Platinum Black by XPS

XPS spectra of Pt black in the as received state showed O and C impurities along with Pt. An in situ treatment by O2 and H2 increased Pt intensity and removed a part of oxygen and carbon impurities. The quasihomogeneous model was used for quantitative evaluation applying atomic sensitivity factors published in the literature (Ref. 1). Decomposition of the O 1s region indicated the presence of adsorbed O, OH, and H2O as well as CO and CO species, whereas the C 1s region could be decomposed to give Pt–C, graphite, CxHy polymer, and oxidized C entitie

The surface state and catalytic properties of Pt black after O₂-H₂ cycles

XPS and UPS of a Pt black catalyst after customary H2-O2 regeneration shows considerable amounts of residual C as well as surface OH/H2O species. Surface C could not be removed even by O2 at 800 K. Oxygenates are stable even after H2 treatment up to 750 K. Their chemical state has been tentatively identified by comparing XPS and UPS results. Catalytic transformations of n-hexane on Pt black treated analogously is reported and the effect of surface species on catalytic properties discussed. Possible consequences of the presence of stable surface OH/H2O species on H2-O2 titrations are mentioned

Carbon accumulation, deactivation and reactivation of Pt catalysts upon exposure to hydrocarbons

The formation and catalytic effect of carbonaceous deposits was studied on monofunctional Pt catalysts: Pt black (PtN, i.e., reduced with hydrazine), Pt/SiO2 (EUROPT-1), Pt on “herringbone” graphite nanofiber (Pt/GNF-H, GNF being able to store hydrogen) and Pt/CeO2 (ceria tending to consume spilt over hydrogen). They were exposed to hexane or t,t-hexa-2,4-diene between 483 and 663 K, with or without H2. Hydrocarbon transformations during these deactivating exposures as well as in subsequent standard test reaction with hexane in hydrogen excess were studied. Carbon accumulation on Pt black after analogous deactivating treatments was also examined by electron spectroscopy (XPS and UPS). The abundance of hydrogen on Pt sites controlled the activity and selectivity containing much PtC species. The amount of surface C could reach 45% causing almost total activity loss, but even 30% C on Pt blacks decreased markedly the catalytic activity, due to massive 3D deposits. “Disordered” carbon selectively poisoned the formation of saturated C6 products and fragmentation. The yield of dehydrogenation to hexenes was a good universal indicator of deactivation for each catalyst. Four regions weredistinguished: “beneficial”, “selective”, “non-selective” and “severe” deactivation

Selectivity of hydrogen chemisorption on clean and lead modified palladium particles; a TPD and photoemission study

This work describes hydrogen chemisorption on clean and lead modified palladium particles obtained from decomposition of PdO. TPD is used as a chemical probe to test the surface properties of several states of metallic palladium relevant in practical selective hydrogenation catalysts. These states differ in oxygen content and the presence of a lead modifier. XPS and UPS data serve as a basis for identifying the surface properties. TPD spectra show a very broad low temperature peak-likely bulk hydride decomposition-and a sharp TPD peak between 330 and 380 K. This latter can be devided into three rather poorly separated subpeaks; addition of Pb does not shift peak maxima but decreases the central subpeak and eliminates the high temperature peak completely. This points to the interaction of Pb with specific surface sites rather than to bulk alloy formation. The enhancement of selectivity in hydrogenation obtained from lead modification is considered as a geometric site blocking effect rather than to arise from a bulk modification of the valence electronic structure of palladium metal

Preparation, characterization and catalytic testing of GePt catalysts

Unsupported and SiO2 supported GePt bimetallic catalysts were prepared by depositing Ge on to Pt underpotentially. Surface-sensitive cyclic voltammetry of Pt black indicated that Ge covered ca. 40–45% of the Pt surface, whereas XPS showed just 96% Pt and 4% Ge (normalized to Pt+Ge=100%). High-resolution Ge map of GePt black obtained by Energy Filtered TEM (EFTEM) showed Ge scattered in the near-surface regions. Both catalysts were tested in hexane (nH) transformation reactions between 543 and 603 K and 60 to 480 Torr H2 pressure (with 10 Torr nH), and compared with the parent Pt catalysts. GePt/SiO2 catalyst was also tested with methylcyclopentane (MCP). Adding Ge to Pt/SiO2 lowered the activity; the opposite effect was observed with GePt black. The selectivities of saturated products on bimetallic catalysts decreased, while those of hydrogenolysis products, benzene and hexenes increased in nH transformations over supported catalyst. The reverse effects were observed over the black samples where addition of Ge prevented accumulation of adventitious carbon. Ring opening was the main reaction with MCP, together with some fragments, benzene and unsaturated hydrocarbons. Ring opening of MCP became more selective with decreasing temperature and increasing hydrogen pressure. Ge on GePt black blocked contiguous Pt sites favoring the formation of coke precursors. The different catalytic behavior of GePt/SiO2 indicated somewhat different Pt–Ge interaction(s)

Palladium-platinum powder catalysts manufactured by colloid synthesis II. Characterization and catalytic tests after oxidizing and reducing treatment

Unsupported Pd, Pt and PdPt bimetallic catalysts were prepared in different atomic ratios using methods of colloid chemistry. They were characterized by XPS, UPS and TEM. Four subsequent treatments with O2 and H2 up to T = 603K were applied in the preparation chamber of the electron spectrometer and before the catalytic runs. Platinum strongly hindered the oxidation of palladium in the bimetallic samples indicating an alloying of the two components. The H2 treatment after O2 led to rather clean metals. These treatments up to 603K decreased the Pt enrichment near to the surface found by XPS, destroying presumably the Pt islands on the surface of a Pd-rich matrix. The particle composition approached thus a homogeneous metal mixture. The catalytic behavior was tested in the hydrogenative ring opening reaction of cis- and trans-methyl-ethyl-cyclopropane (MECP) at 373 K. The product ratios 2-methylpentane/3-methylpentane (2MP/3MP) and 2-methylpentane/n-hexane (2MP/nH) were used to characterize the ring-opening pattern of the samples. The bimetallic catalysts revealed higher activity and completely different selectivities than the monometallic Pt and Pd. Moreover, the 2MP/3MP ratio from trans-MECP and 2MP/nH ratio from cis-MECP increased as the surface Pt enrichment decreased. PdPt catalysts were cleaner than Pd or Pt, their activity higher and selectivity closer to random C C rupture, due, very likely, to the presence of active Pd-Pt ensembles

Preparation, physical characterization and catalytic properties of unsupported Pt–Rh catalyst

Rh was deposited on a parent platinum black catalyst by an underpotential deposition method. Mean particle size and bulk composition of this Rh–Pt sample was determined by TEM and EDS. No individual Rh grains could be observed, but Rh was present in the near-surface regions, according to energy-filtered TEM images. The surface-sensitive cyclic voltammetry indicated 15–20% Rh on the surface. XPS, in turn, detected ∼2–2.5% Rh in the information depth. The Rh–Pt catalyst was tested in methylcyclopentane hydrogenative ring-opening reaction between 468 and 603 K and 8 to 64 kPa H2 pressure (with 1.3 kPa MCP). The parent Pt black as well as a Rh black catalyst was also studied for comparison. MCP produced ring opening and hydrogenolysis products. The ring-opening products (ROP) consisted of 2- and 3-methylpentane (2MP and 3MP) as well as hexane (nH). These were the main products, together with some fragments and unsaturated hydrocarbons. The amount of the latter class increased at higher temperatures. The selectivities of ROP, fragments, and benzene over Rh–Pt catalyst as a function of temperature were between the values observed on Pt and Rh. The hydrogen pressure dependence of selectivities on Rh–Pt was more similar to that observed on Pt. Four subsequent treatments with O2 and H2 up to T = 673 K were applied on the bimetallic catalyst, followed by XPS and catalytic runs, respectively. These treatments promoted structural rearrangement, with XPS detecting less Rh in the near surface region, partly as oxidized Rh after O2 treatment. The catalytic behavior became more Pt-like on these structural and composition changes. We concluded that adding a relatively small amount of Rh to Pt creates bimetallic active sites with properties different from those of its components, behaving as a true bimetallic catalyst

Preparation, physical characterization and catalytic properties of unsupported Pt–Rh catalyst

Rh was deposited on a parent platinum black catalyst by an underpotential deposition method. Mean particle size and bulk composition of this Rh–Pt sample was determined by TEM and EDS. No individual Rh grains could be observed, but Rh was present in the near-surface regions, according to energy-filtered TEM images. The surface-sensitive cyclic voltammetry indicated 15–20% Rh on the surface. XPS, in turn, detected ∼2–2.5% Rh in the information depth. The Rh–Pt catalyst was tested in methylcyclopentane hydrogenative ring-opening reaction between 468 and 603 K and 8 to 64 kPa H2 pressure (with 1.3 kPa MCP). The parent Pt black as well as a Rh black catalyst was also studied for comparison. MCP produced ring opening and hydrogenolysis products. The ring-opening products (ROP) consisted of 2- and 3-methylpentane (2MP and 3MP) as well as hexane (nH). These were the main products, together with some fragments and unsaturated hydrocarbons. The amount of the latter class increased at higher temperatures. The selectivities of ROP, fragments, and benzene over Rh–Pt catalyst as a function of temperature were between the values observed on Pt and Rh. The hydrogen pressure dependence of selectivities on Rh–Pt was more similar to that observed on Pt. Four subsequent treatments with O2 and H2 up to T = 673 K were applied on the bimetallic catalyst, followed by XPS and catalytic runs, respectively. These treatments promoted structural rearrangement, with XPS detecting less Rh in the near surface region, partly as oxidized Rh after O2 treatment. The catalytic behavior became more Pt-like on these structural and composition changes. We concluded that adding a relatively small amount of Rh to Pt creates bimetallic active sites with properties different from those of its components, behaving as a true bimetallic catalyst

Preferential CO oxidation in hydrogen (PROX) on ceria-supported catalysts PART II. Oxidation states and surface species on Pd/CeO2 under reaction conditions, suggested reaction mechanism

The aim of the PROX reaction is to reduce the CO content of hydrogen feed to proton-exchange membrane fuel cells (PEMFCs) by selective oxidation of CO in the presence of excess hydrogen. Both Pt and Pd on ceria are active in CO oxidation (without hydrogen), whereas Pd is poorly active in the presence of hydrogen. In this paper we explore the reasons for such behavior, using the same techniques for Pd/CeO2 as used for Pt/CeO2 in Part I: catalytic tests, in situ DRIFTS, high-pressure XPS, HRTEM, and TDS. We also examine the reaction mechanism of CO oxidation (without hydrogen), which does not occur via exactly the same mechanism on Pt and Pd/CeO2 catalysts. In the presence of hydrogen (PROX) at low temperature (T = 350–380 K), the formation of Pd β-hydride was confirmed by high-pressure in situ XPS. Its formation greatly suppressed the possibility of CO oxidation, because oxygen both from gas-phase and support sites reacted rapidly with hydride H to form water, which readily desorbed from Pd. Nevertheless, CO adsorption was not hampered here. These entities transformed mainly to surface formate and formyl (–CHO) species instead of oxidation as observed by DRIFTS. The participation of a low-temperature water–gas shift type reaction proposed for the platinum system (see Part I) was hindered. Increasing temperature led to decomposition of the hydride phase and a parallel increase in the selectivity toward CO oxidation. This still remained lower on Pd/CeO2 than on Pt/CeO2, however