A series of mesoporous chromium-containing silica tube molecular sieves (CrMCM-41) with variable Si/Cr ratios have been synthesized and characterized by powder X-ray diffraction (XRD), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance UV-visible spectroscopy (UVvis), electron spin resonance (ESR), electron spin-echo modulation (ESEM), Raman spectroscopy, 29 Si magicangle-spinning nuclear magnetic resonance (MAS NMR), and N 2 adsorption measurements. XRD, EPMA, UV-vis, and ESR show that the as-synthesized CrMCM-41 materials have the MCM-41 structure and contain only atomically dispersed Cr(III). FTIR, UV-vis, and Raman reveal that Cr(VI) monochromate exists in calcined CrMCM-41. 29 Si MAS NMR and N 2 adsorption show that part of the chromium is incorporated into the MCM-41 structure. ESR shows that Cr(VI)-O 2exists in calcined CrMCM-41 and transforms to Cr(V) after evacuation above 150 °C. The interaction of Cr(V) with O 2 , CO, C 2 D 4 , ND 3 , CD 3 OH, and D 2 O was studied. Cr(V) occurs at two different sites. At one site, Cr(V) coordinates with one molecule of O 2 , CO, and D 2 O to form square pyramidal complexes and reacts with C 2 D 4 , CD 3 OH, and ND 3 to be reduced to Cr(III). At the other site, Cr(V) is inert to O 2 , CO, and C 2 D 4 while it coordinates with two molecules of ND 3 to form six-coordinated complexes and coordinates with one molecule of CD 3 OH.

CrAPSO-5 molecular sieve was synthesized to incorporate CrIII into the molecular sieve framework using tripropylamine as a structure-directing agent. CrAPSO-5 was characterized by XRD, thermogravimetric analysis and IR spectroscopy. Treatment of as-synthesized CrAPSO-5 with a solution of is CH 3 OHÈHCl e †ective in removing the tripropylamine. EPR shows CrIII with and which is g 1 \ 5.20, g 2 \ 2.00 g 3 \ 0.98, assigned to CrIII in a distorted octahedral coordination. Spectral simulation indicates a zero-Ðeld splitting parameter of 0.50 cm~1. CrIII in ion-exchanged (L)Cr-SAPO-5 also shows octahedral coordination and is located in the main channel. After dehydration, CrIII shows sharper EPR lines with and g M \ 4.00 g A \ 2.00 which are assigned to CrIII in tetrahedral coordination. After calcination and dehydration, the EPR of CrAPSO-5 shows CrV in tetrahedral coordination, characterized by and UVÈVIS spectra g M \ 1.97 g A \ 1.89. support these coordination assignments. Electron spin echo modulation (ESEM) of 31P indicates that CrV substitutes for phosphorus in a framework site in calcined CrAPSO-5 and that CrV is in a six-ring window in dehydrated, oxidized (L)Cr-SAPO-5. Deuterium ESEM indicates, that, in CrAPSO-5, CrV interacts with one molecule of ethylene via a weak r-bond, while in dehydrated, oxidized (L)Cr-SAPO-5, CrV interacts with one molecule of ethylene via a p-bond. These results seem consistent with framework substitution in a phosphorus site in CrAPSO-5.

The increased usage of hydrogen as a next generation clean fuel strongly demands the parallel development of room temperature and low power hydrogen sensors for their safety operation. In this work, we report strong evidence for preferential hydrogen adsorption at edge-sites in an edge oriented vertically aligned 3-D network of MoS 2 flakes at room temperature. The vertically aligned edge-oriented MoS 2 flakes were synthesised by a modified CVD process on a SiO 2 /Si substrate and confirmed by Scanning Electron Microscopy. Raman spectroscopy and PL spectroscopy reveal the signature of few-layer MoS 2 flakes in the sample. The sensor's performance was tested from room temperature to 150 C for 1% hydrogen concentration. The device shows a fast response of 14.3 s even at room temperature. The sensitivity of the device strongly depends on temperature and increases from $1% to $11% as temperature increases. A detail hydrogen sensing mechanism was proposed based on the preferential hydrogen adsorption at MoS 2 edge sites. The proposed gas sensing mechanism was verified by depositing $2-3 nm of ZnO on top of the MoS 2 flakes that partially passivated the edge sites. We found a decrease in the relative response of MoS 2-ZnO hybrid structures. This study provides a strong experimental evidence for the role of MoS 2 edge-sites in the fast hydrogen sensing and a step closer towards room temperature, low power (0.3 mW), hydrogen sensor development.

The local environment of the chromium ion in calcined CrAPSO-11, which may involve framework sites, is compared with the Cr ion environment in solid-state ion-exchanged (S)Cr−SAPO-11 which involves ion exchange into nonframework sites at high temperature. Powder X-ray diffraction confirms that CrAPSO-11 has the SAPO-11 framework and is highly crystalline. The 27Al and 29Si magic-angle-spinning nuclear magnetic resonance spectra are similar to those of corresponding SAPO-11 showing only one type of tetrahedral atom. As-synthesized CrAPSO-11 shows an ESR spectrum assignable to Cr(III), but on calcination this converts to Cr(V). (S)Cr−SAPO-11 forms Cr(V) directly since solid-state ion exchange occurs at high temperature. The UV−vis spectrum of calcined, hydrated CrAPSO-11 is assigned to Cr(V), and after dehydration the coordination for Cr(V) seems consistent with tetrahedral as expected for a framework site. Electron spin resonance (ESR) of calcined, hydrated CrAPSO-11 shows Cr(V) which is consistent with square-pyramidal coordination which gradually converts to Cr(V) in distorted tetrahedral coordination upon dehydration by heating in vacuum. The ESR spectrum of ion-exchanged (S)Cr−SAPO-11 shows Cr(V), but it does not convert to tetrahedral coordination upon heating in vacuum. Hydrogen reduction shows that Cr(V) in (S)Cr−SAPO-11 can be reduced to a several hundred gauss broad ESR signal assignable to Cr(III). Reduction by H2 of calcined CrAPSO-11 does not produce Cr(III) observable by ESR. The interactions between Cr(V) in calcined CrAPSO-11 and (S)Cr−SAPO-11 with D2O, CH3OD, CD3OH, ND3, and pyridine adsorbates are also compared. Differences in the kinetics of absorbate coordination between calcined CrAPSO-11 and (S)Cr−APSO-11 are observed. All these differences support different Cr(V) sites in these two materials. 31P electron spin echo modulation of CrAPSO-11 shows that Cr(V) is surrounded by about 11−12 phosphorus nuclei at 0.58 nm, which is consistent with Cr(V) in CrAPSO-11 being in a framework phosphorus position.

A series of mesoporous siliceous MCM-41 (SiMCM-41) molecular sieves with silicon partially substituted by Al, Ti, and Zr were synthesized hydrothermally. A combination of electron spin resonance (ESR), diffuse reflectance UV−vis spectroscopy (UV−vis), Raman spectroscopy, and temperature-programmed reduction (TPR) was used successfully to characterize the chemical environment of chromium in Cr/MeMCM-41 (Me = Al, Ti, and Zr). UV−vis, ESR, and Raman spectroscopy show that Cr2O3 is present in SiMCM-41, ZrMCM-41, and AlMCM-41. With increasing Al content in AlMCM-41, the amount of Cr2O3 increases, which contrasts with Cr/SiO2·Al2O3. Cr2O3 is reduced in Cr/TiMCM-41 with a Si/Ti ratio of less than 25 compared to higher ratios. The results reveal a strong interaction between chromium and surface titanium centers, which immobilizes the chromium species and reduces the formation of Cr2O3. UV−vis and Raman spectra show that Cr/MeMCM-41 materials are dominated by dichromate. The overall intensity of Cr(V) increases in the order AlMCM-41 < SiMCM-41 < TiMCM-41 < ZrMCM-41. The intensity of Cr2O3 increases in the order of TiMCM-41 < ZrMCM-41 < SiMCM-41 < AlMCM-41. These results show that Ti and Zr stabilize Cr(V) and Cr(VI) compared to Al and Si.