Probing the H2-Induced Restructuring of Pt Nanoclusters by H2-TPD

Subscriber access provided by UNIV OF WESTERN ONTARIO

Article

Probing the H2-induced restructuring of Pt nanoclusters by H2 TPD Olga A. Yakovina, and Alexander S Lisitsyn Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02847 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Probing the H2-induced restructuring of Pt nanoclusters by H2 TPD

Olga A. Yakovina, Alexander S. Lisitsyn*

Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russian Federation

__________________________________________ * Corresponding author. Tel/Fax: +7 383 3269529. E-mail address: [email protected]

1 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Metal clusters with sizes below 1 nm excite great scientific interest, but the main information on their properties still comes from quantum mechanics modeling and costly physical methods of limited availability. We have studied ultradispersed Pt/γ-Al2O3 samples with TPD and complementary adsorption/desorption techniques and observed a strong influence of H2treatment conditions (PH2 ≤ 1 bar; 200 K ≤ T ≤ 470 K) on the H2-TPD profile of Pt/γ-Al2O3. The results corroborate recent theoretical and spectroscopic studies predicting alterations in the structure of Pt nanoclusters under H2 but reveal that the restructuring needs to overcome continuous activation barriers and leads both to an increase in surface coverage and strengthening of Pt–H bonds. This was interpreted as being a consequence of the strong interaction of Pt clusters with the support. The results extend insights into the behavior of supported metal particles and expand the potential of existing experimental techniques.

KEY WORDS Supported catalysts; Nanoclusters; Activated adsorption; Hydrogen-induced effects; Metal– adsorbate interaction; Metal–support interaction.


Page 2 of 37

Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

INTRODUCTION Among the fields that experienced impressive advances, nanoscience and nanotechnology has been undergoing the most explosive growth.1 Particularly, quantum confinement effects impart new physical and chemical properties to metal nanoparticles of less than 2 nm size (so-called “nanoclusters”), thus opening avenues of diverse applications in very different fields.2 However, such tiny objects usually escape detection by conventional techniques and are extremely difficult for study even with modern powerful methods. The high fraction of undercoordinated surface atoms (near 100 %) makes metal clusters highly sensitive to environment, and the recent studies with advanced computational and instrumental techniques3-5 have revealed bond strains, large bond-length disorder, and other anomalies in Pt clusters,6,7 as well as dynamic changes in electronic and geometric structures under the influence of adsorbate and underlying support.8-12 It has direct relevance to many applications, in particular to catalysis, as catalytic performance depends markedly on the size and morphology of metal particles.13 H2-induced effects deserve special attention, as H2 is a reagent or product in the main part of catalytic processes. Unfortunately, the hydrogen is barely detectable by spectroscopic methods, and it lends still greater importance to alternative means. H2 TPD is one of the very few available tools that may help in better understanding the clusters–hydrogen interactions. It is sensitive to the state of metal surface and has become indispensable as a technique in surface-science studies with metal films and single crystals. Here, we have tried to ascertain whether it is possible to follow changes in cluster’s structure by this method and investigated the possible effect from H2-treatment conditions. The experiments were performed with Pt/γ-Al2O3, as it contains subnanometer-sized Pt clusters and is a prototype of most important industrial catalysts.14-16 H2-TPD technique has been widely used in studies of Pt/γ-Al2O3,17-25 but surprisingly little has been learned so far about the theoretically predicted structural transformations of Pt particles.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

EXPERIMENTAL If not indicated otherwise, the supported Pt samples were prepared via adsorption of H2PtCl6 onto γ-Al2O3, followed by in situ calcination (O2, 770 K) and reduction (H2, 570 K). Gas adsorption/desorption experiments were performed on an AutoChem II 2920 (Micromeritics), using gases of a >99.999 % purity. H2-treatment temperatures (TTR) were in the range of 200 K ≤ TTR < 470 K. In experiments on H2-TPD, H2-pretreated sample was purged by Ar at 300 K for 15 min and then heated to 770 K at nominal heating rate of 50 K/min (Ar flow rate 25 cc/min, sample mass 400 mg). If H2 desorption at sub-ambient temperatures was intended, the sample was finally treated by H2 and Ar at 200 K, and the desorption commenced by replacing the cooling mixture with room-temperature water (no temperature-programming). Experimental details and other methods are described in Supporting Information S1-S4.

RESULTS Preliminary examination of Pt/γ-Al2O3 samples by TEM, EDX, XRD, XRF, chemisorption (H2, O2, CO), TPR and TPD confirmed the presence of ultra-small Pt clusters (≤ 1 nm), their resistance to sintering, high purity of the samples and the absence of interference from foreign species during multiple H2-TPD runs (Supporting Information S5). Figure 1 illustrates H2 adsorption on Pt/γ-Al2O3 when the sample is treated by H2 in pulse mode. As expected for instantaneous and strong adsorption, the sample completely absorbs the first several doses of H2 (Figure 1a), and the hydrogen predosed in sub-monolayer quantities desorbs only at high temperatures (Figure 1b). Nevertheless, the amount of H2 adsorbed (Va) was found dependent on H2-pulse size (see H2-TPD results in Figure 2a). Moreover, the measurements of Va with pulse technique showed an expected decrease of Va at elevated temperatures, but the value of Va at sub-ambient temperatures could also decrease (Figure 2b). Figure 3 shows changes in H2-TPD curves after treatments under an H2 flow at different temperatures (TTR). Following heat treatments (10 min in each case), the sample was cooled to 4 ACS Paragon Plus Environment

Page 4 of 37

Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

300 K under H2 and then purged by Ar; the treatments at TTR < 300 K were followed by purging the sample with Ar at the same temperature and heating to 300 K in Ar. One can see in Figure 3a that the amount of H2 desorbed (Vd,300→770K) increases with TTR. Further increasing of TTR, from 370 to 470 K (Figure 3b), marginally influences the Vd value but radically alters the curve shape – the high-temperature desorption peak starts to grow in intensity at the expense of the first one. These trends were affected neither by Ar-purging time before TPD nor heating rate during TPD, and the results were in line with O2-titration data for the samples (Supporting Information S6). Figure 4 provides information about weakly bound hydrogen, which desorbs at 300 K but withstands Ar purge at 200 K. Before the first run shown in Figure 4a, the sample was degassed in Ar at 770 K and then treated at 200 K consecutively in flowing H2 (5 min) and Ar (10 min); following the heating to 300 K during run 1, the sample was cooled again to 200 K (in Ar) and retreated by H2 and Ar under exactly the same conditions. Nevertheless, one can see in Figure 4a that the sample desorbs considerably more H2 in the second run as compared to the first one. Longer treatments under the H2 and Ar flows at 200 K had little effect, but H2 treatments in pulse mode resulted in smaller Vd values (Supporting Information S7). In another series of consecutive runs (Figure 4b), the retreatment by H2 was performed at progressively increasing temperatures (TTR) and was followed by cooling the sample to 200 K in flowing H2. It is seen in Figure 4b that H2 desorption at 200→300 K responds strongly to TTR changes – desorption “tail” progressively vanishes with TTR, and only most weakly bound hydrogen contributes to Vd value at TTR ≥ 370 K. Moreover, the peak height initially decreases with TTR (up to ~370 K) but then increases (370 K < TTR < 470 K; the curves at TTR of 470 and 520 K in Figure 4b practically coincide). In those experiments, the thermal treatments under H2 were followed by ramp-down in the same gas to ambient temperature, with equilibration for 5 min, and then to 200 K. Interestingly, if after the first H2-desorption at 200→300 K we repeated the cooling under H2 from 300 to 200 K to repeat the desorption, there could be appreciable difference between the desorption traces in 5 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the two runs, with the difference being dependent on initial TTR temperature (Figure 4c). In further runs under such conditions, the resulting H2-desorption traces were indistinguishable from that in the second run (Supporting Information S7). All the H2-induced changes could be completely eliminated through thermal desorption of hydrogen, and the degassing in Ar at 770 K allowed the experimental series shown in Figures 14 to be performed in a random order but with essentially the same results. Room-temperature O2titration of adsorbed hydrogen, with subsequent H2-treatments at equal conditions, also provided similar properties for the samples, regardless of previous treatments (Supporting Information S8). The behavior of the samples before and after the titrations was not the same, however. One can see an example in Figure 5, which shows low-temperature desorption from Pt/γ-Al2O3 in four consecutive runs, with equal H2 treatments before each of the runs (flowing H2, 5 min at 300 K, followed by cooling under H2 to 200 K). It should be recalled that analogous experiments with only H2/Ar treatments between the runs showed some difference for the first desorption trace but similarity for the traces in further runs. However, it is seen in Figure 5 that the O2/H2 titrations after the second run initiated further changes, and the sample moves to the state more characteristic for higher TTR (cf. curves in Figure 5 and curves 1-3 in Figure 4b). The decrease in Vd,200→300K value after the titrations was accompanied by an increase in the amount of H2 that desorbs at 300→700 K (Supporting Information S8; see also other examples therein). Pt/γ-Al2O3 samples with different Pt loadings and on the support from different sources showed similar trends, but comparative experiments with Pt/SiO2 samples containing Pt nanoparticles of 2-3 nm in size showed little effect of H2-treatment conditions in the given case (Supporting Information S9).


Page 6 of 37

Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

DISCUSSION

Activated adsorption. Hydrogen molecule does not need activation to dissociate on extended Pt surfaces,26 and the exposure of single crystals and clean metal films to 1-100 Langmuir of H2 at ~100 K is usually sufficient to saturate the surface and have the expected H/Ptsurf ratio of 1.27,28 (1 L corresponds to an exposure of 10−6 torr during one second.) Longer exposures may be necessary to complete the process at still lower temperatures (e.g., >1000 L at 35 K)29, but it only concerns weakly adsorbed species. The gradual filling of less profitable adsorption sites (atop or two-fold sites, where the hydrogen atoms made fewer contacts with metal atoms) and the increasing role of lateral interactions cause weakening of Pt–H bonds with hydrogen coverage, especially in the case of metal nanoparticles, which surface is less ordered.28 This results in broadening of TPD patterns and appearance of more or less distinct peaks in TPD curves.21,30,31 On one hand, the alumina-supported nanoclusters behave similarly (Figure 1), but on the other one, their behavior is different (Figures 2-5). In particular, the observations in Figure 2a show that Pt/γ-Al2O3 samples retain more hydrogen if pretreated at a higher H2 pressure (larger pulse size or, especially, permanent H2 flow). Analogous effect was mentioned in one of previous papers.21 It should not be confused with the Va–PH2 relationship characteristic of adsorption isotherms, which are measured at varying H2 pressure; the data in Figure 2 relate to samples that were purged by Ar after H2 treatments, and Ar-purging time did not influence the difference between H2-TPD curves (Supporting Information S6). Note also that H2-treatment conditions in the given range did not affect properties of SiO2-supported samples (Supporting Information S9). There is an even stronger effect from temperature. According to the data in Figure 2b, H2 pulse chemisorption at ambient temperature results in H/Pt ratio exceeding unity, thus indicating an increased affinity of Pt nanoclusters towards hydrogen. It is in line with many observations for highly-dispersed Pt catalysts in the literature.28,32,33 Nevertheless, it follows from the results 7 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in Figure 3 (cf. Vd values therein) that the amount of hydrogen adsorbed can be further increased upon treating the sample in flowing H2 and combining the treatment with thermal activation. With account of weakly adsorbed hydrogen (which withstands Ar purging at 200 K, as in Figure 4), we could have H/Pt ratios up to 1.8 for 1%Pt/γ-Al2O3 (~10 % more for 0.5%Pt/γ-Al2O3 and 10 % less for 2.5%Pt/γ-Al2O3). It is near the H/Pt values of 1.9-2.1 that were recently derived from XANES data for a 1%Pt/γ-Al2O3 sample under 1-20 atm of H2.34 One might expect that the increase in hydrogen coverage would increase the fraction of weakly bound hydrogen (as in Figure 1b). However, it does not correspond to observations in Figures 3 and 4. The strong shift in H2 desorption to higher temperatures in Figure 3b provides clear evidence for strengthening of Pt–H bonds upon increasing TTR, and the comparison of the data in Figures 3a and 4b leads to the same conclusion. It is seen that increasing of TTR to 370 K increases the Vd values in Figure 3a but decreases those in Figure 4b; in other words, the hydrogen desorbing at 300→770 K (Figure 3a) increases in amounts at the expense of weaker bound one (which desorbs at T ≤ 300 K, Figure 4b). The back increase in Vd,200→300 values upon raising TTR from 370 to 470 K (curves 3-5 in Figure 4b) is probably of the same origin; such conditions could stabilize hydrogen that earlier (TTR ≤ 370 K) left the sample during Ar purging at 200 K. Importantly, the alterations within adsorbed layer take place continuously in the whole temperature range, at least, at 200 K ≤ TTR ≤ 470 K (Figures 3 and 4b). The possibility to initiate them even at sub-ambient temperatures explains the paradoxical (at first sight) difference between H2 traces in Figure 4a. Here, the temperature of H2 pretreatment was restricted by 200 K, but the sample with adsorbed hydrogen was heated to 300 K in the course of desorption run. Obviously, such heating during the first run activates the mechanisms of rearrangements, and a part of desorbing hydrogen gets a chance to readsorb, thereby decreasing the area of experimentally seen desorption trace. The second run starts with already rearranged system and all extra hydrogen has now to evolve. 8 ACS Paragon Plus Environment

Page 8 of 37

Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Thus, the seemingly simple Pt/γ-Al2O3 system exhibits unexpectedly complex behavior, and the observed complexity suggests that the continuous alterations within the adsorbed layer probably result in and, simultaneously, from transformations in some other parts of the system.

Clusters’ restructuring. Hydrogen-induced relaxation of metal surfaces is a common phenomenon, which involves both the topmost and subsurface layers of metal atoms in single crystals27 and impacts bond lengths, strain, and disorder in Pt nanoparticles. 4-7,12 Since early studies,35,36 there have been many reports that micro- and nanometer-sized Pt particles may alter their shapes and surface structures in response to gas surroundings. 37-39 Small Pt particles on the surface of γ-Al2O3 (1-4 nm) undergo noncrystalline-to-crystalline transformations upon heating under H2 (Environmental-TEM data).40 Molecular dynamics and DFT simulations of Pt13 clusters on a (100)-face of γ-Al2O3 predict a reconstruction from a flat (biplanar in the given case) to cuboctahedral morphology upon increasing H/Pt ratio above 1.4.8 Very recently, in situ HERFD–XANES has been used to probe into different structures of the Pt13 cluster on (110) and (100) planes of γ-Al2O3, at different temperatures and pressures (25 and 500 ºC, 1 bar of H2 or vacuum).10 In solutions, the selective surface capping with appropriate ligands enables metal particles to be synthesized in a variety of shapes, including exotic ones, such as a crown.41 However, there are reasons to believe that the shape of small-volume metal particles is controlled by thermodynamics (interfacial energies) rather than kinetics, and the activation barrier for reshaping should be small. Most impressive is HRTEM study by Sun et al.,42 from which it follows that metal particles in nanometer-size range become capable of reversible pseudoelastic deformation: a triangularly shaped Ag nanocrystal (~10 nm) could be pressed mechanically into disc but then rapidly restored its original shape (for a few seconds at room temperature), the process being not induced by electron-beam effects but driven by diffusion of surface atoms. As relaxation time is thought to be proportional to the nanocrystal size to the fourth power,43 we should have had instantaneous response of our Pt clusters to H2-treatment conditions and, so, 9 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 37

would have had indistinguishable desorption curves for all samples (only the treatment at the final stage before TPD would be important, not PH2 or TTR at previous stages). As this is not the case (Figures 3-5), there must be a strong restrictive factor that does not let the tiny clusters act in their own way.

Processes at the metal–support interface. There is ample evidence in the literature that Pt particles strongly interact with alumina surface. The recent study of H2PtCl6-derived sample of 0.35%Pt/γ-Al2O3 by AC-HAADF-STEM has shown that Pt clusters are composed of only 1525 atoms even after reduction (H2) at 700 °C and do not change size when react with n-heptane for several hours at 500 °C.44 Such high resistance to sintering was recognized long ago45 and explains the indispensable role of Pt/γ-Al2O3 catalysts for high-temperature processes in industry (including naphtha reforming, which currently affords all high-octane petrol for automobiles)16. DFT calculations for PtN clusters on chlorinated γ-alumina reveal that the cluster’ energy per metal atom is almost independent of the cluster size (contrary to the common rule for unsupported particles: the bigger the particle, the lower its energy).3,46 Evidently, this is possible only if the clusters adopt a raft-like shape (in full accord with experimental observations)15,44 and Pt atoms form strong bonds with the surface Al/O atoms.3 The necessity to break the strong interaction between Pt atoms and the atoms of the support in order for new Pt–H bonds to be created explains the appearance of activation barriers for hydrogen adsorption and cluster’s restructuring. Mager-Maury et al.8 showed by DFT calculations that the metal–support interaction in Pt13/γ-Al2O3(100) is weakened upon insertion of hydrogen atoms between the Pt cluster and the support, and the number of Pt–(Al/O) bonds reduces to zero for Pt13H34. Their scheme is probably applicable to any ultradispersed Pt/γ-Al2O3 catalyst, yet, with a correction for complexities of real supported systems. In Figure 6, we show two of the possible structures for Pt cluster (corresponding to Pt13H0 and Pt13H20 in Fig. 1 of reference (8)) but indicate the possibility of disordering in support’ surface. The interaction of Pt


Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

atoms with a defective oxide surface should be stronger as compared to perfect ones and will affect the restructuring process and the resulting shape of the cluster. The calculations predict that H2 adsorption onto Pt13/γ-Al2O3(100) proceeds exothermically up to a coverage of 36-40 H atoms per cluster.8 It is close to the H/Pt values for unsupported Pt13 cluster in other theoretical studies.47 However, if it were easy to achieve such a high coverage in Pt/γ-Al2O3 catalysts, then the “freely flying” clusters (with all Pt–(Al/O) bonds disrupted) would rapidly aggregate, but this is at variance with the resistance of the catalysts to sintering. It implies that the approximation of the oxide surface by a perfect crystal plane underestimates the metal–support interaction. In real catalytic systems, not only different planes are exposed ((110) and (100) as the main ones for γ-Al2O3) but each of them has imperfections and defects, which will serve for preferential location of metal precursor and final metal particles (as AlIII sites for Pt1OxCly species)14. Moreover, the oxide supports behave as a chemical reactant during the catalyst preparation and influence the state of metal particles in a variety of ways.48,49 γ-Al2O3 partially dissolves both in acidic and basic media, and this can be prevented neither by precalcination nor pre-washing of the support.50 The dissolution-redeposition phenomena provoke the appearance of less-ordered surfaces and amorphous adlayers, near the metal particles or even over the metal surface (as detected51 for Pd/Al2O3 catalysts), and this should be reflected in the behavior of the particles. Taking these considerations and the experimental data in Figures 1-4 into account, the characteristic features of H2 adsorption onto ultradispersed Pt/γ-Al2O3 can be summarized as follows. The hydrogen adsorbs rapidly onto exposed Pt atoms, which easily relax, but the adsorption needs activation when involves re-arrangements at the metal–support interface. Such rearrangements are kinetically and thermodynamically limited due to the presence of defects and disordered oxide layers on the surface of the support. Only some Pt–(Al/O) bonds brake – depending on H2-treatment conditions, Pt-cluster size, and chemical surroundings of the cluster – and the decreasing number of connections between Pt and (Al/O) atoms does not necessary 11 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

accelerate further restructuring. This results in continuous activation barriers, which cannot be fully overcome, at least, at usual conditions.

Additional factors. In a sense, there is a “struggle” between the hydrogen and the support for Pt atoms, and H2 TPD offers a rare chance to watch such a struggle. It seems natural that a high H2 pressure was in favor of cluster’s restructuring and allowed Pt/γ-Al2O3 to adsorb hydrogen in larger amounts (the treatment in flowing H2 or by large-size H2-pulses, Figure 2a). It is also natural that heat treatments and subsequent cooling under H2, as in Figure 3, provide better conditions for achieving high hydrogen coverage, as compared to pulse chemisorption, under which conditions the H2 desorption prevails at elevated temperatures and lowers the H/Pt values (Figure 2b). Curiously enough, there is also a kind of self-inhibition, when the restructuring cannot be completed under high H2-pressure. Contrary to what is seen in Figure 4a, one can see in Figure 4c, set A, that the sample desorbs an appreciably smaller amount of H2 in repeated run (note an analogous difference between the first and second traces after O2/H2 titrations, runs (3) and (4) in Figure 5). It should be taken into account that during the first H2-treatment at 300 K and until the Ar purge at 200 K, the sample remained under H2 pressure, and Pt clusters probably remained in a transient state abundant in (and stabilized by) weakly-adsorbed species. The desorption of these species in the course of the run (i.e., upon heating to 300 K in Ar) destroys the metastable state and allows the restructuring to be completed. The strong promoting effect of O2/H2 titrations on cluster’s restructuring (as follows from the data in Figure 5 and Supporting Information S8) is also of interest. It may result from the adsorption of oxygen, which is more effective than H2 for reshaping Pt particles,39 but equally it may result from H2O formed during the titrations. The hydroxylation/dehydroxylation of oxide supports does influence the properties and behavior of supported metal particles.52 Flat lying Pt clusters are stabilized on the dehydrated (100) surface of γ-Al2O3, whereas a three-dimensional morphology is favored by the hydroxyl groups of the (110) surface.53 The surface of the support 12 ACS Paragon Plus Environment

Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

in our samples was highly dehydroxylated in the course of degassing at 770 K and should have a strong affinity towards water. The hydroxylation of newly uncovered Al/O sites could compensate them for the “loss” of Pt atoms, thereby facilitating hydrogen adsorption. It is sketched in Figure 6, where H atoms of H2 and H2O molecules are shown in different color, to distinguish them.

Alternatives. The analysis of the results in previous sections has shown that H2-TPD and other desorption data in Figures 1-5 are in agreement between themselves and with the complementary adsorption and titration measurements (Supporting Information S5-S9). They are also in accord with the literature data that have been obtained in recent years in the studies of Pt/γ-Al2O3 system by advanced physical techniques. Together, this lends a firm support to the suggestions and conclusions made, although it is not yet sufficient to leave alternative explanations out of attention. H2-TPD curves reported in the literature for supported catalysts of similar compositions may differ considerably, reflecting an impact from operational factors,28 and some peaks in TPD curves may be purely adventitious (e.g., resulting from decomposition of residual ammonia).54 To have meaningful results, we have selected alumina samples of very high purity and adopted very strict operational protocols (Supporting Information S1-S4). The reproducibility of H2-TPD curves in repeated runs with the same sample and similar results for Pt/γ-Al2O3 samples with different Pt loading and prepared on the supports from different sources provide convincing evidence against significant contribution from impurities or other side-effects (Supporting Information S5-S9 and detailed discussion in Supporting Information S10). Hence, any foreign species, even if presented, behaved as a spectator under our conditions, and the observations in Figures 2-5 reflect an intrinsic property of ultradispersed Pt/γ-Al2O3 system. It may be argued that H2O and thermal treatments promote hydrogen spillover from metal particles onto oxide supports , and the effects observed in Figures 3-5 might be due to that phenomenon (more precisely, to reverse spillover of spilt over hydrogen). However, a key role of 13 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

spillover is very unlikely under the conditions used in this study. Most important is that H2 treatments were conducted for short times (at a minute scale) and at temperatures TTR ≤ 470 K, which are not enough for hydrogen to spill over from Pt onto alumina or back, at least, in significant amounts.55,56 Besides, the TPD results (Figure 3) are well matched with the data on the extra adsorption (Figure 4), at which conditions (sub-ambient temperatures) hydrogen spills over at low/negligible rates even to reducible oxides.57

Final remarks. Activated adsorption of H2 on metal catalysts is not a new phenomenon, and its occurrence for Pt/γ-Al2O3 has been observed in early chemisorption studies.58,59 The effect was sometimes taken into account in dispersion measurements for Pt/γ-Al2O3 (presaturation by H2 at an elevated temperature, followed by measuring desorption isotherm at room temperature).60,61 However, in contrast to the catalysts with early transition metals,62 clear examples for Pt are scarce in the literature and the origin of the effect has not been found out. H2 TPD allows estimation of both the amount and energetics of adsorbed species,24,63 thereby providing richer information as compared to adsorption measurements. On one hand, the system under TPD conditions is the system “on the fly”, i. e., it is changing continuously in the course of desorption process, so that quantitative estimates of thermodynamic characteristics are problematic and physical meaning of the values obtained would be equivocal. It is especially so for metal nanoclusters, as the results show that their structure seriously alters upon H2 adsorption and has to transform back to the initial state during the desorption process. On the other hand, the potential of TPD as a method for studying supported metal catalysts seems underestimated. It does not allow direct observation of events beyond the adsorbed layer, and even the place of location of desorbing species is not certain; however, with a support from complementary techniques and literature data, TPD can provide deeper understanding of the events, as compared to what more powerful and expensive methods could do separately. There have been many studies of Pt/γ-Al2O3 catalysts with H2 TPD, using it for “fingerprints” purposes or to follow the possible effect of promoters and preparation variables. 14 ACS Paragon Plus Environment

Page 15 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

However, only a few previous studies were performed at varying conditions of H2 pretreatments,17-22,55 and all of them were focused on the effect of severe treatments (TTR > 670 K). Ehwald and Leibnitz21 reported a difference in H2-TPD curves for 0.5%Pt/γ-Al2O3 at TTR of 273, 473, and 773 K but tentatively ascribed it to Fe admixtures. Menon and Froment17 presented more such curves, but no regularities are seen either in shapes or areas of the curves at TTR within 300-470 K (judging from Fig. 4 of their paper), and this was probably caused by experimental reasons. Experimentally, H2 TPD is not as easy as adsorption measurements, and more efforts have to be done to find proper conditions and avoid misinterpretation of results. Our findings and the recommendations in Supporting Information may be helpful in analogous studies with other systems.

CONCLUSION The relatively simple adsorption/desorption methods have exhibited a greater potential for probing into metal nanoclusters than could be expected from previous studies. The data obtained confirm the restructuring of Pt clusters under H2 but reveal a complex and delicate interplay between contributing factors. Particularly, the restructuring process appears highly activated for the γ-alumina-supported clusters, and increasing in hydrogen coverage goes along with strengthening of Pt–H bonds. Such trends are likely to be general for supported clusters, and comparative studies with the other platinum-group metals, as well as with other supports which allow obtaining small metal clusters on the surface, would be of interest.

ASSOCIATED CONTENT Supporting Information Details of the sample preparation, desorption experiments, and control of impurities; sample characterization and testing results by the other methods; supplementary information on H2 TPD,


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

low-temperature desorption, and the effect of O2/H2 titrations; testing results of reference Pt/SiO2 and Cl-free Pt/γ-Al2O3 samples and reasoning against side effects.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected]; Tel/Fax: (+7) 383-3269529. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by FASO (project V.45.3.5). We appreciate Drs. I.L. Kraevskaya, A.V. Ishchenko, and D.S. Afanasev for XRF, TEM, EDX, and XRD data.

REFERENCES (1) Li, Y.; Somorjai, G. A. Nanoscale Advances in Catalysis and Energy Applications. Nano Lett. 2010, 10, 2289–2295. (2) Lu, Y.; Chen, W. Sub-nanometre Sized Metal Clusters: From Synthetic Challenges to the Unique Property Discoveries. Chem. Soc. Rev. 2012, 41, 3594–3623. (3) Chizallet, C.; Raybaud, P. Density Functional Theory Simulations of Complex Catalytic Materials in Reactive Environments: Beyond the Ideal Surface at Low Coverage. Catal. Sci. Technol. 2014, 4, 2797-2813. (4) Frenkel, A; Cason, M. W.; Elsen, A; Jung, U.; Small, M.W.; Nuzzo, R.G.; Vila, F.D.; Rehr, J.J.; Stach, E.A.; Yang, J.C. Critical Review: Effects of Complex Interactions on Structure and Dynamics of Supported Metal Catalysts. J. Vac. Sci. Techn. 2014, 32, 020801. (5) Tao, F.; Crozier, P. A. Atomic-Scale Observations of Catalysts Structures under Reaction Conditions and during Catalysis. Chem. Rev. 2016, 116, 3487–3539. 16 ACS Paragon Plus Environment

Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(6) Small, M. W.; Sanchez, S. I.; Marinkovic, N. S.; Frenkel A. I.; Nuzzo, R. G. Influence of Adsorbates on the Electronic Structure, Bond Strain, and Thermal Properties of an AluminaSupported Pt Catalyst. ACS Nano, 2012, 6, 5583–5595. (7) Cuenya, B. R.; Ortigoza, M. A.; Ono, L. K.; Behafarid, F.; Mostafa, S.; Croy, J. R.; Paredis, K.; Shafai, G.; Rahman, T. S.; Li, L.; Zhang, Z.; Yang, J. C. Thermodynamic Properties of Pt Nanoparticles: Size, Shape, Support, and Adsorbate Effects. Phys. Rev. B 2011, 84, 245438. (8) Mager-Maury, C.; Bonnard, G.; Chizallet, C.; Sautet, P.; Raybaud, P. H2-Induced Reconstruction of Supported Pt clusters: Metal–Support Interaction Versus Surface Hydride. ChemCatChem 2011, 3, 200–207. (9) Frenkel, A. I.; Small, M. W.; Smith, J. G.; Nuzzo, R. G.; Kvashnina, K. O.; Tromp, M. An in Situ Study of Bond Strains in 1 nm Pt Catalysts and Their Sensitivities to Cluster−Support and Cluster−Adsorbate Interactions. J. Phys. Chem. C 2013, 117, 23286−23294. (10) Gorczyca, A.; Moizan, V.; Chizallet, C.; Proux, O.; Net, W. D.; Lahera, E.; Hazemann, J.-L.; Raybaud, P.; Joly Y. Monitoring Morphology and Hydrogen Coverage of Nanometric Pt/γAl2O3 Particles by In Situ HERFD–XANES and Quantum Simulations. Angew. Chem. Int. Ed. 2014, 53, 12426–12429. (11) Small, M.W.; Kas, J.J.; Kvashnina, K.O.; Rehr, J.J.; Nuzzo, R.G.; Tromp, M.; Frenkel, A.I. Effects of Adsorbate Coverage and Bond-Length Disorder on the d-Band Center of CarbonSupported Pt Catalysts. ChemPhysChem 2014, 15, 1569–1572. (12) Lei, Y.; Zhao, H.; Rivas, R. D.; Lee, S.; Liu, B.; Lu, J.; Stach, E.; Winans, R. E.; Chapman, K. W.; Greeley, J. P.; Miller, J. T.; Chupas, P. J.; Elam, J. W. Adsorbate-Induced Structural Changes in 1−3 nm Platinum Nanoparticles. J. Am. Chem. Soc. 2014, 136, 9320−9326. (13) Cuenya, B. R.; Behafarid, F. Nanocatalysis: Size- and Shape-Dependent Chemisorption and Catalytic Reactivity. Surf. Sci. Rep. 2015, 70, 135–187. 17 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

(14) Kwak, J. H.; Hu, J.; Mei, D.; Yi, C.-W.; Kim, D. H.; Peden, C. H. F.; Allard, L. F.; Szanyi, J. Coordinatively Unsaturated Al3+ Centers as Binding Sites for Active Catalyst Phases of Platinum on γ-Al2O3. Science 2009, 325, 1670-1673. (15) Sinkler, W.; Sanchez, S.I.; Bradley, S.A.; Wen, J.; Mishra, B.; Kelly, S.D.; Bare, S.R. Aberration-Corrected Transmission Electron Microscopy and In Situ XAFS Structural Characterization of Pt/γ-Al2O3 Nanoparticles. ChemCatChem 2015, 7, 3779–3787. (16) Antos, G. J.; Aitani, A. M.; (Eds) ; Catalytic Naphtha Reforming; Dekker, N.Y., 2004. (17) Menon, P. G.; Froment, G. F. Modification of the Properties of Pt-Al2O3 Catalysts by Hydrogen at High Temperature. J. Catal. 1979, 59, 138–147. (18) Chen, H.-W.; White, J. M. TPD of Deuterium Spillover on Pt/Al2O3. J. Mol. Catal. 1986, 35, 355–364. (19) Levy, P.; Primet, M. States of Hydrogen Adsorption on Platinum-Alumina and Platinum-Ceria Catalysts: A Temperature-Programmed Desorption Study. Appl. Catal. 1991, 70, 263–276. (20) Miller, J. T.; Meyers, B. L.; Modica, F. S.; Lane, G. S.; Vaarkamp, M.; Koningsberger, D. C.; Hydrogen Temperature-Programmed Desorption (H2 TPD) of Supported Platinum Catalysts. J. Catal. 1993, 143, 395-408. (21) Ehwald, H.; Leibnitz, U. Hydrogen Chemisorption on Pt/Al2O3: Dependence of Hydrogen Desorption Behaviour on Its Surface Concentration, on Adsorption Temperature and on Surface Water. Catal. Lett. 1996, 38, 149–155. (22) Ferreira-Aparicio, P.; Guerrero-Ruiz A.; Rodriguez-Ramos, I. Hydrogen Adsorbed Species at the Metal/Support Interface on a Pt/Al2O3 Catalyst. J. Chem. Soc., Faraday Trans. 1997, 93, 3563–3567. (23) Alexeev, O.; Kim, D. W., Graham, G. W.; Shelef, M. ; Gates, B.C. TemperatureProgrammed Desorption of Hydrogen from Platinum Particles on γ-Al2O3: Evidence of Platinum-Catalyzed Dehydroxylation of γ-Al2O3. J. Catal. 1999, 185, 170–181. 18 ACS Paragon Plus Environment

Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(24) Derrouiche, S.; Bianchi, D. Heats of Adsorption Using Temperature Programmed Adsorption Equilibrium: Application to the Adsorption of CO on Cu/Al2O3 and H2 on Pt/Al2O3. Langmuir 2004, 20, 4489-4497. (25) Kalamaras, C. M.; Olympiou, G. G.; Efstathiou, A. M. The Water-Gas Shift Reaction on Pt/γ-Al2O3 Catalyst: Operando SSITKA–DRIFTS–Mass Spectroscopy Studies. Catal. Today 2008, 138, 228–234. (26) Hammer, B.; Norskov, J. K. Why Gold is the Noblest of All the Metals. Nature 1995, 376, 238–240. (27) Samorjai, G.A.; Li, Y. Introduction to Surface Chemistry and Catalysis, 2d Ed.; Wiley, 2010. (28) Bond, G.C. Metal-Catalysed Reactions of Hydrocarbons; Chapters 1-3; Springer, New York, 2005. (29) Pennemann, B.; Oster K.; Wandelt K. Activated Hydrogen Adsorption on the Pt(000) 1×1 surface. Surf. Sci. 1991, 251/252, 877-881. (30) Giannantonio, R.; Ragaini, V.; Magni, P. Dispersion Measurement by the Single Introduction Method Coupled with the Back-Sorption Procedure: A chemisorption and TPD Study of the Different Chemisorbed Hydrogen Species. I. Pt, Ru, and Rh on Alumina. J. Catal. 1994, 46, 103-115. (31) McLeod, A. S.; Gladden, L. F. A Monte Carlo Study of Temperature-Programmed Desorption from Supported-Metal catalysts. Catal. Lett. 1998, 55, 1–6. (32) Ji, Y.; van der Eerden, A. M. J.; Koot, V.; Kooyman, P. J.; Meeldijk, J. D.; Weckhuysen, B. M.; Koningsberger, D. C. Influence of Support Ionicity on the Hydrogen Chemisorption of Pt Particles Dispersed in Y Zeolite: Consequences for Pt Particle Size Determination Using the H/M Method. J. Catal., 2005, 234, 376–384.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

(33) Jensen, C.; Buck, D.; Dilger, H.; Bauer, M.; Phillipp, F.; Roduner E. Maximum Hydrogen Chemisorption on KL Zeolite Supported Pt Clusters. Chem. Commun. 2013, 49, 588590. (34) Mistry, H.; Behafarid, F.; Bare, S.R.; Cuenya, B.R. Pressure-Dependent Effect of Hydrogen Adsorption on Structural and Electronic Properties of Pt/γ-Al2O3 Nanoparticles. ChemCatChem 2014, 6, 348–352. (35) Sushumna, I.; Ruckenstain E. Redispersion of Pt/Alumina via Film Formation. J. Catal. 1987, 106, 77–96. (36) Shi A.-C., Masel R. I. The Effects of Gas Adsorption on Particle Shapes in Supported Platinum Catalysts. J. Catal. 1989, 120, 421–431. (37) Rupprechter, G.; Weilach, C. Spectroscopic Studies of Surface–Gas Interactions and Catalyst Restructuring at Ambient Pressure: Mind the Gap! J. Phys.: Condens. Matter 2008, 20, 184019 (17pp). (38) Cabie, M.; Giorgio, S.; Henry, C. R.; Axet, M. R.; Philippot, K.; Chaudret, B. Direct Observation of the Reversible Changes of the Morphology of Pt Nanoparticles under Gas Environment. J. Phys. Chem. C 2010, 114, 2160–2163. (39) Tao, F. F.; Nguyen, L.; Zhang, S.; Li, Y.; Tang, Y.; Zhang, L.; Frenkel, A. I.; Xia, Y.; Salmeron, M. Formation of Second-Generation Nanoclusters on Metal Nanoparticles Driven by Reactant Gases. Nano Lett. 2016, 16, 5001–5009. (40) Li, L.; Wang, L.-L.; Johnson, D. D.; Zhang, Z.; Sanchez, S. I.; Kang, J. H.; Nuzzo, R. G.; Wang, Q.; Frenkel, A. I.; Li, J.; Ciston, J.; Stach, E. A.; Yang, J. C. Noncrystalline-toCrystalline Transformations in Pt Nanoparticles. J. Am. Chem. Soc. 2013, 135, 13062−13072. (41) Chen, M.; Wu, B.; Yang, J.; Zheng, N. Small Adsorbate-Assisted Shape Control of Pd and Pt Nanocrystals. Adv. Mater. 2012, 24, 862–879.


Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(42) Sun, J; He, L.; Lo, Y.-C.; Xu, T.; Bi, H.; Sun, L.T.; Xhang, Z.; Mao, S.; Li, J. LiquidLike Pseudoelasticity of Sub-10-Nm Crystalline Silver Particles. Nature Mater. 2014, 13, 1007– 1012. (43) Henry, C. D. Deforming like liquid droplets. Nature Mater. 2014, 13, 999-1000. (44) Bradley, S. A.; Sinkler, W.; Blom, D. A.; Bigelow, W.; Voyles, P. M.; Allard, L. F. Behavior of Pt Atoms on Oxide Supports During Reduction Treatments at Elevated Temperatures, Characterized by Aberration Corrected Stem Imaging. Catal. Lett. 2012, 142, 176–182. (45) Dautzenberg, F. M.; Wolters, H. B. M. State of Dispersion of Platinum in AluminaSupported Catalysts. J. Catal. 1978, 51, 26–39. (46) Mager-Maury, C.; Chizallet, C.; Sautet, P.; Raybaud, P. Platinum Nanoclusters Stabilized on γ-Alumina by Chlorine Used As a Capping Surface Ligand: A Density Functional Theory Study. ACS Catal., 2012, 2, 1346–1357. (47) Chen, L.; Zhou, C.; Wu J.; Cheng, H. Hydrogen Adsorption and Desorption on the Pt and Pd Subnano Clusters – A Review. Front. Phys. China 2009, 4, 356–366. (48) Liu, J. Advanced Electron Microscopy of Metal–Support Interactions in Supported Metal Catalysts. ChemCatChem 2011, 3, 934–948. (49) Penner, S.; Armbruster, M. Formation of Intermetallic Compounds by Reactive Metal– Support Interaction: A Frequently Encountered Phenomenon in Catalysis. ChemCatChem 2014, 7, 374–392. (50) Xidong H.; Yongnian, Y.; Jiayu Z. Influence of Soluble Aluminium on the State and Surface Properties of Platinum in a Series of Reduced Platinum/Alumina Catalysts. Appl. Catal. 1988, 40, 291-313. (51) Ivanova, A. S.; Slavinskaya, E.M.; Gulyaev, R.V.; Zaikovskii, V.I.; Stonkus, O.A.; Danilova, I.G.; Plyasova, L.M.; Polukhina, I.A.; Boronin, A.I. Metal–support interactions in Pt/Al2O3 and Pd/Al2O3 catalysts for CO oxidation. Appl. Catal.B 2010, 97, 57–71. 21 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

(52) Sterrer, M.; Freund, H.-J. Towards Realistic Surface Science Models of Heterogeneous Catalysts: Influence of Support Hydroxylation and Catalyst Preparation Method. Catal. Lett. 2013, 143, 375–385. (53) Hu, C. H.; Chizallet, C.; Mager-Maury, C.; Valero, M. C.; Sautet, P.; Toulhoat, H.; Raybaud, P. Modulation of Catalyst Particle Structure upon Support Hydroxylation: Ab Initio Insights into Pd13 and Pt13/γ-Al2O3. J. Catal. 2010, 274, 99–110. (54) Miller, J. T.; Meyers, B. L.; Barr, M. K.; Modica, F. S.; Koningsberger, D.C. Hydrogen Temperature-Programmed Desorptions in Platinum Catalysts: Decomposition and Isotopic Exchange by Spillover Hydrogen of Chemisorbed Ammonia. J. Catal. 1996, 159, 41–49. (55) Kramer, R.; Andre, M. Adsorption of Atomic Hydrogen on Alumina by Hydrogen Spillover. J. Catal. 1979, 58, 287-295. (56) Prins, R.; Palfi, V. K.; Reiher, M. Hydrogen Spillover to Nonreducible Supports. J. Phys. Chem. C 2012, 116, 14274–14283. (57) Prins, R. Hydrogen Spillover. Facts and Fiction. Chem. Rev. 2012, 112, 2714−2738. (58) Dixon, L.T.; Barth, R.; Kokes, R.J.; Gryder, J.W. Hydrogen Adsorption by AluminaSupported Platinum. J. Catal. 1975, 37, 376-382. (59) Candy, J.-P.; Fouilloux, P.; Renouprez, A. J. Hydrogen Adsorption on Platinum Catalysts. Quantitative Determination of the Various Species Population. J.C.S. Faraday I 1980, 76, 616-629. (60) Kip, B. J.; Duivenvoorden F. B. M.; Koningsberger, D. C.; Prins, R. Determination of Metal Particle Size of Highly Dispersed Rh, Ir, and Pt Catalysts by Hydrogen Chemisorption and EXAFS. J. Catal. 1987, 105, 26-38. (61) Vaarkamp, M.; Miller, J. T.; Modica, F. S.; Koningsberger, D. C. On the Relation between Particle Morphology, Structure of the Metal-Support Interface, and Catalytic Properties of Pt/γ-Al2O3. J. Catal. 1996, 294, 294–305.


Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(62) Bartholomew, C.H. Hydrogen Adsorption on Supported Cobalt, Ion, and Nickel. Catal. Lett. 1990, 7, 27–52. (63) Kanervo, J.M.; Keskitalo, T.J.; Slioor, R.I.; Krause, A.O.I. Temperature-Programmed Desorption as a Tool to Extract Quantitative Kinetic or Energetic Information for Porous Catalysts. J. Catal. 2006, 238, 382–393.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure legends:

Figure 1. H2 pulse adsorption (a) and TPD of the hydrogen predosed in submonolayer and excessive quantities (b). In panel (b), the upper X-axis shows time after start of heating, and lower X-axis shows temperature in the middle of the sample; figures (N) specify the number of pulses (70 µl H2 stp each).

Figure 2. H2 TPD from 1%Pt/γ-Al2O3 pretreated at 295 K by H2-pulses of different size (a) and the results of chemisorption measurements by pulse technique at different temperatures (b). During treatments by 70-µl and 14-µl pulses, five such pulses were additionally given after the appearance of H2 in outgoing gas.

Figure 3. H2 TPD from 1%Pt/γ-Al2O3 pretreated in flowing H2 at different temperatures (TTR).

Figure 4. Low-temperature desorption (200→300 K) from 1%Pt/γ-Al2O3 pretreated in flowing H2 at varying conditions.

Figure 5. Changes in low-temperature H2-desorption from 1%Pt/γ-Al2O3 (200→300 K) in successive runs including O2 treatment after the second run.

Figure 6. A simplified scheme of the processes at the metal–support interface. Here, a Pt13 cluster is in contact with γ-Al2O3(100) plane and with amorphous oxide layer (shown in pale color); cross-hatching indicates the possibility of defects in the plane.


Page 24 of 37

Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100x81mm (150 x 150 DPI)

ACS Paragon Plus Environment

Page 26 of 37

Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

100x81mm (150 x 150 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100x109mm (150 x 150 DPI)


Page 28 of 37

Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

100x103mm (150 x 150 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100x109mm (150 x 150 DPI)


Page 30 of 37

Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

100x109mm (150 x 150 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

79x116mm (150 x 150 DPI)


Page 32 of 37

Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

79x116mm (150 x 150 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

79x116mm (150 x 150 DPI)


Page 34 of 37

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

100x109mm (150 x 150 DPI)


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

170x60mm (300 x 300 DPI)


Page 36 of 37

Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

84x47mm (300 x 300 DPI)


Probing the H2-Induced Restructuring of Pt Nanoclusters by H2-TPD

Recommend Documents