(346g) Understanding the Role of Alkali Metals to Promote Pure Nickel and Bimetallic Nickel-Copper Catalysts for Dry Reforming of Methane

Dry reforming of methane (DRM) is one of the reactions in which numerous studies have been conducted to simultaneously convert CO₂ and CH₄ into syngas. Earlier work has shown that bimetallic Ni-Cu catalyst possesses better activity to form coke than conventional pure Ni (111) catalyst. It has been identified that certain promoters such as alkali metals can enhance the activity and minimize carbon deposition on the DRM catalysts. Hence, we examined whether alkali metal promoters (Na and K) promote catalytic ability of the two catalysts for DRM with the help of a first-principles study utilizing density functional theory (DFT). Two different loadings (0.3% and 3%) for the alkali metal promoters were considered and how they influence the stability of two DRM catalysts. In addition to the effects of varying promoter loadings, another factor to determine the energetic stability of the catalytic system was to study the adsorption site of the promoter atoms. This was achieved by studying the relative energy along the adsorption site.

The smallest relative energy for pure Ni (111) was achieved by Na and K atoms at 3% loading of about 8.26 eV and 8.74 eV, respectively, with the promoter atoms either preadsorbing on the top layer of the catalyst slab or replacing one of the present Ni atoms in the top layer. In addition, when two promoters were present in the bimetallic Ni-Cu catalyst exactly the same as the position located in the pure Ni (111), Na and K achieved the smallest relative energy for at -2.58 eV and -2.28 eV, respectively, and the stable catalyst configurations were obtained.

Next, we investigated the carbon sources of CH₄ and CO₂ in the DRM reaction, starting with the adsorption of C atoms at a specific adsorption site to evaluate whether two promoters on catalysts can minimize the deposition of coke. We found that K atoms at 0.3% loading promoted carbon adsorption on top sites of the pure Ni (111) catalysts, whereas for the bimetallic Ni-Cu catalysts, the carbon adsorption at the top and bridge sites was increased. Under the same loading conditions of the K promoter, Na atoms preferred adsorption of C atoms at the top and bridge sites of the bimetallic Ni-Cu system, but not in the pure Ni (111) system. With respect to the adsorption of the DRM intermediate CH₄, both Na and K atoms at 0.3% loading favored adsorption on top and bridge sites of both bimetallic Ni-Cu and pure Ni (111) catalysts.

We therefore expect that the DRM reaction occurring on the Na-promoted pure Ni (111) catalysts would give a better tendency to prevent coke formation.