(493c) DFT Study of the Role of the Promoter in V-Promoted Rh Catalysts for CO Hydrogenation
AIChE Annual Meeting
2014
2014 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
CO Hydrogenation II
Wednesday, November 19, 2014 - 1:10pm to 1:30pm
Unpromoted rhodium
catalysts for CO hydrogenation generally produce methane, however promoters like
V have shown to dramatically increase selectivity towards oxygenates. Density functional theory (DFT) calculations
are performed with the focus on the determination of new pathways at the
interface between the promoter (V) and the active metal (Rh). The goal is to understand the physical
relationship between the promoter and the catalyst under realistic syngas
conversion conditions of high CO surface coverage, incorporation of which has
shown to significantly shift the thermodynamics. Promotion is modeled both as an isolated
metal atom on Rh(211) step edge, and as an oxide
cluster on Rh(111) surface. This work
provides a systematic insight into the observed catalytic performance of the
promoted catalyst viewed as a combined effect of surface structure, adsorbate
coverage and identity of the promoter. Promoter
effects on C‐O cleavage, C‐H
and/or C‐C bond formation reactions are analyzed via
changes to the Rh d-band filling. Chemical promotion through an H-assisted CO
dissociation mechanism via formyl (HCO) as an intermediate species was found to
be a primary pathway for CO activation on V promoted catalyst, due to the high
CO dissociation barriers of 1.61 and 1.93 eV for Rh(111)/VO2 and
RhV(211)
surface, respectively. The observed
decrease in adsorbate binding energy on the undercoordinated Rh step edge could
be attributed to both surface coverage and more predominant V promotion
(∆E = -0.58 eV), the combined effect resulting in a highly reactive surface
that facilitates hydrogenation of CO species (Eact = 0.76 eV) as well
as a particularly low HCO insertion barriers to CHx of 0.52, 0.32
and 0 eV for x = 1, 2, 3, respectively. H-assisted
CO dissociation is thermodynamically favored on Rh(111)/VO2 with a 0.49 eV energy barrier.
The oxide promoted surface provides low dissociation barriers assumed to be due
to the undercoordination
of vanadium atom of the oxide, achieved upon the oxygen removal via H2O
molecule. HCO insertion to CHx,
assumed to be independent of interface, is exothermic with 0.76, 1.01 and 1.41
eV barriers for x = 1, 2, 3, respectively.
V is found to promote H2O
formation due to a repulsive behavior caused by V behaving as a mild Lewis
acidic when embedded in Rh surface and OH intermediate (increased O
electronegativity). Â Â Â
catalysts for CO hydrogenation generally produce methane, however promoters like
V have shown to dramatically increase selectivity towards oxygenates. Density functional theory (DFT) calculations
are performed with the focus on the determination of new pathways at the
interface between the promoter (V) and the active metal (Rh). The goal is to understand the physical
relationship between the promoter and the catalyst under realistic syngas
conversion conditions of high CO surface coverage, incorporation of which has
shown to significantly shift the thermodynamics. Promotion is modeled both as an isolated
metal atom on Rh(211) step edge, and as an oxide
cluster on Rh(111) surface. This work
provides a systematic insight into the observed catalytic performance of the
promoted catalyst viewed as a combined effect of surface structure, adsorbate
coverage and identity of the promoter. Promoter
effects on C‐O cleavage, C‐H
and/or C‐C bond formation reactions are analyzed via
changes to the Rh d-band filling. Chemical promotion through an H-assisted CO
dissociation mechanism via formyl (HCO) as an intermediate species was found to
be a primary pathway for CO activation on V promoted catalyst, due to the high
CO dissociation barriers of 1.61 and 1.93 eV for Rh(111)/VO2 and
RhV(211)
surface, respectively. The observed
decrease in adsorbate binding energy on the undercoordinated Rh step edge could
be attributed to both surface coverage and more predominant V promotion
(∆E = -0.58 eV), the combined effect resulting in a highly reactive surface
that facilitates hydrogenation of CO species (Eact = 0.76 eV) as well
as a particularly low HCO insertion barriers to CHx of 0.52, 0.32
and 0 eV for x = 1, 2, 3, respectively. H-assisted
CO dissociation is thermodynamically favored on Rh(111)/VO2 with a 0.49 eV energy barrier.
The oxide promoted surface provides low dissociation barriers assumed to be due
to the undercoordination
of vanadium atom of the oxide, achieved upon the oxygen removal via H2O
molecule. HCO insertion to CHx,
assumed to be independent of interface, is exothermic with 0.76, 1.01 and 1.41
eV barriers for x = 1, 2, 3, respectively.
V is found to promote H2O
formation due to a repulsive behavior caused by V behaving as a mild Lewis
acidic when embedded in Rh surface and OH intermediate (increased O
electronegativity). Â Â Â