From Electronic Structure to Catalytic Activity: A Single Descriptor for Adsorption and Reactivity on Transition-Metal Carbides

Adsorption and catalytic properties of the polar (111) surface of transition-metal carbides (TMC’s) are investigated by density-functional theory. Atomic and molecular adsorption are rationalized with the concerted-coupling model, in which two types of TMC surface resonances (SR’s) play key roles. The transition-metal derived SR is found to be a single measurable descriptor for the adsorption processes, implying that the Brønsted-Evans-Polanyi relation and scaling relations apply. This gives a picture with implications for ligand and vacancy effects and which has a potential for a broad screening procedure for heterogeneous catalysts.

Figure 1

Upper panel: Calculated atom-projected and total (111) surface DOS’s for a group of TMC’s. The dashed diagonal line connects the mean energies ${\epsilon }_{\mathrm{CCM}}$ of the TMSR’s and illustrates the downward shift of the TMSR energy along the TMC series as the group number of the TM component increases. Lower panel: Difference in surface DOS induced by O adsorption on VC(111).

Figure 2

Adsorption energies of H, N, O, and ${\mathrm{NH}}_x$ ($x=1$, 2, 3) on the considered TMC(111) surfaces vs the descriptor ${\epsilon }_{\mathrm{CCM}}$ (the TMSR center of mass). The adsorption energy is defined as $E_{\mathrm{ads}}^A=E_{\mathrm{TMC}+A}-E_{\mathrm{TMC}}-E_A$ for each adsorbate $A$. (ScC is not included in the linear regressions.) Also included (dashed line) are the transition-state (TS) energies for ${\mathrm{N}}_2$ dissociation, calculated with a fixed bond-length method.

Figure 3

Adsorption energies on TMC(111) surfaces (ScC is not included) for ${\mathrm{NH}}_x$ ($x=1$, 2, 3) vs N atomic adsorption energies (scaling relation). The adsorption energy is defined as $E_{\mathrm{ads}}^{{\mathrm{NH}}_x}=E_{\mathrm{TMC}+{\mathrm{NH}}_x}-E_{\mathrm{TMC}}-E_{{\mathrm{NH}}_x}$. Inset: the linear variation of the activation energy for ${\mathrm{N}}_2$ dissociation ($E_{\mathrm{TS}}$) vs N atomic adsorption energy on TiC, VC, and $\delta$-MoC (BEP relation).