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The catalyzed reaction of carbon dioxide with methane to produce hydrogen and carbon monoxide, CO2 + CH4 → 2CO + 2H2 (∆H=59.1 kcal/mol), has received increasing attention from both an environmental and an industrial perspective. The reaction is attractive because the greenhouse gases (CO2 and CH4) can be converted into a synthesis gas with a low H2/CO ratio, which is suitable for the syntheses of valuable oxygenated chemicals such as aldehydes, methyl alcohol, acetic acid, and dimethyl ether. Ni, Pt, and Ru based catalysts are known to be effective for the CO2-CH4 reaction. One of the major problems is the deactivation of catalysts due to the formation of coke on the catalysts during the reaction. The noble metal based catalysts are less sensitive to coking compared to nickel based catalysts. Nevertheless, it is still worthwhile to develop the nickel-based catalysts resistant to the formation of coke because the noble metals are expensive and of limited availability. The reforming of methane with carbon dioxide over nickel based catalysts has been studied by many investigators. However, there are few reports about fundamental kinetic parameters at present and there are some disagreements on the reaction mechanism including the reaction orders and the activation energy. The catalytic research involves the measurement of reaction rate and the investigation of factors influencing the reaction rate. An accurate description of the reaction rate can be obtained from a rate law which is essential in a reactor design. Although it is difficult to determine the accurate mechanism from an empirical kinetic expression, such empirical kinetic expression may be helpful to understand the catalytic reaction mechanism. The most important consideration in the kinetic experiments is to measure the concentration of reactants or products of interest as a function of time. To measure the concentration, absorption spectroscopy techniques have been generally used. However, their sensitivities may be diminished due to the difficulty in precise measurements of the difference between the incident and the transmitted intensities of radiation. The photoacoustic spectroscopy (PAS) being based on a direct measurement of acoustic waves generated owing to the absorption of optical energy provides highly selective detection and extremely low molecular gas level detection, which enable us to obtain a signal with enough intensity for even time-resolved experiments. Further advantages of this technique can be found in the capability of the elimination of any sophisticated sampling techniques which are inevitable in the conventional techniques. The photoacoustic spectroscopy is thus believed to be a suitable technique for the in situ experiment of a catalytic reaction because it can directly measure the absorption characteristics of the samples of interest. In the previous works, we have shown that the photoacoustic spectroscopy is applicable to the kinetic studies of CO oxidation and water-gas shift reaction over catalysts. In this work, the temporal change in the concentration of CO2 in the catalytic CO2-CH4 reaction was measured in situ as a function of time by using a CO2 laserbased photoacoustic spectroscopy with a suitable difference photoacoustic cell. The measurements were governed at a relatively low reactor pressure so that the technique provided precise kinetic data for the initial reaction stage characterized by high reaction rates. The kinetic data obtained were compared with those of other investigators.

Conversion of CO2to CO with CH4over Ni/SiO2Catalyst: Photoacoustic Measurements of Reaction Rate

Conversion of CO₂to CO with CH₄over Ni/SiO₂Catalyst: Photoacoustic Measurements of Reaction Rate