Display Omitted * Graphene-like coke forms on Pt/Mg(Al)O x during C3H8 dehydrogenation and blocks unselective sites. * Coke precursors migrate from active Pt sites toward the sites of graphene ...assembly. * Burning kinetics of coke is complex due to the mobility of surface species. * Partial oxidation of coke at mild temperatures recovers most active Pt sites. Deposition of graphene-like coke during non-oxidative propane dehydrogenation was investigated on a 0.5%Pt/Mg(Al)O x catalyst. The initial blocking of Pt sites by graphene plays an important role in establishing the excellent steady-state selectivity of Pt-based catalysts toward propylene. Temporal Analysis of Products (TAP) pulse-response experiments was used to demonstrate that during the initial nucleation of graphene-like coke, the blocked active sites are spontaneously recovered on the timescale of minutes after the dosing of the feed is discontinued. These observations suggest that an additional transport process is involved between the generation of coke precursors on Pt dehydrogenation sites and their subsequent assimilation into the growing graphene sheet. After continued exposure to the propane feed under atmospheric pressure flow conditions, extensive deposits of deformed graphene are formed on small Pt nanoparticles and shifted onto the support. Multiple layers of graphite are also formed on large nanoparticles. During subsequent oxidative catalyst regeneration (burning), some of these carbonaceous deposits are readily oxidized in air already at 650K, leading to significant recovery of Pt sites. However, those carbonaceous deposits that are less accessible to activated oxygen resist oxidation up to 800K. Ex situ TEM characterization of incompletely burned samples and isothermal pulsed oxidation provided evidence that transport phenomena on the surface determine the accessibility of graphene-like coke for oxidation at a given temperature.
Complementary to conventional X-ray absorption near edge structure (XANES) and Fourier transformed (FT) extended X-ray absorption fine structure (EXAFS) analysis, the systematic application of ...wavelet transformed (WT) XAS is shown to disclose the physicochemical mechanisms governing Pt–In catalyst formation. The simultaneous k- and R-space resolution of the WT XAS signal allows for the efficient allocation of the elemental nature to each R-space peak. Because of its elemental discrimination capacity, the technique delivers structural models which can subsequently serve as an input for quantitative FT EXAFS modeling. The advantages and limitations of applying WT XAS are demonstrated (1) before and (2) after calcination to 650 °C of a Pt(acac)2 impregnated Mg(In)(Al)O x support and (3) after subsequent H2 reduction to 650 °C. Combined XANES, FT, and WT XAS analysis shows that the acac ligands of the Pt precursor decompose during calcination, leading to atomically dispersed Pt4+ cations on the Mg(In)(Al)O x support. H2 reduction treatment eventually results in the formation of 1.5 nm Pt–In alloyed nanoparticles. Widespread use and systematic application of wavelet-based XAS can potentially reveal in greater detail the intricate mechanisms involved in catalysis, chemistry, and related fields.
Display omitted
•Northernmost NGCS meeting in Tromsø.•Topics covered during NGCS 11.•Statistics of the meeting.•2016 Award for Excellence in Natural Gas Conversion.•Social events and post-symposium ...excursions.
The 11th NGCS meeting took place between 5th and 9th of June 2016, and was hosted in Tromsø/Norway. The latest developments from academic and industrial perspectives were discussed and presented in (1) Production of Synthesis Gas, (2) Synthesis Gas to Fuels and Chemicals, (3) Direct Conversion of Methane, (4) Conversion of Light Paraffins, (5) Natural Gas in Energy Conversion, and (6) Techno-Economic Aspects. Along with the social events and the post-symposium excursions, this NGCS meeting was a memorable symposium for all participants.
Alloyed metal nanocatalysts are of environmental and economic importance in a plethora of chemical technologies. During the catalyst lifetime, supported alloy nanoparticles undergo dynamic changes ...which are well‐recognized but still poorly understood. High‐temperature O2–H2 redox cycling was applied to mimic the lifetime changes in model Pt13In9 nanocatalysts, while monitoring the induced changes by in situ quick X‐ray absorption spectroscopy with one‐second resolution. The different reaction steps involved in repeated Pt13In9 segregation‐alloying are identified and kinetically characterized at the single‐cycle level. Over longer time scales, sintering phenomena are substantiated and the intraparticle structure is revealed throughout the catalyst lifetime. The in situ time‐resolved observation of the dynamic habits of alloyed nanoparticles and their kinetic description can impact catalysis and other fields involving (bi)metallic nanoalloys.
Kinetics captured: The steps involved in repetitive Pt‐In catalyst alloying and segregation are kinetically identified during H2–O2 redox cycling by in situ quick X‐ray absorption spectroscopy (QXAS). Rate laws, coefficients, and activation energies are extracted from QXAS, which allows the construction of the kinetic reaction pathways that steer the dynamic restructuring of alloyed nanocatalysts during their lifetime.
The tandem CO2 hydrogenation to hydrocarbons over mixed metal oxide/zeolite catalysts (OXZEO) is an efficient way of producing value-added hydrocarbons (platform chemicals and fuels) directly from ...CO2 via methanol intermediate in a single reactor. In this contribution, two MAPO-18 zeotypes (M = Mg, Si) were tested and their performance was compared under methanol-to-olefins (MTO) conditions (350 °C, P CH3OH = 0.04 bar, 6.5 gCH3OH h–1 g–1), methanol/CO/H2 cofeed conditions (350 °C, P CH3OH/P CO/P H2 = 1:7.3:21.7 bar, 2.5 gCH3OH h–1 g–1), and tandem CO2 hydrogenation-to-olefin conditions (350 °C, P CO2 /P H2 = 7.5:22.5 bar, 1.4–12.0 gMAPO‑18 h molCO2 –1). In the latter case, the zeotypes were mixed with a fixed amount of ZnO:ZrO2 catalyst, well-known for the conversion of CO2/H2 to methanol. Focus was set on the methanol conversion activity, product selectivity, and performance stability with time-on-stream. In situ and ex situ Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR), sorption experiments, and ab initio molecular dynamics (AIMD) calculations were performed to correlate material performance with material characteristics. The catalytic tests demonstrated the better performance of MgAPO-18 versus SAPO-18 at MTO conditions, the much superior performance of MgAPO-18 under methanol/CO/H2 cofeeds, and yet the increasingly similar performance of the two materials under tandem conditions upon increasing the zeotype-to-oxide ratio in the tandem catalyst bed. In situ FT-IR measurements coupled with AIMD calculations revealed differences in the MTO initiation mechanism between the two materials. SAPO-18 promoted initial CO2 formation, indicative of a formaldehyde-based decarboxylation mechanism, while CO and ketene were the main constituents of the initiation pool in MgAPO-18, suggesting a decarbonylation mechanism. Under tandem CO2 hydrogenation conditions, the presence of high water concentrations and low methanol partial pressure in the reaction medium led to lower, and increasingly similar, methanol turnover frequencies for the zeotypes. Despite both MAPO-18 zeotypes showing signs of activity loss upon storage due to the interaction of the sites with ambient humidity, they presented a remarkable stability after reaching steady state under tandem reaction conditions and after steaming and regeneration cycles at high temperatures. Water adsorption experiments at room temperature confirmed this observation. The faster activity loss observed in the Mg version is assigned to its harder Mg2+-ion character and the higher concentration of CHA defects in the AEI structure, identified by solid-state NMR and XRD. The low stability of a MgAPO-34 zeotype (CHA structure) upon storage corroborated the relationship between CHA defects and instability.
Display omitted
•Graphene-like coke forms on Pt/Mg(Al)Ox during C3H8 dehydrogenation and blocks unselective sites.•Coke precursors migrate from active Pt sites toward the sites of graphene ...assembly.•Burning kinetics of coke is complex due to the mobility of surface species.•Partial oxidation of coke at mild temperatures recovers most active Pt sites.
Deposition of graphene-like coke during non-oxidative propane dehydrogenation was investigated on a 0.5%Pt/Mg(Al)Ox catalyst. The initial blocking of Pt sites by graphene plays an important role in establishing the excellent steady-state selectivity of Pt-based catalysts toward propylene. Temporal Analysis of Products (TAP) pulse-response experiments was used to demonstrate that during the initial nucleation of graphene-like coke, the blocked active sites are spontaneously recovered on the timescale of minutes after the dosing of the feed is discontinued. These observations suggest that an additional transport process is involved between the generation of coke precursors on Pt dehydrogenation sites and their subsequent assimilation into the growing graphene sheet. After continued exposure to the propane feed under atmospheric pressure flow conditions, extensive deposits of deformed graphene are formed on small Pt nanoparticles and shifted onto the support. Multiple layers of graphite are also formed on large nanoparticles. During subsequent oxidative catalyst regeneration (burning), some of these carbonaceous deposits are readily oxidized in air already at 650K, leading to significant recovery of Pt sites. However, those carbonaceous deposits that are less accessible to activated oxygen resist oxidation up to 800K. Ex situ TEM characterization of incompletely burned samples and isothermal pulsed oxidation provided evidence that transport phenomena on the surface determine the accessibility of graphene-like coke for oxidation at a given temperature.
The chemical transformations taking place during the formation of catalytic Pt–Ga alloyed nanoparticles supported on calcined Ga-modified hydrotalcite Mg(Ga)(Al)O x are investigated. The starting ...point is a Pt(acac)2 precursor impregnated onto a Mg(Ga)(Al)O x support. An oxidative treatment first yields Pt nanoparticles, while subsequent reduction efficiently delivers Ga from the support framework to Pt, forming Pt–Ga alloyed clusters. Different steps are discerned in this process based on in situ XAS analysis. During oxidative heating to 350 °C, the initially adsorbed Pt(acac)2 precursor molecules decompose and form atomically dispersed Pt4+ species with 5-/6-fold oxygen coordination. A fraction of the formed Pt–O bonds consists of strong anchoring points between Pt4+ species and support oxygen, decreasing the Pt mobility induced by the basic support. Further calcination to 650 °C leads to scission of these Pt–O support bonds, allowing more mobile Pt species to form 3–11 atom Pt fcc nanoparticles with an oxidized external surface. These subnanometer clusters are proposed to be bound to the Mg(Ga)(Al)O x support by extended 2.5 Å Pt0–(OH)− induced dipole-ion interfacial bonds. Cooling down and subsequent heating in H2 up to 450 °C causes sintering and reduction of these nanoparticles. In the course of this reduction, the proposed 2.5 Å Pt0–(OH)− interfacial bonds are replaced by common 2.0 Å Pt–O bonds. Further heating in H2 to 650 °C causes reduction of framework Ga, allowing Ga transport from the support to the Pt clusters to form 1.5 nm Pt–Ga alloyed nanoparticles. This activated Pt–Ga/Mg(Ga)(Al)O x catalyst is subjected to one O2/H2 redox cycle at 650 °C to mimick the catalyst regeneration procedure. The redox cycle induces an alloy restructuring, leading to a decrease in the degree of Pt–Ga alloying. Wavelet transformed XAScomplemented by XANES and EXAFSis shown to be a key tool to disclose the mechanistic details occurring during Pt–Ga formation.
We describe a strategy for discriminating reaction mechanisms based on the non-steady-state coherency of kinetic characteristics including net production rates as well as gas and surface ...concentrations. The suggested strategy of mechanism discrimination utilizes two novel concepts: instantaneous surface storages and temporal kinetic coherency. The first concept is used to express the catalyst composition during transient experiments through gaseous transformation rates, while the second concept is used to test whether a hypothetical reaction mechanism is consistent with observed kinetic characteristics. In our study, these kinetic characteristics are made available by analyzing exit-flow rate data of Thin-Zone Temporal Analysis of Products (TAP) pulse-response experiments via an algorithm called the Y-Procedure. To illustrate our mechanism discrimination strategy, we used a prototypical multi-route CO oxidation mechanism. The constructed decision tree can be used to distinguish between multiple variations of this mechanism.
•A new strategy was developed for discriminating between catalytic reaction mechanisms.•This strategy is based on temporal coherency of transient kinetic data.•Transient kinetic data include the net production rates R and gas concentrations C.•The R–C data were obtained from thin-zone TAP experiments via the Y-Procedure.
PDF
