y Cu muestra una interacción fuerte con el óxido del metal de transición, probablemente sobre las ..... peaks might be due to contributions of La-Co intermediate.
REVISTA MEXICANA DE FíSICA 47 (4) 367-374
INVESTIGACiÓN
AGOSTO 2001
Surface and bulk modifications of 1.5 mole % SrILa203 catalyst with different Co, Ni, and Cu amounts Sergio L. González-Cortés' Laboratorio de Cinética y Catálisis, Departamento de Química, Facilitad de Ciencias, Universidad de Los Andes Mérida 5 IOIA- VeneZllela Laboratorio de Organometálicos,
Bernardo Fontal Departamento de Química, Facultad de Ciencias, Universidad de Los Andes Mérida
510lA-VelleZllela
Delfín Moronla Escuela de Física, Facultad de Ciencias, Universidad Central de Venezuela Caracas, Venezuela Recibido el 20 de octubre de 1999; aceptado el 19de enero de 200 1 Thc cffccts of Ca, Ni and Cu oxide content on the surface and bulk of the 1.5 mole % Sr/La203 catalysts were studied. Series of t\1/Sr-La catalysts (M = Co, Ni, Cu) were prepared by successive immcrsion impregnation.Thc solids wcrc charactcrizcd hy X-ray diffraction (XRD), infrarcd spcctroscopy (FTIR), temperature-programmed reduction (TPR), electron paramagnetic resonance (EPR), specific surface arca measurerncnts (BET) and therrnogravirnetricanalysis (TOA). The Sr/La203 modified with different Co, Ni, and Cu loading shows a strong metal oxide interaction with La20J. This probably bcgins over anionic vacancies or structure dcfcClSfavoring LaSrMOx solid solulion formalion wilh island.like Slructures according to a Volmer-Webergrowth. h is proposcd {hatthe growlh mode 01' La-Co, La-Ni and La.Cu stoichio. metric phases occur from solid soiutions. The formation 01' only a La.l\'1single phase, probably non-stoichiomctric, incrcases the material porosity whilc diffcrent phases favor the agglomeration or sintering of the particles. Keywords: Surface and bulk modiJication; lanthanurn oxide; strontium; cobalt; nickel; copper
En este trabajo se estudia el efecto de la cantidad de cobalto, níquel y cobre sobre las estructuras superficial y másica del sistema 1.5% molar Sr/La203. Los catalizadores M/Sr-La (M = Co, Ni, Cu) fueron preparados por el método de impregnación con exceso de solución. Los sólidos se caracterizaron por difracción de rayos-X, espectroscopía infrarroja con transformada de Fourier, reducción a temperatura programada, resonancia paramagnética del electrón, medidas del área superficial específica por el método de BET y análisis termogravimétrico. El catalizador Sr/La203 modificado con diferentes cantidades de Co, Ni y Cu muestra una interacción fuerte con el óxido del metal de transición, probablemente sobre las vacancias aniónicas o defectos estructurales que favorecen la formación de la solución sólida LaSrMOx en forma de isla, según el mecanismo de crecimiento de fase de Volmer- Weber. Se propone que el crecimiento de las fases estequiométricas La-Co, La-Ni y La-Cu ocurren a partir de la solución sólida. La porosidad del material se incrementa con la formación de la fase La-M, probablemente no estequiométrica, mientras que fases diferentes favorecen la aglomeración o sinterización de las partículas. Descriptores: Modificaciones superficial y másica; óxido de lantano; estroncio; cobalto; níquel; cobre PAes, 81.10.1'
I. lntroduction Lanthanum oxide, the more basic and with higher ionic cha. racter of the lanthanide series, has been intensively studied nol only as a calalytie SUPPOll[1,2] and promoter [3, 4J, bUl also as H,O and CO, adsorbenl [5-7]. It is well known lhal in rnany catalytic processes the structure defects of this material play an important role in molecular activation. The 010diñcation of lanthanum oxide with strontium, 5r2+, whose ionie radius (Rs, = 1.12Á), is similar to lhal of La3+ (R!'a = 1.14 Á), gives an inerease in eonduetivilY [8,9]. The 1 wt % Sr/La,03 malerial has been profusely sludied sinee lhe initial report by Deboy and Hicks [10] showing thal it transforms methane to higher value products such as ethane and ethylene (C,). Borehert and Baems [11], Kalenik and Wolf [12]and Islam el al. [13,14] have demonstrated lhal the
promoter effect of Sr on La203 can be explained by an increment in oxygen lattice mobility due to the anionic vacancies created in the solid as 5r2+ replaces La3+. This material may be used as support of noble metal cataIYS1S(Ru, Rh, Pt, Ir) fOI the partial oxidalion of melhane lo syngas (CO+H2). However, for industrial purposes is more economical to develop catalysts with lower cost metal s such as Fe, Co, and Ni, which have been reported as a worthwhile aIternative to noble metals [15]. Sorne mixed oxides or perovskites (LaM03, M=Ni, Co, Fe) have already been used for this eatalylie proeess [16-18], however, it is well known that during the activation process at high temperatures (Le., T 2: 600°C), phase segregation and reduction occurs in reducing atmospheres. 11means that the initial mixed oxide is not the active phase during the catalytic process. In view of the increased recent interest in rnethane.selective oxidation processes for syngas production, the present
368
SURFACEAND BULK ~10DIFICATJONSOF 1.5MOLE % SrILa203 CATALYST WlTll DIFFERENT Ca. Ni. ANO Cu AMOUNTS
TAB!.E 1. Composition anJlysis.
of the rre~h calalyst dctermineu by EDX
Element
Compo~ition (mole %)
Sr
1.5 olo0.2
TAB!.E [1. Cry~tal1inc phascs or 1.5 mole % Sr/LalOJ CJtJlysl modilicd wilh Co. Ni anu Cu (10\\1amounl~). whcre C") denotes majorily phase. Cat;:tlyst
[1-10% Ca
phasc
" Ll.:.103
*1.
