Determination of metal particle size of highly dispersed rhodium, iridium and platinum catalysts by hydrogen chemisorption and EXAFS

Koningsberger, D.C.; Kip, BJ; Duivenvoorden, F.B.M.; Prins, R.

doi:10.1021/ja00278a049

Cited by 52 publications

(17 citation statements)

References 2 publications

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“…H* atoms interact repulsively at such coverages (>0.8 ML, mean H*–H* distances of 0.34 nm), as observed on Pt(111) surfaces in previous work . Coverages greater than 1 ML become evident as H 2 pressures increase on all samples (Figure ) even at 473 K and relatively low H 2 pressures (10 kPa), indicative of the expected prevalence of supramonolayer coverages during ambient H 2 chemisorption experiments typically used to determine metal dispersions, as reported previously on Pt/Al 2 O 3 and Pt/SiO 2 (3 nm) − and confirmed by DFT-derived estimates of adsorption free energies on Pt nanoparticles (up to 586 atoms, 2.4 nm) …”

Section: Resultssupporting

confidence: 79%

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

2019

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Density functional theory (DFT) and dihydrogen chemisorption uptakes at temperatures relevant to catalysis are used to determine and interpret adsorption enthalpies and entropies over a broad range of chemisorbed hydrogen (H*) coverages (0.1 ML to saturation) on Pt nanoparticles (1.6, 3.0, and 9.1 nm mean diameters) and Pt(111) surfaces. Heats of adsorption decrease by 30 kJ mol −1 as H 2 coverage increases from nearly bare (0.1 ML) to saturated (∼1 ML) surfaces, because of the preferential saturation of lowcoordination surface atoms by H* at low coverages. Such surface nonuniformity also leads to stronger binding on small Pt particles at all coverages (e.g., 47 kJ mol −1 on 1.6 nm, 40 kJ mol −1 on 3.0 nm, and 37 kJ mol −1 on 9.1 nm particles all at 0.5 ML), because their surfaces expose a larger fraction of low-coordination atoms. As H* approaches monolayer coverages, H*−H* repulsion leads to a sharp decrease in binding energies on all Pt particles. Measured H* entropies decrease with increasing H* coverage and decreasing particle size, but their values (35−65 J mol −1 K −1 ) are much larger than for immobile H* species at all coverages and particle sizes. Two-dimensional gas models in which mobile H* adsorbates move rapidly across uniform (ideal), excluded-area, or DFT-predicted nonuniform potential energy surfaces all give H* entropies ≥30 J mol −1 K −1 , qualitatively consistent with measured values. Larger H* entropies are predicted for uniform (ideal) PES while smaller H* entropies are predicted using nonuniform PES. DFT-derived barriers for H* diffusion between hcp and fcc 3-fold sites on Pt( 111) surface are about 6 kJ mol −1 , a value similar to thermal energies (kT) at temperatures of catalytic relevance, consistent with fast diffusion in the time scale of adsorption−desorption events and isotherm measurements. Mobile adsorbate entropy models are therefore essential for accurate estimates of adsorbate coverages, particularly because vibrational frequencies analyzed by harmonic oscillator approximations lead to large underpredictions of entropies, resulting in DFT-predicted adsorption free energies that are too large to result in high coverageseven at conditions where H* is known to cover the surface. The nonuniformity of Pt surfaces, repulsion among coadsorbates, and high adsorbate mobility starkly contrast with the requirements for Langmuirian descriptions of binding and reactions at surfaces, despite the ubiquitous use and success in practice of the resulting equations in describing adsorption isotherms and reaction rates on surfaces. Such fortuitous agreement is a likely consequence of the versatility of their functional form, taken together with the limited range in pressure and temperature in adsorption and kinetic measurements. The adsorption and kinetic constants derived from such data, however, would differ significantly from theoretical estimates that rigorously account for surface coordination, adsorbate mobility, and coadsorbate repulsion.

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Section: Resultssupporting

confidence: 79%

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

2019

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“…The hydrogen chemisorption capacity decreased with increasing reduction temperature. However, one would expect a higher hydrogen chemisorption capacity based on the lower Pt-Pt coordination number after HTR (38). A decrease in hydrogen chemisorption with increasing reduction temperature has also been observed for Pt/K-LTL and Pt/H-LTL catalysts (10).…”

Section: The Influence Of the Morphology Of The Platinum Particles And The Structure Of The Metal-support Interface On Hydrogen Chemisorpmentioning

confidence: 90%

On the Relation between Particle Morphology, Structure of the Metal-Support Interface, and Catalytic Properties of Pt/γ-Al2O3

Vaarkamp

Miller²,

Modica³

et al. 1996

Journal of Catalysis

189

174

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The relation between the catalytic activity, electronic properties, morphology, and structure of the metal-support interface was studied for Pt/γ -Al 2 O 3 after low (300 • C, LTR) and high temperature reduction (450 • C, HTR). EXAFS revealed that after LTR the platinum particles were three-dimensional, contained 11 Pt atoms on average, and were at a distance of 2.7Å from the support oxygen. During HTR, the morphology of the platinum particles changed from three-dimensional to rafts with a structure similar to Pt(100), as indicated by a decrease in the Pt-Pt coordination number and the absence of the third coordination shell in the EXAFS spectrum. After HTR the Pt-O distance was shortened to 2.2Å due to the desorption of hydrogen from the metal-support interface. The shortening of the Pt-O distance upon HTR treatment agrees with previous studies on zeolite supported platinum. However, zeolite supported platinum particles retained their three-dimensional structure upon HTR. The changes in the structure of the catalyst affected the catalytic, chemisorption, and electronic properties. After HTR the selectivity for hydrogenolysis of both neopentane and methylcyclopentane to methane decreased. At the same time the specific activity for neopentane isomerization and methylcyclopentane ring opening increased. The hydrogen chemisorption capacity after HTR was lower than after LTR. HTR shifted the asymmetric linear CO infrared absorption from 2063 to 2066 cm −1 . Comparison of the Pt L III and L II X-ray absorption edge intensities after LTR and HTR revealed that the number of holes in the d-band of the platinum atoms increased by 9.5% during HTR. It was suggested that the decrease in hydrogen chemisorption capacity, hydrogenolysis selectivity, and number of holes in the d-band are related to the change in the structure of the metal-support interface. The increase in specific activity for isomerization of neopentane and the shift in the CO infrared absorption band with a raise in reduction temperature agreed with reported

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“…The hydrogen-to-metal stoichiometry has been under debate in the literature and it has been proposed that the stoichiometry varies from 0.5 to 2.5 depending on the metal particle size and metal-metal coordination number [19,22,23]. Because of this variation, we do not convert hydrogen uptakes to dispersion values.…”

Section: H 2 Uptakementioning

confidence: 99%

Supported iridium catalysts prepared by atomic layer deposition: effect of reduction and calcination on activity in toluene hydrogenation

Silvennoinen¹,

Jylhä²,

Lindblad

et al. 2007

Catal Lett

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Ligand removal from supported iridium catalysts prepared by atomic layer deposition from Ir(acac) 3 was studied by direct reduction in hydrogen flow and by calcination in oxygen flow followed by reduction in hydrogen flow, as well as by thermogravimetric analysis. Thermal decomposition of acac ligand residuals required high temperatures, and in samples containing no iridium the removal of carbonaceous species was not complete. Metallic iridium particles less than 2 nm in size were formed during direct reduction and larger particles upon calcination followed by reduction. The activity of the catalysts in toluene hydrogenation in most cases depended on particle size.

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Determination of metal particle size of highly dispersed rhodium, iridium and platinum catalysts by hydrogen chemisorption and EXAFS

Abstract: Prins, R. (1986). Determination of metal particle size of highly dispersed Rh, Ir, and Pt catalysts by hydrogen chemisorption and EXAFS.

Cited by 52 publications

References 2 publications

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

Hydrogen Chemisorption Isotherms on Platinum Particles at Catalytic Temperatures: Langmuir and Two-Dimensional Gas Models Revisited

On the Relation between Particle Morphology, Structure of the Metal-Support Interface, and Catalytic Properties of Pt/γ-Al2O3

Supported iridium catalysts prepared by atomic layer deposition: effect of reduction and calcination on activity in toluene hydrogenation

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