SURFACE SPECIES ON Rh/Al₂O₃ DURING CO/H₂ REACTION STUDIED BY TRANSIENT TECHNIQUES

Abstract

Methanation of a 9/1 mixture of Hz/CO at 180"C has been studied by transient techniques in a gradientless microreactor. The use of isotopic techniques and temperature programmed reduction (TPR) made possible the measurement of adsorbed CO, of a reactive carbon C~and of an inactive carbon Cpo As little as 0.04-0.06 monolayers of a heterogeneous layer of C~are present under the reaction conditions studied. About a monolayer of adsorbed CO is present at 18O"C, independent of time on stream in synthesis gas. The inactive CfJ species accumulates slowly with increasing time on stream in synthesis gas even at the low temperature of lSO'C. In addition, appreciable amounts of formate species are observed, exceeding a monolayer and located on the alumina support. The formate does not appear to be in the carbon pathway from CO to CH •. The rate is essentially controlled by the dissociation of surface CO.

Hydrogenation of CO/CO2 mixtures under unsteady-state conditions: Effect of the carbon oxides on the dynamic methanation process

“…The hydrogenation of the second species is temperature dependent and appears as the shoulder following the maximum in particular at 513 K. Similar to the discussion of the back-transient phase for H2O the second maximum, pronounced as a shoulder here, shifts to earlier times with increasing temperature and finally coincides with the first maximum. The presence of highly reactive intermediates adsorbed at the surface is supported by literature reports, which suggest that a solely carbon containing species, denoted as Cα, under steady-state (Gałuszka et al;Alstrup, 1995;Sehested et al, 2005) and unsteady-state (Underwood and Bennett, 1984;Efstathiou and Bennett, 1989;Fujita et al, 1991;Fujita et al, 1993;Fujita and Takezawa, 1997) methanation conditions exists. Fujita et al (Fujita et al, 1993) conducted DRIFTS measurements for transient CO and CO2 hydrogenation experiments and show that the highly reactive species is not present under CO2 methanation conditions, which is consistent with our results.…”

Frequency Response Analysis of the Unsteady-State CO/CO2 Methanation Reaction: An Experimental Study

“…The existence of such species is in accordance with the observations of different authors. [41][42][43]…”

The Periodic Transient Kinetics Method for Investigation of Kinetic Process Dynamics Under Realistic Conditions: Methanation as an Example

“…[51,55] Table 1 illustrates the classification of forced input signals according to the periodicity and indicates references to the application in the context of the unsteady-state CO and CO2 methanation. The aperiodic step changes are mainly used to investigate the transient behavior of the reactants and to gain further insights into the underlying surface processes and reaction mechanisms [22,37,43,53,58]. The forced periodic operation, though, has been investigated for various reaction systems in order to achieve higher conversion [28,29,32,[59][60][61][62] or selectivity [29,63], while an increase in catalyst stability [32] has been reported for different reaction systems, as well.…”

“…This is in accordance to Bundhoo et al, who reported a temporal delay of the formation of CH4 after the adsorption of CO on a Ni catalyst[22].During the back-transient phase the transient molar flow rate of CH4 exhibits positive values, while that for CO and overall carbon is positive at first and turns negative before approaching zero. The positive transient molar flow rate of CH4 is attributed to the hydrogenation of a highly reactive intermediates adsorbed at the surface and subsequent desorption of the CH4 formed, according to literature[43,58,72,74]. Those intermediate are assigned as CHx species, which are converted into CH4 in the following two-step mechanism proposed by Bundhoo et al[22]: First, metallic sites at the catalyst surface are released by desorption or reaction of adsorbed species, which enhances the atomic hydrogen supply.…”