Reactivity of Pt/Ni supported on CeO2-nanorods on methanol steam reforming for H2 production: Steady state and DRIFTS studies

The reactivity of the PtNi supported on CeO₂-nanorods was performance on methanol steam reforming (MSR). CO_ads revealed that outer of the PtNi-catalyst could be mainly Pt-terminated and, CO_ads was slightly attenuated on the surface of the CeO₂-R. The catalytic performance of the bimetallic PtNi/CeO₂-NR catalyst exhibited better methanol conversion and H₂ selectivity than the monometallic samples. The surface species associated with the reaction mechanism from TPD-MSR-DRIFTS identified on the CeO₂-NR sample showed stronger bands associated at the methoxy species complemented with stretching C–H bands, while on the Pt/CeO₂-NR catalyst, the methoxy groups diminish indicating that it decomposes to CO and hydrogen and, new peaks of formate (HCOO^?) groups emerge. This finding suggests that the methoxy groups interacted with the surface oxygen of the support during the reaction to yield formate species and the Pt had important role to promote it as intermediary of the reaction.     

Catalytic and DRIFTS study of the WGS reaction on Pt-based catalysts

The water–gas shift (WGS) activity of Pt/SiO₂, Pt/CeO₂ and Pt/TiO₂ catalysts was studied by in-situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS). Samples contained a similar amount of Pt, between 0.34 and 0.50%, and were characterized by employing a variety of physical and spectroscopic techniques. The catalyst activities were evaluated through both CO conversion versus temperature and CO conversion versus time tests. The DRIFTS spectra were obtained on stream during the WGS reaction at increasing temperatures, from 303 to 573 K. Reduced ceria was the only active support and promoted the WGS reaction on surface bridging OH groups that react with CO to form formate intermediates. Pt/SiO₂ was more active than CeO₂ and catalyzed the WGS reaction through a monofunctional redox mechanism on metallic Pt sites. The CO conversion turnover rate was more than one order of magnitude greater on Pt/CeO₂ than on Pt/SiO₂ showing that the reaction proceeds faster via a bifunctional metal-support mechanism. Platinum on Pt/CeO₂ increased the concentration of OH groups by increasing the ceria reduction extent and also provided a faster pathway for the formation of formate intermediates in comparison to CeO₂ support. Pt/TiO₂ catalysts were clearly more active than Pt/CeO₂. The WGS reaction on Pt/TiO₂ was catalyzed via a bifunctional metal-support mechanism, probably involving the activation of CO and water on the metal and the support, respectively. The role of platinum on Pt/TiO₂ was critical for promoting the reduction of Ti⁴⁺ ions to Ti³⁺ which creates oxygen vacancies in the support to efficiently activate water.     

Mechanistic aspects of the low temperature steam reforming of ethanol over supported Pt catalysts

The influence of the support of Pt catalysts for the reaction of steam reforming of ethanol at low temperatures has been investigated on Al₂O₃, ZrO₂ and CeO₂. It was found that the conversion of ethanol is significantly higher when Pt is dispersed on Al₂O₃ or ZrO₂, compared to CeO₂. Selectivity toward H₂ is higher over ZrO₂-supported catalyst, which is also able to decrease CO production via the water-gas shift reaction. Depending on catalyst employed, interaction of the reaction mixture with the catalyst surface results in the development of a variety of bands attributed to ethoxy, acetate and formate/carbonate species associated with the support, as well as by bands attributed to carbonyl species adsorbed on platinum sites. The oxidation state of Pt seems to affect catalytic activity, which was found to decrease with increasing the population of adsorbed CO species on partially oxidized (Pt^δ+) sites. Evidence is provided that the main reaction pathway ethanol dehydrogenation, through the formation of surface ethoxy species and subsequently acetaldehyde, which is decomposed toward methane, hydrogen and carbon oxides. The population of adsorbed surface species, as well as product distribution in the gas phase varies significantly depending on catalyst reactivity towards the WGS reaction.     

Pt/ZrO2 catalyst for a single-stage water-gas shift reaction: Ti addition effect

Ti modified Pt/ZrO₂ catalysts were prepared to improve the catalytic activity of Pt/ZrO₂ catalyst for a single-stage WGS reaction and the Ti addition effect on ZrO₂ was discussed based on its characterization and WGS reaction test. Ti impregnation into ZrO₂ increased the surface area of the support and the Pt dispersion. The reducibility of the catalyst was enhanced in the controlled Ti impregnation (∼20 wt.%) over Pt/ZrO₂ by the Pt-catalysed reduction of supports, particularly, at the interface between ZrO₂ and TiO₂. The significant CO₂ gas band in the DRIFTS results of Pt/Ti[20]/ZrO₂ indicated that the Ti addition made the formate decomposition rate faster than the Pt/ZrO₂ catalyst, linked with the enhanced Pt dispersion and reducibility of the catalyst. Consequently, Ti impregnation over the ZrO₂ support led to a remarkably enhanced CO conversion and the reaction rate of Pt/Ti[20]/ZrO₂ increased by a factor of about 3 from the bare Pt/ZrO₂ catalyst.     

Enhanced water-gas shift reaction performance of MOF-derived Cu/CeO2 catalysts for hydrogen purification

The water-gas shift (WGS) reaction has received renewed interest because it is one of the key reactions for producing hydrogen and renewable energy in contemporary technologies like fuel cells and bio-refineries. Catalysts play an important role in WGS reaction for achieving high CO conversion and hydrogen generation activity. Thus, the performance and stability of catalysts are vital for the WGS reaction. In the present work, the CuCe metal-organic framework (MOF) is used as a template to derive the nanostructured Cu/CeO₂ catalyst. The influence of CuCe-MOF templated approach on the WGS activity of Cu/CeO₂ has been established. Different Cu doping levels had a significant impact on WGS activity. Amongst, the Ce_0.8Cu_0.2O₂ (Cu2Ce) catalyst had a highest CO conversion (96%). The long-term stability tests further prove that the Cu2Ce catalyst had maintained high CO conversion over 100 h reaction time. XRD and TEM results suggest that different loadings of Cu content have a distinct impact on the dispersion of Cu and the catalytic properties. N₂O chemisorption results suggest that 20 wt.% of Cu loading resulted in high Cu dispersion (52%) compared to other loadings. The H₂-temperature programmed reduction (TPR) revealed that the superior catalytic activity of Cu2Ce catalyst could be attributed to the strong reducibility (i.e. lower redox temperature) derived from CuCe-MOF template. It further suggests well-dispersed copper oxide species at low Cu loadings and crystalline copper oxide species at high Cu loadings. This work emphasizes the significance of Cu/CeO₂ catalysts with exceptional catalytic activity and stability for the WGS process with MOF-precursor.     

Water–gas shift reaction over Pt and Pt–CeOx supported on CexZr1−xO2

The water–gas shift (WGS) reaction was examined over Pt and Pt–CeO_x catalysts supported on Ce_xZr_1−xO₂ (Ce_0.05Zr_0.95O₂, Ce_0.2Zr_0.8O₂, Ce_0.4Zr_0.6O₂, Ce_0.6Zr_0.4O₂, Ce_0.7Zr_0.3O₂ and Ce_0.8Zr_0.2O₂) under severe reaction conditions, viz. 6.7 mol% CO, 6.7 mol% CO₂, and 33.2 mol% H₂O in H₂. The catalysts were characterized with several techniques, including X-ray diffraction (XRD), CO chemisorption, temperature-programmed reduction (TPR) with H₂, temperature-programmed oxidation (TPO), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and bright-field transmission electron microscopy (TEM). Among the supported Pt catalysts tested, Pt/Ce_0.4Zr_0.6O₂ showed the highest WGS activity in all temperature ranges. An improvement in the WGS activity was observed when CeO_x was added with Pt on Ce_xZr_1−xO₂ supports (x = 0.05 and 0.2) due to intimate contact between Pt and CeO_x species. Based on CO chemisorptions and TPR profiles, it has been found that the interaction between Pt species and surface ceria-zirconia species is beneficial to the WGS reaction. A gradual decrease in the catalytic activity with time-on-stream was found over Pt and Pt–CeO_x catalysts supported on Ce_xZr_1−xO₂, which can be explained by a decrease in the Pt dispersion. The participation of surface carbonate species on deactivation appeared to be minor because no improvement in the catalytic activity was found after the regeneration step where the aged catalyst was calcined in 10 mol% O₂ in He at 773 K and subsequently reduced in H₂ at 673 K.     

H2 production from a single stage water–gas shift reaction over Pt/CeO2, Pt/ZrO2, and Pt/Ce(1−x)Zr(x)O2 catalysts

To develop a single stage water–gas shift reaction (WGS) catalyst for compact reformers, Pt/CeO₂, Pt/ZrO₂, and Pt/Ce_(1−x)Zr_(x)O₂ catalysts have been applied for the target reaction. The CeO₂/ZrO₂ ratio was systematically varied to optimize Pt/Ce_(1−x)Zr_(x)O₂ catalysts. Pt/CeO₂ showed the highest turnover frequency (TOF) and the lowest activation energy (E_a) among the catalysts tested in this study. It has been found that the reduction property of the catalyst is more important than the dispersion for a single stage WGS. Pt/CeO₂ catalyst also showed stable catalytic performance. Thus, Pt/CeO₂ can be a promising catalyst for a single stage WGS for compact reformers.     

Hydrogen production by oxidative steam reforming of methanol over Ni/CeO2–ZrO2 catalysts

Single ZrO₂ and mixed CeO₂-ZrO₂ oxides with different CeO₂/ZrO₂ ratios were prepared by the sol-gel method and the CeO₂ by precipitation. The prepared support were impregnated with an aqueous solution of NiCl₂·6H₂O at an appropriate concentration to yield 3 wt.% of nickel respectively in the catalysts. Catalytic materials were characterized by BET (N₂ adsorption-desorption), SEM-EDS, XRD and TPR. The oxidative steam reforming of methanol (OSRM) reaction was investigated on these catalysts for H₂ production as a function of temperature. Depending of the CeO₂/ZrO₂ ratio; the catalysts composition has a significant influence on the surface area (BET), reduction properties and methanol conversion. XRD patterns of the Ni-base catalysts showed well defined diffraction peaks of the metallic Ni except on the Ni/CeO₂ catalyst, suggesting that on this sample all of the active phase was highly dispersed. Ni/Ceria-rich catalysts were vastly active for OSRM, giving a total CH₃OH conversion at 325 °C with GHSV = 0.3 × 10⁵ h⁻¹. They also showed close selectivity toward H₂, with high selectivity to CO₂ in all range of temperatures, this suggests that the reverse WGS reaction does not occur on these samples. It seems that the nickel is the phase mainly responsible of hydrogen production although the CeO₂/ZrO₂ support reduces the CO formation.     

CeO2-promoted Ni/activated carbon catalysts for the water–gas shift (WGS) reaction

The low temperature water–gas shift (WGS) reaction has been studied over carbon-supported nickel catalysts promoted by ceria. To this end, cerium oxide has been dispersed (at different loadings: 10, 20, 30 and 40 wt.%) on the activated carbon surface with the aim of obtaining small ceria particles and a highly available surface area. Furthermore, carbon- and ceria-supported nickel catalysts have also been studied as references. A combination of N₂ adsorption analysis, powder X-ray diffraction, temperature-programmed reduction with H₂, X-ray photoelectron spectroscopy and TEM analysis were used to characterize the Ni–CeO₂ interactions and the CeO₂ dispersion over the activated carbon support. Catalysts were tested in the low temperature WGS reaction with two different feed gas mixtures: the idealized one (with only CO and H₂O) and a slightly harder one (with CO, CO₂, H₂, and H₂O). The obtained results show that there is a clear effect of the ceria loading on the catalytic activity. In both cases, catalysts with 20 and 10 wt.% CeO₂ were the most active materials at low temperature. On the other hand, Ni/C shows a lower activity, this assessing the determinant role of ceria in this reaction. Methane, a product of side reactions, was observed in very low amounts, when CO₂ and H₂ were included in the WGS feed. Nevertheless, our data indicate that the methanation process is mainly due to CO₂, and no CO consumption via methanation takes place at the relevant WGS temperatures. Finally, a stability test was carried out, obtaining CO conversions greater than 40% after 150 h of reaction.     

Electro-catalytic activity of enhanced CO tolerant cerium-promoted Pt/C catalyst for PEM fuel cell anode

Cerium-promoted Pt/C catalysts were prepared by one-pot synthesis process and applied as an anode material for CO tolerance in PEM fuel cell. Its physical properties were characterized by XRD and TEM techniques, which indicated that Pt nano-particles are highly dispersed on the carbon supports. The investigation focused on examining the CO tolerance in sulfur acid solution of Pt–CeO₂/C compared to Pt/C (JM). The hydrogen oxidation activity was strongly depended on the content of the cerium in the Pt catalyst which was detected by CV, LSV, CO-stripping and EIS techniques. Effect of the anode catalyst poisoning on hydrogen oxidation in the presence of CO was studied in single cells. Pt–CeO₂/C catalyst at the appropriate content of 20% Ce presented a very higher CO tolerant activity. A tentative mechanism is proposed for a possible role of a bi-functional synergistic effect between Pt and CeO₂ for the enhanced electro-oxidation of CO. CeO₂-promoted Pt/C catalyst may be one of the attractive candidates as CO tolerance anode material in PEMFC.     

Preferential oxidation of CO in H2 stream on Au/CeO2–TiO2 catalysts

A series of Au catalysts supported on CeO₂–TiO₂ with various CeO₂ contents were prepared. CeO₂–TiO₂ was prepared by incipient-wetness impregnation with aqueous solution of Ce(NO₃)₃ on TiO₂. Gold catalysts were prepared by deposition–precipitation method at pH 7 and 65 °C. The catalysts were characterized by XRD, TEM and XPS. The preferential oxidation of CO in hydrogen stream was carried out in a fixed bed reactor. The catalyst mainly had metallic gold species and small amount of oxidic Au species. The average gold particle size was 2.5 nm. Adding suitable amount of CeO₂ on Au/TiO₂ catalyst could enhance CO oxidation and suppress H₂ oxidation at high reaction temperature (>50 °C). Additives such as La₂O₃, Co₃O₄ and CuO were added to Au/CeO₂–TiO₂ catalyst and tested for the preferential oxidation of CO in hydrogen stream. The addition of CuO on Au/CeO₂–TiO₂ catalyst increased the CO conversion and CO selectivity effectively. Au/CuO–CeO₂–TiO₂ with molar ratio of Cu:Ce:Ti = 0.5:1:9 demonstrated very high CO conversion when the temperature was higher than 65 °C and the CO selectivity also improved substantially. Thus the additive CuO along with the promoter and amorphous oxide ceria and titania not only enhances the electronic interaction, but also stabilizes the nanosize gold particles and thereby enhancing the catalytic activity for PROX reaction to a greater extent.     

Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2–ZrO2

Oxidative steam reforming of ethanol at low oxygen to ethanol ratios was investigated over nickel catalysts on Al₂O₃ supports that were either unpromoted or promoted with CeO₂, ZrO₂ and CeO₂–ZrO₂. The promoted catalysts showed greater activity and a higher hydrogen yield than the unpromoted catalyst. The characterization of the Ni-based catalysts promoted with CeO₂ and/or ZrO₂ showed that the variations induced in the Al₂O₃ by the addition of CeO₂ and/or ZrO₂ alter the catalyst's properties by enhancing Ni dispersion and reducing Ni particle size. The promoters, especially CeO₂–ZrO₂, improved catalytic activity by increasing the H₂ yield and the CO₂/CO and the H₂/CO values while decreasing coke formation. This results from the addition of ZrO₂ into CeO₂. This promoter highlights the advantages of oxygen storage capacity and of mobile oxygen vacancies that increase the number of surface oxygen species. The addition of oxygen facilitates the reaction by regenerating the surface oxygenation of the promoters and by oxidizing surface carbon species and carbon-containing products.     

Comparative study of CuO supported on CeO2, Ce0.8Zr0.2O2 and Ce0.8Al0.2O2 based catalysts in the CO-PROX reaction

CuO supported on CeO_2, Ce_0.8Zr_0.2O₂ and Ce_0.8Al_0.2O₂ based catalysts (6%wt Cu) were synthesized and tested in the preferential oxidation of CO in a H₂-rich stream (CO-PROX). Nanocrystalline supports, CeO₂ and solid solutions of modified CeO₂ with zirconium and aluminum were prepared by a freeze-drying method. CuO was supported by incipient wetness impregnation and calcination at 400 °C. All catalysts exhibit high activity in the CO-PROX reaction and selectivity to CO₂ at low reaction temperature, being the catalyst supported on CeO₂ the more active and stable. The influence of the presence of CO₂ and H₂O was also studied.     

Comparative study of CeO2/CuO and CuO/CeO2 catalysts on catalytic performance for preferential CO oxidation

The CeO₂/CuO and CuO/CeO₂ catalysts were synthesized by the hydrothermal method and characterized via XRD, SEM, H₂-TPR, HRTEM, XPS and N₂ adsorption–desorption techniques. The study shows that the rod-like structure is self-assembled CeO₂, and both hydrothermal time and Ce/Cu molar ratio are important factors when the particle-like CeO₂ is being self-assembled into the rod-like CeO₂. The CuO is key active component in the CO-PROX reaction, and its reduction has a negative influence on the selective oxidation of CO. The advantage of the inverse CeO₂/CuO catalyst is that it still can provide sufficient CuO for CO oxidation before 200 °C in the hydrogen-rich reductive gasses. The traditional CuO/CeO₂ catalyst shows better activity at lower temperature and the inverse CeO₂/CuO catalysts present higher CO₂ selectivity when the CO conversion reaches 100%. The performance of mixed sample verifies that they might be complementary in the CO-PROX system.     

Si-modified Pt/CeO2 catalyst for a single-stage water-gas shift reaction

Si-modified Pt/CeO₂ catalysts were prepared for a water-gas shift (WGS) reaction and the effects of this silica addition on the textural and structural characteristics, reducibility and WGS reaction performance of Pt/CeO₂ were investigated. The surface areas of the prepared catalysts increased and both interplanar spacing and average crystalline size of ceria gradually decreased with Si content, resulting in less crystalline and smaller particles. Si addition up to 20 wt. % facilitated the bulk reduction of ceria by inducing significant hydrogen consumption. The oxygen defects in the support, associated with lower valence state cerium, increased with the Si addition. These modifications offer a promising potential to increase the density of hydroxyl groups on the surface of the ceria and consequently increase the concentration of surface intermediate species. The addition of Si to ceria improved the catalytic performance for the WGS reaction, in spite of its irreducible nature. Pt catalysts supported on Si-modified ceria, with a Si content of 5-10 wt.%, exhibited a 2.5-fold increase in reaction rate and turnover frequency (TOF) compared to that of Pt/CeO₂.     

Hydrogen production from low temperature WGS reaction on co-precipitated Cu–CeO2 catalysts: An optimization of Cu loading

Low temperature water–gas shift (WGS) reaction has been carried out at the gas hourly space velocity of 72,152 h⁻¹ over Cu–CeO₂ catalyst prepared by a co-precipitation method. Cu loading was optimized to obtain highly active co-precipitated Cu–CeO₂ catalysts for low temperature WGS. 80 wt% Cu–CeO₂ exhibited the highest CO conversion as well as the most stable activity (X_CO > 46% at 240 °C for 100 h). The excellent catalytic performance is mainly due to a strong metal to support interaction, resulting in the prevention of Cu sintering.     

Hydrogen production by absorption enhanced water gas shift (AEWGS)

AEWGS is a reaction that combines the WGS reaction and CO₂ capture by a solid absorbent to produce high purity H₂ from synthesis gas in one single step at 600–800 °C. This reactor system, if homogeneous, would not require a catalyst. However, previous research on this concept was not conclusive, since a steel reactor was used and reactor walls were suspected to act as catalyst. Therefore, there is a need to address this issue and to select and evaluate suitable CO₂ absorbents for this concept. AEWGS was studied using a quartz-made fixed-bed reactor at; SV = 3000 h⁻¹, feed; 5% CO, 15% H₂O, balance He–N₂ at 600 °C, 1 atm. CO₂ absorbents tested were CaO*MgO, and Na₂ZrO₃. Empty quartz-reactor tests leaded to conclude that a catalyst is needed for the WGS at temperatures of interest. A 97% H₂ product was obtained with calcined dolomite suggesting this last to act as a WGS catalyst.     

Cooperative effect of Pt and Cu on CeO2 for the CO-PROX reaction under CO2–H2O feed stream

Catalyst improvement for the preferential oxidation of CO (CO-PROX) is essential in developing efficient fuel cell technologies. Here, we investigate the promotion of the Cu/CeO₂ system with Pt, prepared by impregnation and alcohol-reduction methods, in the CO-PROX reaction under ideal and realistic feed compositions. The high Pt dispersion in PtCu/CeO₂ prepared by impregnation led to a CO conversion of 62% and CO₂ selectivity of 83% at 50 °C under a feed stream composed of H₂/CO/O₂, while monometallic Cu/CeO₂ and Pt/CeO₂ showed negligible activity at these conditions. By adding CO₂–H₂O to the feed stream, PtCu/CeO₂ catalysts prepared by both methods presented similar activity. The maximum CO conversion temperature was shifted to 100 °C. Under these conditions, Cu/CeO₂ was inactive, and Pt/CeO₂ showed identical conversion but lower CO₂ selectivity. In-situ XANES revealed that fast oxidation of Cu species at low temperatures is responsible for Cu/CeO₂ deactivation, while preferential adsorption of CO on Pt⁰ sites in PtCu/CeO₂ avoided deactivation. The use of deactivation-resistant Pt sites as complimentary sites for CO activation associated with improved oxygen mobility over Cu–CeO₂ surface proved to be an effective strategy for CO-PROX under H₂O/CO₂ feed stream at low temperatures.     

Kinetics of the water-gas shift reaction over a La0.7Ce0.2FeO3 perovskite-like catalyst using simulated coal-derived syngas at high temperature

The kinetics of the water-gas shift (WGS) reaction over a novel La_0.7Ce_0.2FeO₃ perovskite-like catalyst is investigated using simulated coal-derived syngas at temperatures of 550 °C and 600 °C which are higher than the maximum operating temperature limit for conventional high temperature WGS catalysts. The influences of CO, CO₂, H₂O and H₂ concentration on WGS reaction rate are determined using selected gas compositions that might be encountered in a coal-based gasification system. An empirical power-law rate model used in this study is found to correlate well with experimental data with good accuracy. Kinetics parameters over La_0.7Ce_0.2FeO₃ obtained in this study are mostly in agreement with those previously measured using Fe-Cr based commercial catalysts in a range of relatively lower temperatures (300-500 °C).     

Study on the promotion of FeOx species on water-gas shift reaction in methanol aqueous phase reforming over PtFe/Al2O3 catalyst

Hydrogen production by catalytic aqueous phase reforming (APR) of biomass-derived oxygenates is a promising route to produce hydrogen in a renewable way. As the typical APR catalyst, Pt/Al₂O₃ is confronted with high methanation activity that consumes H₂ and produces undesired CH₄ during the treatment of methanol solution. In contrast, WGS reaction is the main pathway to boost H₂ generation during APR process. In this work, a series of PtFe/Al₂O₃ catalyst with different Fe contents were prepared, characterized and tested for the methanol APR reaction. XRD, TEM, XPS and H₂-TPR technologies were applied to study the change of physicochemical properties with the addition of iron. The restriction on methanation and promotion on WGS reaction were proved as well. With the increase of Fe content, the interaction between Pt and FeO_x species have both positive and negative effects on APR reactivity while the CH₄ selectivity descending continuously from 9.25% to as low as 0.60%. Besides, the effects of reduction temperatures (250–350 °C) of Pt0.5Fe/Al₂O₃ catalyst on APR performances were also investigated. An “Oxygen dynamic transfer cycle” between H₂O/CO1 and FeO_x species with oxygen vacancies is proposed and verified. This indirect WGS reaction through lattice oxygen on FeO_x species that boosting WGS reactivity helps improving H₂ selectivity in the gas products of methanol APR.