Water Gas Shift Reaction Conditions Research Articles

Unravelling the origin of reaction-driven aggregation and fragmentation of atomically dispersed Pt catalyst on ceria support.

Metal-support interaction plays a crucial role in governing the stability and activity of atomically dispersed platinum catalysts on ceria support. The migration and aggregation of platinum atoms during the catalytic reaction leads to the redistribution of active sites. In this study, by utilizing a multimodal characterization scheme, we observed the aggregation of platinum atoms at high temperatures under reverse water gas shift reaction conditions and the subsequent fragmentation of platinum clusters, forming "single atoms" upon cooling. Theoretical simulations of both effects uncovered the roles of carbon monoxide binding on perimeter Pt sites in the clusters and hydrogen coverage in the aggregation and fragmentation mechanisms. This study highlights the complex effects of adsorbate and supports interactions with metal sites in Pt/ceria catalysts that govern their structural transformations under in situ conditions.

Pt-Fe nanoalloy: Structure evolution study and catalytic properties in water gas shift reaction

In the present work we studied the properties of Pt0.5Cu0.5, Pt0.5Fe0.5, Pt0.33Ag0.67, Pt0.6Au0.4 nanoalloys in water gas shift (WGS) reaction for the first time. Cu, Fe, Ag, Au were chosen as modifiers due to the low catalytic activity in undesirable side reactions of carbon oxides methanation. Nanoalloys were synthesized via corresponding double complex salts decomposition, which provided the selective formation of bimetallic nanoparticles. A simulated reformate gas mixture, containing (vol.%) 10 CO, 15 CO2, 30 H2O and 45 H2, was used to evaluate the activity of these systems. Only Pt0.5Fe0.5 nanopowder was active, while Pt-FeOx metal-oxide composite was inert. The positive effect of Pt-Fe alloying on WGS performance was also confirmed for silica supported catalysts. TG and XRD in situ analysis reveal that Pt0.5Fe0.5 is stable toward oxidation under WGS reaction conditions (T < 350°C, reductive atmosphere), while Pt-FeOx undergoes partial reduction but without formation of Pt-Fe alloy nanoparticles.

Structural Evolution of Cu/ZnO Catalysts during Water-Gas Shift Reaction: An In Situ Transmission Electron Microscopy Study.

Supported metal catalysts experience significant structural evolution during the activation process and reaction conditions, which is critical to achieve a desired active surface and interface enabling efficient catalytic processes. However, such dynamic structural information and related mechanistic understandings remain largely elusive owing to the limitation of real-time capturing dynamic information under reaction conditions. Here, using in situ environment transmission electron microscopy, we demonstrate the atomic-scale structural evolution of the model Cu/ZnO catalyst under relevant water-gas shift reaction (WGSR) conditions. Under a CO gas environment, Cu nanoparticles decompose into smaller Cu species and redistribute on ZnO supports with either the crystalline Cu2O or amorphous CuOx phase due to a strong CO-Cu interaction. In addition, we visualize various metal-support interactions between Cu and ZnO under reaction conditions, e.g., ZnO clusters precipitating on Cu nanoparticles, which are critical to understand active sites of Cu/ZnO as catalysts for WGSR. These in situ atomic-scale observations highlight the dynamic interplays between Cu and ZnO that can be extended to other supported metal catalysts.

Carbon monoxide-driven osmium catalyzed reductive amination harvesting WGSR power

First osmium-catalyzed reductive amination under the water gas–shift reaction conditions was developed. Proposed catalytic system demonstrates high performance even at the catalyst loading as low as 0.0625 mol%.

Evaluation of the Oxygen Mobility in CePO4-Supported Catalysts: Mechanistic Implications on the Water–Gas Shift Reaction

The hexagonal and monoclinic phases of CePO4 have been demonstrated to be excellent catalytic supports for Pt-based water–gas shift (WGS) catalysts. Consequently, the elucidation of the WGS reaction mechanism in these materials constitutes a fundamental aspect in order to explain their catalytic behavior. Because the observed WGS reaction path is closely related to the absence or presence of oxygen vacancies in the support, the study of the oxygen mobility in these solids constitutes a key factor for the understanding of the structure of the materials and its influence on the reaction mechanism. In this study, the oxygen mobility in CePO4 supports and the corresponding Pt catalysts has been evaluated by means of isotopic exchange experiments using 18O2 and C18O2 as probe molecules. Results demonstrate that the evaluated solids present a low exchange activity when 18O2 is used, indicating the absence of oxygen vacancies in these solids, thus suggesting a poor influence of the WGS redox mechanism. On the contrary, a high oxygen exchange activity is observed using C18O2, demonstrating that the exchange in these materials takes place through the formation of carbonate-like intermediates, thus suggesting the associative mechanism of the WGS reaction as the preferred path in these solids. Operando diffuse reflectance infrared spectroscopy experiments under WGS reaction conditions confirm these results, proving that the WGS reaction in the studied materials takes place through a formate-mediated associative mechanism.

Rh single atoms on TiO2 dynamically respond to reaction conditions by adapting their site

Single-atom catalysts are widely investigated heterogeneous catalysts; however, the identification of the local environment of single atoms under experimental conditions, as well as operando characterization of their structural changes during catalytic reactions are still challenging. Here, the preferred local coordination of Rh single atoms is investigated on TiO2 during calcination in O2, reduction in H2, CO adsorption, and reverse water gas shift (RWGS) reaction conditions. Theoretical and experimental studies clearly demonstrate that Rh single atoms adapt their local coordination and reactivity in response to various redox conditions. Single-atom catalysts hence do not have static local coordinations, but can switch from inactive to active structure under reaction conditions, hence explaining some conflicting literature accounts. The combination of approaches also elucidates the structure of the catalytic active site during reverse water gas shift. This insight on the real nature of the active site is key for the design of high-performance catalysts.

Elucidation of the Reaction Mechanism for High-Temperature Water Gas Shift over an Industrial-Type Copper-Chromium-Iron Oxide Catalyst.

The water gas shift (WGS) reaction is of paramount importance for the chemical industry, as it constitutes, coupled with methane reforming, the main industrial route to produce hydrogen. Copper-chromium-iron oxide-based catalysts have been widely used for the high-temperature WGS reaction industrially. The WGS reaction mechanism by the CuCrFeO x catalyst has been debated for years, mainly between a "redox" mechanism involving the participation of atomic oxygen from the catalyst and an "associative" mechanism proceeding via a surface formate-like intermediate. In the present work, advanced in situ characterization techniques (infrared spectroscopy, temperature-programmed surface reaction (TPSR), near-ambient pressure XPS (NAP-XPS), and inelastic neutron scattering (INS)) were applied to determine the nature of the catalyst surface and identify surface intermediate species under WGS reaction conditions. The surface of the CuCrFeO x catalyst is found to be dynamic and becomes partially reduced under WGS reaction conditions, forming metallic Cu nanoparticles on Fe3O4. Neither in situ IR not INS spectroscopy detect the presence of surface formate species during WGS. TPSR experiments demonstrate that the evolution of CO2 and H2 from the CO/H2O reactants follows different kinetics than the evolution of CO2 and H2 from HCOOH decomposition (molecule mimicking the associative mechanism). Steady-state isotopic transient kinetic analysis (SSITKA) (CO + H216O → CO + H218O) exhibited significant 16O/18O scrambling, characteristic of a redox mechanism. Computed activation energies for elementary steps for the redox and associative mechanism by density functional theory (DFT) simulations indicate that the redox mechanism is favored over the associative mechanism. The combined spectroscopic, computational, and kinetic evidence in the present study finally resolves the WGS reaction mechanism on the industrial-type high-temperature CuCrFeO x catalyst that is shown to proceed via the redox mechanism.

Novel manganese-promoted inverse CeO2/CuO catalyst: In situ characterization and activity for the water-gas shift reaction

Abstract In this work, the behavior of a novel manganese-promoted inverse CeO2/CuO catalyst under water-gas shift (WGS) reaction conditions is studied. CeO2/CuO and CeO2/Mn-CuO catalysts are characterized by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). Structure/activity relationships and redox characteristics are established on the basis of such results along with those of in situ XRD and X-ray absorption fine structure (XAFS) experiments performed either under WGS or CO-temperature programmed reduction (TPR) conditions. The manganese coprecipitated together with the copper during the preparation of the samples and the Mn-CuO phase strongly affected the size and chemical properties of the catalytically-active ceria nanoparticles. A manganese-induced enhancement on the WGS activity was observed. Differences in the characteristics of the supported ceria particles are proposed as most relevant to explain the observed Mn-promoting effect.

Heterogeneous Iron-Catalyzed Hydrogenation of Nitroarenes under Water-Gas Shift Reaction Conditions

Reduction of various nitroarenes in the presence of heterogeneous iron oxide-based catalyst Fe2O3/NGr@C under water-gas shift reaction (WGSR) conditions has been demonstrated. The catalytic material is prepared in a straightforward manner via deposition/pyrolysis of iron-phenanthroline complex on carbon support. It shows high chemoselectivity towards the reduction of nitroarenes in the presence of other reducible and/or poisoning-capable functional groups. Hydrogenation is achieved using CO/H2O as a hydrogen source. Furthermore, it is demonstrated that the presence of triethylamine additive has a significant positive effect on the rate of reduction.

Cooperative Iron–Oxygen–Copper Catalysis in the Reduction of Benzaldehyde under Water-Gas Shift Reaction Conditions

Two Fe–Cu heterobimetallic complexes have been synthesized from the reactions of the (cyclopentadienone)iron complexes {2,5-(EMe3)2-3,4-(CH2)4(η4-C4C═O)}Fe(CO)3 (E = Si, 1a; E = C, 1b) with (IPr)CuOH (IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene). X-ray crystallographic studies show that these complexes adopt a structure featuring a bridging hydride which can be described by the formula {2,5-(EMe3)2-3,4-(CH2)4(η4-C4C═O)}(CO)2Fe(μ-H)Cu(IPr) (E = Si, 4a′; E = C, 4b′). When they are dissolved in toluene, THF, or cyclohexane, these complexes form a rapidly equilibrated isomeric mixture of 4a′ or 4b′ and the terminal iron hydride {2,5-(EMe3)2-3,4-(CH2)4(η5-C4COCuIPr)}Fe(CO)2H (E = Si, 4a; E = C, 4b). The solution structure for the Me3Si derivative is dominated by 4a. Both 4a and 4b/4b′ react with HCO2H to form a monometallic iron hydride and (IPr)CuOCHO. They also undergo displacement of (IPr)CuH by CO. The heterobimetallic complexes are effective catalysts for the reduction of PhCHO under water-gas shif...

Flame made ceria supported noble metal catalysts for efficient H2 production via the water gas shift reaction

Rh/ceria catalysts were synthesized by flame spray pyrolysis for high temperature water gas shift (WGS) reactions. These catalysts show a high specific surface area due to a high degree of nanocrystallinity. X-ray absorption spectroscopy (XAS) unraveled the formation of small Rh particles under WGS reaction conditions. The catalytic activity was examined at atmospheric pressure by measuring CO conversion as a function of temperature. Some methane formation was observed above 310°C.

In situ characterization of catalysts and membranes in a microchannel under high-temperature water gas shift reaction conditions

Microreactor technology with high heat transfer in combination with stable catalysts is a very attractive approach for reactions involving major heat effects such as methane steam reforming and to some extent, also the high temperature water gas shift (WGS) reaction. For this study Rh/ceria catalysts and an ultrathin hydrogen selective membrane were characterized in situ in a microreactor specially designed for x-ray absorption spectroscopic measurements under WGS conditions. The results of these experiments can serve as a basis for further development of the catalysts and membranes.

Water–gas shift reaction over gold nanoparticles dispersed on nanostructured CeOx–TiO2(110) surfaces: Effects of high ceria coverage

Scanning tunnelling microscopy has been used to study the morphology of an overlayer of ceria in contact with a TiO2(110) substrate. Two types of domains were observed after ceria deposition. An ordered ceria film covered half of the surface and high-resolution imaging suggested a near-c(6×2) relationship to the underlying TiO2(110)-(1×1). The other half of the surface comprised CeOx nanoparticles and reconstructed TiOx supported on TiO2(110)-(1×1). Exposure to a small amount of gold resulted in the formation of isolated gold atoms and small clusters on the ordered ceria film and TiO2(110)-(1×1) areas, which exhibited significant sintering at 500K and showed strong interaction between the sintered gold clusters and the domain boundaries of the ceria film. The Au/CeOx/TiO2(110) model system proved to be a good catalyst for the water–gas shift (WGS) exhibiting much higher turnover frequencies (TOFs) than Cu(111) and Pt(111) benchmarks, or the individual Au/TiO2(110) and Au/CeO2(111) systems. For Au/CeOx/TiO2(110) catalysts, there was a decrease in catalytic activity with increasing ceria coverage that correlates with a reduction in the concentration of Ce3+ formed during WGS reaction conditions.

Improved activity and stability of Pd@CeO2 core–shell catalysts hybridized with multi-walled carbon nanotubes in the water gas shift reaction

An efficient method is presented for preparing hierarchical catalysts composed of multi-walled carbon nanotubes (MWCNTs) and Pd@CeO2 core–shell nanoparticles. These materials were then examined for the water gas shift reaction (WGSR), which demonstrated intimate contact between the constituent parts. The integration of the carbonaceous support improves the stability of the nanoparticles by ordering the dispersion of the inorganic phase and increases the activity by suppressing the deactivation of the active phase that is commonly observed in conventional Pd–CeO2 under reducing conditions, e.g. WGSR conditions. An optimum MWCNTs:Pd@CeO2 ratio exists that affords totally homogeneous structures and provides the best catalytic properties.

Pt States in BaCe0.98Pt0.02O3−δ During Start Up and Shut Down Operations Under Different Conditions: Stability and Activity of Ionic Pt in Water Gas Shift Reaction

Stability of Pt species in doped BaCeO3 perovskites under Water gas shift reaction conditions is studied using X-ray absorption spectroscopy and in situ powder X-ray diffraction. The catalyst BaCe0.98Pt0.02O3–δ has Pt in +2 oxidation state and is found to be stable in the ionic form under reaction conditions as well as under various highly reducing shut down and start up conditions. In-situ XRD under Water Gas shift reaction conditions also reveals the Pt ions to be highly stabilized in lattice and no egress is observed.

Hydrogen Production from the Water-Gas Shift Reaction on Iron Oxide Catalysts

Unsupported and supported iron oxide catalysts were prepared by incipient wetness impregnation method and studied in the water-gas shift reaction (WGSR) in the temperature range 350–450°C. The techniques of characterization employed were BET, X-ray diffraction, acid-base measurements by microcalorimetry and in situ diffuse reflectance infrared Fourier transform spectroscopy. MgO, TiO2, or SiO2 was added in order to (i) obtain a catalyst exempt of chromium oxide and (ii) study the effect of their acid-base properties on catalytic activity of Fe2O3. X-ray diffraction studies, and calorimetric and diffuse reflectance infrared Fourier transform measurements reveal a complete change in the physicochemical properties of the iron oxide catalyst after MgO addition due to the formation of the spinel oxide phase. These results could indicate that the MgFe2O4 phase stabilizes the reduced iron phase, preventing its sintering under realistic WGSR conditions (high H2O partial pressures).

Highly stable bilayer MFI zeolite membranes for high temperature hydrogen separation

Stable inorganic membranes perm-selective to hydrogen will find applications in a number of important industrial processes including water gas shift reaction for hydrogen production. This paper reports a highly stable bilayer MFI zeolite membrane with good hydrogen separation characteristics. The membrane consists of a thin (2μm) ZSM-5 layer on a thick (8μm) silicalite base layer supported on macroporous α-alumina with a yttria stabilized zirconia intermediate barrier layer. The zeolitic pores of the thin ZSM-5 layer were narrowed by catalytic cracking deposition (CCD). At 500°C, the bilayer zeolite membrane exhibits H2 permeance of about 1.2×10−7molm−2s−1Pa−1, with H2 to CO2, CO and H2O vapor selectivity respectively of about 23, 28 and 180. The membrane shows slightly improved separation properties (H2 permeance, H2/CO and H2/CO2 selectivity) as the feed side pressure increases during the test of separation of industrial relevant simulated gas mixture (25% H2: 25% CO: 25% H2O: 25% CO2, with 400ppm H2S) at 500°C for 24 days. Membrane reactor made of the bilayer zeolite membrane shows stable performance under water gas shift reaction conditions (500°C, H2O/CO=3, GHSV=60,000h−1, with a ceria doped iron oxide catalyst) in terms of CO conversion, H2 recovery, and H2 permeance and selectivity of the zeolite membrane. The unprecedented thermal and chemical stability and good separation properties of the bilayer zeolite membrane are related to its unique bilayer structure and synthesis methods.

In situ studies of CeO2-supported Pt, Ru, and Pt–Ru alloy catalysts for the water–gas shift reaction: Active phases and reaction intermediates

The activity and structure of three CeO2-based catalysts (Pt–CeO2, Ru–CeO2, and Pt–Ru alloy-CeO2) active for water–gas shift reaction (WGSR) are studied by in situ X-ray diffraction (XRD), operando X-ray absorption near edge spectroscopy (XANES), operando diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), and high-resolution transmission electron microscopy (HRTEM) in order to understand and thus correlate both bulk and surface dynamics with performance. All systems investigated displayed a high WGS activity. Temperature-resolved XRD (under CO or H2 environment) indicated an additional expansion of the CeO2 crystal lattice at 100–250°C for the catalysts compared to pure CeO2. This extra lattice expansion is due to additional oxygen removal from CeO2 promoted by the deposited Pt, Ru, or Pt–Ru alloy particles; CO showed an accelerated expansion when compared to that of H2. DRIFTS spectra revealed the formation of substantial amounts of formates (HCOO–) on Pt–CeO2 during WGSR, while formates on PtRu–CeO2 and Ru–CeO2 were at a much lower level. For all catalysts, formate species totally disappeared by 350°C. The inhibition of formate formation on PtRu–CeO2 points to a modification of the chemical properties of Pt by alloying with Ru. The fact that the inhibition of Pt-bound formate species does not affect the catalytic activity implies that they are probably merely spectators on Pt, or at least not involved in the main reaction pathway. While the Pt–Ru alloy was not more active than Pt–CeO2, the alloyed catalyst did show a reduced generation of methane under WGSR conditions compared to Ru–CeO2. Pt XANES data confirmed the reduction of Pt in both Pt–CeO2 and PtRu–CeO2 with increasing temperature in the WGSR environment. HRTEM showed that the reduced PtRu–CeO2 catalyst was composed of a Pt–Ru alloy with a mean particle size of 2nm well dispersed over the CeO2 support. Overall, the work indicates that a Pt–Ru alloy supported on CeO2 is an active and selective catalyst for WGSR.

Hydrogen Production in a Large Scale Water–Gas Shift Pd-Based Catalytic Membrane Reactor

Composite palladium and Pd-based membranes represent an appealing technology option to increase the CO conversion and the H2 recovery in the water–gas shift (WGS) reactor as a result of the continuous removal of hydrogen over the course of the reaction. Even though many studies have been performed in this area, their outcome typically represents a proof-of-concept involving reactors with small membrane area. The present study therefore addresses the scaling up of the process to obtain high hydrogen production rates through the use of large surface area, ∼0.02 m2, composite Pd membranes. Two thin, δ < 10 μm, defect-free composite membranes were prepared by the electroless plating method on porous stainless steel tubular supports and tested under pure gases and water–gas shift reaction conditions. Syngas similar to the actual gasifier reacting mixture (40% H2, 42.2% CO, and 17.8% CO2 and steam to carbon ratio varying between 2.5 and 3.5) was fed to the WGS catalytic membrane reactor (WGS-CMR) with a total flow rate up to 1.5 Nm3 h–1, 20 bar maximum pressure, and temperatures ranging 420–440 °C. In the presence of crushed catalyst, CO conversions higher than the equilibrium conversions were obtained within the entire gas hour space velocity (GHSV) range considered in the present study, for pressures between 7 and 20 bar. At a relatively low feed flow rate, GHSV = 1130 h–1, a maximum CO conversion of 98.1% was achieved, with a hydrogen recovery of 81.5% at 440 °C. On the other hand, at the highest GHSVs, the system appeared to be limited by the activity of the ferrochrome catalyst. At 20 bar of absolute pressure in the retentate side, 440 °C, and GHSV = 5650 h–1, the hydrogen production rate was found to be 5.6 Nm3 day–1. Remarkably high hydrogen purity, in excess of 99.97% and 99.2% for the two membranes, respectively, was achieved also in experiments performed with a retentate pressure of 20 bar.

Correction to “Nature of Ptn/TiO2(110) Interface under Water-Gas Shift Reaction Conditions: A Constrained ab Initio Thermodynamics Study”

Correction to “Nature of Pt_<i>n</i>/TiO₂(110) Interface under Water-Gas Shift Reaction Conditions: A Constrained ab Initio Thermodynamics Study”