Factors influencing the temperature-programmed reduction profiles of vanadium pentoxide

Abstract

The temperature-programmed reduction (t.p.r.) of bulk V,O, has been examined as part of a study of the reducibility of V,O,-containing catalysts. T.p.r. profiles have been studied as a function of flow rate, heating rate and sample weight. From experiments at different flow rates it is concluded that the order of the reduction rate in hydrogen is low or even zero. A rule of thumb has been derived to provide an easy check on possible exhaustion of hydrogen in the feed. The influence of sample weight and heating rate is explained in terms of the formation of water in the sample during reduction. The reduction of bulk V,O, to V,O, proceeds in several steps; intermediate species include V,O,, and VO,. The apparent activation energy of ca. 200 kJ mol-l indicates that solid-state diffusion influences the reduction process of V,O,.

Electrical and catalytic properties of some oxides with the fluorite or pyrochlore structure

“…Two groups, B and C, with higher structural order: -In group B, with 0 and 5% Ce substitution, a negative order in CO (-0.3 --0. 4) and a high reaction order in 02 (0.4-0.9) is observed. -Group C, having a Ce content of 10 and 20%, exhibits an order in CO of about zero (-0.1 -0.0) and the reaction order in 02 is 0.5.…”

“…The experiments have been performed in an apparatus described elsewhere [4]. In every experiment 300 mg of fresh catalyst was used and a continuous flow of 9% H 2 in Ar was passed over.…”

“…Other investigations on CO oxidation, using lanthanide oxides and fluorite oxides [1, 2,3] indicated or demonstrated the use of lattice oxygen in the reaction. With temperature programmed reduction it is possible to acquire information on the extraction of oxygen from the crystal lattice [4]. Therefore the reducibility has been tested by TPR measurements as an additional means of characterization.…”

Análise estatística e optimização de perfis de redução termoprogramada (TPR)

“…Porém, altas concentrações diminuem drasticamente a sensibilidade. Concentrações de 3 a 10% são normalmente citadas na literatura para redução de sistemas contendo óxido de cobre 11,12,14 . A influência da vazão de hidrogênio (X 2 ) foi significativa somente para a sensibilidade, a qual se mostrou dependente de todas as variáveis operacionais.…”

“…Apesar da grande importância da interpretação dos perfis de redução em relação às variáveis operacionais (concentração de agente redutor, vazão de gás, taxa de aquecimento e quantidade de amostra), poucos são os trabalhos na literatura que lidam com a influên-cia das condições experimentais nos perfis de redução 10 . Bosh 11 pesquisou a influência das variáveis operacionais nos perfis de redução do V 2 O 5 e Monti 12 analisou a influência dessas variáveis operacionais sobre a temperatura de máximo consumo de H 2 (Tm) na redução do NiO. Esses autores observaram tendên-cias nos efeitos das variáveis operacionais, como o aumento da temperatura de máxima redução com o aumento da taxa de aquecimento, e propuseram um critério, baseado no consumo máximo e mínimo de hidrogênio, para escolha adequada das condições experimentais em TPR.…”