Cu,Zn,Al layered double hydroxides as precursors for copper catalysts in methanol steam reforming – pH-controlled synthesis by microemulsion technique

By co-precipitation inside microemulsion droplets a Cu-based catalyst precursor was prepared with a Cu:Zn:Al ratio of 50:17:33. A pH-controlled synthesis was applied by simultaneous dosing of metal solution and precipitation agent. This technique allows for continuous operation of the synthesis and enables easy and feasible up-scaling. For comparison conventional co-precipitation was applied with the same composition. Both techniques resulted in phase pure layered double hydroxide precursors and finally (after calcination and reduction) in small Cu nanoparticles (8 nm) and ZnAl2O4. By applying the microemulsion technique smaller Cu/ZnAl2O4 aggregates with less embedded Cu particles were obtained. The microemulsion product exhibited a higher BET and specific Cu surface area and also a higher absolute catalytic activity in methanol steam reforming. However, the Cu surface area-normalized, intrinsic activity was lower. This observation was related to differences in interactions of Cu metal and oxide phase

Intermetallic compounds in heterogeneous catalysis - a quickly developing field

The application of intermetallic compounds for understanding in heterogeneous catalysis developed in an excellent way during the last decade. This review provides an overview of concepts and developments revealing the potential of intermetallic compounds in fundamental as well as applied catalysis research. Intermetallic compounds may be considered as platform materials to address current and future catalytic challenges, e.g. in respect to the energy transition

Intermetallic GaPd2_{2} Thin Films for Selective Hydrogenation of Acetylene

The preparation of single‐phase and catalytically active GaPd2 coatings was accomplished via DC magnetron sputtering using an intermetallic sputter target. Thin and uniform layers were deposited on borosilicate glass, Si(111) and planar as well as micro‐structured stainless steel foils. The specimens were examined regarding their phase composition, film morphology and microstructure. Thin films of different layer thickness were catalytically characterized in the semi‐hydrogenation of acetylene, which was conducted at 473 K and a feed gas composition of 0.5 vol.% C2H2, 5 vol.% H2 as well as 50 vol.% C2H4 in helium. Pre‐reduction of the catalyst was found to be essential to enhance the catalytic selectivity. Sputtered GaPd2 showed a high selectivity of 73 % for the hydrogenation to ethylene at conversion levels above 80 %. The surface‐specific activity was strongly increased to 8.97 molacetylene· (A 0· h)–1 compared to bulk‐ or nanoscale GaPd2 (1.93 and 0.30 molacetylene· (A 0· h)–1, respectively) caused by the high specific surface area of the thin films

Properties of Bulk In-Pt Intermetallic Compounds in Methanol Steam Reforming

Catalytic properties of In-Pt intermetallic compounds in methanol steam reforming have been investigated. In2Pt has been proven to be mandatory for a high activity and selectivity. Only small amounts of In2O3 have been found to be beneficial for catalytic performance. Heterogeneous catalysts are often complex materials containing different compounds. While this can lead to highly beneficial interfaces, it is difficult to identify the role of single components. In methanol steam reforming (MSR), the interplay between intermetallic compounds, supporting oxides and redox reactions leads to highly active and CO2-selective materials. Herein, the intrinsic catalytic properties of unsupported In3Pt2, In2Pt, and In7Pt3 as model systems for Pt/In2O3-based catalytic materials in MSR are addressed. In2Pt was identified as the essential compound responsible for the reported excellent CO2-selectivity of 99.5 % at 300 °C in supported systems, showing a CO2-selectivity above 99 % even at 400 °C. Additionally, the partial oxidation of In7Pt3 revealed that too much In2O3 is detrimental for the catalytic properties. The study highlights the crucial role of intermetallic In−Pt compounds in Pt/In2O3 materials with excellent CO2-selectivity

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 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