Dynamics and Thermodynamics of Pd–PdO Phase Transitions: Effects of Pd Cluster Size and Kinetic Implications for Catalytic Methane Combustion

Abstract

Methane oxidation rates uncorrupted by nonchemical effects of transport, taken together with stoichiometric oxygen uptake (oxidation cycle) and evolution (decomposition cycle) data, are used to establish for the first time a set of conditions required for true thermodynamic equilibrium during metal-to-oxide interconversions in small Pd clusters (1.8−8.8 nm). These conditions allow us to assess the intrinsic thermodynamics of small Pd clusters and their catalytic effects in CH 4 oxidation. PdO decomposition in the absence of CH 4 deviates from equilibrium, as this step is limited by the nucleation of an oxygen vacancy ensemble on oxide domains. The nucleation bottleneck is removed by CH 4 during its catalytic sojourns, when CH 4 pressure and the related rates exceed a critical value, because CH 4 effectively removes the oxygen adatoms near an oxygen vacancy site via C−H bond activation on an oxygen−oxygen vacancy site pair that converts the O* adatom to a hydroxyl intermediate, which desorbs as H 2 O in sequential steps. CH 4 oxidation turnovers promote the nucleation of oxygen vacancy ensembles at conditions that maintain the global oxygen equilibration, as confirmed from the absence of CH 4 oxidation rate hysteresis in both Pd oxidation and PdO decomposition cycles and from coincidence of rate and oxygen content profiles during Pd oxidation. A theoretical construction decoupling the inherent cluster size variance from cluster diameter effects shows marked effects of size on bulk phase transition. The bulk phase transition occurs at lower oxygen chemical potentials for the smaller clusters, which confirm their more negative Gibbs free energy for PdO formation than the large structures. The bulk phase transition converts O*−O* adatom sites to Pd 2+ −O 2− ion pairs that are more effective for the kinetically relevant C−H bond activation in CH 4 . These effects of size on the thermodynamics and reactivities of small clusters illustrated in this study are general and extend beyond the Pd−PdO system.

Understanding the Role of SnO2 Support in Water‐Tolerant Methane Combustion: In situ Observation of Pd(OH)2 and Comparison with Pd/Al2O3

“…[8,34,41,42] One should keep in mind, however, that the Pd (re)-oxidation is kinetically limited, and both the kinetics and thermodynamics are particle-size dependent. [41,43,44] A recent operando XAS study of Pd/Al 2 O 3 catalysts with monodispersed Pd particles (3-16 nm range) in wet methane combustion [41] confirmed that at least partially oxidized surface is required for methane combustion. Our study lacks the size uniformity of the Pd particles, but the starting Pd colloidal solution contains 2-nm Pd particles, which sinter to larger particles with 40 % dispersion on Al 2 O 3 and 16 % on SnO 2 with a lower surface area.…”

Platinum Inhibits Low‐Temperature Dry Lean Methane Combustion through Palladium Reduction in Pd−Pt/Al2O3: An In Situ X‐ray Absorption Study

“…The turnover rates increasedb ya no rder of magnitude with at hreefold increasei n the Pd/PdO x ratio, as measuredb yN H 3 temperature-programmed desorption (TPD), and the coexistence of metallicP d with PdO was claimed to be paramount. [7] The conclusion was drawn from oxygen uptake and evolutionm easurements, methane combustion kinetic measurements, and thermodynamic calculations.U sing in situ extended X-ray absorption fine structure (EXAFS) measurements, Grunwaldt et al detected the presence of metallic Pd on aP d/ZrO 2 catalyst at 500-550 8Ca nd suggested its beneficial effect, considering that the activity of prereduced catalyst was highert han that of the oxidized catalyst. [5] Farrauto et al showed al oss of activity when metallic palladium was formed under reducingr eaction conditions.…”

“…[16] In an oxidative atmosphere,P to nag-alumina support is more susceptible to sintering than Pd;t hus, in PdÀPt formulations,P d stabilizes Pt as ar esult of strong interactions between PdO and the support. [13,15,[19][20][21][22][23][24][25] The reactivities of palladium and platinum towards oxygen depend on the temperature and partial pressure of oxygen, [7,17,[26][27][28][29] metal support, [27,30] and nanoparticle size. [13,15,[19][20][21][22][23][24][25] The reactivities of palladium and platinum towards oxygen depend on the temperature and partial pressure of oxygen, [7,17,[26][27][28][29] metal support, [27,30] and nanoparticle size.…”

“…[17,18] In bimetallic catalysts, the chemicalstates of palladium and platinum can be affected by their ratios, preparation methods, and catalyst support to enhance or suppress methane conversion. [7,31] In addition to metallicP da nd Pt, oxide species,s uch as PdO, PdO 2 , PtO, PtO 2 ,a nd Pt 3 O 4 ,m ay be present. [7,31] In addition to metallicP da nd Pt, oxide species,s uch as PdO, PdO 2 , PtO, PtO 2 ,a nd Pt 3 O 4 ,m ay be present.…”

“…[6] The group of Iglesia reported that the methane oxidation rate coefficients at various oxygen pressures at 700 8Cw ere as ingle functiono ft otal oxygen content in Pd/Al 2 O 3 catalysts with nanoparticle sizes from 1.8 to 8.8 nm. [7] The conclusion was drawn from oxygen uptake and evolutionm easurements, methane combustion kinetic measurements, and thermodynamic calculations.U sing in situ extended X-ray absorption fine structure (EXAFS) measurements, Grunwaldt et al detected the presence of metallic Pd on aP d/ZrO 2 catalyst at 500-550 8Ca nd suggested its beneficial effect, considering that the activity of prereduced catalyst was highert han that of the oxidized catalyst. [8] An operando Ramanstudy of the Pd/Al 2 O 3 catalyst in the 25-500 8Cr ange detected only palladium oxide forming under the reactionc onditions; [9] this is consistent with thermodynamic expectations and the kinetic investigation of Pd/ZrO 2 catalysts by Fujimoto et al [10] Thel atter study explained the frequently reported requirement of the coexistence of PdÀPdO x by aM ars-van Krevelen mechanism when CH 4 is activated on site pairs consisting of Oa toms and Ov acancies on the PdO x surface.T hese vacancies can also be easily poisoned by water,w hich is known for its strong inhibitory Palladium-platinum bimetallic catalysts supported on alumina with palladium/platinum molar ratios ranging from 0.25 to 4 are studied in dry lean methane combustion in the temperature range of 200 to 500 8C.…”