Surface Segregation in CuNi Nanoparticle Catalysts During CO2 Hydrogenation: The Role of CO in the Reactant Mixture

Surface segregation and restructuring in size-selected CuNi nanoparticles were investigated via near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at various temperatures in different gas environments. Particularly in focus were structural and morphological changes occurring under CO2 hydrogenation conditions in the presence of carbon monoxide (CO) in the reactant gas mixture. Nickel surface segregation was observed when only CO was present as adsorbate. The segregation trend is inverted in a reaction gas mixture consisting of CO2, H2, and CO, resulting in an increase of copper concentration on the surface. Density functional theory calculations attributed the inversion of the segregation trend to the formation of a stable intermediate on the nanocatalyst surface (CH3O) in the CO-containing reactant mixture, which modifies the nickel segregation energy, thus driving copper to the surface. The promoting role of CO for the synthesis of methanol was demonstrated by catalytic characterization measurements of silica-supported CuNi NPs in a fixed-bed reactor, revealing high methanol selectivity (over 85%) at moderate pressures (20 bar). The results underline the important role of intermediate reaction species in determining the surface composition of bimetallic nanocatalysts and help understand the effect of CO cofeed on the properties of CO2 hydrogenation catalysts.


Catalytic testing
The metal content of the Cu 0.5 Ni 0.5 /SiO 2 powder catalyst was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Perkin Elmer, Optima 8300). For these measurements, about 15 mg of the catalyst powder were dissolved in 2 ml of sulfuric acid (H 2 SO 4 ), 2 ml of nitric acid (HNO 3 ) and 6 ml of hydrochloric acid (HCl) and digested in a microwave digestion system at 180 °C for 30 min (Anton Paar GmbH, Multiwave GO). Before the ICP-OES measurements, the samples were further diluted with water by a factor of 50. The measurements were performed in standard axial mode and the results were analyzed in the software Syngistix.
For the catalytic testing in the fixed-bed reactor, helium was used as an internal standard to calculate the total outlet flow. The molar flow rate of a given component i was calculated taking into account the total outlet flow and the concentration n i obtained with the gas chromatograph for each species: The selectivity (S i ) of a given component i was calculated based on the molar flow rate of the product divided by the sum of the flow rates of all products: The calculated selectivities for CH 3 OH and CH 4 do not take into account any CO production due to the reverse water-gas-shift reaction (RWGS).
The production rate of a given product i is its molar flow rate F i per mass of metal in the catalyst: [mol g -1 min -1 ]. =

Theory calculations
The valence electronic states were expanded in the basis of plane waves with the core-valence interaction represented using the projector augmented wave (PAW) approach and a cutoff of 400 eV. The convergence for the electronic self-consistency is set to 10 -5 eV. Geometry optimization was performed within a conjugate-gradient algorithm with a convergence criterion on forces The adsorption energy of a gas phase molecule on the CuNi system was defined as: , and E(molecule) are the total DFT energies of the CuNi system with the molecule adsorbed on its surface, the bare CuNi system, and the free gas phase molecule, respectively.
When two or more molecules (adsorbates) are present, then the lateral interactions between the adsorbed species are taken into consideration in the calculations. These interactions influence the bonding strength between the adsorbates and the CuNi surface. Thus, the total adsorption energy is in general different from the linear combination of the contributions of the individual species.
The surface segregation energy in the dilute limit, , is defined as the energy difference when moving a single dopant atom from the bulk to the surface, which has been proven to be a typical value to estimate the general trends in surface segregation phenomena in transition metal alloys 1 . In this work it is calculated as follows: , where and E 9 ℎ correspond to the energy values when one site at the surface or in the 9 th atomic layer, respectively, is a Ni dopant, while all other sites are Cu. 9 ℎ is considered as bulk in this work. The molecules which are more stable and more strongly adsorbed on the surface are those which dominate the surface segregation trend.