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Water-Gas Shift Activity of Atomically Dispersed Cationic Platinum versus Metallic Platinum Clusters on Titania Supports Salai Cheettu Ammal, and Andreas Heyden ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02764 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016
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Water-Gas Shift Activity of Atomically Dispersed Cationic Platinum versus Metallic Platinum Clusters on Titania Supports
Salai Cheettu Ammal and Andreas Heyden1
Department of Chemical Engineering, University of South Carolina, 301 South Main Street, Columbia, South Carolina 29208, United States
_______________________________________ 1
Corresponding author. Email:
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ABSTRACT Atomically dispersed supported metal catalysts offer unique opportunities for designing highly selective catalysts and maximizing the utility of precious metals that have potential applications in a wide variety of industrial chemical reactions. Although substantial advances in understanding the origin of the activity of such highly dispersed metal catalysts has been made for a few chemical reactions, the reaction mechanisms and the nature of the active site―small metal clusters versus single atoms―are still highly debated. Using a combination of density functional theory and microkinetic modeling we confirm that a positively charged single Pt atom on TiO2(110) can exhibit a very high low-temperature activity for the water-gas shift reaction (TOF > 0.1 s-1 at 473 K). Comparison of these results with our work on TiO2 supported Pt cluster models provides clear evidence that different active sites are responsible for the experimentally observed activity at low and high temperatures. Finally, we explain why contradictory experimental conclusions have been reported.
KEYWORDS Single Pt cation, Pt-doped titania, Water-Gas Shift, microkinetic modeling, DFT.
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1. INTRODUCTION Single-site heterogeneous catalysts, in which single metal atoms are uniformly distributed on solid surfaces without mutual interaction, enable the advantages of heterogeneous catalysis―avoiding the separation challenge of the catalyst from the reaction medium―to be united with the main advantages of homogeneous catalysis―high utilization of precious metals and high selectivity due to high uniformity of the catalyst, and thus, become increasingly important in industrial applications such as fuel processing, chemicals production, and environmental applications.1-7 Recent reviews have summarized the use of such novel single-site metal and alloy catalysts stabilized on different supports for a variety of reactions including oxidation, hydrogenation, and water gas shift (WGS: CO + H2O ⇌ CO2 + H2).2,3,5,6 These studies demonstrated that the noble metal atoms usually exist as positively charged ions when dispersed onto open supports such as metal oxides or carbon materials. In particular, FlytzaniStephanopoulos and coworkers have shown in a series of studies that atomically dispersed Au and Pt cations on various supports are the active sites for the WGS.2,6-9 The WGS is an important industrial reaction for upgrading H2-rich fuel gas streams for fuel cells and in the production of many chemicals where the H2/CO ratio needs to be adjusted. Over the past decade, it has been shown that Pt-group metals supported on reducible oxides such as TiO2, CeO2, etc. are more active for the WGS at low temperatures (200-300 °C) compared to conventional Cu/ZnO catalysts.10-12 However, the exact nature and function of the active sites in these highly active catalysts is still a matter of debate and disagreements have been reported regarding the active site structure being single metal atoms or small metal nanoparticles. Fabrication of stable single atom catalysts has been a challenging task because of the metal atom’s high mobility which can lead to sintering under practical reaction conditions.13,14 In the
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case of Pt catalysts, it has been even more difficult to prepare isolated Pt atom sites on supports other than ceria without concurrent production of Pt nanoparticles,15 suggesting that the active sites in these catalysts could very well be the Pt nanoparticles and not the isolated Pt atoms. However, a few recent studies reported successful fabrication of single Pt atom catalysts on various supports that showed excellent activity for CO oxidation and WGS at low temperatures.7,16-18 In a recent experimental report, Flytzani-Stephanopoulos and coworkers claimed that a common atomic Pt(II)-O(OH)x species stabilized by sodium on both reducible and irreducible oxide supports and zeolites catalyzes the WGS reaction from ~120 to 400 °C.7 On the contrary, Ding et al. recently reported based on infrared (IR) spectroscopy measurements that on all of these supports only the Pt nanoparticles are active for low temperature CO oxidation and WGS, while isolated Pt atoms behave only as spectators.19 Using a combination of density functional theory (DFT) and microkinetic modeling techniques, we report here the activity of positively charged single Pt atoms on a reducible TiO2(110) surface for the WGS and compare these results to our earlier work on TiO2 supported Pt cluster models20,21 in order to determine under which conditions a specific active site is responsible for the majority of the observed experimental behavior. We find that positively charged single Pt atoms stabilized on a TiO2(110) surface can be as active as Pt clusters for the WGS at low temperatures. 2. MODELS AND METHODS 2.1. Computational Details Experimental studies on Pt nanoparticles supported on TiO2 have used different structures of the TiO2 support such as rutile, anatase or a mixture of anatase and rutile crystallites (Degussa P25) and reported a similar water-gas shift (WGS) activity for these different TiO2 supports.22-24 4 ACS Paragon Plus Environment
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Hence, we chose the rutile TiO2(110) slab model for our calculations. The TiO2(110) surface slab was modeled by a p(4 × 2) unit cell with 17 Å of vacuum and 12 atomic layers with the bottom three layers fixed in their bulk positions. All calculations were performed using the periodic plane wave code VASP 5.3.25-28 The exchange and correlation terms of DFT were described by the Perdew, Burke, and Ezhernof (PBE)29 functional and we used the projector augmented-wave (PAW) potentials30,31 to describe the core-electronic states. A 2×2×1 Monkhorst-Pack (MP)32 kmesh was used to sample the Brillouin zone and the integration over the first Brillouin zone used Gaussian smearing (σ = 0.05 eV) during structural relaxations. The cutoff in the plane wave expansion was 500 eV. Dipole and quadrupole corrections to the energy were taken into account using a modified version of the Makov and Payne method.33 Harris-Foulke-type corrections34 were included for the forces. The climbing image-NEB35 and Dimer methods36,37 were used to optimize the transition state structures. 2.2. Ab Initio Thermodynamic Analysis The formation of a Pt doped TiO2 surface under WGS reaction conditions was described as, + + 2 → + + 2 and the corresponding Gibbs free " # energy was calculated using the equation, ∆ = " #
− 2 % . Here,
!
!
+
! $
" # + 2 % − − ! '
" # and correspond to the zero-point corrected total ! '
energies of the Pt-doped ( and undoped ( surfaces, respectively.
! $
and represent the energies of a TiO2 bulk unit and a Pt atom, respectively. The Gibbs free energies of the gas molecules, % and %
./0 were calculated using ()' , = +,+
./0 12()' , 3 + 45 ln 8 9 :, where +,is the energy calculated using the PBE functional and
∆µgas (T,P0) can be calculated from the rotational, translational, and vibrational partition functions of the gas molecules. The partial pressure of the gas molecules, PCO=0.1, PH2O=0.2, 5 ACS Paragon Plus Environment
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and PH2=0.4 atm, used to calculate the Gibbs free energies were taken from the experimental report by Thinon et. al.22 The stability of Pt ion-doped TiO2 surface models compared to Pt adsorbed structures were also examined using the DFT+U methodology. Since earlier reports38-40 suggested that the reaction energies of doped TiO2 and the defect states of TiO2 are well described by a Hubbard U value between 2 to 3 eV, we used U=2.5 eV for the Ti d orbitals in our calculations.
More detailed information on the calculation of free energies of various
structures considered in the present work can be found in the supporting information. 2.3. Microkinetic Model Harmonic transition state theory was used to calculate the rate constants for the elementary surface reactions and collision theory with a sticking coefficient of 1 was used to calculate the rate constants for adsorption processes. A Master equation was then constructed and solved for the steady-state solution of probability densities, referred here as surface coverages (θ), for the system to occupy each discrete state.20,21 The steady-state equations were solved using the BzzMath library developed by Buzzi-Ferraris41 in order to get the surface coverages. The reaction rate (turnover frequency) of each pathway was calculated using the surface coverages. The apparent activation energy ();; ) was calculated from the overall rates over a temperature range of 473 K – 673 K using the expression, );; = ? @ABCDEFF =
:
,GH
, where the total
pressure (P) and the mole fraction of species i in the reaction mixtures (I ) were kept constant. The reaction order (αi) with respect to species i at a specific temperature (T) is calculated by varying the partial pressure of the species (pi) using the relation, J = 8
= >? @ABCDEFF = KL ;H
:
,;MNH
. The
partial pressures of all other species (pj≠i) are kept constant. Using Campbell’s theory,42-45 the
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R
“degree of rate control” for step i, XRC,i is calculated as, OPQ, = H 8 @
δ@ : δRH S
, where the
H,TMNH
equilibrium constant for step i, Ki, and all other rate constants, kj≠i, are held constant. 3. RESULTS AND DISCUSSION 3.1. Ab Initio Thermodynamic Analysis We start our quest for an appropriate catalyst model by investigating the interaction of a single Pt atom on the TiO2(110) surface under experimental reducing conditions using DFT and constrained thermodynamic calculations. Possible adsorption sites for a Pt atom on a TiO2(110) surface are next to an O atom or an oxygen vacancy and our calculations suggested that such adsorption results in a neutral Pt atom or a negatively charged Pt atom, respectively.46 In addition, Pt could interact with the TiO2 surface by replacing a five- or six-fold coordinated Ti atom (Ti5c and Ti6c) on the surface layer which will result in a positively charged Pt atom on the TiO2 surface. These positively charged Pt atoms on a TiO2 surface, also called a Pt doped TiO2 surface, is the focus of this study. In a recent review, McFarland and Metiu summarized the preparation methods and characterization of different cation doped metal oxides and their catalytic activity for oxidation reactions.47 In another series of studies, Hegde and coworkers have shown that the Pt ion-doped TiO2 catalysts can be synthesized using a solution combustion method and further demonstrated that these catalysts exhibit high catalytic activity for lowtemperature CO oxidation and the WGS reaction.48-50 Since the Ti atoms on the TiO2(110) surface are in a +4 oxidation state, replacing either a Ti5c or Ti6c atom by a Pt atom leads to the formation of a Pt3+/4+ or Pt4+ site, respectively. In order to understand the stability of such doped structures under WGS reaction conditions, Gibbs free energies for the formation of different
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structures were computed as a function of temperature (T) and partial pressures (P) of the hydrogen and water gas molecules (Figure 1).
Figure 1. Gibbs free energies (∆G) for the interaction of a Pt atom on TiO2(110) surface under WGS reaction conditions. Side view of the optimized structures together with the reactions used to calculate Gibbs free energies are provided in the inset. Ti64O128(s) and TiO2(b) are the clean TiO2(110) surface and a TiO2 bulk unit, respectively. QPt is the Bader charge on the Pt atom. Reaction conditions of PH2O=0.2 and PH2=0.4 atm were used to calculate ∆G. 8 ACS Paragon Plus Environment
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Our calculations suggested that simply replacing a surface Ti5c or Ti6c atom by Pt (Ti1xPtTi5cO2
and Ti1-xPtTi6cO2, Figures 1a & 1b) is thermodynamically less favorable than the
adsorption of Pt on a surface oxygen site (TiO2-Pt, Figure 1c) or an oxygen vacancy site (TiO2-xPt, Figure 1d). However, formation of an oxygen vacancy (Ti1-xPtO2-x, Figures 1e & 1f) or addition of H atoms to the TiO2 surface (Ti1-xPtTi5cO2-2H, Figure 1h) while replacing a surface Ti by Pt were found to be thermodynamically more favorable than the adsorption of Pt. In these structures, Pt is connected to four surface oxygens with a square planar geometry and the calculated Bader charge on Pt are close to the Bader charge on Pt in bulk PtO (+0.80).9 These observations agree well with the X-ray photoelectron spectroscopy results reported by Mukri et al.50 which confirmed that the Pt ions in Ti0.97Pt0.03O1.97 are in a +2 oxidation state and the local structure of the Pt2+ ion has a distorted square planar geometry. We note here that we considered multiple possibilities for the formation of a second oxygen vacancy and/or addition of up to six H atoms on the TiO2 surface model when computing the interaction of Pt at different adsorption positions and only present in Figure 1 the most stable structures identified from our calculations. Our results obtained from standard DFT (Figure 1) and also from DFT+U calculations (Figure S1 in the supporting information) clearly indicate that the Pt ion-doped TiO2 surface with two additional H atoms on the surface (Ti1-xPtTi5cO2-2H, Figure 1h) is the most stable structure below 500 K under reducing conditions. Since experimental studies on atomically dispersed Pt anchored on various supports have also suggested that the active site for the WGS is a positively charged Pt atom,2,7-9 we focused in the following our attention on the positively charged Pt atoms on a TiO2(110) surface. Next, we examined the effect of a CO chemical potential on the Pt ion-doped TiO2 surface and the most stable structures identified from our thermodynamic analysis are shown in Figure 2.
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All possible structures considered in the thermodynamic analysis and the reactions used to compute the free energies are summarized in the supporting information. Our calculations revealed that CO binds strongly to the Pt atom in a high oxidation state (Ti1-xPtTi5cO2-CO, Figure S2b) with an adsorption energy of -1.85 eV. If an oxygen vacancy is present in close proximity to the Pt atom, the adsorption energy decreases to -0.64 and -0.99 eV for Pt in the position of the Ti5c (Figure 2a) and Ti6c (Figure 2b) site, respectively. CO adsorption was found to be very weak )=' (" # = -0.07 eV) on the structure with two H atoms (Ti1-xPtO2-2H, Figure S2e in the supporting
information). Further analysis suggested that the most stable structures under WGS reaction conditions have either two CO molecules or one CO and one H atom adsorbed on the Pt atom as shown in Figures 2 (d) and (e). In both cases, the calculated Bader charge on Pt was found to be closer to that of Pt2+. Since Pt2+ prefers to have a square planar structure and two ligands (CO, H) are already adsorbed on Pt in these structures, this Pt is now connected to the TiO2 surface only through two Pt-O bonds. Flytzani-Stephanopoulos and coworkers7 recently demonstrated that single atom-centric Pt sites could be stabilized by Na on a TiO2 support and they observed a transformation of Pt(IV) to Pt(II) upon initiation of the WGS reaction with a change in Pt-O coordination from ~4 to 2. These observations agree with the stable structures, Ti1-xPtO2-2H2CO and Ti1-xPtO2-4H-CO (Figures, 2(d) & (e)) identified from our thermodynamic analysis at temperatures below 500 K. In another study, Ding et al.19 have used infrared (IR) spectroscopy to distinguish Pt single atoms from nanoparticles on various supports and reported that the IR spectra of CO adsorbed on a Pt/TiO2 catalyst revealed two sets of CO bands. It has been suggested that the IR bands corresponding to CO adsorbed on Ptδ+ (~2100 cm-1) are less redshifted from the gas phase value (2143 cm-1) than the IR bands corresponding to adsorbed CO on Pt nanoparticles (2050 to 2080 cm-1). We observed a similar trend in that the CO frequencies
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calculated for the Ti1-xPtO2-4H-CO (2089 cm-1) and Ti1-xPtO2-2H-2CO (2094 cm-1) structures are less red-shifted from our calculated CO gas molecule frequency (2153 cm-1) compared to the linearly adsorbed CO on a Pt cluster (2013 cm-1) that we reported earlier.46 Since Ti1-xPtO2-4HCO is the most stable structure identified from our thermodynamic analysis which also exhibits all characteristics of the active site proposed in the above experimental reports, we chose this structure as our initial catalyst model to investigate the WGS reaction mechanism. Further justification for the stability of this structure is provided in the supporting information.
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Figure 2. Gibbs free energies (∆G) for replacing a Ti atom by Pt on a TiO2(110) surface under WGS reaction conditions. Side view of the optimized structures are given in the inset and QPt is the Bader charge on the Pt atom. Reaction conditions of PCO=0.1, PH2O=0.2, and PH2=0.4 atm were used to calculate ∆G. 3.2. Reaction Pathways of the WGS at TiO2 Supported Pt2+ Sites Possible reaction pathways for the WGS are the redox, associative carboxyl and formate pathways (Figure S7 in the supporting information). On a reducible oxide supported metal catalyst, there is ample evidence that the support oxygen is involved in the reaction mechanism 12 ACS Paragon Plus Environment
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and thus, the reaction occurs via the redox pathway, also known as Mars-van Krevelen mechanism, where CO is first oxidized by a support oxygen and the reduced support is then reoxidized by H2O23,51-56. When surface –OH groups are present, an associative pathway with redox regeneration (or –OH group regeneration) is also a viable pathway in which a carboxyl or formate intermediate formation precedes the production of CO2 and H2.51,54,57-59 Using DFT and microkinetic modeling techniques, we have shown earlier that the WGS reaction at Pt/TiO2 interface sites occurs via a redox pathway20,21 and a combination of redox and associative carboxyl with redox regeneration mechanism operate at the Pt/CeO2 interface.60 In another classical associative mechanism, H2O dissociation occurs either on the metal nanoparticles or on the metal site of the oxide support and the –OH produced from H2O is used in the formation of a carboxyl/formate intermediate. The support oxygen is not removed during this process. We found in our earlier work that such a classical associative mechanism is not favorable on TiO2 supports relative to a redox mechanism since the H2O dissociation on the Ti site is similarly
endothermic (∆" #! = 0.19 YZ)21 as in the case of the Pt(111) surface ∆" #
=
0.52 YZ),61 whereas it is exothermic on oxygen vacancy sites (∆" #! = -0.83 eV).21 Thus, in the following we investigated the redox and the associative carboxyl/formate with redox regeneration pathways on the TiO2 supported single Pt catalyst model (Ti1-xPtO2-4H-CO) identified from our thermodynamic analysis.
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Figure 3. (Free) energy profiles for the redox pathway of the WGS reaction on the Ti1-xPtO24H-CO model (T = 500 K; Pj(gas) = 1 atm). All energies are with reference to the sum of the energies of the initial state (IM1) and the reactant gas molecules. The insets provide a side view of the optimized structures of the intermediates and the color code of the atoms is the same as in Figure 2. 14 ACS Paragon Plus Environment
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Figure 3 illustrates the calculated energy and Gibbs free energy profiles for the redox pathway at a representative temperature of 500 K together with the optimized structures of the intermediates. The transition state (TS) structures are numbered with reference to the elementary steps and are provided in Figure S11 in the supporting information. Since a H atom and a CO molecule are already present in our active Pt site, we refer to our initial reactant structure as (HCO)Pt (Figure 3). The redox pathway of our present model can be described in 9 elementary steps as shown in Figure 4 and Table S1 in the supporting information. In the reaction pathways considered here, three of the surface bridging oxygen atoms (Ob) adjacent to the Pt are involved in the mechanism and thus, we named these oxygen atoms as Ob1, Ob2, and Ob3, as shown in Figure 3. Our calculations revealed that the reaction of adsorbed CO in the reactant structure (HCO)Pt with the neighboring Ob1 to form CO2 and an oxygen vacancy is endothermic by 3.2 eV suggesting that this CO is not reactive. Thus, our redox pathway proceeds with the adsorption of a second CO molecule on Pt which reacts with the surface oxygen forming CO2 and an oxygen vacancy (R1-R3). Since Pt2+ prefers to have the square planar structure, adsorption of a second )=' = CO breaks one of the Pt-O bonds (IM2, Figure 3) and CO adsorbs weakly on the Pt2+ ion (" #
-0.27 eV). The two adsorbed CO molecules in IM2 exhibit different characteristics where one CO is strongly adsorbed with the Pt-C bond distance of 1.85 Å (νCO = 2105 cm-1) similar to that of the CO in IM1 and the second CO is weakly adsorbed with the Pt-C bond distance of 1.99 Å (νCO = 2049 cm-1). The characteristics of the strongly adsorbed CO is maintained in IM3 and IM4 where the weakly adsorbed CO reacts with the surface oxygen to form CO2 and an oxygen vacancy. H2O dissociation at the vacancy followed by H-transfer to Pt and H2 desorption completes the catalytic cycle of the redox pathway. The highest energy state in the free energy
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profile of the redox pathway (Figure 3) was found to be the TS corresponding to the CO2 desorption process (R3).
H
CO
CO
Ob Pt V Ti1-xO2 R3
IM4
-CO2
H Ob
CO H H H H +H2O +H2O O Pt O O b b b Ti1-xO2 R4-R6 IM7 R7
CO
O C H Pt O O b b Ti1-xO2
H
H
H
H Ob Pt V Ob Ti1-xO2
-CO2
CO
R14 H
IM14
Ob
CO H
O C H Pt O O b b Ti1-xO2
IM13 H Carboxyl Pathway -3 -1 Ob R13 Ob Pt Ob Ob 1.4 × 10 s (473 K) Redox Pathway O 1.5 × 10-2 s-1 (573 K) CO Ti O 1-x 2 C H H 3.0 × 10-1 s-1 (473 K) 5.9 × 10-2 s-1 (673 K) IM3 IM8 7.4 × 100 s-1 (573 K) Pt Ob Ob Ob 4.6 × 101 s-1 (673 K) R2 R8 -H2 Ti1-xO2 CO CO CO H H IM12 H H Pt O O O Pt b b b Ob Ob Ob Ti1-xO2 Ti1-xO2 R12 IM9 IM2 CO CO H CO R9 CO H H H Pt O O R1 H Pt O H O O b Ob b b +CO b b Ti1-xO2 Ti1-xO2 Ob Pt Ob Ob R10 Ti1-xO2 R11 IM11 H CO R26-R27 IM1 +CO H IM10 R17 Ob Pt Ob Ob O Ti1-xO2 CO C H H IM24 R24-R25 Ob Pt Ob Ob +H2O Ti1-xO2 CO Formate Pathway IM16 1.4 × 10-1 s-1 (473 K) 0 s-1 (573 K) Pt 1.2 × 10 V O Ob R18 b Ti1-xO2 4.1 × 100 s-1 (673 K) H
IM22
C
H Ob Pt Ob Ob Ti1-xO2
-H2
H
O
CO H
R23 CO H
IM17
Ob Pt V Ob Ti1-xO2 IM21
O H
R22 -CO2
CO H
H
C
Ob Pt Ob Ob Ti1-xO2 IM20
R21
CO H
CO H
O C
Ob Pt Ob Ob Ti1-xO2
R20
IM19
C
O
H R19
Ob Pt Ob Ob Ti1-xO2 IM18
Figure 4. Reaction network of various WGS reaction steps on a positively charged Pt atom supported on a Ti1-xO2(110) surface. 16 ACS Paragon Plus Environment
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Next, we examined the associative pathways with redox regeneration and formation of a carboxyl or formate intermediate. The energy diagrams of these pathways are provided in Figures S8 and S9 in the supporting information and a graphical representation is presented in Figure 4. The highest energy TSs in the carboxyl and formate pathways are the dissociation of the –COOH intermediate (R13) and the –HCOO dissociation (R21), respectively. 3.3. Insights from a Microkinetic Model based on DFT Data and Transition State Theory In order to identify the dominant reaction pathway and rate-controlling steps under experimental reaction conditions, we built a microkinetic reactor model based on DFT data and transition state theory including the 27 elementary steps described in Figure 4. The details of our approach including all elementary rate and equilibrium constants and surface coverages at different temperatures are summarized in Tables S2 and S3 in the supporting information. The rates calculated at different temperatures for the three pathways, shown in the inset of Figure 4, suggest that the classical redox pathway is the dominant pathway in the temperature range of 473-673 K and that high turnover frequencies are possible for this active site. The formate pathway with redox regeneration is preferred over the carboxyl pathway with redox regeneration and its rate is very close to the classical redox pathway at temperatures below 573 K. Next, we calculated an apparent activation barrier of 0.66 eV which is close to the experimental activation energy (0.81 eV) reported for the WGS catalyzed by single site Pt(II) catalysts on various supports both in the presence and absence of sodium.7 Furthermore, our calculations predicted a reaction order of 1 with respect to CO and zero with respect to H2O, H2, and CO2. Campbell’s degree of rate control analysis42-45 suggested that the CO2 desorption process, i.e., formation of a surface oxygen vacancy (R3) is rate-controlling for this catalyst model. Since the formate
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pathway with redox regeneration also contributes to the overall rate at low temperatures, the – HCOO dissociation process (R21) has the second largest effect on the overall rate. Table 1. Turnover Frequency (TOF), Apparent Activation Energy (Eapp), and Reaction Orders Calculated from Microkinetic Reactor Analysis of the WGS Reaction for Different Active Site Models of Pt Supported on a TiO2(110) surface. (PCO=0.1, PH2O=0.2, PCO2=0.1, PH2=0.4 atm)
Catalyst Model
TOF (s-1) 473 K
573 K
673 K
TiO2-Pt8-2CO (edge interface Pt)[a] TiO2-Pt8-2CO (corner interface Pt) [b] Ti1-xO2-Pt-CO4H (Pt2+)[c]
2.0×10-1
2.4×101
4.7×101
Experiment[d]
--
7.8×10 4.5×10
-3
-1
1.0×10 8.6×10
1
0
1.1×10 5.1×10
3
1
Eapp (eV)
Rate limiting step
0.57
Reaction orders (T= 573 K) CO
CO2
H2 O
H2
O-H bond dissociation
0.62
0.00
0.61
-0.19
1.63
CO2 desorption
0.01
0.00
0.01
0.00
0.66
CO2 desorption
1.00
0.00
0.04
0.00
-1
-0.61 -0.30 0.00 0.85 -0.67 6.0×10 20 [a] See ref. . Results presented here slightly differ from the values reported in reference 20 since we used here the same adsorption area per site (Sunit) for all three models which we consider more appropriate for a fair comparison, i.e., Sunit has been halved for the edge interface Pt model from its previous value. [b] See ref. 21. Active site models are provided in the Supporting Information for reference. [c] Current work [d] See ref. 22. In agreement with the experimental reports from Flytzani-Stephanopoulos and coworkers,2,7,9 our results provide clear evidence that atomically dispersed Pt2+ sites supported on TiO2(110) can exhibit high activity for the WGS in the temperature range of 473 – 673 K. However, Ding et al., using IR spectroscopy with CO as a probe molecule, showed the coexistence of single Pt atoms and nanoparticles on many conventional supports such as TiO2 and further claimed that only the Pt nanoparticles are active for CO oxidation and the WGS at low temperatures, i.e., Pt single atoms are not active19. This contradictory conclusion is based on their observation that only the Pt nanoparticle related CO IR peak/spectrum disappears during the WGS reaction at a temperature of up to 300 °C, while no change was observed in the Pt 18 ACS Paragon Plus Environment
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single atom related CO IR peak. To understand this contradicting experimental report, we compared our current results of the single Pt atom model with our earlier results20,21 obtained using a Pt8 cluster model supported on a TiO2(110) surface in Table 1. Two different active sites, Pt along the edge and a corner Pt at the Pt/TiO2 interface were considered in these studies that demonstrated that the redox pathway operates on both active sites. Experimental kinetic data of the WGS on Pt/TiO2 catalysts have been reported by various groups and we have shown in our earlier work that these results are very similar and that our interface model can predict the experimental data at different reaction conditions.20 Thus, we chose only one experimental reaction condition22 to compare the activity of different active site models. Turnover frequencies calculated for different active site models (Table 1) suggest that the interface Pt at the edge of our Pt8/TiO2 model and the single site Pt2+ model are active for the WGS at temperatures below 573 K. The corner interface Pt of the Pt8/TiO2 model becomes active at temperatures above 573 K. In other words, our calculations agree with Ding et al. in that supported Pt nanoparticles are highly active for the WGS―explaining the disappearance of the CO IR peak associated with Pt nanoparticles; however, these results also suggest that positively charged single Pt atoms stabilized on a reducible TiO2 surface can be as active as nanoclusters for the WGS at low temperatures which again agrees with the results reported by Flytzani-Stephanopoulos and coworkers. To understand why Ding et al. claimed that atomically dispersed Pt on titania supports is not active for the WGS, we note that in our single Pt atom model, one CO molecule acts as a ligand and remains strongly attached to the Pt atom throughout the catalytic cycle which explains the non-disappearance of the single Pt atom related CO peak observed by Ding et al. In fact, this strongly adsorbed CO reduces the adsorption strength of the second CO molecule on Pt which reacts easily with the neighboring oxygen atom 19 ACS Paragon Plus Environment
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to form CO2. In other words, the oxide support, one CO and one H atom act as ligands for a positively charged square planar Pt atom. These observations suggest the following potential experiments informed by DFT and microkinetic modeling: (1) It should be possible to identify a Pt carbonyl hydride species under WGS reaction conditions by IR in the frequency range of 2250-2350 cm-1 as informed by DFT-PBE calculations. (2) It should be possible to observe a red shift of about 600 cm-1 for the Pt carbonyl hydride frequency when feeding CO and D2O instead of CO and H2O as observed in the case of Rh carbonyl hydride complexes.62 Next, analysis of our microkinetic models of the WGS at the three active sites provided in Table 1 suggests that the adsorption strength of the reacting CO on Pt correlates with calculated reaction rates and apparent activation barriers. The reacting CO adsorption was found to be )=' )=' strong at corner Pt interface sites (" # = -1.47 eV; \33S = -0.52 eV),21 intermediate at edge Pt )=' )=' )=' interface sites (" # = -0.79 eV; \33S = 0.16 eV),20 and weak on single Pt2+ sites (" # = -0.27 )=' eV; \33S = 0.68 eV). As long as the CO adsorption free energy is positive, CO is not poisoning
the site and calculated rates are high and apparent activation barriers are low. In fact, our calculations predict very similar rates and activation barriers for the single Pt2+ and edge Pt interface sites. The stronger adsorption of CO at the corner Pt interface site seems to be responsible for the low reaction rate predicted at temperatures below 573 K and rates significantly increase at higher temperatures when CO adsorption becomes thermodynamically less favorable. The drastic increase in rate with temperature leads to a high apparent activation barrier for this site. Interestingly, formation of a surface oxygen vacancy during the CO2 desorption process seems to correlate with the rate-controlling step and the reaction orders. The vacancy formation process is thermodynamically favorable only at the interface edge Pt site where it is rapid. In contrast, this process becomes rate controlling for the corner Pt interface site 20 ACS Paragon Plus Environment
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and single Pt2+ site. It is for this reason that only the interface edge Pt active site can reproduce the experimentally observed reaction orders shown in Table 1. Finally, we note here that we used standard DFT with PBE functional to examine the WGS activity of different active sites of Pt/TiO2(110) catalysts. Since the redox pathway was found to be the dominant pathway on all these models, these catalytic cycles involve the formation of an oxygen vacancy on the TiO2(110) surface and we need to address the shortcomings of standard DFT methods based on the generalized gradient approximation (GGA) in describing the electronic structure and energetics of these oxygen vacancy structures. In our earlier work, we examined the effect of including a Hubbard-U term for the d orbitals of Ti by calculating the oxygen vacancy formation energies at the Pt8/TiO2(110) interface with different U values and found that the difference in vacancy formation energies calculated with and without the U parameter are less than 0.2 eV20. In our current work, we find that the oxygen vacancy formation energies for various Pt-doped structures calculated with DFT+U method (UTi = 2.5 eV) are about 0.2-0.3 eV higher than that of standard DFT calculations. Thus, we expect that the trends reported in this paper for different active site models will not be affected by the DFT functional. 4. CONCLUSIONS We find that a redox pathway operates on various types of active sites (single Pt cation and Pt clusters) of Pt/TiO2(110) catalysts. The interface edge Pt and single Pt2+ sites exhibit a high activity for the WGS at temperatures below 573 K while corner Pt interface sites become active at higher temperatures. Calculated reaction rates and apparent activation barriers for the single Pt2+ site are very similar to those at interface edge Pt sites, whereas the rate-controlling process for the single Pt2+ site is similar to the corner Pt interface site. Overall, the adsorption strength of the reacting CO on Pt and the stability of the oxygen vacancy structure play together 21 ACS Paragon Plus Environment
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an important role in determining the activity of these sites. While the CO adsorption strength determines the reaction rate and apparent activation barrier, the ability to form an oxygen vacancy on the oxide support seems to determine the rate-controlling process and the observed reaction orders. Since CO adsorption becomes thermodynamically less favorable and vacancy formation more favorable at higher temperatures, the corner interface Pt site becomes the most active site at temperatures above 573 K. Nevertheless, our findings confirm that single Pt2+ sites stabilized on a reducible surface such as TiO2(110) are active for the WGS at low temperatures where the formation of oxygen vacancies on the TiO2 surface plays a significant role in the WGS activity. Furthermore, our results suggest that the single Pt2+ sites stabilized on TiO2(110) with CO and H as ligands exhibit similar characteristics to homogeneous catalysts and thus, can possess the combined advantages of both homogeneous and heterogeneous catalysts. ASSOCIATED CONTENT Supporting Information Supplementary computational details of the Gibbs free energies calculated for the interaction of a Pt atom with the TiO2(110) surface (Figures S1-S3); migration of a H-Pt-CO unit along the TiO2 surface (Figure S4); different possibilities considered to examine the stability of adsorbed CO in the reactant structure (Figure S5); details of the microkinetic model used with the structure highlighting the atoms displaced for vibrational frequency calculations (Figure S6); possible reaction pathways of the WGS at a three-phase boundary (Figure S7); energy profiles calculated for the carboxyl (Figure S8) and formate (Figure S9) pathways with redox regeneration; structure of active site model of TiO2-Pt8-2CO (Figure S10); transition states involved in the redox pathway of the WGS reaction on the Ti1-xPtO2-4H-CO model (Figure S11); reaction energies and activation barriers for the 27 elementary steps (Table S1); forward rate constants and equilibrium 22 ACS Paragon Plus Environment
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constants calculated at different temperatures (Table S2); and surface coverages of various intermediates calculated at different temperatures (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail:
[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the National Science Foundation under Grant No. CBET-1254352 and in part by XSEDE resources provided by the National Institute for Computational Sciences (NICS), San Diego Supercomputer Center (SDSC), and Texas advanced Computing Center (TACC) under grant number TG-CTS090100. Furthermore, a portion of this research was performed at the U.S. Department of Energy facilities located at the National Energy Research Scientific Computing Center (NERSC).
Finally, computing resources from USC’s High
Performance Computing Group are gratefully acknowledged.
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