Renqin Zhang

Mitigation of CO poisoning on functionalized Pt–TiN surfaces

It has been previously reported that the system of single Pt atoms embedded in N-vacancy (V(N)) sites on the TiN(100) surface (Pt-TiN) could be a promising catalyst for proton exchange membrane fuel cells (PEM FCs). The adsorption of molecules on Pt-TiN is an important step, when it is incorporated as the anode or cathode of PEM FCs. Utilizing first principles calculations based on density functional theory, systematic investigations are performed on the adsorption of several atomic and molecular species on the Pt-TiN system, as well as the co-adsorption of them. The favorable binding sites and adsorption energies of several molecular species, namely carbon dioxide (CO2), carbon monoxide (CO), oxygen (O2), hydrogen (H2), hydroxyl (OH), an oxygen atom (O), and a hydrogen atom (H), are explored. For each, the adsorption energy and preferred binding site are identified and the vibrational frequencies calculated. It is found that CO2, CO and H prefer the Pt top site while OH and O favorably adsorb on the Ti top site. When CO and OH are co-adsorbed on the Pt-TiN(100) surface, OH weakens the adsorption of CO. The weakening effect is enhanced by increasing the coverage of OH. A similar behavior occurs for H and OH co-adsorption on the Pt-TiN(100) surface. Because co-adsorption with OH and H species weakens the adsorption of CO on Pt-TiN, it is expected that the acid and base conditions in PEM FCs could mitigate CO poisoning on functionalized Pt-TiN surfaces.

An atomic-scale view of single-site Pt catalysis for low-temperature CO oxidation

Single-atom catalysts have attracted great attention in recent years due to their high efficiencies and cost savings. However, there is debate concerning the nature of the active site, interaction with the support, and mechanism by which single-atom catalysts operate. Here, using a combined surface science and theory approach, we designed a model system in which we unambiguously show that individual Pt atoms on a well-defined Cu2O film are able to perform CO oxidation at low temperatures. Isotopic labelling studies reveal that oxygen is supplied by the support. Density functional theory rationalizes the reaction mechanism and confirms X-ray photoelectron spectroscopy measurements of the neutral charge state of Pt. Scanning tunnelling microscopy enables visualization of the active site as the reaction progresses, and infrared measurements of the CO stretch frequency are consistent with atomically dispersed Pt atoms. These results serve as a benchmark for characterizing, understanding and designing other single-atom catalysts.Single-atom catalysts are of growing importance, but the nature of their structure and reactivity remains under debate. Here, Sykes and co-workers show that single Pt atoms on a well-defined Cu2O surface are capable of performing low-temperature CO oxidation, and provide data on the binding site and electronic structure of the Pt atoms.

The interaction of reactants, intermediates and products with Cu ions in Cu-SSZ-13 NH3 SCR catalysts: an energetic and ab initio X-ray absorption modeling study

In this contribution, the most likely positions for Cu in Cu-SSZ-13 with a single charge compensating Al atom (ZCu) with a Si:Al ratio of 11:1 were investigated, including the effect of the adsorption of reactants, intermediates, and products that one would find in an NH3 SCR reaction by using first-principles calculations based on density functional theory. The 6-membered ring (6MR) site is the most energetically favorable, while the 8-membered ring (8MR) sites are less favorable with energy differences of about 0.5 eV with respect to the 6MR site for plain ZCu. Upon molecular adsorption, the energy differences between Cu in the 8MR and 6MR sites decrease and, in some cases, almost disappear. For the complex scenarios of NO or CO adsorption, the co-adsorption of 2 NO or 2 CO molecules, as well as NO or CO with OH and H2O, weakens the interaction between adsorbates and Cu. The X-ray absorption near edge structure (XANES) of Cu in Cu-SSZ-13 under different conditions was also modeled from first principles. A small peak feature around 8979.5 eV was found in the K-edge XANES of Cu for a clean ZCu conformation with Cu in the 8MR site, while this feature is absent when Cu is in the 6MR site. We correlate this result for this case as well as for other representative configurations by analyzing the corresponding PDOS of the excited state of Cu while taking into account the core-hole effect. Molecular adsorption onto Cu in the 6MR or 8MR site results in a Cu K-edge XANES that is independent of its location. When NO (or CO, N2) is adsorbed onto Cu in ZCu, a small peak feature in the K-edge XANES appears at 8980 eV, which is induced by the splitting of the Cu 4p state. An analysis of the XANES in the presence of two co-adsorbed species on an isolated Cu ion shows that the K-edge position of Cu has the following order (from low to high energy): clean < M < M + H2O < 2M < M + OH (M denotes NO or CO). As a result, we conclude that (1) XANES can readily distinguish between adsorbates on ZCu due to their different oxidizing capacities, but (2) XANES cannot be used to distinguish between the Cu location and the oxidation state in the presence of adsorbates, except in the presence of H2O and NH3 where such distinctions can be made.

Local Environment Sensitivity of the Cu K-Edge XANES Features in Cu-SSZ-13: An Analysis from First Principles

Cu K-edge X-ray absorption near-edge spectra (XANES) have been widely used to study the properties of Cu-SSZ-13. In this Letter, the sensitivity of the XANES features to the local environment for a Cu+ cation with a linear configuration and a Cu2+ cation with a square-linear configuration in Cu-SSZ-13 is reported. When a Cu+ cation is bonded to H2O or NH3 in a linear configuration, the XANES has a strong peak at around 8983 eV. The intensity of this peak decreases as the linear configuration is broken. As for the Cu2+ cations in a square-planar configuration with a coordination number of 4, two peaks at around 8986 and 8993 eV are found. An intensity decrease for both peaks at around 8986 and 8993 eV is found in an NH3_4_Z2Cu model as the N-Cu-N angle changes from 180 to 100°. We correlate these features to the variation of the 4p state by PDOS analysis. In addition, the feature peaks for both the Cu+ cation and Cu2+ cation do not show a dependence on the Cu-N bond length. We further show that the feature peaks also change when the coordination number of the Cu cation is varied, while these feature peaks are independent of the zeolite topology. These findings help elucidate the experimental XANES features at an atomic and an electronic level.