TY - JOUR
T1 - Influence of the textural characteristics and the preparation conditions of the support on coking resistance and performance of Ni/MgAl2O4 catalysts
AU - Khaleel, Abbas
AU - El Alem, Fatima
AU - Pillantakath, Abdul Rasheed
N1 - Funding Information:
In view of the results presented above, it can be concluded that catalyst performance can be largely governed by the textural properties of the catalyst's support. Textural properties including surface area and pore characteristics have a strong influence on the concentration of active surface sites such as coordinatively unsaturated oxide ions and hydroxyl groups. They also play a key role in removing the carbonaceous species that form during reactions. It is evident that high surface areas combined with adequate interparticle porosity of the catalyst support are two essential characteristics for coking resistance by continuous removal of carbonaceous species as they form. The fact that some crystalline carbon was observed over NiMgAl-CP, which had relatively low pore volume and pore diameter, and considerable coke deposit was observed over NiMgAl-CM, which had the lowest pore volume and the smallest pore diameter, allows suggesting that the interparticle porosity is an important factor in coking resistance. One may refer the coke formation over NiMgAl-CM and NiMgAl-CP to the possible presence of some undetectable amounts of amorphous MgO and/or Al2O3 as impurity on their surfaces. This suggestion can be excluded based on two observations. First, all studied catalysts exhibited similar high initial CH4 conversion and H2 as well as CO selectivity with H2/CO ratio around 2, indicating that they were all active at the beginning without noticeable effect from the support. The fact that the NiMgAl-CM started to show deactivation with time, indicates that methane decomposition started dominating due to the loss of catalytic reforming activity without sufficient regeneration. Second, the rapid and considerable coke formation on NiMgAl-CM, which possessed the lowest pore volume and the smallest pore diameter. If impurities like MgO or Al2O3 existed, their impact, if any, would not be as severe and as rapid as was observed since both materials, MgO and Al2O3 as well as mixtures of them, have already been studied as possible supports and have shown relatively good activity [55]. Therefore, the observed results permit proposing that the interparticle porosity had a major influence here and that the small pore diameters as well as the low pore volume of NiMgAl-CM are the main cause of the considerable coke accumulation. Such impact of small pore diameters was not as severe in the case of NiMgAl-CP since larger pores dominate with smaller portions of their pores below 5 nm as shown in Fig. 2b. A generally accepted mechanism of partial oxidation of methane involves a step of C–H bond activation by reduced Ni particles promoting methane cracking to hydrogen and carbon [56]. If the carbonaceous species that form in this step are not exposed to appropriate oxidation conditions, they grow into larger crystallites that aggregate into larger particles or filaments around the Ni particles and in the interparticle pores of the support. If the pores of the support are too narrow, which is the case in NiMgAl-CM, they may get rapidly filled with carbonaceous species limiting internal diffusion and access by O2 and CO2 molecules, which would oxidize and remove such species as they form. In addition, the accumulated coke blocks some active sites leading to deactivation. On the contrary, the larger volume of wider pores, such as those in NiMgAl-SG catalysts, prevent such carbon accumulation and pore clogging allowing for continuous diffusion of gases over the carbonaceous species and hence their continuous oxidation and removal as they form. In addition, NiMgAl-SG and NiMgAl-CP, showed higher density of basic sites, which was referred to the smaller crystallites of their supports exposing higher concentration of coordinatively unsaturated ions including important basic sites. As described earlier, surface basic sites promote oxidation and removal of carbon deposits by adsorption and activation of CO2 molecules promoting their subsequent reaction with intermediate carbon species giving CO [57,58]. Furthermore, the unique textural characteristics of NiMgAl-SG catalysts allowed the presence of more surface OH groups, which can contribute as a source of oxygen for further oxidation of carbon deposits. The evident role of the physicochemical properties of the supports in the studied catalysts indicates that carbonaceous species that result from methane decomposition over Ni particles migrate to the Ni-support interface allowing for their interaction with the support reactive sites including basic O2− and OH groups yielding CO and H2 as the main products.This work was financially supported by National Water and Energy Center at UAEU, grant code G00002892. The authors also acknowledge the Nanotechnology Research Center (NRC), SRMIST for providing the TEM and the XPS results.
Funding Information:
This work was financially supported by National Water and Energy Center at UAEU, grant code G00002892 . The authors also acknowledge the Nanotechnology Research Center (NRC), SRMIST for providing the TEM and the XPS results.
Publisher Copyright:
© 2022 Elsevier B.V.
PY - 2022/12/1
Y1 - 2022/12/1
N2 - This study aimed at establishing the influence of the textural properties, interparticle pores in particular, of the support in Ni/MgAl2O4 catalyst on other physicochemical properties and on coke formation during partial oxidation of methane. Ni catalysts were prepared over four different MgAl2O4 supports with different surface area and pore characteristics to establish their impact. Mesoporous MgAl2O4 supports were prepared by modified sol-gel and co-precipitation methods, and were compared with a commercial nano-powder counterpart. The catalysts were characterized by various techniques including powder XRD, XRF, N2 sorption, TEM, CO2-TPD, H2-TPR, DRIFTS, TGA, and Raman spectroscopy. The catalysts over the sol-gel prepared supports showed CH4 conversion around 89%, H2 selectivity close to 100%, H2/CO ratio ∼2.05, and negligible carbon deposits. While the catalyst over the support prepared by coprecipitation showed comparable conversion and products selectivity, it showed a slight decrease in conversion that was referred to the formation of a small amount of carbon deposit. On the other hand, the catalyst based on the commercial MgAl2O4 nano-powder showed conversion ∼84%, H2/CO ratio of 2.2, and large amount of coke deposit that lead to unstable conversion and gradual deactivation. The significantly enhanced coking resistance of the catalysts based on the prepared supports was referred to their larger volume of wider mesopores and their higher density of basic sites. On the other hand, the considerable and rapid coke formation over commercial support-based catalyst was referred to the narrow pores as well as lower pore volume, in addition to the lower density of basic sites, of its support. Narrow pores are expected to get rapidly filled with carbonaceous species blocking some active sites, hindering diffusion of products as well as O2 and CO2 access, and limiting oxidation and removal of these species as they form. The results indicate that the dominance of wide interparticle mesopores (>5 nm) of the support are essential to avoid coke accumulation.
AB - This study aimed at establishing the influence of the textural properties, interparticle pores in particular, of the support in Ni/MgAl2O4 catalyst on other physicochemical properties and on coke formation during partial oxidation of methane. Ni catalysts were prepared over four different MgAl2O4 supports with different surface area and pore characteristics to establish their impact. Mesoporous MgAl2O4 supports were prepared by modified sol-gel and co-precipitation methods, and were compared with a commercial nano-powder counterpart. The catalysts were characterized by various techniques including powder XRD, XRF, N2 sorption, TEM, CO2-TPD, H2-TPR, DRIFTS, TGA, and Raman spectroscopy. The catalysts over the sol-gel prepared supports showed CH4 conversion around 89%, H2 selectivity close to 100%, H2/CO ratio ∼2.05, and negligible carbon deposits. While the catalyst over the support prepared by coprecipitation showed comparable conversion and products selectivity, it showed a slight decrease in conversion that was referred to the formation of a small amount of carbon deposit. On the other hand, the catalyst based on the commercial MgAl2O4 nano-powder showed conversion ∼84%, H2/CO ratio of 2.2, and large amount of coke deposit that lead to unstable conversion and gradual deactivation. The significantly enhanced coking resistance of the catalysts based on the prepared supports was referred to their larger volume of wider mesopores and their higher density of basic sites. On the other hand, the considerable and rapid coke formation over commercial support-based catalyst was referred to the narrow pores as well as lower pore volume, in addition to the lower density of basic sites, of its support. Narrow pores are expected to get rapidly filled with carbonaceous species blocking some active sites, hindering diffusion of products as well as O2 and CO2 access, and limiting oxidation and removal of these species as they form. The results indicate that the dominance of wide interparticle mesopores (>5 nm) of the support are essential to avoid coke accumulation.
KW - Mesopores
KW - Ni catalysts
KW - Partial oxidation
KW - Textural properties
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U2 - 10.1016/j.matchemphys.2022.126850
DO - 10.1016/j.matchemphys.2022.126850
M3 - Article
AN - SCOPUS:85139337288
SN - 0254-0584
VL - 292
JO - Materials Chemistry and Physics
JF - Materials Chemistry and Physics
M1 - 126850
ER -