Bond-selective and surface-site-specific dissociation of methane on platinum

I present a molecular beam study of methane dissociation on differ-ent surface sites of several platinum single crystal surfaces (Pt(111), Pt(211), Pt(210), Pt(110)-(2x1)). The experiments were performed in a molecular beam/surface-science apparatus that combines rovibrational state-selective excitation of reactants with detection of the reaction products by reflection absorption infrared spectroscopy (RAIRS), Auger electron spectroscopy (AES) and King and Wells (K&W) technique. First, I explore how the barrier for methane dissociation depends on the Pt surface site (terrace, step, kink, and ridge atoms). For that, I compare the average reaction probabilities of methane on the different crystals using K&W. Moreover, I used the site-specific detection of chemisorbed methyl species by RAIRS to obtain the site-specific reactivity on steps, ridges and terraces. The highest methane reactivity and therefore the lowest barrier was observed for the surface atoms with the lowest coordination. The trans-lational energy dependence of the reactivity shows clear evidence for a direct activated mechanism on all the surface sites studied. The decreasing catalyt-ic activity for Pt atoms with increasing coordination number agrees well with theoretical predictions using first principles quantum theory calculations. Second, I present the role of rovibrational excitation of the incident methane on the chemisorption on different surface sites. For CH4, excitation of one quantum of the antisymmetric C-H stretch vibration (Îœ3) was found to be less efficient than an equivalent amount of translational energy normal to the sur-face for promoting the dissociation on all the surface sites. The vibrational efficacy was seen to be lower for dissociation on the low coordinated sites, in accordance with calculations of transition state geometries. For CH3D, the excitation of the antisymmetric C-H stretch vibration (Îœ4) led to bond selec-tivity on the steps and terraces of the Pt(211) surface. These results show how careful control of the translational energy and rovibrational state of the incident CH3D can be used for site-specific and bond-selective dissociation of methane. As predicted previously by theory but not observed before experi-mentally, the C-H bond selectivity is shown to decrease with increasing translational energy. Third, I explore the effect of surface temperature on the methane dissocia-tion. The sticking coefficient for a direct chemisorption reaction such as CH4 on Pt, might be expected to be independent of surface temperature, because the reaction happens very fast leaving very little time for the molecule to equilibrate with the surface. However, I observed an increase in methane reactivity with increasing surface temperature at low incident CH4 energies. This observation is interpreted by a âloss of coordinationâ from the atoms displaced out of the plane due to their vibrational motion when the surface is heated. The surface-site and quantum-state-specific data presented in this thesis are ideally suitable for testing theoretical models that aim to describe the me-thane-surface reaction at the microscopic level. Moreover, the characteriza-tion of different catalytically active sites for methane dissociation contrib-utes to advancing the understanding of methane reactivity towards real cata-lytic conditions, where surfaces with several different atomic terminations are used.

Quantum-state-resolved reactivity of overtone excited CH4 on Ni(111): Comparing experiment and theory

Rainer Beck, Phil Morten Hundt, Maarten Eduard van Reijzen

Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4 on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH4 in three different vibrational symmetry components, A1, E, and F2, of the 2ν3 antisymmetric stretch overtone vibration as well as in the ν1 + ν3 symmetric plus antisymmetric C–H stretch combination band of F2 symmetry. The quantum state specific dissociation probability S0 (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH4 dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S0 for the most reactive state ν1 + ν3 is an order of magnitude higher than for the least reactive, more energetic 2ν3-E state. Our first principles quantum scattering calculations show that as molecules in the ν1 state approach the surface, the vibrational amplitude becomes localized on the reacting C–H bond, making them very reactive. This behavior results from the weakening of the reacting C–H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C–H stretches. Similarly, we find that overtone normal mode states with more ν1 character are more reactive: S0(2ν1) > S0(ν1 + ν3) > S0(2ν3). The 2ν3 eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν1 and ν1 + ν3 normal modes, being of A1 and F2 symmetry, can contribute to the 2ν3-A1 and 2ν3-F2 eigenstates, respectively, boosting their reactivity over the E component, which contains no ν1 character due to symmetry.

American Institute of Physics2017

Methane dissociation on the steps and terraces of Pt(211) resolved by quantum state and impact site

Rainer Beck, Helen Jane Chadwick, Ana Gutiérrez González

Methane dissociation on the step and terrace sites of a Pt(211) single crystal was studied by reflection absorption infrared spectroscopy (RAIRS) at a surface temperature of 120 K. The C−H stretch RAIRS signal of the chemisorbed methyl product species was used to distinguish between adsorption on step and terrace sites allowing methyl uptake to be monitored as a function of incident kinetic energy for both sites. Our results indicate a direct dissociation mechanism on both sites with higher reactivity on steps than on terraces consistent with a difference in an activation barrier height of at least 30 kJ/mol. State-specific preparation of incident CH4 with one quantum of antisymmetric (ν3) stretch vibration further increases the CH4 reactivity enabling comparison between translational and vibrational activation on both steps and terraces. The reaction is modeled with first principles quantum theory that accurately describes dissociative chemisorption at different sites on the surface.

American Institute of Physics2018

Quantum state-resolved studies of direct and precursor-mediated dissociative chemisorption of silane on Si(100)

Thanh Tung Dang

In this work, I present the results of my studies on the state-resolved reactivity of silane (SiH4) on the Si(100)-(2x1) surface. The results demonstrate a co-existence of both a direct and a precursor mediated mechanisms for the dissociative chemisorptions of SiH4 on Si(100)-(2x1). Above an incident kinetic energy of 34 kJ/mol, where SiH4 chemisorption occurs via the direct mechanism, I find that both translation and vibrational energy activate the chemisorptions. Vibrational energy is slightly less effective than translation energy in promoting the reaction since adding 51.6 kJ/mol of vibrational energy leads to the same reactivity increases as adding 47 kJ/mol of translational energy. Vibrational excitation leads to at most a factor of 4 increase in reactivity of SiH4 on Si(100)-(2x1). While translational activation was reported before [Jones et. al, Chem. Phys. Lett., 1994, 229, 401], this is the first time vibrational activation is observed for SiH4 chemisorption on Si(100)-(2x1). I also investigate mode-specific reactivity by comparing the reactivity of SiH4 excited to two different nearly isoenergetic vibrational states, |2000> and |1100>. The results show that the reactivity of SiH4 excited to the |2000> state is consistently higher than that of the |1100> state over the range of incident kinetic energy studied. This mode-specific reactivity of SiH4 on Si(100)-(2x1) is rationalized in terms of different vibrational amplitude of the |2000> and the |1100> states: the former contains two quanta of Si-H stretch in a single Si-H bond, whereas the latter contains one quantum of Si-H stretch in each of two Si-H bonds. Below 34 kJ/mol, where SiH4 chemisorption on Si(100)x(2x1) occurs via the precursor-mediated mechanism, the effect of internal vibrational energy on the reactivity is found for the first time. While increasing translational energy decreases SiH4 reactivity, vibrational energy increases its reactivity. After the incident SiH4 molecule is trapped in the weakly bound precursor state, its vibrational energy appears to be conserved long enough that it can still promotes the chemisorption reaction once the trapped SiH4 encounters a reactive site. The chemisorption of SiH4 is, however, also weakly activated with vibrational energy in the precursor-mediated mechanism, in which a reactivity enhancement of less than a factor of 2 is observed for vibrationally excited SiH4. The use of isotopic selective excitation in deposition experiments to control the isotopic abundance of Si is not successful due to the fact that SiH4 chemisorption on Si(100)-(2x1) is weakly activated with vibrational energy. With a reactivity enhancement of at most a factor of 4, the relative isotope abundance of 28Si is expected to increase by only 0.6% in the laser excitation experiment. Such a small change in the natural isotopic abundance could not be detected reliably in the limited analysis time by our SIMS setup. However, this experiment can be applied to other system, where vibration excitation is more efficient to promote the reaction. Furthermore, vibrational activation of precursor-mediated gas-surface reaction discovered in this work will open up a new possibility to control gas-surface reaction of physisorbed species on surfaces.

EPFL2007

Quantum state-resolved studies of direct and precursor-mediated dissociative chemisorption of silane on Si(100)

Thanh Tung Dang

EPFL2007