CO2/CH4 reforming reactions

A large number of experimental studies (VA. Tsipouriari, 1999, A.N.J. VanKeulen, 1997) on carbon dioxide reforming of methane have been done all over the world. Adequate experiments (VA. Tsipouriari, 1999, A.N.J. VanKeulen, 1997, J.H. Bitter, 1998) have shown that nickel is the best catalyst for CO2-CH4 reforming reaction. A lot of surface oxygen was found on the catalyst of CO2-CH4 reforming reaction in experiments. 

Figure 1. The diagram of substances relationship in the process of CO2-CH4 reforming reaction.

Figure 2. Each imaginary vibration mode of the pivotal step in the CO2-CH4 reforming reaction.

Both nickel and platinum based catalysts are active for the C02-reforming reaction. The disadvantage of nickel based catalysts is their tendency to form coke and deactivate rapidly [6]. We have shown earlier that Pt/ZrOj is a stable (by virtue of low rate of coke formation) and active catalyst for CO2/CH4 reforming [8]. In this contribution we address the characteristics of Pt/Zr02 catalysts that influence its catalytic behaviour in order to be able understand the mechanism of the catalysed reaction and help in to optimising catalyst. 

The catalytic activity at 993 K for the CH4 reforming reaction with CO2 was determined not only prior to this sequence but also after it. Under these conditions the main reaction did not show deactivation for any of the assayed catalysts. 

Interestingly, this situation is very different when we consider activation of NH3 or H2O by coadsorbed O. This would typically occur in the Ostwald reaction that oxidizes ammonia to NO or the methane reforming reaction in which CH4 reacts with O2 or H2O to give CO, CO2, and H2. 

The catalytic reforming of CH4 by CO2 was carried out in a fixed bed reactor system. The details can be found elsewhere [4]. The characteristics of the catalysts befisre and after the reforming reaction were investigated by N2 physisorption, XRD and SEM. 

The following isotopic labeling experiment was performed in order to quantify the contribution of the direct and indirect reaction routes to CO formation After steady-state reaction with CH4/02/He was achieved, an abrupt switch of the feed from CH4/02/He to an isotopic mixture of CH4/1 02/ C 02/He was made, in which the partial pressures of CH4 and 62 were kept exactly the same as in the ordinary CH4/02/He mixture, so as not to disturb the steady-state condition. However, C 02 was added to the isotopic mixture in an amount corresponding to approximately 10-15% of the CO2 produced during reaction of the mixture. The purpose was to measure the production of C 0 due to reforming of CH4 with C 02 only (indirect reaction scheme) under steady-state conditions of the working catalyst surface. Figure 3 shows the transient responses of and C O... 

The thermodynamic analysis of loannides [193] on SRE in a solid polymer fuel cell indicated that the ethanol steam reforming reaction needs to be carried out in two steps a high-temperature endothermic step (steam reforming), in which ethanol is converted to a gaseous mixtures of H2, CO, CO2, CH4 and unreacted H2O, and a subsequent, low-temperature step (WGSR) in which CO reacts with water to form H2 and CO2. 

SR of ethanol has mainly been conducted under similar conditions as methane SR, which means relatively high temperatures, ambient pressure, and primarily with Ni- or Rh-based catalysts." Ideally, one mole of ethanol is converted into 6 moles of hydrogen (13). During SR, ethanol decomposes mainly through two different routes either by dehydrogenation to acetaldehyde (14) or dehydration to ethylene (15). These two intermediates can be further catalytically reformed to a thermodynamically equilibrated reaction mixture of H2, CO, CO2, CH4 and H2O (12, 16-18). ... 

In additional experiments, a second catalytic monolith was added immediately after the first monolith. Although tiie residence time was doubled in these experiments, neither the water-gas shift reaction (2) or the steam reforming reaction (1) was found to significantly improve the reaction conversion and selectivity. From these data, it is apparent that the primal hurdle to achieving the perfect reactor operation involves the selective oxidation of CH4 to H2 and CO only. If CO2 and H2O are formed, the amount of available O2 is obviously reduced accordingly. From stoichiometry, this results in unreacted CH4 in the product gases since the reforming reaction is too slow to consume this metiiane at these short residence times. Thus, the only way to improve Sh2 and Sep at these short residence times is to maximize the partial oxidation reaction selectivity. 

Using a temperature-programmed surface reaction (TPSR) technique, Li et al. 154) showed that this complete oxidation of methane took place on the NiO catalyst during the CH4/O2 reaction. Weng et al. 145) used in situ microprobe Raman and in situ time-resolved IR spectroscopies to obtain a relationship between the state of the catalyst and the reaction mechanism. These authors showed that RUO2 in the Ru/Si02 catalyst formed easily at 873 K in the presence of a CH4/O2/AJ (2/1/45, molar) mixture and that the dominant pathway to synthesis gas was by the sequence of total oxidation of CH4 followed by reforming of the unconverted CH4 by CO2 and H2O. Thus, these results indicate that the oxidation of methane takes place principally by the combustion mechanism on the oxidized form of this catalyst... 

From these results, the steam reforming reaction (Eq. (1)) and the water-gas shift reaction (Eq. (2)) occur at first to produce CO2 and H2, and then the methanation reaction proceeds to form CH4 during the low temperature catalytic gasification ... 

CH4, a major constituent of natural gases, can be an important carbon resource however, CH4 is a less valuable substance because of its low reactivity. It is important to utilize this stable substance by reaction with other stable and less valuable substances such as CO2 and H2O. Reactions between CH4 and CO2 or H2O lead to CO and H2 [1]. The mixture of CO and H2 is a raw material for synthesis of alcohols, carbonylatlon reactions, and the Fischer-Tropsch synthesis [2]. In this work, we studied basically CH4 reforming by CO2 and H2O over Pd, Pt, and Rh supported on several oxide carriers, and the main object was to stress the effect of acid-base properties of the carriers on the activity of the above metals for the CH4 reforming. 

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