Bogusław Mierzwa

Segregation in model palladium—cobalt clusters

Abstract N-body Sutton—Chen potentials have been developed for fcc Co and the interaction of Co and Pd atoms. These potentials were used to describe the effect of Pd segregation within Pd—Co relaxed clusters. Monte Carlo configurational minimization at room temperature was used to determine the morphology of clusters. For the diffraction patterns averaging over configurations is achieved by molecular dynamics runs. Variation in the position of the 111 peak of the average pattern with cluster alloy composition was compared with X-ray diffraction data for (Pd—Co) silica catalysts. The good fit obtained strongly supports the applicability of the model segregation profile. A key role in the modelling is played by the precise tuning of the Pd—Co potential.

In situ EXAFS study of the alloy catalyst Pd-Co (50%/50%)/SiO2

In situ extended X-ray absorption fine structure (EXAFS) data for nanocrystalline bimetallic Pd-Co catalyst supported on silica, for Pd K and Co K absorption edges, are presented and analysed confirming previously developed atomistic model of segregation within the alloy metal particle. Supporting evidence is presented from dynamical X-ray diffraction (DXRD) data. The confirmed segregation model concerns energy minimised partial surface segregation of Pd in contrary to a complete segregation model that does not fit well the EXAFS data.

Ab initio test of the Warren-Averbach analysis on model palladium nanocrystals

Model powder diffraction patterns were calculated via the Debye formula from atom positions of a range of energy-relaxed closed-shell cubooctahedral clusters. The energy relaxation employed the Sutton–Chen potential scheme with parameters for palladium. The assumed cluster size distribution followed lognormal distribution of a crystallite volume centred with the diameter of 5 nm, as well as two bimodal lognormal distributions centred around 4 nm and 7 nm. These models allowed an in-depth analysis of the Warren–Averbach method of separating strain and size effects in a peak shape Fourier analysis. The atom-displacement distribution in the relaxed clusters could be directly computed, as well as the strain Fourier coefficients. The results showed that in the case of the unimodal size distribution, the method can still be successfully used for obtaining the column length distribution. However, the strain Fourier coefficients obtained from three reflections (002, 004 and 008) cannot be reliably estimated with the Warren–Averbach method. The primary cause is a non-Gaussian strain distribution and a shift of the diffraction maximum, inherent to the nanoparticles, differing for every constituent cluster in the size distribution. For the bimodal size distributions, the obtained column length distributions tend to be shifted towards the centres of the modes and are less sensitive to the larger size mode.

Effect of a Barium Promoter on the Stability and Activity of Carbon‐Supported Cobalt Catalysts for Ammonia Synthesis

Samples of barium‐promoted carbon‐supported cobalt catalysts for ammonia synthesis were prepared; the cobalt content was constant (27.4 wt %) and the amount of barium introduced differed within the range of 0–0.88 mmol gC+Co−1. Turbostratic carbon was used as the support. Several techniques, including X‐ray powder diffraction, TEM, Thermogravimetric analysis–mass spectrometry (methanation studies) and H2 temperature‐programmed desorption were used to examine the properties of the carbon support or catalytic materials. The reaction rate measurements were performed under the following conditions: T=400 °C, p=9.0 MPa, x  NH 3 =8.5 mol %, H2/N2=3:1. It was found that barium is an important component of the cobalt catalysts supported on activated carbon for ammonia synthesis. Barium inhibits the process of carbon support methanation, which also results in a decrease in the sintering of the active phase (cobalt). Moreover, even a small amount of barium significantly activates the cobalt surface in ammonia synthesis, suggesting that barium locates primarily on the surface of cobalt.

Quick low temperature coalescence of Pt nanocrystals on silica exposed to NO – the case of reconstruction driven growth?

We report an operando XRD/MS experiment on nanocrystalline Pt supported on silica, monitoring quick growth of Pt in an NO atmosphere. XRD data following NO flow show structural changes typical of surface reconstruction. Although a small pressure of NO lifts surface reconstruction of a clean surface, a higher pressure of NO causes the surface to reconstruct again. TEM photographs show formation of clusters of Pt nanoparticles, pointing to growth controlled by coalescence. As surface reconstruction is the phenomenon that we always see accompanying quick growth, we postulate a mechanism of self-lifting cyclic reconstruction that drives the transport of whole clusters and their fusion.

Cobalt-lanthanum catalyst precursors for ammonia synthesis: determination of calcination temperature and storage conditions

Abstract A thermal decomposition of a cobalt-lanthanum catalyst precursor containing a mixture of cobalt and lanthanum compounds obtained by co-precipitation were studied using thermal analysis coupled with mass spectrometry (TG-MS). Studies revealed that the calcination in air at 500°C is sufficient to transform the obtained cobalt precipitate into Co3O4, but it leads to only partial decomposition of lanthanum precipitate. In order to obtain Co/La catalyst precursor containing La2O3 the calcination in air at the temperature about 800°C is required. However, it is unfavorable from the point of view of textural properties of the catalyst precursor. A strong effect of storage conditions on the phase composition of the studied cobalt-lanthanum catalyst precursor, caused by the formation of lanthanum hydroxide and lanthanum carbonates from La2O3 when contacting with air, was observed.

Properties and activity of the cobalt catalysts for NH3 synthesis obtained by co-precipitation – the effect of lanthanum addition

Abstract In modern research on catalysts for NH3 synthesis a lot of attention is paid to cobalt. In this work the new catalytic systems based on cobalt are presented. Unsupported cobalt catalysts singly promoted (La or Ba) and doubly promoted (La and Ba) were prepared and tested in NH3 synthesis reaction under commercial synthesis conditions. Characterization studies revealed that lanthanum plays a role of a structural promoter, which improves the surface of catalyst precursors and prevents from sintering during calcination. However, lanthanum has a negative effect on the reduction of cobalt oxide, but the addition of barium promoter (Co/La/Ba catalyst) diminishes the negative impact of La. The co-promotion of cobalt with lanthanum and barium results in the increasing of the active phase surface and improvement of its activity in NH3 synthesis.