Oxidation Of CO In H2 Research Articles

Nanoporous Single-Crystalline Oxide Catalysts for Preferential Oxidation of CO in H2 with an Ultra-wide Temperature Window.

Preferential oxidation of CO in H2 (PROX) reaction is a promising solution to the on-board purification of CO-contaminated H2 fuel for use in next-generation proton-exchange membrane fuel cells (PEMFC). However, achieving high CO selectivity, activity and structural stability across the wide temperature window remains a great challenge. Herein, we fabricate centimeter scale interfacial PROX catalysts grown from nanoporous single-crystalline Pr2 O3 and Nd2 O3 monoliths with lattice surface-deposited Pt clusters at nanoscale. We demonstrate complete and selective removal of CO in H2 over an unprecedented wide temperature window (253-403 K). The monoliths are integrated with an operational PEMFC to purify the H2 fuel contaminated with CO (30 ppm) and enable stable power output for >400 h; over two thousand times longer than without. This work demonstrates that the nanoporous single-crystalline oxide monoliths can simultaneously achieve the stability and overall performance required to realize practically useful PEMFCs.

Nanoporous Single‐Crystalline Oxide Catalysts for Preferential Oxidation of CO in H2 with an Ultra‐wide Temperature Window

AbstractPreferential oxidation of CO in H2 (PROX) reaction is a promising solution to the on‐board purification of CO‐contaminated H2 fuel for use in next‐generation proton‐exchange membrane fuel cells (PEMFC). However, achieving high CO selectivity, activity and structural stability across the wide temperature window remains a great challenge. Herein, we fabricate centimeter scale interfacial PROX catalysts grown from nanoporous single‐crystalline Pr2O3 and Nd2O3 monoliths with lattice surface‐deposited Pt clusters at nanoscale. We demonstrate complete and selective removal of CO in H2 over an unprecedented wide temperature window (253–403 K). The monoliths are integrated with an operational PEMFC to purify the H2 fuel contaminated with CO (30 ppm) and enable stable power output for >400 h; over two thousand times longer than without. This work demonstrates that the nanoporous single‐crystalline oxide monoliths can simultaneously achieve the stability and overall performance required to realize practically useful PEMFCs.

Microwave non-thermal fusion of MOFs derived Cu-O-Ce interface for boosting CO preferential oxidation

Engineering of uniformly-distributed and well-combined multi-component interface is sustainable but challenging for acquiring heterogeneous catalysts. Herein, the low energy consumption of microwave-fuse preparation is imposed on metal-organic frameworks (MOFs) namely Cu-MOF and Ce-MOF to manipulate Cu-O-Ce interface for preferential CO oxidation in H2-rich stream. By comprehensive characterizations, it is discovered that a unique spontaneous-migration of active Cu species onto Ce matrix will be driven by the microwave-fuse to form evenly-distributed and compactly-integrated Cu-O-Ce interface at low temperature, which is 200 °C lower than that required by traditional pyrolysis, clearly indicating the unique non-thermal function of microwave processing on fine-tuning oxide composite interface. In-situ Raman and in-situ DRIFTs further elucidate the improvement on synergistic redox feature along Cu2+-Ovac-Ce3+, leading to 100 % conversion of CO at 75–170 °C. These findings suggest that the unique self-migration of active metal species driven by non-thermal microwave-fuse is worthy of noting as an applicable strategy to acquire efficient multi-component solid interface for advanced heterogeneous catalysis as well as the current system for preferential CO oxidation in H2.

Design of efficient noble metal single-atom and cluster catalysts toward low-temperature preferential oxidation of CO in H2

The preferential oxidation of CO in H2 is attractive for the removal of trace amounts of CO to meet the requirement of proton-exchange membrane fuel cells (PEMFCs) application. The key is to design highly effective catalysts that work well in a wide range of low temperatures. Here, the recent progress in Au and Pt group metal catalysts for the PROX reaction is summarized, covering those with single-atom and cluster dispersed metal species with remarkable performance. Firstly, the progress of some representative catalysts is overviewed, with an emphasis on the strategies for improving low-temperature activity, selectivity, and stability. Then, special attention is focused on the key parameters affecting performance in the PROX reaction. Moreover, the reaction mechanisms in terms of adsorption and activation of reactants are discussed. Finally, the challenges and opportunities are offered for guiding the design of advanced noble metal catalysts toward the PROX process.

Understanding the role of K on PtCo/Al2O3 for preferential oxidation of CO in H2

CO preferential oxidation reaction (CO-PROX) can effectively eliminate CO in H2 rich atmosphere to avoid CO poison the Pt anode of Proton Exchange Membrane Fuel Cell (PEMFC). To match the operation temperature window for PEMFC, PtCo nanoparticles supported on K modified Al2O3 (PtCo/K–Al2O3) were prepared to promote CO-PROX activity. The addition of K species weakened the interaction between PtCo nanoparticle and support, which improved the dispersion of Pt particles and redox property of PtCo/Al2O3. It also facilitated the formation of Pt3Co species and active surface −OH groups, which were involved in CO-PROX reaction. According to in situ DRIFTS spectra, HCO3− and HCOO− were intermediates of PtCo/K–Al2O3 catalyzed CO-PROX at low temperature and high temperature, respectively. Thus, the addition of 1 wt% K to PtCo/Al2O3 (PtCo/1K–Al2O3) could completely oxidize CO in the temperature range of 127–230 °C with O2 selectivity at 50%. The 100% CO conversion temperature window of PtCo/1K–Al2O3 is expanded by 100 °C in comparison of PtCo/Al2O3.

Tailoring Ir-FeOx interactions and catalytic performance in preferential oxidation of CO in H2 via the morphology engineering of anatase TiO2 over Ir-FeOx/TiO2 catalysts

Ir-FeOx/TiO2 catalysts with both Ir and Fe contents of ∼1% wt. employing anatase TiO2 nanocrystals predominantly exposing either {100}, {101}, or {001} facets as the support were prepared and a comprehensive study was conducted on their interactions in pairs that are strongly dependent on the TiO2 morphology. The strongest FeOx-TiO2 interactions occurred over TiO2{001} support while TiO2{101} support contributes to the strongest Ir-TiO2 and Ir-FeOx interactions. Catalytic performance of the Ir-FeOx/TiO2 catalysts in preferential oxidation of CO in H2 is not only affected by the CO reactivity, but also determined by the Ir-FeOx interfaces. Ir-FeOx/TiO2{101} catalyst exhibits the highest CO conversion and best O2 selectivity over the three Ir-FeOx/TiO2 catalysts, which could be ascribed to stronger Ir-FeOx interactions that contribute to the enhancement of a H-spillover effect and subsequently facilitate the transformation of surface carbonaceous intermediates into CO2. These results add the versatility of the morphology engineering strategy in tailoring the structures and catalytic performance of oxide-based catalysts, and broaden the concept of morphology-dependent catalysis of oxide-based nanocrystal catalysts.

Ceria-supported Pd catalysts with different size regimes ranging from single atoms to nanoparticles for the oxidation of CO

Supported metal catalysts are the most widely used in industrial processes and the metal particle size plays a crucial factor in determining the catalytic performance. Herein, CeO2-supported Pd catalysts with different Pd size regimes ranging from single atoms, to nanoclusters (1–2 nm), and to nanoparticles (>2 nm) were used for both CO oxidation and preferential oxidation of CO in H2 (CO-PROX). Compared to Pd nanoclusters and nanoparticles, CeO2-supported single Pd atoms (PdSA/CeO2) are the most intrinsically active in CO oxidation, with an apparent activation energy of ca. 40 kJ mol−1. Results of kinetic investigations and in situ diffuse reflectance infrared Fourier transformed spectroscopy demonstrate the CO oxidation proceeding through a Langmuir-Hinshelwood mechanism with the decomposition of formate species acting dominantly as the rate-determining step. The CO reaction rate is exclusively promoted on PdSA/CeO2 catalysts for the CO-PROX reaction, which could be ascribed to a stronger H-spillover effect on isolated Pd sites to produce bridged-OH on CeO2 surface, simultaneously facilitating the consumptions of bicarbonate and formate species. There results greatly deepen the fundamental understanding of the Pd size regimes over Pd/CeO2 catalysts for the oxidation of CO.

Computationally assisted, surface energy-driven synthesis of Mn-doped Co3O4 fibers with high percentage of reactive facets and enhanced activity for preferential oxidation of CO in H2

Surface engineering is central to heterogeneous catalysis. For Co3O4-based catalysts, the bulk extended Co3O4 {110} surface has been shown to be one of the most active surfaces. However, most available Co3O4 nanocrystals are terminated by the thermodynamically stable {111} and {010} facets, rather than the active {110} facets. The increasing portion of {110}-type facets in the Co3O4-based catalysts correlates with an enhancement of the catalytic activity, and synthesis strategies need to pursue them. Here, with the aid of theoretical calculation, we present doping-assisted growth strategy to modulate the facet termination of Co3O4-based nanocatalysts. Both theoretical and experimental studies demonstrate that Co3O4-based catalysts undergo profound surface termination changes in response to Mn bulk doping. Doping of Mn into Co3O4 lowers the surface energy of {110}-type facets, and reproduces hierarchy of surface energies as {111} > {010} ≈ {110}, providing thermodynamic driving force for morphology change and surface termination transformation. The resulting Mn-doped Co3O4 catalysts are mostly composed of fiber-like shapes, exposing a large fraction of {110} and {010} surfaces. In contrast, Mn-free Co3O4 consists of octahedron-shaped nanoparticles with planar geometries of {111} and {010} as their major and minor specific exposed facets. The Mn-doped Co3O4 catalysts reported herein also show high activity and excellent stability for preferential oxidation of CO in H2 during 40 h of reaction at 60 ℃. We anticipate that our theoretical and experimental findings provide a basis for the design and synthesis of new and high-performance catalysts though surface engineering strategies.

Inverse single-site Fe1(OH)X/Pt(111) model catalyst for preferential oxidation of CO in H2

Inverse oxide/metal model systems are frequently used to investigate catalytic structure-function relationships at an atomic level. By means of a novel atomic layer deposition process, growth of single-site Fe1Ox on a Pt(111) single crystal surface was achieved, as confirmed by scanning tunneling microscopy (STM). The redox properties of the catalyst were characterized by synchrotron radiation based ambient pressure X-ray photoelectron spectroscopy (AP-XPS). After calcination treatment at 373 K in 1 mbar O2 the chemical state of the catalyst was determined as Fe3+. Reduction in 1 mbar H2 at 373 K demonstrates a facile reduction to Fe2+ and complete hydroxylation at significantly lower temperatures than what has been reported for iron oxide nanoparticles. At reaction conditions relevant for preferential oxidation of CO in H2 (PROX), the catalyst exhibits a Fe3+ state (ferric hydroxide) at 298 K while re-oxidation of iron oxide clusters does not occur under the same condition. CO oxidation proceeds on the single-site Fe1(OH)3 through a mechanism including the loss of hydroxyl groups in the temperature range of 373 to 473 K, but no reaction is observed on iron oxide clusters. The results highlight the high flexibility of the single iron atom catalyst in switching oxidation states, not observed for iron oxide nanoparticles under similar reaction conditions, which may indicate a higher intrinsic activity of such single interfacial sites than the conventional metal-oxide interfaces. In summary, our findings of the redox properties on inverse single-site iron oxide model catalyst may provide new insights into applied Fe-Pt catalysis.

Effects of hydrothermal oxidation time of Al on the catalytic performance of Ru/Al@Al2O3 for selective oxidation of CO in H2

The thermal-conducting support is highly desirable for endothermic and exothermic reactions as long as highly dispersed active sites can be maintained. The core-shell Al@Al2O3 supports with different aluminum (Al) contents were prepared from Al particle by controlling the reaction time during hydrothermal surface oxidation and applied as a support to the supported Ru catalysts for preferential CO oxidation in H2 (PROX). The prepared catalysts were characterized by N2-physisorption, X-ray diffraction, CO chemisorption, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), in situ diffuse reflectance infrared Fourier transform spectroscopy after CO adsorption (CO-DRIFTS), and temperature-programmed desorption of ammonia (NH3-TPD) and ethanol (ethanol-TPD). The catalytic activity was dependent on the Al content in the Al@Al2O3 support. The most active Ru/Al@γ-Al2O3 catalyst oxidized CO selectively in H2 over a wide reaction temperature. The surface property of the outermost exterior alumina layer in the Al@γ-Al2O3 support, determined with ethanol TPD, was beneficial for formation of Ru nanoparticles with weak adsorption of CO, probed with CO-DRIFTS, results in the high catalytic performance for PROX.

Bifunctionally faceted Pt/Ru nanoparticles for preferential oxidation of CO in H2

Global hydrogen production from catalytic reforming of hydrocarbons exceeds 50 million tons annually and accounts for 2–5% of global energy consumption. The hydrogen product generally contains 0.5–1 vol% of CO, which must be removed for certain industrial applications. Preferential oxidation (PROX) of CO is regarded as a promising and cost-effective method for hydrogen purification and potentially applicable to new generation fuel cell vehicles. In this work, bifunctionally faceted Pt/Ru nanoparticle catalysts are designed and fabricated via selective atomic layer deposition, which provides an unprecedented capability of tuning nanoparticles’ structure–property relationship for PROX in H2. Pt atomic layers are selectively deposited onto (001) facets of Ru, while leaving neighboring Ru surfaces exposed. By modulating the extent of electron donation with varied Pt coating configurations, it is demonstrated that the bifunctional catalyst with a single monolayer of Pt on Ru shows optimal activity and selectivity. The enhancement of PROX performance originates from the elimination of competitive CO adsorption on Pt, where a neighboring exposed Ru surface could readily provide highly active sites for O2 dissociation.

Synthesis of Quasi‐Bilayer Subnano Metal‐Oxide Interfacial Cluster Catalysts for Advanced Catalysis

Planar metal clusters possess high metal utilization, distinct electronic properties, and catalytic functions from their 3D counterparts. However, synthesis of these materials is challenging due to much elevated surface free energies. Here it is reported that silica supported planar bilayer Pt-CoOx subnano clusters, consisting of approximately one atomic layer of Pt and one CoOx layer on top, can be achieved by employing strong-electrostatic interactions during impregnation and precisely-controlled CoOx coating using atomic layer deposition. Such bilayer structure is unambiguously confirmed by electron microscopy and in situ X-ray absorption fine spectroscopy which is never reported before. This synthetic approach can be extended to another eight permutations of planar metal-oxide subnano clusters. The resulting bilayer catalysts, owing to unique electronic properties and the abundant metal-oxide interfaces created, exhibit excellent catalytic performances in the reactions of preferential oxidation of CO in H2 and selective hydrogenation of acetylene, by showing much higher selectivity and intrinsic activities at least 8 and 48 times greater than those conventional oxide coated 3D metal clusters/nanoparticles, highlighting the advances of bilayer interfacial structure. These findings open a new avenue to design abundant and highly active metal-oxide interfaces for advanced metal catalysis.

Local structure of Pt species dictates remarkable performance on Pt/Al2O3 for preferential oxidation of CO in H2

State-of-the-art production of industrial hydrogen predominantly derives from the reforming of hydrocarbons. However, the unavoidable ∼1 % CO in this hydrogen resource usually poisons proton-exchange-membrane fuel cells (PEMFCs) and other applications. An effective solution to eliminate CO involves the preferential oxidation of CO in H2-rich stream (PROX). Here, we report that ∼1.4 nm Pt nanoparticles supported on inert substrate (Al2O3) can catalyze total CO conversion and CO2 selectivity in a temperature range from −30 to 120 °C. Detailed characterizations indicate that this remarkable performance is determined by the local structure of Pt active centers. The ensemble of Pt(OH) and metallic Pt plays a more critical role compared with PtOx or the only Pt°. On this ensemble, O2 activation is favored while CO adsorption is weakened and H2 adsorption is inhibited, which facilitate the preferential oxidation of CO rather than H2, thus enabling a broad temperature window for 100 % selective CO removal.

Precise Tailoring of Ir-FeOx Interfaces for Improved Catalytic Performance in Preferential Oxidation of Carbon Monoxide in Hydrogen

Preferential oxidation of CO in H2 (PROX) has been regarded as the most promising on-board approach to remove the trace levels of CO (∼1%) present in hydrogen fuels and thus to protect the Pt elect...

Preferential CO Oxidation in H2 over Au/La2O3/Al2O3 Catalysts: The Effect of the Catalyst Reduction Method

We investigated the influence of the reduction method on the preferential oxidation of CO in H2 (CO-PROX) over Au/La2O3/Al2O3 catalysts. An Au/La2O3/Al2O3 sample, prepared using deposition–precipitation with urea, was reduced by chemical reduction with NaBH4 or glycerol. Several techniques, such as diffuse-reflectance infrared Fourier-transform spectroscopy after CO adsorption (CO-DRIFTS), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were used to characterize the catalysts. Additionally, the catalysts were examined by in situ DRIFTS during methanol decomposition. The results reveal that the reduction method affects the average particle size and electronic state of gold, as well as the characteristics of the CO–Au0 interactions. The best CO-PROX performance was observed for the catalyst chemically reduced using NaBH4 with a NaBH4/Au molar ratio of 35. This catalyst contained gold particles with size of ~4 nm, for which the XPS binding energy was lower than that of metallic gold.

Preferential oxidation of CO in H2 on Cu and Cu/CeOx catalysts studied by in situ UV–Vis and mass spectrometry and DFT

Preferential oxidation of CO in H2 was studied by in situ ultraviolet–visible (UV–Vis) and mass spectrometry on flat model Cu and Cu/CeOx catalysts. The experimental findings were interpreted and compared with the results from density functional theory (DFT) calculations of the adsorption and activation energies for the essential reaction steps on Cu(1 1 1). It was found that oxidation of CO preferentially takes place on Cu(0) and that no significant H2 oxidation took place under any of the investigated conditions. The presence of CeOx accelerates Cu(0)-oxidation which leads to catalyst deactivation. In contrast, CeOx promotes the CO oxidation rate on catalysts that were already oxidized to CuOx. The coexistence of CO and H2 is important to sustain the stability of metallic Cu and thereby a high rate of CO2 formation. In pure CO/O2 gas, the metallic phase can only be maintained as long as full O2 conversion is reached. In pure H2/O2, Cu is always partly but never fully oxidized, suggesting that a passivating surface layer is formed. This is also the case for H2 rich gas mixtures with small amounts of CO and O2. The most active surface termination, Cu(0), can therefore not be maintained under the industrially most interesting reaction condition where full conversion of trace amounts of CO in H2 is required. DFT calculations predict that the dissociative H2 adsorption is a key limiting step for hydrogen oxidation on the Cu(1 1 1) surface, especially when the low sticking coefficient is taken into account.

Highlights of the major progress in single-atom catalysis in 2015 and 2016

AbstractThe idea that single metal atoms dispersed on a solid support can act as an efficient heterogeneous catalyst was raised in 2011 when single Pt atoms on an FeOx surface were reported to be active for CO oxidation and preferential oxidation of CO in H2. The last six years have witnessed tremendous progress in the field of single-atom catalysis. Here we introduce the major achievements on this topic in 2015 and 2016. Some particular aspects of single-atom catalysis are discussed in depth, including new approaches in single-atom catalyst (SAC) synthesis, stable gold SACs for various reactions, the high selectivity of Pt and Pd SACs in hydrogenation, and the superior performance of non-noble metal SACs in electrochemistry. These accomplishments will encourage more efforts by researchers to achieve the controllable fabrication of SACs and explore their potential applications.

Identifying the adsorbed active intermediates on Pt surface and promotion of activity through the redox CeO2 in preferential oxidation of CO in H2

A series of non- and modified with niobia and ceria alumina-supported Pt catalysts with different Pt content (0.3–2wt%) were prepared by wetness impregnation method and characterized by N2 adsorption-desorption isotherms, XRD, TPR, DRIFT, in situ EXAFS and ΔXANES. The effect of the content of Pt and promoter on the change of catalyst electronic structure and behaviors in the reaction of preferential oxidation of CO in the presence of H2 was studied. A strong interaction of Pt with CO and O2 in atmospheres of H2/CO and H2/CO/O2 mixtures was detected by ΔXANES. Different steps of activation of reagents molecules were observed. The increase of activity with increasing the Pt content was related to the presence of Pt nanoparticles with high electron density. While, the highest metal dispersion of the low metal-loaded Pt catalyst was responsible for its high CO oxidation selectivity. Addition of ceria to Pt catalysts caused significantly increase of activity and CO oxidation selectivity at lower reaction temperatures. Niobia addition did not improve the catalysts behaviors. The promotion effect of ceria was related to the oxygen mobility at PtOCe interface. The ceria-promoted Pt catalysts with the highest ceria loading of 12wt% exhibited the highest activity and CO oxidation selectivity, but at higher reaction temperatures of about 110°–130°C it was begin slowly to decrease. This was attributed to going on the reverse of WGS reaction and high O2 consumption to H2 oxidation during the PROX-CO reaction on ceria alumina-supported Pt catalysts at high temperatures.

Modeling mass transfer in solid oxide fuel cell anode: II. H2/CO co-oxidation and surface diffusion in synthesis-gas operation

Following the previous work on comparing performance of Fickian, Stefan-Maxwell and dusty-gas model for mass transfer in single fuel system, this article is focused on the electrochemistry and transport in the anode of solid oxide fuel cell using H2H2OCOCO2N2 hybrid fuel. Under the standard framework of the dusty-gas model combined with the Butler-Volmer equation, it carries out a macroscopic area-specific modeling work. More specifically, two variables of hydrogen current fraction and enhancement factor are well defined and solved for the electrochemical co-oxidation of H2 and CO, and the diffusion equivalent circuit model is introduced to describe more comprehensively the resistance of mass transfer including molecular/Knudsen diffusion and surface diffusion. The model has been validated well in full region of VI performance of an experimental anode-supported button cell. An approximate analytical solution of the hydrogen current fraction is also presented for explicit computation. Comparison between the results by different approaches for the effective diffusivity shows the importance of right mass-transfer modeling.

Controlling activity and selectivity using water in the Au-catalysed preferential oxidation of CO in H2.

Industrial hydrogen production through methane steam reforming exceeds 50 million tons annually and accounts for 2-5% of global energy consumption. The hydrogen product, even after processing by the water-gas shift, still typically contains ∼1% CO, which must be removed for many applications. Methanation (CO + 3H2 → CH4 + H2O) is an effective solution to this problem, but consumes 5-15% of the generated hydrogen. The preferential oxidation (PROX) of CO with O2 in hydrogen represents a more-efficient solution. Supported gold nanoparticles, with their high CO-oxidation activity and notoriously low hydrogenation activity, have long been examined as PROX catalysts, but have shown disappointingly low activity and selectivity. Here we show that, under the proper conditions, a commercial Au/Al2O3 catalyst can remove CO to below 10 ppm and still maintain an O2-to-CO2 selectivity of 80-90%. The key to maximizing the catalyst activity and selectivity is to carefully control the feed-flow rate and maintain one to two monolayers of water (a key CO-oxidation co-catalyst) on the catalyst surface.