Nanostructural Features of Pd/C Catalysts Investigated by Physical Methods:  A Reference for Chemisorption Analysis

Abstract

In this work we have determined, on a series of 0.5% Pd/C catalysts, the palladium particle sizes by the following physical techniques: (i) X-ray diffraction (XRD) line broadening (LB) method, associated with the Rietveld method, (ii) small-angle X-ray scattering, and (iii) transmission electron microscopy. The catalysts, suitably aged at different temperatures (673, 773, 873, and 973 K), had significantly different metal dispersions. Since the XRD-LB technique is not able to measure directly very small metal particles or clusters (roughly e25 Å in size), because they give diffuse X-ray scattering spreading out into the background, we have tackled this problem by means of a suitably tailored Rietveld quantitative analysis. This analysis allowed determination of the Pd fraction "visible" in the Voigtian XRD peaks and its average crystallite size using the LB method. As to the nanoparticle size of the undetectable fraction, an average value of 20 Å was assumed, corresponding to the size of a cubooctahedral perfect cluster of the fourth order. Combining all these data, real effective Pd average particle sizes could be calculated and compared with the corresponding values found by CO chemisorption. It was found that a surface Pd/CO stoichiometry of 2 must be assumed, irrespective of Pd dispersion, to get correct values of the average Pd particle size.

From the Lindlar Catalyst to Supported Ligand‐Modified Palladium Nanoparticles: Selectivity Patterns and Accessibility Constraints in the Continuous‐Flow Three‐Phase Hydrogenation of Acetylenic Compounds

“…The interval between successive pulses was minimized to avoid desorption of CO. The palladium dispersion was calculated from the amount of chemisorbed CO, considering an atomic surface density of 1.26×10 19 atoms m −2 and an adsorption stoichiometry of Pd/CO=2 47. X‐ray photoelectron spectroscopy (XPS) was performed on a VG‐Microtech Multilab 3000 spectrometer featuring a hemispheric electron analyzer with 9 channeltrons and non‐monochromatized Al Kα radiation at 1486.6 eV.…”

New Pd–Pt and Pd–Au catalysts for an efficient synthesis of H2O2 from H2 and O2 under very mild conditions

“…In this work we have chosen to perform pulse flow CO chemisorption measurements [39][40][41] on mildly reduced catalysts, in order to mimick the same reductive pretreatment of the catalytic reaction and determine in such a way the amount of active sites that are actually present during the reaction. The chemisorption results, reported in Table 1, show that CO adsorbed in bimetallic Pd0.1Pt and Pd0.2Pt catalysts is very similar and higher than the monometallic sample or the sample with the higher Pt concentration (Pd1.2Pt) indicating that in the latter samples the total adsorbing sites are less abundant and metal dispersion is lower.…”

“…Also for bimetallic palladium-gold samples we have performed pulse flow CO chemisorption measurements [40], in order to determine the amount of active sites that are present during the reaction. We have previously proved [44,45] that under the experimental conditions used gold does not chemisorb CO.…”

Characterisation of Ru/C catalysts for ammonia synthesis by oxygen chemisorption

“…From these data and from the chemisorbed volume of a suitable probe molecule, a reliable value of the apparent chemisorption stoichiometry [17] can be easily calculated. 4 The aim of the present work was to set up and optimise a chemisorption procedure for the determination of Ru dispersion in Ru/C catalysts for ammonia synthesis.…”