Solution combustion synthesis of nanostructured

J Nanopart Res (2018) 20: 214 https://doi.org/10.1007/s11051-018-4312-5

RESEARCH PAPER

Solution combustion synthesis of nanostructured molybdenum carbide H. V. Kirakosyan & Kh. T. Nazaretyan & R. A. Mnatsakanyan & Sofiya V. Aydinyan & S. L. Kharatyan

Received: 7 May 2018 / Accepted: 26 July 2018 / Published online: 14 August 2018 # Springer Nature B.V. 2018

Abstract A novel approach for the preparation of molybdenum carbide by solution combustion synthesis (SCS) combined with subsequent programmed heating of SCS products was proposed using ammonium heptamolybdate (AHM) and organic reducers (glycine, alanine, glucose, etc.) as precursors. It has been shown that SCS temperature and composition of the products are governed by changing the AHM-organic fuel ratio, the type of organic reducer, the rate of gaseous oxygen flow, and quantity of ammonium nitrate. A solution combustion synthesis method allowed to produce molybdenum carbide at the first stage only from the AHMglycine system. In the other studied systems, carburization process was stimulated by the subsequent programmed heating of the SCS product, sometimes with addition of a certain amount of carbon source up to 1200 °C with Vh = 20–100°min−1. The catalytic activity and selectivity of Mo2C was tested on the model H. V. Kirakosyan : K. T. Nazaretyan : R. A. Mnatsakanyan : S. V. Aydinyan : S. L. Kharatyan Laboratory of Kinetics of SHS Processes, A.B. Nalbandyan Institute of Chemical Physics NAS RA, 5/2, P. Sevak Str, 0014 Yerevan, Armenia H. V. Kirakosyan : K. T. Nazaretyan : S. V. Aydinyan : S. L. Kharatyan Department of Inorganic and Analytical Chemistry, Yerevan State University, 1, A. Manoogian Str, 0025 Yerevan, Armenia S. V. Aydinyan (*) Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia e-mail: [email protected]

reaction of isopropyl alcohol conversion. A new phenomenon showing the temperature influence on the selectivity of either propylene or acetone formation was revealed. Keywords Molybdenum carbide . Nanopowder . Catalyst . Solution combustion synthesis . High-speed temperature scanner

Introduction The industrial demand of nanosized transition metal carbides is rapidly increasing thanks to their remarkable chemical, magnetic, electronic, and optical characteristics. The high melting point, good conductivity, thermal stability, excellent corrosion, wear resistance, and catalytic properties make them valid as magnetic data storage materials, in energy conversion and storage technologies, as well as in heterogeneous catalysis (Oyama 1996; Pierson 1996). Nanostructured molybdenum carbide is receiving special attention among metal carbides and stands as a relevant alternative to noble metals in catalysis for utilization in a number of reactions (hydrogen transfer reactions of alkanes, cycloalkanes and longchain alkadiens, desulfurization, methane reforming, ammonia synthesis, water-gas shift reaction, etc.) under severe conditions (Hollak and Gosselink 2013; Zaman and Smith 2012; Lee et al. 1987; Arnold and Christel 2008; Preiss et al. 1998; Lori and Elbaz 2015). Very recently, studies evinced that β-phase molybdenum carbides demonstrate high activity and selectivity for an

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electrochemical hydrogen production process (Zhong et al. 2016; Chen et al. 2013). Because of these multilateral applications, synthesis of nanocrystalline Mo2C utilizing by simple route at lower temperatures is eminently prospective. Traditionally, Mo2C is produced by direct carburization of molybdenum and molybdenum oxide powders at higher temperatures. For the synthesis of Mo 2 C nanopowder, different procedures are adopted (Wan 2016; Vitale et al. 2015; Mnatsakanyan et al. 2016; Cetinkaya and Eroglu 2017; Gao et al. 2014). The structure and crystallite size of carbides and morphology of particles mainly depend on the synthesis temperature, time, type, and concentration of carbon source. The catalytic activity of Mo2C is strongly influenced by surface structure and elemental composition, which are closely associated with the synthesis method and can be improved by constructing proper nanostructures. In this paper, synthesis of Mo2C of nanosized powders through a simple, cheap, and sustainable technology is reported. Recently, exothermic reactions, especially solution combustion synthesis (SCS) or sol-gel combustion synthesis, guarantee potentialities for rapid and direct synthesis of nanomaterials (González-Cortés and Imbert 2013; Wen and Wu 2014; Mukasyan and Dinka 2007; Varma et al. 2016). SCS involves self-sustained redox reactions in a solution of metal-containing oxidizers (typically hydrated metal nitrates) and a fuel, e.g., water-soluble organic amines, acids, and amino acids, mixed on the molecular level. Solution combustion synthesis can be accomplished in an aqueous solution of the oxidizer and fuel, which is sufficiently exothermic to maintain a self-sustained chemical reaction. This form of combustion offers various unique features for material synthesis, such as (i) high specific surface area of products as a consequence of the large amount of gases produced during the synthesis process, (ii) formation of nanocrystalline/amorphous material at high temperature within a short time, and (iii) formation of chelates/stable complexes that in addition to the increased solubility prevent selective precipitation of the metal ions during water removal and promote the formation of highly homogeneous nanostructure of the final product (Djordjevic et al. 1997; Samotus et al. 1991; Gharib et al. 2000; Erri et al. 2004). The SCS approach has been used for the synthesis of a large number of binary and complex oxides (Patil et al. 2008; Segadaes 2006; Toniolo et al. 2007; Dinka and Mukasyan 2005; Saha et al. 2006) and some transition

J Nanopart Res (2018) 20: 214

metals (Ni, Cu, Fe) and alloys (Manukyan et al. 2013; Erri et al. 2008; Kumar et al. 2011). Recently, rare-earth metal sulfides were obtained by SCS in an inert atmosphere using thiourea as a fuel (Tukhtaev et al. 1999; Akbari and Sharifnia 2017). However, to our best knowledge, there are only few reports on the fabrication of carbide nanopowders by the SCS method. WCx/C, WC-Co-Cr3C2-VC, and Fe/C composites were obtained by combining solution combustion synthesis (SCS) with carbothermal reduction (Chen et al. 2017; Chen et al. 2016; Huang et al. 2017). In this work, a novel route to the synthesis of nanosize molybdenum carbide by the solution combustion synthesis (SCS) is addressed. A new thermal analysis setup (high-speed temperature scanner (HSTS-1, IChPh NAS RA)) was used to explore the reaction mechanism at programmed heating as well as to promote carburization/crystallization of the SCS products (Nepapushev et al. 2015; Kamboj et al. 2018; Aydinyan et al. 2018). To investigate acid/base features of catalysts’ surface, the model reaction of isopropyl alcohol (IPA) conversion by dehydrogenation/dehydration is considered one of the most effective methods. Since the IPA reaction can proceed in two directions (propylene or acetone) depending on the dominant activity of the acid or base sites, the temperature dependence of the conversion selectivity makes it possible to draw conclusions about the surface acidity. The presence of redox sites can also promote dehydrogenation of IPA in the presence of oxidants. In this view, the acid/base feature of the carbide surface is important both for the preparation of the supported catalyst and its utilization (Turek et al. 2005; Tanabe and Hölderich 1999; Benyounes et al. 2014).

Experimental For the preparation of precursors’ mixture, ammonium heptamolybdate (NH4)6Mo7O24·4H2O, AHM), together with organic fuel (e.g., glycine, glucose, alanine, melamine), was dissolved in an appropriate amount of deionized water to obtain saturated solution. For the increasing of reaction enthalpy, NH4NO3 was utilized as an auxiliary oxidant to influence on temperature regime and enlarge the area of Mo2C formation. To promote solution combustion reaction and to increase the exothermicity of interaction, regulated flow of gaseous oxygen was applied. Precursor’s homogenous mixture was


filled into the quartz glass and heated on an electrical heater. After water evaporation, a viscous liquid is formed (sol, then gel) which is autoignited at > 200 °C (so-called volume combustion) accompanying by the release of gas and smoke. To follow the reaction in different thermal modes, the temperature-time measurements were made by chromelalumel thermocouple. The initial reagents and obtained nanopowders were characterized by XRD analysis (DRON-3.0 diffractometer), absorption analysis (Micromeritics Gemini VI), high-resolution scanning electron microscopy (HR-SEM Zeiss Ultra 55), and thermal analysis. SC products were heated by HSTS-1 setup by a given 100 min−1 rate up to 1200 °C and kept at this temperature for 2–3 min to stimulate crystallization or/and carburization of the SCS products. Catalytic properties, particularly activity and selectivity of the Mo2C nanopowder, were tested on IPA conversion in a temperature range 130–235 °C. The conversion of IPA was performed in a 4 mm i.d. quartz reactor. Approximately 1 g of catalyst was placed on a quartz wool plug inside the reactor. The reactor was then placed inside a furnace. The temperature of the reactor was measured using a K-type thermocouple placed inside the catalyst bed. The reactor was then brought to the desired temperature in order to measure the catalytic activity. Helium was used as a carrier gas (Vflow = 35 ÷ 90 cm3 min−1). Helium was passed through a series of saturators containing IPA at ambient temperature. He/ IPA mixture was mixed with another stream containing pure He coming through a by-pass line. Helium flow rate coming from the by-pass line was measured using a rotameter and controlled using a needle valve. The catalytic conversion of isopropanol was determined at atmospheric pressure and an alcohol partial pressure of 0.0044 MPa (33 Torr).

Results and discussion Thermodynamic consideration For the thermodynamic analysis of sol-gel combustion processes ISMAN-THERMO software package was used, which allows to calculate both the adiabatic combustion temperature and equilibrium product composition (Shiryaev 1995). Thermodynamic calculations were performed in the AHM-fuel (glucose, glycine) systems at the presence and absence of ammonium

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nitrate and/or oxygen. Calculations ascertain the possibility of molybdenum carbide formation by the sol-gel combustion synthesis from the carefully selected initial mixtures. Moreover, at chosen certain conditions, Mo2C is the only condensed product and its formation is accompanied with a large amount of gas release (about 100–200 l per 1 g Mo2C). Thermodynamic calculations also showed that in addition to the three main gaseous products (CO2, N2, H2O), the formation of which is usually considered in the literature at SCS of various nanomaterials, significant amounts of other gases (CO, CH4, H2) also are formed. Taking into account these circumstances, two tunable parameters (φ1, φ2) were introduced to describe such type of mixtures and reaction stoichiometry. As an example below, the reaction scheme in AHM + glycine system is presented: ðNH4 Þ6 Mo7 O24 4H2 O þ φ1 C2 H5 NO2 þ φ2 O2 ðNH4 NO3 Þ→Mo2 C þ CO=CO2 þ CH4 þ H2 þ H2 O þ N2

ð1Þ

According to thermodynamic consideration in the AHM-φ1C2H5NO2-φ2NH4NO3 system (where φ1 and φ2 are the moles of fuel and oxidizer per 1 mol of AHM), the formation of molybdenum carbide is not observed. When using oxygen as a supplementary oxidizing agent, an optimal range for the formation of molybdenum carbide as the sole condensed product has been found at 600–700 °C temperature interval. Outside the area of Mo2C formation, the reaction condensed products are multiphase and contain mainly MoO2 and MoO3, as well as a mixture of MoO2 and Mo2C. According to thermodynamic calculations in the next system (AHM-φ1C6H12O6-φ2NH4NO3) at a range of φ1 and φ2 parameters (φ1 = 45–60 and φ2 = 90–100), the reaction may yield Mo2C powder within the narrow temperature interval 590–620 °C (Fig. 1a). When using oxygen as a supplementary oxidizing agent, it becomes possible to expand the range of molybdenum carbide formation by the temperature (700–850 °C) and initial mixture composition (φ 1 > 20 and φ 2 = 30–95) (Fig. 1b). In this case also, outside the area of Mo2C formation, the condensed products of interaction are multiphase and contain mainly MoO2, Mo2C, and C.

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solution combustion process was recorded for the whole process in various areas of interaction. SCS product composition and microstructure before and after heating by HSTS-1 setup (Vh = 100 min−1, Tmax = 1200 °C) with and without addition of carbon source were determined. The subsequent HSTS heating after SCS was especially intended to ensure requisite temperature (1000– 1200 °C) and duration for the complete formation of Mo2C, whereas the fast heating prevents Mo2C particle growth.

AHM-glycine system

Fig. 1 Thermodynamic diagrams of the AHM-φ1C6H12О6φ2NH4NO3 (a) and AHM-φ1C6H12О6-φ2O2 (b) systems

Solution combustion synthesis of AHM-fuel systems with subsequent programmed heating Based on preliminary thermodynamic calculations, experiments were performed in a variety of AHM-fuel systems in the presence and absence of ammonium nitrate and oxygen flow. Temperature-time history of

In the AHM-glycine system, the process was initiated in the presence of ammonium nitrate, with and without regulated flow of oxygen. The solution combustion process in the AHMC2H5NO2-NH4NO3-O2 system leads to the formation of molybdenum oxide, MoO2, molybdenum, and αmolybdenum carbide (Fig. 2a (b)). It should be noted that without oxygen flow (low temperatures), molybdenum (VI) is reduced only to MoO2. Hereby, for the complete conversion into molybdenum carbide, carbon was added to the SCS products and heated by a given 100 min−1 rate up to 1200 °C and kept at this temperature for 140 s (Fig. 2 (c)). Both from the results of microstructural (Fig. 3a, b) and adsorption analyses of the product obtained by SCS (MoO2, Mo, Mo2C) and HSTS-1 (Mo2C), it can be revealed that during fast heating process the particle size of product decreases in parallel with increase in the specific surface area (a-mainly MoO2_5.9m2 g−1, bMo2C_6.7m2 g−1). This is due to the fact that at SCS the dwelling time of material being in a hightemperature zone is rather short, so the total time of the process is estimated as a sum of the heating time and the duration of cooling, in other words, by the heating and cooling rates; the higher they are, the lesser time it is to grow the particles. Thus, higher heating and cooling rates lead to a decrease of grain size, because the formed nuclei do not have enough time for the agglomeration and coalescence. In the AHM-C2H5NO2-NH4NO3 system, the reaction is propagated without supplementary flow of oxygen. The solution combustion temperature of the AHM + 60C2H5NO2 + 200NH4NO3 mixture was measured via two chromel-alumel thermocouples; the first showed that maximum temperature in the gel is about


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Fig. 2 SCS thermogram of the AHM + 60C2H5NO2 + 100NH4NO3 + O2 mixture (a). XRD patterns of SCS product (b) and Mo2C obtained at programmed heating of SCS product with addition of carbon (c)

450 °C, while the second recorded the combustion temperature in the gas flame (750 °C) (Fig. 4). XRD spectra of SCS product of the AHM + 60C2H5NO2 + 200NH4NO3 mixture did not show any clear diffraction peaks probably caused by amorphous nature of the product (Fig. 5 (a)). Thus, it was subjected to further heating by HSTS-1 at the rate 100°min−1 up to 1000 °C in one case and in another case up to 1300 °С and kept at these temperatures for 10 min aimed at obtaining either molybdenum carbide or molybdenum. The sample heated up to 1000 °C contains characteristic peaks of molybdenum carbide with trace amounts of molybdenum (Fig. 5 (b)). MoO2 þ C→Mo2 C þ COðCO2 Þ

ð2Þ

(a)

At a sample heating up to 1300 °C, predominantly molybdenum formation was observed with some amounts of molybdenum carbide according to the reaction scheme (Fig. 5 (c)): MoO2 þ Mo2 C→Mo þ COðCO2 Þ

ð3Þ

According to the thermodynamic calculation, at formation of metallic molybdenum by Eq. 3 at temperatures above 1200 °C, Gibbs free energy becomes negative (−1.3 kcal mol−1) which clearly indicates that the reaction is favorable at higher temperatures. AHM-glucose system In the AHM-C6H12O6 system, the process was initiated in the presence of ammonium nitrate leading to the

(b)

Fig. 3 Microstructures of SCS product (a) and Mo2C obtained at programmed heating of SCS product with addition of carbon (b) for mixture AHM + 60C2H5NO2 + 100NH4NO3 + O2

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Fig. 4 SCS thermograms of the AHM + 60C 2 H 5 NO 2 + 200NH4NO3 mixture

predominant formation of molybdenum (IV) oxide. Particularly, in the system AHM-φ1C6H12O6-φ2NH4NO3 depending on the ratio of C6H12O6/NH4NO3, combustion temperature and XRD pattern of the combustion product were changed. As can be seen from the registered thermogram (Fig. 6a), maximum combustion temperature was observed in the AHM + 30C6H12O6 + 200NH4NO3 mixture (Tmax = 530 °C), and XRD pattern of the quenched sample contains only characteristic peaks of MoO2 (Fig. 6b). The corresponding reaction scheme in the AHMglucose-ammonium nitrate system can be presented as follows: ðNH4 Þ6 Mo7 O24 4H2 O þ φ1 C6 H12 O6 þ φ2 NH4 NO3 →MoO2 þ CO=CO2 þ H2 O þ N2

ð4Þ

To stimulate the carbidization of the SCS products, they have been processed in two ways: (i) a certain

Fig. 5 XRD patterns of SCS product (a) and products obtained at programmed heating (b 1000 °C, c 1300 °C)

amount of carbon was added to the SCS product and (ii) the obtained SCS product was impregnated by a certain amount of a saturated solution of glucose. By the next step product was heated at the rate 100°min−1 up to 1100 °C and held for 140 s. As a result, it was completely converted to molybdenum carbide (Fig. 7a). Specific surface area of Mo2C measured by adsorption analysis method was about 9 m2 g−1 when using carbon, while in the presence of glucose it makes 10 m2 g−1. Reduction of the SCS products (MoO2) schematically can be represented by equation: MoO2 þ C→Mo2 C þ CO=CO2

ð5Þ

In the second case brutto-reaction drawn as follows: MoO2 þ C6 H12 O6 →Mo2 C þ CO=CO2 þ H2 O

ð6Þ

Actually, the glucose undergoes decomposition at a temperature below 300 °C, and then the carbon reduces MoO2. According to microstructural examinations, combustion product contains nanoparticles with an average size ~ 50 nm (Fig. 7b). AHM-other fuel systems AHM-alanine, AHM-melamine, AHM-urea, and AHMcitric acid systems were also investigated. In the AHMalanine system, the interaction was conducted under combustion mode also in the simultaneous availability of ammonium nitrate and oxygen flow. Moreover, compared with the use of other reducers, in this case the reaction occurs more rapidly with a higher combustion temperature (Tc = 1230 °C) (Fig. 8a). However, due to the high heating and cooling rates of the process, the obtained product is more fine-grained than the products obtained by other reducing agents. The combustion product represents molybdenum dioxide, which by adding carbon and subsequent programmed heating (Vh = 100 min−1, Tmax = 1200 °C, retention time 140 s) transforms to molybdenum carbide (Fig. 8b, c). When melamine is used as an organic reducer, the solution combustion temperature is about 800 °C (Fig. 9a), and XRD pattern of the quenched sample contains only characteristic peaks of MoO2 (Fig. 9 (b)). Carbidization of the SCS product, i.e., MoO2, was performed in two ways for the comparison of microstructure features. In the first case (I), a certain amount of carbon was added to the SCS product and heated by

Temperature (oC)


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(a)

500

(b)

400 300 200 100 0

0

600

1200

1800 time (s)

2400

Fig. 6 SCS thermogram of the AHM + 30C6H12O6 + 200NH4NO3 system (a). XRD pattern of SCS product (b)

the rate 100°min−1 up to 1200 °C, holding at this temperature for 140 s. The reaction yields molybdenum carbide and molybdenum in small (trace) amounts. Specific surface area of molybdenum carbide measured by adsorption analysis method was about 4 m2 g−1. In another case (II), SCS product was impregnated by the suitable amount of a saturated solution of glucose and heated by HSTS-1 setup with 100°min−1 rate up to 1100 °C and held for 140 s (Fig. 10a). Unlike the previous one, the specific surface area of obtaining Mo2C is about 8.5 m2 g−1 (Fig. 10b). From the results of the microstructural analysis, it is also evident that the Mo2C obtained in the presence of glucose contains more fine-grained particles. Comparative analysis showed that by dint of glycine, alanine, glucose, melamine, urea, and citric acid as fuels, it was possible to obtain molybdenum carbide at the first

stage only in the AHM-glycine system, while in the other systems the carburization step was promoted by programmed heating of the SCS product. Such difference between fuels can be explained by several factors simultaneously influencing on the reaction pathway: first of all, complex-formation with metal ions, decomposition temperature, nature of decomposition products, etc. The higher activity of a −NH2 type ligand compared to that of a −OH group, which in turn is more active than –COOH, was illustrated in Erri et al. (2004) indicating to higher reactivity of amino group containing glycine and ability to produce molybdenum carbide during the SCS process. It was also shown that glycine ensures the formation of more stable complexes with the molybdenum enhancing solubility and preventing selective precipitation of the metal ions during water removal (Djordjevic et al. 1997).

(a)

(b)

Fig. 7 XRD pattern (a) and microstructure (b) of AHM + 30C6H12O6+ 200NH4NO3 product obtained at programmed heating of SCS product with addition of glucose, V = 100 min−1, Tmax = 1100 °C

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(a)

(b)

(c)

Fig. 8 SCS thermogram of the AHM + 30C3H7NO2 + 40NH4NO3 + O2 mixture (a). XRD pattern (b) and SEM image (c) of Mo2C obtained at programmed heating of SCS product with addition of carbon. V = 100/min, Tmax = 1200 °C

is known that IPA undergoes dehydrogenation to form acetone over base sites whereas acid sites favor the dehydration of IPA to propylene (Bej et al. 2003).

Catalytic conversion of isopropyl alcohol in the presence of a α-Mo2C catalyst The acid and base properties and catalytic activity of the obtained Mo2C catalyst were characterized using conversion of isopropyl alcohol (IPA) as a model reaction. It

Temperature (oC)

800

CH3 −CHðOHÞ−CH3 →CH3 −CH ¼ CH2 þ H2 O

ð7Þ

(a)

600 (c)

400 200 0

(b) 0

300

600

time (s)

900

Fig. 9 SCS thermogram of the AHM + 24C3H6N6 + 40NH4NO3 + O2 mixture (a). XRD patterns of SCS product (b) and Mo2C obtained at programmed heating of SCS product with addition of carbon (c)


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(a)

(b)

Fig. 10 Microstructures of Mo2C obtained at programmed heating of MoO2 + 2C (I) (a). MoO2 + 0.5C6H12O6 (II) (b) mixtures

CH3 −CHðOHÞ−CH3 →CH3 −CðOÞ−CH3 þ H2

ð8Þ

The catalyst was typically tested for IPA conversion at 130–235 °C. Acetone and propylene were the main products formed under the reaction conditions employed. Figure 11a shows the temperature dependence of conversion degree of isopropyl alcohol in the presence of Mo2C catalyst. As can be seen from the figure, at Т = 130 °C, the IPA conversion level is 6%, and at T = 235 °C it increases to 90% (contact time 7 s). It was assumed that in the presence of molybdenum carbide, isopropyl alcohol is mainly converted to acetone and hydrogen, as carbides exhibit catalytic properties analogous to platinum group metals because of their unique d-band electronic structure (Krylov 2004; Turek et al. 2005). Last but not least, recent catalytic experiments, back-supported by density functional theory

(DFT) calculations, showed the very high catalytic power of hexagonal α-Mo2C on dehydrogenation reactions and highlighted the connection between surface properties-center of the metal surface d-band, and adsorption and kinetic properties (dos Santos Politi et al. 2013; Chen et al. 2013). In our experiments, it was shown that the conversion of isopropyl alcohol in the presence of Mo2C as a function of temperature can proceed both to the direction of propylene formation and acetone production (Fig. 11b). Thus, at low temperatures, the selectivity of acetone is low and the reaction profits to the formation of propylene. And at 170 °C, the selectivity of acetone is 50%; however, a further increase in temperature leads to a decrease in the selectivity of acetone. Molybdenum carbide and other early transition metal carbides have been reported to possess catalytic

Fig. 11 Dependence of the degree of conversion of isopropyl alcohol (a). The selectivity of acetone (b) vs temperature

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properties that resemble those of Pt-group metals (Hollak and Gosselink 2013). Results from the IPA conversion studies showed that Mo2C possessed significant activity for the dehydrogenation of alcohol, which is indicated the presence of base sites. While basic sites alone can catalyze the dehydrogenation of alcohols, several reports have implicated the activity of concerted acid-base pairs in these reactions (Krylov 2004; Turek et al. 2005). The acid and base sites are likely a consequence of charge transfer from metal to non-metal due to their different electronegativities. Charge transfer would result in electron deficiency (or positive charge) on the Mo atoms and a surplus of electron density (or negative charge) on the carbon atoms. This could lead to the development of Lewis acid and base character over the Mo and C atoms, respectively. Alternately, the acid sites may have been due to oxide domains produced on exposure of the SCS product to oxygen (Qin et al. 2014). Transition metal carbides show activity as metallic, acidic, and bifunctional catalysts in hydrogenation, dehydrogenation, dehydration, and isomerization reactions. In a paper of Sullivan and Bhan (2016) also was shown the presence of hydrogenating and dehydrating centers on the surface of molybdenum carbide. On the solution combustion synthesized Mo2C catalyst, IPA undergoes dehydrogenation to form acetone over base sites whereas solely acid sites favor the dehydration of IPA to propylene depending on temperature indicating both the acid and base sites have played a role in catalyzing these reactions.

Conclusion Solution combustion synthesis combined with the subsequent programmed heating of SCS products generates a novel approach of preparation of nanosize molybdenum carbide, using ammonium heptamolybdate and organic reducers (glycine, alanine, glucose, etc.) as starting materials. SCS temperature and composition of the products were governed by changing the AHMorganic fuel ratio, the type of organic reducer, the rate of gaseous oxygen flow, and quantity of ammonium nitrate. Solution combustion synthesis method allows to produce molybdenum carbide at the first stage only from the AHM-glycine system. In the other studied systems, carburization process is stimulated by the subsequent programmed heating of the SCS product. Microstructural analysis certified that in the presence of


glucose the obtained Mo2C contains more fine-grained particles. Experiments showed that the conversion of isopropyl alcohol in the presence of Mo2C as a function of temperature can proceed both to the direction of propylene formation and acetone production. Acknowledgements The authors gratefully acknowledge the financial support of the State Committee of Science MES of RA (SCS Project 15T-1D196). Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.

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