Density Functional Theory Study of Methanol Steam Reforming on Co(0001) and Co(111) Surfaces

Abstract

We report a periodic density functional theory (DFT) study of the methanol steam reforming (MSR) reaction on the Co(0001) and Co(111) surfaces. Thermochemistry and activation barriers for all elementary steps of two commonly accepted mechanisms, CH2O decomposition and H2COOH formation, were calculated along with the water gas shift (WGS) reaction. The adsorption energies on Co(0001) and Co(111) are within 0.05 eV for all the MSR intermediates examined, which suggests the same catalytic activity for both surfaces. On the basis of both the thermochemistry and barriers, CH2O decomposition into CHO and CO is favored over H2COOH formation on the Co(0001) surface. The strong CO binding on Co(0001) limits its WGS activity to convert CO into CO2. Our results of the MSR and WGS pathways suggest that Co will not show high selectivity toward CO2 for MSR, which matches the limited experimental data available. A simple Langmuir equilibrium model was applied to study the surface coverages on Co. The results show that O* and OH* coverages on Co are higher than on other transition metals such as Pt, Pd, and Cu due to the facile H2O activation on the surface, and reaction steps involving O–H bond breaking and forming may be facilitated by O* and OH*. The results also suggest that Co is more susceptible than other transition metals to oxide formation under steam reforming conditions, especially under high water to alcohol ratios.

Theoretical Insight into the Reaction Mechanism of Ethanol Steam Reforming on Co(0001)

“…A large adsorption energy of −5.81 eV (relative to O(g)) is obtained, consistent with previous theoretical values. 32,43 H*. H* binds strongly with three Co atoms at an fcc hollow site, yielding an adsorption energy of −2.62 eV, which is in accordance with the previous calculation values (−2.81 and −2.63 eV).…”

“…42 The adsorption energy for this species is about −3.46 eV, which is in excellent agreement with previous reports. 25,32,43 O*. The scission of the H−O bond in OH* produces adsorbed oxygen species.…”

Ruthenium–Platinum Catalysts and Direct Methanol Fuel Cells (DMFC): A Review of Theoretical and Experimental Breakthroughs

“…We feel it is pertinent to mention a computational study of methanol steam reforming on Co(0001) and Co(111) surfaces by Luo et al to end the analysis of this section regarding computational breakthroughs in methanol decomposition [74]. Takewaza et al proposed that group VIII metals, such as Ni, Rh, Pd, or Pt, might favor methanol decomposition into CO against steam reforming into CO 2 , and though not included in the mentioned study, cobalt is a group VIII metal [77].…”

“…Luo et al found that coadsorbed O and OH can promote O-H bond breaking or forming reactions and with the participation of O and OH, reaction barriers of all O-H bond breaking or forming can be reduced by 25%-90%. Further, conversion of CO to CO 2 on conversion of CO into COOH or CO 2 before desorption into CO(g) is competitive, and therefore the water gas shift may play also an important role [74].…”

Fischer-Tropsch Synthesis on Multicomponent Catalysts: What Can We Learn from Computer Simulations?

“…Thus, the fate of formaldehyde will be one of the following: (i) decomposition into HCO and H surface species (reverse of reaction 6a); (ii) desorption from the surface; or (iii) further reaction to other oxygenated compounds, e.g., formate, HCOO, species. However, based on the DFT computed barrier for the reverse of reaction 6a on Co(0001), i.e., 0.22 eV (0.13 eV with zero-point energy corrections) [70], formaldehyde desorption or further reaction to other oxygenated compounds seem difficult.…”