Organosulfur adsorbents by self-assembly of titania based ternary metal oxide nanofibers

By doping with secondary and tertiary species, the electron configuration of titanium oxide can be tuned for the selective adsorption of natural gas contaminants such as thiols. In this study, we attempted to coincorporate copper group metals/oxides and lanthanum oxide within titania nanofibers via linear polycondensations of multiple metal acetate complexes. In all cases, a sol–gel synthesis in heptane allowed the nanofibers to randomly pack, forming 3 dimensional network bundles. The resulting nanostructures were characterized using electron microscopy, mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, N2 physisorption and Raman spectroscopy. Multicomponent breakthrough studies with three thiols, H2S, CO2 and CH4 show that doping a TiO2 matrix with copper group metals/ oxides and La2O3 increased the thermal stability of anatase crystallites and nanostructures. We note that Au and Ag2O accumulated on the surfaces of the doped materials, where the La2O3 doping contributed more to the materials thermal stability. The Cu and La doped material was found to be the best adsorbent for thiols with remarkably high selectivity, demonstrating potential applications in industrial gas treatment. In addition, xerogel adsorbents through the random packing of linear structures provide the advantage of a macro-porous bulk material, which is less susceptible to fouling.


Introduction
2][3][4][5][6] Conventional routes for preparing metal oxides, such as the dissolution-precipitation and high-temperature processes, provide limited control over the composition, shape and size of the resulting metal oxides.Despite tremendous efforts made in academic laboratories, [7][8][9][10] many alternative synthetic approaches are less attractive for commercialization because they are oen energy intensive or too complicated for an industrial-scale.2][13][14][15] In this context, the linear polycondensation of Ti-ligand complexes is a convenient route for synthesis of TiO 2 nanobers (Scheme 1). 16,17 one of the most studied metal oxide in the past two decades, 18 TiO 2 has found applications in photocatalysis aer doping with nitrogen, 19 and in conductive glasses aer doping with tungsten. 20Regardless of the application, in many cases it is recognized that the high performance of TiO 2 is due to the partial presence of Ti 3+ ; therefore, it is logical to introduce cations with a lower oxidation state than 4+ into the TiO 2 matrix when exploring new TiO 2 materials.In addition, the anatase phase of TiO 2 is oen more desirable for activity, but is not as thermally stable as many other widely used metal oxides, e.g., SiO 2 and Al 2 O 3 , limiting its applications under higher temperatures.
2][23] While not necessarily considered a stabilizing dopant, TiO 2 modied with the copper group metal/oxides and lanthanum have found applications in CO oxidation, dehydrogenation, 24 CO and O 2 gas sensors, 9 and photocatalysts. 25Following up on previous research with thiol adsorbents, the current study synthesized nanobrous ternary metal oxides of Cu-group/La 2 O 3 /TiO 2 via the sol-gel route and tested their selective adsorption properties.Modication of TiO 2 with other metals or metal oxides changes the electron conguration and has the potential to add multi-functionality, where this approach has been widely utilized in catalysis and semiconductor applications to achieve more desirable performance. 1,26,27Such modications can be achieved by either deposition on the surface or doping within the TiO 2 matrix with other species. 280][31] For an example, the feasibility of making modied 1D TiO 2 structures with well-distributed ZrO 2 and SnO 2 in a one-pot synthesis through simultaneous sol-gel reactions has been demonstrated. 32,33elective separation of thiols from gas streams containing H 2 S and CO 2 is of importance for the conditioning of natural and biogas uids.In the case of amine scrubbing or cyrogenic separation of H 2 S/CO 2 , thiol separation can be energetically inefficient.While zeolitic materials can be used, they are typically hygroscopic and require high regeneration temperatures to remove the water.Thus, adsorbents which are selective towards thiols can be used to isolate thiols and protect further H 2 S/CO 2 separation processes.In this area of materials research, recently Bernini et al. reported the adsorption of thiols on nano-clay modied with Fe(III) phenanthroline complexes; however, the adsorption selectivity of thiols is unknown in the H 2 S and CO 2 containing gas stream. 35Also recently, our laboratories have reported that commercial Au/TiO 2 products are good potential adsorbents for the selective adsorption of thiols while allowing H 2 S and CO 2 slip through; however, the conversion of the anatase TiO 2 support to rutile TiO 2 caused the material capacity to degrade over several thermal regeneration cycles. 34In this study, lanthanum was added in the Cu group species doped TiO 2 to promote the thermal stability via a onepot sol-gel route, which is thought to be more amenable for commercialization.The resulting materials were tested for selective adsorption of thiols in the presence of H 2 S and CO 2 .

Safety note
Because the breakthrough experiments described herein involved toxic H 2 S gas, an H 2 S detector-equipped cabinet and a KOH caustic scrubbing system were used for safety purposes.

Synthesis
Preparation of CuO/La 2 O 3 /TiO 2 .With a molar ratio of Cu : La : Ti of 1 : 3.5 : 100, the CuO/La 2 O 3 /TiO 2 material was synthesized using a sol-gel method.The synthesis was carried out in a 1000 mL three-neck round-bottom ask that was equipped with a condenser, magnetic stirrer and a heating mantle.200 mL of heptane, 6.17 g La(OAc) 3 , 1.29 g CuNO 3 -$2.5H 2 O and 186.04 g acetic acid were added, respectively, and the mixture was heated to 60 C while stirring.The solution was then charged with 158.40 g Ti(O i Pr) 4 , and the yellowish solution was stirred at 60 C until a white colored gel began to form.The gel was aged for two days, followed by eight hours of drying under vacuum and two hours of calcination at 500 C.
The Ag 2 O/La 2 O 3 /TiO 2 and Au/La 2 O 3 /TiO 2 materials were prepared using the same method that was used for the synthesis of CuO/La 2 O 3 /TiO 2 nanobers, except that 5.56 mmol AgNO 3 and HAuCl 4 were used to replace CuNO 3 $2.5H 2 O, respectively.
For comparison purposes, TiO 2 and La 2 O 3 /TiO 2 nanobers were also prepared using synthetic methods that have been reported previously. 16,364.Characterization Mass spectrometry analysis of intermediates was obtained using a Brüker Maldi-TOF Autoex III without using matrix.The N 2 isotherms, BET surface areas and mesopore size distributions of the adsorbents were determined by N 2 adsorption at 77 K (À196 C) on a 3Flex Surface Characterization Analyzer (Micromeritics).Scanning electron microscopy (SEM) images were recorded using a FEI Philips XL30 at 20 kV with platinum coating.The high-resolution transmission electron microscopy (HRTEM) images were obtained using an FEI Tecnai F20 operated at 200 kV.Sample preparation for HRTEM involved grinding to a ne powder before being placed on a nickel grid covered with carbon lm.XRD data was obtained using a Rigaku Multiex diffractometer with a copper target at a speed of 2 min À1 with a step size of 0.02 .XPS analysis was carried out using a Kratos Axis Ultra spectrometer.The binding energy was calibrated with C 1s at 284.8 eV.Electronic spectroscopic data were collected on a 5000 UV-vis Spectrophotometer (Varian) using a diffusive reectance accessory.Sample preparation for UV analysis involved mixing and grinding 50 mg of samples with 2.0 g of BaSO 4 , and then pressing the powder into a pellet.

Thiol adsorption/desorption tests
The experimental breakthrough apparatus was built in-house and has been described with our previous work. 34The gravimetrically prepared gas feed was composed of 0.5% H 2 S and 10% CO 2 (balance CH 4 ).All fresh adsorbents were pretreated at 300 C prior to the rst adsorption/desorption cycle.During the adsorption stage, the feed gas passed through a thiol permeation tube chamber, which added 112 ppm CH 3 SH, 98 ppm C 2 H 5 SH and 49 ppm i-C 3 H 7 SH to the feed.The thiol-containing uid then passed through the adsorbent bed at room temperature (22 C) with a owrate of 5 mL min À1 .Aer the adsorption bed was saturated (indicated by GC analysis results), desorption was carried out at 200 C using the thiol-free feed gas for stripping.An automatic online GC (Varian 3800, equipped with two thermal conductivity detectors and a PFPD detector) was used to analyze the feed gas mixture and the effluent concentrations every 20 min.For H 2 detection, a Varian CP-3800 equipped with a molecular sieve 5A column and a TCD detector was used (argon carrier gas at 20 mL min À1 ).

Data treatment
XPS data were treated using the CasaXPS soware (version 2.3.16).Quantication was carried out on the scans of Cu 2p, Ag 3d, Au 4f, La 3d, Ti 2p, O 1s, and C 1s.The peak areas were background-subtracted before the quantication and peak tting processing.

Sol-gel reactions and mass spectrometry
Before the sol-gel reaction process, the Cu family and La precursors were dissolved in a mixture of heptane and acetic acid.Aer addition of Ti(O i Pr) 4 , the solution was transparent with amber color, however, when Cu 2+ was present the solution was blueish.The sol formation was observed within a few hours as evidenced by a gradual increasing of the opacity of the solution, and then a gel was formed within 10 hours.During the sol-gel reactions, bubbles were occasionally observed and GC analyses of the overhead gas showed the presence of propene.Aer drying in a vacuum oven, the fragile monoliths of the xerogels were obtained.The Cu/La-doped sample was light green, while the Au/La and Ag/La-doped samples were white.Aer calcination at 500 C, the Cu/La doped sample became a darker green, the Ag/La doped sample did not change in color, and the Au/La doped sample became purple which is a sign of formation of Au particles (see ESI Fig. S1 †).
Using the so ionization technique of MALDI, the fragments of the metal-oxo-ligand complexes were analyzed aer ca. 10 min of the sol-gel reaction between acetic acid and metal precursors of Cu 2+ , La 3+ and Ti 4+ (Fig. 1).The fragments are assigned on the basis of m/z and isotope distributions as listed in Table 1.Note that the m/z peaks are overlapping due to the complexity of the three metal species.These data indicate formation of complexes with mixed metal cations (e.g., LaTi 4 -O 3 L 12 and CuLaTi 2 L 12 ), where L ¼ À OAc or À O i Pr.These La 3+ and Cu 2+ incorporated Ti-oxo-ligand intermediates condense and eventually form linear nanostructures.The formation of Tioxo-ligand complexes from the sol-gel reactions of Ti(O i Pr) 4 with acetic acid has been discussed previously, 16 as well as incorporation of Sn into Ti-oxo-ligand complexes. 33It is assumed that the small amount of doped materials in this study will not change the sol-gel chemistry signicantly, 33 and therefore the previously proposed mechanism still applies.Note that TiO 2 can be doped with up to 7% Ce. 37 As we will discuss, it appears La 3+ and Cu 2+ remain in the TiO 2 matrix even aer calcination.
The sol-gel reactions involve: (i) modication of metal precursors with acetate ligands, which are inert and protect metal cations from full oxidation; (ii) esterication and/or dehydration reactions; and (iii) hydration and condensation. 38,39g. 1 Selected m/z signals (in black) from the MALDI experiments (the peak assignments of A-I are listed in Table 2) and the typical positiveion experimental (in blue) and calculated (in red) mass-spectra.The sample was taken from the mixture of Cu(NO 3 ) 2 , La(OAc) 3 and Ti(O i Pr) 4 in heptanes at ca. 10 min after reactions with 5.5 mol equiv. of HOAc at 60 C. L ¼ À OAc or À O i Pr.Modication: Water generation: or Hydration: The hexanuclear complex, Ti 6 O 6 (OAc) 6 (OH) 6, has been found in the initial stage of sol-gel reactions and it is able to condense to linear macromolecules [Ti 6 O 9 (OAc) 6 ] m , where the latter are the building blocks of the nanobers. 16The driving force for nanober formation from the linear macromolecules is the high interfacial energy between the large macromolecules and solvents. 40

Electron microscopy
SEM images indicate that, aer grinding, the overall dimensions of the material agglomerations are on the order of several hundred micrometers, consisting of randomly orientated nanobers.The specic dimensions of the nanobers include diameters of ca.50 nm and lengths from a few hundred nanometers to micrometers (Fig. 2).The particulates appear very porous due to the void space between the bers; indeed, the apparent density of the materials is as low at ca. 0.2 g cm À3 due to the existence of macropores (>50 nm).The brous structures of the CuO/La 2 O 3 /TiO 2 material were maintained at a higher temperature (800 C).Fusion of the nanobers occurred above 800 C (see ESI Fig. S2 † and Table 2 in the section on XRD).
TEM images also show the brous structures of the materials (Fig. 3).In the high-resolution TEM images, the lattice distance of ca.0.35-0.36nm corresponds to the interplanar spacing (d-spacing) of anatase (101) planes (PDF #: 00-021-1272).Doping with a secondary metal may change this signature value (0.35 nm). 36,41,42However, the d-spacing measurement using HRTEM images depends on equipment limitations and there is variation between the sample areas examined.Hence in this research, powder XRD data (as described later) was used, which is more representative of the bulk crystalline materials.A few dark spots are (either metallic or Ag 2 O) in the range of 1-2 nm (in panels d and e) and there are gold particles on the order of 10 nm (in panel g).This was conrmed by EDS analysis equipped on the electron microscope and also by the XPS results (see Section 3.7).The anatase crystallites were less than 10 nm.It is noted that the interactions of different atoms/ domains within the ternary metal oxides would cause disorder of the crystalline phases, where defects on the surface of the materials can be benecial for catalytic applications.2 shows the anatase crystallite sizes calculated using the anatase (101) peaks and the Scherrer equation.For the samples calcined at 500 C, the doping of foreign  species resulted in a decrease of anatase crystallite size.For example, aer doping with La 3+ , the crystallite size decreased from 15.7 nm to 12.3 nm.Aer doping with Cu family elements, the crystallite size further decreased to ca. 10 nm.The same trend can be found for the samples of CuO/La 2 O 3 /TiO 2 that were calcined at 600-1000 C.This indicates that the crystallite size growth may be limited by addition of La and/or Cu species.
With the elevated calcination temperatures, the anatase crystallite sizes for both the CuO/La 2 O 3 /TiO 2 and TiO 2 materials were shown to increase.For the TiO 2 sample, the rutile phase appeared at 600 C (33%), but in the doped samples there was no rutile phase at 600 C and only 23% rutile was produced at 700 C. Addition of CuO has been found to promote the phase transformation from anatase to rutile; 43 however, these results indicate that the anatase stabilizing effect of La 2 O 3 is enough to overcome this effect.

Raman
As shown in Fig. 5 and Table 3, samples show only anatase peaks in the Raman spectra; 44 however, the peak positions and widths change slightly with doping.From Fig. 5, there are no obvious signs of anatase Raman bands decreasing aer La 3+ modication, which is further evidence that the 3.3% La 3+ was incorporated into the TiO 2 matrix rather than being partitioned to the surface as La 2 O 3 .Note that our XRD also did not show a detectable pure La 2 O 3 phase.It has been reported that Raman peak blueshi occurs upon decreased particle size, 45,46 but this does not seem to be the case here.According to the data in Table 2, the anatase crystallite sizes follow a trend of TiO 2 -500 C > La 2 O 3 /TiO 2 > Au/La 2 O 3 /TiO 2 z Ag 2 O/La 2 O 3 /TiO 2 z CuO/La 2 O 3 /TiO 2 .However, the position shi of the Raman peaks is not consistent.For example, when the CuO/La 2 O 3 /TiO 2 sample is compared with the TiO 2 sample, the anatase size is lowered from 17.3 to 9.7 nm and one E g mode blueshis from 145.8 to 150.6 cm À1 ; however, the other mode redshis from 639.5 to 635.4 cm À1 .This suggests that doping of foreign cations affects Ti-O bonds anisotropically. 44The Raman peak broadening can also be observed aer doping; however, it did  not follow a quantitative trend with the change of crystallite size.Raman peak position and broadening have been found with increased oxygen deciency in TiO 2 rutile phase. 47he CuO/La 2 O 3 /TiO 2 samples calcined at 600-1000 C were also examined with Raman, and only anatase and rutile phases were detected.These results agree well with the XRD analysis (see ESI Fig. S5 †).

N 2 physisorption
The N 2 isotherms of the CuO/La 2 O 3 /TiO 2 , Ag 2 O/La 2 O 3 /TiO 2 and Au/La 2 O 3 /TiO 2 materials are shown in Fig. 6a.According to the IUPAC (2014) classication of the isotherms, all three of these curves belong to type IV(a), indicating the presence of mesopores wider than 4 nm.The hysteresis loops fall in the categories between H2(b) and H3, suggesting a large pore size distribution and the presence of macropores. 48On the basis of the electron microscope images in Fig. 1 and 2, the mesopores result from the void space between the crystallites within the nanobers or nanober bundles, while the macropores arise from the void space between the bundles.The pore size distributions calculated using the Barrett-Joyner-Halenda (BJH) model are shown in Fig. 6b.In the pore size region less than 20 nm, the Au/La 2 O 3 / TiO 2 material shows a bimodal distribution at 6.1 and 13.9 nm, the Ag 2 O/La 2 O 3 /TiO 2 material has a wide distribution with the most abundant width at 5.4 nm, but the CuO/La 2 O 3 /TiO 2 sample exhibits one major peak at 7.2 nm.These results suggest that Au and Ag 2 O affected the mesopore structures, likely due to the formation of Au and Ag 2 O particles aer calcination of the xerogels, which is supported by the HRTEM observations.The migration of the surface Au and Ag 2 O phases to form larger domains will naturally block some mesopores.
Table 4 shows the N 2 physisorption results of the CuO/La 2 O 3 / TiO 2 material aer calcination at different temperatures.For comparison, the data for the TiO 2 material are also listed.The results show a slight improvement in thermal stability aer Cu and La doping on TiO 2 , i.e., in the range of 500-700 C the surface area is better maintained.It is noted that the pores became larger with elevated calcination temperature in the range of 500-700/800 C, but then change to smaller pores at higher temperatures.This trend is in agreement with the crystallite size observed by XRD analysis results.According to Table 2, the sizes of the dominant crystallite, rutile, decreased in the temperature range of 800-1000 C. As the particles size decreased, it can be anticipated that the average void space between the particles (estimated as pore size) decreases accordingly.

XPS analysis
According to the compositions of the materials obtained from XPS scans (Table 5), titanium and oxygen concentrations are relatively constant, but Cu, Ag, Au and La concentrations are varied.It is noted that XPS detects a few atomic layers on the solid surface, thus it is considered as a surface characterization technique.When these samples are compared, the La concentrations change in the order of CuO/La 2 O 3 /TiO 2 < Ag 2 O/La 2 O 3 / TiO 2 < Au/La 2 O 3 /TiO 2 ; however, the copper family element concentrations follow the opposite trend, and the total surface concentrations of the dopants (e.g., Cu and La) are in a relatively narrow range of 1.86-2.44%.Less Ag and Au being detected by XPS is attributed to the Au and Ag 2 O being located in the mesopores.Aer deconvolution, the O 1s peak in CuO/La 2 O 3 /TiO 2 arises from both lattice oxygen (at ca.529.2 eV, labeled as O I ) and by some oxygen defects (at ca.531.1 eV, labeled as O II ) (Fig. 7). 37n the case of Ag 2 O/La 2 O 3 /TiO 2 and Au/La 2 O 3 /TiO 2 , a third peak at a lower binding energy of 527.1 eV (labeled as O III ) is observed.It is difficult to assign these three oxygens unambiguously; however, O III in Au/La 2 O 3 /TiO 2 can be assigned to the oxygen atoms adjacent to Au particles (Au is known as electron donor).O III in Ag 2 O/La 2 O 3 /TiO 2 may be due to the oxygen in the Ag 2 O domains.The CuO/La 2 O 3 /TiO 2 sample exhibits only two synthesized peaks, and the O II peak in this sample is signicantly higher than that of the other two counterparts.We  contribute this high O II peak to the combination of doped Cu(II) and La(III).Interestingly, the CuO/La 2 O 3 /TiO 2 material showed the highest level of oxygen defects, which is in line with its highest thiol adsorption capacity which will be discussed later.Aer doping TiO 2 with La and Cu family elements, it is anticipated that binding energy of Ti(IV) would change accordingly due to the electron density variation.The binding energies of Ti(IV) and La(III) in the doped and un-doped samples are listed in Table 6.La 3+ and Cu 2+ doping results in a lower Ti 4+ binding energy, because of the electron deciency induced by incorporating La 3+ ions amongst the TiO 2 matrix.However, Ag 2 O and Au caused a slightly higher binding energy of Ti 4+ , a possible result of electron transfer from Au and Ag 2 O to Ti 4+ .There is a relatively narrow range of deviation in the positions and widths of La 3+ .Determination of the oxidation states of Cu, Ag and Au is provided later by using their binding energies.

Thiol adsorption/desorption
Fig. 8 shows the typical thiol concentration proles during two adsorption/desorption cycles when the CuO/La 2 O 3 /TiO 2 material was used as the adsorbent.The feed gas contains 112, 98 and 49 ppm for CH 3 SH, C 2 H 5 SH and i-C 3 H 7 SH, respectively.For the adsorption stage, all thiol concentrations dropped to undetectable quantities until CH 3 SH rst broke through the adsorption bed, while both H 2 S and CO 2 concentrations remained constant (0.5 and 10 mol%), indicating a selective adsorption towards thiol.The breakthrough time for thiols    gold studies and also the reverse trend with respect to vapour pressure. 34,49When the adsorbent pore structure is not restricted to the adsorbates, it seems that a longer hydrocarbon chain is favorable for both physi-and chemi-sorption due to the same cooperative dispersive forces which contribute to decreased vapour pressure.The initial breakthrough concentrations of i-C 3 H 7 SH and C 2 H 5 SH were larger than their feed concentrations, indicating a competitive adsorption or a single site adsorption model.This is a common phenomenon observed in multi-component adsorption referred to as "roll-up" and has been well documented. 50Aer i-C 3 H 7 SH saturated the adsorbent, the adsorbent bed was switched to regeneration at 200 C using a thiolfree gas mixture (feed gas still contains H 2 S and CO 2 ).The desorption process was quick (less than 20 minutes), where (i) the spike of desorbed thiols were not necessarily captured by automatic GC analysis (the GC injection interval was ca.20 min)   and (ii) the thiol concentrations quickly dropped to near zero.A lower H 2 S concentration can be clearly observed in Fig. 8 at the regeneration and feed stages: however, this is not quite denitive evidence of H 2 S replacing the adsorbed thiols during regeneration and/or when the adsorbent bed was cooled back down to room temperature (feed stage).There is a very small back pressure difference when this instrument switches modes, due to a slight difference in gas restriction when the rotary valve is turned.A similar drop in CO 2 during this regeneration time was also observed.
The thiol adsorption capacities of each adsorbent are listed in Table 7.Compared to the previously study of a commercial Au/TiO 2 adsorbent (0.46 mmol g À1 ), 34 the Au/La 2 O 3 /TiO 2 material in the current study showed less adsorption capacity for thiols, possibly due to a larger Au particle size and subsequently lower surface area on Au.Furthermore, the Ag 2 O/La 2 O 3 /TiO 2 material showed an even lower adsorption capacity than the Au/ La 2 O 3 /TiO 2 sample; however, the CuO/La 2 O 3 /TiO 2 nanobers exhibited a very high initial thiol adsorption capacity.
Using the pore volume of the CuO/La 2 O 3 /TiO 2 material, obtained from N 2 physisorption, and the density of 2-propanethiol (0.82 g mL À1 at 25 C), the calculated maximum adsorption capacity of 2-propanethiol is 4.20 mmol g À1 , comparable with the measured data in the rst two cycles (see Table 7).It is noted that the thiol adsorption capacity of CuO/La 2 O 3 /TiO 2 nanobers was less aer the rst two adsorption/desorption cycles (which can be due to fouling and/or a contribution from some irreversible chemi-sorption).Aer the rst two cycles, the capacity stabilised at $1.45 mmol g À1 in subsequent cycles.Much more extensive cycling in the presence of trace water will need to be performed before determining if this material has the stability required for an industrial application; however, it is the highest performing material tested so far in our investigations.The adsorption capacity of thiols by CuO/La 2 O 3 /TiO 2 nanobers is more than three times than that observed on Au/TiO 2 , 34 and it is comparable with the results of thiol adsorption capacity of an Fe(III) complex supported on nano-clays. 35he strong adsorption of thiols on the adsorbents was also conrmed by using SEM/EDS analysis (see ESI †).
To determine if chemical adsorption occurred in the early cycles, the effluent was further examined by a GC capable of H 2 analysis.It was found that H 2 was produced during the initial adsorption, conrming a chemical component of the adsorption prole.The generation of H 2 can be written as and where A represents adsorption sites.In the regeneration stage, the replacement of adsorbed thiols with H 2 S can be written as It is noted that chemical adsorption can only occur on the adsorption surface up to a monolayer.However, according to Table 7, more than a monolayer of thiols were adsorbed on the surface of the CuO/La 2 O 3 /TiO 2 nanobers, thus indicating that both chemi-and physi-adsorption occurred.When the pure TiO 2 nanobers were tested for thiol adsorption, no activity was observed.These results suggest that, by doping TiO 2 with CuO and La 2 O 3 ; the whole surface has become active for thiol adsorption, similar to zeolites for water and carbon dioxide.We note that these experiments have been performed in an oxygen free stream, whereas, if oxygen was present, sulde oxidation would likely be a signicant mechanism which could lead to fouling.
In order to study the impact of the adsorbed thiols, the spent adsorbents were compared with the corresponding fresh samples using XPS (Fig. 9).In this gure, all binding energy intensities for the used samples were multiplied by ten so that the peaks could be observed on the same scale as the corresponding fresh samples.The spectra show that Au was partially oxidized aer thiol adsorption, while the oxidation states of Ag + and Cu 2+ were partially reduced.A recent report has shown that adsorbed thiols can be reduced by Cu 2+ . 51In addition, CuO is known to react with H 2 S to form CuS. 52 In this study, however, aer examining S 2p binding energy of the used samples, there was no sign of formation of sulde aer 10 adsorption/ desorption cycles.Note that we have assumed that some exposure to air during transportation to the XPS would not reoxidized any potential sulde aer the adsorption experiments.An explanation for the absence of sulde is that Cu 2+ is embedded in the TiO 2 matrix.The signicant copper family peak intensity drop of these species suggests that thiolates were adsorbed onto these elements.
In order to examine the thermal stability of the adsorbents, the spent adsorbents also were characterized with XRD (see ESI Table S1 †).Compared to the fresh materials, there was no change of anatase crystallite sizes aer multiple adsorption/ desorption cycles, indicating good thermal stability of the adsorbents due to the La doping.

General comments
On the basis of adsorption capacities of thiols on samples CuO/ La 2 O 3 /TiO 2 , Ag 2 O/La 2 O 3 /TiO 2 , and Au/La 2 O 3 /TiO 2 nanobers in this research and Au/TiO 2 nanobers in our previous work, we observed that most adsorbents adsorb thiols using active domains but the newly prepared CuO/La 2 O 3 /TiO 2 material is an exception.Instead of using noble metal surfaces, the CuO/ La 2 O 3 /TiO 2 nanobers appear to adsorb thiols using a signicant portion, if not all, of the surface and micro/mesopores.There are a few possible explanations for the high performance of the CuO/La 2 O 3 /TiO 2 nanobers.First, incorporation of Cu 2+ and La 3+ within the TiO 2 matrix results in electron deciency, which causes a reduction of electron density in the bulk, and thereby increases capacity for thiol adsorption.Indeed, the oxygen defect of CuO/La 2 O 3 /TiO 2 was as high as 15%, while those of Ag 2 O/La 2 O 3 /TiO 2 and Au/La 2 O 3 /TiO 2 were 2.7 and 6.1, respectively (Table 5).In addition, Cu + /Cu 2+ redox chemistry may play a role in stabilization of the different charges of the adsorbate.

Conclusions
This research has expanded the exploration of materials formed in the one-pot synthesis of one-dimensional ternary metal oxides by sol-gel reactions of soluble metal precursors with acetic acid.We report the preparation of three materials, where copper group metals/oxides and lanthanum oxide have been coincorporated within titania nanobers in an effort to increase thermal stability and thiol selectivity.Electron microscopy revealed randomly orientated brous nanostructures, allowing formation of both mesopore and macropore in all materials studied.While mesopores insured a relatively high surface area, the macropores allow for a design with a low pressure drop and less susceptibility for fouling, which is essential for many industrial applications.The mass-spectra of the solution at the initial sol-gel reaction stage showed evidence of multiple metal species in one metals-oxo-acetate complex, leading to a homogeneous distribution of the metal species in the linear macromolecules in the xerogels (before calcination).The characterization of the ternary metal CuO/La 2 O 3 /TiO 2 material showed evidence of Cu 2+ and La 3+ being doped within the TiO 2 matrix versus partitioning to the surface.Au existed as metallic nanoparticles at the surface and Ag 2 O also seemed to be isolated from the La-doped TiO 2 matrix.While all three of the doped TiO 2 nanober materials were found to selectively adsorb thiols from an H 2 S containing gas stream and were regenerable at mild conditions (200 C), the CuO/La 2 O 3 /TiO 2 material showed much larger adsorption capacity when compared to any of the other samples prepared in the current study.The thiol adsorption capacity of CuO/La 2 O 3 /TiO 2 (1.45 mmol g À1 ) is more than three times than that of Au/TiO 2 (0.46 mmol g À1 ) under the same conditions.Future work will involve the investigation of the effect of dopant concentration on adsorption capacity in an effort to maximize the performance of these materials and the long term adsorption/desorption cycling in the presence of water.

23
Fig. 4 shows the XRD patterns of the CuO/La 2 O 3 /TiO 2 , Ag 2 O/ La 2 O 3 /TiO 2 and Au/La 2 O 3 /TiO 2 materials.From these samples only TiO 2 anatase phase and Au crystals were detected.Absence of CuO, Ag 2 O and La 2 O 3 suggest that (i) the minor materials are amorphous, (ii) the dopants are incorporated into the TiO 2 nanober crystal phase, or (iii) they are below the detection limit of XRD.Table2shows the anatase crystallite sizes calculated using the anatase (101) peaks and the Scherrer equation.For the samples calcined at 500 C, the doping of foreign

Fig. 3
Fig. 3 TEM images of CuO/La 2 O 3 /TiO 2 (a and b), Ag 2 O/La 2 O 3 /TiO 2 (c and d), and Au/La 2 O 3 /TiO 2 (e and f).The white arrows in panels (c, d and e) show the metallic particles.

a
Molar ratio of Cu (or Ag/Au) in percentage.b Normalized O I peak centered at ca. 529.2 AE 0.5 eV corresponding to the lattice oxygen O 2À .c Normalized O II peak centered at ca. 531.1 AE 0.5 eV corresponding to the oxygen defects.d Normalized O III peak centered at 527.1 AE 0.5 eV c. followed a trend of i-C 3 H 7 SH > C 2 H 5 SH > CH 3 SH, indicating that i-C 3 H 7 SH has the strongest adsorption capacity.These ndings match thiol adsorption trends observed on previous

Fig. 7
Fig. 7 Deconvolution of O 1s peaks in the samples of CuO/La 2 O 3 /TiO 2 (a), Ag 2 O/La 2 O 3 /TiO 2 (b) and Au/La 2 O 3 /TiO 2 (c).O I , O II , and O III are contributed by lattice oxygen, oxygen defects by doping, and electron rich oxygen, respectively.

Fig. 8
Fig.8The gas-phase concentrations for three thiols in a synthetic sour gas stream for two adsorption and desorption cycles.The left axis is the concentration of CH 3 SH (blue), C 2 H 5 SH (red) and i-C 3 H 7 SH (green).The right axis is the H 2 S concentration (purple).

Table 1
Proposed molecular fragments corresponding to the mass spectra in Fig.1.L ¼ À OAc or À O i Pr

Table 2
Anatase (101) position and the Scherrer crystallite size The crystallite diameters were estimated using Scherrer equation and powder XRD data (estimated uncertainty of 0.4 nm).b Rutile (110).c The number in the bracket shows the crystallite size of the corresponding undoped TiO 2 nanobers (see ESI).

Table 3
Wave numbers and full width half maximum (in parenthesis) of the fresh samples measured by Raman

Table 4
The specific BET surface area, pore size and pore volume of CuO/La 2 O 3 /TiO 2 and TiO 2 nanofibers calcined at 500-1000 C a Adsorption average pore diameter.b Single point adsorption total volume.

Table 5
Quantification results of XPS analysis

Table 6
Binding energy and full width at half maximum of Ti 2p and La 3d a Binding energy.b Binding energy difference between the doped and neat TiO 2 .c Full width at half maximum.

Table 7
Adsorption capacity for thiols in a sour gas on CuO/La 2 O 3 / TiO 2 , Ag 2 O/La 2 O 3 /TiO 2 and Au/La 2 O 3 /TiO 2 nanofibers with selected regeneration temperatures a Aer 3 cycles, the capacity stabilized at $1.45 mmol g À1 level.Brackets contain the mass of adsorbent in the bed.