Nature of the N–Pd Interaction in Nitrogen-Doped Carbon Nanotube Catalysts

Abstract

In this work, the geometric and electronic structure of N species in N-doped carbon nanotubes (NCNTs) is derived by X-ray photoemission (XPS) and absorption spectroscopy (NEXAFS) of the N 1s core excitation. Substitutional N species in pyridine-like configuration and another form of N with higher thermal stability are found in NCNTs. The structural configuration of the high thermally stable N species, in the literature often referred to as graphitic N, is assessed in this work by a combined theoretical and experimental study as a 3-fold substitutional N species in an NCNT basic structural unit (BSU). Furthermore, the nature of the interaction of those N species with a Pd metal center immobilized onto NCNTs is of σ-type donation from the filled π-orbital of the N atom to the empty d-orbital of the Pd atom and a π back-donation from the filled Pd atomic d-orbital to the π* antibonding orbital of the N atom. We have found that the interaction of pyridine N with Pd is characterized by a charge transfer typical of a covalent chemical bond with partial ionic character, consistent with the chemical shift observed in the Pd 3d core level of divalent Pd. Graphitic N sites interact with Pd by a covalent bond without any charge redistribution. In this case, the electronic state of the Pd corresponds to metallic Pd nanoparticles electronically modified by the interaction with the support. The catalytic reactivity of these samples in hydrogenation, CO oxidation, and oxygen reduction reaction (ORR) allowed clarifying some aspects of the metal carbon support interaction in catalysis.

Single Isolated Pd2+ Cations Supported on N-Doped Carbon as Active Sites for Hydrogen Production from Formic Acid Decomposition

“…Hence, the atoms seen by aberrationcorrected STEM are in the Pd 2+ state stabilized by nitrogen of the support. Very recently, Arrigo et al 27 observed similar changes in NEXAFS spectra of nitrogen after Pd deposition and assigned them to the formation of a covalent bond of Pd with pyridinic nitrogen with partial ionic character consistent with the shift of the binding energy of Pd 3d close to the Pd 2+ state.…”

“…The N 1s lines at ~398.1 (N1), ~399.8 (N2), ~400.7 (N3) and ~402.4 eV (N4) were attributed to pyridinic, pyrrolic, graphitic and oxidized nitrogen species, respectively. 25,27,34,35 The first three species are present in approximately the same concentrations (~30% , Table S2, Supporting information). For the samples with the reduced deposited Pd, a better fit of the N 1s spectra could be achieved accepting the presence of one more type of nitrogen species with a binding energy of 398.7-…”

“…The XPS measurements of the Pd electronic state in the ex situ reduced supported samples are shown in Figure 7b. The Pd 3d spectrum consisted of 2 doublets with 3d 5/2 positions at about 335.9 and about 338.0 eV typical for supported Pd metallic clusters and ionic Pd 2+ species, 27 respectively. So, in contrast to the unsupported Pd powder, the Pd 2+ species were present in a significant concentration in the N-free and N-doped samples in spite of the As the unsupported Pd powder does not show the presence of the Pd 2+ species after the same reduction (Figure 7a), the presence of this oxidized Pd form in the supported catalysts was assigned to highly dispersed Pd species strongly interacting with the support through oxygen or nitrogen atoms.…”

“…24 Arrigo et al 27 also noticed that coordination of Pd 2+ species to nitrogen of the support complicates the reduction of these species with hydrogen. Stonkus et al 29 confirmed that deposition of Pd on N-doped carbon nanofibers leads to a change of redox properties of Pd as compared to those for Pd supported on N-free carbon nanofibers.…”

“…The presence of nitrogen leads to stabilization of a supported phase, changing its chemical and electronic properties or even to creation of novel active sites for catalytic reactions. [24][25][26][27][28][29] Recently, Arrigo et al 27 reported a study of interaction of N-doped carbon nanotubes with Pd species deposited from Pd(NO 3 ) 2 and Na 2 PdCl 4 and of reactivity of the obtained catalysts in different reactions like oxygen reduction, CO oxidation and hydrogenation. Our present work is focused to understanding of the nature of Pd species active in decomposition of formic acid and the character of interaction of these Pd species with the N-doped carbon surface.…”