In Situ Surface Studies of Site-Isolated Hydrogenation Catalysts – The Intermetallic Compound PdGa

Selective acetylene hydrogenation is an important method for removing traces of acetylene in the ethylene feed for the production of polyethylene. Typical catalysts, like Pd dispersed on metal oxides are widely used for this reaction and show a limited selectivity and long-term stability. This can be attributed to the presence of active-sites ensembles on the catalyst surface. This drawback can be overcome by using the intermetallic compound PdGa which possesses palladium atoms in the crystal structure well isolated from each other by a gallium shell. PdGa shows higher selectivity and increased long-term stability compared to the commercial catalysts, including PdAg alloys. In the present work the surface of the intermetallic compound PdGa was probed by in situ XPS as well as CO adsorption using FTIR spectroscopy. The XPS investigation before hydrogenation revealed a significant modification of the Pd electronic state in the intermetallic compound compared to Pd metal: the Pd3d5/2 peak is shifted by 1 eV to higher binding energy. In situ XPS measurements, performed at ~1 mbar pressure, showed a high stability of the Pd surface states without appearance of any additional components or significant shifts of the Pd3d5/2 peak when applying the reactive atmosphere and temperature (1.0 mbar of H2 + 0.1 mbar of C2H2 at 120 ºC). This is in contrast to Pd metal for which the formation of an additional Pd component during alkyne hydrogenation was detected. Investigation of carbon and palladium depth profiles for PdGa indicates the absence of a subsurface carbon-containing phase, distinguishing this material decidedly from metallic palladium catalysts. The adsorption of CO on the PdGa compound at room temperature results in the appearance of only one band with a maximum at 2047 cm-1, which corresponds to linear Pd–CO carbonyls. It should be mentioned that the observed band (2047 cm–1) is shifted to lower wavenumbers compared to the respective CO (on-top) species forming upon adsorption on metallic palladium (2100-2080 cm–1), which is an indication for the modification of the Pd electronic states by covalent bonding in the investigated intermetallic compound. The absence of bands due to bridged carbonyls in the observed spectra and the fact that the observed band is not coverage dependent indicated that the active sites in PdGa are really isolated. Characterization of PdGa by FTIR and in situ XPS revealed high surface stability during the reaction of acetylene hydrogenation and confirms the isolation of the active Pd site on the surface. In combination with modified electronic Pd states due to covalent bonding in the intermetallic compound it leads to superior catalytic properties like high selectivity and long-term stability during the partial hydrogenation of acetylene

In Situ Studies of Site-Isolated Hydrogenation Catalysts – The Intermetallic Compound PdGa

Selective acetylene hydrogenation is an important method for removing traces of acetylene in the ethylene feed for the production of polyethylene. Typical catalysts show a limited selectivity and long-term stability. This can be attributed to the presence of active-site ensembles. This drawback can be overcome by using the intermetallic compound PdGa which possesses palladium atoms in the crystal structure well isolated by a gallium shell. PdGa shows higher selectivity and increased long-term stability compared to commercial catalysts. The XPS investigation before the reaction revealed a significant modification of the Pd electronic state in the intermetallic compound compared to Pd metal: the Pd3d5/2 peak is shifted by 1 eV to higher binding energy. In situ XPS measurements showed a high stability of the Pd surface states without appearance of any additional components or significant shifts of the Pd3d5/2 peak when applying the reactive atmosphere and temperature (1.0 mbar H2 0.1 mbar C2H2 at 120 ºC). This is in contrast to Pd metal for which the formation of an additional Pd component during alkyne hydrogenation was reported recently. The adsorption of CO on PdGa at room temperature results in the appearance of only one band with a maximum at 2047 cm-1, which should correspond to linearly bound CO (Pd–CO). It should be mentioned that the observed band (2047 cm–1) is shifted to lower wavenumbers compared to the respective CO (on-top) species forming upon adsorption on metallic palladium (2100-2080 cm–1), which may be an indication for the modification of the Pd electronic states by covalent bonding in the investigated intermetallic compound. The absence of bands due to bridged carbonyls in the spectra and the fact that the observed band is not coverage dependent indicates that the active sites in PdGa are really isolated. Characterization of PdGa revealed high surface stability during the hydrogenation of acetylene and confirms the isolation of the active Pd sites on the surface. In combination with the modified electronic Pd states – perhaps due to the covalent bonding – it leads to superior catalytic properties high selectivity and long-term stability

Pinning the catalytic centre: A new concept for catalysts development

A new concept for the development of catalysts is introduced. Utilizing ordered Pd-Ga intermetallic compounds high selectivity and long-term stability in the acetylene hydrogenation can be achieved. The high selectivity observed for all compounds (Pd3Ga7, PdGa, Pd2Ga) indicates that not only the surface geometric site isolation but also the suppression of hydride formation through a covalent bonding interaction absent in conventional Pd alloys are important for achieving superior catalytic properties. Our results demonstrate that structurally well-defined intermetallic compounds exhibit a high potential in heterogeneous catalysis and are promising candidates for industrial applications

In situ Surface Characterization of the Intermetallic Compound PdGa – A Highly Selective Hydrogenation Catalyst

The structurally well-defined intermetallic compound PdGa – a highly selective catalyst for the semi-hydrogenation of acetylene – was characterized by Fourier transform infrared spectroscopy (FTIR) in situ X-ray photoelectron spectroscopy and in situ Prompt gamma activation analysis. A strong modification of the electronic states in PdGa compared to elemental Pd was revealed as well as the complete isolation of the Pd atoms on the surface of PdGa. In situ investigations proved the high stability of the surface, thus excluding segregation phenomena (common for alloys) or sub-surface chemistry involving C and/or H atoms (known for elemental Pd). By suppressing the sub-surface chemistry, the electronic modification as well as the site isolation lead to the high selectivity and long-term stability of PdGa in the semi-hydrogenation of acetylene

Glass-like ordering and spatial inhomogeneity of magnetic structure in Ba3FeRu2O9 : The role of Fe/Ru-site disorder

Several doped 6H hexagonal ruthenates, having the general formula Ba3MRu2O9, have been studied over a significant period of time in order to understand the unusual magnetism of ruthenium metal. However, among them, the M=Fe compound appears different since it is observed that unlike others, the 3d Fe ions and 4d Ru ions can easily exchange their crystallographic positions and as a result many possible magnetic interactions become realizable. The present study involving several experimental methods on this compound establish that the magnetic structure of Ba3FeRu2O9 is indeed very different from all other 6H ruthenates. Local structural study reveals that the possible Fe/Ru-site disorder further extends to create local chemical inhomogeneity, affecting the high temperature magnetism of this material. There is a gradual decrease of 57Fe M\"ossbauer spectral intensity with decreasing temperature (below 100 K), which reveals that there is a large spread in the magnetic ordering temperatures, corresponding to many spatially inhomogeneous regions. However, finally at about 25 K, the whole compound is found to take up a global glass-like magnetic ordering.Comment: 24 page, 7 figure