2/Al2O3 catalyst obtained on the basis of the mixed bimetallic heteropoly acid (HPA) H4[SiMo3W9O40]. As reference samples, monometallic MoS2 and WS2 catalysts have been prepared from the corresponding supported analogues, as well as a Mo + WS2 sample based on a mechanical mixture of monometallic HPA in the atomic ratio of Mo/W = 1/3. The catalytic properties of the synthesized catalysts have been studied in model reactions of hydrodesulfurization (HDS) of dibenzthiophene (DBT) and hydrogenation (HYD) of naphthalene in a flow unit. It has been shown that the catalytic activity of the samples in both the DBT HDS and naphthalene HYD reactions increases in the following order: MoS2 < WS2 < Mo + WS2$$ \ll $$ MoWS2. It has been found that the bulk tungsten-containing catalysts exhibit higher specific catalytic activity than the supported counterparts. Increased values of hydrogen uptake according to the results of hydrogen temperature-programmed reduction for the bulk catalysts indicate an increase in the number of active sites and the formation of a more effective active phase compared to supported catalysts.

Influence of Oxygen-Containing Compounds on Conversion and Selectivity of Dibenzotiophene and Naphthaline on Bulk and Supplied Co(Ni)MoS2 Catalysts

The supported CoMoS2/Al2O3 and NiMoS2/Al2O3 catalysts were prepared by impregnating of alumina to incipient wetness with aqueous solutions of 12-molybdophosphoric heteropoly acid and nickel or cobalt citrates. A bulk Ref-MoS2 catalyst was obtained by thermal decomposition of ammonium tetratiomolybdate. The synthesized catalysts were investigated by low temperature nitrogen adsorption and high resolution transmission electron microscopy. The catalytic properties were studied in the dibenzothiophene hydrodesulfurization and naphthalene hydrogenation in the presence of dodecanoic acid or guaiacol in a flow unit with a microreactor under hydrogen pressure. It was shown that the bulk catalyst Ref-MoS2 has minimal sensitivity to dodecanoic acid and guaiacol during the combined hydrotreatment of dibenzothiophene and naphthalene. The effective adsorption constants of dodecanoic acid and guaiacol were calculated using the Langmuir—Hinshelwood model.

Three-dimensionally ordered macroporous bulk catalysts with enhanced catalytic performance for thiophene hydrodesulfurization

The microstructure tuning of bulk catalysts would significantly influence their physical/chemical properties for the subsequent applications. In this work, the three-dimensionally ordered macroporous (3DOM) bulk catalysts (CoMo, CoMoW and CoW) were successfully fabricated by employing the poly-(methyl methacrylate) (PMMA) as hard template. The prepared bulk catalysts were characterized by means of techniques, such as TG/DTG, XRD, low-temperature N2 adsorption-desorption, SEM, etc. The characterization and catalytic performance evaluation results indicated that the unique hierarchical pore system of the 3DOM bulk catalysts could not only supply more available surface area, thus more active sites, but also facilitate the diffusion of the reactants/products to/from the active sites of the bulk catalyst. Moreover, the 3DOM structure of the bulk catalysts was retained after the pre-sulfurization process and hydrodesulfurization reaction, illustrating the relatively high stability of prepared bulk catalysts in the experimental investigation. The 3DOM CoMo bulk catalyst exhibited superior catalytic performance with thiophene conversion of 99.2% at 360 °C in comparison with the CoW and CoMoW bulk catalysts.

Designing Catalytic Sites on Surfaces with Optimal H-Atom Binding via Atom Doping Using the Inverse Molecular Design Approach.

It remains a general challenge to computationally design optimal catalytic structures based on earth-abundant metals for hydrogenation. Here, we demonstrate an effective computational approach based on inverse molecular design to deterministically design optimal catalytic sites on the Cu(100) surface through the doping of Fe and/or Zn, and a stable Zn-doped Cu(100) surface was found with minimal binding energy to H atoms. By the calculations at the level of density functional theory, the optimized catalyst sites are verified to be valid on the Cu(100) surface in an infinite periodic system. We analyze the electronic structure cause of the optimal binding sites using the analysis of the density of states. In addition, we use a Cu29Zn3 atomic cluster, where such an optimum catalytic site is valid on the Cu(100) surface, to understand the role of doped Zn atoms on lowering the H atom binding energy. We found that in the atomic cluster, the atomic orbitals of surface Zn-atoms show less participation in the binding of H atoms, compared to the atomic orbitals of surface Cu atoms. Our study provides valuable chemistry insights on designing catalytic structures using earth-abundant metals, and it may lead to the development of novel Cu-based earth-abundant alloys in bulk, nanoparticles, atomic clusters, or single-atom catalysts for important catalytic applications such as lignin degradation or CO2 conversion.

New Insights into the Catalytic Activity of Cobalt Orthophosphate Co3 (PO4 )2 from Charge Density Analysis.

An extensive characterization of Co3(PO4)2 was performed by topological analysis according to Bader‘s Quantum Theory of Atoms in Molecules from the experimentally and theoretically determined electron density. This study sheds light on the reactivity of cobalt orthophosphate as a solid‐state heterogeneous oxidative‐dehydration and ‐dehydrogenation catalyst. Various faces of the bulk catalyst were identified as possible reactive sites given their topological properties. The charge accumulations and depletions around the two independent five‐ and sixfold‐coordinated cobalt atoms, found in the topological analysis, are correlated to the orientation and population of the d‐orbitals. It is shown that the (011) face has the best structural features for catalysis. Fivefold‐coordinated ions in close proximity to advantageously oriented vacant coordination sites and electron depletions suit the oxygen lone pairs of the reactant, mainly for chemisorption. This is confirmed both from the multipole refinement as well as from density functional theory calculations. Nearby basic phosphate ions are readily available for C−H activation.

Optimization of a fluidized bed reactor for methane decomposition over Fe/Al2O3 catalysts: Activity and regeneration studies

Catalytic methane decomposition was investigated over 40 wt% Fe/Al2O3 catalyst in fluidized bed reactor (FLBR). After optimization of FLBR conditions in terms of catalyst bulk density, particle size, minimum fluidization velocity, and the catalyst bed height, the catalyst activity and stability tests were conducted by comparison with a fixed bed reactor (FBR). Although a similar stable methane conversion was obtained over both reactors, the pressure drop during 35 min operation of FBR was 9 times higher than that of FLBR, which indicated the possibility of continuous operation of methane decomposition process over FLBR. Further, the influence of the space velocity, feed dilution and regeneration on catalysts reactivity was studied in FLBR to conclude that a reaction condition of 12 L/gcat∙h, feed of 20%H2–80%CH4 and CO2-regeneration of deactivated catalysts may be favourable for operating methane decomposition in FLBR continually and effectively to provide stable hydrogen.