In Situ FTIR Spectroscopy for Scanning Accessible Active Sites in Defect-Engineered UiO-66.

Evolution of acid and basic sites in UiO-66 and UiO-66-NH2 metal-organic frameworks: FTIR study by probe molecules

Low temperature CO adsorption on a Pd-H-ZSM-5 sample results in formation of Pd3+(CO)2 species (2221 and 2200 cm−1). The dicarbonyl structure has been proven by coadsorption of 12CO and 13CO: mixed Pd3+(13CO)(12CO) species absorb at 2210 and 2158 cm−1. Evacuation of the sample at increasing temperatures leads to reduction of palladium and formation of different (poly)carbonyls of Pd2+, followed by Pd+–CO (2126 cm−1) and Pd0–CO (2105 cm−1) species. Adsorption of NO on the activated sample leads mainly to formation of Pd2+–NO linear nitrosyls (1881 cm−1), a small amount of NO+ (2137 cm−1), and Pd2+(H2O)(NO) species (1839 cm−1). Coadsorption of NO and O2 results in formation of (i) species typical of the H-ZSM-5 support (NO+, N2O3, NO2 and N2O4) and (ii) nitrates formed with the participation of palladium. Coadsorption of NO and excess CO results in appearance of mixed carbonyl–nitrosyl species of Pd2+ (νCO at 2146 cm−1 and νNO at 1813–1791 cm−1). The latter are decomposed easily losing their CO ligand. The results suggest a high coordinative unsaturation of the cationic palladium sites in Pd-H-ZSM-5.

Low temperature CO adsorption followed by IR spectroscopy (IR/CO) characterization was used to depict the MoS2 morphology change with sulfidation temperature on MoS2/Al2O3 catalyst. It is found that the morphology of MoS2 slabs on MoS2/Al2O3 catalyst under typical sulfidation temperature range (573 to 723 K) is a truncated triangle exposing both the M-edge and S-edge. Moreover, the IR/CO data indicate that the truncation degree (ratio of S-edge/M-edge) of MoS2 slabs gradually increases with increasing sulfidation temperature. This finding is in line with density functional theory calculation on model catalysts, providing IR evidence of MoS2 morphology change with sulfidation temperature on an Al2O3-supported catalyst. As a further step, it is also found that the MoS2 morphology is strongly influenced by MoS2–Al2O3 interactions under the same sulfidation temperature.

Apparent surface coverage (ASC) of a series of tungsten-alumina catalysts was determined by the electrophoretic migration method (EM). A linear increase in W coverage with increasing W loading (up to 13.1 wt.% W) was observed. The results were in close agreement with those obtained previously by ion scattering spectroscopy (ISS) and low temperature CO adsorption.

FTIR Study of Low-Temperature CO Adsorption on Cu-ZSM-5: Evidence of the Formation of Cu2+(CO)2 Species

Two Au/TiO2 samples with different gold loadings (0.7 and 4.0 wt.% Au) were prepared by deposition-precipitation with urea and calcined at 673 K. TEM revealed gold particles of 3.2 and 3.9 nm for the 0.7 and 4.0 wt.% samples, respectively. The samples were subjected to different red-ox treatments and then the state of gold was determined by the FTIR spectra of CO adsorbed at low temperature. Several kinds of gold carbonyl species were detected during the experiments: (i) Au0–CO at around 2107 cm−1; (ii) Au+–CO at ca. 2175 cm−1; (iii) Auδ+–CO in the region of 2140–2137 cm−1 and (iv) Auδ′+–CO (δ′ > δ) at around 2155 cm−1. The 4.0 wt.% sample contained mainly metallic gold after evacuation at 673 K. Subsequent interaction with oxygen at 373 K leads to oxidation of a fraction of the surface metallic gold sites to Auδ+ sites. These sites were considered as cations located on the surface of the metal particles with a partially positive charge δ+ (0 < δ < 1) because of electron transfer from the gold bulk. Evacuation at 673 K leads to back reduction of the Auδ+ sites to metallic gold. The oxidation of gold particles was more efficient when performed with a NO + O2 mixture. It resulted in creation of Auδ′+ sites with a higher positive charge than that of the Auδ+ sites. In this case the oxidation involved a higher number of Au0 sites. A similar treatment of the 0.7 wt.% Au sample, however, resulted in formation of “isolated” Au+ species. The results indicate that small metal particles are more easily oxidized by a NO + O2 mixture. A model of the formation of the different sites, explaining well the experimental results, is proposed.

Two infrared OH stretching bands are produced after interaction of CO with H-ZSM-5 bridging hydroxyls. These two bands are not due to heterogeneity of the acid sites but arise from Fermi-resonance.

Developing efficient, stable, and inexpensive catalysts for the preferential CO oxidation in H₂ (CO-PROX) over a wide temperature range in the presence of CO₂ and H₂O is indispensable for the hydrogen purification process. Herein, CuO was introduced to the CeMnO₂-supported Pt catalyst to modulate the oxygen activation capacity and provide the available number of active sites in CO-PROX. One part of the CuO species doped into CeO₂ strongly interacts with Ce, thus enhancing the oxygen transfer capacity of the catalyst. The other part of CuO species located on the surface of the catalyst provides extra Cu⁺ sites available for low-temperature CO adsorption. This synergistic interaction with Pt sites further enhances CO and O₂ activation, broadening the temperature window of high activity. The optimal Pt-10CuO/CeMnO₂ catalyst exhibits complete CO conversion (CO/O₂ ratio of 1:1) within the practical temperature range of 130–190 °C, even in the presence of CO₂ and H₂O, and remains stable at 150 °C for 76 h testing without any deactivation. This work will give a novel approach for the design of highly efficient inexpensive catalysts for industrial preferential oxidation of CO in H₂, especially in the presence of CO₂ and H₂O.

Adsorption of IR probe molecules (CO and NO) has been used for characterization of a Pt-Cr(2)O(3)/WO(3)-ZrO(2) sample. For better assignments of the results obtained 'blank' experiments with WO(3)-ZrO(2), Pt/WO(3)-ZrO(2) and Cr(2)O(3)/WO(3)-ZrO(2) were also performed. The Cr(2)O(3)/WO(3)-ZrO(2) sample was prepared via sulfate route and surface sulfates were detected on the activated sample. These species keep ca. 50% of their initial concentration even after reduction by hydrogen at 773 K. The amount of Lewis acid sites on the activated sample, as measured by CO adsorption, is relatively low and is associated mainly with coordiantively unsaturated (c.u.s.) Zr(4+) cations. Reduction of the sample generates Cr(3+) ions and the concentration of the Lewis acid sites continuously increases with the reduction temperature up to 773 K. At this reduction temperature, however, the strength of the acid sites decreases due to partial removal of the sulfates. While CO is not an efficient probe to discriminate between Zr(4+) and Cr(3+) sites, NO is selectively strongly adsorbed on Cr(3+) cations which allows a more precise monitoring of the evolution of the Cr(3+) sites during reduction. The activated Pt-Cr(2)O(3)/WO(3)-ZrO(2) sample also shows presence of sulfates but these species are practically absent after reduction with hydrogen at 773 K. It was found that platinum promotes the reduction of chromium and tungsten species as well. CO adsorption on activated sample reveals the existence of cationic platinum. However, reduction with hydrogen even at room temperature generates metallic Pt. Its amount continuously increases with the reduction temperature up to 573 K and then, due to the SMSI effect and/or agglomeration, the amount of platinum accessible to adsorption decreases. Comparison with the results obtained with the Pt/WO(3)-ZrO(2) sample shows that chromium and/or sulfates stabilize the cationic form of platinum.

Low temperature adsorption of CO on Pt(111): disequilibrium and the occupation of three-fold hollow sites

Preparation and enhanced CO2 adsorption capacity of UiO-66/graphene oxide composites

Simulated diesel oxidation catalysts (DOCs) consisting of 2.5% Pd were prepared on γ-Al2O3 and lanthana-stabilized γ-Al2O3; it was found that the La-containing catalyst had higher CO conversion and lower onset temperature for CO oxidation (∼100 °C). Aberration-corrected STEM showed that the La–alumina support helped to stabilize Pd in smaller particles and clusters, increasing dispersion from 17 to 26%. The higher dispersion was responsible, in part, for the improved CO oxidation rate; at 140 °C, the turnover frequency (TOF) was improved from 0.0019 to 0.0095 s–1 with the addition of La. This TOF increase appears to be tied to facile redox behavior of the Pd/La–alumina catalyst, which was evident in the results of in situ X-ray absorption spectroscopy (XAS) and FTIR spectroscopy. In these experiments, both catalysts were calcined at 500 °C to form PdO and then reduced to Pd metal at 140 °C in the presence of CO. When the CO-covered catalyst was exposed to CO oxidation reaction conditions at 140 °C, the 2.5% Pd/Al2O3 catalyst remained nearly fully reduced, and the surface coverage of CO did not change, indicating irreversible CO adsorption and very low reactivity toward oxygen. On the other hand, the more active 2.5% Pd/La–Al2O3 catalyst was more reactive toward oxygen, with a portion of the Pd becoming oxidized when the gas phase was switched from pure CO to the reaction mixture. There was a drop in surface coverage of CO when switching from pure CO to the reaction mixture on the Pd/La–alumina. The results suggest that the role of the La–alumina support is 2-fold, increasing the dispersion of Pd by forming small, stable Pd particles and allowing a portion of the Pd to exhibit facile redox behavior at low temperatures, making the Pd less susceptible to poisoning by CO. This work provides insights into factors that could lead to improved low-temperature CO oxidation reactivity in Pd-based automotive exhaust catalysts.

Spatially resolved operando HERFD-XANES (high energy resolution fluorescence detected X-ray absorption near edge structure) complemented by CO concentration gradient profiles along the catalyst bed (SpaciPro) was used to identify the dominant reaction paths for the low and high temperature CO oxidation on Pt/CeO2 and Pt/Al2O3. At low temperatures, features associated with CO adsorption on Pt were found for both catalysts. During the oxidation reaction light-off, the evolution of the spectral and catalytic profile diverged along the catalyst bed. The CO oxidation rate was high on Pt/CeO2 from the beginning of the catalyst bed with CO being adsorbed on Pt, whereas low CO conversion due to strong CO poisoning was found on Pt/Al2O3. This correlation of the CO concentration gradient with unique insight by HERFD-XANES gave direct proof of the crucial contribution of the Pt-CeO2 perimeter sites overcoming the CO self-inhibition effect at low temperatures.

Using STM, infrared absorption reflection spectroscopy experiments and density functional calculations we show that low temperature adsorption of CO on gold surfaces modified by vacancy islands leads to morphological changes and the formation of nanosized Au particles. These results demonstrate a dynamic response of a surface during adsorption with consequences for the surface reactivity.

Synthesis and doping of alkali metals on MOF-74 for CO2 and CH4 pure and binary mixtures adsorption