Simon Penner

How to Control the Selectivity of Palladium-based Catalysts in Hydrogenation Reactions: The Role of Subsurface Chemistry

Discussed are the recent experimental and theoretical results on palladium‐based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi‐method and multi‐material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub‐surface of Pd metal was observed by in situ spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H2 partial pressure) alkyne fragmentation leads to formation of a PdC surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub‐surface increases considerably the selectivity of alkyne semi‐hydrogenation over Pd‐based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that PdC hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over in situ formed bulk PdH. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such PdxGay catalysts show extended stability under in situ conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd3Ga7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga2O3, upon heating of Pd/Ga2O3 in hydrogen, was shown to lead to formation of PdGa intermetallic compounds at moderate temperatures. In this way, Pd5Ga2 and Pd2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively.

In Situ FT-IR Spectroscopic Study of CO2 and CO Adsorption on Y2O3, ZrO2, and Yttria-Stabilized ZrO2

In situ FT-IR spectroscopy was exploited to study the adsorption of CO2 and CO on commercially available yttria-stabilized ZrO2 (8 mol % Y, YSZ-8), Y2O3, and ZrO2. All three oxides were pretreated at high temperatures (1173 K) in air, which leads to effective dehydroxylation of pure ZrO2. Both Y2O3 and YSZ-8 show a much higher reactivity toward CO and CO2 adsorption than ZrO2 because of more facile rehydroxylation of Y-containing phases. Several different carbonate species have been observed following CO2 adsorption on Y2O3 and YSZ-8, which are much more strongly bound on the former, due to formation of higher-coordinated polydentate carbonate species upon annealing. As the crucial factor governing the formation of carbonates, the presence of reactive (basic) surface hydroxyl groups on Y-centers was identified. Therefore, chemisorption of CO2 most likely includes insertion of the CO2 molecule into a reactive surface hydroxyl group and the subsequent formation of a bicarbonate species. Formate formation following CO adsorption has been observed on all three oxides but is less pronounced on ZrO2 due to effective dehydroxylation of the surface during high-temperature treatment. The latter generally causes suppression of the surface reactivity of ZrO2 samples regarding reactions involving CO or CO2 as reaction intermediates.

Comparison of the reactivity of different Pd-O species in CO oxidation.

The reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd(5)O(4) surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO(x) suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s(-1). The Pd(5)O(4) surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10(-6) mbar CO at T > 500 K led only to partial reduction toward PdO(x) suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd(5)O(4) surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.

High CO2 Selectivity in Methanol Steam Reforming through ZnPd/ZnO Teamwork

Methanol steam reforming (MSR; CH3OH+H2O! 3H2 +CO2) is considered an important building block in the future energy infrastructure to provide clean hydrogen for fuel cell applications. The suppression of CO is the greatest challenge, as the subsequently used fuel cell catalysts only tolerate CO concentrations of up to 50 ppm. Pd/ZnO catalysts have been shown to compete with Cu-based catalysts, and have set benchmarks at about 1000 ppm CO. The formation of the intermetallic compound ZnPd on the ZnO support is held responsible for the high CO2 selectivity. However, no proof of the origin of high CO2 selectivity has been reported thus far. Unraveling the structural properties that account for the selectivity of this catalyst would definitely aid in improving MSR catalysts for practical application. Recently, detailed characterization of the single constituents (unsupported ZnPd and pure ZnO) in MSR resulted in the justified proposal of a bifunctional synergism between intermetallic and oxidic species, which leads to a highly active interface that is necessary for the outstanding selectivity of ZnPd/ZnO catalysts. The bulk composition of ZnPd affects its surface composition and also determines the oxidizability of Zn on the surface. The presence of oxidized Zn in near-surface regions has been shown to be inevitably linked to a high CO2-selectivity in inverse model catalyst studies of near-surface intermetallic phases. The knowledge transfer from these model systems to high-performance catalysts represents a great step towards understanding MSR. Herein we reveal the origin of the high CO2 selectivity of ZnO-supported ZnPd particles in MSR by linking the catalytic properties of ZnPd/ZnO, especially in the initial phase on-stream, with aberration-corrected high-resolution transmission electron microscopy (HRTEM) imaging of the catalyst at different stages of the MSR reaction. ZnPd/ZnO was first examined by TEM and scanning TEM (STEM) after reductive treatment at 773 K (state I). The sample consists of intermetallic particles (ca. 5–50 nm in size) that can be clearly discriminated from the ZnO support (Supporting Information, Figure S3). Figure 1 shows a repre-

Platinum nanocrystals supported by silica, alumina and ceria: metal–support interaction due to high-temperature reduction in hydrogen

Regular Pt nanoparticles, obtained by epitaxial deposition on NaCl surfaces, were supported by thin films of silica, alumina and ceria and sub- jected to hydrogen reduction at temperatures up to 1073 K. The changes in morphology and composition were followed by (HR)TEM, electron diffraction and EELS, and the results were supported by theoretical calculations. The structural changes of the Pt particles upon reduction at 773 K and above are surprisingly similar despite the differing chemical properties of the three supports. Some platelet- and cube-like geometries exhibit double lattice periodicities in high resolution images and electron diffraction patterns. With increasing reduction larger aggregates of more complex appearance and structure are formed. Surface reconstruction under hydrogen and alloy formation are considered as responsible for this effect. Most likely, the first step is identical on all three systems and consists in the topotactic formation of Pt rich Pt3Me (Me = Si, Al, Ce) under the influence of hydrogen, followed by transformation into diverging structures of lower Pt content and different crystallography. Density func- tional calculations were performed for deriving energies of formation of PtMe and Pt 3Me compounds.

Hydrogen Production by Methanol Steam Reforming on Copper Boosted by Zinc-Assisted Water Activation

For use of polymer electrolyte membrane fuel cells (PEMFC) in mobile power applications, an efficient source of CO-depleted hydrogen is needed. To avoid technical and safety problems of hydrogen handling, storage, and transport, methanol can be used as practical and abundant energy carrier for on-board H2 generation, as it has the advantage of a high energy density. Hydrogen generation from methanol can be performed by catalytic methanol steam reforming (MSR): CH3OH+H2O→CO2+3 H2. Methanol conversion must be carried out with very high CO2/H2 selectivity to avoid CO poisoning of the fuel-cell anode. A number of promising selective MSR catalysts are already available. Apart from advanced copper-based catalysts,1, 2 special attention is presently paid to the highly MSR-selective reduced state of Pd/ZnO,3 containing a particularily stable intermetallic PdZn (1:1) active phase.3, 4 Therefore, we recently studied related “inverse” near-surface PdZn intermetallic phases, showing that three-dimensional PdZn active site ensembles are equally important for selective dehydrogenation of methanol (thus avoiding CO) and for efficient water activation.5 For the less costly Cu/ZnO catalysts, originally designed for methanol synthesis, improvements towards a technical MSR application regarding sintering stability, pyrophoricity, and selectivity are still required. Empirical development of Cu/ZnO catalyst preparation and activation has aimed in a particularily large Cu0–ZnO contact.6 Nevertheless, it is very difficult to derive an unambiguous causality for the role of the contact on technical catalysts. It is known that zinc leads to an improvement in the desired properties, but a clear assignment of a predominant promotional effect (both from the theoretical and experimental side) is still missing. In the Cu/ZnO literature, seemingly incompatible model interpretations can be found, involving the “metallic copper model”,7 the “special site model”,8 the “morphology model”,7, 9 the “spillover model”,10 and last but not least the “Cu-Zn alloy model”.8, 11 Consequently, the Cu-ZnO(H) contact most likely constitutes a combination of promotional effects. The central aim of our study is to highlight the aspect of zinc-promoted water activation. This is achieved by using an ultrahigh-vacuum (UHV) “inverse” model catalyst approach, which, in contrast to investigations on real catalyst systems, allows the zinc segregation behavior and the changes in redox chemistry of both copper and zinc to be better followed. This provides a solid basis for directional promotion of microkinetic steps leading to enhanced CO2 selectivity.

Growth and decomposition of aligned and ordered PdO nanoparticles

The formation, thermal decomposition, and reduction of small PdO particles were studied by high-resolution transmission electron microscopy and selected area electron diffraction. Well-defined Pd particles (mean size of 5-7 nm) were grown epitaxially on NaCl (001) surfaces and subsequently covered by a layer of amorphous SiO2 (25 nm), prepared by reactive deposition of SiO in 10(-2) Pa O2. The resulting films were exposed to molecular O2 in the temperature range of 373-673 K, and the growth of PdO was studied. The formation of a PdO phase starts at 623 K and is almost completed at 673 K. The high-resolution experiments suggest a topotactic growth of PdO crystallites on top of the original Pd particles. Subsequent reaction of the PdO in 10 mbar CO for 15 min and thermal decomposition in 1 bar He for 1 h were also investigated in the temperature range from 373 to 573 K. Reductive treatments in CO up to 493 K do not cause a significant change in the PdO structure. The reduction of PdO starts at 503 K and is completed at 523 K. In contrast, PdO decomposes in 1 bar He at around 573 K. The mechanism of PdO growth and decay is discussed and compared to results of previous studies on other metals, e.g., on rhodium.

Formation of Intermetallic Compounds by Reactive Metal-Support Interaction: A Frequently Encountered Phenomenon in Catalysis

This Review discusses the structural and catalytic aspects of the recently introduced reactive metal–support interaction. This special term was coined to account for the inability of the original concept of the strong metal–support interaction to accurately describe the structural, compositional, and electronic changes frequently occurring in oxide‐supported metal particle catalysts at very high temperatures upon reduction in hydrogen, in many cases leading to intermetallic compound or substitutional alloy formation. This inaccuracy predominantly refers to the requirement of full reversibility upon oxidation and mild reduction for a “strong” metal–support interaction. A close look at the formation of oxide‐supported intermetallic compounds upon high‐temperature reduction reveals that these compounds are very common in catalysis and the situation is much more complex compared to unsupported intermetallic compounds due to the presence of the intermetallic compound–oxide interface.

Rh and Pt nanoparticles supported by CeO2: Metal–support interaction upon high-temperature reduction observed by electron microscopy

Pt and Rh nanoparticles obtained by epitaxial growth on NaCl (001) were coated with a crystalline ceria support and subjected to systematic hydrogen reduction in the temperature range 573 K ≤ T ≤ 973 K. The structural and morphological changes were examined by high resolution electron microscopy and selected area electron diffraction. Metal–support interaction between Pt particles and ceria becomes apparent after reduction in hydrogen at 723 K and results in alloy formation and reconstruction of the metal into regular cube-like Pt3Ce particles which are stable up to 973 K. In contrast, the reduction of the comparable rhodium-ceria system leads to larger particles of undefined shape which start to develop below 673 K and increase in size up to reduction at 973 K. Electron diffraction reveals again alloy formation, but unlike on Pt, several alloy phases (Rh3Ce, Rh2Ce and RhCe) are developed simultaneously and a defined topotactic growth does not occur. The thermal stability of the resulting phases and their resistance toward oxidation are discussed.

From Oxide-Supported Palladium to Intermetallic Palladium Phases: Consequences for Methanol Steam Reforming

This Minireview summarizes the fundamental results of a comparative inverse‐model versus real‐model catalyst approach toward methanol steam reforming (MSR) on the highly CO2‐selective H2‐reduced states of supported Pd/ZnO, Pd/Ga2O3, and Pd/In2O3 catalysts. Our model approach was extended to the related Pd/GeO2 and Pd/SnO2 systems, which showed previously unknown MSR performance. This approach allowed us to determine salient CO2‐selectivity‐guiding structural and electronic effects on the molecular level, to establish a knowledge‐based approach for the optimization of CO2 selectivity. Regarding the inverse‐model catalysts, in situ X‐ray photoelectron spectroscopy (in situ XPS) studies on near‐surface intermetallic PdZn, PdGa, and PdIn phases (NSIP), as well as bulk Pd2Ga, under realistic MSR conditions were performed alongside catalytic testing. To highlight the importance of a specifically prepared bulk intermetallicoxide interface, unsupported bulk intermetallic compounds of PdxGay were chosen as additional MSR model compounds, which allowed us to clearly deduce, for example, the water‐activating role of the special Pd2Ga‐β‐Ga2O3 intermetallicoxide interaction. The inverse‐model studies were complemented by their related “real‐model” experiments. Structure–activity and structure–selectivity correlations were performed on epitaxially ordered PdZn, Pd5Ga2, PdIn, Pd3Sn2, and Pd2Ge nanoparticles that were embedded in thin crystalline films of their respective oxides. The reductively activated “thin‐film model catalysts” that were prepared by sequential Pd and oxide deposition onto NaCl(001) exhibited the required large bimetaloxide interface and the highly epitaxial ordering that was required for (HR)TEM studies and for identification of the structural and catalytic (bi)metalsupport interactions. To fully understand the bimetalsupport interactions in the supported systems, our studies were extended to the MeOH‐ and formaldehyde‐reforming properties of the clean supporting oxides. From a direct comparison of the “isolated” MSR performance of the purely bimetallic surfaces to that of the “isolated” oxide surfaces and of the “bimetaloxide contact” systems, a pronounced “bimetaloxide synergy” toward optimum CO2 activity/selectivity was most evident. Moreover, the system‐specific mechanisms that led to undesired CO formation and to spoiling of the CO2 selectivity could be extracted.