First-Principle Microkinetic Modeling of Ethanol Dehydrogenation on Metal Catalyst Surfaces in Non-oxidative Environment: Design of Bimetallic Alloys

Abstract

An ab-initio microkinetic model (MKM) was developed to understand the reactivity trend in transitional metal catalysts which are often used for non-oxidative conversion of alcohols to produce aldehydes. The MKM utilized ethanol reaction on the step sites of the catalyst, to calculate the turnover frequency (TOFs) for the production of acetaldehyde at 423 K. At these conditions, Cu showed the highest activity (TOF ~ 102 s−1), followed by Pt (TOF ~ 101 s−1). This explains why Cu, being the cheaper metal, is the ‘catalyst of choice’ for this reaction. Pd was the other most active (TOF ~ 10− 1 s− 1) catalyst, however, its activity was lesser than that of Cu and Pt. Rest of the metals screened (Co, Ni, Ag, Au, Rh, Ru, Re) showed low or negligible conversion for this reaction at 423 K. Interestingly, in the non-oxidative environment, the three most reactive metals (Cu, Pt and Pd) were also selective towards acetaldehyde production. At a higher temperature (473 K) and assuming around 10% conversion over the length of a packed-bed reactor, an appreciable TOF (~ 10− 2 s− 1) towards the aldehyde was calculated for other metals such as Co, Ni, Ag and Au. While Pt was the most active catalyst (TOF ~ 104 s−1), the TOF on Cu remained high (TOF ~ 103 s−1), to yield the desired aldehyde product. The trend in calculated TOF at 473 K followed the order; Pt > Cu > Pd > Co > Ni > Ag > Au > Rh > Ru > Re. Out of all the metals studied only Ag and Au were found to be selective towards ethylene, rest other metals were selective towards the aldehyde product. The surface of most of the metal catalysts (Pt, Pd, Co, Ni, Rh, Ru, Re) was covered with the intermediate CH3CO at both the temperatures (423 K and 473 K) studied. In contrast, only the surface of Cu was covered with the intermediate CH3CH2O, suggesting difficult C–H bond activation on Cu. Motivated from this monometallic reactivity trend, surface design of bimetallic alloys, based on Cu were explored for this reaction. Alloying Cu with a metal commonly used for C–H bond activation (such as Ni, Pd, Pt, Rh) was calculated to show high reactivity (TOF > 103 s−1) for the aldehyde production. Following a similar rationale, one may think of tuning the reactivity of Ni itself by allowing the reaction to stop at CH3CHO, before undergoing further C–H bond activation. We suggest Ni alloying with Sn for moderating the surface catalytic activity of Ni, which was calculated to show high TOF (> 103 s−1) for this reaction.