Understanding the Reduction of Transition-Metal Phosphates to Transition-Metal Phosphides by Combining Temperature-Programmed Reduction and Infrared Spectroscopy.

Temperature-programmed reduction of transition-metal phosphate precursors is the most commonly used method for the preparation of transition-metal phosphides (a new class of versatile metal catalysts), but the reduction processes are still not clear. Here we describe the construction of a temperature-programmed reduction-infrared spectroscopy apparatus (TPR-IR) to analyze the gas flows during the reduction of nickel, molybdenum, and tungsten phosphates. PH3 and Pn+ species already appeared in the gas flows at low temperature (ca. 200 °C), and PH3 was involved in the formation of phosphides. The emission of PH3 and Pn+ during the reduction of the molybdenum and tungsten phosphates was smaller than that of the nickel phosphate. Ni2 P drastically accelerated the formation of the PH3 and Pn+ . These results explain why excess phosphorus is needed for the preparation of Ni2 P, and also demonstrate that the TPR-IR technique is an efficient method to understand the complex processes of catalyst preparation.

Location of the Spinel Vacancies in γ-Al2 O3.

A non-spinel model for the structure of γ-Al2 O3 , with 25 % of the Al3+ cations at tetrahedral positions, has been the subject of wide interest. However, 17 O NMR measurements and, more recently, 27 Al NMR measurements have shown that there are considerably more Al3+ cations at tetrahedral positions. This means that the Al3+ vacancies in γ-Al2 O3 are not at tetrahedral but at octahedral positions, as in isostructural γ-Fe2 O3 and in accordance with density functional theory predictions. This has consequences with regard to the surface structure of γ-Al2 O3 , and thus, for catalysis.

Disproportionation of hypophosphite and phosphite.

The disproportionation of NaH2PO2, NaH2PO3, Na2HPO3 and their mixtures was studied by TG-DSC, XRD and 31P NMR. NaH2PO2 reacted in three steps to yield PH3, Na5P3O10 and H2 by disproportionation and oxidation with water released in condensation reactions. In the first step at 310 °C NaH2PO2 reacted to yield PH3, Na2H2P2O5, Na2HPO3, Na4P2O7 and H2. H2 rather than H2O was the coproduct of the disproportionation, because H2O oxidized hypophosphite and phosphite at elevated temperatures, in agreement with DFT results that show that the reaction of H3PO3 with H2O is exothermic. The oxidation of phosphite by H2O has a twofold effect on gas formation. First, it diminishes gas formation because water is not released and, second, because H2O oxidizes phosphite, less phosphite is available for disproportionation to phosphate and, thus, less PH3 is formed. As a consequence, the maximum PH3 efficiency in the disproportionation of NaH2PO2 was not 50% but 40%. Phosphate was never observed, only Na4P2O7, which appeared as a primary product of the oxidation of Na2H2P2O5 by H2O. Na2HPO3 did not react below 450 °C when heated alone, but in the presence of compounds which release H2O it was oxidized to Na4P2O7 below 400 °C. NaH2PO3 first reacted to yield PH3 and Na2H2P2O5 and then to Na5P3O10 and Na3P3O9. Heating a 1 : 1 mixture of NaH2PO3 and Na2HPO3 led to PH3, Na2H2P2O5 and Na2HPO3, then to Na4P2O7 and eventually to Na5P3O10 and Na3P3O9. In all cases some red phosphorus was formed by the decomposition of PH3, especially at higher temperatures.

Facile Preparation of Ni2P with a Sulfur-Containing Surface Layer by Low-Temperature Reduction of Ni2P2S6.

Preparation of Ni2P by temperature-programmed reduction (TPR) of a phosphate precursor is challenging because the P-O bond is strong. An alternative approach to synthesizing Ni2P, by reduction of nickel hexathiodiphosphate (Ni2P2S6), is presented. Conversion of Ni2P2S6 into Ni2P occurs at 200-220 °C, a temperature much lower than that required by the conventional TPR method (typically 500 °C). A sulfur-containing layer with a thickness of about 4.7 nm, composed of tiny crystallites, was observed at the surface of the obtained Ni2 P catalyst (Ni2P-S). This is a direct observation of the sulfur-containing layer of Ni2P, or the so-called nickel phosphosulfide phase. Both the hydrodesulfurization activity and the selective hydrogenation performance of Ni2P-S were superior to that of the catalyst prepared by the TPR method, suggesting a positive role of sulfur on the surface of Ni2P-S. These features render Ni2P-S a legitimate alternative non-precious metal catalyst for hydrogenation reactions.

CoSi particles on silica support as a highly active and selective catalyst for naphthalene hydrogenation.

CoSi particles on a silica support, synthesized by metal organic chemical vapor deposition (MOCVD) of Co(SiCl(3))(CO)(4) as a precursor at atmospheric pressure and moderate temperature in a fluidized bed reactor, show high catalytic activity and selectivity in naphthalene hydrogenation.

TRAPDOR double-resonance and high-resolution MAS NMR for structural and template studies in zeolite ZSM-5.

A combination of (27)Al magic-angle spinning (MAS)/multiple-quantum (MQ) MAS, and (27)Al-{(14)N} TRAnsfer of Population in DOuble-Resonance (TRAPDOR) nuclear magnetic resonance (NMR) was used to study aluminium environments in zeolite ZSM-5. (27)Al-{(14)N} TRAPDOR experiments, in combination with (14)N NMR were employed to show that the two tetrahedral peaks observed in the (27)Al MAS/3Q-MAS spectra of as-synthesized ZSM-5 are due to aluminium atoms occupying crystallographically inequivalent T-sites. A (13)C-{(27)Al} TRAPDOR experiment was used to study the template, tetrapropyl ammonium bromide (TPABr), in the three-dimensional pore system of ZSM-5. The inequivalency of the methyl groups of TPA was observed in the (13)C-{(27)Al} TRAPDOR spectra of as-synthesized ZSM-5 and the motion of the methyl end of the propyl chain appeared to be more restricted in the sinusoidal channel than in the straight channel.

Time-resolved in situ XAS study of the preparation of supported gold clusters.

Incipient-wetness impregnation of gamma-Al(2)O(3) with HAuCl(4) and subsequent removal of chlorine with NaOH, and deposition-precipitation of HAuCl(4) on TiO(2) at pH 7 resulted in supported Au(3+) species. Time-resolved in situ XAS at the Au L(3) edge showed that the Al(2)O(3)-supported oxidic or hydroxidic species were reduced in hydrogen at 440 K to yield small metallic gold clusters. The Au(3+) precursor decomposed to metallic gold in inert atmosphere at 573 K and in oxidizing atmosphere above 623 K. In all atmospheres, initially small clusters were formed that gradually grew with increasing temperature. The TiO(2)-supported species were considerably less stable. In hydrogen and carbon monoxide, Au(0) clusters of 1 to 1.5 nm were formed at room temperature, which was the lowest temperature studied. In inert and oxidizing atmosphere, the Au(3+) precursor decomposed fully to metallic gold at 530 K, as shown by XAS and temperature-programmed experiments. Large clusters were obtained already in the initial stage of reduction. Residual chlorine inhibited the reduction and led to sintering of the gold clusters. Exposure of the TiO(2)-supported catalyst precursor to light or the X-ray beam led to partial reduction, and STEM showed that storage of the reduced gold clusters under ambient conditions led to agglomeration and bimodal cluster-size distributions.

Analysis of in situ EXAFS data of supported metal catalysts using the third and fourth cumulant.

X-Ray absorption spectra of supported Pt catalysts with various Pt cluster sizes were collected between 77 and 673 K, in inert and hydrogen atmospheres. When analyzing these spectra with the standard EXAFS equation, a Pt-Pt bond contraction and a large increase in the inner potential correction were observed with increasing temperature. These errors are up to 0.08 A and 10 eV for clusters of 1 nm diameter. They were corrected by including the third and fourth cumulants as fit parameters. Fit guidelines were developed to analyze EXAFS data of supported metal catalysts collected at elevated temperatures, allowing for asymmetry and broadening or sharpening of the pair distribution function. These comprise fixing fit parameters, using different k-weightings and identifying trends in a series of experiments. By fitting the EXAFS spectra using these guidelines, it was determined that in small Pt clusters the Pt-Pt bond is 0.10 A shorter than in bulk Pt. These contracted bonds relax to distances near that of bulk Pt upon hydrogen chemisorption.

Multinuclear solid-state high-resolution and 13C -{27Al} double-resonance magic-angle spinning NMR studies on aluminum alkoxides.

A combination of 27Al magic-angle spinning (MAS)/multiple quantum (MQ)-MAS, 13C-1H CPMAS, and 13C-{27Al} transfer of population in double-resonance (TRAPDOR) nuclear magnetic resonance (NMR) were used for the structural elucidation of the aluminum alkoxides aluminum ethoxide, aluminum isopropoxide, and aluminum tertiarybutoxide. Aluminum alkoxides exist as oligomers with aluminum in different coordinations. High-resolution 27Al MAS NMR experiments with high-spinning speed distinguished the aluminum atoms in different environments. The 27Al MAS NMR spectrum gave well-resolved powder patterns with different coordinations. Z-filter MQ-MAS was performed to obtain the number and types of aluminum environments in the oligomeric structure. 13C-1H CPMAS chemical shifts resolved the different carbon species (-CH3, =CH2, =CH-, and =C=) in the structures. 13C-{27Al} TRAPDOR experiments were employed to obtain relative Al-C dipolar interactions and to distinguish between terminal and bridging alkoxides in the crystallographic structures. The complete characterization of selected aluminum alkoxides using advanced NMR methods has evidenced the tetrameric structure for aluminum isopropoxide and the dimeric structure for aluminum tertiary-butoxide, as reported in the literature, and proposed a polymeric structure for aluminum ethoxide.

New synthesis method for nickel phosphide hydrotreating catalysts.

Nickel phosphide particles on silica and alumina support were prepared from metal or metal oxide particles by treatment with phosphine and hydrogen at moderate temperature, resulting in small particle sizes equivalent to that of the precursor particle size.

An in situ and operando X-ray absorption spectroscopy setup for measuring sub-monolayer model and powder catalysts.

A new spectroscopic cell has been designed for studying model catalysts using in situ or operando X-ray absorption spectroscopy. The setup allows gas treatment and can be used between 100 and 870 K. Pressures from 10(-3) Pa up to 300 kPa can be applied. Measurements on model systems in this particular pressure range are a valuable extension of the commonly used UHV characterization techniques. Using this setup, we were able to analyze the Au L3 EXAFS of a silica wafer covered with sub-monolayer concentrations of gold (0.05 ML). By modifying the sample holder, powder catalysts can also be analyzed under plug-flow conditions. As an example, the reduction of a Au/SiO2 powder catalyst prepared from HAuCl4 was followed.

Effect of temperature on aluminum coordination in zeolites H-Y and H-USY and amorphous silica-alumina: an in situ Al K edge XANES study.

In situ Al K edge XANES spectroscopy shows that the fraction of octahedrally coordinated aluminum in amorphous silica-alumina (ASA) and ultrastable Y zeolite (USY) decreases with increasing temperature under vacuum. In H-USY, about 10% of the aluminum remains octahedrally coordinated at 673 K, while, in ASA, virtually all the octahedrally coordinated aluminum is converted to tetrahedral coordination. In crystalline nonsteamed protonic zeolites, the fraction of octahedrally coordinated aluminum decreased to zero at 300 K. This is ascribed to the greater flexibility of the amorphous silica-alumina network in hosting water molecules and to the high concentration of silanol groups, which stabilize the hydrogen bonds. A large fraction of the nonframework aluminum in USY is amorphous silica-alumina.

Local structure of the zeolitic catalytically active site during reaction.

The structural changes of the catalytic active site that occur during catalytic reaction in an acidic zeolite are detected. The local structure of the zeolitic Brønsted active site is a distorted tetrahedrally coordinated aluminum that has three short and one long aluminum-oxygen bond. Using in situ Al K edge X-ray absorption spectroscopy, the adsorption of a reactive intermediate in the oligomerization of ethene changed the local structure of the catalytic active site; the long aluminum oxygen bond is partially relaxed. At increasingly higher temperature, extensive coking of the catalyst frees the Brønsted acid site from the reactive intermediate, restoring the asymmetric coordination. These measurements show that application of in situ Al K edge spectroscopy provides fundamental insight into the structure of zeolitic catalytically active sites during catalytic action.

Functional-Group-Directed Diastereoselective Hydrogenation of Aromatic Compounds. 1.

Diastereoselective liquid phase hydrogenation of a series of monosubstituted indane and tetralin substrates was studied on supported rhodium catalysts. Predominantly the cis-cis diastereomer, obtained by hydrogenation from the diastereoface opposite the substituent (at the stereogenic center), and the cis-trans diastereomer, obtained by hydrogenation from the diastereoface on the same side as the substituent, were formed. The diastereoselectivity between the two isomers was dependent on the steric repulsion or the electronic attraction of the substituent with the surface of the catalyst. The hydroxyl group did not exhibit a strong attraction (haptophilicity), and the cis-cis diastereomer was obtained as the major product. The amino group exhibited a very high haptophilicity, yielding primarily the cis-trans diastereomer. The diastereoselectivity obtained in the hydrogenation of all the substrates was influenced on addition of bases to the reaction mixture. In the case of alcoholic substrates, the selectivity to the cis-trans diastereomer could be substantially increased with alkaline hydroxide additives.