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Characteristic Raman spectra for different samples: (I) MoO 2 after 10 h, (II) and (III) MoO x C y after 6 and 4 h of the reaction under reductive mixture of H 2 /n-C 7 at 370 C and (IV) starting metal oxide (MoO 3 ).
Source publication
A non-isothermal kinetic study of the oxidation of “carbon-modified MoO3” in the temperature range of 150–550°C by simultaneous TGA–DTA was investigated. During the oxidation process, two thermal events were detected, which are associated with the oxidation of carbon in MoOxCy and MoO2 to MoO3. The model-free and model-fitting kinetic approaches ha...
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... Fig. 2, four selected Raman spectra of MoO 3 and MoO x C y at 4, 6 and 10 h are shown. Raman bands at 471, 666, 820 and 996 cm À1 correspond to orthorhombic MoO 3 , while bands at 461, 495, 571, 589 and 744 cm À1 are characteristic of monoclinic MoO 2 obtained during the experiments, which is in agreement with our XRD results and previous ...
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... four selected Raman spectra of MoO 3 and MoO x C y at 4, 6 and 10 h are shown. Raman bands at 471, 666, 820 and 996 cm À1 correspond to orthorhombic MoO 3 , while bands at 461, 495, 571, 589 and 744 cm À1 are characteristic of monoclinic MoO 2 obtained during the experiments, which is in agreement with our XRD results and previous work [10,11]. Fig. 2 shows the evolution of the Raman spectra at different reaction times of the reduction process of the MoO 3 phase and its conversion into MoO 2 . The bands at 820 and 996 cm À1 significantly decreased in intensity and resolution as the reaction time is increased. Moreover, Fig. 2 illustrates that (MoO x C y ) phase is highly disordered, ...
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... is in agreement with our XRD results and previous work [10,11]. Fig. 2 shows the evolution of the Raman spectra at different reaction times of the reduction process of the MoO 3 phase and its conversion into MoO 2 . The bands at 820 and 996 cm À1 significantly decreased in intensity and resolution as the reaction time is increased. Moreover, Fig. 2 illustrates that (MoO x C y ) phase is highly disordered, which is in agreement with X-ray results (Fig. 1); this is due to the increase of oxygen defects and the incorporation of carbon in the oxide matrix (MoO 3 ). An increase, of oxygen vacancy concentration and the degree of substitution of oxygen by carbon atoms cause the ...
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... the other hand, after the treatment of MoO 3 with the reductive mixture, the Raman bands at 996 and 820 cm À1 decreased in intensity until they completely disappeared, with the formation of one new band at 744 cm À1 attributed to the bridging of Mo-O vibrations in the MoO 2 [10,11] (Fig. 2). These changes are associated with the degree of crystallization of the samples and the lower oxygen/metal ...
Citations
... The increase in weight could also be associated with the oxidation of segregated carbon forming deposits on the catalyst surface during treatment with oxidizing mixture [16]. ...
The present paper reports a study of molybdenum carbide as heterogeneous catalyst in the hydrodeoxygenation (HDO) reaction of bio-oils from acacia wood or empty fruit bunches of palm oil (EFB) obtained by hydrothermal liquefaction process. Mass molybdenum carbide was prepared using commercial MoO3 as precursor through the carburization methodology. The solid was characterized by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. For comparison purposes, the HDO reaction was also performed using a NiMo-based commercial catalyst, which was exposed to sulfidation previous to catalytic test. The reactions were performed in a batch-type reactor under reducing atmosphere (H2) at 350 °C and autogenous pressure. The hexagonal β-Mo2C phase obtained during the carburization process was highly active in the deoxygenation reaction, which resulted in enhanced HDO of bio-oils whose heating value is much higher than the heating value of untreated acacia (from 27.6 to 41.1 MJ/kg) and EFB bio-oils (from 29.1 to 36.9 MJ/kg). The higher heating value of the upgraded bio-oil with Mo2C and the ease recovery of the catalyst for recycling are promising advantages for the material to be used in relevant processes as an alternative to sulfided catalysts. On the other hand, the upgraded bio-oils have physicochemical characteristics suitable for their coupling in traditional fossil fuel production processes.
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... Los estudios relacionados con la cinética heterogénea en la actualidad pueden ser abordados de diversas maneras. Esta diversidad está sustentada en la variedad de regímenes de calentamiento, métodos, metodologías de cálculo a emplear, equipamiento, material utilizado y particularidades de las reacciones químicas objeto de estudio, lo que ha sido demostrado en los innumerables estudios cinéticos realizados [1,2]. ...
... Integrando la ecuación (1) y aproximando la integral de temperatura (la cual no tiene solución exacta) mediante la serie asintótica y tomando hasta el primer término de la serie, al aplicar logaritmos y reordenar, para un programa de calentamiento no isotérmico se tiene la ecuación (2): ...
... Y al graficar el miembro izquierdo vs. 1000/T para cada función f(α) se obtiene el triplete cinético. Debe escogerse entre todos los posibles modelos cinéticos el que tenga un coeficiente de correlación (r) más cercano a 1, así como (Z) que debe estar entre 10 11 y 10 14 [2]. ...
The kinetic study of orange peel pyrolysis is realized in this paper. The pyrolysisis a complex process because it involves simultaneous chemical reactions, thence the difficulty to realize a kinetic study of the material. We use several equations: Coats (integral), Sharp, Achar (differentials) and isoconversional (free kinetic). The discussion integrated of the kinetic results obtains a larger representability of the same, which results necessary in the treatment of natural materials. The study kinetic is determined for models D4 and F1. The rank of activation energy are 80-90 kJ/mol (free kinetic) and 25-237 kJ/mol (model fitting method) because simultaneous reactions occur and demonstrates the biggest precision of the isoconversional method.
... According to Fisler and Mackwell [37], the experiments with a platinum tracer show that fayalite grows at the quartz-fayalite boundary and the process is controlled by iron and oxygen diffusion. The collected TG-DTA data were used to determine the kinetic parameters (the activation energy and the preexponential factor) of the oxidation processes as described elsewhere [38][39][40][41][42][43][44][45][46][47][48][49][50][51]. The kinetic analysis is based on the assumption that the rate constant (k) is described by the Arrhenius equation: ...
The kinetics of air copper slag oxidation under nonisothermal conditions is studied using simultaneous TG–DTA at a varying heating rate of slag and flow rate of the oxidizing gas flux. The values of the kinetic parameters, activation energy and pre-exponential factor, have been determined based on: data from DTA by the methods of Kissinger and Ozawa; data from TG using an isoconversion method and the computation procedures of Ozawa–Flynn–Wall and Kissinger–Akahira–Sunose. No relationship between the kinetic parameters and the oxidation gas flow rate has been established. The changes of the phase composition with temperature are investigated by X-ray powder diffraction analysis on the basis of data obtained for the products formed at the different stages of the oxidation process. The morphology of the oxidized slag as well as the elements distribution is studied by electron microscopy and EDS analysis.
... The experimental TGA curves were analysed to evaluate rate constants, activation energies and the most probable reaction mechanisms. This methodology has been described previously (Sharp et al., 1966;Hancock and Sharp, 1972;Criado et al., 1990;Cumhrera and Sanchez-Bajo, 1995;Aleman-Vazquez et al., 2004;Go et al., 2008). Kinetic analysis of solid-state or heterogeneous process is usually based on a single step kinetic equation: ...
The purpose of this study was to investigate the possibility of separating pyrometallurgical copper (fayalite) slag by oxidation in a synthetic air atmosphere into ferrous and silicate phases suitable for resources to be recovered from them. Isothermal oxidation kinetics and the most probable reaction models are studied in the temperature range of 773 to 1173 K using thermogravimetric analysis data and the classical Johnson-Mehl-Avrami-Yerofeev-Kolmogorov (JMAYK) equation. Depending on the model applied, the activation energies of fayalite slag oxidation in the temperature range of 773-973 K were: D1: 37.55 kJ mol(-1); D2: 43.27 kJ mol(-1); D3: 50.52 kJ mol(-1); and D4: 45.65 kJ mol(-1). Depending on the model applied, the activation energies of fayalite slag oxidation in the temperature range of 1073-1173 K were: F1: 20.48 kJ mol(-1); R2: 20.45 kJ mol(-1); R3: 20.18 kJ mol(-1); A2: 21.54 kJ mol( -1) and A3: 22.34 kJ mol(-1). The transformation of fayalite to hematite and amorphous silica was completed after 1435, 1350 and 1080 s at temperatures of 1073, 1123 and 1173 K, respectively. The following oxidation products were identified by X-ray diffraction: (1) fayalite, hematite and magnesioferrite (Fe(2)MgO(4)) in the temperature range 773 to 973 K; and (2) hematite and magnesium iron oxide (Mg(1.55)Fe( 1.6)O(4)) in the temperature range of 1073 to 1173 K.
... When the chains break they become shorter and as a consequence more stables, then a higher activation energy would be needed to initiate any process. This result reveals typical behavior for competitive reaction [23]. On the other hand, during the last stage, associated to lignin degradation process (curve III), were also detected different changes undergone by the E a . ...
Thermal degradation of orange peel was studied in dynamic air atmosphere by means of simultaneous TG-DSC and TG-FTIR analysis.
According to the obtained thermal profiles, the orange peel degradation occurred in at least three steps associated with its
three main components (hemicellulose, cellulose and lignin). The volatiles compounds evolved out at 150–400 °C and the gas
products were mainly CO2, CO, and CH4. A mixture of acids, aldehydes or ketones C=O, alkanes C–C, ethers C–O–C and H2O was also detected. The E
α on α dependence reveled the existence of different and simultaneous processes suggesting that the combustion reaction is
controlled by oxygen accessibility, motivated by the high evolution low-molecular-mass gases and volatile organic compounds.
These results could explain the non-autocatalytic character of the reactions during the decomposition process.
... The molybdenum oxides are highly important compounds in catalysis. Since the catalytically active phase appears to be neither MoO 3 nor MoO 2 but rather partially reduced molybdenum mixed sub-oxide, the molybdenum oxides have been investigated in great detail in previous papers [1][2][3][4][5][6]. The reports found in the literature widely concurred with two general explanations concerning the reaction pathway for this kind of reactions. ...
... The high resolution thermo-gravimetric data (Hi-Res TGA) were collected under different heating programs. They were processed according to an isoconversion procedure [5,22] to obtain the dependence of the activation energy E a (a) on the transformation degree (a). For this purpose, the sample obtained after 24 h of reaction was selected as a representative example (Fig. 6). ...
... While in the last stage of reaction (VI) assigned to simultaneous oxidation of molybdenum sub-oxides, MoO 2 to MoO 3 . The profile of this curve shows a complex dependence of E a (a) on (a), and reveals the character of a competitive reaction controlled by the diffusion of the oxygen in the bulk solid [5,[34][35][36]. According to our experimental results the ''carbon-modified molybdenum sub-oxide'' catalyst during the isomerization process should be viewed as a fairly complex system in which not only molybdenum sub-oxides are present but also different types of carbon (ordered and disordered) are found. ...
The phase composition and the physico-chemical nature of the “carbon-modified molybdenum sub-oxide” catalyst during the isomerization of n-heptane were studied. It was found that different phases of molybdenum sub-oxides coexist, MoO2, MoO, besides MoO3−x. Our results indicate that MoO is present as dispersed nano-structures inside the bulk catalysts. The thermal oxidation study revealed that the O2 is consumed to oxidize part of the reduced oxide as well as carbonaceous deposits simultaneously, giving an additional evidence of the mixture of oxide phases during reaction. A thermo-kinetic study of the different oxidation process is included and the results are discussed in terms of the activation energy for the oxidation of each phase. The structural and thermal characterization has revealed the complexity of the system, but has also opened a window to a new insight into the understanding of MoO3 catalyst.Graphical abstractThe formation of molybdenum sub-oxides during the reduction of the MoO3 catalyst has been extensively studied. However, the nature of the surface species during isomerization processes is not yet fully understood. The purpose of this work is to show a more realistic chemical approach to the complexity of this system and to raise a novel understanding of its catalytic behavior during isomerization reactions.
... It is common to find various and simultaneous events in thermal analyses of the carbon-modified MoO 3 system (obtained under the present conditions). 27 Observation of the different events is largely influenced by the conditions of the reaction, surface morphology, and MoO 3 particle size. 18,27 It can be observed that at least two thermal events are clearly defined, each one related to oxygen uptake throughout oxidation of either MoO 3-x or carbon according to the following equations ...
... 27 Observation of the different events is largely influenced by the conditions of the reaction, surface morphology, and MoO 3 particle size. 18,27 It can be observed that at least two thermal events are clearly defined, each one related to oxygen uptake throughout oxidation of either MoO 3-x or carbon according to the following equations ...
MoO3 transformations under isomerization process conditions were studied. The products obtained after different times under stream (H2/n-heptane mixture, 18.5 bar, at 370 degrees C) were characterized by X-ray diffraction, Raman spectroscopy, thermal analysis, and high-resolution transmission electron microscopy (HRTEM). Theoretical quantum calculations were carried out with the aim of understanding the paradox of the real active phase in isomerization reactions. Theoretical calculations predict the existence of a metallic-like MoO phase with a structure that matches the X-ray diffraction experimental results. From experimental and simulated HRTEM images it was possible to identify the presence of small MoO cubic crystallites inside MoOx matrix phases. These results also support the previously proposed idea that isomerization reactions take place as a result of the existence of a bifunctional catalyst. The Raman and thermo-programmed oxidation (TPO) analyses show the existence of at least two types of carbonaceous deposits which tend to increase its ordering with the increase of time under stream. The carbon K edge in electron energy loss spectroscopy (EELS) of a sample after 24 h under stream shows that these carbonaceous deposits consist of a mixture of sp2- and sp3-hybridized carbons.
... La introducción y desarrollo de nuevos dispositivos de regulación y control de alta precisión en el mercado de instrumentación ha motivado que los métodos convencionales isotérmicos se vean desplazados por los no isotérmicos y hasta ahora, resultan más populares para el estudio cinético de reacciones heterogéneas o en estado sólido [1,7]. Sin embargo, hay que decir que estos métodos presentan como uno de sus aspectos negativos, la compleja dependencia de la velocidad de la reacción con la temperatura, lo que hace que el tratamiento analítico de los datos sea complicado y propenso a errores de baja y/o sobre estimación [8,10]. ...
... Muchos autores [1,2,4,6] recomiendan el uso de bajas velocidades de calentamiento en orden de obtener una mejor correspondencia entre los resultados de experimentos isotérmicos y no isotérmicos, al elevar la temperatura. En nuestra opinión recomendamos los métodos de análisis térmico inverso [4,[21][22][23][24], los que permiten realizar un proceso cuasi-isotérmico con las ventajas de uno no isotérmico, lo que presupone el establecimiento de bases teóricas que, aunque sujetas a controversia, permiten aumentar las posibilidades de aplicación de ambos métodos y disminuir su inconsistencia [8,25]. ...
... El segundo problema fundamental es que los resultados de cada aplicación del método de ajuste de modelos suministra únicamente un simple par de parámetros de Arrhenius. Una alternativa para buscar una mejor correspondencia e interpretación en el estudio cinética de reacciones térmicamente activadas en estado sólido, se ha discutido en varios trabajos mediante el empleo de métodos de cinética libre y del uso combinado de estos criterios con los métodos de ajuste matemáticos de modelos [8,9,16,18,[27][28][29]. ...
Resumen
Este trabajo aborda algunos de los rasgos que caracterizan la cinética no isotérmica en reacciones heterogéneas o en estado sólido, en términos formales y experimentales. Estas reacciones no pueden ser asumidas siguiendo una simple ley de velocidad similar a los criterios establecidos para las reacciones homogéneas (gases y líquidos). La complejidad de estas reacciones en muchos casos puede provocar que tanto los parámetros cinéticos (energía de activación y factor de frecuencia), así como el mecanismo de la reacción cambien en el transcurso de la reacción, característica que le imprime carácter global o aparente a los parámetros cinéticos evaluados. Se discute la limitada validez del método de ajuste de modelos matemáticos para describir las características esenciales de un proceso térmicamente activado y la importancia del empleo de un criterio de cinética libre, tal como el método de isoconversión para describir la dependencia de la energía de activación en función del grado de avance de la reacción.
Abstract:
This work describe some common aspects that distinguish to the non-isothermal kinetics in heterogeneous reactions or in solid state. Those reactions generally can not be assumed to follow simple rate laws that are applicable to homogeneous reactions (gas or liquid phases). The complexity those reactions in many cases case, generate change in the kinetic parameters as well as reaction mechanism during the course of the process, feature that gives it a global or apparent character to these parameters. Showing, the limited validity of the application of the model-fitting method for describes the essentials characteristics of a process.
Catalytic dehydration of glycerol over solid catalysts provides a sustainable and economic route to producing acrolein. Unsupported MoP catalyst was prepared by temperature-programmed-reduction method and used for the gas-phase glycerol dehydration. The catalyst was characterized by means of XRD, SEM, TEM, NH3-TPD, Py-IR, XPS, TG-DSC, and N2 adsorption-desorption. It is indicated that the acidic P-OH and Mo-OH groups, which are usually associated with the weak or moderate strength of Brønsted acid sites, were presented on the surface of MoP. The effects of reaction temperature and carrier gas flow rate on glycerol conversion and acrolein selectivity were investigated over MoP. The selectivity to acrolein was up to 89.0% and kept constant in a 50-h run at 240 °C. The gradual decrease of glycerol conversion might be related to the insoluble carbonaceous materials deposited on the catalyst surface.
In the current energy scenario, electrochemical capacitors hold the promise to fulfil requirements of high power densities, short charging-discharging times, and long lifetimes. For such supercapacitors, MoO3 is one of the most potent candidate as a negative electrode material, due to a very high theoretical charge density ascribed to easily-switchable multiple oxidation states. Nevertheless, the extremely poor electrochemical stability of MoO3 in aqueous electrolytes prevents any practical use. In the present work, we outline a strategy to overcome the poor stability of MoO3 electrodes: we produce by thermal phosphidation an amorphous MoP protective shell on Mo oxide nanotube arrays. This not only leads to an outstanding stability of the electrode but the phosphidized hybrid structures even maintains a capacitance as high as the initial capacitance (i.e. measured before deterioration) of unprotected pure oxide structures: the phosphidized hybrid structure, used as binder-free electrodes, can deliver a non-intercalation-based volumetric capacitance of over 2000 F cm⁻³ with an outstanding capacitance retention of ∼93% over 10,000 charging/discharging cycles.
