Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary CH Bonds

Abstract

Achieving selective C-H bond cleavage is critical for developing catalytic processes that transform small alkanes to value-added products. The present study clarifies the molecular-level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C-H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ-complexes preferentially adopt geometries on PdO(101) in which only primary C-H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H-Pd dative bond forms at each CH3 group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C-H bonds.

PdO self-assembly on zeolite SSZ-13 with rows of O3Al(IV)OH selectively incorporated in PdO(101) facets for moisture-resistant methane oxidation

“…Surprisingly, we found that dissociation of only one water molecule on (101) PdO is energetically unfavourable (Fig. 6) compared to undissociated water molecule, which is in agreement with pioneering studies by Weaver and co-workers [25][26][27][28][29][30][31][32]. The dissociated state is higher by 11 kJ/mol-Pd than the undissociated one.…”

“…It was determined that some optimal (not too small, >10 nm) size of PdO nanoparticles is required to effectively catalyze methane and hydrocarbon combustion in the studies by Chin and co-workers [21][22][23][24]. Moreover, XPS and DFT single-crystal studies of Weaver and co-workers [24][25][26][27][28][29][30][31][32] unambiguously identified that PdO (101) facet, containing coordinatively unsaturated tri-coordinate Pd(II)1O3 sites, is critical in inducing catalytic activity for both alkane and methane combustion. These particular tri-coordinate Pd(II) sites are able to form a σ-complex with methane: formation of such a complex creates a pre-requisite site with close geometric proximity between CH4 and PdO required for further heterolytic activation of methane C-H bond on the Pd-O pair, the latter representing the rate-determining step of methane combustion.…”

“…PdO (100) surface was shown to be largely inactive by the same authors. Since the first step of methane activation involves the formation of a sigma-complex on 3-coordinate Pd sites present only on (101) facets with the following (ratedetermining) heterolytic cleavage of C-H bond on Pd-O pair [21][22][23][24][25][26][27][28][29][30][31][32], it is thus not surprising that larger particles with well-defined facets are more active for methane combustion. However, the main issue of supported and unsupported PdO nanoparticles is their deactivation in the presence of water vapor (always present in >2-3% in the vehicle exhaust) [11][12][13][14][15][16][17][18][19][20].…”

A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane

“…They are considerably shorter than for d 6 -propane weakly bound to an open Fe site in the MOF Fe 2 (dobdc) (∼3 Å), as analyzed by powder neutron diffraction at 4 K. 41 The 1,3-motif is similar to that proposed for propane coordination to a PdO(101) surface, albeit spanning two Pd sites. 45 The propane σ-complex Mn(η 5 -C 5 H 5 )(CO) 2 (propane) has been characterized in solution using in situ NMR spectroscopy at low temperature (133 K). 16 Given the quality of the refinement, hydrogen atoms were not located, and so the hapticity of the Rh···H–C interaction in [1-propane][BAr F 4 ] (e.g., η 1 or η 2 ) was interrogated using computational techniques ( section 2.3 ).…”

Ultra-Low Amounts of Palladium (0.005-0.05 Wt% Pd) Supported on Titania: Remarkable Low-Temperature Activity for NO Reduction with CO and Structure-Function Property Relationships in Methane Oxidation

“…3 A and B). It is well-known from pioneering studies of Weaver [38][39][40][41][42][43] and co-workers that for Pd catalysts to be active for methane and hydrocarbon oxidation, coordinatively unsaturated tri-coordinate sites on (101) planes of PdO nanoparticles should be present or, alternatively, contiguous coordinatively unsaturated sites on Pd metal surface. Thus, it is generally accepted in the literature and further validated in the combined theory-experimental studies of Chin, Iglesia and others [45][46][47] that nanoparticles of PdO of certain size exposing most reactive (101) facets are most active for methane combustion.…”