Deactivation Of Pt Research Articles

Infrared reflection absorption spectroscopy of the adsorbed structures of a protic ionic liquid on the low-index planes of Pt

Modification with a protonic ionic liquid containing [7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD)][bis(pentafluoroethanesulfonyl)imide (beti)] enhanced the activities of the cathodic reactions of fuel cells (oxygen reduction reactions (ORR)) on Pt(111) and Pt(110), whereas it deactivated Pt(100). To elucidate the reaction mechanism, the ORR inhibiting species (PtOH) and the adsorption geometries of [MTBD][beti] on the low-index planes of Pt were studied using infrared reflection absorption spectroscopy (IRRAS). After modification with [MTBD][beti], the intensity of the bands representing PtOH decreased in the spectra of all the low-index planes. The adsorbed MTBD was tilted toward Pt(111) and vertically adsorbed onto Pt(110). On Pt(100), MTBD was adsorbed with its molecular plane parallel to the surface. Bands representing adsorbed beti were observed in the spectra of all surfaces. The increased ORR activities of the Pt(111) and Pt(110) surfaces were attributed to the decrease in PtOH formation after [MTBD][beti] modification and the low steric hindrance of the vertically adsorbed MTBD. Meanwhile, the deactivation of Pt(100) can be attributed to the increased steric hindrance of the parallel adsorption geometry of MTBD.

Pt : Ge Ratio as a Lever of Activity and Selectivity Control of Supported PtGe Clusters in Thermal Dehydrogenation**

AbstractEthylene is a key molecule in the chemical industry and it can be obtained through the catalytic dehydrogenation of ethane. Pt‐based catalysts show high performance toward alkane dehydrogenation, but suffer from coke formation and sintering that deactivate the catalyst. Ge was recently discovered to be a promising alloying element that suppresses deactivation of Pt while preserving its catalytic activity toward alkane dehydrogenation. This work explores the effect of the Ge content in supported PtGe cluster alloys, on the activity toward ethane dehydrogenation, selectivity against deeper dehydrogenation and coking, and sintering resistance. The model proposed herein is a tetrameric Pt cluster supported on magnesia, with varying amounts of added Ge. The phase diagram for these clusters was computed using global optimization at the density functional theory level, and under the paradigm of a statistical ensemble of many states populated by clusters at catalytic temperatures. The phase diagram shows that various Ge contents should be synthetically accessible, with Pt4Ge/MgO and Pt4Ge4/MgO being the most likely phases. The subsequent adsorption and mechanistic studies show that the clusters with the 1 : 4 Ge to Pt ratio (Pt4Ge/MgO) feature the largest resistance to sintering and best selectivity in the ethane dehydrogenation toward ethylene. Clusters without Ge are too active and easily coke, whereas clusters with higher Ge content start losing the catalytic activity toward ethane dehydrogenation. Thus, Ge concentration is a lever of control of Pt cluster stability and selectivity, and of cluster catalyst design. The effect of the Ge concentration on the cluster properties is explained on the basis of the electronic structure.

Does the composition in PtGe clusters play any role in fighting CO poisoning?

The high catalytic activity of Pt is accompanied by a high affinity for CO, making it extremely susceptible to poisoning. Such CO poisoning limits the use of proton exchange membrane fuel cells. In this work, using global minima search techniques and exhaustive electronic structure characterization, the dopant concentration is pinpointed as a crucial factor to improve the CO tolerance of Pt catalysts. By investigating the PtGe nanoclusters of different sizes and compositions, we found that, for those clusters with roughly the same amount of Pt and Ge, the binding to CO is weakened significantly. The uniqueness of the PtGe equimolar clusters is traced down to the electronic effects. The strong covalency and electrostatic stabilization arising from the advantageous Pt-Ge mixing make the equimolar clusters highly resistant toward CO poisoning and therefore more durable. Importantly, the novel catalysts not only are more resistant to deactivation but also remain catalytically active toward hydrogen oxidation. Representative clusters are additionally deposited on graphene with a pentagon-octagon-pentagon (5-8-5) reconstructed divacancy. The remarkable results of free-standing clusters hold true for surface mounted clusters, in which the interaction with CO is dramatically weakened for those compounds with a Pt:Ge ratio of 1:1. Our results demonstrate that Ge can be a promising alloying agent to mitigate the deactivation of Pt and that the dopant concentration is a critical factor in the design of advanced catalysts.

Syngas production via CO2 reforming of methane over noble metal (Ru, Pt, and Pd) doped LaAlO3 perovskite catalyst

Recently perovskite materials have received extensive attention for dry reforming of methane (DRM) reaction due to their stable ordered crystalline structure. In this work, materials with ionic substitutions of Ru, Pt, and Pd in LaAlO3 perovskite were synthesized using solution combustion synthesis route. The characterizations of catalysts were carried out by XRD, TEM, XPS, STEM, and BET surface area analyser. H2-TPR was performed to examine the strong metal support interaction between active metal and support that improve the resistance of the perovskite structure. The catalytic activity of these catalysts was examined for DRM. Stable conversion of both CH4 and CO2 were observed on stability test experiments for 50 h over Ru substituted LaAlO3 catalyst. This indicates that it is a highly stable and coke resistant catalyst for DRM. TEM and STEM results show the deactivation of Pt and Pd incorporated LaAlO3 due to sintering and carbon deposition. The apparent activation energy was determined from Arrhenius plot which was found to be 86 kJ mol−1 and 72 kJ mol−1 over LaAl0.98Ru0.02O3-δ for CH4 and CO2 consumption, respectively. DRM reaction mechanism was proposed via formation of oxycarbonates over LaAl0.98Ru0.02O3-δ.

Effects of different pollutants on the performance and deactivation of Pt–Pd catalysts in a pilot-scale waste incineration system

ABSTRACTA commercial Pt–Pd catalyst was applied in a pilot-scale waste incineration system. The characteristics of reacted catalysts and the concentrations of different pollutants were analyzed. Moreover, the effects of different pollutants on the performance and deactivation of catalyst were investigated. Experimental results showed that the activity of Pt–Pd catalysts for the removal of total hydrocarbons (THCs) was influenced by the contents of heavy metals (lead, cadmium, and mercury), chlorine, sulfur, and fly ash in the flue gas. The main pollutants that influenced the removal efficiency of THCs were lead (Pb), cadmium (Cd), and mercury (Hg). The reasons of catalyst deactivation included the deposition of carbon and fly ash on the catalyst surface, the transformation of different metals (Pb and Cd) to form metal oxides (PbO, PbO2, and CdO) to adsorb and block the active sites, and the chemical reactions of sulfur with active metals to form sulfate or sulfide (PdS2).

Phosphoric Acid Tolerant Oxygen Reduction Reaction Catalysts for High-Temperature Polymer Electrolyte Fuel Cells

The limitations of low-temperature polymer electrolyte fuel cells (LT-PEFCs) can be addressed by elevating the operating temperature above 100 °C. High-temperature (HT-) PEFC systems benefit from a simplified heat and water management, a higher tolerance toward fuel impurities such as carbon monoxide and faster oxygen reduction reaction (ORR) kinetics [1,2]. Operating temperatures higher than 100 °C involve the usage of other electrolytes than conventional perfluorosulfonic acid membranes (e.g. Nafion) due to the absence of liquid water in the system and the resulting loss of proton conductivity in the membrane. For PEFC operation in the range of 150-200 °C, humidification-independent phosphoric acid doped polybenzimidazole (PBI) is typically used as membrane material [1,2]. However, the usage of phosphoric acid determines also the activity of the catalysts due to the strong adsorption of phosphate species on the platinum surface and the corresponding reduced number of available active sites. In order to overcome the significant deactivation of Pt, the employed catalyst system needs to be modified. It was shown that combining Pt with Au via electroplating results in increased ORR activity also in the presence of phosphoric acid [3]. We present a facile and straight-forward synthesis of Pt-Au nanoparticles supported on high surface area carbon with varying metal stoichiometry. The as-prepared Pt-Au/C catalysts are further benchmarked to a standard Pt/C catalyst for usage as cathode catalysts in HT-PEFC systems. The ORR activity and stability of the catalyst systems are evaluated ex situ by means of cyclic voltammetry, Levich-Analysis and accelerated stress tests using a rotating disk electrode (RDE) setup. Furthermore, the catalyst systems are characterized in situ by means of polarization curves, continuous operation, accelerated stress tests and electrochemical impedance spectroscopy measurements at single cell and stack level. The as-prepared Pt-Au/C catalysts show increased tolerance toward phosphoric acid in comparison to the standard Pt/C (see Figure 1 and Figure 2). Acknowledgment Financial support was provided by The Climate and Energy Fund of the Austrian Federal Government and The Austrian Research Promotion Agency (FFG) through the program Energieforschung (e!Mission). [1] T. Ossiander, M. Perchthaler, C. Heinzl, F. Schönberger, P. Völk, M. Welsch, A. Chromik, V. Hacker, C. Scheu, Influence of membrane type and molecular weight distribution on the degradation of PBI-based HTPEM fuel cells, J. Memb. Sci. 509 (2016) 27–35. [2] A. Schenk, C. Grimmer, M. Perchthaler, S. Weinberger, B. Pichler, C. Heinzl, C. Scheu, F.-A. Mautner, B. Bitschnau, V. Hacker, Platinum–cobalt catalysts for the oxygen reduction reaction in high temperature proton exchange membrane fuel cells – Long term behavior under ex-situ and in-situ conditions, J. Power Sources. 266 (2014) 313–322. [3] J.E. Lim, U.J. Lee, S.H. Ahn, E. Cho, H.J. Kim, J.H. Jang, H. Son, S.K. Kim, Oxygen reduction reaction on electrodeposited PtAu alloy catalysts in the presence of phosphoric acid, Appl. Catal. B Environ. 165 (2015) 495–502. Figure 1

Pyrolyzed Stable Ferrocene-Co-Corrole N4 As a Promising Non-Precious Electrocatalyst for Oxygen Reduction Reaction

The limited natural abundance, high cost and carbon monoxide deactivation of Pt and other noble metals possess a major barrier in its applications for hydrogen or methanol fuel cells and therefore; development of alternative electrocatalytic materials based on non-precious metal-N4 is a key challenge in the current growing demand for the clean-energy fuel cell research. In this regard, in our group, a new cobalt-corrole molecule functionalized with ferrocene has been successfully synthesized and its structural characterization was done by UV-Vis, NMR and HR-Mass spectroscopic studies. The molecular structure of the complex was also confirmed by single crystal X-ray diffraction and explained with structural orientation of the atoms and attached moieties. Furthermore, this molecule mixed with activated carbon was pyrolyzed at different temperatures and among the resulting electrocatalysts, particularly at 500 °C was shown to be performed as promising ORR active catalyst for the PEM Fuel-Cell application. The unique Co-corrole integrated with mono substituted peripheral Fe complex with its pyrolyzed form of low symmetric structure provides a bimetallic (Co and Fe) active centre and facilitates the oxygen reduction reaction (ORR) via a 4-electron pathway. The prepared carbon black supported catalyst at 500 °C pyrolyzed temperature exhibits higher electron transfer number 3.96 for ORR in acidic medium with better stability, which is superior to the previously reported pyrolyzed Co-corroles. The enhancement of the ORR activity of the well characterized Co-corrole with peripheral Fe-metallocene bimetallic N4 macrocyclic complex provides a new prospect for the next-generation of non-precious metal-N4 electrocatalysts for fuel cell application. In this presentation, I will talk more in details of the electrochemical experimental process of the catalyst and its characterization using different analytical techniques. Key words: N4-macrocyclic, corrole, oxygen reduction reaction, electrocatalysts, fuel cell

Benzene alkylation with ethane in ethylbenzene over a PtH-MFI catalyst: Kinetic and IR investigation of the catalyst deactivation

This study demonstrates that the deactivation of the highly acidic PtH-MFI catalyst in the reaction of benzene alkylation with ethane in ethylbenzene (EB) proceeds in two modes. The first mode corresponds to the selective deactivation of the Pt sites, which are highly active in the hydrogenolysis reactions. This deactivation occurs during the first ∼4h of the catalyst operation and results in the significant improvement of the catalyst selectivity toward EB. The second, much slower, deactivation process is observed during tens of hours and is responsible for the deactivation of Pt and acid sites with similar rates. This deactivation mode leads to the re-distribution of the reaction products in a way that is very similar to the effect of contact time on the product distribution. The analysis of the IR spectra of the fresh and coked catalyst samples reveals that coke species are formed both inside and outside of the zeolite channels. It is shown that these coke species block the catalyst Brønsted acid sites directly but do not block the zeolite channels up to the coke content of 5–6wt.% in the catalyst. The coke located on the external surface has a major effect on the large Pt particles (∼10nm) present in the catalyst, as follows from the IR study of the CO adsorption. Our work also indicates that there are at least two different Pt sites in the Pt particles, possibly responsible for the different reaction types, namely dehydrogenation and hydrogenolysis. These two different Pt sites are affected by coke deposits differently, and therefore, their deactivation can explain the observed two modes of the catalyst deactivation.

Electrochemical and DFT Analysis of Deactivation of Pt Supported on Niobia

The catalytic deactivation of platinum supported on NbRuyOz mixed metal oxide support has been investigated. 60% Pt/NbRuyOz was synthesized in a single step using spray pyrolysis, followed by reductive thermal post-treatment. The catalyst was deposited onto a tabbed, Au TEM grid and rapidly cycled between anodic and cathodic potentials in 0.1M HClO4 and 1M KOH to observe mechanisms of corrosion and deactivation. After 3600 cycles, TEM shows a thin film of oxygen-rich niobia with the potential to block ion transport covering the catalyst and support particles. Density Functional Theory calculations indicate that the rate of oxygen diffusion across the Pt(111) surface is in good agreement with experimental observations of the period of catalyst deactivation (several weeks) when exposed to open atmosphere. This suggests that the mechanism of catalytic deactivation could involve the capture and release of oxygen as Nb2O5 breaks down into reduced fragments of NbOx.

Electrochemical and DFT Analysis of Deactivation of Pt Supported on Niobia

not Available.

Analysis of sulfur poisoning on a PEM fuel cell electrode

The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer electrolyte membrane (PEM) fuel cell. At 70 °C, sequential hydrogen sulfide (H 2S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H 2S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H 2/O 2 polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H 2S on Pt anode is presented. H 2S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H 2S. As an implication, the local potential of a PEM fuel cell anode exposed to H 2S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.

Role of Bi promotion and solvent in platinum-catalyzed alcohol oxidation probed by in situ X-ray absorption and ATR-IR spectroscopy

Modification of 5 wt% Pt/Al(2)O(3) by Bi (0.9 wt%) affords a drastic improvement of catalytic activity in the liquid phase aerobic oxidation of benzyl alcohol. The nature of the solvent employed, cyclohexane or toluene, seems to influence the catalytic activity as well. We have investigated the catalysts under working conditions using in situ X-ray absorption spectroscopy (XAS) and attenuated total reflection infrared spectroscopy (ATR-IR), aiming at uncovering the roles of the metal promoter and the reaction medium. XAS confirms that Bi is oxidized more easily than Pt, maintaining the catalytic activity of the metallic Pt sites for a longer period of time. Interestingly, toluene contrary to cyclohexane reduced Pt to a large extent. The freshly reduced noble metal sites seem to directly interact with the solvent, inducing an immediate poisoning of the material and limiting its performance. This behaviour is not observed in the presence of Bi, whose geometric effect (site blocking) is interpreted as additionally limiting the adsorption of toluene and the premature deactivation of Pt. ATR-IR spectroscopy during CO adsorption on Pt and during reaction indicates that Bi is located rather on extended surfaces than on step or kink sites. Side products, CO and benzoate species, appearing during the reaction reveal that the geometric suppression of undesired reactions does not occur to the same extent on Pt-based catalysts as on Pd, suggesting that decarbonylation of the produced aldehyde on Pt may occur also on sites other than the (111) terraces.

Reasons for the deactivation of Pt 0/TiO 2 photocatalyst treated by inert gas N 2

The effect of N 2 treatment on the photocatalytic activity of Pt 0/TiO 2 was investigated. The results showed that after treatment at 500 °C in N 2, 70% of the photocatalytic activity of 1.0 wt.% Pt 0/TiO 2 was lost by the evaluation of photocatalytic oxidation reaction of C 3H 6. Transmission electron microscopy (TEM) and Ar + ion sputtering tests revealed that in the course of high-temperature N 2 treatment, the size of Pt 0 particles on TiO 2 increases and a strong interaction between metal and support, i.e. Pt 0 particles encapsulated by Ti x O y , happens, which are the reasons for the deactivation of Pt 0/TiO 2 photocatalyst treated by high-temperature N 2.

Spatial control over atomic layer deposition using microcontact-printed resists

Area-selective thin film growth by atomic layer deposition (ALD) has been achieved on octadecyltrichlorosilane (ODTS) patterned substrates. Patterned hydrophobic self-assembled monolayers (SAMs) were first transferred to silicon and yttria-stabilized zirconia (YSZ) substrates by microcontact printing. Subsequently, films of either HfO 2 or Pt were grown selectively on the SAM-free regions of the surface, while ALD was blocked in regions where ODTS was present. The deposited pattern was readily observed through scanning electron microscopy and scanning Auger imaging, demonstrating that soft lithography is a simple and promising method to achieve area-selective ALD. The selectivity of the soft lithography-based method and the subsequent pattern resolution was compared for Pt versus HfO 2. It was found that using ODTS films, it is easier to achieve complete deactivation of Pt than HfO 2.

One-step conversion of n-butane to isobutene over H-beta supported Pt and Pt,M (M = Cu, In, Sn) catalysts: An investigation on the role of the second metal

The one-step transformation of n-butane to isobutene was studied over H-beta zeolite supported Pt and Pt,M (M = Cu, In, Sn) catalysts. Catalytic performance of monometallic Pt/H-beta samples resulted to be affected by the support acidity, a lower number of acid sites leading to higher iso- and n-butenes selectivities and lower by-products formation. Addition of Cu, In or Sn to Pt enhanced both isobutene and n-butenes selectivities, which were in the order: Pt,In > Pt,Sn ≫ Pt,Cu > Pt. All Pt,M samples exhibited also a higher stability than the corresponding monometallic Pt samples, the sequence of deactivation rates being: Pt > In,Pt > Cu,Pt ≈ Sn,Pt. On the basis of characterization results it was stated that the addition of Cu, In or Sn to Pt affects the n-butane dehydroisomerization modifying both the surface structure of Pt clusters and the support acidity. In particular the observed order of isobutene selectivity was related to the degree of Pt–M interaction leading to a dilution of Pt clusters, which inhibits hydrogenolysis reactions and enhances dehydrogenation processes. The decrease in the number of acid sites caused by addition of the second metal was instead accounted for the improved resistance to deactivation of Pt,M catalysts.

On the Issue of the Deactivation of Au–Ceria and Pt–Ceria Water–Gas Shift Catalysts in Practical Fuel‐Cell Applications

In recent years, nanostructured Au catalysts have attracted a lot of attention as potential candidates for the removal of poisonous carbon monoxide from synthesis gas streams, thus upgrading hydrogen gas to be used as fuel at the anode of lowtemperature fuel cells. The interest in Au nanoparticles has been motivated by the pioneering work of Haruta et al., who demonstrated the extremely high activity of finely dispersed Au on reducible oxide supports for CO oxidation. 2] Gold oxide systems have also been evaluated as potential catalysts for the water–gas shift (WGS) reaction and the preferential oxidation of CO in H2-rich fuel-gas streams. [9–16] Among these catalysts, the combination of Au with nanoscale cerium oxide (ceria) has been shown to be the most active lowtemperature WGS catalyst and one of the most selective preferential oxidation (PROX) catalysts for application at 80–120 8C. Nanocrystalline ceria has unique properties as an oxide support, as it increases the activity of Au toward CO oxidation and the WGS reaction by more than two orders of magnitude relative to microcrystalline ceria. Similar findings have been reported by Gorte and co-workers for Ptgroup metals on nanoscale ceria. Evidence in the literature has been mounting on the participation of the surface oxygen atoms of ceria in the catalysis of CO oxidation by dioxygen or water. Deactivation with time-on-stream and/or in shut-down/ restart operation currently plagues all known WGS catalysts based on ceria or copper oxide. Kim and Thompson reported fast deactivation of their Au–ceria catalyst, which they attributed to the blockage of the active sites by carbonates and/or formates formed during the WGS reaction. However, this fast extensive initial decline of activity of Au–ceria in the WGS reaction was not observed in a less H2-rich gas composition. [5] In their report on the deactivation of Pt–ceria with time-on-stream, Zalc et al. claimed that overreduction of ceria cannot be avoided, thus rendering ceriasupported catalysts impractical for the WGS reaction in fuelcell systems. Furthermore, in cyclic operation, including shut-down to room temperature and start-up of reactors, severe deactivation has been reported both for Pt–ceria and Au–ceria WGS catalysts. The activity of Pd–ceria and Pt– ceria for the WGS reaction can be partially recovered by reoxidation of the catalyst in oxygen at high temperatures (600 8C), but this process is not practical in the continuous operation of a fuel-cell system. To the best of our knowledge, no in situ remedy of the deactivation problem has been reported. Herein, we present a new way to stabilize the activity of Au–ceria catalysts for the WGS and PROX reactions under realistic operating conditions. We started from the observation that Au–ceria catalysts have excellent stability with time-on-stream under PROX conditions at 120 8C. We further studied this type of catalyst in shut-down/start-up cycles of the PROX reactor under conditions that allowed water condensation on the catalyst. Interestingly, this did not affect the subsequent catalyst activity at 120 8C (Figure 1). After reaching steady state at 120 8C in the PROX reaction, the 0.28 AuCe(Gd)Ox catalyst sample was cooled to room temperature and held under a continuous flow of the same gas mixture for 2 h before being reheated to 120 8C. Stable CO conversion was recovered. The sample was then cooled down to room temperature and held in the flowing full gas stream for 6 h; again, no drop of activity was found after reheating at 120 8C. Excellent performance in the shut-down/start-up cycling of the PROX reactor was also observed with another Au–ceria (La-doped) sample (0.57 AuCe(La)Ox ; Figure 1). Remarkable stability was further found for the 0.28 AuCe(Gd)Ox sample at higher conversions of CO when working with a higher O2/CO ratio [*] W. Deng, Dr. M. Flytzani-Stephanopoulos Department of Chemical and Biological Engineering Tufts University Medford, MA 02155 (USA) Fax: (+1)617-627-3991 E-mail: mflytzan@tufts.edu

Preparation and Characterization of Cu/Pt/BEA Catalyst for Low Temperature CO Oxidation

The low temperature CO oxidation over Pt-based bimetallic catalysts was investigated. Various additives such as Ni, Cu, Pd, Ag and Au were added to the Pt/BEA catalyst by an ion-exchange method and the addition of Cu to the calcined Pt/BEA catalyst was found to remarkably improve the catalytic performance. A calcination step for the Pt/BEA before Cu ion-exchange was especially important in forming the active phases for CO oxidation. XRD results indicated that the metallic Pt was not considered to be related to the active site for CO oxidation. Moreover, interactions between Pt and Cu on the Cu/Pt*/BEA catalyst could be confirmed by CO-TPD. Pt was found to be an active component for CO oxidation, but is easily deactivated by the adsorption of CO molecules. It is thought that Cu, interacting with Pt, plays an important role in preventing the deactivation of Pt.

Deactivation of Pt·PdO/Al 2O 3 in catalytic combustion of methane

Catalytic combustion is being widely studied as a means of controlling the NO x of combustors. However, the poor durability of palladium-based catalysts hinders the development of catalytic combustors for the methane combustion reaction. The effects of the addition of platinum to PdO/Al 2O 3 on catalyst deactivation at 1073 K were studied using a fixed-bed flow reactor at atmospheric pressure. The adaptability of various deactivation equations to the data of methane combustion was investigated. The deactivation behavior of the catalysts could be expressed by an equation based on two kinds of deactivation species. The addition of Pt prolongs the lifetime of the PdO/Al 2O 3 catalyst by decreasing the initial activity. This is due not to the transformation of PdO to Pd–Pt but to the prevention of the growth of PdO and Pd–Pt particles during the reaction.

Fischer–Tropsch synthesis: effect of water on the deactivation of Pt promoted Co/Al 2O 3 catalysts

The effect of water on the deactivation of a Pt promoted Co/Al 2O 3 catalyst [15%Co-Pt(0.5%)/Al 2O 3] during Fischer–Tropsch (FT) synthesis was studied in a continuously stirred tank reactor (CSTR) by (1) varying the conversion through changes of the space velocity and (2) by including additional water into the feed gas. At higher space velocities (>2 SL/gcat/h), the catalyst was exposed to low water partial pressures and exhibited stable activity. The runs with lower space velocities resulted in higher water partial pressures and higher catalyst deactivation rates. The addition of small amounts of water slightly decreased CO conversion and the effect was reversible. Increasing the amount of added water to provide P H 2O / P CO∼1 in the feed resulted in a permanent deactivation of the catalyst. The higher CO 2 selectivity indicated that cobalt oxide or another catalytic form of cobalt such as cobalt aluminate was formed under higher water partial pressure.

Sulfur-Tolerant Pt-Supported Zeolite Catalysts for Benzene Hydrogenation: I. Influence of the Support

Benzene hydrogenation was investigated over a series of Pt-supported MOR and LTL zeolites in the presence of varying thiophene concentrations. Higher activity and a higher sulfur resistance were observed for Pt supported on partially alkali-exchanged zeolites compared to completely alkali-exchanged and purely acidic zeolites. This is attributed to the contribution of catalytically active sites on the metal and on Brønsted acid sites in the vicinity or at the perimeter of the metal particles. Increasing temperature and hydrogen partial pressure increased the sulfur tolerance of the catalysts. Although the deactivation of Pt by thiophene during benzene hydrogenation was higher for Pt/KH-LTL compared to Pt/NaH-MOR, the higher overall hydrogenation activity for benzene in the presence of thiophene was observed for Pt/KH-LTL. This is attributed to the high Pt dispersion and the closer vicinity of Pt and Si(OH)Al groups in LTL. As a consequence, the benzene hydrogenation route on Brønsted acid sites with spillover of hydrogen from Pt is more effective on Pt/KH-LTL than on Pt/NaH-MOR.