Relationships between oxygenate and hydrocarbon formation during CO hydrogenation on Rh/SiO2: Use of multiproduct SSITKA

In the present research, the mechanism of C1 and C2 hydrocarbon and oxygenate formation during CO hydrogenation on Rh/SiO2 were for the first time investigated in detail using multiproduct SSITKA (steady-state isotopic transient kinetic analysis). This was also the first effort to explore at the site level the relationship between similar products [e.g., EtOH (ethanol) vs. AcH (acetaldehyde)] on Rh/SiO2. A fixed-bed differential reactor was used for CO hydrogenation and SSITKA at 250 °C and 1.8 atm. The selectivity to CH4 was higher than any other product, but the surface reaction residence time for CH4 formation was not the shortest among all the products. The surface reaction residence time for C2 hydrocarbons was longer than that for any other product (C1–C2). Even though the selectivities to AcH and EtOH were similar, their surface reaction residence times differed significantly. Based on the SSITKA results, MeOH and CH4 appear to be produced on completely different active sites. Moreover, C2 hydrocarbons do not appear to be formed from adsorbed AcH. It is likely, however, that all C2 products share at least one intermediate with CH4, but none with MeOH. Several recently proposed pathways for EtOH and AcH formation are presented and compared to our results. The secondary reaction of AcH to form EtOH on the same sites does not appear to be a dominant pathway for EtOH formation. However, the precise mechanism for EtOH formation still needs further investigation.