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

Surface Spectroscopy and Catalytic Properties of Model Platinum Catalysts Exposed to Hydrocarbons

Hydrocarbonaceous deposits are normally present on Pt during hydrocarbon reactions. Carbon deposition is dehydrogenated during evacuation and appear as “graphitic”,“polymeric” and “deactivating” carbon. The latter may correspond to “disordered” carbon. C atoms on Pt can also be present. Activity and selectivities of “skeletal” reactions of hexane (isomerization, C5-cyclization, aromatization, fragmentation) are influenced by the amount and chemical state of carbon. This depends of the temperature of treatment and the presence of H2

Surface and structural properties of Pt/CeO2 catalyst under preferential CO oxidation in hydrogen (PROX)

Preferential oxidation of CO in the presence of excess hydrogen was studied on Pt/CeO2 with 5% metal loading. Catalytic data were similar to those observed on 1% Pt/CeO2 earlier [Wootsch et al. J. Catal. 225 (2004) 259]. The optimum temperature region is T373 K; conversion and selectivity of CO oxidation strongly decreased at higher temperatures. High-pressure XPS indicated CO adsorbed on platinum particles and significant amount of water on the ceria surface. The top-most ceria surface re-oxidized as small amount of oxygen (3%) was introduced into the H2/CO feed. Despite this surface re-oxidation, high-resolution TEM after reaction indicated oxygen deficient ceria bulk structure, in which the defects formed a super-cell, with CeO1.695 structure. The defective ceria is suggested to play an important role stabilizing the hydrogen bonded surface water, which (i) suppresses further hydrogen oxidation and (ii) reacts at the metal/support interface with linearly adsorbed CO in a low temperature water-gas-shift type reaction to produce CO2

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

Preparation of gold catalysts for the preferential CO oxidation reaction in the presence of hydrogen (PROX)

SSCI-VIDE+CARE+CDSInternational audienceNon

Preferential CO oxidation in hydrogen PROX on unsupported PtSn catalyst

PROX reaction was studied over an unsupported PtSn catalyst. CO2 and H2O were produced from a H2 flow containing 1% CO, and 0.4 to 2% O2. PtSn pretreated with O2 and H2 contained more Pt in the near-surface region and was more active towards CO oxidation than a sample pretreated in H2 only. In-situ XPS was also carried out during PROX reaction. Sn 3d indicated the presence of Sn-oxides, near to the surface region, along with Pt3Sn. Pt 4f peak showed metallic Pt under all conditions. Water and OH enrichment was observed in the O 1s spectrum, taken during PROX reaction

Preparation of gold catalysts for the preferential CO oxidation reaction in the presence of hydrogen (PROX)

SSCI-VIDE+CARE+CDSInternational audienceNon

Preferential oxidation of carbon monoxide in the presence of hydrogen (PROX) over ceria-zirconia supported noble metal catalysts

SSCI-VIDE+CARE+CDSInternational audienceNon