Reduction of Nitric Oxide with Hydrogen on Catalysts of Singly Dispersed Bimetallic Sites Pt₁Co_m and Pd₁Co_n

The bimetallic catalyst has been one of the main categories of heterogeneous catalysts for chemical production and energy transformation. Isolation of the continuously packed bimetallic sites of a bimetallic catalyst forms singly dispersed bimetallic sites which have distinctly different chemical environment and electronic state and thus exhibit a different catalytic performance. Two types of catalysts consisting of singly dispersed bimetallic sites Pt₁Co_m or Pd₁Co_n (m and n are the average coordination numbers of Co to a Pt or Pd atom) were prepared through a deposition or impregnation with a following controlled calcination and reduction to form Pt₁Co_m or Pd₁Co_n sites. These bimetallic sites are separately anchored on a nonmetallic support. Each site only consists of a few metal atoms. Single dispersions of these isolated bimetallic sites were identified with scanning transmission electron microscopy. Extended X-ray absorption fine structure spectroscopy (EXAFS) revealed the chemical bonding of single atom Pt₁ (or Pd₁) to Co atoms and thus confirmed the formation of bimetallic sites, Pt₁Co_m and Pd₁Co_n. Reduction of NO with H₂ was used as a probing reaction to test the catalytic performance on this type of catalyst. Selectivity in reducing nitric oxide to N₂ on Pt₁Co_m at 150 °C is 98%. Pd₁Co_n is active for reduction of NO with a selectivity of 98% at 250°C. In situ studies of surface chemistry with ambient-pressure X-ray photoelectron spectroscopy and coordination environment of Pt and Pd atoms with EXAFS showed that chemical state and coordination environment of Pt₁Co_m and Pd₁Co_n remain during catalysis up to 250 and 300°C, respectively. The correlation of surface chemistries and structures of these catalysts with their corresponding catalytic activities and selectivities suggests a method to develop new bimetallic catalysts and a new type of single site catalysts.