Temperature-programmed Desorption Studies Research Articles

Surface‐modified Ag@Ru‐P25 for photocatalytic CO2 conversion with high selectivity over CH4 formation at the solid–gas interface

AbstractSystematic optimization of the photocatalyst and investigation of the role of each component is important to maximizing catalytic activity and comprehending the photocatalytic conversion of CO2 reduction to solar fuels. A surface‐modified Ag@Ru‐P25 photocatalyst with H2O2 treatment was designed in this study to convert CO2 and H2O vapor into highly selective CH4. Ru doping followed by Ag nanoparticles (NPs) cocatalyst deposition on P25 (TiO2) enhances visible light absorption and charge separation, whereas H2O2 treatment modifies the surface of the photocatalyst with hydroxyl (–OH) groups and promotes CO2 adsorption. High‐resonance transmission electron microscopy, X‐ray photoelectron spectroscopy, X‐ray absorption near‐edge structure, and extended X‐ray absorption fine structure techniques were used to analyze the surface and chemical composition of the photocatalyst, while thermogravimetric analysis, CO2 adsorption isotherm, and temperature programmed desorption study were performed to examine the significance of H2O2 treatment in increasing CO2 reduction activity. The optimized Ag1.0@Ru1.0‐P25 photocatalyst performed excellent CO2 reduction activity into CO, CH4, and C2H6 with a ~95% selectivity of CH4, where the activity was ~135 times higher than that of pristine TiO2 (P25). For the first time, this work explored the effect of H2O2 treatment on the photocatalyst that dramatically increases CO2 reduction activity.

Mechanistic Insights into Nonoxidative Ethanol Dehydrogenation on NiCu Single-Atom Alloys

Ethanol dehydrogenation presents a promising pathway toward the production of acetaldehyde, a valuable building block in chemicals production. Under nonoxidative conditions, the reaction is facilitated by supported Cu nanoparticles, which afford reasonable activity and high selectivity. The stability issues associated with Cu nanoparticle sintering can be addressed by the addition of small amounts of Ni, which further boost reactivity while retaining selectivity. Despite the promise of NiCu single-atom alloys for nonoxidative ethanol dehydrogenation, little is known about the role of each component and the pathway of this mechanistically complex process. Herein, kinetic investigations from reactor tests identify C–H bond scission as the rate-limiting step, while 1-hydroxyethyl is detected as the intermediate via IR spectroscopy. Temperature-programmed desorption studies are employed to examine the effect of Ni coverage and to demonstrate that Ni atoms activate ethanol selectively at lower temperatures, resulting in higher acetaldehyde yield than pure Cu. Temperature-programmed desorption experiments also reveal the spillover of intermediates from the Ni atom to neighboring Cu sites as a relevant step in the reaction pathway. Density functional theory calculations are used to investigate the reaction energetics and to confirm that C–H bond scission is the initial reaction step, while a clear effect of H2 partial pressure on the reaction pathway is realized. Further, counter to the expected behavior that all reaction steps take place on the Ni atoms, our degree of rate control analysis reveals that a mechanism involving spillover of the 1-hydroxyethyl intermediate from the Ni atom to the Cu surface, where it will dehydrogenate further, is more likely. Our combined kinetic, spectroscopic, and theoretical approach sheds light on this complex reaction mechanism and represents a promising method for the understanding and designing of highly active, selective, and stable single-atom alloys for other multistep catalytic processes.

Hydrogenation of Formate Species Using Atomic Hydrogen on a Cu(111) Model Catalyst.

The reaction mechanism of the CH3OH synthesis by the hydrogenation of CO2 on Cu catalysts is unclear because of the challenge in experimentally detecting reaction intermediates formed by the hydrogenation of adsorbed formate (HCOOa). Thus, the objective of this study is to clarify the reaction mechanism of the CH3OH synthesis by establishing the kinetic natures of intermediates formed by the hydrogenation of adsorbed HCOOa on Cu(111). We exposed HCOOa on Cu(111) to atomic hydrogen at low temperatures of 200-250 K and observed the species using infrared reflection absorption (IRA) spectroscopy and temperature-programmed desorption (TPD) studies. In the IRA spectra, a new peak was observed upon the exposure of HCOOa on Cu(111) to atomic hydrogen at 200 K and was assigned to the adsorbed dioxymethylene (H2COOa) species. The intensity of the new peak gradually decreased with heating from 200 to 290 K, whereas the IR peaks representing HCOOa species increased correspondingly. In addition, small amounts of formaldehyde (HCHO), which were formed by the exposure of HCOOa species to atomic hydrogen, were detected in the TPD studies. Therefore, H2COOa is formed via hydrogenation by atomic hydrogen, which thermally decomposes at ∼250 K on Cu(111). We propose a potential diagram of the CH3OH synthesis via H2COOa from CO2 on Cu surfaces, with the aid of density functional theory calculations and literature data, in which the hydrogenation of bidentate HCOOa to H2COOa is potentially the rate-determining step and accounts for the apparent activation energy of the methanol synthesis from CO2 on Cu surfaces.

CuO-Fe(III)-Zeolite-Y as efficient catalyst for oxidative alcohol-amine coupling reactions

CuO nanocatalyst with layered structure was supported over Fe-exchanged zeolite-Y (CuO-Fe(III)-Y) for selective synthesis of amides by oxidative coupling of aromatic primary alcohols and amines. The CuO-Fe(III)-Y catalyst was characterized by different physicochemical and spectroscopic techniques. The amide bond formation reaction was found to occur in absence of any external base and reagents. The temperature-programmed desorption (TPD) study predicted the development of strong basic sites in the CuO-Fe(III)-Y catalyst that boosted the dehydrogenative coupling process. The activity of the material in such coupling reaction was found to be comparable to those of other costly metal catalysts under different reaction conditions. The product yield of the reaction was found to be substrate-dependent giving moderate to maximum yield of 85%. The anomalies in the reaction appeared in case of 5-nitro-2‑chloro-benzylalcohol that undergoes a CN coupling product through CCl bond activation. The mechanism of the reaction was believed to proceed through FeIV=O species that helped in the activation of the benzylic CH bond.

Interaction of Methanol over CsCl- and KCl-Doped η-Alumina and the Attenuation of Dimethyl Ether Formation

As part of a program to investigate aspects of surface chemistry relevant to methyl chloride synthesis catalysis, the interaction of methanol with η-alumina doped with either CsCl or KCl in the range 0.01–1.0 mmol g(cat)–1 is investigated by a combination of diffuse reflectance infrared Fourier transform spectroscopy and temperature-programed desorption (TPD). Infrared spectra (IR) recorded at 293 K show that increasing the concentration of the group 1 metal chloride progressively decreases the surface concentration of associatively chemisorbed methanol and changes the environment in which the adsorbed methanol resides. For CsCl concentrations of ≥0.6 mmol g(cat)–1, chemisorbed methoxy species dominate the IR spectrum, while TPD studies show that the amount of methanol adsorbed onto the surface, and subsequently desorbed unchanged, changes relatively little. In the TPD experiments, some of the adsorbed methanol reacts to give dimethyl ether (DME) which then desorbs; for dopant concentrations of 1.0 mmol g(cat)–1, DME formation is suppressed to below the limit of detection. Unexpectedly, the presence of formate species generated at 293 K is also observed spectroscopically, characterized by a νasym(COO) mode which exhibits a hypsochromic shift relative to potassium formate; surface concentrations of formate are higher at higher loadings of group 1 metal chloride. Temperature-programed IR spectroscopy shows that the room-temperature formate species desorbs, decomposes, or migrates on warming to 653 K. Thermal ramping of the methanol-saturated surface also results in formate production but one that exhibits an IR profile in agreement with earlier observations and literature values. Increasing the concentrations of the group 1 metal chloride progressively decreases the presence of the thermally induced formate moiety. The study not only reinforces the concept of group 1 metal chloride additives progressively rendering ineffective those Lewis acid sites present at the η-alumina surface which convey discrete reaction characteristics [e.g., (i) dimerization of methanol to form DME and (ii) an activated methoxy → formate transition] but also suggests the generation of reactive sites not present in the undoped alumina.

On the various Cu-redox pathways and O2-mediated Bronsted-to-Lewis adsorbed-NH3 redistribution under SCR half-cycle conditions

The Standard SCR reaction on Cu-SSZ-13 is a complex redox process facilitated by a reduction half cycle (RHC) and an oxidation half cycle (OHC). It is generally accepted that RHC requires the simultaneous presence of NO and NH3, while OHC requires both NO and O2; however understanding of how these individual reactants impact the Cu redox cycle is limited. In this study, we provide experimental investigation of NO-only, NH3-only and NO+NH3 RHC, and their relative rates. Simple kinetic models were developed to understand the dependance of NO-only and NH3-only RHC on Cu speciation. Various OHC routes including NO+O2 and O2-only OHC, and a previously unreported H2O-mediated OHC pathway were also investigated. Detailed NH3 temperature-programmed desorption studies showed that RHC selectively consumes NH3 adsorbed on Lewis-acid sites, and that part of the remaining Bronsted-bound NH3 moves to the Lewis-acid sites during subsequent OHC, thereby resulting in a repartitioning of adsorbed NH3.

Enhanced hydrogen absorption and desorption properties of MgH2 with graphene and vanadium disulfide

Magnesium hydride (MgH2) is the most prominent carrier for storing hydrogen in solid-state mode. However, their slow kinetics and high thermodynamics become an obstacle in hydrogen storage. The present study elaborates on the catalytic effect of graphene (Gr) and vanadium disulfide (VS2) on MgH2 to enhance its hydrogen sorption kinetic. The temperature-programmed desorption study shows that the onset desorption temperature of MgH2 catalyzed by VS2 and MgH2 catalyzed by Gr is 289 °C and 300 °C, respectively. These desorption temperatures are 87 °C and 76 °C lower than the desorption temperature of pristine MgH2. The rapid rehydrogenation kinetics for the MgH2 catalyzed by VS2 have been found at a temperature of 300 °C under 15 atm H2 pressure by absorbing ∼4.04 wt% of hydrogen within 1 min, whereas the MgH2 catalyzed by Gr takes ∼3 min for absorbing the same amount of hydrogen under the similar temperature and pressure conditions. The faster release of hydrogen was also observed in MgH2 catalyzed by VS2 than MgH2 catalyzed by Gr and pristine MgH2. MgH2 catalyzed by VS2 releases ∼2.54 wt% of hydrogen within 10 min, while MgH2 catalyzed by Gr takes ∼30 min to release the same amount of hydrogen. Furthermore, MgH2 catalyzed by VS2 also persists in the excellent cyclic stability and reversibility up to 25 cycles.

A temperature programmed desorption study of interactions between water and hydrophobes at cryogenic temperatures.

It is considered that hydrophobic solutes dissolve in water via the formation of icelike cages in the first hydration shell. However, this conventional picture is currently under debate. We have investigated how hydrophobic species, such as D2, Ne, Ar, Xe, CH4, and C3H8, interact with water in composite films of amorphous solid water (ASW) based on temperature programmed desorption (TPD). The D2 and Ne species tend to be incorporated in ASW without being caged, whereas two distinct peaks assignable to the caged species are identifiable for the other solutes examined here. The low-temperature peak is observed preferentially for Ar and CH4 prior to crystallization. The hydrophobes are thought to be encapsulated in porous ASW films via reorganization of the hydrogen bond network up to 100 K; most of them are released in a liquidlike phase that occurs immediately before crystallization at ca. 160 K. The nature of hydrophobic hydration at cryogenic temperature appears to differ from that in normal water at room temperature because the former resembles crystalline ices in the local hydrogen-bond structure rather than the latter. No ordered structures assignable to clathrate hydrates were identified before and after crystallization.

Infrared reflection absorption spectroscopy and temperature-programmed desorption studies of CO adsorption on Ni/CeO2(111) thin films: The role of the ceria support

Ceria-supported Ni has shown unique catalytic activity due to unique properties of small Ni particles and strong metal-support interaction. Identification of adsorption sites and understanding the chemical interaction over Ni-ceria at the fundamental level provide crucial insights into the reaction pathways of complex catalytic processes. In this study, to probe the surface sites, the adsorption of CO was carried out with model Ni/ceria systems consisting of Ni nanoparticles vapor-deposited on well-ordered CeOx(111) (1.5 < x < 2) thin films using infrared reflection absorption spectroscopy (IRRAS) and temperature-programmed desorption (TPD) under ultrahigh vacuum (UHV) conditions. Fully oxidized CeO2(111) (Ce4+) and partially reduced CeO1.75(111) (Ce4+/Ce3+) thin films were grown on Ru(0001) to examine the role of the ceria support. Ni with low coverages (e.g., 0.2 ML) grows forming small two-dimensional particles on ceria at 300 K, which develop into three-dimensional clusters after heating to 700 K. In the absence of Ni, CO adsorption at 1 mTorr at 100 K shows distinct IR bands at 2158 cm−1 on CeO2 and 2165 cm−1 on CeO1.75. Bridging and atop IR bands associated with CO adsorption over metallic Ni were observed on the Ni-CeO1.75 surface at 300 K under UHV conditions. CO adsorption over Ni0 was also observed over as-deposited Ni on CeO2. However, a new IR band at 2146 cm−1 due to CO adsorption over Ni2+ species was detected at 100 K over the annealed Ni particles on CeO2. CO IRRAS data suggest the oxidation of Ni to Ni2+ on CeO2 and the formation of predominant Ni2+ species with heating, which is further confirmed with CO TPD data and previous x-ray photoelectron spectroscopy results.

Understanding size-dependent hydrogenation of dimethyl oxalate to methyl glycolate over Ag catalysts

Ag catalysts are promising for the selective hydrogenation of dimethyl oxalate (DMO) to methyl glycolate (MG), where the size of Ag nanoparticles strongly dominates their catalytic performances. Fundamental understandings on the size dependence, especially at the level of active sites, are still lacking but of great importance for the rational design of Ag-based catalysts for this reaction. Herein, a series of catalysts consisting of Ag nanoparticles sized from 2.9 to 8.8 nm anchored on amine-derivatized mesoporous silica nanospheres (Ag/AS) were synthesized and employed to understand the size-dependent DMO hydrogenation to MG. A volcano-shape trend between the hydrogenation activity and the increasing Ag particle size is observed for the differently sized Ag/AS catalysts, indicating a remarkable size dependence. The origin of the size dependence is further understood based on the cuboctahedron model of Ag nanoparticle, and the step-likes active sites are identified as the dominant ones. Results combining with the X-ray photoelectron spectroscopy, in situ Fourier transform infrared spectroscopy and temperature programmed desorption studies, indicate that the hydrogenation of DMO to MG on Ag/AS catalysts is mainly dominated by the number of active sites when the Ag particle size ≥ 5.3 nm, but by the electronic properties when the Ag particle size < 5.3 nm. The balance between the number of active sites and electronic structure gives rise to a maximal reaction rate in term of apparent TOF on the Ag/AS catalyst with Ag particle size of 5.3 nm. These understandings may guide the design of highly efficient Ag catalysts for the coal-based MG production.

Influence of Lewis and Brønsted acidic sites on graphitic carbon nitride catalyst for aqueous phase conversion of biomass derived monosaccharides to 5-hydroxymethylfurfural

Conversion of saccharides to 5-hydroxymethylfurfural is one of the fundamental steps in the synthesis of biodiesel. In this work, we report on the influence of Lewis and Brønsted acidic sites on metal-free graphitic carbon nitride catalysts for the conversion of monosaccharides to 5-hydroxymethylfurfural. We have examined the Lewis acidic sites by doping boron onto graphitic carbon nitride, and under optimal conditions, a maximal 91% yield of 5-hydroxymethylfurfural has been obtained from fructose. Subsequently, the boron doped graphitic carbon nitride was functionalized with Brønsted acidic group to prepare sulfonic acid functionalized boron doped graphitic carbon nitride, which possesses both Lewis and Brønsted acidic sites. However, the use of sulfonic acid functionalized boron doped graphitic carbon nitride as catalyst resulted in a lower yield (78%) of 5-hydroxymethylfurfural from fructose. In addition, time-dependent nuclear magnetic resonance spectroscopic study, temperature-programmed desorption study, and green metrics calculations have been performed to substantiate our findings. Overall, this work provides valuable insights on the influence of Lewis and Brønsted acidic sites on graphitic carbon nitride for biomass conversion reactions.

Oxygen effect in NO direct decomposition over K/Co-Mg-Mn-Al mixed oxide catalyst–Temperature programmed desorption study

• The role of oxygen in NO decomposition was studied over Co-Mg-Mn-Al mixed oxide. • Transition metals in octahedral coordination represent sites for NO chemisorption. • Potassium ensures NO oxidation to surface nitrite intermediates. • Oxygen inhibition was ascribed to formation of stable surface nitrate species. Co-Mg-Mn-Al mixed oxide with or without K was prepared and characterized by XPS, TPR H 2 , TPD NO, TPD O 2 and TPD NO+O 2 . NO decomposition was performed over catalyst activated in different atmosphere (N 2 , He, O 2 ). Presence of oxygen during activation changed the course of surface stabilization but did not affect NO conversion achieved at steady state. O 2 desorbed as a reaction product does not affect the reaction rate for NO inlet concentration of 400 - 8000 ppm, but presence of O 2 in inlet gas mixture (0.05 - 3 mol.%) has significant inhibiting effect. Transition metals in octahedral coordination represent sites for NO chemisorption, while potassium ensures NO oxidation to surface nitrites, which are indispensable reactive species. Oxygen desorption was the slowest reaction step, while N 2 desorption was fast. Oxygen inhibition was ascribed to oxidation of NO* species on the octahedral Co 3+ and Mn 3+ to more stable surface nitrates making NO* species as limiting reactant.

Temperature-programmed desorption study of catalytic carbonization behavior during the preparation of ordered mesoporous carbon

Temperature-programmed desorption study of catalytic carbonization behavior during the preparation of ordered mesoporous carbon

Structural and Kinetics Understanding of Support Effects in Pd-Catalyzed Semi-Hydrogenation of Acetylene

In this study, the support effects on the Pd-catalyzed semi-hydrogenation of acetylene have been investigated from the structural and kinetic perspectives. According to the results of kinetic analysis and X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, temperature-programmed hydride decomposition, and in situ X-ray diffraction measurements, using carbon nanotubes as support for Pd nanocatalysts with various sizes instead of α-Al 2 O 3 decreases the Pd 0 3d binding energy and suppresses the formation of undesirable palladium hydride species, thus increasing the ethylene yield. Furthermore, X-ray absorption spectroscopy, high-resolution transmission electron microscopy, and C 2 H 4 temperature-programmed desorption studies combined with density-functional theory calculations reveal the existence of a unique Pd local environment, containing subsurface carbon atoms, that produces positive geometric effects on the acetylene conversion reaction. Therefore, tailoring the Pd local environment and electronic properties represents an effective strategy for the fabrication and design of highly active and selective Pd semi-hydrogenation catalysts.

Alkali/alkaline earth ion-exchanged and palladium dispersed MCM-22 zeolite as a potential catalyst for eugenol isomerization and Heck coupling reactions

Alkali and alkaline earth metal ions (Na+, K+, Cs+, Mg2+) exchanged MCM-22 zeolites were prepared and subsequently palladium (2 wt.%; Pd) was dispersed on above exchanged MCM-22 zeolite materials. All the MCM-22 materials were systematically characterized by FTIR, powder X-ray diffraction, N2 sorption analysis and temperature-programmed desorption (TPD) of CO2. The XRD pattern and FTIR data confirmed the existence of the MCM-22 framework structure even after exchanging bulky metal ions and palladium loading. TPD studies using CO2 supports that the cesium and magnesium incorporated MCM-22 possess a strong and large number of basic sites. The alkali and alkaline-earth metal ions exchanged MCM-22 catalysts were explored for industrially important eugenol isomerisation, whereas the palladium containing MCM-22 materials were utilised for Heck coupling reaction of styrene with iodobenzene. The Cs-MCM-22 showed the best activity for the eugenol isomerization with the isoeugenol yield of 76%. The Cs/Pd-MCM-22 was shown as promising heterogeneous catalyst for Heck coupling reaction of styrene with iodobenzene and yield 99% stilbene. For both isomerization and Heck coupling reaction, the catalysts retain their activities even after several runs. Among different alkali and alkaline earth ion-exchanged MCM-22 materials, cesium containing MCM-22 was shown as a promising catalyst for isomerization of lignin-derived biomass model compound viz., eugenol. Subsequently, the palladium dispersed CsMCM-22 was found to be the potential catalyst for Heck coupling under ambient conditions.

Porous magnesia-alumina composite nanoparticle for biodiesel production

A facile, aerosol-based controlled synthesis of porous Mg-Al-O composite nanoparticle is demonstrated for developing solid base catalysts with high performance for transesterification of soybean oil to biodiesel. Pore size, crystallite size and chemical composition of the Mg-Al-O composite nanoparticle are tunable by design. A significantly higher surface area (by 1.8 times) and a smaller pore size (i.e., decreased by 20%) of the Mg-Al-O composite particle achieved by using the aerosol-based synthesis than the conventional method. Hybridization with Al2O3 remarkably increased surface area of the MgO particle by decreasing pore size using homogenous Al precursor or increasing pore volume via choosing heterogeneous Al precursor. The fatty acid methyl esters (FAME) yield catalyzed by Mg-Al-O composite particle was significantly higher in comparison to the results without catalysts (i.e., a maximum of 3.4×). The FAME yield was proportional to methanol-to-oil molar ratio, and the highest yield was identified at Mg/Al = 4, in accordance to the highest number of strong basic site quantified via a CO2-based temperature-programmed desorption study. Operation stability (i.e., FAME yield declined by <4%) and chemical stability (i.e., mass leaching <0.06% of total catalyst mass) were sufficiently high for the synthesized porous Mg-Al-O composite nanoparticle based on the 3-cycle test. The work establishes a prototype study of developing porous Mg-Al-O composite nanostructure by applying a gas-phase evaporation-induced self-assembly to achieve high activity and operation stability. The study also shows promise to further enhance strong basicity and corresponding catalytic activity through mechanistic understanding of the designed composite nanocatalyst.

Response surface optimization and process design for glycidol synthesis using potassium modified rice husk silica

Glycerol, an inexpensive by-product from biodiesel production can be converted into many useful products notably glycidol, which has a wide range of uses. In this study, glycidol synthesis has been done using a biowaste mediated catalyst in a single step process. Silica and potassium incorporated silica were synthesized from biowaste rice husk. These catalysts were characterized by different spectroscopic techniques. Basic sites in the catalysts were estimated using temperature-programmed desorption study. Four operational parameters were optimized using Box Behnken Design (BBD) of response surface methodology (RSM). Potassium incorporated rice husk was found to be one of the best catalysts for glycidol production with 60.8% glycerol conversion and 62.9% selectivity within one hour of reaction time.

Adsorption of Azobenzene on Hexagonal Boron Nitride Nanomesh Supported by Rh(111).

Adsorption properties of azobenzene, the prototypical molecular switch, were investigated on a hexagonal boron nitride (h-BN) monolayer (“nanomesh”) prepared on Rh(111). The h-BN layer was produced by decomposing borazine (B3N3H6) at 1000–1050 K. Temperature-programmed desorption (TPD) studies revealed that azobenzene molecules adsorbed on the “wire” and “pore” regions desorb at slightly different temperatures. Angle-resolved high-resolution electron energy loss spectroscopy (HREELS) measurements demonstrated that the first molecular layer is characterized predominantly by an adsorption geometry with the molecular plane parallel to the surface. Scanning tunneling microscopy (STM) indicated a clear preference for adsorption in the pores, manifesting a templating effect, but in some cases one-dimensional molecular stripes also form, implying attractive molecule–molecule interaction. Density functional theory (DFT) calculations provided further details regarding the adsorption energetics and bonding and confirmed the experimental findings that the molecules adsorb with the phenyl rings parallel to the surface, preferentially in the pores, and indicated also the presence of an attractive molecule–molecule interaction.

A facile route to synthesize mesoporous ZSM-5 with hexagonal arrays using P123 triblock copolymer

Mesoporous ZSM-5 samples were synthesized using Pluronic P123 triblock copolymer (EO20PO70EO20) as a supramolecular template through a pH adjusting method. The resulting materials were characterized using X-ray diffraction, transmission electron microscopy (TEM), nitrogen adsorption/desorption, scanning electron microscopy (SEM), solid-state nuclear magnetic resonance (MAS-NMR) and ammonia temperature programmed desorption (TPD) analyses. Adjusting the synthesis pH allowed obtaining a well-crystallized ZSM-5 material with meso-channels of size of 8–10 nm as calculated from non-local density functional theory (NLDFT) and also supported by TEM images. This strategy performs the growth of zeolite crystals with a mesostructured pore lattice in a cooperative manner, avoiding both a phase separation between the surfactant and zeolite units during crystallization and the formation of a separate amorphous mesoporous material. In addition, ammonia TPD studies revealed that the acidity of the mesoporous ZSM-5 material is comparable with the microporous conventional one. It was found from NH3-TPD profiles that in the case of the mesoporous ZSM-5 samples some of the strong acid sites have been transformed into weak acid sites which might be attributed to the acid sites present on the crystal external surface in mesopores.

The reaction of propylene to propylene-oxide on CeO2: An FTIR spectroscopy and temperature programmed desorption study.

The potential of CeO2 as an epoxidation catalyst is studied for the reaction of propylene with hydrogen peroxide (H2O2) by Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD). Adsorption and decomposition of H2O2 and propylene oxide (PO) are also explored to determine their surface chemistry and thermal stability. Hydrogen peroxide adsorbed dissociatively on CeO2 forming adsorbed peroxo (O-O) species, as observed through vibrational features at 890 cm-1 and (830-855) cm-1 (FTIR). The signal at 890 cm-1 disappeared when a pulse of propylene was passed through the catalyst, and at the same time, adsorbed PO was observed (a sharp IR mode at 827 cm-1; ring deformation). The reaction between gas phase propylene and adsorbed peroxide species suggested the Eley-Rideal type mechanism. The absence of a ring opening reaction of PO at room temperature may indicate that CeO2 can be a suitable oxide for epoxidation of hydrocarbons. PO started to decompose above 323 K, as observed from FTIR and TPD results. TPD spectra of PO show its desorption at 365 K, with a small fraction decomposing into acetaldehyde and formaldehyde due to partial decomposition, while CO2 and CO are released at higher temperatures. Adsorbed acetate, formate, and carbonate species, formed due to further reactions of aldehydes, are observed during the thermal reaction (FTIR).