Tuning Selectivity in the Direct Conversion of Methane to Methanol: Bimetallic Synergistic Effects on the Cleavage of C–H and O–H Bonds over NiCu/CeO2 Catalysts

The efficient activation of methane and the simultaneous water dissociation are crucial in many catalytic reactions on oxide-supported transition metal catalysts. On very low-loaded Ni/CeO2 surfaces, methane easily fully decomposes, CH4 → C + 4H, and water dissociates, H2O→ OH + H. However, in important reactions such as the direct oxidation of methane to methanol (MTM), where complex interplay exists between reactants (CH4, O2), it is desirable to avoid the complete dehydrogenation of methane to carbon. Remarkably, the barrier for the activation of C–H bonds in CHx (x = 1–3) species on Ni/CeO2 surfaces can be manipulated by adding Cu, forming bimetallic NiCu clusters, whereas the ease for cleavage of O–H bonds in water is not affected by ensemble effects, as obtained from density functional theory-based calculations. CH4 activation occurs only on Ni sites, and H2O activation occurs on both Ni and Cu sites. The MTM reaction pathway for the example of the Ni3Cu1/CeO2 model catalyst predicts a higher selectivity and a lower activation barrier for methanol production, compared with that for Ni4/CeO2. These findings point toward a possible strategy to design active and stable catalysts which can be employed for methane activation and conversions.

reported in this work, we have found only one imaginary frequency, and the full geometry optimizations starting from its back and forward nearest congurations (along the reaction path) ended in a non-dissociated and dissociated state, respectively.
In the calculated potential energy proles, the energy barrier, E Barrier = E T S E IS , equals the dierence between the energy of the transition state, E T S , and the initial (molecularly chemisorbed) state, E IS , whereas the eective or apparent energy barrier is given by the energy of the transition state, E T S , referenced to gas-phase CH 4 and the clean surface.

Models Stability
The results in Figure S1 indicate that as the size of the clusters increases, the stability per atom increases, in line with previous work. 10 The supported at Ni 4 , Cu 4 and Ni 3 Cu 1 clusters are slightly less stable than the corresponding pyramidal ones, for which only the atoms in direct contact with the support are oxidized. In addition, the formation energies of In all cases, the formation energy is negative, which indicates that the formation of these structures are energetically favored. Although this information is relevant, it would be incomplete without the calculation of diusion barriers of adsorbed Cu 1 species to attach to Ni 3 clusters, forming Ni 3 Cu 1 at clusters, followed by the pathway for the formation of Ni 3 Cu 1 .pyr from Ni 3 Cu 1 .at (see Figure S1). The results in Figure S1 reveal that the diusion of an isolated Cu 1 species and attachment to the Ni 3 cluster to form the Ni 3 Cu 1 .at cluster is likely to occur since the barrier is as low as 0.33 eV, however, the formation of a pyramidal S3 Ni 3 Cu 1 .pyr structure (Ni 3 Cu 1 .pyr.1) from the Ni 3 Cu 1 .at has a barrier of 0.85 eV, which is 2.57 times higher. In short, kinetic considerations also support the choice of the model systems in this study. Figure S1: Adsorbed Ni 4−x Cu x (x=0 to 4) on CeO 2 (111). The average adsorption energy of Ni 4−x Cu x species is listed below each structure in eV per metallic atom. The pathway for the formation of the Ni 3 Cu 1 .at cluster from the adsorbed Ni 3 cluster and the diusion of Cu 1 species, followed by the pathway for the formation of a Ni 3 Cu 1 .pyr from the Ni 3 Cu 1 .at is shown. Ni and Cu atoms are depicted in blue and brown, respectively, while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray. Table S1: Bader charges, computed as dierences with respect to the nominal charges of the corresponding isolated atoms. (a) Supported clusters Ni 4−x Cu x .CeO 2 . (b) Bimetallic nanoparticle in the gas phase (Ni 4−x Cu x .gas), i.e., the free-standing Ni 4−x Cu x clusters, which result from the removal of the CeO 2 (111) support from the Ni 4−x Cu x .CeO 2 systems, without further optimization of the geometry. Each atom is labeled following the numbering in Figure  1 in the main text. Numbers in blue (brown) refer to the Ni (Cu) species.    The predicted E Barrier values correspond to the activation energy barrier calculated as the energy dierence between the predicted energy of the transition state and the calculated energy of the initial state. The model catalysts whose E TS energy is less than zero is related to the fact that on them CH 4 binds relatively strongly, so that if the barrier for the rst H abstraction from the chemisorbed CH 4 molecule is suciently low, E TS will be negative when referenced to gas-phase CH 4 and the clean surface.    Table S4: Energy (in eV) and geometrical parameters for the molecular initial state (IS) structure of the adsorption of CH 4 on Ni 4−x Cu x .CeO 2 . Distances between the carbon atom and the bimetallic particle (CB), as well as between the carbon atom and the hydrogen atoms (CH), are indicated (in pm). The charge gained by the C atom upon adsorption of CH 4 with respect to molecule in the gas phase is also indicated. All energies are relative to CH 4 in the gas phase and the corresponding clean surfaces.  Figure S5: Initial, transition and nal state structures for the non-cooperative rst dehydrogenation of CH 4 on Ni 4 .CeO 2 (cf. Figure S11). Representative distances are indicated in pm. Ni atoms are depicted in blue, while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray. S10 Figure S6: Initial, transition and nal state structures for the non-cooperative rst dehydrogenation of CH 4 on Cu 4 .CeO 2 (cf. Figure S12). Representative distances are indicated in pm. Cu atoms are depicted in brown, while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray. Figure S7: Total density of states (DOS) of the bimetallic particle and dz 2 -projected density of states for each metallic atom in the particle. S12 Figure S8: Initial, transition and nal state structures for the non-cooperative rst dehydrogenation of CH 4 on Ni 3 Cu 1 .CeO 2 (cf. Figure S13). Representative distances are indicated in pm. Ni and Cu atoms are depicted in blue and brown,respectively, while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray. Figure S9: Initial, transition and nal state structures for the non-cooperative rst dehydrogenation of CH 4 on Ni 2 Cu 2 .CeO 2 (cf. Figure S14). Representative distances are indicated in pm. Ni and Cu atoms are depicted in blue and brown,respectively,while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray. S13 Figure S10: Initial, transition and nal state structures for the non-cooperative rst dehydrogenation of CH 4 on Ni 1 Cu 3 .CeO 2 (cf. Figure S15). Representative distances are indicated in pm. Ni and Cu atoms are depicted in blue and brown,respectively, while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray.          Figure 3 in the main text.    Figure S20). Selected distances are indicated in pm. Ni and Cu atoms are depicted in blue and brown,respectively, while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray.
S23 Figure S26: Initial, transition and nal state structures for the non-cooperative rst dehydrogenation of H 2 O on Ni 2 Cu 2 .CeO 2 (cf. Figure S21). Selected distances are indicated in pm. Ni and Cu atoms are depicted in blue and brown,respectively,while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray. Figure S27: Initial, transition and nal state structures for the non-cooperative rst dehydrogenation of H 2 O on Ni 1 Cu 3 .CeO 2 (cf. Figure S22). Selected distances are indicated in pm. Ni and Cu atoms are depicted in blue and brown,respectively, while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray.
S24 Figure S28: Initial, transition and nal state structures for the non-cooperative rst dehydrogenation of H 2 O on Cu 4 .CeO 2 (cf. Figure S23). Selected distances are indicated in pm. Cu atoms are depicted in brown, while surface/subsurface oxygen atoms are in red/green, Ce 4+ in white, and Ce 3+ in gray.