3.1. Effect of Reaction Temperature
Homogeneous (empty reactor) steam reforming of ethanol was primarily investigated. It was observed that for temperature values ranging from 200 to 700°C the main reaction product was acetaldehyde, carbon oxides and methane Fig. 2, which appear at temperature above 400°C. It is see that as temperature increases up to about 700°C the concentration of acetaldehyde passes through maximum, while a carbon oxides concentration increased with increasing temperature. For the same temperature interval the methane concentration was almost constant (Fig. 2). The ethanol conversion increases with increasing temperature and attains 16% at 700°C. Furthermore, a considerable carbon imbalance was observed in the temperature range 400-700°C and the visual inspection showed, after a 2 h operation the wall of reactor was covered with carbon.
In ref.  we have already discussed the effect of temperature on ethanol conversion and products distribution upon ethanol decomposition over Pd/C catalyst in steam presence. It was shown that when water-ethanol mixture (H20/C2H50H molar ratio
8.1-1.04) was fed into the reactor at WHSV 1600-2200 cm3/g-h, the reaction products were H2, CO, C02 and CH4, and 100% ethanol conversion was attained at 330-360°C. Analysis of the temperature dependencies of the outlet H2, CO, C02 and CH4 concentrations proves that within the whole temperature interval the concentrations obeyed the following equations:
[H2] = [CH4] + [C02]
[CH4] = [C02] + [CO]
This means that CH4, CO and H2 were the products of ethanol decomposition by reaction:
C2H5OH = CH4 + CO + H2 (3)
while C02 was the product of the water gas-shift reaction:
CO + H20 = C02 + H2 (4)
It is known [7-10], however, that ethanol decomposition to CH4, CO and H2 over metallic palladium containing catalysts proceeds by reactions:
C2H5OH -> CH3CHO + H2 (5)
CH3CHO -> CH4 + CO
with the limiting stage being the ethanol dehydration, whereas acetaldehyde decomposition proceeding at high rate (Fig. 3).
The knowledge of the ethanol decomposition mechanism allows to offer the usage of the bimetallic, bifunctional catalyst for an intensification of process in a two layer fixed bed reactor and for price reduction of the catalyst in the first layer. Ethanol-to-acetaldehyde dehydration will proceed on the first component of such catalyst, with acetaldehyde decomposition to carbon oxide and methane proceeding on the second one (Fig. 4). As reaction (3) effectively and with high selectivity proceeds on I-B group metals , copper might be used as the first component of the first layer. As the second component of the first layer it is necessary to retain Pd, which as against Co and Ni, does not give rise to the surface carbon deposition [4, 11, 12].
Fig. 5 shows comparative temperature dependences of ethanol conversion during its decomposition over bimetallic catalysts 5wt. %Cu-lwt.% Pd/C and 5wt. %Cu-lwt.% Pd/Ce02-Zr02, and over lwt.% Pd/C. One can see that total conversion of ethanol is reached at 330° C over the bimetallic catalysts, whereas for the Pd/C catalyst at this temperature the conversion does not exceed 80 %. The reaction products for those catalysts were H2, CO, C02, CH4 and CH3CFIO, except for Cu-Pd – catalysts at 330°C, were CH3CFIO was absent. The data of Fig. 5a prove the bimetallic catalysts being essentially more active and selective then the catalyst containing Pd only. Moreover, bimetallic catalysts increase the water gas shift reaction. Fig. 5b shows that CO
selectivity for bimetallic catalysts decreased with increasing temperature. Selectivity Pd/C at experimental condition was stable.
Fig. 5a. Effect of temperature on ethanol conversion upon ethanol decomposition over bimetallic catalysts
5wt.% Cu – lwt.% Pd, and over lwt.% Pd/C. Experimental conditions: WHSV = 5000 cm3/h-g, inlet
composition 8 vol.% C2H5OH + 92 vol.% H20.
Fig. 5b. Effect of temperature on CO selectivity (b) upon ethanol decomposition over bimetallic catalysts 5wt.% Cu-lwt.% Pd, and over lwt.% Pd/C. Experimental conditions: WHSV = 5000 cm3/h-g, inlet composition
8 vol.% C2H5OH + 92 vol.% H20.
To estimate the feasibility to use Pd-Cu/Ce02-Zr02 catalyst in ethanol decomposition in steam presence, it seemed reasonable to determine its stability under the reaction conditions.
Under experimental conditions, (temperature 275°C, ethanol conversion was ca. 99% (100% ethanol conversion was attained at 280°C)); only H2, CH4, CO, C02 and
C2H5OH were detected at the reactor outlet. The outlet concentrations were stable for 50 h. There is no carbon imbalance (within +5% accuracy) was observed during this experimental run.
Thus, the use of the bifunctional, bimetallic catalyst as the catalyst for the first layer of the two-layer reactor for ethanol decomposition proved to be quite promising for the steam reforming of ethanol to syngas.