Estudo do efeito do cloreto em catalisadores de paládio usando a análise no infravermelho e de fotoelétrons excitados por raios x

Abstract

Recebido em 24/4/01; aceito em 4/9/01 INFRARED AND X-RAY PHOTOELECTRON SPECTROSCOPY STUDY OF CHLORIDE EFFECT IN PALLADIUM CATALYSTS. Chloride poisoning is known as having an inhibitor effect in the activity of metal catalysis. In this work in situ infrared spectroscopy (FTIR) of adsorbed carbon monoxide and x-ray photoelectron spectroscopy (XPS) were used to investigate the effect of chloride presence in the electronic metal density in the d subshell of palladium dispersed on alumina. The chloride poisoning effect was interpreted as an electronic effect since a weak back-bonded Pd-CO was formed due to the decrease in the electronic density of the d subshell of palladium, which could be also detected by the higher Pd 3d 5/2 binding energy in the chloride presence. A similar poisoning effect was also observed for chloride free Pd/Al 2 O 3 reduced at 500 o C, and it was interpreted based on the interaction of metal with the alumina surface. The use of molybdena/alumina binary system as support, yield a contrary effect due to the metal-support interaction.Keywords: chloride poisoning effect; infrared; XPS. INTRODUÇÃOAs propriedades catalíticas de catalisadores metálicos suportados são geralmente influenciadas pela presença de aditivos, que podem ser tanto promotores 1,2 como também inibidores [3][4][5][6] . Alguns inibidores como cloreto, as vezes, podem estar presente no catalisador, não como aditivos, mas sim, como resíduo dos sais metálicos precursores do catalisador, e podem produzir a diminuição na atividade catalítica 3,7,8 .A influência do cloreto na atividade catalítica pode ser interpretada de duas formas: i) Como um efeito estérico, onde o haleto coordena-se aos sítios que são necessários a adsorção e ativação de outras espécies 9-11 . Nesse caso o cloreto produz o bloqueio dos sítios metálicos resultando na diminuição da acessibilidade dos reagentes a esses sítios 11,12 . ii) A outra interpretação é considerar o envenenamento do cloreto como um efeito eletrônico. Nesse caso cloreto metálico pode ser formado 13,14 . Para metais nobres como paládio, esses haletos são mais estáveis que os óxidos 13,15 . A atividade catalítica portanto diminui, pois os haletos metálicos são menos ativos que as partículas metálicas.Nesse trabalho, nossos resultados evidenciam que a presença de cloreto no catalisador metálico produz efeitos eletrônicos e que quando esse é reduzido a temperatura de 500 o C, a interação do paládio disperso com o oxigênio do suporte alumina pode também resultar em um efeito semelhante ao do cloreto. Uma redução parcial do efeito do envenenamento por cloreto pode ser obtida quando usa-se molibdênio/alumina como suporte. As interpretações foram feitas a partir da análise através da espectroscopia de fotoelétrons excitados por raios-X e também na análise das ligações por retro-doação pi entre o metal e a carbonila, que dependem da densidade eletrônica do metal. Essas ligações podem ser monitoradas a partir da espectroscopia no infravermelho. PARTE EXPERIMENTAL Síntese do Pd/Al 2 O 3 (sem cloreto)O su...

Pd-γAl2O3 applied to triglycerides hydrogenation with supercritical propaneExperimental and theoretical catalysts characterization

“…The shift to a higher frequencies of the linear band observed for A and B samples (Fig. 2(a)) and b)) was previously reported [17,18,53] and was attributed to an electronic effect on Pd due to charge transfer from the metal to O atoms in Al 2 O 3 . The latter is evidenced by the lower retro donation of d electrons of the metal to the p anti-bonding orbital of the carbon in the CO molecule directly influencing the vibrating energy of the C O bond.…”

“…An identical trend in activity and selectivity was found in this work, diminishing the isomerization products and increasing the production of saturated compounds when catalysts modified with chlorine or with small cluster sizes were used. The shifts to higher wavelengths of the FTIR bands of CO adsorption for A, B and C + Cl catalysts could be explained by less retrodonation of the d electrons of Pd atoms due to the electron density depression caused by interaction of this band with the O of the alumina support or by chlorine atoms adsorbed in a bridge configuration on Pd, as demonstrated by the theoretical calculations and reported in the literature [53]. The C catalyst exhibits the most interesting change in the activity and selectivity results due to chlorine addition.…”

“…It is reported that the chlorine-modified catalysts are difficult to reduce. This effect is attributed to an electronic effect upon the metal [53]. The FTIR spectrum of the C + Cl catalyst reduced at 473 K after 2 h and 700 Torr of H 2 is shown in Fig.…”

“…This effect is attributed by some authors to the strong adsorption of high-unsaturated chains present in the fatty acids as happens for lighter di-alquenes and acetylenic compounds [17,61]. They proposed the electronic modification of the small Pd clusters due to the strong interaction with the oxygen of the alumina [53]. The chlorine modification reduces the specific activity by 30% for B catalysts and 42% for C catalysts, but in the case of A catalysts the activity remains almost at the same level.…”

“…It was reported that chlorine combines with the Pd atoms and alters their adsorption properties [54]. Some authors using XPS technique, informed increments of 0.9 eV of the Pd 3d 5/2 energy band for 1% Pd/g-Al 2 O 3 (150 m 2 g À1 ) prepared from PdCl 2 in HCl solution, [53]. The A + Cl catalyst activity could be explained by the adsorption of chlorine on Al 2 O 3 rather than on Pd.…”