Catalytic Reactivity of CuNi Alloys toward H₂O and CO Dissociation for an Efficient Water–Gas Shift: A DFT Study

Abstract

Density functional theory (DFT) calculations are employed to study H 2 O and CO dissociations on a set of CuNi bimetallic surfaces aiming at exploring the optimal Ni ensemble on Cu(111) for an efficient waterÀgas shift (WGS) process, i.e., splitting H 2 O with high reactivity and avoiding CO activation. We found that Ni additives in the Cu(111) surface layer including a Ni monomer can remarkably enhance water splitting. Meanwhile, H 2 O dissociation barriers (E act ) are strongly correlated with the H adsorption energies (E ad ): the larger E ad , the smaller E act . Moreover, H 2 O dissociation may be more practical via the dissociated H* rather than OH closing to Ni atoms. For the scission of the CÀO bond, the process is unfavorable on Ni monomers, though it is obviously promoted on Ni dimers, trimers, and other ensembles with higher Ni content. It is deduced that the selectivity of the CuÀNi bimetallic catalysts toward WGS would decrease with increasing Ni concentration. These findings suggest that the bimetallic CuNi catalysts with highly dispersed Ni ensembles containing lower Ni concentration should exhibit high performance for the WGS process.

Do Ni/Cu and Cu/Ni Alloys have Different Catalytic Performances towards Water‐Gas Shift? A Density Functional Theory Investigation

“…The calculated barriers and reaction heats on pure metals are very close to recently reported values; Gan et al, [9] Phatak et al [21] , and Zhu et al [26] have reported a barrier of approximately 0.90 eV on Ni(111) for water dissociation. Our result on Cu is also consistent with earlier reports, that is, 1.28 eV by Wang and Nakamura, [22] 1.31 eV by Gan et al, [9] 1.40 eV by Wang et al [27] at the DFT-GGA and PW91 level, and 1.36 eV by Gokhale et al [7] at the DFT-GGA and PBE level.…”

“…Summarizing the previous endeavors on the WGS process catalyzed by Cu/Ni alloys, we found that some researchers [9,11] suggested adding Ni to Cu (denoted as Cu/Ni hereafter), whereas other researchers [19,20] believed that adding Cu to Ni (denoted as Ni/Cu hereafter) would enhance the catalytic performance. Thus, whether Cu/Ni or Ni/Cu is more efficient and suitable for catalyzing the WGS process can be confusing.…”

“…It is well-known that water dissociation is generally recognized as the rate-limiting step for the entire WGS process and Ni is more reactive toward H 2 O dissociation than Cu. [7][8][9] Therefore, Ni-based catalysts can be considered as alternatives. Unfortunately, compared with Cu, the presence of Ni also promotes CO adsorption, [9] carbon deposition, [10] and an undesirable side reaction (methanation).…”

“…[7][8][9] Therefore, Ni-based catalysts can be considered as alternatives. Unfortunately, compared with Cu, the presence of Ni also promotes CO adsorption, [9] carbon deposition, [10] and an undesirable side reaction (methanation). [11] In the case of lowtemperature fuel cells, a strong CO-electrode interaction is detrimental, because the CO level must be reduced for cell tolerance.…”

“…Theoretically, Gan et al [9] studied H 2 O and CO dissociations on a set of Cu/Ni surfaces (adding Ni to a Cu surface) by using density functional theory (DFT) calculations. They suggested that the bimetallic Cu/Ni catalysts with highly dispersed Ni ensembles containing low Ni concentration should exhibit high performance towards the WGS process.…”