Steam reforming of methane over Ni-substituted Sr hexaaluminates
Catalysis for Sustainable Energy
Introduction Natural gas, specifically methane, is one of the most important and available energy resources  . Catalytic steam reforming of methane (SRM) is well-established process for converting hydrocarbons into synthetic gas in industry. The SRM is currently the most cost effective and highly developed method for production of hydrogen. CH 4 + H 2 O = CO + 3H 2 ; ∆H 298 0 = 206 kJ/mol The SRM process is highly endothermic, and the reaction requires high temperatures of 700 -800°C [1, 2]
... 700 -800°C [1, 2] . Besides, the reaction is always followed by the water gas shift reaction (WGS) yielding CO 2 as by-product CO + H 2 O = CO 2 + H 2 ; ∆H 298 0 = -41 kJ/mol It should be noted that H 2 O/CH 4 ratio influences product composition. In particular, an excess amount of steam prevents coking of the catalyst surface. At the same time, high reaction temperatures and the use of excess steam increase the amount of energy consumed and, consequently, the hydrogen production cost. The development of new more active steam reforming catalysts with a high coking stability is required . Supported noble metals Pd and Pt have successfully been employed as highly active catalysts for steam reforming of methane to synthesis gas ; however, their industrial application is not reasonable due to their high price. Traditional Steam reforming of methane over Ni-substituted Sr hexaaluminates. Abstract Ni-substituted Sr-aluminates Sr 1-x Ni x Al 11.2+x Ni 0.8-x O 19-δ (x = 0; 0.2; 0.4; 0.8) obtained by a precipitation method and calcined at 1200°C have been characterized by different physicochemical techniques and their catalytic properties have been tested in steam reformation of methane. It has been shown that substitution of Al 3+ and/or Sr 2+ by Ni 2+ in the aluminate structure results in changes of phase composition, specific surface area, and reducibility of samples. It has been established that the samples are not completely reduced in the temperature range of 30-900°C. The Sr 1-x Ni x Al 11.2+x Ni 0.8-x O 19-δ (x = 0; 0.2; 0.4) catalysts are active and stable in the steam reforming of methane at 700oC: residual amount of methane is (1.1±1.0) vol.%, while the Sr 1-x Ni x Al 11.2+x Ni 0.8-x O 19-δ (x = 0.8) sample is rapidly deactivated by coking. Keywords Ni-substituted aluminates • Steam reforming • Methane • Catalyst © Versita Sp. z o.o. supported nickel catalysts have also been reported to be effective for this reaction, but they suffer from serious deactivation due to carbon deposition, nickel particles sintering and phase transformation. Therefore, it is very important to improve the state of the Ni catalyst in order to overcome these disadvantages . Among different catalysts, nickel supported on inexpensive, thermally stable a-alumina has been widely employed as the conventional industrial steam reforming catalyst . Alumina is widely used as an oxide support, but its surface area significantly decreases with the transition from metastable g-phase into the a-phase. A high surface area is desired to maintain high dispersion of catalytic components. Some researchers have greatly enhanced the catalytic activity and stability of Ni catalysts by adding some components to the alumina support to inhibit sintering and phase transformation. Basic oxide additives, such as CaO, MgO, La 2 O 3 , BaO, have been reported to aid in maintaining catalytic activity and suppressing the deactivation, owing to the interaction between NiO and supports . The use of hexaaluminates AAl 12 O 19 in the development of high temperature catalytic systems has been of interest primarily due to their high sintering stability . Their high thermal stability is related to the lamellar structure that consists of Al 2 O 3 -containing spinel block, intercalated by mirror planes, in which the large cations (Ba, Sr, La, etc.) are located (Figure 1) . The substitution of catalytically active metals into the lattice of hexaaluminate compounds is an area of active interest of reforming catalyst applications. 3 Unauthenticated Download Date | 2/25/20 1:37 PM M.V. Bukhtiyarova et al. 12 The Ni-containing supported catalyst (8 wt.% NiO/Mg-Al-O, support Mg-Al-O was calcined at 1200°C) prepared by impregnation of support by Ni(NO 3 ) 2 solution followed by treatment at 500°C for 4 hours was used for comparison . Catalyst characterization Elemental analysis was performed using the inductively couple plasma (ICP) -atomic absorption spectroscopy with an accuracy of 0.01 -0.03% . XRD studies were carried out using a URD-63 (Germany) diffractometer with CuK a (l = 1.5418 Å) monochromatic radiation. Diffraction profiles were recorded both in a continuous mode and in a step-by-step scanning mode at 0.05-0.1° in 2q-step and dwell time of 20-30 sec depending on the crystallinity of a sample. The phase identification was performed by comparison of the measured set of the interfacial distances d i and the corresponding intensities of the diffraction maxima I i with those found in the database JCPDS (PCPDF Win. Ver 1.30, JCPDS ICDD, Swarthmore, PA, USA, 1997). The specific surface area was determined with an accuracy of ±10% by the argon thermal desorption . Temperature-programmed reduction (TPR) of samples by hydrogen was carried out in the flow reactor. The samples of 0.2 g with the average granule size of 0.25-0.5 mm were previously pretreated in oxygen at 500°C for 30 minutes, and then they were cooled to room temperature. The 10 vol.% H 2 /Ar stream (40 ml/min) was passed over the sample while it was heated from 40 to 900°C at the heating rate of 10°C/min. 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